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1=head1 NAME
2
3perlhack - How to hack at the Perl internals
4
5=head1 DESCRIPTION
6
7This document attempts to explain how Perl development takes place,
8and ends with some suggestions for people wanting to become bona fide
9porters.
10
11The perl5-porters mailing list is where the Perl standard distribution
12is maintained and developed. The list can get anywhere from 10 to 150
13messages a day, depending on the heatedness of the debate. Most days
14there are two or three patches, extensions, features, or bugs being
15discussed at a time.
16
f8e3975a 17A searchable archive of the list is at either:
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18
19 http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
20
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21or
22
23 http://archive.develooper.com/perl5-porters@perl.org/
24
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25List subscribers (the porters themselves) come in several flavours.
26Some are quiet curious lurkers, who rarely pitch in and instead watch
27the ongoing development to ensure they're forewarned of new changes or
28features in Perl. Some are representatives of vendors, who are there
29to make sure that Perl continues to compile and work on their
30platforms. Some patch any reported bug that they know how to fix,
31some are actively patching their pet area (threads, Win32, the regexp
32engine), while others seem to do nothing but complain. In other
33words, it's your usual mix of technical people.
34
35Over this group of porters presides Larry Wall. He has the final word
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36in what does and does not change in the Perl language. Various
37releases of Perl are shepherded by a ``pumpking'', a porter
38responsible for gathering patches, deciding on a patch-by-patch
39feature-by-feature basis what will and will not go into the release.
40For instance, Gurusamy Sarathy is the pumpking for the 5.6 release of
41Perl.
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42
43In addition, various people are pumpkings for different things. For
44instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
45pumpkin, and Tom Christiansen is the documentation pumpking.
46
47Larry sees Perl development along the lines of the US government:
48there's the Legislature (the porters), the Executive branch (the
49pumpkings), and the Supreme Court (Larry). The legislature can
50discuss and submit patches to the executive branch all they like, but
51the executive branch is free to veto them. Rarely, the Supreme Court
52will side with the executive branch over the legislature, or the
53legislature over the executive branch. Mostly, however, the
54legislature and the executive branch are supposed to get along and
55work out their differences without impeachment or court cases.
56
57You might sometimes see reference to Rule 1 and Rule 2. Larry's power
58as Supreme Court is expressed in The Rules:
59
60=over 4
61
62=item 1
63
64Larry is always by definition right about how Perl should behave.
65This means he has final veto power on the core functionality.
66
67=item 2
68
69Larry is allowed to change his mind about any matter at a later date,
70regardless of whether he previously invoked Rule 1.
71
72=back
73
74Got that? Larry is always right, even when he was wrong. It's rare
75to see either Rule exercised, but they are often alluded to.
76
77New features and extensions to the language are contentious, because
78the criteria used by the pumpkings, Larry, and other porters to decide
79which features should be implemented and incorporated are not codified
80in a few small design goals as with some other languages. Instead,
81the heuristics are flexible and often difficult to fathom. Here is
82one person's list, roughly in decreasing order of importance, of
83heuristics that new features have to be weighed against:
84
85=over 4
86
87=item Does concept match the general goals of Perl?
88
89These haven't been written anywhere in stone, but one approximation
90is:
91
92 1. Keep it fast, simple, and useful.
93 2. Keep features/concepts as orthogonal as possible.
94 3. No arbitrary limits (platforms, data sizes, cultures).
95 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
96 5. Either assimilate new technologies, or build bridges to them.
97
98=item Where is the implementation?
99
100All the talk in the world is useless without an implementation. In
101almost every case, the person or people who argue for a new feature
102will be expected to be the ones who implement it. Porters capable
103of coding new features have their own agendas, and are not available
104to implement your (possibly good) idea.
105
106=item Backwards compatibility
107
108It's a cardinal sin to break existing Perl programs. New warnings are
109contentious--some say that a program that emits warnings is not
110broken, while others say it is. Adding keywords has the potential to
111break programs, changing the meaning of existing token sequences or
112functions might break programs.
113
114=item Could it be a module instead?
115
116Perl 5 has extension mechanisms, modules and XS, specifically to avoid
117the need to keep changing the Perl interpreter. You can write modules
118that export functions, you can give those functions prototypes so they
119can be called like built-in functions, you can even write XS code to
120mess with the runtime data structures of the Perl interpreter if you
121want to implement really complicated things. If it can be done in a
122module instead of in the core, it's highly unlikely to be added.
123
124=item Is the feature generic enough?
125
126Is this something that only the submitter wants added to the language,
127or would it be broadly useful? Sometimes, instead of adding a feature
128with a tight focus, the porters might decide to wait until someone
129implements the more generalized feature. For instance, instead of
130implementing a ``delayed evaluation'' feature, the porters are waiting
131for a macro system that would permit delayed evaluation and much more.
132
133=item Does it potentially introduce new bugs?
134
135Radical rewrites of large chunks of the Perl interpreter have the
136potential to introduce new bugs. The smaller and more localized the
137change, the better.
138
139=item Does it preclude other desirable features?
140
141A patch is likely to be rejected if it closes off future avenues of
142development. For instance, a patch that placed a true and final
143interpretation on prototypes is likely to be rejected because there
144are still options for the future of prototypes that haven't been
145addressed.
146
147=item Is the implementation robust?
148
149Good patches (tight code, complete, correct) stand more chance of
150going in. Sloppy or incorrect patches might be placed on the back
151burner until the pumpking has time to fix, or might be discarded
152altogether without further notice.
153
154=item Is the implementation generic enough to be portable?
155
156The worst patches make use of a system-specific features. It's highly
157unlikely that nonportable additions to the Perl language will be
158accepted.
159
160=item Is there enough documentation?
161
162Patches without documentation are probably ill-thought out or
163incomplete. Nothing can be added without documentation, so submitting
164a patch for the appropriate manpages as well as the source code is
165always a good idea. If appropriate, patches should add to the test
166suite as well.
167
168=item Is there another way to do it?
169
170Larry said ``Although the Perl Slogan is I<There's More Than One Way
171to Do It>, I hesitate to make 10 ways to do something''. This is a
172tricky heuristic to navigate, though--one man's essential addition is
173another man's pointless cruft.
174
175=item Does it create too much work?
176
177Work for the pumpking, work for Perl programmers, work for module
178authors, ... Perl is supposed to be easy.
179
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180=item Patches speak louder than words
181
182Working code is always preferred to pie-in-the-sky ideas. A patch to
183add a feature stands a much higher chance of making it to the language
184than does a random feature request, no matter how fervently argued the
185request might be. This ties into ``Will it be useful?'', as the fact
186that someone took the time to make the patch demonstrates a strong
187desire for the feature.
188
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189=back
190
191If you're on the list, you might hear the word ``core'' bandied
192around. It refers to the standard distribution. ``Hacking on the
193core'' means you're changing the C source code to the Perl
194interpreter. ``A core module'' is one that ships with Perl.
195
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196=head2 Keeping in sync
197
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198The source code to the Perl interpreter, in its different versions, is
199kept in a repository managed by a revision control system (which is
200currently the Perforce program, see http://perforce.com/). The
201pumpkings and a few others have access to the repository to check in
202changes. Periodically the pumpking for the development version of Perl
203will release a new version, so the rest of the porters can see what's
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204changed. The current state of the main trunk of repository, and patches
205that describe the individual changes that have happened since the last
206public release are available at this location:
207
208 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
209
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210If you are a member of the perl5-porters mailing list, it is a good
211thing to keep in touch with the most recent changes. If not only to
212verify if what you would have posted as a bug report isn't already
213solved in the most recent available perl development branch, also
214known as perl-current, bleading edge perl, bleedperl or bleadperl.
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215
216Needless to say, the source code in perl-current is usually in a perpetual
217state of evolution. You should expect it to be very buggy. Do B<not> use
218it for any purpose other than testing and development.
e8cd7eae 219
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220Keeping in sync with the most recent branch can be done in several ways,
221but the most convenient and reliable way is using B<rsync>, available at
222ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
223branch by FTP.)
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224
225If you choose to keep in sync using rsync, there are two approaches
3e148164 226to doing so:
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227
228=over 4
229
230=item rsync'ing the source tree
231
3e148164 232Presuming you are in the directory where your perl source resides
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233and you have rsync installed and available, you can `upgrade' to
234the bleadperl using:
235
236 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
237
238This takes care of updating every single item in the source tree to
239the latest applied patch level, creating files that are new (to your
240distribution) and setting date/time stamps of existing files to
241reflect the bleadperl status.
242
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243Note that this will not delete any files that were in '.' before
244the rsync. Once you are sure that the rsync is running correctly,
245run it with the --delete and the --dry-run options like this:
246
247 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
248
249This will I<simulate> an rsync run that also deletes files not
250present in the bleadperl master copy. Observe the results from
251this run closely. If you are sure that the actual run would delete
252no files precious to you, you could remove the '--dry-run' option.
253
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254You can than check what patch was the latest that was applied by
255looking in the file B<.patch>, which will show the number of the
256latest patch.
257
258If you have more than one machine to keep in sync, and not all of
259them have access to the WAN (so you are not able to rsync all the
260source trees to the real source), there are some ways to get around
261this problem.
262
263=over 4
264
265=item Using rsync over the LAN
266
267Set up a local rsync server which makes the rsynced source tree
3e148164 268available to the LAN and sync the other machines against this
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269directory.
270
271From http://rsync.samba.org/README.html:
272
273 "Rsync uses rsh or ssh for communication. It does not need to be
274 setuid and requires no special privileges for installation. It
3958b146 275 does not require an inetd entry or a daemon. You must, however,
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276 have a working rsh or ssh system. Using ssh is recommended for
277 its security features."
