=for comment
Consistent formatting of this file is achieved with:
- perl ./Porting/podtidy pod/perlrepository.pod
+ perl ./Porting/podtidy pod/perlgit.pod
=head1 NAME
-perlrepository - Using the Perl source repository
+perlgit - Detailed information about git and the Perl repository
-=head1 SYNOPSIS
+=head1 DESCRIPTION
-All of Perl's source code is kept centrally in a Git repository at
-I<perl5.git.perl.org>. The repository contains many Perl revisions from
-Perl 1 onwards and all the revisions from Perforce, the version control
-system we were using previously. This repository is accessible in
-different ways.
-
-The full repository takes up about 80MB of disk space. A check out of
-the blead branch (that is, the main development branch, which contains
-bleadperl, the development version of perl 5) takes up about 160MB of
-disk space (including the repository). A build of bleadperl takes up
-about 200MB (including the repository and the check out).
-
-=head1 Getting access to the repository
-
-=head2 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
-
-=head2 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 (which uses port 9418):
-
- % git clone git://perl5.git.perl.org/perl.git perl-git
-
-This clones the repository and makes a local copy in the F<perl-git>
-directory.
-
-If your local network does not allow you to use port 9418, then you can
-fetch a copy of the repository over HTTP (this is at least 4x slower):
-
- % git clone http://perl5.git.perl.org/perl.git perl-http
-
-This clones the repository and makes a local copy in the F<perl-http>
-directory.
-
-=head2 Write access to the repository
-
-If you are a committer, then you can fetch a copy of the repository
-that you can push back on with:
-
- % git clone ssh://perl5.git.perl.org/perl.git perl-ssh
-
-This clones the repository and makes a local copy in the F<perl-ssh>
-directory.
-
-If you cloned using the git protocol, which is faster than ssh, then
-you will need to modify the URL for the origin remote to enable
-pushing. To do that edit F<.git/config> with git-config(1) like this:
-
- % git config remote.origin.url ssh://perl5.git.perl.org/perl.git
-
-You can also set up your user name and e-mail address. Most people do
-this once globally in their F<~/.gitconfig> by doing something like:
-
- % git config --global user.name "Ævar Arnfjörð Bjarmason"
- % git config --global user.email avarab@gmail.com
-
-However if you'd like to override that just for perl then execute then
-execute something like the following in F<perl-git>:
-
- % git config user.email avar@cpan.org
+This document provides details on using git to develop Perl. If you are
+just interested in working on a quick patch, see L<perlhack> first.
+This document is intended for people who are regular contributors to
+Perl, including those with write access to the git repository.
-It is also possible to keep C<origin> as a git remote, and add a new
-remote for ssh access:
+=head1 CLONING THE REPOSITORY
- % git remote add camel perl5.git.perl.org:/perl.git
+All of Perl's source code is kept centrally in a Git repository at
+I<perl5.git.perl.org>.
-This allows you to update your local repository by pulling from
-C<origin>, which is faster and doesn't require you to authenticate, and
-to push your changes back with the C<camel> remote:
+You can make a read-only clone of the repository by running:
- % git fetch camel
- % git push camel
+ % git clone git://perl5.git.perl.org/perl.git perl
-The C<fetch> command just updates the C<camel> refs, as the objects
-themselves should have been fetched when pulling from C<origin>.
+This uses the git protocol (port 9418).
-=head2 A note on camel and dromedary
+If you cannot use the git protocol for firewall reasons, you can also
+clone via http, though this is much slower:
-The committers have SSH access to the two servers that serve
-C<perl5.git.perl.org>. One is C<perl5.git.perl.org> itself (I<camel>),
-which is the 'master' repository. The second one is
-C<users.perl5.git.perl.org> (I<dromedary>), which can be used for
-general testing and development. Dromedary syncs the git tree from
-camel every few minutes, you should not push there. Both machines also
-have a full CPAN mirror in /srv/CPAN, please use this. To share files
-with the general public, dromedary serves your ~/public_html/ as
-C<http://users.perl5.git.perl.org/~yourlogin/>
+ % git clone http://perl5.git.perl.org/perl.git perl
-These hosts have fairly strict firewalls to the outside. Outgoing, only
-rsync, ssh and git are allowed. For http and ftp, you can use
-http://webproxy:3128 as proxy. Incoming, the firewall tries to detect
-attacks and blocks IP addresses with suspicious activity. This
-sometimes (but very rarely) has false positives and you might get
-blocked. The quickest way to get unblocked is to notify the admins.
-
-These two boxes are owned, hosted, and operated by booking.com. You can
-reach the sysadmins in #p5p on irc.perl.org or via mail to
-C<perl5-porters@perl.org>
-
-=head1 Overview of the repository
+=head1 WORKING WITH THE REPOSITORY
Once you have changed into the repository directory, you can inspect
-it.
-
-After a clone the repository will contain a single local branch, which
-will be the current branch as well, as indicated by the asterisk.
+it. After a clone the repository will contain a single local branch,
+which will be the current branch as well, as indicated by the asterisk.
% git branch
* blead
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
- # modified: pod/perlrepository.pod
+ # modified: pod/perlgit.pod
#
# Changed but not updated:
# (use "git add <file>..." to update what will be committed)
#
- # modified: pod/perlrepository.pod
+ # modified: pod/perlgit.pod
#
# Untracked files:
# (use "git add <file>..." to include in what will be committed)
is also what you see as a template if you do not provide a message to
C<git commit>.
-Assuming that you'd like to commit all the changes you've just made as
-a a single atomic unit, run this command:
-
- % git commit -a
-
-(That C<-a> tells git to add every file you've changed to this commit.
-New files aren't automatically added to your commit when you use
-C<commit -a> If you want to add files or to commit some, but not all of
-your changes, have a look at the documentation for C<git add>.)
-
-Git will start up your favorite text editor, so that you can craft a
-commit message for your change. See L</Commit message> below for more
-information about what makes a good commit message.
-
-Once you've finished writing your commit message and exited your
-editor, git will write your change to disk and tell you something like
-this:
+=head2 Patch workflow
- Created commit daf8e63: explain git status and stuff about remotes
- 1 files changed, 83 insertions(+), 3 deletions(-)
-
-
-If you re-run C<git status>, you should see something like this:
-
- % git status
- # On branch blead
- # Your branch is ahead of 'origin/blead' by 2 commits.
- #
- # Untracked files:
- # (use "git add <file>..." to include in what will be committed)
- #
- # deliberate.untracked
- nothing added to commit but untracked files present (use "git add" to track)
+First, please read L<perlhack> for details on hacking the Perl core.
+That document covers many details on how to create a good patch.
-
-When in doubt, before you do anything else, check your status and read
-it carefully, many questions are answered directly by the git status
-output.
-
-=head1 Submitting a patch
-
-If you have a patch in mind for Perl, you should first get a copy of
-the repository:
-
- % git clone git://perl5.git.perl.org/perl.git perl-git
-
-Then change into the directory:
-
- % cd perl-git
-
-Alternatively, if you already have a Perl repository, you should ensure
-that you're on the I<blead> branch, and your repository is up to date:
+If you already have a Perl repository, you should ensure that you're on
+the I<blead> branch, and your repository is up to date:
% git checkout blead
% git pull
It's preferable to patch against the latest blead version, since this
is where new development occurs for all changes other than critical bug
-fixes. Critical bug fix patches should be made against the relevant
+fixes. Critical bug fix patches should be made against the relevant
maint branches, or should be submitted with a note indicating all the
branches where the fix should be applied.
Lesley Binks <lesley.binks@gmail.com>
Lincoln D. Stein <lstein@cshl.org>
-For changes significant enough to warrant a F<pod/perldelta.pod> entry, the
-porters would greatly appreciate it if you submit an entry along with your
-actual change. Significant changes include, but are not limited to:
-
-=over 4
-
-=item *
-
-adding, deprecating, or removing core features
-
-=item *
-
-adding, deprecating, removing, or upgrading core or dual-life modules
-
-=item *
-
-adding new core tests
-
-=item *
-
-fixing security issues and user-visible bugs in the core
-
-=item *
-
-changes that might break existing code, either on the perl or C level
-
-=item *
-
-significant performance improvements
-
-=item *
-
-adding, removing, or significantly changing documentation in the F<pod/>
-directory
+Now commit your change locally:
-=item *
+ % git commit -a -m 'Rename Leon Brocard to Orange Brocard'
+ Created commit 6196c1d: Rename Leon Brocard to Orange Brocard
+ 1 files changed, 1 insertions(+), 1 deletions(-)
-important platform-specific changes
+The C<-a> option is used to include all files that git tracks that you
+have changed. If at this time, you only want to commit some of the
+files you have worked on, you can omit the C<-a> and use the command
+C<S<git add I<FILE ...>>> before doing the commit. C<S<git add
+--interactive>> allows you to even just commit portions of files
+instead of all the changes in them.
-=back
+The C<-m> option is used to specify the commit message. If you omit it,
+git will open a text editor for you to compose the message
+interactively. This is useful when the changes are more complex than
+the sample given here, and, depending on the editor, to know that the
+first line of the commit message doesn't exceed the 50 character legal
+maximum.
