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 To keep in sync with the most recent branch can be done in several
222 ways, but the most convenient and reliable way is using B<rsync>,
223 available at ftp://rsync.samba.org/pub/rsync/ (other ways include ftp).
225 If you choose to keep in sync using rsync, there are two approaches
230 =item rsync'ing the source tree
232 Presuming you are in the directory where your perl source resides,
233 and you have rsync installed and available, you can `upgrade' to
236 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
238 This takes care of updating every single item in the source tree to
239 the latest applied patch level, creating files that are new (to your
240 distribution) and setting date/time stamps of existing files to
241 reflect the bleadperl status.
243 You can than check what patch was the latest that was applied by
244 looking in the file B<.patch>, which will show the number of the
247 If you have more than one machine to keep in sync, and not all of
248 them have access to the WAN (so you are not able to rsync all the
249 source trees to the real source), there are some ways to get around
254 =item Using rsync over the LAN
256 Set up a local rsync server which makes the rsynced source tree
257 available to the LAN, and sync the other machines towards this
260 From http://rsync.samba.org/README.html:
262 "Rsync uses rsh or ssh for communication. It does not need to be
263 setuid and requires no special privileges for installation. It
264 does not require a inetd entry or a deamon. You must, however,
265 have a working rsh or ssh system. Using ssh is recommended for
266 its security features."
268 =item Using pushing over the NFS
270 Having the other systems mounted over the NFS, you can take an
271 active pushing approach, in checking the just updated tree against
272 the other not-yet synced trees. An example would be:
276 Though this is not perfect. It could be improved with checking
277 file checksums before updating. Not all NFS systems support
278 reliable utime support (when used over the NFS).
282 =item rsync'ing the patches
284 The source tree is maintained by the pumpking who applies patches to
285 the files in the tree. These patches are either created by the
286 pumpking himself using C<diff -c> after updating the file manually or
287 by applying patches sent in by posters on the perl5-porters list.
288 These patches are also saved and rsync'able, so you can apply them
289 yourself to the source files.
291 Presuming you are in a directory where your patches reside, you can
292 get them in sync with:
294 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
296 This makes sure the latest available patch is downloaded to your
299 It's then up to you to apply these patches, using something like:
301 # last=`ls -rt1 *.gz | tail -1`
302 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
303 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
305 # patch -p1 -N <../perl-current-diffs/blead.patch
307 or, since this is only a hint towards how it works, use CPAN-patchaperl
308 from Andreas König to have better control over the patching process.
312 =head3 Why rsync the source tree
318 Since you don't have to apply the patches yourself, you are sure all
319 files in the source tree are in the right state.
321 =item It's more recent
323 According to Gurusamy Sarathy:
325 "... The rsync mirror is automatic and syncs with the repository
328 Updating the patch area still requires manual intervention
329 (with all the goofiness that implies, which you've noted) and
330 is typically on a daily cycle. Making this process automatic
331 is on my tuit list, but don't ask me when."
333 =item It's more reliable
335 Well, since the patches are updated by hand, I don't have to say no
336 more ... (see Sarathy's remark).
340 =head3 Why rsync the patches
346 If you have more than one machine that you want to keep in track with
347 bleadperl, it's easier to rsync the patches only once and than apply
348 them to all the source trees on the different machines.
350 In case you try to keep in pace on 5 different machines, for which
351 only one of them has access to the WAN, rsync'ing all the source
352 tree's should than be done 5 times over the NFS, whereas having
353 rsync'ed the patches only once, I can apply them to all the source
354 trees automatically. Need I say more ;-)
356 =item It's a good reference
358 If you do not only like to have the most recent development branch,
359 but also like to B<fix> bugs, or extend features, you want to dive
360 into the sources. If you are a seasoned perl core diver, you don't
361 need no manuals, tips, roadmaps, perlguts.pod or other aids to find
362 your way around. But if you are a starter, the patches may help you
363 in finding where you should start and how to change the bits that
366 The file B<Changes> is updated on occasions the pumpking sees as his
367 own little sync points. On those occasions, he releases a tar-ball of
368 the current source tree (i.e. perl@7582.tar.gz), which will be an
369 excellent point to start with when choosing to use the 'rsync the
370 patches' scheme. Starting with perl@7582, which means a set of source
371 files on which the latest applied patch is number 7582, you apply all
372 succeeding patches available from than on (7583, 7584, ...).
374 You can use the patches later as a kind of search archive.
378 =item Finding a start point
380 If you want to fix/change the behaviour of function/feature Foo, just
381 scan the patches for patches that mention Foo either in the subject,
382 the comments, or the body of the fix. A good change the patch shows
383 you the files that are affected by that patch which are very likely
384 to be the starting point of your journey into the guts of perl.
386 =item Finding how to fix a bug
388 If you've found I<where> the function/feature Foo misbehaves, but you
389 don't know how to fix it (but you do know the change you want to
390 make), you can, again, peruse the patches for similar changes and
391 look how others apply the fix.
393 =item Finding the source of misbehaviour
395 When you keep in sync with bleadperl, the pumpking would love to
396 I<see> that the community efforts realy work. So after each of his
397 sync points, you are to 'make test' to check if everything is still
398 in working order. If it is, you do 'make ok', which will send an OK
399 report to perlbug@perl.org. (If you do not have access to a mailer
400 from the sytem you just finished successfully 'make test', you can
401 do 'make okfile', which creates the file C<perl.ok>, which you can
402 than take to your favourite mailer and mail yourself).
