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PERLHACK(1)			 Perl Programmers Reference Guide		      PERLHACK(1)

       perlhack - How to hack on Perl

       This document explains how Perl development works. It includes details about the Perl 5
       Porters email list, the Perl repository, the Perlbug bug tracker, patch guidelines, and
       commentary on Perl development philosophy.

       If you just want to submit a single small patch like a pod fix, a test for a bug, comment
       fixes, etc., it's easy! Here's how:

       o   Check out the source repository

	   The perl source is in a git repository. You can clone the repository with the
	   following command:

	     % git clone git://perl5.git.perl.org/perl.git perl

       o   Make your change

	   Hack, hack, hack.

       o   Test your change

	   You can run all the tests with the following commands:

	     % ./Configure -des -Dusedevel
	     % make test

	   Keep hacking until the tests pass.

       o   Commit your change

	   Committing your work will save the change on your local system:

	     % git commit -a -m 'Commit message goes here'

	   Make sure the commit message describes your change in a single sentence. For example,
	   "Fixed spelling errors in perlhack.pod".

       o   Send your change to perlbug

	   The next step is to submit your patch to the Perl core ticket system via email.

	   Assuming your patch consists of a single git commit, the following writes the file as
	   a MIME attachment, and sends it with a meaningful subject:

	     % git format-patch -1 --attach
	     % perlbug -s "[PATCH] $(git log -1 --oneline HEAD)" -f 0001-*.patch

	   The perlbug program will ask you a few questions about your email address and the
	   patch you're submitting. Once you've answered them it will submit your patch via

       o   Thank you

	   The porters appreciate the time you spent helping to make Perl better.  Thank you!

       If you want to report a bug in Perl, you must use the perlbug command line tool. This tool
       will ensure that your bug report includes all the relevant system and configuration

       To browse existing Perl bugs and patches, you can use the web interface at

       Please check the archive of the perl5-porters list (see below) and/or the bug tracking
       system before submitting a bug report. Often, you'll find that the bug has been reported

       You can log in to the bug tracking system and comment on existing bug reports. If you have
       additional information regarding an existing bug, please add it. This will help the
       porters fix the bug.

       The perl5-porters (p5p) mailing list is where the Perl standard distribution is maintained
       and developed. The people who maintain Perl are also referred to as the "Perl 5 Porters",
       "p5p" or just the "porters".

       A searchable archive of the list is available at
       <http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/>. There is also another archive
       at http://archive.develooper.com/perl5-porters@perl.org/

   perl-changes mailing list
       The perl5-changes mailing list receives a copy of each patch that gets submitted to the
       maintenance and development branches of the perl repository. See
       <http://lists.perl.org/list/perl5-changes.html> for subscription and archive information.

   #p5p on IRC
       Many porters are also active on the <irc://irc.perl.org/#p5p> channel.  Feel free to join
       the channel and ask questions about hacking on the Perl core.

       All of Perl's source code is kept centrally in a Git repository at perl5.git.perl.org. The
       repository contains many Perl revisions from Perl 1 onwards and all the revisions from
       Perforce, the previous version control system.

       For much more detail on using git with the Perl repository, please see perlgit.

   Read access via Git
       You will need a copy of Git for your computer. You can fetch a copy of the repository
       using the git protocol:

	 % git clone git://perl5.git.perl.org/perl.git perl

       This clones the repository and makes a local copy in the perl directory.

       If you cannot use the git protocol for firewall reasons, you can also clone via http,
       though this is much slower:

	 % git clone http://perl5.git.perl.org/perl.git perl

   Read access via the web
       You may access the repository over the web. This allows you to browse the tree, see recent
       commits, subscribe to RSS feeds for the changes, search for particular commits and more.
       You may access it at <http://perl5.git.perl.org/perl.git>. A mirror of the repository is
       found at <http://github.com/mirrors/perl>.

   Read access via rsync
       You can also choose to use rsync to get a copy of the current source tree for the
       bleadperl branch and all maintenance branches:

	   % rsync -avz rsync://perl5.git.perl.org/perl-current .
	   % rsync -avz rsync://perl5.git.perl.org/perl-5.12.x .
	   % rsync -avz rsync://perl5.git.perl.org/perl-5.10.x .
	   % rsync -avz rsync://perl5.git.perl.org/perl-5.8.x .
	   % rsync -avz rsync://perl5.git.perl.org/perl-5.6.x .
	   % rsync -avz rsync://perl5.git.perl.org/perl-5.005xx .

