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 is the pumpking for the 5.8 release, and
42 Hugo van der Sanden will be the pumpking for the 5.10 release.
44 In addition, various people are pumpkings for different things. For
45 instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
48 Larry sees Perl development along the lines of the US government:
49 there's the Legislature (the porters), the Executive branch (the
50 pumpkings), and the Supreme Court (Larry). The legislature can
51 discuss and submit patches to the executive branch all they like, but
52 the executive branch is free to veto them. Rarely, the Supreme Court
53 will side with the executive branch over the legislature, or the
54 legislature over the executive branch. Mostly, however, the
55 legislature and the executive branch are supposed to get along and
56 work out their differences without impeachment or court cases.
58 You might sometimes see reference to Rule 1 and Rule 2. Larry's power
59 as Supreme Court is expressed in The Rules:
65 Larry is always by definition right about how Perl should behave.
66 This means he has final veto power on the core functionality.
70 Larry is allowed to change his mind about any matter at a later date,
71 regardless of whether he previously invoked Rule 1.
75 Got that? Larry is always right, even when he was wrong. It's rare
76 to see either Rule exercised, but they are often alluded to.
78 New features and extensions to the language are contentious, because
79 the criteria used by the pumpkings, Larry, and other porters to decide
80 which features should be implemented and incorporated are not codified
81 in a few small design goals as with some other languages. Instead,
82 the heuristics are flexible and often difficult to fathom. Here is
83 one person's list, roughly in decreasing order of importance, of
84 heuristics that new features have to be weighed against:
88 =item Does concept match the general goals of Perl?
90 These haven't been written anywhere in stone, but one approximation
93 1. Keep it fast, simple, and useful.
94 2. Keep features/concepts as orthogonal as possible.
95 3. No arbitrary limits (platforms, data sizes, cultures).
96 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
97 5. Either assimilate new technologies, or build bridges to them.
99 =item Where is the implementation?
101 All the talk in the world is useless without an implementation. In
102 almost every case, the person or people who argue for a new feature
103 will be expected to be the ones who implement it. Porters capable
104 of coding new features have their own agendas, and are not available
105 to implement your (possibly good) idea.
107 =item Backwards compatibility
109 It's a cardinal sin to break existing Perl programs. New warnings are
110 contentious--some say that a program that emits warnings is not
111 broken, while others say it is. Adding keywords has the potential to
112 break programs, changing the meaning of existing token sequences or
113 functions might break programs.
115 =item Could it be a module instead?
117 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
118 the need to keep changing the Perl interpreter. You can write modules
119 that export functions, you can give those functions prototypes so they
120 can be called like built-in functions, you can even write XS code to
121 mess with the runtime data structures of the Perl interpreter if you
122 want to implement really complicated things. If it can be done in a
123 module instead of in the core, it's highly unlikely to be added.
125 =item Is the feature generic enough?
127 Is this something that only the submitter wants added to the language,
128 or would it be broadly useful? Sometimes, instead of adding a feature
129 with a tight focus, the porters might decide to wait until someone
130 implements the more generalized feature. For instance, instead of
131 implementing a ``delayed evaluation'' feature, the porters are waiting
132 for a macro system that would permit delayed evaluation and much more.
134 =item Does it potentially introduce new bugs?
136 Radical rewrites of large chunks of the Perl interpreter have the
137 potential to introduce new bugs. The smaller and more localized the
140 =item Does it preclude other desirable features?
142 A patch is likely to be rejected if it closes off future avenues of
143 development. For instance, a patch that placed a true and final
144 interpretation on prototypes is likely to be rejected because there
145 are still options for the future of prototypes that haven't been
148 =item Is the implementation robust?
150 Good patches (tight code, complete, correct) stand more chance of
151 going in. Sloppy or incorrect patches might be placed on the back
152 burner until the pumpking has time to fix, or might be discarded
153 altogether without further notice.
155 =item Is the implementation generic enough to be portable?
157 The worst patches make use of a system-specific features. It's highly
158 unlikely that nonportable additions to the Perl language will be
161 =item Is the implementation tested?
163 Patches which change behaviour (fixing bugs or introducing new features)
164 must include regression tests to verify that everything works as expected.
165 Without tests provided by the original author, how can anyone else changing
166 perl in the future be sure that they haven't unwittingly broken the behaviour
167 the patch implements? And without tests, how can the patch's author be
168 confident that his/her hard work put into the patch won't be accidentally
169 thrown away by someone in the future?
171 =item Is there enough documentation?
173 Patches without documentation are probably ill-thought out or
174 incomplete. Nothing can be added without documentation, so submitting
175 a patch for the appropriate manpages as well as the source code is
178 =item Is there another way to do it?
180 Larry said ``Although the Perl Slogan is I<There's More Than One Way
181 to Do It>, I hesitate to make 10 ways to do something''. This is a
182 tricky heuristic to navigate, though--one man's essential addition is
183 another man's pointless cruft.
185 =item Does it create too much work?
187 Work for the pumpking, work for Perl programmers, work for module
188 authors, ... Perl is supposed to be easy.
190 =item Patches speak louder than words
192 Working code is always preferred to pie-in-the-sky ideas. A patch to
193 add a feature stands a much higher chance of making it to the language
194 than does a random feature request, no matter how fervently argued the
195 request might be. This ties into ``Will it be useful?'', as the fact
196 that someone took the time to make the patch demonstrates a strong
197 desire for the feature.
201 If you're on the list, you might hear the word ``core'' bandied
202 around. It refers to the standard distribution. ``Hacking on the
203 core'' means you're changing the C source code to the Perl
204 interpreter. ``A core module'' is one that ships with Perl.
206 =head2 Keeping in sync
208 The source code to the Perl interpreter, in its different versions, is
209 kept in a repository managed by a revision control system ( which is
210 currently the Perforce program, see http://perforce.com/ ). The
211 pumpkings and a few others have access to the repository to check in
212 changes. Periodically the pumpking for the development version of Perl
213 will release a new version, so the rest of the porters can see what's
214 changed. The current state of the main trunk of repository, and patches
215 that describe the individual changes that have happened since the last
216 public release are available at this location:
218 http://public.activestate.com/gsar/APC/
219 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
221 If you're looking for a particular change, or a change that affected
222 a particular set of files, you may find the B<Perl Repository Browser>
225 http://public.activestate.com/cgi-bin/perlbrowse
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 Keeping in sync with the most recent branch can be done in several ways,
243 but the most convenient and reliable way is using B<rsync>, available at
244 ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
247 If you choose to keep in sync using rsync, there are two approaches
252 =item rsync'ing the source tree
254 Presuming you are in the directory where your perl source resides
255 and you have rsync installed and available, you can `upgrade' to
258 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
260 This takes care of updating every single item in the source tree to
261 the latest applied patch level, creating files that are new (to your
262 distribution) and setting date/time stamps of existing files to
263 reflect the bleadperl status.
265 Note that this will not delete any files that were in '.' before
266 the rsync. Once you are sure that the rsync is running correctly,
267 run it with the --delete and the --dry-run options like this:
269 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
271 This will I<simulate> an rsync run that also deletes files not
272 present in the bleadperl master copy. Observe the results from
273 this run closely. If you are sure that the actual run would delete
274 no files precious to you, you could remove the '--dry-run' option.
276 You can than check what patch was the latest that was applied by
277 looking in the file B<.patch>, which will show the number of the
280 If you have more than one machine to keep in sync, and not all of
281 them have access to the WAN (so you are not able to rsync all the
282 source trees to the real source), there are some ways to get around
287 =item Using rsync over the LAN
289 Set up a local rsync server which makes the rsynced source tree
290 available to the LAN and sync the other machines against this
293 From http://rsync.samba.org/README.html :
295 "Rsync uses rsh or ssh for communication. It does not need to be
296 setuid and requires no special privileges for installation. It
297 does not require an inetd entry or a daemon. You must, however,
298 have a working rsh or ssh system. Using ssh is recommended for
299 its security features."
301 =item Using pushing over the NFS
303 Having the other systems mounted over the NFS, you can take an
304 active pushing approach by checking the just updated tree against
305 the other not-yet synced trees. An example would be
314 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
317 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
319 foreach my $host (keys %remote) {
320 unless (-d $remote{$host}) {
321 print STDERR "Cannot Xsync for host $host\n";
324 foreach my $file (keys %MF) {
325 my $rfile = "$remote{$host}/$file";
326 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
327 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
328 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
329 printf "%4s %-34s %8d %9d %8d %9d\n",
330 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
332 copy ($file, $rfile);
333 utime time, $MF{$file}[2], $rfile;
334 chmod $MF{$file}[0], $rfile;
338 though this is not perfect. It could be improved with checking
339 file checksums before updating. Not all NFS systems support
340 reliable utime support (when used over the NFS).
