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PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression. They fall into two major
categories: list operators and named unary operators. These differ in their precedence relationship
with a following comma. (See the precedence table in perlop.) List operators take more than one
argument, while unary operators can never take more than one argument. Thus, a comma terminates the
argument of a unary operator, but merely separates the arguments of a list operator. A unary
operator generally provides a scalar context to its argument, while a list operator may provide
either scalar or list contexts for its arguments. If it does both, scalar arguments come first and
list argument follow, and there can only ever be one such list argument. For instance, splice() has
three scalar arguments followed by a list, whereas gethostbyname() has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context
for elements of the list) are shown with LIST as an argument. Such a list may consist of any
combination of scalar arguments or list values; the list values will be included in the list as if
each individual element were interpolated at that point in the list, forming a longer single-dimensional singledimensional
dimensional list value. Commas should separate literal elements of the LIST.
Any function in the list below may be used either with or without parentheses around its arguments.
(The syntax descriptions omit the parentheses.) If you use parentheses, the simple but occasionally
surprising rule is this: It looks like a function, therefore it is a function, and precedence doesn't
matter. Otherwise it's a list operator or unary operator, and precedence does matter. Whitespace
between the function and left parenthesis doesn't count, so sometimes you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For example, the third line above
produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither unary nor list operators.
These include such functions as "time" and "endpwent". For example, "time+86_400" always means
"time() + 86_400".
For functions that can be used in either a scalar or list context, nonabortive failure is generally
indicated in a scalar context by returning the undefined value, and in a list context by returning
the empty list.
Remember the following important rule: There is no rule that relates the behavior of an expression in
list context to its behavior in scalar context, or vice versa. It might do two totally different
things. Each operator and function decides which sort of value would be most appropriate to return
in scalar context. Some operators return the length of the list that would have been returned in
list context. Some operators return the first value in the list. Some operators return the last
value in the list. Some operators return a count of successful operations. In general, they do what
you want, unless you want consistency.
A named array in scalar context is quite different from what would at first glance appear to be a
list in scalar context. You can't get a list like "(1,2,3)" into being in scalar context, because
the compiler knows the context at compile time. It would generate the scalar comma operator there,
not the list construction version of the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the same name
(like chown(2), fork(2), closedir(2), etc.) all return true when they succeed and "undef" otherwise,
as is usually mentioned in the descriptions below. This is different from the C interfaces, which
return "-1" on failure. Exceptions to this rule are "wait", "waitpid", and "syscall". System calls
also set the special $! variable on failure. Other functions do not, except accidentally.
Extension modules can also hook into the Perl parser to define new kinds of keyword-headed
expression. These may look like functions, but may also look completely different. The syntax
following the keyword is defined entirely by the extension. If you are an implementor, see
"PL_keyword_plugin" in perlapi for the mechanism. If you are using such a module, see the module's
documentation for details of the syntax that it defines.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like some keywords and named
operators) arranged by category. Some functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst", "length", "oct", "ord", "pack",
"q//", "qq//", "reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt", "srand"
Functions for real @ARRAYs
"each", "keys", "pop", "push", "shift", "splice", "unshift", "values"
Functions for list data
"grep", "join", "map", "qw//", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof", "fileno", "flock", "format",
"getc", "print", "printf", "read", "readdir", "rewinddir", "say", "seek", "seekdir", "select",
"syscall", "sysread", "sysseek", "syswrite", "tell", "telldir", "truncate", "warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat", "mkdir",
"open", "opendir", "readlink", "rename", "rmdir", "stat", "symlink", "sysopen", "umask",
"unlink", "utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto", "last", "next", "redo",
"return", "sub", "wantarray"
Keywords related to switch
"break", "continue", "given", "when", "default"
(These are available only if you enable the "switch" feature. See feature and "Switch
statements" in perlsyn.)
Keywords related to scoping
"caller", "import", "local", "my", "our", "state", "package", "use"
("state" is available only if the "state" feature is enabled. See feature.)
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our", "reset", "scalar", "state", "undef",
"wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx//", "setpgrp",
"setpriority", "sleep", "system", "times", "wait", "waitpid"
Keywords related to Perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientation
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen", "recv",
"send", "setsockopt", "shutdown", "socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop", "shmctl", "shmget",
"shmread", "shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent", "getgrgid", "getgrnam",
"getlogin", "getpwent", "getpwnam", "getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent", "getnetbyaddr",
"getnetbyname", "getnetent", "getprotobyname", "getprotobynumber", "getprotoent",
"getservbyname", "getservbyport", "getservent", "sethostent", "setnetent", "setprotoent",
"setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "break", "chomp", "chr", "continue", "default", "exists", "formline", "given",
"glob", "import", "lc", "lcfirst", "lock", "map", "my", "no", "our", "prototype", "qr//", "qw//",
"qx//", "readline", "readpipe", "ref", "sub"*, "sysopen", "tie", "tied", "uc", "ucfirst",
"untie", "use", "when"
* "sub" was a keyword in Perl 4, but in Perl 5 it is an operator, which can be used in
expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix
environments, the functionality of some Unix system calls may not be available, or details of the
available functionality may differ slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump", "endgrent",
"endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork",
"getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr", "getnetbyname",
"getnetent", "getppid", "getpgrp", "getpriority", "getprotobynumber", "getprotoent", "getpwent",
"getpwnam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link",
"lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select",
"semctl", "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp", "setpriority",
"setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
"socket", "socketpair", "stat", "symlink", "syscall", "sysopen", "system", "times", "truncate",
"umask", "unlink", "utime", "wait", "waitpid"
For more information about the portability of these functions, see perlport and other available
platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X DIRHANDLE
-X A file test, where X is one of the letters listed below. This unary operator takes one argument,
either a filename, a filehandle, or a dirhandle, and tests the associated file to see if
something is true about it. If the argument is omitted, tests $_, except for "-t", which tests
STDIN. Unless otherwise documented, it returns 1 for true and '' for false, or the undefined
value if the file doesn't exist. Despite the funny names, precedence is the same as any other
named unary operator. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by
default based solely on the mode of the file and the uids and gids of the user. There may be
other reasons you can't actually read, write, or execute the file: for example network filesystem
access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable
formats. Note that the use of these six specific operators to verify if some operation is
possible is usually a mistake, because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests
always return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run
by the superuser may thus need to do a stat() to determine the actual mode of the file, or
temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results
than the bare stat() mode bits. When under the "use filetest 'access'" the above-mentioned
filetests test whether the permission can (not) be granted using the access(2) family of system
calls. Also note that the "-x" and "-X" may under this pragma return true even if there are no
execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to
the underlying system calls' definitions. Note also that, due to the implementation of "use
filetest 'access'", the "_" special filehandle won't cache the results of the file tests when
this pragma is in effect. Read the documentation for the "filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still works as
expected, however: only single letters following a minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The first block or so of the file is examined for
odd characters such as strange control codes or characters with the high bit set. If too many
strange characters (>30%) are found, it's a "-B" file; otherwise it's a "-T" file. Also, any
file containing a zero byte in the first block is considered a binary file. If "-T" or "-B" is
used on a filehandle, the current IO buffer is examined rather than the first block. Both "-T"
and "-B" return true on an empty file, or a file at EOF when testing a filehandle. Because you
have to read a file to do the "-T" test, on most occasions you want to use a "-f" against the
file first, as in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat" operators) are given the special
filehandle consisting of a solitary underline, then the stat structure of the previous file test
(or stat operator) is used, saving a system call. (This doesn't work with "-t", and you need to
remember that lstat() and "-l" leave values in the stat structure for the symbolic link, not the
real file.) (Also, if the stat buffer was filled by an "lstat" call, "-T" and "-B" will reset it
with the results of "stat _"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file test operators, in a
way that "-f -w -x $file" is equivalent to "-x $file && -w _ && -f _". (This is only fancy fancy:
if you use the return value of "-f $file" as an argument to another filetest operator, no special
magic will happen.)
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as accept(2) does. Returns the packed address if it
succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in
perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm
Arranges to have a SIGALRM delivered to this process after the specified number of wallclock
seconds has elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some
machines, unfortunately, the elapsed time may be up to one second less or more than you specified
because of how seconds are counted, and process scheduling may delay the delivery of the signal
even further.)
Only one timer may be counting at once. Each call disables the previous timer, and an argument
of 0 may be supplied to cancel the previous timer without starting a new one. The returned value
is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting
from Perl 5.8 part of the standard distribution) provides ualarm(). You may also use Perl's
four-argument version of select() leaving the first three arguments undefined, or you might be
able to use the "syscall" interface to access setitimer(2) if your system supports it. See
perlfaq8 for details.
It is usually a mistake to intermix "alarm" and "sleep" calls, because "sleep" may be internally
implemented on your system with "alarm".
If you want to use "alarm" to time out a system call you need to use an "eval"/"die" pair. You
can't rely on the alarm causing the system call to fail with $! set to "EINTR" because Perl sets
up signal handlers to restart system calls on some systems. Using "eval"/"die" always works,
modulo the caveats given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar
relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3) manpage for
more information.
bind SOCKET,NAME
Binds a network address to a socket, just as bind(2) does. Returns true if it succeeded, false
otherwise. NAME should be a packed address of the appropriate type for the socket. See the
examples in "Sockets: Client/Server Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the
run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression,
the value is taken as the name of the filehandle. Returns true on success, otherwise it returns
"undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based systems) binmode() is necessary when you're
not working with a text file. For the sake of portability it is a good idea to always use it
when appropriate, and to never use it when it isn't appropriate. Also, people can set their I/O
to be by default UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data, like for example images.
If LAYER is present it is a single string, but may contain multiple directives. The directives
alter the behaviour of the filehandle. When LAYER is present using binmode on a text file makes
sense.
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing binary
data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to
Unicode characters). Note that, despite what may be implied in "Programming Perl" (the Camel,
3rd edition) or elsewhere, ":raw" is not simply the inverse of ":crlf". Other layers that would
affect the binary nature of the stream are also disabled. See PerlIO, perlrun, and the discussion
about the PERLIO environment variable.
The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are called I/O
layers. The "open" pragma can be used to establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as "DISCIPLINE" in "Programming Perl,
3rd Edition". However, since the publishing of this book, by many known as "Camel III", the
consensus of the naming of this functionality has moved from "discipline" to "layer". All
documentation of this version of Perl therefore refers to "layers" rather than to "disciplines".
Now back to the regularly scheduled documentation...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(utf8)". ":utf8" just marks the data as
UTF-8 without further checking, while ":encoding(utf8)" checks the data for actually being valid
UTF-8. More details can be found in PerlIO::encoding.
In general, binmode() should be called after open() but before any I/O is done on the filehandle.
Calling binmode() normally flushes any pending buffered output data (and perhaps pending input
data) on the handle. An exception to this is the ":encoding" layer that changes the default
character encoding of the handle, see open. The ":encoding" layer sometimes needs to be called
in mid-stream, and it doesn't flush the stream. The ":encoding" also implicitly pushes on top of
itself the ":utf8" layer because internally Perl operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all work together to
let the programmer treat a single character ("\n") as the line terminator, irrespective of the
external representation. On many operating systems, the native text file representation matches
the internal representation, but on some platforms the external representation of "\n" is made up
of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a single character to end each line
in the external representation of text (even though that single character is CARRIAGE RETURN on
Mac OS and LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS and the various
flavors of MS-Windows your program sees a "\n" as a simple "\cJ", but what's stored in text files
are the two characters "\cM\cJ". That means that, if you don't use binmode() on these systems,
"\cM\cJ" sequences on disk will be converted to "\n" on input, and any "\n" in your program will
be converted back to "\cM\cJ" on output. This is what you want for text files, but it can be
disastrous for binary files.
Another consequence of using binmode() (on some systems) is that special end-of-file markers will
be seen as part of the data stream. For systems from the Microsoft family this means that if
your binary data contains "\cZ", the I/O subsystem will regard it as the end of the file, unless
you use binmode().
binmode() is important not only for readline() and print() operations, but also when using
read(), seek(), sysread(), syswrite() and tell() (see perlport for more details). See the $/ and
"$\" variables in perlvar for how to manually set your input and output line-termination
sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME
package. If CLASSNAME is omitted, the current package is used. Because a "bless" is often the
last thing in a constructor, it returns the reference for convenience. Always use the two-argument twoargument
argument version if a derived class might inherit the function doing the blessing. See perltoot
and perlobj for more about the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all
lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase
names. To prevent confusion, you may wish to avoid such package names as well. Make sure that
CLASSNAME is a true value.
See "Perl Modules" in perlmod.
break
Break out of a "given()" block.
This keyword is enabled by the "switch" feature: see feature for more information.
caller EXPR
caller
Returns the context of the current subroutine call. In scalar context, returns the caller's
package name if there is a caller (that is, if we're in a subroutine or "eval" or "require") and
the undefined value otherwise. In list context, returns
# 0 1 2
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The
value of EXPR indicates how many call frames to go back before the current one.
# 0 1 2 3 4
($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine call, but an "eval". In such a
case additional elements $evaltext and $is_require are set: $is_require is true if the frame is
created by a "require" or "use" statement, $evaltext contains the text of the "eval EXPR"
statement. In particular, for an "eval BLOCK" statement, $subroutine is "(eval)", but $evaltext
is undefined. (Note also that each "use" statement creates a "require" frame inside an "eval
EXPR" frame.) $subroutine may also be "(unknown)" if this particular subroutine happens to have
been deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The
$hints and $bitmask values are subject to change between versions of Perl, and are not meant for
external use.
$hinthash is a reference to a hash containing the value of "%^H" when the caller was compiled, or
"undef" if "%^H" was empty. Do not modify the values of this hash, as they are the actual values
stored in the optree.
Furthermore, when called from within the DB package, caller returns more detailed information: it
sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before "caller" had a chance to
get the information. That means that caller(N) might not return information about the call frame
you expect it to, for "N > 1". In particular, @DB::args might have information from the previous
time "caller" was called.
Also be aware that setting @DB::args is best effort, intended for debugging or generating
backtraces, and should not be relied upon. In particular, as @_ contains aliases to the caller's
arguments, Perl does not take a copy of @_, so @DB::args will contain modifications the
subroutine makes to @_ or its contents, not the original values at call time. @DB::args, like @_,
does not hold explicit references to its elements, so under certain cases its elements may have
become freed and reallocated for other variables or temporary values. Finally, a side effect of
the current implementation means that the effects of "shift @_" can normally be undone (but not
"pop @_" or other splicing, and not if a reference to @_ has been taken, and subject to the
caveat about reallocated elements), so @DB::args is actually a hybrid of the current state and
initial state of @_. Buyer beware.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory
specified by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}.
(Under VMS, the variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) If neither is
set, "chdir" does nothing. It returns true on success, false otherwise. See the example under
"die".
On systems that support fchdir(2), you may pass a filehandle or directory handle as argument. On
systems that don't support fchdir(2), passing handles raises an exception.
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numerical
mode, which should probably be an octal number, and which definitely should not be a string of
octal digits: 0644 is okay, but "0644" is not. Returns the number of files successfully changed.
See also "oct", if all you have is a string.
$cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
$mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
$mode = "0644"; chmod oct($mode), "foo"; # this is better
$mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass filehandles among the files. On systems that
don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as
globs or glob references to be recognized; barewords are considered filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the "Fcntl" module:
use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.
chomp VARIABLE
chomp( LIST )
chomp
This safer version of "chop" removes any trailing string that corresponds to the current value of
$/ (also known as $INPUT_RECORD_SEPARATOR in the "English" module). It returns the total number
of characters removed from all its arguments. It's often used to remove the newline from the end
of an input record when you're worried that the final record may be missing its newline. When in
paragraph mode ("$/ = """), it removes all trailing newlines from the string. When in slurp mode
("$/ = undef") or fixed-length record mode ($/ is a reference to an integer or the like, see
perlvar) chomp() won't remove anything. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is
returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable.
This is because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as
"chomp( $cwd = `pwd` )" which you might expect. Similarly, "chomp $a, $b" is interpreted as
"chomp($a), $b" rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop
Chops off the last character of a string and returns the character chopped. It is much more
efficient than "s/.$//s" because it neither scans nor copies the string. If VARIABLE is omitted,
chops $_. If VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is returned.
Note that "chop" returns the last character. To return all but the last character, use
"substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the
numeric uid and gid, in that order. A value of -1 in either position is interpreted by most
systems to leave that value unchanged. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass filehandles among the files. On systems that
don't support fchown(2), passing filehandles raises an exception. Filehandles must be passed as
globs or glob references to be recognized; barewords are considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the
superuser, although you should be able to change the group to any of your secondary groups. On
insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On
POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the character set. For example, "chr(65)" is
"A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face.
Negative values give the Unicode replacement character (chr(0xfffd)), except under the bytes
pragma, where the low eight bits of the value (truncated to an integer) are used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8
for backward compatibility reasons.
See perlunicode for more about Unicode.
chroot FILENAME
chroot
This function works like the system call by the same name: it makes the named directory the new
root directory for all further pathnames that begin with a "/" by your process and all its
children. (It doesn't change your current working directory, which is unaffected.) For security
reasons, this call is restricted to the superuser. If FILENAME is omitted, does a "chroot" to
$_.
close FILEHANDLE
close
Closes the file or pipe associated with the filehandle, flushes the IO buffers, and closes the
system file descriptor. Returns true if those operations have succeeded and if no error was
reported by any PerlIO layer. Closes the currently selected filehandle if the argument is
omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it,
because "open" closes it for you. (See "open".) However, an explicit "close" on an input file
resets the line counter ($.), while the implicit close done by "open" does not.
If the filehandle came from a piped open, "close" returns false if one of the other syscalls
involved fails or if its program exits with non-zero status. If the only problem was that the
program exited non-zero, $! will be set to 0. Closing a pipe also waits for the process
executing on the pipe to exit--in case you wish to look at the output of the pipe afterwards--and
implicitly puts the exit status value of that command into $? and "${^CHILD_ERROR_NATIVE}".
