PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1)
NAME
perlhack - How to hack on Perl
DESCRIPTION
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.
SUPER QUICK PATCH GUIDE
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:
· 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
· Make your change
Hack, hack, hack.
· Test your change
You can run all the tests with the following commands:
% ./Configure -des -Dusedevel
% make test
Keep hacking until the tests pass.
· 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".
· 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 email.
· Thank you
The porters appreciate the time you spent helping to make Perl better. Thank you!
PERL 5 PORTERS
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/
<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/>. There is also another archive at
http://archive.develooper.com/[email protected]/ <http://archive.develooper.com/[email protected]/>.
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
<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.
GETTING THE PERL SOURCE
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.)
Submitting patches
If you have a small patch to submit, please submit it via perlbug. You can also send email directly to
[email protected]. 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 [email protected] 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 list.
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.
understand what you were trying to do, how you were trying to do it, and why the change matters to Perl.
· 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.
· What
Your commit message should describe what part of the Perl core you're changing and what you expect your
patch to do.
· 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.
Style
In general, please follow the particular style of the code you are patching.
In particular, follow these general guidelines for patching Perl sources:
· 8-wide tabs (no exceptions!)
· 4-wide indents for code, 2-wide indents for nested CPP #defines
· Try hard not to exceed 79-columns
· ANSI C prototypes
· Uncuddled elses and "K&R" style for indenting control constructs
· Avoid assignments in conditionals, but if they're unavoidable, use extra paren, e.g. "if (a && (b = c))
..."
· "return foo;" rather than "return(foo);"
· "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
<[email protected]>):
· Know what you're testing. Read the docs, and the source.
· Tend to fail, not succeed.
· Interpret results strictly.
· Use unrelated features (this will flush out bizarre interactions).
· Use non-standard idioms (otherwise you are not testing TIMTOWTDI).
· 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).
· Give meaningful error messages when a test fails.
· Avoid using qx// and system() unless you are testing for them. If you do use them, make sure that you
cover _all_ perl platforms.
· Unlink any temporary files you create.
· Promote unforeseen warnings to errors with $SIG{__WARN__}.
· Be sure to use the libraries and modules shipped with the version being tested, not those that were
already installed.
· Add comments to the code explaining what you are testing for.
· Make updating the '1..42' string unnecessary. Or make sure that you update it.
· 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.
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:
· Adding, deprecating, or removing core features
· Adding, deprecating, removing, or upgrading core or dual-life modules
· Adding new core tests
· Fixing security issues and user-visible bugs in the core
· Changes that might break existing code, either on the perl or C level
· Significant performance improvements
· Adding, removing, or significantly changing documentation in the pod/ directory
· 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
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 addressed.
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?
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 feature.
TESTING
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>.
· 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.
· t/cmd, t/run, t/io and t/op
Now that basic require() and subroutines are tested, you can use the t/test.pl library.
You can also use certain libraries like Config conditionally, but be sure to skip the test gracefully if
it's not there.
· 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 systems.
· test_porting
This runs some basic sanity tests on the source tree and helps catch basic errors before you submit a
patch.
· coretest
Run perl on all core tests (t/* and lib/[a-z]* pragma tests).
specifically fixed, of course).
(Not available on Win32)
· minitest
Run miniperl on t/base, t/comp, t/cmd, t/run, t/io, t/op, t/uni and t/mro tests.
· 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.
· 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 t/harness.
· utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will succeed.
(Not available on Win32)
· minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with different versions of this encoding.
"make utest.utf16" runs the test suite with a combination of "-utf8" and "-utf16" arguments to t/TEST.
(Not available on Win32)
· 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"
· test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal test.
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
or
./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.
· -v
Run the tests under verbose mode so you can see what tests were run, and debug output.
· -torture
Run the torture tests as well as the normal set.
· -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.
· -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:
· 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.
· PERL_DESTRUCT_LEVEL=2
Other environment variables that may influence tests
· 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.
· PERL_TEST_NOVREXX
Setting this variable skips the vrexx.t tests for OS2::REXX.
· PERL_TEST_NUMCONVERTS
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.
MORE READING FOR GUTS HACKERS
To hack on the Perl guts, you'll need to read the following things:
· perlsource
An overview of the Perl source tree. This will help you find the files you're looking for.
· perlinterp
An overview of the Perl interpreter source code and some details on how Perl does what it does.
· 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 works.
· 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.
· 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:
<http://search.cpan.org/dist/illguts/>
· perlxstut and perlxs
holder, but most of it applies to anyone wanting to go about Perl development.
· 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.
CPAN TESTERS AND PERL SMOKERS
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/>.
WHAT NEXT?
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
· 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...
· 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.
· 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 editions:
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.
AUTHOR
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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