3 perlhack - How to hack at the Perl internals
7 This document attempts to explain how Perl development takes place,
8 and ends with some suggestions for people wanting to become bona fide
11 The perl5-porters mailing list is where the Perl standard distribution
12 is maintained and developed. The list can get anywhere from 10 to 150
13 messages a day, depending on the heatedness of the debate. Most days
14 there are two or three patches, extensions, features, or bugs being
17 A searchable archive of the list is at either:
19 http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
23 http://archive.develooper.com/perl5-porters@perl.org/
25 List subscribers (the porters themselves) come in several flavours.
26 Some are quiet curious lurkers, who rarely pitch in and instead watch
27 the ongoing development to ensure they're forewarned of new changes or
28 features in Perl. Some are representatives of vendors, who are there
29 to make sure that Perl continues to compile and work on their
30 platforms. Some patch any reported bug that they know how to fix,
31 some are actively patching their pet area (threads, Win32, the regexp
32 engine), while others seem to do nothing but complain. In other
33 words, it's your usual mix of technical people.
35 Over this group of porters presides Larry Wall. He has the final word
36 in what does and does not change in the Perl language. Various
37 releases of Perl are shepherded by a "pumpking", a porter
38 responsible for gathering patches, deciding on a patch-by-patch,
39 feature-by-feature basis what will and will not go into the release.
40 For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of
41 Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and
42 Rafael Garcia-Suarez holds the pumpking crown for the 5.10 release.
44 In addition, various people are pumpkings for different things. For
45 instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the
46 I<Configure> pumpkin up till the 5.8 release. For the 5.10 release
47 H.Merijn Brand took over.
49 Larry sees Perl development along the lines of the US government:
50 there's the Legislature (the porters), the Executive branch (the
51 pumpkings), and the Supreme Court (Larry). The legislature can
52 discuss and submit patches to the executive branch all they like, but
53 the executive branch is free to veto them. Rarely, the Supreme Court
54 will side with the executive branch over the legislature, or the
55 legislature over the executive branch. Mostly, however, the
56 legislature and the executive branch are supposed to get along and
57 work out their differences without impeachment or court cases.
59 You might sometimes see reference to Rule 1 and Rule 2. Larry's power
60 as Supreme Court is expressed in The Rules:
66 Larry is always by definition right about how Perl should behave.
67 This means he has final veto power on the core functionality.
71 Larry is allowed to change his mind about any matter at a later date,
72 regardless of whether he previously invoked Rule 1.
76 Got that? Larry is always right, even when he was wrong. It's rare
77 to see either Rule exercised, but they are often alluded to.
79 New features and extensions to the language are contentious, because
80 the criteria used by the pumpkings, Larry, and other porters to decide
81 which features should be implemented and incorporated are not codified
82 in a few small design goals as with some other languages. Instead,
83 the heuristics are flexible and often difficult to fathom. Here is
84 one person's list, roughly in decreasing order of importance, of
85 heuristics that new features have to be weighed against:
89 =item Does concept match the general goals of Perl?
91 These haven't been written anywhere in stone, but one approximation
94 1. Keep it fast, simple, and useful.
95 2. Keep features/concepts as orthogonal as possible.
96 3. No arbitrary limits (platforms, data sizes, cultures).
97 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
98 5. Either assimilate new technologies, or build bridges to them.
100 =item Where is the implementation?
102 All the talk in the world is useless without an implementation. In
103 almost every case, the person or people who argue for a new feature
104 will be expected to be the ones who implement it. Porters capable
105 of coding new features have their own agendas, and are not available
106 to implement your (possibly good) idea.
108 =item Backwards compatibility
110 It's a cardinal sin to break existing Perl programs. New warnings are
111 contentious--some say that a program that emits warnings is not
112 broken, while others say it is. Adding keywords has the potential to
113 break programs, changing the meaning of existing token sequences or
114 functions might break programs.
116 =item Could it be a module instead?
118 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
119 the need to keep changing the Perl interpreter. You can write modules
120 that export functions, you can give those functions prototypes so they
121 can be called like built-in functions, you can even write XS code to
122 mess with the runtime data structures of the Perl interpreter if you
123 want to implement really complicated things. If it can be done in a
124 module instead of in the core, it's highly unlikely to be added.
126 =item Is the feature generic enough?
128 Is this something that only the submitter wants added to the language,
129 or would it be broadly useful? Sometimes, instead of adding a feature
130 with a tight focus, the porters might decide to wait until someone
131 implements the more generalized feature. For instance, instead of
132 implementing a "delayed evaluation" feature, the porters are waiting
133 for a macro system that would permit delayed evaluation and much more.
135 =item Does it potentially introduce new bugs?
137 Radical rewrites of large chunks of the Perl interpreter have the
138 potential to introduce new bugs. The smaller and more localized the
141 =item Does it preclude other desirable features?
143 A patch is likely to be rejected if it closes off future avenues of
144 development. For instance, a patch that placed a true and final
145 interpretation on prototypes is likely to be rejected because there
146 are still options for the future of prototypes that haven't been
149 =item Is the implementation robust?
151 Good patches (tight code, complete, correct) stand more chance of
152 going in. Sloppy or incorrect patches might be placed on the back
153 burner until the pumpking has time to fix, or might be discarded
154 altogether without further notice.
156 =item Is the implementation generic enough to be portable?
158 The worst patches make use of a system-specific features. It's highly
159 unlikely that non-portable additions to the Perl language will be
162 =item Is the implementation tested?
164 Patches which change behaviour (fixing bugs or introducing new features)
165 must include regression tests to verify that everything works as expected.
166 Without tests provided by the original author, how can anyone else changing
167 perl in the future be sure that they haven't unwittingly broken the behaviour
168 the patch implements? And without tests, how can the patch's author be
169 confident that his/her hard work put into the patch won't be accidentally
170 thrown away by someone in the future?
172 =item Is there enough documentation?
174 Patches without documentation are probably ill-thought out or
175 incomplete. Nothing can be added without documentation, so submitting
176 a patch for the appropriate manpages as well as the source code is
179 =item Is there another way to do it?
181 Larry said "Although the Perl Slogan is I<There's More Than One Way
182 to Do It>, I hesitate to make 10 ways to do something". This is a
183 tricky heuristic to navigate, though--one man's essential addition is
184 another man's pointless cruft.
186 =item Does it create too much work?
188 Work for the pumpking, work for Perl programmers, work for module
189 authors, ... Perl is supposed to be easy.
191 =item Patches speak louder than words
193 Working code is always preferred to pie-in-the-sky ideas. A patch to
194 add a feature stands a much higher chance of making it to the language
195 than does a random feature request, no matter how fervently argued the
196 request might be. This ties into "Will it be useful?", as the fact
197 that someone took the time to make the patch demonstrates a strong
198 desire for the feature.
202 If you're on the list, you might hear the word "core" bandied
203 around. It refers to the standard distribution. "Hacking on the
204 core" means you're changing the C source code to the Perl
205 interpreter. "A core module" is one that ships with Perl.
207 =head2 Keeping in sync
209 The source code to the Perl interpreter, in its different versions, is
210 kept in a repository managed by the git revision control system. The
211 pumpkings and a few others have write access to the repository to check in
214 How to clone and use the git perl repository is described in L<perlrepository>.
216 You can also choose to use rsync to get a copy of the current source tree
217 for the bleadperl branch and all maintenance branches :
219 $ rsync -avz rsync://perl5.git.perl.org/APC/perl-current .
220 $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.10.x .
221 $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.8.x .
222 $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.6.x .
223 $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.005xx .
225 (Add the C<--delete> option to remove leftover files)
227 You may also want to subscribe to the perl5-changes mailing list to
228 receive a copy of each patch that gets submitted to the maintenance
229 and development "branches" of the perl repository. See
230 http://lists.perl.org/ for subscription information.
232 If you are a member of the perl5-porters mailing list, it is a good
233 thing to keep in touch with the most recent changes. If not only to
234 verify if what you would have posted as a bug report isn't already
235 solved in the most recent available perl development branch, also
236 known as perl-current, bleading edge perl, bleedperl or bleadperl.
238 Needless to say, the source code in perl-current is usually in a perpetual
239 state of evolution. You should expect it to be very buggy. Do B<not> use
240 it for any purpose other than testing and development.
242 =head2 Perlbug administration
244 There is a single remote administrative interface for modifying bug status,
245 category, open issues etc. using the B<RT> bugtracker system, maintained
246 by Robert Spier. Become an administrator, and close any bugs you can get
247 your sticky mitts on:
249 http://bugs.perl.org/
251 To email the bug system administrators:
253 "perlbug-admin" <perlbug-admin@perl.org>
255 =head2 Submitting patches
257 Always submit patches to I<perl5-porters@perl.org>. If you're
258 patching a core module and there's an author listed, send the author a
259 copy (see L<Patching a core module>). This lets other porters review
260 your patch, which catches a surprising number of errors in patches.
261 Please patch against the latest B<development> version. (e.g., even if
262 you're fixing a bug in the 5.8 track, patch against the C<blead> branch in
265 If changes are accepted, they are applied to the development branch. Then
266 the maintenance pumpking decides which of those patches is to be
267 backported to the maint branch. Only patches that survive the heat of the
268 development branch get applied to maintenance versions.
270 Your patch should update the documentation and test suite. See
271 L<Writing a test>. If you have added or removed files in the distribution,
272 edit the MANIFEST file accordingly, sort the MANIFEST file using
273 C<make manisort>, and include those changes as part of your patch.
275 Patching documentation also follows the same order: if accepted, a patch
276 is first applied to B<development>, and if relevant then it's backported
277 to B<maintenance>. (With an exception for some patches that document
278 behaviour that only appears in the maintenance branch, but which has
279 changed in the development version.)
281 To report a bug in Perl, use the program I<perlbug> which comes with
282 Perl (if you can't get Perl to work, send mail to the address
283 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
284 I<perlbug> feeds into the automated bug-tracking system, access to
285 which is provided through the web at http://rt.perl.org/rt3/ . It
286 often pays to check the archives of the perl5-porters mailing list to
287 see whether the bug you're reporting has been reported before, and if
288 so whether it was considered a bug. See above for the location of
289 the searchable archives.
291 The CPAN testers ( http://testers.cpan.org/ ) are a group of
292 volunteers who test CPAN modules on a variety of platforms. Perl
293 Smokers ( http://www.nntp.perl.org/group/perl.daily-build and
294 http://www.nntp.perl.org/group/perl.daily-build.reports/ )
295 automatically test Perl source releases on platforms with various
296 configurations. Both efforts welcome volunteers. In order to get
297 involved in smoke testing of the perl itself visit
298 L<http://search.cpan.org/dist/Test-Smoke>. In order to start smoke
299 testing CPAN modules visit L<http://search.cpan.org/dist/CPANPLUS-YACSmoke/>
300 or L<http://search.cpan.org/dist/minismokebox/> or
301 L<http://search.cpan.org/dist/CPAN-Reporter/>.
303 It's a good idea to read and lurk for a while before chipping in.
304 That way you'll get to see the dynamic of the conversations, learn the
305 personalities of the players, and hopefully be better prepared to make
306 a useful contribution when do you speak up.
308 If after all this you still think you want to join the perl5-porters
309 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
310 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
312 To hack on the Perl guts, you'll need to read the following things:
318 This is of paramount importance, since it's the documentation of what
319 goes where in the Perl source. Read it over a couple of times and it
320 might start to make sense - don't worry if it doesn't yet, because the
321 best way to study it is to read it in conjunction with poking at Perl
322 source, and we'll do that later on.
324 Gisle Aas's illustrated perlguts (also known as I<illguts>) is wonderful,
325 although a little out of date with regard to some size details; the
326 various SV structures have since been reworked for smaller memory footprint.
327 The fundamentals are right however, and the pictures are very helpful.
329 L<http://www.perl.org/tpc/1998/Perl_Language_and_Modules/Perl%20Illustrated/>
331 =item L<perlxstut> and L<perlxs>
333 A working knowledge of XSUB programming is incredibly useful for core
334 hacking; XSUBs use techniques drawn from the PP code, the portion of the
335 guts that actually executes a Perl program. It's a lot gentler to learn
336 those techniques from simple examples and explanation than from the core
341 The documentation for the Perl API explains what some of the internal
342 functions do, as well as the many macros used in the source.
344 =item F<Porting/pumpkin.pod>
346 This is a collection of words of wisdom for a Perl porter; some of it is
347 only useful to the pumpkin holder, but most of it applies to anyone
348 wanting to go about Perl development.
