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 is the pumpking for the 5.6 release of
43 In addition, various people are pumpkings for different things. For
44 instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
45 pumpkin, and Tom Christiansen is the documentation pumpking.
47 Larry sees Perl development along the lines of the US government:
48 there's the Legislature (the porters), the Executive branch (the
49 pumpkings), and the Supreme Court (Larry). The legislature can
50 discuss and submit patches to the executive branch all they like, but
51 the executive branch is free to veto them. Rarely, the Supreme Court
52 will side with the executive branch over the legislature, or the
53 legislature over the executive branch. Mostly, however, the
54 legislature and the executive branch are supposed to get along and
55 work out their differences without impeachment or court cases.
57 You might sometimes see reference to Rule 1 and Rule 2. Larry's power
58 as Supreme Court is expressed in The Rules:
64 Larry is always by definition right about how Perl should behave.
65 This means he has final veto power on the core functionality.
69 Larry is allowed to change his mind about any matter at a later date,
70 regardless of whether he previously invoked Rule 1.
74 Got that? Larry is always right, even when he was wrong. It's rare
75 to see either Rule exercised, but they are often alluded to.
77 New features and extensions to the language are contentious, because
78 the criteria used by the pumpkings, Larry, and other porters to decide
79 which features should be implemented and incorporated are not codified
80 in a few small design goals as with some other languages. Instead,
81 the heuristics are flexible and often difficult to fathom. Here is
82 one person's list, roughly in decreasing order of importance, of
83 heuristics that new features have to be weighed against:
87 =item Does concept match the general goals of Perl?
89 These haven't been written anywhere in stone, but one approximation
92 1. Keep it fast, simple, and useful.
93 2. Keep features/concepts as orthogonal as possible.
94 3. No arbitrary limits (platforms, data sizes, cultures).
95 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
96 5. Either assimilate new technologies, or build bridges to them.
98 =item Where is the implementation?
100 All the talk in the world is useless without an implementation. In
101 almost every case, the person or people who argue for a new feature
102 will be expected to be the ones who implement it. Porters capable
103 of coding new features have their own agendas, and are not available
104 to implement your (possibly good) idea.
106 =item Backwards compatibility
108 It's a cardinal sin to break existing Perl programs. New warnings are
109 contentious--some say that a program that emits warnings is not
110 broken, while others say it is. Adding keywords has the potential to
111 break programs, changing the meaning of existing token sequences or
112 functions might break programs.
114 =item Could it be a module instead?
116 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
117 the need to keep changing the Perl interpreter. You can write modules
118 that export functions, you can give those functions prototypes so they
119 can be called like built-in functions, you can even write XS code to
120 mess with the runtime data structures of the Perl interpreter if you
121 want to implement really complicated things. If it can be done in a
122 module instead of in the core, it's highly unlikely to be added.
124 =item Is the feature generic enough?
126 Is this something that only the submitter wants added to the language,
127 or would it be broadly useful? Sometimes, instead of adding a feature
128 with a tight focus, the porters might decide to wait until someone
129 implements the more generalized feature. For instance, instead of
130 implementing a ``delayed evaluation'' feature, the porters are waiting
131 for a macro system that would permit delayed evaluation and much more.
133 =item Does it potentially introduce new bugs?
135 Radical rewrites of large chunks of the Perl interpreter have the
136 potential to introduce new bugs. The smaller and more localized the
139 =item Does it preclude other desirable features?
141 A patch is likely to be rejected if it closes off future avenues of
142 development. For instance, a patch that placed a true and final
143 interpretation on prototypes is likely to be rejected because there
144 are still options for the future of prototypes that haven't been
147 =item Is the implementation robust?
149 Good patches (tight code, complete, correct) stand more chance of
150 going in. Sloppy or incorrect patches might be placed on the back
151 burner until the pumpking has time to fix, or might be discarded
152 altogether without further notice.
154 =item Is the implementation generic enough to be portable?
156 The worst patches make use of a system-specific features. It's highly
157 unlikely that nonportable additions to the Perl language will be
160 =item Is there enough documentation?
162 Patches without documentation are probably ill-thought out or
163 incomplete. Nothing can be added without documentation, so submitting
164 a patch for the appropriate manpages as well as the source code is
165 always a good idea. If appropriate, patches should add to the test
168 =item Is there another way to do it?
170 Larry said ``Although the Perl Slogan is I<There's More Than One Way
171 to Do It>, I hesitate to make 10 ways to do something''. This is a
172 tricky heuristic to navigate, though--one man's essential addition is
173 another man's pointless cruft.
175 =item Does it create too much work?
177 Work for the pumpking, work for Perl programmers, work for module
178 authors, ... Perl is supposed to be easy.
180 =item Patches speak louder than words
182 Working code is always preferred to pie-in-the-sky ideas. A patch to
183 add a feature stands a much higher chance of making it to the language
184 than does a random feature request, no matter how fervently argued the
185 request might be. This ties into ``Will it be useful?'', as the fact
186 that someone took the time to make the patch demonstrates a strong
187 desire for the feature.
191 If you're on the list, you might hear the word ``core'' bandied
192 around. It refers to the standard distribution. ``Hacking on the
193 core'' means you're changing the C source code to the Perl
194 interpreter. ``A core module'' is one that ships with Perl.
196 =head2 Keeping in sync
198 The source code to the Perl interpreter, in its different versions, is
199 kept in a repository managed by a revision control system (which is
200 currently the Perforce program, see http://perforce.com/). The
201 pumpkings and a few others have access to the repository to check in
202 changes. Periodically the pumpking for the development version of Perl
203 will release a new version, so the rest of the porters can see what's
204 changed. The current state of the main trunk of repository, and patches
205 that describe the individual changes that have happened since the last
206 public release are available at this location:
208 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
210 If you are a member of the perl5-porters mailing list, it is a good
211 thing to keep in touch with the most recent changes. If not only to
212 verify if what you would have posted as a bug report isn't already
213 solved in the most recent available perl development branch, also
214 known as perl-current, bleading edge perl, bleedperl or bleadperl.
216 Needless to say, the source code in perl-current is usually in a perpetual
217 state of evolution. You should expect it to be very buggy. Do B<not> use
218 it for any purpose other than testing and development.
220 Keeping in sync with the most recent branch can be done in several ways,
221 but the most convenient and reliable way is using B<rsync>, available at
222 ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
225 If you choose to keep in sync using rsync, there are two approaches
230 =item rsync'ing the source tree
232 Presuming you are in the directory where your perl source resides
233 and you have rsync installed and available, you can `upgrade' to
236 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
238 This takes care of updating every single item in the source tree to
239 the latest applied patch level, creating files that are new (to your
240 distribution) and setting date/time stamps of existing files to
241 reflect the bleadperl status.
243 Note that this will not delete any files that were in '.' before
244 the rsync. Once you are sure that the rsync is running correctly,
245 run it with the --delete and the --dry-run options like this:
247 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
249 This will I<simulate> an rsync run that also deletes files not
250 present in the bleadperl master copy. Observe the results from
251 this run closely. If you are sure that the actual run would delete
252 no files precious to you, you could remove the '--dry-run' option.
254 You can than check what patch was the latest that was applied by
255 looking in the file B<.patch>, which will show the number of the
258 If you have more than one machine to keep in sync, and not all of
259 them have access to the WAN (so you are not able to rsync all the
260 source trees to the real source), there are some ways to get around
265 =item Using rsync over the LAN
267 Set up a local rsync server which makes the rsynced source tree
268 available to the LAN and sync the other machines against this
271 From http://rsync.samba.org/README.html:
273 "Rsync uses rsh or ssh for communication. It does not need to be
274 setuid and requires no special privileges for installation. It
275 does not require an inetd entry or a daemon. You must, however,
276 have a working rsh or ssh system. Using ssh is recommended for
277 its security features."
