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 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
220 If you are a member of the perl5-porters mailing list, it is a good
221 thing to keep in touch with the most recent changes. If not only to
222 verify if what you would have posted as a bug report isn't already
223 solved in the most recent available perl development branch, also
224 known as perl-current, bleading edge perl, bleedperl or bleadperl.
226 Needless to say, the source code in perl-current is usually in a perpetual
227 state of evolution. You should expect it to be very buggy. Do B<not> use
228 it for any purpose other than testing and development.
230 Keeping in sync with the most recent branch can be done in several ways,
231 but the most convenient and reliable way is using B<rsync>, available at
232 ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
235 If you choose to keep in sync using rsync, there are two approaches
240 =item rsync'ing the source tree
242 Presuming you are in the directory where your perl source resides
243 and you have rsync installed and available, you can `upgrade' to
246 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
248 This takes care of updating every single item in the source tree to
249 the latest applied patch level, creating files that are new (to your
250 distribution) and setting date/time stamps of existing files to
251 reflect the bleadperl status.
253 Note that this will not delete any files that were in '.' before
254 the rsync. Once you are sure that the rsync is running correctly,
255 run it with the --delete and the --dry-run options like this:
257 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
259 This will I<simulate> an rsync run that also deletes files not
260 present in the bleadperl master copy. Observe the results from
261 this run closely. If you are sure that the actual run would delete
262 no files precious to you, you could remove the '--dry-run' option.
264 You can than check what patch was the latest that was applied by
265 looking in the file B<.patch>, which will show the number of the
268 If you have more than one machine to keep in sync, and not all of
269 them have access to the WAN (so you are not able to rsync all the
270 source trees to the real source), there are some ways to get around
275 =item Using rsync over the LAN
277 Set up a local rsync server which makes the rsynced source tree
278 available to the LAN and sync the other machines against this
281 From http://rsync.samba.org/README.html :
283 "Rsync uses rsh or ssh for communication. It does not need to be
284 setuid and requires no special privileges for installation. It
285 does not require an inetd entry or a daemon. You must, however,
286 have a working rsh or ssh system. Using ssh is recommended for
287 its security features."
289 =item Using pushing over the NFS
291 Having the other systems mounted over the NFS, you can take an
292 active pushing approach by checking the just updated tree against
293 the other not-yet synced trees. An example would be
302 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
305 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
307 foreach my $host (keys %remote) {
308 unless (-d $remote{$host}) {
309 print STDERR "Cannot Xsync for host $host\n";
312 foreach my $file (keys %MF) {
313 my $rfile = "$remote{$host}/$file";
314 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
315 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
316 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
317 printf "%4s %-34s %8d %9d %8d %9d\n",
318 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
320 copy ($file, $rfile);
321 utime time, $MF{$file}[2], $rfile;
322 chmod $MF{$file}[0], $rfile;
326 though this is not perfect. It could be improved with checking
327 file checksums before updating. Not all NFS systems support
328 reliable utime support (when used over the NFS).
332 =item rsync'ing the patches
334 The source tree is maintained by the pumpking who applies patches to
335 the files in the tree. These patches are either created by the
336 pumpking himself using C<diff -c> after updating the file manually or
337 by applying patches sent in by posters on the perl5-porters list.
338 These patches are also saved and rsync'able, so you can apply them
339 yourself to the source files.
341 Presuming you are in a directory where your patches reside, you can
342 get them in sync with
344 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
346 This makes sure the latest available patch is downloaded to your
349 It's then up to you to apply these patches, using something like
351 # last=`ls -t *.gz | sed q`
352 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
353 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
355 # patch -p1 -N <../perl-current-diffs/blead.patch
357 or, since this is only a hint towards how it works, use CPAN-patchaperl
358 from Andreas König to have better control over the patching process.
362 =head2 Why rsync the source tree
366 =item It's easier to rsync the source tree
368 Since you don't have to apply the patches yourself, you are sure all
369 files in the source tree are in the right state.
371 =item It's more recent
373 According to Gurusamy Sarathy:
375 "... The rsync mirror is automatic and syncs with the repository
378 "Updating the patch area still requires manual intervention
379 (with all the goofiness that implies, which you've noted) and
380 is typically on a daily cycle. Making this process automatic
381 is on my tuit list, but don't ask me when."
383 =item It's more reliable
385 Well, since the patches are updated by hand, I don't have to say any
386 more ... (see Sarathy's remark).
390 =head2 Why rsync the patches
394 =item It's easier to rsync the patches
396 If you have more than one machine that you want to keep in track with
397 bleadperl, it's easier to rsync the patches only once and then apply
398 them to all the source trees on the different machines.
400 In case you try to keep in pace on 5 different machines, for which
401 only one of them has access to the WAN, rsync'ing all the source
402 trees should than be done 5 times over the NFS. Having
403 rsync'ed the patches only once, I can apply them to all the source
404 trees automatically. Need you say more ;-)
406 =item It's a good reference
408 If you do not only like to have the most recent development branch,
409 but also like to B<fix> bugs, or extend features, you want to dive
410 into the sources. If you are a seasoned perl core diver, you don't
411 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
412 your way around. But if you are a starter, the patches may help you
413 in finding where you should start and how to change the bits that
416 The file B<Changes> is updated on occasions the pumpking sees as his
417 own little sync points. On those occasions, he releases a tar-ball of
418 the current source tree (i.e. perl@7582.tar.gz), which will be an
419 excellent point to start with when choosing to use the 'rsync the
420 patches' scheme. Starting with perl@7582, which means a set of source
421 files on which the latest applied patch is number 7582, you apply all
422 succeeding patches available from then on (7583, 7584, ...).
424 You can use the patches later as a kind of search archive.
428 =item Finding a start point
430 If you want to fix/change the behaviour of function/feature Foo, just
431 scan the patches for patches that mention Foo either in the subject,
432 the comments, or the body of the fix. A good chance the patch shows
433 you the files that are affected by that patch which are very likely
434 to be the starting point of your journey into the guts of perl.
436 =item Finding how to fix a bug
438 If you've found I<where> the function/feature Foo misbehaves, but you
439 don't know how to fix it (but you do know the change you want to
440 make), you can, again, peruse the patches for similar changes and
441 look how others apply the fix.
443 =item Finding the source of misbehaviour
445 When you keep in sync with bleadperl, the pumpking would love to
446 I<see> that the community efforts really work. So after each of his
447 sync points, you are to 'make test' to check if everything is still
448 in working order. If it is, you do 'make ok', which will send an OK
449 report to perlbug@perl.org. (If you do not have access to a mailer
450 from the system you just finished successfully 'make test', you can
451 do 'make okfile', which creates the file C<perl.ok>, which you can
452 than take to your favourite mailer and mail yourself).
454 But of course, as always, things will not always lead to a success
455 path, and one or more test do not pass the 'make test'. Before
456 sending in a bug report (using 'make nok' or 'make nokfile'), check
457 the mailing list if someone else has reported the bug already and if
458 so, confirm it by replying to that message. If not, you might want to
459 trace the source of that misbehaviour B<before> sending in the bug,
460 which will help all the other porters in finding the solution.
462 Here the saved patches come in very handy. You can check the list of
463 patches to see which patch changed what file and what change caused
464 the misbehaviour. If you note that in the bug report, it saves the
465 one trying to solve it, looking for that point.
469 If searching the patches is too bothersome, you might consider using
470 perl's bugtron to find more information about discussions and
471 ramblings on posted bugs.
