3 perlhack - How to hack at the Perl internals
7 This document attempts to explain how Perl development takes place,
8 and ends with some suggestions for people wanting to become bona fide
11 The perl5-porters mailing list is where the Perl standard distribution
12 is maintained and developed. The list can get anywhere from 10 to 150
13 messages a day, depending on the heatedness of the debate. Most days
14 there are two or three patches, extensions, features, or bugs being
17 A searchable archive of the list is at either:
19 http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
23 http://archive.develooper.com/perl5-porters@perl.org/
25 List subscribers (the porters themselves) come in several flavours.
26 Some are quiet curious lurkers, who rarely pitch in and instead watch
27 the ongoing development to ensure they're forewarned of new changes or
28 features in Perl. Some are representatives of vendors, who are there
29 to make sure that Perl continues to compile and work on their
30 platforms. Some patch any reported bug that they know how to fix,
31 some are actively patching their pet area (threads, Win32, the regexp
32 engine), while others seem to do nothing but complain. In other
33 words, it's your usual mix of technical people.
35 Over this group of porters presides Larry Wall. He has the final word
36 in what does and does not change in the Perl language. Various
37 releases of Perl are shepherded by a "pumpking", a porter
38 responsible for gathering patches, deciding on a patch-by-patch,
39 feature-by-feature basis what will and will not go into the release.
40 For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of
41 Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and
42 Rafael Garcia-Suarez holds the pumpking crown for the 5.10 release.
44 In addition, various people are pumpkings for different things. For
45 instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the
46 I<Configure> pumpkin up till the 5.8 release. For the 5.10 release
47 H.Merijn Brand took over.
49 Larry sees Perl development along the lines of the US government:
50 there's the Legislature (the porters), the Executive branch (the
51 pumpkings), and the Supreme Court (Larry). The legislature can
52 discuss and submit patches to the executive branch all they like, but
53 the executive branch is free to veto them. Rarely, the Supreme Court
54 will side with the executive branch over the legislature, or the
55 legislature over the executive branch. Mostly, however, the
56 legislature and the executive branch are supposed to get along and
57 work out their differences without impeachment or court cases.
59 You might sometimes see reference to Rule 1 and Rule 2. Larry's power
60 as Supreme Court is expressed in The Rules:
66 Larry is always by definition right about how Perl should behave.
67 This means he has final veto power on the core functionality.
71 Larry is allowed to change his mind about any matter at a later date,
72 regardless of whether he previously invoked Rule 1.
76 Got that? Larry is always right, even when he was wrong. It's rare
77 to see either Rule exercised, but they are often alluded to.
79 New features and extensions to the language are contentious, because
80 the criteria used by the pumpkings, Larry, and other porters to decide
81 which features should be implemented and incorporated are not codified
82 in a few small design goals as with some other languages. Instead,
83 the heuristics are flexible and often difficult to fathom. Here is
84 one person's list, roughly in decreasing order of importance, of
85 heuristics that new features have to be weighed against:
89 =item Does concept match the general goals of Perl?
91 These haven't been written anywhere in stone, but one approximation
94 1. Keep it fast, simple, and useful.
95 2. Keep features/concepts as orthogonal as possible.
96 3. No arbitrary limits (platforms, data sizes, cultures).
97 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
98 5. Either assimilate new technologies, or build bridges to them.
100 =item Where is the implementation?
102 All the talk in the world is useless without an implementation. In
103 almost every case, the person or people who argue for a new feature
104 will be expected to be the ones who implement it. Porters capable
105 of coding new features have their own agendas, and are not available
106 to implement your (possibly good) idea.
108 =item Backwards compatibility
110 It's a cardinal sin to break existing Perl programs. New warnings are
111 contentious--some say that a program that emits warnings is not
112 broken, while others say it is. Adding keywords has the potential to
113 break programs, changing the meaning of existing token sequences or
114 functions might break programs.
116 =item Could it be a module instead?
118 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
119 the need to keep changing the Perl interpreter. You can write modules
120 that export functions, you can give those functions prototypes so they
121 can be called like built-in functions, you can even write XS code to
122 mess with the runtime data structures of the Perl interpreter if you
123 want to implement really complicated things. If it can be done in a
124 module instead of in the core, it's highly unlikely to be added.
126 =item Is the feature generic enough?
128 Is this something that only the submitter wants added to the language,
129 or would it be broadly useful? Sometimes, instead of adding a feature
130 with a tight focus, the porters might decide to wait until someone
131 implements the more generalized feature. For instance, instead of
132 implementing a "delayed evaluation" feature, the porters are waiting
133 for a macro system that would permit delayed evaluation and much more.
135 =item Does it potentially introduce new bugs?
137 Radical rewrites of large chunks of the Perl interpreter have the
138 potential to introduce new bugs. The smaller and more localized the
141 =item Does it preclude other desirable features?
143 A patch is likely to be rejected if it closes off future avenues of
144 development. For instance, a patch that placed a true and final
145 interpretation on prototypes is likely to be rejected because there
146 are still options for the future of prototypes that haven't been
149 =item Is the implementation robust?
151 Good patches (tight code, complete, correct) stand more chance of
152 going in. Sloppy or incorrect patches might be placed on the back
153 burner until the pumpking has time to fix, or might be discarded
154 altogether without further notice.
156 =item Is the implementation generic enough to be portable?
158 The worst patches make use of a system-specific features. It's highly
159 unlikely that nonportable additions to the Perl language will be
162 =item Is the implementation tested?
164 Patches which change behaviour (fixing bugs or introducing new features)
165 must include regression tests to verify that everything works as expected.
166 Without tests provided by the original author, how can anyone else changing
167 perl in the future be sure that they haven't unwittingly broken the behaviour
168 the patch implements? And without tests, how can the patch's author be
169 confident that his/her hard work put into the patch won't be accidentally
170 thrown away by someone in the future?
172 =item Is there enough documentation?
174 Patches without documentation are probably ill-thought out or
175 incomplete. Nothing can be added without documentation, so submitting
176 a patch for the appropriate manpages as well as the source code is
179 =item Is there another way to do it?
181 Larry said "Although the Perl Slogan is I<There's More Than One Way
182 to Do It>, I hesitate to make 10 ways to do something". This is a
183 tricky heuristic to navigate, though--one man's essential addition is
184 another man's pointless cruft.
186 =item Does it create too much work?
188 Work for the pumpking, work for Perl programmers, work for module
189 authors, ... Perl is supposed to be easy.
191 =item Patches speak louder than words
193 Working code is always preferred to pie-in-the-sky ideas. A patch to
194 add a feature stands a much higher chance of making it to the language
195 than does a random feature request, no matter how fervently argued the
196 request might be. This ties into "Will it be useful?", as the fact
197 that someone took the time to make the patch demonstrates a strong
198 desire for the feature.
202 If you're on the list, you might hear the word "core" bandied
203 around. It refers to the standard distribution. "Hacking on the
204 core" means you're changing the C source code to the Perl
205 interpreter. "A core module" is one that ships with Perl.
207 =head2 Keeping in sync
209 The source code to the Perl interpreter, in its different versions, is
210 kept in a repository managed by a revision control system ( which is
211 currently the Perforce program, see http://perforce.com/ ). The
212 pumpkings and a few others have access to the repository to check in
213 changes. Periodically the pumpking for the development version of Perl
214 will release a new version, so the rest of the porters can see what's
215 changed. The current state of the main trunk of repository, and patches
216 that describe the individual changes that have happened since the last
217 public release are available at this location:
219 http://public.activestate.com/pub/apc/
220 ftp://public.activestate.com/pub/apc/
222 If you're looking for a particular change, or a change that affected
223 a particular set of files, you may find the B<Perl Repository Browser>
226 http://public.activestate.com/cgi-bin/perlbrowse
228 You may also want to subscribe to the perl5-changes mailing list to
229 receive a copy of each patch that gets submitted to the maintenance
230 and development "branches" of the perl repository. See
231 http://lists.perl.org/ for subscription information.
233 If you are a member of the perl5-porters mailing list, it is a good
234 thing to keep in touch with the most recent changes. If not only to
235 verify if what you would have posted as a bug report isn't already
236 solved in the most recent available perl development branch, also
237 known as perl-current, bleading edge perl, bleedperl or bleadperl.
239 Needless to say, the source code in perl-current is usually in a perpetual
240 state of evolution. You should expect it to be very buggy. Do B<not> use
241 it for any purpose other than testing and development.
243 Keeping in sync with the most recent branch can be done in several ways,
244 but the most convenient and reliable way is using B<rsync>, available at
245 ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
248 If you choose to keep in sync using rsync, there are two approaches
253 =item rsync'ing the source tree
255 Presuming you are in the directory where your perl source resides
256 and you have rsync installed and available, you can "upgrade" to
259 # rsync -avz rsync://public.activestate.com/perl-current/ .
261 This takes care of updating every single item in the source tree to
262 the latest applied patch level, creating files that are new (to your
263 distribution) and setting date/time stamps of existing files to
264 reflect the bleadperl status.
266 Note that this will not delete any files that were in '.' before
267 the rsync. Once you are sure that the rsync is running correctly,
268 run it with the --delete and the --dry-run options like this:
270 # rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .
272 This will I<simulate> an rsync run that also deletes files not
273 present in the bleadperl master copy. Observe the results from
274 this run closely. If you are sure that the actual run would delete
275 no files precious to you, you could remove the '--dry-run' option.
277 You can than check what patch was the latest that was applied by
278 looking in the file B<.patch>, which will show the number of the
281 If you have more than one machine to keep in sync, and not all of
282 them have access to the WAN (so you are not able to rsync all the
283 source trees to the real source), there are some ways to get around
288 =item Using rsync over the LAN
290 Set up a local rsync server which makes the rsynced source tree
291 available to the LAN and sync the other machines against this
294 From http://rsync.samba.org/README.html :
296 "Rsync uses rsh or ssh for communication. It does not need to be
297 setuid and requires no special privileges for installation. It
298 does not require an inetd entry or a daemon. You must, however,
299 have a working rsh or ssh system. Using ssh is recommended for
300 its security features."