-Please make sure you add the perldelta entry to the right section within
-F<pod/perldelta.pod>. More information on how to write good perldelta entries is
-available in the C<Style> section of F<Porting/how_to_write_a_perldelta.pod>.
+Once you've finished writing your commit message and exited your
+editor, git will write your change to disk and tell you something like
+this:
-Now commit your change locally:
+ Created commit daf8e63: explain git status and stuff about remotes
+ 1 files changed, 83 insertions(+), 3 deletions(-)
- % git commit -a -m 'Rename Leon Brocard to Orange Brocard'
- Created commit 6196c1d: Rename Leon Brocard to Orange Brocard
- 1 files changed, 1 insertions(+), 1 deletions(-)
+If you re-run C<git status>, you should see something like this:
-The C<-a> option is used to include all files that git tracks that you have
-changed. If at this time, you only want to commit some of the files you have
-worked on, you can omit the C<-a> and use the command C<S<git add I<FILE ...>>>
-before doing the commit. C<S<git add --interactive>> allows you to even just
-commit portions of files instead of all the changes in them.
+ % git status
+ # On branch blead
+ # Your branch is ahead of 'origin/blead' by 2 commits.
+ #
+ # Untracked files:
+ # (use "git add <file>..." to include in what will be committed)
+ #
+ # deliberate.untracked
+ nothing added to commit but untracked files present (use "git add" to track)
-The C<-m> option is used to specify the commit message. If you omit it, git
-will open a text editor for you to compose the message interactively. This
-is useful when the changes are more complex than the sample given here, and,
-depending on the editor, to know that the first line of the commit message
-doesn't exceed the 50 character legal maximum.
+When in doubt, before you do anything else, check your status and read
+it carefully, many questions are answered directly by the git status
+output.
You can examine your last commit with:
L<perlbug@perl.org|mailto:perlbug@perl.org> with a description of your
changes, and include this patch file as an attachment. In addition to
being tracked by RT, mail to perlbug will automatically be forwarded to
-perl5-porters (with manual moderation, so please be patient). You should only
-send patches to L<perl5-porters@perl.org|mailto:perl5-porters@perl.org>
-directly if the patch is not ready to be applied, but intended for discussion.
+perl5-porters (with manual moderation, so please be patient). You
+should only send patches to
+L<perl5-porters@perl.org|mailto:perl5-porters@perl.org> directly if the
+patch is not ready to be applied, but intended for discussion.
See the next section for how to configure and use git to send these
emails for you.
% git branch -D orange
Deleted branch orange.
+=head2 Committing your changes
+
+Assuming that you'd like to commit all the changes you've made as a a
+single atomic unit, run this command:
+
+ % git commit -a
+
+(That C<-a> tells git to add every file you've changed to this commit.
+New files aren't automatically added to your commit when you use
+C<commit -a> If you want to add files or to commit some, but not all of
+your changes, have a look at the documentation for C<git add>.)
+
+Git will start up your favorite text editor, so that you can craft a
+commit message for your change. See L<perlhack/Commit message> for more
+information about what makes a good commit message.
+
+Once you've finished writing your commit message and exited your
+editor, git will write your change to disk and tell you something like
+this:
+
+ Created commit daf8e63: explain git status and stuff about remotes
+ 1 files changed, 83 insertions(+), 3 deletions(-)
+
+If you re-run C<git status>, you should see something like this:
+
+ % git status
+ # On branch blead
+ # Your branch is ahead of 'origin/blead' by 2 commits.
+ #
+ # Untracked files:
+ # (use "git add <file>..." to include in what will be committed)
+ #
+ # deliberate.untracked
+ nothing added to commit but untracked files present (use "git add" to track)
+
+When in doubt, before you do anything else, check your status and read
+it carefully, many questions are answered directly by the git status
+output.
+
=head2 Using git to send patch emails
+Please read L<perlhack> first in order to figure out where your patches
+should be sent.
+
In your ~/git/perl repository, set the destination email to perl's bug
tracker:
$ git config sendemail.to perlbug@perl.org
-Or maybe perl5-porters (discussed above):
+Or maybe perl5-porters:
$ git config sendemail.to perl5-porters@perl.org
$ git config --global sendemail.smtpuser YOURUSERNAME@gmail.com
With this configuration, you will be prompted for your gmail password
-when you run 'git send-email'. You can also configure
+when you run 'git send-email'. You can also configure
C<sendemail.smtppass> with your password if you don't care about having
your password in the .gitconfig file.
Be aware that many files in the distribution are derivative--avoid
patching them, because git won't see the changes to them, and the build
-process will overwrite them. Patch the originals instead. Most
+process will overwrite them. Patch the originals instead. Most
utilities (like perldoc) are in this category, i.e. patch
F<utils/perldoc.PL> rather than F<utils/perldoc>. Similarly, don't
create patches for files under $src_root/ext from their copies found in
-$install_root/lib. If you are unsure about the proper location of a
+$install_root/lib. If you are unsure about the proper location of a
file that may have gotten copied while building the source
distribution, consult the C<MANIFEST>.
-=for XXX
-What should we recommend about binary files now? Do we need anything?
-
-=head2 Getting your patch accepted
-
-If you are submitting a code patch there are several things that you
-need to do.
-
-=over 4
-
-=item Commit message
-
-As you craft each patch you intend to submit to the Perl core, it's
-important to write a good commit message.
-
-The first line of the commit message should be a short description and
-should skip the full stop. It should be no longer than the subject line
-of an E-Mail, 50 characters being a good rule of thumb.
-
-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 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 let a programmer
-with a reasonable familiarity with 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.
-
-=over 4
-
-=item What
-
-Your commit message should describe what part of the Perl core you're
-changing and what you expect your patch to do.
-
-=item Why
-
-Perhaps most importantly, 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.
-
-=item 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.
-
-=back
-
-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.
-
-=item 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.
-
-=item 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
- no C++ style (//) comments
- mark places that need to be revisited with XXX (and revisit often!)
- opening brace lines up with "if" when conditional spans multiple
- lines; should be at end-of-line otherwise
- in function definitions, name starts in column 0 (return value is on
- previous line)
- single space after keywords that are followed by parens, no space
- between function name and following paren
- 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.
-
-=item Testsuite
-
-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 testsuite additions should generally follow these guidelines
-(courtesy of Gurusamy Sarathy <gsar@activestate.com>):
-
- 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:
- - All optional arguments
- - Return values in various contexts (boolean, scalar, list, lvalue)
- - Use both global and lexical variables
- - Don't forget the exceptional, pathological cases.
-
-=back
-
-=head1 Accepting a patch
-
-If you have received a patch file generated using the above section,
-you should try out the patch.
-
-First we need to create a temporary new branch for these changes and
-switch into it:
-
- % git checkout -b experimental
-
-Patches that were formatted by C<git format-patch> are applied with
-C<git am>:
-
- % git am 0001-Rename-Leon-Brocard-to-Orange-Brocard.patch
- Applying Rename Leon Brocard to Orange Brocard
-
-If just a raw diff is provided, it is also possible use this two-step
-process:
-
- % git apply bugfix.diff
- % git commit -a -m "Some fixing" --author="That Guy <that.guy@internets.com>"
-
-Now we can inspect the change:
-
- % git show HEAD
- commit b1b3dab48344cff6de4087efca3dbd63548ab5e2
- Author: Leon Brocard <acme@astray.com>
- Date: Fri Dec 19 17:02:59 2008 +0000
-
- Rename Leon Brocard to Orange Brocard
-
- diff --git a/AUTHORS b/AUTHORS
- index 293dd70..722c93e 100644
- --- a/AUTHORS
- +++ b/AUTHORS
- @@ -541,7 +541,7 @@ Lars Hecking <lhecking@nmrc.ucc.ie>
- Laszlo Molnar <laszlo.molnar@eth.ericsson.se>
- Leif Huhn <leif@hale.dkstat.com>
- Len Johnson <lenjay@ibm.net>
- -Leon Brocard <acme@astray.com>
- +Orange Brocard <acme@astray.com>
- Les Peters <lpeters@aol.net>
- Lesley Binks <lesley.binks@gmail.com>
- Lincoln D. Stein <lstein@cshl.org>
-
-If you are a committer to Perl and you think the patch is good, you can
-then merge it into blead then push it out to the main repository:
-
- % git checkout blead
- % git merge experimental
- % git push
-
-If you want to delete your temporary branch, you may do so with:
-
- % git checkout blead
- % git branch -d experimental
- error: The branch 'experimental' is not an ancestor of your current HEAD.
- If you are sure you want to delete it, run 'git branch -D experimental'.
- % git branch -D experimental
- Deleted branch experimental.
-
-=head1 Cleaning a working directory
+=head2 Cleaning a working directory
The command C<git clean> can with varying arguments be used as a
replacement for C<make clean>.
If you want to cancel one or several commits, you can use C<git reset>.
-=head1 Bisecting
+=head2 Bisecting
C<git> provides a built-in way to determine, with a binary search in
the history, which commit should be blamed for introducing a given bug.
C<git help bisect> has much more information on how you can tweak your
binary searches.
+=head1 Topic branches and rewriting history
-=head1 Submitting a patch via GitHub
-
-GitHub is a website that makes it easy to fork and publish projects
-with Git. First you should set up a GitHub account and log in.
