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:
19 http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
21 The list is also archived under the usenet group name
22 C<perl.porters-gw> at:
26 List subscribers (the porters themselves) come in several flavours.
27 Some are quiet curious lurkers, who rarely pitch in and instead watch
28 the ongoing development to ensure they're forewarned of new changes or
29 features in Perl. Some are representatives of vendors, who are there
30 to make sure that Perl continues to compile and work on their
31 platforms. Some patch any reported bug that they know how to fix,
32 some are actively patching their pet area (threads, Win32, the regexp
33 engine), while others seem to do nothing but complain. In other
34 words, it's your usual mix of technical people.
36 Over this group of porters presides Larry Wall. He has the final word
37 in what does and does not change in the Perl language. Various
38 releases of Perl are shepherded by a ``pumpking'', a porter
39 responsible for gathering patches, deciding on a patch-by-patch
40 feature-by-feature basis what will and will not go into the release.
41 For instance, Gurusamy Sarathy is the pumpking for the 5.6 release of
44 In addition, various people are pumpkings for different things. For
45 instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
46 pumpkin, and Tom Christiansen is the documentation pumpking.
48 Larry sees Perl development along the lines of the US government:
49 there's the Legislature (the porters), the Executive branch (the
50 pumpkings), and the Supreme Court (Larry). The legislature can
51 discuss and submit patches to the executive branch all they like, but
52 the executive branch is free to veto them. Rarely, the Supreme Court
53 will side with the executive branch over the legislature, or the
54 legislature over the executive branch. Mostly, however, the
55 legislature and the executive branch are supposed to get along and
56 work out their differences without impeachment or court cases.
58 You might sometimes see reference to Rule 1 and Rule 2. Larry's power
59 as Supreme Court is expressed in The Rules:
65 Larry is always by definition right about how Perl should behave.
66 This means he has final veto power on the core functionality.
70 Larry is allowed to change his mind about any matter at a later date,
71 regardless of whether he previously invoked Rule 1.
75 Got that? Larry is always right, even when he was wrong. It's rare
76 to see either Rule exercised, but they are often alluded to.
78 New features and extensions to the language are contentious, because
79 the criteria used by the pumpkings, Larry, and other porters to decide
80 which features should be implemented and incorporated are not codified
81 in a few small design goals as with some other languages. Instead,
82 the heuristics are flexible and often difficult to fathom. Here is
83 one person's list, roughly in decreasing order of importance, of
84 heuristics that new features have to be weighed against:
88 =item Does concept match the general goals of Perl?
90 These haven't been written anywhere in stone, but one approximation
93 1. Keep it fast, simple, and useful.
94 2. Keep features/concepts as orthogonal as possible.
95 3. No arbitrary limits (platforms, data sizes, cultures).
96 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
97 5. Either assimilate new technologies, or build bridges to them.
99 =item Where is the implementation?
101 All the talk in the world is useless without an implementation. In
102 almost every case, the person or people who argue for a new feature
103 will be expected to be the ones who implement it. Porters capable
104 of coding new features have their own agendas, and are not available
105 to implement your (possibly good) idea.
107 =item Backwards compatibility
109 It's a cardinal sin to break existing Perl programs. New warnings are
110 contentious--some say that a program that emits warnings is not
111 broken, while others say it is. Adding keywords has the potential to
112 break programs, changing the meaning of existing token sequences or
113 functions might break programs.
115 =item Could it be a module instead?
117 Perl 5 has extension mechanisms, modules and XS, specifically to avoid
118 the need to keep changing the Perl interpreter. You can write modules
119 that export functions, you can give those functions prototypes so they
120 can be called like built-in functions, you can even write XS code to
121 mess with the runtime data structures of the Perl interpreter if you
122 want to implement really complicated things. If it can be done in a
123 module instead of in the core, it's highly unlikely to be added.
125 =item Is the feature generic enough?
127 Is this something that only the submitter wants added to the language,
128 or would it be broadly useful? Sometimes, instead of adding a feature
129 with a tight focus, the porters might decide to wait until someone
130 implements the more generalized feature. For instance, instead of
131 implementing a ``delayed evaluation'' feature, the porters are waiting
132 for a macro system that would permit delayed evaluation and much more.
134 =item Does it potentially introduce new bugs?
136 Radical rewrites of large chunks of the Perl interpreter have the
137 potential to introduce new bugs. The smaller and more localized the
140 =item Does it preclude other desirable features?
142 A patch is likely to be rejected if it closes off future avenues of
143 development. For instance, a patch that placed a true and final
144 interpretation on prototypes is likely to be rejected because there
145 are still options for the future of prototypes that haven't been
148 =item Is the implementation robust?
150 Good patches (tight code, complete, correct) stand more chance of
151 going in. Sloppy or incorrect patches might be placed on the back
152 burner until the pumpking has time to fix, or might be discarded
153 altogether without further notice.
155 =item Is the implementation generic enough to be portable?
157 The worst patches make use of a system-specific features. It's highly
158 unlikely that nonportable additions to the Perl language will be
161 =item Is there enough documentation?
163 Patches without documentation are probably ill-thought out or
164 incomplete. Nothing can be added without documentation, so submitting
165 a patch for the appropriate manpages as well as the source code is
166 always a good idea. If appropriate, patches should add to the test
169 =item Is there another way to do it?
171 Larry said ``Although the Perl Slogan is I<There's More Than One Way
172 to Do It>, I hesitate to make 10 ways to do something''. This is a
173 tricky heuristic to navigate, though--one man's essential addition is
174 another man's pointless cruft.
176 =item Does it create too much work?
178 Work for the pumpking, work for Perl programmers, work for module
179 authors, ... Perl is supposed to be easy.
181 =item Patches speak louder than words
183 Working code is always preferred to pie-in-the-sky ideas. A patch to
184 add a feature stands a much higher chance of making it to the language
185 than does a random feature request, no matter how fervently argued the
186 request might be. This ties into ``Will it be useful?'', as the fact
187 that someone took the time to make the patch demonstrates a strong
188 desire for the feature.
192 If you're on the list, you might hear the word ``core'' bandied
193 around. It refers to the standard distribution. ``Hacking on the
194 core'' means you're changing the C source code to the Perl
195 interpreter. ``A core module'' is one that ships with Perl.
197 =head2 Keeping in sync
199 The source code to the Perl interpreter, in its different versions, is
200 kept in a repository managed by a revision control system (which is
201 currently the Perforce program, see http://perforce.com/). The
202 pumpkings and a few others have access to the repository to check in
203 changes. Periodically the pumpking for the development version of Perl
204 will release a new version, so the rest of the porters can see what's
205 changed. The current state of the main trunk of repository, and patches
206 that describe the individual changes that have happened since the last
207 public release are available at this location:
209 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
211 If you are a member of the perl5-porters mailing list, it is a good
212 thing to keep in touch with the most recent changes. If not only to
213 verify if what you would have posted as a bug report isn't already
214 solved in the most recent available perl development branch, also
215 known as perl-current, bleading edge perl, bleedperl or bleadperl.