350 =item The perl5-porters FAQ
352 This should be available from http://dev.perl.org/perl5/docs/p5p-faq.html .
353 It contains hints on reading perl5-porters, information on how
354 perl5-porters works and how Perl development in general works.
358 =head2 Finding Your Way Around
360 Perl maintenance can be split into a number of areas, and certain people
361 (pumpkins) will have responsibility for each area. These areas sometimes
362 correspond to files or directories in the source kit. Among the areas are:
368 Modules shipped as part of the Perl core live in various subdirectories, where
369 two are dedicated to core-only modules, and two are for the dual-life modules
370 which live on CPAN and may be maintained separately with respect to the Perl
373 lib/ is for pure-Perl modules, which exist in the core only.
375 ext/ is for XS extensions, and modules with special Makefile.PL requirements, which exist in the core only.
377 cpan/ is for dual-life modules, where the CPAN module is canonical (should be patched first).
379 dist/ is for dual-life modules, where the blead source is canonical.
383 There are tests for nearly all the modules, built-ins and major bits
384 of functionality. Test files all have a .t suffix. Module tests live
385 in the F<lib/> and F<ext/> directories next to the module being
386 tested. Others live in F<t/>. See L<Writing a test>
390 Documentation maintenance includes looking after everything in the
391 F<pod/> directory, (as well as contributing new documentation) and
392 the documentation to the modules in core.
396 The configure process is the way we make Perl portable across the
397 myriad of operating systems it supports. Responsibility for the
398 configure, build and installation process, as well as the overall
399 portability of the core code rests with the configure pumpkin - others
400 help out with individual operating systems.
402 The files involved are the operating system directories, (F<win32/>,
403 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
404 and F<Makefile>, as well as the metaconfig files which generate
405 F<Configure>. (metaconfig isn't included in the core distribution.)
409 And of course, there's the core of the Perl interpreter itself. Let's
410 have a look at that in a little more detail.
414 Before we leave looking at the layout, though, don't forget that
415 F<MANIFEST> contains not only the file names in the Perl distribution,
416 but short descriptions of what's in them, too. For an overview of the
417 important files, try this:
419 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
421 =head2 Elements of the interpreter
423 The work of the interpreter has two main stages: compiling the code
424 into the internal representation, or bytecode, and then executing it.
425 L<perlguts/Compiled code> explains exactly how the compilation stage
428 Here is a short breakdown of perl's operation:
434 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
435 This is very high-level code, enough to fit on a single screen, and it
436 resembles the code found in L<perlembed>; most of the real action takes
439 F<perlmain.c> is generated by L<writemain> from F<miniperlmain.c> at
440 make time, so you should make perl to follow this along.
442 First, F<perlmain.c> allocates some memory and constructs a Perl
443 interpreter, along these lines:
445 1 PERL_SYS_INIT3(&argc,&argv,&env);
447 3 if (!PL_do_undump) {
448 4 my_perl = perl_alloc();
451 7 perl_construct(my_perl);
452 8 PL_perl_destruct_level = 0;
455 Line 1 is a macro, and its definition is dependent on your operating
456 system. Line 3 references C<PL_do_undump>, a global variable - all
457 global variables in Perl start with C<PL_>. This tells you whether the
458 current running program was created with the C<-u> flag to perl and then
459 F<undump>, which means it's going to be false in any sane context.
461 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
462 interpreter. It's quite a simple function, and the guts of it looks like
465 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
467 Here you see an example of Perl's system abstraction, which we'll see
468 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
469 own C<malloc> as defined in F<malloc.c> if you selected that option at
472 Next, in line 7, we construct the interpreter using perl_construct,
473 also in F<perl.c>; this sets up all the special variables that Perl
474 needs, the stacks, and so on.
476 Now we pass Perl the command line options, and tell it to go:
478 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
482 exitstatus = perl_destruct(my_perl);
486 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
487 in F<perl.c>, which processes the command line options, sets up any
488 statically linked XS modules, opens the program and calls C<yyparse> to
493 The aim of this stage is to take the Perl source, and turn it into an op
494 tree. We'll see what one of those looks like later. Strictly speaking,
495 there's three things going on here.
497 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
498 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
499 B<is> a YACC grammar for Perl!) The job of the parser is to take your
500 code and "understand" it, splitting it into sentences, deciding which
501 operands go with which operators and so on.
503 The parser is nobly assisted by the lexer, which chunks up your input
504 into tokens, and decides what type of thing each token is: a variable
505 name, an operator, a bareword, a subroutine, a core function, and so on.
506 The main point of entry to the lexer is C<yylex>, and that and its
507 associated routines can be found in F<toke.c>. Perl isn't much like
508 other computer languages; it's highly context sensitive at times, it can
509 be tricky to work out what sort of token something is, or where a token
510 ends. As such, there's a lot of interplay between the tokeniser and the
511 parser, which can get pretty frightening if you're not used to it.
513 As the parser understands a Perl program, it builds up a tree of
514 operations for the interpreter to perform during execution. The routines
515 which construct and link together the various operations are to be found
516 in F<op.c>, and will be examined later.
520 Now the parsing stage is complete, and the finished tree represents
521 the operations that the Perl interpreter needs to perform to execute our
522 program. Next, Perl does a dry run over the tree looking for
523 optimisations: constant expressions such as C<3 + 4> will be computed
524 now, and the optimizer will also see if any multiple operations can be
525 replaced with a single one. For instance, to fetch the variable C<$foo>,
526 instead of grabbing the glob C<*foo> and looking at the scalar
527 component, the optimizer fiddles the op tree to use a function which
528 directly looks up the scalar in question. The main optimizer is C<peep>
529 in F<op.c>, and many ops have their own optimizing functions.
533 Now we're finally ready to go: we have compiled Perl byte code, and all
534 that's left to do is run it. The actual execution is done by the
535 C<runops_standard> function in F<run.c>; more specifically, it's done by
536 these three innocent looking lines:
538 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
542 You may be more comfortable with the Perl version of that:
544 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
546 Well, maybe not. Anyway, each op contains a function pointer, which
547 stipulates the function which will actually carry out the operation.
548 This function will return the next op in the sequence - this allows for
549 things like C<if> which choose the next op dynamically at run time.
550 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
551 execution if required.
553 The actual functions called are known as PP code, and they're spread
554 between four files: F<pp_hot.c> contains the "hot" code, which is most
555 often used and highly optimized, F<pp_sys.c> contains all the
556 system-specific functions, F<pp_ctl.c> contains the functions which
557 implement control structures (C<if>, C<while> and the like) and F<pp.c>
558 contains everything else. These are, if you like, the C code for Perl's
559 built-in functions and operators.
561 Note that each C<pp_> function is expected to return a pointer to the next
562 op. Calls to perl subs (and eval blocks) are handled within the same
563 runops loop, and do not consume extra space on the C stack. For example,
564 C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or C<CxEVAL> block
565 struct onto the context stack which contain the address of the op
566 following the sub call or eval. They then return the first op of that sub
567 or eval block, and so execution continues of that sub or block. Later, a
568 C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB> or C<CxEVAL>,
569 retrieves the return op from it, and returns it.
571 =item Exception handing
573 Perl's exception handing (i.e. C<die> etc.) is built on top of the low-level
574 C<setjmp()>/C<longjmp()> C-library functions. These basically provide a
575 way to capture the current PC and SP registers and later restore them; i.e.
576 a C<longjmp()> continues at the point in code where a previous C<setjmp()>
577 was done, with anything further up on the C stack being lost. This is why
578 code should always save values using C<SAVE_FOO> rather than in auto
581 The perl core wraps C<setjmp()> etc in the macros C<JMPENV_PUSH> and
582 C<JMPENV_JUMP>. The basic rule of perl exceptions is that C<exit>, and
583 C<die> (in the absence of C<eval>) perform a C<JMPENV_JUMP(2)>, while
584 C<die> within C<eval> does a C<JMPENV_JUMP(3)>.
586 At entry points to perl, such as C<perl_parse()>, C<perl_run()> and
587 C<call_sv(cv, G_EVAL)> each does a C<JMPENV_PUSH>, then enter a runops
588 loop or whatever, and handle possible exception returns. For a 2 return,
589 final cleanup is performed, such as popping stacks and calling C<CHECK> or
590 C<END> blocks. Amongst other things, this is how scope cleanup still
591 occurs during an C<exit>.
593 If a C<die> can find a C<CxEVAL> block on the context stack, then the
594 stack is popped to that level and the return op in that block is assigned
595 to C<PL_restartop>; then a C<JMPENV_JUMP(3)> is performed. This normally
596 passes control back to the guard. In the case of C<perl_run> and
597 C<call_sv>, a non-null C<PL_restartop> triggers re-entry to the runops
598 loop. The is the normal way that C<die> or C<croak> is handled within an
601 Sometimes ops are executed within an inner runops loop, such as tie, sort
602 or overload code. In this case, something like
604 sub FETCH { eval { die } }
606 would cause a longjmp right back to the guard in C<perl_run>, popping both
607 runops loops, which is clearly incorrect. One way to avoid this is for the
608 tie code to do a C<JMPENV_PUSH> before executing C<FETCH> in the inner
609 runops loop, but for efficiency reasons, perl in fact just sets a flag,
610 using C<CATCH_SET(TRUE)>. The C<pp_require>, C<pp_entereval> and
611 C<pp_entertry> ops check this flag, and if true, they call C<docatch>,
612 which does a C<JMPENV_PUSH> and starts a new runops level to execute the
613 code, rather than doing it on the current loop.
615 As a further optimisation, on exit from the eval block in the C<FETCH>,
616 execution of the code following the block is still carried on in the inner
617 loop. When an exception is raised, C<docatch> compares the C<JMPENV>
618 level of the C<CxEVAL> with C<PL_top_env> and if they differ, just
619 re-throws the exception. In this way any inner loops get popped.
623 1: eval { tie @a, 'A' };
629 To run this code, C<perl_run> is called, which does a C<JMPENV_PUSH> then
630 enters a runops loop. This loop executes the eval and tie ops on line 1,
631 with the eval pushing a C<CxEVAL> onto the context stack.
633 The C<pp_tie> does a C<CATCH_SET(TRUE)>, then starts a second runops loop
634 to execute the body of C<TIEARRAY>. When it executes the entertry op on
635 line 3, C<CATCH_GET> is true, so C<pp_entertry> calls C<docatch> which
636 does a C<JMPENV_PUSH> and starts a third runops loop, which then executes
637 the die op. At this point the C call stack looks like this:
640 Perl_runops # third loop
644 Perl_runops # second loop
648 Perl_runops # first loop
653 and the context and data stacks, as shown by C<-Dstv>, look like:
657 CX 1: EVAL => AV() PV("A"\0)
665 The die pops the first C<CxEVAL> off the context stack, sets
666 C<PL_restartop> from it, does a C<JMPENV_JUMP(3)>, and control returns to
667 the top C<docatch>. This then starts another third-level runops level,
668 which executes the nextstate, pushmark and die ops on line 4. At the point
669 that the second C<pp_die> is called, the C call stack looks exactly like
670 that above, even though we are no longer within an inner eval; this is
671 because of the optimization mentioned earlier. However, the context stack
672 now looks like this, ie with the top CxEVAL popped:
676 CX 1: EVAL => AV() PV("A"\0)
682 The die on line 4 pops the context stack back down to the CxEVAL, leaving
688 As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
689 C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:
693 Perl_runops # second loop
697 Perl_runops # first loop
702 In this case, because the C<JMPENV> level recorded in the C<CxEVAL>
703 differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
704 and the C stack unwinds to:
709 Because C<PL_restartop> is non-null, C<run_body> starts a new runops loop
710 and execution continues.
714 =head2 Internal Variable Types
716 You should by now have had a look at L<perlguts>, which tells you about
717 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
720 These variables are used not only to represent Perl-space variables, but
721 also any constants in the code, as well as some structures completely
722 internal to Perl. The symbol table, for instance, is an ordinary Perl
723 hash. Your code is represented by an SV as it's read into the parser;
724 any program files you call are opened via ordinary Perl filehandles, and
727 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
728 Perl program. Let's see, for instance, how Perl treats the constant
731 % perl -MDevel::Peek -e 'Dump("hello")'
732 1 SV = PV(0xa041450) at 0xa04ecbc
734 3 FLAGS = (POK,READONLY,pPOK)
735 4 PV = 0xa0484e0 "hello"\0
739 Reading C<Devel::Peek> output takes a bit of practise, so let's go
740 through it line by line.