-
-Perl's git repository is mirrored on GitHub at this page:
-
- http://github.com/mirrors/perl/tree/blead
-
-Visit the page and click the "fork" button. This clones the Perl git
-repository for you and provides you with "Your Clone URL" from which
-you should clone:
-
- % git clone git@github.com:USERNAME/perl.git perl-github
+Individual committers should create topic branches under
+B<yourname>/B<some_descriptive_name>. Other committers should check
+with a topic branch's creator before making any change to it.
-The same patch as above, using github might look like this:
+The simplest way to create a remote topic branch that works on all
+versions of git is to push the current head as a new branch on the
+remote, then check it out locally:
- % cd perl-github
- % git remote add upstream git://perl5.git.perl.org/perl.git
- % git pull upstream blead
- % git checkout -b orange
- % perl -pi -e 's{Leon Brocard}{Orange Brocard}' AUTHORS
- % git commit -a -m 'Rename Leon Brocard to Orange Brocard'
- % git push origin orange
+ $ branch="$yourname/$some_descriptive_name"
+ $ git push origin HEAD:$branch
+ $ git checkout -b $branch origin/$branch
-The orange branch has been pushed to GitHub, so you should now send an
-email (see L</Submitting a patch>) with a description of your changes
-and the following information:
+Users of git 1.7 or newer can do it in a more obvious manner:
- http://github.com/USERNAME/perl/tree/orange
- git://github.com/USERNAME/perl.git branch orange
+ $ branch="$yourname/$some_descriptive_name"
+ $ git checkout -b $branch
+ $ git push origin -u $branch
-=head1 Merging from a branch via GitHub
+If you are not the creator of B<yourname>/B<some_descriptive_name>, you
+might sometimes find that the original author has edited the branch's
+history. There are lots of good reasons for this. Sometimes, an author
+might simply be rebasing the branch onto a newer source point.
+Sometimes, an author might have found an error in an early commit which
+they wanted to fix before merging the branch to blead.
-If someone has provided a branch via GitHub and you are a committer,
-you should use the following in your perl-ssh directory:
+Currently the master repository is configured to forbid
+non-fast-forward merges. This means that the branches within can not be
+rebased and pushed as a single step.
- % git remote add avar git://github.com/avar/perl.git
- % git fetch avar
+The only way you will ever be allowed to rebase or modify the history
+of a pushed branch is to delete it and push it as a new branch under
+the same name. Please think carefully about doing this. It may be
+better to sequentially rename your branches so that it is easier for
+others working with you to cherry-pick their local changes onto the new
+version. (XXX: needs explanation).
-Now you can see the differences between the branch and blead:
+If you want to rebase a personal topic branch, you will have to delete
+your existing topic branch and push as a new version of it. You can do
+this via the following formula (see the explanation about C<refspec>'s
+in the git push documentation for details) after you have rebased your
+branch:
- % git diff avar/orange
+ # first rebase
+ $ git checkout $user/$topic
+ $ git fetch
+ $ git rebase origin/blead
-And you can see the commits:
+ # then "delete-and-push"
+ $ git push origin :$user/$topic
+ $ git push origin $user/$topic
- % git log avar/orange
+B<NOTE:> it is forbidden at the repository level to delete any of the
+"primary" branches. That is any branch matching
+C<m!^(blead|maint|perl)!>. Any attempt to do so will result in git
+producing an error like this:
-If you approve of a specific commit, you can cherry pick it:
+ $ git push origin :blead
+ *** It is forbidden to delete blead/maint branches in this repository
+ error: hooks/update exited with error code 1
+ error: hook declined to update refs/heads/blead
+ To ssh://perl5.git.perl.org/perl
+ ! [remote rejected] blead (hook declined)
+ error: failed to push some refs to 'ssh://perl5.git.perl.org/perl'
- % git cherry-pick 0c24b290ae02b2ab3304f51d5e11e85eb3659eae
+As a matter of policy we do B<not> edit the history of the blead and
+maint-* branches. If a typo (or worse) sneaks into a commit to blead or
+maint-*, we'll fix it in another commit. The only types of updates
+allowed on these branches are "fast-forward's", where all history is
+preserved.
-Or you could just merge the whole branch if you like it all:
+Annotated tags in the canonical perl.git repository will never be
+deleted or modified. Think long and hard about whether you want to push
+a local tag to perl.git before doing so. (Pushing unannotated tags is
+not allowed.)
- % git merge avar/orange
+=head3 Grafts
-And then push back to the repository:
+The perl history contains one mistake which was not caught in the
+conversion: a merge was recorded in the history between blead and
+maint-5.10 where no merge actually occurred. Due to the nature of git,
+this is now impossible to fix in the public repository. You can remove
+this mis-merge locally by adding the following line to your
+C<.git/info/grafts> file:
- % git push
+ 296f12bbbbaa06de9be9d09d3dcf8f4528898a49 434946e0cb7a32589ed92d18008aaa1d88515930
+It is particularly important to have this graft line if any bisecting
+is done in the area of the "merge" in question.
-=head1 Topic branches and rewriting history
+=head2 Topic branches and rewriting history
Individual committers should create topic branches under
B<yourname>/B<some_descriptive_name>. Other committers should check
they wanted to fix before merging the branch to blead.
Currently the master repository is configured to forbid
-non-fast-forward merges. This means that the branches within can not
-be rebased and pushed as a single step.
+non-fast-forward merges. This means that the branches within can not be
+rebased and pushed as a single step.
The only way you will ever be allowed to rebase or modify the history
of a pushed branch is to delete it and push it as a new branch under
a local tag to perl.git before doing so. (Pushing unannotated tags is
not allowed.)
-=head1 Committing to blead
+=head1 WRITE ACCESS TO THE GIT REPOSITORY
+
+Once you have write access, you will need to modify the URL for the
+origin remote to enable pushing. Edit F<.git/config> with the
+git-config(1) command:
+
+ % git config remote.origin.url ssh://perl5.git.perl.org/perl.git
+
+You can also set up your user name and e-mail address. Most people do
+this once globally in their F<~/.gitconfig> by doing something like:
+
+ % git config --global user.name "Ævar Arnfjörð Bjarmason"
+ % git config --global user.email avarab@gmail.com
+
+However if you'd like to override that just for perl then execute then
+execute something like the following in F<perl>:
+
+ % git config user.email avar@cpan.org
+
+It is also possible to keep C<origin> as a git remote, and add a new
+remote for ssh access:
+
+ % git remote add camel perl5.git.perl.org:/perl.git
+
+This allows you to update your local repository by pulling from
+C<origin>, which is faster and doesn't require you to authenticate, and
+to push your changes back with the C<camel> remote:
+
+ % git fetch camel
+ % git push camel
+
+The C<fetch> command just updates the C<camel> refs, as the objects
+themselves should have been fetched when pulling from C<origin>.
+=head1 Accepting a patch
+
+If you have received a patch file generated using the above section,
+you should try out the patch.
+
+First we need to create a temporary new branch for these changes and
+switch into it:
+
+ % git checkout -b experimental
+
+Patches that were formatted by C<git format-patch> are applied with
+C<git am>:
+
+ % git am 0001-Rename-Leon-Brocard-to-Orange-Brocard.patch
+ Applying Rename Leon Brocard to Orange Brocard
+
+If just a raw diff is provided, it is also possible use this two-step
+process:
+
+ % git apply bugfix.diff
+ % git commit -a -m "Some fixing" --author="That Guy <that.guy@internets.com>"
-If you are a committer, you have the right to commit changes directly to
-the 'blead' branch, which will become the next production release of Perl.
+Now we can inspect the change:
+
+ % git show HEAD
+ commit b1b3dab48344cff6de4087efca3dbd63548ab5e2
+ Author: Leon Brocard <acme@astray.com>
+ Date: Fri Dec 19 17:02:59 2008 +0000
+
+ Rename Leon Brocard to Orange Brocard
+
+ diff --git a/AUTHORS b/AUTHORS
+ index 293dd70..722c93e 100644
+ --- a/AUTHORS
+ +++ b/AUTHORS
+ @@ -541,7 +541,7 @@ Lars Hecking <lhecking@nmrc.ucc.ie>
+ Laszlo Molnar <laszlo.molnar@eth.ericsson.se>
+ Leif Huhn <leif@hale.dkstat.com>
+ Len Johnson <lenjay@ibm.net>
+ -Leon Brocard <acme@astray.com>
+ +Orange Brocard <acme@astray.com>
+ Les Peters <lpeters@aol.net>
+ Lesley Binks <lesley.binks@gmail.com>
+ Lincoln D. Stein <lstein@cshl.org>
+
+If you are a committer to Perl and you think the patch is good, you can
+then merge it into blead then push it out to the main repository:
+
+ % git checkout blead
+ % git merge experimental
+ % git push
+
+If you want to delete your temporary branch, you may do so with:
+
+ % git checkout blead
+ % git branch -d experimental
+ error: The branch 'experimental' is not an ancestor of your current HEAD.
+ If you are sure you want to delete it, run 'git branch -D experimental'.
+ % git branch -D experimental
+ Deleted branch experimental.
+
+=head2 Committing to blead
+
+The 'blead' branch will become the next production release of Perl.