217 Needless to say, the source code in perl-current is usually in a perpetual
218 state of evolution. You should expect it to be very buggy. Do B<not> use
219 it for any purpose other than testing and development.
221 Keeping in sync with the most recent branch can be done in several ways,
222 but the most convenient and reliable way is using B<rsync>, available at
223 ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
226 If you choose to keep in sync using rsync, there are two approaches
231 =item rsync'ing the source tree
233 Presuming you are in the directory where your perl source resides
234 and you have rsync installed and available, you can `upgrade' to
237 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
239 This takes care of updating every single item in the source tree to
240 the latest applied patch level, creating files that are new (to your
241 distribution) and setting date/time stamps of existing files to
242 reflect the bleadperl status.
244 Note that this will not delete any files that were in '.' before
245 the rsync. Once you are sure that the rsync is running correctly,
246 run it with the --delete and the --dry-run options like this:
248 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
250 This will I<simulate> an rsync run that also deletes files not
251 present in the bleadperl master copy. Observe the results from
252 this run closely. If you are sure that the actual run would delete
253 no files precious to you, you could remove the '--dry-run' option.
255 You can than check what patch was the latest that was applied by
256 looking in the file B<.patch>, which will show the number of the
259 If you have more than one machine to keep in sync, and not all of
260 them have access to the WAN (so you are not able to rsync all the
261 source trees to the real source), there are some ways to get around
266 =item Using rsync over the LAN
268 Set up a local rsync server which makes the rsynced source tree
269 available to the LAN and sync the other machines against this
272 From http://rsync.samba.org/README.html:
274 "Rsync uses rsh or ssh for communication. It does not need to be
275 setuid and requires no special privileges for installation. It
276 does not require an inetd entry or a daemon. You must, however,
277 have a working rsh or ssh system. Using ssh is recommended for
278 its security features."
280 =item Using pushing over the NFS
282 Having the other systems mounted over the NFS, you can take an
283 active pushing approach by checking the just updated tree against
284 the other not-yet synced trees. An example would be
293 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
296 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
298 foreach my $host (keys %remote) {
299 unless (-d $remote{$host}) {
300 print STDERR "Cannot Xsync for host $host\n";
303 foreach my $file (keys %MF) {
304 my $rfile = "$remote{$host}/$file";
305 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
306 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
307 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
308 printf "%4s %-34s %8d %9d %8d %9d\n",
309 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
311 copy ($file, $rfile);
312 utime time, $MF{$file}[2], $rfile;
313 chmod $MF{$file}[0], $rfile;
317 though this is not perfect. It could be improved with checking
318 file checksums before updating. Not all NFS systems support
319 reliable utime support (when used over the NFS).
323 =item rsync'ing the patches
325 The source tree is maintained by the pumpking who applies patches to
326 the files in the tree. These patches are either created by the
327 pumpking himself using C<diff -c> after updating the file manually or
328 by applying patches sent in by posters on the perl5-porters list.
329 These patches are also saved and rsync'able, so you can apply them
330 yourself to the source files.
332 Presuming you are in a directory where your patches reside, you can
333 get them in sync with
335 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
337 This makes sure the latest available patch is downloaded to your
340 It's then up to you to apply these patches, using something like
342 # last=`ls -rt1 *.gz | tail -1`
343 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
344 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
346 # patch -p1 -N <../perl-current-diffs/blead.patch
348 or, since this is only a hint towards how it works, use CPAN-patchaperl
349 from Andreas König to have better control over the patching process.
353 =head3 Why rsync the source tree
359 Since you don't have to apply the patches yourself, you are sure all
360 files in the source tree are in the right state.
362 =item It's more recent
364 According to Gurusamy Sarathy:
366 "... The rsync mirror is automatic and syncs with the repository
369 "Updating the patch area still requires manual intervention
370 (with all the goofiness that implies, which you've noted) and
371 is typically on a daily cycle. Making this process automatic
372 is on my tuit list, but don't ask me when."
374 =item It's more reliable
376 Well, since the patches are updated by hand, I don't have to say any
377 more ... (see Sarathy's remark).
381 =head3 Why rsync the patches
387 If you have more than one machine that you want to keep in track with
388 bleadperl, it's easier to rsync the patches only once and then apply
389 them to all the source trees on the different machines.
391 In case you try to keep in pace on 5 different machines, for which
392 only one of them has access to the WAN, rsync'ing all the source
393 trees should than be done 5 times over the NFS. Having
394 rsync'ed the patches only once, I can apply them to all the source
395 trees automatically. Need you say more ;-)
397 =item It's a good reference
399 If you do not only like to have the most recent development branch,
400 but also like to B<fix> bugs, or extend features, you want to dive
401 into the sources. If you are a seasoned perl core diver, you don't
402 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
403 your way around. But if you are a starter, the patches may help you
404 in finding where you should start and how to change the bits that
407 The file B<Changes> is updated on occasions the pumpking sees as his
408 own little sync points. On those occasions, he releases a tar-ball of
409 the current source tree (i.e. perl@7582.tar.gz), which will be an
410 excellent point to start with when choosing to use the 'rsync the
411 patches' scheme. Starting with perl@7582, which means a set of source
412 files on which the latest applied patch is number 7582, you apply all
413 succeeding patches available from then on (7583, 7584, ...).
415 You can use the patches later as a kind of search archive.
419 =item Finding a start point
421 If you want to fix/change the behaviour of function/feature Foo, just
422 scan the patches for patches that mention Foo either in the subject,
423 the comments, or the body of the fix. A good chance the patch shows
424 you the files that are affected by that patch which are very likely
425 to be the starting point of your journey into the guts of perl.
427 =item Finding how to fix a bug
429 If you've found I<where> the function/feature Foo misbehaves, but you
430 don't know how to fix it (but you do know the change you want to
431 make), you can, again, peruse the patches for similar changes and
432 look how others apply the fix.
434 =item Finding the source of misbehaviour
436 When you keep in sync with bleadperl, the pumpking would love to
437 I<see> that the community efforts really work. So after each of his
438 sync points, you are to 'make test' to check if everything is still
439 in working order. If it is, you do 'make ok', which will send an OK
440 report to perlbug@perl.org. (If you do not have access to a mailer
441 from the system you just finished successfully 'make test', you can
442 do 'make okfile', which creates the file C<perl.ok>, which you can
443 than take to your favourite mailer and mail yourself).
445 But of course, as always, things will not always lead to a success
446 path, and one or more test do not pass the 'make test'. Before
447 sending in a bug report (using 'make nok' or 'make nokfile'), check
448 the mailing list if someone else has reported the bug already and if
449 so, confirm it by replying to that message. If not, you might want to
450 trace the source of that misbehaviour B<before> sending in the bug,
451 which will help all the other porters in finding the solution.
453 Here the saved patches come in very handy. You can check the list of
454 patches to see which patch changed what file and what change caused
455 the misbehaviour. If you note that in the bug report, it saves the
456 one trying to solve it, looking for that point.