       (Add the "--delete" option to remove leftover files.)

       To get a full list of the available sync points:

	   % rsync perl5.git.perl.org::

   Write access via git
       If you have a commit bit, please see perlgit for more details on using git.

       If you're planning to do more extensive work than a single small fix, we encourage you to
       read the documentation below. This will help you focus your work and make your patches
       easier to incorporate into the Perl source.

   Submitting patches
       If you have a small patch to submit, please submit it via perlbug. You can also send email
       directly to perlbug@perl.org. Please note that messages sent to perlbug may be held in a
       moderation queue, so you won't receive a response immediately.

       You'll know your submission has been processed when you receive an email from our ticket
       tracking system. This email will give you a ticket number. Once your patch has made it to
       the ticket tracking system, it will also be sent to the perl5-porters@perl.org list.

       Patches are reviewed and discussed on the p5p list. Simple, uncontroversial patches will
       usually be applied without any discussion.  When the patch is applied, the ticket will be
       updated and you will receive email. In addition, an email will be sent to the p5p list.

       In other cases, the patch will need more work or discussion. That will happen on the p5p

       You are encouraged to participate in the discussion and advocate for your patch. Sometimes
       your patch may get lost in the shuffle. It's appropriate to send a reminder email to p5p
       if no action has been taken in a month. Please remember that the Perl 5 developers are all
       volunteers, and be polite.

       Changes are always applied directly to the main development branch, called "blead". Some
       patches may be backported to a maintenance branch.  If you think your patch is appropriate
       for the maintenance branch, please explain why when you submit it.

   Getting your patch accepted
       If you are submitting a code patch there are several things that you can do to help the
       Perl 5 Porters accept your patch.

       Patch style

       If you used git to check out the Perl source, then using "git format-patch" will produce a
       patch in a style suitable for Perl. The "format-patch" command produces one patch file for
       each commit you made. If you prefer to send a single patch for all commits, you can use
       "git diff".

	 % git checkout blead
	 % git pull
	 % git diff blead my-branch-name

       This produces a patch based on the difference between blead and your current branch. It's
       important to make sure that blead is up to date before producing the diff, that's why we
       call "git pull" first.

       We strongly recommend that you use git if possible. It will make your life easier, and
       ours as well.

       However, if you're not using git, you can still produce a suitable patch. You'll need a
       pristine copy of the Perl source to diff against.  The porters prefer unified diffs. Using
       GNU "diff", you can produce a diff like this:

	 % diff -Npurd perl.pristine perl.mine

       Make sure that you "make realclean" in your copy of Perl to remove any build artifacts, or
       you may get a confusing result.

       Commit message

       As you craft each patch you intend to submit to the Perl core, it's important to write a
       good commit message. This is especially important if your submission will consist of a
       series of commits.

       The first line of the commit message should be a short description without a period. It
       should be no longer than the subject line of an email, 50 characters being a good rule of

       A lot of Git tools (Gitweb, GitHub, git log --pretty=oneline, ...) will only display the
       first line (cut off at 50 characters) when presenting commit summaries.

       The commit message should include a description of the problem that the patch corrects or
       new functionality that the patch adds.

       As a general rule of thumb, your commit message should help a programmer who knows the
       Perl core quickly understand what you were trying to do, how you were trying to do it, and
       why the change matters to Perl.

       o   Why

	   Your commit message should describe why the change you are making is important. When
	   someone looks at your change in six months or six years, your intent should be clear.

	   If you're deprecating a feature with the intent of later simplifying another bit of
	   code, say so. If you're fixing a performance problem or adding a new feature to
	   support some other bit of the core, mention that.

       o   What

	   Your commit message should describe what part of the Perl core you're changing and
	   what you expect your patch to do.

       o   How

	   While it's not necessary for documentation changes, new tests or trivial patches, it's
	   often worth explaining how your change works.  Even if it's clear to you today, it may
	   not be clear to a porter next month or next year.

       A commit message isn't intended to take the place of comments in your code. Commit
       messages should describe the change you made, while code comments should describe the
       current state of the code.

       If you've just implemented a new feature, complete with doc, tests and well-commented
       code, a brief commit message will often suffice. If, however, you've just changed a single
       character deep in the parser or lexer, you might need to write a small novel to ensure
       that future readers understand what you did and why you did it.