Closing the read end of a pipe before the process writing to it at the other end is done writing
results in the writer receiving a SIGPIPE. If the other end can't handle that, be sure to read
all the data before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the
real filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just like connect(2). Returns true if it succeeded,
false otherwise. NAME should be a packed address of the appropriate type for the socket. See
the examples in "Sockets: Client/Server Communication" in perlipc.
continue BLOCK
continue
"continue" is actually a flow control statement rather than a function. If there is a "continue"
BLOCK attached to a BLOCK (typically in a "while" or "foreach"), it is always executed just
before the conditional is about to be evaluated again, just like the third part of a "for" loop
in C. Thus it can be used to increment a loop variable, even when the loop has been continued
via the "next" statement (which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block; "last" and "redo" behave as if
they had been executed within the main block. So will "next", but since it will execute a
"continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is equivalent to using an empty one, logically enough, so "next"
goes directly back to check the condition at the top of the loop.
If the "switch" feature is enabled, "continue" is also a function that exits the current "when"
(or "default") block and falls through to the next one. See feature and "Switch statements" in
perlsyn for more information.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the "Math::Trig::acos()" function, or use this
relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming that you
actually have a version there that has not been extirpated as a potential munition).
crypt() is a one-way hash function. The PLAINTEXT and SALT is turned into a short string, called
a digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but
there is no (known) way to get the original PLAINTEXT from the hash. Small changes in the
PLAINTEXT or SALT will result in large changes in the digest.
There is no decrypt function. This function isn't all that useful for cryptography (for that,
look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is a bit of a misnomer.
Instead it is primarily used to check if two pieces of text are the same without having to
transmit or store the text itself. An example is checking if a correct password is given. The
digest of the password is stored, not the password itself. The user types in a password that is
crypt()'d with the same salt as the stored digest. If the two digests match the password is
correct.
When verifying an existing digest string you should use the digest as the salt (like
"crypt($plain, $digest) eq $digest"). The SALT used to create the digest is visible as part of
the digest. This ensures crypt() will hash the new string with the same salt as the digest.
This allows your code to work with the standard crypt and with more exotic implementations. In
other words, do not assume anything about the returned string itself, or how many bytes in the
digest matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11
bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of PLAINTEXT mattered. But
alternative hashing schemes (like MD5), higher level security schemes (like C2), and
implementations on non-Unix platforms may produce different strings.
When choosing a new salt create a random two character string whose characters come from the set
"[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This
set of characters is just a recommendation; the characters allowed in the salt depend solely on
your system's crypt library, and Perl can't restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this program knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for hashing large quantities of data, not least of all because
you can't get the information back. Look at the Digest module for more robust algorithms.
If using crypt() on a Unicode string (which potentially has characters with codepoints above
255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the
string back to an eight-bit byte string before calling crypt() (on that copy). If that works,
good. If not, crypt() dies with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name
of the hash. (Unlike normal "open", the first argument is not a filehandle, even though it looks
like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If
the database does not exist, it is created with protection specified by MASK (as modified by the
"umask"). If your system supports only the older DBM functions, you may make only one "dbmopen"
call in your program. In older versions of Perl, if your system had neither DBM nor ndbm,
calling "dbmopen" produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them.
If you want to test whether you can write, either use file tests or try setting a dummy hash
entry inside an "eval" to trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on large DBM
files. You may prefer to use the "each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the various dbm
approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value "undef".
If EXPR is not present, $_ is checked.
Many operations return "undef" to indicate failure, end of file, system error, uninitialized
variable, and other exceptional conditions. This function allows you to distinguish "undef" from
other values. (A simple Boolean test will not distinguish among "undef", zero, the empty string,
and "0", which are all equally false.) Note that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: "pop" returns "undef" when its argument is
an empty array, or when the element to return happens to be "undef".
You may also use "defined(&func)" to check whether subroutine &func has ever been defined. The
return value is unaffected by any forward declarations of &func. A subroutine that is not
defined may still be callable: its package may have an "AUTOLOAD" method that makes it spring
into existence the first time that it is called; see perlsub.
Use of "defined" on aggregates (hashes and arrays) is deprecated. It used to report whether
memory for that aggregate has ever been allocated. This behavior may disappear in future
versions of Perl. You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined, not whether the key
exists in the hash. Use "exists" for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are surprised to discover that the number 0
and "" (the zero-length string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds and $1 is defined, although it matched "nothing". It didn't really
fail to match anything. Rather, it matched something that happened to be zero characters long.
This is all very above-board and honest. When a function returns an undefined value, it's an
admission that it couldn't give you an honest answer. So you should use "defined" only when
questioning the integrity of what you're trying to do. At other times, a simple comparison to 0
or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies an element or slice of a hash, "delete" deletes the specified
elements from that hash so that exists() on that element no longer returns true. Setting a hash
element to the undefined value does not remove its key, but deleting it does; see "exists".
It returns the value or values deleted in list context, or the last such element in scalar
context. The return list's length always matches that of the argument list: deleting non-existent nonexistent
existent elements returns the undefined value in their corresponding positions.
delete() may also be used on arrays and array slices, but its behavior is less straightforward.
Although exists() will return false for deleted entries, deleting array elements never changes
indices of existing values; use shift() or splice() for that. However, if all deleted elements
fall at the end of an array, the array's size shrinks to the position of the highest element that
still tests true for exists(), or to 0 if none do.
Be aware that calling delete on array values is deprecated and likely to be removed in a future
version of Perl.
Deleting from %ENV modifies the environment. Deleting from a hash tied to a DBM file deletes the
entry from the DBM file. Deleting from a "tied" hash or array may not necessarily return
anything; it depends on the implementation of the "tied" package's DELETE method, which may do
whatever it pleases.
The "delete local EXPR" construct localizes the deletion to the current block at run time. Until
the block exits, elements locally deleted temporarily no longer exist. See "Localized deletion
of elements of composite types" in perlsub.
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the
customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its final operation is an element or slice of an
aggregate:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
"die" raises an exception. Inside an "eval" the error message is stuffed into $@ and the "eval"
is terminated with the undefined value. If the exception is outside of all enclosing "eval"s,
then the uncaught exception prints LIST to "STDERR" and exits with a non-zero value. If you need
to exit the process with a specific exit code, see exit.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the current script line number and input
line number (if any) are also printed, and a newline is supplied. Note that the "input line
number" (also known as "chunk") is subject to whatever notion of "line" happens to be currently
in effect, and is also available as the special variable $.. See "$/" in perlvar and "$." in
perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the
string "at foo line 123" is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
If the output is empty and $@ already contains a value (typically from a previous eval) that
value is reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If the output is empty and $@ contains an object reference that has a "PROPAGATE" method, that
method will be called with additional file and line number parameters. The return value replaces
the value in $@. i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };" were called.
If $@ is empty then the string "Died" is used.
If an uncaught exception results in interpreter exit, the exit code is determined from the values
of $! and $? with this pseudocode:
exit $! if $!; # errno
exit $? >> 8 if $? >> 8; # child exit status
exit 255; # last resort
The intent is to squeeze as much possible information about the likely cause into the limited
space of the system exit code. However, as $! is the value of C's "errno", which can be set by
any system call, this means that the value of the exit code used by "die" can be non-predictable,
so should not be relied upon, other than to be non-zero.
You can also call "die" with a reference argument, and if this is trapped within an "eval", $@
contains that reference. This permits more elaborate exception handling using objects that
maintain arbitrary state about the exception. Such a scheme is sometimes preferable to matching
particular string values of $@ with regular expressions. Because $@ is a global variable and
"eval" may be used within object implementations, be careful that analyzing the error object
doesn't replace the reference in the global variable. It's easiest to make a local copy of the
reference before any manipulations. Here's an example:
use Scalar::Util "blessed";
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because Perl stringifies uncaught exception messages before display, you'll probably want to
overload stringification operations on exception objects. See overload for details about that.
You can arrange for a callback to be run just before the "die" does its deed, by setting the
$SIG{__DIE__} hook. The associated handler is called with the error text and can change the
error message, if it sees fit, by calling "die" again. See "$SIG{expr}" in perlvar for details
on setting %SIG entries, and "eval BLOCK" for some examples. Although this feature was to be run
only right before your program was to exit, this is not currently so: the $SIG{__DIE__} hook is
currently called even inside eval()ed blocks/strings! If one wants the hook to do nothing in
such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at
a distance, this counterintuitive behavior may be fixed in a future release.
See also exit(), warn(), and the Carp module.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands
indicated by BLOCK. When modified by the "while" or "until" loop modifier, executes the BLOCK
once before testing the loop condition. (On other statements the loop modifiers test the
conditional first.)
"do BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo"
cannot be used to leave or restart the block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See perlsub.
do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error
messages, searches the @INC directories, and updates %INC if the file is found. See "Predefined
Names" in perlvar for these variables. It also differs in that code evaluated with "do FILENAME"
cannot see lexicals in the enclosing scope; "eval STRING" does. It's the same, however, in that
it does reparse the file every time you call it, so you probably don't want to do this inside a
loop.
If "do" cannot read the file, it returns undef and sets $! to the error. If "do" can read the
file but cannot compile it, it returns undef and sets an error message in $@. If the file is
successfully compiled, "do" returns the value of the last expression evaluated.
Inclusion of library modules is better done with the "use" and "require" operators, which also do
automatic error checking and raise an exception if there's a problem.
You might like to use "do" to read in a program configuration file. Manual error checking can be
done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump
This function causes an immediate core dump. See also the -u command-line switch in perlrun,
which does the same thing. Primarily this is so that you can use the undump program (not
supplied) to turn your core dump into an executable binary after having initialized all your
variables at the beginning of the program. When the new binary is executed it will begin by
executing a "goto LABEL" (with all the restrictions that "goto" suffers). Think of it as a goto
with an intervening core dump and reincarnation. If "LABEL" is omitted, restarts the program
from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is
reincarnated, with possible resulting confusion by Perl.
This function is now largely obsolete, mostly because it's very hard to convert a core file into
an executable. That's why you should now invoke it as "CORE::dump()", if you don't want to be
warned against a possible typo.
each HASH
each ARRAY
When called in list context, returns a 2-element list consisting of the key and value for the
next element of a hash, or the index and value for the next element of an array, so that you can
iterate over it. When called in scalar context, returns only the key (not the value) in a hash,
or the index in an array.
Hash entries are returned in an apparently random order. The actual random order is subject to
change in future versions of Perl, but it is guaranteed to be in the same order as either the
"keys" or "values" function would produce on the same (unmodified) hash. Since Perl 5.8.2 the
ordering can be different even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
After "each" has returned all entries from the hash or array, the next call to "each" returns the
empty list in list context and "undef" in scalar context. The next call following that one
restarts iteration. Each hash or array has its own internal iterator, accessed by "each",
"keys", and "values". The iterator is implicitly reset when "each" has reached the end as just
described; it can be explicitly reset by calling "keys" or "values" on the hash or array. If you
add or delete a hash's elements while iterating over it, entries may be skipped or duplicated--so
don't do that. Exception: It is always safe to delete the item most recently returned by
"each()", so the following code works properly:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
This prints out your environment like the printenv(1) program, but in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open.
FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function
actually reads a character and then "ungetc"s it, so isn't useful in an interactive context.) Do
not read from a terminal file (or call "eof(FILEHANDLE)" on it) after end-of-file is reached.
File types such as terminals may lose the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is
different. It refers to the pseudo file formed from the files listed on the command line and
accessed via the "<>" operator. Since "<>" isn't explicitly opened, as a normal filehandle is,
an "eof()" before "<>" has been used will cause @ARGV to be examined to determine if input is
available. Similarly, an "eof()" after "<>" has returned end-of-file will assume you are
processing another @ARGV list, and if you haven't set @ARGV, will read input from "STDIN"; see
"I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each file, "eof()"
will detect the end of only the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl, because the input operators typically
return "undef" when they run out of data, or if there was an error.
eval EXPR
eval BLOCK
eval
In the first form, the return value of EXPR is parsed and executed as if it were a little Perl
program. The value of the expression (which is itself determined within scalar context) is first
parsed, and if there weren't any errors, executed in the lexical context of the current Perl
program, so that any variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the "eval" executes. If EXPR is omitted, evaluates $_.
This form is typically used to delay parsing and subsequent execution of the text of EXPR until
run time.
In the second form, the code within the BLOCK is parsed only once--at the same time the code
surrounding the "eval" itself was parsed--and executed within the context of the current Perl
program. This form is typically used to trap exceptions more efficiently than the first (see
below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; miniprogram;
program; a return statement may be also used, just as with subroutines. The expression providing
the return value is evaluated in void, scalar, or list context, depending on the context of the
"eval" itself. See "wantarray" for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a "die" statement is executed, "eval" returns an
undefined value in scalar context or an empty list in list context, and $@ is set to the error
message. If there was no error, $@ is guaranteed to be the empty string. Beware that using
"eval" neither silences Perl from printing warnings to STDERR, nor does it stuff the text of
warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility, or
turn off warnings inside the BLOCK or EXPR using "no warnings 'all'". See "warn", perlvar,
warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is useful for determining whether a
particular feature (such as "socket" or "symlink") is implemented. It is also Perl's exception
trapping mechanism, where the die operator is used to raise exceptions.
If you want to trap errors when loading an XS module, some problems with the binary interface
(such as Perl version skew) may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is set.
See perlrun.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors
without incurring the penalty of recompiling each time. The error, if any, is still returned in
$@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in libraries does have some issues. Due to the
current arguably broken state of "__DIE__" hooks, you may wish not to trigger any "__DIE__" hooks
that user code may have installed. You can use the "local $SIG{__DIE__}" construct for this
purpose, as this example shows:
# a private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again, which has the
effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a
future release.
With an "eval", you should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the variable $x.
(Although case 2 has misleading double quotes making the reader wonder what else might be
happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x',
which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons,
but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a
place where normally you would like to use double quotes, except that in this particular
situation, you can just use symbolic references instead, as in case 6.
The assignment to $@ occurs before restoration of localised variables, which means a temporary is
required if you want to mask some but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # DOES NOT WORK
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}
"eval BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo"
cannot be used to leave or restart the block.
An "eval ''" executed within the "DB" package doesn't see the usual surrounding lexical scope,
but rather the scope of the first non-DB piece of code that called it. You don't normally need to
worry about this unless you are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never returns; use "system" instead of "exec"
if you want it to return. It fails and returns false only if the command does not exist and it
is executed directly instead of via your system's command shell (see below).
Since it's a common mistake to use "exec" instead of "system", Perl warns you if there is a
following statement that isn't "die", "warn", or "exit" (if "-w" is set--but you always do that,
right?). If you really want to follow an "exec" with some other statement, you can use one of
these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array with more than one value,
calls execvp(3) with the arguments in LIST. If there is only one scalar argument or an array
with one element in it, the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on
Unix platforms, but varies on other platforms). If there are no shell metacharacters in the
argument, it is split into words and passed directly to "execvp", which is more efficient.
Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are
executing about its own name, you can specify the program you actually want to run as an
"indirect object" (without a comma) in front of the LIST. (This always forces interpretation of
the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results are subject to its quirks and
capabilities. See "`STRING`" in perlop for details.
Using an indirect object with "exec" or "system" is also more secure. This usage (which also
works fine with system()) forces interpretation of the arguments as a multivalued list, even if
the list had just one argument. That way you're safe from the shell expanding wildcards or
splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it
"surprise" an argument. The second version didn't; it tried to run a program named "echo
surprise", didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl attempts to flush all files opened for output before the exec, but
this may not be supported on some platforms (see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles to
avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods on your
objects.
exists EXPR
Given an expression that specifies an element of a hash, returns true if the specified element in
the hash has ever been initialized, even if the corresponding value is undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
exists may also be called on array elements, but its behavior is much less obvious, and is
strongly tied to the use of "delete" on arrays. Be aware that calling exists on array values is
deprecated and likely to be removed in a future version of Perl.
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and defined if it exists, but the
reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified
subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for
exists or defined does not count as declaring it. Note that a subroutine that does not exist may
still be callable: its package may have an "AUTOLOAD" method that makes it spring into existence
the first time that it is called; see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or
array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the mostly deeply nested array or hash will not spring into existence just because its
existence was tested, any intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
will spring into existence due to the existence test for the $key element above. This happens
anywhere the arrow operator is used, including even here:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be
an lvalue context may be fixed in a future release.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values
for EXPR are 0 for success and 1 for error; other values are subject to interpretation depending
on the environment in which the Perl program is running. For example, exiting 69
(EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item
undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap
whatever error happened. Use "die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first,
but these "END" routines may not themselves abort the exit. Likewise any object destructors that
need to be called are called before the real exit. If this is a problem, you can call
"POSIX:_exit($status)" to avoid END and destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives
"exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value returned work just
like "ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". Like "ioctl", it maps a 0
return from the system call into "0 but true" in Perl. This string is true in boolean context
and 0 in numeric context. It is also exempt from the normal -w warnings on improper numeric
conversions.
Note that "fcntl" raises an exception if used on a machine that doesn't implement fcntl(2). See
the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.
Here's an example of setting a filehandle named "REMOTE" to be non-blocking at the system level.
You'll have to negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. This
is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-handling
operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle,
generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of "open" may return undefined even
though they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on
failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2)
locking, or lockf(3). "flock" is Perl's portable file locking interface, although it locks
entire files only, not records.
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely
until the lock is granted, and that its locks merely advisory. Such discretionary locks are more
flexible, but offer fewer guarantees. This means that programs that do not also use "flock" may
modify files locked with "flock". See perlport, your port's specific documentation, or your
system-specific local manpages for details. It's best to assume traditional behavior if you're
writing portable programs. (But if you're not, you should as always feel perfectly free to write
for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These
constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you
import them from the Fcntl module, either individually, or as a group using the ':flock' tag.
LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a
previously requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX then "flock"
returns immediately rather than blocking waiting for the lock; check the return status to see if
you got it.