460 If searching the patches is too bothersome, you might consider using
461 perl's bugtron to find more information about discussions and
462 ramblings on posted bugs.
466 If you want to get the best of both worlds, rsync both the source
467 tree for convenience, reliability and ease and rsync the patches
470 =head2 Submitting patches
472 Always submit patches to I<perl5-porters@perl.org>. This lets other
473 porters review your patch, which catches a surprising number of errors
474 in patches. Either use the diff program (available in source code
475 form from I<ftp://ftp.gnu.org/pub/gnu/>), or use Johan Vromans'
476 I<makepatch> (available from I<CPAN/authors/id/JV/>). Unified diffs
477 are preferred, but context diffs are accepted. Do not send RCS-style
478 diffs or diffs without context lines. More information is given in
479 the I<Porting/patching.pod> file in the Perl source distribution.
480 Please patch against the latest B<development> version (e.g., if
481 you're fixing a bug in the 5.005 track, patch against the latest
482 5.005_5x version). Only patches that survive the heat of the
483 development branch get applied to maintenance versions.
485 Your patch should update the documentation and test suite.
487 To report a bug in Perl, use the program I<perlbug> which comes with
488 Perl (if you can't get Perl to work, send mail to the address
489 I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
490 I<perlbug> feeds into the automated bug-tracking system, access to
491 which is provided through the web at I<http://bugs.perl.org/>. It
492 often pays to check the archives of the perl5-porters mailing list to
493 see whether the bug you're reporting has been reported before, and if
494 so whether it was considered a bug. See above for the location of
495 the searchable archives.
497 The CPAN testers (I<http://testers.cpan.org/>) are a group of
498 volunteers who test CPAN modules on a variety of platforms. Perl Labs
499 (I<http://labs.perl.org/>) automatically tests Perl source releases on
500 platforms and gives feedback to the CPAN testers mailing list. Both
501 efforts welcome volunteers.
503 It's a good idea to read and lurk for a while before chipping in.
504 That way you'll get to see the dynamic of the conversations, learn the
505 personalities of the players, and hopefully be better prepared to make
506 a useful contribution when do you speak up.
508 If after all this you still think you want to join the perl5-porters
509 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
510 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
512 To hack on the Perl guts, you'll need to read the following things:
518 This is of paramount importance, since it's the documentation of what
519 goes where in the Perl source. Read it over a couple of times and it
520 might start to make sense - don't worry if it doesn't yet, because the
521 best way to study it is to read it in conjunction with poking at Perl
522 source, and we'll do that later on.
524 You might also want to look at Gisle Aas's illustrated perlguts -
525 there's no guarantee that this will be absolutely up-to-date with the
526 latest documentation in the Perl core, but the fundamentals will be
527 right. (http://gisle.aas.no/perl/illguts/)
529 =item L<perlxstut> and L<perlxs>
531 A working knowledge of XSUB programming is incredibly useful for core
532 hacking; XSUBs use techniques drawn from the PP code, the portion of the
533 guts that actually executes a Perl program. It's a lot gentler to learn
534 those techniques from simple examples and explanation than from the core
539 The documentation for the Perl API explains what some of the internal
540 functions do, as well as the many macros used in the source.
542 =item F<Porting/pumpkin.pod>
544 This is a collection of words of wisdom for a Perl porter; some of it is
545 only useful to the pumpkin holder, but most of it applies to anyone
546 wanting to go about Perl development.
548 =item The perl5-porters FAQ
550 This is posted to perl5-porters at the beginning on every month, and
551 should be available from http://perlhacker.org/p5p-faq; alternatively,
552 you can get the FAQ emailed to you by sending mail to
553 C<perl5-porters-faq@perl.org>. It contains hints on reading
554 perl5-porters, information on how perl5-porters works and how Perl
555 development in general works.
559 =head2 Finding Your Way Around
561 Perl maintenance can be split into a number of areas, and certain people
562 (pumpkins) will have responsibility for each area. These areas sometimes
563 correspond to files or directories in the source kit. Among the areas are:
569 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
570 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
571 contains the core XS modules.
575 Documentation maintenance includes looking after everything in the
576 F<pod/> directory, (as well as contributing new documentation) and
577 the documentation to the modules in core.
581 The configure process is the way we make Perl portable across the
582 myriad of operating systems it supports. Responsibility for the
583 configure, build and installation process, as well as the overall
584 portability of the core code rests with the configure pumpkin - others
585 help out with individual operating systems.
587 The files involved are the operating system directories, (F<win32/>,
588 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
589 and F<Makefile>, as well as the metaconfig files which generate
590 F<Configure>. (metaconfig isn't included in the core distribution.)
594 And of course, there's the core of the Perl interpreter itself. Let's
595 have a look at that in a little more detail.
599 Before we leave looking at the layout, though, don't forget that
600 F<MANIFEST> contains not only the file names in the Perl distribution,
601 but short descriptions of what's in them, too. For an overview of the
602 important files, try this:
604 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
606 =head2 Elements of the interpreter
608 The work of the interpreter has two main stages: compiling the code
609 into the internal representation, or bytecode, and then executing it.
610 L<perlguts/Compiled code> explains exactly how the compilation stage
613 Here is a short breakdown of perl's operation:
619 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
620 This is very high-level code, enough to fit on a single screen, and it
621 resembles the code found in L<perlembed>; most of the real action takes
624 First, F<perlmain.c> allocates some memory and constructs a Perl
627 1 PERL_SYS_INIT3(&argc,&argv,&env);
629 3 if (!PL_do_undump) {
630 4 my_perl = perl_alloc();
633 7 perl_construct(my_perl);
634 8 PL_perl_destruct_level = 0;
637 Line 1 is a macro, and its definition is dependent on your operating
638 system. Line 3 references C<PL_do_undump>, a global variable - all
639 global variables in Perl start with C<PL_>. This tells you whether the
640 current running program was created with the C<-u> flag to perl and then
641 F<undump>, which means it's going to be false in any sane context.
643 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
644 interpreter. It's quite a simple function, and the guts of it looks like
647 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
649 Here you see an example of Perl's system abstraction, which we'll see
650 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
651 own C<malloc> as defined in F<malloc.c> if you selected that option at
654 Next, in line 7, we construct the interpreter; this sets up all the
655 special variables that Perl needs, the stacks, and so on.
657 Now we pass Perl the command line options, and tell it to go:
659 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
661 exitstatus = perl_run(my_perl);
665 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
666 in F<perl.c>, which processes the command line options, sets up any
667 statically linked XS modules, opens the program and calls C<yyparse> to
672 The aim of this stage is to take the Perl source, and turn it into an op
673 tree. We'll see what one of those looks like later. Strictly speaking,
674 there's three things going on here.
676 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
677 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
678 B<is> a YACC grammar for Perl!) The job of the parser is to take your
679 code and `understand' it, splitting it into sentences, deciding which
680 operands go with which operators and so on.
