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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.
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40For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of
41Perl, and Jarkko Hietaniemi is the pumpking for the 5.8 release, and
42Hugo van der Sanden will be the pumpking for the 5.10 release.
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43
44In addition, various people are pumpkings for different things. For
45instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
caf100c0 46pumpkin.
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47
48Larry sees Perl development along the lines of the US government:
49there's the Legislature (the porters), the Executive branch (the
50pumpkings), and the Supreme Court (Larry). The legislature can
51discuss and submit patches to the executive branch all they like, but
52the executive branch is free to veto them. Rarely, the Supreme Court
53will side with the executive branch over the legislature, or the
54legislature over the executive branch. Mostly, however, the
55legislature and the executive branch are supposed to get along and
56work out their differences without impeachment or court cases.
57
58You might sometimes see reference to Rule 1 and Rule 2. Larry's power
59as Supreme Court is expressed in The Rules:
60
61=over 4
62
63=item 1
64
65Larry is always by definition right about how Perl should behave.
66This means he has final veto power on the core functionality.
67
68=item 2
69
70Larry is allowed to change his mind about any matter at a later date,
71regardless of whether he previously invoked Rule 1.
72
73=back
74
75Got that? Larry is always right, even when he was wrong. It's rare
76to see either Rule exercised, but they are often alluded to.
77
78New features and extensions to the language are contentious, because
79the criteria used by the pumpkings, Larry, and other porters to decide
80which features should be implemented and incorporated are not codified
81in a few small design goals as with some other languages. Instead,
82the heuristics are flexible and often difficult to fathom. Here is
83one person's list, roughly in decreasing order of importance, of
84heuristics that new features have to be weighed against:
85
86=over 4
87
88=item Does concept match the general goals of Perl?
89
90These haven't been written anywhere in stone, but one approximation
91is:
92
93 1. Keep it fast, simple, and useful.
94 2. Keep features/concepts as orthogonal as possible.
95 3. No arbitrary limits (platforms, data sizes, cultures).
96 4. Keep it open and exciting to use/patch/advocate Perl everywhere.
97 5. Either assimilate new technologies, or build bridges to them.
98
99=item Where is the implementation?
100
101All the talk in the world is useless without an implementation. In
102almost every case, the person or people who argue for a new feature
103will be expected to be the ones who implement it. Porters capable
104of coding new features have their own agendas, and are not available
105to implement your (possibly good) idea.
106
107=item Backwards compatibility
108
109It's a cardinal sin to break existing Perl programs. New warnings are
110contentious--some say that a program that emits warnings is not
111broken, while others say it is. Adding keywords has the potential to
112break programs, changing the meaning of existing token sequences or
113functions might break programs.
114
115=item Could it be a module instead?
116
117Perl 5 has extension mechanisms, modules and XS, specifically to avoid
118the need to keep changing the Perl interpreter. You can write modules
119that export functions, you can give those functions prototypes so they
120can be called like built-in functions, you can even write XS code to
121mess with the runtime data structures of the Perl interpreter if you
122want to implement really complicated things. If it can be done in a
123module instead of in the core, it's highly unlikely to be added.
124
125=item Is the feature generic enough?
126
127Is this something that only the submitter wants added to the language,
128or would it be broadly useful? Sometimes, instead of adding a feature
129with a tight focus, the porters might decide to wait until someone
130implements the more generalized feature. For instance, instead of
131implementing a ``delayed evaluation'' feature, the porters are waiting
132for a macro system that would permit delayed evaluation and much more.
133
134=item Does it potentially introduce new bugs?
135
136Radical rewrites of large chunks of the Perl interpreter have the
137potential to introduce new bugs. The smaller and more localized the
138change, the better.
139
140=item Does it preclude other desirable features?
141
142A patch is likely to be rejected if it closes off future avenues of
143development. For instance, a patch that placed a true and final
144interpretation on prototypes is likely to be rejected because there
145are still options for the future of prototypes that haven't been
146addressed.
147
148=item Is the implementation robust?
149
150Good patches (tight code, complete, correct) stand more chance of
151going in. Sloppy or incorrect patches might be placed on the back
152burner until the pumpking has time to fix, or might be discarded
153altogether without further notice.
154
155=item Is the implementation generic enough to be portable?
156
157The worst patches make use of a system-specific features. It's highly
158unlikely that nonportable additions to the Perl language will be
159accepted.
160
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161=item Is the implementation tested?
162
163Patches which change behaviour (fixing bugs or introducing new features)
164must include regression tests to verify that everything works as expected.
165Without tests provided by the original author, how can anyone else changing
166perl in the future be sure that they haven't unwittingly broken the behaviour
167the patch implements? And without tests, how can the patch's author be
9d077eaa 168confident that his/her hard work put into the patch won't be accidentally
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169thrown away by someone in the future?
170
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171=item Is there enough documentation?
172
173Patches without documentation are probably ill-thought out or
174incomplete. Nothing can be added without documentation, so submitting
175a patch for the appropriate manpages as well as the source code is
a936dd3c 176always a good idea.
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177
178=item Is there another way to do it?
179
180Larry said ``Although the Perl Slogan is I<There's More Than One Way
181to Do It>, I hesitate to make 10 ways to do something''. This is a
182tricky heuristic to navigate, though--one man's essential addition is
183another man's pointless cruft.
184
185=item Does it create too much work?
186
187Work for the pumpking, work for Perl programmers, work for module
188authors, ... Perl is supposed to be easy.
189
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190=item Patches speak louder than words
191
192Working code is always preferred to pie-in-the-sky ideas. A patch to
193add a feature stands a much higher chance of making it to the language
194than does a random feature request, no matter how fervently argued the
195request might be. This ties into ``Will it be useful?'', as the fact
196that someone took the time to make the patch demonstrates a strong
197desire for the feature.
198
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199=back
200
201If you're on the list, you might hear the word ``core'' bandied
202around. It refers to the standard distribution. ``Hacking on the
203core'' means you're changing the C source code to the Perl
204interpreter. ``A core module'' is one that ships with Perl.
205
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206=head2 Keeping in sync
207
e8cd7eae 208The source code to the Perl interpreter, in its different versions, is
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209kept in a repository managed by a revision control system ( which is
210currently the Perforce program, see http://perforce.com/ ). The
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211pumpkings and a few others have access to the repository to check in
212changes. Periodically the pumpking for the development version of Perl
213will release a new version, so the rest of the porters can see what's
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214changed. The current state of the main trunk of repository, and patches
215that describe the individual changes that have happened since the last
216public release are available at this location:
217
0cfb3454 218 http://public.activestate.com/gsar/APC/
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219 ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
220
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221If you're looking for a particular change, or a change that affected
222a particular set of files, you may find the B<Perl Repository Browser>
223useful:
224
225 http://public.activestate.com/cgi-bin/perlbrowse
226
227You may also want to subscribe to the perl5-changes mailing list to
228receive a copy of each patch that gets submitted to the maintenance
229and development "branches" of the perl repository. See
230http://lists.perl.org/ for subscription information.
231
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232If you are a member of the perl5-porters mailing list, it is a good
233thing to keep in touch with the most recent changes. If not only to
234verify if what you would have posted as a bug report isn't already
235solved in the most recent available perl development branch, also
236known as perl-current, bleading edge perl, bleedperl or bleadperl.
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237
238Needless to say, the source code in perl-current is usually in a perpetual
239state of evolution. You should expect it to be very buggy. Do B<not> use
240it for any purpose other than testing and development.
e8cd7eae 241
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242Keeping in sync with the most recent branch can be done in several ways,
243but the most convenient and reliable way is using B<rsync>, available at
244ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
245branch by FTP.)
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246
247If you choose to keep in sync using rsync, there are two approaches
3e148164 248to doing so:
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249
250=over 4
251
252=item rsync'ing the source tree
253
3e148164 254Presuming you are in the directory where your perl source resides
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255and you have rsync installed and available, you can `upgrade' to
256the bleadperl using:
257
258 # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
259
260This takes care of updating every single item in the source tree to
261the latest applied patch level, creating files that are new (to your
262distribution) and setting date/time stamps of existing files to
263reflect the bleadperl status.
264
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265Note that this will not delete any files that were in '.' before
266the rsync. Once you are sure that the rsync is running correctly,
267run it with the --delete and the --dry-run options like this:
268
269 # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
270
271This will I<simulate> an rsync run that also deletes files not
272present in the bleadperl master copy. Observe the results from
273this run closely. If you are sure that the actual run would delete
274no files precious to you, you could remove the '--dry-run' option.
275
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276You can than check what patch was the latest that was applied by
277looking in the file B<.patch>, which will show the number of the
278latest patch.
279
280If you have more than one machine to keep in sync, and not all of
281them have access to the WAN (so you are not able to rsync all the
282source trees to the real source), there are some ways to get around
283this problem.
284
285=over 4
286
287=item Using rsync over the LAN
288
289Set up a local rsync server which makes the rsynced source tree
3e148164 290available to the LAN and sync the other machines against this
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291directory.
292
1577cd80 293From http://rsync.samba.org/README.html :
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294
295 "Rsync uses rsh or ssh for communication. It does not need to be
296 setuid and requires no special privileges for installation. It
3958b146 297 does not require an inetd entry or a daemon. You must, however,
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298 have a working rsh or ssh system. Using ssh is recommended for
299 its security features."
300
301=item Using pushing over the NFS
302
303Having the other systems mounted over the NFS, you can take an
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304active pushing approach by checking the just updated tree against
305the other not-yet synced trees. An example would be
306
307 #!/usr/bin/perl -w
308
309 use strict;
310 use File::Copy;
311
312 my %MF = map {
313 m/(\S+)/;
314 $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
315 } `cat MANIFEST`;
316
317 my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
318
319 foreach my $host (keys %remote) {
320 unless (-d $remote{$host}) {
321 print STDERR "Cannot Xsync for host $host\n";
322 next;
323 }
324 foreach my $file (keys %MF) {
325 my $rfile = "$remote{$host}/$file";
326 my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
327 defined $size or ($mode, $size, $mtime) = (0, 0, 0);
328 $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
329 printf "%4s %-34s %8d %9d %8d %9d\n",
330 $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
331 unlink $rfile;
332 copy ($file, $rfile);
333 utime time, $MF{$file}[2], $rfile;
334 chmod $MF{$file}[0], $rfile;
335 }
336 }
337
338though this is not perfect. It could be improved with checking
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339file checksums before updating. Not all NFS systems support
340reliable utime support (when used over the NFS).
341
342=back
343
344=item rsync'ing the patches
345
346The source tree is maintained by the pumpking who applies patches to
347the files in the tree. These patches are either created by the
348pumpking himself using C<diff -c> after updating the file manually or
349by applying patches sent in by posters on the perl5-porters list.
350These patches are also saved and rsync'able, so you can apply them
351yourself to the source files.