404 But of course, as allways, things will not allways lead to a success
405 path, and one or more test do not pass the 'make test'. Before
406 sending in a bug report (using 'make nok' or 'make nokfile'), check
407 the mailing list if someone else has reported the bug already and if
408 so, confirm it by replying to that message. If not, you might want to
409 trace the source of that misbehaviour B<before> sending in the bug,
410 which will help all the other porters in finding the solution.
412 Here the saved patches come in very handy. You can check in there
413 which patch changed what file and what change caused the
414 misbehaviour. If you note that in the bug report, it saves the one
415 trying to solve it, looking for that point.
419 If searching the patches is too bothersome, you might consider using
420 perl's bugtron to find more information about discussions and
421 ramblings on posted bugs.
425 =head2 Submitting patches
427 Always submit patches to I<perl5-porters@perl.org>. This lets other
428 porters review your patch, which catches a surprising number of errors
429 in patches. Either use the diff program (available in source code
430 form from I<ftp://ftp.gnu.org/pub/gnu/>), or use Johan Vromans'
431 I<makepatch> (available from I<CPAN/authors/id/JV/>). Unified diffs
432 are preferred, but context diffs are accepted. Do not send RCS-style
433 diffs or diffs without context lines. More information is given in
434 the I<Porting/patching.pod> file in the Perl source distribution.
435 Please patch against the latest B<development> version (e.g., if
436 you're fixing a bug in the 5.005 track, patch against the latest
437 5.005_5x version). Only patches that survive the heat of the
438 development branch get applied to maintenance versions.
440 Your patch should update the documentation and test suite.
442 To report a bug in Perl, use the program I<perlbug> which comes with
443 Perl (if you can't get Perl to work, send mail to the address
444 I<perlbug@perl.com> or I<perlbug@perl.org>). Reporting bugs through
445 I<perlbug> feeds into the automated bug-tracking system, access to
446 which is provided through the web at I<http://bugs.perl.org/>. It
447 often pays to check the archives of the perl5-porters mailing list to
448 see whether the bug you're reporting has been reported before, and if
449 so whether it was considered a bug. See above for the location of
450 the searchable archives.
452 The CPAN testers (I<http://testers.cpan.org/>) are a group of
453 volunteers who test CPAN modules on a variety of platforms. Perl Labs
454 (I<http://labs.perl.org/>) automatically tests Perl source releases on
455 platforms and gives feedback to the CPAN testers mailing list. Both
456 efforts welcome volunteers.
458 It's a good idea to read and lurk for a while before chipping in.
459 That way you'll get to see the dynamic of the conversations, learn the
460 personalities of the players, and hopefully be better prepared to make
461 a useful contribution when do you speak up.
463 If after all this you still think you want to join the perl5-porters
464 mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
465 unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
467 To hack on the Perl guts, you'll need to read the following things:
473 This is of paramount importance, since it's the documentation of what
474 goes where in the Perl source. Read it over a couple of times and it
475 might start to make sense - don't worry if it doesn't yet, because the
476 best way to study it is to read it in conjunction with poking at Perl
477 source, and we'll do that later on.
479 You might also want to look at Gisle Aas's illustrated perlguts -
480 there's no guarantee that this will be absolutely up-to-date with the
481 latest documentation in the Perl core, but the fundamentals will be
482 right. (http://gisle.aas.no/perl/illguts/)
484 =item L<perlxstut> and L<perlxs>
486 A working knowledge of XSUB programming is incredibly useful for core
487 hacking; XSUBs use techniques drawn from the PP code, the portion of the
488 guts that actually executes a Perl program. It's a lot gentler to learn
489 those techniques from simple examples and explanation than from the core
494 The documentation for the Perl API explains what some of the internal
495 functions do, as well as the many macros used in the source.
497 =item F<Porting/pumpkin.pod>
499 This is a collection of words of wisdom for a Perl porter; some of it is
500 only useful to the pumpkin holder, but most of it applies to anyone
501 wanting to go about Perl development.
503 =item The perl5-porters FAQ
505 This is posted to perl5-porters at the beginning on every month, and
506 should be available from http://perlhacker.org/p5p-faq; alternatively,
507 you can get the FAQ emailed to you by sending mail to
508 C<perl5-porters-faq@perl.org>. It contains hints on reading
509 perl5-porters, information on how perl5-porters works and how Perl
510 development in general works.
514 =head2 Finding Your Way Around
516 Perl maintenance can be split into a number of areas, and certain people
517 (pumpkins) will have responsibility for each area. These areas sometimes
518 correspond to files or directories in the source kit. Among the areas are:
524 Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
525 subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
526 contains the core XS modules.
530 Documentation maintenance includes looking after everything in the
531 F<pod/> directory, (as well as contributing new documentation) and
532 the documentation to the modules in core.
536 The configure process is the way we make Perl portable across the
537 myriad of operating systems it supports. Responsibility for the
538 configure, build and installation process, as well as the overall
539 portability of the core code rests with the configure pumpkin - others
540 help out with individual operating systems.
542 The files involved are the operating system directories, (F<win32/>,
543 F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
544 and F<Makefile>, as well as the metaconfig files which generate
545 F<Configure>. (metaconfig isn't included in the core distribution.)
549 And of course, there's the core of the Perl interpreter itself. Let's
550 have a look at that in a little more detail.