742 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
743 memory. SVs themselves are very simple structures, but they contain a
744 pointer to a more complex structure. In this case, it's a PV, a
745 structure which holds a string value, at location C<0xa041450>. Line 2
746 is the reference count; there are no other references to this data, so
749 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
750 read-only SV (because it's a constant) and the data is a PV internally.
751 Next we've got the contents of the string, starting at location
754 Line 5 gives us the current length of the string - note that this does
755 B<not> include the null terminator. Line 6 is not the length of the
756 string, but the length of the currently allocated buffer; as the string
757 grows, Perl automatically extends the available storage via a routine
760 You can get at any of these quantities from C very easily; just add
761 C<Sv> to the name of the field shown in the snippet, and you've got a
762 macro which will return the value: C<SvCUR(sv)> returns the current
763 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
764 C<SvPV(sv, len)> returns the string itself with its length, and so on.
765 More macros to manipulate these properties can be found in L<perlguts>.
767 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
770 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
775 6 junk = SvPV_force(sv, tlen);
776 7 SvGROW(sv, tlen + len + 1);
779 10 Move(ptr,SvPVX(sv)+tlen,len,char);
781 12 *SvEND(sv) = '\0';
782 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
786 This is a function which adds a string, C<ptr>, of length C<len> onto
787 the end of the PV stored in C<sv>. The first thing we do in line 6 is
788 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
789 macro to force a PV. As a side effect, C<tlen> gets set to the current
790 value of the PV, and the PV itself is returned to C<junk>.
792 In line 7, we make sure that the SV will have enough room to accommodate
793 the old string, the new string and the null terminator. If C<LEN> isn't
794 big enough, C<SvGROW> will reallocate space for us.
796 Now, if C<junk> is the same as the string we're trying to add, we can
797 grab the string directly from the SV; C<SvPVX> is the address of the PV
800 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
801 memory around: we move the string C<ptr> to the end of the PV - that's
802 the start of the PV plus its current length. We're moving C<len> bytes
803 of type C<char>. After doing so, we need to tell Perl we've extended the
804 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
805 macro which gives us the end of the string, so that needs to be a
808 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
809 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
810 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware
811 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
812 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
813 data if taint mode is turned on.
815 AVs and HVs are more complicated, but SVs are by far the most common
816 variable type being thrown around. Having seen something of how we
817 manipulate these, let's go on and look at how the op tree is
822 First, what is the op tree, anyway? The op tree is the parsed
823 representation of your program, as we saw in our section on parsing, and
824 it's the sequence of operations that Perl goes through to execute your
825 program, as we saw in L</Running>.
827 An op is a fundamental operation that Perl can perform: all the built-in
828 functions and operators are ops, and there are a series of ops which
829 deal with concepts the interpreter needs internally - entering and
830 leaving a block, ending a statement, fetching a variable, and so on.
832 The op tree is connected in two ways: you can imagine that there are two
833 "routes" through it, two orders in which you can traverse the tree.
834 First, parse order reflects how the parser understood the code, and
835 secondly, execution order tells perl what order to perform the
838 The easiest way to examine the op tree is to stop Perl after it has
839 finished parsing, and get it to dump out the tree. This is exactly what
840 the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
841 and L<B::Debug|B::Debug> do.
843 Let's have a look at how Perl sees C<$a = $b + $c>:
845 % perl -MO=Terse -e '$a=$b+$c'
846 1 LISTOP (0x8179888) leave
847 2 OP (0x81798b0) enter
848 3 COP (0x8179850) nextstate
849 4 BINOP (0x8179828) sassign
850 5 BINOP (0x8179800) add [1]
851 6 UNOP (0x81796e0) null [15]
852 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
853 8 UNOP (0x81797e0) null [15]
854 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
855 10 UNOP (0x816b4f0) null [15]
856 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
858 Let's start in the middle, at line 4. This is a BINOP, a binary
859 operator, which is at location C<0x8179828>. The specific operator in
860 question is C<sassign> - scalar assignment - and you can find the code
861 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
862 binary operator, it has two children: the add operator, providing the
863 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
866 Line 10 is the null op: this does exactly nothing. What is that doing
867 there? If you see the null op, it's a sign that something has been
868 optimized away after parsing. As we mentioned in L</Optimization>,
869 the optimization stage sometimes converts two operations into one, for
870 example when fetching a scalar variable. When this happens, instead of
871 rewriting the op tree and cleaning up the dangling pointers, it's easier
872 just to replace the redundant operation with the null op. Originally,
873 the tree would have looked like this:
875 10 SVOP (0x816b4f0) rv2sv [15]
876 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
878 That is, fetch the C<a> entry from the main symbol table, and then look
879 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
880 happens to do both these things.
882 The right hand side, starting at line 5 is similar to what we've just
883 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
886 Now, what's this about?
888 1 LISTOP (0x8179888) leave
889 2 OP (0x81798b0) enter
890 3 COP (0x8179850) nextstate
892 C<enter> and C<leave> are scoping ops, and their job is to perform any
893 housekeeping every time you enter and leave a block: lexical variables
894 are tidied up, unreferenced variables are destroyed, and so on. Every
895 program will have those first three lines: C<leave> is a list, and its
896 children are all the statements in the block. Statements are delimited
897 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
898 the ops to be performed for each statement being the children of
899 C<nextstate>. C<enter> is a single op which functions as a marker.
901 That's how Perl parsed the program, from top to bottom:
914 However, it's impossible to B<perform> the operations in this order:
915 you have to find the values of C<$b> and C<$c> before you add them
916 together, for instance. So, the other thread that runs through the op
917 tree is the execution order: each op has a field C<op_next> which points
918 to the next op to be run, so following these pointers tells us how perl
919 executes the code. We can traverse the tree in this order using
920 the C<exec> option to C<B::Terse>:
922 % perl -MO=Terse,exec -e '$a=$b+$c'
923 1 OP (0x8179928) enter
924 2 COP (0x81798c8) nextstate
925 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
926 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
927 5 BINOP (0x8179878) add [1]
928 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
929 7 BINOP (0x81798a0) sassign
930 8 LISTOP (0x8179900) leave
932 This probably makes more sense for a human: enter a block, start a
933 statement. Get the values of C<$b> and C<$c>, and add them together.
934 Find C<$a>, and assign one to the other. Then leave.
936 The way Perl builds up these op trees in the parsing process can be
937 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
938 piece we need to construct the tree for C<$a = $b + $c>
940 1 term : term ASSIGNOP term
941 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
943 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
945 If you're not used to reading BNF grammars, this is how it works: You're
946 fed certain things by the tokeniser, which generally end up in upper
947 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
948 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
949 "terminal symbols", because you can't get any simpler than them.
951 The grammar, lines one and three of the snippet above, tells you how to
952 build up more complex forms. These complex forms, "non-terminal symbols"
953 are generally placed in lower case. C<term> here is a non-terminal
954 symbol, representing a single expression.
956 The grammar gives you the following rule: you can make the thing on the
957 left of the colon if you see all the things on the right in sequence.
958 This is called a "reduction", and the aim of parsing is to completely
959 reduce the input. There are several different ways you can perform a
960 reduction, separated by vertical bars: so, C<term> followed by C<=>
961 followed by C<term> makes a C<term>, and C<term> followed by C<+>
962 followed by C<term> can also make a C<term>.
964 So, if you see two terms with an C<=> or C<+>, between them, you can
965 turn them into a single expression. When you do this, you execute the
966 code in the block on the next line: if you see C<=>, you'll do the code
967 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
968 which contributes to the op tree.
971 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
973 What this does is creates a new binary op, and feeds it a number of
974 variables. The variables refer to the tokens: C<$1> is the first token in
975 the input, C<$2> the second, and so on - think regular expression
976 backreferences. C<$$> is the op returned from this reduction. So, we
977 call C<newBINOP> to create a new binary operator. The first parameter to
978 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
979 operator, so we want the type to be C<ADDOP>. We could specify this
980 directly, but it's right there as the second token in the input, so we
981 use C<$2>. The second parameter is the op's flags: 0 means "nothing
982 special". Then the things to add: the left and right hand side of our
983 expression, in scalar context.
987 When perl executes something like C<addop>, how does it pass on its
988 results to the next op? The answer is, through the use of stacks. Perl
989 has a number of stacks to store things it's currently working on, and
990 we'll look at the three most important ones here.
996 Arguments are passed to PP code and returned from PP code using the
997 argument stack, C<ST>. The typical way to handle arguments is to pop
998 them off the stack, deal with them how you wish, and then push the result
999 back onto the stack. This is how, for instance, the cosine operator
1004 value = Perl_cos(value);
1007 We'll see a more tricky example of this when we consider Perl's macros
1008 below. C<POPn> gives you the NV (floating point value) of the top SV on
1009 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1010 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1011 should be extended if necessary - it can't be necessary here, because we
1012 know there's room for one more item on the stack, since we've just
1013 removed one! The C<XPUSH*> macros at least guarantee safety.
1015 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1016 the first element in your portion of the stack, and C<TOP*> gives you
1017 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1018 negation of an integer:
1022 Just set the integer value of the top stack entry to its negation.
1024 Argument stack manipulation in the core is exactly the same as it is in
1025 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1026 description of the macros used in stack manipulation.
1030 I say "your portion of the stack" above because PP code doesn't
1031 necessarily get the whole stack to itself: if your function calls
1032 another function, you'll only want to expose the arguments aimed for the
1033 called function, and not (necessarily) let it get at your own data. The
1034 way we do this is to have a "virtual" bottom-of-stack, exposed to each
1035 function. The mark stack keeps bookmarks to locations in the argument
1036 stack usable by each function. For instance, when dealing with a tied
1037 variable, (internally, something with "P" magic) Perl has to call
1038 methods for accesses to the tied variables. However, we need to separate
1039 the arguments exposed to the method to the argument exposed to the
1040 original function - the store or fetch or whatever it may be. Here's
1041 roughly how the tied C<push> is implemented; see C<av_push> in F<av.c>:
1045 3 PUSHs(SvTIED_obj((SV*)av, mg));
1049 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1052 Let's examine the whole implementation, for practice:
1056 Push the current state of the stack pointer onto the mark stack. This is
1057 so that when we've finished adding items to the argument stack, Perl
1058 knows how many things we've added recently.
1061 3 PUSHs(SvTIED_obj((SV*)av, mg));
1064 We're going to add two more items onto the argument stack: when you have
1065 a tied array, the C<PUSH> subroutine receives the object and the value
1066 to be pushed, and that's exactly what we have here - the tied object,
1067 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1071 Next we tell Perl to update the global stack pointer from our internal
1072 variable: C<dSP> only gave us a local copy, not a reference to the global.
1075 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1078 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1079 variables are tidied up, everything that has been localised gets
1080 its previous value returned, and so on. Think of them as the C<{> and
1081 C<}> of a Perl block.
1083 To actually do the magic method call, we have to call a subroutine in
1084 Perl space: C<call_method> takes care of that, and it's described in
1085 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1086 going to discard its return value. The call_method() function
1087 removes the top element of the mark stack, so there is nothing for
1088 the caller to clean up.
1092 C doesn't have a concept of local scope, so perl provides one. We've
1093 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1094 stack implements the C equivalent of, for example:
1101 See L<perlguts/Localising Changes> for how to use the save stack.
1105 =head2 Millions of Macros
1107 One thing you'll notice about the Perl source is that it's full of
1108 macros. Some have called the pervasive use of macros the hardest thing
1109 to understand, others find it adds to clarity. Let's take an example,
1110 the code which implements the addition operator:
1114 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1117 6 SETn( left + right );
1122 Every line here (apart from the braces, of course) contains a macro. The
1123 first line sets up the function declaration as Perl expects for PP code;
1124 line 3 sets up variable declarations for the argument stack and the
1125 target, the return value of the operation. Finally, it tries to see if
1126 the addition operation is overloaded; if so, the appropriate subroutine
1129 Line 5 is another variable declaration - all variable declarations start
1130 with C<d> - which pops from the top of the argument stack two NVs (hence
1131 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1132 C<rl>. These are the two operands to the addition operator. Next, we
1133 call C<SETn> to set the NV of the return value to the result of adding
1134 the two values. This done, we return - the C<RETURN> macro makes sure
1135 that our return value is properly handled, and we pass the next operator
1136 to run back to the main run loop.
1138 Most of these macros are explained in L<perlapi>, and some of the more
1139 important ones are explained in L<perlxs> as well. Pay special attention
1140 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1141 the C<[pad]THX_?> macros.