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or
unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that
FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most if
not all systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing
semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to
use LOCK_SH and requires that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would need to
use the more system-specific "fcntl" for that. If you like you can force Perl to ignore your
system's flock(2) function, and so provide its own fcntl(2)-based emulation, by passing the
switch "-Ud_flock" to the Configure program when you configure Perl.
Here's a mailbox appender for BSD systems.
use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
# and, in case someone appended while we were waiting...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);
On systems that support a real flock(2), locks are inherited across fork() calls, whereas those
that must resort to the more capricious fcntl(2) function lose their locks, making it seriously
harder to write servers.
See also DB_File for other flock() examples.
fork
Does a fork(2) system call to create a new process running the same program at the same point.
It returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is
unsuccessful. File descriptors (and sometimes locks on those descriptors) are shared, while
everything else is copied. On most systems supporting fork(), great care has gone into making it
extremely efficient (for example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.
Beginning with v5.6.0, Perl attempts to flush all files opened for output before forking the
child process, but this may not be supported on some platforms (see perlport). To be safe, you
may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on
any open handles to avoid duplicate output.
If you "fork" without ever waiting on your children, you will accumulate zombies. On some
systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc for more
examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are
actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a
CGI script or a backgrounded job launched from a remote shell) won't think you're done. You
should reopen those to /dev/null if it's any issue.
format
Declare a picture format for use by the "write" function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may call it, too. It formats (see
perlform) a list of values according to the contents of PICTURE, placing the output into the
format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a "write" is done,
the contents of $^A are written to some filehandle. You could also read $^A and then set $^A
back to "". Note that a format typically does one "formline" per line of form, but the
"formline" function itself doesn't care how many newlines are embedded in the PICTURE. This
means that the "~" and "~~" tokens treat the entire PICTURE as a single line. You may therefore
need to use multiple formlines to implement a single record format, just like the "format"
compiler.
Be careful if you put double quotes around the picture, because an "@" character may be taken to
mean the beginning of an array name. "formline" always returns true. See perlform for other
examples.
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at
end of file or if there was an error (in the latter case $! is set). If FILEHANDLE is omitted,
reads from STDIN. This is not particularly efficient. However, it cannot be used by itself to
fetch single characters without waiting for the user to hit enter. For that, try something more
like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an exercise to the reader.
The "POSIX::getattr" function can do this more portably on systems purporting POSIX compliance.
See also the "Term::ReadKey" module from your nearest CPAN site; details on CPAN can be found on
"CPAN" in perlmodlib.
getlogin
This implements the C library function of the same name, which on most systems returns the
current login from /etc/utmp, if any. If it returns the empty string, use "getpwuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a PID of 0 to get the current
process group for the current process. Will raise an exception if used on a machine that doesn't
implement getpgrp(2). If PID is omitted, returns process group of current process. Note that
the POSIX version of "getpgrp" does not accept a PID argument, so only "PID==0" is truly
portable.
getppid
Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions "getpid()" and "getppid()" return different
values from different threads. In order to be portable, this behavior is not reflected by the
Perl-level function "getppid()", that returns a consistent value across threads. If you want to
call the underlying "getppid()", you may use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a user. (See getpriority(2).)
Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines are the same as their counterparts in the system C library. In list context, the
return values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get an empty list.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as
opposed to the login name) and other information pertaining to the user. Beware, however, that
in many system users are able to change this information and therefore it cannot be trusted and
therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password
and login shell, are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you
get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.)
For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special in that they are unsupported on
many systems. If the $quota is unsupported, it is an empty scalar. If it is supported, it
usually encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If
it is supported it usually encodes some administrative comment about the user. In some systems
the $quota field may be $change or $age, fields that have to do with password aging. In some
systems the $comment field may be $class. The $expire field, if present, encodes the expiration
period of the account or the password. For the availability and the exact meaning of these
fields in your system, please consult your getpwnam(3) documentation and your pwd.h file. You
can also find out from within Perl what your $quota and $comment fields mean and whether you have
the $expire field by using the "Config" module and the values "d_pwquota", "d_pwage",
"d_pwchange", "d_pwcomment", and "d_pwexpire". Shadow password files are supported only if your
vendor has implemented them in the intuitive fashion that calling the regular C library routines
gets the shadow versions if you're running under privilege or if there exists the shadow(3)
functions as found in System V (this includes Solaris and Linux.) Those systems that implement a
proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the
members of the group.
For the gethost*() functions, if the "h_errno" variable is supported in C, it will be returned to
you via $? if the function call fails. The @addrs value returned by a successful call is a list
of raw addresses returned by the corresponding library call. In the Internet domain, each
address is four bytes long; you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket;
$packed_ip = gethostbyname("www.perl.org");
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}
Make sure <gethostbyname()> is called in SCALAR context and that its return value is checked for
definedness.
If you get tired of remembering which element of the return list contains which return value, by-name byname
name interfaces are provided in standard modules: "File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime", and "User::grent". These
override the normal built-ins, supplying versions that return objects with the appropriate names
for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid), they aren't, because a
"File::stat" object is different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you don't know
the address because you have several different IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a given LEVEL. Options may exist at
multiple protocol levels depending on the socket type, but at least the uppermost socket level
SOL_SOCKET (defined in the "Socket" module) will exist. To query options at another level the
protocol number of the appropriate protocol controlling the option should be supplied. For
example, to indicate that an option is to be interpreted by the TCP protocol, LEVEL should be set
to the protocol number of TCP, which you can get using "getprotobyname".
The function returns a packed string representing the requested socket option, or "undef" on
error, with the reason for the error placed in $!). Just what is in the packed string depends on
LEVEL and OPTNAME; consult getsockopt(2) for details. A common case is that the option is an
integer, in which case the result is a packed integer, which you can decode using "unpack" with
the "i" (or "I") format.
An example to test whether Nagle's algorithm is turned on on a socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "getsockopt TCP_NODELAY: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob
In list context, returns a (possibly empty) list of filename expansions on the value of EXPR such
as the standard Unix shell /bin/csh would do. In scalar context, glob iterates through such
filename expansions, returning undef when the list is exhausted. This is the internal function
implementing the "<*.c>" operator, but you can use it directly. If EXPR is omitted, $_ is used.
The "<*.c>" operator is discussed in more detail in "I/O Operators" in perlop.
Note that "glob" splits its arguments on whitespace and treats each segment as separate pattern.
As such, "glob("*.c *.h")" matches all files with a .c or .h extension. The expression "glob(".*
*")" matchs all files in the current working directory.
If non-empty braces are the only wildcard characters used in the "glob", no filenames are
matched, but potentially many strings are returned. For example, this produces nine strings, one
for each pairing of fruits and colors:
@many = glob "{apple,tomato,cherry}={green,yellow,red}";
Beginning with v5.6.0, this operator is implemented using the standard "File::Glob" extension.
See File::Glob for details, including "bsd_glob" which does not treat whitespace as a pattern
separator.
gmtime EXPR
gmtime
Works just like localtime but the returned values are localized for the standard Greenwich time
zone.
Note: when called in list context, $isdst, the last value returned by gmtime is always 0. There
is no Daylight Saving Time in GMT.
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL and resumes execution there. It
can't be used to get out of a block or subroutine given to "sort". It can be used to go almost
anywhere else within the dynamic scope, including out of subroutines, but it's usually better to
use some other construct such as "last" or "die". The author of Perl has never felt the need to
use this form of "goto" (in Perl, that is; C is another matter). (The difference is that C does
not offer named loops combined with loop control. Perl does, and this replaces most structured
uses of "goto" in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be resolved dynamically. This allows
for computed "goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for
maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
Use of "goto-LABEL" or "goto-EXPR" to jump into a construct is deprecated and will issue a
warning. Even then, it may not be used to go into any construct that requires initialization,
such as a subroutine or a "foreach" loop. It also can't be used to go into a construct that is
optimized away.
The "goto-&NAME" form is quite different from the other forms of "goto". In fact, it isn't a
goto in the normal sense at all, and doesn't have the stigma associated with other gotos.
Instead, it exits the current subroutine (losing any changes set by local()) and immediately
calls in its place the named subroutine using the current value of @_. This is used by
"AUTOLOAD" subroutines that wish to load another subroutine and then pretend that the other
subroutine had been called in the first place (except that any modifications to @_ in the current
subroutine are propagated to the other subroutine.) After the "goto", not even "caller" will be
able to tell that this routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code
reference, or a block that evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it
is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and
returns the list value consisting of those elements for which the expression evaluated to true.
In scalar context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause bizarre results if the elements of LIST are not
variables. Similarly, grep returns aliases into the original list, much as a for loop's index
variable aliases the list elements. That is, modifying an element of a list returned by grep
(for example, in a "foreach", "map" or another "grep") actually modifies the element in the
original list. This is usually something to be avoided when writing clear code.
If $_ is lexical in the scope where the "grep" appears (because it has been declared with "my
$_") then, in addition to being locally aliased to the list elements, $_ keeps being lexical
inside the block; i.e., it can't be seen from the outside, avoiding any potential side-effects.
See also "map" for a list composed of the results of the BLOCK or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding value. (To convert strings that
might start with either 0, "0x", or "0b", see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a
warning. Leading whitespace is not stripped, unlike oct(). To present something as hex, look
into "printf", "sprintf", or "unpack".
import LIST
There is no builtin "import" function. It is just an ordinary method (subroutine) defined (or
inherited) by modules that wish to export names to another module. The "use" function calls the
"import" method for the package used. See also "use", perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within another, but without the wildcard-like behavior
of a full regular-expression pattern match. It returns the position of the first occurrence of
SUBSTR in STR at or after POSITION. If POSITION is omitted, starts searching from the beginning
of the string. POSITION before the beginning of the string or after its end is treated as if it
were the beginning or the end, respectively. POSITION and the return value are based at 0 (or
whatever you've set the $[ variable to--but don't do that). If the substring is not found,
"index" returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use this
function for rounding: one because it truncates towards 0, and two because machine
representations of floating-point numbers can sometimes produce counterintuitive results. For
example, "int(-6.725/0.025)" produces -268 rather than the correct -269; that's because it's
really more like -268.99999999999994315658 instead. Usually, the "sprintf", "printf", or the
"POSIX::floor" and "POSIX::ceil" functions will serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the
correct definitions you'll have to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help
you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION;
a C pointer to the string value of SCALAR will be passed as the third argument of the actual
"ioctl" call. (If SCALAR has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the
scalar before using it.) The "pack" and "unpack" functions may be needed to manipulate the
values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the
actual value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of
EXPR, and returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its first argument. Compare
"split".
keys HASH
keys ARRAY
Returns a list consisting of all the keys of the named hash, or the indices of an array. (In
scalar context, returns the number of keys or indices.)
The keys of a hash are returned in an apparently random order. The actual random order is
subject to change in future versions of Perl, but it is guaranteed to be the same order as either
the "values" or "each" function produces (given that the hash has not been modified). Since Perl
5.8.1 the ordering is different even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling keys() resets the HASH or ARRAY's internal iterator (see "each"). In
particular, calling keys() in void context resets the iterator with no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the hash, so modifying them will not
affect the original hash. Compare "values".
To sort a hash by value, you'll need to use a "sort" function. Here's a descending numeric sort
of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
Used as an lvalue, "keys" allows you to increase the number of hash buckets allocated for the
given hash. This can gain you a measure of efficiency if you know the hash is going to get big.
(This is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds
up to the next power of two. These buckets will be retained even if you do "%hash = ()", use
"undef %hash" if you want to free the storage while %hash is still in scope. You can't shrink
the number of buckets allocated for the hash using "keys" in this way (but you needn't worry
about doing this by accident, as trying has no effect). "keys @array" in an lvalue context is a
syntax error.
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the number of processes successfully signaled
(which is not necessarily the same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process, but "kill" checks whether it's possible to
send a signal to it (that means, to be brief, that the process is owned by the same user, or we
are the super-user). This is useful to check that a child process is still alive (even if only
as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this
construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. That
means you usually want to use positive not negative signals. You may also use a signal name in
quotes.
The behavior of kill when a PROCESS number is zero or negative depends on the operating system.
For example, on POSIX-conforming systems, zero will signal the current process group and -1 will
signal all processes.
See "Signals" in perlipc for more details.
last LABEL
last
The "last" command is like the "break" statement in C (as used in loops); it immediately exits
the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing
loop. The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block that returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "last"
can be used to effect an early exit out of such a block.
See also "continue" for an illustration of how "last", "next", and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal function implementing the "\L" escape
in double-quoted strings.
If EXPR is omitted, uses $_.
What gets returned depends on several factors:
If "use bytes" is in effect:
On EBCDIC platforms
The results are what the C language system call "tolower()" returns.
On ASCII platforms
The results follow ASCII semantics. Only characters "A-Z" change, to "a-z" respectively.
Otherwise, If EXPR has the UTF8 flag set
If the current package has a subroutine named "ToLower", it will be used to change the case
(See "User-Defined Case Mappings" in perlunicode.) Otherwise Unicode semantics are used for
the case change.
Otherwise, if "use locale" is in effect
Respects current LC_CTYPE locale. See perllocale.
Otherwise, if "use feature 'unicode_strings'" is in effect:
Unicode semantics are used for the case change. Any subroutine named "ToLower" will not be
used.
Otherwise:
On EBCDIC platforms
The results are what the C language system call "tolower()" returns.
On ASCII platforms
ASCII semantics are used for the case change. The lowercase of any character outside the
ASCII range is the character itself.
lcfirst EXPR
lcfirst
Returns the value of EXPR with the first character lowercased. This is the internal function
implementing the "\l" escape in double-quoted strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma, such as in a locale, as "lc" does.
length EXPR
length
Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of $_.
If EXPR is undefined, returns "undef".
This function cannot be used on an entire array or hash to find out how many elements these have.
For that, use "scalar @array" and "scalar keys %hash", respectively.
Like all Perl character operations, length() normally deals in logical characters, not physical
bytes. For how many bytes a string encoded as UTF-8 would take up, use
"length(Encode::encode_utf8(EXPR))" (you'll have to "use Encode" first). See Encode and
perlunicode.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen(2) system call does. Returns true if it succeeded, false
otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead, because "local" isn't what most people think
of as "local". See "Private Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more
than one value is listed, the list must be placed in parentheses. See "Temporary Values via
local()" in perlsub for details, including issues with tied arrays and hashes.
The "delete local EXPR" construct can also be used to localize the deletion of array/hash
elements to the current block. See "Localized deletion of elements of composite types" in
perlsub.
localtime EXPR
localtime
Converts a time as returned by the time function to a 9-element list with the time analyzed for
the local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour
are the seconds, minutes, and hours of the specified time.
$mday is the day of the month, and $mon is the month itself, in the range 0..11 with 0 indicating
January and 11 indicating December. This makes it easy to get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just the last two digits of the year. That is,
$year is 123 in year 2023. The proper way to get a 4-digit year is simply:
$year += 1900;
Otherwise you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
To get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
$wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the
day of the year, in the range 0..364 (or 0..365 in leap years.)
$isdst is true if the specified time occurs during Daylight Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current time (as returned by time(3)).
In scalar context, "localtime()" returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a Perl builtin. For GMT instead of local time
use the "gmtime" builtin. See also the "Time::Local" module (to convert the second, minutes,
hours, ... back to the integer value returned by time()), and the POSIX module's strftime(3) and
mktime(3) functions.
To get somewhat similar but locale dependent date strings, set up your locale environment
variables appropriately (please see perllocale) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day of the week and the month of the year, may
not necessarily be three characters wide.
See "localtime" in perlport for portability concerns.
The Time::gmtime and Time::localtime modules provides a convenient, by-name access mechanism to
the gmtime() and localtime() functions, respectively.
For a comprehensive date and time representation look at the DateTime module on CPAN.
lock THING
This function places an advisory lock on a shared variable, or referenced object contained in
THING until the lock goes out of scope.
lock() is a "weak keyword" : this means that if you've defined a function by this name (before
any calls to it), that function will be called instead. If you are not under "use
threads::shared" this does nothing. See threads::shared.
log EXPR
log Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns log of $_. To get
the log of another base, use basic algebra: The base-N log of a number is equal to the natural
log of that number divided by the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat EXPR
lstat
Does the same thing as the "stat" function (including setting the special "_" filehandle) but
stats a symbolic link instead of the file the symbolic link points to. If symbolic links are
unimplemented on your system, a normal "stat" is done. For much more detailed information,
please see the documentation for "stat".
If EXPR is omitted, stats $_.
m// The match operator. See "Regexp Quote-Like Operators" in perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and
returns the list value composed of the results of each such evaluation. In scalar context,
returns the total number of elements so generated. Evaluates BLOCK or EXPR in list context, so
each element of LIST may produce zero, one, or more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach (@array) {
$hash{get_a_key_for($_)} = $_;
}
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause bizarre results if the elements of LIST are not
variables. Using a regular "foreach" loop for this purpose would be clearer in most cases. See
also "grep" for an array composed of those items of the original list for which the BLOCK or EXPR
evaluates to true.
If $_ is lexical in the scope where the "map" appears (because it has been declared with "my
$_"), then, in addition to being locally aliased to the list elements, $_ keeps being lexical
inside the block; that is, it can't be seen from the outside, avoiding any potential side-effects. sideeffects.
effects.
"{" starts both hash references and blocks, so "map { ..." could be either the start of map BLOCK
LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing "}" it has to take a
guess at which it's dealing with based on what it finds just after the "{". Usually it gets it
right, but if it doesn't it won't realize something is wrong until it gets to the "}" and
encounters the missing (or unexpected) comma. The syntax error will be reported close to the "}",
but you'll need to change something near the "{" such as using a unary "+" to give Perl some
help:
%hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_" => 1) } @array # this also works
%hash = map { lc($_) => 1 } @array # as does this.
%hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
@hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
to get a list of anonymous hashes each with only one entry apiece.
mkdir FILENAME,MASK
mkdir FILENAME
mkdir
Creates the directory specified by FILENAME, with permissions specified by MASK (as modified by
"umask"). If it succeeds it returns true, otherwise it returns false and sets $! (errno). If
omitted, MASK defaults to 0777. If omitted, FILENAME defaults to $_.
In general, it is better to create directories with permissive MASK, and let the user modify that
with their "umask", than it is to supply a restrictive MASK and give the user no way to be more
permissive. The exceptions to this rule are when the file or directory should be kept private
(mail files, for instance). The perlfunc(1) entry on "umask" discusses the choice of MASK in
more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing
slashes. Some operating and filesystems do not get this right, so Perl automatically removes all
trailing slashes to keep everyone happy.
To recursively create a directory structure, look at the "mkpath" function of the File::Path
module.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable
that will hold the returned "msqid_ds" structure. Returns like "ioctl": the undefined value for
error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in
perlipc, "IPC::SysV", and "IPC::Semaphore" documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue id, or the undefined value
if there is an error. See also "SysV IPC" in perlipc and "IPC::SysV" and "IPC::Msg"
documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from message queue ID into variable
VAR with a maximum message size of SIZE. Note that when a message is received, the message type
as a native long integer will be the first thing in VAR, followed by the actual message. This
packing may be opened with "unpack("l! a*")". Taints the variable. Returns true if successful,
or false if there is an error. See also "SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Msg"
documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must
begin with the native long integer message type, and be followed by the length of the actual
message, and finally the message itself. This kind of packing can be achieved with "pack("l!
a*", $type, $message)". Returns true if successful, or false if there is an error. See also
"IPC::SysV" and "IPC::SysV::Msg" documentation.
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
A "my" declares the listed variables to be local (lexically) to the enclosing block, file, or
"eval". If more than one value is listed, the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound
to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or
starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See "Private Variables via
my()" in perlsub for details, and fields, attributes, and Attribute::Handlers.
next LABEL
next
The "next" command is like the "continue" statement in C; it starts the next iteration of the
loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the above, it would get executed even on discarded
lines. If LABEL is omitted, the command refers to the innermost enclosing loop.
"next" cannot be used to exit a block which returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "next"
will exit such a block early.
See also "continue" for an illustration of how "last", "next", and "redo" work.
no MODULE VERSION LIST
no MODULE VERSION
no MODULE LIST
no MODULE
no VERSION
See the "use" function, of which "no" is the opposite.
oct EXPR
oct Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to
start off with "0x", interprets it as a hex string. If EXPR starts off with "0b", it is
interpreted as a binary string. Leading whitespace is ignored in all three cases.) The
following will handle decimal, binary, octal, and hex in standard Perl notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf() or
printf():
$dec_perms = (stat("filename"))[2] & 07777;
$oct_perm_str = sprintf "%o", $perms;
The oct() function is commonly used when a string such as 644 needs to be converted into a file
mode, for example. Although Perl automatically converts strings into numbers as needed, this
automatic conversion assumes base 10.
Leading white space is ignored without warning, as too are any trailing non-digits, such as a
decimal point ("oct" only handles non-negative integers, not negative integers or floating
point).
open FILEHANDLE,EXPR
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE.
Simple examples to open a file for reading:
open(my $fh, '<', "input.txt") or die $!;
and for writing:
open(my $fh, '>', "output.txt") or die $!;
(The following is a comprehensive reference to open(): for a gentler introduction you may
consider perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array or hash element) the variable is assigned
a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression, its value is
used as the name of the real filehandle wanted. (This is considered a symbolic reference, so
"use strict 'refs'" should not be in effect.)
If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename.
(Note that lexical variables--those declared with "my"--will not work for this purpose; so if
you're using "my", specify EXPR in your call to open.)
If three or more arguments are specified then the mode of opening and the filename are separate.
If MODE is '<' or nothing, the file is opened for input. If MODE is '>', the file is truncated
and opened for output, being created if necessary. If MODE is '>>', the file is opened for
appending, again being created if necessary.
You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access
to the file; thus '+<' is almost always preferred for read/write updates--the '+>' mode would
clobber the file first. You can't usually use either read-write mode for updating textfiles,
since they have variable length records. See the -i switch in perlrun for a better approach.
The file is created with permissions of 0666 modified by the process's "umask" value.
These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
In the two-argument (and one-argument) form of the call, the mode and filename should be
concatenated (in that order), possibly separated by spaces. You may omit the mode in these forms
when that mode is '<'.
If the filename begins with '|', the filename is interpreted as a command to which output is to
be piped, and if the filename ends with a '|', the filename is interpreted as a command that
pipes output to us. See "Using open() for IPC" in perlipc for more examples of this. (You are
not allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3,
and "Bidirectional Communication with Another Process" in perlipc for alternatives.)
For three or more arguments if MODE is '|-', the filename is interpreted as a command to which
output is to be piped, and if MODE is '-|', the filename is interpreted as a command that pipes
output to us. In the two-argument (and one-argument) form, one should replace dash ('-') with
the command. See "Using open() for IPC" in perlipc for more examples of this. (You are not
allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and
"Bidirectional Communication" in perlipc for alternatives.)
In the form of pipe opens taking three or more arguments, if LIST is specified (extra arguments
after the command name) then LIST becomes arguments to the command invoked if the platform
supports it. The meaning of "open" with more than three arguments for non-pipe modes is not yet
defined, but experimental "layers" may give extra LIST arguments meaning.
In the two-argument (and one-argument) form, opening '<-' or '-' opens STDIN and opening '>-'
opens STDOUT.
You may use the three-argument form of open to specify I/O layers (sometimes referred to as
"disciplines") to apply to the handle that affect how the input and output are processed (see
open and PerlIO for more details). For example:
open(my $fh, "<:encoding(UTF-8)", "filename")
|| die "can't open UTF-8 encoded filename: $!";
opens the UTF-8 encoded file containing Unicode characters; see perluniintro. Note that if layers
are specified in the three-argument form, then default layers stored in ${^OPEN} (see perlvar;
usually set by the open pragma or the switch -CioD) are ignored.
Open returns nonzero on success, the undefined value otherwise. If the "open" involved a pipe,
the return value happens to be the pid of the subprocess.
If you're running Perl on a system that distinguishes between text files and binary files, then
you should check out "binmode" for tips for dealing with this. The key distinction between
systems that need "binmode" and those that don't is their text file formats. Systems like Unix,
Mac OS, and Plan 9, that end lines with a single character and encode that character in C as "\n"
do not need "binmode". The rest need it.
When opening a file, it's seldom a good idea to continue if the request failed, so "open" is
frequently used with "die". Even if "die" won't do what you want (say, in a CGI script, where
you want to format a suitable error message (but there are modules that can help with that
problem)) always check the return value from opening a file.
As a special case the 3-arg form with a read/write mode and the third argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also using "+<" works for symmetry, but you
really should consider writing something to the temporary file first. You will need to seek() to
do the reading.
Since v5.8.0, Perl has built using PerlIO by default. Unless you've changed this (i.e.,
Configure -Uuseperlio), you can open filehandles directly to Perl scalars via:
open($fh, '>', \$variable) || ..
To (re)open "STDOUT" or "STDERR" as an in-memory file, close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
General examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(my $dbase, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(my $dbase, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in-memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will appear in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
See perliol for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify an EXPR beginning with '>&', in which case
the rest of the string is interpreted as the name of a filehandle (or file descriptor, if
numeric) to be duped (as dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>",
and "+<". The mode you specify should match the mode of the original filehandle. (Duping a
filehandle does not take into account any existing contents of IO buffers.) If you use the 3-arg
form then you can pass either a number, the name of a filehandle or the normal "reference to a
glob".
Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then Perl will do
an equivalent of C's "fdopen" of that file descriptor (and not call dup(2)); this is more
parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful (besides being parsimonious) for example when
something is dependent on file descriptors, like for example locking using flock(). If you do
just "open(A, '>>&B')", the filehandle A will not have the same file descriptor as B, and
therefore flock(A) will not flock(B), and vice versa. But with "open(A, '>>&=B')" the
filehandles will share the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl will be using the standard C libraries'
fdopen() to implement the "=" functionality. On many Unix systems fdopen() fails when file
descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is most
often the default.
You can see whether Perl has been compiled with PerlIO or not by running "perl -V" and looking
for "useperlio=" line. If "useperlio" is "define", you have PerlIO, otherwise you don't.
If you open a pipe on the command '-', i.e., either '|-' or '-|' with 2-arguments (or 1-argument)
form of open(), then there is an implicit fork done, and the return value of open is the pid of
the child within the parent process, and 0 within the child process. (Use "defined($pid)" to
determine whether the open was successful.) The filehandle behaves normally for the parent, but
I/O to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child
process, the filehandle isn't opened--I/O happens from/to the new STDOUT/STDIN. Typically this
is used like the normal piped open when you want to exercise more control over just how the pipe
command gets executed, such as when running setuid and you don't want to have to scan shell
commands for metacharacters.
The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as "list form", which is not yet supported on all
platforms. A good rule of thumb is that if your platform has true "fork()" (in other words, if
your platform is Unix) you can use the list form.
See "Safe Pipe Opens" in perlipc for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any
operation that may do a fork, but this may not be supported on some platforms (see perlport). To
be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F. See "$^F" in perlvar.
Closing any piped filehandle causes the parent process to wait for the child to finish, and
returns the status value in $? and "${^CHILD_ERROR_NATIVE}".
The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing
whitespace deleted, and the normal redirection characters honored. This property, known as
"magic open", can often be used to good effect. A user could specify a filename of "rsh cat file
|", or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One should conscientiously choose between the
magic and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form "rsh cat file |", but will not work on a
filename that happens to have a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your system), then you should use the "sysopen"
function, which involves no such magic (but may use subtly different filemodes than Perl open(),
which is mapped to C fopen()). This is another way to protect your filenames from
interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package (or one of its subclasses, such as "IO::File"
or "IO::Socket"), you can generate anonymous filehandles that have the scope of whatever
variables hold references to them, and automatically close whenever and however you leave that
scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = IO::File->new;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See "seek" for some details about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir", "rewinddir", and
"closedir". Returns true if successful. DIRHANDLE may be an expression whose value can be used
as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar
variable (or array or hash element), the variable is assigned a reference to a new anonymous
dirhandle. DIRHANDLEs have their own namespace separate from FILEHANDLEs.
See example at "readdir".
ord EXPR
ord Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, or Unicode) value of the
first character of EXPR. If EXPR is omitted, uses $_.
For the reverse, see "chr". See perlunicode for more about Unicode.
our EXPR
our TYPE EXPR
our EXPR : ATTRS
our TYPE EXPR : ATTRS
"our" associates a simple name with a package variable in the current package for use within the
current scope. When "use strict 'vars'" is in effect, "our" lets you use declared global
variables without qualifying them with package names, within the lexical scope of the "our"
declaration. In this way "our" differs from "use vars", which is package scoped.
Unlike "my", which both allocates storage for a variable and associates a simple name with that
storage for use within the current scope, "our" associates a simple name with a package variable
in the current package, for use within the current scope. In other words, "our" has the same
scoping rules as "my", but does not necessarily create a variable.
If more than one value is listed, the list must be placed in parentheses.
our $foo;
our($bar, $baz);
An "our" declaration declares a global variable that will be visible across its entire lexical
scope, even across package boundaries. The package in which the variable is entered is
determined at the point of the declaration, not at the point of use. This means the following
behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the same lexical scope are allowed if they are
in different packages. If they happen to be in the same package, Perl will emit warnings if you
have asked for them, just like multiple "my" declarations. Unlike a second "my" declaration,
which will bind the name to a fresh variable, a second "our" declaration in the same package, in
the same scope, is merely redundant.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30
An "our" declaration may also have a list of attributes associated with it.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound
to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or
starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See "Private Variables via
my()" in perlsub for details, and fields, attributes, and Attribute::Handlers.
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The
resulting string is the concatenation of the converted values. Typically, each converted value
looks like its machine-level representation. For example, on 32-bit machines an integer may be
represented by a sequence of 4 bytes, which will in Perl be presented as a string that's 4
characters long.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Raises an exception otherwise.)
i A signed integer value.
I A unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in native format.
d A double-precision float in native format.
F A Perl internal floating-point value (NV) in native format
D A float of long-double precision in native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Raises an exception otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in character mode
and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
details). Its bytes represent an unsigned integer in base 128,
most significant digit first, with as few digits as possible. Bit
eight (the high bit) is set on each byte except the last.
x A null byte (a.k.a ASCII NUL, "\000", chr(0))
X Back up a byte.
@ Null-fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null-fill or truncate to absolute position specified by the value.
( Start of a ()-group.
One or more modifiers below may optionally follow certain letters in the TEMPLATE (the second
column lists letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
xX Make x and X act as alignment commands.
nNvV Treat integers as signed instead of unsigned.
@. Specify position as byte offset in the internal
representation of the packed string. Efficient but
dangerous.
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()" groups to force a particular byte-order on all
components in that group, including all its subgroups.
The following rules apply:
• Each letter may optionally be followed by a number indicating the repeat count. A numeric
repeat count may optionally be enclosed in brackets, as in "pack("C[80]", @arr)". The repeat
count gobbles that many values from the LIST when used with all format types other than "a",
"A", "Z", "b", "B", "h", "H", "@", ".", "x", "X", and "P", where it means something else,
dscribed below. Supplying a "*" for the repeat count instead of a number means to use
however many items are left, except for:
• "@", "x", and "X", where it is equivalent to 0.
• <.>, where it means relative to the start of the string.
• "u", where it is equivalent to 1 (or 45, which here is equivalent).
One can replace a numeric repeat count with a template letter enclosed in brackets to use the
packed byte length of the bracketed template for the repeat count.
For example, the template "x[L]" skips as many bytes as in a packed long, and the template
"$t X[$t] $t" unpacks twice whatever $t (when variable-expanded) unpacks. If the template in
brackets contains alignment commands (such as "x![d]"), its packed length is calculated as if
the start of the template had the maximal possible alignment.
When used with "Z", a "*" as the repeat count is guaranteed to add a trailing null byte, so
the resulting string is always one byte longer than the byte length of the item itself.
When used with "@", the repeat count represents an offset from the start of the innermost
"()" group.
When used with ".", the repeat count determines the starting position to calculate the value
offset as follows:
• If the repeat count is 0, it's relative to the current position.
• If the repeat count is "*", the offset is relative to the start of the packed string.
• And if it's an integer n, the offset is relative to the start of the nth innermost "()"
group, or to the start of the string if n is bigger then the group level.
The repeat count for "u" is interpreted as the maximal number of bytes to encode per line of
output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65.
• The "a", "A", and "Z" types gobble just one value, but pack it as a string of length count,
padding with nulls or spaces as needed. When unpacking, "A" strips trailing whitespace and
nulls, "Z" strips everything after the first null, and "a" returns data without any sort of
trimming.
If the value to pack is too long, the result is truncated. If it's too long and an explicit
count is provided, "Z" packs only "$count-1" bytes, followed by a null byte. Thus "Z" always
packs a trailing null, except for when the count is 0.
• Likewise, the "b" and "B" formats pack a string that's that many bits long. Each such format
generates 1 bit of the result.
Each result bit is based on the least-significant bit of the corresponding input character,
i.e., on "ord($char)%2". In particular, characters "0" and "1" generate bits 0 and 1, as do
characters "\000" and "\001".
Starting from the beginning of the input string, each 8-tuple of characters is converted to 1
character of output. With format "b", the first character of the 8-tuple determines the
least-significant bit of a character; with format "B", it determines the most-significant bit
of a character.
If the length of the input string is not evenly divisible by 8, the remainder is packed as if
the input string were padded by null characters at the end. Similarly during unpacking,
"extra" bits are ignored.
If the input string is longer than needed, remaining characters are ignored.
A "*" for the repeat count uses all characters of the input field. On unpacking, bits are
converted to a string of "0"s and "1"s.
• The "h" and "H" formats pack a string that many nybbles (4-bit groups, representable as
hexadecimal digits, "0".."9" "a".."f") long.
For each such format, pack() generates 4 bits of the result. With non-alphabetical
characters, the result is based on the 4 least-significant bits of the input character, i.e.,
on "ord($char)%16". In particular, characters "0" and "1" generate nybbles 0 and 1, as do
bytes "\0" and "\1". For characters "a".."f" and "A".."F", the result is compatible with the
usual hexadecimal digits, so that "a" and "A" both generate the nybble "0xa==10". Do not use
any characters but these with this format.
Starting from the beginning of the template to pack(), each pair of characters is converted
to 1 character of output. With format "h", the first character of the pair determines the
least-significant nybble of the output character; with format "H", it determines the most-significant mostsignificant
significant nybble.
If the length of the input string is not even, it behaves as if padded by a null character at
the end. Similarly, "extra" nybbles are ignored during unpacking.
If the input string is longer than needed, extra characters are ignored.
A "*" for the repeat count uses all characters of the input field. For unpack(), nybbles are
converted to a string of hexadecimal digits.
• The "p" format packs a pointer to a null-terminated string. You are responsible for ensuring
that the string is not a temporary value, as that could potentially get deallocated before
you got around to using the packed result. The "P" format packs a pointer to a structure of
the size indicated by the length. A null pointer is created if the corresponding value for
"p" or "P" is "undef"; similarly with unpack(), where a null pointer unpacks into "undef".
If your system has a strange pointer size--meaning a pointer is neither as big as an int nor
as big as a long--it may not be possible to pack or unpack pointers in big- or little-endian
byte order. Attempting to do so raises an exception.
• The "/" template character allows packing and unpacking of a sequence of items where the
packed structure contains a packed item count followed by the packed items themselves. This
is useful when the structure you're unpacking has encoded the sizes or repeat counts for some
of its fields within the structure itself as separate fields.
For "pack", you write length-item"/"sequence-item, and the length-item describes how the
length value is packed. Formats likely to be of most use are integer-packing ones like "n"
for Java strings, "w" for ASN.1 or SNMP, and "N" for Sun XDR.