302 =item Using pushing over the NFS
304 Having the other systems mounted over the NFS, you can take an
305 active pushing approach by checking the just updated tree against
306 the other not-yet synced trees. An example would be
315 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
318 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
320 foreach my $host (keys %remote) {
321 unless (-d $remote{$host}) {
322 print STDERR "Cannot Xsync for host $host\n";
325 foreach my $file (keys %MF) {
326 my $rfile = "$remote{$host}/$file";
327 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
328 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
329 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
330 printf "%4s %-34s %8d %9d %8d %9d\n",
331 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
333 copy ($file, $rfile);
334 utime time, $MF{$file}[2], $rfile;
335 chmod $MF{$file}[0], $rfile;
339 though this is not perfect. It could be improved with checking
340 file checksums before updating. Not all NFS systems support
341 reliable utime support (when used over the NFS).
345 =item rsync'ing the patches
347 The source tree is maintained by the pumpking who applies patches to
348 the files in the tree. These patches are either created by the
349 pumpking himself using C<diff -c> after updating the file manually or
350 by applying patches sent in by posters on the perl5-porters list.
351 These patches are also saved and rsync'able, so you can apply them
352 yourself to the source files.
354 Presuming you are in a directory where your patches reside, you can
355 get them in sync with
357 # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
359 This makes sure the latest available patch is downloaded to your
362 It's then up to you to apply these patches, using something like
364 # last=`ls -t *.gz | sed q`
365 # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
366 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
368 # patch -p1 -N <../perl-current-diffs/blead.patch
370 or, since this is only a hint towards how it works, use CPAN-patchaperl
371 from Andreas König to have better control over the patching process.
375 =head2 Why rsync the source tree
379 =item It's easier to rsync the source tree
381 Since you don't have to apply the patches yourself, you are sure all
382 files in the source tree are in the right state.
384 =item It's more reliable
386 While both the rsync-able source and patch areas are automatically
387 updated every few minutes, keep in mind that applying patches may
388 sometimes mean careful hand-holding, especially if your version of
389 the C<patch> program does not understand how to deal with new files,
390 files with 8-bit characters, or files without trailing newlines.
394 =head2 Why rsync the patches
398 =item It's easier to rsync the patches
400 If you have more than one machine that you want to keep in track with
401 bleadperl, it's easier to rsync the patches only once and then apply
402 them to all the source trees on the different machines.
404 In case you try to keep in pace on 5 different machines, for which
405 only one of them has access to the WAN, rsync'ing all the source
406 trees should than be done 5 times over the NFS. Having
407 rsync'ed the patches only once, I can apply them to all the source
408 trees automatically. Need you say more ;-)
410 =item It's a good reference
412 If you do not only like to have the most recent development branch,
413 but also like to B<fix> bugs, or extend features, you want to dive
414 into the sources. If you are a seasoned perl core diver, you don't
415 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
416 your way around. But if you are a starter, the patches may help you
417 in finding where you should start and how to change the bits that
420 The file B<Changes> is updated on occasions the pumpking sees as his
421 own little sync points. On those occasions, he releases a tar-ball of
422 the current source tree (i.e. perl@7582.tar.gz), which will be an
423 excellent point to start with when choosing to use the 'rsync the
424 patches' scheme. Starting with perl@7582, which means a set of source
425 files on which the latest applied patch is number 7582, you apply all
426 succeeding patches available from then on (7583, 7584, ...).
428 You can use the patches later as a kind of search archive.
432 =item Finding a start point
434 If you want to fix/change the behaviour of function/feature Foo, just
435 scan the patches for patches that mention Foo either in the subject,
436 the comments, or the body of the fix. A good chance the patch shows
437 you the files that are affected by that patch which are very likely
438 to be the starting point of your journey into the guts of perl.
440 =item Finding how to fix a bug
442 If you've found I<where> the function/feature Foo misbehaves, but you
443 don't know how to fix it (but you do know the change you want to
444 make), you can, again, peruse the patches for similar changes and
445 look how others apply the fix.
447 =item Finding the source of misbehaviour
449 When you keep in sync with bleadperl, the pumpking would love to
450 I<see> that the community efforts really work. So after each of his
451 sync points, you are to 'make test' to check if everything is still
452 in working order. If it is, you do 'make ok', which will send an OK
453 report to perlbug@perl.org. (If you do not have access to a mailer
454 from the system you just finished successfully 'make test', you can
455 do 'make okfile', which creates the file C<perl.ok>, which you can
456 than take to your favourite mailer and mail yourself).
458 But of course, as always, things will not always lead to a success
459 path, and one or more test do not pass the 'make test'. Before
460 sending in a bug report (using 'make nok' or 'make nokfile'), check
461 the mailing list if someone else has reported the bug already and if
462 so, confirm it by replying to that message. If not, you might want to
463 trace the source of that misbehaviour B<before> sending in the bug,
464 which will help all the other porters in finding the solution.
466 Here the saved patches come in very handy. You can check the list of
467 patches to see which patch changed what file and what change caused
468 the misbehaviour. If you note that in the bug report, it saves the
469 one trying to solve it, looking for that point.
473 If searching the patches is too bothersome, you might consider using
474 perl's bugtron to find more information about discussions and
475 ramblings on posted bugs.
477 If you want to get the best of both worlds, rsync both the source
478 tree for convenience, reliability and ease and rsync the patches
483 =head2 Working with the source
485 Because you cannot use the Perforce client, you cannot easily generate
486 diffs against the repository, nor will merges occur when you update
487 via rsync. If you edit a file locally and then rsync against the
488 latest source, changes made in the remote copy will I<overwrite> your
491 The best way to deal with this is to maintain a tree of symlinks to
492 the rsync'd source. Then, when you want to edit a file, you remove
493 the symlink, copy the real file into the other tree, and edit it. You
494 can then diff your edited file against the original to generate a
495 patch, and you can safely update the original tree.
497 Perl's F<Configure> script can generate this tree of symlinks for you.
498 The following example assumes that you have used rsync to pull a copy
499 of the Perl source into the F<perl-rsync> directory. In the directory
500 above that one, you can execute the following commands:
504 ../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g"
506 This will start the Perl configuration process. After a few prompts,
507 you should see something like this:
509 Symbolic links are supported.
511 Checking how to test for symbolic links...
512 Your builtin 'test -h' may be broken.
513 Trying external '/usr/bin/test -h'.
514 You can test for symbolic links with '/usr/bin/test -h'.
516 Creating the symbolic links...
517 (First creating the subdirectories...)
518 (Then creating the symlinks...)
520 The specifics may vary based on your operating system, of course.
521 After you see this, you can abort the F<Configure> script, and you
522 will see that the directory you are in has a tree of symlinks to the
523 F<perl-rsync> directories and files.
525 If you plan to do a lot of work with the Perl source, here are some
526 Bourne shell script functions that can make your life easier:
539 if [ -L $1.orig ]; then
545 Replace "vi" with your favorite flavor of editor.
547 Here is another function which will quickly generate a patch for the
548 files which have been edited in your symlink tree:
552 for f in `find . -name '*.orig' | sed s,^\./,,`
554 case `echo $f | sed 's,.orig$,,;s,.*\.,,'` in
556 pod) diffopts='-F^=' ;;
559 diff -du $diffopts $f `echo $f | sed 's,.orig$,,'`
563 This function produces patches which include enough context to make
564 your changes obvious. This makes it easier for the Perl pumpking(s)
565 to review them when you send them to the perl5-porters list, and that
566 means they're more likely to get applied.
568 This function assumed a GNU diff, and may require some tweaking for
571 =head2 Perlbug administration
573 There is a single remote administrative interface for modifying bug status,
574 category, open issues etc. using the B<RT> I<bugtracker> system, maintained
575 by I<Robert Spier>. Become an administrator, and close any bugs you can get
576 your sticky mitts on:
580 The bugtracker mechanism for B<perl5> bugs in particular is at:
582 http://bugs6.perl.org/perlbug
584 To email the bug system administrators:
586 "perlbug-admin" <perlbug-admin@perl.org>
589 =head2 Submitting patches
591 Always submit patches to I<perl5-porters@perl.org>. If you're
592 patching a core module and there's an author listed, send the author a
593 copy (see L<Patching a core module>). This lets other porters review
594 your patch, which catches a surprising number of errors in patches.
595 Either use the diff program (available in source code form from
596 ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' I<makepatch>
597 (available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
598 but context diffs are accepted. Do not send RCS-style diffs or diffs
599 without context lines. More information is given in the
600 I<Porting/patching.pod> file in the Perl source distribution. Please
601 patch against the latest B<development> version (e.g., if you're
602 fixing a bug in the 5.005 track, patch against the latest 5.005_5x
603 version). Only patches that survive the heat of the development
604 branch get applied to maintenance versions.
606 Your patch should update the documentation and test suite. See
609 To report a bug in Perl, use the program I<perlbug> which comes with
610 Perl (if you can't get Perl to work, send mail to the address
611 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
612 I<perlbug> feeds into the automated bug-tracking system, access to
613 which is provided through the web at http://bugs.perl.org/ . It
614 often pays to check the archives of the perl5-porters mailing list to
615 see whether the bug you're reporting has been reported before, and if
616 so whether it was considered a bug. See above for the location of
617 the searchable archives.
619 The CPAN testers ( http://testers.cpan.org/ ) are a group of
620 volunteers who test CPAN modules on a variety of platforms. Perl
621 Smokers ( http://archives.develooper.com/daily-build@perl.org/ )
622 automatically tests Perl source releases on platforms with various
623 configurations. Both efforts welcome volunteers.
625 It's a good idea to read and lurk for a while before chipping in.
626 That way you'll get to see the dynamic of the conversations, learn the
627 personalities of the players, and hopefully be better prepared to make
628 a useful contribution when do you speak up.
630 If after all this you still think you want to join the perl5-porters
631 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
632 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
634 To hack on the Perl guts, you'll need to read the following things:
640 This is of paramount importance, since it's the documentation of what
641 goes where in the Perl source. Read it over a couple of times and it
642 might start to make sense - don't worry if it doesn't yet, because the
643 best way to study it is to read it in conjunction with poking at Perl
644 source, and we'll do that later on.
646 You might also want to look at Gisle Aas's illustrated perlguts -
647 there's no guarantee that this will be absolutely up-to-date with the
648 latest documentation in the Perl core, but the fundamentals will be
649 right. ( http://gisle.aas.no/perl/illguts/ )
651 =item L<perlxstut> and L<perlxs>
653 A working knowledge of XSUB programming is incredibly useful for core
654 hacking; XSUBs use techniques drawn from the PP code, the portion of the
655 guts that actually executes a Perl program. It's a lot gentler to learn
656 those techniques from simple examples and explanation than from the core
661 The documentation for the Perl API explains what some of the internal
662 functions do, as well as the many macros used in the source.