1143 =head2 The .i Targets
1145 You can expand the macros in a F<foo.c> file by saying
1149 which will expand the macros using cpp. Don't be scared by the results.
1151 =head1 SOURCE CODE STATIC ANALYSIS
1153 Various tools exist for analysing C source code B<statically>, as
1154 opposed to B<dynamically>, that is, without executing the code.
1155 It is possible to detect resource leaks, undefined behaviour, type
1156 mismatches, portability problems, code paths that would cause illegal
1157 memory accesses, and other similar problems by just parsing the C code
1158 and looking at the resulting graph, what does it tell about the
1159 execution and data flows. As a matter of fact, this is exactly
1160 how C compilers know to give warnings about dubious code.
1164 The good old C code quality inspector, C<lint>, is available in
1165 several platforms, but please be aware that there are several
1166 different implementations of it by different vendors, which means that
1167 the flags are not identical across different platforms.
1169 There is a lint variant called C<splint> (Secure Programming Lint)
1170 available from http://www.splint.org/ that should compile on any
1173 There are C<lint> and <splint> targets in Makefile, but you may have
1174 to diddle with the flags (see above).
1178 Coverity (http://www.coverity.com/) is a product similar to lint and
1179 as a testbed for their product they periodically check several open
1180 source projects, and they give out accounts to open source developers
1181 to the defect databases.
1183 =head2 cpd (cut-and-paste detector)
1185 The cpd tool detects cut-and-paste coding. If one instance of the
1186 cut-and-pasted code changes, all the other spots should probably be
1187 changed, too. Therefore such code should probably be turned into a
1188 subroutine or a macro.
1190 cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
1191 (http://pmd.sourceforge.net/). pmd was originally written for static
1192 analysis of Java code, but later the cpd part of it was extended to
1193 parse also C and C++.
1195 Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
1196 pmd-X.Y.jar from it, and then run that on source code thusly:
1198 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
1200 You may run into memory limits, in which case you should use the -Xmx option:
1206 Though much can be written about the inconsistency and coverage
1207 problems of gcc warnings (like C<-Wall> not meaning "all the
1208 warnings", or some common portability problems not being covered by
1209 C<-Wall>, or C<-ansi> and C<-pedantic> both being a poorly defined
1210 collection of warnings, and so forth), gcc is still a useful tool in
1211 keeping our coding nose clean.
1213 The C<-Wall> is by default on.
1215 The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
1216 always, but unfortunately they are not safe on all platforms, they can
1217 for example cause fatal conflicts with the system headers (Solaris
1218 being a prime example). If Configure C<-Dgccansipedantic> is used,
1219 the C<cflags> frontend selects C<-ansi -pedantic> for the platforms
1220 where they are known to be safe.
1222 Starting from Perl 5.9.4 the following extra flags are added:
1236 C<-Wdeclaration-after-statement>
1240 The following flags would be nice to have but they would first need
1241 their own Augean stablemaster:
1255 C<-Wstrict-prototypes>
1259 The C<-Wtraditional> is another example of the annoying tendency of
1260 gcc to bundle a lot of warnings under one switch -- it would be
1261 impossible to deploy in practice because it would complain a lot -- but
1262 it does contain some warnings that would be beneficial to have available
1263 on their own, such as the warning about string constants inside macros
1264 containing the macro arguments: this behaved differently pre-ANSI
1265 than it does in ANSI, and some C compilers are still in transition,
1266 AIX being an example.
1268 =head2 Warnings of other C compilers
1270 Other C compilers (yes, there B<are> other C compilers than gcc) often
1271 have their "strict ANSI" or "strict ANSI with some portability extensions"
1272 modes on, like for example the Sun Workshop has its C<-Xa> mode on
1273 (though implicitly), or the DEC (these days, HP...) has its C<-std1>
1278 You can compile a special debugging version of Perl, which allows you
1279 to use the C<-D> option of Perl to tell more about what Perl is doing.
1280 But sometimes there is no alternative than to dive in with a debugger,
1281 either to see the stack trace of a core dump (very useful in a bug
1282 report), or trying to figure out what went wrong before the core dump
1283 happened, or how did we end up having wrong or unexpected results.
1285 =head2 Poking at Perl
1287 To really poke around with Perl, you'll probably want to build Perl for
1288 debugging, like this:
1290 ./Configure -d -D optimize=-g
1293 C<-g> is a flag to the C compiler to have it produce debugging
1294 information which will allow us to step through a running program,
1295 and to see in which C function we are at (without the debugging
1296 information we might see only the numerical addresses of the functions,
1297 which is not very helpful).
1299 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1300 enables all the internal debugging code in Perl. There are a whole bunch
1301 of things you can debug with this: L<perlrun> lists them all, and the
1302 best way to find out about them is to play about with them. The most
1303 useful options are probably
1305 l Context (loop) stack processing
1307 o Method and overloading resolution
1308 c String/numeric conversions
1310 Some of the functionality of the debugging code can be achieved using XS
1313 -Dr => use re 'debug'
1314 -Dx => use O 'Debug'
1316 =head2 Using a source-level debugger
1318 If the debugging output of C<-D> doesn't help you, it's time to step
1319 through perl's execution with a source-level debugger.
1325 We'll use C<gdb> for our examples here; the principles will apply to
1326 any debugger (many vendors call their debugger C<dbx>), but check the
1327 manual of the one you're using.
1331 To fire up the debugger, type
1335 Or if you have a core dump:
1339 You'll want to do that in your Perl source tree so the debugger can read
1340 the source code. You should see the copyright message, followed by the
1345 C<help> will get you into the documentation, but here are the most
1352 Run the program with the given arguments.
1354 =item break function_name
1356 =item break source.c:xxx
1358 Tells the debugger that we'll want to pause execution when we reach
1359 either the named function (but see L<perlguts/Internal Functions>!) or the given
1360 line in the named source file.
1364 Steps through the program a line at a time.
1368 Steps through the program a line at a time, without descending into
1373 Run until the next breakpoint.
1377 Run until the end of the current function, then stop again.
1381 Just pressing Enter will do the most recent operation again - it's a
1382 blessing when stepping through miles of source code.
1386 Execute the given C code and print its results. B<WARNING>: Perl makes
1387 heavy use of macros, and F<gdb> does not necessarily support macros
1388 (see later L</"gdb macro support">). You'll have to substitute them
1389 yourself, or to invoke cpp on the source code files
1390 (see L</"The .i Targets">)
1391 So, for instance, you can't say
1393 print SvPV_nolen(sv)
1397 print Perl_sv_2pv_nolen(sv)
1401 You may find it helpful to have a "macro dictionary", which you can
1402 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1403 recursively apply those macros for you.
1405 =head2 gdb macro support
1407 Recent versions of F<gdb> have fairly good macro support, but
1408 in order to use it you'll need to compile perl with macro definitions
1409 included in the debugging information. Using F<gcc> version 3.1, this
1410 means configuring with C<-Doptimize=-g3>. Other compilers might use a
1411 different switch (if they support debugging macros at all).
1413 =head2 Dumping Perl Data Structures
1415 One way to get around this macro hell is to use the dumping functions in
1416 F<dump.c>; these work a little like an internal
1417 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1418 that you can't get at from Perl. Let's take an example. We'll use the
1419 C<$a = $b + $c> we used before, but give it a bit of context:
1420 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1422 What about C<pp_add>, the function we examined earlier to implement the
1425 (gdb) break Perl_pp_add
1426 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1428 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1429 With the breakpoint in place, we can run our program:
1431 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1433 Lots of junk will go past as gdb reads in the relevant source files and
1434 libraries, and then:
1436 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1437 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1442 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1443 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1446 #define dPOPTOPnnrl_ul NV right = POPn; \
1447 SV *leftsv = TOPs; \
1448 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1450 C<POPn> takes the SV from the top of the stack and obtains its NV either
1451 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1452 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1453 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1454 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1456 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1457 convert it. If we step again, we'll find ourselves there:
1459 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1463 We can now use C<Perl_sv_dump> to investigate the SV:
1465 SV = PV(0xa057cc0) at 0xa0675d0
1468 PV = 0xa06a510 "6XXXX"\0
1473 We know we're going to get C<6> from this, so let's finish the
1477 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1478 0x462669 in Perl_pp_add () at pp_hot.c:311
1481 We can also dump out this op: the current op is always stored in
1482 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1483 similar output to L<B::Debug|B::Debug>.
1486 13 TYPE = add ===> 14
1488 FLAGS = (SCALAR,KIDS)
1490 TYPE = null ===> (12)
1492 FLAGS = (SCALAR,KIDS)
1494 11 TYPE = gvsv ===> 12
1500 # finish this later #
1504 All right, we've now had a look at how to navigate the Perl sources and
1505 some things you'll need to know when fiddling with them. Let's now get
1506 on and create a simple patch. Here's something Larry suggested: if a
1507 C<U> is the first active format during a C<pack>, (for example,
1508 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1511 If you are working with a git clone of the Perl repository, you will want to
1512 create a branch for your changes. This will make creating a proper patch much
1513 simpler. See the L<perlrepository> for details on how to do this.
1515 How do we prepare to fix this up? First we locate the code in question -
1516 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1517 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1518 altering this file, let's copy it to F<pp.c~>.
1520 [Well, it was in F<pp.c> when this tutorial was written. It has now been
1521 split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1523 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1524 loop over the pattern, taking each format character in turn into
1525 C<datum_type>. Then for each possible format character, we swallow up
1526 the other arguments in the pattern (a field width, an asterisk, and so
1527 on) and convert the next chunk input into the specified format, adding
1528 it onto the output SV C<cat>.
1530 How do we know if the C<U> is the first format in the C<pat>? Well, if
1531 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1532 test whether we're still at the start of the string. So, here's where
1536 register char *pat = SvPVx(*++MARK, fromlen);
1537 register char *patend = pat + fromlen;
1542 We'll have another string pointer in there:
1545 register char *pat = SvPVx(*++MARK, fromlen);
1546 register char *patend = pat + fromlen;
1552 And just before we start the loop, we'll set C<patcopy> to be the start
1557 sv_setpvn(cat, "", 0);
1559 while (pat < patend) {
1561 Now if we see a C<U> which was at the start of the string, we turn on
1562 the C<UTF8> flag for the output SV, C<cat>:
1564 + if (datumtype == 'U' && pat==patcopy+1)
1566 if (datumtype == '#') {
1567 while (pat < patend && *pat != '\n')
1570 Remember that it has to be C<patcopy+1> because the first character of
1571 the string is the C<U> which has been swallowed into C<datumtype!>
1573 Oops, we forgot one thing: what if there are spaces at the start of the
1574 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1575 character, even though it's not the first thing in the pattern. In this
1576 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1578 if (isSPACE(datumtype))
1583 if (isSPACE(datumtype)) {
1588 OK. That's the C part done. Now we must do two additional things before
1589 this patch is ready to go: we've changed the behaviour of Perl, and so
1590 we must document that change. We must also provide some more regression
1591 tests to make sure our patch works and doesn't create a bug somewhere
1592 else along the line.
1594 The regression tests for each operator live in F<t/op/>, and so we
1595 make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1596 tests to the end. First, we'll test that the C<U> does indeed create
1599 t/op/pack.t has a sensible ok() function, but if it didn't we could
1600 use the one from t/test.pl.
1602 require './test.pl';
1603 plan( tests => 159 );
1607 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1608 print "ok $test\n"; $test++;
1610 we can write the more sensible (see L<Test::More> for a full
1611 explanation of is() and other testing functions).
1613 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
1614 "U* produces Unicode" );
1616 Now we'll test that we got that space-at-the-beginning business right:
1618 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
1619 " with spaces at the beginning" );
1621 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1622 the first active format:
1624 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
1625 "U* not first isn't Unicode" );
1627 Mustn't forget to change the number of tests which appears at the top,
1628 or else the automated tester will get confused. This will either look
1635 plan( tests => 156 );
1637 We now compile up Perl, and run it through the test suite. Our new
1640 Finally, the documentation. The job is never done until the paperwork is
1641 over, so let's describe the change we've just made. The relevant place
1642 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1643 this text in the description of C<pack>:
1647 If the pattern begins with a C<U>, the resulting string will be treated
1648 as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
1649 with an initial C<U0>, and the bytes that follow will be interpreted as
1650 Unicode characters. If you don't want this to happen, you can begin your
1651 pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
1652 string, and then follow this with a C<U*> somewhere in your pattern.
