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293 lines
13 KiB
Plaintext
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=head1 NAME
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perlhack - How to hack at the Perl internals
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=head1 DESCRIPTION
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This document attempts to explain how Perl development takes place,
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and ends with some suggestions for people wanting to become bona fide
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porters.
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The perl5-porters mailing list is where the Perl standard distribution
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is maintained and developed. The list can get anywhere from 10 to 150
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messages a day, depending on the heatedness of the debate. Most days
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there are two or three patches, extensions, features, or bugs being
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discussed at a time.
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A searchable archive of the list is at:
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http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
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The list is also archived under the usenet group name
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C<perl.porters-gw> at:
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http://www.deja.com/
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List subscribers (the porters themselves) come in several flavours.
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Some are quiet curious lurkers, who rarely pitch in and instead watch
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the ongoing development to ensure they're forewarned of new changes or
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features in Perl. Some are representatives of vendors, who are there
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to make sure that Perl continues to compile and work on their
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platforms. Some patch any reported bug that they know how to fix,
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some are actively patching their pet area (threads, Win32, the regexp
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engine), while others seem to do nothing but complain. In other
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words, it's your usual mix of technical people.
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Over this group of porters presides Larry Wall. He has the final word
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in what does and does not change in the Perl language. Various
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releases of Perl are shepherded by a ``pumpking'', a porter
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responsible for gathering patches, deciding on a patch-by-patch
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feature-by-feature basis what will and will not go into the release.
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For instance, Gurusamy Sarathy is the pumpking for the 5.6 release of
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Perl.
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In addition, various people are pumpkings for different things. For
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instance, Andy Dougherty and Jarkko Hietaniemi share the I<Configure>
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pumpkin, and Tom Christiansen is the documentation pumpking.
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Larry sees Perl development along the lines of the US government:
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there's the Legislature (the porters), the Executive branch (the
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pumpkings), and the Supreme Court (Larry). The legislature can
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discuss and submit patches to the executive branch all they like, but
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the executive branch is free to veto them. Rarely, the Supreme Court
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will side with the executive branch over the legislature, or the
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legislature over the executive branch. Mostly, however, the
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legislature and the executive branch are supposed to get along and
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work out their differences without impeachment or court cases.
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You might sometimes see reference to Rule 1 and Rule 2. Larry's power
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as Supreme Court is expressed in The Rules:
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=over 4
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=item 1
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Larry is always by definition right about how Perl should behave.
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This means he has final veto power on the core functionality.
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=item 2
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Larry is allowed to change his mind about any matter at a later date,
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regardless of whether he previously invoked Rule 1.
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=back
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Got that? Larry is always right, even when he was wrong. It's rare
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to see either Rule exercised, but they are often alluded to.
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New features and extensions to the language are contentious, because
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the criteria used by the pumpkings, Larry, and other porters to decide
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which features should be implemented and incorporated are not codified
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in a few small design goals as with some other languages. Instead,
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the heuristics are flexible and often difficult to fathom. Here is
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one person's list, roughly in decreasing order of importance, of
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heuristics that new features have to be weighed against:
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=over 4
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=item Does concept match the general goals of Perl?
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These haven't been written anywhere in stone, but one approximation
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is:
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1. Keep it fast, simple, and useful.
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2. Keep features/concepts as orthogonal as possible.
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3. No arbitrary limits (platforms, data sizes, cultures).
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4. Keep it open and exciting to use/patch/advocate Perl everywhere.
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5. Either assimilate new technologies, or build bridges to them.
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=item Where is the implementation?
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All the talk in the world is useless without an implementation. In
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almost every case, the person or people who argue for a new feature
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will be expected to be the ones who implement it. Porters capable
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of coding new features have their own agendas, and are not available
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to implement your (possibly good) idea.
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=item Backwards compatibility
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It's a cardinal sin to break existing Perl programs. New warnings are
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contentious--some say that a program that emits warnings is not
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broken, while others say it is. Adding keywords has the potential to
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break programs, changing the meaning of existing token sequences or
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functions might break programs.
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=item Could it be a module instead?
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Perl 5 has extension mechanisms, modules and XS, specifically to avoid
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the need to keep changing the Perl interpreter. You can write modules
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that export functions, you can give those functions prototypes so they
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can be called like built-in functions, you can even write XS code to
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mess with the runtime data structures of the Perl interpreter if you
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want to implement really complicated things. If it can be done in a
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module instead of in the core, it's highly unlikely to be added.
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=item Is the feature generic enough?
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Is this something that only the submitter wants added to the language,
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or would it be broadly useful? Sometimes, instead of adding a feature
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with a tight focus, the porters might decide to wait until someone
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implements the more generalized feature. For instance, instead of
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implementing a ``delayed evaluation'' feature, the porters are waiting
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for a macro system that would permit delayed evaluation and much more.
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=item Does it potentially introduce new bugs?
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Radical rewrites of large chunks of the Perl interpreter have the
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potential to introduce new bugs. The smaller and more localized the
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change, the better.
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=item Does it preclude other desirable features?
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A patch is likely to be rejected if it closes off future avenues of
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development. For instance, a patch that placed a true and final
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interpretation on prototypes is likely to be rejected because there
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are still options for the future of prototypes that haven't been
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addressed.
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=item Is the implementation robust?
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Good patches (tight code, complete, correct) stand more chance of
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going in. Sloppy or incorrect patches might be placed on the back
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burner until the pumpking has time to fix, or might be discarded
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altogether without further notice.
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=item Is the implementation generic enough to be portable?