473 If you want to get the best of both worlds, rsync both the source
474 tree for convenience, reliability and ease and rsync the patches
480 =head2 Perlbug remote interface
484 There are three (3) remote administrative interfaces for modifying bug
485 status, category, etc. In all cases an admin must be first registered
486 with the Perlbug database by sending an email request to
487 richard@perl.org or bugmongers@perl.org.
489 The main requirement is the willingness to classify, (with the
490 emphasis on closing where possible :), outstanding bugs. Further
491 explanation can be garnered from the web at http://bugs.perl.org/ , or
492 by asking on the admin mailing list at: bugmongers@perl.org
494 For more info on the web see
496 http://bugs.perl.org/perlbug.cgi?req=spec
502 =item 1 http://bugs.perl.org
504 Login via the web, (remove B<admin/> if only browsing), where interested Cc's, tests, patches and change-ids, etc. may be assigned.
506 http://bugs.perl.org/admin/index.html
509 =item 2 bugdb@perl.org
511 Where the subject line is used for commands:
514 Subject: -a close bugid1 bugid2 aix install
520 =item 3 commands_and_bugdids@bugs.perl.org
522 Where the address itself is the source for the commands:
524 To: close_bugid1_bugid2_aix@bugs.perl.org
526 To: help@bugs.perl.org
529 =item notes, patches, tests
531 For patches and tests, the message body is assigned to the appropriate bug/s and forwarded to p5p for their attention.
533 To: test_<bugid1>_aix_close@bugs.perl.org
534 Subject: this is a test for the (now closed) aix bug
536 Test is the body of the mail
540 =head2 Submitting patches
542 Always submit patches to I<perl5-porters@perl.org>. If you're
543 patching a core module and there's an author listed, send the author a
544 copy (see L<Patching a core module>). This lets other porters review
545 your patch, which catches a surprising number of errors in patches.
546 Either use the diff program (available in source code form from
547 ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' I<makepatch>
548 (available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
549 but context diffs are accepted. Do not send RCS-style diffs or diffs
550 without context lines. More information is given in the
551 I<Porting/patching.pod> file in the Perl source distribution. Please
552 patch against the latest B<development> version (e.g., if you're
553 fixing a bug in the 5.005 track, patch against the latest 5.005_5x
554 version). Only patches that survive the heat of the development
555 branch get applied to maintenance versions.
557 Your patch should update the documentation and test suite. See
560 To report a bug in Perl, use the program I<perlbug> which comes with
561 Perl (if you can't get Perl to work, send mail to the address
562 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
563 I<perlbug> feeds into the automated bug-tracking system, access to
564 which is provided through the web at http://bugs.perl.org/ . It
565 often pays to check the archives of the perl5-porters mailing list to
566 see whether the bug you're reporting has been reported before, and if
567 so whether it was considered a bug. See above for the location of
568 the searchable archives.
570 The CPAN testers ( http://testers.cpan.org/ ) are a group of
571 volunteers who test CPAN modules on a variety of platforms. Perl Labs
572 ( http://labs.perl.org/ ) automatically tests Perl source releases on
573 platforms and gives feedback to the CPAN testers mailing list. Both
574 efforts welcome volunteers.
576 It's a good idea to read and lurk for a while before chipping in.
577 That way you'll get to see the dynamic of the conversations, learn the
578 personalities of the players, and hopefully be better prepared to make
579 a useful contribution when do you speak up.
581 If after all this you still think you want to join the perl5-porters
582 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
583 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
585 To hack on the Perl guts, you'll need to read the following things:
591 This is of paramount importance, since it's the documentation of what
592 goes where in the Perl source. Read it over a couple of times and it
593 might start to make sense - don't worry if it doesn't yet, because the
594 best way to study it is to read it in conjunction with poking at Perl
595 source, and we'll do that later on.
597 You might also want to look at Gisle Aas's illustrated perlguts -
598 there's no guarantee that this will be absolutely up-to-date with the
599 latest documentation in the Perl core, but the fundamentals will be
600 right. ( http://gisle.aas.no/perl/illguts/ )
602 =item L<perlxstut> and L<perlxs>
604 A working knowledge of XSUB programming is incredibly useful for core
605 hacking; XSUBs use techniques drawn from the PP code, the portion of the
606 guts that actually executes a Perl program. It's a lot gentler to learn
607 those techniques from simple examples and explanation than from the core
612 The documentation for the Perl API explains what some of the internal
613 functions do, as well as the many macros used in the source.
615 =item F<Porting/pumpkin.pod>
617 This is a collection of words of wisdom for a Perl porter; some of it is
618 only useful to the pumpkin holder, but most of it applies to anyone
619 wanting to go about Perl development.
621 =item The perl5-porters FAQ
623 This is posted to perl5-porters at the beginning on every month, and
624 should be available from http://perlhacker.org/p5p-faq ; alternatively,
625 you can get the FAQ emailed to you by sending mail to
626 C<perl5-porters-faq@perl.org>. It contains hints on reading
627 perl5-porters, information on how perl5-porters works and how Perl
628 development in general works.
632 =head2 Finding Your Way Around
634 Perl maintenance can be split into a number of areas, and certain people
635 (pumpkins) will have responsibility for each area. These areas sometimes
636 correspond to files or directories in the source kit. Among the areas are:
642 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
643 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
644 contains the core XS modules.
648 There are tests for nearly all the modules, built-ins and major bits
649 of functionality. Test files all have a .t suffix. Module tests live
650 in the F<lib/> and F<ext/> directories next to the module being
651 tested. Others live in F<t/>. See L<Writing a test>
655 Documentation maintenance includes looking after everything in the
656 F<pod/> directory, (as well as contributing new documentation) and
657 the documentation to the modules in core.
661 The configure process is the way we make Perl portable across the
662 myriad of operating systems it supports. Responsibility for the
663 configure, build and installation process, as well as the overall
664 portability of the core code rests with the configure pumpkin - others
665 help out with individual operating systems.
667 The files involved are the operating system directories, (F<win32/>,
668 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
669 and F<Makefile>, as well as the metaconfig files which generate
670 F<Configure>. (metaconfig isn't included in the core distribution.)
674 And of course, there's the core of the Perl interpreter itself. Let's
675 have a look at that in a little more detail.
679 Before we leave looking at the layout, though, don't forget that
680 F<MANIFEST> contains not only the file names in the Perl distribution,
681 but short descriptions of what's in them, too. For an overview of the
682 important files, try this:
684 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
686 =head2 Elements of the interpreter
688 The work of the interpreter has two main stages: compiling the code
689 into the internal representation, or bytecode, and then executing it.
690 L<perlguts/Compiled code> explains exactly how the compilation stage
693 Here is a short breakdown of perl's operation:
699 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
700 This is very high-level code, enough to fit on a single screen, and it
701 resembles the code found in L<perlembed>; most of the real action takes
704 First, F<perlmain.c> allocates some memory and constructs a Perl
707 1 PERL_SYS_INIT3(&argc,&argv,&env);
709 3 if (!PL_do_undump) {
710 4 my_perl = perl_alloc();
713 7 perl_construct(my_perl);
714 8 PL_perl_destruct_level = 0;
717 Line 1 is a macro, and its definition is dependent on your operating
718 system. Line 3 references C<PL_do_undump>, a global variable - all
719 global variables in Perl start with C<PL_>. This tells you whether the
720 current running program was created with the C<-u> flag to perl and then
721 F<undump>, which means it's going to be false in any sane context.
723 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
724 interpreter. It's quite a simple function, and the guts of it looks like
727 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
729 Here you see an example of Perl's system abstraction, which we'll see
730 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
731 own C<malloc> as defined in F<malloc.c> if you selected that option at
734 Next, in line 7, we construct the interpreter; this sets up all the
735 special variables that Perl needs, the stacks, and so on.