344 =item rsync'ing the patches
346 The source tree is maintained by the pumpking who applies patches to
347 the files in the tree. These patches are either created by the
348 pumpking himself using C<diff -c> after updating the file manually or
349 by applying patches sent in by posters on the perl5-porters list.
350 These patches are also saved and rsync'able, so you can apply them
351 yourself to the source files.
353 Presuming you are in a directory where your patches reside, you can
354 get them in sync with
356 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
358 This makes sure the latest available patch is downloaded to your
361 It's then up to you to apply these patches, using something like
363 # last=`ls -t *.gz | sed q`
364 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
365 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
367 # patch -p1 -N <../perl-current-diffs/blead.patch
369 or, since this is only a hint towards how it works, use CPAN-patchaperl
370 from Andreas König to have better control over the patching process.
374 =head2 Why rsync the source tree
378 =item It's easier to rsync the source tree
380 Since you don't have to apply the patches yourself, you are sure all
381 files in the source tree are in the right state.
383 =item It's more reliable
385 While both the rsync-able source and patch areas are automatically
386 updated every few minutes, keep in mind that applying patches may
387 sometimes mean careful hand-holding, especially if your version of
388 the C<patch> program does not understand how to deal with new files,
389 files with 8-bit characters, or files without trailing newlines.
393 =head2 Why rsync the patches
397 =item It's easier to rsync the patches
399 If you have more than one machine that you want to keep in track with
400 bleadperl, it's easier to rsync the patches only once and then apply
401 them to all the source trees on the different machines.
403 In case you try to keep in pace on 5 different machines, for which
404 only one of them has access to the WAN, rsync'ing all the source
405 trees should than be done 5 times over the NFS. Having
406 rsync'ed the patches only once, I can apply them to all the source
407 trees automatically. Need you say more ;-)
409 =item It's a good reference
411 If you do not only like to have the most recent development branch,
412 but also like to B<fix> bugs, or extend features, you want to dive
413 into the sources. If you are a seasoned perl core diver, you don't
414 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
415 your way around. But if you are a starter, the patches may help you
416 in finding where you should start and how to change the bits that
419 The file B<Changes> is updated on occasions the pumpking sees as his
420 own little sync points. On those occasions, he releases a tar-ball of
421 the current source tree (i.e. perl@7582.tar.gz), which will be an
422 excellent point to start with when choosing to use the 'rsync the
423 patches' scheme. Starting with perl@7582, which means a set of source
424 files on which the latest applied patch is number 7582, you apply all
425 succeeding patches available from then on (7583, 7584, ...).
427 You can use the patches later as a kind of search archive.
431 =item Finding a start point
433 If you want to fix/change the behaviour of function/feature Foo, just
434 scan the patches for patches that mention Foo either in the subject,
435 the comments, or the body of the fix. A good chance the patch shows
436 you the files that are affected by that patch which are very likely
437 to be the starting point of your journey into the guts of perl.
439 =item Finding how to fix a bug
441 If you've found I<where> the function/feature Foo misbehaves, but you
442 don't know how to fix it (but you do know the change you want to
443 make), you can, again, peruse the patches for similar changes and
444 look how others apply the fix.
446 =item Finding the source of misbehaviour
448 When you keep in sync with bleadperl, the pumpking would love to
449 I<see> that the community efforts really work. So after each of his
450 sync points, you are to 'make test' to check if everything is still
451 in working order. If it is, you do 'make ok', which will send an OK
452 report to perlbug@perl.org. (If you do not have access to a mailer
453 from the system you just finished successfully 'make test', you can
454 do 'make okfile', which creates the file C<perl.ok>, which you can
455 than take to your favourite mailer and mail yourself).
457 But of course, as always, things will not always lead to a success
458 path, and one or more test do not pass the 'make test'. Before
459 sending in a bug report (using 'make nok' or 'make nokfile'), check
460 the mailing list if someone else has reported the bug already and if
461 so, confirm it by replying to that message. If not, you might want to
462 trace the source of that misbehaviour B<before> sending in the bug,
463 which will help all the other porters in finding the solution.
465 Here the saved patches come in very handy. You can check the list of
466 patches to see which patch changed what file and what change caused
467 the misbehaviour. If you note that in the bug report, it saves the
468 one trying to solve it, looking for that point.
472 If searching the patches is too bothersome, you might consider using
473 perl's bugtron to find more information about discussions and
474 ramblings on posted bugs.
476 If you want to get the best of both worlds, rsync both the source
477 tree for convenience, reliability and ease and rsync the patches
483 =head2 Perlbug remote interface
487 There are three (3) remote administrative interfaces for modifying bug
488 status, category, etc. In all cases an admin must be first registered
489 with the Perlbug database by sending an email request to
490 richard@perl.org or bugmongers@perl.org.
492 The main requirement is the willingness to classify, (with the
493 emphasis on closing where possible :), outstanding bugs. Further
494 explanation can be garnered from the web at http://bugs.perl.org/ , or
495 by asking on the admin mailing list at: bugmongers@perl.org
497 For more info on the web see
499 http://bugs.perl.org/perlbug.cgi?req=spec
501 =item 1 http://bugs.perl.org
503 Login via the web, (remove B<admin/> if only browsing), where interested Cc's, tests, patches and change-ids, etc. may be assigned.
505 http://bugs.perl.org/admin/index.html
508 =item 2 bugdb@perl.org
510 Where the subject line is used for commands:
513 Subject: -a close bugid1 bugid2 aix install
519 =item 3 commands_and_bugdids@bugs.perl.org
521 Where the address itself is the source for the commands:
523 To: close_bugid1_bugid2_aix@bugs.perl.org
525 To: help@bugs.perl.org
528 =item notes, patches, tests
530 For patches and tests, the message body is assigned to the appropriate bug/s and forwarded to p5p for their attention.
532 To: test_<bugid1>_aix_close@bugs.perl.org
533 Subject: this is a test for the (now closed) aix bug
535 Test is the body of the mail
539 =head2 Submitting patches
541 Always submit patches to I<perl5-porters@perl.org>. If you're
542 patching a core module and there's an author listed, send the author a
543 copy (see L<Patching a core module>). This lets other porters review
544 your patch, which catches a surprising number of errors in patches.
545 Either use the diff program (available in source code form from
546 ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' I<makepatch>
547 (available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
548 but context diffs are accepted. Do not send RCS-style diffs or diffs
549 without context lines. More information is given in the
550 I<Porting/patching.pod> file in the Perl source distribution. Please
551 patch against the latest B<development> version (e.g., if you're
552 fixing a bug in the 5.005 track, patch against the latest 5.005_5x
553 version). Only patches that survive the heat of the development
554 branch get applied to maintenance versions.
556 Your patch should update the documentation and test suite. See
559 To report a bug in Perl, use the program I<perlbug> which comes with
560 Perl (if you can't get Perl to work, send mail to the address
561 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
562 I<perlbug> feeds into the automated bug-tracking system, access to
563 which is provided through the web at http://bugs.perl.org/ . It
564 often pays to check the archives of the perl5-porters mailing list to
565 see whether the bug you're reporting has been reported before, and if
566 so whether it was considered a bug. See above for the location of
567 the searchable archives.
569 The CPAN testers ( http://testers.cpan.org/ ) are a group of
570 volunteers who test CPAN modules on a variety of platforms. Perl
571 Smokers ( http://archives.develooper.com/daily-build@perl.org/ )
572 automatically tests Perl source releases on platforms with various
573 configurations. Both efforts welcome volunteers.
575 It's a good idea to read and lurk for a while before chipping in.
576 That way you'll get to see the dynamic of the conversations, learn the
577 personalities of the players, and hopefully be better prepared to make
578 a useful contribution when do you speak up.
580 If after all this you still think you want to join the perl5-porters
581 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
582 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
584 To hack on the Perl guts, you'll need to read the following things:
590 This is of paramount importance, since it's the documentation of what
591 goes where in the Perl source. Read it over a couple of times and it
592 might start to make sense - don't worry if it doesn't yet, because the
593 best way to study it is to read it in conjunction with poking at Perl
594 source, and we'll do that later on.