554 Before we leave looking at the layout, though, don't forget that
555 F<MANIFEST> contains not only the file names in the Perl distribution,
556 but short descriptions of what's in them, too. For an overview of the
557 important files, try this:
559 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
561 =head2 Elements of the interpreter
563 The work of the interpreter has two main stages: compiling the code
564 into the internal representation, or bytecode, and then executing it.
565 L<perlguts/Compiled code> explains exactly how the compilation stage
568 Here is a short breakdown of perl's operation:
574 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
575 This is very high-level code, enough to fit on a single screen, and it
576 resembles the code found in L<perlembed>; most of the real action takes
579 First, F<perlmain.c> allocates some memory and constructs a Perl
582 1 PERL_SYS_INIT3(&argc,&argv,&env);
584 3 if (!PL_do_undump) {
585 4 my_perl = perl_alloc();
588 7 perl_construct(my_perl);
589 8 PL_perl_destruct_level = 0;
592 Line 1 is a macro, and its definition is dependent on your operating
593 system. Line 3 references C<PL_do_undump>, a global variable - all
594 global variables in Perl start with C<PL_>. This tells you whether the
595 current running program was created with the C<-u> flag to perl and then
596 F<undump>, which means it's going to be false in any sane context.
598 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
599 interpreter. It's quite a simple function, and the guts of it looks like
602 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
604 Here you see an example of Perl's system abstraction, which we'll see
605 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
606 own C<malloc> as defined in F<malloc.c> if you selected that option at
609 Next, in line 7, we construct the interpreter; this sets up all the
610 special variables that Perl needs, the stacks, and so on.
612 Now we pass Perl the command line options, and tell it to go:
614 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
616 exitstatus = perl_run(my_perl);
620 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
621 in F<perl.c>, which processes the command line options, sets up any
622 statically linked XS modules, opens the program and calls C<yyparse> to
627 The aim of this stage is to take the Perl source, and turn it into an op
628 tree. We'll see what one of those looks like later. Strictly speaking,
629 there's three things going on here.
631 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
632 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
633 B<is> a YACC grammar for Perl!) The job of the parser is to take your
634 code and `understand' it, splitting it into sentences, deciding which
635 operands go with which operators and so on.
637 The parser is nobly assisted by the lexer, which chunks up your input
638 into tokens, and decides what type of thing each token is: a variable
639 name, an operator, a bareword, a subroutine, a core function, and so on.
640 The main point of entry to the lexer is C<yylex>, and that and its
641 associated routines can be found in F<toke.c>. Perl isn't much like
642 other computer languages; it's highly context sensitive at times, it can
643 be tricky to work out what sort of token something is, or where a token
644 ends. As such, there's a lot of interplay between the tokeniser and the
645 parser, which can get pretty frightening if you're not used to it.
647 As the parser understands a Perl program, it builds up a tree of
648 operations for the interpreter to perform during execution. The routines
649 which construct and link together the various operations are to be found
650 in F<op.c>, and will be examined later.
654 Now the parsing stage is complete, and the finished tree represents
655 the operations that the Perl interpreter needs to perform to execute our
656 program. Next, Perl does a dry run over the tree looking for
657 optimisations: constant expressions such as C<3 + 4> will be computed
658 now, and the optimizer will also see if any multiple operations can be
659 replaced with a single one. For instance, to fetch the variable C<$foo>,
660 instead of grabbing the glob C<*foo> and looking at the scalar
661 component, the optimizer fiddles the op tree to use a function which
662 directly looks up the scalar in question. The main optimizer is C<peep>
663 in F<op.c>, and many ops have their own optimizing functions.
667 Now we're finally ready to go: we have compiled Perl byte code, and all
668 that's left to do is run it. The actual execution is done by the
669 C<runops_standard> function in F<run.c>; more specifically, it's done by
670 these three innocent looking lines:
672 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
676 You may be more comfortable with the Perl version of that:
678 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
680 Well, maybe not. Anyway, each op contains a function pointer, which
681 stipulates the function which will actually carry out the operation.
682 This function will return the next op in the sequence - this allows for
683 things like C<if> which choose the next op dynamically at run time.
684 The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
685 execution if required.
687 The actual functions called are known as PP code, and they're spread
688 between four files: F<pp_hot.c> contains the `hot' code, which is most
689 often used and highly optimized, F<pp_sys.c> contains all the
690 system-specific functions, F<pp_ctl.c> contains the functions which
691 implement control structures (C<if>, C<while> and the like) and F<pp.c>
692 contains everything else. These are, if you like, the C code for Perl's
693 built-in functions and operators.
697 =head2 Internal Variable Types
699 You should by now have had a look at L<perlguts>, which tells you about
700 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
703 These variables are used not only to represent Perl-space variables, but
704 also any constants in the code, as well as some structures completely
705 internal to Perl. The symbol table, for instance, is an ordinary Perl
706 hash. Your code is represented by an SV as it's read into the parser;
707 any program files you call are opened via ordinary Perl filehandles, and
710 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
711 Perl program. Let's see, for instance, how Perl treats the constant
714 % perl -MDevel::Peek -e 'Dump("hello")'
715 1 SV = PV(0xa041450) at 0xa04ecbc
717 3 FLAGS = (POK,READONLY,pPOK)
718 4 PV = 0xa0484e0 "hello"\0
722 Reading C<Devel::Peek> output takes a bit of practise, so let's go
723 through it line by line.