737 Now we pass Perl the command line options, and tell it to go:
739 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
741 exitstatus = perl_run(my_perl);
745 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
746 in F<perl.c>, which processes the command line options, sets up any
747 statically linked XS modules, opens the program and calls C<yyparse> to
752 The aim of this stage is to take the Perl source, and turn it into an op
753 tree. We'll see what one of those looks like later. Strictly speaking,
754 there's three things going on here.
756 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
757 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
758 B<is> a YACC grammar for Perl!) The job of the parser is to take your
759 code and `understand' it, splitting it into sentences, deciding which
760 operands go with which operators and so on.
762 The parser is nobly assisted by the lexer, which chunks up your input
763 into tokens, and decides what type of thing each token is: a variable
764 name, an operator, a bareword, a subroutine, a core function, and so on.
765 The main point of entry to the lexer is C<yylex>, and that and its
766 associated routines can be found in F<toke.c>. Perl isn't much like
767 other computer languages; it's highly context sensitive at times, it can
768 be tricky to work out what sort of token something is, or where a token
769 ends. As such, there's a lot of interplay between the tokeniser and the
770 parser, which can get pretty frightening if you're not used to it.
772 As the parser understands a Perl program, it builds up a tree of
773 operations for the interpreter to perform during execution. The routines
774 which construct and link together the various operations are to be found
775 in F<op.c>, and will be examined later.
779 Now the parsing stage is complete, and the finished tree represents
780 the operations that the Perl interpreter needs to perform to execute our
781 program. Next, Perl does a dry run over the tree looking for
782 optimisations: constant expressions such as C<3 + 4> will be computed
783 now, and the optimizer will also see if any multiple operations can be
784 replaced with a single one. For instance, to fetch the variable C<$foo>,
785 instead of grabbing the glob C<*foo> and looking at the scalar
786 component, the optimizer fiddles the op tree to use a function which
787 directly looks up the scalar in question. The main optimizer is C<peep>
788 in F<op.c>, and many ops have their own optimizing functions.
792 Now we're finally ready to go: we have compiled Perl byte code, and all
793 that's left to do is run it. The actual execution is done by the
794 C<runops_standard> function in F<run.c>; more specifically, it's done by
795 these three innocent looking lines:
797 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
801 You may be more comfortable with the Perl version of that:
803 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
805 Well, maybe not. Anyway, each op contains a function pointer, which
806 stipulates the function which will actually carry out the operation.
807 This function will return the next op in the sequence - this allows for
808 things like C<if> which choose the next op dynamically at run time.
809 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
810 execution if required.
812 The actual functions called are known as PP code, and they're spread
813 between four files: F<pp_hot.c> contains the `hot' code, which is most
814 often used and highly optimized, F<pp_sys.c> contains all the
815 system-specific functions, F<pp_ctl.c> contains the functions which
816 implement control structures (C<if>, C<while> and the like) and F<pp.c>
817 contains everything else. These are, if you like, the C code for Perl's
818 built-in functions and operators.
822 =head2 Internal Variable Types
824 You should by now have had a look at L<perlguts>, which tells you about
825 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
828 These variables are used not only to represent Perl-space variables, but
829 also any constants in the code, as well as some structures completely
830 internal to Perl. The symbol table, for instance, is an ordinary Perl
831 hash. Your code is represented by an SV as it's read into the parser;
832 any program files you call are opened via ordinary Perl filehandles, and
835 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
836 Perl program. Let's see, for instance, how Perl treats the constant
839 % perl -MDevel::Peek -e 'Dump("hello")'
840 1 SV = PV(0xa041450) at 0xa04ecbc
842 3 FLAGS = (POK,READONLY,pPOK)
843 4 PV = 0xa0484e0 "hello"\0
847 Reading C<Devel::Peek> output takes a bit of practise, so let's go
848 through it line by line.
850 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
851 memory. SVs themselves are very simple structures, but they contain a
852 pointer to a more complex structure. In this case, it's a PV, a
853 structure which holds a string value, at location C<0xa041450>. Line 2
854 is the reference count; there are no other references to this data, so
857 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
858 read-only SV (because it's a constant) and the data is a PV internally.
859 Next we've got the contents of the string, starting at location
862 Line 5 gives us the current length of the string - note that this does
863 B<not> include the null terminator. Line 6 is not the length of the
864 string, but the length of the currently allocated buffer; as the string
865 grows, Perl automatically extends the available storage via a routine
868 You can get at any of these quantities from C very easily; just add
869 C<Sv> to the name of the field shown in the snippet, and you've got a
870 macro which will return the value: C<SvCUR(sv)> returns the current
871 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
872 C<SvPV(sv, len)> returns the string itself with its length, and so on.
873 More macros to manipulate these properties can be found in L<perlguts>.
875 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
878 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
883 6 junk = SvPV_force(sv, tlen);
884 7 SvGROW(sv, tlen + len + 1);
887 10 Move(ptr,SvPVX(sv)+tlen,len,char);
889 12 *SvEND(sv) = '\0';
890 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
894 This is a function which adds a string, C<ptr>, of length C<len> onto
895 the end of the PV stored in C<sv>. The first thing we do in line 6 is
896 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
897 macro to force a PV. As a side effect, C<tlen> gets set to the current
898 value of the PV, and the PV itself is returned to C<junk>.
900 In line 7, we make sure that the SV will have enough room to accommodate
901 the old string, the new string and the null terminator. If C<LEN> isn't
902 big enough, C<SvGROW> will reallocate space for us.
904 Now, if C<junk> is the same as the string we're trying to add, we can
905 grab the string directly from the SV; C<SvPVX> is the address of the PV
908 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
909 memory around: we move the string C<ptr> to the end of the PV - that's
910 the start of the PV plus its current length. We're moving C<len> bytes
911 of type C<char>. After doing so, we need to tell Perl we've extended the
912 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
913 macro which gives us the end of the string, so that needs to be a
916 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
917 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
918 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
919 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
920 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
921 data if taint mode is turned on.
923 AVs and HVs are more complicated, but SVs are by far the most common
924 variable type being thrown around. Having seen something of how we
925 manipulate these, let's go on and look at how the op tree is
930 First, what is the op tree, anyway? The op tree is the parsed
931 representation of your program, as we saw in our section on parsing, and
932 it's the sequence of operations that Perl goes through to execute your
933 program, as we saw in L</Running>.
935 An op is a fundamental operation that Perl can perform: all the built-in
936 functions and operators are ops, and there are a series of ops which
937 deal with concepts the interpreter needs internally - entering and
938 leaving a block, ending a statement, fetching a variable, and so on.
940 The op tree is connected in two ways: you can imagine that there are two
941 "routes" through it, two orders in which you can traverse the tree.