596 You might also want to look at Gisle Aas's illustrated perlguts -
597 there's no guarantee that this will be absolutely up-to-date with the
598 latest documentation in the Perl core, but the fundamentals will be
599 right. ( http://gisle.aas.no/perl/illguts/ )
601 =item L<perlxstut> and L<perlxs>
603 A working knowledge of XSUB programming is incredibly useful for core
604 hacking; XSUBs use techniques drawn from the PP code, the portion of the
605 guts that actually executes a Perl program. It's a lot gentler to learn
606 those techniques from simple examples and explanation than from the core
611 The documentation for the Perl API explains what some of the internal
612 functions do, as well as the many macros used in the source.
614 =item F<Porting/pumpkin.pod>
616 This is a collection of words of wisdom for a Perl porter; some of it is
617 only useful to the pumpkin holder, but most of it applies to anyone
618 wanting to go about Perl development.
620 =item The perl5-porters FAQ
622 This is posted to perl5-porters at the beginning on every month, and
623 should be available from http://perlhacker.org/p5p-faq ; alternatively,
624 you can get the FAQ emailed to you by sending mail to
625 C<perl5-porters-faq@perl.org>. It contains hints on reading
626 perl5-porters, information on how perl5-porters works and how Perl
627 development in general works.
631 =head2 Finding Your Way Around
633 Perl maintenance can be split into a number of areas, and certain people
634 (pumpkins) will have responsibility for each area. These areas sometimes
635 correspond to files or directories in the source kit. Among the areas are:
641 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
642 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
643 contains the core XS modules.
647 There are tests for nearly all the modules, built-ins and major bits
648 of functionality. Test files all have a .t suffix. Module tests live
649 in the F<lib/> and F<ext/> directories next to the module being
650 tested. Others live in F<t/>. See L<Writing a test>
654 Documentation maintenance includes looking after everything in the
655 F<pod/> directory, (as well as contributing new documentation) and
656 the documentation to the modules in core.
660 The configure process is the way we make Perl portable across the
661 myriad of operating systems it supports. Responsibility for the
662 configure, build and installation process, as well as the overall
663 portability of the core code rests with the configure pumpkin - others
664 help out with individual operating systems.
666 The files involved are the operating system directories, (F<win32/>,
667 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
668 and F<Makefile>, as well as the metaconfig files which generate
669 F<Configure>. (metaconfig isn't included in the core distribution.)
673 And of course, there's the core of the Perl interpreter itself. Let's
674 have a look at that in a little more detail.
678 Before we leave looking at the layout, though, don't forget that
679 F<MANIFEST> contains not only the file names in the Perl distribution,
680 but short descriptions of what's in them, too. For an overview of the
681 important files, try this:
683 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
685 =head2 Elements of the interpreter
687 The work of the interpreter has two main stages: compiling the code
688 into the internal representation, or bytecode, and then executing it.
689 L<perlguts/Compiled code> explains exactly how the compilation stage
692 Here is a short breakdown of perl's operation:
698 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
699 This is very high-level code, enough to fit on a single screen, and it
700 resembles the code found in L<perlembed>; most of the real action takes
703 First, F<perlmain.c> allocates some memory and constructs a Perl
706 1 PERL_SYS_INIT3(&argc,&argv,&env);
708 3 if (!PL_do_undump) {
709 4 my_perl = perl_alloc();
712 7 perl_construct(my_perl);
713 8 PL_perl_destruct_level = 0;
716 Line 1 is a macro, and its definition is dependent on your operating
717 system. Line 3 references C<PL_do_undump>, a global variable - all
718 global variables in Perl start with C<PL_>. This tells you whether the
719 current running program was created with the C<-u> flag to perl and then
720 F<undump>, which means it's going to be false in any sane context.
722 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
723 interpreter. It's quite a simple function, and the guts of it looks like
726 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
728 Here you see an example of Perl's system abstraction, which we'll see
729 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
730 own C<malloc> as defined in F<malloc.c> if you selected that option at
733 Next, in line 7, we construct the interpreter; this sets up all the
734 special variables that Perl needs, the stacks, and so on.
736 Now we pass Perl the command line options, and tell it to go:
738 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
740 exitstatus = perl_run(my_perl);
744 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
745 in F<perl.c>, which processes the command line options, sets up any
746 statically linked XS modules, opens the program and calls C<yyparse> to
751 The aim of this stage is to take the Perl source, and turn it into an op
752 tree. We'll see what one of those looks like later. Strictly speaking,
753 there's three things going on here.
755 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
756 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
757 B<is> a YACC grammar for Perl!) The job of the parser is to take your
758 code and `understand' it, splitting it into sentences, deciding which
759 operands go with which operators and so on.
761 The parser is nobly assisted by the lexer, which chunks up your input
762 into tokens, and decides what type of thing each token is: a variable
763 name, an operator, a bareword, a subroutine, a core function, and so on.
764 The main point of entry to the lexer is C<yylex>, and that and its
765 associated routines can be found in F<toke.c>. Perl isn't much like
766 other computer languages; it's highly context sensitive at times, it can
767 be tricky to work out what sort of token something is, or where a token
768 ends. As such, there's a lot of interplay between the tokeniser and the
769 parser, which can get pretty frightening if you're not used to it.
771 As the parser understands a Perl program, it builds up a tree of
772 operations for the interpreter to perform during execution. The routines
773 which construct and link together the various operations are to be found
774 in F<op.c>, and will be examined later.
778 Now the parsing stage is complete, and the finished tree represents
779 the operations that the Perl interpreter needs to perform to execute our
780 program. Next, Perl does a dry run over the tree looking for
781 optimisations: constant expressions such as C<3 + 4> will be computed
782 now, and the optimizer will also see if any multiple operations can be
783 replaced with a single one. For instance, to fetch the variable C<$foo>,
784 instead of grabbing the glob C<*foo> and looking at the scalar
785 component, the optimizer fiddles the op tree to use a function which
786 directly looks up the scalar in question. The main optimizer is C<peep>
787 in F<op.c>, and many ops have their own optimizing functions.
791 Now we're finally ready to go: we have compiled Perl byte code, and all
792 that's left to do is run it. The actual execution is done by the
793 C<runops_standard> function in F<run.c>; more specifically, it's done by
794 these three innocent looking lines:
796 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
800 You may be more comfortable with the Perl version of that:
802 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
804 Well, maybe not. Anyway, each op contains a function pointer, which
805 stipulates the function which will actually carry out the operation.
806 This function will return the next op in the sequence - this allows for
807 things like C<if> which choose the next op dynamically at run time.
808 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
809 execution if required.
811 The actual functions called are known as PP code, and they're spread
812 between four files: F<pp_hot.c> contains the `hot' code, which is most
813 often used and highly optimized, F<pp_sys.c> contains all the
814 system-specific functions, F<pp_ctl.c> contains the functions which
815 implement control structures (C<if>, C<while> and the like) and F<pp.c>
816 contains everything else. These are, if you like, the C code for Perl's
817 built-in functions and operators.
821 =head2 Internal Variable Types
823 You should by now have had a look at L<perlguts>, which tells you about
824 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
827 These variables are used not only to represent Perl-space variables, but
828 also any constants in the code, as well as some structures completely
829 internal to Perl. The symbol table, for instance, is an ordinary Perl
830 hash. Your code is represented by an SV as it's read into the parser;
831 any program files you call are opened via ordinary Perl filehandles, and
834 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
835 Perl program. Let's see, for instance, how Perl treats the constant
838 % perl -MDevel::Peek -e 'Dump("hello")'
839 1 SV = PV(0xa041450) at 0xa04ecbc
841 3 FLAGS = (POK,READONLY,pPOK)
842 4 PV = 0xa0484e0 "hello"\0
846 Reading C<Devel::Peek> output takes a bit of practise, so let's go
847 through it line by line.
849 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
850 memory. SVs themselves are very simple structures, but they contain a
851 pointer to a more complex structure. In this case, it's a PV, a
852 structure which holds a string value, at location C<0xa041450>. Line 2
853 is the reference count; there are no other references to this data, so
856 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
857 read-only SV (because it's a constant) and the data is a PV internally.