725 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
726 memory. SVs themselves are very simple structures, but they contain a
727 pointer to a more complex structure. In this case, it's a PV, a
728 structure which holds a string value, at location C<0xa041450>. Line 2
729 is the reference count; there are no other references to this data, so
732 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
733 read-only SV (because it's a constant) and the data is a PV internally.
734 Next we've got the contents of the string, starting at location
737 Line 5 gives us the current length of the string - note that this does
738 B<not> include the null terminator. Line 6 is not the length of the
739 string, but the length of the currently allocated buffer; as the string
740 grows, Perl automatically extends the available storage via a routine
743 You can get at any of these quantities from C very easily; just add
744 C<Sv> to the name of the field shown in the snippet, and you've got a
745 macro which will return the value: C<SvCUR(sv)> returns the current
746 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
747 C<SvPV(sv, len)> returns the string itself with its length, and so on.
748 More macros to manipulate these properties can be found in L<perlguts>.
750 Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
753 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
758 6 junk = SvPV_force(sv, tlen);
759 7 SvGROW(sv, tlen + len + 1);
762 10 Move(ptr,SvPVX(sv)+tlen,len,char);
764 12 *SvEND(sv) = '\0';
765 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
769 This is a function which adds a string, C<ptr>, of length C<len> onto
770 the end of the PV stored in C<sv>. The first thing we do in line 6 is
771 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
772 macro to force a PV. As a side effect, C<tlen> gets set to the current
773 value of the PV, and the PV itself is returned to C<junk>.
775 In line 7, we make sure that the SV will have enough room to accommodate
776 the old string, the new string and the null terminator. If C<LEN> isn't
777 big enough, C<SvGROW> will reallocate space for us.
779 Now, if C<junk> is the same as the string we're trying to add, we can
780 grab the string directly from the SV; C<SvPVX> is the address of the PV
783 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
784 memory around: we move the string C<ptr> to the end of the PV - that's
785 the start of the PV plus its current length. We're moving C<len> bytes
786 of type C<char>. After doing so, we need to tell Perl we've extended the
787 string, by altering C<CUR> to reflect the new length. C<SvEND> is a
788 macro which gives us the end of the string, so that needs to be a
791 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
792 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
793 want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
794 version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
795 and turns on POK. The final C<SvTAINT> is a macro which launders tainted
796 data if taint mode is turned on.
798 AVs and HVs are more complicated, but SVs are by far the most common
799 variable type being thrown around. Having seen something of how we
800 manipulate these, let's go on and look at how the op tree is
805 First, what is the op tree, anyway? The op tree is the parsed
806 representation of your program, as we saw in our section on parsing, and
807 it's the sequence of operations that Perl goes through to execute your
808 program, as we saw in L</Running>.
810 An op is a fundamental operation that Perl can perform: all the built-in
811 functions and operators are ops, and there are a series of ops which
812 deal with concepts the interpreter needs internally - entering and
813 leaving a block, ending a statement, fetching a variable, and so on.
815 The op tree is connected in two ways: you can imagine that there are two
816 "routes" through it, two orders in which you can traverse the tree.
817 First, parse order reflects how the parser understood the code, and
818 secondly, execution order tells perl what order to perform the
821 The easiest way to examine the op tree is to stop Perl after it has
822 finished parsing, and get it to dump out the tree. This is exactly what
823 the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
825 Let's have a look at how Perl sees C<$a = $b + $c>:
827 % perl -MO=Terse -e '$a=$b+$c'
828 1 LISTOP (0x8179888) leave
829 2 OP (0x81798b0) enter
830 3 COP (0x8179850) nextstate
831 4 BINOP (0x8179828) sassign
832 5 BINOP (0x8179800) add [1]
833 6 UNOP (0x81796e0) null [15]
834 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
835 8 UNOP (0x81797e0) null [15]
836 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
837 10 UNOP (0x816b4f0) null [15]
838 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
840 Let's start in the middle, at line 4. This is a BINOP, a binary
841 operator, which is at location C<0x8179828>. The specific operator in
842 question is C<sassign> - scalar assignment - and you can find the code
843 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
844 binary operator, it has two children: the add operator, providing the
845 result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
848 Line 10 is the null op: this does exactly nothing. What is that doing
849 there? If you see the null op, it's a sign that something has been
850 optimized away after parsing. As we mentioned in L</Optimization>,
851 the optimization stage sometimes converts two operations into one, for
852 example when fetching a scalar variable. When this happens, instead of
853 rewriting the op tree and cleaning up the dangling pointers, it's easier
854 just to replace the redundant operation with the null op. Originally,
855 the tree would have looked like this:
857 10 SVOP (0x816b4f0) rv2sv [15]
858 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
860 That is, fetch the C<a> entry from the main symbol table, and then look
861 at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
862 happens to do both these things.