942 First, parse order reflects how the parser understood the code, and
943 secondly, execution order tells perl what order to perform the
946 The easiest way to examine the op tree is to stop Perl after it has
947 finished parsing, and get it to dump out the tree. This is exactly what
948 the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
950 Let's have a look at how Perl sees C<$a = $b + $c>:
952 % perl -MO=Terse -e '$a=$b+$c'
953 1 LISTOP (0x8179888) leave
954 2 OP (0x81798b0) enter
955 3 COP (0x8179850) nextstate
956 4 BINOP (0x8179828) sassign
957 5 BINOP (0x8179800) add [1]
958 6 UNOP (0x81796e0) null [15]
959 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
960 8 UNOP (0x81797e0) null [15]
961 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
962 10 UNOP (0x816b4f0) null [15]
963 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
965 Let's start in the middle, at line 4. This is a BINOP, a binary
966 operator, which is at location C<0x8179828>. The specific operator in
967 question is C<sassign> - scalar assignment - and you can find the code
968 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
969 binary operator, it has two children: the add operator, providing the
970 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
973 Line 10 is the null op: this does exactly nothing. What is that doing
974 there? If you see the null op, it's a sign that something has been
975 optimized away after parsing. As we mentioned in L</Optimization>,
976 the optimization stage sometimes converts two operations into one, for
977 example when fetching a scalar variable. When this happens, instead of
978 rewriting the op tree and cleaning up the dangling pointers, it's easier
979 just to replace the redundant operation with the null op. Originally,
980 the tree would have looked like this:
982 10 SVOP (0x816b4f0) rv2sv [15]
983 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
985 That is, fetch the C<a> entry from the main symbol table, and then look
986 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
987 happens to do both these things.
989 The right hand side, starting at line 5 is similar to what we've just
990 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
993 Now, what's this about?
995 1 LISTOP (0x8179888) leave
996 2 OP (0x81798b0) enter
997 3 COP (0x8179850) nextstate
999 C<enter> and C<leave> are scoping ops, and their job is to perform any
1000 housekeeping every time you enter and leave a block: lexical variables
1001 are tidied up, unreferenced variables are destroyed, and so on. Every
1002 program will have those first three lines: C<leave> is a list, and its
1003 children are all the statements in the block. Statements are delimited
1004 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
1005 the ops to be performed for each statement being the children of
1006 C<nextstate>. C<enter> is a single op which functions as a marker.
1008 That's how Perl parsed the program, from top to bottom:
1021 However, it's impossible to B<perform> the operations in this order:
1022 you have to find the values of C<$b> and C<$c> before you add them
1023 together, for instance. So, the other thread that runs through the op
1024 tree is the execution order: each op has a field C<op_next> which points
1025 to the next op to be run, so following these pointers tells us how perl
1026 executes the code. We can traverse the tree in this order using
1027 the C<exec> option to C<B::Terse>:
1029 % perl -MO=Terse,exec -e '$a=$b+$c'
1030 1 OP (0x8179928) enter
1031 2 COP (0x81798c8) nextstate
1032 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
1033 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
1034 5 BINOP (0x8179878) add [1]
1035 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
1036 7 BINOP (0x81798a0) sassign
1037 8 LISTOP (0x8179900) leave
1039 This probably makes more sense for a human: enter a block, start a
1040 statement. Get the values of C<$b> and C<$c>, and add them together.
1041 Find C<$a>, and assign one to the other. Then leave.
1043 The way Perl builds up these op trees in the parsing process can be
1044 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1045 piece we need to construct the tree for C<$a = $b + $c>
1047 1 term : term ASSIGNOP term
1048 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1050 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1052 If you're not used to reading BNF grammars, this is how it works: You're
1053 fed certain things by the tokeniser, which generally end up in upper
1054 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1055 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1056 `terminal symbols', because you can't get any simpler than them.
1058 The grammar, lines one and three of the snippet above, tells you how to
1059 build up more complex forms. These complex forms, `non-terminal symbols'
1060 are generally placed in lower case. C<term> here is a non-terminal
1061 symbol, representing a single expression.
1063 The grammar gives you the following rule: you can make the thing on the
1064 left of the colon if you see all the things on the right in sequence.
1065 This is called a "reduction", and the aim of parsing is to completely
1066 reduce the input. There are several different ways you can perform a
1067 reduction, separated by vertical bars: so, C<term> followed by C<=>
1068 followed by C<term> makes a C<term>, and C<term> followed by C<+>
1069 followed by C<term> can also make a C<term>.
1071 So, if you see two terms with an C<=> or C<+>, between them, you can
1072 turn them into a single expression. When you do this, you execute the
1073 code in the block on the next line: if you see C<=>, you'll do the code
1074 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1075 which contributes to the op tree.
1078 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1080 What this does is creates a new binary op, and feeds it a number of
1081 variables. The variables refer to the tokens: C<$1> is the first token in
1082 the input, C<$2> the second, and so on - think regular expression
1083 backreferences. C<$$> is the op returned from this reduction. So, we
1084 call C<newBINOP> to create a new binary operator. The first parameter to
1085 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1086 operator, so we want the type to be C<ADDOP>. We could specify this
1087 directly, but it's right there as the second token in the input, so we
1088 use C<$2>. The second parameter is the op's flags: 0 means `nothing
1089 special'. Then the things to add: the left and right hand side of our
1090 expression, in scalar context.
1094 When perl executes something like C<addop>, how does it pass on its
1095 results to the next op? The answer is, through the use of stacks. Perl
1096 has a number of stacks to store things it's currently working on, and
1097 we'll look at the three most important ones here.
1101 =item Argument stack
1103 Arguments are passed to PP code and returned from PP code using the
1104 argument stack, C<ST>. The typical way to handle arguments is to pop
1105 them off the stack, deal with them how you wish, and then push the result
1106 back onto the stack. This is how, for instance, the cosine operator
1111 value = Perl_cos(value);
1114 We'll see a more tricky example of this when we consider Perl's macros
1115 below. C<POPn> gives you the NV (floating point value) of the top SV on
1116 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1117 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1118 should be extended if necessary - it can't be necessary here, because we
1119 know there's room for one more item on the stack, since we've just
1120 removed one! The C<XPUSH*> macros at least guarantee safety.
1122 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1123 the first element in your portion of the stack, and C<TOP*> gives you
1124 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1125 negation of an integer:
1129 Just set the integer value of the top stack entry to its negation.
1131 Argument stack manipulation in the core is exactly the same as it is in
1132 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1133 description of the macros used in stack manipulation.
1137 I say `your portion of the stack' above because PP code doesn't
1138 necessarily get the whole stack to itself: if your function calls
1139 another function, you'll only want to expose the arguments aimed for the
1140 called function, and not (necessarily) let it get at your own data. The
1141 way we do this is to have a `virtual' bottom-of-stack, exposed to each
1142 function. The mark stack keeps bookmarks to locations in the argument
1143 stack usable by each function. For instance, when dealing with a tied
1144 variable, (internally, something with `P' magic) Perl has to call
1145 methods for accesses to the tied variables. However, we need to separate
1146 the arguments exposed to the method to the argument exposed to the
1147 original function - the store or fetch or whatever it may be. Here's how
1148 the tied C<push> is implemented; see C<av_push> in F<av.c>:
1152 3 PUSHs(SvTIED_obj((SV*)av, mg));
1156 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1160 The lines which concern the mark stack are the first, fifth and last
1161 lines: they save away, restore and remove the current position of the
1164 Let's examine the whole implementation, for practice:
1168 Push the current state of the stack pointer onto the mark stack. This is
1169 so that when we've finished adding items to the argument stack, Perl
1170 knows how many things we've added recently.
1173 3 PUSHs(SvTIED_obj((SV*)av, mg));
1176 We're going to add two more items onto the argument stack: when you have
1177 a tied array, the C<PUSH> subroutine receives the object and the value
1178 to be pushed, and that's exactly what we have here - the tied object,
1179 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1183 Next we tell Perl to make the change to the global stack pointer: C<dSP>
1184 only gave us a local copy, not a reference to the global.