858 Next we've got the contents of the string, starting at location
861 Line 5 gives us the current length of the string - note that this does
862 B<not> include the null terminator. Line 6 is not the length of the
863 string, but the length of the currently allocated buffer; as the string
864 grows, Perl automatically extends the available storage via a routine
867 You can get at any of these quantities from C very easily; just add
868 C<Sv> to the name of the field shown in the snippet, and you've got a
869 macro which will return the value: C<SvCUR(sv)> returns the current
870 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
871 C<SvPV(sv, len)> returns the string itself with its length, and so on.
872 More macros to manipulate these properties can be found in L<perlguts>.
874 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
877 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
882 6 junk = SvPV_force(sv, tlen);
883 7 SvGROW(sv, tlen + len + 1);
886 10 Move(ptr,SvPVX(sv)+tlen,len,char);
888 12 *SvEND(sv) = '\0';
889 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
893 This is a function which adds a string, C<ptr>, of length C<len> onto
894 the end of the PV stored in C<sv>. The first thing we do in line 6 is
895 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
896 macro to force a PV. As a side effect, C<tlen> gets set to the current
897 value of the PV, and the PV itself is returned to C<junk>.
899 In line 7, we make sure that the SV will have enough room to accommodate
900 the old string, the new string and the null terminator. If C<LEN> isn't
901 big enough, C<SvGROW> will reallocate space for us.
903 Now, if C<junk> is the same as the string we're trying to add, we can
904 grab the string directly from the SV; C<SvPVX> is the address of the PV
907 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
908 memory around: we move the string C<ptr> to the end of the PV - that's
909 the start of the PV plus its current length. We're moving C<len> bytes
910 of type C<char>. After doing so, we need to tell Perl we've extended the
911 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
912 macro which gives us the end of the string, so that needs to be a
915 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
916 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
917 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
918 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
919 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
920 data if taint mode is turned on.
922 AVs and HVs are more complicated, but SVs are by far the most common
923 variable type being thrown around. Having seen something of how we
924 manipulate these, let's go on and look at how the op tree is
929 First, what is the op tree, anyway? The op tree is the parsed
930 representation of your program, as we saw in our section on parsing, and
931 it's the sequence of operations that Perl goes through to execute your
932 program, as we saw in L</Running>.
934 An op is a fundamental operation that Perl can perform: all the built-in
935 functions and operators are ops, and there are a series of ops which
936 deal with concepts the interpreter needs internally - entering and
937 leaving a block, ending a statement, fetching a variable, and so on.
939 The op tree is connected in two ways: you can imagine that there are two
940 "routes" through it, two orders in which you can traverse the tree.
941 First, parse order reflects how the parser understood the code, and
942 secondly, execution order tells perl what order to perform the
945 The easiest way to examine the op tree is to stop Perl after it has
946 finished parsing, and get it to dump out the tree. This is exactly what
947 the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
949 Let's have a look at how Perl sees C<$a = $b + $c>:
951 % perl -MO=Terse -e '$a=$b+$c'
952 1 LISTOP (0x8179888) leave
953 2 OP (0x81798b0) enter
954 3 COP (0x8179850) nextstate
955 4 BINOP (0x8179828) sassign
956 5 BINOP (0x8179800) add [1]
957 6 UNOP (0x81796e0) null [15]
958 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
959 8 UNOP (0x81797e0) null [15]
960 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
961 10 UNOP (0x816b4f0) null [15]
962 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
964 Let's start in the middle, at line 4. This is a BINOP, a binary
965 operator, which is at location C<0x8179828>. The specific operator in
966 question is C<sassign> - scalar assignment - and you can find the code
967 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
968 binary operator, it has two children: the add operator, providing the
969 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
972 Line 10 is the null op: this does exactly nothing. What is that doing
973 there? If you see the null op, it's a sign that something has been
974 optimized away after parsing. As we mentioned in L</Optimization>,
975 the optimization stage sometimes converts two operations into one, for
976 example when fetching a scalar variable. When this happens, instead of
977 rewriting the op tree and cleaning up the dangling pointers, it's easier
978 just to replace the redundant operation with the null op. Originally,
979 the tree would have looked like this:
981 10 SVOP (0x816b4f0) rv2sv [15]
982 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
984 That is, fetch the C<a> entry from the main symbol table, and then look
985 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
986 happens to do both these things.
988 The right hand side, starting at line 5 is similar to what we've just
989 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
992 Now, what's this about?
994 1 LISTOP (0x8179888) leave
995 2 OP (0x81798b0) enter
996 3 COP (0x8179850) nextstate
998 C<enter> and C<leave> are scoping ops, and their job is to perform any
999 housekeeping every time you enter and leave a block: lexical variables
1000 are tidied up, unreferenced variables are destroyed, and so on. Every
1001 program will have those first three lines: C<leave> is a list, and its
1002 children are all the statements in the block. Statements are delimited
1003 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
1004 the ops to be performed for each statement being the children of
1005 C<nextstate>. C<enter> is a single op which functions as a marker.
1007 That's how Perl parsed the program, from top to bottom:
1020 However, it's impossible to B<perform> the operations in this order:
1021 you have to find the values of C<$b> and C<$c> before you add them
1022 together, for instance. So, the other thread that runs through the op
1023 tree is the execution order: each op has a field C<op_next> which points
1024 to the next op to be run, so following these pointers tells us how perl
1025 executes the code. We can traverse the tree in this order using
1026 the C<exec> option to C<B::Terse>:
1028 % perl -MO=Terse,exec -e '$a=$b+$c'
1029 1 OP (0x8179928) enter
1030 2 COP (0x81798c8) nextstate
1031 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
1032 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
1033 5 BINOP (0x8179878) add [1]
1034 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
1035 7 BINOP (0x81798a0) sassign
1036 8 LISTOP (0x8179900) leave
1038 This probably makes more sense for a human: enter a block, start a
1039 statement. Get the values of C<$b> and C<$c>, and add them together.
1040 Find C<$a>, and assign one to the other. Then leave.
1042 The way Perl builds up these op trees in the parsing process can be
1043 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1044 piece we need to construct the tree for C<$a = $b + $c>
1046 1 term : term ASSIGNOP term
1047 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1049 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1051 If you're not used to reading BNF grammars, this is how it works: You're
1052 fed certain things by the tokeniser, which generally end up in upper
1053 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1054 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1055 `terminal symbols', because you can't get any simpler than them.
1057 The grammar, lines one and three of the snippet above, tells you how to
1058 build up more complex forms. These complex forms, `non-terminal symbols'
1059 are generally placed in lower case. C<term> here is a non-terminal
1060 symbol, representing a single expression.
1062 The grammar gives you the following rule: you can make the thing on the
1063 left of the colon if you see all the things on the right in sequence.
1064 This is called a "reduction", and the aim of parsing is to completely
1065 reduce the input. There are several different ways you can perform a
1066 reduction, separated by vertical bars: so, C<term> followed by C<=>
1067 followed by C<term> makes a C<term>, and C<term> followed by C<+>
1068 followed by C<term> can also make a C<term>.
1070 So, if you see two terms with an C<=> or C<+>, between them, you can
1071 turn them into a single expression. When you do this, you execute the
1072 code in the block on the next line: if you see C<=>, you'll do the code
1073 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1074 which contributes to the op tree.
1077 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1079 What this does is creates a new binary op, and feeds it a number of
1080 variables. The variables refer to the tokens: C<$1> is the first token in
1081 the input, C<$2> the second, and so on - think regular expression
1082 backreferences. C<$$> is the op returned from this reduction. So, we
1083 call C<newBINOP> to create a new binary operator. The first parameter to
1084 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1085 operator, so we want the type to be C<ADDOP>. We could specify this
1086 directly, but it's right there as the second token in the input, so we
1087 use C<$2>. The second parameter is the op's flags: 0 means `nothing
1088 special'. Then the things to add: the left and right hand side of our
1089 expression, in scalar context.
1093 When perl executes something like C<addop>, how does it pass on its
1094 results to the next op? The answer is, through the use of stacks. Perl
1095 has a number of stacks to store things it's currently working on, and
1096 we'll look at the three most important ones here.
1100 =item Argument stack
1102 Arguments are passed to PP code and returned from PP code using the
1103 argument stack, C<ST>. The typical way to handle arguments is to pop
1104 them off the stack, deal with them how you wish, and then push the result
1105 back onto the stack. This is how, for instance, the cosine operator
1110 value = Perl_cos(value);
1113 We'll see a more tricky example of this when we consider Perl's macros
1114 below. C<POPn> gives you the NV (floating point value) of the top SV on
1115 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1116 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1117 should be extended if necessary - it can't be necessary here, because we
1118 know there's room for one more item on the stack, since we've just
1119 removed one! The C<XPUSH*> macros at least guarantee safety.