279 =item Using pushing over the NFS
281 Having the other systems mounted over the NFS, you can take an
282 active pushing approach by checking the just updated tree against
283 the other not-yet synced trees. An example would be
292 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
295 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
297 foreach my $host (keys %remote) {
298 unless (-d $remote{$host}) {
299 print STDERR "Cannot Xsync for host $host\n";
302 foreach my $file (keys %MF) {
303 my $rfile = "$remote{$host}/$file";
304 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
305 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
306 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
307 printf "%4s %-34s %8d %9d %8d %9d\n",
308 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
310 copy ($file, $rfile);
311 utime time, $MF{$file}[2], $rfile;
312 chmod $MF{$file}[0], $rfile;
316 though this is not perfect. It could be improved with checking
317 file checksums before updating. Not all NFS systems support
318 reliable utime support (when used over the NFS).
322 =item rsync'ing the patches
324 The source tree is maintained by the pumpking who applies patches to
325 the files in the tree. These patches are either created by the
326 pumpking himself using C<diff -c> after updating the file manually or
327 by applying patches sent in by posters on the perl5-porters list.
328 These patches are also saved and rsync'able, so you can apply them
329 yourself to the source files.
331 Presuming you are in a directory where your patches reside, you can
332 get them in sync with
334 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
336 This makes sure the latest available patch is downloaded to your
339 It's then up to you to apply these patches, using something like
341 # last=`ls -rt1 *.gz | tail -1`
342 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
343 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
345 # patch -p1 -N <../perl-current-diffs/blead.patch
347 or, since this is only a hint towards how it works, use CPAN-patchaperl
348 from Andreas König to have better control over the patching process.
352 =head2 Why rsync the source tree
356 =item It's easier to rsync the source tree
358 Since you don't have to apply the patches yourself, you are sure all
359 files in the source tree are in the right state.
361 =item It's more recent
363 According to Gurusamy Sarathy:
365 "... The rsync mirror is automatic and syncs with the repository
368 "Updating the patch area still requires manual intervention
369 (with all the goofiness that implies, which you've noted) and
370 is typically on a daily cycle. Making this process automatic
371 is on my tuit list, but don't ask me when."
373 =item It's more reliable
375 Well, since the patches are updated by hand, I don't have to say any
376 more ... (see Sarathy's remark).
380 =head2 Why rsync the patches
384 =item It's easier to rsync the patches
386 If you have more than one machine that you want to keep in track with
387 bleadperl, it's easier to rsync the patches only once and then apply
388 them to all the source trees on the different machines.
390 In case you try to keep in pace on 5 different machines, for which
391 only one of them has access to the WAN, rsync'ing all the source
392 trees should than be done 5 times over the NFS. Having
393 rsync'ed the patches only once, I can apply them to all the source
394 trees automatically. Need you say more ;-)
396 =item It's a good reference
398 If you do not only like to have the most recent development branch,
399 but also like to B<fix> bugs, or extend features, you want to dive
400 into the sources. If you are a seasoned perl core diver, you don't
401 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
402 your way around. But if you are a starter, the patches may help you
403 in finding where you should start and how to change the bits that
406 The file B<Changes> is updated on occasions the pumpking sees as his
407 own little sync points. On those occasions, he releases a tar-ball of
408 the current source tree (i.e. perl@7582.tar.gz), which will be an
409 excellent point to start with when choosing to use the 'rsync the
410 patches' scheme. Starting with perl@7582, which means a set of source
411 files on which the latest applied patch is number 7582, you apply all
412 succeeding patches available from then on (7583, 7584, ...).
414 You can use the patches later as a kind of search archive.
418 =item Finding a start point
420 If you want to fix/change the behaviour of function/feature Foo, just
421 scan the patches for patches that mention Foo either in the subject,
422 the comments, or the body of the fix. A good chance the patch shows
423 you the files that are affected by that patch which are very likely
424 to be the starting point of your journey into the guts of perl.
426 =item Finding how to fix a bug
428 If you've found I<where> the function/feature Foo misbehaves, but you
429 don't know how to fix it (but you do know the change you want to
430 make), you can, again, peruse the patches for similar changes and
431 look how others apply the fix.
433 =item Finding the source of misbehaviour
435 When you keep in sync with bleadperl, the pumpking would love to
436 I<see> that the community efforts really work. So after each of his
437 sync points, you are to 'make test' to check if everything is still
438 in working order. If it is, you do 'make ok', which will send an OK
439 report to perlbug@perl.org. (If you do not have access to a mailer
440 from the system you just finished successfully 'make test', you can
441 do 'make okfile', which creates the file C<perl.ok>, which you can
442 than take to your favourite mailer and mail yourself).
444 But of course, as always, things will not always lead to a success
445 path, and one or more test do not pass the 'make test'. Before
446 sending in a bug report (using 'make nok' or 'make nokfile'), check
447 the mailing list if someone else has reported the bug already and if
448 so, confirm it by replying to that message. If not, you might want to
449 trace the source of that misbehaviour B<before> sending in the bug,
450 which will help all the other porters in finding the solution.
452 Here the saved patches come in very handy. You can check the list of
453 patches to see which patch changed what file and what change caused
454 the misbehaviour. If you note that in the bug report, it saves the
455 one trying to solve it, looking for that point.
459 If searching the patches is too bothersome, you might consider using
460 perl's bugtron to find more information about discussions and
461 ramblings on posted bugs.
463 If you want to get the best of both worlds, rsync both the source
464 tree for convenience, reliability and ease and rsync the patches
470 =head2 Perlbug remote interface
474 There are three (3) remote administrative interfaces for modifying bug status, category, etc. In all cases an admin must be first registered with the Perlbug database by sending an email request to richard@perl.org or bugmongers@perl.org.
476 The main requirement is the willingness to classify, (with the emphasis on closing where possible :), outstanding bugs. Further explanation can be garnered from the web at http://bugs.perl.org/, or by asking on the admin mailing list at: bugmongers@perl.org
478 For more info on the web see
480 http://bugs.perl.org/perlbug.cgi?req=spec
486 =item 1 http://bugs.perl.org
488 Login via the web, (remove B<admin/> if only browsing), where interested Cc's, tests, patches and change-ids, etc. may be assigned.
490 http://bugs.perl.org/admin/index.html
493 =item 2 bugdb@perl.org
495 Where the subject line is used for commands:
498 Subject: -a close bugid1 bugid2 aix install
504 =item 3 commands_and_bugdids@bugs.perl.org
506 Where the address itself is the source for the commands:
508 To: close_bugid1_bugid2_aix@bugs.perl.org
510 To: help@bugs.perl.org
513 =item notes, patches, tests
515 For patches and tests, the message body is assigned to the appropriate bug/s and forwarded to p5p for their attention.
517 To: test_<bugid1>_aix_close@bugs.perl.org
518 Subject: this is a test for the (now closed) aix bug
520 Test is the body of the mail
524 =head2 Submitting patches
526 Always submit patches to I<perl5-porters@perl.org>. If you're
527 patching a core module and there's an author listed, send the author a
528 copy (see L<Patching a core module>). This lets other porters review
529 your patch, which catches a surprising number of errors in patches.
530 Either use the diff program (available in source code form from
531 I<ftp://ftp.gnu.org/pub/gnu/>), or use Johan Vromans' I<makepatch>
532 (available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
533 but context diffs are accepted. Do not send RCS-style diffs or diffs
534 without context lines. More information is given in the
535 I<Porting/patching.pod> file in the Perl source distribution. Please
536 patch against the latest B<development> version (e.g., if you're
537 fixing a bug in the 5.005 track, patch against the latest 5.005_5x
538 version). Only patches that survive the heat of the development
539 branch get applied to maintenance versions.
541 Your patch should update the documentation and test suite. See
544 To report a bug in Perl, use the program I<perlbug> which comes with
545 Perl (if you can't get Perl to work, send mail to the address
546 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
547 I<perlbug> feeds into the automated bug-tracking system, access to
548 which is provided through the web at I<http://bugs.perl.org/>. It
549 often pays to check the archives of the perl5-porters mailing list to
550 see whether the bug you're reporting has been reported before, and if
551 so whether it was considered a bug. See above for the location of
552 the searchable archives.