1187 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1190 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1191 variables are tidied up, everything that has been localised gets
1192 its previous value returned, and so on. Think of them as the C<{> and
1193 C<}> of a Perl block.
1195 To actually do the magic method call, we have to call a subroutine in
1196 Perl space: C<call_method> takes care of that, and it's described in
1197 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1198 going to discard its return value.
1202 Finally, we remove the value we placed on the mark stack, since we
1203 don't need it any more.
1207 C doesn't have a concept of local scope, so perl provides one. We've
1208 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1209 stack implements the C equivalent of, for example:
1216 See L<perlguts/Localising Changes> for how to use the save stack.
1220 =head2 Millions of Macros
1222 One thing you'll notice about the Perl source is that it's full of
1223 macros. Some have called the pervasive use of macros the hardest thing
1224 to understand, others find it adds to clarity. Let's take an example,
1225 the code which implements the addition operator:
1229 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1232 6 SETn( left + right );
1237 Every line here (apart from the braces, of course) contains a macro. The
1238 first line sets up the function declaration as Perl expects for PP code;
1239 line 3 sets up variable declarations for the argument stack and the
1240 target, the return value of the operation. Finally, it tries to see if
1241 the addition operation is overloaded; if so, the appropriate subroutine
1244 Line 5 is another variable declaration - all variable declarations start
1245 with C<d> - which pops from the top of the argument stack two NVs (hence
1246 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1247 C<rl>. These are the two operands to the addition operator. Next, we
1248 call C<SETn> to set the NV of the return value to the result of adding
1249 the two values. This done, we return - the C<RETURN> macro makes sure
1250 that our return value is properly handled, and we pass the next operator
1251 to run back to the main run loop.
1253 Most of these macros are explained in L<perlapi>, and some of the more
1254 important ones are explained in L<perlxs> as well. Pay special attention
1255 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1256 the C<[pad]THX_?> macros.
1258 =head2 Poking at Perl
1260 To really poke around with Perl, you'll probably want to build Perl for
1261 debugging, like this:
1263 ./Configure -d -D optimize=-g
1266 C<-g> is a flag to the C compiler to have it produce debugging
1267 information which will allow us to step through a running program.
1268 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1269 enables all the internal debugging code in Perl. There are a whole bunch
1270 of things you can debug with this: L<perlrun> lists them all, and the
1271 best way to find out about them is to play about with them. The most
1272 useful options are probably
1274 l Context (loop) stack processing
1276 o Method and overloading resolution
1277 c String/numeric conversions
1279 Some of the functionality of the debugging code can be achieved using XS
1282 -Dr => use re 'debug'
1283 -Dx => use O 'Debug'
1285 =head2 Using a source-level debugger
1287 If the debugging output of C<-D> doesn't help you, it's time to step
1288 through perl's execution with a source-level debugger.
1294 We'll use C<gdb> for our examples here; the principles will apply to any
1295 debugger, but check the manual of the one you're using.
1299 To fire up the debugger, type
1303 You'll want to do that in your Perl source tree so the debugger can read
1304 the source code. You should see the copyright message, followed by the
1309 C<help> will get you into the documentation, but here are the most
1316 Run the program with the given arguments.
1318 =item break function_name
1320 =item break source.c:xxx
1322 Tells the debugger that we'll want to pause execution when we reach
1323 either the named function (but see L<perlguts/Internal Functions>!) or the given
1324 line in the named source file.
1328 Steps through the program a line at a time.
1332 Steps through the program a line at a time, without descending into
1337 Run until the next breakpoint.
1341 Run until the end of the current function, then stop again.
1345 Just pressing Enter will do the most recent operation again - it's a
1346 blessing when stepping through miles of source code.
1350 Execute the given C code and print its results. B<WARNING>: Perl makes
1351 heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1352 substitute them yourself. So, for instance, you can't say
1354 print SvPV_nolen(sv)
1358 print Perl_sv_2pv_nolen(sv)
1360 You may find it helpful to have a "macro dictionary", which you can
1361 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1362 recursively apply the macros for you.
1366 =head2 Dumping Perl Data Structures
1368 One way to get around this macro hell is to use the dumping functions in
1369 F<dump.c>; these work a little like an internal
1370 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1371 that you can't get at from Perl. Let's take an example. We'll use the
1372 C<$a = $b + $c> we used before, but give it a bit of context:
1373 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1375 What about C<pp_add>, the function we examined earlier to implement the
1378 (gdb) break Perl_pp_add
1379 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1381 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1382 With the breakpoint in place, we can run our program:
1384 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1386 Lots of junk will go past as gdb reads in the relevant source files and
1387 libraries, and then:
1389 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1390 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1395 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1396 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1399 #define dPOPTOPnnrl_ul NV right = POPn; \
1400 SV *leftsv = TOPs; \
1401 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1403 C<POPn> takes the SV from the top of the stack and obtains its NV either
1404 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1405 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1406 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1407 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1409 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1410 convert it. If we step again, we'll find ourselves there:
1412 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1416 We can now use C<Perl_sv_dump> to investigate the SV:
1418 SV = PV(0xa057cc0) at 0xa0675d0
1421 PV = 0xa06a510 "6XXXX"\0
1426 We know we're going to get C<6> from this, so let's finish the
1430 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1431 0x462669 in Perl_pp_add () at pp_hot.c:311
1434 We can also dump out this op: the current op is always stored in
1435 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1436 similar output to L<B::Debug|B::Debug>.
1439 13 TYPE = add ===> 14
1441 FLAGS = (SCALAR,KIDS)
1443 TYPE = null ===> (12)
1445 FLAGS = (SCALAR,KIDS)
1447 11 TYPE = gvsv ===> 12
1453 # finish this later #
1457 All right, we've now had a look at how to navigate the Perl sources and
1458 some things you'll need to know when fiddling with them. Let's now get
1459 on and create a simple patch. Here's something Larry suggested: if a
1460 C<U> is the first active format during a C<pack>, (for example,
1461 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1464 How do we prepare to fix this up? First we locate the code in question -
1465 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1466 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1467 altering this file, let's copy it to F<pp.c~>.
1469 [Well, it was in F<pp.c> when this tutorial was written. It has now been
1470 split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1472 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1473 loop over the pattern, taking each format character in turn into
1474 C<datum_type>. Then for each possible format character, we swallow up
1475 the other arguments in the pattern (a field width, an asterisk, and so
1476 on) and convert the next chunk input into the specified format, adding
1477 it onto the output SV C<cat>.
1479 How do we know if the C<U> is the first format in the C<pat>? Well, if
1480 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1481 test whether we're still at the start of the string. So, here's where
1485 register char *pat = SvPVx(*++MARK, fromlen);
1486 register char *patend = pat + fromlen;
1491 We'll have another string pointer in there:
1494 register char *pat = SvPVx(*++MARK, fromlen);
1495 register char *patend = pat + fromlen;
1501 And just before we start the loop, we'll set C<patcopy> to be the start
1506 sv_setpvn(cat, "", 0);
1508 while (pat < patend) {
1510 Now if we see a C<U> which was at the start of the string, we turn on
1511 the UTF8 flag for the output SV, C<cat>:
1513 + if (datumtype == 'U' && pat==patcopy+1)
1515 if (datumtype == '#') {
1516 while (pat < patend && *pat != '\n')
1519 Remember that it has to be C<patcopy+1> because the first character of
1520 the string is the C<U> which has been swallowed into C<datumtype!>
1522 Oops, we forgot one thing: what if there are spaces at the start of the
1523 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1524 character, even though it's not the first thing in the pattern. In this
1525 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1527 if (isSPACE(datumtype))
1532 if (isSPACE(datumtype)) {
1537 OK. That's the C part done. Now we must do two additional things before
1538 this patch is ready to go: we've changed the behaviour of Perl, and so
1539 we must document that change. We must also provide some more regression
1540 tests to make sure our patch works and doesn't create a bug somewhere
1541 else along the line.