1121 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1122 the first element in your portion of the stack, and C<TOP*> gives you
1123 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1124 negation of an integer:
1128 Just set the integer value of the top stack entry to its negation.
1130 Argument stack manipulation in the core is exactly the same as it is in
1131 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1132 description of the macros used in stack manipulation.
1136 I say `your portion of the stack' above because PP code doesn't
1137 necessarily get the whole stack to itself: if your function calls
1138 another function, you'll only want to expose the arguments aimed for the
1139 called function, and not (necessarily) let it get at your own data. The
1140 way we do this is to have a `virtual' bottom-of-stack, exposed to each
1141 function. The mark stack keeps bookmarks to locations in the argument
1142 stack usable by each function. For instance, when dealing with a tied
1143 variable, (internally, something with `P' magic) Perl has to call
1144 methods for accesses to the tied variables. However, we need to separate
1145 the arguments exposed to the method to the argument exposed to the
1146 original function - the store or fetch or whatever it may be. Here's how
1147 the tied C<push> is implemented; see C<av_push> in F<av.c>:
1151 3 PUSHs(SvTIED_obj((SV*)av, mg));
1155 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1159 The lines which concern the mark stack are the first, fifth and last
1160 lines: they save away, restore and remove the current position of the
1163 Let's examine the whole implementation, for practice:
1167 Push the current state of the stack pointer onto the mark stack. This is
1168 so that when we've finished adding items to the argument stack, Perl
1169 knows how many things we've added recently.
1172 3 PUSHs(SvTIED_obj((SV*)av, mg));
1175 We're going to add two more items onto the argument stack: when you have
1176 a tied array, the C<PUSH> subroutine receives the object and the value
1177 to be pushed, and that's exactly what we have here - the tied object,
1178 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1182 Next we tell Perl to make the change to the global stack pointer: C<dSP>
1183 only gave us a local copy, not a reference to the global.
1186 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1189 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1190 variables are tidied up, everything that has been localised gets
1191 its previous value returned, and so on. Think of them as the C<{> and
1192 C<}> of a Perl block.
1194 To actually do the magic method call, we have to call a subroutine in
1195 Perl space: C<call_method> takes care of that, and it's described in
1196 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1197 going to discard its return value.
1201 Finally, we remove the value we placed on the mark stack, since we
1202 don't need it any more.
1206 C doesn't have a concept of local scope, so perl provides one. We've
1207 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1208 stack implements the C equivalent of, for example:
1215 See L<perlguts/Localising Changes> for how to use the save stack.
1219 =head2 Millions of Macros
1221 One thing you'll notice about the Perl source is that it's full of
1222 macros. Some have called the pervasive use of macros the hardest thing
1223 to understand, others find it adds to clarity. Let's take an example,
1224 the code which implements the addition operator:
1228 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1231 6 SETn( left + right );
1236 Every line here (apart from the braces, of course) contains a macro. The
1237 first line sets up the function declaration as Perl expects for PP code;
1238 line 3 sets up variable declarations for the argument stack and the
1239 target, the return value of the operation. Finally, it tries to see if
1240 the addition operation is overloaded; if so, the appropriate subroutine
1243 Line 5 is another variable declaration - all variable declarations start
1244 with C<d> - which pops from the top of the argument stack two NVs (hence
1245 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1246 C<rl>. These are the two operands to the addition operator. Next, we
1247 call C<SETn> to set the NV of the return value to the result of adding
1248 the two values. This done, we return - the C<RETURN> macro makes sure
1249 that our return value is properly handled, and we pass the next operator
1250 to run back to the main run loop.
1252 Most of these macros are explained in L<perlapi>, and some of the more
1253 important ones are explained in L<perlxs> as well. Pay special attention
1254 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1255 the C<[pad]THX_?> macros.
1257 =head2 Poking at Perl
1259 To really poke around with Perl, you'll probably want to build Perl for
1260 debugging, like this:
1262 ./Configure -d -D optimize=-g
1265 C<-g> is a flag to the C compiler to have it produce debugging
1266 information which will allow us to step through a running program.
1267 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1268 enables all the internal debugging code in Perl. There are a whole bunch
1269 of things you can debug with this: L<perlrun> lists them all, and the
1270 best way to find out about them is to play about with them. The most
1271 useful options are probably
1273 l Context (loop) stack processing
1275 o Method and overloading resolution
1276 c String/numeric conversions
1278 Some of the functionality of the debugging code can be achieved using XS
1281 -Dr => use re 'debug'
1282 -Dx => use O 'Debug'
1284 =head2 Using a source-level debugger
1286 If the debugging output of C<-D> doesn't help you, it's time to step
1287 through perl's execution with a source-level debugger.
1293 We'll use C<gdb> for our examples here; the principles will apply to any
1294 debugger, but check the manual of the one you're using.
1298 To fire up the debugger, type
1302 You'll want to do that in your Perl source tree so the debugger can read
1303 the source code. You should see the copyright message, followed by the
1308 C<help> will get you into the documentation, but here are the most
1315 Run the program with the given arguments.
1317 =item break function_name
1319 =item break source.c:xxx
1321 Tells the debugger that we'll want to pause execution when we reach
1322 either the named function (but see L<perlguts/Internal Functions>!) or the given
1323 line in the named source file.
1327 Steps through the program a line at a time.
1331 Steps through the program a line at a time, without descending into
1336 Run until the next breakpoint.
1340 Run until the end of the current function, then stop again.
1344 Just pressing Enter will do the most recent operation again - it's a
1345 blessing when stepping through miles of source code.
1349 Execute the given C code and print its results. B<WARNING>: Perl makes
1350 heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1351 substitute them yourself. So, for instance, you can't say
1353 print SvPV_nolen(sv)
1357 print Perl_sv_2pv_nolen(sv)
1359 You may find it helpful to have a "macro dictionary", which you can
1360 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1361 recursively apply the macros for you.
1365 =head2 Dumping Perl Data Structures
1367 One way to get around this macro hell is to use the dumping functions in
1368 F<dump.c>; these work a little like an internal
1369 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1370 that you can't get at from Perl. Let's take an example. We'll use the
1371 C<$a = $b + $c> we used before, but give it a bit of context:
1372 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1374 What about C<pp_add>, the function we examined earlier to implement the
1377 (gdb) break Perl_pp_add
1378 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1380 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1381 With the breakpoint in place, we can run our program:
1383 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1385 Lots of junk will go past as gdb reads in the relevant source files and
1386 libraries, and then:
1388 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1389 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1394 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1395 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1398 #define dPOPTOPnnrl_ul NV right = POPn; \
1399 SV *leftsv = TOPs; \
1400 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1402 C<POPn> takes the SV from the top of the stack and obtains its NV either
1403 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1404 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1405 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1406 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1408 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1409 convert it. If we step again, we'll find ourselves there:
1411 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1415 We can now use C<Perl_sv_dump> to investigate the SV:
1417 SV = PV(0xa057cc0) at 0xa0675d0
1420 PV = 0xa06a510 "6XXXX"\0
1425 We know we're going to get C<6> from this, so let's finish the
1429 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1430 0x462669 in Perl_pp_add () at pp_hot.c:311
1433 We can also dump out this op: the current op is always stored in
1434 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1435 similar output to L<B::Debug|B::Debug>.
1438 13 TYPE = add ===> 14
1440 FLAGS = (SCALAR,KIDS)
1442 TYPE = null ===> (12)
1444 FLAGS = (SCALAR,KIDS)
1446 11 TYPE = gvsv ===> 12
1452 # finish this later #
1456 All right, we've now had a look at how to navigate the Perl sources and
1457 some things you'll need to know when fiddling with them. Let's now get
1458 on and create a simple patch. Here's something Larry suggested: if a
1459 C<U> is the first active format during a C<pack>, (for example,
1460 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1463 How do we prepare to fix this up? First we locate the code in question -
1464 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1465 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1466 altering this file, let's copy it to F<pp.c~>.
1468 [Well, it was in F<pp.c> when this tutorial was written. It has now been
1469 split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1471 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1472 loop over the pattern, taking each format character in turn into
1473 C<datum_type>. Then for each possible format character, we swallow up
1474 the other arguments in the pattern (a field width, an asterisk, and so
1475 on) and convert the next chunk input into the specified format, adding
1476 it onto the output SV C<cat>.