278
279=item Using pushing over the NFS
280
281Having the other systems mounted over the NFS, you can take an
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282active pushing approach by checking the just updated tree against
283the other not-yet synced trees. An example would be
284
285 #!/usr/bin/perl -w
286
287 use strict;
288 use File::Copy;
289
290 my %MF = map {
291 m/(\S+)/;
292 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
293 } `cat MANIFEST`;
294
295 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
296
297 foreach my $host (keys %remote) {
298 unless (-d $remote{$host}) {
299 print STDERR "Cannot Xsync for host $host\n";
300 next;
301 }
302 foreach my $file (keys %MF) {
303 my $rfile = "$remote{$host}/$file";
304 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
305 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
306 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
307 printf "%4s %-34s %8d %9d %8d %9d\n",
308 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
309 unlink $rfile;
310 copy ($file, $rfile);
311 utime time, $MF{$file}[2], $rfile;
312 chmod $MF{$file}[0], $rfile;
313 }
314 }
315
316though this is not perfect. It could be improved with checking
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317file checksums before updating. Not all NFS systems support
318reliable utime support (when used over the NFS).
319
320=back
321
322=item rsync'ing the patches
323
324The source tree is maintained by the pumpking who applies patches to
325the files in the tree. These patches are either created by the
326pumpking himself using C<diff -c> after updating the file manually or
327by applying patches sent in by posters on the perl5-porters list.
328These patches are also saved and rsync'able, so you can apply them
329yourself to the source files.
330
331Presuming you are in a directory where your patches reside, you can
3e148164 332get them in sync with
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333
334 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
335
336This makes sure the latest available patch is downloaded to your
337patch directory.
338
3e148164 339It's then up to you to apply these patches, using something like
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340
341 # last=`ls -rt1 *.gz | tail -1`
342 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
343 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
344 # cd ../perl-current
345 # patch -p1 -N <../perl-current-diffs/blead.patch
346
347or, since this is only a hint towards how it works, use CPAN-patchaperl
348from Andreas K├Ânig to have better control over the patching process.
349
350=back
351
f7e1e956 352=head2 Why rsync the source tree
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353
354=over 4
355
10f58044 356=item It's easier to rsync the source tree
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357
358Since you don't have to apply the patches yourself, you are sure all
359files in the source tree are in the right state.
360
361=item It's more recent
362
363According to Gurusamy Sarathy:
364
365 "... The rsync mirror is automatic and syncs with the repository
366 every five minutes.
367
3e148164 368 "Updating the patch area still requires manual intervention
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369 (with all the goofiness that implies, which you've noted) and
370 is typically on a daily cycle. Making this process automatic
371 is on my tuit list, but don't ask me when."
372
373=item It's more reliable
374
3e148164 375Well, since the patches are updated by hand, I don't have to say any
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376more ... (see Sarathy's remark).
377
378=back
379
f7e1e956 380=head2 Why rsync the patches
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381
382=over 4
383
10f58044 384=item It's easier to rsync the patches
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385
386If you have more than one machine that you want to keep in track with
3e148164 387bleadperl, it's easier to rsync the patches only once and then apply
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388them to all the source trees on the different machines.
389
390In case you try to keep in pace on 5 different machines, for which
391only one of them has access to the WAN, rsync'ing all the source
3e148164 392trees should than be done 5 times over the NFS. Having
a1f349fd 393rsync'ed the patches only once, I can apply them to all the source
3e148164 394trees automatically. Need you say more ;-)
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395
396=item It's a good reference
397
398If you do not only like to have the most recent development branch,
399but also like to B<fix> bugs, or extend features, you want to dive
400into the sources. If you are a seasoned perl core diver, you don't
401need no manuals, tips, roadmaps, perlguts.pod or other aids to find
402your way around. But if you are a starter, the patches may help you
403in finding where you should start and how to change the bits that
404bug you.
405
406The file B<Changes> is updated on occasions the pumpking sees as his
407own little sync points. On those occasions, he releases a tar-ball of
408the current source tree (i.e. perl@7582.tar.gz), which will be an
409excellent point to start with when choosing to use the 'rsync the
410patches' scheme. Starting with perl@7582, which means a set of source
411files on which the latest applied patch is number 7582, you apply all
f18956b7 412succeeding patches available from then on (7583, 7584, ...).
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413
414You can use the patches later as a kind of search archive.
415
416=over 4
417
418=item Finding a start point
419
420If you want to fix/change the behaviour of function/feature Foo, just
421scan the patches for patches that mention Foo either in the subject,
3e148164 422the comments, or the body of the fix. A good chance the patch shows
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423you the files that are affected by that patch which are very likely
424to be the starting point of your journey into the guts of perl.
425
426=item Finding how to fix a bug
427
428If you've found I<where> the function/feature Foo misbehaves, but you
429don't know how to fix it (but you do know the change you want to
430make), you can, again, peruse the patches for similar changes and
431look how others apply the fix.
432
433=item Finding the source of misbehaviour
434
435When you keep in sync with bleadperl, the pumpking would love to
3958b146 436I<see> that the community efforts really work. So after each of his
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437sync points, you are to 'make test' to check if everything is still
438in working order. If it is, you do 'make ok', which will send an OK
439report to perlbug@perl.org. (If you do not have access to a mailer
3e148164 440from the system you just finished successfully 'make test', you can
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441do 'make okfile', which creates the file C<perl.ok>, which you can
442than take to your favourite mailer and mail yourself).
443
3958b146 444But of course, as always, things will not always lead to a success
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445path, and one or more test do not pass the 'make test'. Before
446sending in a bug report (using 'make nok' or 'make nokfile'), check
447the mailing list if someone else has reported the bug already and if
448so, confirm it by replying to that message. If not, you might want to
449trace the source of that misbehaviour B<before> sending in the bug,
450which will help all the other porters in finding the solution.
451
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452Here the saved patches come in very handy. You can check the list of
453patches to see which patch changed what file and what change caused
454the misbehaviour. If you note that in the bug report, it saves the
455one trying to solve it, looking for that point.
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456
457=back
458
459If searching the patches is too bothersome, you might consider using
460perl's bugtron to find more information about discussions and
461ramblings on posted bugs.
462
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463If you want to get the best of both worlds, rsync both the source
464tree for convenience, reliability and ease and rsync the patches
465for reference.
466
52315700
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467=back
468
469
470=head2 Perlbug remote interface
471
472=over 4
473
474There are three (3) remote administrative interfaces for modifying bug status, category, etc. In all cases an admin must be first registered with the Perlbug database by sending an email request to richard@perl.org or bugmongers@perl.org.
475
476The main requirement is the willingness to classify, (with the emphasis on closing where possible :), outstanding bugs. Further explanation can be garnered from the web at http://bugs.perl.org/, or by asking on the admin mailing list at: bugmongers@perl.org
477
478For more info on the web see
479
480 http://bugs.perl.org/perlbug.cgi?req=spec
481
482
483B<The interfaces:>
484
485
486=item 1 http://bugs.perl.org
487
488Login via the web, (remove B<admin/> if only browsing), where interested Cc's, tests, patches and change-ids, etc. may be assigned.
489
490 http://bugs.perl.org/admin/index.html
491
492
493=item 2 bugdb@perl.org
494
495Where the subject line is used for commands:
496
497 To: bugdb@perl.org
498 Subject: -a close bugid1 bugid2 aix install
499
500 To: bugdb@perl.org
501 Subject: -h
502
503
504=item 3 commands_and_bugdids@bugs.perl.org
505
506Where the address itself is the source for the commands:
507
508 To: close_bugid1_bugid2_aix@bugs.perl.org
509
510 To: help@bugs.perl.org
511
512
513=item notes, patches, tests
514
515For patches and tests, the message body is assigned to the appropriate bug/s and forwarded to p5p for their attention.
516
517 To: test_<bugid1>_aix_close@bugs.perl.org
518 Subject: this is a test for the (now closed) aix bug
519
520 Test is the body of the mail
521
522=back
523
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524=head2 Submitting patches
525
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526Always submit patches to I<perl5-porters@perl.org>. If you're
527patching a core module and there's an author listed, send the author a
528copy (see L<Patching a core module>). This lets other porters review
529your patch, which catches a surprising number of errors in patches.
530Either use the diff program (available in source code form from
531I<ftp://ftp.gnu.org/pub/gnu/>), or use Johan Vromans' I<makepatch>
532(available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
533but context diffs are accepted. Do not send RCS-style diffs or diffs
534without context lines. More information is given in the
535I<Porting/patching.pod> file in the Perl source distribution. Please
536patch against the latest B<development> version (e.g., if you're
537fixing a bug in the 5.005 track, patch against the latest 5.005_5x
538version). Only patches that survive the heat of the development
539branch get applied to maintenance versions.
540
541Your patch should update the documentation and test suite. See
542L<Writing a test>.
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543
544To report a bug in Perl, use the program I<perlbug> which comes with
545Perl (if you can't get Perl to work, send mail to the address
f18956b7 546I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
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547I<perlbug> feeds into the automated bug-tracking system, access to
548which is provided through the web at I<http://bugs.perl.org/>. It
549often pays to check the archives of the perl5-porters mailing list to
550see whether the bug you're reporting has been reported before, and if
551so whether it was considered a bug. See above for the location of
552the searchable archives.
553
554The CPAN testers (I<http://testers.cpan.org/>) are a group of
555volunteers who test CPAN modules on a variety of platforms. Perl Labs
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556(I<http://labs.perl.org/>) automatically tests Perl source releases on
557platforms and gives feedback to the CPAN testers mailing list. Both
558efforts welcome volunteers.
e8cd7eae 559
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560It's a good idea to read and lurk for a while before chipping in.
561That way you'll get to see the dynamic of the conversations, learn the
562personalities of the players, and hopefully be better prepared to make
563a useful contribution when do you speak up.