Before pushing I<any> local change to blead, it's incredibly important
that you do a few things, lest other committers come after you with
=item *
-Make sure you have a good commit message. See L</Commit message> elsewhere in
-this document
+Make sure you have a good commit message. See L<perlhack/Commit
+message> for details.
=item *
-Run the test suite. You might not think that one typo fix would break
-a test file. You'd be wrong. Here's an example of where not running the
-suite caused problems. A patch was submitted that added a couple of
-tests to an existing .t. It couldn't possibly affect anything else, so
+Run the test suite. You might not think that one typo fix would break a
+test file. You'd be wrong. Here's an example of where not running the
+suite caused problems. A patch was submitted that added a couple of
+tests to an existing .t. It couldn't possibly affect anything else, so
no need to test beyond the single affected .t, right? But, the
submitter's email address had changed since the last of their
-submissions, and this caused other tests to fail. Running the test
+submissions, and this caused other tests to fail. Running the test
target given in the next item would have caught this problem.
=item *
=back
-=head1 Committing to maintenance versions
+=head2 Committing to maintenance versions
Maintenance versions should only be altered to add critical bug fixes,
see L<perlpolicy>.
B<-x> option to C<git cherry-pick> in order to record the SHA1 of the
original commit in the new commit message.
-Before pushing any change to a maint version, make sure you've satisfied
-the steps in L</Committing to blead> above.
+Before pushing any change to a maint version, make sure you've
+satisfied the steps in L</Committing to blead> above.
-=head1 Grafts
+=head2 Grafts
The perl history contains one mistake which was not caught in the
conversion: a merge was recorded in the history between blead and
-maint-5.10 where no merge actually occurred. Due to the nature of git,
-this is now impossible to fix in the public repository. You can remove
+maint-5.10 where no merge actually occurred. Due to the nature of git,
+this is now impossible to fix in the public repository. You can remove
this mis-merge locally by adding the following line to your
C<.git/info/grafts> file:
It is particularly important to have this graft line if any bisecting
is done in the area of the "merge" in question.
-=head1 SEE ALSO
+=head2 Merging from a branch via GitHub
-=over
+While we don't encourage the submission of patches via GitHub, that
+will still happen. Here is a guide to merging patches from a GitHub
+repository.
-=item *
+ % git remote add avar git://github.com/avar/perl.git
+ % git fetch avar
-The git documentation, accessible via the C<git help> command
+Now you can see the differences between the branch and blead:
-=item *
+ % git diff avar/orange
-L<perlpolicy> - Perl core development policy
+And you can see the commits:
-=back
+ % git log avar/orange
-=cut
+If you approve of a specific commit, you can cherry pick it:
+
+ % git cherry-pick 0c24b290ae02b2ab3304f51d5e11e85eb3659eae
+
+Or you could just merge the whole branch if you like it all:
+
+ % git merge avar/orange
+
+And then push back to the repository:
+
+ % git push
+
+=head2 A note on camel and dromedary
+
+The committers have SSH access to the two servers that serve
+C<perl5.git.perl.org>. One is C<perl5.git.perl.org> itself (I<camel>),
+which is the 'master' repository. The second one is
+C<users.perl5.git.perl.org> (I<dromedary>), which can be used for
+general testing and development. Dromedary syncs the git tree from
+camel every few minutes, you should not push there. Both machines also
+have a full CPAN mirror in /srv/CPAN, please use this. To share files
+with the general public, dromedary serves your ~/public_html/ as
+C<http://users.perl5.git.perl.org/~yourlogin/>
+
+These hosts have fairly strict firewalls to the outside. Outgoing, only
+rsync, ssh and git are allowed. For http and ftp, you can use
+http://webproxy:3128 as proxy. Incoming, the firewall tries to detect
+attacks and blocks IP addresses with suspicious activity. This
+sometimes (but very rarely) has false positives and you might get
+blocked. The quickest way to get unblocked is to notify the admins.
+
+These two boxes are owned, hosted, and operated by booking.com. You can
+reach the sysadmins in #p5p on irc.perl.org or via mail to
+C<perl5-porters@perl.org>.
-=head1 NAME
-
-perlhack - How to hack at the Perl internals
-
-=head1 DESCRIPTION
-
-This document attempts to explain how Perl development takes place,
-and ends with some suggestions for people wanting to become bona fide
-porters.
-
-=head1 HOW PERL DEVELOPMENT HAPPENS
-
-=head2 Perl 5 Porters
-
-The perl5-porters mailing list is where the Perl standard distribution
-is maintained and developed. The list can get anywhere from 10 to 150
-messages a day, depending on the heatedness of the debate. Most days
-there are two or three patches, extensions, features, or bugs being
-discussed at a time.
-
-A searchable archive of the list is at either:
-
- http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
-
-or
-
- http://archive.develooper.com/perl5-porters@perl.org/
-
-List subscribers (the porters themselves) come in several flavours.
-Some are quiet curious lurkers, who rarely pitch in and instead watch
-the ongoing development to ensure they're forewarned of new changes or
-features in Perl. Some are representatives of vendors, who are there
-to make sure that Perl continues to compile and work on their
-platforms. Some patch any reported bug that they know how to fix,
-some are actively patching their pet area (threads, Win32, the regexp
-engine), while others seem to do nothing but complain. In other
-words, it's your usual mix of technical people.
-
-Over this group of porters presides Larry Wall. He has the final word
-in what does and does not change in the Perl language. Various
-releases of Perl are shepherded by a "pumpking", a porter
-responsible for gathering patches, deciding on a patch-by-patch,
-feature-by-feature basis what will and will not go into the release.
-For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of
-Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and
-Rafael Garcia-Suarez holds the pumpking crown for the 5.10 release.
-
-In addition, various people are pumpkings for different things. For
-instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the
-I<Configure> pumpkin up till the 5.8 release. For the 5.10 release
-H.Merijn Brand took over.
-
-Larry sees Perl development along the lines of the US government:
-there's the Legislature (the porters), the Executive branch (the
-pumpkings), and the Supreme Court (Larry). The legislature can
-discuss and submit patches to the executive branch all they like, but
-the executive branch is free to veto them. Rarely, the Supreme Court
-will side with the executive branch over the legislature, or the
-legislature over the executive branch. Mostly, however, the
-legislature and the executive branch are supposed to get along and
-work out their differences without impeachment or court cases.
-
-You might sometimes see reference to Rule 1 and Rule 2. Larry's power
-as Supreme Court is expressed in The Rules:
-
-=over 4
-
-=item 1
-
-Larry is always by definition right about how Perl should behave.
-This means he has final veto power on the core functionality.
-
-=item 2
-
-Larry is allowed to change his mind about any matter at a later date,
-regardless of whether he previously invoked Rule 1.
-
-=back
-
-Got that? Larry is always right, even when he was wrong. It's rare
-to see either Rule exercised, but they are often alluded to.
-
-=head2 What makes for a good patch?
-
-New features and extensions to the language are contentious, because
-the criteria used by the pumpkings, Larry, and other porters to decide
-which features should be implemented and incorporated are not codified
-in a few small design goals as with some other languages. Instead,
-the heuristics are flexible and often difficult to fathom. Here is
-one person's list, roughly in decreasing order of importance, of
-heuristics that new features have to be weighed against:
-
-=over 4
-
-=item Does concept match the general goals of Perl?
-
-These haven't been written anywhere in stone, but one approximation
-is:
-
- 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.
-
-=item 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.
-
-=item Backwards compatibility
-
-It's a cardinal sin to break existing Perl programs. New warnings are
-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.
-
-=item 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. If it can be done in a
-module instead of in the core, it's highly unlikely to be added.
-
-=item Is the feature generic enough?
-
-Is this something that only the submitter wants added to the language,
-or would it be 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. For instance, instead of
-implementing a "delayed evaluation" feature, the porters are waiting
-for a macro system that would permit delayed evaluation and much more.
-
-=item Does it potentially introduce new bugs?
-
-Radical rewrites of large chunks of the Perl interpreter have the
-potential to introduce new bugs. The smaller and more localized the
-change, the better.
-
-=item 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.
-
-=item 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.
-
-=item Is the implementation generic enough to be portable?
-
-The worst patches make use of a system-specific features. It's highly
-unlikely that non-portable additions to the Perl language will be
-accepted.
-
-=item 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?
-
-=item Is there enough documentation?
-
-Patches without documentation are probably ill-thought out or
-incomplete. Nothing can be added without documentation, so submitting
-a patch for the appropriate manpages as well as the source code is
-always a good idea.
-
-=item Is there another way to do it?
-
-Larry said "Although the Perl Slogan is I<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.
-
-=item Does it create too much work?
-
-Work for the pumpking, work for Perl programmers, work for module
-authors, ... Perl is supposed to be easy.
-
-=item 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 feature.
-
-=back
-
-If you're on the list, you might hear the word "core" bandied
-around. It refers to the standard distribution. "Hacking on the
-core" means you're changing the C source code to the Perl
-interpreter. "A core module" is one that ships with Perl.
-
-=head2 Getting the Perl source
-
-The source code to the Perl interpreter, in its different versions, is
-kept in a repository managed by the git revision control system. The
-pumpkings and a few others have write access to the repository to check in
-changes.
