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PERLAPIO(1) Perl Programmers Reference Guide PERLAPIO(1)
NAME
perlapio - perl's IO abstraction interface.
SYNOPSIS
#define PERLIO_NOT_STDIO 0 /* For co-existence with stdio only */
#include <perlio.h> /* Usually via #include <perl.h> */
PerlIO *PerlIO_stdin(void);
PerlIO *PerlIO_stdout(void);
PerlIO *PerlIO_stderr(void);
PerlIO *PerlIO_open(const char *path,const char *mode);
PerlIO *PerlIO_fdopen(int fd, const char *mode);
PerlIO *PerlIO_reopen(const char *path, const char *mode, PerlIO *old); /* deprecated */
int PerlIO_close(PerlIO *f);
int PerlIO_stdoutf(const char *fmt,...)
int PerlIO_puts(PerlIO *f,const char *string);
int PerlIO_putc(PerlIO *f,int ch);
int PerlIO_write(PerlIO *f,const void *buf,size_t numbytes);
int PerlIO_printf(PerlIO *f, const char *fmt,...);
int PerlIO_vprintf(PerlIO *f, const char *fmt, va_list args);
int PerlIO_flush(PerlIO *f);
int PerlIO_eof(PerlIO *f);
int PerlIO_error(PerlIO *f);
void PerlIO_clearerr(PerlIO *f);
int PerlIO_getc(PerlIO *d);
int PerlIO_ungetc(PerlIO *f,int ch);
int PerlIO_read(PerlIO *f, void *buf, size_t numbytes);
int PerlIO_fileno(PerlIO *f);
void PerlIO_setlinebuf(PerlIO *f);
Off_t PerlIO_tell(PerlIO *f);
int PerlIO_seek(PerlIO *f, Off_t offset, int whence);
void PerlIO_rewind(PerlIO *f);
int PerlIO_getpos(PerlIO *f, SV *save); /* prototype changed */
int PerlIO_setpos(PerlIO *f, SV *saved); /* prototype changed */
int PerlIO_fast_gets(PerlIO *f);
int PerlIO_has_cntptr(PerlIO *f);
int PerlIO_get_cnt(PerlIO *f);
char *PerlIO_get_ptr(PerlIO *f);
void PerlIO_set_ptrcnt(PerlIO *f, char *ptr, int count);
int PerlIO_canset_cnt(PerlIO *f); /* deprecated */
void PerlIO_set_cnt(PerlIO *f, int count); /* deprecated */
int PerlIO_has_base(PerlIO *f);
char *PerlIO_get_base(PerlIO *f);
int PerlIO_get_bufsiz(PerlIO *f);
PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
FILE *PerlIO_exportFILE(PerlIO *f, int flags);
FILE *PerlIO_findFILE(PerlIO *f);
void PerlIO_releaseFILE(PerlIO *f,FILE *stdio);
int PerlIO_apply_layers(PerlIO *f, const char *mode, const char *layers);
int PerlIO_binmode(PerlIO *f, int ptype, int imode, const char *layers);
void PerlIO_debug(const char *fmt,...)
DESCRIPTION
Perl's source code, and extensions that want maximum portability, should use the above functions
instead of those defined in ANSI C's stdio.h. The perl headers (in particular "perlio.h") will
"#define" them to the I/O mechanism selected at Configure time.
The functions are modeled on those in stdio.h, but parameter order has been "tidied up a little".
"PerlIO *" takes the place of FILE *. Like FILE * it should be treated as opaque (it is probably safe
to assume it is a pointer to something).
There are currently three implementations:
1. USE_STDIO
All above are #define'd to stdio functions or are trivial wrapper functions which call stdio. In
this case only PerlIO * is a FILE *. This has been the default implementation since the
abstraction was introduced in perl5.003_02.
2. USE_SFIO
A "legacy" implementation in terms of the "sfio" library. Used for some specialist applications
on Unix machines ("sfio" is not widely ported away from Unix). Most of above are #define'd to
the sfio functions. PerlIO * is in this case Sfio_t *.
3. USE_PERLIO
Introduced just after perl5.7.0, this is a re-implementation of the above abstraction which
allows perl more control over how IO is done as it decouples IO from the way the operating system
and C library choose to do things. For USE_PERLIO PerlIO * has an extra layer of indirection - it
is a pointer-to-a-pointer. This allows the PerlIO * to remain with a known value while swapping
the implementation around underneath at run time. In this case all the above are true (but very
simple) functions which call the underlying implementation.
This is the only implementation for which "PerlIO_apply_layers()" does anything "interesting".