1478 How do we know if the C<U> is the first format in the C<pat>? Well, if
1479 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1480 test whether we're still at the start of the string. So, here's where
1484 register char *pat = SvPVx(*++MARK, fromlen);
1485 register char *patend = pat + fromlen;
1490 We'll have another string pointer in there:
1493 register char *pat = SvPVx(*++MARK, fromlen);
1494 register char *patend = pat + fromlen;
1500 And just before we start the loop, we'll set C<patcopy> to be the start
1505 sv_setpvn(cat, "", 0);
1507 while (pat < patend) {
1509 Now if we see a C<U> which was at the start of the string, we turn on
1510 the UTF8 flag for the output SV, C<cat>:
1512 + if (datumtype == 'U' && pat==patcopy+1)
1514 if (datumtype == '#') {
1515 while (pat < patend && *pat != '\n')
1518 Remember that it has to be C<patcopy+1> because the first character of
1519 the string is the C<U> which has been swallowed into C<datumtype!>
1521 Oops, we forgot one thing: what if there are spaces at the start of the
1522 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1523 character, even though it's not the first thing in the pattern. In this
1524 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1526 if (isSPACE(datumtype))
1531 if (isSPACE(datumtype)) {
1536 OK. That's the C part done. Now we must do two additional things before
1537 this patch is ready to go: we've changed the behaviour of Perl, and so
1538 we must document that change. We must also provide some more regression
1539 tests to make sure our patch works and doesn't create a bug somewhere
1540 else along the line.
1542 The regression tests for each operator live in F<t/op/>, and so we
1543 make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1544 tests to the end. First, we'll test that the C<U> does indeed create
1547 t/op/pack.t has a sensible ok() function, but if it didn't we could
1548 use the one from t/test.pl.
1550 require './test.pl';
1551 plan( tests => 159 );
1555 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1556 print "ok $test\n"; $test++;
1558 we can write the more sensible (see L<Test::More> for a full
1559 explanation of is() and other testing functions).
1561 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
1562 "U* produces unicode" );
1564 Now we'll test that we got that space-at-the-beginning business right:
1566 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
1567 " with spaces at the beginning" );
1569 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1570 the first active format:
1572 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
1573 "U* not first isn't unicode" );
1575 Mustn't forget to change the number of tests which appears at the top,
1576 or else the automated tester will get confused. This will either look
1583 plan( tests => 156 );
1585 We now compile up Perl, and run it through the test suite. Our new
1588 Finally, the documentation. The job is never done until the paperwork is
1589 over, so let's describe the change we've just made. The relevant place
1590 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1591 this text in the description of C<pack>:
1595 If the pattern begins with a C<U>, the resulting string will be treated
1596 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1597 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1598 characters. If you don't want this to happen, you can begin your pattern
1599 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1600 string, and then follow this with a C<U*> somewhere in your pattern.
1602 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1603 that if we're making major changes, we should copy the entire directory
1604 to somewhere safe before we begin fiddling, and then do
1606 diff -ruN old new > patch
1608 However, we know which files we've changed, and we can simply do this:
1610 diff -u pp.c~ pp.c > patch
1611 diff -u t/op/pack.t~ t/op/pack.t >> patch
1612 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1614 We end up with a patch looking a little like this:
1616 --- pp.c~ Fri Jun 02 04:34:10 2000
1617 +++ pp.c Fri Jun 16 11:37:25 2000
1618 @@ -4375,6 +4375,7 @@
1621 register char *pat = SvPVx(*++MARK, fromlen);
1623 register char *patend = pat + fromlen;
1626 @@ -4405,6 +4406,7 @@
1629 And finally, we submit it, with our rationale, to perl5-porters. Job
1632 =head2 Patching a core module
1634 This works just like patching anything else, with an extra
1635 consideration. Many core modules also live on CPAN. If this is so,
1636 patch the CPAN version instead of the core and send the patch off to
1637 the module maintainer (with a copy to p5p). This will help the module
1638 maintainer keep the CPAN version in sync with the core version without
1639 constantly scanning p5p.
1641 =head2 Adding a new function to the core
1643 If, as part of a patch to fix a bug, or just because you have an
1644 especially good idea, you decide to add a new function to the core,
1645 discuss your ideas on p5p well before you start work. It may be that
1646 someone else has already attempted to do what you are considering and
1647 can give lots of good advice or even provide you with bits of code
1648 that they already started (but never finished).
1650 You have to follow all of the advice given above for patching. It is
1651 extremely important to test any addition thoroughly and add new tests
1652 to explore all boundary conditions that your new function is expected
1653 to handle. If your new function is used only by one module (e.g. toke),
1654 then it should probably be named S_your_function (for static); on the
1655 other hand, if you expect it to accessible from other functions in
1656 Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1659 The location of any new code is also an important consideration. Don't
1660 just create a new top level .c file and put your code there; you would
1661 have to make changes to Configure (so the Makefile is created properly),
1662 as well as possibly lots of include files. This is strictly pumpking
1665 It is better to add your function to one of the existing top level
1666 source code files, but your choice is complicated by the nature of
1667 the Perl distribution. Only the files that are marked as compiled
1668 static are located in the perl executable. Everything else is located
1669 in the shared library (or DLL if you are running under WIN32). So,
1670 for example, if a function was only used by functions located in
1671 toke.c, then your code can go in toke.c. If, however, you want to call
1672 the function from universal.c, then you should put your code in another
1673 location, for example util.c.
1675 In addition to writing your c-code, you will need to create an
1676 appropriate entry in embed.pl describing your function, then run
1677 'make regen_headers' to create the entries in the numerous header
1678 files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1679 for information on the various options that you can set in embed.pl.
1680 You will forget to do this a few (or many) times and you will get
1681 warnings during the compilation phase. Make sure that you mention
1682 this when you post your patch to P5P; the pumpking needs to know this.
1684 When you write your new code, please be conscious of existing code
1685 conventions used in the perl source files. See L<perlstyle> for
1686 details. Although most of the guidelines discussed seem to focus on
1687 Perl code, rather than c, they all apply (except when they don't ;).
1688 See also I<Porting/patching.pod> file in the Perl source distribution
1689 for lots of details about both formatting and submitting patches of
1692 Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1693 Test on as many platforms as you can find. Test as many perl
1694 Configure options as you can (e.g. MULTIPLICITY). If you have
1695 profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1696 below for how to use them to further test your code. Remember that
1697 most of the people on P5P are doing this on their own time and
1698 don't have the time to debug your code.
1700 =head2 Writing a test
1702 Every module and built-in function has an associated test file (or
1703 should...). If you add or change functionality, you have to write a
1704 test. If you fix a bug, you have to write a test so that bug never
1705 comes back. If you alter the docs, it would be nice to test what the
1706 new documentation says.
1708 In short, if you submit a patch you probably also have to patch the
1711 For modules, the test file is right next to the module itself.
1712 F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1713 so there are some snags (and it would be wonderful for you to brush
1714 them out), but it basically works that way. Everything else lives in
1721 Testing of the absolute basic functionality of Perl. Things like
1722 C<if>, basic file reads and writes, simple regexes, etc. These are
1723 run first in the test suite and if any of them fail, something is
1728 These test the basic control structures, C<if/else>, C<while>,
1733 Tests basic issues of how Perl parses and compiles itself.
1737 Tests for built-in IO functions, including command line arguments.
1741 The old home for the module tests, you shouldn't put anything new in
1742 here. There are still some bits and pieces hanging around in here
1743 that need to be moved. Perhaps you could move them? Thanks!
1747 Tests for perl's built in functions that don't fit into any of the
1752 Tests for POD directives. There are still some tests for the Pod
1753 modules hanging around in here that need to be moved out into F<lib/>.
1757 Testing features of how perl actually runs, including exit codes and
1758 handling of PERL* environment variables.
1762 The core uses the same testing style as the rest of Perl, a simple
1763 "ok/not ok" run through Test::Harness, but there are a few special
1766 There are three ways to write a test in the core. Test::More,
1767 t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1768 decision of which to use depends on what part of the test suite you're
1769 working on. This is a measure to prevent a high-level failure (such
1770 as Config.pm breaking) from causing basic functionality tests to fail.