-
-How to clone and use the git perl repository is described in L<perlrepository>.
-
-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 C<--delete> option to remove leftover files)
-
-To get a full list of the available sync points:
-
- $ rsync perl5.git.perl.org::
-
-You may also want to subscribe to the perl5-changes mailing list to
-receive a copy of each patch that gets submitted to the maintenance
-and development "branches" of the perl repository. See
-http://lists.perl.org/ for subscription information.
-
-If you are a member of the perl5-porters mailing list, it is a good
-thing to keep in touch with the most recent changes. If not only to
-verify if what you would have posted as a bug report isn't already
-solved in the most recent available perl development branch, also
-known as perl-current, bleeding edge perl, bleedperl or bleadperl.
-
-Needless to say, the source code in perl-current is usually in a perpetual
-state of evolution. You should expect it to be very buggy. Do B<not> use
-it for any purpose other than testing and development.
-
-=head2 Bug tracking with Perlbug
-
-There is a single remote administrative interface for modifying bug status,
-category, open issues etc. using the B<RT> bugtracker system, maintained
-by Robert Spier. Become an administrator, and close any bugs you can get
-your sticky mitts on:
-
- http://bugs.perl.org/
-
-To email the bug system administrators:
-
- "perlbug-admin" <perlbug-admin@perl.org>
-
-=head2 Submitting patches
-
-Always submit patches to I<perl5-porters@perl.org>. If you're
-patching a core module and there's an author listed, send the author a
-copy (see L<Patching a core module>). This lets other porters review
-your patch, which catches a surprising number of errors in patches.
-Please patch against the latest B<development> version. (e.g., even if
-you're fixing a bug in the 5.8 track, patch against the C<blead> branch in
-the git repository.)
-
-If changes are accepted, they are applied to the development branch. Then
-the maintenance pumpking decides which of those patches is to be
-backported to the maint branch. Only patches that survive the heat of the
-development branch get applied to maintenance versions.
-
-Your patch should update the documentation and test suite. See
-L<TESTING>. If you have added or removed files in the distribution,
-edit the MANIFEST file accordingly, sort the MANIFEST file using
-C<make manisort>, and include those changes as part of your patch.
-
-Patching documentation also follows the same order: if accepted, a patch
-is first applied to B<development>, and if relevant then it's backported
-to B<maintenance>. (With an exception for some patches that document
-behaviour that only appears in the maintenance branch, but which has
-changed in the development version.)
-
-To report a bug in Perl, use the program L<perlbug> which comes with
-Perl (if you can't get Perl to work, send mail to the address
-I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through
-I<perlbug> feeds into the automated bug-tracking system, access to
-which is provided through the web at http://rt.perl.org/rt3/ . It
-often pays to check the archives of the perl5-porters mailing list to
-see whether the bug you're reporting has been reported before, and if
-so whether it was considered a bug. See above for the location of
-the searchable archives.
-
-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
-L<http://search.cpan.org/dist/Test-Smoke>. In order to start smoke
-testing CPAN modules visit L<http://search.cpan.org/dist/CPANPLUS-YACSmoke/>
-or L<http://search.cpan.org/dist/minismokebox/> or
-L<http://search.cpan.org/dist/CPAN-Reporter/>.
-
-It's a good idea to read and lurk for a while before chipping in.
-That way you'll get to see the dynamic of the conversations, learn the
-personalities of the players, and hopefully be better prepared to make
-a useful contribution when do you speak up.
-
-If after all this you still think you want to join the perl5-porters
-mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
-unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
-
-=head2 Patching a core module
-
-This works just like patching anything else, with an extra
-consideration. Many core modules also live on CPAN. If this is so,
-patch the CPAN version instead of the core and send the patch off to
-the module maintainer (with a copy to p5p). This will help the module
-maintainer keep the CPAN version in sync with the core version without
-constantly scanning p5p.
-
-The list of maintainers of core modules is usefully documented in
-F<Porting/Maintainers.pl>.
-
-=head2 Adding a new function to the core
-
-If, as part of a patch to fix a bug, or just because you have an
-especially good idea, you decide to add a new function to the core,
-discuss your ideas on p5p well before you start work. It may be that
-someone else has already attempted to do what you are considering and
-can give lots of good advice or even provide you with bits of code
-that they already started (but never finished).
-
-You have to follow all of the advice given above for patching. It is
-extremely important to test any addition thoroughly and add new tests
-to explore all boundary conditions that your new function is expected
-to handle. If your new function is used only by one module (e.g. toke),
-then it should probably be named S_your_function (for static); on the
-other hand, if you expect it to accessible from other functions in
-Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions>
-for more details.
-
-The location of any new code is also an important consideration. Don't
-just create a new top level .c file and put your code there; you would
-have to make changes to Configure (so the Makefile is created properly),
-as well as possibly lots of include files. This is strictly pumpking
-business.
-
-It is better to add your function to one of the existing top level
-source code files, but your choice is complicated by the nature of
-the Perl distribution. Only the files that are marked as compiled
-static are located in the perl executable. Everything else is located
-in the shared library (or DLL if you are running under WIN32). So,
-for example, if a function was only used by functions located in
-toke.c, then your code can go in toke.c. If, however, you want to call
-the function from universal.c, then you should put your code in another
-location, for example util.c.
-
-In addition to writing your c-code, you will need to create an
-appropriate entry in embed.pl describing your function, then run
-'make regen_headers' to create the entries in the numerous header
-files that perl needs to compile correctly. See L<perlguts/Internal Functions>
-for information on the various options that you can set in embed.pl.
-You will forget to do this a few (or many) times and you will get
-warnings during the compilation phase. Make sure that you mention
-this when you post your patch to P5P; the pumpking needs to know this.
-
-When you write your new code, please be conscious of existing code
-conventions used in the perl source files. See L<perlstyle> for
-details. Although most of the guidelines discussed seem to focus on
-Perl code, rather than c, they all apply (except when they don't ;).
-Also see L<perlrepository> for lots of details about both formatting and
-submitting patches of your changes.
-
-Lastly, TEST TEST TEST TEST TEST any code before posting to p5p.
-Test on as many platforms as you can find. Test as many perl
-Configure options as you can (e.g. MULTIPLICITY). If you have
-profiling or memory tools, see L<MEMORY DEBUGGERS> and L<PROFILING>
-below for how to use them to further test your code. Remember that
-most of the people on P5P are doing this on their own time and
-don't have the time to debug your code.
-
-=head2 Background reading
-
-To hack on the Perl guts, you'll need to read the following things:
-
-=over 3
-
-=item L<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 I<illguts>, has very
-helpful pictures:
-
-L<http://search.cpan.org/dist/illguts/>
-
-=item L<perlxstut> and L<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.
-
-=item L<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.
-
-=item F<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 development.
-
-=item 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.
-
-=back
-
-=head1 UNDERSTANDING THE SOURCE
-
-=head2 Finding your way around
-
-Perl maintenance can be split into a number of areas, and certain people
-(pumpkins) will have responsibility for each area. These areas sometimes
-correspond to files or directories in the source kit. Among the areas are:
-
-=over 3
-
-=item Core modules
-
-Modules shipped as part of the Perl core live in various subdirectories, where
-two are dedicated to core-only modules, and two are for the dual-life modules
-which live on CPAN and may be maintained separately with respect to the Perl
-core:
-
- lib/ is for pure-Perl modules, which exist in the core only.
-
- ext/ is for XS extensions, and modules with special Makefile.PL
- requirements, which exist in the core only.
-
- cpan/ is for dual-life modules, where the CPAN module is
- canonical (should be patched first).
-
- dist/ is for dual-life modules, where the blead source is
- canonical.
-
-For some dual-life modules it has not been discussed if the CPAN version or the
-blead source is canonical. Until that is done, those modules should be in
-F<cpan/>.
-
-=item Tests
-
-There are tests for nearly all the modules, built-ins and major bits
-of functionality. Test files all have a .t suffix. Module tests live
-in the F<lib/> and F<ext/> directories next to the module being
-tested. Others live in F<t/>. See L<TESTING>
-
-=item Documentation
-
-Documentation maintenance includes looking after everything in the
-F<pod/> directory, (as well as contributing new documentation) and
-the documentation to the modules in core.
-
-=item Configure
-
-The Configure process is the way we make Perl portable across the
-myriad of operating systems it supports. Responsibility for the
-Configure, build and installation process, as well as the overall
-portability of the core code rests with the Configure pumpkin -
-others help out with individual operating systems.
-
-The three files that fall under his/her responsibility are Configure,
-config_h.SH, and Porting/Glossary (and a whole bunch of small related
-files that are less important here). The Configure pumpkin decides how
-patches to these are dealt with. Currently, the Configure pumpkin will
-accept patches in most common formats, even directly to these files.
-Other committers are allowed to commit to these files under the strict
-condition that they will inform the Configure pumpkin, either on IRC
-(if he/she happens to be around) or through (personal) e-mail.
-
-The files involved are the operating system directories, (F<win32/>,
-F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
-and F<Makefile>, as well as the metaconfig files which generate
-F<Configure>. (metaconfig isn't included in the core distribution.)
-
-See http://perl5.git.perl.org/metaconfig.git/blob/HEAD:/README for a
-description of the full process involved.
