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PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1)
NAME
perlhack - How to hack at the Perl internals
DESCRIPTION
This document attempts to explain how Perl development takes place, and ends with some suggestions
for people wanting to become bona fide porters.
The perl5-porters mailing list is where the Perl standard distribution is maintained and developed.
The list can get anywhere from 10 to 150 messages a day, depending on the heatedness of the debate.
Most days there are two or three patches, extensions, features, or bugs being discussed at a time.
A searchable archive of the list is at either:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
or
http://archive.develooper.com/perl5-porters@perl.org/
List subscribers (the porters themselves) come in several flavours. Some are quiet curious lurkers,
who rarely pitch in and instead watch the ongoing development to ensure they're forewarned of new
changes or features in Perl. Some are representatives of vendors, who are there to make sure that
Perl continues to compile and work on their platforms. Some patch any reported bug that they know
how to fix, some are actively patching their pet area (threads, Win32, the regexp engine), while
others seem to do nothing but complain. In other words, it's your usual mix of technical people.
Over this group of porters presides Larry Wall. He has the final word in what does and does not
change in the Perl language. Various releases of Perl are shepherded by a "pumpking", a porter
responsible for gathering patches, deciding on a patch-by-patch, feature-by-feature basis what will
and will not go into the release. For instance, Gurusamy Sarathy was the pumpking for the 5.6
release of Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and Rafael Garcia-Suarez
holds the pumpking crown for the 5.10 release.
In addition, various people are pumpkings for different things. For instance, Andy Dougherty and
Jarkko Hietaniemi did a grand job as the Configure pumpkin up till the 5.8 release. For the 5.10
release H.Merijn Brand took over.
Larry sees Perl development along the lines of the US government: there's the Legislature (the
porters), the Executive branch (the pumpkings), and the Supreme Court (Larry). The legislature can
discuss and submit patches to the executive branch all they like, but the executive branch is free to
veto them. Rarely, the Supreme Court will side with the executive branch over the legislature, or
the legislature over the executive branch. Mostly, however, the legislature and the executive branch
are supposed to get along and work out their differences without impeachment or court cases.
You might sometimes see reference to Rule 1 and Rule 2. Larry's power as Supreme Court is expressed
in The Rules:
1. Larry is always by definition right about how Perl should behave. This means he has final veto
power on the core functionality.
2. Larry is allowed to change his mind about any matter at a later date, regardless of whether he
previously invoked Rule 1.
Got that? Larry is always right, even when he was wrong. It's rare to see either Rule exercised,
but they are often alluded to.
New features and extensions to the language are contentious, because the criteria used by the
pumpkings, Larry, and other porters to decide which features should be implemented and incorporated
are not codified in a few small design goals as with some other languages. Instead, the heuristics
are flexible and often difficult to fathom. Here is one person's list, roughly in decreasing order
of importance, of heuristics that new features have to be weighed against:
Does concept match the general goals of Perl?
These haven't been written anywhere in stone, but one approximation is:
1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate Perl everywhere.
5. Either assimilate new technologies, or build bridges to them.
Where is the implementation?
All the talk in the world is useless without an implementation. In almost every case, the person
or people who argue for a new feature will be expected to be the ones who implement it. Porters
capable of coding new features have their own agendas, and are not available to implement your
(possibly good) idea.
Backwards compatibility
It's a cardinal sin to break existing Perl programs. New warnings are contentious--some say that
a program that emits warnings is not broken, while others say it is. Adding keywords has the
potential to break programs, changing the meaning of existing token sequences or functions might
break programs.
Could it be a module instead?
Perl 5 has extension mechanisms, modules and XS, specifically to avoid the need to keep changing
the Perl interpreter. You can write modules that export functions, you can give those functions
prototypes so they can be called like built-in functions, you can even write XS code to mess with
the runtime data structures of the Perl interpreter if you want to implement really complicated
things. If it can be done in a module instead of in the core, it's highly unlikely to be added.
Is the feature generic enough?
Is this something that only the submitter wants added to the language, or would it be broadly
useful? Sometimes, instead of adding a feature with a tight focus, the porters might decide to
wait until someone implements the more generalized feature. For instance, instead of
implementing a "delayed evaluation" feature, the porters are waiting for a macro system that
would permit delayed evaluation and much more.
Does it potentially introduce new bugs?
Radical rewrites of large chunks of the Perl interpreter have the potential to introduce new
bugs. The smaller and more localized the change, the better.
Does it preclude other desirable features?
A patch is likely to be rejected if it closes off future avenues of development. For instance, a
patch that placed a true and final interpretation on prototypes is likely to be rejected because
there are still options for the future of prototypes that haven't been addressed.
Is the implementation robust?
Good patches (tight code, complete, correct) stand more chance of going in. Sloppy or incorrect
patches might be placed on the back burner until the pumpking has time to fix, or might be
discarded altogether without further notice.
Is the implementation generic enough to be portable?
The worst patches make use of a system-specific features. It's highly unlikely that non-portable
additions to the Perl language will be accepted.
Is the implementation tested?
Patches which change behaviour (fixing bugs or introducing new features) must include regression
tests to verify that everything works as expected. Without tests provided by the original
author, how can anyone else changing perl in the future be sure that they haven't unwittingly
broken the behaviour the patch implements? And without tests, how can the patch's author be
confident that his/her hard work put into the patch won't be accidentally thrown away by someone
in the future?
Is there enough documentation?
Patches without documentation are probably ill-thought out or incomplete. Nothing can be added
without documentation, so submitting a patch for the appropriate manpages as well as the source
code is always a good idea.
Is there another way to do it?
Larry said "Although the Perl Slogan is There's More Than One Way to Do It, I hesitate to make 10
ways to do something". This is a tricky heuristic to navigate, though--one man's essential
addition is another man's pointless cruft.
Does it create too much work?
Work for the pumpking, work for Perl programmers, work for module authors, ... Perl is supposed
to be easy.
Patches speak louder than words
Working code is always preferred to pie-in-the-sky ideas. A patch to add a feature stands a much
higher chance of making it to the language than does a random feature request, no matter how
fervently argued the request might be. This ties into "Will it be useful?", as the fact that
someone took the time to make the patch demonstrates a strong desire for the feature.
If you're on the list, you might hear the word "core" bandied around. It refers to the standard
distribution. "Hacking on the core" means you're changing the C source code to the Perl interpreter.
"A core module" is one that ships with Perl.
Keeping in sync
The source code to the Perl interpreter, in its different versions, is kept in a repository managed
by the git revision control system. The pumpkings and a few others have write access to the
repository to check in changes.
How to clone and use the git perl repository is described in perlrepository.
You can also choose to use rsync to get a copy of the current source tree for the bleadperl branch
and all maintenance branches:
$ rsync -avz rsync://perl5.git.perl.org/perl-current .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.12.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.10.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.8.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.6.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.005xx .
(Add the "--delete" option to remove leftover files)
To get a full list of the available sync points:
$ rsync perl5.git.perl.org::
You may also want to subscribe to the perl5-changes mailing list to receive a copy of each patch that
gets submitted to the maintenance and development "branches" of the perl repository. See
http://lists.perl.org/ for subscription information.
If you are a member of the perl5-porters mailing list, it is a good thing to keep in touch with the
most recent changes. If not only to verify if what you would have posted as a bug report isn't
already solved in the most recent available perl development branch, also known as perl-current,
bleading edge perl, bleedperl or bleadperl.
Needless to say, the source code in perl-current is usually in a perpetual state of evolution. You
should expect it to be very buggy. Do not use it for any purpose other than testing and development.
Perlbug administration
There is a single remote administrative interface for modifying bug status, category, open issues
etc. using the RT bugtracker system, maintained by Robert Spier. Become an administrator, and close
any bugs you can get your sticky mitts on:
http://bugs.perl.org/
To email the bug system administrators:
"perlbug-admin" <perlbug-admin@perl.org>
Submitting patches
Always submit patches to perl5-porters@perl.org. If you're patching a core module and there's an
author listed, send the author a copy (see "Patching a core module"). This lets other porters review
your patch, which catches a surprising number of errors in patches. Please patch against the latest
development version. (e.g., even if you're fixing a bug in the 5.8 track, patch against the "blead"
branch in the git repository.)
If changes are accepted, they are applied to the development branch. Then the maintenance pumpking
decides which of those patches is to be backported to the maint branch. Only patches that survive
the heat of the development branch get applied to maintenance versions.
Your patch should update the documentation and test suite. See "Writing a test". If you have added
or removed files in the distribution, edit the MANIFEST file accordingly, sort the MANIFEST file
using "make manisort", and include those changes as part of your patch.
Patching documentation also follows the same order: if accepted, a patch is first applied to
development, and if relevant then it's backported to maintenance. (With an exception for some patches
that document behaviour that only appears in the maintenance branch, but which has changed in the
development version.)
To report a bug in Perl, use the program perlbug which comes with Perl (if you can't get Perl to
work, send mail to the address perlbug@perl.org or perlbug@perl.com). Reporting bugs through perlbug
feeds into the automated bug-tracking system, access to which is provided through the web at
http://rt.perl.org/rt3/ . It often pays to check the archives of the perl5-porters mailing list to
see whether the bug you're reporting has been reported before, and if so whether it was considered a
bug. See above for the location of the searchable archives.
The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers who test CPAN modules on a
variety of platforms. Perl Smokers ( http://www.nntp.perl.org/group/perl.daily-build and
http://www.nntp.perl.org/group/perl.daily-build.reports/ ) automatically test Perl source releases on
platforms with various configurations. Both efforts welcome volunteers. In order to get involved in
smoke testing of the perl itself visit http://search.cpan.org/dist/Test-Smoke
<http://search.cpan.org/dist/Test-Smoke>. In order to start smoke testing CPAN modules visit
http://search.cpan.org/dist/CPANPLUS-YACSmoke/ <http://search.cpan.org/dist/CPANPLUS-YACSmoke/> or
<http://search.cpan.org/dist/minismokebox/> or http://search.cpan.org/dist/CPAN-Reporter/
<http://search.cpan.org/dist/CPAN-Reporter/>.
It's a good idea to read and lurk for a while before chipping in. That way you'll get to see the
dynamic of the conversations, learn the personalities of the players, and hopefully be better
prepared to make a useful contribution when do you speak up.
If after all this you still think you want to join the perl5-porters mailing list, send mail to
perl5-porters-subscribe@perl.org. To unsubscribe, send mail to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following things:
perlguts
This is of paramount importance, since it's the documentation of what goes where in the Perl
source. Read it over a couple of times and it might start to make sense - don't worry if it
doesn't yet, because the best way to study it is to read it in conjunction with poking at Perl
source, and we'll do that later on.
Gisle Aas's "illustrated perlguts", also known as illguts, has very helpful pictures:
<http://search.cpan.org/dist/illguts/>
perlxstut and perlxs
A working knowledge of XSUB programming is incredibly useful for core hacking; XSUBs use
techniques drawn from the PP code, the portion of the guts that actually executes a Perl program.
It's a lot gentler to learn those techniques from simple examples and explanation than from the
core itself.
perlapi
The documentation for the Perl API explains what some of the internal functions do, as well as the
many macros used in the source.
Porting/pumpkin.pod
This is a collection of words of wisdom for a Perl porter; some of it is only useful to the
pumpkin holder, but most of it applies to anyone wanting to go about Perl development.