864 The right hand side, starting at line 5 is similar to what we've just
865 seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
868 Now, what's this about?
870 1 LISTOP (0x8179888) leave
871 2 OP (0x81798b0) enter
872 3 COP (0x8179850) nextstate
874 C<enter> and C<leave> are scoping ops, and their job is to perform any
875 housekeeping every time you enter and leave a block: lexical variables
876 are tidied up, unreferenced variables are destroyed, and so on. Every
877 program will have those first three lines: C<leave> is a list, and its
878 children are all the statements in the block. Statements are delimited
879 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
880 the ops to be performed for each statement being the children of
881 C<nextstate>. C<enter> is a single op which functions as a marker.
883 That's how Perl parsed the program, from top to bottom:
896 However, it's impossible to B<perform> the operations in this order:
897 you have to find the values of C<$b> and C<$c> before you add them
898 together, for instance. So, the other thread that runs through the op
899 tree is the execution order: each op has a field C<op_next> which points
900 to the next op to be run, so following these pointers tells us how perl
901 executes the code. We can traverse the tree in this order using
902 the C<exec> option to C<B::Terse>:
904 % perl -MO=Terse,exec -e '$a=$b+$c'
905 1 OP (0x8179928) enter
906 2 COP (0x81798c8) nextstate
907 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
908 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
909 5 BINOP (0x8179878) add [1]
910 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
911 7 BINOP (0x81798a0) sassign
912 8 LISTOP (0x8179900) leave
914 This probably makes more sense for a human: enter a block, start a
915 statement. Get the values of C<$b> and C<$c>, and add them together.
916 Find C<$a>, and assign one to the other. Then leave.
918 The way Perl builds up these op trees in the parsing process can be
919 unravelled by examining F<perly.y>, the YACC grammar. Let's take the
920 piece we need to construct the tree for C<$a = $b + $c>
922 1 term : term ASSIGNOP term
923 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
925 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
927 If you're not used to reading BNF grammars, this is how it works: You're
928 fed certain things by the tokeniser, which generally end up in upper
929 case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
930 code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
931 `terminal symbols', because you can't get any simpler than them.
933 The grammar, lines one and three of the snippet above, tells you how to
934 build up more complex forms. These complex forms, `non-terminal symbols'
935 are generally placed in lower case. C<term> here is a non-terminal
936 symbol, representing a single expression.
938 The grammar gives you the following rule: you can make the thing on the
939 left of the colon if you see all the things on the right in sequence.
940 This is called a "reduction", and the aim of parsing is to completely
941 reduce the input. There are several different ways you can perform a
942 reduction, separated by vertical bars: so, C<term> followed by C<=>
943 followed by C<term> makes a C<term>, and C<term> followed by C<+>
944 followed by C<term> can also make a C<term>.
946 So, if you see two terms with an C<=> or C<+>, between them, you can
947 turn them into a single expression. When you do this, you execute the
948 code in the block on the next line: if you see C<=>, you'll do the code
949 in line 2. If you see C<+>, you'll do the code in line 4. It's this code
950 which contributes to the op tree.
953 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
955 What this does is creates a new binary op, and feeds it a number of
956 variables. The variables refer to the tokens: C<$1> is the first token in
957 the input, C<$2> the second, and so on - think regular expression
958 backreferences. C<$$> is the op returned from this reduction. So, we
959 call C<newBINOP> to create a new binary operator. The first parameter to
960 C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
961 operator, so we want the type to be C<ADDOP>. We could specify this
962 directly, but it's right there as the second token in the input, so we
963 use C<$2>. The second parameter is the op's flags: 0 means `nothing
964 special'. Then the things to add: the left and right hand side of our
965 expression, in scalar context.
969 When perl executes something like C<addop>, how does it pass on its
970 results to the next op? The answer is, through the use of stacks. Perl
971 has a number of stacks to store things it's currently working on, and
972 we'll look at the three most important ones here.
978 Arguments are passed to PP code and returned from PP code using the
979 argument stack, C<ST>. The typical way to handle arguments is to pop
980 them off the stack, deal with them how you wish, and then push the result
981 back onto the stack. This is how, for instance, the cosine operator
986 value = Perl_cos(value);
989 We'll see a more tricky example of this when we consider Perl's macros
990 below. C<POPn> gives you the NV (floating point value) of the top SV on
991 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
992 the result back as an NV. The C<X> in C<XPUSHn> means that the stack
993 should be extended if necessary - it can't be necessary here, because we
994 know there's room for one more item on the stack, since we've just
995 removed one! The C<XPUSH*> macros at least guarantee safety.
997 Alternatively, you can fiddle with the stack directly: C<SP> gives you
998 the first element in your portion of the stack, and C<TOP*> gives you
999 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1000 negation of an integer:
1004 Just set the integer value of the top stack entry to its negation.
1006 Argument stack manipulation in the core is exactly the same as it is in
1007 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1008 description of the macros used in stack manipulation.
1012 I say `your portion of the stack' above because PP code doesn't
1013 necessarily get the whole stack to itself: if your function calls
1014 another function, you'll only want to expose the arguments aimed for the
1015 called function, and not (necessarily) let it get at your own data. The
1016 way we do this is to have a `virtual' bottom-of-stack, exposed to each
1017 function. The mark stack keeps bookmarks to locations in the argument
1018 stack usable by each function. For instance, when dealing with a tied
1019 variable, (internally, something with `P' magic) Perl has to call
1020 methods for accesses to the tied variables. However, we need to separate
1021 the arguments exposed to the method to the argument exposed to the
1022 original function - the store or fetch or whatever it may be. Here's how
1023 the tied C<push> is implemented; see C<av_push> in F<av.c>:
1027 3 PUSHs(SvTIED_obj((SV*)av, mg));
1031 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1035 The lines which concern the mark stack are the first, fifth and last
1036 lines: they save away, restore and remove the current position of the
1039 Let's examine the whole implementation, for practice:
1043 Push the current state of the stack pointer onto the mark stack. This is
1044 so that when we've finished adding items to the argument stack, Perl
1045 knows how many things we've added recently.