1654 =head2 Patching a core module
1656 This works just like patching anything else, with an extra
1657 consideration. Many core modules also live on CPAN. If this is so,
1658 patch the CPAN version instead of the core and send the patch off to
1659 the module maintainer (with a copy to p5p). This will help the module
1660 maintainer keep the CPAN version in sync with the core version without
1661 constantly scanning p5p.
1663 The list of maintainers of core modules is usefully documented in
1664 F<Porting/Maintainers.pl>.
1666 =head2 Adding a new function to the core
1668 If, as part of a patch to fix a bug, or just because you have an
1669 especially good idea, you decide to add a new function to the core,
1670 discuss your ideas on p5p well before you start work. It may be that
1671 someone else has already attempted to do what you are considering and
1672 can give lots of good advice or even provide you with bits of code
1673 that they already started (but never finished).
1675 You have to follow all of the advice given above for patching. It is
1676 extremely important to test any addition thoroughly and add new tests
1677 to explore all boundary conditions that your new function is expected
1678 to handle. If your new function is used only by one module (e.g. toke),
1679 then it should probably be named S_your_function (for static); on the
1680 other hand, if you expect it to accessible from other functions in
1681 Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1684 The location of any new code is also an important consideration. Don't
1685 just create a new top level .c file and put your code there; you would
1686 have to make changes to Configure (so the Makefile is created properly),
1687 as well as possibly lots of include files. This is strictly pumpking
1690 It is better to add your function to one of the existing top level
1691 source code files, but your choice is complicated by the nature of
1692 the Perl distribution. Only the files that are marked as compiled
1693 static are located in the perl executable. Everything else is located
1694 in the shared library (or DLL if you are running under WIN32). So,
1695 for example, if a function was only used by functions located in
1696 toke.c, then your code can go in toke.c. If, however, you want to call
1697 the function from universal.c, then you should put your code in another
1698 location, for example util.c.
1700 In addition to writing your c-code, you will need to create an
1701 appropriate entry in embed.pl describing your function, then run
1702 'make regen_headers' to create the entries in the numerous header
1703 files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1704 for information on the various options that you can set in embed.pl.
1705 You will forget to do this a few (or many) times and you will get
1706 warnings during the compilation phase. Make sure that you mention
1707 this when you post your patch to P5P; the pumpking needs to know this.
1709 When you write your new code, please be conscious of existing code
1710 conventions used in the perl source files. See L<perlstyle> for
1711 details. Although most of the guidelines discussed seem to focus on
1712 Perl code, rather than c, they all apply (except when they don't ;).
1713 Also see I<perlrepository> for lots of details about both formatting and
1714 submitting patches of your changes.
1716 Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1717 Test on as many platforms as you can find. Test as many perl
1718 Configure options as you can (e.g. MULTIPLICITY). If you have
1719 profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1720 below for how to use them to further test your code. Remember that
1721 most of the people on P5P are doing this on their own time and
1722 don't have the time to debug your code.
1724 =head2 Writing a test
1726 Every module and built-in function has an associated test file (or
1727 should...). If you add or change functionality, you have to write a
1728 test. If you fix a bug, you have to write a test so that bug never
1729 comes back. If you alter the docs, it would be nice to test what the
1730 new documentation says.
1732 In short, if you submit a patch you probably also have to patch the
1735 For modules, the test file is right next to the module itself.
1736 F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1737 so there are some snags (and it would be wonderful for you to brush
1738 them out), but it basically works that way. Everything else lives in
1741 If you add a new test directory under F<t/>, it is imperative that you
1742 add that directory to F<t/HARNESS> and F<t/TEST>.
1748 Testing of the absolute basic functionality of Perl. Things like
1749 C<if>, basic file reads and writes, simple regexes, etc. These are
1750 run first in the test suite and if any of them fail, something is
1755 These test the basic control structures, C<if/else>, C<while>,
1760 Tests basic issues of how Perl parses and compiles itself.
1764 Tests for built-in IO functions, including command line arguments.
1768 The old home for the module tests, you shouldn't put anything new in
1769 here. There are still some bits and pieces hanging around in here
1770 that need to be moved. Perhaps you could move them? Thanks!
1774 Tests for perl's method resolution order implementations
1779 Tests for perl's built in functions that don't fit into any of the
1784 Tests for regex related functions or behaviour. (These used to live
1789 Testing features of how perl actually runs, including exit codes and
1790 handling of PERL* environment variables.
1794 Tests for the core support of Unicode.
1798 Windows-specific tests.
1802 A test suite for the s2p converter.
1806 The core uses the same testing style as the rest of Perl, a simple
1807 "ok/not ok" run through Test::Harness, but there are a few special
1810 There are three ways to write a test in the core. Test::More,
1811 t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1812 decision of which to use depends on what part of the test suite you're
1813 working on. This is a measure to prevent a high-level failure (such
1814 as Config.pm breaking) from causing basic functionality tests to fail.
1820 Since we don't know if require works, or even subroutines, use ad hoc
1821 tests for these two. Step carefully to avoid using the feature being
1824 =item t/cmd t/run t/io t/op
1826 Now that basic require() and subroutines are tested, you can use the
1827 t/test.pl library which emulates the important features of Test::More
1828 while using a minimum of core features.
1830 You can also conditionally use certain libraries like Config, but be
1831 sure to skip the test gracefully if it's not there.
1835 Now that the core of Perl is tested, Test::More can be used. You can
1836 also use the full suite of core modules in the tests.
1840 When you say "make test" Perl uses the F<t/TEST> program to run the
1841 test suite (except under Win32 where it uses F<t/harness> instead.)
1842 All tests are run from the F<t/> directory, B<not> the directory
1843 which contains the test. This causes some problems with the tests
1844 in F<lib/>, so here's some opportunity for some patching.
1846 You must be triply conscious of cross-platform concerns. This usually
1847 boils down to using File::Spec and avoiding things like C<fork()> and
1848 C<system()> unless absolutely necessary.
1850 =head2 Special Make Test Targets
1852 There are various special make targets that can be used to test Perl
1853 slightly differently than the standard "test" target. Not all them
1854 are expected to give a 100% success rate. Many of them have several
1855 aliases, and many of them are not available on certain operating
1862 Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests).
1864 (Not available on Win32)
1868 Run all the tests through B::Deparse. Not all tests will succeed.
1870 (Not available on Win32)
1872 =item test.taintwarn
1874 Run all tests with the B<-t> command-line switch. Not all tests
1875 are expected to succeed (until they're specifically fixed, of course).
1877 (Not available on Win32)
1881 Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
1882 F<t/op>, F<t/uni> and F<t/mro> tests.
1884 =item test.valgrind check.valgrind utest.valgrind ucheck.valgrind
1886 (Only in Linux) Run all the tests using the memory leak + naughty
1887 memory access tool "valgrind". The log files will be named
1888 F<testname.valgrind>.
1890 =item test.third check.third utest.third ucheck.third
1892 (Only in Tru64) Run all the tests using the memory leak + naughty
1893 memory access tool "Third Degree". The log files will be named
1894 F<perl.3log.testname>.
1896 =item test.torture torturetest
1898 Run all the usual tests and some extra tests. As of Perl 5.8.0 the
1899 only extra tests are Abigail's JAPHs, F<t/japh/abigail.t>.
1901 You can also run the torture test with F<t/harness> by giving
1902 C<-torture> argument to F<t/harness>.
1904 =item utest ucheck test.utf8 check.utf8
1906 Run all the tests with -Mutf8. Not all tests will succeed.
1908 (Not available on Win32)
1910 =item minitest.utf16 test.utf16
1912 Runs the tests with UTF-16 encoded scripts, encoded with different
1913 versions of this encoding.
1915 C<make utest.utf16> runs the test suite with a combination of C<-utf8> and
1916 C<-utf16> arguments to F<t/TEST>.
1918 (Not available on Win32)
1922 Run the test suite with the F<t/harness> controlling program, instead of
1923 F<t/TEST>. F<t/harness> is more sophisticated, and uses the
1924 L<Test::Harness> module, thus using this test target supposes that perl
1925 mostly works. The main advantage for our purposes is that it prints a
1926 detailed summary of failed tests at the end. Also, unlike F<t/TEST>, it
1927 doesn't redirect stderr to stdout.
1929 Note that under Win32 F<t/harness> is always used instead of F<t/TEST>, so
1930 there is no special "test_harness" target.
1932 Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES
1933 environment variables to control the behaviour of F<t/harness>. This means
1936 nmake test TEST_FILES="op/*.t"
1937 nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
1939 =item Parallel tests
1941 The core distribution can now run its regression tests in parallel on
1942 Unix-like platforms. Instead of running C<make test>, set C<TEST_JOBS> in
1943 your environment to the number of tests to run in parallel, and run
1944 C<make test_harness>. On a Bourne-like shell, this can be done as
1946 TEST_JOBS=3 make test_harness # Run 3 tests in parallel
1948 An environment variable is used, rather than parallel make itself, because
1949 L<TAP::Harness> needs to be able to schedule individual non-conflicting test
1950 scripts itself, and there is no standard interface to C<make> utilities to
1951 interact with their job schedulers.
1953 Note that currently some test scripts may fail when run in parallel (most
1954 notably C<ext/IO/t/io_dir.t>). If necessary run just the failing scripts
1955 again sequentially and see if the failures go away.
1956 =item test-notty test_notty
1958 Sets PERL_SKIP_TTY_TEST to true before running normal test.
1962 =head2 Running tests by hand
1964 You can run part of the test suite by hand by using one the following
1965 commands from the F<t/> directory :
1967 ./perl -I../lib TEST list-of-.t-files
1971 ./perl -I../lib harness list-of-.t-files
1973 (if you don't specify test scripts, the whole test suite will be run.)
1975 =head3 Using t/harness for testing
1977 If you use C<harness> for testing you have several command line options
1978 available to you. The arguments are as follows, and are in the order
1979 that they must appear if used together.
1981 harness -v -torture -re=pattern LIST OF FILES TO TEST
1982 harness -v -torture -re LIST OF PATTERNS TO MATCH
1984 If C<LIST OF FILES TO TEST> is omitted the file list is obtained from
1985 the manifest. The file list may include shell wildcards which will be
1992 Run the tests under verbose mode so you can see what tests were run,
1997 Run the torture tests as well as the normal set.
2001 Filter the file list so that all the test files run match PATTERN.
2002 Note that this form is distinct from the B<-re LIST OF PATTERNS> form below
2003 in that it allows the file list to be provided as well.
2005 =item -re LIST OF PATTERNS
2007 Filter the file list so that all the test files run match
2008 /(LIST|OF|PATTERNS)/. Note that with this form the patterns
2009 are joined by '|' and you cannot supply a list of files, instead
2010 the test files are obtained from the MANIFEST.
2014 You can run an individual test by a command similar to
2016 ./perl -I../lib patho/to/foo.t
2018 except that the harnesses set up some environment variables that may
2019 affect the execution of the test :
2025 indicates that we're running this test part of the perl core test suite.
2026 This is useful for modules that have a dual life on CPAN.
2028 =item PERL_DESTRUCT_LEVEL=2
2030 is set to 2 if it isn't set already (see L</PERL_DESTRUCT_LEVEL>)
2034 (used only by F<t/TEST>) if set, overrides the path to the perl executable
2035 that should be used to run the tests (the default being F<./perl>).
2037 =item PERL_SKIP_TTY_TEST
2039 if set, tells to skip the tests that need a terminal. It's actually set
2040 automatically by the Makefile, but can also be forced artificially by
2041 running 'make test_notty'.
2045 =head3 Other environment variables that may influence tests
2049 =item PERL_TEST_Net_Ping
2051 Setting this variable runs all the Net::Ping modules tests,
2052 otherwise some tests that interact with the outside world are skipped.
2055 =item PERL_TEST_NOVREXX
2057 Setting this variable skips the vrexx.t tests for OS2::REXX.
2059 =item PERL_TEST_NUMCONVERTS
2061 This sets a variable in op/numconvert.t.
2065 See also the documentation for the Test and Test::Harness modules,
2066 for more environment variables that affect testing.
2068 =head2 Common problems when patching Perl source code
2070 Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
2071 some cases we have to take pre-ANSI requirements into consideration.