-
-=item Interpreter
-
-And of course, there's the core of the Perl interpreter itself. Let's
-have a look at that in a little more detail.
-
-=back
-
-Before we leave looking at the layout, though, don't forget that
-F<MANIFEST> contains not only the file names in the Perl distribution,
-but short descriptions of what's in them, too. For an overview of the
-important files, try this:
-
- perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
-
-=head2 Elements of the interpreter
-
-The work of the interpreter has two main stages: compiling the code
-into the internal representation, or bytecode, and then executing it.
-L<perlguts/Compiled code> explains exactly how the compilation stage
-happens.
-
-Here is a short breakdown of perl's operation:
-
-=over 3
-
-=item Startup
-
-The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
-This is very high-level code, enough to fit on a single screen, and it
-resembles the code found in L<perlembed>; most of the real action takes
-place in F<perl.c>
-
-F<perlmain.c> is generated by C<ExtUtils::Miniperl> from F<miniperlmain.c> at
-make time, so you should make perl to follow this along.
-
-First, F<perlmain.c> allocates some memory and constructs a Perl
-interpreter, along these lines:
-
- 1 PERL_SYS_INIT3(&argc,&argv,&env);
- 2
- 3 if (!PL_do_undump) {
- 4 my_perl = perl_alloc();
- 5 if (!my_perl)
- 6 exit(1);
- 7 perl_construct(my_perl);
- 8 PL_perl_destruct_level = 0;
- 9 }
-
-Line 1 is a macro, and its definition is dependent on your operating
-system. Line 3 references C<PL_do_undump>, a global variable - all
-global variables in Perl start with C<PL_>. This tells you whether the
-current running program was created with the C<-u> flag to perl and then
-F<undump>, which means it's going to be false in any sane context.
-
-Line 4 calls a function in F<perl.c> to allocate memory for a Perl
-interpreter. It's quite a simple function, and the guts of it looks like
-this:
-
- my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
-
-Here you see an example of Perl's system abstraction, which we'll see
-later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
-own C<malloc> as defined in F<malloc.c> if you selected that option at
-configure time.
-
-Next, in line 7, we construct the interpreter using perl_construct,
-also in F<perl.c>; this sets up all the special variables that Perl
-needs, the stacks, and so on.
-
-Now we pass Perl the command line options, and tell it to go:
-
- exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
- if (!exitstatus)
- perl_run(my_perl);
-
- exitstatus = perl_destruct(my_perl);
-
- perl_free(my_perl);
-
-C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
-in F<perl.c>, which processes the command line options, sets up any
-statically linked XS modules, opens the program and calls C<yyparse> to
-parse it.
-
-=item Parsing
-
-The aim of this stage is to take the Perl source, and turn it into an op
-tree. We'll see what one of those looks like later. Strictly speaking,
-there's three things going on here.
-
-C<yyparse>, the parser, lives in F<perly.c>, although you're better off
-reading the original YACC input in F<perly.y>. (Yes, Virginia, there
-B<is> a YACC grammar for Perl!) The job of the parser is to take your
-code and "understand" it, splitting it into sentences, deciding which
-operands go with which operators and so on.
-
-The parser is nobly assisted by the lexer, which chunks up your input
-into tokens, and decides what type of thing each token is: a variable
-name, an operator, a bareword, a subroutine, a core function, and so on.
-The main point of entry to the lexer is C<yylex>, and that and its
-associated routines can be found in F<toke.c>. Perl isn't much like
-other computer languages; it's highly context sensitive at times, it can
-be tricky to work out what sort of token something is, or where a token
-ends. As such, there's a lot of interplay between the tokeniser and the
-parser, which can get pretty frightening if you're not used to it.
-
-As the parser understands a Perl program, it builds up a tree of
-operations for the interpreter to perform during execution. The routines
-which construct and link together the various operations are to be found
-in F<op.c>, and will be examined later.
-
-=item Optimization
-
-Now the parsing stage is complete, and the finished tree represents
-the operations that the Perl interpreter needs to perform to execute our
-program. Next, Perl does a dry run over the tree looking for
-optimisations: constant expressions such as C<3 + 4> will be computed
-now, and the optimizer will also see if any multiple operations can be
-replaced with a single one. For instance, to fetch the variable C<$foo>,
-instead of grabbing the glob C<*foo> and looking at the scalar
-component, the optimizer fiddles the op tree to use a function which
-directly looks up the scalar in question. The main optimizer is C<peep>
-in F<op.c>, and many ops have their own optimizing functions.
-
-=item Running
-
-Now we're finally ready to go: we have compiled Perl byte code, and all
-that's left to do is run it. The actual execution is done by the
-C<runops_standard> function in F<run.c>; more specifically, it's done by
-these three innocent looking lines:
-
- while ((PL_op = PL_op->op_ppaddr(aTHX))) {
- PERL_ASYNC_CHECK();
- }
-
-You may be more comfortable with the Perl version of that:
-
- PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
-
-Well, maybe not. Anyway, each op contains a function pointer, which
-stipulates the function which will actually carry out the operation.
-This function will return the next op in the sequence - this allows for
-things like C<if> which choose the next op dynamically at run time.
-The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
-execution if required.
-
-The actual functions called are known as PP code, and they're spread
-between four files: F<pp_hot.c> contains the "hot" code, which is most
-often used and highly optimized, F<pp_sys.c> contains all the
-system-specific functions, F<pp_ctl.c> contains the functions which
-implement control structures (C<if>, C<while> and the like) and F<pp.c>
-contains everything else. These are, if you like, the C code for Perl's
-built-in functions and operators.
-
-Note that each C<pp_> function is expected to return a pointer to the next
-op. Calls to perl subs (and eval blocks) are handled within the same
-runops loop, and do not consume extra space on the C stack. For example,
-C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or C<CxEVAL> block
-struct onto the context stack which contain the address of the op
-following the sub call or eval. They then return the first op of that sub
-or eval block, and so execution continues of that sub or block. Later, a
-C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB> or C<CxEVAL>,
-retrieves the return op from it, and returns it.
-
-=item Exception handing
-
-Perl's exception handing (i.e. C<die> etc.) is built on top of the low-level
-C<setjmp()>/C<longjmp()> C-library functions. These basically provide a
-way to capture the current PC and SP registers and later restore them; i.e.
-a C<longjmp()> continues at the point in code where a previous C<setjmp()>
-was done, with anything further up on the C stack being lost. This is why
-code should always save values using C<SAVE_FOO> rather than in auto
-variables.
-
-The perl core wraps C<setjmp()> etc in the macros C<JMPENV_PUSH> and
-C<JMPENV_JUMP>. The basic rule of perl exceptions is that C<exit>, and
-C<die> (in the absence of C<eval>) perform a C<JMPENV_JUMP(2)>, while
-C<die> within C<eval> does a C<JMPENV_JUMP(3)>.
-
-At entry points to perl, such as C<perl_parse()>, C<perl_run()> and
-C<call_sv(cv, G_EVAL)> each does a C<JMPENV_PUSH>, then enter a runops
-loop or whatever, and handle possible exception returns. For a 2 return,
-final cleanup is performed, such as popping stacks and calling C<CHECK> or
-C<END> blocks. Amongst other things, this is how scope cleanup still
-occurs during an C<exit>.
-
-If a C<die> can find a C<CxEVAL> block on the context stack, then the
-stack is popped to that level and the return op in that block is assigned
-to C<PL_restartop>; then a C<JMPENV_JUMP(3)> is performed. This normally
-passes control back to the guard. In the case of C<perl_run> and
-C<call_sv>, a non-null C<PL_restartop> triggers re-entry to the runops
-loop. The is the normal way that C<die> or C<croak> is handled within an
-C<eval>.
-
-Sometimes ops are executed within an inner runops loop, such as tie, sort
-or overload code. In this case, something like
-
- sub FETCH { eval { die } }
-
-would cause a longjmp right back to the guard in C<perl_run>, popping both
-runops loops, which is clearly incorrect. One way to avoid this is for the
-tie code to do a C<JMPENV_PUSH> before executing C<FETCH> in the inner
-runops loop, but for efficiency reasons, perl in fact just sets a flag,
-using C<CATCH_SET(TRUE)>. The C<pp_require>, C<pp_entereval> and
-C<pp_entertry> ops check this flag, and if true, they call C<docatch>,
-which does a C<JMPENV_PUSH> and starts a new runops level to execute the
-code, rather than doing it on the current loop.
-
-As a further optimisation, on exit from the eval block in the C<FETCH>,
-execution of the code following the block is still carried on in the inner
-loop. When an exception is raised, C<docatch> compares the C<JMPENV>
-level of the C<CxEVAL> with C<PL_top_env> and if they differ, just
-re-throws the exception. In this way any inner loops get popped.
-
-Here's an example.
-
- 1: eval { tie @a, 'A' };
- 2: sub A::TIEARRAY {
- 3: eval { die };
- 4: die;
- 5: }
-
-To run this code, C<perl_run> is called, which does a C<JMPENV_PUSH> then
-enters a runops loop. This loop executes the eval and tie ops on line 1,
-with the eval pushing a C<CxEVAL> onto the context stack.