1048 3 PUSHs(SvTIED_obj((SV*)av, mg));
1051 We're going to add two more items onto the argument stack: when you have
1052 a tied array, the C<PUSH> subroutine receives the object and the value
1053 to be pushed, and that's exactly what we have here - the tied object,
1054 retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1058 Next we tell Perl to make the change to the global stack pointer: C<dSP>
1059 only gave us a local copy, not a reference to the global.
1062 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1065 C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1066 variables are tidied up, everything that has been localised gets
1067 its previous value returned, and so on. Think of them as the C<{> and
1068 C<}> of a Perl block.
1070 To actually do the magic method call, we have to call a subroutine in
1071 Perl space: C<call_method> takes care of that, and it's described in
1072 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1073 going to discard its return value.
1077 Finally, we remove the value we placed on the mark stack, since we
1078 don't need it any more.
1082 C doesn't have a concept of local scope, so perl provides one. We've
1083 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1084 stack implements the C equivalent of, for example:
1091 See L<perlguts/Localising Changes> for how to use the save stack.
1095 =head2 Millions of Macros
1097 One thing you'll notice about the Perl source is that it's full of
1098 macros. Some have called the pervasive use of macros the hardest thing
1099 to understand, others find it adds to clarity. Let's take an example,
1100 the code which implements the addition operator:
1104 3 djSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1107 6 SETn( left + right );
1112 Every line here (apart from the braces, of course) contains a macro. The
1113 first line sets up the function declaration as Perl expects for PP code;
1114 line 3 sets up variable declarations for the argument stack and the
1115 target, the return value of the operation. Finally, it tries to see if
1116 the addition operation is overloaded; if so, the appropriate subroutine
1119 Line 5 is another variable declaration - all variable declarations start
1120 with C<d> - which pops from the top of the argument stack two NVs (hence
1121 C<nn>) and puts them into the variables C<right> and C<left>, hence the
1122 C<rl>. These are the two operands to the addition operator. Next, we
1123 call C<SETn> to set the NV of the return value to the result of adding
1124 the two values. This done, we return - the C<RETURN> macro makes sure
1125 that our return value is properly handled, and we pass the next operator
1126 to run back to the main run loop.
1128 Most of these macros are explained in L<perlapi>, and some of the more
1129 important ones are explained in L<perlxs> as well. Pay special attention
1130 to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1131 the C<[pad]THX_?> macros.
1134 =head2 Poking at Perl
1136 To really poke around with Perl, you'll probably want to build Perl for
1137 debugging, like this:
1139 ./Configure -d -D optimize=-g
1142 C<-g> is a flag to the C compiler to have it produce debugging
1143 information which will allow us to step through a running program.
1144 F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1145 enables all the internal debugging code in Perl. There are a whole bunch
1146 of things you can debug with this: L<perlrun> lists them all, and the
1147 best way to find out about them is to play about with them. The most
1148 useful options are probably
1150 l Context (loop) stack processing
1152 o Method and overloading resolution
1153 c String/numeric conversions
1155 Some of the functionality of the debugging code can be achieved using XS
1158 -Dr => use re 'debug'
1159 -Dx => use O 'Debug'
1161 =head2 Using a source-level debugger
1163 If the debugging output of C<-D> doesn't help you, it's time to step
1164 through perl's execution with a source-level debugger.
1170 We'll use C<gdb> for our examples here; the principles will apply to any
1171 debugger, but check the manual of the one you're using.
1175 To fire up the debugger, type
1179 You'll want to do that in your Perl source tree so the debugger can read
1180 the source code. You should see the copyright message, followed by the
1185 C<help> will get you into the documentation, but here are the most
1192 Run the program with the given arguments.
1194 =item break function_name
1196 =item break source.c:xxx
1198 Tells the debugger that we'll want to pause execution when we reach
1199 either the named function (but see L</Function names>!) or the given
1200 line in the named source file.
1204 Steps through the program a line at a time.
1208 Steps through the program a line at a time, without descending into
1213 Run until the next breakpoint.
1217 Run until the end of the current function, then stop again.
1221 Just pressing Enter will do the most recent operation again - it's a
1222 blessing when stepping through miles of source code.
1226 Execute the given C code and print its results. B<WARNING>: Perl makes
1227 heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1228 substitute them yourself. So, for instance, you can't say
1230 print SvPV_nolen(sv)
1234 print Perl_sv_2pv_nolen(sv)
1236 You may find it helpful to have a "macro dictionary", which you can
1237 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1238 recursively apply the macros for you.
1242 =head2 Dumping Perl Data Structures
1244 One way to get around this macro hell is to use the dumping functions in
1245 F<dump.c>; these work a little like an internal
1246 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1247 that you can't get at from Perl. Let's take an example. We'll use the
1248 C<$a = $b + $c> we used before, but give it a bit of context:
1249 C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1251 What about C<pp_add>, the function we examined earlier to implement the
1254 (gdb) break Perl_pp_add
1255 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1257 Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Function Names>.