352
353Presuming you are in a directory where your patches reside, you can
3e148164 354get them in sync with
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355
356 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
357
358This makes sure the latest available patch is downloaded to your
359patch directory.
360
3e148164 361It's then up to you to apply these patches, using something like
a1f349fd 362
df3477ff 363 # last=`ls -t *.gz | sed q`
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364 # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
365 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
366 # cd ../perl-current
367 # patch -p1 -N <../perl-current-diffs/blead.patch
368
369or, since this is only a hint towards how it works, use CPAN-patchaperl
370from Andreas K├Ânig to have better control over the patching process.
371
372=back
373
f7e1e956 374=head2 Why rsync the source tree
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375
376=over 4
377
10f58044 378=item It's easier to rsync the source tree
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379
380Since you don't have to apply the patches yourself, you are sure all
381files in the source tree are in the right state.
382
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383=item It's more reliable
384
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385While both the rsync-able source and patch areas are automatically
386updated every few minutes, keep in mind that applying patches may
387sometimes mean careful hand-holding, especially if your version of
388the C<patch> program does not understand how to deal with new files,
389files with 8-bit characters, or files without trailing newlines.
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390
391=back
392
f7e1e956 393=head2 Why rsync the patches
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394
395=over 4
396
10f58044 397=item It's easier to rsync the patches
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398
399If you have more than one machine that you want to keep in track with
3e148164 400bleadperl, it's easier to rsync the patches only once and then apply
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401them to all the source trees on the different machines.
402
403In case you try to keep in pace on 5 different machines, for which
404only one of them has access to the WAN, rsync'ing all the source
3e148164 405trees should than be done 5 times over the NFS. Having
a1f349fd 406rsync'ed the patches only once, I can apply them to all the source
3e148164 407trees automatically. Need you say more ;-)
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408
409=item It's a good reference
410
411If you do not only like to have the most recent development branch,
412but also like to B<fix> bugs, or extend features, you want to dive
413into the sources. If you are a seasoned perl core diver, you don't
414need no manuals, tips, roadmaps, perlguts.pod or other aids to find
415your way around. But if you are a starter, the patches may help you
416in finding where you should start and how to change the bits that
417bug you.
418
419The file B<Changes> is updated on occasions the pumpking sees as his
420own little sync points. On those occasions, he releases a tar-ball of
421the current source tree (i.e. perl@7582.tar.gz), which will be an
422excellent point to start with when choosing to use the 'rsync the
423patches' scheme. Starting with perl@7582, which means a set of source
424files on which the latest applied patch is number 7582, you apply all
f18956b7 425succeeding patches available from then on (7583, 7584, ...).
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426
427You can use the patches later as a kind of search archive.
428
429=over 4
430
431=item Finding a start point
432
433If you want to fix/change the behaviour of function/feature Foo, just
434scan the patches for patches that mention Foo either in the subject,
3e148164 435the comments, or the body of the fix. A good chance the patch shows
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436you the files that are affected by that patch which are very likely
437to be the starting point of your journey into the guts of perl.
438
439=item Finding how to fix a bug
440
441If you've found I<where> the function/feature Foo misbehaves, but you
442don't know how to fix it (but you do know the change you want to
443make), you can, again, peruse the patches for similar changes and
444look how others apply the fix.
445
446=item Finding the source of misbehaviour
447
448When you keep in sync with bleadperl, the pumpking would love to
3958b146 449I<see> that the community efforts really work. So after each of his
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450sync points, you are to 'make test' to check if everything is still
451in working order. If it is, you do 'make ok', which will send an OK
452report to perlbug@perl.org. (If you do not have access to a mailer
3e148164 453from the system you just finished successfully 'make test', you can
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454do 'make okfile', which creates the file C<perl.ok>, which you can
455than take to your favourite mailer and mail yourself).
456
3958b146 457But of course, as always, things will not always lead to a success
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458path, and one or more test do not pass the 'make test'. Before
459sending in a bug report (using 'make nok' or 'make nokfile'), check
460the mailing list if someone else has reported the bug already and if
461so, confirm it by replying to that message. If not, you might want to
462trace the source of that misbehaviour B<before> sending in the bug,
463which will help all the other porters in finding the solution.
464
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465Here the saved patches come in very handy. You can check the list of
466patches to see which patch changed what file and what change caused
467the misbehaviour. If you note that in the bug report, it saves the
468one trying to solve it, looking for that point.
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469
470=back
471
472If searching the patches is too bothersome, you might consider using
473perl's bugtron to find more information about discussions and
474ramblings on posted bugs.
475
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476If you want to get the best of both worlds, rsync both the source
477tree for convenience, reliability and ease and rsync the patches
478for reference.
479
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480=back
481
482
3fd28c4e 483=head2 Perlbug administration
52315700 484
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485There is a single remote administrative interface for modifying bug status,
486category, open issues etc. using the B<RT> I<bugtracker> system, maintained
487by I<Robert Spier>. Become an administrator, and close any bugs you can get
488your sticky mitts on:
52315700 489
3fd28c4e 490 http://rt.perl.org
52315700 491
3fd28c4e 492The bugtracker mechanism for B<perl5> bugs in particular is at:
52315700 493
3fd28c4e 494 http://bugs6.perl.org/perlbug
52315700 495
3fd28c4e 496To email the bug system administrators:
52315700 497
3fd28c4e 498 "perlbug-admin" <perlbug-admin@perl.org>
52315700 499
52315700 500
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501=head2 Submitting patches
502
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503Always submit patches to I<perl5-porters@perl.org>. If you're
504patching a core module and there's an author listed, send the author a
505copy (see L<Patching a core module>). This lets other porters review
506your patch, which catches a surprising number of errors in patches.
507Either use the diff program (available in source code form from
f224927c 508ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' I<makepatch>
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509(available from I<CPAN/authors/id/JV/>). Unified diffs are preferred,
510but context diffs are accepted. Do not send RCS-style diffs or diffs
511without context lines. More information is given in the
512I<Porting/patching.pod> file in the Perl source distribution. Please
513patch against the latest B<development> version (e.g., if you're
514fixing a bug in the 5.005 track, patch against the latest 5.005_5x
515version). Only patches that survive the heat of the development
516branch get applied to maintenance versions.
517
518Your patch should update the documentation and test suite. See
519L<Writing a test>.
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520
521To report a bug in Perl, use the program I<perlbug> which comes with
522Perl (if you can't get Perl to work, send mail to the address
f18956b7 523I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
e8cd7eae 524I<perlbug> feeds into the automated bug-tracking system, access to
f224927c 525which is provided through the web at http://bugs.perl.org/ . It
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526often pays to check the archives of the perl5-porters mailing list to
527see whether the bug you're reporting has been reported before, and if
528so whether it was considered a bug. See above for the location of
529the searchable archives.
530
f224927c 531The CPAN testers ( http://testers.cpan.org/ ) are a group of
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532volunteers who test CPAN modules on a variety of platforms. Perl
533Smokers ( http://archives.develooper.com/daily-build@perl.org/ )
534automatically tests Perl source releases on platforms with various
535configurations. Both efforts welcome volunteers.
e8cd7eae 536
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537It's a good idea to read and lurk for a while before chipping in.
538That way you'll get to see the dynamic of the conversations, learn the
539personalities of the players, and hopefully be better prepared to make
540a useful contribution when do you speak up.
541
542If after all this you still think you want to join the perl5-porters
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543mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
544unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
e8cd7eae 545
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546To hack on the Perl guts, you'll need to read the following things:
547
548=over 3
549
550=item L<perlguts>
551
552This is of paramount importance, since it's the documentation of what
553goes where in the Perl source. Read it over a couple of times and it
554might start to make sense - don't worry if it doesn't yet, because the
555best way to study it is to read it in conjunction with poking at Perl
556source, and we'll do that later on.
557
558You might also want to look at Gisle Aas's illustrated perlguts -
559there's no guarantee that this will be absolutely up-to-date with the
560latest documentation in the Perl core, but the fundamentals will be
1577cd80 561right. ( http://gisle.aas.no/perl/illguts/ )
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562
563=item L<perlxstut> and L<perlxs>
564
565A working knowledge of XSUB programming is incredibly useful for core
566hacking; XSUBs use techniques drawn from the PP code, the portion of the
567guts that actually executes a Perl program. It's a lot gentler to learn
568those techniques from simple examples and explanation than from the core
569itself.
570
571=item L<perlapi>
572
573The documentation for the Perl API explains what some of the internal
574functions do, as well as the many macros used in the source.
575
576=item F<Porting/pumpkin.pod>
577
578This is a collection of words of wisdom for a Perl porter; some of it is
579only useful to the pumpkin holder, but most of it applies to anyone
580wanting to go about Perl development.
581
582=item The perl5-porters FAQ
583
7d7d5695
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584This should be available from http://simon-cozens.org/writings/p5p-faq ;
585alternatively, you can get the FAQ emailed to you by sending mail to
586C<perl5-porters-faq@perl.org>. It contains hints on reading perl5-porters,
587information on how perl5-porters works and how Perl development in general
588works.
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589
590=back
591
592=head2 Finding Your Way Around
593
594Perl maintenance can be split into a number of areas, and certain people
595(pumpkins) will have responsibility for each area. These areas sometimes
596correspond to files or directories in the source kit. Among the areas are:
597
598=over 3
599
600=item Core modules
601
602Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
603subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
604contains the core XS modules.
605
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606=item Tests
607
608There are tests for nearly all the modules, built-ins and major bits
609of functionality. Test files all have a .t suffix. Module tests live
610in the F<lib/> and F<ext/> directories next to the module being
611tested. Others live in F<t/>. See L<Writing a test>
612
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613=item Documentation
614
615Documentation maintenance includes looking after everything in the
616F<pod/> directory, (as well as contributing new documentation) and
617the documentation to the modules in core.
618
619=item Configure
620
621The configure process is the way we make Perl portable across the
622myriad of operating systems it supports. Responsibility for the
623configure, build and installation process, as well as the overall
624portability of the core code rests with the configure pumpkin - others
625help out with individual operating systems.
626
627The files involved are the operating system directories, (F<win32/>,
628F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
629and F<Makefile>, as well as the metaconfig files which generate
630F<Configure>. (metaconfig isn't included in the core distribution.)
631
632=item Interpreter
633
634And of course, there's the core of the Perl interpreter itself. Let's
635have a look at that in a little more detail.
636
637=back
638
639Before we leave looking at the layout, though, don't forget that
640F<MANIFEST> contains not only the file names in the Perl distribution,
641but short descriptions of what's in them, too. For an overview of the
642important files, try this:
643
644 perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
645
646=head2 Elements of the interpreter
647
648The work of the interpreter has two main stages: compiling the code
649into the internal representation, or bytecode, and then executing it.