       Comments, Comments, Comments

       Be sure to adequately comment your code. While commenting every line is unnecessary,
       anything that takes advantage of side effects of operators, that creates changes that will
       be felt outside of the function being patched, or that others may find confusing should be
       documented. If you are going to err, it is better to err on the side of adding too many
       comments than too few.

       The best comments explain why the code does what it does, not what it does.


       In general, please follow the particular style of the code you are patching.

       In particular, follow these general guidelines for patching Perl sources:

       o   8-wide tabs (no exceptions!)

       o   4-wide indents for code, 2-wide indents for nested CPP #defines

       o   Try hard not to exceed 79-columns

       o   ANSI C prototypes

       o   Uncuddled elses and "K&R" style for indenting control constructs

       o   No C++ style (//) comments

       o   Mark places that need to be revisited with XXX (and revisit often!)

       o   Opening brace lines up with "if" when conditional spans multiple lines; should be at
	   end-of-line otherwise

       o   In function definitions, name starts in column 0 (return value is on previous line)

       o   Single space after keywords that are followed by parens, no space between function
	   name and following paren

       o   Avoid assignments in conditionals, but if they're unavoidable, use extra paren, e.g.
	   "if (a && (b = c)) ..."

       o   "return foo;" rather than "return(foo);"

       o   "if (!foo) ..." rather than "if (foo == FALSE) ..." etc.

       Test suite

       If your patch changes code (rather than just changing documentation), you should also
       include one or more test cases which illustrate the bug you're fixing or validate the new
       functionality you're adding. In general, you should update an existing test file rather
       than create a new one.

       Your test suite additions should generally follow these guidelines (courtesy of Gurusamy
       Sarathy <gsar@activestate.com>):

       o   Know what you're testing. Read the docs, and the source.

       o   Tend to fail, not succeed.

       o   Interpret results strictly.

       o   Use unrelated features (this will flush out bizarre interactions).

       o   Use non-standard idioms (otherwise you are not testing TIMTOWTDI).

       o   Avoid using hardcoded test numbers whenever possible (the EXPECTED/GOT found in
	   t/op/tie.t is much more maintainable, and gives better failure reports).

       o   Give meaningful error messages when a test fails.

       o   Avoid using qx// and system() unless you are testing for them. If you do use them,
	   make sure that you cover _all_ perl platforms.

       o   Unlink any temporary files you create.

       o   Promote unforeseen warnings to errors with $SIG{__WARN__}.

       o   Be sure to use the libraries and modules shipped with the version being tested, not
	   those that were already installed.

       o   Add comments to the code explaining what you are testing for.

       o   Make updating the '1..42' string unnecessary. Or make sure that you update it.

       o   Test _all_ behaviors of a given operator, library, or function.

	   Test all optional arguments.

	   Test return values in various contexts (boolean, scalar, list, lvalue).

	   Use both global and lexical variables.

	   Don't forget the exceptional, pathological cases.

   Patching a core module
       This works just like patching anything else, with one extra consideration.

       Modules in the cpan/ directory of the source tree are maintained outside of the Perl core.
       When the author updates the module, the updates are simply copied into the core.  See that
       module's documentation or its listing on <http://search.cpan.org/> for more information on
       reporting bugs and submitting patches.

       In most cases, patches to modules in cpan/ should be sent upstream and should not be
       applied to the Perl core individually.  If a patch to a file in cpan/ absolutely cannot
       wait for the fix to be made upstream, released to CPAN and copied to blead, you must add
       (or update) a "CUSTOMIZED" entry in the "Porting/Maintainers.pl" file to flag that a local
       modification has been made.  See "Porting/Maintainers.pl" for more details.

       In contrast, modules in the dist/ directory are maintained in the core.

   Updating perldelta
       For changes significant enough to warrant a pod/perldelta.pod entry, the porters will
       greatly appreciate it if you submit a delta entry along with your actual change.
       Significant changes include, but are not limited to:

       o   Adding, deprecating, or removing core features

       o   Adding, deprecating, removing, or upgrading core or dual-life modules

       o   Adding new core tests

       o   Fixing security issues and user-visible bugs in the core

       o   Changes that might break existing code, either on the perl or C level

       o   Significant performance improvements

       o   Adding, removing, or significantly changing documentation in the pod/ directory

       o   Important platform-specific changes

       Please make sure you add the perldelta entry to the right section within
       pod/perldelta.pod. More information on how to write good perldelta entries is available in
       the "Style" section of Porting/how_to_write_a_perldelta.pod.