The perl5-porters FAQ
This should be available from http://dev.perl.org/perl5/docs/p5p-faq.html . It contains hints on
reading perl5-porters, information on how perl5-porters works and how Perl development in general
works.
Finding Your Way Around
Perl maintenance can be split into a number of areas, and certain people (pumpkins) will have
responsibility for each area. These areas sometimes correspond to files or directories in the source
kit. Among the areas are:
Core modules
Modules shipped as part of the Perl core live in various subdirectories, where two are dedicated
to core-only modules, and two are for the dual-life modules which live on CPAN and may be
maintained separately with respect to the Perl core:
lib/ is for pure-Perl modules, which exist in the core only.
ext/ is for XS extensions, and modules with special Makefile.PL
requirements, which exist in the core only.
cpan/ is for dual-life modules, where the CPAN module is
canonical (should be patched first).
dist/ is for dual-life modules, where the blead source is
canonical.
For some dual-life modules it has not been discussed if the CPAN version or the blead source is
canonical. Until that is done, those modules should be in cpan/.
Tests
There are tests for nearly all the modules, built-ins and major bits of functionality. Test files
all have a .t suffix. Module tests live in the lib/ and ext/ directories next to the module being
tested. Others live in t/. See "Writing a test"
Documentation
Documentation maintenance includes looking after everything in the pod/ directory, (as well as
contributing new documentation) and the documentation to the modules in core.
Configure
The Configure process is the way we make Perl portable across the myriad of operating systems it
supports. Responsibility for the Configure, build and installation process, as well as the overall
portability of the core code rests with the Configure pumpkin - others help out with individual
operating systems.
The three files that fall under his/her responsibility are Configure, config_h.SH, and
Porting/Glossary (and a whole bunch of small related files that are less important here). The
Configure pumpkin decides how patches to these are dealt with. Currently, the Configure pumpkin
will accept patches in most common formats, even directly to these files. Other committers are
allowed to commit to these files under the strict condition that they will inform the Configure
pumpkin, either on IRC (if he/she happens to be around) or through (personal) e-mail.
The files involved are the operating system directories, (win32/, os2/, vms/ and so on) the shell
scripts which generate config.h and Makefile, as well as the metaconfig files which generate
Configure. (metaconfig isn't included in the core distribution.)
See http://perl5.git.perl.org/metaconfig.git/blob/HEAD:/README for a description of the full
process involved.
Interpreter
And of course, there's the core of the Perl interpreter itself. Let's have a look at that in a
little more detail.
Before we leave looking at the layout, though, don't forget that MANIFEST contains not only the file
names in the Perl distribution, but short descriptions of what's in them, too. For an overview of the
important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
Elements of the interpreter
The work of the interpreter has two main stages: compiling the code into the internal representation,
or bytecode, and then executing it. "Compiled code" in perlguts explains exactly how the compilation
stage happens.
Here is a short breakdown of perl's operation:
Startup
The action begins in perlmain.c. (or miniperlmain.c for miniperl) This is very high-level code,
enough to fit on a single screen, and it resembles the code found in perlembed; most of the real
action takes place in perl.c
perlmain.c is generated by writemain from miniperlmain.c at make time, so you should make perl to
follow this along.
First, perlmain.c allocates some memory and constructs a Perl interpreter, along these lines:
1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(1);
7 perl_construct(my_perl);
8 PL_perl_destruct_level = 0;
9 }
Line 1 is a macro, and its definition is dependent on your operating system. Line 3 references
"PL_do_undump", a global variable - all global variables in Perl start with "PL_". This tells you
whether the current running program was created with the "-u" flag to perl and then undump, which
means it's going to be false in any sane context.
Line 4 calls a function in perl.c to allocate memory for a Perl interpreter. It's quite a simple
function, and the guts of it looks like this:
my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
Here you see an example of Perl's system abstraction, which we'll see later: "PerlMem_malloc" is
either your system's "malloc", or Perl's own "malloc" as defined in malloc.c if you selected that
option at configure time.
Next, in line 7, we construct the interpreter using perl_construct, also in perl.c; this sets up
all the special variables that Perl needs, the stacks, and so on.
Now we pass Perl the command line options, and tell it to go:
exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
if (!exitstatus)
perl_run(my_perl);
exitstatus = perl_destruct(my_perl);
perl_free(my_perl);
"perl_parse" is actually a wrapper around "S_parse_body", as defined in perl.c, which processes
the command line options, sets up any statically linked XS modules, opens the program and calls
"yyparse" to parse it.
Parsing
The aim of this stage is to take the Perl source, and turn it into an op tree. We'll see what one
of those looks like later. Strictly speaking, there's three things going on here.
"yyparse", the parser, lives in perly.c, although you're better off reading the original YACC
input in perly.y. (Yes, Virginia, there is a YACC grammar for Perl!) The job of the parser is to
take your code and "understand" it, splitting it into sentences, deciding which operands go with
which operators and so on.
The parser is nobly assisted by the lexer, which chunks up your input into tokens, and decides
what type of thing each token is: a variable name, an operator, a bareword, a subroutine, a core
function, and so on. The main point of entry to the lexer is "yylex", and that and its associated
routines can be found in toke.c. Perl isn't much like other computer languages; it's highly
context sensitive at times, it can be tricky to work out what sort of token something is, or where
a token ends. As such, there's a lot of interplay between the tokeniser and the parser, which can
get pretty frightening if you're not used to it.
As the parser understands a Perl program, it builds up a tree of operations for the interpreter to
perform during execution. The routines which construct and link together the various operations
are to be found in op.c, and will be examined later.
Optimization
Now the parsing stage is complete, and the finished tree represents the operations that the Perl
interpreter needs to perform to execute our program. Next, Perl does a dry run over the tree
looking for optimisations: constant expressions such as "3 + 4" will be computed now, and the
optimizer will also see if any multiple operations can be replaced with a single one. For
instance, to fetch the variable $foo, instead of grabbing the glob *foo and looking at the scalar
component, the optimizer fiddles the op tree to use a function which directly looks up the scalar
in question. The main optimizer is "peep" in op.c, and many ops have their own optimizing
functions.
Running
Now we're finally ready to go: we have compiled Perl byte code, and all that's left to do is run
it. The actual execution is done by the "runops_standard" function in run.c; more specifically,
it's done by these three innocent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of that:
PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function pointer, which stipulates the function which
will actually carry out the operation. This function will return the next op in the sequence -this sequencethis
this allows for things like "if" which choose the next op dynamically at run time. The
"PERL_ASYNC_CHECK" makes sure that things like signals interrupt execution if required.
The actual functions called are known as PP code, and they're spread between four files: pp_hot.c
contains the "hot" code, which is most often used and highly optimized, pp_sys.c contains all the
system-specific functions, pp_ctl.c contains the functions which implement control structures
("if", "while" and the like) and pp.c contains everything else. These are, if you like, the C code
for Perl's built-in functions and operators.
Note that each "pp_" function is expected to return a pointer to the next op. Calls to perl subs
(and eval blocks) are handled within the same runops loop, and do not consume extra space on the C
stack. For example, "pp_entersub" and "pp_entertry" just push a "CxSUB" or "CxEVAL" block struct
onto the context stack which contain the address of the op following the sub call or eval. They
then return the first op of that sub or eval block, and so execution continues of that sub or
block. Later, a "pp_leavesub" or "pp_leavetry" op pops the "CxSUB" or "CxEVAL", retrieves the
return op from it, and returns it.
Exception handing
Perl's exception handing (i.e. "die" etc.) is built on top of the low-level "setjmp()"/"longjmp()"
C-library functions. These basically provide a way to capture the current PC and SP registers and
later restore them; i.e. a "longjmp()" continues at the point in code where a previous "setjmp()"
was done, with anything further up on the C stack being lost. This is why code should always save
values using "SAVE_FOO" rather than in auto variables.
The perl core wraps "setjmp()" etc in the macros "JMPENV_PUSH" and "JMPENV_JUMP". The basic rule
of perl exceptions is that "exit", and "die" (in the absence of "eval") perform a JMPENV_JUMP(2),
while "die" within "eval" does a JMPENV_JUMP(3).
At entry points to perl, such as "perl_parse()", "perl_run()" and "call_sv(cv, G_EVAL)" each does
a "JMPENV_PUSH", then enter a runops loop or whatever, and handle possible exception returns. For
a 2 return, final cleanup is performed, such as popping stacks and calling "CHECK" or "END"
blocks. Amongst other things, this is how scope cleanup still occurs during an "exit".
If a "die" can find a "CxEVAL" block on the context stack, then the stack is popped to that level
and the return op in that block is assigned to "PL_restartop"; then a JMPENV_JUMP(3) is performed.
This normally passes control back to the guard. In the case of "perl_run" and "call_sv", a non-null nonnull
null "PL_restartop" triggers re-entry to the runops loop. The is the normal way that "die" or
"croak" is handled within an "eval".
Sometimes ops are executed within an inner runops loop, such as tie, sort or overload code. In
this case, something like
sub FETCH { eval { die } }
would cause a longjmp right back to the guard in "perl_run", popping both runops loops, which is
clearly incorrect. One way to avoid this is for the tie code to do a "JMPENV_PUSH" before
executing "FETCH" in the inner runops loop, but for efficiency reasons, perl in fact just sets a
flag, using "CATCH_SET(TRUE)". The "pp_require", "pp_entereval" and "pp_entertry" ops check this
flag, and if true, they call "docatch", which does a "JMPENV_PUSH" and starts a new runops level
to execute the code, rather than doing it on the current loop.
As a further optimisation, on exit from the eval block in the "FETCH", execution of the code
following the block is still carried on in the inner loop. When an exception is raised, "docatch"
compares the "JMPENV" level of the "CxEVAL" with "PL_top_env" and if they differ, just re-throws
the exception. In this way any inner loops get popped.
Here's an example.
1: eval { tie @a, 'A' };
2: sub A::TIEARRAY {
3: eval { die };
4: die;
5: }
To run this code, "perl_run" is called, which does a "JMPENV_PUSH" then enters a runops loop. This
loop executes the eval and tie ops on line 1, with the eval pushing a "CxEVAL" onto the context
stack.
The "pp_tie" does a "CATCH_SET(TRUE)", then starts a second runops loop to execute the body of
"TIEARRAY". When it executes the entertry op on line 3, "CATCH_GET" is true, so "pp_entertry"
calls "docatch" which does a "JMPENV_PUSH" and starts a third runops loop, which then executes the
die op. At this point the C call stack looks like this:
Perl_pp_die
Perl_runops # third loop
S_docatch_body
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
and the context and data stacks, as shown by "-Dstv", look like:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
CX 1: EVAL => *
retop=nextstate
The die pops the first "CxEVAL" off the context stack, sets "PL_restartop" from it, does a
JMPENV_JUMP(3), and control returns to the top "docatch". This then starts another third-level
runops level, which executes the nextstate, pushmark and die ops on line 4. At the point that the
second "pp_die" is called, the C call stack looks exactly like that above, even though we are no
longer within an inner eval; this is because of the optimization mentioned earlier. However, the
context stack now looks like this, ie with the top CxEVAL popped:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
The die on line 4 pops the context stack back down to the CxEVAL, leaving it as:
STACK 0: MAIN
CX 0: BLOCK =>
As usual, "PL_restartop" is extracted from the "CxEVAL", and a JMPENV_JUMP(3) done, which pops the
C stack back to the docatch:
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
In this case, because the "JMPENV" level recorded in the "CxEVAL" differs from the current one,
"docatch" just does a JMPENV_JUMP(3) and the C stack unwinds to:
perl_run
main
Because "PL_restartop" is non-null, "run_body" starts a new runops loop and execution continues.