650L<perlguts/Compiled code> explains exactly how the compilation stage
651happens.
652
653Here is a short breakdown of perl's operation:
654
655=over 3
656
657=item Startup
658
659The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
660This is very high-level code, enough to fit on a single screen, and it
661resembles the code found in L<perlembed>; most of the real action takes
662place in F<perl.c>
663
664First, F<perlmain.c> allocates some memory and constructs a Perl
665interpreter:
666
667 1 PERL_SYS_INIT3(&argc,&argv,&env);
668 2
669 3 if (!PL_do_undump) {
670 4 my_perl = perl_alloc();
671 5 if (!my_perl)
672 6 exit(1);
673 7 perl_construct(my_perl);
674 8 PL_perl_destruct_level = 0;
675 9 }
676
677Line 1 is a macro, and its definition is dependent on your operating
678system. Line 3 references C<PL_do_undump>, a global variable - all
679global variables in Perl start with C<PL_>. This tells you whether the
680current running program was created with the C<-u> flag to perl and then
681F<undump>, which means it's going to be false in any sane context.
682
683Line 4 calls a function in F<perl.c> to allocate memory for a Perl
684interpreter. It's quite a simple function, and the guts of it looks like
685this:
686
687 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
688
689Here you see an example of Perl's system abstraction, which we'll see
690later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
691own C<malloc> as defined in F<malloc.c> if you selected that option at
692configure time.
693
694Next, in line 7, we construct the interpreter; this sets up all the
695special variables that Perl needs, the stacks, and so on.
696
697Now we pass Perl the command line options, and tell it to go:
698
699 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
700 if (!exitstatus) {
701 exitstatus = perl_run(my_perl);
702 }
703
704
705C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
706in F<perl.c>, which processes the command line options, sets up any
707statically linked XS modules, opens the program and calls C<yyparse> to
708parse it.
709
710=item Parsing
711
712The aim of this stage is to take the Perl source, and turn it into an op
713tree. We'll see what one of those looks like later. Strictly speaking,
714there's three things going on here.
715
716C<yyparse>, the parser, lives in F<perly.c>, although you're better off
717reading the original YACC input in F<perly.y>. (Yes, Virginia, there
718B<is> a YACC grammar for Perl!) The job of the parser is to take your
719code and `understand' it, splitting it into sentences, deciding which
720operands go with which operators and so on.
721
722The parser is nobly assisted by the lexer, which chunks up your input
723into tokens, and decides what type of thing each token is: a variable
724name, an operator, a bareword, a subroutine, a core function, and so on.
725The main point of entry to the lexer is C<yylex>, and that and its
726associated routines can be found in F<toke.c>. Perl isn't much like
727other computer languages; it's highly context sensitive at times, it can
728be tricky to work out what sort of token something is, or where a token
729ends. As such, there's a lot of interplay between the tokeniser and the
730parser, which can get pretty frightening if you're not used to it.
731
732As the parser understands a Perl program, it builds up a tree of
733operations for the interpreter to perform during execution. The routines
734which construct and link together the various operations are to be found
735in F<op.c>, and will be examined later.
736
737=item Optimization
738
739Now the parsing stage is complete, and the finished tree represents
740the operations that the Perl interpreter needs to perform to execute our
741program. Next, Perl does a dry run over the tree looking for
742optimisations: constant expressions such as C<3 + 4> will be computed
743now, and the optimizer will also see if any multiple operations can be
744replaced with a single one. For instance, to fetch the variable C<$foo>,
745instead of grabbing the glob C<*foo> and looking at the scalar
746component, the optimizer fiddles the op tree to use a function which
747directly looks up the scalar in question. The main optimizer is C<peep>
748in F<op.c>, and many ops have their own optimizing functions.
749
750=item Running
751
752Now we're finally ready to go: we have compiled Perl byte code, and all
753that's left to do is run it. The actual execution is done by the
754C<runops_standard> function in F<run.c>; more specifically, it's done by
755these three innocent looking lines:
756
757 while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
758 PERL_ASYNC_CHECK();
759 }
760
761You may be more comfortable with the Perl version of that:
762
763 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
764
765Well, maybe not. Anyway, each op contains a function pointer, which
766stipulates the function which will actually carry out the operation.
767This function will return the next op in the sequence - this allows for
768things like C<if> which choose the next op dynamically at run time.
769The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
770execution if required.
771
772The actual functions called are known as PP code, and they're spread
773between four files: F<pp_hot.c> contains the `hot' code, which is most
774often used and highly optimized, F<pp_sys.c> contains all the
775system-specific functions, F<pp_ctl.c> contains the functions which
776implement control structures (C<if>, C<while> and the like) and F<pp.c>
777contains everything else. These are, if you like, the C code for Perl's
778built-in functions and operators.
779
780=back
781
782=head2 Internal Variable Types
783
784You should by now have had a look at L<perlguts>, which tells you about
785Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
786that now.
787
788These variables are used not only to represent Perl-space variables, but
789also any constants in the code, as well as some structures completely
790internal to Perl. The symbol table, for instance, is an ordinary Perl
791hash. Your code is represented by an SV as it's read into the parser;
792any program files you call are opened via ordinary Perl filehandles, and
793so on.
794
795The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
796Perl program. Let's see, for instance, how Perl treats the constant
797C<"hello">.
798
799 % perl -MDevel::Peek -e 'Dump("hello")'
800 1 SV = PV(0xa041450) at 0xa04ecbc
801 2 REFCNT = 1
802 3 FLAGS = (POK,READONLY,pPOK)
803 4 PV = 0xa0484e0 "hello"\0
804 5 CUR = 5
805 6 LEN = 6
806
807Reading C<Devel::Peek> output takes a bit of practise, so let's go
808through it line by line.
809
810Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
811memory. SVs themselves are very simple structures, but they contain a
812pointer to a more complex structure. In this case, it's a PV, a
813structure which holds a string value, at location C<0xa041450>. Line 2
814is the reference count; there are no other references to this data, so
815it's 1.
816
817Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
818read-only SV (because it's a constant) and the data is a PV internally.
819Next we've got the contents of the string, starting at location
820C<0xa0484e0>.
821
822Line 5 gives us the current length of the string - note that this does
823B<not> include the null terminator. Line 6 is not the length of the
824string, but the length of the currently allocated buffer; as the string
825grows, Perl automatically extends the available storage via a routine
826called C<SvGROW>.
827
828You can get at any of these quantities from C very easily; just add
829C<Sv> to the name of the field shown in the snippet, and you've got a
830macro which will return the value: C<SvCUR(sv)> returns the current
831length of the string, C<SvREFCOUNT(sv)> returns the reference count,
832C<SvPV(sv, len)> returns the string itself with its length, and so on.
833More macros to manipulate these properties can be found in L<perlguts>.
834
835Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
836
837 1 void
838 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
839 3 {
840 4 STRLEN tlen;
841 5 char *junk;
842
843 6 junk = SvPV_force(sv, tlen);
844 7 SvGROW(sv, tlen + len + 1);
845 8 if (ptr == junk)
846 9 ptr = SvPVX(sv);
847 10 Move(ptr,SvPVX(sv)+tlen,len,char);
848 11 SvCUR(sv) += len;
849 12 *SvEND(sv) = '\0';
850 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
851 14 SvTAINT(sv);
852 15 }
853
854This is a function which adds a string, C<ptr>, of length C<len> onto
855the end of the PV stored in C<sv>. The first thing we do in line 6 is
856make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
857macro to force a PV. As a side effect, C<tlen> gets set to the current
858value of the PV, and the PV itself is returned to C<junk>.
859
b1866b2d 860In line 7, we make sure that the SV will have enough room to accommodate
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861the old string, the new string and the null terminator. If C<LEN> isn't
862big enough, C<SvGROW> will reallocate space for us.
863
864Now, if C<junk> is the same as the string we're trying to add, we can
865grab the string directly from the SV; C<SvPVX> is the address of the PV
866in the SV.
867
868Line 10 does the actual catenation: the C<Move> macro moves a chunk of
869memory around: we move the string C<ptr> to the end of the PV - that's
870the start of the PV plus its current length. We're moving C<len> bytes
871of type C<char>. After doing so, we need to tell Perl we've extended the
872string, by altering C<CUR> to reflect the new length. C<SvEND> is a
873macro which gives us the end of the string, so that needs to be a
874C<"\0">.
875
876Line 13 manipulates the flags; since we've changed the PV, any IV or NV
877values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
1e54db1a 878want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware
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879version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
880and turns on POK. The final C<SvTAINT> is a macro which launders tainted
881data if taint mode is turned on.
882
883AVs and HVs are more complicated, but SVs are by far the most common
884variable type being thrown around. Having seen something of how we
885manipulate these, let's go on and look at how the op tree is
886constructed.
887
888=head2 Op Trees
889
890First, what is the op tree, anyway? The op tree is the parsed
891representation of your program, as we saw in our section on parsing, and
892it's the sequence of operations that Perl goes through to execute your
893program, as we saw in L</Running>.
894
895An op is a fundamental operation that Perl can perform: all the built-in
896functions and operators are ops, and there are a series of ops which
897deal with concepts the interpreter needs internally - entering and
898leaving a block, ending a statement, fetching a variable, and so on.
899
900The op tree is connected in two ways: you can imagine that there are two
901"routes" through it, two orders in which you can traverse the tree.
902First, parse order reflects how the parser understood the code, and
903secondly, execution order tells perl what order to perform the
904operations in.
905
906The easiest way to examine the op tree is to stop Perl after it has
907finished parsing, and get it to dump out the tree. This is exactly what
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908the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
909and L<B::Debug|B::Debug> do.
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910
911Let's have a look at how Perl sees C<$a = $b + $c>:
912
913 % perl -MO=Terse -e '$a=$b+$c'
914 1 LISTOP (0x8179888) leave
915 2 OP (0x81798b0) enter
916 3 COP (0x8179850) nextstate
917 4 BINOP (0x8179828) sassign
918 5 BINOP (0x8179800) add [1]
919 6 UNOP (0x81796e0) null [15]
920 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
921 8 UNOP (0x81797e0) null [15]
922 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
923 10 UNOP (0x816b4f0) null [15]
924 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
925
926Let's start in the middle, at line 4. This is a BINOP, a binary
927operator, which is at location C<0x8179828>. The specific operator in
928question is C<sassign> - scalar assignment - and you can find the code
929which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
930binary operator, it has two children: the add operator, providing the
931result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
932line 10.
933
934Line 10 is the null op: this does exactly nothing. What is that doing
935there? If you see the null op, it's a sign that something has been
936optimized away after parsing. As we mentioned in L</Optimization>,
937the optimization stage sometimes converts two operations into one, for
938example when fetching a scalar variable. When this happens, instead of
939rewriting the op tree and cleaning up the dangling pointers, it's easier
940just to replace the redundant operation with the null op. Originally,
941the tree would have looked like this:
942
943 10 SVOP (0x816b4f0) rv2sv [15]
944 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
945
946That is, fetch the C<a> entry from the main symbol table, and then look
947at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
948happens to do both these things.