564
565If after all this you still think you want to join the perl5-porters
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566mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
567unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
e8cd7eae 568
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569To hack on the Perl guts, you'll need to read the following things:
570
571=over 3
572
573=item L<perlguts>
574
575This is of paramount importance, since it's the documentation of what
576goes where in the Perl source. Read it over a couple of times and it
577might start to make sense - don't worry if it doesn't yet, because the
578best way to study it is to read it in conjunction with poking at Perl
579source, and we'll do that later on.
580
581You might also want to look at Gisle Aas's illustrated perlguts -
582there's no guarantee that this will be absolutely up-to-date with the
583latest documentation in the Perl core, but the fundamentals will be
584right. (http://gisle.aas.no/perl/illguts/)
585
586=item L<perlxstut> and L<perlxs>
587
588A working knowledge of XSUB programming is incredibly useful for core
589hacking; XSUBs use techniques drawn from the PP code, the portion of the
590guts that actually executes a Perl program. It's a lot gentler to learn
591those techniques from simple examples and explanation than from the core
592itself.
593
594=item L<perlapi>
595
596The documentation for the Perl API explains what some of the internal
597functions do, as well as the many macros used in the source.
598
599=item F<Porting/pumpkin.pod>
600
601This is a collection of words of wisdom for a Perl porter; some of it is
602only useful to the pumpkin holder, but most of it applies to anyone
603wanting to go about Perl development.
604
605=item The perl5-porters FAQ
606
607This is posted to perl5-porters at the beginning on every month, and
608should be available from http://perlhacker.org/p5p-faq; alternatively,
609you can get the FAQ emailed to you by sending mail to
610C<perl5-porters-faq@perl.org>. It contains hints on reading
611perl5-porters, information on how perl5-porters works and how Perl
612development in general works.
613
614=back
615
616=head2 Finding Your Way Around
617
618Perl maintenance can be split into a number of areas, and certain people
619(pumpkins) will have responsibility for each area. These areas sometimes
620correspond to files or directories in the source kit. Among the areas are:
621
622=over 3
623
624=item Core modules
625
626Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
627subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
628contains the core XS modules.
629
f7e1e956
MS
630=item Tests
631
632There are tests for nearly all the modules, built-ins and major bits
633of functionality. Test files all have a .t suffix. Module tests live
634in the F<lib/> and F<ext/> directories next to the module being
635tested. Others live in F<t/>. See L<Writing a test>
636
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637=item Documentation
638
639Documentation maintenance includes looking after everything in the
640F<pod/> directory, (as well as contributing new documentation) and
641the documentation to the modules in core.
642
643=item Configure
644
645The configure process is the way we make Perl portable across the
646myriad of operating systems it supports. Responsibility for the
647configure, build and installation process, as well as the overall
648portability of the core code rests with the configure pumpkin - others
649help out with individual operating systems.
650
651The files involved are the operating system directories, (F<win32/>,
652F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
653and F<Makefile>, as well as the metaconfig files which generate
654F<Configure>. (metaconfig isn't included in the core distribution.)
655
656=item Interpreter
657
658And of course, there's the core of the Perl interpreter itself. Let's
659have a look at that in a little more detail.
660
661=back
662
663Before we leave looking at the layout, though, don't forget that
664F<MANIFEST> contains not only the file names in the Perl distribution,
665but short descriptions of what's in them, too. For an overview of the
666important files, try this:
667
668 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
669
670=head2 Elements of the interpreter
671
672The work of the interpreter has two main stages: compiling the code
673into the internal representation, or bytecode, and then executing it.
674L<perlguts/Compiled code> explains exactly how the compilation stage
675happens.
676
677Here is a short breakdown of perl's operation:
678
679=over 3
680
681=item Startup
682
683The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
684This is very high-level code, enough to fit on a single screen, and it
685resembles the code found in L<perlembed>; most of the real action takes
686place in F<perl.c>
687
688First, F<perlmain.c> allocates some memory and constructs a Perl
689interpreter:
690
691 1 PERL_SYS_INIT3(&argc,&argv,&env);
692 2
693 3 if (!PL_do_undump) {
694 4 my_perl = perl_alloc();
695 5 if (!my_perl)
696 6 exit(1);
697 7 perl_construct(my_perl);
698 8 PL_perl_destruct_level = 0;
699 9 }
700
701Line 1 is a macro, and its definition is dependent on your operating
702system. Line 3 references C<PL_do_undump>, a global variable - all
703global variables in Perl start with C<PL_>. This tells you whether the
704current running program was created with the C<-u> flag to perl and then
705F<undump>, which means it's going to be false in any sane context.
706
707Line 4 calls a function in F<perl.c> to allocate memory for a Perl
708interpreter. It's quite a simple function, and the guts of it looks like
709this:
710
711 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
712
713Here you see an example of Perl's system abstraction, which we'll see
714later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
715own C<malloc> as defined in F<malloc.c> if you selected that option at
716configure time.
717
718Next, in line 7, we construct the interpreter; this sets up all the
719special variables that Perl needs, the stacks, and so on.
720
721Now we pass Perl the command line options, and tell it to go:
722
723 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
724 if (!exitstatus) {
725 exitstatus = perl_run(my_perl);
726 }
727
728
729C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
730in F<perl.c>, which processes the command line options, sets up any
731statically linked XS modules, opens the program and calls C<yyparse> to
732parse it.
733
734=item Parsing
735
736The aim of this stage is to take the Perl source, and turn it into an op
737tree. We'll see what one of those looks like later. Strictly speaking,
738there's three things going on here.
739
740C<yyparse>, the parser, lives in F<perly.c>, although you're better off
741reading the original YACC input in F<perly.y>. (Yes, Virginia, there
742B<is> a YACC grammar for Perl!) The job of the parser is to take your
743code and `understand' it, splitting it into sentences, deciding which
744operands go with which operators and so on.
745
746The parser is nobly assisted by the lexer, which chunks up your input
747into tokens, and decides what type of thing each token is: a variable
748name, an operator, a bareword, a subroutine, a core function, and so on.
749The main point of entry to the lexer is C<yylex>, and that and its
750associated routines can be found in F<toke.c>. Perl isn't much like
751other computer languages; it's highly context sensitive at times, it can
752be tricky to work out what sort of token something is, or where a token
753ends. As such, there's a lot of interplay between the tokeniser and the
754parser, which can get pretty frightening if you're not used to it.
755
756As the parser understands a Perl program, it builds up a tree of
757operations for the interpreter to perform during execution. The routines
758which construct and link together the various operations are to be found
759in F<op.c>, and will be examined later.
760
761=item Optimization
762
763Now the parsing stage is complete, and the finished tree represents
764the operations that the Perl interpreter needs to perform to execute our
765program. Next, Perl does a dry run over the tree looking for
766optimisations: constant expressions such as C<3 + 4> will be computed
767now, and the optimizer will also see if any multiple operations can be
768replaced with a single one. For instance, to fetch the variable C<$foo>,
769instead of grabbing the glob C<*foo> and looking at the scalar
770component, the optimizer fiddles the op tree to use a function which
771directly looks up the scalar in question. The main optimizer is C<peep>
772in F<op.c>, and many ops have their own optimizing functions.
773
774=item Running
775
776Now we're finally ready to go: we have compiled Perl byte code, and all
777that's left to do is run it. The actual execution is done by the
778C<runops_standard> function in F<run.c>; more specifically, it's done by
779these three innocent looking lines:
780
781 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
782 PERL_ASYNC_CHECK();
783 }
784
785You may be more comfortable with the Perl version of that:
786
787 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
788
789Well, maybe not. Anyway, each op contains a function pointer, which
790stipulates the function which will actually carry out the operation.
791This function will return the next op in the sequence - this allows for
792things like C<if> which choose the next op dynamically at run time.
793The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
794execution if required.
795
796The actual functions called are known as PP code, and they're spread
797between four files: F<pp_hot.c> contains the `hot' code, which is most
798often used and highly optimized, F<pp_sys.c> contains all the
799system-specific functions, F<pp_ctl.c> contains the functions which
800implement control structures (C<if>, C<while> and the like) and F<pp.c>
801contains everything else. These are, if you like, the C code for Perl's
802built-in functions and operators.
803
804=back
805
806=head2 Internal Variable Types
807
808You should by now have had a look at L<perlguts>, which tells you about
809Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
810that now.
811
812These variables are used not only to represent Perl-space variables, but
813also any constants in the code, as well as some structures completely
814internal to Perl. The symbol table, for instance, is an ordinary Perl
815hash. Your code is represented by an SV as it's read into the parser;
816any program files you call are opened via ordinary Perl filehandles, and
817so on.
818
819The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
820Perl program. Let's see, for instance, how Perl treats the constant
821C<"hello">.
822
823 % perl -MDevel::Peek -e 'Dump("hello")'
824 1 SV = PV(0xa041450) at 0xa04ecbc
825 2 REFCNT = 1
826 3 FLAGS = (POK,READONLY,pPOK)
827 4 PV = 0xa0484e0 "hello"\0
828 5 CUR = 5
829 6 LEN = 6
830
831Reading C<Devel::Peek> output takes a bit of practise, so let's go
832through it line by line.
833
834Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
835memory. SVs themselves are very simple structures, but they contain a
836pointer to a more complex structure. In this case, it's a PV, a
837structure which holds a string value, at location C<0xa041450>. Line 2
838is the reference count; there are no other references to this data, so
839it's 1.
840
841Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
842read-only SV (because it's a constant) and the data is a PV internally.
843Next we've got the contents of the string, starting at location
844C<0xa0484e0>.
845
846Line 5 gives us the current length of the string - note that this does
847B<not> include the null terminator. Line 6 is not the length of the
848string, but the length of the currently allocated buffer; as the string
849grows, Perl automatically extends the available storage via a routine
850called C<SvGROW>.