The USE_PERLIO implementation is described in perliol.
Because "perlio.h" is a thin layer (for efficiency) the semantics of these functions are somewhat
dependent on the underlying implementation. Where these variations are understood they are noted
below.
Unless otherwise noted, functions return 0 on success, or a negative value (usually "EOF" which is
usually -1) and set "errno" on error.
PerlIO_stdin(), PerlIO_stdout(), PerlIO_stderr()
Use these rather than "stdin", "stdout", "stderr". They are written to look like "function calls"
rather than variables because this makes it easier to make them function calls if platform cannot
export data to loaded modules, or if (say) different "threads" might have different values.
PerlIO_open(path, mode), PerlIO_fdopen(fd,mode)
These correspond to fopen()/fdopen() and the arguments are the same. Return "NULL" and set
"errno" if there is an error. There may be an implementation limit on the number of open
handles, which may be lower than the limit on the number of open files - "errno" may not be set
when "NULL" is returned if this limit is exceeded.
PerlIO_reopen(path,mode,f)
While this currently exists in all three implementations perl itself does not use it. As perl
does not use it, it is not well tested.
Perl prefers to "dup" the new low-level descriptor to the descriptor used by the existing PerlIO.
This may become the behaviour of this function in the future.
PerlIO_printf(f,fmt,...), PerlIO_vprintf(f,fmt,a)
These are fprintf()/vfprintf() equivalents.
PerlIO_stdoutf(fmt,...)
This is printf() equivalent. printf is #defined to this function, so it is (currently) legal to
use "printf(fmt,...)" in perl sources.
PerlIO_read(f,buf,count), PerlIO_write(f,buf,count)
These correspond functionally to fread() and fwrite() but the arguments and return values are
different. The PerlIO_read() and PerlIO_write() signatures have been modeled on the more sane
low level read() and write() functions instead: The "file" argument is passed first, there is
only one "count", and the return value can distinguish between error and "EOF".
Returns a byte count if successful (which may be zero or positive), returns negative value and
sets "errno" on error. Depending on implementation "errno" may be "EINTR" if operation was
interrupted by a signal.
PerlIO_close(f)
Depending on implementation "errno" may be "EINTR" if operation was interrupted by a signal.
PerlIO_puts(f,s), PerlIO_putc(f,c)
These correspond to fputs() and fputc(). Note that arguments have been revised to have "file"
first.
PerlIO_ungetc(f,c)
This corresponds to ungetc(). Note that arguments have been revised to have "file" first.
Arranges that next read operation will return the byte c. Despite the implied "character" in the
name only values in the range 0..0xFF are defined. Returns the byte c on success or -1 ("EOF") on
error. The number of bytes that can be "pushed back" may vary, only 1 character is certain, and
then only if it is the last character that was read from the handle.
PerlIO_getc(f)
This corresponds to getc(). Despite the c in the name only byte range 0..0xFF is supported.
Returns the character read or -1 ("EOF") on error.
PerlIO_eof(f)
This corresponds to feof(). Returns a true/false indication of whether the handle is at end of
file. For terminal devices this may or may not be "sticky" depending on the implementation. The
flag is cleared by PerlIO_seek(), or PerlIO_rewind().
PerlIO_error(f)
This corresponds to ferror(). Returns a true/false indication of whether there has been an IO
error on the handle.
PerlIO_fileno(f)
This corresponds to fileno(), note that on some platforms, the meaning of "fileno" may not match
Unix. Returns -1 if the handle has no open descriptor associated with it.
PerlIO_clearerr(f)
This corresponds to clearerr(), i.e., clears 'error' and (usually) 'eof' flags for the "stream".
Does not return a value.
PerlIO_flush(f)
This corresponds to fflush(). Sends any buffered write data to the underlying file. If called
with "NULL" this may flush all open streams (or core dump with some USE_STDIO implementations).
Calling on a handle open for read only, or on which last operation was a read of some kind may
lead to undefined behaviour on some USE_STDIO implementations. The USE_PERLIO (layers)
implementation tries to behave better: it flushes all open streams when passed "NULL", and
attempts to retain data on read streams either in the buffer or by seeking the handle to the
current logical position.
PerlIO_seek(f,offset,whence)
This corresponds to fseek(). Sends buffered write data to the underlying file, or discards any
buffered read data, then positions the file descriptor as specified by offset and whence (sic).
This is the correct thing to do when switching between read and write on the same handle (see
issues with PerlIO_flush() above). Offset is of type "Off_t" which is a perl Configure value
which may not be same as stdio's "off_t".