For "pack", sequence-item may have a repeat count, in which case the minimum of that and the
number of available items is used as the argument for length-item. If it has no repeat count
or uses a '*', the number of available items is used.
For "unpack", an internal stack of integer arguments unpacked so far is used. You write
"/"sequence-item and the repeat count is obtained by popping off the last element from the
stack. The sequence-item must not have a repeat count.
If sequence-item refers to a string type ("A", "a", or "Z"), the length-item is the string
length, not the number of strings. With an explicit repeat count for pack, the packed string
is adjusted to that length. For example:
unpack("W/a", "\04Gurusamy") gives ("Guru")
unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
pack("a/W2", ord("a") .. ord("z")) gives "2ab"
The length-item is not returned explicitly from "unpack".
Supplying a count to the length-item format letter is only useful with "A", "a", or "Z".
Packing with a length-item of "a" or "Z" may introduce "\000" characters, which Perl does not
regard as legal in numeric strings.
• The integer types "s", "S", "l", and "L" may be followed by a "!" modifier to specify native
shorts or longs. As shown in the example above, a bare "l" means exactly 32 bits, although
the native "long" as seen by the local C compiler may be larger. This is mainly an issue on
64-bit platforms. You can see whether using "!" makes any difference this way:
printf "format s is %d, s! is %d\n",
length pack("s"), length pack("s!");
printf "format l is %d, l! is %d\n",
length pack("l"), length pack("l!");
"i!" and "I!" are also allowed, but only for completeness' sake: they are identical to "i"
and "I".
The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform
where Perl was built are also available from the command line:
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';
or programmatically via the "Config" module:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
$Config{longlongsize} is undefined on systems without long long support.
• The integer formats "s", "S", "i", "I", "l", "L", "j", and "J" are inherently non-portable
between processors and operating systems because they obey native byteorder and endianness.
For example, a 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
(arranged in and handled by the CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, Intel and VAX CPUs are little-endian, while everybody else, including Motorola
m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian. Alpha and MIPS can be either:
Digital/Compaq used/uses them in little-endian mode, but SGI/Cray uses them in big-endian
mode.
The names big-endian and little-endian are comic references to the egg-eating habits of the
little-endian Lilliputians and the big-endian Blefuscudians from the classic Jonathan Swift
satire, Gulliver's Travels. This entered computer lingo via the paper "On Holy Wars and a
Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can determine your system endianness with this incantation:
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built is also available via Config:
use Config;
print "$Config{byteorder}\n";
or from the command line:
$ perl -V:byteorder
Byteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-endian.
For portably packed integers, either use the formats "n", "N", "v", and "V" or else use the
">" and "<" modifiers described immediately below. See also perlport.
• Starting with Perl 5.9.2, integer and floating-point formats, along with the "p" and "P"
formats and "()" groups, may all be followed by the ">" or "<" endianness modifiers to
respectively enforce big- or little-endian byte-order. These modifiers are especially useful
given how "n", "N", "v" and "V" don't cover signed integers, 64-bit integers, or floating-
point values.
Here are some concerns to keep in mind when using endianness modifier:
• Exchanging signed integers between different platforms works only when all platforms
store them in the same format. Most platforms store signed integers in two's-complement
notation, so usually this is not an issue.
• The ">" or "<" modifiers can only be used on floating-point formats on big- or little-
endian machines. Otherwise, attempting to use them raises an exception.
• Forcing big- or little-endian byte-order on floating-point values for data exchange can
work only if all platforms use the same binary representation such as IEEE floating-point. floatingpoint.
point. Even if all platforms are using IEEE, there may still be subtle differences.
Being able to use ">" or "<" on floating-point values can be useful, but also dangerous
if you don't know exactly what you're doing. It is not a general way to portably store
floating-point values.
• When using ">" or "<" on a "()" group, this affects all types inside the group that
accept byte-order modifiers, including all subgroups. It is silently ignored for all
other types. You are not allowed to override the byte-order within a group that already
has a byte-order modifier suffix.
• Real numbers (floats and doubles) are in native machine format only. Due to the multiplicity
of floating-point formats and the lack of a standard "network" representation for them, no
facility for interchange has been made. This means that packed floating-point data written
on one machine may not be readable on another, even if both use IEEE floating-point
arithmetic (because the endianness of the memory representation is not part of the IEEE
spec). See also perlport.
If you know exactly what you're doing, you can use the ">" or "<" modifiers to force big- or
little-endian byte-order on floating-point values.
Because Perl uses doubles (or long doubles, if configured) internally for all numeric
calculation, converting from double into float and thence to double again loses precision, so
"unpack("f", pack("f", $foo)") will not in general equal $foo.
• Pack and unpack can operate in two modes: character mode ("C0" mode) where the packed string
is processed per character, and UTF-8 mode ("U0" mode) where the packed string is processed
in its UTF-8-encoded Unicode form on a byte-by-byte basis. Character mode is the default
unless the format string starts with "U". You can always switch mode mid-format with an
explicit "C0" or "U0" in the format. This mode remains in effect until the next mode change,
or until the end of the "()" group it (directly) applies to.
• You must yourself do any alignment or padding by inserting, for example, enough "x"es while
packing. There is no way for pack() and unpack() to know where characters are going to or
coming from, so they handle their output and input as flat sequences of characters.
• A "()" group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count
either as postfix, or for unpack(), also via the "/" template character. Within each
repetition of a group, positioning with "@" starts over at 0. Therefore, the result of
pack("@1A((@2A)@3A)", qw[X Y Z])
is the string "\0X\0\0YZ".
• "x" and "X" accept the "!" modifier to act as alignment commands: they jump forward or back
to the closest position aligned at a multiple of "count" characters. For example, to pack()
or unpack() a C structure like
struct {
char c; /* one signed, 8-bit character */
double d;
char cc[2];
}
one may need to use the template "c x![d] d c[2]". This assumes that doubles must be aligned
to the size of double.
For alignment commands, a "count" of 0 is equivalent to a "count" of 1; both are no-ops.
• "n", "N", "v" and "V" accept the "!" modifier to represent signed 16-/32-bit integers in
big-/little-endian order. This is portable only when all platforms sharing packed data use
the same binary representation for signed integers; for example, when all platforms use
two's-complement representation.
• Comments can be embedded in a TEMPLATE using "#" through the end of line. White space can
separate pack codes from each other, but modifiers and repeat counts must follow immediately.
Breaking complex templates into individual line-by-line components, suitably annotated, can
do as much to improve legibility and maintainability of pack/unpack formats as "/x" can for
complicated pattern matches.
• If TEMPLATE requires more arguments that pack() is given, pack() assumes additional ""
arguments. If TEMPLATE requires fewer arguments than given, extra arguments are ignored.
Examples:
$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the UTF-8
# bytes because the U at the start of the format caused a switch to
# U0-mode, so the UTF-8 bytes get joined into characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. On EBCDIC systems, the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same
The same template may generally also be used in unpack().
package NAMESPACE VERSION
package NAMESPACE
Declares the compilation unit as being in the given namespace. The scope of the package
declaration is from the declaration itself through the end of the enclosing block, file, or eval
(the same as the "my" operator). All further unqualified dynamic identifiers will be in this
namespace. A package statement affects dynamic variables only, including those you've used
"local" on, but not lexical variables, which are created with "my" (or "our" (or "state")).
Typically it would be the first declaration in a file included by "require" or "use". You can
switch into a package in more than one place, since this only determines which default symbol
table the compiler uses for the rest of that block. You can refer to identifiers in other
packages than the current one by prefixing the identifier with the package name and a double
colon, as in $SomePack::var or "ThatPack::INPUT_HANDLE". If package name is omitted, the "main"
package as assumed. That is, $::sail is equivalent to $main::sail (as well as to "$main'sail",
still seen in ancient code, mostly from Perl 4).
If VERSION is provided, "package" sets the $VERSION variable in the given namespace to a version
object with the VERSION provided. VERSION must be a "strict" style version number as defined by
the version module: a positive decimal number (integer or decimal-fraction) without
exponentiation or else a dotted-decimal v-string with a leading 'v' character and at least three
components. You should set $VERSION only once per package.
See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub
for other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system call. Note that if you set up a
loop of piped processes, deadlock can occur unless you are very careful. In addition, note that
Perl's pipes use IO buffering, so you may need to set $| to flush your WRITEHANDLE after each
command, depending on the application.
See IPC::Open2, IPC::Open3, and "Bidirectional Communication" in perlipc for examples of such
things.
On systems that support a close-on-exec flag on files, that flag is set on all newly opened file
descriptors whose "fileno"s are higher than the current value of $^F (by default 2 for "STDERR").
See "$^F" in perlvar.
pop ARRAY
pop Pops and returns the last value of the array, shortening the array by one element.
Returns the undefined value if the array is empty, although this may also happen at other times.
If ARRAY is omitted, pops the @ARGV array in the main program, but the @_ array in subroutines,
just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search left off for the variable in question ($_ is
used when the variable is not specified). Note that 0 is a valid match offset. "undef"
indicates that the search position is reset (usually due to match failure, but can also be
because no match has yet been run on the scalar). "pos" directly accesses the location used by
the regexp engine to store the offset, so assigning to "pos" will change that offset, and so will
also influence the "\G" zero-width assertion in regular expressions. Because a failed "m//gc"
match doesn't reset the offset, the return from "pos" won't change either in this case. See
perlre and perlop.
print FILEHANDLE LIST
print LIST
print
Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar
variable containing the name of or a reference to the filehandle, thus introducing one level of
indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be
misinterpreted as an operator unless you interpose a "+" or put parentheses around the
arguments.) If FILEHANDLE is omitted, prints to standard output by default, or to the last
selected output channel; see "select". If LIST is also omitted, prints $_ to the currently
selected output handle. To set the default output handle to something other than STDOUT use the
select operation. The current value of $, (if any) is printed between each LIST item. The
current value of "$\" (if any) is printed after the entire LIST has been printed. Because print
takes a LIST, anything in the LIST is evaluated in list context, and any subroutine that you call
will have one or more of its expressions evaluated in list context. Also be careful not to
follow the print keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print; put parentheses around all the arguments (or
interpose a "+", but that doesn't look as good).
Note that if you're storing FILEHANDLEs in an array, or if you're using any other expression more
complex than a scalar variable to retrieve it, you will have to use a block returning the
filehandle value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for more on
signal handling.
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that "$\" (the output record
separator) is not appended. The first argument of the list will be interpreted as the "printf"
format. See "sprintf" for an explanation of the format argument. If "use locale" is in effect,
and POSIX::setlocale() has been called, the character used for the decimal separator in formatted
floating-point numbers is affected by the LC_NUMERIC locale. See perllocale and POSIX.
Don't fall into the trap of using a "printf" when a simple "print" would do. The "print" is more
efficient and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string (or "undef" if the function has no prototype).
FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve.
If FUNCTION is a string starting with "CORE::", the rest is taken as a name for a Perl builtin.
If the builtin is not overridable (such as "qw//") or if its arguments cannot be adequately
expressed by a prototype (such as "system"), prototype() returns "undef", because the builtin
does not really behave like a Perl function. Otherwise, the string describing the equivalent
prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of
ARRAY increases by the length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the number of elements in the array following the completed
"push".
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See "Quote-Like Operators" in perlop.
qr/STRING/
Regexp-like quote. See "Regexp Quote-Like Operators" in perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word" characters backslashed. (That is, all characters
not matching "/[A-Za-z_0-9]/" will be preceded by a backslash in the returned string, regardless
of any locale settings.) This is the internal function implementing the "\Q" escape in double-
quoted strings.
If EXPR is omitted, uses $_.
quotemeta (and "\Q" ... "\E") are useful when interpolating strings into regular expressions,
because by default an interpolated variable will be considered a mini-regular expression. For
example:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};
Will cause $sentence to become 'The big bad wolf jumped over...'.
On the other hand:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};
Or:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};
Will both leave the sentence as is. Normally, when accepting string input from the user,
quotemeta() or "\Q" must be used.
rand EXPR
rand
Returns a random fractional number greater than or equal to 0 and less than the value of EXPR.
(EXPR should be positive.) If EXPR is omitted, the value 1 is used. Currently EXPR with the
value 0 is also special-cased as 1 (this was undocumented before Perl 5.8.0 and is subject to
change in future versions of Perl). Automatically calls "srand" unless "srand" has already been
called. See also "srand".
Apply "int()" to the value returned by "rand()" if you want random integers instead of random
fractional numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns numbers that are too large or too small, then
your version of Perl was probably compiled with the wrong number of RANDBITS.)
"rand()" is not cryptographically secure. You should not rely on it in security-sensitive
situations. As of this writing, a number of third-party CPAN modules offer random number
generators intended by their authors to be cryptographically secure, including:
Math::Random::Secure, Math::Random::MT::Perl, and Math::TrulyRandom.
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE.
Returns the number of characters actually read, 0 at end of file, or undef if there was an error
(in the latter case $! is also set). SCALAR will be grown or shrunk so that the last character
actually read is the last character of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other than the
beginning. A negative OFFSET specifies placement at that many characters counting backwards from
the end of the string. A positive OFFSET greater than the length of SCALAR results in the string
being padded to the required size with "\0" bytes before the result of the read is appended.
The call is implemented in terms of either Perl's or your system's native fread(3) library
function. To get a true read(2) system call, see "sysread".
Note the characters: depending on the status of the filehandle, either (8-bit) bytes or
characters are read. By default all filehandles operate on bytes, but for example if the
filehandle has been opened with the ":utf8" I/O layer (see "open", and the "open" pragma, open),
the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the
":encoding" pragma: in that case pretty much any characters can be read.
readdir DIRHANDLE
Returns the next directory entry for a directory opened by "opendir". If used in list context,
returns all the rest of the entries in the directory. If there are no more entries, returns the
undefined value in scalar context and the empty list in list context.
If you're planning to filetest the return values out of a "readdir", you'd better prepend the
directory in question. Otherwise, because we didn't "chdir" there, it would have been testing
the wrong file.
opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;
As of Perl 5.11.2 you can use a bare "readdir" in a "while" loop, which will set $_ on every
iteration.
opendir(my $dh, $some_dir) || die;
while(readdir $dh) {
print "$some_dir/$_\n";
}
closedir $dh;
readline EXPR
readline
Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR is not
provided). In scalar context, each call reads and returns the next line until end-of-file is
reached, whereupon the subsequent call returns "undef". In list context, reads until end-of-file
is reached and returns a list of lines. Note that the notion of "line" used here is whatever you
may have defined with $/ or $INPUT_RECORD_SEPARATOR). See "$/" in perlvar.
When $/ is set to "undef", when "readline" is in scalar context (i.e., file slurp mode), and when
an empty file is read, it returns '' the first time, followed by "undef" subsequently.
This is the internal function implementing the "<EXPR>" operator, but you can use it directly.
The "<EXPR>" operator is discussed in more detail in "I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If "readline" encounters an operating system error, $! will be set with the corresponding error
message. It can be helpful to check $! when you are reading from filehandles you don't trust,
such as a tty or a socket. The following example uses the operator form of "readline" and dies
if the result is not defined.
while ( ! eof($fh) ) {
defined( $_ = <$fh> ) or die "readline failed: $!";
...
}
Note that you have can't handle "readline" errors that way with the "ARGV" filehandle. In that
case, you have to open each element of @ARGV yourself since "eof" handles "ARGV" differently.
foreach my $arg (@ARGV) {
open(my $fh, $arg) or warn "Can't open $arg: $!";
while ( ! eof($fh) ) {
defined( $_ = <$fh> )
or die "readline failed for $arg: $!";
...
}
}
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links are implemented. If not, raises an
exception. If there is a system error, returns the undefined value and sets $! (errno). If EXPR
is omitted, uses $_.
readpipe EXPR
readpipe
EXPR is executed as a system command. The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially multi-line) string. In list context,
returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR). This
is the internal function implementing the "qx/EXPR/" operator, but you can use it directly. The
"qx/EXPR/" operator is discussed in more detail in "I/O Operators" in perlop. If EXPR is
omitted, uses $_.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH characters of data into variable
SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length
actually read. Takes the same flags as the system call of the same name. Returns the address of
the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an
error, returns the undefined value. This call is actually implemented in terms of recvfrom(2)
system call. See "UDP: Message Passing" in perlipc for examples.
Note the characters: depending on the status of the socket, either (8-bit) bytes or characters
are received. By default all sockets operate on bytes, but for example if the socket has been
changed using binmode() to operate with the ":encoding(utf8)" I/O layer (see the "open" pragma,
open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the
":encoding" pragma: in that case pretty much any characters can be read.
redo LABEL
redo
The "redo" command restarts the loop block without evaluating the conditional again. The
"continue" block, if any, is not executed. If the LABEL is omitted, the command refers to the
innermost enclosing loop. Programs that want to lie to themselves about what was just input
normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block that returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "redo"
inside such a block will effectively turn it into a looping construct.
See also "continue" for an illustration of how "last", "next", and "redo" work.
ref EXPR
ref Returns a non-empty string if EXPR is a reference, the empty string otherwise. If EXPR is not
specified, $_ will be used. The value returned depends on the type of thing the reference is a
reference to. Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp
If the referenced object has been blessed into a package, then that package name is returned
instead. You can think of "ref" as a "typeof" operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
The return value "LVALUE" indicates a reference to an lvalue that is not a variable. You get this
from taking the reference of function calls like "pos()" or "substr()". "VSTRING" is returned if
the reference points to a version string.
The result "Regexp" indicates that the argument is a regular expression resulting from "qr//".
See also perlref.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for
success, false otherwise.
Behavior of this function varies wildly depending on your system implementation. For example, it
will usually not work across file system boundaries, even though the system mv command sometimes
compensates for this. Other restrictions include whether it works on directories, open files, or
pre-existing files. Check perlport and either the rename(2) manpage or equivalent system
documentation for details.
For a platform independent "move" function look at the File::Copy module.
require VERSION
require EXPR
require
Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by
$_ if EXPR is not supplied.
VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a
literal of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION). An exception is
raised if VERSION is greater than the version of the current Perl interpreter. Compare with
"use", which can do a similar check at compile time.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads
to misleading error messages under earlier versions of Perl that do not support this syntax. The
equivalent numeric version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, "require" demands that a library file be included if it hasn't already been included.
The file is included via the do-FILE mechanism, which is essentially just a variety of "eval"
with the caveat that lexical variables in the invoking script will be invisible to the included
code. Has semantics similar to the following subroutine:
sub require {
my ($filename) = @_;
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
die "Compilation failed in require";
}
my ($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
if ($@) {
$INC{$filename} = undef;
die $@;
} elsif (!$result) {
delete $INC{$filename};
die "$filename did not return true value";
} else {
return $result;
}
}
Note that the file will not be included twice under the same specified name.
The file must return true as the last statement to indicate successful execution of any
initialization code, so it's customary to end such a file with "1;" unless you're sure it'll
return true otherwise. But it's better just to put the "1;", in case you add more statements.
If EXPR is a bareword, the require assumes a ".pm" extension and replaces "::" with "/" in the
filename for you, to make it easy to load standard modules. This form of loading of modules does
not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the "Foo/Bar.pm" file in the directories specified in
the @INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file in the @INC array and will complain about
not finding "Foo::Bar" there. In this case you can do:
eval "require $class";
Now that you understand how "require" looks for files with a bareword argument, there is a little
extra functionality going on behind the scenes. Before "require" looks for a ".pm" extension, it
will first look for a similar filename with a ".pmc" extension. If this file is found, it will be
loaded in place of any file ending in a ".pm" extension.
You can also insert hooks into the import facility, by putting Perl code directly into the @INC
array. There are three forms of hooks: subroutine references, array references and blessed
objects.
Subroutine references are the simplest case. When the inclusion system walks through @INC and
encounters a subroutine, this subroutine gets called with two parameters, the first a reference
to itself, and the second the name of the file to be included (e.g., "Foo/Bar.pm"). The
subroutine should return either nothing or else a list of up to three values in the following
order:
1. A filehandle, from which the file will be read.
2. A reference to a subroutine. If there is no filehandle (previous item), then this subroutine
is expected to generate one line of source code per call, writing the line into $_ and
returning 1, then returning 0 at end of file. If there is a filehandle, then the subroutine
will be called to act as a simple source filter, with the line as read in $_. Again, return
1 for each valid line, and 0 after all lines have been returned.
3. Optional state for the subroutine. The state is passed in as $_[1]. A reference to the
subroutine itself is passed in as $_[0].
If an empty list, "undef", or nothing that matches the first 3 values above is returned, then
"require" looks at the remaining elements of @INC. Note that this filehandle must be a real
filehandle (strictly a typeglob or reference to a typeglob, blessed or unblessed); tied
filehandles will be ignored and return value processing will stop there.
If the hook is an array reference, its first element must be a subroutine reference. This
subroutine is called as above, but the first parameter is the array reference. This lets you
indirectly pass arguments to the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
If the hook is an object, it must provide an INC method that will be called as above, the first
parameter being the object itself. (Note that you must fully qualify the sub's name, as
unqualified "INC" is always forced into package "main".) Here is a typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, Foo->new(...);
These hooks are also permitted to set the %INC entry corresponding to the files they have loaded.
See "%INC" in perlvar.
For a yet-more-powerful import facility, see "use" and perlmod.
reset EXPR
reset
Generally used in a "continue" block at the end of a loop to clear variables and reset "??"
searches so that they work again. The expression is interpreted as a list of single characters
(hyphens allowed for ranges). All variables and arrays beginning with one of those letters are
reset to their pristine state. If the expression is omitted, one-match searches ("?pattern?")
are reset to match again. Only resets variables or searches in the current package. Always
returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays and your
%ENV hash. Resets only package variables; lexical variables are unaffected, but they clean
themselves up on scope exit anyway, so you'll probably want to use them instead. See "my".
return EXPR
return
Returns from a subroutine, "eval", or "do FILE" with the value given in EXPR. Evaluation of EXPR
may be in list, scalar, or void context, depending on how the return value will be used, and the
context may vary from one execution to the next (see "wantarray"). If no EXPR is given, returns
an empty list in list context, the undefined value in scalar context, and (of course) nothing at
all in void context.
(In the absence of an explicit "return", a subroutine, eval, or do FILE automatically returns the
value of the last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of the elements of LIST in the opposite order.
In scalar context, concatenates the elements of LIST and returns a string value with all
characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world
print scalar reverse "dlrow ,", "olleH"; # Hello, world
Used without arguments in scalar context, reverse() reverses $_.
$_ = "dlrow ,olleH";
print reverse; # No output, list context
print scalar reverse; # Hello, world
Note that reversing an array to itself (as in "@a = reverse @a") will preserve non-existent
elements whenever possible, i.e., for non magical arrays or tied arrays with "EXISTS" and
"DELETE" methods.
This operator is also handy for inverting a hash, although there are some caveats. If a value is
duplicated in the original hash, only one of those can be represented as a key in the inverted
hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a
large hash, such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the "readdir" routine on
DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the position of the last occurrence of SUBSTR in
STR. If POSITION is specified, returns the last occurrence beginning at or before that position.
rmdir FILENAME
rmdir
Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it
returns true, otherwise it returns false and sets $! (errno). If FILENAME is omitted, uses $_.
To remove a directory tree recursively ("rm -rf" on Unix) look at the "rmtree" function of the
File::Path module.
s///
The substitution operator. See "Regexp Quote-Like Operators" in perlop.
say FILEHANDLE LIST
say LIST
say Just like "print", but implicitly appends a newline. "say LIST" is simply an abbreviation for "{
local $\ = "\n"; print LIST }".
This keyword is available only when the "say" feature is enabled: see feature.
scalar EXPR
Forces EXPR to be interpreted in scalar context and returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be interpolated in list context because
in practice, this is never needed. If you really wanted to do so, however, you could use the
construction "@{[ (some expression) ]}", but usually a simple "(some expression)" suffices.
Because "scalar" is a unary operator, if you accidentally use for EXPR a parenthesized list, this
behaves as a scalar comma expression, evaluating all but the last element in void context and
returning the final element evaluated in scalar context. This is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See perlop for more details on unary operators and the comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the "fseek" call of "stdio". FILEHANDLE may be an
expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the
new position in bytes to POSITION, 1 to set it to the current position plus POSITION, and 2 to
set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants
"SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file, current position, end of the file)
from the Fcntl module. Returns 1 on success, 0 otherwise.
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":encoding(utf8)" open layer), tell() will return byte offsets, not character offsets
(because implementing that would render seek() and tell() rather slow).
If you want to position the file for "sysread" or "syswrite", don't use "seek", because buffering
makes its effect on the file's read-write position unpredictable and non-portable. Use "sysseek"
instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch
between reading and writing. Amongst other things, this may have the effect of calling stdio's
clearerr(3). A WHENCE of 1 ("SEEK_CUR") is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating "tail -f". Once you hit EOF on your read and then
sleep for a while, you (probably) have to stick in a dummy seek() to reset things. The "seek"
doesn't change the position, but it does clear the end-of-file condition on the handle, so that
the next "<FILE>" makes Perl try again to read something. (We hope.)
If that doesn't work (some I/O implementations are particularly cantankerous), you might need
something like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine on DIRHANDLE. POS must be a value returned
by "telldir". "seekdir" also has the same caveats about possible directory compaction as the
corresponding system library routine.
select FILEHANDLE
select
Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new current
default filehandle for output. This has two effects: first, a "write" or a "print" without a
filehandle will default to this FILEHANDLE. Second, references to variables related to output
will refer to this output channel. For example, if you have to set the top of form format for
more than one output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write
the last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) syscall with the bit masks specified, which can be constructed using
"fileno" and "vec", along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles, you may wish to write a subroutine like this:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft, so calling select() in scalar
context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be
fractional. Note: not all implementations are capable of returning the $timeleft. If not, they
always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after signals (say, SIGALRM) is implementation-dependent. implementationdependent.
dependent. See also perlport for notes on the portability of "select".
On error, "select" behaves like select(2): it returns -1 and sets $!.
On some Unixes, select(2) may report a socket file descriptor as "ready for reading" when no data
is available, and thus a subsequent read blocks. This can be avoided if you always use O_NONBLOCK
on the socket. See select(2) and fcntl(2) for further details.
WARNING: One should not attempt to mix buffered I/O (like "read" or <FH>) with "select", except
as permitted by POSIX, and even then only on POSIX systems. You have to use "sysread" instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a
variable that will hold the returned semid_ds structure or semaphore value array. Returns like
"ioctl": the undefined value for error, ""0 but true"" for zero, or the actual return value
otherwise. The ARG must consist of a vector of native short integers, which may be created with
"pack("s!",(0)x$nsem)". See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::Semaphore"
documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined value if
there is an error. See also "SysV IPC" in perlipc, "IPC::SysV", "IPC::SysV::Semaphore"
documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop(2) for semaphore operations such as signalling and waiting.
OPSTRING must be a packed array of semop structures. Each semop structure can be generated with
"pack("s!3", $semnum, $semop, $semflag)". The length of OPSTRING implies the number of semaphore
operations. Returns true if successful, or false if there is an error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See also "SysV IPC" in perlipc, "IPC::SysV", and
"IPC::SysV::Semaphore" documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes
the same flags as the system call of the same name. On unconnected sockets, you must specify a
destination to send to, in which case it does a sendto(2) syscall. Returns the number of
characters sent, or the undefined value on error. The sendmsg(2) syscall is currently
unimplemented. See "UDP: Message Passing" in perlipc for examples.
Note the characters: depending on the status of the socket, either (8-bit) bytes or characters
are sent. By default all sockets operate on bytes, but for example if the socket has been
changed using binmode() to operate with the ":encoding(utf8)" I/O layer (see "open", or the
"open" pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes.
Similarly for the ":encoding" pragma: in that case pretty much any characters can be sent.
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0 for the current process. Raises an
exception when used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If
the arguments are omitted, it defaults to "0,0". Note that the BSD 4.2 version of "setpgrp" does
not accept any arguments, so only "setpgrp(0,0)" is portable. See also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user. (See setpriority(2).)
Raises an exception when used on a machine that doesn't implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there is an error. Use integer constants
provided by the "Socket" module for LEVEL and OPNAME. Values for LEVEL can also be obtained from
getprotobyname. OPTVAL might either be a packed string or an integer. An integer OPTVAL is
shorthand for pack("i", OPTVAL).
An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
shift ARRAY
shift
Shifts the first value of the array off and returns it, shortening the array by 1 and moving
everything down. If there are no elements in the array, returns the undefined value. If ARRAY
is omitted, shifts the @_ array within the lexical scope of subroutines and formats, and the
@ARGV array outside a subroutine and also within the lexical scopes established by the "eval
STRING", "BEGIN {}", "INIT {}", "CHECK {}", "UNITCHECK {}" and "END {}" constructs.
See also "unshift", "push", and "pop". "shift" and "unshift" do the same thing to the left end
of an array that "pop" and "push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable
that will hold the returned "shmid_ds" structure. Returns like ioctl: the undefined value for
error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in
perlipc and "IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined
value if there is an error. See also "SysV IPC" in perlipc and "IPC::SysV" documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by
attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable
that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if
STRING is too short, nulls are written to fill out SIZE bytes. Return true if successful, or
false if there is an error. shmread() taints the variable. See also "SysV IPC" in perlipc,
"IPC::SysV" documentation, and the "IPC::Shareable" module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation
as in the syscall of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other side you're done writing but not done
reading, or vice versa. It's also a more insistent form of close because it also disables the
file descriptor in any forked copies in other processes.
Returns 1 for success; on error, returns "undef" if the first argument is not a valid filehandle,
or returns 0 and sets $! for any other failure.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_.
For the inverse sine operation, you may use the "Math::Trig::asin" function, or use this
relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep
Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is given.
Returns the integer number of seconds actually slept.
May be interrupted if the process receives a signal such as "SIGALRM".
eval {
local $SIG{ALARM} = sub { die "Alarm!\n" };
sleep;
};
die $@ unless $@ eq "Alarm!\n";
You probably cannot mix "alarm" and "sleep" calls, because "sleep" is often implemented using
"alarm".
On some older systems, it may sleep up to a full second less than what you requested, depending
on how it counts seconds. Most modern systems always sleep the full amount. They may appear to
sleep longer than that, however, because your process might not be scheduled right away in a busy
multitasking system.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting
from Perl 5.8 part of the standard distribution) provides usleep(). You may also use Perl's
four-argument version of select() leaving the first three arguments undefined, or you might be
able to use the "syscall" interface to access setitimer(2) if your system supports it. See
perlfaq8 for details.
See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and
PROTOCOL are specified the same as for the syscall of the same name. You should "use Socket"
first to get the proper definitions imported. See the examples in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F. See "$^F" in perlvar.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE,
and PROTOCOL are specified the same as for the syscall of the same name. If unimplemented,
raises an exception. Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened
file descriptors, as determined by the value of $^F. See "$^F" in perlvar.
Some systems defined "pipe" in terms of "socketpair", in which a call to "pipe(Rdr, Wtr)" is
essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using
IP sockets to localhost if your system implements sockets but not socketpair.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the sorted list value. In scalar context, the
behaviour of "sort()" is undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard string comparison order. If SUBNAME is
specified, it gives the name of a subroutine that returns an integer less than, equal to, or
greater than 0, depending on how the elements of the list are to be ordered. (The "<=>" and
"cmp" operators are extremely useful in such routines.) SUBNAME may be a scalar variable name
(unsubscripted), in which case the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.
If the subroutine's prototype is "($$)", the elements to be compared are passed by reference in
@_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements
to be compared are passed into the subroutine as the package global variables $a and $b (see
example below). Note that in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.
The values to be compared are always passed by reference and should not be modified.
You also cannot exit out of the sort block or subroutine using any of the loop control operators
described in perlsyn or with "goto".
When "use locale" is in effect, "sort LIST" sorts LIST according to the current collation locale.
See perllocale.
sort() returns aliases into the original list, much as a for loop's index variable aliases the
list elements. That is, modifying an element of a list returned by sort() (for example, in a
"foreach", "map" or "grep") actually modifies the element in the original list. This is usually
something to be avoided when writing clear code.
Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not
stable, and could go quadratic. (A stable sort preserves the input order of elements that
compare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length
N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort
implementation was replaced with a stable mergesort algorithm whose worst-case behavior is
O(NlogN). But benchmarks indicated that for some inputs, on some platforms, the original
quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt
control of the underlying algorithm may not persist into future Perls, but the ability to
characterize the input or output in implementation independent ways quite probably will. See the
sort pragma.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
my @new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my @nums = @caps = ();
for (@old) {
push @nums, ( /=(\d+)/ ? $1 : undef );
push @caps, uc($_);
}
my @new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
Warning: syntactical care is required when sorting the list returned from a function. If you want
to sort the list returned by the function call "find_records(@key)", you can use:
@contact = sort { $a cmp $b } find_records @key;
@contact = sort +find_records(@key);
@contact = sort &find_records(@key);
@contact = sort(find_records(@key));
If instead you want to sort the array @key with the comparison routine "find_records()" then you
can use:
@contact = sort { find_records() } @key;
@contact = sort find_records(@key);
@contact = sort(find_records @key);
@contact = sort(find_records (@key));
If you're using strict, you must not declare $a and $b as lexicals. They are package globals.
That means that if you're in the "main" package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a and $::b), but if you're in the "FooPack"
package, it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns inconsistent results (sometimes
saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are
not well-defined.
Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), and because "sort"
raises an exception unless the result of a comparison is defined, when sorting with a comparison
function like "$a <=> $b", be careful about lists that might contain a "NaN". The following
example takes advantage that "NaN != NaN" to eliminate any "NaN"s from the input list.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the
elements of LIST, if any. In list context, returns the elements removed from the array. In
scalar context, returns the last element removed, or "undef" if no elements are removed. The
array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end
of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is
negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the
array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of
the array, Perl issues a warning, and splices at the end of the array.
The following equivalences hold (assuming "$[ == 0 and $#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split
Splits the string EXPR into a list of strings and returns that list. By default, empty leading
fields are preserved, and empty trailing ones are deleted. (If all fields are empty, they are
considered to be trailing.)
In scalar context, returns the number of fields found.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace
(after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter
separating the fields. (Note that the delimiter may be longer than one character.)
If LIMIT is specified and positive, it represents the maximum number of fields the EXPR will be
split into, though the actual number of fields returned depends on the number of times PATTERN
matches within EXPR. If LIMIT is unspecified or zero, trailing null fields are stripped (which
potential users of "pop" would do well to remember). If LIMIT is negative, it is treated as if
an arbitrarily large LIMIT had been specified. Note that splitting an EXPR that evaluates to the
empty string always returns the empty list, regardless of the LIMIT specified.
A pattern matching the empty string (not to be confused with an empty pattern "//", which is just
one member of the set of patterns matching the epmty string), splits EXPR into individual
characters. For example:
print join(':', split(/ */, 'hi there')), "\n";
produces the output 'h:i:t:h:e:r:e'.
As a special case for "split", the empty pattern "//" specifically matches the empty string; this
is not be confused with the normal use of an empty pattern to mean the last successful match. So
to split a string into individual characters, the following:
print join(':', split(//, 'hi there')), "\n";
produces the output 'h:i: :t:h:e:r:e'.
Empty leading fields are produced when there are positive-width matches at the beginning of the
string; a zero-width match at the beginning of the string does not produce an empty field. For
example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other hand, are produced
when there is a match at the end of the string (and when LIMIT is given and is not 0), regardless
of the length of the match. For example:
print join(':', split(//, 'hi there!', -1)), "\n";
print join(':', split(/\W/, 'hi there!', -1)), "\n";
produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively, both with an empty
trailing field.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the
number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have
been 4 by default. In time critical applications it behooves you not to split into more fields
than you really need.