1258 With the breakpoint in place, we can run our program:
1260 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1262 Lots of junk will go past as gdb reads in the relevant source files and
1263 libraries, and then:
1265 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1266 309 djSP; dATARGET; tryAMAGICbin(add,opASSIGN);
1271 We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1272 arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1275 #define dPOPTOPnnrl_ul NV right = POPn; \
1276 SV *leftsv = TOPs; \
1277 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1279 C<POPn> takes the SV from the top of the stack and obtains its NV either
1280 directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1281 C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1282 C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1283 C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1285 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1286 convert it. If we step again, we'll find ourselves there:
1288 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1292 We can now use C<Perl_sv_dump> to investigate the SV:
1294 SV = PV(0xa057cc0) at 0xa0675d0
1297 PV = 0xa06a510 "6XXXX"\0
1302 We know we're going to get C<6> from this, so let's finish the
1306 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1307 0x462669 in Perl_pp_add () at pp_hot.c:311
1310 We can also dump out this op: the current op is always stored in
1311 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1312 similar output to L<B::Debug|B::Debug>.
1315 13 TYPE = add ===> 14
1317 FLAGS = (SCALAR,KIDS)
1319 TYPE = null ===> (12)
1321 FLAGS = (SCALAR,KIDS)
1323 11 TYPE = gvsv ===> 12
1329 < finish this later >
1333 All right, we've now had a look at how to navigate the Perl sources and
1334 some things you'll need to know when fiddling with them. Let's now get
1335 on and create a simple patch. Here's something Larry suggested: if a
1336 C<U> is the first active format during a C<pack>, (for example,
1337 C<pack "U3C8", @stuff>) then the resulting string should be treated as
1340 How do we prepare to fix this up? First we locate the code in question -
1341 the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1342 files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1343 altering this file, let's copy it to F<pp.c~>.
1345 Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1346 loop over the pattern, taking each format character in turn into
1347 C<datum_type>. Then for each possible format character, we swallow up
1348 the other arguments in the pattern (a field width, an asterisk, and so
1349 on) and convert the next chunk input into the specified format, adding
1350 it onto the output SV C<cat>.
1352 How do we know if the C<U> is the first format in the C<pat>? Well, if
1353 we have a pointer to the start of C<pat> then, if we see a C<U> we can
1354 test whether we're still at the start of the string. So, here's where
1358 register char *pat = SvPVx(*++MARK, fromlen);
1359 register char *patend = pat + fromlen;
1364 We'll have another string pointer in there:
1367 register char *pat = SvPVx(*++MARK, fromlen);
1368 register char *patend = pat + fromlen;
1374 And just before we start the loop, we'll set C<patcopy> to be the start
1379 sv_setpvn(cat, "", 0);
1381 while (pat < patend) {
1383 Now if we see a C<U> which was at the start of the string, we turn on
1384 the UTF8 flag for the output SV, C<cat>:
1386 + if (datumtype == 'U' && pat==patcopy+1)
1388 if (datumtype == '#') {
1389 while (pat < patend && *pat != '\n')
1392 Remember that it has to be C<patcopy+1> because the first character of
1393 the string is the C<U> which has been swallowed into C<datumtype!>
1395 Oops, we forgot one thing: what if there are spaces at the start of the
1396 pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1397 character, even though it's not the first thing in the pattern. In this
1398 case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1400 if (isSPACE(datumtype))
1405 if (isSPACE(datumtype)) {
1410 OK. That's the C part done. Now we must do two additional things before
1411 this patch is ready to go: we've changed the behaviour of Perl, and so
1412 we must document that change. We must also provide some more regression
1413 tests to make sure our patch works and doesn't create a bug somewhere
1414 else along the line.
1416 The regression tests for each operator live in F<t/op/>, and so we make
1417 a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our tests
1418 to the end. First, we'll test that the C<U> does indeed create Unicode
1421 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1422 print "ok $test\n"; $test++;
1424 Now we'll test that we got that space-at-the-beginning business right:
1426 print 'not ' unless "1.20.300.4000" eq
1427 sprintf "%vd", pack(" U*",1,20,300,4000);
1428 print "ok $test\n"; $test++;
1430 And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1431 the first active format:
1433 print 'not ' unless v1.20.300.4000 ne
1434 sprintf "%vd", pack("C0U*",1,20,300,4000);
1435 print "ok $test\n"; $test++;
1437 Mustn't forget to change the number of tests which appears at the top, or
1438 else the automated tester will get confused:
1443 We now compile up Perl, and run it through the test suite. Our new
1446 Finally, the documentation. The job is never done until the paperwork is
1447 over, so let's describe the change we've just made. The relevant place
1448 is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1449 this text in the description of C<pack>:
1453 If the pattern begins with a C<U>, the resulting string will be treated
1454 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1455 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1456 characters. If you don't want this to happen, you can begin your pattern
1457 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1458 string, and then follow this with a C<U*> somewhere in your pattern.
1460 All done. Now let's create the patch. F<Porting/patching.pod> tells us
1461 that if we're making major changes, we should copy the entire directory
1462 to somewhere safe before we begin fiddling, and then do
1464 diff -ruN old new > patch
1466 However, we know which files we've changed, and we can simply do this:
1468 diff -u pp.c~ pp.c > patch
1469 diff -u t/op/pack.t~ t/op/pack.t >> patch
1470 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1472 We end up with a patch looking a little like this:
1474 --- pp.c~ Fri Jun 02 04:34:10 2000
1475 +++ pp.c Fri Jun 16 11:37:25 2000
1476 @@ -4375,6 +4375,7 @@
1479 register char *pat = SvPVx(*++MARK, fromlen);
1481 register char *patend = pat + fromlen;
1484 @@ -4405,6 +4406,7 @@
1487 And finally, we submit it, with our rationale, to perl5-porters. Job
1490 =head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1492 Sometimes it helps to use external tools while debugging and
1493 testing Perl. This section tries to guide you through using
1494 some common testing and debugging tools with Perl. This is
1495 meant as a guide to interfacing these tools with Perl, not
1496 as any kind of guide to the use of the tools themselves.