851
852You can get at any of these quantities from C very easily; just add
853C<Sv> to the name of the field shown in the snippet, and you've got a
854macro which will return the value: C<SvCUR(sv)> returns the current
855length of the string, C<SvREFCOUNT(sv)> returns the reference count,
856C<SvPV(sv, len)> returns the string itself with its length, and so on.
857More macros to manipulate these properties can be found in L<perlguts>.
858
859Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
860
861 1 void
862 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
863 3 {
864 4 STRLEN tlen;
865 5 char *junk;
866
867 6 junk = SvPV_force(sv, tlen);
868 7 SvGROW(sv, tlen + len + 1);
869 8 if (ptr == junk)
870 9 ptr = SvPVX(sv);
871 10 Move(ptr,SvPVX(sv)+tlen,len,char);
872 11 SvCUR(sv) += len;
873 12 *SvEND(sv) = '\0';
874 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
875 14 SvTAINT(sv);
876 15 }
877
878This is a function which adds a string, C<ptr>, of length C<len> onto
879the end of the PV stored in C<sv>. The first thing we do in line 6 is
880make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
881macro to force a PV. As a side effect, C<tlen> gets set to the current
882value of the PV, and the PV itself is returned to C<junk>.
883
b1866b2d 884In line 7, we make sure that the SV will have enough room to accommodate
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SC
885the old string, the new string and the null terminator. If C<LEN> isn't
886big enough, C<SvGROW> will reallocate space for us.
887
888Now, if C<junk> is the same as the string we're trying to add, we can
889grab the string directly from the SV; C<SvPVX> is the address of the PV
890in the SV.
891
892Line 10 does the actual catenation: the C<Move> macro moves a chunk of
893memory around: we move the string C<ptr> to the end of the PV - that's
894the start of the PV plus its current length. We're moving C<len> bytes
895of type C<char>. After doing so, we need to tell Perl we've extended the
896string, by altering C<CUR> to reflect the new length. C<SvEND> is a
897macro which gives us the end of the string, so that needs to be a
898C<"\0">.
899
900Line 13 manipulates the flags; since we've changed the PV, any IV or NV
901values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
902want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
903version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
904and turns on POK. The final C<SvTAINT> is a macro which launders tainted
905data if taint mode is turned on.
906
907AVs and HVs are more complicated, but SVs are by far the most common
908variable type being thrown around. Having seen something of how we
909manipulate these, let's go on and look at how the op tree is
910constructed.
911
912=head2 Op Trees
913
914First, what is the op tree, anyway? The op tree is the parsed
915representation of your program, as we saw in our section on parsing, and
916it's the sequence of operations that Perl goes through to execute your
917program, as we saw in L</Running>.
918
919An op is a fundamental operation that Perl can perform: all the built-in
920functions and operators are ops, and there are a series of ops which
921deal with concepts the interpreter needs internally - entering and
922leaving a block, ending a statement, fetching a variable, and so on.
923
924The op tree is connected in two ways: you can imagine that there are two
925"routes" through it, two orders in which you can traverse the tree.
926First, parse order reflects how the parser understood the code, and
927secondly, execution order tells perl what order to perform the
928operations in.
929
930The easiest way to examine the op tree is to stop Perl after it has
931finished parsing, and get it to dump out the tree. This is exactly what
932the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
933
934Let's have a look at how Perl sees C<$a = $b + $c>:
935
936 % perl -MO=Terse -e '$a=$b+$c'
937 1 LISTOP (0x8179888) leave
938 2 OP (0x81798b0) enter
939 3 COP (0x8179850) nextstate
940 4 BINOP (0x8179828) sassign
941 5 BINOP (0x8179800) add [1]
942 6 UNOP (0x81796e0) null [15]
943 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
944 8 UNOP (0x81797e0) null [15]
945 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
946 10 UNOP (0x816b4f0) null [15]
947 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
948
949Let's start in the middle, at line 4. This is a BINOP, a binary
950operator, which is at location C<0x8179828>. The specific operator in
951question is C<sassign> - scalar assignment - and you can find the code
952which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
953binary operator, it has two children: the add operator, providing the
954result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
955line 10.
956
957Line 10 is the null op: this does exactly nothing. What is that doing
958there? If you see the null op, it's a sign that something has been
959optimized away after parsing. As we mentioned in L</Optimization>,
960the optimization stage sometimes converts two operations into one, for
961example when fetching a scalar variable. When this happens, instead of
962rewriting the op tree and cleaning up the dangling pointers, it's easier
963just to replace the redundant operation with the null op. Originally,
964the tree would have looked like this:
965
966 10 SVOP (0x816b4f0) rv2sv [15]
967 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
968
969That is, fetch the C<a> entry from the main symbol table, and then look
970at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
971happens to do both these things.
972
973The right hand side, starting at line 5 is similar to what we've just
974seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
975two C<gvsv>s.
976
977Now, what's this about?
978
979 1 LISTOP (0x8179888) leave
980 2 OP (0x81798b0) enter
981 3 COP (0x8179850) nextstate
982
983C<enter> and C<leave> are scoping ops, and their job is to perform any
984housekeeping every time you enter and leave a block: lexical variables
985are tidied up, unreferenced variables are destroyed, and so on. Every
986program will have those first three lines: C<leave> is a list, and its
987children are all the statements in the block. Statements are delimited
988by C<nextstate>, so a block is a collection of C<nextstate> ops, with
989the ops to be performed for each statement being the children of
990C<nextstate>. C<enter> is a single op which functions as a marker.
991
992That's how Perl parsed the program, from top to bottom:
993
994 Program
995 |
996 Statement
997 |
998 =
999 / \
1000 / \
1001 $a +
1002 / \
1003 $b $c
1004
1005However, it's impossible to B<perform> the operations in this order:
1006you have to find the values of C<$b> and C<$c> before you add them
1007together, for instance. So, the other thread that runs through the op
1008tree is the execution order: each op has a field C<op_next> which points
1009to the next op to be run, so following these pointers tells us how perl
1010executes the code. We can traverse the tree in this order using
1011the C<exec> option to C<B::Terse>:
1012
1013 % perl -MO=Terse,exec -e '$a=$b+$c'
1014 1 OP (0x8179928) enter
1015 2 COP (0x81798c8) nextstate
1016 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
1017 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
1018 5 BINOP (0x8179878) add [1]
1019 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
1020 7 BINOP (0x81798a0) sassign
1021 8 LISTOP (0x8179900) leave
1022
1023This probably makes more sense for a human: enter a block, start a
1024statement. Get the values of C<$b> and C<$c>, and add them together.
1025Find C<$a>, and assign one to the other. Then leave.
1026
1027The way Perl builds up these op trees in the parsing process can be
1028unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1029piece we need to construct the tree for C<$a = $b + $c>
1030
1031 1 term : term ASSIGNOP term
1032 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1033 3 | term ADDOP term
1034 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1035
1036If you're not used to reading BNF grammars, this is how it works: You're
1037fed certain things by the tokeniser, which generally end up in upper
1038case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1039code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1040`terminal symbols', because you can't get any simpler than them.
1041
1042The grammar, lines one and three of the snippet above, tells you how to
1043build up more complex forms. These complex forms, `non-terminal symbols'
1044are generally placed in lower case. C<term> here is a non-terminal
1045symbol, representing a single expression.
1046
1047The grammar gives you the following rule: you can make the thing on the
1048left of the colon if you see all the things on the right in sequence.
1049This is called a "reduction", and the aim of parsing is to completely
1050reduce the input. There are several different ways you can perform a
1051reduction, separated by vertical bars: so, C<term> followed by C<=>
1052followed by C<term> makes a C<term>, and C<term> followed by C<+>
1053followed by C<term> can also make a C<term>.
1054
1055So, if you see two terms with an C<=> or C<+>, between them, you can
1056turn them into a single expression. When you do this, you execute the
1057code in the block on the next line: if you see C<=>, you'll do the code
1058in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1059which contributes to the op tree.
1060
1061 | term ADDOP term
1062 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1063
1064What this does is creates a new binary op, and feeds it a number of
1065variables. The variables refer to the tokens: C<$1> is the first token in
1066the input, C<$2> the second, and so on - think regular expression
1067backreferences. C<$$> is the op returned from this reduction. So, we
1068call C<newBINOP> to create a new binary operator. The first parameter to
1069C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1070operator, so we want the type to be C<ADDOP>. We could specify this
1071directly, but it's right there as the second token in the input, so we
1072use C<$2>. The second parameter is the op's flags: 0 means `nothing
1073special'. Then the things to add: the left and right hand side of our
1074expression, in scalar context.
1075
1076=head2 Stacks
1077
1078When perl executes something like C<addop>, how does it pass on its
1079results to the next op? The answer is, through the use of stacks. Perl
1080has a number of stacks to store things it's currently working on, and
1081we'll look at the three most important ones here.
1082
1083=over 3
1084
1085=item Argument stack
1086
1087Arguments are passed to PP code and returned from PP code using the
1088argument stack, C<ST>. The typical way to handle arguments is to pop
1089them off the stack, deal with them how you wish, and then push the result
1090back onto the stack. This is how, for instance, the cosine operator
1091works:
1092
1093 NV value;
1094 value = POPn;
1095 value = Perl_cos(value);
1096 XPUSHn(value);
1097
1098We'll see a more tricky example of this when we consider Perl's macros
1099below. C<POPn> gives you the NV (floating point value) of the top SV on
1100the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1101the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1102should be extended if necessary - it can't be necessary here, because we
1103know there's room for one more item on the stack, since we've just
1104removed one! The C<XPUSH*> macros at least guarantee safety.