664 =item F<Porting/pumpkin.pod>
666 This is a collection of words of wisdom for a Perl porter; some of it is
667 only useful to the pumpkin holder, but most of it applies to anyone
668 wanting to go about Perl development.
670 =item The perl5-porters FAQ
672 This should be available from http://simon-cozens.org/writings/p5p-faq ;
673 alternatively, you can get the FAQ emailed to you by sending mail to
674 C<perl5-porters-faq@perl.org>. It contains hints on reading perl5-porters,
675 information on how perl5-porters works and how Perl development in general
680 =head2 Finding Your Way Around
682 Perl maintenance can be split into a number of areas, and certain people
683 (pumpkins) will have responsibility for each area. These areas sometimes
684 correspond to files or directories in the source kit. Among the areas are:
690 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
691 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
692 contains the core XS modules.
696 There are tests for nearly all the modules, built-ins and major bits
697 of functionality. Test files all have a .t suffix. Module tests live
698 in the F<lib/> and F<ext/> directories next to the module being
699 tested. Others live in F<t/>. See L<Writing a test>
703 Documentation maintenance includes looking after everything in the
704 F<pod/> directory, (as well as contributing new documentation) and
705 the documentation to the modules in core.
709 The configure process is the way we make Perl portable across the
710 myriad of operating systems it supports. Responsibility for the
711 configure, build and installation process, as well as the overall
712 portability of the core code rests with the configure pumpkin - others
713 help out with individual operating systems.
715 The files involved are the operating system directories, (F<win32/>,
716 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
717 and F<Makefile>, as well as the metaconfig files which generate
718 F<Configure>. (metaconfig isn't included in the core distribution.)
722 And of course, there's the core of the Perl interpreter itself. Let's
723 have a look at that in a little more detail.
727 Before we leave looking at the layout, though, don't forget that
728 F<MANIFEST> contains not only the file names in the Perl distribution,
729 but short descriptions of what's in them, too. For an overview of the
730 important files, try this:
732 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
734 =head2 Elements of the interpreter
736 The work of the interpreter has two main stages: compiling the code
737 into the internal representation, or bytecode, and then executing it.
738 L<perlguts/Compiled code> explains exactly how the compilation stage
741 Here is a short breakdown of perl's operation:
747 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
748 This is very high-level code, enough to fit on a single screen, and it
749 resembles the code found in L<perlembed>; most of the real action takes
752 First, F<perlmain.c> allocates some memory and constructs a Perl
755 1 PERL_SYS_INIT3(&argc,&argv,&env);
757 3 if (!PL_do_undump) {
758 4 my_perl = perl_alloc();
761 7 perl_construct(my_perl);
762 8 PL_perl_destruct_level = 0;
765 Line 1 is a macro, and its definition is dependent on your operating
766 system. Line 3 references C<PL_do_undump>, a global variable - all
767 global variables in Perl start with C<PL_>. This tells you whether the
768 current running program was created with the C<-u> flag to perl and then
769 F<undump>, which means it's going to be false in any sane context.
771 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
772 interpreter. It's quite a simple function, and the guts of it looks like
775 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
777 Here you see an example of Perl's system abstraction, which we'll see
778 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
779 own C<malloc> as defined in F<malloc.c> if you selected that option at
782 Next, in line 7, we construct the interpreter; this sets up all the
783 special variables that Perl needs, the stacks, and so on.
785 Now we pass Perl the command line options, and tell it to go:
787 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
789 exitstatus = perl_run(my_perl);
793 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
794 in F<perl.c>, which processes the command line options, sets up any
795 statically linked XS modules, opens the program and calls C<yyparse> to
800 The aim of this stage is to take the Perl source, and turn it into an op
801 tree. We'll see what one of those looks like later. Strictly speaking,
802 there's three things going on here.
804 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
805 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
806 B<is> a YACC grammar for Perl!) The job of the parser is to take your
807 code and "understand" it, splitting it into sentences, deciding which
808 operands go with which operators and so on.
810 The parser is nobly assisted by the lexer, which chunks up your input
811 into tokens, and decides what type of thing each token is: a variable
812 name, an operator, a bareword, a subroutine, a core function, and so on.
813 The main point of entry to the lexer is C<yylex>, and that and its
814 associated routines can be found in F<toke.c>. Perl isn't much like
815 other computer languages; it's highly context sensitive at times, it can
816 be tricky to work out what sort of token something is, or where a token
817 ends. As such, there's a lot of interplay between the tokeniser and the
818 parser, which can get pretty frightening if you're not used to it.
820 As the parser understands a Perl program, it builds up a tree of
821 operations for the interpreter to perform during execution. The routines
822 which construct and link together the various operations are to be found
823 in F<op.c>, and will be examined later.
827 Now the parsing stage is complete, and the finished tree represents
828 the operations that the Perl interpreter needs to perform to execute our
829 program. Next, Perl does a dry run over the tree looking for
830 optimisations: constant expressions such as C<3 + 4> will be computed
831 now, and the optimizer will also see if any multiple operations can be
832 replaced with a single one. For instance, to fetch the variable C<$foo>,
833 instead of grabbing the glob C<*foo> and looking at the scalar
834 component, the optimizer fiddles the op tree to use a function which
835 directly looks up the scalar in question. The main optimizer is C<peep>
836 in F<op.c>, and many ops have their own optimizing functions.
840 Now we're finally ready to go: we have compiled Perl byte code, and all
841 that's left to do is run it. The actual execution is done by the
842 C<runops_standard> function in F<run.c>; more specifically, it's done by
843 these three innocent looking lines:
845 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
849 You may be more comfortable with the Perl version of that:
851 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
853 Well, maybe not. Anyway, each op contains a function pointer, which
854 stipulates the function which will actually carry out the operation.
855 This function will return the next op in the sequence - this allows for
856 things like C<if> which choose the next op dynamically at run time.
857 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
858 execution if required.
860 The actual functions called are known as PP code, and they're spread
861 between four files: F<pp_hot.c> contains the "hot" code, which is most
862 often used and highly optimized, F<pp_sys.c> contains all the
863 system-specific functions, F<pp_ctl.c> contains the functions which
864 implement control structures (C<if>, C<while> and the like) and F<pp.c>
865 contains everything else. These are, if you like, the C code for Perl's
866 built-in functions and operators.
868 Note that each C<pp_> function is expected to return a pointer to the next
869 op. Calls to perl subs (and eval blocks) are handled within the same
870 runops loop, and do not consume extra space on the C stack. For example,
871 C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or C<CxEVAL> block
872 struct onto the context stack which contain the address of the op
873 following the sub call or eval. They then return the first op of that sub
874 or eval block, and so execution continues of that sub or block. Later, a
875 C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB> or C<CxEVAL>,
876 retrieves the return op from it, and returns it.
878 =item Exception handing
880 Perl's exception handing (i.e. C<die> etc) is built on top of the low-level
881 C<setjmp()>/C<longjmp()> C-library functions. These basically provide a
882 way to capture the current PC and SP registers and later restore them; i.e.
883 a C<longjmp()> continues at the point in code where a previous C<setjmp()>
884 was done, with anything further up on the C stack being lost. This is why
885 code should always save values using C<SAVE_FOO> rather than in auto
888 The perl core wraps C<setjmp()> etc in the macros C<JMPENV_PUSH> and
889 C<JMPENV_JUMP>. The basic rule of perl exceptions is that C<exit>, and
890 C<die> (in the absence of C<eval>) perform a C<JMPENV_JUMP(2)>, while
891 C<die> within C<eval> does a C<JMPENV_JUMP(3)>.
893 At entry points to perl, such as C<perl_parse()>, C<perl_run()> and
894 C<call_sv(cv, G_EVAL)> each does a C<JMPENV_PUSH>, then enter a runops
895 loop or whatever, and handle possible exception returns. For a 2 return,
896 final cleanup is performed, such as popping stacks and calling C<CHECK> or
897 C<END> blocks. Amongst other things, this is how scope cleanup still
898 occurs during an C<exit>.
900 If a C<die> can find a C<CxEVAL> block on the context stack, then the
901 stack is popped to that level and the return op in that block is assigned
902 to C<PL_restartop>; then a C<JMPENV_JUMP(3)> is performed. This normally
903 passes control back to the guard. In the case of C<perl_run> and
904 C<call_sv>, a non-null C<PL_restartop> triggers re-entry to the runops
905 loop. The is the normal way that C<die> or C<croak> is handled within an
908 Sometimes ops are executed within an inner runops loop, such as tie, sort
909 or overload code. In this case, something like
911 sub FETCH { eval { die } }
913 would cause a longjmp right back to the guard in C<perl_run>, popping both
914 runops loops, which is clearly incorrect. One way to avoid this is for the
915 tie code to do a C<JMPENV_PUSH> before executing C<FETCH> in the inner
916 runops loop, but for efficiency reasons, perl in fact just sets a flag,
917 using C<CATCH_SET(TRUE)>. The C<pp_require>, C<pp_entereval> and
918 C<pp_entertry> ops check this flag, and if true, they call C<docatch>,
919 which does a C<JMPENV_PUSH> and starts a new runops level to execute the
920 code, rather than doing it on the current loop.
922 As a further optimisation, on exit from the eval block in the C<FETCH>,
923 execution of the code following the block is still carried on in the inner
924 loop. When an exception is raised, C<docatch> compares the C<JMPENV>
925 level of the C<CxEVAL> with C<PL_top_env> and if they differ, just
926 re-throws the exception. In this way any inner loops get popped.
930 1: eval { tie @a, 'A' };
936 To run this code, C<perl_run> is called, which does a C<JMPENV_PUSH> then
937 enters a runops loop. This loop executes the eval and tie ops on line 1,
938 with the eval pushing a C<CxEVAL> onto the context stack.