682 The parser is nobly assisted by the lexer, which chunks up your input
683 into tokens, and decides what type of thing each token is: a variable
684 name, an operator, a bareword, a subroutine, a core function, and so on.
685 The main point of entry to the lexer is C<yylex>, and that and its
686 associated routines can be found in F<toke.c>. Perl isn't much like
687 other computer languages; it's highly context sensitive at times, it can
688 be tricky to work out what sort of token something is, or where a token
689 ends. As such, there's a lot of interplay between the tokeniser and the
690 parser, which can get pretty frightening if you're not used to it.
692 As the parser understands a Perl program, it builds up a tree of
693 operations for the interpreter to perform during execution. The routines
694 which construct and link together the various operations are to be found
695 in F<op.c>, and will be examined later.
699 Now the parsing stage is complete, and the finished tree represents
700 the operations that the Perl interpreter needs to perform to execute our
701 program. Next, Perl does a dry run over the tree looking for
702 optimisations: constant expressions such as C<3 + 4> will be computed
703 now, and the optimizer will also see if any multiple operations can be
704 replaced with a single one. For instance, to fetch the variable C<$foo>,
705 instead of grabbing the glob C<*foo> and looking at the scalar
706 component, the optimizer fiddles the op tree to use a function which
707 directly looks up the scalar in question. The main optimizer is C<peep>
708 in F<op.c>, and many ops have their own optimizing functions.
712 Now we're finally ready to go: we have compiled Perl byte code, and all
713 that's left to do is run it. The actual execution is done by the
714 C<runops_standard> function in F<run.c>; more specifically, it's done by
715 these three innocent looking lines:
717 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
721 You may be more comfortable with the Perl version of that:
723 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
725 Well, maybe not. Anyway, each op contains a function pointer, which
726 stipulates the function which will actually carry out the operation.
727 This function will return the next op in the sequence - this allows for
728 things like C<if> which choose the next op dynamically at run time.
729 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
730 execution if required.
732 The actual functions called are known as PP code, and they're spread
733 between four files: F<pp_hot.c> contains the `hot' code, which is most
734 often used and highly optimized, F<pp_sys.c> contains all the
735 system-specific functions, F<pp_ctl.c> contains the functions which
736 implement control structures (C<if>, C<while> and the like) and F<pp.c>
737 contains everything else. These are, if you like, the C code for Perl's
738 built-in functions and operators.
742 =head2 Internal Variable Types
744 You should by now have had a look at L<perlguts>, which tells you about
745 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
748 These variables are used not only to represent Perl-space variables, but
749 also any constants in the code, as well as some structures completely
750 internal to Perl. The symbol table, for instance, is an ordinary Perl
751 hash. Your code is represented by an SV as it's read into the parser;
752 any program files you call are opened via ordinary Perl filehandles, and
755 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
756 Perl program. Let's see, for instance, how Perl treats the constant
759 % perl -MDevel::Peek -e 'Dump("hello")'
760 1 SV = PV(0xa041450) at 0xa04ecbc
762 3 FLAGS = (POK,READONLY,pPOK)
763 4 PV = 0xa0484e0 "hello"\0
767 Reading C<Devel::Peek> output takes a bit of practise, so let's go
768 through it line by line.
770 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
771 memory. SVs themselves are very simple structures, but they contain a
772 pointer to a more complex structure. In this case, it's a PV, a
773 structure which holds a string value, at location C<0xa041450>. Line 2
774 is the reference count; there are no other references to this data, so
777 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
778 read-only SV (because it's a constant) and the data is a PV internally.
779 Next we've got the contents of the string, starting at location
782 Line 5 gives us the current length of the string - note that this does
783 B<not> include the null terminator. Line 6 is not the length of the
784 string, but the length of the currently allocated buffer; as the string
785 grows, Perl automatically extends the available storage via a routine
788 You can get at any of these quantities from C very easily; just add
789 C<Sv> to the name of the field shown in the snippet, and you've got a
790 macro which will return the value: C<SvCUR(sv)> returns the current
791 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
792 C<SvPV(sv, len)> returns the string itself with its length, and so on.
793 More macros to manipulate these properties can be found in L<perlguts>.
795 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
798 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
803 6 junk = SvPV_force(sv, tlen);
804 7 SvGROW(sv, tlen + len + 1);
807 10 Move(ptr,SvPVX(sv)+tlen,len,char);
809 12 *SvEND(sv) = '\0';
810 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
814 This is a function which adds a string, C<ptr>, of length C<len> onto
815 the end of the PV stored in C<sv>. The first thing we do in line 6 is
816 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
817 macro to force a PV. As a side effect, C<tlen> gets set to the current
818 value of the PV, and the PV itself is returned to C<junk>.
820 In line 7, we make sure that the SV will have enough room to accommodate
821 the old string, the new string and the null terminator. If C<LEN> isn't
822 big enough, C<SvGROW> will reallocate space for us.
824 Now, if C<junk> is the same as the string we're trying to add, we can
825 grab the string directly from the SV; C<SvPVX> is the address of the PV
828 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
829 memory around: we move the string C<ptr> to the end of the PV - that's
830 the start of the PV plus its current length. We're moving C<len> bytes
831 of type C<char>. After doing so, we need to tell Perl we've extended the
832 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
833 macro which gives us the end of the string, so that needs to be a
836 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
837 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
838 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
839 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
840 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
841 data if taint mode is turned on.
843 AVs and HVs are more complicated, but SVs are by far the most common
844 variable type being thrown around. Having seen something of how we
845 manipulate these, let's go on and look at how the op tree is
850 First, what is the op tree, anyway? The op tree is the parsed
851 representation of your program, as we saw in our section on parsing, and
852 it's the sequence of operations that Perl goes through to execute your
853 program, as we saw in L</Running>.
855 An op is a fundamental operation that Perl can perform: all the built-in
856 functions and operators are ops, and there are a series of ops which
857 deal with concepts the interpreter needs internally - entering and
858 leaving a block, ending a statement, fetching a variable, and so on.
860 The op tree is connected in two ways: you can imagine that there are two
861 "routes" through it, two orders in which you can traverse the tree.