1105
1106Alternatively, you can fiddle with the stack directly: C<SP> gives you
1107the first element in your portion of the stack, and C<TOP*> gives you
1108the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1109negation of an integer:
1110
1111 SETi(-TOPi);
1112
1113Just set the integer value of the top stack entry to its negation.
1114
1115Argument stack manipulation in the core is exactly the same as it is in
1116XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1117description of the macros used in stack manipulation.
1118
1119=item Mark stack
1120
1121I say `your portion of the stack' above because PP code doesn't
1122necessarily get the whole stack to itself: if your function calls
1123another function, you'll only want to expose the arguments aimed for the
1124called function, and not (necessarily) let it get at your own data. The
1125way we do this is to have a `virtual' bottom-of-stack, exposed to each
1126function. The mark stack keeps bookmarks to locations in the argument
1127stack usable by each function. For instance, when dealing with a tied
1128variable, (internally, something with `P' magic) Perl has to call
1129methods for accesses to the tied variables. However, we need to separate
1130the arguments exposed to the method to the argument exposed to the
1131original function - the store or fetch or whatever it may be. Here's how
1132the tied C<push> is implemented; see C<av_push> in F<av.c>:
1133
1134 1 PUSHMARK(SP);
1135 2 EXTEND(SP,2);
1136 3 PUSHs(SvTIED_obj((SV*)av, mg));
1137 4 PUSHs(val);
1138 5 PUTBACK;
1139 6 ENTER;
1140 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1141 8 LEAVE;
1142 9 POPSTACK;
13a2d996 1143
a422fd2d
SC
1144The lines which concern the mark stack are the first, fifth and last
1145lines: they save away, restore and remove the current position of the
1146argument stack.
1147
1148Let's examine the whole implementation, for practice:
1149
1150 1 PUSHMARK(SP);
1151
1152Push the current state of the stack pointer onto the mark stack. This is
1153so that when we've finished adding items to the argument stack, Perl
1154knows how many things we've added recently.
1155
1156 2 EXTEND(SP,2);
1157 3 PUSHs(SvTIED_obj((SV*)av, mg));
1158 4 PUSHs(val);
1159
1160We're going to add two more items onto the argument stack: when you have
1161a tied array, the C<PUSH> subroutine receives the object and the value
1162to be pushed, and that's exactly what we have here - the tied object,
1163retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1164
1165 5 PUTBACK;
1166
1167Next we tell Perl to make the change to the global stack pointer: C<dSP>
1168only gave us a local copy, not a reference to the global.
1169
1170 6 ENTER;
1171 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1172 8 LEAVE;
1173
1174C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1175variables are tidied up, everything that has been localised gets
1176its previous value returned, and so on. Think of them as the C<{> and
1177C<}> of a Perl block.
1178
1179To actually do the magic method call, we have to call a subroutine in
1180Perl space: C<call_method> takes care of that, and it's described in
1181L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1182going to discard its return value.
1183
1184 9 POPSTACK;
1185
1186Finally, we remove the value we placed on the mark stack, since we
1187don't need it any more.
1188
1189=item Save stack
1190
1191C doesn't have a concept of local scope, so perl provides one. We've
1192seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1193stack implements the C equivalent of, for example:
1194
1195 {
1196 local $foo = 42;
1197 ...
1198 }
1199
1200See L<perlguts/Localising Changes> for how to use the save stack.
1201
1202=back
1203
1204=head2 Millions of Macros
1205
1206One thing you'll notice about the Perl source is that it's full of
1207macros. Some have called the pervasive use of macros the hardest thing
1208to understand, others find it adds to clarity. Let's take an example,
1209the code which implements the addition operator:
1210
1211 1 PP(pp_add)
1212 2 {
39644a26 1213 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
a422fd2d
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1214 4 {
1215 5 dPOPTOPnnrl_ul;
1216 6 SETn( left + right );
1217 7 RETURN;
1218 8 }
1219 9 }
1220
1221Every line here (apart from the braces, of course) contains a macro. The
1222first line sets up the function declaration as Perl expects for PP code;
1223line 3 sets up variable declarations for the argument stack and the
1224target, the return value of the operation. Finally, it tries to see if
1225the addition operation is overloaded; if so, the appropriate subroutine
1226is called.
1227
1228Line 5 is another variable declaration - all variable declarations start
1229with C<d> - which pops from the top of the argument stack two NVs (hence
1230C<nn>) and puts them into the variables C<right> and C<left>, hence the
1231C<rl>. These are the two operands to the addition operator. Next, we
1232call C<SETn> to set the NV of the return value to the result of adding
1233the two values. This done, we return - the C<RETURN> macro makes sure
1234that our return value is properly handled, and we pass the next operator
1235to run back to the main run loop.
1236
1237Most of these macros are explained in L<perlapi>, and some of the more
1238important ones are explained in L<perlxs> as well. Pay special attention
1239to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1240the C<[pad]THX_?> macros.
1241
1242
1243=head2 Poking at Perl
1244
1245To really poke around with Perl, you'll probably want to build Perl for
1246debugging, like this:
1247
1248 ./Configure -d -D optimize=-g
1249 make
1250
1251C<-g> is a flag to the C compiler to have it produce debugging
1252information which will allow us to step through a running program.
1253F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1254enables all the internal debugging code in Perl. There are a whole bunch
1255of things you can debug with this: L<perlrun> lists them all, and the
1256best way to find out about them is to play about with them. The most
1257useful options are probably
1258
1259 l Context (loop) stack processing
1260 t Trace execution
1261 o Method and overloading resolution
1262 c String/numeric conversions
1263
1264Some of the functionality of the debugging code can be achieved using XS
1265modules.
13a2d996 1266
a422fd2d
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1267 -Dr => use re 'debug'
1268 -Dx => use O 'Debug'
1269
1270=head2 Using a source-level debugger
1271
1272If the debugging output of C<-D> doesn't help you, it's time to step
1273through perl's execution with a source-level debugger.
1274
1275=over 3
1276
1277=item *
1278
1279We'll use C<gdb> for our examples here; the principles will apply to any
1280debugger, but check the manual of the one you're using.
1281
1282=back
1283
1284To fire up the debugger, type
1285
1286 gdb ./perl
1287
1288You'll want to do that in your Perl source tree so the debugger can read
1289the source code. You should see the copyright message, followed by the
1290prompt.
1291
1292 (gdb)
1293
1294C<help> will get you into the documentation, but here are the most
1295useful commands:
1296
1297=over 3
1298
1299=item run [args]
1300
1301Run the program with the given arguments.
1302
1303=item break function_name
1304
1305=item break source.c:xxx
1306
1307Tells the debugger that we'll want to pause execution when we reach
cea6626f 1308either the named function (but see L<perlguts/Internal Functions>!) or the given
a422fd2d
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1309line in the named source file.
1310
1311=item step
1312
1313Steps through the program a line at a time.
1314
1315=item next
1316
1317Steps through the program a line at a time, without descending into
1318functions.
1319
1320=item continue
1321
1322Run until the next breakpoint.
1323
1324=item finish
1325
1326Run until the end of the current function, then stop again.
1327
13a2d996 1328=item 'enter'
a422fd2d
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1329
1330Just pressing Enter will do the most recent operation again - it's a
1331blessing when stepping through miles of source code.
1332
1333=item print
1334
1335Execute the given C code and print its results. B<WARNING>: Perl makes
1336heavy use of macros, and F<gdb> is not aware of macros. You'll have to
1337substitute them yourself. So, for instance, you can't say
1338
1339 print SvPV_nolen(sv)
1340
1341but you have to say
1342
1343 print Perl_sv_2pv_nolen(sv)
1344
1345You may find it helpful to have a "macro dictionary", which you can
1346produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
1347recursively apply the macros for you.
1348
1349=back
1350
1351=head2 Dumping Perl Data Structures
1352
1353One way to get around this macro hell is to use the dumping functions in
1354F<dump.c>; these work a little like an internal
1355L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1356that you can't get at from Perl. Let's take an example. We'll use the
1357C<$a = $b + $c> we used before, but give it a bit of context:
1358C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1359
1360What about C<pp_add>, the function we examined earlier to implement the
1361C<+> operator:
1362
1363 (gdb) break Perl_pp_add
1364 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1365
cea6626f 1366Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
a422fd2d
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1367With the breakpoint in place, we can run our program:
1368
1369 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1370
1371Lots of junk will go past as gdb reads in the relevant source files and
1372libraries, and then:
1373
1374 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
39644a26 1375 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
a422fd2d
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1376 (gdb) step
1377 311 dPOPTOPnnrl_ul;
1378 (gdb)
1379
1380We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1381arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1382slightly expand it:
1383
1384 #define dPOPTOPnnrl_ul NV right = POPn; \
1385 SV *leftsv = TOPs; \
1386 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1387
1388C<POPn> takes the SV from the top of the stack and obtains its NV either
1389directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1390C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1391C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1392C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1393
1394Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1395convert it. If we step again, we'll find ourselves there:
1396
1397 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1398 1669 if (!sv)
1399 (gdb)
1400
1401We can now use C<Perl_sv_dump> to investigate the SV:
1402
1403 SV = PV(0xa057cc0) at 0xa0675d0
1404 REFCNT = 1
1405 FLAGS = (POK,pPOK)
1406 PV = 0xa06a510 "6XXXX"\0
1407 CUR = 5
1408 LEN = 6
1409 $1 = void
1410
1411We know we're going to get C<6> from this, so let's finish the
1412subroutine:
1413
1414 (gdb) finish
1415 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1416 0x462669 in Perl_pp_add () at pp_hot.c:311
1417 311 dPOPTOPnnrl_ul;
1418
1419We can also dump out this op: the current op is always stored in
1420C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1421similar output to L<B::Debug|B::Debug>.