949
950The right hand side, starting at line 5 is similar to what we've just
951seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
952two C<gvsv>s.
953
954Now, what's this about?
955
956 1 LISTOP (0x8179888) leave
957 2 OP (0x81798b0) enter
958 3 COP (0x8179850) nextstate
959
960C<enter> and C<leave> are scoping ops, and their job is to perform any
961housekeeping every time you enter and leave a block: lexical variables
962are tidied up, unreferenced variables are destroyed, and so on. Every
963program will have those first three lines: C<leave> is a list, and its
964children are all the statements in the block. Statements are delimited
965by C<nextstate>, so a block is a collection of C<nextstate> ops, with
966the ops to be performed for each statement being the children of
967C<nextstate>. C<enter> is a single op which functions as a marker.
968
969That's how Perl parsed the program, from top to bottom:
970
971 Program
972 |
973 Statement
974 |
975 =
976 / \
977 / \
978 $a +
979 / \
980 $b $c
981
982However, it's impossible to B<perform> the operations in this order:
983you have to find the values of C<$b> and C<$c> before you add them
984together, for instance. So, the other thread that runs through the op
985tree is the execution order: each op has a field C<op_next> which points
986to the next op to be run, so following these pointers tells us how perl
987executes the code. We can traverse the tree in this order using
988the C<exec> option to C<B::Terse>:
989
990 % perl -MO=Terse,exec -e '$a=$b+$c'
991 1 OP (0x8179928) enter
992 2 COP (0x81798c8) nextstate
993 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
994 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
995 5 BINOP (0x8179878) add [1]
996 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
997 7 BINOP (0x81798a0) sassign
998 8 LISTOP (0x8179900) leave
999
1000This probably makes more sense for a human: enter a block, start a
1001statement. Get the values of C<$b> and C<$c>, and add them together.
1002Find C<$a>, and assign one to the other. Then leave.
1003
1004The way Perl builds up these op trees in the parsing process can be
1005unravelled by examining F<perly.y>, the YACC grammar. Let's take the
1006piece we need to construct the tree for C<$a = $b + $c>
1007
1008 1 term : term ASSIGNOP term
1009 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
1010 3 | term ADDOP term
1011 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1012
1013If you're not used to reading BNF grammars, this is how it works: You're
1014fed certain things by the tokeniser, which generally end up in upper
1015case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
1016code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
1017`terminal symbols', because you can't get any simpler than them.
1018
1019The grammar, lines one and three of the snippet above, tells you how to
1020build up more complex forms. These complex forms, `non-terminal symbols'
1021are generally placed in lower case. C<term> here is a non-terminal
1022symbol, representing a single expression.
1023
1024The grammar gives you the following rule: you can make the thing on the
1025left of the colon if you see all the things on the right in sequence.
1026This is called a "reduction", and the aim of parsing is to completely
1027reduce the input. There are several different ways you can perform a
1028reduction, separated by vertical bars: so, C<term> followed by C<=>
1029followed by C<term> makes a C<term>, and C<term> followed by C<+>
1030followed by C<term> can also make a C<term>.
1031
1032So, if you see two terms with an C<=> or C<+>, between them, you can
1033turn them into a single expression. When you do this, you execute the
1034code in the block on the next line: if you see C<=>, you'll do the code
1035in line 2. If you see C<+>, you'll do the code in line 4. It's this code
1036which contributes to the op tree.
1037
1038 | term ADDOP term
1039 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
1040
1041What this does is creates a new binary op, and feeds it a number of
1042variables. The variables refer to the tokens: C<$1> is the first token in
1043the input, C<$2> the second, and so on - think regular expression
1044backreferences. C<$$> is the op returned from this reduction. So, we
1045call C<newBINOP> to create a new binary operator. The first parameter to
1046C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
1047operator, so we want the type to be C<ADDOP>. We could specify this
1048directly, but it's right there as the second token in the input, so we
1049use C<$2>. The second parameter is the op's flags: 0 means `nothing
1050special'. Then the things to add: the left and right hand side of our
1051expression, in scalar context.
1052
1053=head2 Stacks
1054
1055When perl executes something like C<addop>, how does it pass on its
1056results to the next op? The answer is, through the use of stacks. Perl
1057has a number of stacks to store things it's currently working on, and
1058we'll look at the three most important ones here.
1059
1060=over 3
1061
1062=item Argument stack
1063
1064Arguments are passed to PP code and returned from PP code using the
1065argument stack, C<ST>. The typical way to handle arguments is to pop
1066them off the stack, deal with them how you wish, and then push the result
1067back onto the stack. This is how, for instance, the cosine operator
1068works:
1069
1070 NV value;
1071 value = POPn;
1072 value = Perl_cos(value);
1073 XPUSHn(value);
1074
1075We'll see a more tricky example of this when we consider Perl's macros
1076below. C<POPn> gives you the NV (floating point value) of the top SV on
1077the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
1078the result back as an NV. The C<X> in C<XPUSHn> means that the stack
1079should be extended if necessary - it can't be necessary here, because we
1080know there's room for one more item on the stack, since we've just
1081removed one! The C<XPUSH*> macros at least guarantee safety.
1082
1083Alternatively, you can fiddle with the stack directly: C<SP> gives you
1084the first element in your portion of the stack, and C<TOP*> gives you
1085the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
1086negation of an integer:
1087
1088 SETi(-TOPi);
1089
1090Just set the integer value of the top stack entry to its negation.
1091
1092Argument stack manipulation in the core is exactly the same as it is in
1093XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
1094description of the macros used in stack manipulation.
1095
1096=item Mark stack
1097
1098I say `your portion of the stack' above because PP code doesn't
1099necessarily get the whole stack to itself: if your function calls
1100another function, you'll only want to expose the arguments aimed for the
1101called function, and not (necessarily) let it get at your own data. The
1102way we do this is to have a `virtual' bottom-of-stack, exposed to each
1103function. The mark stack keeps bookmarks to locations in the argument
1104stack usable by each function. For instance, when dealing with a tied
1105variable, (internally, something with `P' magic) Perl has to call
1106methods for accesses to the tied variables. However, we need to separate
1107the arguments exposed to the method to the argument exposed to the
1108original function - the store or fetch or whatever it may be. Here's how
1109the tied C<push> is implemented; see C<av_push> in F<av.c>:
1110
1111 1 PUSHMARK(SP);
1112 2 EXTEND(SP,2);
1113 3 PUSHs(SvTIED_obj((SV*)av, mg));
1114 4 PUSHs(val);
1115 5 PUTBACK;
1116 6 ENTER;
1117 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1118 8 LEAVE;
1119 9 POPSTACK;
13a2d996 1120
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1121The lines which concern the mark stack are the first, fifth and last
1122lines: they save away, restore and remove the current position of the
1123argument stack.
1124
1125Let's examine the whole implementation, for practice:
1126
1127 1 PUSHMARK(SP);
1128
1129Push the current state of the stack pointer onto the mark stack. This is
1130so that when we've finished adding items to the argument stack, Perl
1131knows how many things we've added recently.
1132
1133 2 EXTEND(SP,2);
1134 3 PUSHs(SvTIED_obj((SV*)av, mg));
1135 4 PUSHs(val);
1136
1137We're going to add two more items onto the argument stack: when you have
1138a tied array, the C<PUSH> subroutine receives the object and the value
1139to be pushed, and that's exactly what we have here - the tied object,
1140retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
1141
1142 5 PUTBACK;
1143
1144Next we tell Perl to make the change to the global stack pointer: C<dSP>
1145only gave us a local copy, not a reference to the global.
1146
1147 6 ENTER;
1148 7 call_method("PUSH", G_SCALAR|G_DISCARD);
1149 8 LEAVE;
1150
1151C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
1152variables are tidied up, everything that has been localised gets
1153its previous value returned, and so on. Think of them as the C<{> and
1154C<}> of a Perl block.
1155
1156To actually do the magic method call, we have to call a subroutine in
1157Perl space: C<call_method> takes care of that, and it's described in
1158L<perlcall>. We call the C<PUSH> method in scalar context, and we're
1159going to discard its return value.
1160
1161 9 POPSTACK;
1162
1163Finally, we remove the value we placed on the mark stack, since we
1164don't need it any more.
1165
1166=item Save stack
1167
1168C doesn't have a concept of local scope, so perl provides one. We've
1169seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
1170stack implements the C equivalent of, for example:
1171
1172 {
1173 local $foo = 42;
1174 ...
1175 }
1176
1177See L<perlguts/Localising Changes> for how to use the save stack.
1178
1179=back
1180
1181=head2 Millions of Macros
1182
1183One thing you'll notice about the Perl source is that it's full of
1184macros. Some have called the pervasive use of macros the hardest thing
1185to understand, others find it adds to clarity. Let's take an example,
1186the code which implements the addition operator:
1187
1188 1 PP(pp_add)
1189 2 {
39644a26 1190 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
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1191 4 {
1192 5 dPOPTOPnnrl_ul;
1193 6 SETn( left + right );
1194 7 RETURN;
1195 8 }
1196 9 }
1197
1198Every line here (apart from the braces, of course) contains a macro. The
1199first line sets up the function declaration as Perl expects for PP code;
1200line 3 sets up variable declarations for the argument stack and the
1201target, the return value of the operation. Finally, it tries to see if
1202the addition operation is overloaded; if so, the appropriate subroutine
1203is called.
1204
1205Line 5 is another variable declaration - all variable declarations start
1206with C<d> - which pops from the top of the argument stack two NVs (hence
1207C<nn>) and puts them into the variables C<right> and C<left>, hence the
1208C<rl>. These are the two operands to the addition operator. Next, we
1209call C<SETn> to set the NV of the return value to the result of adding
1210the two values. This done, we return - the C<RETURN> macro makes sure
1211that our return value is properly handled, and we pass the next operator
1212to run back to the main run loop.
1213
1214Most of these macros are explained in L<perlapi>, and some of the more
1215important ones are explained in L<perlxs> as well. Pay special attention
1216to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
1217the C<[pad]THX_?> macros.
1218
52d59bef
JH
1219=head2 The .i Targets
1220
1221You can expand the macros in a F<foo.c> file by saying
1222
1223 make foo.i
1224
1225which will expand the macros using cpp. Don't be scared by the results.
1226
a422fd2d
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1227=head2 Poking at Perl
1228
1229To really poke around with Perl, you'll probably want to build Perl for
1230debugging, like this:
1231
1232 ./Configure -d -D optimize=-g
1233 make
1234
1235C<-g> is a flag to the C compiler to have it produce debugging
1236information which will allow us to step through a running program.