554 The CPAN testers (I<http://testers.cpan.org/>) are a group of
555 volunteers who test CPAN modules on a variety of platforms. Perl Labs
556 (I<http://labs.perl.org/>) automatically tests Perl source releases on
557 platforms and gives feedback to the CPAN testers mailing list. Both
558 efforts welcome volunteers.
560 It's a good idea to read and lurk for a while before chipping in.
561 That way you'll get to see the dynamic of the conversations, learn the
562 personalities of the players, and hopefully be better prepared to make
563 a useful contribution when do you speak up.
565 If after all this you still think you want to join the perl5-porters
566 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
567 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
569 To hack on the Perl guts, you'll need to read the following things:
575 This is of paramount importance, since it's the documentation of what
576 goes where in the Perl source. Read it over a couple of times and it
577 might start to make sense - don't worry if it doesn't yet, because the
578 best way to study it is to read it in conjunction with poking at Perl
579 source, and we'll do that later on.
581 You might also want to look at Gisle Aas's illustrated perlguts -
582 there's no guarantee that this will be absolutely up-to-date with the
583 latest documentation in the Perl core, but the fundamentals will be
584 right. (http://gisle.aas.no/perl/illguts/)
586 =item L<perlxstut> and L<perlxs>
588 A working knowledge of XSUB programming is incredibly useful for core
589 hacking; XSUBs use techniques drawn from the PP code, the portion of the
590 guts that actually executes a Perl program. It's a lot gentler to learn
591 those techniques from simple examples and explanation than from the core
596 The documentation for the Perl API explains what some of the internal
597 functions do, as well as the many macros used in the source.
599 =item F<Porting/pumpkin.pod>
601 This is a collection of words of wisdom for a Perl porter; some of it is
602 only useful to the pumpkin holder, but most of it applies to anyone
603 wanting to go about Perl development.
605 =item The perl5-porters FAQ
607 This is posted to perl5-porters at the beginning on every month, and
608 should be available from http://perlhacker.org/p5p-faq; alternatively,
609 you can get the FAQ emailed to you by sending mail to
610 C<perl5-porters-faq@perl.org>. It contains hints on reading
611 perl5-porters, information on how perl5-porters works and how Perl
612 development in general works.
616 =head2 Finding Your Way Around
618 Perl maintenance can be split into a number of areas, and certain people
619 (pumpkins) will have responsibility for each area. These areas sometimes
620 correspond to files or directories in the source kit. Among the areas are:
626 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
627 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
628 contains the core XS modules.
632 There are tests for nearly all the modules, built-ins and major bits
633 of functionality. Test files all have a .t suffix. Module tests live
634 in the F<lib/> and F<ext/> directories next to the module being
635 tested. Others live in F<t/>. See L<Writing a test>
639 Documentation maintenance includes looking after everything in the
640 F<pod/> directory, (as well as contributing new documentation) and
641 the documentation to the modules in core.
645 The configure process is the way we make Perl portable across the
646 myriad of operating systems it supports. Responsibility for the
647 configure, build and installation process, as well as the overall
648 portability of the core code rests with the configure pumpkin - others
649 help out with individual operating systems.
651 The files involved are the operating system directories, (F<win32/>,
652 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
653 and F<Makefile>, as well as the metaconfig files which generate
654 F<Configure>. (metaconfig isn't included in the core distribution.)
658 And of course, there's the core of the Perl interpreter itself. Let's
659 have a look at that in a little more detail.
663 Before we leave looking at the layout, though, don't forget that
664 F<MANIFEST> contains not only the file names in the Perl distribution,
665 but short descriptions of what's in them, too. For an overview of the
666 important files, try this:
668 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
670 =head2 Elements of the interpreter
672 The work of the interpreter has two main stages: compiling the code
673 into the internal representation, or bytecode, and then executing it.
674 L<perlguts/Compiled code> explains exactly how the compilation stage
677 Here is a short breakdown of perl's operation:
683 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
684 This is very high-level code, enough to fit on a single screen, and it
685 resembles the code found in L<perlembed>; most of the real action takes
688 First, F<perlmain.c> allocates some memory and constructs a Perl
691 1 PERL_SYS_INIT3(&argc,&argv,&env);
693 3 if (!PL_do_undump) {
694 4 my_perl = perl_alloc();
697 7 perl_construct(my_perl);
698 8 PL_perl_destruct_level = 0;
701 Line 1 is a macro, and its definition is dependent on your operating
702 system. Line 3 references C<PL_do_undump>, a global variable - all
703 global variables in Perl start with C<PL_>. This tells you whether the
704 current running program was created with the C<-u> flag to perl and then
705 F<undump>, which means it's going to be false in any sane context.
707 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
708 interpreter. It's quite a simple function, and the guts of it looks like
711 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
713 Here you see an example of Perl's system abstraction, which we'll see
714 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
715 own C<malloc> as defined in F<malloc.c> if you selected that option at
718 Next, in line 7, we construct the interpreter; this sets up all the
719 special variables that Perl needs, the stacks, and so on.
721 Now we pass Perl the command line options, and tell it to go:
723 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
725 exitstatus = perl_run(my_perl);
729 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
730 in F<perl.c>, which processes the command line options, sets up any
731 statically linked XS modules, opens the program and calls C<yyparse> to
736 The aim of this stage is to take the Perl source, and turn it into an op
737 tree. We'll see what one of those looks like later. Strictly speaking,
738 there's three things going on here.
740 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
741 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
742 B<is> a YACC grammar for Perl!) The job of the parser is to take your
743 code and `understand' it, splitting it into sentences, deciding which
744 operands go with which operators and so on.
746 The parser is nobly assisted by the lexer, which chunks up your input
747 into tokens, and decides what type of thing each token is: a variable
748 name, an operator, a bareword, a subroutine, a core function, and so on.
749 The main point of entry to the lexer is C<yylex>, and that and its
750 associated routines can be found in F<toke.c>. Perl isn't much like
751 other computer languages; it's highly context sensitive at times, it can
752 be tricky to work out what sort of token something is, or where a token
753 ends. As such, there's a lot of interplay between the tokeniser and the
754 parser, which can get pretty frightening if you're not used to it.
756 As the parser understands a Perl program, it builds up a tree of
757 operations for the interpreter to perform during execution. The routines
758 which construct and link together the various operations are to be found
759 in F<op.c>, and will be examined later.
763 Now the parsing stage is complete, and the finished tree represents
764 the operations that the Perl interpreter needs to perform to execute our
765 program. Next, Perl does a dry run over the tree looking for
766 optimisations: constant expressions such as C<3 + 4> will be computed
767 now, and the optimizer will also see if any multiple operations can be
768 replaced with a single one. For instance, to fetch the variable C<$foo>,
769 instead of grabbing the glob C<*foo> and looking at the scalar
770 component, the optimizer fiddles the op tree to use a function which
771 directly looks up the scalar in question. The main optimizer is C<peep>
772 in F<op.c>, and many ops have their own optimizing functions.
776 Now we're finally ready to go: we have compiled Perl byte code, and all
777 that's left to do is run it. The actual execution is done by the
778 C<runops_standard> function in F<run.c>; more specifically, it's done by
779 these three innocent looking lines:
781 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
785 You may be more comfortable with the Perl version of that:
787 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
789 Well, maybe not. Anyway, each op contains a function pointer, which
790 stipulates the function which will actually carry out the operation.
791 This function will return the next op in the sequence - this allows for
792 things like C<if> which choose the next op dynamically at run time.
793 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
794 execution if required.
796 The actual functions called are known as PP code, and they're spread
797 between four files: F<pp_hot.c> contains the `hot' code, which is most
798 often used and highly optimized, F<pp_sys.c> contains all the
799 system-specific functions, F<pp_ctl.c> contains the functions which
800 implement control structures (C<if>, C<while> and the like) and F<pp.c>
801 contains everything else. These are, if you like, the C code for Perl's
802 built-in functions and operators.