-
-The C<pp_tie> does a C<CATCH_SET(TRUE)>, then starts a second runops loop
-to execute the body of C<TIEARRAY>. When it executes the entertry op on
-line 3, C<CATCH_GET> is true, so C<pp_entertry> calls C<docatch> which
-does a C<JMPENV_PUSH> and starts a third runops loop, which then executes
-the die op. At this point the C call stack looks like this:
-
- Perl_pp_die
- Perl_runops # third loop
- S_docatch_body
- S_docatch
- Perl_pp_entertry
- Perl_runops # second loop
- S_call_body
- Perl_call_sv
- Perl_pp_tie
- Perl_runops # first loop
- S_run_body
- perl_run
- main
-
-and the context and data stacks, as shown by C<-Dstv>, look like:
-
- STACK 0: MAIN
- CX 0: BLOCK =>
- CX 1: EVAL => AV() PV("A"\0)
- retop=leave
- STACK 1: MAGIC
- CX 0: SUB =>
- retop=(null)
- CX 1: EVAL => *
- retop=nextstate
-
-The die pops the first C<CxEVAL> off the context stack, sets
-C<PL_restartop> from it, does a C<JMPENV_JUMP(3)>, and control returns to
-the top C<docatch>. This then starts another third-level runops level,
-which executes the nextstate, pushmark and die ops on line 4. At the point
-that the second C<pp_die> is called, the C call stack looks exactly like
-that above, even though we are no longer within an inner eval; this is
-because of the optimization mentioned earlier. However, the context stack
-now looks like this, ie with the top CxEVAL popped:
-
- STACK 0: MAIN
- CX 0: BLOCK =>
- CX 1: EVAL => AV() PV("A"\0)
- retop=leave
- STACK 1: MAGIC
- CX 0: SUB =>
- retop=(null)
-
-The die on line 4 pops the context stack back down to the CxEVAL, leaving
-it as:
-
- STACK 0: MAIN
- CX 0: BLOCK =>
-
-As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
-C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:
-
- S_docatch
- Perl_pp_entertry
- Perl_runops # second loop
- S_call_body
- Perl_call_sv
- Perl_pp_tie
- Perl_runops # first loop
- S_run_body
- perl_run
- main
-
-In this case, because the C<JMPENV> level recorded in the C<CxEVAL>
-differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
-and the C stack unwinds to:
-
- perl_run
- main
-
-Because C<PL_restartop> is non-null, C<run_body> starts a new runops loop
-and execution continues.
-
-=back
-
-=head2 Internal Variable Types
-
-You should by now have had a look at L<perlguts>, which tells you about
-Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
-that now.
-
-These variables are used not only to represent Perl-space variables, but
-also any constants in the code, as well as some structures completely
-internal to Perl. The symbol table, for instance, is an ordinary Perl
-hash. Your code is represented by an SV as it's read into the parser;
-any program files you call are opened via ordinary Perl filehandles, and
-so on.
-
-The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
-Perl program. Let's see, for instance, how Perl treats the constant
-C<"hello">.
-
- % perl -MDevel::Peek -e 'Dump("hello")'
- 1 SV = PV(0xa041450) at 0xa04ecbc
- 2 REFCNT = 1
- 3 FLAGS = (POK,READONLY,pPOK)
- 4 PV = 0xa0484e0 "hello"\0
- 5 CUR = 5
- 6 LEN = 6
-
-Reading C<Devel::Peek> output takes a bit of practise, so let's go
-through it line by line.
-
-Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
-memory. SVs themselves are very simple structures, but they contain a
-pointer to a more complex structure. In this case, it's a PV, a
-structure which holds a string value, at location C<0xa041450>. Line 2
-is the reference count; there are no other references to this data, so
-it's 1.
-
-Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
-read-only SV (because it's a constant) and the data is a PV internally.
-Next we've got the contents of the string, starting at location
-C<0xa0484e0>.
-
-Line 5 gives us the current length of the string - note that this does
-B<not> include the null terminator. Line 6 is not the length of the
-string, but the length of the currently allocated buffer; as the string
-grows, Perl automatically extends the available storage via a routine
-called C<SvGROW>.
-
-You can get at any of these quantities from C very easily; just add
-C<Sv> to the name of the field shown in the snippet, and you've got a
-macro which will return the value: C<SvCUR(sv)> returns the current
-length of the string, C<SvREFCOUNT(sv)> returns the reference count,
-C<SvPV(sv, len)> returns the string itself with its length, and so on.
-More macros to manipulate these properties can be found in L<perlguts>.
-
-Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c>
-
- 1 void
- 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
- 3 {
- 4 STRLEN tlen;
- 5 char *junk;
-
- 6 junk = SvPV_force(sv, tlen);
- 7 SvGROW(sv, tlen + len + 1);
- 8 if (ptr == junk)
- 9 ptr = SvPVX(sv);
- 10 Move(ptr,SvPVX(sv)+tlen,len,char);
- 11 SvCUR(sv) += len;
- 12 *SvEND(sv) = '\0';
- 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
- 14 SvTAINT(sv);
- 15 }
-
-This is a function which adds a string, C<ptr>, of length C<len> onto
-the end of the PV stored in C<sv>. The first thing we do in line 6 is
-make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
-macro to force a PV. As a side effect, C<tlen> gets set to the current
-value of the PV, and the PV itself is returned to C<junk>.
-
-In line 7, we make sure that the SV will have enough room to accommodate
-the old string, the new string and the null terminator. If C<LEN> isn't
-big enough, C<SvGROW> will reallocate space for us.
-
-Now, if C<junk> is the same as the string we're trying to add, we can
-grab the string directly from the SV; C<SvPVX> is the address of the PV
-in the SV.
-
-Line 10 does the actual catenation: the C<Move> macro moves a chunk of
-memory around: we move the string C<ptr> to the end of the PV - that's
-the start of the PV plus its current length. We're moving C<len> bytes
-of type C<char>. After doing so, we need to tell Perl we've extended the
-string, by altering C<CUR> to reflect the new length. C<SvEND> is a
-macro which gives us the end of the string, so that needs to be a
-C<"\0">.
-
-Line 13 manipulates the flags; since we've changed the PV, any IV or NV
-values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
-want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware
-version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
-and turns on POK. The final C<SvTAINT> is a macro which launders tainted
-data if taint mode is turned on.
-
-AVs and HVs are more complicated, but SVs are by far the most common
-variable type being thrown around. Having seen something of how we
-manipulate these, let's go on and look at how the op tree is
-constructed.
-
-=head2 Op Trees
-
-First, what is the op tree, anyway? The op tree is the parsed
-representation of your program, as we saw in our section on parsing, and
-it's the sequence of operations that Perl goes through to execute your
-program, as we saw in L</Running>.
-
-An op is a fundamental operation that Perl can perform: all the built-in
-functions and operators are ops, and there are a series of ops which
-deal with concepts the interpreter needs internally - entering and
-leaving a block, ending a statement, fetching a variable, and so on.
-
-The op tree is connected in two ways: you can imagine that there are two
-"routes" through it, two orders in which you can traverse the tree.
-First, parse order reflects how the parser understood the code, and
-secondly, execution order tells perl what order to perform the
-operations in.
-
-The easiest way to examine the op tree is to stop Perl after it has
-finished parsing, and get it to dump out the tree. This is exactly what
-the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
-and L<B::Debug|B::Debug> do.
-
-Let's have a look at how Perl sees C<$a = $b + $c>:
-
- % perl -MO=Terse -e '$a=$b+$c'
- 1 LISTOP (0x8179888) leave
- 2 OP (0x81798b0) enter
- 3 COP (0x8179850) nextstate
- 4 BINOP (0x8179828) sassign
- 5 BINOP (0x8179800) add [1]
- 6 UNOP (0x81796e0) null [15]
- 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
- 8 UNOP (0x81797e0) null [15]
- 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
- 10 UNOP (0x816b4f0) null [15]
- 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
-
-Let's start in the middle, at line 4. This is a BINOP, a binary
-operator, which is at location C<0x8179828>. The specific operator in
-question is C<sassign> - scalar assignment - and you can find the code
-which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
-binary operator, it has two children: the add operator, providing the
-result of C<$b+$c>, is uppermost on line 5, and the left hand side is on
-line 10.
-
-Line 10 is the null op: this does exactly nothing. What is that doing
-there? If you see the null op, it's a sign that something has been
-optimized away after parsing. As we mentioned in L</Optimization>,
-the optimization stage sometimes converts two operations into one, for
-example when fetching a scalar variable. When this happens, instead of
-rewriting the op tree and cleaning up the dangling pointers, it's easier
-just to replace the redundant operation with the null op. Originally,
-the tree would have looked like this:
-
- 10 SVOP (0x816b4f0) rv2sv [15]
- 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
-
-That is, fetch the C<a> entry from the main symbol table, and then look
-at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
-happens to do both these things.
-
-The right hand side, starting at line 5 is similar to what we've just
-seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together
-two C<gvsv>s.
-
-Now, what's this about?