If the PATTERN contains parentheses, additional list elements are created from each matching
substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email message in $header, you could split it up
into fields and their values this way:
$header =~ s/\n(?=\s)//g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an expression to specify patterns that vary at
runtime. (To do runtime compilation only once, use "/$variable/o".)
As a special case, specifying a PATTERN of space (' ') will split on white space just as "split"
with no arguments does. Thus, "split(' ')" can be used to emulate awk's default behavior,
whereas "split(/ /)" will give you as many initial null fields (empty string) as there are
leading spaces. A "split" on "/\s+/" is like a "split(' ')" except that any leading whitespace
produces a null first field. A "split" with no arguments really does a "split(' ', $_)"
internally.
A PATTERN of "/^/" is treated as if it were "/^/m", since it isn't much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing parentheses that are not matched in a "split()"
will be set to "undef" when returned:
@fields = split /(A)|B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf" conventions of the C library function "sprintf".
See below for more details and see sprintf(3) or printf(3) on your system for an explanation of
the general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting: it emulates the C function sprintf(3), but doesn't use it
except for floating-point numbers, and even then only standard modifiers are allowed. Non-standard Nonstandard
standard extensions in your local sprintf(3) are therefore unavailable from Perl.
Unlike "printf", "sprintf" does not do what you probably mean when you pass it an array as your
first argument. The array is given scalar context, and instead of using the 0th element of the
array as the format, Perl will use the count of elements in the array as the format, which is
almost never useful.
Perl's "sprintf" permits the following universally-known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary
but widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the scientific notation produced by %e, %E, %g and %G
for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three
or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either
"1.23e99" or "1.23e099".
Between the "%" and the format letter, you may specify several additional attributes controlling
the interpretation of the format. In order, these are:
format parameter index
An explicit format parameter index, such as "2$". By default sprintf will format the next
unused argument in the list, but this allows you to take the arguments out of order:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix non-negative number with a space
+ prefix non-negative number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# ensure the leading "0" for any octal,
prefix non-zero hexadecimal with "0x" or "0X",
prefix non-zero binary with "0b" or "0B"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#o>', 12; # prints "<014>"
printf '<%#x>', 12; # prints "<0xc>"
printf '<%#X>', 12; # prints "<0XC>"
printf '<%#b>', 12; # prints "<0b1100>"
printf '<%#B>', 12; # prints "<0B1100>"
When a space and a plus sign are given as the flags at once, a plus sign is used to prefix a
positive number.
printf '<%+ d>', 12; # prints "<+12>"
printf '<% +d>', 12; # prints "<+12>"
When the # flag and a precision are given in the %o conversion, the precision is incremented
if it's necessary for the leading "0".
printf '<%#.5o>', 012; # prints "<00012>"
printf '<%#.5o>', 012345; # prints "<012345>"
printf '<%#.0o>', 0; # prints "<0>"
vector flag
This flag tells Perl to interpret the supplied string as a vector of integers, one for each
character in the string. Perl applies the format to each integer in turn, then joins the
resulting strings with a separator (a dot "." by default). This can be useful for displaying
ordinal values of characters in arbitrary strings:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override the string to use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for the join string using
something like "*2$v"; for example:
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
(minimum) width
Arguments are usually formatted to be only as wide as required to display the given value.
You can override the width by putting a number here, or get the width from the next argument
(with "*") or from a specified argument (e.g., with "*2$"):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
printf '<%*s>', 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf '<%2s>', "long"; # prints "<long>" (does not truncate)
If a field width obtained through "*" is negative, it has the same effect as the "-" flag:
left-justification.
precision, or maximum width
You can specify a precision (for numeric conversions) or a maximum width (for string
conversions) by specifying a "." followed by a number. For floating-point formats except 'g'
and 'G', this specifies how many places right of the decimal point to show (the default being
6). For example:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For "g" and "G", this specifies the maximum number of digits to show, including thoe prior to
the decimal point and those after it; for example:
# These examples are subject to system-specific variation.
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision implies that the output of the number itself
should be zero-padded to this width, where the 0 flag is ignored:
printf '<%.6d>', 1; # prints "<000001>"
printf '<%+.6d>', 1; # prints "<+000001>"
printf '<%-10.6d>', 1; # prints "<000001 >"
printf '<%10.6d>', 1; # prints "< 000001>"
printf '<%010.6d>', 1; # prints "< 000001>"
printf '<%+10.6d>', 1; # prints "< +000001>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
printf '<%10.6x>', 1; # prints "< 000001>"
printf '<%010.6x>', 1; # prints "< 000001>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"
For string conversions, specifying a precision truncates the string to fit the specified
width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using ".*":
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
If a precision obtained through "*" is negative, it counts as having no precision at all.
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*s>', 0, "string"; # prints "<>"
printf '<%.*s>', -1, "string"; # prints "<string>"
printf '<%.*d>', 1, 0; # prints "<0>"
printf '<%.*d>', 0, 0; # prints "<>"
printf '<%.*d>', -1, 0; # prints "<0>"
You cannot currently get the precision from a specified number, but it is intended that this
will be possible in the future, for example using ".*2$":
printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
size
For numeric conversions, you can specify the size to interpret the number as using "l", "h",
"V", "q", "L", or "ll". For integer conversions ("d u o x X b i D U O"), numbers are usually
assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits),
but you can override this to use instead one of the standard C types, as supported by the
compiler used to build Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will raise an exception if Perl does not understand "quads" in your installation.
(This requires either that the platform natively support quads, or that Perl were
specifically compiled to support quads.) You can find out whether your Perl supports quads
via Config:
use Config;
if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
print "Nice quads!\n";
}
For floating-point conversions ("e f g E F G"), numbers are usually assumed to be the default
floating-point size on your platform (double or long double), but you can force "long double"
with "q", "L", or "ll" if your platform supports them. You can find out whether your Perl
supports long doubles via Config:
use Config;
print "long doubles\n" if $Config{d_longdbl} eq "define";
You can find out whether Perl considers "long double" to be the default floating-point size
to use on your platform via Config:
use Config;
if ($Config{uselongdouble} eq "define") {
print "long doubles by default\n";
}
It can also be that long doubles and doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl code, but is supported for compatibility with
XS code. It means "use the standard size for a Perl integer or floating-point number", which
is the default.
order of arguments
Normally, sprintf() takes the next unused argument as the value to format for each format
specification. If the format specification uses "*" to require additional arguments, these
are consumed from the argument list in the order they appear in the format specification
before the value to format. Where an argument is specified by an explicit index, this does
not affect the normal order for the arguments, even when the explicitly specified index would
have been the next argument.
So:
printf "<%*.*s>", $a, $b, $c;
uses $a for the width, $b for the precision, and $c as the value to format; while:
printf "<%*1$.*s>", $a, $b;
would use $a for the width and precision, and $b as the value to format.
Here are some more examples; be aware that when using an explicit index, the "$" may need
escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If "use locale" is in effect and POSIX::setlocale() has been called, the character used for the
decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale. See
perllocale and POSIX.
sqrt EXPR
sqrt
Return the positive square root of EXPR. If EXPR is omitted, uses $_. Works only for non-negative nonnegative
negative operands unless you've loaded the "Math::Complex" module.
use Math::Complex;
print sqrt(-4); # prints 2i
srand EXPR
srand
Sets the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand" function so that "rand" can produce a different
sequence each time you run your program.
If srand() is not called explicitly, it is called implicitly at the first use of the "rand"
operator. However, this was not true of versions of Perl before 5.004, so if your script will
run under older Perl versions, it should call "srand".
Most programs won't even call srand() at all, except those that need a cryptographically-strong
starting point rather than the generally acceptable default, which is based on time of day,
process ID, and memory allocation, or the /dev/urandom device if available. You may also want to
call srand() after a fork() to avoid child processes sharing the same seed value as the parent
(and consequently each other).
You can call srand($seed) with the same $seed to reproduce the same sequence from rand(), but
this is usually reserved for generating predictable results for testing or debugging. Otherwise,
don't call srand() more than once in your program.
Do not call srand() (i.e., without an argument) more than once per process. The internal state
of the random number generator should contain more entropy than can be provided by any seed, so
calling srand() again actually loses randomness.
Most implementations of "srand" take an integer and will silently truncate decimal numbers. This
means "srand(42)" will usually produce the same results as "srand(42.1)". To be safe, always
pass "srand" an integer.
In versions of Perl prior to 5.004 the default seed was just the current "time". This isn't a
particularly good seed, so many old programs supply their own seed value (often "time ^ $$" or
"time ^ ($$ + ($$ << 15))"), but that isn't necessary any more.
For cryptographic purposes, however, you need something much more random than the default seed.
Checksumming the compressed output of one or more rapidly changing operating system status
programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
If you're particularly concerned with this, search the CPAN for random number generator modules
instead of rolling out your own.
Frequently called programs (like CGI scripts) that simply use
time ^ $$
for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
stat DIRHANDLE
stat
Returns a 13-element list giving the status info for a file, either the file opened via
FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns the empty
list if "stat" fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are the meanings of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-
portable. In particular, you cannot expect it to be a "creation time", see "Files and
Filesystems" in perlport for details.
If "stat" is passed the special filehandle consisting of an underline, no stat is done, but the
current contents of the stat structure from the last "stat", "lstat", or filetest are returned.
Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is negative under NFS.)
Because the mode contains both the file type and its permissions, you should mask off the file
type portion and (s)printf using a "%o" if you want to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value indicating success or failure, and, if
successful, sets the information associated with the special filehandle "_".
The File::stat module provides a convenient, by-name access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and functions ("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_directory = S_ISDIR($mode);
You could write the last two using the "-u" and "-d" operators. Commonly available "S_IF*"
constants are:
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the "S_IF*" functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with (for example) S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details about the "S_*" constants.
To get status info for a symbolic link instead of the target file behind the link, use the
"lstat" function.
state EXPR
state TYPE EXPR
state EXPR : ATTRS
state TYPE EXPR : ATTRS
"state" declares a lexically scoped variable, just like "my" does. However, those variables will
never be reinitialized, contrary to lexical variables that are reinitialized each time their
enclosing block is entered.
"state" variables are enabled only when the "use feature "state"" pragma is in effect. See
feature.
study SCALAR
study
Takes extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern
matches on the string before it is next modified. This may or may not save time, depending on
the nature and number of patterns you are searching on, and on the distribution of character
frequencies in the string to be searched; you probably want to compare run times with and without
it to see which runs faster. Those loops that scan for many short constant strings (including
the constant parts of more complex patterns) will benefit most. You may have only one "study"
active at a time: if you study a different scalar the first is "unstudied". (The way "study"
works is this: a linked list of every character in the string to be searched is made, so we know,
for example, where all the 'k' characters are. From each search string, the rarest character is
selected, based on some static frequency tables constructed from some C programs and English
text. Only those places that contain this "rarest" character are examined.)
For example, here is a loop that inserts index producing entries before any line containing a
certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only locations in $_ that contain "f" will be looked at, because
"f" is rarer than "o". In general, this is a big win except in pathological cases. The only
question is whether it saves you more time than it took to build the linked list in the first
place.
Note that if you have to look for strings that you don't know till runtime, you can build an
entire loop as a string and "eval" that to avoid recompiling all your patterns all the time.
Together with undefining $/ to input entire files as one record, this can be quite fast, often
faster than specialized programs like fgrep(1). The following scans a list of files (@files) for
a list of words (@words), and prints out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward
declaration. Without a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created.
See perlsub and perlref for details about subroutines and references, and attributes and
Attribute::Handlers for more information about attributes.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever
you've set $[ to (but don't do that). If OFFSET is negative (or more precisely, less than $[),
starts that far from the end of the string. If LENGTH is omitted, returns everything to the end
of the string. If LENGTH is negative, leaves that many characters off the end of the string.
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr
You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If
you assign something shorter than LENGTH, the string will shrink, and if you assign something
longer than LENGTH, the string will grow to accommodate it. To keep the string the same length,
you may need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within
the string is returned. If the substring is beyond either end of the string, substr() returns
the undefined value and produces a warning. When used as an lvalue, specifying a substring that
is entirely outside the string raises an exception. Here's an example showing the behavior for
boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exception
An alternative to using substr() as an lvalue is to specify the replacement string as the 4th
argument. This allows you to replace parts of the EXPR and return what was there before in one
operation, just as you can with splice().
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the 3-arg version of substr() acts as a 'magic bullet'; each
time it is assigned to, it remembers which part of the original string is being modified; for
example:
$x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
Prior to Perl version 5.9.1, the result of using an lvalue multiple times was unspecified.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0
otherwise. On systems that don't support symbolic links, raises an exception. To check for
that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall NUMBER, LIST
Calls the system call specified as the first element of the list, passing the remaining elements
as arguments to the system call. If unimplemented, raises an exception. The arguments are
interpreted as follows: if a given argument is numeric, the argument is passed as an int. If
not, the pointer to the string value is passed. You are responsible to make sure a string is
pre-extended long enough to receive any result that might be written into a string. You can't
use a string literal (or other read-only string) as an argument to "syscall" because Perl has to
assume that any string pointer might be written through. If your integer arguments are not
literals and have never been interpreted in a numeric context, you may need to add 0 to them to
force them to look like numbers. This emulates the "syswrite" function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your syscall, which in practice
should (usually) suffice.
Syscall returns whatever value returned by the system call it calls. If the system call fails,
"syscall" returns "-1" and sets $! (errno). Note that some system calls can legitimately return
"-1". The proper way to handle such calls is to assign "$!=0;" before the call and check the
value of $! if syscall returns "-1".
There's a problem with "syscall(&SYS_pipe)": it returns the file number of the read end of the
pipe it creates. There is no way to retrieve the file number of the other end. You can avoid
this problem by using "pipe" instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If
FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This
function calls the underlying operating system's "open" function with the parameters FILENAME,
MODE, PERMS.
The possible values and flag bits of the MODE parameter are system-dependent; they are available
via the standard module "Fcntl". See the documentation of your operating system's "open" to see
which values and flag bits are available. You may combine several flags using the "|"-operator.
Some of the most common values are "O_RDONLY" for opening the file in read-only mode, "O_WRONLY"
for opening the file in write-only mode, and "O_RDWR" for opening the file in read-write mode.
For historical reasons, some values work on almost every system supported by Perl: 0 means read-only, readonly,
only, 1 means write-only, and 2 means read/write. We know that these values do not work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the "open" call creates it (typically because
MODE includes the "O_CREAT" flag), then the value of PERMS specifies the permissions of the newly
created file. If you omit the PERMS argument to "sysopen", Perl uses the octal value 0666.
These permission values need to be in octal, and are modified by your process's current "umask".
In many systems the "O_EXCL" flag is available for opening files in exclusive mode. This is not
locking: exclusiveness means here that if the file already exists, sysopen() fails. "O_EXCL" may
not work on network filesystems, and has no effect unless the "O_CREAT" flag is set as well.
Setting "O_CREAT|O_EXCL" prevents the file from being opened if it is a symbolic link. It does
not protect against symbolic links in the file's path.
Sometimes you may want to truncate an already-existing file. This can be done using the
"O_TRUNC" flag. The behavior of "O_TRUNC" with "O_RDONLY" is undefined.
You should seldom if ever use 0644 as argument to "sysopen", because that takes away the user's
option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on "umask"
for more on this.
Note that "sysopen" depends on the fdopen() C library function. On many Unix systems, fdopen()
is known to fail when file descriptors exceed a certain value, typically 255. If you need more
file descriptors than that, consider rebuilding Perl to use the "sfio" library, or perhaps using
the POSIX::open() function.
See perlopentut for a kinder, gentler explanation of opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using
the read(2). It bypasses buffered IO, so mixing this with other kinds of reads, "print",
"write", "seek", "tell", or "eof" can cause confusion because the perlio or stdio layers usually
buffers data. Returns the number of bytes actually read, 0 at end of file, or undef if there was
an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last
byte actually read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other than the
beginning. A negative OFFSET specifies placement at that many characters counting backwards from
the end of the string. A positive OFFSET greater than the length of SCALAR results in the string
being padded to the required size with "\0" bytes before the result of the read is appended.
There is no syseof() function, which is ok, since eof() doesn't work well on device files (like
ttys) anyway. Use sysread() and check for a return value for 0 to decide whether you're done.
Note that if the filehandle has been marked as ":utf8" Unicode characters are read instead of
bytes (the LENGTH, OFFSET, and the return value of sysread() are in Unicode characters). The
":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode", "open", and the
"open" pragma, open.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an expression whose
value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to
POSITION, 1 to set the it to the current position plus POSITION, and 2 to set it to EOF plus
POSITION (typically negative).
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":encoding(utf8)" I/O layer), tell() will return byte offsets, not character offsets
(because implementing that would render sysseek() unacceptably slow).
sysseek() bypasses normal buffered IO, so mixing this with reads (other than "sysread", for
example "<>" or read()) "print", "write", "seek", "tell", or "eof" may cause confusion.
For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the
file, current position, end of the file) from the Fcntl module. Use of the constants is also
more portable than relying on 0, 1, and 2. For example to define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A position of zero is returned as
the string "0 but true"; thus "sysseek" returns true on success and false on failure, yet you can
still easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except that a fork is done first, and the parent
process waits for the child process to exit. Note that argument processing varies depending on
the number of arguments. If there is more than one argument in LIST, or if LIST is an array with
more than one value, starts the program given by the first element of the list with arguments
given by the rest of the list. If there is only one scalar argument, the argument is checked for
shell metacharacters, and if there are any, the entire argument is passed to the system's command
shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other platforms). If
there are no shell metacharacters in the argument, it is split into words and passed directly to
"execvp", which is more efficient.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any
operation that may do a fork, but this may not be supported on some platforms (see perlport). To
be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
The return value is the exit status of the program as returned by the "wait" call. To get the
actual exit value, shift right by eight (see below). See also "exec". This is not what you want
to use to capture the output from a command, for that you should use merely backticks or "qx//",
as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the
program or an error of the wait(2) system call (inspect $! for the reason).