PerlIO_tell(f)
This corresponds to ftell(). Returns the current file position, or (Off_t) -1 on error. May
just return value system "knows" without making a system call or checking the underlying file
descriptor (so use on shared file descriptors is not safe without a PerlIO_seek()). Return value
is of type "Off_t" which is a perl Configure value which may not be same as stdio's "off_t".
PerlIO_getpos(f,p), PerlIO_setpos(f,p)
These correspond (loosely) to fgetpos() and fsetpos(). Rather than stdio's Fpos_t they expect a
"Perl Scalar Value" to be passed. What is stored there should be considered opaque. The layout of
the data may vary from handle to handle. When not using stdio or if platform does not have the
stdio calls then they are implemented in terms of PerlIO_tell() and PerlIO_seek().
PerlIO_rewind(f)
This corresponds to rewind(). It is usually defined as being
PerlIO_seek(f,(Off_t)0L, SEEK_SET);
PerlIO_clearerr(f);
PerlIO_tmpfile()
This corresponds to tmpfile(), i.e., returns an anonymous PerlIO or NULL on error. The system
will attempt to automatically delete the file when closed. On Unix the file is usually
"unlink"-ed just after it is created so it does not matter how it gets closed. On other systems
the file may only be deleted if closed via PerlIO_close() and/or the program exits via "exit".
Depending on the implementation there may be "race conditions" which allow other processes access
to the file, though in general it will be safer in this regard than ad. hoc. schemes.
PerlIO_setlinebuf(f)
This corresponds to setlinebuf(). Does not return a value. What constitutes a "line" is
implementation dependent but usually means that writing "\n" flushes the buffer. What happens
with things like "this\nthat" is uncertain. (Perl core uses it only when "dumping"; it has
nothing to do with $| auto-flush.)
Co-existence with stdio
There is outline support for co-existence of PerlIO with stdio. Obviously if PerlIO is implemented
in terms of stdio there is no problem. However in other cases then mechanisms must exist to create a
FILE * which can be passed to library code which is going to use stdio calls.
The first step is to add this line:
#define PERLIO_NOT_STDIO 0
before including any perl header files. (This will probably become the default at some point). That
prevents "perlio.h" from attempting to #define stdio functions onto PerlIO functions.
XS code is probably better using "typemap" if it expects FILE * arguments. The standard typemap will
be adjusted to comprehend any changes in this area.
PerlIO_importFILE(f,mode)
Used to get a PerlIO * from a FILE *.
The mode argument should be a string as would be passed to fopen/PerlIO_open. If it is NULL then
- for legacy support - the code will (depending upon the platform and the implementation) either
attempt to empirically determine the mode in which f is open, or use "r+" to indicate a
read/write stream.
Once called the FILE * should ONLY be closed by calling "PerlIO_close()" on the returned PerlIO
*.
The PerlIO is set to textmode. Use PerlIO_binmode if this is not the desired mode.
This is not the reverse of PerlIO_exportFILE().
PerlIO_exportFILE(f,mode)
Given a PerlIO * create a 'native' FILE * suitable for passing to code expecting to be compiled
and linked with ANSI C stdio.h. The mode argument should be a string as would be passed to
fopen/PerlIO_open. If it is NULL then - for legacy support - the FILE * is opened in same mode
as the PerlIO *.
The fact that such a FILE * has been 'exported' is recorded, (normally by pushing a new :stdio
"layer" onto the PerlIO *), which may affect future PerlIO operations on the original PerlIO *.
You should not call "fclose()" on the file unless you call "PerlIO_releaseFILE()" to disassociate
it from the PerlIO *. (Do not use PerlIO_importFILE() for doing the disassociation.)
Calling this function repeatedly will create a FILE * on each call (and will push an :stdio layer
each time as well).
PerlIO_releaseFILE(p,f)
Calling PerlIO_releaseFILE informs PerlIO that all use of FILE * is complete. It is removed from
the list of 'exported' FILE *s, and the associated PerlIO * should revert to its original
behaviour.
Use this to disassociate a file from a PerlIO * that was associated using PerlIO_exportFILE().
PerlIO_findFILE(f)
Returns a native FILE * used by a stdio layer. If there is none, it will create one with
PerlIO_exportFILE. In either case the FILE * should be considered as belonging to PerlIO
subsystem and should only be closed by calling "PerlIO_close()".
"Fast gets" Functions
In addition to standard-like API defined so far above there is an "implementation" interface which
allows perl to get at internals of PerlIO. The following calls correspond to the various FILE_xxx
macros determined by Configure - or their equivalent in other implementations. This section is really
of interest to only those concerned with detailed perl-core behaviour, implementing a PerlIO mapping
or writing code which can make use of the "read ahead" that has been done by the IO system in the
same way perl does. Note that any code that uses these interfaces must be prepared to do things the
traditional way if a handle does not support them.