806 =head2 Internal Variable Types
808 You should by now have had a look at L<perlguts>, which tells you about
809 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
812 These variables are used not only to represent Perl-space variables, but
813 also any constants in the code, as well as some structures completely
814 internal to Perl. The symbol table, for instance, is an ordinary Perl
815 hash. Your code is represented by an SV as it's read into the parser;
816 any program files you call are opened via ordinary Perl filehandles, and
819 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
820 Perl program. Let's see, for instance, how Perl treats the constant
823 % perl -MDevel::Peek -e 'Dump("hello")'
824 1 SV = PV(0xa041450) at 0xa04ecbc
826 3 FLAGS = (POK,READONLY,pPOK)
827 4 PV = 0xa0484e0 "hello"\0
831 Reading C<Devel::Peek> output takes a bit of practise, so let's go
832 through it line by line.
834 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
835 memory. SVs themselves are very simple structures, but they contain a
836 pointer to a more complex structure. In this case, it's a PV, a
837 structure which holds a string value, at location C<0xa041450>. Line 2
838 is the reference count; there are no other references to this data, so
841 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
842 read-only SV (because it's a constant) and the data is a PV internally.
843 Next we've got the contents of the string, starting at location
846 Line 5 gives us the current length of the string - note that this does
847 B<not> include the null terminator. Line 6 is not the length of the
848 string, but the length of the currently allocated buffer; as the string
849 grows, Perl automatically extends the available storage via a routine
852 You can get at any of these quantities from C very easily; just add
853 C<Sv> to the name of the field shown in the snippet, and you've got a
854 macro which will return the value: C<SvCUR(sv)> returns the current
855 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
856 C<SvPV(sv, len)> returns the string itself with its length, and so on.
857 More macros to manipulate these properties can be found in L<perlguts>.
859 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
862 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
867 6 junk = SvPV_force(sv, tlen);
868 7 SvGROW(sv, tlen + len + 1);
871 10 Move(ptr,SvPVX(sv)+tlen,len,char);
873 12 *SvEND(sv) = '\0';
874 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
878 This is a function which adds a string, C<ptr>, of length C<len> onto
879 the end of the PV stored in C<sv>. The first thing we do in line 6 is
880 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
881 macro to force a PV. As a side effect, C<tlen> gets set to the current
882 value of the PV, and the PV itself is returned to C<junk>.
884 In line 7, we make sure that the SV will have enough room to accommodate
885 the old string, the new string and the null terminator. If C<LEN> isn't
886 big enough, C<SvGROW> will reallocate space for us.
888 Now, if C<junk> is the same as the string we're trying to add, we can
889 grab the string directly from the SV; C<SvPVX> is the address of the PV
892 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
893 memory around: we move the string C<ptr> to the end of the PV - that's
894 the start of the PV plus its current length. We're moving C<len> bytes
895 of type C<char>. After doing so, we need to tell Perl we've extended the
896 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
897 macro which gives us the end of the string, so that needs to be a
900 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
901 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
902 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
903 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
904 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
905 data if taint mode is turned on.
907 AVs and HVs are more complicated, but SVs are by far the most common
908 variable type being thrown around. Having seen something of how we
909 manipulate these, let's go on and look at how the op tree is
914 First, what is the op tree, anyway? The op tree is the parsed
915 representation of your program, as we saw in our section on parsing, and
916 it's the sequence of operations that Perl goes through to execute your
917 program, as we saw in L</Running>.
919 An op is a fundamental operation that Perl can perform: all the built-in
920 functions and operators are ops, and there are a series of ops which
921 deal with concepts the interpreter needs internally - entering and
922 leaving a block, ending a statement, fetching a variable, and so on.
924 The op tree is connected in two ways: you can imagine that there are two
925 "routes" through it, two orders in which you can traverse the tree.
926 First, parse order reflects how the parser understood the code, and
927 secondly, execution order tells perl what order to perform the
930 The easiest way to examine the op tree is to stop Perl after it has
931 finished parsing, and get it to dump out the tree. This is exactly what
932 the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
934 Let's have a look at how Perl sees C<$a = $b + $c>:
936 % perl -MO=Terse -e '$a=$b+$c'
937 1 LISTOP (0x8179888) leave
938 2 OP (0x81798b0) enter
939 3 COP (0x8179850) nextstate
940 4 BINOP (0x8179828) sassign
941 5 BINOP (0x8179800) add [1]
942 6 UNOP (0x81796e0) null [15]
943 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
944 8 UNOP (0x81797e0) null [15]
945 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
946 10 UNOP (0x816b4f0) null [15]
947 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
949 Let's start in the middle, at line 4. This is a BINOP, a binary
950 operator, which is at location C<0x8179828>. The specific operator in
951 question is C<sassign> - scalar assignment - and you can find the code
952 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
953 binary operator, it has two children: the add operator, providing the
954 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
957 Line 10 is the null op: this does exactly nothing. What is that doing
958 there? If you see the null op, it's a sign that something has been
959 optimized away after parsing. As we mentioned in L</Optimization>,
960 the optimization stage sometimes converts two operations into one, for
961 example when fetching a scalar variable. When this happens, instead of
962 rewriting the op tree and cleaning up the dangling pointers, it's easier
963 just to replace the redundant operation with the null op. Originally,
964 the tree would have looked like this:
966 10 SVOP (0x816b4f0) rv2sv [15]
967 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
969 That is, fetch the C<a> entry from the main symbol table, and then look
970 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
971 happens to do both these things.
973 The right hand side, starting at line 5 is similar to what we've just
974 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
977 Now, what's this about?
979 1 LISTOP (0x8179888) leave
980 2 OP (0x81798b0) enter
981 3 COP (0x8179850) nextstate
983 C<enter> and C<leave> are scoping ops, and their job is to perform any
984 housekeeping every time you enter and leave a block: lexical variables
985 are tidied up, unreferenced variables are destroyed, and so on. Every
986 program will have those first three lines: C<leave> is a list, and its
987 children are all the statements in the block. Statements are delimited
988 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
989 the ops to be performed for each statement being the children of
990 C<nextstate>. C<enter> is a single op which functions as a marker.
992 That's how Perl parsed the program, from top to bottom:
1005 However, it's impossible to B<perform> the operations in this order:
1006 you have to find the values of C<$b> and C<$c> before you add them
1007 together, for instance. So, the other thread that runs through the op
1008 tree is the execution order: each op has a field C<op_next> which points
1009 to the next op to be run, so following these pointers tells us how perl
1010 executes the code. We can traverse the tree in this order using
1011 the C<exec> option to C<B::Terse>:
1013 % perl -MO=Terse,exec -e '$a=$b+$c'
1014 1 OP (0x8179928) enter
1015 2 COP (0x81798c8) nextstate
1016 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
1017 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
1018 5 BINOP (0x8179878) add [1]
1019 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
1020 7 BINOP (0x81798a0) sassign
1021 8 LISTOP (0x8179900) leave
1023 This probably makes more sense for a human: enter a block, start a
1024 statement. Get the values of C<$b> and C<$c>, and add them together.
1025 Find C<$a>, and assign one to the other. Then leave.
1027 The way Perl builds up these op trees in the parsing process can be
1028 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1029 piece we need to construct the tree for C<$a = $b + $c>
1031 1 term : term ASSIGNOP term
1032 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1034 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1036 If you're not used to reading BNF grammars, this is how it works: You're
1037 fed certain things by the tokeniser, which generally end up in upper
1038 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1039 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1040 `terminal symbols', because you can't get any simpler than them.
1042 The grammar, lines one and three of the snippet above, tells you how to
1043 build up more complex forms. These complex forms, `non-terminal symbols'
1044 are generally placed in lower case. C<term> here is a non-terminal
1045 symbol, representing a single expression.
1047 The grammar gives you the following rule: you can make the thing on the
1048 left of the colon if you see all the things on the right in sequence.
1049 This is called a "reduction", and the aim of parsing is to completely
1050 reduce the input. There are several different ways you can perform a
1051 reduction, separated by vertical bars: so, C<term> followed by C<=>
1052 followed by C<term> makes a C<term>, and C<term> followed by C<+>
1053 followed by C<term> can also make a C<term>.