If you'd like to make "system" (and many other bits of Perl) die on error, have a look at the
autodie pragma.
Like "exec", "system" allows you to lie to a program about its name if you use the "system
PROGRAM LIST" syntax. Again, see "exec".
Since "SIGINT" and "SIGQUIT" are ignored during the execution of "system", if you expect your
program to terminate on receipt of these signals you will need to arrange to do so yourself based
on the return value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
If you'd like to manually inspect "system"'s failure, you can check all possible failure modes by
inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
Alternatively, you may inspect the value of "${^CHILD_ERROR_NATIVE}" with the "W*()" calls from
the POSIX module.
When "system"'s arguments are executed indirectly by the shell, results and return codes are
subject to its quirks. See "`STRING`" in perlop and "exec" for details.
Since "system" does a "fork" and "wait" it may affect a "SIGCHLD" handler. See perlipc for
details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using
write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses buffered IO, so mixing
this with reads (other than sysread()), "print", "write", "seek", "tell", or "eof" may cause
confusion because the perlio and stdio layers usually buffers data. Returns the number of bytes
actually written, or "undef" if there was an error (in this case the errno variable $! is also
set). If the LENGTH is greater than the data available in the SCALAR after the OFFSET, only as
much data as is available will be written.
An OFFSET may be specified to write the data from some part of the string other than the
beginning. A negative OFFSET specifies writing that many characters counting backwards from the
end of the string. If SCALAR is of length zero, you can only use an OFFSET of 0.
Warning: If the filehandle is marked ":utf8", Unicode characters encoded in UTF-8 are written
instead of bytes, and the LENGTH, OFFSET, and return value of syswrite() are in (UTF-8 encoded
Unicode) characters. The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See
"binmode", "open", and the "open" pragma, open.
tell FILEHANDLE
tell
Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an
expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted,
assumes the file last read.
Note the in bytes: even if the filehandle has been set to operate on characters (for example by
using the ":encoding(utf8)" open layer), tell() will return byte offsets, not character offsets
(because that would render seek() and tell() rather slow).
The return value of tell() for the standard streams like the STDIN depends on the operating
system: it may return -1 or something else. tell() on pipes, fifos, and sockets usually returns
-1.
There is no "systell" function. Use "sysseek(FH, 0, 1)" for that.
Do not use tell() (or other buffered I/O operations) on a filehandle that has been manipulated by
sysread(), syswrite() or sysseek(). Those functions ignore the buffering, while tell() does not.
telldir DIRHANDLE
Returns the current position of the "readdir" routines on DIRHANDLE. Value may be given to
"seekdir" to access a particular location in a directory. "telldir" has the same caveats about
possible directory compaction as the corresponding system library routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will provide the implementation for the
variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a
class implementing objects of correct type. Any additional arguments are passed to the "new"
method of the class (meaning "TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically
these are arguments such as might be passed to the "dbm_open()" function of C. The object
returned by the "new" method is also returned by the "tie" function, which would be useful if you
want to access other methods in CLASSNAME.
Note that functions such as "keys" and "values" may return huge lists when used on large objects,
like DBM files. You may prefer to use the "each" function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a filehandle should have the following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array,
Tie::Scalar, and Tie::Handle.
Unlike "dbmopen", the "tie" function will not "use" or "require" a module for you; you need to do
that explicitly yourself. See DB_File or the Config module for interesting "tie"
implementations.
For further details see perltie, "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value that was originally
returned by the "tie" call that bound the variable to a package.) Returns the undefined value if
VARIABLE isn't tied to a package.
time
Returns the number of non-leap seconds since whatever time the system considers to be the epoch,
suitable for feeding to "gmtime" and "localtime". On most systems the epoch is 00:00:00 UTC,
January 1, 1970; a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 1904
in the current local time zone for its epoch.
For measuring time in better granularity than one second, you may use either the Time::HiRes
module (from CPAN, and starting from Perl 5.8 part of the standard distribution), or if you have
gettimeofday(2), you may be able to use the "syscall" interface of Perl. See perlfaq8 for
details.
For date and time processing look at the many related modules on CPAN. For a comprehensive date
and time representation look at the DateTime module.
times
Returns a four-element list giving the user and system times, in seconds, for this process and
the children of this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
Children's times are only included for terminated children.
tr///
The transliteration operator. Same as "y///". See "Quote and Quote-like Operators" in perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Raises an
exception if truncate isn't implemented on your system. Returns true if successful, the
undefined value otherwise.
The behavior is undefined if LENGTH is greater than the length of the file.
The position in the file of FILEHANDLE is left unchanged. You may want to call seek before
writing to the file.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal function implementing the "\U"
escape in double-quoted strings. It does not attempt to do titlecase mapping on initial letters.
See "ucfirst" for that.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma, such as in a locale, as "lc" does.
ucfirst EXPR
ucfirst
Returns the value of EXPR with the first character in uppercase (titlecase in Unicode). This is
the internal function implementing the "\u" escape in double-quoted strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma, such as in a locale, as "lc" does.
umask EXPR
umask
Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted,
merely returns the current umask.
The Unix permission "rwxr-x---" is represented as three sets of three bits, or three octal
digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The "umask" value is
such a number representing disabled permissions bits. The permission (or "mode") values you pass
"mkdir" or "sysopen" are modified by your umask, so even if you tell "sysopen" to create a file
with permissions 0777, if your umask is 0022 then the file will actually be created with
permissions 0755. If your "umask" were 0027 (group can't write; others can't read, write, or
execute), then passing "sysopen" 0666 would create a file with mode 0640 ("0666 &~ 027" is 0640).
Here's some advice: supply a creation mode of 0666 for regular files (in "sysopen") and one of
0777 for directories (in "mkdir") and executable files. This gives users the freedom of choice:
if they want protected files, they might choose process umasks of 022, 027, or even the
particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better
left to the user. The exception to this is when writing files that should be kept private: mail
files, web browser cookies, .rhosts files, and so on.
If umask(2) is not implemented on your system and you are trying to restrict access for yourself
(i.e., "(EXPR & 0700) > 0"), raises an exception. If umask(2) is not implemented and you are not
trying to restrict access for yourself, returns "undef".
Remember that a umask is a number, usually given in octal; it is not a string of octal digits.
See also "oct", if all you have is a string.
undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array
(using "@"), a hash (using "%"), a subroutine (using "&"), or a typeglob (using "*"). Saying
"undef $hash{$key}" will probably not do what you expect on most predefined variables or DBM list
values, so don't do that; see delete. Always returns the undefined value. You can omit the
EXPR, in which case nothing is undefined, but you still get an undefined value that you could,
for instance, return from a subroutine, assign to a variable, or pass as a parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink
Deletes a list of files. On success, it returns the number of files it successfully deleted. On
failure, it returns false and sets $! (errno):
my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";
On error, "unlink" will not tell you which files it could not remove. If you want to know which
files you could not remove, try them one at a time:
foreach my $file ( @goners ) {
unlink $file or warn "Could not unlink $file: $!";
}
Note: "unlink" will not attempt to delete directories unless you are superuser and the -U flag is
supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can
inflict damage on your filesystem. Finally, using "unlink" on directories is not supported on
many operating systems. Use "rmdir" instead.
If LIST is omitted, "unlink" uses $_.
unpack TEMPLATE,EXPR
unpack TEMPLATE
"unpack" does the reverse of "pack": it takes a string and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)
If EXPR is omitted, unpacks the $_ string. See perlpacktut for an introduction to this function.
The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately
to a value. Typically, either the string is a result of "pack", or the characters of the string
represent a C structure of some kind.
The TEMPLATE has the same format as in the "pack" function. Here's a subroutine that does
substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items themselves. Default is a
16-bit checksum. Checksum is calculated by summing numeric values of expanded values (for string
fields the sum of "ord($char)" is taken, for bit fields the sum of zeroes and ones).
For example, the following computes the same number as the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32W*",<>) % 65535;
};
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since Perl has no way of checking whether the
value passed to "unpack()" corresponds to a valid memory location, passing a pointer value that's
not known to be valid is likely to have disastrous consequences.
If there are more pack codes or if the repeat count of a field or a group is larger than what the
remainder of the input string allows, the result is not well defined: the repeat count may be
decreased, or "unpack()" may produce empty strings or zeros, or it may raise an exception. If
the input string is longer than one described by the TEMPLATE, the remainder of that input string
is ignored.
See "pack" for more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a package. (See "tie".) Has no effect if the variable
is not tied.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of a "push", depending on how you look at it.
Prepends list to the front of the array, and returns the new number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in
the same order. Use "reverse" to do the reverse.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from the named module, generally by aliasing
certain subroutine or variable names into your package. It is exactly equivalent to
BEGIN { require Module; Module->import( LIST ); }
except that Module must be a bareword.
In the peculiar "use VERSION" form, VERSION may be either a positive decimal fraction such as
5.006, which will be compared to $], or a v-string of the form v5.6.1, which will be compared to
$^V (aka $PERL_VERSION). An exception is raised if VERSION is greater than the version of the
current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with
"require", which can do a similar check at run time. Symmetrically, "no VERSION" allows you to
specify that you want a version of Perl older than the specified one.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads
to misleading error messages under earlier versions of Perl (that is, prior to 5.6.0) that do not
support this syntax. The equivalent numeric version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current Perl version before "use"ing library
modules that won't work with older versions of Perl. (We try not to do this more than we have
to.)
Also, if the specified Perl version is greater than or equal to 5.9.5, "use VERSION" will also
load the "feature" pragma and enable all features available in the requested version. See
feature. Similarly, if the specified Perl version is greater than or equal to 5.11.0, strictures
are enabled lexically as with "use strict" (except that the strict.pm file is not actually
loaded).
The "BEGIN" forces the "require" and "import" to happen at compile time. The "require" makes
sure the module is loaded into memory if it hasn't been yet. The "import" is not a builtin; it's
just an ordinary static method call into the "Module" package to tell the module to import the
list of features back into the current package. The module can implement its "import" method any
way it likes, though most modules just choose to derive their "import" method via inheritance
from the "Exporter" class that is defined in the "Exporter" module. See Exporter. If no
"import" method can be found then the call is skipped, even if there is an AUTOLOAD method.
If you do not want to call the package's "import" method (for instance, to stop your namespace
from being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the "use" will call the VERSION
method in class Module with the given version as an argument. The default VERSION method,
inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the
variable $Module::VERSION.
Again, there is a distinction between omitting LIST ("import" called with no arguments) and an
explicit empty LIST "()" ("import" not called). Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this
way. Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into the current block scope (like "strict" or
"integer", unlike ordinary modules, which import symbols into the current package (which are
effective through the end of the file).
Because "use" takes effect at compile time, it doesn't respect the ordinary flow control of the
code being compiled. In particular, putting a "use" inside the false branch of a conditional
doesn't prevent it from being processed. If a module or pragma only needs to be loaded
conditionally, this can be done using the if pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);
There's a corresponding "no" command that unimports meanings imported by "use", i.e., it calls
"unimport Module LIST" instead of "import". It behaves just as "import" does with VERSION, an
omitted or empty LIST, or no unimport method being found.
no integer;
no strict 'refs';
no warnings;
Care should be taken when using the "no VERSION" form of "no". It is only meant to be used to
assert that the running perl is of a earlier version than its argument and not to undo the
feature-enabling side effects of "use VERSION".
See perlmodlib for a list of standard modules and pragmas. See perlrun for the "-M" and "-m"
command-line options to Perl that give "use" functionality from the command-line.
utime LIST
Changes the access and modification times on each file of a list of files. The first two
elements of the list must be the NUMERICAL access and modification times, in that order. Returns
the number of files successfully changed. The inode change time of each file is set to the
current time. For example, this code has the same effect as the Unix touch(1) command when the
files already exist and belong to the user running the program:
#!/usr/bin/perl
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
Since Perl 5.7.2, if the first two elements of the list are "undef", the utime(2) syscall from
your C library is called with a null second argument. On most systems, this will set the file's
access and modification times to the current time (i.e., equivalent to the example above) and
will work even on files you don't own provided you have write permission:
for $file (@ARGV) {
utime(undef, undef, $file)
|| warn "couldn't touch $file: $!";
}
Under NFS this will use the time of the NFS server, not the time of the local machine. If there
is a time synchronization problem, the NFS server and local machine will have different times.
The Unix touch(1) command will in fact normally use this form instead of the one shown in the
first example.
Passing only one of the first two elements as "undef" is equivalent to passing a 0 and will not
have the effect described when both are "undef". This also triggers an uninitialized warning.
On systems that support futimes(2), you may pass filehandles among the files. On systems that
don't support futimes(2), passing filehandles raises an exception. Filehandles must be passed as
globs or glob references to be recognized; barewords are considered filenames.
values HASH
values ARRAY
Returns a list consisting of all the values of the named hash, or the values of an array. (In a
scalar context, returns the number of values.)
The values are returned in an apparently random order. The actual random order is subject to
change in future versions of Perl, but it is guaranteed to be the same order as either the "keys"
or "each" function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is
different even between different runs of Perl for security reasons (see "Algorithmic Complexity
Attacks" in perlsec).
As a side effect, calling values() resets the HASH or ARRAY's internal iterator, see "each". (In
particular, calling values() in void context resets the iterator with no other overhead. Apart
from resetting the iterator, "values @array" in list context is the same as plain @array. We
recommend that you use void context "keys @array" for this, but reasoned that it taking "values
@array" out would require more documentation than leaving it in.)
Note that the values are not copied, which means modifying them will modify the contents of the
hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
See also "keys", "each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of elements of width BITS, and returns the
value of the element specified by OFFSET as an unsigned integer. BITS therefore specifies the
number of bits that are reserved for each element in the bit vector. This must be a power of two
from 1 to 32 (or 64, if your platform supports that).
If BITS is 8, "elements" coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each
group is converted to a number as with pack()/unpack() with big-endian formats "n"/"N" (and
analogously for BITS==64). See "pack" for details.
If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into
8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04,
0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single input byte "chr(0x36)" into two
groups gives a list "(0x6, 0x3)"; breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)".
"vec" may also be assigned to, in which case parentheses are needed to give the expression the
correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is returned. If an element off the
end of the string is written to, Perl will first extend the string with sufficiently many zero
bytes. It is an error to try to write off the beginning of the string (i.e., negative OFFSET).
If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag set), this is
ignored by "vec", and it operates on the internal byte string, not the conceptual character
string, even if you only have characters with values less than 256.
Strings created with "vec" can also be manipulated with the logical operators "|", "&", "^", and
"~". These operators will assume a bit vector operation is desired when both operands are
strings. See "Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the
string after each step. Note that this code works in the same way on big-endian or little-endian
machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the "*".
Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it runs, the example above should print the
following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------vec($_, -----------------------------------------------------------------vec($_,
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait
Behaves like wait(2) on your system: it waits for a child process to terminate and returns the
pid of the deceased process, or "-1" if there are no child processes. The status is returned in
$? and "${^CHILD_ERROR_NATIVE}". Note that a return value of "-1" could mean that child
processes are being automatically reaped, as described in perlipc.
If you use wait in your handler for $SIG{CHLD} it may accidently wait for the child created by
qx() or system(). See perlipc for details.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the pid of the deceased process, or
"-1" if there is no such child process. On some systems, a value of 0 indicates that there are
processes still running. The status is returned in $? and "${^CHILD_ERROR_NATIVE}". If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} while $kid > 0;
then you can do a non-blocking wait for all pending zombie processes. Non-blocking wait is
available on machines supporting either the waitpid(2) or wait4(2) syscalls. However, waiting
for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call
by remembering the status values of processes that have exited but have not been harvested by the
Perl script yet.)
Note that on some systems, a return value of "-1" could mean that child processes are being
automatically reaped. See perlipc for details, and for other examples.
wantarray
Returns true if the context of the currently executing subroutine or "eval" is looking for a list
value. Returns false if the context is looking for a scalar. Returns the undefined value if the
context is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
"wantarray()"'s result is unspecified in the top level of a file, in a "BEGIN", "UNITCHECK",
"CHECK", "INIT" or "END" block, or in a "DESTROY" method.
This function should have been named wantlist() instead.
warn LIST
Prints the value of LIST to STDERR. If the last element of LIST does not end in a newline, it
appends the same file/line number text as "die" does.
If the output is empty and $@ already contains a value (typically from a previous eval) that
value is used after appending "\t...caught" to $@. This is useful for staying almost, but not
entirely similar to "die".
If $@ is empty then the string "Warning: Something's wrong" is used.
No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's
responsibility to deal with the message as it sees fit (like, for instance, converting it into a
"die"). Most handlers must therefore arrange to actually display the warnings that they are not
prepared to deal with, by calling "warn" again in the handler. Note that this is quite safe and
will not produce an endless loop, since "__WARN__" hooks are not called from inside one.
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which
don't suppress the error text, but can instead call "die" again to change it).
Using a "__WARN__" handler provides a powerful way to silence all warnings (even the so-called
mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries, and for more examples. See the Carp module for
other kinds of warnings using its carp() and cluck() functions.
write FILEHANDLE
write EXPR
write
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format
associated with that file. By default the format for a file is the one having the same name as
the filehandle, but the format for the current output channel (see the "select" function) may be
set explicitly by assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if there is insufficient room on the current
page for the formatted record, the page is advanced by writing a form feed, a special top-of-page
format is used to format the new page header, and then the record is written. By default the
top-of-page format is the name of the filehandle with "_TOP" appended, but it may be dynamically
set to the format of your choice by assigning the name to the $^ variable while the filehandle is
selected. The number of lines remaining on the current page is in variable "$-", which can be
set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out
as STDOUT but may be changed by the "select" operator. If the FILEHANDLE is an EXPR, then the
expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at
run time. For more on formats, see perlform.
Note that write is not the opposite of "read". Unfortunately.
y///
The transliteration operator. Same as "tr///". See "Quote and Quote-like Operators" in perlop.
perl v5.12.5 2012-11-03 PERLFUNC(1)
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