Internal Variable Types
You should by now have had a look at perlguts, which tells you about Perl's internal variable types:
SVs, HVs, AVs and the rest. If not, do that now.
These variables are used not only to represent Perl-space variables, but also any constants in the
code, as well as some structures completely internal to Perl. The symbol table, for instance, is an
ordinary Perl hash. Your code is represented by an SV as it's read into the parser; any program files
you call are opened via ordinary Perl filehandles, and so on.
The core Devel::Peek module lets us examine SVs from a Perl program. Let's see, for instance, how
Perl treats the constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
Reading "Devel::Peek" output takes a bit of practise, so let's go through it line by line.
Line 1 tells us we're looking at an SV which lives at 0xa04ecbc in memory. SVs themselves are very
simple structures, but they contain a pointer to a more complex structure. In this case, it's a PV, a
structure which holds a string value, at location 0xa041450. Line 2 is the reference count; there
are no other references to this data, so it's 1.
Line 3 are the flags for this SV - it's OK to use it as a PV, it's a read-only SV (because it's a
constant) and the data is a PV internally. Next we've got the contents of the string, starting at
location 0xa0484e0.
Line 5 gives us the current length of the string - note that this does not include the null
terminator. Line 6 is not the length of the string, but the length of the currently allocated buffer;
as the string grows, Perl automatically extends the available storage via a routine called "SvGROW".
You can get at any of these quantities from C very easily; just add "Sv" to the name of the field
shown in the snippet, and you've got a macro which will return the value: "SvCUR(sv)" returns the
current length of the string, "SvREFCOUNT(sv)" returns the reference count, "SvPV(sv, len)" returns
the string itself with its length, and so on. More macros to manipulate these properties can be
found in perlguts.
Let's take an example of manipulating a PV, from "sv_catpvn", in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\0';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length "len" onto the end of the PV stored in "sv".
The first thing we do in line 6 is make sure that the SV has a valid PV, by calling the "SvPV_force"
macro to force a PV. As a side effect, "tlen" gets set to the current value of the PV, and the PV
itself is returned to "junk".
In line 7, we make sure that the SV will have enough room to accommodate the old string, the new
string and the null terminator. If "LEN" isn't big enough, "SvGROW" will reallocate space for us.
Now, if "junk" is the same as the string we're trying to add, we can grab the string directly from
the SV; "SvPVX" is the address of the PV in the SV.
Line 10 does the actual catenation: the "Move" macro moves a chunk of memory around: we move the
string "ptr" to the end of the PV - that's the start of the PV plus its current length. We're moving
"len" bytes of type "char". After doing so, we need to tell Perl we've extended the string, by
altering "CUR" to reflect the new length. "SvEND" is a macro which gives us the end of the string, so
that needs to be a "\0".
Line 13 manipulates the flags; since we've changed the PV, any IV or NV values will no longer be
valid: if we have "$a=10; $a.="6";" we don't want to use the old IV of 10. "SvPOK_only_utf8" is a
special UTF-8-aware version of "SvPOK_only", a macro which turns off the IOK and NOK flags and turns
on POK. The final "SvTAINT" is a macro which launders tainted data if taint mode is turned on.
AVs and HVs are more complicated, but SVs are by far the most common variable type being thrown
around. Having seen something of how we manipulate these, let's go on and look at how the op tree is
constructed.
Op Trees
First, what is the op tree, anyway? The op tree is the parsed representation of your program, as we
saw in our section on parsing, and it's the sequence of operations that Perl goes through to execute
your program, as we saw in "Running".
An op is a fundamental operation that Perl can perform: all the built-in functions and operators are
ops, and there are a series of ops which deal with concepts the interpreter needs internally -entering internallyentering
entering and leaving a block, ending a statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that there are two "routes" through it, two
orders in which you can traverse the tree. First, parse order reflects how the parser understood the
code, and secondly, execution order tells perl what order to perform the operations in.
The easiest way to examine the op tree is to stop Perl after it has finished parsing, and get it to
dump out the tree. This is exactly what the compiler backends B::Terse, B::Concise and B::Debug do.
Let's have a look at how Perl sees "$a = $b + $c":
% perl -MO=Terse -e '$a=$b+$c'
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
Let's start in the middle, at line 4. This is a BINOP, a binary operator, which is at location
0x8179828. The specific operator in question is "sassign" - scalar assignment - and you can find the
code which implements it in the function "pp_sassign" in pp_hot.c. As a binary operator, it has two
children: the add operator, providing the result of "$b+$c", is uppermost on line 5, and the left
hand side is on line 10.
Line 10 is the null op: this does exactly nothing. What is that doing there? If you see the null op,
it's a sign that something has been optimized away after parsing. As we mentioned in "Optimization",
the optimization stage sometimes converts two operations into one, for example when fetching a scalar
variable. When this happens, instead of rewriting the op tree and cleaning up the dangling pointers,
it's easier just to replace the redundant operation with the null op. Originally, the tree would have
looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the "a" entry from the main symbol table, and then look at the scalar component of it:
"gvsv" ("pp_gvsv" into pp_hot.c) happens to do both these things.
The right hand side, starting at line 5 is similar to what we've just seen: we have the "add" op
("pp_add" also in pp_hot.c) add together two "gvsv"s.
Now, what's this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
"enter" and "leave" are scoping ops, and their job is to perform any housekeeping every time you
enter and leave a block: lexical variables are tidied up, unreferenced variables are destroyed, and
so on. Every program will have those first three lines: "leave" is a list, and its children are all
the statements in the block. Statements are delimited by "nextstate", so a block is a collection of
"nextstate" ops, with the ops to be performed for each statement being the children of "nextstate".
"enter" is a single op which functions as a marker.
That's how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c
However, it's impossible to perform the operations in this order: you have to find the values of $b
and $c before you add them together, for instance. So, the other thread that runs through the op tree
is the execution order: each op has a field "op_next" which points to the next op to be run, so
following these pointers tells us how perl executes the code. We can traverse the tree in this order
using the "exec" option to "B::Terse":
% perl -MO=Terse,exec -e '$a=$b+$c'
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block, start a statement. Get the values of $b
and $c, and add them together. Find $a, and assign one to the other. Then leave.
The way Perl builds up these op trees in the parsing process can be unravelled by examining perly.y,
the YACC grammar. Let's take the piece we need to construct the tree for "$a = $b + $c"
1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
If you're not used to reading BNF grammars, this is how it works: You're fed certain things by the
tokeniser, which generally end up in upper case. Here, "ADDOP", is provided when the tokeniser sees
"+" in your code. "ASSIGNOP" is provided when "=" is used for assigning. These are "terminal
symbols", because you can't get any simpler than them.
The grammar, lines one and three of the snippet above, tells you how to build up more complex forms.
These complex forms, "non-terminal symbols" are generally placed in lower case. "term" here is a non-terminal nonterminal
terminal symbol, representing a single expression.
The grammar gives you the following rule: you can make the thing on the left of the colon if you see
all the things on the right in sequence. This is called a "reduction", and the aim of parsing is to
completely reduce the input. There are several different ways you can perform a reduction, separated
by vertical bars: so, "term" followed by "=" followed by "term" makes a "term", and "term" followed
by "+" followed by "term" can also make a "term".
So, if you see two terms with an "=" or "+", between them, you can turn them into a single
expression. When you do this, you execute the code in the block on the next line: if you see "=",
you'll do the code in line 2. If you see "+", you'll do the code in line 4. It's this code which
contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a number of variables. The variables refer to
the tokens: $1 is the first token in the input, $2 the second, and so on - think regular expression
backreferences. $$ is the op returned from this reduction. So, we call "newBINOP" to create a new
binary operator. The first parameter to "newBINOP", a function in op.c, is the op type. It's an
addition operator, so we want the type to be "ADDOP". We could specify this directly, but it's right
there as the second token in the input, so we use $2. The second parameter is the op's flags: 0 means
"nothing special". Then the things to add: the left and right hand side of our expression, in scalar
context.
Stacks
When perl executes something like "addop", how does it pass on its results to the next op? The answer
is, through the use of stacks. Perl has a number of stacks to store things it's currently working on,
and we'll look at the three most important ones here.
Argument stack
Arguments are passed to PP code and returned from PP code using the argument stack, "ST". The
typical way to handle arguments is to pop them off the stack, deal with them how you wish, and
then push the result back onto the stack. This is how, for instance, the cosine operator works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
We'll see a more tricky example of this when we consider Perl's macros below. "POPn" gives you the
NV (floating point value) of the top SV on the stack: the $x in "cos($x)". Then we compute the
cosine, and push the result back as an NV. The "X" in "XPUSHn" means that the stack should be
extended if necessary - it can't be necessary here, because we know there's room for one more item
on the stack, since we've just removed one! The "XPUSH*" macros at least guarantee safety.
Alternatively, you can fiddle with the stack directly: "SP" gives you the first element in your
portion of the stack, and "TOP*" gives you the top SV/IV/NV/etc. on the stack. So, for instance,
to do unary negation of an integer:
SETi(-TOPi);
Just set the integer value of the top stack entry to its negation.
Argument stack manipulation in the core is exactly the same as it is in XSUBs - see perlxstut,
perlxs and perlguts for a longer description of the macros used in stack manipulation.
Mark stack
I say "your portion of the stack" above because PP code doesn't necessarily get the whole stack to
itself: if your function calls another function, you'll only want to expose the arguments aimed
for the called function, and not (necessarily) let it get at your own data. The way we do this is
to have a "virtual" bottom-of-stack, exposed to each function. The mark stack keeps bookmarks to
locations in the argument stack usable by each function. For instance, when dealing with a tied
variable, (internally, something with "P" magic) Perl has to call methods for accesses to the tied
variables. However, we need to separate the arguments exposed to the method to the argument
exposed to the original function - the store or fetch or whatever it may be. Here's roughly how
the tied "push" is implemented; see "av_push" in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
Let's examine the whole implementation, for practice:
1 PUSHMARK(SP);
Push the current state of the stack pointer onto the mark stack. This is so that when we've
finished adding items to the argument stack, Perl knows how many things we've added recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
We're going to add two more items onto the argument stack: when you have a tied array, the "PUSH"
subroutine receives the object and the value to be pushed, and that's exactly what we have here -the herethe
the tied object, retrieved with "SvTIED_obj", and the value, the SV "val".
5 PUTBACK;
Next we tell Perl to update the global stack pointer from our internal variable: "dSP" only gave
us a local copy, not a reference to the global.
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
"ENTER" and "LEAVE" localise a block of code - they make sure that all variables are tidied up,
everything that has been localised gets its previous value returned, and so on. Think of them as
the "{" and "}" of a Perl block.
To actually do the magic method call, we have to call a subroutine in Perl space: "call_method"
takes care of that, and it's described in perlcall. We call the "PUSH" method in scalar context,
and we're going to discard its return value. The call_method() function removes the top element
of the mark stack, so there is nothing for the caller to clean up.
Save stack
C doesn't have a concept of local scope, so perl provides one. We've seen that "ENTER" and "LEAVE"
are used as scoping braces; the save stack implements the C equivalent of, for example:
{
local $foo = 42;
...
}
See "Localising Changes" in perlguts for how to use the save stack.