   What makes for a good patch?
       New features and extensions to the language can be contentious. There is no specific set
       of criteria which determine what features get added, but here are some questions to
       consider when developing a patch:

       Does the concept match the general goals of Perl?

       Our goals include, but are not limited to:

       1.  Keep it fast, simple, and useful.

       2.  Keep features/concepts as orthogonal as possible.

       3.  No arbitrary limits (platforms, data sizes, cultures).

       4.  Keep it open and exciting to use/patch/advocate Perl everywhere.

       5.  Either assimilate new technologies, or build bridges to them.

       Where is the implementation?

       All the talk in the world is useless without an implementation. In almost every case, the
       person or people who argue for a new feature will be expected to be the ones who implement
       it. Porters capable of coding new features have their own agendas, and are not available
       to implement your (possibly good) idea.

       Backwards compatibility

       It's a cardinal sin to break existing Perl programs. New warnings can be contentious--some
       say that a program that emits warnings is not broken, while others say it is. Adding
       keywords has the potential to break programs, changing the meaning of existing token
       sequences or functions might break programs.

       The Perl 5 core includes mechanisms to help porters make backwards incompatible changes
       more compatible such as the feature and deprecate modules. Please use them when

       Could it be a module instead?

       Perl 5 has extension mechanisms, modules and XS, specifically to avoid the need to keep
       changing the Perl interpreter. You can write modules that export functions, you can give
       those functions prototypes so they can be called like built-in functions, you can even
       write XS code to mess with the runtime data structures of the Perl interpreter if you want
       to implement really complicated things.

       Whenever possible, new features should be prototyped in a CPAN module before they will be
       considered for the core.

       Is the feature generic enough?

       Is this something that only the submitter wants added to the language, or is it broadly
       useful?	Sometimes, instead of adding a feature with a tight focus, the porters might
       decide to wait until someone implements the more generalized feature.

       Does it potentially introduce new bugs?

       Radical rewrites of large chunks of the Perl interpreter have the potential to introduce
       new bugs.

       How big is it?

       The smaller and more localized the change, the better. Similarly, a series of small
       patches is greatly preferred over a single large patch.

       Does it preclude other desirable features?

       A patch is likely to be rejected if it closes off future avenues of development. For
       instance, a patch that placed a true and final interpretation on prototypes is likely to
       be rejected because there are still options for the future of prototypes that haven't been

       Is the implementation robust?

       Good patches (tight code, complete, correct) stand more chance of going in. Sloppy or
       incorrect patches might be placed on the back burner until the pumpking has time to fix,
       or might be discarded altogether without further notice.

       Is the implementation generic enough to be portable?

       The worst patches make use of system-specific features. It's highly unlikely that non-
       portable additions to the Perl language will be accepted.

       Is the implementation tested?

       Patches which change behaviour (fixing bugs or introducing new features) must include
       regression tests to verify that everything works as expected.

       Without tests provided by the original author, how can anyone else changing perl in the
       future be sure that they haven't unwittingly broken the behaviour the patch implements?
       And without tests, how can the patch's author be confident that his/her hard work put into
       the patch won't be accidentally thrown away by someone in the future?

       Is there enough documentation?

       Patches without documentation are probably ill-thought out or incomplete. No features can
       be added or changed without documentation, so submitting a patch for the appropriate pod
       docs as well as the source code is important.

       Is there another way to do it?

       Larry said "Although the Perl Slogan is There's More Than One Way to Do It, I hesitate to
       make 10 ways to do something". This is a tricky heuristic to navigate, though--one man's
       essential addition is another man's pointless cruft.

       Does it create too much work?

       Work for the pumpking, work for Perl programmers, work for module authors, ... Perl is
       supposed to be easy.

       Patches speak louder than words

       Working code is always preferred to pie-in-the-sky ideas. A patch to add a feature stands
       a much higher chance of making it to the language than does a random feature request, no
       matter how fervently argued the request might be. This ties into "Will it be useful?", as
       the fact that someone took the time to make the patch demonstrates a strong desire for the

       The core uses the same testing style as the rest of Perl, a simple "ok/not ok" run through
       Test::Harness, but there are a few special considerations.