1543 The regression tests for each operator live in F<t/op/>, and so we
1544 make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1545 tests to the end. First, we'll test that the C<U> does indeed create
1548 t/op/pack.t has a sensible ok() function, but if it didn't we could
1549 use the one from t/test.pl.
1551 require './test.pl';
1552 plan( tests => 159 );
1556 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1557 print "ok $test\n"; $test++;
1559 we can write the more sensible (see L<Test::More> for a full
1560 explanation of is() and other testing functions).
1562 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
1563 "U* produces unicode" );
1565 Now we'll test that we got that space-at-the-beginning business right:
1567 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
1568 " with spaces at the beginning" );
1570 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1571 the first active format:
1573 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
1574 "U* not first isn't unicode" );
1576 Mustn't forget to change the number of tests which appears at the top,
1577 or else the automated tester will get confused. This will either look
1584 plan( tests => 156 );
1586 We now compile up Perl, and run it through the test suite. Our new
1589 Finally, the documentation. The job is never done until the paperwork is
1590 over, so let's describe the change we've just made. The relevant place
1591 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1592 this text in the description of C<pack>:
1596 If the pattern begins with a C<U>, the resulting string will be treated
1597 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1598 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1599 characters. If you don't want this to happen, you can begin your pattern
1600 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1601 string, and then follow this with a C<U*> somewhere in your pattern.
1603 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1604 that if we're making major changes, we should copy the entire directory
1605 to somewhere safe before we begin fiddling, and then do
1607 diff -ruN old new > patch
1609 However, we know which files we've changed, and we can simply do this:
1611 diff -u pp.c~ pp.c > patch
1612 diff -u t/op/pack.t~ t/op/pack.t >> patch
1613 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1615 We end up with a patch looking a little like this:
1617 --- pp.c~ Fri Jun 02 04:34:10 2000
1618 +++ pp.c Fri Jun 16 11:37:25 2000
1619 @@ -4375,6 +4375,7 @@
1622 register char *pat = SvPVx(*++MARK, fromlen);
1624 register char *patend = pat + fromlen;
1627 @@ -4405,6 +4406,7 @@
1630 And finally, we submit it, with our rationale, to perl5-porters. Job
1633 =head2 Patching a core module
1635 This works just like patching anything else, with an extra
1636 consideration. Many core modules also live on CPAN. If this is so,
1637 patch the CPAN version instead of the core and send the patch off to
1638 the module maintainer (with a copy to p5p). This will help the module
1639 maintainer keep the CPAN version in sync with the core version without
1640 constantly scanning p5p.
1642 =head2 Adding a new function to the core
1644 If, as part of a patch to fix a bug, or just because you have an
1645 especially good idea, you decide to add a new function to the core,
1646 discuss your ideas on p5p well before you start work. It may be that
1647 someone else has already attempted to do what you are considering and
1648 can give lots of good advice or even provide you with bits of code
1649 that they already started (but never finished).
1651 You have to follow all of the advice given above for patching. It is
1652 extremely important to test any addition thoroughly and add new tests
1653 to explore all boundary conditions that your new function is expected
1654 to handle. If your new function is used only by one module (e.g. toke),
1655 then it should probably be named S_your_function (for static); on the
1656 other hand, if you expect it to accessible from other functions in
1657 Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1660 The location of any new code is also an important consideration. Don't
1661 just create a new top level .c file and put your code there; you would
1662 have to make changes to Configure (so the Makefile is created properly),
1663 as well as possibly lots of include files. This is strictly pumpking
1666 It is better to add your function to one of the existing top level
1667 source code files, but your choice is complicated by the nature of
1668 the Perl distribution. Only the files that are marked as compiled
1669 static are located in the perl executable. Everything else is located
1670 in the shared library (or DLL if you are running under WIN32). So,
1671 for example, if a function was only used by functions located in
1672 toke.c, then your code can go in toke.c. If, however, you want to call
1673 the function from universal.c, then you should put your code in another
1674 location, for example util.c.
1676 In addition to writing your c-code, you will need to create an
1677 appropriate entry in embed.pl describing your function, then run
1678 'make regen_headers' to create the entries in the numerous header
1679 files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1680 for information on the various options that you can set in embed.pl.
1681 You will forget to do this a few (or many) times and you will get
1682 warnings during the compilation phase. Make sure that you mention
1683 this when you post your patch to P5P; the pumpking needs to know this.
1685 When you write your new code, please be conscious of existing code
1686 conventions used in the perl source files. See <perlstyle> for
1687 details. Although most of the guidelines discussed seem to focus on
1688 Perl code, rather than c, they all apply (except when they don't ;).
1689 See also I<Porting/patching.pod> file in the Perl source distribution
1690 for lots of details about both formatting and submitting patches of
1693 Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1694 Test on as many platforms as you can find. Test as many perl
1695 Configure options as you can (e.g. MULTIPLICITY). If you have
1696 profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1697 below for how to use them to further test your code. Remember that
1698 most of the people on P5P are doing this on their own time and
1699 don't have the time to debug your code.
1701 =head2 Writing a test
1703 Every module and built-in function has an associated test file (or
1704 should...). If you add or change functionality, you have to write a
1705 test. If you fix a bug, you have to write a test so that bug never
1706 comes back. If you alter the docs, it would be nice to test what the
1707 new documentation says.
1709 In short, if you submit a patch you probably also have to patch the
1712 For modules, the test file is right next to the module itself.
1713 F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1714 so there are some snags (and it would be wonderful for you to brush
1715 them out), but it basically works that way. Everything else lives in
1722 Testing of the absolute basic functionality of Perl. Things like
1723 C<if>, basic file reads and writes, simple regexes, etc. These are
1724 run first in the test suite and if any of them fail, something is
1729 These test the basic control structures, C<if/else>, C<while>,
1734 Tests basic issues of how Perl parses and compiles itself.
1738 Tests for built-in IO functions, including command line arguments.
1742 The old home for the module tests, you shouldn't put anything new in
1743 here. There are still some bits and pieces hanging around in here
1744 that need to be moved. Perhaps you could move them? Thanks!
1748 Tests for perl's built in functions that don't fit into any of the
1753 Tests for POD directives. There are still some tests for the Pod
1754 modules hanging around in here that need to be moved out into F<lib/>.
1758 Testing features of how perl actually runs, including exit codes and
1759 handling of PERL* environment variables.
1763 The core uses the same testing style as the rest of Perl, a simple
1764 "ok/not ok" run through Test::Harness, but there are a few special
1767 There are three ways to write a test in the core. Test::More,
1768 t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1769 decision of which to use depends on what part of the test suite you're
1770 working on. This is a measure to prevent a high-level failure (such
1771 as Config.pm breaking) from causing basic functionality tests to fail.
1777 Since we don't know if require works, or even subroutines, use ad hoc
1778 tests for these two. Step carefully to avoid using the feature being
1781 =item t/cmd t/run t/io t/op
1783 Now that basic require() and subroutines are tested, you can use the
1784 t/test.pl library which emulates the important features of Test::More
1785 while using a minimum of core features.
1787 You can also conditionally use certain libraries like Config, but be
1788 sure to skip the test gracefully if it's not there.
1792 Now that the core of Perl is tested, Test::More can be used. You can
1793 also use the full suite of core modules in the tests.