1055 So, if you see two terms with an C<=> or C<+>, between them, you can
1056 turn them into a single expression. When you do this, you execute the
1057 code in the block on the next line: if you see C<=>, you'll do the code
1058 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1059 which contributes to the op tree.
1062 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1064 What this does is creates a new binary op, and feeds it a number of
1065 variables. The variables refer to the tokens: C<$1> is the first token in
1066 the input, C<$2> the second, and so on - think regular expression
1067 backreferences. C<$$> is the op returned from this reduction. So, we
1068 call C<newBINOP> to create a new binary operator. The first parameter to
1069 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1070 operator, so we want the type to be C<ADDOP>. We could specify this
1071 directly, but it's right there as the second token in the input, so we
1072 use C<$2>. The second parameter is the op's flags: 0 means `nothing
1073 special'. Then the things to add: the left and right hand side of our
1074 expression, in scalar context.
1078 When perl executes something like C<addop>, how does it pass on its
1079 results to the next op? The answer is, through the use of stacks. Perl
1080 has a number of stacks to store things it's currently working on, and
1081 we'll look at the three most important ones here.
1085 =item Argument stack
1087 Arguments are passed to PP code and returned from PP code using the
1088 argument stack, C<ST>. The typical way to handle arguments is to pop
1089 them off the stack, deal with them how you wish, and then push the result
1090 back onto the stack. This is how, for instance, the cosine operator
1095 value = Perl_cos(value);
1098 We'll see a more tricky example of this when we consider Perl's macros
1099 below. C<POPn> gives you the NV (floating point value) of the top SV on
1100 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1101 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1102 should be extended if necessary - it can't be necessary here, because we
1103 know there's room for one more item on the stack, since we've just
1104 removed one! The C<XPUSH*> macros at least guarantee safety.
1106 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1107 the first element in your portion of the stack, and C<TOP*> gives you
1108 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1109 negation of an integer:
1113 Just set the integer value of the top stack entry to its negation.
1115 Argument stack manipulation in the core is exactly the same as it is in
1116 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1117 description of the macros used in stack manipulation.
1121 I say `your portion of the stack' above because PP code doesn't
1122 necessarily get the whole stack to itself: if your function calls
1123 another function, you'll only want to expose the arguments aimed for the
1124 called function, and not (necessarily) let it get at your own data. The
1125 way we do this is to have a `virtual' bottom-of-stack, exposed to each
1126 function. The mark stack keeps bookmarks to locations in the argument
1127 stack usable by each function. For instance, when dealing with a tied
1128 variable, (internally, something with `P' magic) Perl has to call
1129 methods for accesses to the tied variables. However, we need to separate
1130 the arguments exposed to the method to the argument exposed to the
1131 original function - the store or fetch or whatever it may be. Here's how
1132 the tied C<push> is implemented; see C<av_push> in F<av.c>:
1136 3 PUSHs(SvTIED_obj((SV*)av, mg));
1140 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1144 The lines which concern the mark stack are the first, fifth and last
1145 lines: they save away, restore and remove the current position of the
1148 Let's examine the whole implementation, for practice:
1152 Push the current state of the stack pointer onto the mark stack. This is
1153 so that when we've finished adding items to the argument stack, Perl
1154 knows how many things we've added recently.
1157 3 PUSHs(SvTIED_obj((SV*)av, mg));
1160 We're going to add two more items onto the argument stack: when you have
1161 a tied array, the C<PUSH> subroutine receives the object and the value
1162 to be pushed, and that's exactly what we have here - the tied object,
1163 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1167 Next we tell Perl to make the change to the global stack pointer: C<dSP>
1168 only gave us a local copy, not a reference to the global.
1171 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1174 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1175 variables are tidied up, everything that has been localised gets
1176 its previous value returned, and so on. Think of them as the C<{> and
1177 C<}> of a Perl block.
1179 To actually do the magic method call, we have to call a subroutine in
1180 Perl space: C<call_method> takes care of that, and it's described in
1181 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1182 going to discard its return value.
1186 Finally, we remove the value we placed on the mark stack, since we
1187 don't need it any more.
1191 C doesn't have a concept of local scope, so perl provides one. We've
1192 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1193 stack implements the C equivalent of, for example:
1200 See L<perlguts/Localising Changes> for how to use the save stack.
1204 =head2 Millions of Macros
1206 One thing you'll notice about the Perl source is that it's full of
1207 macros. Some have called the pervasive use of macros the hardest thing
1208 to understand, others find it adds to clarity. Let's take an example,
1209 the code which implements the addition operator:
1213 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1216 6 SETn( left + right );
1221 Every line here (apart from the braces, of course) contains a macro. The
1222 first line sets up the function declaration as Perl expects for PP code;
1223 line 3 sets up variable declarations for the argument stack and the
1224 target, the return value of the operation. Finally, it tries to see if
1225 the addition operation is overloaded; if so, the appropriate subroutine
1228 Line 5 is another variable declaration - all variable declarations start
1229 with C<d> - which pops from the top of the argument stack two NVs (hence
1230 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1231 C<rl>. These are the two operands to the addition operator. Next, we
1232 call C<SETn> to set the NV of the return value to the result of adding
1233 the two values. This done, we return - the C<RETURN> macro makes sure
1234 that our return value is properly handled, and we pass the next operator
1235 to run back to the main run loop.
1237 Most of these macros are explained in L<perlapi>, and some of the more
1238 important ones are explained in L<perlxs> as well. Pay special attention
1239 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1240 the C<[pad]THX_?> macros.
1243 =head2 Poking at Perl
1245 To really poke around with Perl, you'll probably want to build Perl for
1246 debugging, like this:
1248 ./Configure -d -D optimize=-g
1251 C<-g> is a flag to the C compiler to have it produce debugging
1252 information which will allow us to step through a running program.
1253 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1254 enables all the internal debugging code in Perl. There are a whole bunch
1255 of things you can debug with this: L<perlrun> lists them all, and the
1256 best way to find out about them is to play about with them. The most
1257 useful options are probably
1259 l Context (loop) stack processing
1261 o Method and overloading resolution
1262 c String/numeric conversions
1264 Some of the functionality of the debugging code can be achieved using XS
1267 -Dr => use re 'debug'
1268 -Dx => use O 'Debug'
1270 =head2 Using a source-level debugger
1272 If the debugging output of C<-D> doesn't help you, it's time to step
1273 through perl's execution with a source-level debugger.
1279 We'll use C<gdb> for our examples here; the principles will apply to any
1280 debugger, but check the manual of the one you're using.
1284 To fire up the debugger, type
1288 You'll want to do that in your Perl source tree so the debugger can read
1289 the source code. You should see the copyright message, followed by the
1294 C<help> will get you into the documentation, but here are the most
1301 Run the program with the given arguments.
1303 =item break function_name
1305 =item break source.c:xxx
1307 Tells the debugger that we'll want to pause execution when we reach
1308 either the named function (but see L<perlguts/Internal Functions>!) or the given
1309 line in the named source file.
1313 Steps through the program a line at a time.
1317 Steps through the program a line at a time, without descending into
1322 Run until the next breakpoint.
1326 Run until the end of the current function, then stop again.
1330 Just pressing Enter will do the most recent operation again - it's a
1331 blessing when stepping through miles of source code.
1335 Execute the given C code and print its results. B<WARNING>: Perl makes
1336 heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1337 substitute them yourself. So, for instance, you can't say
1339 print SvPV_nolen(sv)
1343 print Perl_sv_2pv_nolen(sv)
1345 You may find it helpful to have a "macro dictionary", which you can
1346 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1347 recursively apply the macros for you.