862 First, parse order reflects how the parser understood the code, and
863 secondly, execution order tells perl what order to perform the
866 The easiest way to examine the op tree is to stop Perl after it has
867 finished parsing, and get it to dump out the tree. This is exactly what
868 the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
870 Let's have a look at how Perl sees C<$a = $b + $c>:
872 % perl -MO=Terse -e '$a=$b+$c'
873 1 LISTOP (0x8179888) leave
874 2 OP (0x81798b0) enter
875 3 COP (0x8179850) nextstate
876 4 BINOP (0x8179828) sassign
877 5 BINOP (0x8179800) add [1]
878 6 UNOP (0x81796e0) null [15]
879 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
880 8 UNOP (0x81797e0) null [15]
881 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
882 10 UNOP (0x816b4f0) null [15]
883 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
885 Let's start in the middle, at line 4. This is a BINOP, a binary
886 operator, which is at location C<0x8179828>. The specific operator in
887 question is C<sassign> - scalar assignment - and you can find the code
888 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
889 binary operator, it has two children: the add operator, providing the
890 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
893 Line 10 is the null op: this does exactly nothing. What is that doing
894 there? If you see the null op, it's a sign that something has been
895 optimized away after parsing. As we mentioned in L</Optimization>,
896 the optimization stage sometimes converts two operations into one, for
897 example when fetching a scalar variable. When this happens, instead of
898 rewriting the op tree and cleaning up the dangling pointers, it's easier
899 just to replace the redundant operation with the null op. Originally,
900 the tree would have looked like this:
902 10 SVOP (0x816b4f0) rv2sv [15]
903 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
905 That is, fetch the C<a> entry from the main symbol table, and then look
906 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
907 happens to do both these things.
909 The right hand side, starting at line 5 is similar to what we've just
910 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
913 Now, what's this about?
915 1 LISTOP (0x8179888) leave
916 2 OP (0x81798b0) enter
917 3 COP (0x8179850) nextstate
919 C<enter> and C<leave> are scoping ops, and their job is to perform any
920 housekeeping every time you enter and leave a block: lexical variables
921 are tidied up, unreferenced variables are destroyed, and so on. Every
922 program will have those first three lines: C<leave> is a list, and its
923 children are all the statements in the block. Statements are delimited
924 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
925 the ops to be performed for each statement being the children of
926 C<nextstate>. C<enter> is a single op which functions as a marker.
928 That's how Perl parsed the program, from top to bottom:
941 However, it's impossible to B<perform> the operations in this order:
942 you have to find the values of C<$b> and C<$c> before you add them
943 together, for instance. So, the other thread that runs through the op
944 tree is the execution order: each op has a field C<op_next> which points
945 to the next op to be run, so following these pointers tells us how perl
946 executes the code. We can traverse the tree in this order using
947 the C<exec> option to C<B::Terse>:
949 % perl -MO=Terse,exec -e '$a=$b+$c'
950 1 OP (0x8179928) enter
951 2 COP (0x81798c8) nextstate
952 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
953 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
954 5 BINOP (0x8179878) add [1]
955 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
956 7 BINOP (0x81798a0) sassign
957 8 LISTOP (0x8179900) leave
959 This probably makes more sense for a human: enter a block, start a
960 statement. Get the values of C<$b> and C<$c>, and add them together.
961 Find C<$a>, and assign one to the other. Then leave.
963 The way Perl builds up these op trees in the parsing process can be
964 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
965 piece we need to construct the tree for C<$a = $b + $c>
967 1 term : term ASSIGNOP term
968 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
970 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
972 If you're not used to reading BNF grammars, this is how it works: You're
973 fed certain things by the tokeniser, which generally end up in upper
974 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
975 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
976 `terminal symbols', because you can't get any simpler than them.
978 The grammar, lines one and three of the snippet above, tells you how to
979 build up more complex forms. These complex forms, `non-terminal symbols'
980 are generally placed in lower case. C<term> here is a non-terminal
981 symbol, representing a single expression.
983 The grammar gives you the following rule: you can make the thing on the
984 left of the colon if you see all the things on the right in sequence.
985 This is called a "reduction", and the aim of parsing is to completely
986 reduce the input. There are several different ways you can perform a
987 reduction, separated by vertical bars: so, C<term> followed by C<=>
988 followed by C<term> makes a C<term>, and C<term> followed by C<+>
989 followed by C<term> can also make a C<term>.
991 So, if you see two terms with an C<=> or C<+>, between them, you can
992 turn them into a single expression. When you do this, you execute the
993 code in the block on the next line: if you see C<=>, you'll do the code
994 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
995 which contributes to the op tree.
998 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1000 What this does is creates a new binary op, and feeds it a number of
1001 variables. The variables refer to the tokens: C<$1> is the first token in
1002 the input, C<$2> the second, and so on - think regular expression
1003 backreferences. C<$$> is the op returned from this reduction. So, we
1004 call C<newBINOP> to create a new binary operator. The first parameter to
1005 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1006 operator, so we want the type to be C<ADDOP>. We could specify this
1007 directly, but it's right there as the second token in the input, so we
1008 use C<$2>. The second parameter is the op's flags: 0 means `nothing
1009 special'. Then the things to add: the left and right hand side of our
1010 expression, in scalar context.
1014 When perl executes something like C<addop>, how does it pass on its
1015 results to the next op? The answer is, through the use of stacks. Perl
1016 has a number of stacks to store things it's currently working on, and
1017 we'll look at the three most important ones here.
1021 =item Argument stack
1023 Arguments are passed to PP code and returned from PP code using the
1024 argument stack, C<ST>. The typical way to handle arguments is to pop
1025 them off the stack, deal with them how you wish, and then push the result
1026 back onto the stack. This is how, for instance, the cosine operator
1031 value = Perl_cos(value);
1034 We'll see a more tricky example of this when we consider Perl's macros
1035 below. C<POPn> gives you the NV (floating point value) of the top SV on
1036 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1037 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1038 should be extended if necessary - it can't be necessary here, because we
1039 know there's room for one more item on the stack, since we've just
1040 removed one! The C<XPUSH*> macros at least guarantee safety.
1042 Alternatively, you can fiddle with the stack directly: C<SP> gives you
1043 the first element in your portion of the stack, and C<TOP*> gives you
1044 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1045 negation of an integer:
1049 Just set the integer value of the top stack entry to its negation.
1051 Argument stack manipulation in the core is exactly the same as it is in
1052 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1053 description of the macros used in stack manipulation.
1057 I say `your portion of the stack' above because PP code doesn't
1058 necessarily get the whole stack to itself: if your function calls
1059 another function, you'll only want to expose the arguments aimed for the
1060 called function, and not (necessarily) let it get at your own data. The
1061 way we do this is to have a `virtual' bottom-of-stack, exposed to each
1062 function. The mark stack keeps bookmarks to locations in the argument
1063 stack usable by each function. For instance, when dealing with a tied
1064 variable, (internally, something with `P' magic) Perl has to call
1065 methods for accesses to the tied variables. However, we need to separate
1066 the arguments exposed to the method to the argument exposed to the
1067 original function - the store or fetch or whatever it may be. Here's how
1068 the tied C<push> is implemented; see C<av_push> in F<av.c>:
1072 3 PUSHs(SvTIED_obj((SV*)av, mg));
1076 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1080 The lines which concern the mark stack are the first, fifth and last
1081 lines: they save away, restore and remove the current position of the
1084 Let's examine the whole implementation, for practice:
1088 Push the current state of the stack pointer onto the mark stack. This is
1089 so that when we've finished adding items to the argument stack, Perl
1090 knows how many things we've added recently.