1237F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
1238enables all the internal debugging code in Perl. There are a whole bunch
1239of things you can debug with this: L<perlrun> lists them all, and the
1240best way to find out about them is to play about with them. The most
1241useful options are probably
1242
1243 l Context (loop) stack processing
1244 t Trace execution
1245 o Method and overloading resolution
1246 c String/numeric conversions
1247
1248Some of the functionality of the debugging code can be achieved using XS
1249modules.
13a2d996 1250
a422fd2d
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1251 -Dr => use re 'debug'
1252 -Dx => use O 'Debug'
1253
1254=head2 Using a source-level debugger
1255
1256If the debugging output of C<-D> doesn't help you, it's time to step
1257through perl's execution with a source-level debugger.
1258
1259=over 3
1260
1261=item *
1262
1263We'll use C<gdb> for our examples here; the principles will apply to any
1264debugger, but check the manual of the one you're using.
1265
1266=back
1267
1268To fire up the debugger, type
1269
1270 gdb ./perl
1271
1272You'll want to do that in your Perl source tree so the debugger can read
1273the source code. You should see the copyright message, followed by the
1274prompt.
1275
1276 (gdb)
1277
1278C<help> will get you into the documentation, but here are the most
1279useful commands:
1280
1281=over 3
1282
1283=item run [args]
1284
1285Run the program with the given arguments.
1286
1287=item break function_name
1288
1289=item break source.c:xxx
1290
1291Tells the debugger that we'll want to pause execution when we reach
cea6626f 1292either the named function (but see L<perlguts/Internal Functions>!) or the given
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1293line in the named source file.
1294
1295=item step
1296
1297Steps through the program a line at a time.
1298
1299=item next
1300
1301Steps through the program a line at a time, without descending into
1302functions.
1303
1304=item continue
1305
1306Run until the next breakpoint.
1307
1308=item finish
1309
1310Run until the end of the current function, then stop again.
1311
13a2d996 1312=item 'enter'
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1313
1314Just pressing Enter will do the most recent operation again - it's a
1315blessing when stepping through miles of source code.
1316
1317=item print
1318
1319Execute the given C code and print its results. B<WARNING>: Perl makes
52d59bef
JH
1320heavy use of macros, and F<gdb> does not necessarily support macros
1321(see later L</"gdb macro support">). You'll have to substitute them
1322yourself, or to invoke cpp on the source code files
1323(see L</"The .i Targets">)
1324So, for instance, you can't say
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1325
1326 print SvPV_nolen(sv)
1327
1328but you have to say
1329
1330 print Perl_sv_2pv_nolen(sv)
1331
ffc145e8
RK
1332=back
1333
a422fd2d
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1334You may find it helpful to have a "macro dictionary", which you can
1335produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
52d59bef
JH
1336recursively apply those macros for you.
1337
1338=head2 gdb macro support
a422fd2d 1339
52d59bef 1340Recent versions of F<gdb> have fairly good macro support, but
ea031e66
RGS
1341in order to use it you'll need to compile perl with macro definitions
1342included in the debugging information. Using F<gcc> version 3.1, this
1343means configuring with C<-Doptimize=-g3>. Other compilers might use a
1344different switch (if they support debugging macros at all).
1345
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1346=head2 Dumping Perl Data Structures
1347
1348One way to get around this macro hell is to use the dumping functions in
1349F<dump.c>; these work a little like an internal
1350L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
1351that you can't get at from Perl. Let's take an example. We'll use the
1352C<$a = $b + $c> we used before, but give it a bit of context:
1353C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
1354
1355What about C<pp_add>, the function we examined earlier to implement the
1356C<+> operator:
1357
1358 (gdb) break Perl_pp_add
1359 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
1360
cea6626f 1361Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
a422fd2d
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1362With the breakpoint in place, we can run our program:
1363
1364 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
1365
1366Lots of junk will go past as gdb reads in the relevant source files and
1367libraries, and then:
1368
1369 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
39644a26 1370 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
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1371 (gdb) step
1372 311 dPOPTOPnnrl_ul;
1373 (gdb)
1374
1375We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
1376arranges for two C<NV>s to be placed into C<left> and C<right> - let's
1377slightly expand it:
1378
1379 #define dPOPTOPnnrl_ul NV right = POPn; \
1380 SV *leftsv = TOPs; \
1381 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
1382
1383C<POPn> takes the SV from the top of the stack and obtains its NV either
1384directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
1385C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
1386C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
1387C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
1388
1389Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
1390convert it. If we step again, we'll find ourselves there:
1391
1392 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1393 1669 if (!sv)
1394 (gdb)
1395
1396We can now use C<Perl_sv_dump> to investigate the SV:
1397
1398 SV = PV(0xa057cc0) at 0xa0675d0
1399 REFCNT = 1
1400 FLAGS = (POK,pPOK)
1401 PV = 0xa06a510 "6XXXX"\0
1402 CUR = 5
1403 LEN = 6
1404 $1 = void
1405
1406We know we're going to get C<6> from this, so let's finish the
1407subroutine:
1408
1409 (gdb) finish
1410 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
1411 0x462669 in Perl_pp_add () at pp_hot.c:311
1412 311 dPOPTOPnnrl_ul;
1413
1414We can also dump out this op: the current op is always stored in
1415C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
1416similar output to L<B::Debug|B::Debug>.
1417
1418 {
1419 13 TYPE = add ===> 14
1420 TARG = 1
1421 FLAGS = (SCALAR,KIDS)
1422 {
1423 TYPE = null ===> (12)
1424 (was rv2sv)
1425 FLAGS = (SCALAR,KIDS)
1426 {
1427 11 TYPE = gvsv ===> 12
1428 FLAGS = (SCALAR)
1429 GV = main::b
1430 }
1431 }
1432
10f58044 1433# finish this later #
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1434
1435=head2 Patching
1436
1437All right, we've now had a look at how to navigate the Perl sources and
1438some things you'll need to know when fiddling with them. Let's now get
1439on and create a simple patch. Here's something Larry suggested: if a
1440C<U> is the first active format during a C<pack>, (for example,
1441C<pack "U3C8", @stuff>) then the resulting string should be treated as
1e54db1a 1442UTF-8 encoded.
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1443
1444How do we prepare to fix this up? First we locate the code in question -
1445the C<pack> happens at runtime, so it's going to be in one of the F<pp>
1446files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
1447altering this file, let's copy it to F<pp.c~>.
1448
a6ec74c1
JH
1449[Well, it was in F<pp.c> when this tutorial was written. It has now been
1450split off with C<pp_unpack> to its own file, F<pp_pack.c>]
1451
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1452Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
1453loop over the pattern, taking each format character in turn into
1454C<datum_type>. Then for each possible format character, we swallow up
1455the other arguments in the pattern (a field width, an asterisk, and so
1456on) and convert the next chunk input into the specified format, adding
1457it onto the output SV C<cat>.
1458
1459How do we know if the C<U> is the first format in the C<pat>? Well, if
1460we have a pointer to the start of C<pat> then, if we see a C<U> we can
1461test whether we're still at the start of the string. So, here's where
1462C<pat> is set up:
1463
1464 STRLEN fromlen;
1465 register char *pat = SvPVx(*++MARK, fromlen);
1466 register char *patend = pat + fromlen;
1467 register I32 len;
1468 I32 datumtype;
1469 SV *fromstr;
1470
1471We'll have another string pointer in there:
1472
1473 STRLEN fromlen;
1474 register char *pat = SvPVx(*++MARK, fromlen);
1475 register char *patend = pat + fromlen;
1476 + char *patcopy;
1477 register I32 len;
1478 I32 datumtype;
1479 SV *fromstr;
1480
1481And just before we start the loop, we'll set C<patcopy> to be the start
1482of C<pat>:
1483
1484 items = SP - MARK;
1485 MARK++;
1486 sv_setpvn(cat, "", 0);
1487 + patcopy = pat;
1488 while (pat < patend) {
1489
1490Now if we see a C<U> which was at the start of the string, we turn on
1e54db1a 1491the C<UTF8> flag for the output SV, C<cat>:
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1492
1493 + if (datumtype == 'U' && pat==patcopy+1)
1494 + SvUTF8_on(cat);
1495 if (datumtype == '#') {
1496 while (pat < patend && *pat != '\n')
1497 pat++;
1498
1499Remember that it has to be C<patcopy+1> because the first character of
1500the string is the C<U> which has been swallowed into C<datumtype!>
1501
1502Oops, we forgot one thing: what if there are spaces at the start of the
1503pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
1504character, even though it's not the first thing in the pattern. In this
1505case, we have to advance C<patcopy> along with C<pat> when we see spaces:
1506
1507 if (isSPACE(datumtype))
1508 continue;
1509
1510needs to become
1511
1512 if (isSPACE(datumtype)) {
1513 patcopy++;
1514 continue;
1515 }
1516
1517OK. That's the C part done. Now we must do two additional things before
1518this patch is ready to go: we've changed the behaviour of Perl, and so
1519we must document that change. We must also provide some more regression
1520tests to make sure our patch works and doesn't create a bug somewhere
1521else along the line.
1522
b23b8711
MS
1523The regression tests for each operator live in F<t/op/>, and so we
1524make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
1525tests to the end. First, we'll test that the C<U> does indeed create
1526Unicode strings.
1527
1528t/op/pack.t has a sensible ok() function, but if it didn't we could
35c336e6 1529use the one from t/test.pl.
b23b8711 1530
35c336e6
MS
1531 require './test.pl';
1532 plan( tests => 159 );
b23b8711
MS
1533
1534so instead of this:
a422fd2d
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1535
1536 print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
1537 print "ok $test\n"; $test++;
1538
35c336e6
MS
1539we can write the more sensible (see L<Test::More> for a full
1540explanation of is() and other testing functions).
b23b8711 1541
35c336e6 1542 is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
812f5127 1543 "U* produces unicode" );
b23b8711 1544
a422fd2d
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1545Now we'll test that we got that space-at-the-beginning business right:
1546
35c336e6 1547 is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
812f5127 1548 " with spaces at the beginning" );
a422fd2d
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1549
1550And finally we'll test that we don't make Unicode strings if C<U> is B<not>
1551the first active format:
1552
35c336e6 1553 isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
812f5127 1554 "U* not first isn't unicode" );
a422fd2d 1555
35c336e6
MS
1556Mustn't forget to change the number of tests which appears at the top,
1557or else the automated tester will get confused. This will either look
1558like this:
a422fd2d 1559
35c336e6
MS
1560 print "1..156\n";
1561
1562or this:
1563
1564 plan( tests => 156 );
a422fd2d
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1565
1566We now compile up Perl, and run it through the test suite. Our new
1567tests pass, hooray!