Millions of Macros
One thing you'll notice about the Perl source is that it's full of macros. Some have called the
pervasive use of macros the hardest thing to understand, others find it adds to clarity. Let's take
an example, the code which implements the addition operator:
1 PP(pp_add)
2 {
3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
4 {
5 dPOPTOPnnrl_ul;
6 SETn( left + right );
7 RETURN;
8 }
9 }
Every line here (apart from the braces, of course) contains a macro. The first line sets up the
function declaration as Perl expects for PP code; line 3 sets up variable declarations for the
argument stack and the target, the return value of the operation. Finally, it tries to see if the
addition operation is overloaded; if so, the appropriate subroutine is called.
Line 5 is another variable declaration - all variable declarations start with "d" - which pops from
the top of the argument stack two NVs (hence "nn") and puts them into the variables "right" and
"left", hence the "rl". These are the two operands to the addition operator. Next, we call "SETn" to
set the NV of the return value to the result of adding the two values. This done, we return - the
"RETURN" macro makes sure that our return value is properly handled, and we pass the next operator to
run back to the main run loop.
Most of these macros are explained in perlapi, and some of the more important ones are explained in
perlxs as well. Pay special attention to "Background and PERL_IMPLICIT_CONTEXT" in perlguts for
information on the "[pad]THX_?" macros.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by the results.
SOURCE CODE STATIC ANALYSIS
Various tools exist for analysing C source code statically, as opposed to dynamically, that is,
without executing the code. It is possible to detect resource leaks, undefined behaviour, type
mismatches, portability problems, code paths that would cause illegal memory accesses, and other
similar problems by just parsing the C code and looking at the resulting graph, what does it tell
about the execution and data flows. As a matter of fact, this is exactly how C compilers know to
give warnings about dubious code.
lint, splint
The good old C code quality inspector, "lint", is available in several platforms, but please be aware
that there are several different implementations of it by different vendors, which means that the
flags are not identical across different platforms.
There is a lint variant called "splint" (Secure Programming Lint) available from
http://www.splint.org/ that should compile on any Unix-like platform.
There are "lint" and <splint> targets in Makefile, but you may have to diddle with the flags (see
above).
Coverity
Coverity (http://www.coverity.com/) is a product similar to lint and as a testbed for their product
they periodically check several open source projects, and they give out accounts to open source
developers to the defect databases.
cpd (cut-and-paste detector)
The cpd tool detects cut-and-paste coding. If one instance of the cut-and-pasted code changes, all
the other spots should probably be changed, too. Therefore such code should probably be turned into
a subroutine or a macro.
cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project (http://pmd.sourceforge.net/).
pmd was originally written for static analysis of Java code, but later the cpd part of it was
extended to parse also C and C++.
Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the pmd-X.Y.jar from it, and then
run that on source code thusly:
java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
You may run into memory limits, in which case you should use the -Xmx option:
java -Xmx512M ...
gcc warnings
Though much can be written about the inconsistency and coverage problems of gcc warnings (like
"-Wall" not meaning "all the warnings", or some common portability problems not being covered by
"-Wall", or "-ansi" and "-pedantic" both being a poorly defined collection of warnings, and so
forth), gcc is still a useful tool in keeping our coding nose clean.
The "-Wall" is by default on.
The "-ansi" (and its sidekick, "-pedantic") would be nice to be on always, but unfortunately they are
not safe on all platforms, they can for example cause fatal conflicts with the system headers
(Solaris being a prime example). If Configure "-Dgccansipedantic" is used, the "cflags" frontend
selects "-ansi -pedantic" for the platforms where they are known to be safe.
Starting from Perl 5.9.4 the following extra flags are added:
• "-Wendif-labels"
• "-Wextra"
• "-Wdeclaration-after-statement"
The following flags would be nice to have but they would first need their own Augean stablemaster:
• "-Wpointer-arith"
• "-Wshadow"
• "-Wstrict-prototypes"
The "-Wtraditional" is another example of the annoying tendency of gcc to bundle a lot of warnings
under one switch (it would be impossible to deploy in practice because it would complain a lot) but
it does contain some warnings that would be beneficial to have available on their own, such as the
warning about string constants inside macros containing the macro arguments: this behaved differently
pre-ANSI than it does in ANSI, and some C compilers are still in transition, AIX being an example.
Warnings of other C compilers
Other C compilers (yes, there are other C compilers than gcc) often have their "strict ANSI" or
"strict ANSI with some portability extensions" modes on, like for example the Sun Workshop has its
"-Xa" mode on (though implicitly), or the DEC (these days, HP...) has its "-std1" mode on.
DEBUGGING
You can compile a special debugging version of Perl, which allows you to use the "-D" option of Perl
to tell more about what Perl is doing. But sometimes there is no alternative than to dive in with a
debugger, either to see the stack trace of a core dump (very useful in a bug report), or trying to
figure out what went wrong before the core dump happened, or how did we end up having wrong or
unexpected results.
Poking at Perl
To really poke around with Perl, you'll probably want to build Perl for debugging, like this:
./Configure -d -D optimize=-g
make
"-g" is a flag to the C compiler to have it produce debugging information which will allow us to step
through a running program, and to see in which C function we are at (without the debugging
information we might see only the numerical addresses of the functions, which is not very helpful).
Configure will also turn on the "DEBUGGING" compilation symbol which enables all the internal
debugging code in Perl. There are a whole bunch of things you can debug with this: perlrun lists them
all, and the best way to find out about them is to play about with them. The most useful options are
probably
l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be achieved using XS modules.
-Dr => use re 'debug'
-Dx => use O 'Debug'
Using a source-level debugger
If the debugging output of "-D" doesn't help you, it's time to step through perl's execution with a
source-level debugger.
• We'll use "gdb" for our examples here; the principles will apply to any debugger (many vendors
call their debugger "dbx"), but check the manual of the one you're using.
To fire up the debugger, type
gdb ./perl
Or if you have a core dump:
gdb ./perl core
You'll want to do that in your Perl source tree so the debugger can read the source code. You should
see the copyright message, followed by the prompt.
(gdb)
"help" will get you into the documentation, but here are the most useful commands:
run [args]
Run the program with the given arguments.
break function_name
break source.c:xxx
Tells the debugger that we'll want to pause execution when we reach either the named function (but
see "Internal Functions" in perlguts!) or the given line in the named source file.
step
Steps through the program a line at a time.
next
Steps through the program a line at a time, without descending into functions.
continue
Run until the next breakpoint.
finish
Run until the end of the current function, then stop again.
'enter'
Just pressing Enter will do the most recent operation again - it's a blessing when stepping
through miles of source code.
print
Execute the given C code and print its results. WARNING: Perl makes heavy use of macros, and gdb
does not necessarily support macros (see later "gdb macro support"). You'll have to substitute
them yourself, or to invoke cpp on the source code files (see "The .i Targets") So, for instance,
you can't say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a "macro dictionary", which you can produce by saying "cpp -dM perl.c
| sort". Even then, cpp won't recursively apply those macros for you.
gdb macro support
Recent versions of gdb have fairly good macro support, but in order to use it you'll need to compile
perl with macro definitions included in the debugging information. Using gcc version 3.1, this means
configuring with "-Doptimize=-g3". Other compilers might use a different switch (if they support
debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping functions in dump.c; these work a little
like an internal Devel::Peek, but they also cover OPs and other structures that you can't get at from
Perl. Let's take an example. We'll use the "$a = $b + $c" we used before, but give it a bit of
context: "$b = "6XXXX"; $c = 2.3;". Where's a good place to stop and poke around?
What about "pp_add", the function we examined earlier to implement the "+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions" in perlguts. With the
breakpoint in place, we can run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
Lots of junk will go past as gdb reads in the relevant source files and libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(gdb) step
311 dPOPTOPnnrl_ul;
(gdb)
We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul" arranges for two "NV"s to be
placed into "left" and "right" - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
"POPn" takes the SV from the top of the stack and obtains its NV either directly (if "SvNOK" is set)
or by calling the "sv_2nv" function. "TOPs" takes the next SV from the top of the stack - yes,
"POPn" uses "TOPs" - but doesn't remove it. We then use "SvNV" to get the NV from "leftsv" in the
same way as before - yes, "POPn" uses "SvNV".
Since we don't have an NV for $b, we'll have to use "sv_2nv" to convert it. If we step again, we'll
find ourselves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
We know we're going to get 6 from this, so let's finish the subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always stored in "PL_op", and we can dump it with
"Perl_op_dump". This'll give us similar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (12)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
11 TYPE = gvsv ===> 12
FLAGS = (SCALAR)
GV = main::b
}
}
# finish this later #
Patching
All right, we've now had a look at how to navigate the Perl sources and some things you'll need to
know when fiddling with them. Let's now get on and create a simple patch. Here's something Larry
suggested: if a "U" is the first active format during a "pack", (for example, "pack "U3C8", @stuff")
then the resulting string should be treated as UTF-8 encoded.
If you are working with a git clone of the Perl repository, you will want to create a branch for your
changes. This will make creating a proper patch much simpler. See the perlrepository for details on
how to do this.
How do we prepare to fix this up? First we locate the code in question - the "pack" happens at
runtime, so it's going to be in one of the pp files. Sure enough, "pp_pack" is in pp.c. Since we're
going to be altering this file, let's copy it to pp.c~.
[Well, it was in pp.c when this tutorial was written. It has now been split off with "pp_unpack" to
its own file, pp_pack.c]
Now let's look over "pp_pack": we take a pattern into "pat", and then loop over the pattern, taking
each format character in turn into "datum_type". Then for each possible format character, we swallow
up the other arguments in the pattern (a field width, an asterisk, and so on) and convert the next
chunk input into the specified format, adding it onto the output SV "cat".
How do we know if the "U" is the first format in the "pat"? Well, if we have a pointer to the start
of "pat" then, if we see a "U" we can test whether we're still at the start of the string. So, here's
where "pat" is set up:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
SV *fromstr;
We'll have another string pointer in there:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
+ char *patcopy;
register I32 len;
I32 datumtype;
SV *fromstr;
And just before we start the loop, we'll set "patcopy" to be the start of "pat":
items = SP - MARK;
MARK++;
sv_setpvn(cat, "", 0);
+ patcopy = pat;
while (pat < patend) {
Now if we see a "U" which was at the start of the string, we turn on the "UTF8" flag for the output
SV, "cat":
+ if (datumtype == 'U' && pat==patcopy+1)
+ SvUTF8_on(cat);
if (datumtype == '#') {
while (pat < patend && *pat != '\n')
pat++;
Remember that it has to be "patcopy+1" because the first character of the string is the "U" which has
been swallowed into "datumtype!"
Oops, we forgot one thing: what if there are spaces at the start of the pattern? "pack(" U*",
@stuff)" will have "U" as the first active character, even though it's not the first thing in the
pattern. In this case, we have to advance "patcopy" along with "pat" when we see spaces:
if (isSPACE(datumtype))
continue;
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
OK. That's the C part done. Now we must do two additional things before this patch is ready to go:
we've changed the behaviour of Perl, and so we must document that change. We must also provide some
more regression tests to make sure our patch works and doesn't create a bug somewhere else along the
line.