       There are three ways to write a test in the core. Test::More, t/test.pl and ad hoc "print
       $test ? "ok 42\n" : "not ok 42\n"". The decision of which to use depends on what part of
       the test suite you're working on. This is a measure to prevent a high-level failure (such
       as Config.pm breaking) from causing basic functionality tests to fail.

       The t/test.pl library provides some of the features of Test::More, but avoids loading most
       modules and uses as few core features as possible.

       If you write your own test, use the Test Anything Protocol <http://testanything.org>.

       o   t/base and t/comp

	   Since we don't know if require works, or even subroutines, use ad hoc tests for these
	   two. Step carefully to avoid using the feature being tested.

       o   t/cmd, t/run, t/io and t/op

	   Now that basic require() and subroutines are tested, you can use the t/test.pl

	   You can also use certain libraries like Config conditionally, but be sure to skip the
	   test gracefully if it's not there.

       o   Everything else

	   Now that the core of Perl is tested, Test::More can and should be used. You can also
	   use the full suite of core modules in the tests.

       When you say "make test", Perl uses the t/TEST program to run the test suite (except under
       Win32 where it uses t/harness instead). All tests are run from the t/ directory, not the
       directory which contains the test. This causes some problems with the tests in lib/, so
       here's some opportunity for some patching.

       You must be triply conscious of cross-platform concerns. This usually boils down to using
       File::Spec and avoiding things like "fork()" and "system()" unless absolutely necessary.

   Special "make test" targets
       There are various special make targets that can be used to test Perl slightly differently
       than the standard "test" target. Not all them are expected to give a 100% success rate.
       Many of them have several aliases, and many of them are not available on certain operating

       o   test_porting

	   This runs some basic sanity tests on the source tree and helps catch basic errors
	   before you submit a patch.

       o   coretest

	   Run perl on all core tests (t/* and lib/[a-z]* pragma tests).

	   (Not available on Win32)

       o   test.deparse

	   Run all the tests through B::Deparse. Not all tests will succeed.

	   (Not available on Win32)

       o   test.taintwarn

	   Run all tests with the -t command-line switch. Not all tests are expected to succeed
	   (until they're specifically fixed, of course).

	   (Not available on Win32)

       o   minitest

	   Run miniperl on t/base, t/comp, t/cmd, t/run, t/io, t/op, t/uni and t/mro tests.

       o   test.valgrind check.valgrind utest.valgrind ucheck.valgrind

	   (Only in Linux) Run all the tests using the memory leak + naughty memory access tool
	   "valgrind". The log files will be named testname.valgrind.

       o   test.torture torturetest

	   Run all the usual tests and some extra tests. As of Perl 5.8.0, the only extra tests
	   are Abigail's JAPHs, t/japh/abigail.t.

	   You can also run the torture test with t/harness by giving "-torture" argument to

       o   utest ucheck test.utf8 check.utf8

	   Run all the tests with -Mutf8. Not all tests will succeed.

	   (Not available on Win32)

       o   minitest.utf16 test.utf16

	   Runs the tests with UTF-16 encoded scripts, encoded with different versions of this

	   "make utest.utf16" runs the test suite with a combination of "-utf8" and "-utf16"
	   arguments to t/TEST.

	   (Not available on Win32)

       o   test_harness

	   Run the test suite with the t/harness controlling program, instead of t/TEST.
	   t/harness is more sophisticated, and uses the Test::Harness module, thus using this
	   test target supposes that perl mostly works. The main advantage for our purposes is
	   that it prints a detailed summary of failed tests at the end. Also, unlike t/TEST, it
	   doesn't redirect stderr to stdout.

	   Note that under Win32 t/harness is always used instead of t/TEST, so there is no
	   special "test_harness" target.

	   Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES environment
	   variables to control the behaviour of t/harness. This means you can say

	       nmake test TEST_FILES="op/*.t"
	       nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"

       o   test-notty test_notty

	   Sets PERL_SKIP_TTY_TEST to true before running normal test.

   Parallel tests
       The core distribution can now run its regression tests in parallel on Unix-like platforms.
       Instead of running "make test", set "TEST_JOBS" in your environment to the number of tests
       to run in parallel, and run "make test_harness". On a Bourne-like shell, this can be done

	   TEST_JOBS=3 make test_harness  # Run 3 tests in parallel

       An environment variable is used, rather than parallel make itself, because TAP::Harness
       needs to be able to schedule individual non-conflicting test scripts itself, and there is
       no standard interface to "make" utilities to interact with their job schedulers.