1351 =head2 Dumping Perl Data Structures
1353 One way to get around this macro hell is to use the dumping functions in
1354 F<dump.c>; these work a little like an internal
1355 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1356 that you can't get at from Perl. Let's take an example. We'll use the
1357 C<$a = $b + $c> we used before, but give it a bit of context:
1358 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1360 What about C<pp_add>, the function we examined earlier to implement the
1363 (gdb) break Perl_pp_add
1364 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1366 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1367 With the breakpoint in place, we can run our program:
1369 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1371 Lots of junk will go past as gdb reads in the relevant source files and
1372 libraries, and then:
1374 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1375 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1380 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1381 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1384 #define dPOPTOPnnrl_ul NV right = POPn; \
1385 SV *leftsv = TOPs; \
1386 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1388 C<POPn> takes the SV from the top of the stack and obtains its NV either
1389 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1390 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1391 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1392 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1394 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1395 convert it. If we step again, we'll find ourselves there:
1397 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1401 We can now use C<Perl_sv_dump> to investigate the SV:
1403 SV = PV(0xa057cc0) at 0xa0675d0
1406 PV = 0xa06a510 "6XXXX"\0
1411 We know we're going to get C<6> from this, so let's finish the
1415 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1416 0x462669 in Perl_pp_add () at pp_hot.c:311
1419 We can also dump out this op: the current op is always stored in
1420 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1421 similar output to L<B::Debug|B::Debug>.
1424 13 TYPE = add ===> 14
1426 FLAGS = (SCALAR,KIDS)
1428 TYPE = null ===> (12)
1430 FLAGS = (SCALAR,KIDS)
1432 11 TYPE = gvsv ===> 12
1438 # finish this later #
1442 All right, we've now had a look at how to navigate the Perl sources and
1443 some things you'll need to know when fiddling with them. Let's now get
1444 on and create a simple patch. Here's something Larry suggested: if a
1445 C<U> is the first active format during a C<pack>, (for example,
1446 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1449 How do we prepare to fix this up? First we locate the code in question -
1450 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1451 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1452 altering this file, let's copy it to F<pp.c~>.
1454 [Well, it was in F<pp.c> when this tutorial was written. It has now been
1455 split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1457 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1458 loop over the pattern, taking each format character in turn into
1459 C<datum_type>. Then for each possible format character, we swallow up
1460 the other arguments in the pattern (a field width, an asterisk, and so
1461 on) and convert the next chunk input into the specified format, adding
1462 it onto the output SV C<cat>.
1464 How do we know if the C<U> is the first format in the C<pat>? Well, if
1465 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1466 test whether we're still at the start of the string. So, here's where
1470 register char *pat = SvPVx(*++MARK, fromlen);
1471 register char *patend = pat + fromlen;
1476 We'll have another string pointer in there:
1479 register char *pat = SvPVx(*++MARK, fromlen);
1480 register char *patend = pat + fromlen;
1486 And just before we start the loop, we'll set C<patcopy> to be the start
1491 sv_setpvn(cat, "", 0);
1493 while (pat < patend) {
1495 Now if we see a C<U> which was at the start of the string, we turn on
1496 the UTF8 flag for the output SV, C<cat>:
1498 + if (datumtype == 'U' && pat==patcopy+1)
1500 if (datumtype == '#') {
1501 while (pat < patend && *pat != '\n')
1504 Remember that it has to be C<patcopy+1> because the first character of
1505 the string is the C<U> which has been swallowed into C<datumtype!>
1507 Oops, we forgot one thing: what if there are spaces at the start of the
1508 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1509 character, even though it's not the first thing in the pattern. In this
1510 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1512 if (isSPACE(datumtype))
1517 if (isSPACE(datumtype)) {
1522 OK. That's the C part done. Now we must do two additional things before
1523 this patch is ready to go: we've changed the behaviour of Perl, and so
1524 we must document that change. We must also provide some more regression
1525 tests to make sure our patch works and doesn't create a bug somewhere
1526 else along the line.
1528 The regression tests for each operator live in F<t/op/>, and so we
1529 make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1530 tests to the end. First, we'll test that the C<U> does indeed create
1533 t/op/pack.t has a sensible ok() function, but if it didn't we could
1538 my($ok, $name) = @_;
1540 # You have to do it this way or VMS will get confused.
1541 print $ok ? "ok $test - $name\n" : "not ok $test - $name\n";
1543 printf "# Failed test at line %d\n", (caller)[2] unless $ok;
1551 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1552 print "ok $test\n"; $test++;
1554 we can write the (somewhat) more sensible:
1556 ok( "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000),
1557 "U* produces unicode" );
1559 Now we'll test that we got that space-at-the-beginning business right:
1561 ok( "1.20.300.4000" eq sprintf "%vd", pack(" U*",1,20,300,4000),
1562 " with spaces at the beginning" );
1564 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1565 the first active format:
1567 ok( v1.20.300.4000 ne sprintf "%vd", pack("C0U*",1,20,300,4000),
1568 "U* not first isn't unicode" );
1570 Mustn't forget to change the number of tests which appears at the top, or
1571 else the automated tester will get confused:
1576 We now compile up Perl, and run it through the test suite. Our new
1579 Finally, the documentation. The job is never done until the paperwork is
1580 over, so let's describe the change we've just made. The relevant place
1581 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1582 this text in the description of C<pack>:
1586 If the pattern begins with a C<U>, the resulting string will be treated
1587 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1588 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1589 characters. If you don't want this to happen, you can begin your pattern
1590 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1591 string, and then follow this with a C<U*> somewhere in your pattern.
1593 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1594 that if we're making major changes, we should copy the entire directory
1595 to somewhere safe before we begin fiddling, and then do
1597 diff -ruN old new > patch
1599 However, we know which files we've changed, and we can simply do this:
1601 diff -u pp.c~ pp.c > patch
1602 diff -u t/op/pack.t~ t/op/pack.t >> patch
1603 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1605 We end up with a patch looking a little like this:
1607 --- pp.c~ Fri Jun 02 04:34:10 2000
1608 +++ pp.c Fri Jun 16 11:37:25 2000
1609 @@ -4375,6 +4375,7 @@
1612 register char *pat = SvPVx(*++MARK, fromlen);
1614 register char *patend = pat + fromlen;
1617 @@ -4405,6 +4406,7 @@
1620 And finally, we submit it, with our rationale, to perl5-porters. Job
1623 =head2 Patching a core module
1625 This works just like patching anything else, with an extra
1626 consideration. Many core modules also live on CPAN. If this is so,
1627 patch the CPAN version instead of the core and send the patch off to
1628 the module maintainer (with a copy to p5p). This will help the module
1629 maintainer keep the CPAN version in sync with the core version without
1630 constantly scanning p5p.
1632 =head2 Adding a new function to the core
1634 If, as part of a patch to fix a bug, or just because you have an
1635 especially good idea, you decide to add a new function to the core,
1636 discuss your ideas on p5p well before you start work. It may be that
1637 someone else has already attempted to do what you are considering and
1638 can give lots of good advice or even provide you with bits of code
1639 that they already started (but never finished).
1641 You have to follow all of the advice given above for patching. It is
1642 extremely important to test any addition thoroughly and add new tests
1643 to explore all boundary conditions that your new function is expected
1644 to handle. If your new function is used only by one module (e.g. toke),
1645 then it should probably be named S_your_function (for static); on the
1646 other hand, if you expect it to accessable from other functions in
1647 Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1650 The location of any new code is also an important consideration. Don't
1651 just create a new top level .c file and put your code there; you would
1652 have to make changes to Configure (so the Makefile is created properly),
1653 as well as possibly lots of include files. This is strictly pumpking
1656 It is better to add your function to one of the existing top level
1657 source code files, but your choice is complicated by the nature of
1658 the Perl distribution. Only the files that are marked as compiled
1659 static are located in the perl executable. Everything else is located
1660 in the shared library (or DLL if you are running under WIN32). So,
1661 for example, if a function was only used by functions located in
1662 toke.c, then your code can go in toke.c. If, however, you want to call
1663 the function from universal.c, then you should put your code in another
1664 location, for example util.c.
1666 In addition to writing your c-code, you will need to create an
1667 appropriate entry in embed.pl describing your function, then run
1668 'make regen_headers' to create the entries in the numerous header
1669 files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1670 for information on the various options that you can set in embed.pl.