The regression tests for each operator live in t/op/, and so we make a copy of t/op/pack.t to
t/op/pack.t~. Now we can add our tests to the end. First, we'll test that the "U" does indeed create
Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didn't we could use the one from t/test.pl.
require './test.pl';
plan( tests => 159 );
so instead of this:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
we can write the more sensible (see Test::More for a full explanation of is() and other testing
functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces Unicode" );
Now we'll test that we got that space-at-the-beginning business right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
And finally we'll test that we don't make Unicode strings if "U" is not the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isn't Unicode" );
Mustn't forget to change the number of tests which appears at the top, or else the automated tester
will get confused. This will either look like this:
print "1..156\n";
or this:
plan( tests => 156 );
We now compile up Perl, and run it through the test suite. Our new tests pass, hooray!
Finally, the documentation. The job is never done until the paperwork is over, so let's describe the
change we've just made. The relevant place is pod/perlfunc.pod; again, we make a copy, and then we'll
insert this text in the description of "pack":
=item *
If the pattern begins with a C<U>, the resulting string will be treated
as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
with an initial C<U0>, and the bytes that follow will be interpreted as
Unicode characters. If you don't want this to happen, you can begin
your pattern with C<C0> (or anything else) to force Perl not to UTF-8
encode your string, and then follow this with a C<U*> somewhere in your
pattern.
Patching a core module
This works just like patching anything else, with an extra consideration. Many core modules also
live on CPAN. If this is so, patch the CPAN version instead of the core and send the patch off to
the module maintainer (with a copy to p5p). This will help the module maintainer keep the CPAN
version in sync with the core version without constantly scanning p5p.
The list of maintainers of core modules is usefully documented in Porting/Maintainers.pl.
Adding a new function to the core
If, as part of a patch to fix a bug, or just because you have an especially good idea, you decide to
add a new function to the core, discuss your ideas on p5p well before you start work. It may be that
someone else has already attempted to do what you are considering and can give lots of good advice or
even provide you with bits of code that they already started (but never finished).
You have to follow all of the advice given above for patching. It is extremely important to test any
addition thoroughly and add new tests to explore all boundary conditions that your new function is
expected to handle. If your new function is used only by one module (e.g. toke), then it should
probably be named S_your_function (for static); on the other hand, if you expect it to accessible
from other functions in Perl, you should name it Perl_your_function. See "Internal Functions" in
perlguts for more details.
The location of any new code is also an important consideration. Don't just create a new top level
.c file and put your code there; you would have to make changes to Configure (so the Makefile is
created properly), as well as possibly lots of include files. This is strictly pumpking business.
It is better to add your function to one of the existing top level source code files, but your choice
is complicated by the nature of the Perl distribution. Only the files that are marked as compiled
static are located in the perl executable. Everything else is located in the shared library (or DLL
if you are running under WIN32). So, for example, if a function was only used by functions located
in toke.c, then your code can go in toke.c. If, however, you want to call the function from
universal.c, then you should put your code in another location, for example util.c.
In addition to writing your c-code, you will need to create an appropriate entry in embed.pl
describing your function, then run 'make regen_headers' to create the entries in the numerous header
files that perl needs to compile correctly. See "Internal Functions" in perlguts for information on
the various options that you can set in embed.pl. You will forget to do this a few (or many) times
and you will get warnings during the compilation phase. Make sure that you mention this when you
post your patch to P5P; the pumpking needs to know this.
When you write your new code, please be conscious of existing code conventions used in the perl
source files. See perlstyle for details. Although most of the guidelines discussed seem to focus on
Perl code, rather than c, they all apply (except when they don't ;). Also see perlrepository for
lots of details about both formatting and submitting patches of your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to p5p. Test on as many platforms as you
can find. Test as many perl Configure options as you can (e.g. MULTIPLICITY). If you have profiling
or memory tools, see "EXTERNAL TOOLS FOR DEBUGGING PERL" below for how to use them to further test
your code. Remember that most of the people on P5P are doing this on their own time and don't have
the time to debug your code.
Writing a test
Every module and built-in function has an associated test file (or should...). If you add or change
functionality, you have to write a test. If you fix a bug, you have to write a test so that bug
never comes back. If you alter the docs, it would be nice to test what the new documentation says.
In short, if you submit a patch you probably also have to patch the tests.
For modules, the test file is right next to the module itself. lib/strict.t tests lib/strict.pm.
This is a recent innovation, so there are some snags (and it would be wonderful for you to brush them
out), but it basically works that way. Everything else lives in t/.
If you add a new test directory under t/, it is imperative that you add that directory to t/HARNESS
and t/TEST.
t/base/
Testing of the absolute basic functionality of Perl. Things like "if", basic file reads and
writes, simple regexes, etc. These are run first in the test suite and if any of them fail,
something is really broken.
t/cmd/
These test the basic control structures, "if/else", "while", subroutines, etc.
t/comp/
Tests basic issues of how Perl parses and compiles itself.
t/io/
Tests for built-in IO functions, including command line arguments.
t/lib/
The old home for the module tests, you shouldn't put anything new in here. There are still some
bits and pieces hanging around in here that need to be moved. Perhaps you could move them?
Thanks!
t/mro/
Tests for perl's method resolution order implementations (see mro).
t/op/
Tests for perl's built in functions that don't fit into any of the other directories.
t/re/
Tests for regex related functions or behaviour. (These used to live in t/op).
t/run/
Testing features of how perl actually runs, including exit codes and handling of PERL* environment
variables.
t/uni/
Tests for the core support of Unicode.
t/win32/
Windows-specific tests.
t/x2p
A test suite for the s2p converter.
The core uses the same testing style as the rest of Perl, a simple "ok/not ok" run through
Test::Harness, but there are a few special considerations.
There are three ways to write a test in the core. Test::More, t/test.pl and ad hoc "print $test ?
"ok 42\n" : "not ok 42\n"". The decision of which to use depends on what part of the test suite
you're working on. This is a measure to prevent a high-level failure (such as Config.pm breaking)
from causing basic functionality tests to fail. If you write your own test, use the Test Anything
Protocol.
t/base t/comp
Since we don't know if require works, or even subroutines, use ad hoc tests for these two. Step
carefully to avoid using the feature being tested.
t/cmd t/run t/io t/op
Now that basic require() and subroutines are tested, you can use the t/test.pl library which
emulates the important features of Test::More while using a minimum of core features.
You can also conditionally use certain libraries like Config, but be sure to skip the test
gracefully if it's not there.
t/lib ext lib
Now that the core of Perl is tested, Test::More can be used. You can also use the full suite of
core modules in the tests.
When you say "make test" Perl uses the t/TEST program to run the test suite (except under Win32 where
it uses t/harness instead.) All tests are run from the t/ directory, not the directory which
contains the test. This causes some problems with the tests in lib/, so here's some opportunity for
some patching.
You must be triply conscious of cross-platform concerns. This usually boils down to using File::Spec
and avoiding things like "fork()" and "system()" unless absolutely necessary.
Special Make Test Targets
There are various special make targets that can be used to test Perl slightly differently than the
standard "test" target. Not all them are expected to give a 100% success rate. Many of them have
several aliases, and many of them are not available on certain operating systems.
coretest
Run perl on all core tests (t/* and lib/[a-z]* pragma tests).
(Not available on Win32)
test.deparse
Run all the tests through B::Deparse. Not all tests will succeed.
(Not available on Win32)
test.taintwarn
Run all tests with the -t command-line switch. Not all tests are expected to succeed (until
they're specifically fixed, of course).
(Not available on Win32)
minitest
Run miniperl on t/base, t/comp, t/cmd, t/run, t/io, t/op, t/uni and t/mro tests.
test.valgrind check.valgrind utest.valgrind ucheck.valgrind
(Only in Linux) Run all the tests using the memory leak + naughty memory access tool "valgrind".
The log files will be named testname.valgrind.
test.third check.third utest.third ucheck.third
(Only in Tru64) Run all the tests using the memory leak + naughty memory access tool "Third
Degree". The log files will be named perl.3log.testname.
test.torture torturetest
Run all the usual tests and some extra tests. As of Perl 5.8.0 the only extra tests are
Abigail's JAPHs, t/japh/abigail.t.
You can also run the torture test with t/harness by giving "-torture" argument to t/harness.
utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will succeed.
(Not available on Win32)
minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with different versions of this encoding.
"make utest.utf16" runs the test suite with a combination of "-utf8" and "-utf16" arguments to
t/TEST.
(Not available on Win32)
test_harness
Run the test suite with the t/harness controlling program, instead of t/TEST. t/harness is more
sophisticated, and uses the Test::Harness module, thus using this test target supposes that perl
mostly works. The main advantage for our purposes is that it prints a detailed summary of failed
tests at the end. Also, unlike t/TEST, it doesn't redirect stderr to stdout.
Note that under Win32 t/harness is always used instead of t/TEST, so there is no special
"test_harness" target.
Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES environment variables to
control the behaviour of t/harness. This means you can say
nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
Parallel tests
The core distribution can now run its regression tests in parallel on Unix-like platforms.
Instead of running "make test", set "TEST_JOBS" in your environment to the number of tests to run
in parallel, and run "make test_harness". On a Bourne-like shell, this can be done as
TEST_JOBS=3 make test_harness # Run 3 tests in parallel
An environment variable is used, rather than parallel make itself, because TAP::Harness needs to
be able to schedule individual non-conflicting test scripts itself, and there is no standard
interface to "make" utilities to interact with their job schedulers.
Note that currently some test scripts may fail when run in parallel (most notably
"ext/IO/t/io_dir.t"). If necessary run just the failing scripts again sequentially and see if the
failures go away. =item test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal test.
Running tests by hand
You can run part of the test suite by hand by using one the following commands from the t/ directory
:
./perl -I../lib TEST list-of-.t-files
or
./perl -I../lib harness list-of-.t-files
(if you don't specify test scripts, the whole test suite will be run.)
Using t/harness for testing
If you use "harness" for testing you have several command line options available to you. The
arguments are as follows, and are in the order that they must appear if used together.
harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH
If "LIST OF FILES TO TEST" is omitted the file list is obtained from the manifest. The file list may
include shell wildcards which will be expanded out.
-v Run the tests under verbose mode so you can see what tests were run, and debug output.
-torture
Run the torture tests as well as the normal set.
-re=PATTERN
Filter the file list so that all the test files run match PATTERN. Note that this form is
distinct from the -re LIST OF PATTERNS form below in that it allows the file list to be provided
as well.
-re LIST OF PATTERNS
Filter the file list so that all the test files run match /(LIST|OF|PATTERNS)/. Note that with
this form the patterns are joined by '|' and you cannot supply a list of files, instead the test
files are obtained from the MANIFEST.
You can run an individual test by a command similar to
./perl -I../lib patho/to/foo.t
except that the harnesses set up some environment variables that may affect the execution of the test
:
PERL_CORE=1
indicates that we're running this test part of the perl core test suite. This is useful for
modules that have a dual life on CPAN.
PERL_DESTRUCT_LEVEL=2
is set to 2 if it isn't set already (see "PERL_DESTRUCT_LEVEL")
PERL
(used only by t/TEST) if set, overrides the path to the perl executable that should be used to
run the tests (the default being ./perl).
PERL_SKIP_TTY_TEST
if set, tells to skip the tests that need a terminal. It's actually set automatically by the
Makefile, but can also be forced artificially by running 'make test_notty'.
Other environment variables that may influence tests
PERL_TEST_Net_Ping
Setting this variable runs all the Net::Ping modules tests, otherwise some tests that interact
with the outside world are skipped. See perl58delta.
PERL_TEST_NOVREXX
Setting this variable skips the vrexx.t tests for OS2::REXX.
PERL_TEST_NUMCONVERTS
This sets a variable in op/numconvert.t.