1422
1423 {
1424 13 TYPE = add ===> 14
1425 TARG = 1
1426 FLAGS = (SCALAR,KIDS)
1427 {
1428 TYPE = null ===> (12)
1429 (was rv2sv)
1430 FLAGS = (SCALAR,KIDS)
1431 {
1432 11 TYPE = gvsv ===> 12
1433 FLAGS = (SCALAR)
1434 GV = main::b
1435 }
1436 }
1437
10f58044 1438# finish this later #
a422fd2d
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1439
1440=head2 Patching
1441
1442All right, we've now had a look at how to navigate the Perl sources and
1443some things you'll need to know when fiddling with them. Let's now get
1444on and create a simple patch. Here's something Larry suggested: if a
1445C<U> is the first active format during a C<pack>, (for example,
1446C<pack "U3C8", @stuff>) then the resulting string should be treated as
1447UTF8 encoded.
1448
1449How do we prepare to fix this up? First we locate the code in question -
1450the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1451files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1452altering this file, let's copy it to F<pp.c~>.
1453
a6ec74c1
JH
1454[Well, it was in F<pp.c> when this tutorial was written. It has now been
1455split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1456
a422fd2d
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1457Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1458loop over the pattern, taking each format character in turn into
1459C<datum_type>. Then for each possible format character, we swallow up
1460the other arguments in the pattern (a field width, an asterisk, and so
1461on) and convert the next chunk input into the specified format, adding
1462it onto the output SV C<cat>.
1463
1464How do we know if the C<U> is the first format in the C<pat>? Well, if
1465we have a pointer to the start of C<pat> then, if we see a C<U> we can
1466test whether we're still at the start of the string. So, here's where
1467C<pat> is set up:
1468
1469 STRLEN fromlen;
1470 register char *pat = SvPVx(*++MARK, fromlen);
1471 register char *patend = pat + fromlen;
1472 register I32 len;
1473 I32 datumtype;
1474 SV *fromstr;
1475
1476We'll have another string pointer in there:
1477
1478 STRLEN fromlen;
1479 register char *pat = SvPVx(*++MARK, fromlen);
1480 register char *patend = pat + fromlen;
1481 + char *patcopy;
1482 register I32 len;
1483 I32 datumtype;
1484 SV *fromstr;
1485
1486And just before we start the loop, we'll set C<patcopy> to be the start
1487of C<pat>:
1488
1489 items = SP - MARK;
1490 MARK++;
1491 sv_setpvn(cat, "", 0);
1492 + patcopy = pat;
1493 while (pat < patend) {
1494
1495Now if we see a C<U> which was at the start of the string, we turn on
1496the UTF8 flag for the output SV, C<cat>:
1497
1498 + if (datumtype == 'U' && pat==patcopy+1)
1499 + SvUTF8_on(cat);
1500 if (datumtype == '#') {
1501 while (pat < patend && *pat != '\n')
1502 pat++;
1503
1504Remember that it has to be C<patcopy+1> because the first character of
1505the string is the C<U> which has been swallowed into C<datumtype!>
1506
1507Oops, we forgot one thing: what if there are spaces at the start of the
1508pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1509character, even though it's not the first thing in the pattern. In this
1510case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1511
1512 if (isSPACE(datumtype))
1513 continue;
1514
1515needs to become
1516
1517 if (isSPACE(datumtype)) {
1518 patcopy++;
1519 continue;
1520 }
1521
1522OK. That's the C part done. Now we must do two additional things before
1523this patch is ready to go: we've changed the behaviour of Perl, and so
1524we must document that change. We must also provide some more regression
1525tests to make sure our patch works and doesn't create a bug somewhere
1526else along the line.
1527
b23b8711
MS
1528The regression tests for each operator live in F<t/op/>, and so we
1529make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1530tests to the end. First, we'll test that the C<U> does indeed create
1531Unicode strings.
1532
1533t/op/pack.t has a sensible ok() function, but if it didn't we could
35c336e6 1534use the one from t/test.pl.
b23b8711 1535
35c336e6
MS
1536 require './test.pl';
1537 plan( tests => 159 );
b23b8711
MS
1538
1539so instead of this:
a422fd2d
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1540
1541 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1542 print "ok $test\n"; $test++;
1543
35c336e6
MS
1544we can write the more sensible (see L<Test::More> for a full
1545explanation of is() and other testing functions).
b23b8711 1546
35c336e6 1547 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
812f5127 1548 "U* produces unicode" );
b23b8711 1549
a422fd2d
SC
1550Now we'll test that we got that space-at-the-beginning business right:
1551
35c336e6 1552 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
812f5127 1553 " with spaces at the beginning" );
a422fd2d
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1554
1555And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1556the first active format:
1557
35c336e6 1558 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
812f5127 1559 "U* not first isn't unicode" );
a422fd2d 1560
35c336e6
MS
1561Mustn't forget to change the number of tests which appears at the top,
1562or else the automated tester will get confused. This will either look
1563like this:
a422fd2d 1564
35c336e6
MS
1565 print "1..156\n";
1566
1567or this:
1568
1569 plan( tests => 156 );
a422fd2d
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1570
1571We now compile up Perl, and run it through the test suite. Our new
1572tests pass, hooray!
1573
1574Finally, the documentation. The job is never done until the paperwork is
1575over, so let's describe the change we've just made. The relevant place
1576is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1577this text in the description of C<pack>:
1578
1579 =item *
1580
1581 If the pattern begins with a C<U>, the resulting string will be treated
1582 as Unicode-encoded. You can force UTF8 encoding on in a string with an
1583 initial C<U0>, and the bytes that follow will be interpreted as Unicode
1584 characters. If you don't want this to happen, you can begin your pattern
1585 with C<C0> (or anything else) to force Perl not to UTF8 encode your
1586 string, and then follow this with a C<U*> somewhere in your pattern.
1587
1588All done. Now let's create the patch. F<Porting/patching.pod> tells us
1589that if we're making major changes, we should copy the entire directory
1590to somewhere safe before we begin fiddling, and then do
13a2d996 1591
a422fd2d
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1592 diff -ruN old new > patch
1593
1594However, we know which files we've changed, and we can simply do this:
1595
1596 diff -u pp.c~ pp.c > patch
1597 diff -u t/op/pack.t~ t/op/pack.t >> patch
1598 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1599
1600We end up with a patch looking a little like this:
1601
1602 --- pp.c~ Fri Jun 02 04:34:10 2000
1603 +++ pp.c Fri Jun 16 11:37:25 2000
1604 @@ -4375,6 +4375,7 @@
1605 register I32 items;
1606 STRLEN fromlen;
1607 register char *pat = SvPVx(*++MARK, fromlen);
1608 + char *patcopy;
1609 register char *patend = pat + fromlen;
1610 register I32 len;
1611 I32 datumtype;
1612 @@ -4405,6 +4406,7 @@
1613 ...
1614
1615And finally, we submit it, with our rationale, to perl5-porters. Job
1616done!
1617
f7e1e956
MS
1618=head2 Patching a core module
1619
1620This works just like patching anything else, with an extra
1621consideration. Many core modules also live on CPAN. If this is so,
1622patch the CPAN version instead of the core and send the patch off to
1623the module maintainer (with a copy to p5p). This will help the module
1624maintainer keep the CPAN version in sync with the core version without
1625constantly scanning p5p.
1626
acbe17fc
JP
1627=head2 Adding a new function to the core
1628
1629If, as part of a patch to fix a bug, or just because you have an
1630especially good idea, you decide to add a new function to the core,
1631discuss your ideas on p5p well before you start work. It may be that
1632someone else has already attempted to do what you are considering and
1633can give lots of good advice or even provide you with bits of code
1634that they already started (but never finished).
1635
1636You have to follow all of the advice given above for patching. It is
1637extremely important to test any addition thoroughly and add new tests
1638to explore all boundary conditions that your new function is expected
1639to handle. If your new function is used only by one module (e.g. toke),
1640then it should probably be named S_your_function (for static); on the
1641other hand, if you expect it to accessable from other functions in
1642Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1643for more details.
1644
1645The location of any new code is also an important consideration. Don't
1646just create a new top level .c file and put your code there; you would
1647have to make changes to Configure (so the Makefile is created properly),
1648as well as possibly lots of include files. This is strictly pumpking
1649business.
1650
1651It is better to add your function to one of the existing top level
1652source code files, but your choice is complicated by the nature of
1653the Perl distribution. Only the files that are marked as compiled
1654static are located in the perl executable. Everything else is located
1655in the shared library (or DLL if you are running under WIN32). So,
1656for example, if a function was only used by functions located in
1657toke.c, then your code can go in toke.c. If, however, you want to call
1658the function from universal.c, then you should put your code in another
1659location, for example util.c.
1660
1661In addition to writing your c-code, you will need to create an
1662appropriate entry in embed.pl describing your function, then run
1663'make regen_headers' to create the entries in the numerous header
1664files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1665for information on the various options that you can set in embed.pl.
1666You will forget to do this a few (or many) times and you will get
1667warnings during the compilation phase. Make sure that you mention
1668this when you post your patch to P5P; the pumpking needs to know this.
1669
1670When you write your new code, please be conscious of existing code
1671conventions used in the perl source files. See <perlstyle> for
1672details. Although most of the guidelines discussed seem to focus on
1673Perl code, rather than c, they all apply (except when they don't ;).
1674See also I<Porting/patching.pod> file in the Perl source distribution
1675for lots of details about both formatting and submitting patches of
1676your changes.
1677
1678Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1679Test on as many platforms as you can find. Test as many perl
1680Configure options as you can (e.g. MULTIPLICITY). If you have
1681profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
1682below for how to use them to futher test your code. Remember that
1683most of the people on P5P are doing this on their own time and
1684don't have the time to debug your code.
f7e1e956
MS
1685
1686=head2 Writing a test
1687
1688Every module and built-in function has an associated test file (or
1689should...). If you add or change functionality, you have to write a
1690test. If you fix a bug, you have to write a test so that bug never
1691comes back. If you alter the docs, it would be nice to test what the
1692new documentation says.