940 The C<pp_tie> does a C<CATCH_SET(TRUE)>, then starts a second runops loop
941 to execute the body of C<TIEARRAY>. When it executes the entertry op on
942 line 3, C<CATCH_GET> is true, so C<pp_entertry> calls C<docatch> which
943 does a C<JMPENV_PUSH> and starts a third runops loop, which then executes
944 the die op. At this point the C call stack looks like this:
947 Perl_runops # third loop
951 Perl_runops # second loop
955 Perl_runops # first loop
960 and the context and data stacks, as shown by C<-Dstv>, look like:
964 CX 1: EVAL => AV() PV("A"\0)
972 The die pops the first C<CxEVAL> off the context stack, sets
973 C<PL_restartop> from it, does a C<JMPENV_JUMP(3)>, and control returns to
974 the top C<docatch>. This then starts another third-level runops level,
975 which executes the nextstate, pushmark and die ops on line 4. At the point
976 that the second C<pp_die> is called, the C call stack looks exactly like
977 that above, even though we are no longer within an inner eval; this is
978 because of the optimization mentioned earlier. However, the context stack
979 now looks like this, ie with the top CxEVAL popped:
983 CX 1: EVAL => AV() PV("A"\0)
989 The die on line 4 pops the context stack back down to the CxEVAL, leaving
995 As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
996 C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:
1000 Perl_runops # second loop
1004 Perl_runops # first loop
1009 In this case, because the C<JMPENV> level recorded in the C<CxEVAL>
1010 differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
1011 and the C stack unwinds to:
1016 Because C<PL_restartop> is non-null, C<run_body> starts a new runops loop
1017 and execution continues.
1021 =head2 Internal Variable Types
1023 You should by now have had a look at L<perlguts>, which tells you about
1024 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
1027 These variables are used not only to represent Perl-space variables, but
1028 also any constants in the code, as well as some structures completely
1029 internal to Perl. The symbol table, for instance, is an ordinary Perl
1030 hash. Your code is represented by an SV as it's read into the parser;
1031 any program files you call are opened via ordinary Perl filehandles, and
1034 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
1035 Perl program. Let's see, for instance, how Perl treats the constant
1038 % perl -MDevel::Peek -e 'Dump("hello")'
1039 1 SV = PV(0xa041450) at 0xa04ecbc
1041 3 FLAGS = (POK,READONLY,pPOK)
1042 4 PV = 0xa0484e0 "hello"\0
1046 Reading C<Devel::Peek> output takes a bit of practise, so let's go
1047 through it line by line.
1049 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
1050 memory. SVs themselves are very simple structures, but they contain a
1051 pointer to a more complex structure. In this case, it's a PV, a
1052 structure which holds a string value, at location C<0xa041450>. Line 2
1053 is the reference count; there are no other references to this data, so
1056 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
1057 read-only SV (because it's a constant) and the data is a PV internally.
1058 Next we've got the contents of the string, starting at location
1061 Line 5 gives us the current length of the string - note that this does
1062 B<not> include the null terminator. Line 6 is not the length of the
1063 string, but the length of the currently allocated buffer; as the string
1064 grows, Perl automatically extends the available storage via a routine
1067 You can get at any of these quantities from C very easily; just add
1068 C<Sv> to the name of the field shown in the snippet, and you've got a
1069 macro which will return the value: C<SvCUR(sv)> returns the current
1070 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
1071 C<SvPV(sv, len)> returns the string itself with its length, and so on.
1072 More macros to manipulate these properties can be found in L<perlguts>.
1074 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
1077 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
1082 6 junk = SvPV_force(sv, tlen);
1083 7 SvGROW(sv, tlen + len + 1);
1086 10 Move(ptr,SvPVX(sv)+tlen,len,char);
1087 11 SvCUR(sv) += len;
1088 12 *SvEND(sv) = '\0';
1089 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
1093 This is a function which adds a string, C<ptr>, of length C<len> onto
1094 the end of the PV stored in C<sv>. The first thing we do in line 6 is
1095 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
1096 macro to force a PV. As a side effect, C<tlen> gets set to the current
1097 value of the PV, and the PV itself is returned to C<junk>.
1099 In line 7, we make sure that the SV will have enough room to accommodate
1100 the old string, the new string and the null terminator. If C<LEN> isn't
1101 big enough, C<SvGROW> will reallocate space for us.
1103 Now, if C<junk> is the same as the string we're trying to add, we can
1104 grab the string directly from the SV; C<SvPVX> is the address of the PV
1107 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
1108 memory around: we move the string C<ptr> to the end of the PV - that's
1109 the start of the PV plus its current length. We're moving C<len> bytes
1110 of type C<char>. After doing so, we need to tell Perl we've extended the
1111 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
1112 macro which gives us the end of the string, so that needs to be a
1115 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
1116 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
1117 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware
1118 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
1119 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
1120 data if taint mode is turned on.
1122 AVs and HVs are more complicated, but SVs are by far the most common
1123 variable type being thrown around. Having seen something of how we
1124 manipulate these, let's go on and look at how the op tree is
1129 First, what is the op tree, anyway? The op tree is the parsed
1130 representation of your program, as we saw in our section on parsing, and
1131 it's the sequence of operations that Perl goes through to execute your
1132 program, as we saw in L</Running>.
1134 An op is a fundamental operation that Perl can perform: all the built-in
1135 functions and operators are ops, and there are a series of ops which
1136 deal with concepts the interpreter needs internally - entering and
1137 leaving a block, ending a statement, fetching a variable, and so on.
1139 The op tree is connected in two ways: you can imagine that there are two
1140 "routes" through it, two orders in which you can traverse the tree.
1141 First, parse order reflects how the parser understood the code, and
1142 secondly, execution order tells perl what order to perform the
1145 The easiest way to examine the op tree is to stop Perl after it has
1146 finished parsing, and get it to dump out the tree. This is exactly what
1147 the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
1148 and L<B::Debug|B::Debug> do.
1150 Let's have a look at how Perl sees C<$a = $b + $c>:
1152 % perl -MO=Terse -e '$a=$b+$c'
1153 1 LISTOP (0x8179888) leave
1154 2 OP (0x81798b0) enter
1155 3 COP (0x8179850) nextstate
1156 4 BINOP (0x8179828) sassign
1157 5 BINOP (0x8179800) add [1]
1158 6 UNOP (0x81796e0) null [15]
1159 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
1160 8 UNOP (0x81797e0) null [15]
1161 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
1162 10 UNOP (0x816b4f0) null [15]
1163 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
1165 Let's start in the middle, at line 4. This is a BINOP, a binary
1166 operator, which is at location C<0x8179828>. The specific operator in
1167 question is C<sassign> - scalar assignment - and you can find the code
1168 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
1169 binary operator, it has two children: the add operator, providing the
1170 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
1173 Line 10 is the null op: this does exactly nothing. What is that doing
1174 there? If you see the null op, it's a sign that something has been
1175 optimized away after parsing. As we mentioned in L</Optimization>,
1176 the optimization stage sometimes converts two operations into one, for
1177 example when fetching a scalar variable. When this happens, instead of
1178 rewriting the op tree and cleaning up the dangling pointers, it's easier
1179 just to replace the redundant operation with the null op. Originally,
1180 the tree would have looked like this:
1182 10 SVOP (0x816b4f0) rv2sv [15]
1183 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
1185 That is, fetch the C<a> entry from the main symbol table, and then look
1186 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
1187 happens to do both these things.
1189 The right hand side, starting at line 5 is similar to what we've just
1190 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
1193 Now, what's this about?
1195 1 LISTOP (0x8179888) leave
1196 2 OP (0x81798b0) enter
1197 3 COP (0x8179850) nextstate
1199 C<enter> and C<leave> are scoping ops, and their job is to perform any
1200 housekeeping every time you enter and leave a block: lexical variables
1201 are tidied up, unreferenced variables are destroyed, and so on. Every
1202 program will have those first three lines: C<leave> is a list, and its
1203 children are all the statements in the block. Statements are delimited
1204 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
1205 the ops to be performed for each statement being the children of
1206 C<nextstate>. C<enter> is a single op which functions as a marker.
1208 That's how Perl parsed the program, from top to bottom:
1221 However, it's impossible to B<perform> the operations in this order:
1222 you have to find the values of C<$b> and C<$c> before you add them
1223 together, for instance. So, the other thread that runs through the op
1224 tree is the execution order: each op has a field C<op_next> which points
1225 to the next op to be run, so following these pointers tells us how perl
1226 executes the code. We can traverse the tree in this order using
1227 the C<exec> option to C<B::Terse>:
1229 % perl -MO=Terse,exec -e '$a=$b+$c'
1230 1 OP (0x8179928) enter
1231 2 COP (0x81798c8) nextstate
1232 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
1233 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
1234 5 BINOP (0x8179878) add [1]
1235 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
1236 7 BINOP (0x81798a0) sassign
1237 8 LISTOP (0x8179900) leave
1239 This probably makes more sense for a human: enter a block, start a
1240 statement. Get the values of C<$b> and C<$c>, and add them together.
1241 Find C<$a>, and assign one to the other. Then leave.
1243 The way Perl builds up these op trees in the parsing process can be
1244 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1245 piece we need to construct the tree for C<$a = $b + $c>
1247 1 term : term ASSIGNOP term
1248 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1250 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1252 If you're not used to reading BNF grammars, this is how it works: You're
1253 fed certain things by the tokeniser, which generally end up in upper
1254 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1255 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1256 "terminal symbols", because you can't get any simpler than them.
1258 The grammar, lines one and three of the snippet above, tells you how to
1259 build up more complex forms. These complex forms, "non-terminal symbols"
1260 are generally placed in lower case. C<term> here is a non-terminal
1261 symbol, representing a single expression.
1263 The grammar gives you the following rule: you can make the thing on the
1264 left of the colon if you see all the things on the right in sequence.
1265 This is called a "reduction", and the aim of parsing is to completely
1266 reduce the input. There are several different ways you can perform a
1267 reduction, separated by vertical bars: so, C<term> followed by C<=>
1268 followed by C<term> makes a C<term>, and C<term> followed by C<+>
1269 followed by C<term> can also make a C<term>.
1271 So, if you see two terms with an C<=> or C<+>, between them, you can
1272 turn them into a single expression. When you do this, you execute the
1273 code in the block on the next line: if you see C<=>, you'll do the code
1274 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1275 which contributes to the op tree.
1278 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1280 What this does is creates a new binary op, and feeds it a number of
1281 variables. The variables refer to the tokens: C<$1> is the first token in
1282 the input, C<$2> the second, and so on - think regular expression
1283 backreferences. C<$$> is the op returned from this reduction. So, we
1284 call C<newBINOP> to create a new binary operator. The first parameter to
1285 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1286 operator, so we want the type to be C<ADDOP>. We could specify this
1287 directly, but it's right there as the second token in the input, so we
1288 use C<$2>. The second parameter is the op's flags: 0 means "nothing
1289 special". Then the things to add: the left and right hand side of our
1290 expression, in scalar context.