1093 3 PUSHs(SvTIED_obj((SV*)av, mg));
1096 We're going to add two more items onto the argument stack: when you have
1097 a tied array, the C<PUSH> subroutine receives the object and the value
1098 to be pushed, and that's exactly what we have here - the tied object,
1099 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1103 Next we tell Perl to make the change to the global stack pointer: C<dSP>
1104 only gave us a local copy, not a reference to the global.
1107 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1110 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1111 variables are tidied up, everything that has been localised gets
1112 its previous value returned, and so on. Think of them as the C<{> and
1113 C<}> of a Perl block.
1115 To actually do the magic method call, we have to call a subroutine in
1116 Perl space: C<call_method> takes care of that, and it's described in
1117 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1118 going to discard its return value.
1122 Finally, we remove the value we placed on the mark stack, since we
1123 don't need it any more.
1127 C doesn't have a concept of local scope, so perl provides one. We've
1128 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1129 stack implements the C equivalent of, for example:
1136 See L<perlguts/Localising Changes> for how to use the save stack.
1140 =head2 Millions of Macros
1142 One thing you'll notice about the Perl source is that it's full of
1143 macros. Some have called the pervasive use of macros the hardest thing
1144 to understand, others find it adds to clarity. Let's take an example,
1145 the code which implements the addition operator:
1149 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1152 6 SETn( left + right );
1157 Every line here (apart from the braces, of course) contains a macro. The
1158 first line sets up the function declaration as Perl expects for PP code;
1159 line 3 sets up variable declarations for the argument stack and the
1160 target, the return value of the operation. Finally, it tries to see if
1161 the addition operation is overloaded; if so, the appropriate subroutine
1164 Line 5 is another variable declaration - all variable declarations start
1165 with C<d> - which pops from the top of the argument stack two NVs (hence
1166 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1167 C<rl>. These are the two operands to the addition operator. Next, we
1168 call C<SETn> to set the NV of the return value to the result of adding
1169 the two values. This done, we return - the C<RETURN> macro makes sure
1170 that our return value is properly handled, and we pass the next operator
1171 to run back to the main run loop.
1173 Most of these macros are explained in L<perlapi>, and some of the more
1174 important ones are explained in L<perlxs> as well. Pay special attention
1175 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1176 the C<[pad]THX_?> macros.
1179 =head2 Poking at Perl
1181 To really poke around with Perl, you'll probably want to build Perl for
1182 debugging, like this:
1184 ./Configure -d -D optimize=-g
1187 C<-g> is a flag to the C compiler to have it produce debugging
1188 information which will allow us to step through a running program.
1189 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1190 enables all the internal debugging code in Perl. There are a whole bunch
1191 of things you can debug with this: L<perlrun> lists them all, and the
1192 best way to find out about them is to play about with them. The most
1193 useful options are probably
1195 l Context (loop) stack processing
1197 o Method and overloading resolution
1198 c String/numeric conversions
1200 Some of the functionality of the debugging code can be achieved using XS
1203 -Dr => use re 'debug'
1204 -Dx => use O 'Debug'
1206 =head2 Using a source-level debugger
1208 If the debugging output of C<-D> doesn't help you, it's time to step
1209 through perl's execution with a source-level debugger.
1215 We'll use C<gdb> for our examples here; the principles will apply to any
1216 debugger, but check the manual of the one you're using.
1220 To fire up the debugger, type
1224 You'll want to do that in your Perl source tree so the debugger can read
1225 the source code. You should see the copyright message, followed by the
1230 C<help> will get you into the documentation, but here are the most
1237 Run the program with the given arguments.
1239 =item break function_name
1241 =item break source.c:xxx
1243 Tells the debugger that we'll want to pause execution when we reach
1244 either the named function (but see L<perlguts/Internal Functions>!) or the given
1245 line in the named source file.
1249 Steps through the program a line at a time.
1253 Steps through the program a line at a time, without descending into
1258 Run until the next breakpoint.
1262 Run until the end of the current function, then stop again.
1266 Just pressing Enter will do the most recent operation again - it's a
1267 blessing when stepping through miles of source code.
1271 Execute the given C code and print its results. B<WARNING>: Perl makes
1272 heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1273 substitute them yourself. So, for instance, you can't say
1275 print SvPV_nolen(sv)
1279 print Perl_sv_2pv_nolen(sv)
1281 You may find it helpful to have a "macro dictionary", which you can
1282 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1283 recursively apply the macros for you.
1287 =head2 Dumping Perl Data Structures
1289 One way to get around this macro hell is to use the dumping functions in
1290 F<dump.c>; these work a little like an internal
1291 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1292 that you can't get at from Perl. Let's take an example. We'll use the
1293 C<$a = $b + $c> we used before, but give it a bit of context:
1294 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1296 What about C<pp_add>, the function we examined earlier to implement the
1299 (gdb) break Perl_pp_add
1300 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1302 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
1303 With the breakpoint in place, we can run our program:
1305 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1307 Lots of junk will go past as gdb reads in the relevant source files and
1308 libraries, and then:
1310 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1311 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1316 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1317 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1320 #define dPOPTOPnnrl_ul NV right = POPn; \
1321 SV *leftsv = TOPs; \
1322 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1324 C<POPn> takes the SV from the top of the stack and obtains its NV either
1325 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1326 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1327 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1328 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1330 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1331 convert it. If we step again, we'll find ourselves there:
1333 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1337 We can now use C<Perl_sv_dump> to investigate the SV:
1339 SV = PV(0xa057cc0) at 0xa0675d0
1342 PV = 0xa06a510 "6XXXX"\0
1347 We know we're going to get C<6> from this, so let's finish the
1351 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1352 0x462669 in Perl_pp_add () at pp_hot.c:311
1355 We can also dump out this op: the current op is always stored in
1356 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1357 similar output to L<B::Debug|B::Debug>.
1360 13 TYPE = add ===> 14
1362 FLAGS = (SCALAR,KIDS)
1364 TYPE = null ===> (12)
1366 FLAGS = (SCALAR,KIDS)
1368 11 TYPE = gvsv ===> 12
1374 < finish this later >
1378 All right, we've now had a look at how to navigate the Perl sources and
1379 some things you'll need to know when fiddling with them. Let's now get
1380 on and create a simple patch. Here's something Larry suggested: if a
1381 C<U> is the first active format during a C<pack>, (for example,
1382 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1385 How do we prepare to fix this up? First we locate the code in question -
1386 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1387 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1388 altering this file, let's copy it to F<pp.c~>.
1390 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1391 loop over the pattern, taking each format character in turn into
1392 C<datum_type>. Then for each possible format character, we swallow up
1393 the other arguments in the pattern (a field width, an asterisk, and so
1394 on) and convert the next chunk input into the specified format, adding
1395 it onto the output SV C<cat>.