2072 You don't care about some particular platform having broken Perl?
2073 I hear there is still a strong demand for J2EE programmers.
2075 =head2 Perl environment problems
2081 Not compiling with threading
2083 Compiling with threading (-Duseithreads) completely rewrites
2084 the function prototypes of Perl. You better try your changes
2085 with that. Related to this is the difference between "Perl_-less"
2086 and "Perl_-ly" APIs, for example:
2088 Perl_sv_setiv(aTHX_ ...);
2091 The first one explicitly passes in the context, which is needed for e.g.
2092 threaded builds. The second one does that implicitly; do not get them
2093 mixed. If you are not passing in a aTHX_, you will need to do a dTHX
2094 (or a dVAR) as the first thing in the function.
2096 See L<perlguts/"How multiple interpreters and concurrency are supported">
2097 for further discussion about context.
2101 Not compiling with -DDEBUGGING
2103 The DEBUGGING define exposes more code to the compiler,
2104 therefore more ways for things to go wrong. You should try it.
2108 Introducing (non-read-only) globals
2110 Do not introduce any modifiable globals, truly global or file static.
2111 They are bad form and complicate multithreading and other forms of
2112 concurrency. The right way is to introduce them as new interpreter
2113 variables, see F<intrpvar.h> (at the very end for binary compatibility).
2115 Introducing read-only (const) globals is okay, as long as you verify
2116 with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
2117 BSD-style output) that the data you added really is read-only.
2118 (If it is, it shouldn't show up in the output of that command.)
2120 If you want to have static strings, make them constant:
2122 static const char etc[] = "...";
2124 If you want to have arrays of constant strings, note carefully
2125 the right combination of C<const>s:
2127 static const char * const yippee[] =
2128 {"hi", "ho", "silver"};
2130 There is a way to completely hide any modifiable globals (they are all
2131 moved to heap), the compilation setting C<-DPERL_GLOBAL_STRUCT_PRIVATE>.
2132 It is not normally used, but can be used for testing, read more
2133 about it in L<perlguts/"Background and PERL_IMPLICIT_CONTEXT">.
2137 Not exporting your new function
2139 Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
2140 function that is part of the public API (the shared Perl library)
2141 to be explicitly marked as exported. See the discussion about
2142 F<embed.pl> in L<perlguts>.
2146 Exporting your new function
2148 The new shiny result of either genuine new functionality or your
2149 arduous refactoring is now ready and correctly exported. So what
2150 could possibly go wrong?
2152 Maybe simply that your function did not need to be exported in the
2153 first place. Perl has a long and not so glorious history of exporting
2154 functions that it should not have.
2156 If the function is used only inside one source code file, make it
2157 static. See the discussion about F<embed.pl> in L<perlguts>.
2159 If the function is used across several files, but intended only for
2160 Perl's internal use (and this should be the common case), do not
2161 export it to the public API. See the discussion about F<embed.pl>
2166 =head2 Portability problems
2168 The following are common causes of compilation and/or execution
2169 failures, not common to Perl as such. The C FAQ is good bedtime
2170 reading. Please test your changes with as many C compilers and
2171 platforms as possible -- we will, anyway, and it's nice to save
2172 oneself from public embarrassment.
2174 If using gcc, you can add the C<-std=c89> option which will hopefully
2175 catch most of these unportabilities. (However it might also catch
2176 incompatibilities in your system's header files.)
2178 Use the Configure C<-Dgccansipedantic> flag to enable the gcc
2179 C<-ansi -pedantic> flags which enforce stricter ANSI rules.
2181 If using the C<gcc -Wall> note that not all the possible warnings
2182 (like C<-Wunitialized>) are given unless you also compile with C<-O>.
2184 Note that if using gcc, starting from Perl 5.9.5 the Perl core source
2185 code files (the ones at the top level of the source code distribution,
2186 but not e.g. the extensions under ext/) are automatically compiled
2187 with as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>,
2188 and a selection of C<-W> flags (see cflags.SH).
2190 Also study L<perlport> carefully to avoid any bad assumptions
2191 about the operating system, filesystems, and so forth.
2193 You may once in a while try a "make microperl" to see whether we
2194 can still compile Perl with just the bare minimum of interfaces.
2197 Do not assume an operating system indicates a certain compiler.
2203 Casting pointers to integers or casting integers to pointers
2205 void castaway(U8* p)
2211 void castaway(U8* p)
2215 Both are bad, and broken, and unportable. Use the PTR2IV()
2216 macro that does it right. (Likewise, there are PTR2UV(), PTR2NV(),
2217 INT2PTR(), and NUM2PTR().)
2221 Casting between data function pointers and data pointers
2223 Technically speaking casting between function pointers and data
2224 pointers is unportable and undefined, but practically speaking
2225 it seems to work, but you should use the FPTR2DPTR() and DPTR2FPTR()
2226 macros. Sometimes you can also play games with unions.
2230 Assuming sizeof(int) == sizeof(long)
2232 There are platforms where longs are 64 bits, and platforms where ints
2233 are 64 bits, and while we are out to shock you, even platforms where
2234 shorts are 64 bits. This is all legal according to the C standard.
2235 (In other words, "long long" is not a portable way to specify 64 bits,
2236 and "long long" is not even guaranteed to be any wider than "long".)
2238 Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
2239 Avoid things like I32 because they are B<not> guaranteed to be
2240 I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
2241 guaranteed to be B<int> or B<long>. If you really explicitly need
2242 64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
2246 Assuming one can dereference any type of pointer for any type of data
2249 long pony = *p; /* BAD */
2251 Many platforms, quite rightly so, will give you a core dump instead
2252 of a pony if the p happens not be correctly aligned.
2258 (int)*p = ...; /* BAD */
2260 Simply not portable. Get your lvalue to be of the right type,
2261 or maybe use temporary variables, or dirty tricks with unions.
2265 Assume B<anything> about structs (especially the ones you
2266 don't control, like the ones coming from the system headers)
2272 That a certain field exists in a struct
2276 That no other fields exist besides the ones you know of
2280 That a field is of certain signedness, sizeof, or type
2284 That the fields are in a certain order
2290 While C guarantees the ordering specified in the struct definition,
2291 between different platforms the definitions might differ
2297 That the sizeof(struct) or the alignments are the same everywhere
2303 There might be padding bytes between the fields to align the fields -
2304 the bytes can be anything
2308 Structs are required to be aligned to the maximum alignment required
2309 by the fields - which for native types is for usually equivalent to
2310 sizeof() of the field
2318 Assuming the character set is ASCIIish
2320 Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
2321 This is transparent for the most part, but because the character sets
2322 differ, you shouldn't use numeric (decimal, octal, nor hex) constants
2323 to refer to characters. You can safely say 'A', but not 0x41.
2324 You can safely say '\n', but not \012.
2325 If a character doesn't have a trivial input form, you can
2326 create a #define for it in both C<utfebcdic.h> and C<utf8.h>, so that
2327 it resolves to different values depending on the character set being used.
2328 (There are three different EBCDIC character sets defined in C<utfebcdic.h>,
2329 so it might be best to insert the #define three times in that file.)
2331 Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper case
2332 alphabetic characters. That is not true in EBCDIC. Nor for 'a' to 'z'.
2333 But '0' - '9' is an unbroken range in both systems. Don't assume anything
2336 Many of the comments in the existing code ignore the possibility of EBCDIC,
2337 and may be wrong therefore, even if the code works.
2338 This is actually a tribute to the successful transparent insertion of being
2339 able to handle EBCDIC without having to change pre-existing code.
2341 UTF-8 and UTF-EBCDIC are two different encodings used to represent Unicode
2342 code points as sequences of bytes. Macros
2343 with the same names (but different definitions)
2344 in C<utf8.h> and C<utfebcdic.h>
2345 are used to allow the calling code to think that there is only one such
2347 This is almost always referred to as C<utf8>, but it means the EBCDIC version
2348 as well. Again, comments in the code may well be wrong even if the code itself
2350 For example, the concept of C<invariant characters> differs between ASCII and
2352 On ASCII platforms, only characters that do not have the high-order
2353 bit set (i.e. whose ordinals are strict ASCII, 0 - 127)
2354 are invariant, and the documentation and comments in the code
2356 often referring to something like, say, C<hibit>.
2357 The situation differs and is not so simple on EBCDIC machines, but as long as
2358 the code itself uses the C<NATIVE_IS_INVARIANT()> macro appropriately, it
2359 works, even if the comments are wrong.
2363 Assuming the character set is just ASCII
2365 ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
2366 characters have different meanings depending on the locale. Absent a locale,
2367 currently these extra characters are generally considered to be unassigned,
2368 and this has presented some problems.
2369 This is scheduled to be changed in 5.12 so that these characters will
2370 be considered to be Latin-1 (ISO-8859-1).
2374 Mixing #define and #ifdef
2376 #define BURGLE(x) ... \
2377 #ifdef BURGLE_OLD_STYLE /* BAD */
2378 ... do it the old way ... \
2380 ... do it the new way ... \
2383 You cannot portably "stack" cpp directives. For example in the above
2384 you need two separate BURGLE() #defines, one for each #ifdef branch.
2388 Adding non-comment stuff after #endif or #else
2392 #else !SNOSH /* BAD */
2394 #endif SNOSH /* BAD */
2396 The #endif and #else cannot portably have anything non-comment after
2397 them. If you want to document what is going (which is a good idea
2398 especially if the branches are long), use (C) comments:
2406 The gcc option C<-Wendif-labels> warns about the bad variant
2407 (by default on starting from Perl 5.9.4).
2411 Having a comma after the last element of an enum list
2419 is not portable. Leave out the last comma.
2421 Also note that whether enums are implicitly morphable to ints
2422 varies between compilers, you might need to (int).
2428 // This function bamfoodles the zorklator. /* BAD */
2430 That is C99 or C++. Perl is C89. Using the //-comments is silently
2431 allowed by many C compilers but cranking up the ANSI C89 strictness
2432 (which we like to do) causes the compilation to fail.
2436 Mixing declarations and code
2441 set_zorkmids(n); /* BAD */
2444 That is C99 or C++. Some C compilers allow that, but you shouldn't.
2446 The gcc option C<-Wdeclaration-after-statements> scans for such problems
2447 (by default on starting from Perl 5.9.4).
2451 Introducing variables inside for()
2453 for(int i = ...; ...; ...) { /* BAD */
2455 That is C99 or C++. While it would indeed be awfully nice to have that
2456 also in C89, to limit the scope of the loop variable, alas, we cannot.
2460 Mixing signed char pointers with unsigned char pointers
2462 int foo(char *s) { ... }
2464 unsigned char *t = ...; /* Or U8* t = ... */
2467 While this is legal practice, it is certainly dubious, and downright
2468 fatal in at least one platform: for example VMS cc considers this a
2469 fatal error. One cause for people often making this mistake is that a
2470 "naked char" and therefore dereferencing a "naked char pointer" have
2471 an undefined signedness: it depends on the compiler and the flags of
2472 the compiler and the underlying platform whether the result is signed
2473 or unsigned. For this very same reason using a 'char' as an array
2478 Macros that have string constants and their arguments as substrings of
2479 the string constants
2481 #define FOO(n) printf("number = %d\n", n) /* BAD */
2484 Pre-ANSI semantics for that was equivalent to
2486 printf("10umber = %d\10");
2488 which is probably not what you were expecting. Unfortunately at least
2489 one reasonably common and modern C compiler does "real backward
2490 compatibility" here, in AIX that is what still happens even though the
2491 rest of the AIX compiler is very happily C89.
2495 Using printf formats for non-basic C types
2498 printf("i = %d\n", i); /* BAD */
2500 While this might by accident work in some platform (where IV happens
2501 to be an C<int>), in general it cannot. IV might be something larger.
2502 Even worse the situation is with more specific types (defined by Perl's
2503 configuration step in F<config.h>):
2506 printf("who = %d\n", who); /* BAD */
2508 The problem here is that Uid_t might be not only not C<int>-wide
2509 but it might also be unsigned, in which case large uids would be
2510 printed as negative values.
2512 There is no simple solution to this because of printf()'s limited
2513 intelligence, but for many types the right format is available as
2514 with either 'f' or '_f' suffix, for example:
2516 IVdf /* IV in decimal */
2517 UVxf /* UV is hexadecimal */
2519 printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
2521 Uid_t_f /* Uid_t in decimal */
2523 printf("who = %"Uid_t_f"\n", who);
2525 Or you can try casting to a "wide enough" type:
2527 printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
2529 Also remember that the C<%p> format really does require a void pointer:
2532 printf("p = %p\n", (void*)p);
2534 The gcc option C<-Wformat> scans for such problems.
2538 Blindly using variadic macros
2540 gcc has had them for a while with its own syntax, and C99 brought
2541 them with a standardized syntax. Don't use the former, and use
2542 the latter only if the HAS_C99_VARIADIC_MACROS is defined.