1294 When perl executes something like C<addop>, how does it pass on its
1295 results to the next op? The answer is, through the use of stacks. Perl
1296 has a number of stacks to store things it's currently working on, and
1297 we'll look at the three most important ones here.
1301 =item Argument stack
1303 Arguments are passed to PP code and returned from PP code using the
1304 argument stack, C<ST>. The typical way to handle arguments is to pop
1305 them off the stack, deal with them how you wish, and then push the result
1306 back onto the stack. This is how, for instance, the cosine operator
1311 value = Perl_cos(value);
1314 We'll see a more tricky example of this when we consider Perl's macros
1315 below. C<POPn> gives you the NV (floating point value) of the top SV on
1316 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1317 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1318 should be extended if necessary - it can't be necessary here, because we
1319 know there's room for one more item on the stack, since we've just
1320 removed one! The C<XPUSH*> macros at least guarantee safety.
1322 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1323 the first element in your portion of the stack, and C<TOP*> gives you
1324 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1325 negation of an integer:
1329 Just set the integer value of the top stack entry to its negation.
1331 Argument stack manipulation in the core is exactly the same as it is in
1332 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1333 description of the macros used in stack manipulation.
1337 I say "your portion of the stack" above because PP code doesn't
1338 necessarily get the whole stack to itself: if your function calls
1339 another function, you'll only want to expose the arguments aimed for the
1340 called function, and not (necessarily) let it get at your own data. The
1341 way we do this is to have a "virtual" bottom-of-stack, exposed to each
1342 function. The mark stack keeps bookmarks to locations in the argument
1343 stack usable by each function. For instance, when dealing with a tied
1344 variable, (internally, something with "P" magic) Perl has to call
1345 methods for accesses to the tied variables. However, we need to separate
1346 the arguments exposed to the method to the argument exposed to the
1347 original function - the store or fetch or whatever it may be. Here's
1348 roughly how the tied C<push> is implemented; see C<av_push> in F<av.c>:
1352 3 PUSHs(SvTIED_obj((SV*)av, mg));
1356 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1359 Let's examine the whole implementation, for practice:
1363 Push the current state of the stack pointer onto the mark stack. This is
1364 so that when we've finished adding items to the argument stack, Perl
1365 knows how many things we've added recently.
1368 3 PUSHs(SvTIED_obj((SV*)av, mg));
1371 We're going to add two more items onto the argument stack: when you have
1372 a tied array, the C<PUSH> subroutine receives the object and the value
1373 to be pushed, and that's exactly what we have here - the tied object,
1374 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1378 Next we tell Perl to update the global stack pointer from our internal
1379 variable: C<dSP> only gave us a local copy, not a reference to the global.
1382 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1385 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1386 variables are tidied up, everything that has been localised gets
1387 its previous value returned, and so on. Think of them as the C<{> and
1388 C<}> of a Perl block.
1390 To actually do the magic method call, we have to call a subroutine in
1391 Perl space: C<call_method> takes care of that, and it's described in
1392 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1393 going to discard its return value. The call_method() function
1394 removes the top element of the mark stack, so there is nothing for
1395 the caller to clean up.
1399 C doesn't have a concept of local scope, so perl provides one. We've
1400 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1401 stack implements the C equivalent of, for example:
1408 See L<perlguts/Localising Changes> for how to use the save stack.
1412 =head2 Millions of Macros
1414 One thing you'll notice about the Perl source is that it's full of
1415 macros. Some have called the pervasive use of macros the hardest thing
1416 to understand, others find it adds to clarity. Let's take an example,
1417 the code which implements the addition operator:
1421 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1424 6 SETn( left + right );
1429 Every line here (apart from the braces, of course) contains a macro. The
1430 first line sets up the function declaration as Perl expects for PP code;
1431 line 3 sets up variable declarations for the argument stack and the
1432 target, the return value of the operation. Finally, it tries to see if
1433 the addition operation is overloaded; if so, the appropriate subroutine
1436 Line 5 is another variable declaration - all variable declarations start
1437 with C<d> - which pops from the top of the argument stack two NVs (hence
1438 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1439 C<rl>. These are the two operands to the addition operator. Next, we
1440 call C<SETn> to set the NV of the return value to the result of adding
1441 the two values. This done, we return - the C<RETURN> macro makes sure
1442 that our return value is properly handled, and we pass the next operator
1443 to run back to the main run loop.
1445 Most of these macros are explained in L<perlapi>, and some of the more
1446 important ones are explained in L<perlxs> as well. Pay special attention
1447 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1448 the C<[pad]THX_?> macros.
1450 =head2 The .i Targets
1452 You can expand the macros in a F<foo.c> file by saying
1456 which will expand the macros using cpp. Don't be scared by the results.
1458 =head2 Poking at Perl
1460 To really poke around with Perl, you'll probably want to build Perl for
1461 debugging, like this:
1463 ./Configure -d -D optimize=-g
1466 C<-g> is a flag to the C compiler to have it produce debugging
1467 information which will allow us to step through a running program.
1468 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1469 enables all the internal debugging code in Perl. There are a whole bunch
1470 of things you can debug with this: L<perlrun> lists them all, and the
1471 best way to find out about them is to play about with them. The most
1472 useful options are probably
1474 l Context (loop) stack processing
1476 o Method and overloading resolution
1477 c String/numeric conversions
1479 Some of the functionality of the debugging code can be achieved using XS
1482 -Dr => use re 'debug'
1483 -Dx => use O 'Debug'
1485 =head2 Using a source-level debugger
1487 If the debugging output of C<-D> doesn't help you, it's time to step
1488 through perl's execution with a source-level debugger.
1494 We'll use C<gdb> for our examples here; the principles will apply to any
1495 debugger, but check the manual of the one you're using.
1499 To fire up the debugger, type
1503 You'll want to do that in your Perl source tree so the debugger can read
1504 the source code. You should see the copyright message, followed by the
1509 C<help> will get you into the documentation, but here are the most
1516 Run the program with the given arguments.
1518 =item break function_name
1520 =item break source.c:xxx
1522 Tells the debugger that we'll want to pause execution when we reach
1523 either the named function (but see L<perlguts/Internal Functions>!) or the given
1524 line in the named source file.
1528 Steps through the program a line at a time.
1532 Steps through the program a line at a time, without descending into
1537 Run until the next breakpoint.
1541 Run until the end of the current function, then stop again.
1545 Just pressing Enter will do the most recent operation again - it's a
1546 blessing when stepping through miles of source code.
1550 Execute the given C code and print its results. B<WARNING>: Perl makes
1551 heavy use of macros, and F<gdb> does not necessarily support macros
1552 (see later L</"gdb macro support">). You'll have to substitute them
1553 yourself, or to invoke cpp on the source code files
1554 (see L</"The .i Targets">)
1555 So, for instance, you can't say
1557 print SvPV_nolen(sv)
1561 print Perl_sv_2pv_nolen(sv)
1565 You may find it helpful to have a "macro dictionary", which you can
1566 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1567 recursively apply those macros for you.
1569 =head2 gdb macro support
1571 Recent versions of F<gdb> have fairly good macro support, but
1572 in order to use it you'll need to compile perl with macro definitions
1573 included in the debugging information. Using F<gcc> version 3.1, this
1574 means configuring with C<-Doptimize=-g3>. Other compilers might use a
1575 different switch (if they support debugging macros at all).
1577 =head2 Dumping Perl Data Structures
1579 One way to get around this macro hell is to use the dumping functions in
1580 F<dump.c>; these work a little like an internal
1581 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1582 that you can't get at from Perl. Let's take an example. We'll use the
1583 C<$a = $b + $c> we used before, but give it a bit of context:
1584 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1586 What about C<pp_add>, the function we examined earlier to implement the
1589 (gdb) break Perl_pp_add
1590 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1592 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1593 With the breakpoint in place, we can run our program:
1595 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1597 Lots of junk will go past as gdb reads in the relevant source files and
1598 libraries, and then:
1600 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1601 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1606 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1607 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1610 #define dPOPTOPnnrl_ul NV right = POPn; \
1611 SV *leftsv = TOPs; \
1612 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1614 C<POPn> takes the SV from the top of the stack and obtains its NV either
1615 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1616 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1617 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1618 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1620 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1621 convert it. If we step again, we'll find ourselves there:
1623 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1627 We can now use C<Perl_sv_dump> to investigate the SV:
1629 SV = PV(0xa057cc0) at 0xa0675d0
1632 PV = 0xa06a510 "6XXXX"\0
1637 We know we're going to get C<6> from this, so let's finish the
1641 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1642 0x462669 in Perl_pp_add () at pp_hot.c:311
1645 We can also dump out this op: the current op is always stored in
1646 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1647 similar output to L<B::Debug|B::Debug>.
1650 13 TYPE = add ===> 14
1652 FLAGS = (SCALAR,KIDS)
1654 TYPE = null ===> (12)
1656 FLAGS = (SCALAR,KIDS)
1658 11 TYPE = gvsv ===> 12
1664 # finish this later #
1668 All right, we've now had a look at how to navigate the Perl sources and
1669 some things you'll need to know when fiddling with them. Let's now get
1670 on and create a simple patch. Here's something Larry suggested: if a
1671 C<U> is the first active format during a C<pack>, (for example,
1672 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1675 How do we prepare to fix this up? First we locate the code in question -
1676 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1677 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1678 altering this file, let's copy it to F<pp.c~>.
1680 [Well, it was in F<pp.c> when this tutorial was written. It has now been
1681 split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1683 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1684 loop over the pattern, taking each format character in turn into
1685 C<datum_type>. Then for each possible format character, we swallow up
1686 the other arguments in the pattern (a field width, an asterisk, and so
1687 on) and convert the next chunk input into the specified format, adding
1688 it onto the output SV C<cat>.