1693
1694In short, if you submit a patch you probably also have to patch the
1695tests.
1696
1697For modules, the test file is right next to the module itself.
1698F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1699so there are some snags (and it would be wonderful for you to brush
1700them out), but it basically works that way. Everything else lives in
1701F<t/>.
1702
1703=over 3
1704
1705=item F<t/base/>
1706
1707Testing of the absolute basic functionality of Perl. Things like
1708C<if>, basic file reads and writes, simple regexes, etc. These are
1709run first in the test suite and if any of them fail, something is
1710I<really> broken.
1711
1712=item F<t/cmd/>
1713
1714These test the basic control structures, C<if/else>, C<while>,
35c336e6 1715subroutines, etc.
f7e1e956
MS
1716
1717=item F<t/comp/>
1718
1719Tests basic issues of how Perl parses and compiles itself.
1720
1721=item F<t/io/>
1722
1723Tests for built-in IO functions, including command line arguments.
1724
1725=item F<t/lib/>
1726
1727The old home for the module tests, you shouldn't put anything new in
1728here. There are still some bits and pieces hanging around in here
1729that need to be moved. Perhaps you could move them? Thanks!
1730
1731=item F<t/op/>
1732
1733Tests for perl's built in functions that don't fit into any of the
1734other directories.
1735
1736=item F<t/pod/>
1737
1738Tests for POD directives. There are still some tests for the Pod
1739modules hanging around in here that need to be moved out into F<lib/>.
1740
1741=item F<t/run/>
1742
1743Testing features of how perl actually runs, including exit codes and
1744handling of PERL* environment variables.
1745
1746=back
1747
1748The core uses the same testing style as the rest of Perl, a simple
1749"ok/not ok" run through Test::Harness, but there are a few special
1750considerations.
1751
35c336e6
MS
1752There are three ways to write a test in the core. Test::More,
1753t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1754decision of which to use depends on what part of the test suite you're
1755working on. This is a measure to prevent a high-level failure (such
1756as Config.pm breaking) from causing basic functionality tests to fail.
1757
1758=over 4
1759
1760=item t/base t/comp
1761
1762Since we don't know if require works, or even subroutines, use ad hoc
1763tests for these two. Step carefully to avoid using the feature being
1764tested.
1765
1766=item t/cmd t/run t/io t/op
1767
1768Now that basic require() and subroutines are tested, you can use the
1769t/test.pl library which emulates the important features of Test::More
1770while using a minimum of core features.
1771
1772You can also conditionally use certain libraries like Config, but be
1773sure to skip the test gracefully if it's not there.
1774
1775=item t/lib ext lib
1776
1777Now that the core of Perl is tested, Test::More can be used. You can
1778also use the full suite of core modules in the tests.
1779
1780=back
f7e1e956
MS
1781
1782When you say "make test" Perl uses the F<t/TEST> program to run the
1783test suite. All tests are run from the F<t/> directory, B<not> the
1784directory which contains the test. This causes some problems with the
1785tests in F<lib/>, so here's some opportunity for some patching.
1786
1787You must be triply conscious of cross-platform concerns. This usually
1788boils down to using File::Spec and avoiding things like C<fork()> and
1789C<system()> unless absolutely necessary.
1790
1791
902b9dbf
MF
1792=head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1793
1794Sometimes it helps to use external tools while debugging and
1795testing Perl. This section tries to guide you through using
1796some common testing and debugging tools with Perl. This is
1797meant as a guide to interfacing these tools with Perl, not
1798as any kind of guide to the use of the tools themselves.
1799
1800=head2 Rational Software's Purify
1801
1802Purify is a commercial tool that is helpful in identifying
1803memory overruns, wild pointers, memory leaks and other such
1804badness. Perl must be compiled in a specific way for
1805optimal testing with Purify. Purify is available under
1806Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1807
1808The only currently known leaks happen when there are
1809compile-time errors within eval or require. (Fixing these
1810is non-trivial, unfortunately, but they must be fixed
1811eventually.)
1812
1813=head2 Purify on Unix
1814
1815On Unix, Purify creates a new Perl binary. To get the most
1816benefit out of Purify, you should create the perl to Purify
1817using:
1818
1819 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1820 -Uusemymalloc -Dusemultiplicity
1821
1822where these arguments mean:
1823
1824=over 4
1825
1826=item -Accflags=-DPURIFY
1827
1828Disables Perl's arena memory allocation functions, as well as
1829forcing use of memory allocation functions derived from the
1830system malloc.
1831
1832=item -Doptimize='-g'
1833
1834Adds debugging information so that you see the exact source
1835statements where the problem occurs. Without this flag, all
1836you will see is the source filename of where the error occurred.
1837
1838=item -Uusemymalloc
1839
1840Disable Perl's malloc so that Purify can more closely monitor
1841allocations and leaks. Using Perl's malloc will make Purify
1842report most leaks in the "potential" leaks category.
1843
1844=item -Dusemultiplicity
1845
1846Enabling the multiplicity option allows perl to clean up
1847thoroughly when the interpreter shuts down, which reduces the
1848number of bogus leak reports from Purify.
1849
1850=back
1851
1852Once you've compiled a perl suitable for Purify'ing, then you
1853can just:
1854
1855 make pureperl
1856
1857which creates a binary named 'pureperl' that has been Purify'ed.
1858This binary is used in place of the standard 'perl' binary
1859when you want to debug Perl memory problems.
1860
1861As an example, to show any memory leaks produced during the
1862standard Perl testset you would create and run the Purify'ed
1863perl as:
1864
1865 make pureperl
1866 cd t
1867 ../pureperl -I../lib harness
1868
1869which would run Perl on test.pl and report any memory problems.
1870
1871Purify outputs messages in "Viewer" windows by default. If
1872you don't have a windowing environment or if you simply
1873want the Purify output to unobtrusively go to a log file
1874instead of to the interactive window, use these following
1875options to output to the log file "perl.log":
1876
1877 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
1878 -log-file=perl.log -append-logfile=yes"
1879
1880If you plan to use the "Viewer" windows, then you only need this option:
1881
1882 setenv PURIFYOPTIONS "-chain-length=25"
1883
1884=head2 Purify on NT
1885
1886Purify on Windows NT instruments the Perl binary 'perl.exe'
1887on the fly. There are several options in the makefile you
1888should change to get the most use out of Purify:
1889
1890=over 4
1891
1892=item DEFINES
1893
1894You should add -DPURIFY to the DEFINES line so the DEFINES
1895line looks something like:
1896
1897 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
1898
1899to disable Perl's arena memory allocation functions, as
1900well as to force use of memory allocation functions derived
1901from the system malloc.
1902
1903=item USE_MULTI = define
1904
1905Enabling the multiplicity option allows perl to clean up
1906thoroughly when the interpreter shuts down, which reduces the
1907number of bogus leak reports from Purify.
1908
1909=item #PERL_MALLOC = define
1910
1911Disable Perl's malloc so that Purify can more closely monitor
1912allocations and leaks. Using Perl's malloc will make Purify
1913report most leaks in the "potential" leaks category.
1914
1915=item CFG = Debug
1916
1917Adds debugging information so that you see the exact source
1918statements where the problem occurs. Without this flag, all
1919you will see is the source filename of where the error occurred.
1920
1921=back
1922
1923As an example, to show any memory leaks produced during the
1924standard Perl testset you would create and run Purify as:
1925
1926 cd win32
1927 make
1928 cd ../t
1929 purify ../perl -I../lib harness
1930
1931which would instrument Perl in memory, run Perl on test.pl,
1932then finally report any memory problems.
1933
09187cb1
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1934=head2 Compaq's/Digital's Third Degree
1935
1936Third Degree is a tool for memory leak detection and memory access checks.
1937It is one of the many tools in the ATOM toolkit. The toolkit is only
1938available on Tru64 (formerly known as Digital UNIX formerly known as
1939DEC OSF/1).
1940
1941When building Perl, you must first run Configure with -Doptimize=-g
1942and -Uusemymalloc flags, after that you can use the make targets
51a35ef1
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1943"perl.third" and "test.third". (What is required is that Perl must be
1944compiled using the C<-g> flag, you may need to re-Configure.)
09187cb1 1945
64cea5fd 1946The short story is that with "atom" you can instrument the Perl
83f0ef60 1947executable to create a new executable called F<perl.third>. When the
4ae3d70a 1948instrumented executable is run, it creates a log of dubious memory
83f0ef60 1949traffic in file called F<perl.3log>. See the manual pages of atom and
4ae3d70a
JH
1950third for more information. The most extensive Third Degree
1951documentation is available in the Compaq "Tru64 UNIX Programmer's
1952Guide", chapter "Debugging Programs with Third Degree".
64cea5fd 1953
83f0ef60 1954The "test.third" leaves a lot of files named F<perl.3log.*> in the t/
64cea5fd
JH
1955subdirectory. There is a problem with these files: Third Degree is so
1956effective that it finds problems also in the system libraries.
83f0ef60
JH
1957Therefore there are certain types of errors that you should ignore in
1958your debugging. Errors with stack traces matching
64cea5fd
JH
1959
1960 __actual_atof|__catgets|_doprnt|__exc_|__exec|_findio|__localtime|setlocale|__sia_|__strxfrm
1961
1962(all in libc.so) are known to be non-serious. You can also
1963ignore the combinations
1964
1965 Perl_gv_fetchfile() calling strcpy()
1966 S_doopen_pmc() calling strcmp()
1967
1968causing "rih" (reading invalid heap) errors.