1797 When you say "make test" Perl uses the F<t/TEST> program to run the
1798 test suite. All tests are run from the F<t/> directory, B<not> the
1799 directory which contains the test. This causes some problems with the
1800 tests in F<lib/>, so here's some opportunity for some patching.
1802 You must be triply conscious of cross-platform concerns. This usually
1803 boils down to using File::Spec and avoiding things like C<fork()> and
1804 C<system()> unless absolutely necessary.
1807 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1809 Sometimes it helps to use external tools while debugging and
1810 testing Perl. This section tries to guide you through using
1811 some common testing and debugging tools with Perl. This is
1812 meant as a guide to interfacing these tools with Perl, not
1813 as any kind of guide to the use of the tools themselves.
1815 =head2 Rational Software's Purify
1817 Purify is a commercial tool that is helpful in identifying
1818 memory overruns, wild pointers, memory leaks and other such
1819 badness. Perl must be compiled in a specific way for
1820 optimal testing with Purify. Purify is available under
1821 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1823 The only currently known leaks happen when there are
1824 compile-time errors within eval or require. (Fixing these
1825 is non-trivial, unfortunately, but they must be fixed
1828 =head2 Purify on Unix
1830 On Unix, Purify creates a new Perl binary. To get the most
1831 benefit out of Purify, you should create the perl to Purify
1834 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1835 -Uusemymalloc -Dusemultiplicity
1837 where these arguments mean:
1841 =item -Accflags=-DPURIFY
1843 Disables Perl's arena memory allocation functions, as well as
1844 forcing use of memory allocation functions derived from the
1847 =item -Doptimize='-g'
1849 Adds debugging information so that you see the exact source
1850 statements where the problem occurs. Without this flag, all
1851 you will see is the source filename of where the error occurred.
1855 Disable Perl's malloc so that Purify can more closely monitor
1856 allocations and leaks. Using Perl's malloc will make Purify
1857 report most leaks in the "potential" leaks category.
1859 =item -Dusemultiplicity
1861 Enabling the multiplicity option allows perl to clean up
1862 thoroughly when the interpreter shuts down, which reduces the
1863 number of bogus leak reports from Purify.
1867 Once you've compiled a perl suitable for Purify'ing, then you
1872 which creates a binary named 'pureperl' that has been Purify'ed.
1873 This binary is used in place of the standard 'perl' binary
1874 when you want to debug Perl memory problems.
1876 To minimize the number of memory leak false alarms
1877 (see L</PERL_DESTRUCT_LEVEL>), set environment variable
1878 PERL_DESTRUCT_LEVEL to 2.
1880 setenv PERL_DESTRUCT_LEVEL 2
1882 In Bourne-type shells:
1884 PERL_DESTRUCT_LEVEL=2
1885 export PERL_DESTRUCT_LEVEL
1887 As an example, to show any memory leaks produced during the
1888 standard Perl testset you would create and run the Purify'ed
1893 ../pureperl -I../lib harness
1895 which would run Perl on test.pl and report any memory problems.
1897 Purify outputs messages in "Viewer" windows by default. If
1898 you don't have a windowing environment or if you simply
1899 want the Purify output to unobtrusively go to a log file
1900 instead of to the interactive window, use these following
1901 options to output to the log file "perl.log":
1903 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1904 -log-file=perl.log -append-logfile=yes"
1906 If you plan to use the "Viewer" windows, then you only need this option:
1908 setenv PURIFYOPTIONS "-chain-length=25"
1910 In Bourne-type shells:
1912 PURIFY_OPTIONS="..."
1913 export PURIFY_OPTIONS
1915 or if you have the "env" utility:
1917 env PURIFY_OPTIONS="..." ../pureperl ...
1921 Purify on Windows NT instruments the Perl binary 'perl.exe'
1922 on the fly. There are several options in the makefile you
1923 should change to get the most use out of Purify:
1929 You should add -DPURIFY to the DEFINES line so the DEFINES
1930 line looks something like:
1932 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1934 to disable Perl's arena memory allocation functions, as
1935 well as to force use of memory allocation functions derived
1936 from the system malloc.
1938 =item USE_MULTI = define
1940 Enabling the multiplicity option allows perl to clean up
1941 thoroughly when the interpreter shuts down, which reduces the
1942 number of bogus leak reports from Purify.
1944 =item #PERL_MALLOC = define
1946 Disable Perl's malloc so that Purify can more closely monitor
1947 allocations and leaks. Using Perl's malloc will make Purify
1948 report most leaks in the "potential" leaks category.
1952 Adds debugging information so that you see the exact source
1953 statements where the problem occurs. Without this flag, all
1954 you will see is the source filename of where the error occurred.
1958 As an example, to show any memory leaks produced during the
1959 standard Perl testset you would create and run Purify as:
1964 purify ../perl -I../lib harness
1966 which would instrument Perl in memory, run Perl on test.pl,
1967 then finally report any memory problems.
1969 =head2 Compaq's/Digital's Third Degree
1971 Third Degree is a tool for memory leak detection and memory access checks.
1972 It is one of the many tools in the ATOM toolkit. The toolkit is only
1973 available on Tru64 (formerly known as Digital UNIX formerly known as
1976 When building Perl, you must first run Configure with -Doptimize=-g
1977 and -Uusemymalloc flags, after that you can use the make targets
1978 "perl.third" and "test.third". (What is required is that Perl must be
1979 compiled using the C<-g> flag, you may need to re-Configure.)
1981 The short story is that with "atom" you can instrument the Perl
1982 executable to create a new executable called F<perl.third>. When the
1983 instrumented executable is run, it creates a log of dubious memory
1984 traffic in file called F<perl.3log>. See the manual pages of atom and
1985 third for more information. The most extensive Third Degree
1986 documentation is available in the Compaq "Tru64 UNIX Programmer's
1987 Guide", chapter "Debugging Programs with Third Degree".
1989 The "test.third" leaves a lot of files named F<perl.3log.*> in the t/
1990 subdirectory. There is a problem with these files: Third Degree is so
1991 effective that it finds problems also in the system libraries.
1992 Therefore there are certain types of errors that you should ignore in
1993 your debugging. Errors with stack traces matching
1995 __actual_atof|__catgets|_doprnt|__exc_|__exec|_findio|__localtime|setlocale|__sia_|__strxfrm
1997 (all in libc.so) are known to be non-serious. You can also
1998 ignore the combinations
2000 Perl_gv_fetchfile() calling strcpy()
2001 S_doopen_pmc() calling strcmp()
2003 causing "rih" (reading invalid heap) errors.
2005 There are also leaks that for given certain definition of a leak,
2006 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
2008 =head2 PERL_DESTRUCT_LEVEL
2010 If you want to run any of the tests yourself manually using the
2011 pureperl or perl.third executables, please note that by default
2012 perl B<does not> explicitly cleanup all the memory it has allocated
2013 (such as global memory arenas) but instead lets the exit() of
2014 the whole program "take care" of such allocations, also known
2015 as "global destruction of objects".
2017 There is a way to tell perl to do complete cleanup: set the
2018 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
2019 The t/TEST wrapper does set this to 2, and this is what you
2020 need to do too, if you don't want to see the "global leaks":
2021 For example, for "third-degreed" Perl:
2023 env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
2027 Depending on your platform there are various of profiling Perl.
2029 There are two commonly used techniques of profiling executables:
2030 I<statistical time-sampling> and I<basic-block counting>.
2032 The first method takes periodically samples of the CPU program
2033 counter, and since the program counter can be correlated with the code
2034 generated for functions, we get a statistical view of in which
2035 functions the program is spending its time. The caveats are that very
2036 small/fast functions have lower probability of showing up in the
2037 profile, and that periodically interrupting the program (this is
2038 usually done rather frequently, in the scale of milliseconds) imposes
2039 an additional overhead that may skew the results. The first problem
2040 can be alleviated by running the code for longer (in general this is a
2041 good idea for profiling), the second problem is usually kept in guard
2042 by the profiling tools themselves.