1690 How do we know if the C<U> is the first format in the C<pat>? Well, if
1691 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1692 test whether we're still at the start of the string. So, here's where
1696 register char *pat = SvPVx(*++MARK, fromlen);
1697 register char *patend = pat + fromlen;
1702 We'll have another string pointer in there:
1705 register char *pat = SvPVx(*++MARK, fromlen);
1706 register char *patend = pat + fromlen;
1712 And just before we start the loop, we'll set C<patcopy> to be the start
1717 sv_setpvn(cat, "", 0);
1719 while (pat < patend) {
1721 Now if we see a C<U> which was at the start of the string, we turn on
1722 the C<UTF8> flag for the output SV, C<cat>:
1724 + if (datumtype == 'U' && pat==patcopy+1)
1726 if (datumtype == '#') {
1727 while (pat < patend && *pat != '\n')
1730 Remember that it has to be C<patcopy+1> because the first character of
1731 the string is the C<U> which has been swallowed into C<datumtype!>
1733 Oops, we forgot one thing: what if there are spaces at the start of the
1734 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1735 character, even though it's not the first thing in the pattern. In this
1736 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1738 if (isSPACE(datumtype))
1743 if (isSPACE(datumtype)) {
1748 OK. That's the C part done. Now we must do two additional things before
1749 this patch is ready to go: we've changed the behaviour of Perl, and so
1750 we must document that change. We must also provide some more regression
1751 tests to make sure our patch works and doesn't create a bug somewhere
1752 else along the line.
1754 The regression tests for each operator live in F<t/op/>, and so we
1755 make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1756 tests to the end. First, we'll test that the C<U> does indeed create
1759 t/op/pack.t has a sensible ok() function, but if it didn't we could
1760 use the one from t/test.pl.
1762 require './test.pl';
1763 plan( tests => 159 );
1767 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1768 print "ok $test\n"; $test++;
1770 we can write the more sensible (see L<Test::More> for a full
1771 explanation of is() and other testing functions).
1773 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
1774 "U* produces unicode" );
1776 Now we'll test that we got that space-at-the-beginning business right:
1778 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
1779 " with spaces at the beginning" );
1781 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1782 the first active format:
1784 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
1785 "U* not first isn't unicode" );
1787 Mustn't forget to change the number of tests which appears at the top,
1788 or else the automated tester will get confused. This will either look
1795 plan( tests => 156 );
1797 We now compile up Perl, and run it through the test suite. Our new
1800 Finally, the documentation. The job is never done until the paperwork is
1801 over, so let's describe the change we've just made. The relevant place
1802 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1803 this text in the description of C<pack>:
1807 If the pattern begins with a C<U>, the resulting string will be treated
1808 as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
1809 with an initial C<U0>, and the bytes that follow will be interpreted as
1810 Unicode characters. If you don't want this to happen, you can begin your
1811 pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
1812 string, and then follow this with a C<U*> somewhere in your pattern.
1814 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1815 that if we're making major changes, we should copy the entire directory
1816 to somewhere safe before we begin fiddling, and then do
1818 diff -ruN old new > patch
1820 However, we know which files we've changed, and we can simply do this:
1822 diff -u pp.c~ pp.c > patch
1823 diff -u t/op/pack.t~ t/op/pack.t >> patch
1824 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1826 We end up with a patch looking a little like this:
1828 --- pp.c~ Fri Jun 02 04:34:10 2000
1829 +++ pp.c Fri Jun 16 11:37:25 2000
1830 @@ -4375,6 +4375,7 @@
1833 register char *pat = SvPVx(*++MARK, fromlen);
1835 register char *patend = pat + fromlen;
1838 @@ -4405,6 +4406,7 @@
1841 And finally, we submit it, with our rationale, to perl5-porters. Job
1844 =head2 Patching a core module
1846 This works just like patching anything else, with an extra
1847 consideration. Many core modules also live on CPAN. If this is so,
1848 patch the CPAN version instead of the core and send the patch off to
1849 the module maintainer (with a copy to p5p). This will help the module
1850 maintainer keep the CPAN version in sync with the core version without
1851 constantly scanning p5p.
1853 The list of maintainers of core modules is usefully documented in
1854 F<Porting/Maintainers.pl>.
1856 =head2 Adding a new function to the core
1858 If, as part of a patch to fix a bug, or just because you have an
1859 especially good idea, you decide to add a new function to the core,
1860 discuss your ideas on p5p well before you start work. It may be that
1861 someone else has already attempted to do what you are considering and
1862 can give lots of good advice or even provide you with bits of code
1863 that they already started (but never finished).
1865 You have to follow all of the advice given above for patching. It is
1866 extremely important to test any addition thoroughly and add new tests
1867 to explore all boundary conditions that your new function is expected
1868 to handle. If your new function is used only by one module (e.g. toke),
1869 then it should probably be named S_your_function (for static); on the
1870 other hand, if you expect it to accessible from other functions in
1871 Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1874 The location of any new code is also an important consideration. Don't
1875 just create a new top level .c file and put your code there; you would
1876 have to make changes to Configure (so the Makefile is created properly),
1877 as well as possibly lots of include files. This is strictly pumpking
1880 It is better to add your function to one of the existing top level
1881 source code files, but your choice is complicated by the nature of
1882 the Perl distribution. Only the files that are marked as compiled
1883 static are located in the perl executable. Everything else is located
1884 in the shared library (or DLL if you are running under WIN32). So,
1885 for example, if a function was only used by functions located in
1886 toke.c, then your code can go in toke.c. If, however, you want to call
1887 the function from universal.c, then you should put your code in another
1888 location, for example util.c.
1890 In addition to writing your c-code, you will need to create an
1891 appropriate entry in embed.pl describing your function, then run
1892 'make regen_headers' to create the entries in the numerous header
1893 files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1894 for information on the various options that you can set in embed.pl.
1895 You will forget to do this a few (or many) times and you will get
1896 warnings during the compilation phase. Make sure that you mention
1897 this when you post your patch to P5P; the pumpking needs to know this.
1899 When you write your new code, please be conscious of existing code
1900 conventions used in the perl source files. See L<perlstyle> for
1901 details. Although most of the guidelines discussed seem to focus on
1902 Perl code, rather than c, they all apply (except when they don't ;).
1903 See also I<Porting/patching.pod> file in the Perl source distribution
1904 for lots of details about both formatting and submitting patches of
1907 Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1908 Test on as many platforms as you can find. Test as many perl
1909 Configure options as you can (e.g. MULTIPLICITY). If you have
1910 profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1911 below for how to use them to further test your code. Remember that
1912 most of the people on P5P are doing this on their own time and
1913 don't have the time to debug your code.
1915 =head2 Writing a test
1917 Every module and built-in function has an associated test file (or
1918 should...). If you add or change functionality, you have to write a
1919 test. If you fix a bug, you have to write a test so that bug never
1920 comes back. If you alter the docs, it would be nice to test what the
1921 new documentation says.
1923 In short, if you submit a patch you probably also have to patch the
1926 For modules, the test file is right next to the module itself.
1927 F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1928 so there are some snags (and it would be wonderful for you to brush
1929 them out), but it basically works that way. Everything else lives in
1936 Testing of the absolute basic functionality of Perl. Things like
1937 C<if>, basic file reads and writes, simple regexes, etc. These are
1938 run first in the test suite and if any of them fail, something is
1943 These test the basic control structures, C<if/else>, C<while>,
1948 Tests basic issues of how Perl parses and compiles itself.
1952 Tests for built-in IO functions, including command line arguments.
1956 The old home for the module tests, you shouldn't put anything new in
1957 here. There are still some bits and pieces hanging around in here
1958 that need to be moved. Perhaps you could move them? Thanks!
1962 Tests for perl's built in functions that don't fit into any of the
1967 Tests for POD directives. There are still some tests for the Pod
1968 modules hanging around in here that need to be moved out into F<lib/>.
1972 Testing features of how perl actually runs, including exit codes and
1973 handling of PERL* environment variables.
1977 Tests for the core support of Unicode.
1981 Windows-specific tests.
1985 A test suite for the s2p converter.
1989 The core uses the same testing style as the rest of Perl, a simple
1990 "ok/not ok" run through Test::Harness, but there are a few special
1993 There are three ways to write a test in the core. Test::More,
1994 t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1995 decision of which to use depends on what part of the test suite you're
1996 working on. This is a measure to prevent a high-level failure (such
1997 as Config.pm breaking) from causing basic functionality tests to fail.
2003 Since we don't know if require works, or even subroutines, use ad hoc
2004 tests for these two. Step carefully to avoid using the feature being
2007 =item t/cmd t/run t/io t/op
2009 Now that basic require() and subroutines are tested, you can use the
2010 t/test.pl library which emulates the important features of Test::More
2011 while using a minimum of core features.
2013 You can also conditionally use certain libraries like Config, but be
2014 sure to skip the test gracefully if it's not there.
2018 Now that the core of Perl is tested, Test::More can be used. You can
2019 also use the full suite of core modules in the tests.
2023 When you say "make test" Perl uses the F<t/TEST> program to run the
2024 test suite (except under Win32 where it uses F<t/harness> instead.)
2025 All tests are run from the F<t/> directory, B<not> the directory
2026 which contains the test. This causes some problems with the tests
2027 in F<lib/>, so here's some opportunity for some patching.
2029 You must be triply conscious of cross-platform concerns. This usually
2030 boils down to using File::Spec and avoiding things like C<fork()> and
2031 C<system()> unless absolutely necessary.
2033 =head2 Special Make Test Targets
2035 There are various special make targets that can be used to test Perl
2036 slightly differently than the standard "test" target. Not all them
2037 are expected to give a 100% success rate. Many of them have several
2038 aliases, and many of them are not available on certain operating
2045 Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests).
2047 (Not available on Win32)
2051 Run all the tests through B::Deparse. Not all tests will succeed.
2053 (Not available on Win32)
2055 =item test.taintwarn
2057 Run all tests with the B<-t> command-line switch. Not all tests
2058 are expected to succeed (until they're specifically fixed, of course).
2060 (Not available on Win32)
2064 Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
2065 F<t/op>, and F<t/uni> tests.
2067 =item test.valgrind check.valgrind utest.valgrind ucheck.valgrind
2069 (Only in Linux) Run all the tests using the memory leak + naughty
2070 memory access tool "valgrind". The log files will be named
2071 F<testname.valgrind>.
2073 =item test.third check.third utest.third ucheck.third
2075 (Only in Tru64) Run all the tests using the memory leak + naughty
2076 memory access tool "Third Degree". The log files will be named
2077 F<perl.3log.testname>.
2079 =item test.torture torturetest
2081 Run all the usual tests and some extra tests. As of Perl 5.8.0 the
2082 only extra tests are Abigail's JAPHs, F<t/japh/abigail.t>.