2546 Blindly passing va_list
2548 Not all platforms support passing va_list to further varargs (stdarg)
2549 functions. The right thing to do is to copy the va_list using the
2550 Perl_va_copy() if the NEED_VA_COPY is defined.
2554 Using gcc statement expressions
2556 val = ({...;...;...}); /* BAD */
2558 While a nice extension, it's not portable. The Perl code does
2559 admittedly use them if available to gain some extra speed
2560 (essentially as a funky form of inlining), but you shouldn't.
2564 Binding together several statements in a macro
2566 Use the macros STMT_START and STMT_END.
2574 Testing for operating systems or versions when should be testing for features
2576 #ifdef __FOONIX__ /* BAD */
2580 Unless you know with 100% certainty that quux() is only ever available
2581 for the "Foonix" operating system B<and> that is available B<and>
2582 correctly working for B<all> past, present, B<and> future versions of
2583 "Foonix", the above is very wrong. This is more correct (though still
2584 not perfect, because the below is a compile-time check):
2590 How does the HAS_QUUX become defined where it needs to be? Well, if
2591 Foonix happens to be UNIXy enough to be able to run the Configure
2592 script, and Configure has been taught about detecting and testing
2593 quux(), the HAS_QUUX will be correctly defined. In other platforms,
2594 the corresponding configuration step will hopefully do the same.
2596 In a pinch, if you cannot wait for Configure to be educated,
2597 or if you have a good hunch of where quux() might be available,
2598 you can temporarily try the following:
2600 #if (defined(__FOONIX__) || defined(__BARNIX__))
2610 But in any case, try to keep the features and operating systems separate.
2614 =head2 Problematic System Interfaces
2620 malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
2621 allocate at least one byte. (In general you should rarely need to
2622 work at this low level, but instead use the various malloc wrappers.)
2626 snprintf() - the return type is unportable. Use my_snprintf() instead.
2630 =head2 Security problems
2632 Last but not least, here are various tips for safer coding.
2640 Or we will publicly ridicule you. Seriously.
2644 Do not use strcpy() or strcat() or strncpy() or strncat()
2646 Use my_strlcpy() and my_strlcat() instead: they either use the native
2647 implementation, or Perl's own implementation (borrowed from the public
2648 domain implementation of INN).
2652 Do not use sprintf() or vsprintf()
2654 If you really want just plain byte strings, use my_snprintf()
2655 and my_vsnprintf() instead, which will try to use snprintf() and
2656 vsnprintf() if those safer APIs are available. If you want something
2657 fancier than a plain byte string, use SVs and Perl_sv_catpvf().
2661 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
2663 Sometimes it helps to use external tools while debugging and
2664 testing Perl. This section tries to guide you through using
2665 some common testing and debugging tools with Perl. This is
2666 meant as a guide to interfacing these tools with Perl, not
2667 as any kind of guide to the use of the tools themselves.
2669 B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or
2670 Third Degree greatly slows down the execution: seconds become minutes,
2671 minutes become hours. For example as of Perl 5.8.1, the
2672 ext/Encode/t/Unicode.t takes extraordinarily long to complete under
2673 e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
2674 than six hours, even on a snappy computer-- the said test must be
2675 doing something that is quite unfriendly for memory debuggers. If you
2676 don't feel like waiting, that you can simply kill away the perl
2679 B<NOTE 2>: To minimize the number of memory leak false alarms (see
2680 L</PERL_DESTRUCT_LEVEL> for more information), you have to have
2681 environment variable PERL_DESTRUCT_LEVEL set to 2. The F<TEST>
2682 and harness scripts do that automatically. But if you are running
2683 some of the tests manually-- for csh-like shells:
2685 setenv PERL_DESTRUCT_LEVEL 2
2687 and for Bourne-type shells:
2689 PERL_DESTRUCT_LEVEL=2
2690 export PERL_DESTRUCT_LEVEL
2692 or in UNIXy environments you can also use the C<env> command:
2694 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
2696 B<NOTE 3>: There are known memory leaks when there are compile-time
2697 errors within eval or require, seeing C<S_doeval> in the call stack
2698 is a good sign of these. Fixing these leaks is non-trivial,
2699 unfortunately, but they must be fixed eventually.
2701 B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
2702 unless Perl is built with the Configure option
2703 C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
2705 =head2 Rational Software's Purify
2707 Purify is a commercial tool that is helpful in identifying
2708 memory overruns, wild pointers, memory leaks and other such
2709 badness. Perl must be compiled in a specific way for
2710 optimal testing with Purify. Purify is available under
2711 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
2713 =head2 Purify on Unix
2715 On Unix, Purify creates a new Perl binary. To get the most
2716 benefit out of Purify, you should create the perl to Purify
2719 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
2720 -Uusemymalloc -Dusemultiplicity
2722 where these arguments mean:
2726 =item -Accflags=-DPURIFY
2728 Disables Perl's arena memory allocation functions, as well as
2729 forcing use of memory allocation functions derived from the
2732 =item -Doptimize='-g'
2734 Adds debugging information so that you see the exact source
2735 statements where the problem occurs. Without this flag, all
2736 you will see is the source filename of where the error occurred.
2740 Disable Perl's malloc so that Purify can more closely monitor
2741 allocations and leaks. Using Perl's malloc will make Purify
2742 report most leaks in the "potential" leaks category.
2744 =item -Dusemultiplicity
2746 Enabling the multiplicity option allows perl to clean up
2747 thoroughly when the interpreter shuts down, which reduces the
2748 number of bogus leak reports from Purify.
2752 Once you've compiled a perl suitable for Purify'ing, then you
2757 which creates a binary named 'pureperl' that has been Purify'ed.
2758 This binary is used in place of the standard 'perl' binary
2759 when you want to debug Perl memory problems.
2761 As an example, to show any memory leaks produced during the
2762 standard Perl testset you would create and run the Purify'ed
2767 ../pureperl -I../lib harness
2769 which would run Perl on test.pl and report any memory problems.
2771 Purify outputs messages in "Viewer" windows by default. If
2772 you don't have a windowing environment or if you simply
2773 want the Purify output to unobtrusively go to a log file
2774 instead of to the interactive window, use these following
2775 options to output to the log file "perl.log":
2777 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
2778 -log-file=perl.log -append-logfile=yes"
2780 If you plan to use the "Viewer" windows, then you only need this option:
2782 setenv PURIFYOPTIONS "-chain-length=25"
2784 In Bourne-type shells:
2787 export PURIFYOPTIONS
2789 or if you have the "env" utility:
2791 env PURIFYOPTIONS="..." ../pureperl ...
2795 Purify on Windows NT instruments the Perl binary 'perl.exe'
2796 on the fly. There are several options in the makefile you
2797 should change to get the most use out of Purify:
2803 You should add -DPURIFY to the DEFINES line so the DEFINES
2804 line looks something like:
2806 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
2808 to disable Perl's arena memory allocation functions, as
2809 well as to force use of memory allocation functions derived
2810 from the system malloc.
2812 =item USE_MULTI = define
2814 Enabling the multiplicity option allows perl to clean up
2815 thoroughly when the interpreter shuts down, which reduces the
2816 number of bogus leak reports from Purify.
2818 =item #PERL_MALLOC = define
2820 Disable Perl's malloc so that Purify can more closely monitor
2821 allocations and leaks. Using Perl's malloc will make Purify
2822 report most leaks in the "potential" leaks category.
2826 Adds debugging information so that you see the exact source
2827 statements where the problem occurs. Without this flag, all
2828 you will see is the source filename of where the error occurred.
2832 As an example, to show any memory leaks produced during the
2833 standard Perl testset you would create and run Purify as:
2838 purify ../perl -I../lib harness
2840 which would instrument Perl in memory, run Perl on test.pl,
2841 then finally report any memory problems.
2845 The excellent valgrind tool can be used to find out both memory leaks
2846 and illegal memory accesses. As of version 3.3.0, Valgrind only
2847 supports Linux on x86, x86-64 and PowerPC. The special "test.valgrind"
2848 target can be used to run the tests under valgrind. Found errors
2849 and memory leaks are logged in files named F<testfile.valgrind>.
2851 Valgrind also provides a cachegrind tool, invoked on perl as:
2853 VG_OPTS=--tool=cachegrind make test.valgrind
2855 As system libraries (most notably glibc) are also triggering errors,
2856 valgrind allows to suppress such errors using suppression files. The
2857 default suppression file that comes with valgrind already catches a lot
2858 of them. Some additional suppressions are defined in F<t/perl.supp>.
2860 To get valgrind and for more information see
2862 http://developer.kde.org/~sewardj/
2864 =head2 Compaq's/Digital's/HP's Third Degree
2866 Third Degree is a tool for memory leak detection and memory access checks.
2867 It is one of the many tools in the ATOM toolkit. The toolkit is only
2868 available on Tru64 (formerly known as Digital UNIX formerly known as
2871 When building Perl, you must first run Configure with -Doptimize=-g
2872 and -Uusemymalloc flags, after that you can use the make targets
2873 "perl.third" and "test.third". (What is required is that Perl must be
2874 compiled using the C<-g> flag, you may need to re-Configure.)
2876 The short story is that with "atom" you can instrument the Perl
2877 executable to create a new executable called F<perl.third>. When the
2878 instrumented executable is run, it creates a log of dubious memory
2879 traffic in file called F<perl.3log>. See the manual pages of atom and
2880 third for more information. The most extensive Third Degree
2881 documentation is available in the Compaq "Tru64 UNIX Programmer's
2882 Guide", chapter "Debugging Programs with Third Degree".
2884 The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/
2885 subdirectory. There is a problem with these files: Third Degree is so
2886 effective that it finds problems also in the system libraries.
2887 Therefore you should used the Porting/thirdclean script to cleanup
2888 the F<*.3log> files.
2890 There are also leaks that for given certain definition of a leak,
2891 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
2893 =head2 PERL_DESTRUCT_LEVEL
2895 If you want to run any of the tests yourself manually using e.g.
2896 valgrind, or the pureperl or perl.third executables, please note that
2897 by default perl B<does not> explicitly cleanup all the memory it has
2898 allocated (such as global memory arenas) but instead lets the exit()
2899 of the whole program "take care" of such allocations, also known as
2900 "global destruction of objects".
2902 There is a way to tell perl to do complete cleanup: set the
2903 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
2904 The t/TEST wrapper does set this to 2, and this is what you
2905 need to do too, if you don't want to see the "global leaks":
2906 For example, for "third-degreed" Perl:
2908 env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
2910 (Note: the mod_perl apache module uses also this environment variable
2911 for its own purposes and extended its semantics. Refer to the mod_perl
2912 documentation for more information. Also, spawned threads do the
2913 equivalent of setting this variable to the value 1.)
2915 If, at the end of a run you get the message I<N scalars leaked>, you can
2916 recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the addresses
2917 of all those leaked SVs to be dumped along with details as to where each
2918 SV was originally allocated. This information is also displayed by
2919 Devel::Peek. Note that the extra details recorded with each SV increases
2920 memory usage, so it shouldn't be used in production environments. It also
2921 converts C<new_SV()> from a macro into a real function, so you can use
2922 your favourite debugger to discover where those pesky SVs were allocated.
2924 If you see that you're leaking memory at runtime, but neither valgrind
2925 nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
2926 leaking SVs that are still reachable and will be properly cleaned up
2927 during destruction of the interpreter. In such cases, using the C<-Dm>
2928 switch can point you to the source of the leak. If the executable was
2929 built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV allocations
2930 in addition to memory allocations. Each SV allocation has a distinct
2931 serial number that will be written on creation and destruction of the SV.
2932 So if you're executing the leaking code in a loop, you need to look for
2933 SVs that are created, but never destroyed between each cycle. If such an
2934 SV is found, set a conditional breakpoint within C<new_SV()> and make it
2935 break only when C<PL_sv_serial> is equal to the serial number of the
2936 leaking SV. Then you will catch the interpreter in exactly the state
2937 where the leaking SV is allocated, which is sufficient in many cases to
2938 find the source of the leak.
2940 As C<-Dm> is using the PerlIO layer for output, it will by itself
2941 allocate quite a bunch of SVs, which are hidden to avoid recursion.
2942 You can bypass the PerlIO layer if you use the SV logging provided
2943 by C<-DPERL_MEM_LOG> instead.
2947 If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
2948 through logging functions, which is handy for breakpoint setting.