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The worst patches make use of a system-specific features. It's highly
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unlikely that nonportable additions to the Perl language will be
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accepted.
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=item Is there enough documentation?
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Patches without documentation are probably ill-thought out or
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incomplete. Nothing can be added without documentation, so submitting
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a patch for the appropriate manpages as well as the source code is
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always a good idea. If appropriate, patches should add to the test
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suite as well.
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=item Is there another way to do it?
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Larry said ``Although the Perl Slogan is I<There's More Than One Way
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to Do It>, I hesitate to make 10 ways to do something''. This is a
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tricky heuristic to navigate, though--one man's essential addition is
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another man's pointless cruft.
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=item Does it create too much work?
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Work for the pumpking, work for Perl programmers, work for module
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authors, ... Perl is supposed to be easy.
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=item Patches speak louder than words
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Working code is always preferred to pie-in-the-sky ideas. A patch to
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add a feature stands a much higher chance of making it to the language
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than does a random feature request, no matter how fervently argued the
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request might be. This ties into ``Will it be useful?'', as the fact
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that someone took the time to make the patch demonstrates a strong
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desire for the feature.
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=back
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If you're on the list, you might hear the word ``core'' bandied
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around. It refers to the standard distribution. ``Hacking on the
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core'' means you're changing the C source code to the Perl
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interpreter. ``A core module'' is one that ships with Perl.
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The source code to the Perl interpreter, in its different versions, is
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kept in a repository managed by a revision control system (which is
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currently the Perforce program, see http://perforce.com/). The
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pumpkings and a few others have access to the repository to check in
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changes. Periodically the pumpking for the development version of Perl
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will release a new version, so the rest of the porters can see what's
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changed. The current state of the main trunk of repository, and patches
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that describe the individual changes that have happened since the last
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public release are available at this location:
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ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
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Selective parts are also visible via the rsync protocol. To get all
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the individual changes to the mainline since the last development
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release, use the following command:
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rsync -avuz rsync://ftp.linux.activestate.com/perl-diffs perl-diffs
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Use this to get the latest source tree in full:
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rsync -avuz rsync://ftp.linux.activestate.com/perl-current perl-current
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Needless to say, the source code in perl-current is usually in a perpetual
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state of evolution. You should expect it to be very buggy. Do B<not> use
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it for any purpose other than testing and development.
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Always submit patches to I<perl5-porters@perl.org>. This lets other
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porters review your patch, which catches a surprising number of errors
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in patches. Either use the diff program (available in source code
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form from I<ftp://ftp.gnu.org/pub/gnu/>), or use Johan Vromans'
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I<makepatch> (available from I<CPAN/authors/id/JV/>). Unified diffs
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are preferred, but context diffs are accepted. Do not send RCS-style
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diffs or diffs without context lines. More information is given in
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the I<Porting/patching.pod> file in the Perl source distribution.
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Please patch against the latest B<development> version (e.g., if
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you're fixing a bug in the 5.005 track, patch against the latest
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5.005_5x version). Only patches that survive the heat of the
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development branch get applied to maintenance versions.
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Your patch should update the documentation and test suite.
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To report a bug in Perl, use the program I<perlbug> which comes with
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Perl (if you can't get Perl to work, send mail to the address
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I<perlbug@perl.com> or I<perlbug@perl.org>). Reporting bugs through
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I<perlbug> feeds into the automated bug-tracking system, access to
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which is provided through the web at I<http://bugs.perl.org/>. It
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often pays to check the archives of the perl5-porters mailing list to
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see whether the bug you're reporting has been reported before, and if
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so whether it was considered a bug. See above for the location of
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the searchable archives.
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The CPAN testers (I<http://testers.cpan.org/>) are a group of
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volunteers who test CPAN modules on a variety of platforms. Perl Labs
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(I<http://labs.perl.org/>) automatically tests Perl source releases on
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platforms and gives feedback to the CPAN testers mailing list. Both
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efforts welcome volunteers.
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To become an active and patching Perl porter, you'll need to learn how
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Perl works on the inside. Chip Salzenberg, a pumpking, has written
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articles on Perl internals for The Perl Journal
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(I<http://www.tpj.com/>) which explain how various parts of the Perl
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interpreter work. The C<perlguts> manpage explains the internal data
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structures. And, of course, the C source code (sometimes sparsely
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commented, sometimes commented well) is a great place to start (begin
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with C<perl.c> and see where it goes from there). A lot of the style
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of the Perl source is explained in the I<Porting/pumpkin.pod> file in
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the source distribution.
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It is essential that you be comfortable using a good debugger
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(e.g. gdb, dbx) before you can patch perl. Stepping through perl
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as it executes a script is perhaps the best (if sometimes tedious)
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way to gain a precise understanding of the overall architecture of
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the language.
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If you build a version of the Perl interpreter with C<-DDEBUGGING>,
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Perl's B<-D> command line flag will cause copious debugging information
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to be emitted (see the C<perlrun> manpage). If you build a version of
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Perl with compiler debugging information (e.g. with the C compiler's
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C<-g> option instead of C<-O>) then you can step through the execution
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of the interpreter with your favourite C symbolic debugger, setting
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breakpoints on particular functions.
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It's a good idea to read and lurk for a while before chipping in.
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That way you'll get to see the dynamic of the conversations, learn the
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personalities of the players, and hopefully be better prepared to make
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a useful contribution when do you speak up.
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If after all this you still think you want to join the perl5-porters
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mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
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unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
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=head1 AUTHOR
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This document was written by Nathan Torkington, and is maintained by
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the perl5-porters mailing list.
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