       Note that currently some test scripts may fail when run in parallel (most notably
       ext/IO/t/io_dir.t). If necessary, run just the failing scripts again sequentially and see
       if the failures go away.

   Running tests by hand
       You can run part of the test suite by hand by using one of the following commands from the
       t/ directory:

	   ./perl -I../lib TEST list-of-.t-files


	   ./perl -I../lib harness list-of-.t-files

       (If you don't specify test scripts, the whole test suite will be run.)

   Using t/harness for testing
       If you use "harness" for testing, you have several command line options available to you.
       The arguments are as follows, and are in the order that they must appear if used together.

	   harness -v -torture -re=pattern LIST OF FILES TO TEST
	   harness -v -torture -re LIST OF PATTERNS TO MATCH

       If "LIST OF FILES TO TEST" is omitted, the file list is obtained from the manifest. The
       file list may include shell wildcards which will be expanded out.

       o   -v

	   Run the tests under verbose mode so you can see what tests were run, and debug output.

       o   -torture

	   Run the torture tests as well as the normal set.

       o   -re=PATTERN

	   Filter the file list so that all the test files run match PATTERN. Note that this form
	   is distinct from the -re LIST OF PATTERNS form below in that it allows the file list
	   to be provided as well.

       o   -re LIST OF PATTERNS

	   Filter the file list so that all the test files run match /(LIST|OF|PATTERNS)/. Note
	   that with this form the patterns are joined by '|' and you cannot supply a list of
	   files, instead the test files are obtained from the MANIFEST.

       You can run an individual test by a command similar to

	   ./perl -I../lib path/to/foo.t

       except that the harnesses set up some environment variables that may affect the execution
       of the test:

       o   PERL_CORE=1

	   indicates that we're running this test as part of the perl core test suite. This is
	   useful for modules that have a dual life on CPAN.


	   is set to 2 if it isn't set already (see "PERL_DESTRUCT_LEVEL" in perlhacktips).

       o   PERL

	   (used only by t/TEST) if set, overrides the path to the perl executable that should be
	   used to run the tests (the default being ./perl).


	   if set, tells to skip the tests that need a terminal. It's actually set automatically
	   by the Makefile, but can also be forced artificially by running 'make test_notty'.

       Other environment variables that may influence tests

       o   PERL_TEST_Net_Ping

	   Setting this variable runs all the Net::Ping modules tests, otherwise some tests that
	   interact with the outside world are skipped. See perl58delta.


	   Setting this variable skips the vrexx.t tests for OS2::REXX.


	   This sets a variable in op/numconvert.t.

       See also the documentation for the Test and Test::Harness modules, for more environment
       variables that affect testing.

       To hack on the Perl guts, you'll need to read the following things:

       o   perlsource

	   An overview of the Perl source tree. This will help you find the files you're looking

       o   perlinterp

	   An overview of the Perl interpreter source code and some details on how Perl does what
	   it does.

       o   perlhacktut

	   This document walks through the creation of a small patch to Perl's C code. If you're
	   just getting started with Perl core hacking, this will help you understand how it

       o   perlhacktips

	   More details on hacking the Perl core. This document focuses on lower level details
	   such as how to write tests, compilation issues, portability, debugging, etc.

	   If you plan on doing serious C hacking, make sure to read this.

       o   perlguts

	   This is of paramount importance, since it's the documentation of what goes where in
	   the Perl source. Read it over a couple of times and it might start to make sense -
	   don't worry if it doesn't yet, because the best way to study it is to read it in
	   conjunction with poking at Perl source, and we'll do that later on.

	   Gisle Aas's "illustrated perlguts", also known as illguts, has very helpful pictures:


       o   perlxstut and perlxs

	   A working knowledge of XSUB programming is incredibly useful for core hacking; XSUBs
	   use techniques drawn from the PP code, the portion of the guts that actually executes
	   a Perl program. It's a lot gentler to learn those techniques from simple examples and
	   explanation than from the core itself.

       o   perlapi

	   The documentation for the Perl API explains what some of the internal functions do, as
	   well as the many macros used in the source.

       o   Porting/pumpkin.pod

	   This is a collection of words of wisdom for a Perl porter; some of it is only useful
	   to the pumpkin holder, but most of it applies to anyone wanting to go about Perl

       o   The perl5-porters FAQ

	   This should be available from http://dev.perl.org/perl5/docs/p5p-faq.html . It
	   contains hints on reading perl5-porters, information on how perl5-porters works and
	   how Perl development in general works.