2084 You can also run the torture test with F<t/harness> by giving
2085 C<-torture> argument to F<t/harness>.
2087 =item utest ucheck test.utf8 check.utf8
2089 Run all the tests with -Mutf8. Not all tests will succeed.
2091 (Not available on Win32)
2093 =item minitest.utf16 test.utf16
2095 Runs the tests with UTF-16 encoded scripts, encoded with different
2096 versions of this encoding.
2098 C<make utest.utf16> runs the test suite with a combination of C<-utf8> and
2099 C<-utf16> arguments to F<t/TEST>.
2101 (Not available on Win32)
2105 Run the test suite with the F<t/harness> controlling program, instead of
2106 F<t/TEST>. F<t/harness> is more sophisticated, and uses the
2107 L<Test::Harness> module, thus using this test target supposes that perl
2108 mostly works. The main advantage for our purposes is that it prints a
2109 detailed summary of failed tests at the end. Also, unlike F<t/TEST>, it
2110 doesn't redirect stderr to stdout.
2112 Note that under Win32 F<t/harness> is always used instead of F<t/TEST>, so
2113 there is no special "test_harness" target.
2115 Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES
2116 environment variables to control the behaviour of F<t/harness>. This means
2119 nmake test TEST_FILES="op/*.t"
2120 nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
2122 =item test-notty test_notty
2124 Sets PERL_SKIP_TTY_TEST to true before running normal test.
2128 =head2 Running tests by hand
2130 You can run part of the test suite by hand by using one the following
2131 commands from the F<t/> directory :
2133 ./perl -I../lib TEST list-of-.t-files
2137 ./perl -I../lib harness list-of-.t-files
2139 (if you don't specify test scripts, the whole test suite will be run.)
2141 =head3 Using t/harness for testing
2143 If you use C<harness> for testing you have several command line options
2144 available to you. The arguments are as follows, and are in the order
2145 that they must appear if used together.
2147 harness -v -torture -re=pattern LIST OF FILES TO TEST
2148 harness -v -torture -re LIST OF PATTERNS TO MATCH
2150 If C<LIST OF FILES TO TEST> is omitted the file list is obtained from
2151 the manifest. The file list may include shell wildcards which will be
2158 Run the tests under verbose mode so you can see what tests were run,
2163 Run the torture tests as well as the normal set.
2167 Filter the file list so that all the test files run match PATTERN.
2168 Note that this form is distinct from the B<-re LIST OF PATTERNS> form below
2169 in that it allows the file list to be provided as well.
2171 =item -re LIST OF PATTERNS
2173 Filter the file list so that all the test files run match
2174 /(LIST|OF|PATTERNS)/. Note that with this form the patterns
2175 are joined by '|' and you cannot supply a list of files, instead
2176 the test files are obtained from the MANIFEST.
2180 You can run an individual test by a command similar to
2182 ./perl -I../lib patho/to/foo.t
2184 except that the harnesses set up some environment variables that may
2185 affect the execution of the test :
2191 indicates that we're running this test part of the perl core test suite.
2192 This is useful for modules that have a dual life on CPAN.
2194 =item PERL_DESTRUCT_LEVEL=2
2196 is set to 2 if it isn't set already (see L</PERL_DESTRUCT_LEVEL>)
2200 (used only by F<t/TEST>) if set, overrides the path to the perl executable
2201 that should be used to run the tests (the default being F<./perl>).
2203 =item PERL_SKIP_TTY_TEST
2205 if set, tells to skip the tests that need a terminal. It's actually set
2206 automatically by the Makefile, but can also be forced artificially by
2207 running 'make test_notty'.
2211 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
2213 Sometimes it helps to use external tools while debugging and
2214 testing Perl. This section tries to guide you through using
2215 some common testing and debugging tools with Perl. This is
2216 meant as a guide to interfacing these tools with Perl, not
2217 as any kind of guide to the use of the tools themselves.
2219 B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or
2220 Third Degree greatly slows down the execution: seconds become minutes,
2221 minutes become hours. For example as of Perl 5.8.1, the
2222 ext/Encode/t/Unicode.t takes extraordinarily long to complete under
2223 e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
2224 than six hours, even on a snappy computer-- the said test must be
2225 doing something that is quite unfriendly for memory debuggers. If you
2226 don't feel like waiting, that you can simply kill away the perl
2229 B<NOTE 2>: To minimize the number of memory leak false alarms (see
2230 L</PERL_DESTRUCT_LEVEL> for more information), you have to have
2231 environment variable PERL_DESTRUCT_LEVEL set to 2. The F<TEST>
2232 and harness scripts do that automatically. But if you are running
2233 some of the tests manually-- for csh-like shells:
2235 setenv PERL_DESTRUCT_LEVEL 2
2237 and for Bourne-type shells:
2239 PERL_DESTRUCT_LEVEL=2
2240 export PERL_DESTRUCT_LEVEL
2242 or in UNIXy environments you can also use the C<env> command:
2244 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
2246 B<NOTE 3>: There are known memory leaks when there are compile-time
2247 errors within eval or require, seeing C<S_doeval> in the call stack
2248 is a good sign of these. Fixing these leaks is non-trivial,
2249 unfortunately, but they must be fixed eventually.
2251 =head2 Rational Software's Purify
2253 Purify is a commercial tool that is helpful in identifying
2254 memory overruns, wild pointers, memory leaks and other such
2255 badness. Perl must be compiled in a specific way for
2256 optimal testing with Purify. Purify is available under
2257 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
2259 =head2 Purify on Unix
2261 On Unix, Purify creates a new Perl binary. To get the most
2262 benefit out of Purify, you should create the perl to Purify
2265 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
2266 -Uusemymalloc -Dusemultiplicity
2268 where these arguments mean:
2272 =item -Accflags=-DPURIFY
2274 Disables Perl's arena memory allocation functions, as well as
2275 forcing use of memory allocation functions derived from the
2278 =item -Doptimize='-g'
2280 Adds debugging information so that you see the exact source
2281 statements where the problem occurs. Without this flag, all
2282 you will see is the source filename of where the error occurred.
2286 Disable Perl's malloc so that Purify can more closely monitor
2287 allocations and leaks. Using Perl's malloc will make Purify
2288 report most leaks in the "potential" leaks category.
2290 =item -Dusemultiplicity
2292 Enabling the multiplicity option allows perl to clean up
2293 thoroughly when the interpreter shuts down, which reduces the
2294 number of bogus leak reports from Purify.
2298 Once you've compiled a perl suitable for Purify'ing, then you
2303 which creates a binary named 'pureperl' that has been Purify'ed.
2304 This binary is used in place of the standard 'perl' binary
2305 when you want to debug Perl memory problems.
2307 As an example, to show any memory leaks produced during the
2308 standard Perl testset you would create and run the Purify'ed
2313 ../pureperl -I../lib harness
2315 which would run Perl on test.pl and report any memory problems.
2317 Purify outputs messages in "Viewer" windows by default. If
2318 you don't have a windowing environment or if you simply
2319 want the Purify output to unobtrusively go to a log file
2320 instead of to the interactive window, use these following
2321 options to output to the log file "perl.log":
2323 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
2324 -log-file=perl.log -append-logfile=yes"
2326 If you plan to use the "Viewer" windows, then you only need this option:
2328 setenv PURIFYOPTIONS "-chain-length=25"
2330 In Bourne-type shells:
2333 export PURIFYOPTIONS
2335 or if you have the "env" utility:
2337 env PURIFYOPTIONS="..." ../pureperl ...
2341 Purify on Windows NT instruments the Perl binary 'perl.exe'
2342 on the fly. There are several options in the makefile you
2343 should change to get the most use out of Purify:
2349 You should add -DPURIFY to the DEFINES line so the DEFINES
2350 line looks something like:
2352 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
2354 to disable Perl's arena memory allocation functions, as
2355 well as to force use of memory allocation functions derived
2356 from the system malloc.
2358 =item USE_MULTI = define
2360 Enabling the multiplicity option allows perl to clean up
2361 thoroughly when the interpreter shuts down, which reduces the
2362 number of bogus leak reports from Purify.
2364 =item #PERL_MALLOC = define
2366 Disable Perl's malloc so that Purify can more closely monitor
2367 allocations and leaks. Using Perl's malloc will make Purify
2368 report most leaks in the "potential" leaks category.
2372 Adds debugging information so that you see the exact source
2373 statements where the problem occurs. Without this flag, all
2374 you will see is the source filename of where the error occurred.
2378 As an example, to show any memory leaks produced during the
2379 standard Perl testset you would create and run Purify as:
2384 purify ../perl -I../lib harness
2386 which would instrument Perl in memory, run Perl on test.pl,
2387 then finally report any memory problems.
2391 The excellent valgrind tool can be used to find out both memory leaks
2392 and illegal memory accesses. As of August 2003 it unfortunately works
2393 only on x86 (ELF) Linux. The special "test.valgrind" target can be used
2394 to run the tests under valgrind. Found errors and memory leaks are
2395 logged in files named F<test.valgrind>.
2397 As system libraries (most notably glibc) are also triggering errors,
2398 valgrind allows to suppress such errors using suppression files. The
2399 default suppression file that comes with valgrind already catches a lot
2400 of them. Some additional suppressions are defined in F<t/perl.supp>.
2402 To get valgrind and for more information see
2404 http://developer.kde.org/~sewardj/
2406 =head2 Compaq's/Digital's/HP's Third Degree
2408 Third Degree is a tool for memory leak detection and memory access checks.
2409 It is one of the many tools in the ATOM toolkit. The toolkit is only
2410 available on Tru64 (formerly known as Digital UNIX formerly known as
2413 When building Perl, you must first run Configure with -Doptimize=-g
2414 and -Uusemymalloc flags, after that you can use the make targets
2415 "perl.third" and "test.third". (What is required is that Perl must be
2416 compiled using the C<-g> flag, you may need to re-Configure.)
2418 The short story is that with "atom" you can instrument the Perl
2419 executable to create a new executable called F<perl.third>. When the
2420 instrumented executable is run, it creates a log of dubious memory
2421 traffic in file called F<perl.3log>. See the manual pages of atom and
2422 third for more information. The most extensive Third Degree
2423 documentation is available in the Compaq "Tru64 UNIX Programmer's
2424 Guide", chapter "Debugging Programs with Third Degree".
2426 The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/
2427 subdirectory. There is a problem with these files: Third Degree is so
2428 effective that it finds problems also in the system libraries.