1671 You will forget to do this a few (or many) times and you will get
1672 warnings during the compilation phase. Make sure that you mention
1673 this when you post your patch to P5P; the pumpking needs to know this.
1675 When you write your new code, please be conscious of existing code
1676 conventions used in the perl source files. See <perlstyle> for
1677 details. Although most of the guidelines discussed seem to focus on
1678 Perl code, rather than c, they all apply (except when they don't ;).
1679 See also I<Porting/patching.pod> file in the Perl source distribution
1680 for lots of details about both formatting and submitting patches of
1683 Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1684 Test on as many platforms as you can find. Test as many perl
1685 Configure options as you can (e.g. MULTIPLICITY). If you have
1686 profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1687 below for how to use them to futher test your code. Remember that
1688 most of the people on P5P are doing this on their own time and
1689 don't have the time to debug your code.
1691 =head2 Writing a test
1693 Every module and built-in function has an associated test file (or
1694 should...). If you add or change functionality, you have to write a
1695 test. If you fix a bug, you have to write a test so that bug never
1696 comes back. If you alter the docs, it would be nice to test what the
1697 new documentation says.
1699 In short, if you submit a patch you probably also have to patch the
1702 For modules, the test file is right next to the module itself.
1703 F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1704 so there are some snags (and it would be wonderful for you to brush
1705 them out), but it basically works that way. Everything else lives in
1712 Testing of the absolute basic functionality of Perl. Things like
1713 C<if>, basic file reads and writes, simple regexes, etc. These are
1714 run first in the test suite and if any of them fail, something is
1719 These test the basic control structures, C<if/else>, C<while>,
1724 Tests basic issues of how Perl parses and compiles itself.
1728 Tests for built-in IO functions, including command line arguments.
1732 The old home for the module tests, you shouldn't put anything new in
1733 here. There are still some bits and pieces hanging around in here
1734 that need to be moved. Perhaps you could move them? Thanks!
1738 Tests for perl's built in functions that don't fit into any of the
1743 Tests for POD directives. There are still some tests for the Pod
1744 modules hanging around in here that need to be moved out into F<lib/>.
1748 Testing features of how perl actually runs, including exit codes and
1749 handling of PERL* environment variables.
1753 The core uses the same testing style as the rest of Perl, a simple
1754 "ok/not ok" run through Test::Harness, but there are a few special
1757 For most libraries and extensions, you'll want to use the Test::More
1758 library rather than rolling your own test functions. If a module test
1759 doesn't use Test::More, consider rewriting it so it does. For the
1760 rest it's best to use a simple C<print "ok $test_num\n"> style to avoid
1761 broken core functionality from causing the whole test to collapse.
1763 When you say "make test" Perl uses the F<t/TEST> program to run the
1764 test suite. All tests are run from the F<t/> directory, B<not> the
1765 directory which contains the test. This causes some problems with the
1766 tests in F<lib/>, so here's some opportunity for some patching.
1768 You must be triply conscious of cross-platform concerns. This usually
1769 boils down to using File::Spec and avoiding things like C<fork()> and
1770 C<system()> unless absolutely necessary.
1773 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1775 Sometimes it helps to use external tools while debugging and
1776 testing Perl. This section tries to guide you through using
1777 some common testing and debugging tools with Perl. This is
1778 meant as a guide to interfacing these tools with Perl, not
1779 as any kind of guide to the use of the tools themselves.
1781 =head2 Rational Software's Purify
1783 Purify is a commercial tool that is helpful in identifying
1784 memory overruns, wild pointers, memory leaks and other such
1785 badness. Perl must be compiled in a specific way for
1786 optimal testing with Purify. Purify is available under
1787 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1789 The only currently known leaks happen when there are
1790 compile-time errors within eval or require. (Fixing these
1791 is non-trivial, unfortunately, but they must be fixed
1794 =head2 Purify on Unix
1796 On Unix, Purify creates a new Perl binary. To get the most
1797 benefit out of Purify, you should create the perl to Purify
1800 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1801 -Uusemymalloc -Dusemultiplicity
1803 where these arguments mean:
1807 =item -Accflags=-DPURIFY
1809 Disables Perl's arena memory allocation functions, as well as
1810 forcing use of memory allocation functions derived from the
1813 =item -Doptimize='-g'
1815 Adds debugging information so that you see the exact source
1816 statements where the problem occurs. Without this flag, all
1817 you will see is the source filename of where the error occurred.
1821 Disable Perl's malloc so that Purify can more closely monitor
1822 allocations and leaks. Using Perl's malloc will make Purify
1823 report most leaks in the "potential" leaks category.
1825 =item -Dusemultiplicity
1827 Enabling the multiplicity option allows perl to clean up
1828 thoroughly when the interpreter shuts down, which reduces the
1829 number of bogus leak reports from Purify.
1833 Once you've compiled a perl suitable for Purify'ing, then you
1838 which creates a binary named 'pureperl' that has been Purify'ed.
1839 This binary is used in place of the standard 'perl' binary
1840 when you want to debug Perl memory problems.
1842 As an example, to show any memory leaks produced during the
1843 standard Perl testset you would create and run the Purify'ed
1848 ../pureperl -I../lib harness
1850 which would run Perl on test.pl and report any memory problems.
1852 Purify outputs messages in "Viewer" windows by default. If
1853 you don't have a windowing environment or if you simply
1854 want the Purify output to unobtrusively go to a log file
1855 instead of to the interactive window, use these following
1856 options to output to the log file "perl.log":
1858 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1859 -log-file=perl.log -append-logfile=yes"
1861 If you plan to use the "Viewer" windows, then you only need this option:
1863 setenv PURIFYOPTIONS "-chain-length=25"
1867 Purify on Windows NT instruments the Perl binary 'perl.exe'
1868 on the fly. There are several options in the makefile you
1869 should change to get the most use out of Purify:
1875 You should add -DPURIFY to the DEFINES line so the DEFINES
1876 line looks something like:
1878 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1880 to disable Perl's arena memory allocation functions, as
1881 well as to force use of memory allocation functions derived
1882 from the system malloc.
1884 =item USE_MULTI = define
1886 Enabling the multiplicity option allows perl to clean up
1887 thoroughly when the interpreter shuts down, which reduces the
1888 number of bogus leak reports from Purify.
1890 =item #PERL_MALLOC = define
1892 Disable Perl's malloc so that Purify can more closely monitor
1893 allocations and leaks. Using Perl's malloc will make Purify
1894 report most leaks in the "potential" leaks category.
1898 Adds debugging information so that you see the exact source
1899 statements where the problem occurs. Without this flag, all
1900 you will see is the source filename of where the error occurred.
1904 As an example, to show any memory leaks produced during the
1905 standard Perl testset you would create and run Purify as:
1910 purify ../perl -I../lib harness
1912 which would instrument Perl in memory, run Perl on test.pl,
1913 then finally report any memory problems.
1915 =head2 Compaq's/Digital's Third Degree
1917 Third Degree is a tool for memory leak detection and memory access checks.
1918 It is one of the many tools in the ATOM toolkit. The toolkit is only
1919 available on Tru64 (formerly known as Digital UNIX formerly known as
1922 When building Perl, you must first run Configure with -Doptimize=-g
1923 and -Uusemymalloc flags, after that you can use the make targets
1924 "perl.third" and "test.third". (What is required is that Perl must be
1925 compiled using the C<-g> flag, you may need to re-Configure.)
1927 The short story is that with "atom" you can instrument the Perl
1928 executable to create a new executable called F<perl.third>. When the
1929 instrumented executable is run, it creates a log of dubious memory
1930 traffic in file called F<perl.3log>. See the manual pages of atom and
1931 third for more information. The most extensive Third Degree
1932 documentation is available in the Compaq "Tru64 UNIX Programmer's
1933 Guide", chapter "Debugging Programs with Third Degree".
1935 The "test.third" leaves a lot of files named F<perl.3log.*> in the t/
1936 subdirectory. There is a problem with these files: Third Degree is so
1937 effective that it finds problems also in the system libraries.