See also the documentation for the Test and Test::Harness modules, for more environment variables
that affect testing.
Common problems when patching Perl source code
Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In some cases we have to take pre-ANSI preANSI
ANSI requirements into consideration. You don't care about some particular platform having broken
Perl? I hear there is still a strong demand for J2EE programmers.
Perl environment problems
• Not compiling with threading
Compiling with threading (-Duseithreads) completely rewrites the function prototypes of Perl.
You better try your changes with that. Related to this is the difference between "Perl_-less"
and "Perl_-ly" APIs, for example:
Perl_sv_setiv(aTHX_ ...);
sv_setiv(...);
The first one explicitly passes in the context, which is needed for e.g. threaded builds. The
second one does that implicitly; do not get them mixed. If you are not passing in a aTHX_, you
will need to do a dTHX (or a dVAR) as the first thing in the function.
See "How multiple interpreters and concurrency are supported" in perlguts for further discussion
about context.
• Not compiling with -DDEBUGGING
The DEBUGGING define exposes more code to the compiler, therefore more ways for things to go
wrong. You should try it.
• Introducing (non-read-only) globals
Do not introduce any modifiable globals, truly global or file static. They are bad form and
complicate multithreading and other forms of concurrency. The right way is to introduce them as
new interpreter variables, see intrpvar.h (at the very end for binary compatibility).
Introducing read-only (const) globals is okay, as long as you verify with e.g. "nm
libperl.a|egrep -v ' [TURtr] '" (if your "nm" has BSD-style output) that the data you added
really is read-only. (If it is, it shouldn't show up in the output of that command.)
If you want to have static strings, make them constant:
static const char etc[] = "...";
If you want to have arrays of constant strings, note carefully the right combination of "const"s:
static const char * const yippee[] =
{"hi", "ho", "silver"};
There is a way to completely hide any modifiable globals (they are all moved to heap), the
compilation setting "-DPERL_GLOBAL_STRUCT_PRIVATE". It is not normally used, but can be used for
testing, read more about it in "Background and PERL_IMPLICIT_CONTEXT" in perlguts.
• Not exporting your new function
Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any function that is part of the
public API (the shared Perl library) to be explicitly marked as exported. See the discussion
about embed.pl in perlguts.
• Exporting your new function
The new shiny result of either genuine new functionality or your arduous refactoring is now ready
and correctly exported. So what could possibly go wrong?
Maybe simply that your function did not need to be exported in the first place. Perl has a long
and not so glorious history of exporting functions that it should not have.
If the function is used only inside one source code file, make it static. See the discussion
about embed.pl in perlguts.
If the function is used across several files, but intended only for Perl's internal use (and this
should be the common case), do not export it to the public API. See the discussion about
embed.pl in perlguts.
Portability problems
The following are common causes of compilation and/or execution failures, not common to Perl as such.
The C FAQ is good bedtime reading. Please test your changes with as many C compilers and platforms
as possible; we will, anyway, and it's nice to save oneself from public embarrassment.
If using gcc, you can add the "-std=c89" option which will hopefully catch most of these
unportabilities. (However it might also catch incompatibilities in your system's header files.)
Use the Configure "-Dgccansipedantic" flag to enable the gcc "-ansi -pedantic" flags which enforce
stricter ANSI rules.
If using the "gcc -Wall" note that not all the possible warnings (like "-Wunitialized") are given
unless you also compile with "-O".
Note that if using gcc, starting from Perl 5.9.5 the Perl core source code files (the ones at the top
level of the source code distribution, but not e.g. the extensions under ext/) are automatically
compiled with as many as possible of the "-std=c89", "-ansi", "-pedantic", and a selection of "-W"
flags (see cflags.SH).
Also study perlport carefully to avoid any bad assumptions about the operating system, filesystems,
and so forth.
You may once in a while try a "make microperl" to see whether we can still compile Perl with just the
bare minimum of interfaces. (See README.micro.)
Do not assume an operating system indicates a certain compiler.
• Casting pointers to integers or casting integers to pointers
void castaway(U8* p)
{
IV i = p;
or
void castaway(U8* p)
{
IV i = (IV)p;
Both are bad, and broken, and unportable. Use the PTR2IV() macro that does it right. (Likewise,
there are PTR2UV(), PTR2NV(), INT2PTR(), and NUM2PTR().)
• Casting between data function pointers and data pointers
Technically speaking casting between function pointers and data pointers is unportable and
undefined, but practically speaking it seems to work, but you should use the FPTR2DPTR() and
DPTR2FPTR() macros. Sometimes you can also play games with unions.
• Assuming sizeof(int) == sizeof(long)
There are platforms where longs are 64 bits, and platforms where ints are 64 bits, and while we
are out to shock you, even platforms where shorts are 64 bits. This is all legal according to
the C standard. (In other words, "long long" is not a portable way to specify 64 bits, and "long
long" is not even guaranteed to be any wider than "long".)
Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth. Avoid things like I32
because they are not guaranteed to be exactly 32 bits, they are at least 32 bits, nor are they
guaranteed to be int or long. If you really explicitly need 64-bit variables, use I64 and U64,
but only if guarded by HAS_QUAD.
• Assuming one can dereference any type of pointer for any type of data
char *p = ...;
long pony = *p; /* BAD */
Many platforms, quite rightly so, will give you a core dump instead of a pony if the p happens
not be correctly aligned.
• Lvalue casts
(int)*p = ...; /* BAD */
Simply not portable. Get your lvalue to be of the right type, or maybe use temporary variables,
or dirty tricks with unions.
• Assume anything about structs (especially the ones you don't control, like the ones coming from
the system headers)
• That a certain field exists in a struct
• That no other fields exist besides the ones you know of
• That a field is of certain signedness, sizeof, or type
• That the fields are in a certain order
• While C guarantees the ordering specified in the struct definition, between
different platforms the definitions might differ
• That the sizeof(struct) or the alignments are the same everywhere
• There might be padding bytes between the fields to align the fields - the bytes
can be anything
• Structs are required to be aligned to the maximum alignment required by the
fields - which for native types is for usually equivalent to sizeof() of the
field
• Assuming the character set is ASCIIish
Perl can compile and run under EBCDIC platforms. See perlebcdic. This is transparent for the
most part, but because the character sets differ, you shouldn't use numeric (decimal, octal, nor
hex) constants to refer to characters. You can safely say 'A', but not 0x41. You can safely say
'\n', but not \012. If a character doesn't have a trivial input form, you can create a #define
for it in both "utfebcdic.h" and "utf8.h", so that it resolves to different values depending on
the character set being used. (There are three different EBCDIC character sets defined in
"utfebcdic.h", so it might be best to insert the #define three times in that file.)
Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper case alphabetic
characters. That is not true in EBCDIC. Nor for 'a' to 'z'. But '0' - '9' is an unbroken range
in both systems. Don't assume anything about other ranges.
Many of the comments in the existing code ignore the possibility of EBCDIC, and may be wrong
therefore, even if the code works. This is actually a tribute to the successful transparent
insertion of being able to handle EBCDIC without having to change pre-existing code.
UTF-8 and UTF-EBCDIC are two different encodings used to represent Unicode code points as
sequences of bytes. Macros with the same names (but different definitions) in "utf8.h" and
"utfebcdic.h" are used to allow the calling code to think that there is only one such encoding.
This is almost always referred to as "utf8", but it means the EBCDIC version as well. Again,
comments in the code may well be wrong even if the code itself is right. For example, the
concept of "invariant characters" differs between ASCII and EBCDIC. On ASCII platforms, only
characters that do not have the high-order bit set (i.e. whose ordinals are strict ASCII, 0 -127) 0127)
127) are invariant, and the documentation and comments in the code may assume that, often
referring to something like, say, "hibit". The situation differs and is not so simple on EBCDIC
machines, but as long as the code itself uses the "NATIVE_IS_INVARIANT()" macro appropriately, it
works, even if the comments are wrong.
• Assuming the character set is just ASCII
ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra characters have
different meanings depending on the locale. Absent a locale, currently these extra characters
are generally considered to be unassigned, and this has presented some problems. This is being
changed starting in 5.12 so that these characters will be considered to be Latin-1 (ISO-8859-1).
• Mixing #define and #ifdef
#define BURGLE(x) ... \
#ifdef BURGLE_OLD_STYLE /* BAD */
... do it the old way ... \
#else
... do it the new way ... \
#endif
You cannot portably "stack" cpp directives. For example in the above you need two separate
BURGLE() #defines, one for each #ifdef branch.
• Adding non-comment stuff after #endif or #else
#ifdef SNOSH
...
#else !SNOSH /* BAD */
...
#endif SNOSH /* BAD */
The #endif and #else cannot portably have anything non-comment after them. If you want to
document what is going (which is a good idea especially if the branches are long), use (C)
comments:
#ifdef SNOSH
...
#else /* !SNOSH */
...
#endif /* SNOSH */
The gcc option "-Wendif-labels" warns about the bad variant (by default on starting from Perl
5.9.4).
• Having a comma after the last element of an enum list
enum color {
CERULEAN,
CHARTREUSE,
CINNABAR, /* BAD */
};
is not portable. Leave out the last comma.
Also note that whether enums are implicitly morphable to ints varies between compilers, you might
need to (int).
• Using //-comments
// This function bamfoodles the zorklator. /* BAD */
That is C99 or C++. Perl is C89. Using the //-comments is silently allowed by many C compilers
but cranking up the ANSI C89 strictness (which we like to do) causes the compilation to fail.
• Mixing declarations and code
void zorklator()
{
int n = 3;
set_zorkmids(n); /* BAD */
int q = 4;
That is C99 or C++. Some C compilers allow that, but you shouldn't.
The gcc option "-Wdeclaration-after-statements" scans for such problems (by default on starting
from Perl 5.9.4).
• Introducing variables inside for()
for(int i = ...; ...; ...) { /* BAD */
That is C99 or C++. While it would indeed be awfully nice to have that also in C89, to limit the
scope of the loop variable, alas, we cannot.
• Mixing signed char pointers with unsigned char pointers
int foo(char *s) { ... }
...
unsigned char *t = ...; /* Or U8* t = ... */
foo(t); /* BAD */
While this is legal practice, it is certainly dubious, and downright fatal in at least one
platform: for example VMS cc considers this a fatal error. One cause for people often making
this mistake is that a "naked char" and therefore dereferencing a "naked char pointer" have an
undefined signedness: it depends on the compiler and the flags of the compiler and the underlying
platform whether the result is signed or unsigned. For this very same reason using a 'char' as
an array index is bad.
• Macros that have string constants and their arguments as substrings of the string constants
#define FOO(n) printf("number = %d\n", n) /* BAD */
FOO(10);
Pre-ANSI semantics for that was equivalent to
printf("10umber = %d\10");
which is probably not what you were expecting. Unfortunately at least one reasonably common and
modern C compiler does "real backward compatibility" here, in AIX that is what still happens even
though the rest of the AIX compiler is very happily C89.
• Using printf formats for non-basic C types
IV i = ...;
printf("i = %d\n", i); /* BAD */
While this might by accident work in some platform (where IV happens to be an "int"), in general
it cannot. IV might be something larger. Even worse the situation is with more specific types
(defined by Perl's configuration step in config.h):
Uid_t who = ...;
printf("who = %d\n", who); /* BAD */
The problem here is that Uid_t might be not only not "int"-wide but it might also be unsigned, in
which case large uids would be printed as negative values.