1776 Since we don't know if require works, or even subroutines, use ad hoc
1777 tests for these two. Step carefully to avoid using the feature being
1780 =item t/cmd t/run t/io t/op
1782 Now that basic require() and subroutines are tested, you can use the
1783 t/test.pl library which emulates the important features of Test::More
1784 while using a minimum of core features.
1786 You can also conditionally use certain libraries like Config, but be
1787 sure to skip the test gracefully if it's not there.
1791 Now that the core of Perl is tested, Test::More can be used. You can
1792 also use the full suite of core modules in the tests.
1796 When you say "make test" Perl uses the F<t/TEST> program to run the
1797 test suite. All tests are run from the F<t/> directory, B<not> the
1798 directory which contains the test. This causes some problems with the
1799 tests in F<lib/>, so here's some opportunity for some patching.
1801 You must be triply conscious of cross-platform concerns. This usually
1802 boils down to using File::Spec and avoiding things like C<fork()> and
1803 C<system()> unless absolutely necessary.
1805 =head2 Special Make Test Targets
1807 There are various special make targets that can be used to test Perl
1808 slightly differently than the standard "test" target. Not all them
1809 are expected to give a 100% success rate. Many of them have several
1816 Run F<perl> on all but F<lib/*> tests.
1820 Run all the tests through the B::Deparse. Not all tests will succeed.
1824 Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
1825 F<t/op>, and F<t/uni> tests.
1827 =item test.third check.third utest.third ucheck.third
1829 (Only in Tru64) Run all the tests using the memory leak + naughty
1830 memory access tool "Third Degree". The log files will be named
1831 F<perl3.log.testname>.
1833 =item test.torture torturetest
1835 Run all the usual tests and some extra tests. As of Perl 5.8.0 the
1836 only extra tests are Abigail's JAPHs, t/japh/abigail.t.
1838 You can also run the torture test with F<t/harness> by giving
1839 C<-torture> argument to F<t/harness>.
1841 =item utest ucheck test.utf8 check.utf8
1843 Run all the tests with -Mutf8. Not all tests will succeed.
1847 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1849 Sometimes it helps to use external tools while debugging and
1850 testing Perl. This section tries to guide you through using
1851 some common testing and debugging tools with Perl. This is
1852 meant as a guide to interfacing these tools with Perl, not
1853 as any kind of guide to the use of the tools themselves.
1855 =head2 Rational Software's Purify
1857 Purify is a commercial tool that is helpful in identifying
1858 memory overruns, wild pointers, memory leaks and other such
1859 badness. Perl must be compiled in a specific way for
1860 optimal testing with Purify. Purify is available under
1861 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1863 The only currently known leaks happen when there are
1864 compile-time errors within eval or require. (Fixing these
1865 is non-trivial, unfortunately, but they must be fixed
1868 =head2 Purify on Unix
1870 On Unix, Purify creates a new Perl binary. To get the most
1871 benefit out of Purify, you should create the perl to Purify
1874 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1875 -Uusemymalloc -Dusemultiplicity
1877 where these arguments mean:
1881 =item -Accflags=-DPURIFY
1883 Disables Perl's arena memory allocation functions, as well as
1884 forcing use of memory allocation functions derived from the
1887 =item -Doptimize='-g'
1889 Adds debugging information so that you see the exact source
1890 statements where the problem occurs. Without this flag, all
1891 you will see is the source filename of where the error occurred.
1895 Disable Perl's malloc so that Purify can more closely monitor
1896 allocations and leaks. Using Perl's malloc will make Purify
1897 report most leaks in the "potential" leaks category.
1899 =item -Dusemultiplicity
1901 Enabling the multiplicity option allows perl to clean up
1902 thoroughly when the interpreter shuts down, which reduces the
1903 number of bogus leak reports from Purify.
1907 Once you've compiled a perl suitable for Purify'ing, then you
1912 which creates a binary named 'pureperl' that has been Purify'ed.
1913 This binary is used in place of the standard 'perl' binary
1914 when you want to debug Perl memory problems.
1916 To minimize the number of memory leak false alarms
1917 (see L</PERL_DESTRUCT_LEVEL>), set environment variable
1918 PERL_DESTRUCT_LEVEL to 2.
1920 setenv PERL_DESTRUCT_LEVEL 2
1922 In Bourne-type shells:
1924 PERL_DESTRUCT_LEVEL=2
1925 export PERL_DESTRUCT_LEVEL
1927 As an example, to show any memory leaks produced during the
1928 standard Perl testset you would create and run the Purify'ed
1933 ../pureperl -I../lib harness
1935 which would run Perl on test.pl and report any memory problems.
1937 Purify outputs messages in "Viewer" windows by default. If
1938 you don't have a windowing environment or if you simply
1939 want the Purify output to unobtrusively go to a log file
1940 instead of to the interactive window, use these following
1941 options to output to the log file "perl.log":
1943 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1944 -log-file=perl.log -append-logfile=yes"
1946 If you plan to use the "Viewer" windows, then you only need this option:
1948 setenv PURIFYOPTIONS "-chain-length=25"
1950 In Bourne-type shells:
1953 export PURIFYOPTIONS
1955 or if you have the "env" utility:
1957 env PURIFYOPTIONS="..." ../pureperl ...
1961 Purify on Windows NT instruments the Perl binary 'perl.exe'
1962 on the fly. There are several options in the makefile you
1963 should change to get the most use out of Purify:
1969 You should add -DPURIFY to the DEFINES line so the DEFINES
1970 line looks something like:
1972 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1974 to disable Perl's arena memory allocation functions, as
1975 well as to force use of memory allocation functions derived
1976 from the system malloc.
1978 =item USE_MULTI = define
1980 Enabling the multiplicity option allows perl to clean up
1981 thoroughly when the interpreter shuts down, which reduces the
1982 number of bogus leak reports from Purify.
1984 =item #PERL_MALLOC = define
1986 Disable Perl's malloc so that Purify can more closely monitor
1987 allocations and leaks. Using Perl's malloc will make Purify
1988 report most leaks in the "potential" leaks category.
1992 Adds debugging information so that you see the exact source
1993 statements where the problem occurs. Without this flag, all
1994 you will see is the source filename of where the error occurred.
1998 As an example, to show any memory leaks produced during the
1999 standard Perl testset you would create and run Purify as:
2004 purify ../perl -I../lib harness
2006 which would instrument Perl in memory, run Perl on test.pl,
2007 then finally report any memory problems.
2009 B<NOTE>: as of Perl 5.8.0, the ext/Encode/t/Unicode.t takes
2010 extraordinarily long (hours?) to complete under Purify. It has been
2011 theorized that it would eventually finish, but nobody has so far been
2012 patient enough :-) (This same extreme slowdown has been seen also with
2013 the Third Degree tool, so the said test must be doing something that
2014 is quite unfriendly for memory debuggers.) It is suggested that you
2015 simply kill away that testing process.
2017 =head2 Compaq's/Digital's/HP's Third Degree
2019 Third Degree is a tool for memory leak detection and memory access checks.
2020 It is one of the many tools in the ATOM toolkit. The toolkit is only
2021 available on Tru64 (formerly known as Digital UNIX formerly known as
2024 When building Perl, you must first run Configure with -Doptimize=-g
2025 and -Uusemymalloc flags, after that you can use the make targets
2026 "perl.third" and "test.third". (What is required is that Perl must be
2027 compiled using the C<-g> flag, you may need to re-Configure.)
2029 The short story is that with "atom" you can instrument the Perl
2030 executable to create a new executable called F<perl.third>. When the
2031 instrumented executable is run, it creates a log of dubious memory
2032 traffic in file called F<perl.3log>. See the manual pages of atom and
2033 third for more information. The most extensive Third Degree
2034 documentation is available in the Compaq "Tru64 UNIX Programmer's
2035 Guide", chapter "Debugging Programs with Third Degree".
2037 The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/
2038 subdirectory. There is a problem with these files: Third Degree is so
2039 effective that it finds problems also in the system libraries.
2040 Therefore you should used the Porting/thirdclean script to cleanup
2041 the F<*.3log> files.
2043 There are also leaks that for given certain definition of a leak,
2044 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
2046 =head2 PERL_DESTRUCT_LEVEL
2048 If you want to run any of the tests yourself manually using the
2049 pureperl or perl.third executables, please note that by default
2050 perl B<does not> explicitly cleanup all the memory it has allocated
2051 (such as global memory arenas) but instead lets the exit() of
2052 the whole program "take care" of such allocations, also known
2053 as "global destruction of objects".