1498 =head2 Rational Software's Purify
1500 Purify is a commercial tool that is helpful in identifying
1501 memory overruns, wild pointers, memory leaks and other such
1502 badness. Perl must be compiled in a specific way for
1503 optimal testing with Purify. Purify is available under
1504 Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1506 The only currently known leaks happen when there are
1507 compile-time errors within eval or require. (Fixing these
1508 is non-trivial, unfortunately, but they must be fixed
1511 =head2 Purify on Unix
1513 On Unix, Purify creates a new Perl binary. To get the most
1514 benefit out of Purify, you should create the perl to Purify
1517 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1518 -Uusemymalloc -Dusemultiplicity
1520 where these arguments mean:
1524 =item -Accflags=-DPURIFY
1526 Disables Perl's arena memory allocation functions, as well as
1527 forcing use of memory allocation functions derived from the
1530 =item -Doptimize='-g'
1532 Adds debugging information so that you see the exact source
1533 statements where the problem occurs. Without this flag, all
1534 you will see is the source filename of where the error occurred.
1538 Disable Perl's malloc so that Purify can more closely monitor
1539 allocations and leaks. Using Perl's malloc will make Purify
1540 report most leaks in the "potential" leaks category.
1542 =item -Dusemultiplicity
1544 Enabling the multiplicity option allows perl to clean up
1545 thoroughly when the interpreter shuts down, which reduces the
1546 number of bogus leak reports from Purify.
1550 Once you've compiled a perl suitable for Purify'ing, then you
1555 which creates a binary named 'pureperl' that has been Purify'ed.
1556 This binary is used in place of the standard 'perl' binary
1557 when you want to debug Perl memory problems.
1559 As an example, to show any memory leaks produced during the
1560 standard Perl testset you would create and run the Purify'ed
1565 ../pureperl -I../lib harness
1567 which would run Perl on test.pl and report any memory problems.
1569 Purify outputs messages in "Viewer" windows by default. If
1570 you don't have a windowing environment or if you simply
1571 want the Purify output to unobtrusively go to a log file
1572 instead of to the interactive window, use these following
1573 options to output to the log file "perl.log":
1575 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1576 -log-file=perl.log -append-logfile=yes"
1578 If you plan to use the "Viewer" windows, then you only need this option:
1580 setenv PURIFYOPTIONS "-chain-length=25"
1584 Purify on Windows NT instruments the Perl binary 'perl.exe'
1585 on the fly. There are several options in the makefile you
1586 should change to get the most use out of Purify:
1592 You should add -DPURIFY to the DEFINES line so the DEFINES
1593 line looks something like:
1595 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1597 to disable Perl's arena memory allocation functions, as
1598 well as to force use of memory allocation functions derived
1599 from the system malloc.
1601 =item USE_MULTI = define
1603 Enabling the multiplicity option allows perl to clean up
1604 thoroughly when the interpreter shuts down, which reduces the
1605 number of bogus leak reports from Purify.
1607 =item #PERL_MALLOC = define
1609 Disable Perl's malloc so that Purify can more closely monitor
1610 allocations and leaks. Using Perl's malloc will make Purify
1611 report most leaks in the "potential" leaks category.
1615 Adds debugging information so that you see the exact source
1616 statements where the problem occurs. Without this flag, all
1617 you will see is the source filename of where the error occurred.
1621 As an example, to show any memory leaks produced during the
1622 standard Perl testset you would create and run Purify as:
1627 purify ../perl -I../lib harness
1629 which would instrument Perl in memory, run Perl on test.pl,
1630 then finally report any memory problems.
1634 We've had a brief look around the Perl source, an overview of the stages
1635 F<perl> goes through when it's running your code, and how to use a
1636 debugger to poke at the Perl guts. We took a very simple problem and
1637 demonstrated how to solve it fully - with documentation, regression
1638 tests, and finally a patch for submission to p5p. Finally, we talked
1639 about how to use external tools to debug and test Perl.
1641 I'd now suggest you read over those references again, and then, as soon
1642 as possible, get your hands dirty. The best way to learn is by doing,
1649 Subscribe to perl5-porters, follow the patches and try and understand
1650 them; don't be afraid to ask if there's a portion you're not clear on -
1651 who knows, you may unearth a bug in the patch...
1655 Keep up to date with the bleeding edge Perl distributions and get
1656 familiar with the changes. Try and get an idea of what areas people are
1657 working on and the changes they're making.
1661 Do read the README associated with your operating system, i.e. README.aix
1662 on the IBM AIX OS. Don't hesitate to supply patches to that README if
1663 you find anything missing or changed over a new OS release.
1667 Find an area of Perl that seems interesting to you, and see if you can
1668 work out how it works. Scan through the source, and step over it in the
1669 debugger. Play, poke, investigate, fiddle! You'll probably get to
1670 understand not just your chosen area but a much wider range of F<perl>'s
1671 activity as well, and probably sooner than you'd think.
1677 =item I<The Road goes ever on and on, down from the door where it began.>
1681 If you can do these things, you've started on the long road to Perl porting.
1682 Thanks for wanting to help make Perl better - and happy hacking!
1686 This document was written by Nathan Torkington, and is maintained by
1687 the perl5-porters mailing list.