PerlIO_fast_gets(f)
Returns true if implementation has all the interfaces required to allow perl's "sv_gets" to
"bypass" normal IO mechanism. This can vary from handle to handle.
PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
PerlIO_canset_cnt(f) && \
'Can set pointer into buffer'
PerlIO_has_cntptr(f)
Implementation can return pointer to current position in the "buffer" and a count of bytes
available in the buffer. Do not use this - use PerlIO_fast_gets.
PerlIO_get_cnt(f)
Return count of readable bytes in the buffer. Zero or negative return means no more bytes
available.
PerlIO_get_ptr(f)
Return pointer to next readable byte in buffer, accessing via the pointer (dereferencing) is only
safe if PerlIO_get_cnt() has returned a positive value. Only positive offsets up to value
returned by PerlIO_get_cnt() are allowed.
PerlIO_set_ptrcnt(f,p,c)
Set pointer into buffer, and a count of bytes still in the buffer. Should be used only to set
pointer to within range implied by previous calls to "PerlIO_get_ptr" and "PerlIO_get_cnt". The
two values must be consistent with each other (implementation may only use one or the other or
may require both).
PerlIO_canset_cnt(f)
Implementation can adjust its idea of number of bytes in the buffer. Do not use this - use
PerlIO_fast_gets.
PerlIO_set_cnt(f,c)
Obscure - set count of bytes in the buffer. Deprecated. Only usable if PerlIO_canset_cnt()
returns true. Currently used in only doio.c to force count less than -1 to -1. Perhaps should
be PerlIO_set_empty or similar. This call may actually do nothing if "count" is deduced from
pointer and a "limit". Do not use this - use PerlIO_set_ptrcnt().
PerlIO_has_base(f)
Returns true if implementation has a buffer, and can return pointer to whole buffer and its size.
Used by perl for -T / -B tests. Other uses would be very obscure...
PerlIO_get_base(f)
Return start of buffer. Access only positive offsets in the buffer up to the value returned by
PerlIO_get_bufsiz().
PerlIO_get_bufsiz(f)
Return the total number of bytes in the buffer, this is neither the number that can be read, nor
the amount of memory allocated to the buffer. Rather it is what the operating system and/or
implementation happened to "read()" (or whatever) last time IO was requested.
Other Functions
PerlIO_apply_layers(f,mode,layers)
The new interface to the USE_PERLIO implementation. The layers ":crlf" and ":raw" are only ones
allowed for other implementations and those are silently ignored. (As of perl5.8 ":raw" is
deprecated.) Use PerlIO_binmode() below for the portable case.
PerlIO_binmode(f,ptype,imode,layers)
The hook used by perl's "binmode" operator. ptype is perl's character for the kind of IO:
'<' read
'>' write
'+' read/write
imode is "O_BINARY" or "O_TEXT".
layers is a string of layers to apply, only ":crlf" makes sense in the non USE_PERLIO case. (As
of perl5.8 ":raw" is deprecated in favour of passing NULL.)
Portable cases are:
PerlIO_binmode(f,ptype,O_BINARY,NULL);
and
PerlIO_binmode(f,ptype,O_TEXT,":crlf");
On Unix these calls probably have no effect whatsoever. Elsewhere they alter "\n" to CR,LF
translation and possibly cause a special text "end of file" indicator to be written or honoured
on read. The effect of making the call after doing any IO to the handle depends on the
implementation. (It may be ignored, affect any data which is already buffered as well, or only
apply to subsequent data.)
PerlIO_debug(fmt,...)
PerlIO_debug is a printf()-like function which can be used for debugging. No return value. Its
main use is inside PerlIO where using real printf, warn() etc. would recursively call PerlIO and
be a problem.
PerlIO_debug writes to the file named by $ENV{'PERLIO_DEBUG'} typical use might be
Bourne shells (sh, ksh, bash, zsh, ash, ...):
PERLIO_DEBUG=/dev/tty ./perl somescript some args
Csh/Tcsh:
setenv PERLIO_DEBUG /dev/tty
./perl somescript some args
If you have the "env" utility:
env PERLIO_DEBUG=/dev/tty ./perl somescript some args
Win32:
set PERLIO_DEBUG=CON
perl somescript some args
If $ENV{'PERLIO_DEBUG'} is not set PerlIO_debug() is a no-op.
perl v5.16.2 2012-10-11 PERLAPIO(1)
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