1397 How do we know if the C<U> is the first format in the C<pat>? Well, if
1398 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1399 test whether we're still at the start of the string. So, here's where
1403 register char *pat = SvPVx(*++MARK, fromlen);
1404 register char *patend = pat + fromlen;
1409 We'll have another string pointer in there:
1412 register char *pat = SvPVx(*++MARK, fromlen);
1413 register char *patend = pat + fromlen;
1419 And just before we start the loop, we'll set C<patcopy> to be the start
1424 sv_setpvn(cat, "", 0);
1426 while (pat < patend) {
1428 Now if we see a C<U> which was at the start of the string, we turn on
1429 the UTF8 flag for the output SV, C<cat>:
1431 + if (datumtype == 'U' && pat==patcopy+1)
1433 if (datumtype == '#') {
1434 while (pat < patend && *pat != '\n')
1437 Remember that it has to be C<patcopy+1> because the first character of
1438 the string is the C<U> which has been swallowed into C<datumtype!>
1440 Oops, we forgot one thing: what if there are spaces at the start of the
1441 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1442 character, even though it's not the first thing in the pattern. In this
1443 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1445 if (isSPACE(datumtype))
1450 if (isSPACE(datumtype)) {
1455 OK. That's the C part done. Now we must do two additional things before
1456 this patch is ready to go: we've changed the behaviour of Perl, and so
1457 we must document that change. We must also provide some more regression
1458 tests to make sure our patch works and doesn't create a bug somewhere
1459 else along the line.
1461 The regression tests for each operator live in F<t/op/>, and so we make
1462 a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our tests
1463 to the end. First, we'll test that the C<U> does indeed create Unicode
1466 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1467 print "ok $test\n"; $test++;
1469 Now we'll test that we got that space-at-the-beginning business right:
1471 print 'not ' unless "1.20.300.4000" eq
1472 sprintf "%vd", pack(" U*",1,20,300,4000);
1473 print "ok $test\n"; $test++;
1475 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1476 the first active format:
1478 print 'not ' unless v1.20.300.4000 ne
1479 sprintf "%vd", pack("C0U*",1,20,300,4000);
1480 print "ok $test\n"; $test++;
1482 Mustn't forget to change the number of tests which appears at the top, or
1483 else the automated tester will get confused:
1488 We now compile up Perl, and run it through the test suite. Our new
1491 Finally, the documentation. The job is never done until the paperwork is
1492 over, so let's describe the change we've just made. The relevant place
1493 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1494 this text in the description of C<pack>:
1498 If the pattern begins with a C<U>, the resulting string will be treated
1499 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1500 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1501 characters. If you don't want this to happen, you can begin your pattern
1502 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1503 string, and then follow this with a C<U*> somewhere in your pattern.
1505 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1506 that if we're making major changes, we should copy the entire directory
1507 to somewhere safe before we begin fiddling, and then do
1509 diff -ruN old new > patch
1511 However, we know which files we've changed, and we can simply do this:
1513 diff -u pp.c~ pp.c > patch
1514 diff -u t/op/pack.t~ t/op/pack.t >> patch
1515 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1517 We end up with a patch looking a little like this:
1519 --- pp.c~ Fri Jun 02 04:34:10 2000
1520 +++ pp.c Fri Jun 16 11:37:25 2000
1521 @@ -4375,6 +4375,7 @@
1524 register char *pat = SvPVx(*++MARK, fromlen);
1526 register char *patend = pat + fromlen;
1529 @@ -4405,6 +4406,7 @@
1532 And finally, we submit it, with our rationale, to perl5-porters. Job
1535 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1537 Sometimes it helps to use external tools while debugging and
1538 testing Perl. This section tries to guide you through using
1539 some common testing and debugging tools with Perl. This is
1540 meant as a guide to interfacing these tools with Perl, not
1541 as any kind of guide to the use of the tools themselves.
1543 =head2 Rational Software's Purify
1545 Purify is a commercial tool that is helpful in identifying
1546 memory overruns, wild pointers, memory leaks and other such
1547 badness. Perl must be compiled in a specific way for
1548 optimal testing with Purify. Purify is available under
1549 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1551 The only currently known leaks happen when there are
1552 compile-time errors within eval or require. (Fixing these
1553 is non-trivial, unfortunately, but they must be fixed
1556 =head2 Purify on Unix
1558 On Unix, Purify creates a new Perl binary. To get the most
1559 benefit out of Purify, you should create the perl to Purify
1562 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1563 -Uusemymalloc -Dusemultiplicity
1565 where these arguments mean:
1569 =item -Accflags=-DPURIFY
1571 Disables Perl's arena memory allocation functions, as well as
1572 forcing use of memory allocation functions derived from the
1575 =item -Doptimize='-g'
1577 Adds debugging information so that you see the exact source
1578 statements where the problem occurs. Without this flag, all
1579 you will see is the source filename of where the error occurred.
1583 Disable Perl's malloc so that Purify can more closely monitor
1584 allocations and leaks. Using Perl's malloc will make Purify
1585 report most leaks in the "potential" leaks category.
1587 =item -Dusemultiplicity
1589 Enabling the multiplicity option allows perl to clean up
1590 thoroughly when the interpreter shuts down, which reduces the
1591 number of bogus leak reports from Purify.
1595 Once you've compiled a perl suitable for Purify'ing, then you
1600 which creates a binary named 'pureperl' that has been Purify'ed.
1601 This binary is used in place of the standard 'perl' binary
1602 when you want to debug Perl memory problems.
1604 As an example, to show any memory leaks produced during the
1605 standard Perl testset you would create and run the Purify'ed
1610 ../pureperl -I../lib harness
1612 which would run Perl on test.pl and report any memory problems.
1614 Purify outputs messages in "Viewer" windows by default. If
1615 you don't have a windowing environment or if you simply
1616 want the Purify output to unobtrusively go to a log file
1617 instead of to the interactive window, use these following
1618 options to output to the log file "perl.log":
1620 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1621 -log-file=perl.log -append-logfile=yes"
1623 If you plan to use the "Viewer" windows, then you only need this option:
1625 setenv PURIFYOPTIONS "-chain-length=25"
1629 Purify on Windows NT instruments the Perl binary 'perl.exe'
1630 on the fly. There are several options in the makefile you
1631 should change to get the most use out of Purify:
1637 You should add -DPURIFY to the DEFINES line so the DEFINES
1638 line looks something like:
1640 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1642 to disable Perl's arena memory allocation functions, as
1643 well as to force use of memory allocation functions derived
1644 from the system malloc.
1646 =item USE_MULTI = define
1648 Enabling the multiplicity option allows perl to clean up
1649 thoroughly when the interpreter shuts down, which reduces the
1650 number of bogus leak reports from Purify.
1652 =item #PERL_MALLOC = define
1654 Disable Perl's malloc so that Purify can more closely monitor
1655 allocations and leaks. Using Perl's malloc will make Purify
1656 report most leaks in the "potential" leaks category.
1660 Adds debugging information so that you see the exact source
1661 statements where the problem occurs. Without this flag, all
1662 you will see is the source filename of where the error occurred.
1666 As an example, to show any memory leaks produced during the
1667 standard Perl testset you would create and run Purify as:
1672 purify ../perl -I../lib harness
1674 which would instrument Perl in memory, run Perl on test.pl,
1675 then finally report any memory problems.