There is no simple solution to this because of printf()'s limited intelligence, but for many
types the right format is available as with either 'f' or '_f' suffix, for example:
IVdf /* IV in decimal */
UVxf /* UV is hexadecimal */
printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
Uid_t_f /* Uid_t in decimal */
printf("who = %"Uid_t_f"\n", who);
Or you can try casting to a "wide enough" type:
printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
Also remember that the %p format really does require a void pointer:
U8* p = ...;
printf("p = %p\n", (void*)p);
The gcc option "-Wformat" scans for such problems.
• Blindly using variadic macros
gcc has had them for a while with its own syntax, and C99 brought them with a standardized
syntax. Don't use the former, and use the latter only if the HAS_C99_VARIADIC_MACROS is defined.
• Blindly passing va_list
Not all platforms support passing va_list to further varargs (stdarg) functions. The right thing
to do is to copy the va_list using the Perl_va_copy() if the NEED_VA_COPY is defined.
• Using gcc statement expressions
val = ({...;...;...}); /* BAD */
While a nice extension, it's not portable. The Perl code does admittedly use them if available
to gain some extra speed (essentially as a funky form of inlining), but you shouldn't.
• Binding together several statements in a macro
Use the macros STMT_START and STMT_END.
STMT_START {
...
} STMT_END
• Testing for operating systems or versions when should be testing for features
#ifdef __FOONIX__ /* BAD */
foo = quux();
#endif
Unless you know with 100% certainty that quux() is only ever available for the "Foonix" operating
system and that is available and correctly working for all past, present, and future versions of
"Foonix", the above is very wrong. This is more correct (though still not perfect, because the
below is a compile-time check):
#ifdef HAS_QUUX
foo = quux();
#endif
How does the HAS_QUUX become defined where it needs to be? Well, if Foonix happens to be Unixy
enough to be able to run the Configure script, and Configure has been taught about detecting and
testing quux(), the HAS_QUUX will be correctly defined. In other platforms, the corresponding
configuration step will hopefully do the same.
In a pinch, if you cannot wait for Configure to be educated, or if you have a good hunch of where
quux() might be available, you can temporarily try the following:
#if (defined(__FOONIX__) || defined(__BARNIX__))
# define HAS_QUUX
#endif
...
#ifdef HAS_QUUX
foo = quux();
#endif
But in any case, try to keep the features and operating systems separate.
Problematic System Interfaces
• malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable allocate at least one byte.
(In general you should rarely need to work at this low level, but instead use the various malloc
wrappers.)
• snprintf() - the return type is unportable. Use my_snprintf() instead.
Security problems
Last but not least, here are various tips for safer coding.
• Do not use gets()
Or we will publicly ridicule you. Seriously.
• Do not use strcpy() or strcat() or strncpy() or strncat()
Use my_strlcpy() and my_strlcat() instead: they either use the native implementation, or Perl's
own implementation (borrowed from the public domain implementation of INN).
• Do not use sprintf() or vsprintf()
If you really want just plain byte strings, use my_snprintf() and my_vsnprintf() instead, which
will try to use snprintf() and vsnprintf() if those safer APIs are available. If you want
something fancier than a plain byte string, use SVs and Perl_sv_catpvf().
EXTERNAL TOOLS FOR DEBUGGING PERL
Sometimes it helps to use external tools while debugging and testing Perl. This section tries to
guide you through using some common testing and debugging tools with Perl. This is meant as a guide
to interfacing these tools with Perl, not as any kind of guide to the use of the tools themselves.
NOTE 1: Running under memory debuggers such as Purify, valgrind, or Third Degree greatly slows down
the execution: seconds become minutes, minutes become hours. For example as of Perl 5.8.1, the
ext/Encode/t/Unicode.t takes extraordinarily long to complete under e.g. Purify, Third Degree, and
valgrind. Under valgrind it takes more than six hours, even on a snappy computer. The said test must
be doing something that is quite unfriendly for memory debuggers. If you don't feel like waiting,
that you can simply kill away the perl process.
NOTE 2: To minimize the number of memory leak false alarms (see "PERL_DESTRUCT_LEVEL" for more
information), you have to set the environment variable PERL_DESTRUCT_LEVEL to 2.
For csh-like shells:
setenv PERL_DESTRUCT_LEVEL 2
For Bourne-type shells:
PERL_DESTRUCT_LEVEL=2
export PERL_DESTRUCT_LEVEL
In Unixy environments you can also use the "env" command:
env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
NOTE 3: There are known memory leaks when there are compile-time errors within eval or require,
seeing "S_doeval" in the call stack is a good sign of these. Fixing these leaks is non-trivial,
unfortunately, but they must be fixed eventually.
NOTE 4: DynaLoader will not clean up after itself completely unless Perl is built with the Configure
option "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".
Rational Software's Purify
Purify is a commercial tool that is helpful in identifying memory overruns, wild pointers, memory
leaks and other such badness. Perl must be compiled in a specific way for optimal testing with
Purify. Purify is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most benefit out of Purify, you should create
the perl to Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
-Accflags=-DPURIFY
Disables Perl's arena memory allocation functions, as well as forcing use of memory allocation
functions derived from the system malloc.
-Doptimize='-g'
Adds debugging information so that you see the exact source statements where the problem occurs.
Without this flag, all you will see is the source filename of where the error occurred.
-Uusemymalloc
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using
Perl's malloc will make Purify report most leaks in the "potential" leaks category.
-Dusemultiplicity
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts
down, which reduces the number of bogus leak reports from Purify.
Once you've compiled a perl suitable for Purify'ing, then you can just:
make pureperl
which creates a binary named 'pureperl' that has been Purify'ed. This binary is used in place of the
standard 'perl' binary when you want to debug Perl memory problems.
As an example, to show any memory leaks produced during the standard Perl testset you would create
and run the Purify'ed perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory problems.
Purify outputs messages in "Viewer" windows by default. If you don't have a windowing environment or
if you simply want the Purify output to unobtrusively go to a log file instead of to the interactive
window, use these following options to output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the "Viewer" windows, then you only need this option:
setenv PURIFYOPTIONS "-chain-length=25"
In Bourne-type shells:
PURIFYOPTIONS="..."
export PURIFYOPTIONS
or if you have the "env" utility:
env PURIFYOPTIONS="..." ../pureperl ...
Purify on NT
Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly. There are several options in
the makefile you should change to get the most use out of Purify:
DEFINES
You should add -DPURIFY to the DEFINES line so the DEFINES line looks something like:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as well as to force use of memory allocation
functions derived from the system malloc.
USE_MULTI = define
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts
down, which reduces the number of bogus leak reports from Purify.
#PERL_MALLOC = define
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using
Perl's malloc will make Purify report most leaks in the "potential" leaks category.
CFG = Debug
Adds debugging information so that you see the exact source statements where the problem occurs.
Without this flag, all you will see is the source filename of where the error occurred.
As an example, to show any memory leaks produced during the standard Perl testset you would create
and run Purify as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl, then finally report any memory problems.
valgrind
The excellent valgrind tool can be used to find out both memory leaks and illegal memory accesses.
As of version 3.3.0, Valgrind only supports Linux on x86, x86-64 and PowerPC. The special
"test.valgrind" target can be used to run the tests under valgrind. Found errors and memory leaks
are logged in files named testfile.valgrind.
Valgrind also provides a cachegrind tool, invoked on perl as:
VG_OPTS=--tool=cachegrind make test.valgrind
As system libraries (most notably glibc) are also triggering errors, valgrind allows to suppress such
errors using suppression files. The default suppression file that comes with valgrind already catches
a lot of them. Some additional suppressions are defined in t/perl.supp.
To get valgrind and for more information see
http://developer.kde.org/~sewardj/
Compaq's/Digital's/HP's Third Degree
Third Degree is a tool for memory leak detection and memory access checks. It is one of the many
tools in the ATOM toolkit. The toolkit is only available on Tru64 (formerly known as Digital UNIX
formerly known as DEC OSF/1).
When building Perl, you must first run Configure with -Doptimize=-g and -Uusemymalloc flags, after
that you can use the make targets "perl.third" and "test.third". (What is required is that Perl must
be compiled using the "-g" flag, you may need to re-Configure.)
The short story is that with "atom" you can instrument the Perl executable to create a new executable
called perl.third. When the instrumented executable is run, it creates a log of dubious memory
traffic in file called perl.3log. See the manual pages of atom and third for more information. The
most extensive Third Degree documentation is available in the Compaq "Tru64 UNIX Programmer's Guide",
chapter "Debugging Programs with Third Degree".
The "test.third" leaves a lot of files named foo_bar.3log in the t/ subdirectory. There is a problem
with these files: Third Degree is so effective that it finds problems also in the system libraries.
Therefore you should used the Porting/thirdclean script to cleanup the *.3log files.
There are also leaks that for given certain definition of a leak, aren't. See "PERL_DESTRUCT_LEVEL"
for more information.
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using e.g. valgrind, or the pureperl or
perl.third executables, please note that by default perl does not explicitly cleanup all the memory
it has allocated (such as global memory arenas) but instead lets the exit() of the whole program
"take care" of such allocations, also known as "global destruction of objects".
There is a way to tell perl to do complete cleanup: set the environment variable PERL_DESTRUCT_LEVEL
to a non-zero value. The t/TEST wrapper does set this to 2, and this is what you need to do too, if
you don't want to see the "global leaks": For example, for "third-degreed" Perl:
env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
(Note: the mod_perl apache module uses also this environment variable for its own purposes and
extended its semantics. Refer to the mod_perl documentation for more information. Also, spawned
threads do the equivalent of setting this variable to the value 1.)
If, at the end of a run you get the message N scalars leaked, you can recompile with
"-DDEBUG_LEAKING_SCALARS", which will cause the addresses of all those leaked SVs to be dumped along
with details as to where each SV was originally allocated. This information is also displayed by
Devel::Peek. Note that the extra details recorded with each SV increases memory usage, so it
shouldn't be used in production environments. It also converts "new_SV()" from a macro into a real
function, so you can use your favourite debugger to discover where those pesky SVs were allocated.
If you see that you're leaking memory at runtime, but neither valgrind nor "-DDEBUG_LEAKING_SCALARS"
will find anything, you're probably leaking SVs that are still reachable and will be properly cleaned
up during destruction of the interpreter. In such cases, using the "-Dm" switch can point you to the
source of the leak. If the executable was built with "-DDEBUG_LEAKING_SCALARS", "-Dm" will output SV
allocations in addition to memory allocations. Each SV allocation has a distinct serial number that
will be written on creation and destruction of the SV. So if you're executing the leaking code in a
loop, you need to look for SVs that are created, but never destroyed between each cycle. If such an
SV is found, set a conditional breakpoint within "new_SV()" and make it break only when
"PL_sv_serial" is equal to the serial number of the leaking SV. Then you will catch the interpreter
in exactly the state where the leaking SV is allocated, which is sufficient in many cases to find the
source of the leak.
As "-Dm" is using the PerlIO layer for output, it will by itself allocate quite a bunch of SVs, which
are hidden to avoid recursion. You can bypass the PerlIO layer if you use the SV logging provided by
"-DPERL_MEM_LOG" instead.
PERL_MEM_LOG
If compiled with "-DPERL_MEM_LOG", both memory and SV allocations go through logging functions, which
is handy for breakpoint setting.