1568
1569Finally, the documentation. The job is never done until the paperwork is
1570over, so let's describe the change we've just made. The relevant place
1571is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
1572this text in the description of C<pack>:
1573
1574 =item *
1575
1576 If the pattern begins with a C<U>, the resulting string will be treated
1e54db1a
JH
1577 as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
1578 with an initial C<U0>, and the bytes that follow will be interpreted as
1579 Unicode characters. If you don't want this to happen, you can begin your
1580 pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
a422fd2d
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1581 string, and then follow this with a C<U*> somewhere in your pattern.
1582
1583All done. Now let's create the patch. F<Porting/patching.pod> tells us
1584that if we're making major changes, we should copy the entire directory
1585to somewhere safe before we begin fiddling, and then do
13a2d996 1586
a422fd2d
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1587 diff -ruN old new > patch
1588
1589However, we know which files we've changed, and we can simply do this:
1590
1591 diff -u pp.c~ pp.c > patch
1592 diff -u t/op/pack.t~ t/op/pack.t >> patch
1593 diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
1594
1595We end up with a patch looking a little like this:
1596
1597 --- pp.c~ Fri Jun 02 04:34:10 2000
1598 +++ pp.c Fri Jun 16 11:37:25 2000
1599 @@ -4375,6 +4375,7 @@
1600 register I32 items;
1601 STRLEN fromlen;
1602 register char *pat = SvPVx(*++MARK, fromlen);
1603 + char *patcopy;
1604 register char *patend = pat + fromlen;
1605 register I32 len;
1606 I32 datumtype;
1607 @@ -4405,6 +4406,7 @@
1608 ...
1609
1610And finally, we submit it, with our rationale, to perl5-porters. Job
1611done!
1612
f7e1e956
MS
1613=head2 Patching a core module
1614
1615This works just like patching anything else, with an extra
1616consideration. Many core modules also live on CPAN. If this is so,
1617patch the CPAN version instead of the core and send the patch off to
1618the module maintainer (with a copy to p5p). This will help the module
1619maintainer keep the CPAN version in sync with the core version without
1620constantly scanning p5p.
1621
acbe17fc
JP
1622=head2 Adding a new function to the core
1623
1624If, as part of a patch to fix a bug, or just because you have an
1625especially good idea, you decide to add a new function to the core,
1626discuss your ideas on p5p well before you start work. It may be that
1627someone else has already attempted to do what you are considering and
1628can give lots of good advice or even provide you with bits of code
1629that they already started (but never finished).
1630
1631You have to follow all of the advice given above for patching. It is
1632extremely important to test any addition thoroughly and add new tests
1633to explore all boundary conditions that your new function is expected
1634to handle. If your new function is used only by one module (e.g. toke),
1635then it should probably be named S_your_function (for static); on the
210b36aa 1636other hand, if you expect it to accessible from other functions in
acbe17fc
JP
1637Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
1638for more details.
1639
1640The location of any new code is also an important consideration. Don't
1641just create a new top level .c file and put your code there; you would
1642have to make changes to Configure (so the Makefile is created properly),
1643as well as possibly lots of include files. This is strictly pumpking
1644business.
1645
1646It is better to add your function to one of the existing top level
1647source code files, but your choice is complicated by the nature of
1648the Perl distribution. Only the files that are marked as compiled
1649static are located in the perl executable. Everything else is located
1650in the shared library (or DLL if you are running under WIN32). So,
1651for example, if a function was only used by functions located in
1652toke.c, then your code can go in toke.c. If, however, you want to call
1653the function from universal.c, then you should put your code in another
1654location, for example util.c.
1655
1656In addition to writing your c-code, you will need to create an
1657appropriate entry in embed.pl describing your function, then run
1658'make regen_headers' to create the entries in the numerous header
1659files that perl needs to compile correctly. See L<perlguts/Internal Functions>
1660for information on the various options that you can set in embed.pl.
1661You will forget to do this a few (or many) times and you will get
1662warnings during the compilation phase. Make sure that you mention
1663this when you post your patch to P5P; the pumpking needs to know this.
1664
1665When you write your new code, please be conscious of existing code
884bad00 1666conventions used in the perl source files. See L<perlstyle> for
acbe17fc
JP
1667details. Although most of the guidelines discussed seem to focus on
1668Perl code, rather than c, they all apply (except when they don't ;).
1669See also I<Porting/patching.pod> file in the Perl source distribution
1670for lots of details about both formatting and submitting patches of
1671your changes.
1672
1673Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
1674Test on as many platforms as you can find. Test as many perl
1675Configure options as you can (e.g. MULTIPLICITY). If you have
1676profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL>
210b36aa 1677below for how to use them to further test your code. Remember that
acbe17fc
JP
1678most of the people on P5P are doing this on their own time and
1679don't have the time to debug your code.
f7e1e956
MS
1680
1681=head2 Writing a test
1682
1683Every module and built-in function has an associated test file (or
1684should...). If you add or change functionality, you have to write a
1685test. If you fix a bug, you have to write a test so that bug never
1686comes back. If you alter the docs, it would be nice to test what the
1687new documentation says.
1688
1689In short, if you submit a patch you probably also have to patch the
1690tests.
1691
1692For modules, the test file is right next to the module itself.
1693F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
1694so there are some snags (and it would be wonderful for you to brush
1695them out), but it basically works that way. Everything else lives in
1696F<t/>.
1697
1698=over 3
1699
1700=item F<t/base/>
1701
1702Testing of the absolute basic functionality of Perl. Things like
1703C<if>, basic file reads and writes, simple regexes, etc. These are
1704run first in the test suite and if any of them fail, something is
1705I<really> broken.
1706
1707=item F<t/cmd/>
1708
1709These test the basic control structures, C<if/else>, C<while>,
35c336e6 1710subroutines, etc.
f7e1e956
MS
1711
1712=item F<t/comp/>
1713
1714Tests basic issues of how Perl parses and compiles itself.
1715
1716=item F<t/io/>
1717
1718Tests for built-in IO functions, including command line arguments.
1719
1720=item F<t/lib/>
1721
1722The old home for the module tests, you shouldn't put anything new in
1723here. There are still some bits and pieces hanging around in here
1724that need to be moved. Perhaps you could move them? Thanks!
1725
1726=item F<t/op/>
1727
1728Tests for perl's built in functions that don't fit into any of the
1729other directories.
1730
1731=item F<t/pod/>
1732
1733Tests for POD directives. There are still some tests for the Pod
1734modules hanging around in here that need to be moved out into F<lib/>.
1735
1736=item F<t/run/>
1737
1738Testing features of how perl actually runs, including exit codes and
1739handling of PERL* environment variables.
1740
244d9cb7
RGS
1741=item F<t/uni/>
1742
1743Tests for the core support of Unicode.
1744
1745=item F<t/win32/>
1746
1747Windows-specific tests.
1748
1749=item F<t/x2p>
1750
1751A test suite for the s2p converter.
1752
f7e1e956
MS
1753=back
1754
1755The core uses the same testing style as the rest of Perl, a simple
1756"ok/not ok" run through Test::Harness, but there are a few special
1757considerations.
1758
35c336e6
MS
1759There are three ways to write a test in the core. Test::More,
1760t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The
1761decision of which to use depends on what part of the test suite you're
1762working on. This is a measure to prevent a high-level failure (such
1763as Config.pm breaking) from causing basic functionality tests to fail.
1764
1765=over 4
1766
1767=item t/base t/comp
1768
1769Since we don't know if require works, or even subroutines, use ad hoc
1770tests for these two. Step carefully to avoid using the feature being
1771tested.
1772
1773=item t/cmd t/run t/io t/op
1774
1775Now that basic require() and subroutines are tested, you can use the
1776t/test.pl library which emulates the important features of Test::More
1777while using a minimum of core features.
1778
1779You can also conditionally use certain libraries like Config, but be
1780sure to skip the test gracefully if it's not there.
1781
1782=item t/lib ext lib
1783
1784Now that the core of Perl is tested, Test::More can be used. You can
1785also use the full suite of core modules in the tests.
1786
1787=back
f7e1e956
MS
1788
1789When you say "make test" Perl uses the F<t/TEST> program to run the
1790test suite. All tests are run from the F<t/> directory, B<not> the
1791directory which contains the test. This causes some problems with the
1792tests in F<lib/>, so here's some opportunity for some patching.
1793
1794You must be triply conscious of cross-platform concerns. This usually
1795boils down to using File::Spec and avoiding things like C<fork()> and
1796C<system()> unless absolutely necessary.
1797
e018f8be
JH
1798=head2 Special Make Test Targets
1799
1800There are various special make targets that can be used to test Perl
1801slightly differently than the standard "test" target. Not all them
1802are expected to give a 100% success rate. Many of them have several
1803aliases.
1804
1805=over 4
1806
1807=item coretest
1808
7d7d5695 1809Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests).
e018f8be
JH
1810
1811=item test.deparse
1812
b26492ee
RGS
1813Run all the tests through B::Deparse. Not all tests will succeed.
1814
1815=item test.taintwarn
1816
1817Run all tests with the B<-t> command-line switch. Not all tests
1818are expected to succeed (until they're specifically fixed, of course).
e018f8be
JH
1819
1820=item minitest
1821
1822Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
1823F<t/op>, and F<t/uni> tests.
1824
7a834142
JH
1825=item test.valgrind check.valgrind utest.valgrind ucheck.valgrind
1826
1827(Only in Linux) Run all the tests using the memory leak + naughty
1828memory access tool "valgrind". The log files will be named
1829F<testname.valgrind>.
1830
e018f8be
JH
1831=item test.third check.third utest.third ucheck.third
1832
1833(Only in Tru64) Run all the tests using the memory leak + naughty
1834memory access tool "Third Degree". The log files will be named
1835F<perl3.log.testname>.
1836
1837=item test.torture torturetest
1838
1839Run all the usual tests and some extra tests. As of Perl 5.8.0 the
244d9cb7 1840only extra tests are Abigail's JAPHs, F<t/japh/abigail.t>.
e018f8be
JH
1841
1842You can also run the torture test with F<t/harness> by giving
1843C<-torture> argument to F<t/harness>.
1844
1845=item utest ucheck test.utf8 check.utf8
1846
1847Run all the tests with -Mutf8. Not all tests will succeed.
1848
244d9cb7
RGS
1849=item test_harness
1850
1851Run the test suite with the F<t/harness> controlling program, instead of
1852F<t/TEST>. F<t/harness> is more sophisticated, and uses the
1853L<Test::Harness> module, thus using this test target supposes that perl
1854mostly works. The main advantage for our purposes is that it prints a
00bf5cd9
RGS
1855detailed summary of failed tests at the end. Also, unlike F<t/TEST>, it
1856doesn't redirect stderr to stdout.