2429 Therefore you should used the Porting/thirdclean script to cleanup
2430 the F<*.3log> files.
2432 There are also leaks that for given certain definition of a leak,
2433 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
2435 =head2 PERL_DESTRUCT_LEVEL
2437 If you want to run any of the tests yourself manually using e.g.
2438 valgrind, or the pureperl or perl.third executables, please note that
2439 by default perl B<does not> explicitly cleanup all the memory it has
2440 allocated (such as global memory arenas) but instead lets the exit()
2441 of the whole program "take care" of such allocations, also known as
2442 "global destruction of objects".
2444 There is a way to tell perl to do complete cleanup: set the
2445 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
2446 The t/TEST wrapper does set this to 2, and this is what you
2447 need to do too, if you don't want to see the "global leaks":
2448 For example, for "third-degreed" Perl:
2450 env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
2452 (Note: the mod_perl apache module uses also this environment variable
2453 for its own purposes and extended its semantics. Refer to the mod_perl
2454 documentation for more information. Also, spawned threads do the
2455 equivalent of setting this variable to the value 1.)
2457 If, at the end of a run you get the message I<N scalars leaked>, you can
2458 recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the addresses
2459 of all those leaked SVs to be dumped along with details as to where each
2460 SV was originally allocated. This information is also displayed by
2461 Devel::Peek. Note that the extra details recorded with each SV increases
2462 memory usage, so it shouldn't be used in production environments. It also
2463 converts C<new_SV()> from a macro into a real function, so you can use
2464 your favourite debugger to discover where those pesky SVs were allocated.
2468 If compiled with C<-DPERL_MEM_LOG>, all Newx() and Renew() allocations
2469 and Safefree() in the Perl core go through logging functions, which is
2470 handy for breakpoint setting. If also compiled with C<-DPERL_MEM_LOG_STDERR>,
2471 the allocations and frees are logged to STDERR (or more precisely, to the
2472 file descriptor 2) in these logging functions, with the calling source code
2473 file and line number (and C function name, if supported by the C compiler).
2475 This logging is somewhat similar to C<-Dm> but independent of C<-DDEBUGGING>,
2476 and at a higher level (the C<-Dm> is directly at the point of C<malloc()>,
2477 while the C<PERL_MEM_LOG> is at the level of C<New()>).
2481 Depending on your platform there are various of profiling Perl.
2483 There are two commonly used techniques of profiling executables:
2484 I<statistical time-sampling> and I<basic-block counting>.
2486 The first method takes periodically samples of the CPU program
2487 counter, and since the program counter can be correlated with the code
2488 generated for functions, we get a statistical view of in which
2489 functions the program is spending its time. The caveats are that very
2490 small/fast functions have lower probability of showing up in the
2491 profile, and that periodically interrupting the program (this is
2492 usually done rather frequently, in the scale of milliseconds) imposes
2493 an additional overhead that may skew the results. The first problem
2494 can be alleviated by running the code for longer (in general this is a
2495 good idea for profiling), the second problem is usually kept in guard
2496 by the profiling tools themselves.
2498 The second method divides up the generated code into I<basic blocks>.
2499 Basic blocks are sections of code that are entered only in the
2500 beginning and exited only at the end. For example, a conditional jump
2501 starts a basic block. Basic block profiling usually works by
2502 I<instrumenting> the code by adding I<enter basic block #nnnn>
2503 book-keeping code to the generated code. During the execution of the
2504 code the basic block counters are then updated appropriately. The
2505 caveat is that the added extra code can skew the results: again, the
2506 profiling tools usually try to factor their own effects out of the
2509 =head2 Gprof Profiling
2511 gprof is a profiling tool available in many UNIX platforms,
2512 it uses F<statistical time-sampling>.
2514 You can build a profiled version of perl called "perl.gprof" by
2515 invoking the make target "perl.gprof" (What is required is that Perl
2516 must be compiled using the C<-pg> flag, you may need to re-Configure).
2517 Running the profiled version of Perl will create an output file called
2518 F<gmon.out> is created which contains the profiling data collected
2519 during the execution.
2521 The gprof tool can then display the collected data in various ways.
2522 Usually gprof understands the following options:
2528 Suppress statically defined functions from the profile.
2532 Suppress the verbose descriptions in the profile.
2536 Exclude the given routine and its descendants from the profile.
2540 Display only the given routine and its descendants in the profile.
2544 Generate a summary file called F<gmon.sum> which then may be given
2545 to subsequent gprof runs to accumulate data over several runs.
2549 Display routines that have zero usage.
2553 For more detailed explanation of the available commands and output
2554 formats, see your own local documentation of gprof.
2556 =head2 GCC gcov Profiling
2558 Starting from GCC 3.0 I<basic block profiling> is officially available
2561 You can build a profiled version of perl called F<perl.gcov> by
2562 invoking the make target "perl.gcov" (what is required that Perl must
2563 be compiled using gcc with the flags C<-fprofile-arcs
2564 -ftest-coverage>, you may need to re-Configure).
2566 Running the profiled version of Perl will cause profile output to be
2567 generated. For each source file an accompanying ".da" file will be
2570 To display the results you use the "gcov" utility (which should
2571 be installed if you have gcc 3.0 or newer installed). F<gcov> is
2572 run on source code files, like this
2576 which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2577 contain the source code annotated with relative frequencies of
2578 execution indicated by "#" markers.
2580 Useful options of F<gcov> include C<-b> which will summarise the
2581 basic block, branch, and function call coverage, and C<-c> which
2582 instead of relative frequencies will use the actual counts. For
2583 more information on the use of F<gcov> and basic block profiling
2584 with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2586 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2588 and its section titled "8. gcov: a Test Coverage Program"
2590 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2592 =head2 Pixie Profiling
2594 Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2595 UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
2596 I<basic-block counting>.
2598 You can build a profiled version of perl called F<perl.pixie> by
2599 invoking the make target "perl.pixie" (what is required is that Perl
2600 must be compiled using the C<-g> flag, you may need to re-Configure).
2602 In Tru64 a file called F<perl.Addrs> will also be silently created,
2603 this file contains the addresses of the basic blocks. Running the
2604 profiled version of Perl will create a new file called "perl.Counts"
2605 which contains the counts for the basic block for that particular
2608 To display the results you use the F<prof> utility. The exact
2609 incantation depends on your operating system, "prof perl.Counts" in
2610 IRIX, and "prof -pixie -all -L. perl" in Tru64.
2612 In IRIX the following prof options are available:
2618 Reports the most heavily used lines in descending order of use.
2619 Useful for finding the hotspot lines.
2623 Groups lines by procedure, with procedures sorted in descending order of use.
2624 Within a procedure, lines are listed in source order.
2625 Useful for finding the hotspots of procedures.
2629 In Tru64 the following options are available:
2635 Procedures sorted in descending order by the number of cycles executed
2636 in each procedure. Useful for finding the hotspot procedures.
2637 (This is the default option.)
2641 Lines sorted in descending order by the number of cycles executed in
2642 each line. Useful for finding the hotspot lines.
2644 =item -i[nvocations]
2646 The called procedures are sorted in descending order by number of calls
2647 made to the procedures. Useful for finding the most used procedures.
2651 Grouped by procedure, sorted by cycles executed per procedure.
2652 Useful for finding the hotspots of procedures.
2656 The compiler emitted code for these lines, but the code was unexecuted.
2660 Unexecuted procedures.
2664 For further information, see your system's manual pages for pixie and prof.
2666 =head2 Miscellaneous tricks
2672 Those debugging perl with the DDD frontend over gdb may find the
2675 You can extend the data conversion shortcuts menu, so for example you
2676 can display an SV's IV value with one click, without doing any typing.
2677 To do that simply edit ~/.ddd/init file and add after:
2679 ! Display shortcuts.
2680 Ddd*gdbDisplayShortcuts: \
2681 /t () // Convert to Bin\n\
2682 /d () // Convert to Dec\n\
2683 /x () // Convert to Hex\n\
2684 /o () // Convert to Oct(\n\
2686 the following two lines:
2688 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
2689 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
2691 so now you can do ivx and pvx lookups or you can plug there the
2692 sv_peek "conversion":
2694 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
2696 (The my_perl is for threaded builds.)
2697 Just remember that every line, but the last one, should end with \n\
2699 Alternatively edit the init file interactively via:
2700 3rd mouse button -> New Display -> Edit Menu
2702 Note: you can define up to 20 conversion shortcuts in the gdb
2707 If you see in a debugger a memory area mysteriously full of 0xABABABAB
2708 or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros,
2715 We've had a brief look around the Perl source, an overview of the stages
2716 F<perl> goes through when it's running your code, and how to use a
2717 debugger to poke at the Perl guts. We took a very simple problem and
2718 demonstrated how to solve it fully - with documentation, regression
2719 tests, and finally a patch for submission to p5p. Finally, we talked
2720 about how to use external tools to debug and test Perl.
2722 I'd now suggest you read over those references again, and then, as soon
2723 as possible, get your hands dirty. The best way to learn is by doing,
2730 Subscribe to perl5-porters, follow the patches and try and understand
2731 them; don't be afraid to ask if there's a portion you're not clear on -
2732 who knows, you may unearth a bug in the patch...
2736 Keep up to date with the bleeding edge Perl distributions and get
2737 familiar with the changes. Try and get an idea of what areas people are
2738 working on and the changes they're making.
2742 Do read the README associated with your operating system, e.g. README.aix
2743 on the IBM AIX OS. Don't hesitate to supply patches to that README if
2744 you find anything missing or changed over a new OS release.
2748 Find an area of Perl that seems interesting to you, and see if you can
2749 work out how it works. Scan through the source, and step over it in the
2750 debugger. Play, poke, investigate, fiddle! You'll probably get to
2751 understand not just your chosen area but a much wider range of F<perl>'s
2752 activity as well, and probably sooner than you'd think.
2758 =item I<The Road goes ever on and on, down from the door where it began.>
2762 If you can do these things, you've started on the long road to Perl porting.
2763 Thanks for wanting to help make Perl better - and happy hacking!
2767 This document was written by Nathan Torkington, and is maintained by
2768 the perl5-porters mailing list.