2055 There is a way to tell perl to do complete cleanup: set the
2056 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
2057 The t/TEST wrapper does set this to 2, and this is what you
2058 need to do too, if you don't want to see the "global leaks":
2059 For example, for "third-degreed" Perl:
2061 env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
2063 (Note: the mod_perl apache module uses also this environment variable
2064 for its own purposes and extended its semantics. Refer to the mod_perl
2065 documentation for more information.)
2069 Depending on your platform there are various of profiling Perl.
2071 There are two commonly used techniques of profiling executables:
2072 I<statistical time-sampling> and I<basic-block counting>.
2074 The first method takes periodically samples of the CPU program
2075 counter, and since the program counter can be correlated with the code
2076 generated for functions, we get a statistical view of in which
2077 functions the program is spending its time. The caveats are that very
2078 small/fast functions have lower probability of showing up in the
2079 profile, and that periodically interrupting the program (this is
2080 usually done rather frequently, in the scale of milliseconds) imposes
2081 an additional overhead that may skew the results. The first problem
2082 can be alleviated by running the code for longer (in general this is a
2083 good idea for profiling), the second problem is usually kept in guard
2084 by the profiling tools themselves.
2086 The second method divides up the generated code into I<basic blocks>.
2087 Basic blocks are sections of code that are entered only in the
2088 beginning and exited only at the end. For example, a conditional jump
2089 starts a basic block. Basic block profiling usually works by
2090 I<instrumenting> the code by adding I<enter basic block #nnnn>
2091 book-keeping code to the generated code. During the execution of the
2092 code the basic block counters are then updated appropriately. The
2093 caveat is that the added extra code can skew the results: again, the
2094 profiling tools usually try to factor their own effects out of the
2097 =head2 Gprof Profiling
2099 gprof is a profiling tool available in many UNIX platforms,
2100 it uses F<statistical time-sampling>.
2102 You can build a profiled version of perl called "perl.gprof" by
2103 invoking the make target "perl.gprof" (What is required is that Perl
2104 must be compiled using the C<-pg> flag, you may need to re-Configure).
2105 Running the profiled version of Perl will create an output file called
2106 F<gmon.out> is created which contains the profiling data collected
2107 during the execution.
2109 The gprof tool can then display the collected data in various ways.
2110 Usually gprof understands the following options:
2116 Suppress statically defined functions from the profile.
2120 Suppress the verbose descriptions in the profile.
2124 Exclude the given routine and its descendants from the profile.
2128 Display only the given routine and its descendants in the profile.
2132 Generate a summary file called F<gmon.sum> which then may be given
2133 to subsequent gprof runs to accumulate data over several runs.
2137 Display routines that have zero usage.
2141 For more detailed explanation of the available commands and output
2142 formats, see your own local documentation of gprof.
2144 =head2 GCC gcov Profiling
2146 Starting from GCC 3.0 I<basic block profiling> is officially available
2149 You can build a profiled version of perl called F<perl.gcov> by
2150 invoking the make target "perl.gcov" (what is required that Perl must
2151 be compiled using gcc with the flags C<-fprofile-arcs
2152 -ftest-coverage>, you may need to re-Configure).
2154 Running the profiled version of Perl will cause profile output to be
2155 generated. For each source file an accompanying ".da" file will be
2158 To display the results you use the "gcov" utility (which should
2159 be installed if you have gcc 3.0 or newer installed). F<gcov> is
2160 run on source code files, like this
2164 which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2165 contain the source code annotated with relative frequencies of
2166 execution indicated by "#" markers.
2168 Useful options of F<gcov> include C<-b> which will summarise the
2169 basic block, branch, and function call coverage, and C<-c> which
2170 instead of relative frequencies will use the actual counts. For
2171 more information on the use of F<gcov> and basic block profiling
2172 with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2174 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2176 and its section titled "8. gcov: a Test Coverage Program"
2178 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2180 =head2 Pixie Profiling
2182 Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2183 UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
2184 I<basic-block counting>.
2186 You can build a profiled version of perl called F<perl.pixie> by
2187 invoking the make target "perl.pixie" (what is required is that Perl
2188 must be compiled using the C<-g> flag, you may need to re-Configure).
2190 In Tru64 a file called F<perl.Addrs> will also be silently created,
2191 this file contains the addresses of the basic blocks. Running the
2192 profiled version of Perl will create a new file called "perl.Counts"
2193 which contains the counts for the basic block for that particular
2196 To display the results you use the F<prof> utility. The exact
2197 incantation depends on your operating system, "prof perl.Counts" in
2198 IRIX, and "prof -pixie -all -L. perl" in Tru64.
2200 In IRIX the following prof options are available:
2206 Reports the most heavily used lines in descending order of use.
2207 Useful for finding the hotspot lines.
2211 Groups lines by procedure, with procedures sorted in descending order of use.
2212 Within a procedure, lines are listed in source order.
2213 Useful for finding the hotspots of procedures.
2217 In Tru64 the following options are available:
2223 Procedures sorted in descending order by the number of cycles executed
2224 in each procedure. Useful for finding the hotspot procedures.
2225 (This is the default option.)
2229 Lines sorted in descending order by the number of cycles executed in
2230 each line. Useful for finding the hotspot lines.
2232 =item -i[nvocations]
2234 The called procedures are sorted in descending order by number of calls
2235 made to the procedures. Useful for finding the most used procedures.
2239 Grouped by procedure, sorted by cycles executed per procedure.
2240 Useful for finding the hotspots of procedures.
2244 The compiler emitted code for these lines, but the code was unexecuted.
2248 Unexecuted procedures.
2252 For further information, see your system's manual pages for pixie and prof.
2254 =head2 Miscellaneous tricks
2260 Those debugging perl with the DDD frontend over gdb may find the
2263 You can extend the data conversion shortcuts menu, so for example you
2264 can display an SV's IV value with one click, without doing any typing.
2265 To do that simply edit ~/.ddd/init file and add after:
2267 ! Display shortcuts.
2268 Ddd*gdbDisplayShortcuts: \
2269 /t () // Convert to Bin\n\
2270 /d () // Convert to Dec\n\
2271 /x () // Convert to Hex\n\
2272 /o () // Convert to Oct(\n\
2274 the following two lines:
2276 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
2277 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
2279 so now you can do ivx and pvx lookups or you can plug there the
2280 sv_peek "conversion":
2282 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
2284 (The my_perl is for threaded builds.)
2285 Just remember that every line, but the last one, should end with \n\
2287 Alternatively edit the init file interactively via:
2288 3rd mouse button -> New Display -> Edit Menu
2290 Note: you can define up to 20 conversion shortcuts in the gdb
2295 If you see in a debugger a memory area mysteriously full of 0xabababab,
2296 you may be seeing the effect of the Poison() macro, see L<perlclib>.
2302 We've had a brief look around the Perl source, an overview of the stages
2303 F<perl> goes through when it's running your code, and how to use a
2304 debugger to poke at the Perl guts. We took a very simple problem and
2305 demonstrated how to solve it fully - with documentation, regression
2306 tests, and finally a patch for submission to p5p. Finally, we talked
2307 about how to use external tools to debug and test Perl.
2309 I'd now suggest you read over those references again, and then, as soon
2310 as possible, get your hands dirty. The best way to learn is by doing,
2317 Subscribe to perl5-porters, follow the patches and try and understand
2318 them; don't be afraid to ask if there's a portion you're not clear on -
2319 who knows, you may unearth a bug in the patch...
2323 Keep up to date with the bleeding edge Perl distributions and get
2324 familiar with the changes. Try and get an idea of what areas people are
2325 working on and the changes they're making.
2329 Do read the README associated with your operating system, e.g. README.aix
2330 on the IBM AIX OS. Don't hesitate to supply patches to that README if
2331 you find anything missing or changed over a new OS release.
2335 Find an area of Perl that seems interesting to you, and see if you can
2336 work out how it works. Scan through the source, and step over it in the
2337 debugger. Play, poke, investigate, fiddle! You'll probably get to
2338 understand not just your chosen area but a much wider range of F<perl>'s
2339 activity as well, and probably sooner than you'd think.
2345 =item I<The Road goes ever on and on, down from the door where it began.>
2349 If you can do these things, you've started on the long road to Perl porting.
2350 Thanks for wanting to help make Perl better - and happy hacking!
2354 This document was written by Nathan Torkington, and is maintained by
2355 the perl5-porters mailing list.