244d9cb7
RGS
1857
1858=back
1859
1860=head2 Running tests by hand
1861
1862You can run part of the test suite by hand by using one the following
1863commands from the F<t/> directory :
1864
1865 ./perl -I../lib TEST list-of-.t-files
1866
1867or
1868
1869 ./perl -I../lib harness list-of-.t-files
1870
1871(if you don't specify test scripts, the whole test suite will be run.)
1872
1873You can run an individual test by a command similar to
1874
1875 ./perl -I../lib patho/to/foo.t
1876
1877except that the harnesses set up some environment variables that may
1878affect the execution of the test :
1879
1880=over 4
1881
1882=item PERL_CORE=1
1883
1884indicates that we're running this test part of the perl core test suite.
1885This is useful for modules that have a dual life on CPAN.
1886
1887=item PERL_DESTRUCT_LEVEL=2
1888
1889is set to 2 if it isn't set already (see L</PERL_DESTRUCT_LEVEL>)
1890
1891=item PERL
1892
1893(used only by F<t/TEST>) if set, overrides the path to the perl executable
1894that should be used to run the tests (the default being F<./perl>).
1895
1896=item PERL_SKIP_TTY_TEST
1897
1898if set, tells to skip the tests that need a terminal. It's actually set
1899automatically by the Makefile, but can also be forced artificially by
1900running 'make test_notty'.
1901
e018f8be 1902=back
f7e1e956 1903
902b9dbf
MF
1904=head1 EXTERNAL TOOLS FOR DEBUGGING PERL
1905
1906Sometimes it helps to use external tools while debugging and
1907testing Perl. This section tries to guide you through using
1908some common testing and debugging tools with Perl. This is
1909meant as a guide to interfacing these tools with Perl, not
1910as any kind of guide to the use of the tools themselves.
1911
a958818a
JH
1912B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or
1913Third Degree greatly slows down the execution: seconds become minutes,
1914minutes become hours. For example as of Perl 5.8.1, the
1915ext/Encode/t/Unicode.t takes extraordinarily long to complete under
1916e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
1917than six hours, even on a snappy computer-- the said test must be
1918doing something that is quite unfriendly for memory debuggers. If you
1919don't feel like waiting, that you can simply kill away the perl
1920process.
1921
1922B<NOTE 2>: To minimize the number of memory leak false alarms (see
1923L</PERL_DESTRUCT_LEVEL> for more information), you have to have
1924environment variable PERL_DESTRUCT_LEVEL set to 2. The F<TEST>
1925and harness scripts do that automatically. But if you are running
1926some of the tests manually-- for csh-like shells:
1927
1928 setenv PERL_DESTRUCT_LEVEL 2
1929
1930and for Bourne-type shells:
1931
1932 PERL_DESTRUCT_LEVEL=2
1933 export PERL_DESTRUCT_LEVEL
1934
1935or in UNIXy environments you can also use the C<env> command:
1936
1937 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
a1b65709 1938
37c0adeb
JH
1939B<NOTE 3>: There are known memory leaks when there are compile-time
1940errors within eval or require, seeing C<S_doeval> in the call stack
1941is a good sign of these. Fixing these leaks is non-trivial,
1942unfortunately, but they must be fixed eventually.
1943
902b9dbf
MF
1944=head2 Rational Software's Purify
1945
1946Purify is a commercial tool that is helpful in identifying
1947memory overruns, wild pointers, memory leaks and other such
1948badness. Perl must be compiled in a specific way for
1949optimal testing with Purify. Purify is available under
1950Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
1951
902b9dbf
MF
1952=head2 Purify on Unix
1953
1954On Unix, Purify creates a new Perl binary. To get the most
1955benefit out of Purify, you should create the perl to Purify
1956using:
1957
1958 sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
1959 -Uusemymalloc -Dusemultiplicity
1960
1961where these arguments mean:
1962
1963=over 4
1964
1965=item -Accflags=-DPURIFY
1966
1967Disables Perl's arena memory allocation functions, as well as
1968forcing use of memory allocation functions derived from the
1969system malloc.
1970
1971=item -Doptimize='-g'
1972
1973Adds debugging information so that you see the exact source
1974statements where the problem occurs. Without this flag, all
1975you will see is the source filename of where the error occurred.
1976
1977=item -Uusemymalloc
1978
1979Disable Perl's malloc so that Purify can more closely monitor
1980allocations and leaks. Using Perl's malloc will make Purify
1981report most leaks in the "potential" leaks category.
1982
1983=item -Dusemultiplicity
1984
1985Enabling the multiplicity option allows perl to clean up
1986thoroughly when the interpreter shuts down, which reduces the
1987number of bogus leak reports from Purify.
1988
1989=back
1990
1991Once you've compiled a perl suitable for Purify'ing, then you
1992can just:
1993
1994 make pureperl
1995
1996which creates a binary named 'pureperl' that has been Purify'ed.
1997This binary is used in place of the standard 'perl' binary
1998when you want to debug Perl memory problems.
1999
2000As an example, to show any memory leaks produced during the
2001standard Perl testset you would create and run the Purify'ed
2002perl as:
2003
2004 make pureperl
2005 cd t
2006 ../pureperl -I../lib harness
2007
2008which would run Perl on test.pl and report any memory problems.
2009
2010Purify outputs messages in "Viewer" windows by default. If
2011you don't have a windowing environment or if you simply
2012want the Purify output to unobtrusively go to a log file
2013instead of to the interactive window, use these following
2014options to output to the log file "perl.log":
2015
2016 setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
2017 -log-file=perl.log -append-logfile=yes"
2018
2019If you plan to use the "Viewer" windows, then you only need this option:
2020
2021 setenv PURIFYOPTIONS "-chain-length=25"
2022
c406981e
JH
2023In Bourne-type shells:
2024
98631ff8
JL
2025 PURIFYOPTIONS="..."
2026 export PURIFYOPTIONS
c406981e
JH
2027
2028or if you have the "env" utility:
2029
98631ff8 2030 env PURIFYOPTIONS="..." ../pureperl ...
c406981e 2031
902b9dbf
MF
2032=head2 Purify on NT
2033
2034Purify on Windows NT instruments the Perl binary 'perl.exe'
2035on the fly. There are several options in the makefile you
2036should change to get the most use out of Purify:
2037
2038=over 4
2039
2040=item DEFINES
2041
2042You should add -DPURIFY to the DEFINES line so the DEFINES
2043line looks something like:
2044
2045 DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
2046
2047to disable Perl's arena memory allocation functions, as
2048well as to force use of memory allocation functions derived
2049from the system malloc.
2050
2051=item USE_MULTI = define
2052
2053Enabling the multiplicity option allows perl to clean up
2054thoroughly when the interpreter shuts down, which reduces the
2055number of bogus leak reports from Purify.
2056
2057=item #PERL_MALLOC = define
2058
2059Disable Perl's malloc so that Purify can more closely monitor
2060allocations and leaks. Using Perl's malloc will make Purify
2061report most leaks in the "potential" leaks category.
2062
2063=item CFG = Debug
2064
2065Adds debugging information so that you see the exact source
2066statements where the problem occurs. Without this flag, all
2067you will see is the source filename of where the error occurred.
2068
2069=back
2070
2071As an example, to show any memory leaks produced during the
2072standard Perl testset you would create and run Purify as:
2073
2074 cd win32
2075 make
2076 cd ../t
2077 purify ../perl -I../lib harness
2078
2079which would instrument Perl in memory, run Perl on test.pl,
2080then finally report any memory problems.
2081
7a834142
JH
2082=head2 valgrind
2083
2084The excellent valgrind tool can be used to find out both memory leaks
2085and illegal memory accesses. As of August 2003 it unfortunately works
2086only on x86 (ELF) Linux. The special "test.valgrind" target can be used
d44161bf
MHM
2087to run the tests under valgrind. Found errors and memory leaks are
2088logged in files named F<test.valgrind>.
2089
2090As system libraries (most notably glibc) are also triggering errors,
2091valgrind allows to suppress such errors using suppression files. The
2092default suppression file that comes with valgrind already catches a lot
2093of them. Some additional suppressions are defined in F<t/perl.supp>.
7a834142
JH
2094
2095To get valgrind and for more information see
2096
2097 http://developer.kde.org/~sewardj/
2098
f134cc4e 2099=head2 Compaq's/Digital's/HP's Third Degree
09187cb1
JH
2100
2101Third Degree is a tool for memory leak detection and memory access checks.
2102It is one of the many tools in the ATOM toolkit. The toolkit is only
2103available on Tru64 (formerly known as Digital UNIX formerly known as
2104DEC OSF/1).
2105
2106When building Perl, you must first run Configure with -Doptimize=-g
2107and -Uusemymalloc flags, after that you can use the make targets
51a35ef1
JH
2108"perl.third" and "test.third". (What is required is that Perl must be
2109compiled using the C<-g> flag, you may need to re-Configure.)
09187cb1 2110
64cea5fd 2111The short story is that with "atom" you can instrument the Perl
83f0ef60 2112executable to create a new executable called F<perl.third>. When the
4ae3d70a 2113instrumented executable is run, it creates a log of dubious memory
83f0ef60 2114traffic in file called F<perl.3log>. See the manual pages of atom and
4ae3d70a
JH
2115third for more information. The most extensive Third Degree
2116documentation is available in the Compaq "Tru64 UNIX Programmer's
2117Guide", chapter "Debugging Programs with Third Degree".
64cea5fd 2118
9c54ecba 2119The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/
64cea5fd
JH
2120subdirectory. There is a problem with these files: Third Degree is so
2121effective that it finds problems also in the system libraries.
9c54ecba
JH
2122Therefore you should used the Porting/thirdclean script to cleanup
2123the F<*.3log> files.
64cea5fd
JH
2124
2125There are also leaks that for given certain definition of a leak,
2126aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
2127
2128=head2 PERL_DESTRUCT_LEVEL
2129
a958818a
JH
2130If you want to run any of the tests yourself manually using e.g.
2131valgrind, or the pureperl or perl.third executables, please note that
2132by default perl B<does not> explicitly cleanup all the memory it has
2133allocated (such as global memory arenas) but instead lets the exit()
2134of the whole program "take care" of such allocations, also known as
2135"global destruction of objects".
64cea5fd
JH
2136
2137There is a way to tell perl to do complete cleanup: set the
2138environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
2139The t/TEST wrapper does set this to 2, and this is what you
2140need to do too, if you don't want to see the "global leaks":
1f56d61a 2141For example, for "third-degreed" Perl:
64cea5fd 2142
1f56d61a 2143 env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
09187cb1 2144
414f2397
RGS
2145(Note: the mod_perl apache module uses also this environment variable
2146for its own purposes and extended its semantics. Refer to the mod_perl
287a822c
RGS
2147documentation for more information. Also, spawned threads do the
2148equivalent of setting this variable to the value 1.)