1677 =head2 Compaq's/Digital's Third Degree
1679 Third Degree is a tool for memory leak detection and memory access checks.
1680 It is one of the many tools in the ATOM toolkit. The toolkit is only
1681 available on Tru64 (formerly known as Digital UNIX formerly known as
1684 When building Perl, you must first run Configure with -Doptimize=-g
1685 and -Uusemymalloc flags, after that you can use the make targets
1686 "perl.third" and "test.third".
1688 The short story is that with "atom" you can instrument the Perl
1689 executable to create a new executable called F<perl.third>. When the
1690 instrumented executable is run, it creates a log of dubious memory
1691 traffic in file called F<perl.3log>. See the manual pages of atom and
1692 third for more information. The most extensive Third Degree
1693 documentation is available in the Compaq "Tru64 UNIX Programmer's
1694 Guide", chapter "Debugging Programs with Third Degree".
1696 The "test.third" leaves a lot of files named F<perl.3log.*> in the t/
1697 subdirectory. There is a problem with these files: Third Degree is so
1698 effective that it finds problems also in the system libraries.
1699 Therefore there are certain types of errors that you should ignore in
1700 your debugging. Errors with stack traces matching
1702 __actual_atof|__catgets|_doprnt|__exc_|__exec|_findio|__localtime|setlocale|__sia_|__strxfrm
1704 (all in libc.so) are known to be non-serious. You can also
1705 ignore the combinations
1707 Perl_gv_fetchfile() calling strcpy()
1708 S_doopen_pmc() calling strcmp()
1710 causing "rih" (reading invalid heap) errors.
1712 There are also leaks that for given certain definition of a leak,
1713 aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
1715 =head2 PERL_DESTRUCT_LEVEL
1717 If you want to run any of the tests yourself manually using the
1718 pureperl or perl.third executables, please note that by default
1719 perl B<does not> explicitly cleanup all the memory it has allocated
1720 (such as global memory arenas) but instead lets the exit() of
1721 the whole program "take care" of such allocations, also known
1722 as "global destruction of objects".
1724 There is a way to tell perl to do complete cleanup: set the
1725 environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
1726 The t/TEST wrapper does set this to 2, and this is what you
1727 need to do too, if you don't want to see the "global leaks":
1729 PERL_DESTRUCT_LEVEL=2 ./perl.third t/foo/bar.t
1731 =head2 Gprof Profiling
1733 gprof is a profiling tool available in many UNIX platforms.
1734 The profiling is based on statistical time-sampling; this means that
1735 some routines, especially those executing really fast, may be missed.
1737 You can build a profiled version of perl called "perl.gprof" by
1738 invoking the make target "perl.gprof". Running the profiled version
1739 of Perl will create an output file called F<gmon.out> is created which
1740 contains the profiling data collected during the execution.
1742 The gprof tool can then display the collected data in various ways.
1743 Usually gprof understands the following options:
1749 Suppress statically defined functions from the profile.
1753 Suppress the verbose descriptions in the profile.
1757 Exclude the given routine and its descendants from the profile.
1761 Display only the given routine and its descendants in the profile.
1765 Generate a summary file called F<gmon.sum> which then may be given
1766 to subsequent gprof runs to accumulate data over several runs.
1770 Display routines that have zero usage.
1774 For more detailed explanation of the available commands and output
1775 formats, see your own local documentation of gprof.
1777 =head2 Pixie Profiling
1779 Pixie is a profiling tool available on IRIX and Tru64
1780 (aka Digital UNIX aka DEC OSF/1) platforms. Pixie does its profiling
1781 using "basic-block counting". A basic block is a program region that
1782 is entered only at the beginning and exited only at the end.
1784 You can build a profiled version of perl called F<perl.pixie> by
1785 invoking the make target "perl.pixie" (in Tru64 a file called
1786 F<perl.Addrs> will also be silently created, this file contains the
1787 addresses of the basic blocks). Running the profiled version of Perl
1788 will create a new file called "perl.Counts" which contains the counts
1789 for the basic block for that particular program execution.
1791 To display the results you must use the "prof" utility. The exact
1792 incantation depends on your operating system, "prof perl.Counts" in
1793 IRIX, and "prof -pixie -all -L. perl" in Tru64.
1795 In IRIX the following prof options are available:
1801 Reports the most heavily used lines in descending order of use.
1802 Useful for finding the hotspot lines.
1806 Groups lines by procedure, with procedures sorted in descending order of use.
1807 Within a procedure, lines are listed in source order.
1808 Useful for finding the hotspots of procedures.
1812 In Tru64 the following options are available:
1818 Procedures sorted in descending order by the number of cycles executed
1819 in each procedure. Useful for finding the hotspot procedures.
1820 (This is the default option.)
1824 Lines sorted in descending order by the number of cycles executed in
1825 each line. Useful for finding the hotspot lines.
1827 =item -i[nvocations]
1829 The called procedures are sorted in descending order by number of calls
1830 made to the procedures. Useful for finding the most used procedures.
1834 Grouped by procedure, sorted by cycles executed per procedure.
1835 Useful for finding the hotspots of procedures.
1839 The compiler emitted code for these lines, but the code was unexecuted.
1843 Unexecuted procedures.
1847 For further information, see your system's manual pages for pixie and prof.
1851 We've had a brief look around the Perl source, an overview of the stages
1852 F<perl> goes through when it's running your code, and how to use a
1853 debugger to poke at the Perl guts. We took a very simple problem and
1854 demonstrated how to solve it fully - with documentation, regression
1855 tests, and finally a patch for submission to p5p. Finally, we talked
1856 about how to use external tools to debug and test Perl.
1858 I'd now suggest you read over those references again, and then, as soon
1859 as possible, get your hands dirty. The best way to learn is by doing,
1866 Subscribe to perl5-porters, follow the patches and try and understand
1867 them; don't be afraid to ask if there's a portion you're not clear on -
1868 who knows, you may unearth a bug in the patch...
1872 Keep up to date with the bleeding edge Perl distributions and get
1873 familiar with the changes. Try and get an idea of what areas people are
1874 working on and the changes they're making.
1878 Do read the README associated with your operating system, e.g. README.aix
1879 on the IBM AIX OS. Don't hesitate to supply patches to that README if
1880 you find anything missing or changed over a new OS release.
1884 Find an area of Perl that seems interesting to you, and see if you can
1885 work out how it works. Scan through the source, and step over it in the
1886 debugger. Play, poke, investigate, fiddle! You'll probably get to
1887 understand not just your chosen area but a much wider range of F<perl>'s
1888 activity as well, and probably sooner than you'd think.
1894 =item I<The Road goes ever on and on, down from the door where it began.>
1898 If you can do these things, you've started on the long road to Perl porting.
1899 Thanks for wanting to help make Perl better - and happy hacking!
1903 This document was written by Nathan Torkington, and is maintained by
1904 the perl5-porters mailing list.