1969
1970There are also leaks that for given certain definition of a leak,
1971aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
1972
1973=head2 PERL_DESTRUCT_LEVEL
1974
1975If you want to run any of the tests yourself manually using the
1976pureperl or perl.third executables, please note that by default
1977perl B<does not> explicitly cleanup all the memory it has allocated
1978(such as global memory arenas) but instead lets the exit() of
1979the whole program "take care" of such allocations, also known
1980as "global destruction of objects".
1981
1982There is a way to tell perl to do complete cleanup: set the
1983environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
1984The t/TEST wrapper does set this to 2, and this is what you
1985need to do too, if you don't want to see the "global leaks":
1986
1987 PERL_DESTRUCT_LEVEL=2 ./perl.third t/foo/bar.t
09187cb1 1988
51a35ef1
JH
1989=head2 Profiling
1990
1991Depending on your platform there are various of profiling Perl.
1992
1993There are two commonly used techniques of profiling executables:
10f58044 1994I<statistical time-sampling> and I<basic-block counting>.
51a35ef1
JH
1995
1996The first method takes periodically samples of the CPU program
1997counter, and since the program counter can be correlated with the code
1998generated for functions, we get a statistical view of in which
1999functions the program is spending its time. The caveats are that very
2000small/fast functions have lower probability of showing up in the
2001profile, and that periodically interrupting the program (this is
2002usually done rather frequently, in the scale of milliseconds) imposes
2003an additional overhead that may skew the results. The first problem
2004can be alleviated by running the code for longer (in general this is a
2005good idea for profiling), the second problem is usually kept in guard
2006by the profiling tools themselves.
2007
10f58044 2008The second method divides up the generated code into I<basic blocks>.
51a35ef1
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2009Basic blocks are sections of code that are entered only in the
2010beginning and exited only at the end. For example, a conditional jump
2011starts a basic block. Basic block profiling usually works by
10f58044 2012I<instrumenting> the code by adding I<enter basic block #nnnn>
51a35ef1
JH
2013book-keeping code to the generated code. During the execution of the
2014code the basic block counters are then updated appropriately. The
2015caveat is that the added extra code can skew the results: again, the
2016profiling tools usually try to factor their own effects out of the
2017results.
2018
83f0ef60
JH
2019=head2 Gprof Profiling
2020
51a35ef1
JH
2021gprof is a profiling tool available in many UNIX platforms,
2022it uses F<statistical time-sampling>.
83f0ef60
JH
2023
2024You can build a profiled version of perl called "perl.gprof" by
51a35ef1
JH
2025invoking the make target "perl.gprof" (What is required is that Perl
2026must be compiled using the C<-pg> flag, you may need to re-Configure).
2027Running the profiled version of Perl will create an output file called
2028F<gmon.out> is created which contains the profiling data collected
2029during the execution.
83f0ef60
JH
2030
2031The gprof tool can then display the collected data in various ways.
2032Usually gprof understands the following options:
2033
2034=over 4
2035
2036=item -a
2037
2038Suppress statically defined functions from the profile.
2039
2040=item -b
2041
2042Suppress the verbose descriptions in the profile.
2043
2044=item -e routine
2045
2046Exclude the given routine and its descendants from the profile.
2047
2048=item -f routine
2049
2050Display only the given routine and its descendants in the profile.
2051
2052=item -s
2053
2054Generate a summary file called F<gmon.sum> which then may be given
2055to subsequent gprof runs to accumulate data over several runs.
2056
2057=item -z
2058
2059Display routines that have zero usage.
2060
2061=back
2062
2063For more detailed explanation of the available commands and output
2064formats, see your own local documentation of gprof.
2065
51a35ef1
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2066=head2 GCC gcov Profiling
2067
10f58044 2068Starting from GCC 3.0 I<basic block profiling> is officially available
51a35ef1
JH
2069for the GNU CC.
2070
2071You can build a profiled version of perl called F<perl.gcov> by
2072invoking the make target "perl.gcov" (what is required that Perl must
2073be compiled using gcc with the flags C<-fprofile-arcs
2074-ftest-coverage>, you may need to re-Configure).
2075
2076Running the profiled version of Perl will cause profile output to be
2077generated. For each source file an accompanying ".da" file will be
2078created.
2079
2080To display the results you use the "gcov" utility (which should
2081be installed if you have gcc 3.0 or newer installed). F<gcov> is
2082run on source code files, like this
2083
2084 gcov sv.c
2085
2086which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2087contain the source code annotated with relative frequencies of
2088execution indicated by "#" markers.
2089
2090Useful options of F<gcov> include C<-b> which will summarise the
2091basic block, branch, and function call coverage, and C<-c> which
2092instead of relative frequencies will use the actual counts. For
2093more information on the use of F<gcov> and basic block profiling
2094with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2095
2096 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2097
2098and its section titled "8. gcov: a Test Coverage Program"
2099
2100 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2101
4ae3d70a
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2102=head2 Pixie Profiling
2103
51a35ef1
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2104Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2105UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
10f58044 2106I<basic-block counting>.
4ae3d70a 2107
83f0ef60 2108You can build a profiled version of perl called F<perl.pixie> by
51a35ef1
JH
2109invoking the make target "perl.pixie" (what is required is that Perl
2110must be compiled using the C<-g> flag, you may need to re-Configure).
2111
2112In Tru64 a file called F<perl.Addrs> will also be silently created,
2113this file contains the addresses of the basic blocks. Running the
2114profiled version of Perl will create a new file called "perl.Counts"
2115which contains the counts for the basic block for that particular
2116program execution.
4ae3d70a 2117
51a35ef1 2118To display the results you use the F<prof> utility. The exact
4ae3d70a
JH
2119incantation depends on your operating system, "prof perl.Counts" in
2120IRIX, and "prof -pixie -all -L. perl" in Tru64.
2121
6c41479b
JH
2122In IRIX the following prof options are available:
2123
2124=over 4
2125
2126=item -h
2127
2128Reports the most heavily used lines in descending order of use.
6e36760b 2129Useful for finding the hotspot lines.
6c41479b
JH
2130
2131=item -l
2132
2133Groups lines by procedure, with procedures sorted in descending order of use.
2134Within a procedure, lines are listed in source order.
6e36760b 2135Useful for finding the hotspots of procedures.
6c41479b
JH
2136
2137=back
2138
2139In Tru64 the following options are available:
2140
2141=over 4
2142
3958b146 2143=item -p[rocedures]
6c41479b 2144
3958b146 2145Procedures sorted in descending order by the number of cycles executed
6e36760b 2146in each procedure. Useful for finding the hotspot procedures.
6c41479b
JH
2147(This is the default option.)
2148
24000d2f 2149=item -h[eavy]
6c41479b 2150
6e36760b
JH
2151Lines sorted in descending order by the number of cycles executed in
2152each line. Useful for finding the hotspot lines.
6c41479b 2153
24000d2f 2154=item -i[nvocations]
6c41479b 2155
6e36760b
JH
2156The called procedures are sorted in descending order by number of calls
2157made to the procedures. Useful for finding the most used procedures.
6c41479b 2158
24000d2f 2159=item -l[ines]
6c41479b
JH
2160
2161Grouped by procedure, sorted by cycles executed per procedure.
6e36760b 2162Useful for finding the hotspots of procedures.
6c41479b
JH
2163
2164=item -testcoverage
2165
2166The compiler emitted code for these lines, but the code was unexecuted.
2167
24000d2f 2168=item -z[ero]
6c41479b
JH
2169
2170Unexecuted procedures.
2171
aa500c9e 2172=back
6c41479b
JH
2173
2174For further information, see your system's manual pages for pixie and prof.
4ae3d70a 2175
a422fd2d
SC
2176=head2 CONCLUSION
2177
2178We've had a brief look around the Perl source, an overview of the stages
2179F<perl> goes through when it's running your code, and how to use a
902b9dbf
MF
2180debugger to poke at the Perl guts. We took a very simple problem and
2181demonstrated how to solve it fully - with documentation, regression
2182tests, and finally a patch for submission to p5p. Finally, we talked
2183about how to use external tools to debug and test Perl.
a422fd2d
SC
2184
2185I'd now suggest you read over those references again, and then, as soon
2186as possible, get your hands dirty. The best way to learn is by doing,
2187so:
2188
2189=over 3
2190
2191=item *
2192
2193Subscribe to perl5-porters, follow the patches and try and understand
2194them; don't be afraid to ask if there's a portion you're not clear on -
2195who knows, you may unearth a bug in the patch...
2196
2197=item *
2198
2199Keep up to date with the bleeding edge Perl distributions and get
2200familiar with the changes. Try and get an idea of what areas people are
2201working on and the changes they're making.
2202
2203=item *
2204
3e148164 2205Do read the README associated with your operating system, e.g. README.aix
a1f349fd
MB
2206on the IBM AIX OS. Don't hesitate to supply patches to that README if
2207you find anything missing or changed over a new OS release.
2208
2209=item *
2210
a422fd2d
SC
2211Find an area of Perl that seems interesting to you, and see if you can
2212work out how it works. Scan through the source, and step over it in the
2213debugger. Play, poke, investigate, fiddle! You'll probably get to
2214understand not just your chosen area but a much wider range of F<perl>'s
2215activity as well, and probably sooner than you'd think.
2216
2217=back
2218
2219=over 3
2220
2221=item I<The Road goes ever on and on, down from the door where it began.>
2222
2223=back
2224
2225If you can do these things, you've started on the long road to Perl porting.
2226Thanks for wanting to help make Perl better - and happy hacking!
2227
e8cd7eae
GS
2228=head1 AUTHOR
2229
2230This document was written by Nathan Torkington, and is maintained by
2231the perl5-porters mailing list.
2232