5a6c59ef
DM
2149
2150If, at the end of a run you get the message I<N scalars leaked>, you can
2151recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause
2152the addresses of all those leaked SVs to be dumped; it also converts
2153C<new_SV()> from a macro into a real function, so you can use your
2154favourite debugger to discover where those pesky SVs were allocated.
414f2397 2155
51a35ef1
JH
2156=head2 Profiling
2157
2158Depending on your platform there are various of profiling Perl.
2159
2160There are two commonly used techniques of profiling executables:
10f58044 2161I<statistical time-sampling> and I<basic-block counting>.
51a35ef1
JH
2162
2163The first method takes periodically samples of the CPU program
2164counter, and since the program counter can be correlated with the code
2165generated for functions, we get a statistical view of in which
2166functions the program is spending its time. The caveats are that very
2167small/fast functions have lower probability of showing up in the
2168profile, and that periodically interrupting the program (this is
2169usually done rather frequently, in the scale of milliseconds) imposes
2170an additional overhead that may skew the results. The first problem
2171can be alleviated by running the code for longer (in general this is a
2172good idea for profiling), the second problem is usually kept in guard
2173by the profiling tools themselves.
2174
10f58044 2175The second method divides up the generated code into I<basic blocks>.
51a35ef1
JH
2176Basic blocks are sections of code that are entered only in the
2177beginning and exited only at the end. For example, a conditional jump
2178starts a basic block. Basic block profiling usually works by
10f58044 2179I<instrumenting> the code by adding I<enter basic block #nnnn>
51a35ef1
JH
2180book-keeping code to the generated code. During the execution of the
2181code the basic block counters are then updated appropriately. The
2182caveat is that the added extra code can skew the results: again, the
2183profiling tools usually try to factor their own effects out of the
2184results.
2185
83f0ef60
JH
2186=head2 Gprof Profiling
2187
51a35ef1
JH
2188gprof is a profiling tool available in many UNIX platforms,
2189it uses F<statistical time-sampling>.
83f0ef60
JH
2190
2191You can build a profiled version of perl called "perl.gprof" by
51a35ef1
JH
2192invoking the make target "perl.gprof" (What is required is that Perl
2193must be compiled using the C<-pg> flag, you may need to re-Configure).
2194Running the profiled version of Perl will create an output file called
2195F<gmon.out> is created which contains the profiling data collected
2196during the execution.
83f0ef60
JH
2197
2198The gprof tool can then display the collected data in various ways.
2199Usually gprof understands the following options:
2200
2201=over 4
2202
2203=item -a
2204
2205Suppress statically defined functions from the profile.
2206
2207=item -b
2208
2209Suppress the verbose descriptions in the profile.
2210
2211=item -e routine
2212
2213Exclude the given routine and its descendants from the profile.
2214
2215=item -f routine
2216
2217Display only the given routine and its descendants in the profile.
2218
2219=item -s
2220
2221Generate a summary file called F<gmon.sum> which then may be given
2222to subsequent gprof runs to accumulate data over several runs.
2223
2224=item -z
2225
2226Display routines that have zero usage.
2227
2228=back
2229
2230For more detailed explanation of the available commands and output
2231formats, see your own local documentation of gprof.
2232
51a35ef1
JH
2233=head2 GCC gcov Profiling
2234
10f58044 2235Starting from GCC 3.0 I<basic block profiling> is officially available
51a35ef1
JH
2236for the GNU CC.
2237
2238You can build a profiled version of perl called F<perl.gcov> by
2239invoking the make target "perl.gcov" (what is required that Perl must
2240be compiled using gcc with the flags C<-fprofile-arcs
2241-ftest-coverage>, you may need to re-Configure).
2242
2243Running the profiled version of Perl will cause profile output to be
2244generated. For each source file an accompanying ".da" file will be
2245created.
2246
2247To display the results you use the "gcov" utility (which should
2248be installed if you have gcc 3.0 or newer installed). F<gcov> is
2249run on source code files, like this
2250
2251 gcov sv.c
2252
2253which will cause F<sv.c.gcov> to be created. The F<.gcov> files
2254contain the source code annotated with relative frequencies of
2255execution indicated by "#" markers.
2256
2257Useful options of F<gcov> include C<-b> which will summarise the
2258basic block, branch, and function call coverage, and C<-c> which
2259instead of relative frequencies will use the actual counts. For
2260more information on the use of F<gcov> and basic block profiling
2261with gcc, see the latest GNU CC manual, as of GCC 3.0 see
2262
2263 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
2264
2265and its section titled "8. gcov: a Test Coverage Program"
2266
2267 http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
2268
4ae3d70a
JH
2269=head2 Pixie Profiling
2270
51a35ef1
JH
2271Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
2272UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
10f58044 2273I<basic-block counting>.
4ae3d70a 2274
83f0ef60 2275You can build a profiled version of perl called F<perl.pixie> by
51a35ef1
JH
2276invoking the make target "perl.pixie" (what is required is that Perl
2277must be compiled using the C<-g> flag, you may need to re-Configure).
2278
2279In Tru64 a file called F<perl.Addrs> will also be silently created,
2280this file contains the addresses of the basic blocks. Running the
2281profiled version of Perl will create a new file called "perl.Counts"
2282which contains the counts for the basic block for that particular
2283program execution.
4ae3d70a 2284
51a35ef1 2285To display the results you use the F<prof> utility. The exact
4ae3d70a
JH
2286incantation depends on your operating system, "prof perl.Counts" in
2287IRIX, and "prof -pixie -all -L. perl" in Tru64.
2288
6c41479b
JH
2289In IRIX the following prof options are available:
2290
2291=over 4
2292
2293=item -h
2294
2295Reports the most heavily used lines in descending order of use.
6e36760b 2296Useful for finding the hotspot lines.
6c41479b
JH
2297
2298=item -l
2299
2300Groups lines by procedure, with procedures sorted in descending order of use.
2301Within a procedure, lines are listed in source order.
6e36760b 2302Useful for finding the hotspots of procedures.
6c41479b
JH
2303
2304=back
2305
2306In Tru64 the following options are available:
2307
2308=over 4
2309
3958b146 2310=item -p[rocedures]
6c41479b 2311
3958b146 2312Procedures sorted in descending order by the number of cycles executed
6e36760b 2313in each procedure. Useful for finding the hotspot procedures.
6c41479b
JH
2314(This is the default option.)
2315
24000d2f 2316=item -h[eavy]
6c41479b 2317
6e36760b
JH
2318Lines sorted in descending order by the number of cycles executed in
2319each line. Useful for finding the hotspot lines.
6c41479b 2320
24000d2f 2321=item -i[nvocations]
6c41479b 2322
6e36760b
JH
2323The called procedures are sorted in descending order by number of calls
2324made to the procedures. Useful for finding the most used procedures.
6c41479b 2325
24000d2f 2326=item -l[ines]
6c41479b
JH
2327
2328Grouped by procedure, sorted by cycles executed per procedure.
6e36760b 2329Useful for finding the hotspots of procedures.
6c41479b
JH
2330
2331=item -testcoverage
2332
2333The compiler emitted code for these lines, but the code was unexecuted.
2334
24000d2f 2335=item -z[ero]
6c41479b
JH
2336
2337Unexecuted procedures.
2338
aa500c9e 2339=back
6c41479b
JH
2340
2341For further information, see your system's manual pages for pixie and prof.
4ae3d70a 2342
b8ddf6b3
SB
2343=head2 Miscellaneous tricks
2344
2345=over 4
2346
2347=item *
2348
cc177e1a 2349Those debugging perl with the DDD frontend over gdb may find the
b8ddf6b3
SB
2350following useful:
2351
2352You can extend the data conversion shortcuts menu, so for example you
2353can display an SV's IV value with one click, without doing any typing.
2354To do that simply edit ~/.ddd/init file and add after:
2355
2356 ! Display shortcuts.
2357 Ddd*gdbDisplayShortcuts: \
2358 /t () // Convert to Bin\n\
2359 /d () // Convert to Dec\n\
2360 /x () // Convert to Hex\n\
2361 /o () // Convert to Oct(\n\
2362
2363the following two lines:
2364
2365 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
2366 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
2367
2368so now you can do ivx and pvx lookups or you can plug there the
2369sv_peek "conversion":
2370
2371 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
2372
2373(The my_perl is for threaded builds.)
2374Just remember that every line, but the last one, should end with \n\
2375
2376Alternatively edit the init file interactively via:
23773rd mouse button -> New Display -> Edit Menu
2378
2379Note: you can define up to 20 conversion shortcuts in the gdb
2380section.
2381
9965345d
JH
2382=item *
2383
2384If you see in a debugger a memory area mysteriously full of 0xabababab,
2385you may be seeing the effect of the Poison() macro, see L<perlclib>.
2386
b8ddf6b3
SB
2387=back
2388
a422fd2d
SC
2389=head2 CONCLUSION
2390
2391We've had a brief look around the Perl source, an overview of the stages
2392F<perl> goes through when it's running your code, and how to use a
902b9dbf
MF
2393debugger to poke at the Perl guts. We took a very simple problem and
2394demonstrated how to solve it fully - with documentation, regression
2395tests, and finally a patch for submission to p5p. Finally, we talked
2396about how to use external tools to debug and test Perl.
a422fd2d
SC
2397
2398I'd now suggest you read over those references again, and then, as soon
2399as possible, get your hands dirty. The best way to learn is by doing,
2400so:
2401
2402=over 3
2403
2404=item *
2405
2406Subscribe to perl5-porters, follow the patches and try and understand
2407them; don't be afraid to ask if there's a portion you're not clear on -
2408who knows, you may unearth a bug in the patch...
2409
2410=item *
2411
2412Keep up to date with the bleeding edge Perl distributions and get
2413familiar with the changes. Try and get an idea of what areas people are
2414working on and the changes they're making.
2415
2416=item *
2417
3e148164 2418Do read the README associated with your operating system, e.g. README.aix
a1f349fd
MB
2419on the IBM AIX OS. Don't hesitate to supply patches to that README if
2420you find anything missing or changed over a new OS release.
2421
2422=item *
2423
a422fd2d
SC
2424Find an area of Perl that seems interesting to you, and see if you can
2425work out how it works. Scan through the source, and step over it in the
2426debugger. Play, poke, investigate, fiddle! You'll probably get to
2427understand not just your chosen area but a much wider range of F<perl>'s
2428activity as well, and probably sooner than you'd think.
2429
2430=back
2431
2432=over 3
2433
2434=item I<The Road goes ever on and on, down from the door where it began.>
2435
2436=back
2437
2438If you can do these things, you've started on the long road to Perl porting.
2439Thanks for wanting to help make Perl better - and happy hacking!
2440
e8cd7eae
GS
2441=head1 AUTHOR
2442
2443This document was written by Nathan Torkington, and is maintained by
2444the perl5-porters mailing list.
2445