2950 Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging
2951 functions read $ENV{PERL_MEM_LOG} to determine whether to log the
2952 event, and if so how:
2954 $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
2955 $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
2956 $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
2957 $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
2959 Memory logging is somewhat similar to C<-Dm> but is independent of
2960 C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(),
2961 and Safefree() are logged with the caller's source code file and line
2962 number (and C function name, if supported by the C compiler). In
2963 contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging
2966 Since the logging doesn't use PerlIO, all SV allocations are logged
2967 and no extra SV allocations are introduced by enabling the logging.
2968 If compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for
2969 each SV allocation is also logged.
2973 Depending on your platform there are various of profiling Perl.
2975 There are two commonly used techniques of profiling executables:
2976 I<statistical time-sampling> and I<basic-block counting>.
2978 The first method takes periodically samples of the CPU program
2979 counter, and since the program counter can be correlated with the code
2980 generated for functions, we get a statistical view of in which
2981 functions the program is spending its time. The caveats are that very
2982 small/fast functions have lower probability of showing up in the
2983 profile, and that periodically interrupting the program (this is
2984 usually done rather frequently, in the scale of milliseconds) imposes
2985 an additional overhead that may skew the results. The first problem
2986 can be alleviated by running the code for longer (in general this is a
2987 good idea for profiling), the second problem is usually kept in guard
2988 by the profiling tools themselves.
2990 The second method divides up the generated code into I<basic blocks>.
2991 Basic blocks are sections of code that are entered only in the
2992 beginning and exited only at the end. For example, a conditional jump
2993 starts a basic block. Basic block profiling usually works by
2994 I<instrumenting> the code by adding I<enter basic block #nnnn>
2995 book-keeping code to the generated code. During the execution of the
2996 code the basic block counters are then updated appropriately. The
2997 caveat is that the added extra code can skew the results: again, the
2998 profiling tools usually try to factor their own effects out of the
3001 =head2 Gprof Profiling
3003 gprof is a profiling tool available in many UNIX platforms,
3004 it uses F<statistical time-sampling>.
3006 You can build a profiled version of perl called "perl.gprof" by
3007 invoking the make target "perl.gprof" (What is required is that Perl
3008 must be compiled using the C<-pg> flag, you may need to re-Configure).
3009 Running the profiled version of Perl will create an output file called
3010 F<gmon.out> is created which contains the profiling data collected
3011 during the execution.
3013 The gprof tool can then display the collected data in various ways.
3014 Usually gprof understands the following options:
3020 Suppress statically defined functions from the profile.
3024 Suppress the verbose descriptions in the profile.
3028 Exclude the given routine and its descendants from the profile.
3032 Display only the given routine and its descendants in the profile.
3036 Generate a summary file called F<gmon.sum> which then may be given
3037 to subsequent gprof runs to accumulate data over several runs.
3041 Display routines that have zero usage.
3045 For more detailed explanation of the available commands and output
3046 formats, see your own local documentation of gprof.
3050 $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
3051 $ ./perl.gprof someprog # creates gmon.out in current directory
3052 $ gprof ./perl.gprof > out
3055 =head2 GCC gcov Profiling
3057 Starting from GCC 3.0 I<basic block profiling> is officially available
3060 You can build a profiled version of perl called F<perl.gcov> by
3061 invoking the make target "perl.gcov" (what is required that Perl must
3062 be compiled using gcc with the flags C<-fprofile-arcs
3063 -ftest-coverage>, you may need to re-Configure).
3065 Running the profiled version of Perl will cause profile output to be
3066 generated. For each source file an accompanying ".da" file will be
3069 To display the results you use the "gcov" utility (which should
3070 be installed if you have gcc 3.0 or newer installed). F<gcov> is
3071 run on source code files, like this
3075 which will cause F<sv.c.gcov> to be created. The F<.gcov> files
3076 contain the source code annotated with relative frequencies of
3077 execution indicated by "#" markers.
3079 Useful options of F<gcov> include C<-b> which will summarise the
3080 basic block, branch, and function call coverage, and C<-c> which
3081 instead of relative frequencies will use the actual counts. For
3082 more information on the use of F<gcov> and basic block profiling
3083 with gcc, see the latest GNU CC manual, as of GCC 3.0 see
3085 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
3087 and its section titled "8. gcov: a Test Coverage Program"
3089 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
3093 $ sh Configure -des -Doptimize='-g' -Accflags='-fprofile-arcs -ftest-coverage' \
3094 -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
3095 $ rm -f regexec.c.gcov regexec.gcda
3098 $ view regexec.c.gcov
3100 =head2 Pixie Profiling
3102 Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
3103 UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
3104 I<basic-block counting>.
3106 You can build a profiled version of perl called F<perl.pixie> by
3107 invoking the make target "perl.pixie" (what is required is that Perl
3108 must be compiled using the C<-g> flag, you may need to re-Configure).
3110 In Tru64 a file called F<perl.Addrs> will also be silently created,
3111 this file contains the addresses of the basic blocks. Running the
3112 profiled version of Perl will create a new file called "perl.Counts"
3113 which contains the counts for the basic block for that particular
3116 To display the results you use the F<prof> utility. The exact
3117 incantation depends on your operating system, "prof perl.Counts" in
3118 IRIX, and "prof -pixie -all -L. perl" in Tru64.
3120 In IRIX the following prof options are available:
3126 Reports the most heavily used lines in descending order of use.
3127 Useful for finding the hotspot lines.
3131 Groups lines by procedure, with procedures sorted in descending order of use.
3132 Within a procedure, lines are listed in source order.
3133 Useful for finding the hotspots of procedures.
3137 In Tru64 the following options are available:
3143 Procedures sorted in descending order by the number of cycles executed
3144 in each procedure. Useful for finding the hotspot procedures.
3145 (This is the default option.)
3149 Lines sorted in descending order by the number of cycles executed in
3150 each line. Useful for finding the hotspot lines.
3152 =item -i[nvocations]
3154 The called procedures are sorted in descending order by number of calls
3155 made to the procedures. Useful for finding the most used procedures.
3159 Grouped by procedure, sorted by cycles executed per procedure.
3160 Useful for finding the hotspots of procedures.
3164 The compiler emitted code for these lines, but the code was unexecuted.
3168 Unexecuted procedures.
3172 For further information, see your system's manual pages for pixie and prof.
3174 =head2 Miscellaneous tricks
3180 Those debugging perl with the DDD frontend over gdb may find the
3183 You can extend the data conversion shortcuts menu, so for example you
3184 can display an SV's IV value with one click, without doing any typing.
3185 To do that simply edit ~/.ddd/init file and add after:
3187 ! Display shortcuts.
3188 Ddd*gdbDisplayShortcuts: \
3189 /t () // Convert to Bin\n\
3190 /d () // Convert to Dec\n\
3191 /x () // Convert to Hex\n\
3192 /o () // Convert to Oct(\n\
3194 the following two lines:
3196 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
3197 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
3199 so now you can do ivx and pvx lookups or you can plug there the
3200 sv_peek "conversion":
3202 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
3204 (The my_perl is for threaded builds.)
3205 Just remember that every line, but the last one, should end with \n\
3207 Alternatively edit the init file interactively via:
3208 3rd mouse button -> New Display -> Edit Menu
3210 Note: you can define up to 20 conversion shortcuts in the gdb
3215 If you see in a debugger a memory area mysteriously full of 0xABABABAB
3216 or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros,
3221 Under ithreads the optree is read only. If you want to enforce this, to check
3222 for write accesses from buggy code, compile with C<-DPL_OP_SLAB_ALLOC> to
3223 enable the OP slab allocator and C<-DPERL_DEBUG_READONLY_OPS> to enable code
3224 that allocates op memory via C<mmap>, and sets it read-only at run time.
3225 Any write access to an op results in a C<SIGBUS> and abort.
3227 This code is intended for development only, and may not be portable even to
3228 all Unix variants. Also, it is an 80% solution, in that it isn't able to make
3229 all ops read only. Specifically it
3235 Only sets read-only on all slabs of ops at C<CHECK> time, hence ops allocated
3236 later via C<require> or C<eval> will be re-write
3240 Turns an entire slab of ops read-write if the refcount of any op in the slab
3241 needs to be decreased.
3245 Turns an entire slab of ops read-write if any op from the slab is freed.
3249 It's not possible to turn the slabs to read-only after an action requiring
3250 read-write access, as either can happen during op tree building time, so
3251 there may still be legitimate write access.
3253 However, as an 80% solution it is still effective, as currently it catches
3254 a write access during the generation of F<Config.pm>, which means that we
3255 can't yet build F<perl> with this enabled.
3262 We've had a brief look around the Perl source, how to maintain quality
3263 of the source code, an overview of the stages F<perl> goes through
3264 when it's running your code, how to use debuggers to poke at the Perl
3265 guts, and finally how to analyse the execution of Perl. We took a very
3266 simple problem and demonstrated how to solve it fully - with
3267 documentation, regression tests, and finally a patch for submission to
3268 p5p. Finally, we talked about how to use external tools to debug and
3271 I'd now suggest you read over those references again, and then, as soon
3272 as possible, get your hands dirty. The best way to learn is by doing,
3279 Subscribe to perl5-porters, follow the patches and try and understand
3280 them; don't be afraid to ask if there's a portion you're not clear on -
3281 who knows, you may unearth a bug in the patch...
3285 Keep up to date with the bleeding edge Perl distributions and get
3286 familiar with the changes. Try and get an idea of what areas people are
3287 working on and the changes they're making.
3291 Do read the README associated with your operating system, e.g. README.aix
3292 on the IBM AIX OS. Don't hesitate to supply patches to that README if
3293 you find anything missing or changed over a new OS release.
3297 Find an area of Perl that seems interesting to you, and see if you can
3298 work out how it works. Scan through the source, and step over it in the
3299 debugger. Play, poke, investigate, fiddle! You'll probably get to
3300 understand not just your chosen area but a much wider range of F<perl>'s
3301 activity as well, and probably sooner than you'd think.
3307 =item I<The Road goes ever on and on, down from the door where it began.>
3311 If you can do these things, you've started on the long road to Perl porting.
3312 Thanks for wanting to help make Perl better - and happy hacking!
3314 =head2 Metaphoric Quotations
3316 If you recognized the quote about the Road above, you're in luck.
3318 Most software projects begin each file with a literal description of each
3319 file's purpose. Perl instead begins each with a literary allusion to that
3322 Like chapters in many books, all top-level Perl source files (along with a
3323 few others here and there) begin with an epigramic inscription that alludes,
3324 indirectly and metaphorically, to the material you're about to read.
3326 Quotations are taken from writings of J.R.R Tolkien pertaining to his
3327 Legendarium, almost always from I<The Lord of the Rings>. Chapters and
3328 page numbers are given using the following editions:
3334 I<The Hobbit>, by J.R.R. Tolkien. The hardcover, 70th-anniversary
3335 edition of 2007 was used, published in the UK by Harper Collins Publishers
3336 and in the US by the Houghton Mifflin Company.
3340 I<The Lord of the Rings>, by J.R.R. Tolkien. The hardcover,
3341 50th-anniversary edition of 2004 was used, published in the UK by Harper
3342 Collins Publishers and in the US by the Houghton Mifflin Company.
3346 I<The Lays of Beleriand>, by J.R.R. Tolkien and published posthumously by his
3347 son and literary executor, C.J.R. Tolkien, being the 3rd of the 12 volumes
3348 in Christopher's mammoth I<History of Middle Earth>. Page numbers derive
3349 from the hardcover edition, first published in 1983 by George Allen &
3350 Unwin; no page numbers changed for the special 3-volume omnibus edition of
3351 2002 or the various trade-paper editions, all again now by Harper Collins
3352 or Houghton Mifflin.
3356 Other JRRT books fair game for quotes would thus include I<The Adventures of
3357 Tom Bombadil>, I<The Silmarillion>, I<Unfinished Tales>, and I<The Tale of
3358 the Children of Hurin>, all but the first posthumously assembled by CJRT.
3359 But I<The Lord of the Rings> itself is perfectly fine and probably best to
3360 quote from, provided you can find a suitable quote there.
3362 So if you were to supply a new, complete, top-level source file to add to
3363 Perl, you should conform to this peculiar practice by yourself selecting an
3364 appropriate quotation from Tolkien, retaining the original spelling and
3365 punctuation and using the same format the rest of the quotes are in.
3366 Indirect and oblique is just fine; remember, it's a metaphor, so being meta
3367 is, after all, what it's for.
3371 This document was written by Nathan Torkington, and is maintained by
3372 the perl5-porters mailing list.