1938 Therefore there are certain types of errors that you should ignore in
1939 your debugging. Errors with stack traces matching
1941 __actual_atof|__catgets|_doprnt|__exc_|__exec|_findio|__localtime|setlocale|__sia_|__strxfrm
1943 (all in libc.so) are known to be non-serious. You can also
1944 ignore the combinations
1946 Perl_gv_fetchfile() calling strcpy()
1947 S_doopen_pmc() calling strcmp()
1949 causing "rih" (reading invalid heap) errors.
1951 There are also leaks that for given certain definition of a leak,
1952 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
1954 =head2 PERL_DESTRUCT_LEVEL
1956 If you want to run any of the tests yourself manually using the
1957 pureperl or perl.third executables, please note that by default
1958 perl B<does not> explicitly cleanup all the memory it has allocated
1959 (such as global memory arenas) but instead lets the exit() of
1960 the whole program "take care" of such allocations, also known
1961 as "global destruction of objects".
1963 There is a way to tell perl to do complete cleanup: set the
1964 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
1965 The t/TEST wrapper does set this to 2, and this is what you
1966 need to do too, if you don't want to see the "global leaks":
1968 PERL_DESTRUCT_LEVEL=2 ./perl.third t/foo/bar.t
1972 Depending on your platform there are various of profiling Perl.
1974 There are two commonly used techniques of profiling executables:
1975 I<statistical time-sampling> and I<basic-block counting>.
1977 The first method takes periodically samples of the CPU program
1978 counter, and since the program counter can be correlated with the code
1979 generated for functions, we get a statistical view of in which
1980 functions the program is spending its time. The caveats are that very
1981 small/fast functions have lower probability of showing up in the
1982 profile, and that periodically interrupting the program (this is
1983 usually done rather frequently, in the scale of milliseconds) imposes
1984 an additional overhead that may skew the results. The first problem
1985 can be alleviated by running the code for longer (in general this is a
1986 good idea for profiling), the second problem is usually kept in guard
1987 by the profiling tools themselves.
1989 The second method divides up the generated code into I<basic blocks>.
1990 Basic blocks are sections of code that are entered only in the
1991 beginning and exited only at the end. For example, a conditional jump
1992 starts a basic block. Basic block profiling usually works by
1993 I<instrumenting> the code by adding I<enter basic block #nnnn>
1994 book-keeping code to the generated code. During the execution of the
1995 code the basic block counters are then updated appropriately. The
1996 caveat is that the added extra code can skew the results: again, the
1997 profiling tools usually try to factor their own effects out of the
2000 =head2 Gprof Profiling
2002 gprof is a profiling tool available in many UNIX platforms,
2003 it uses F<statistical time-sampling>.
2005 You can build a profiled version of perl called "perl.gprof" by
2006 invoking the make target "perl.gprof" (What is required is that Perl
2007 must be compiled using the C<-pg> flag, you may need to re-Configure).
2008 Running the profiled version of Perl will create an output file called
2009 F<gmon.out> is created which contains the profiling data collected
2010 during the execution.
2012 The gprof tool can then display the collected data in various ways.
2013 Usually gprof understands the following options:
2019 Suppress statically defined functions from the profile.
2023 Suppress the verbose descriptions in the profile.
2027 Exclude the given routine and its descendants from the profile.
2031 Display only the given routine and its descendants in the profile.
2035 Generate a summary file called F<gmon.sum> which then may be given
2036 to subsequent gprof runs to accumulate data over several runs.
2040 Display routines that have zero usage.
2044 For more detailed explanation of the available commands and output
2045 formats, see your own local documentation of gprof.
2047 =head2 GCC gcov Profiling
2049 Starting from GCC 3.0 I<basic block profiling> is officially available
2052 You can build a profiled version of perl called F<perl.gcov> by
2053 invoking the make target "perl.gcov" (what is required that Perl must
2054 be compiled using gcc with the flags C<-fprofile-arcs
2055 -ftest-coverage>, you may need to re-Configure).
2057 Running the profiled version of Perl will cause profile output to be
2058 generated. For each source file an accompanying ".da" file will be
2061 To display the results you use the "gcov" utility (which should
2062 be installed if you have gcc 3.0 or newer installed). F<gcov> is
2063 run on source code files, like this
2067 which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2068 contain the source code annotated with relative frequencies of
2069 execution indicated by "#" markers.
2071 Useful options of F<gcov> include C<-b> which will summarise the
2072 basic block, branch, and function call coverage, and C<-c> which
2073 instead of relative frequencies will use the actual counts. For
2074 more information on the use of F<gcov> and basic block profiling
2075 with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2077 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2079 and its section titled "8. gcov: a Test Coverage Program"
2081 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2083 =head2 Pixie Profiling
2085 Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2086 UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
2087 I<basic-block counting>.
2089 You can build a profiled version of perl called F<perl.pixie> by
2090 invoking the make target "perl.pixie" (what is required is that Perl
2091 must be compiled using the C<-g> flag, you may need to re-Configure).
2093 In Tru64 a file called F<perl.Addrs> will also be silently created,
2094 this file contains the addresses of the basic blocks. Running the
2095 profiled version of Perl will create a new file called "perl.Counts"
2096 which contains the counts for the basic block for that particular
2099 To display the results you use the F<prof> utility. The exact
2100 incantation depends on your operating system, "prof perl.Counts" in
2101 IRIX, and "prof -pixie -all -L. perl" in Tru64.
2103 In IRIX the following prof options are available:
2109 Reports the most heavily used lines in descending order of use.
2110 Useful for finding the hotspot lines.
2114 Groups lines by procedure, with procedures sorted in descending order of use.
2115 Within a procedure, lines are listed in source order.
2116 Useful for finding the hotspots of procedures.
2120 In Tru64 the following options are available:
2126 Procedures sorted in descending order by the number of cycles executed
2127 in each procedure. Useful for finding the hotspot procedures.
2128 (This is the default option.)
2132 Lines sorted in descending order by the number of cycles executed in
2133 each line. Useful for finding the hotspot lines.
2135 =item -i[nvocations]
2137 The called procedures are sorted in descending order by number of calls
2138 made to the procedures. Useful for finding the most used procedures.
2142 Grouped by procedure, sorted by cycles executed per procedure.
2143 Useful for finding the hotspots of procedures.
2147 The compiler emitted code for these lines, but the code was unexecuted.
2151 Unexecuted procedures.
2155 For further information, see your system's manual pages for pixie and prof.
2159 We've had a brief look around the Perl source, an overview of the stages
2160 F<perl> goes through when it's running your code, and how to use a
2161 debugger to poke at the Perl guts. We took a very simple problem and
2162 demonstrated how to solve it fully - with documentation, regression
2163 tests, and finally a patch for submission to p5p. Finally, we talked
2164 about how to use external tools to debug and test Perl.
2166 I'd now suggest you read over those references again, and then, as soon
2167 as possible, get your hands dirty. The best way to learn is by doing,
2174 Subscribe to perl5-porters, follow the patches and try and understand
2175 them; don't be afraid to ask if there's a portion you're not clear on -
2176 who knows, you may unearth a bug in the patch...
2180 Keep up to date with the bleeding edge Perl distributions and get
2181 familiar with the changes. Try and get an idea of what areas people are
2182 working on and the changes they're making.
2186 Do read the README associated with your operating system, e.g. README.aix
2187 on the IBM AIX OS. Don't hesitate to supply patches to that README if
2188 you find anything missing or changed over a new OS release.
2192 Find an area of Perl that seems interesting to you, and see if you can
2193 work out how it works. Scan through the source, and step over it in the
2194 debugger. Play, poke, investigate, fiddle! You'll probably get to
2195 understand not just your chosen area but a much wider range of F<perl>'s
2196 activity as well, and probably sooner than you'd think.
2202 =item I<The Road goes ever on and on, down from the door where it began.>
2206 If you can do these things, you've started on the long road to Perl porting.
2207 Thanks for wanting to help make Perl better - and happy hacking!
2211 This document was written by Nathan Torkington, and is maintained by
2212 the perl5-porters mailing list.