2044 The second method divides up the generated code into I<basic blocks>.
2045 Basic blocks are sections of code that are entered only in the
2046 beginning and exited only at the end. For example, a conditional jump
2047 starts a basic block. Basic block profiling usually works by
2048 I<instrumenting> the code by adding I<enter basic block #nnnn>
2049 book-keeping code to the generated code. During the execution of the
2050 code the basic block counters are then updated appropriately. The
2051 caveat is that the added extra code can skew the results: again, the
2052 profiling tools usually try to factor their own effects out of the
2055 =head2 Gprof Profiling
2057 gprof is a profiling tool available in many UNIX platforms,
2058 it uses F<statistical time-sampling>.
2060 You can build a profiled version of perl called "perl.gprof" by
2061 invoking the make target "perl.gprof" (What is required is that Perl
2062 must be compiled using the C<-pg> flag, you may need to re-Configure).
2063 Running the profiled version of Perl will create an output file called
2064 F<gmon.out> is created which contains the profiling data collected
2065 during the execution.
2067 The gprof tool can then display the collected data in various ways.
2068 Usually gprof understands the following options:
2074 Suppress statically defined functions from the profile.
2078 Suppress the verbose descriptions in the profile.
2082 Exclude the given routine and its descendants from the profile.
2086 Display only the given routine and its descendants in the profile.
2090 Generate a summary file called F<gmon.sum> which then may be given
2091 to subsequent gprof runs to accumulate data over several runs.
2095 Display routines that have zero usage.
2099 For more detailed explanation of the available commands and output
2100 formats, see your own local documentation of gprof.
2102 =head2 GCC gcov Profiling
2104 Starting from GCC 3.0 I<basic block profiling> is officially available
2107 You can build a profiled version of perl called F<perl.gcov> by
2108 invoking the make target "perl.gcov" (what is required that Perl must
2109 be compiled using gcc with the flags C<-fprofile-arcs
2110 -ftest-coverage>, you may need to re-Configure).
2112 Running the profiled version of Perl will cause profile output to be
2113 generated. For each source file an accompanying ".da" file will be
2116 To display the results you use the "gcov" utility (which should
2117 be installed if you have gcc 3.0 or newer installed). F<gcov> is
2118 run on source code files, like this
2122 which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2123 contain the source code annotated with relative frequencies of
2124 execution indicated by "#" markers.
2126 Useful options of F<gcov> include C<-b> which will summarise the
2127 basic block, branch, and function call coverage, and C<-c> which
2128 instead of relative frequencies will use the actual counts. For
2129 more information on the use of F<gcov> and basic block profiling
2130 with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2132 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2134 and its section titled "8. gcov: a Test Coverage Program"
2136 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2138 =head2 Pixie Profiling
2140 Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2141 UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
2142 I<basic-block counting>.
2144 You can build a profiled version of perl called F<perl.pixie> by
2145 invoking the make target "perl.pixie" (what is required is that Perl
2146 must be compiled using the C<-g> flag, you may need to re-Configure).
2148 In Tru64 a file called F<perl.Addrs> will also be silently created,
2149 this file contains the addresses of the basic blocks. Running the
2150 profiled version of Perl will create a new file called "perl.Counts"
2151 which contains the counts for the basic block for that particular
2154 To display the results you use the F<prof> utility. The exact
2155 incantation depends on your operating system, "prof perl.Counts" in
2156 IRIX, and "prof -pixie -all -L. perl" in Tru64.
2158 In IRIX the following prof options are available:
2164 Reports the most heavily used lines in descending order of use.
2165 Useful for finding the hotspot lines.
2169 Groups lines by procedure, with procedures sorted in descending order of use.
2170 Within a procedure, lines are listed in source order.
2171 Useful for finding the hotspots of procedures.
2175 In Tru64 the following options are available:
2181 Procedures sorted in descending order by the number of cycles executed
2182 in each procedure. Useful for finding the hotspot procedures.
2183 (This is the default option.)
2187 Lines sorted in descending order by the number of cycles executed in
2188 each line. Useful for finding the hotspot lines.
2190 =item -i[nvocations]
2192 The called procedures are sorted in descending order by number of calls
2193 made to the procedures. Useful for finding the most used procedures.
2197 Grouped by procedure, sorted by cycles executed per procedure.
2198 Useful for finding the hotspots of procedures.
2202 The compiler emitted code for these lines, but the code was unexecuted.
2206 Unexecuted procedures.
2210 For further information, see your system's manual pages for pixie and prof.
2212 =head2 Miscellaneous tricks
2218 Those debugging perl with the DDD frontend over gdb may find the
2221 You can extend the data conversion shortcuts menu, so for example you
2222 can display an SV's IV value with one click, without doing any typing.
2223 To do that simply edit ~/.ddd/init file and add after:
2225 ! Display shortcuts.
2226 Ddd*gdbDisplayShortcuts: \
2227 /t () // Convert to Bin\n\
2228 /d () // Convert to Dec\n\
2229 /x () // Convert to Hex\n\
2230 /o () // Convert to Oct(\n\
2232 the following two lines:
2234 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
2235 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
2237 so now you can do ivx and pvx lookups or you can plug there the
2238 sv_peek "conversion":
2240 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
2242 (The my_perl is for threaded builds.)
2243 Just remember that every line, but the last one, should end with \n\
2245 Alternatively edit the init file interactively via:
2246 3rd mouse button -> New Display -> Edit Menu
2248 Note: you can define up to 20 conversion shortcuts in the gdb
2255 We've had a brief look around the Perl source, an overview of the stages
2256 F<perl> goes through when it's running your code, and how to use a
2257 debugger to poke at the Perl guts. We took a very simple problem and
2258 demonstrated how to solve it fully - with documentation, regression
2259 tests, and finally a patch for submission to p5p. Finally, we talked
2260 about how to use external tools to debug and test Perl.
2262 I'd now suggest you read over those references again, and then, as soon
2263 as possible, get your hands dirty. The best way to learn is by doing,
2270 Subscribe to perl5-porters, follow the patches and try and understand
2271 them; don't be afraid to ask if there's a portion you're not clear on -
2272 who knows, you may unearth a bug in the patch...
2276 Keep up to date with the bleeding edge Perl distributions and get
2277 familiar with the changes. Try and get an idea of what areas people are
2278 working on and the changes they're making.
2282 Do read the README associated with your operating system, e.g. README.aix
2283 on the IBM AIX OS. Don't hesitate to supply patches to that README if
2284 you find anything missing or changed over a new OS release.
2288 Find an area of Perl that seems interesting to you, and see if you can
2289 work out how it works. Scan through the source, and step over it in the
2290 debugger. Play, poke, investigate, fiddle! You'll probably get to
2291 understand not just your chosen area but a much wider range of F<perl>'s
2292 activity as well, and probably sooner than you'd think.
2298 =item I<The Road goes ever on and on, down from the door where it began.>
2302 If you can do these things, you've started on the long road to Perl porting.
2303 Thanks for wanting to help make Perl better - and happy hacking!
2307 This document was written by Nathan Torkington, and is maintained by
2308 the perl5-porters mailing list.