Unless "-DPERL_MEM_LOG_NOIMPL" is also compiled, the logging functions read $ENV{PERL_MEM_LOG} to
determine whether to log the event, and if so how:
$ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
$ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
$ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
$ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
Memory logging is somewhat similar to "-Dm" but is independent of "-DDEBUGGING", and at a higher
level; all uses of Newx(), Renew(), and Safefree() are logged with the caller's source code file and
line number (and C function name, if supported by the C compiler). In contrast, "-Dm" is directly at
the point of "malloc()". SV logging is similar.
Since the logging doesn't use PerlIO, all SV allocations are logged and no extra SV allocations are
introduced by enabling the logging. If compiled with "-DDEBUG_LEAKING_SCALARS", the serial number
for each SV allocation is also logged.
Profiling
Depending on your platform there are various of profiling Perl.
There are two commonly used techniques of profiling executables: statistical time-sampling and basic-block basicblock
block counting.
The first method takes periodically samples of the CPU program counter, and since the program counter
can be correlated with the code generated for functions, we get a statistical view of in which
functions the program is spending its time. The caveats are that very small/fast functions have
lower probability of showing up in the profile, and that periodically interrupting the program (this
is usually done rather frequently, in the scale of milliseconds) imposes an additional overhead that
may skew the results. The first problem can be alleviated by running the code for longer (in general
this is a good idea for profiling), the second problem is usually kept in guard by the profiling
tools themselves.
The second method divides up the generated code into basic blocks. Basic blocks are sections of code
that are entered only in the beginning and exited only at the end. For example, a conditional jump
starts a basic block. Basic block profiling usually works by instrumenting the code by adding enter
basic block #nnnn book-keeping code to the generated code. During the execution of the code the
basic block counters are then updated appropriately. The caveat is that the added extra code can
skew the results: again, the profiling tools usually try to factor their own effects out of the
results.
Gprof Profiling
gprof is a profiling tool available in many Unix platforms, it uses statistical time-sampling.
You can build a profiled version of perl called "perl.gprof" by invoking the make target "perl.gprof"
(What is required is that Perl must be compiled using the "-pg" flag, you may need to re-Configure).
Running the profiled version of Perl will create an output file called gmon.out is created which
contains the profiling data collected during the execution.
The gprof tool can then display the collected data in various ways. Usually gprof understands the
following options:
-a Suppress statically defined functions from the profile.
-b Suppress the verbose descriptions in the profile.
-e routine
Exclude the given routine and its descendants from the profile.
-f routine
Display only the given routine and its descendants in the profile.
-s Generate a summary file called gmon.sum which then may be given to subsequent gprof runs to
accumulate data over several runs.
-z Display routines that have zero usage.
For more detailed explanation of the available commands and output formats, see your own local
documentation of gprof.
quick hint:
$ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
$ ./perl.gprof someprog # creates gmon.out in current directory
$ gprof ./perl.gprof > out
$ view out
GCC gcov Profiling
Starting from GCC 3.0 basic block profiling is officially available for the GNU CC.
You can build a profiled version of perl called perl.gcov by invoking the make target "perl.gcov"
(what is required that Perl must be compiled using gcc with the flags "-fprofile-arcs
-ftest-coverage", you may need to re-Configure).
Running the profiled version of Perl will cause profile output to be generated. For each source file
an accompanying ".da" file will be created.
To display the results you use the "gcov" utility (which should be installed if you have gcc 3.0 or
newer installed). gcov is run on source code files, like this
gcov sv.c
which will cause sv.c.gcov to be created. The .gcov files contain the source code annotated with
relative frequencies of execution indicated by "#" markers.
Useful options of gcov include "-b" which will summarise the basic block, branch, and function call
coverage, and "-c" which instead of relative frequencies will use the actual counts. For more
information on the use of gcov and basic block profiling with gcc, see the latest GNU CC manual, as
of GCC 3.0 see
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
and its section titled "8. gcov: a Test Coverage Program"
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
quick hint:
$ sh Configure -des -Doptimize='-g' -Accflags='-fprofile-arcs -ftest-coverage' \
-Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
$ rm -f regexec.c.gcov regexec.gcda
$ ./perl.gcov
$ gcov regexec.c
$ view regexec.c.gcov
Pixie Profiling
Pixie is a profiling tool available on IRIX and Tru64 (aka Digital UNIX aka DEC OSF/1) platforms.
Pixie does its profiling using basic-block counting.
You can build a profiled version of perl called perl.pixie by invoking the make target "perl.pixie"
(what is required is that Perl must be compiled using the "-g" flag, you may need to re-Configure).
In Tru64 a file called perl.Addrs will also be silently created, this file contains the addresses of
the basic blocks. Running the profiled version of Perl will create a new file called "perl.Counts"
which contains the counts for the basic block for that particular program execution.
To display the results you use the prof utility. The exact incantation depends on your operating
system, "prof perl.Counts" in IRIX, and "prof -pixie -all -L. perl" in Tru64.
In IRIX the following prof options are available:
-h Reports the most heavily used lines in descending order of use. Useful for finding the hotspot
lines.
-l Groups lines by procedure, with procedures sorted in descending order of use. Within a
procedure, lines are listed in source order. Useful for finding the hotspots of procedures.
In Tru64 the following options are available:
-p[rocedures]
Procedures sorted in descending order by the number of cycles executed in each procedure. Useful
for finding the hotspot procedures. (This is the default option.)
-h[eavy]
Lines sorted in descending order by the number of cycles executed in each line. Useful for
finding the hotspot lines.
-i[nvocations]
The called procedures are sorted in descending order by number of calls made to the procedures.
Useful for finding the most used procedures.
-l[ines]
Grouped by procedure, sorted by cycles executed per procedure. Useful for finding the hotspots
of procedures.
-testcoverage
The compiler emitted code for these lines, but the code was unexecuted.
-z[ero]
Unexecuted procedures.
For further information, see your system's manual pages for pixie and prof.
Miscellaneous tricks
• Those debugging perl with the DDD frontend over gdb may find the following useful:
You can extend the data conversion shortcuts menu, so for example you can display an SV's IV
value with one click, without doing any typing. To do that simply edit ~/.ddd/init file and add
after:
! Display shortcuts.
Ddd*gdbDisplayShortcuts: \
/t () // Convert to Bin\n\
/d () // Convert to Dec\n\
/x () // Convert to Hex\n\
/o () // Convert to Oct(\n\
the following two lines:
((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
so now you can do ivx and pvx lookups or you can plug there the sv_peek "conversion":
Perl_sv_peek(my_perl, (SV*)()) // sv_peek
(The my_perl is for threaded builds.) Just remember that every line, but the last one, should
end with \n\
Alternatively edit the init file interactively via: 3rd mouse button -> New Display -> Edit Menu
Note: you can define up to 20 conversion shortcuts in the gdb section.
• If you see in a debugger a memory area mysteriously full of 0xABABABAB or 0xEFEFEFEF, you may be
seeing the effect of the Poison() macros, see perlclib.
• Under ithreads the optree is read only. If you want to enforce this, to check for write accesses
from buggy code, compile with "-DPL_OP_SLAB_ALLOC" to enable the OP slab allocator and
"-DPERL_DEBUG_READONLY_OPS" to enable code that allocates op memory via "mmap", and sets it read-only readonly
only at run time. Any write access to an op results in a "SIGBUS" and abort.
This code is intended for development only, and may not be portable even to all Unix variants.
Also, it is an 80% solution, in that it isn't able to make all ops read only. Specifically it
1. Only sets read-only on all slabs of ops at "CHECK" time, hence ops allocated later via
"require" or "eval" will be re-write
2. Turns an entire slab of ops read-write if the refcount of any op in the slab needs to be
decreased.
3. Turns an entire slab of ops read-write if any op from the slab is freed.
It's not possible to turn the slabs to read-only after an action requiring read-write access, as
either can happen during op tree building time, so there may still be legitimate write access.
However, as an 80% solution it is still effective, as currently it catches a write access during
the generation of Config.pm, which means that we can't yet build perl with this enabled.
CONCLUSION
We've had a brief look around the Perl source, how to maintain quality of the source code, an
overview of the stages perl goes through when it's running your code, how to use debuggers to poke at
the Perl guts, and finally how to analyse the execution of Perl. We took a very simple problem and
demonstrated how to solve it fully - with documentation, regression tests, and finally a patch for
submission to p5p. Finally, we talked about how to use external tools to debug and test Perl.
I'd now suggest you read over those references again, and then, as soon as possible, get your hands
dirty. The best way to learn is by doing, so:
• Subscribe to perl5-porters, follow the patches and try and understand them; don't be afraid to ask
if there's a portion you're not clear on - who knows, you may unearth a bug in the patch...
• Keep up to date with the bleeding edge Perl distributions and get familiar with the changes. Try
and get an idea of what areas people are working on and the changes they're making.
• Do read the README associated with your operating system, e.g. README.aix on the IBM AIX OS. Don't
hesitate to supply patches to that README if you find anything missing or changed over a new OS
release.
• Find an area of Perl that seems interesting to you, and see if you can work out how it works. Scan
through the source, and step over it in the debugger. Play, poke, investigate, fiddle! You'll
probably get to understand not just your chosen area but a much wider range of perl's activity as
well, and probably sooner than you'd think.
The Road goes ever on and on, down from the door where it began.
If you can do these things, you've started on the long road to Perl porting. Thanks for wanting to
help make Perl better - and happy hacking!
Metaphoric Quotations
If you recognized the quote about the Road above, you're in luck.
Most software projects begin each file with a literal description of each file's purpose. Perl
instead begins each with a literary allusion to that file's purpose.
Like chapters in many books, all top-level Perl source files (along with a few others here and there)
begin with an epigramic inscription that alludes, indirectly and metaphorically, to the material
you're about to read.
Quotations are taken from writings of J.R.R Tolkien pertaining to his Legendarium, almost always from
The Lord of the Rings. Chapters and page numbers are given using the following editions:
• The Hobbit, by J.R.R. Tolkien. The hardcover, 70th-anniversary edition of 2007 was used,
published in the UK by Harper Collins Publishers and in the US by the Houghton Mifflin Company.
• The Lord of the Rings, by J.R.R. Tolkien. The hardcover, 50th-anniversary edition of 2004 was
used, published in the UK by Harper Collins Publishers and in the US by the Houghton Mifflin
Company.
• The Lays of Beleriand, by J.R.R. Tolkien and published posthumously by his son and literary
executor, C.J.R. Tolkien, being the 3rd of the 12 volumes in Christopher's mammoth History of
Middle Earth. Page numbers derive from the hardcover edition, first published in 1983 by George
Allen & Unwin; no page numbers changed for the special 3-volume omnibus edition of 2002 or the
various trade-paper editions, all again now by Harper Collins or Houghton Mifflin.
Other JRRT books fair game for quotes would thus include The Adventures of Tom Bombadil, The
Silmarillion, Unfinished Tales, and The Tale of the Children of Hurin, all but the first posthumously
assembled by CJRT. But The Lord of the Rings itself is perfectly fine and probably best to quote
from, provided you can find a suitable quote there.
So if you were to supply a new, complete, top-level source file to add to Perl, you should conform to
this peculiar practice by yourself selecting an appropriate quotation from Tolkien, retaining the
original spelling and punctuation and using the same format the rest of the quotes are in. Indirect
and oblique is just fine; remember, it's a metaphor, so being meta is, after all, what it's for.
AUTHOR
This document was written by Nathan Torkington, and is maintained by the perl5-porters mailing list.
SEE ALSO
perlrepository
perl v5.12.5 2012-11-03 PERLHACK(1)
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