       The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers who test CPAN
       modules on a variety of platforms.

       Perl Smokers ( http://www.nntp.perl.org/group/perl.daily-build/ and
       http://www.nntp.perl.org/group/perl.daily-build.reports/ ) automatically test Perl source
       releases on platforms with various configurations.

       Both efforts welcome volunteers. In order to get involved in smoke testing of the perl
       itself visit http://search.cpan.org/dist/Test-Smoke/ <http://search.cpan.org/dist/Test-
       Smoke/>. In order to start smoke testing CPAN modules visit
       http://search.cpan.org/dist/CPANPLUS-YACSmoke/ <http://search.cpan.org/dist/CPANPLUS-
       YACSmoke/> or <http://search.cpan.org/dist/minismokebox/> or
       http://search.cpan.org/dist/CPAN-Reporter/ <http://search.cpan.org/dist/CPAN-Reporter/>.

       If you've read all the documentation in the document and the ones listed above, you're
       more than ready to hack on Perl.

       Here's some more recommendations

       o   Subscribe to perl5-porters, follow the patches and try and understand them; don't be
	   afraid to ask if there's a portion you're not clear on - who knows, you may unearth a
	   bug in the patch...

       o   Do read the README associated with your operating system, e.g.  README.aix on the IBM
	   AIX OS. Don't hesitate to supply patches to that README if you find anything missing
	   or changed over a new OS release.

       o   Find an area of Perl that seems interesting to you, and see if you can work out how it
	   works. Scan through the source, and step over it in the debugger. Play, poke,
	   investigate, fiddle! You'll probably get to understand not just your chosen area but a
	   much wider range of perl's activity as well, and probably sooner than you'd think.

   "The Road goes ever on and on, down from the door where it began."
       If you can do these things, you've started on the long road to Perl porting. Thanks for
       wanting to help make Perl better - and happy hacking!

   Metaphoric Quotations
       If you recognized the quote about the Road above, you're in luck.

       Most software projects begin each file with a literal description of each file's purpose.
       Perl instead begins each with a literary allusion to that file's purpose.

       Like chapters in many books, all top-level Perl source files (along with a few others here
       and there) begin with an epigrammatic inscription that alludes, indirectly and
       metaphorically, to the material you're about to read.

       Quotations are taken from writings of J.R.R. Tolkien pertaining to his Legendarium, almost
       always from The Lord of the Rings. Chapters and page numbers are given using the following

       o   The Hobbit, by J.R.R. Tolkien. The hardcover, 70th-anniversary edition of 2007 was
	   used, published in the UK by Harper Collins Publishers and in the US by the Houghton
	   Mifflin Company.

       o   The Lord of the Rings, by J.R.R. Tolkien. The hardcover, 50th-anniversary edition of
	   2004 was used, published in the UK by Harper Collins Publishers and in the US by the
	   Houghton Mifflin Company.

       o   The Lays of Beleriand, by J.R.R. Tolkien and published posthumously by his son and
	   literary executor, C.J.R. Tolkien, being the 3rd of the 12 volumes in Christopher's
	   mammoth History of Middle Earth. Page numbers derive from the hardcover edition, first
	   published in 1983 by George Allen & Unwin; no page numbers changed for the special
	   3-volume omnibus edition of 2002 or the various trade-paper editions, all again now by
	   Harper Collins or Houghton Mifflin.

       Other JRRT books fair game for quotes would thus include The Adventures of Tom Bombadil,
       The Silmarillion, Unfinished Tales, and The Tale of the Children of Hurin, all but the
       first posthumously assembled by CJRT. But The Lord of the Rings itself is perfectly fine
       and probably best to quote from, provided you can find a suitable quote there.

       So if you were to supply a new, complete, top-level source file to add to Perl, you should
       conform to this peculiar practice by yourself selecting an appropriate quotation from
       Tolkien, retaining the original spelling and punctuation and using the same format the
       rest of the quotes are in. Indirect and oblique is just fine; remember, it's a metaphor,
       so being meta is, after all, what it's for.

       This document was originally written by Nathan Torkington, and is maintained by the
       perl5-porters mailing list.

perl v5.16.3				    2013-03-04				      PERLHACK(1)

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