-
- 1 LISTOP (0x8179888) leave
- 2 OP (0x81798b0) enter
- 3 COP (0x8179850) nextstate
-
-C<enter> and C<leave> are scoping ops, and their job is to perform any
-housekeeping every time you enter and leave a block: lexical variables
-are tidied up, unreferenced variables are destroyed, and so on. Every
-program will have those first three lines: C<leave> is a list, and its
-children are all the statements in the block. Statements are delimited
-by C<nextstate>, so a block is a collection of C<nextstate> ops, with
-the ops to be performed for each statement being the children of
-C<nextstate>. C<enter> is a single op which functions as a marker.
-
-That's how Perl parsed the program, from top to bottom:
-
- Program
- |
- Statement
- |
- =
- / \
- / \
- $a +
- / \
- $b $c
-
-However, it's impossible to B<perform> the operations in this order:
-you have to find the values of C<$b> and C<$c> before you add them
-together, for instance. So, the other thread that runs through the op
-tree is the execution order: each op has a field C<op_next> which points
-to the next op to be run, so following these pointers tells us how perl
-executes the code. We can traverse the tree in this order using
-the C<exec> option to C<B::Terse>:
-
- % perl -MO=Terse,exec -e '$a=$b+$c'
- 1 OP (0x8179928) enter
- 2 COP (0x81798c8) nextstate
- 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
- 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
- 5 BINOP (0x8179878) add [1]
- 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
- 7 BINOP (0x81798a0) sassign
- 8 LISTOP (0x8179900) leave
-
-This probably makes more sense for a human: enter a block, start a
-statement. Get the values of C<$b> and C<$c>, and add them together.
-Find C<$a>, and assign one to the other. Then leave.
-
-The way Perl builds up these op trees in the parsing process can be
-unravelled by examining F<perly.y>, the YACC grammar. Let's take the
-piece we need to construct the tree for C<$a = $b + $c>
-
- 1 term : term ASSIGNOP term
- 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
- 3 | term ADDOP term
- 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
-
-If you're not used to reading BNF grammars, this is how it works: You're
-fed certain things by the tokeniser, which generally end up in upper
-case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your
-code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are
-"terminal symbols", because you can't get any simpler than them.
-
-The grammar, lines one and three of the snippet above, tells you how to
-build up more complex forms. These complex forms, "non-terminal symbols"
-are generally placed in lower case. C<term> here is a non-terminal
-symbol, representing a single expression.
-
-The grammar gives you the following rule: you can make the thing on the
-left of the colon if you see all the things on the right in sequence.
-This is called a "reduction", and the aim of parsing is to completely
-reduce the input. There are several different ways you can perform a
-reduction, separated by vertical bars: so, C<term> followed by C<=>
-followed by C<term> makes a C<term>, and C<term> followed by C<+>
-followed by C<term> can also make a C<term>.
-
-So, if you see two terms with an C<=> or C<+>, between them, you can
-turn them into a single expression. When you do this, you execute the
-code in the block on the next line: if you see C<=>, you'll do the code
-in line 2. If you see C<+>, you'll do the code in line 4. It's this code
-which contributes to the op tree.
-
- | term ADDOP term
- { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
-
-What this does is creates a new binary op, and feeds it a number of
-variables. The variables refer to the tokens: C<$1> is the first token in
-the input, C<$2> the second, and so on - think regular expression
-backreferences. C<$$> is the op returned from this reduction. So, we
-call C<newBINOP> to create a new binary operator. The first parameter to
-C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
-operator, so we want the type to be C<ADDOP>. We could specify this
-directly, but it's right there as the second token in the input, so we
-use C<$2>. The second parameter is the op's flags: 0 means "nothing
-special". Then the things to add: the left and right hand side of our
-expression, in scalar context.
-
-=head2 Stacks
-
-When perl executes something like C<addop>, how does it pass on its
-results to the next op? The answer is, through the use of stacks. Perl
-has a number of stacks to store things it's currently working on, and
-we'll look at the three most important ones here.
-
-=over 3
-
-=item Argument stack
-
-Arguments are passed to PP code and returned from PP code using the
-argument stack, C<ST>. The typical way to handle arguments is to pop
-them off the stack, deal with them how you wish, and then push the result
-back onto the stack. This is how, for instance, the cosine operator
-works:
-
- NV value;
- value = POPn;
- value = Perl_cos(value);
- XPUSHn(value);
-
-We'll see a more tricky example of this when we consider Perl's macros
-below. C<POPn> gives you the NV (floating point value) of the top SV on
-the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
-the result back as an NV. The C<X> in C<XPUSHn> means that the stack
-should be extended if necessary - it can't be necessary here, because we
-know there's room for one more item on the stack, since we've just
-removed one! The C<XPUSH*> macros at least guarantee safety.
-
-Alternatively, you can fiddle with the stack directly: C<SP> gives you
-the first element in your portion of the stack, and C<TOP*> gives you
-the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
-negation of an integer:
-
- SETi(-TOPi);
-
-Just set the integer value of the top stack entry to its negation.
-
-Argument stack manipulation in the core is exactly the same as it is in
-XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
-description of the macros used in stack manipulation.
-
-=item Mark stack
-
-I say "your portion of the stack" above because PP code doesn't
-necessarily get the whole stack to itself: if your function calls
-another function, you'll only want to expose the arguments aimed for the
-called function, and not (necessarily) let it get at your own data. The
-way we do this is to have a "virtual" bottom-of-stack, exposed to each
-function. The mark stack keeps bookmarks to locations in the argument
-stack usable by each function. For instance, when dealing with a tied
-variable, (internally, something with "P" magic) Perl has to call
-methods for accesses to the tied variables. However, we need to separate
-the arguments exposed to the method to the argument exposed to the
-original function - the store or fetch or whatever it may be. Here's
-roughly how the tied C<push> is implemented; see C<av_push> in F<av.c>:
-
- 1 PUSHMARK(SP);
- 2 EXTEND(SP,2);
- 3 PUSHs(SvTIED_obj((SV*)av, mg));
- 4 PUSHs(val);
- 5 PUTBACK;
- 6 ENTER;
- 7 call_method("PUSH", G_SCALAR|G_DISCARD);
- 8 LEAVE;
-
-Let's examine the whole implementation, for practice:
-
- 1 PUSHMARK(SP);
-
-Push the current state of the stack pointer onto the mark stack. This is
-so that when we've finished adding items to the argument stack, Perl
-knows how many things we've added recently.
-
- 2 EXTEND(SP,2);
- 3 PUSHs(SvTIED_obj((SV*)av, mg));
- 4 PUSHs(val);
-
-We're going to add two more items onto the argument stack: when you have
-a tied array, the C<PUSH> subroutine receives the object and the value
-to be pushed, and that's exactly what we have here - the tied object,
-retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
-
- 5 PUTBACK;
-
-Next we tell Perl to update the global stack pointer from our internal
-variable: C<dSP> only gave us a local copy, not a reference to the global.
-
- 6 ENTER;
- 7 call_method("PUSH", G_SCALAR|G_DISCARD);
- 8 LEAVE;
-
-C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
-variables are tidied up, everything that has been localised gets
-its previous value returned, and so on. Think of them as the C<{> and
-C<}> of a Perl block.
-
-To actually do the magic method call, we have to call a subroutine in
-Perl space: C<call_method> takes care of that, and it's described in
-L<perlcall>. We call the C<PUSH> method in scalar context, and we're
-going to discard its return value. The call_method() function
-removes the top element of the mark stack, so there is nothing for
-the caller to clean up.
-
-=item Save stack
-
-C doesn't have a concept of local scope, so perl provides one. We've
-seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
-stack implements the C equivalent of, for example:
-
- {
- local $foo = 42;
- ...
- }
-
-See L<perlguts/Localising Changes> for how to use the save stack.
-
-=back
-
-=head2 Millions of Macros
-
-One thing you'll notice about the Perl source is that it's full of
-macros. Some have called the pervasive use of macros the hardest thing
-to understand, others find it adds to clarity. Let's take an example,
-the code which implements the addition operator:
-
- 1 PP(pp_add)
- 2 {
- 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
- 4 {
- 5 dPOPTOPnnrl_ul;
- 6 SETn( left + right );
- 7 RETURN;
- 8 }
- 9 }
-
-Every line here (apart from the braces, of course) contains a macro. The
-first line sets up the function declaration as Perl expects for PP code;
-line 3 sets up variable declarations for the argument stack and the
-target, the return value of the operation. Finally, it tries to see if
-the addition operation is overloaded; if so, the appropriate subroutine
-is called.
-
-Line 5 is another variable declaration - all variable declarations start
-with C<d> - which pops from the top of the argument stack two NVs (hence
-C<nn>) and puts them into the variables C<right> and C<left>, hence the
-C<rl>. These are the two operands to the addition operator. Next, we
-call C<SETn> to set the NV of the return value to the result of adding
-the two values. This done, we return - the C<RETURN> macro makes sure
-that our return value is properly handled, and we pass the next operator
-to run back to the main run loop.
-
-Most of these macros are explained in L<perlapi>, and some of the more
-important ones are explained in L<perlxs> as well. Pay special attention
-to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
-the C<[pad]THX_?> macros.
-
-=head2 The .i Targets
-
-You can expand the macros in a F<foo.c> file by saying
-
- make foo.i
-
-which will expand the macros using cpp. Don't be scared by the results.
-
-=head1 TESTING
-
-Every module and built-in function has an associated test file (or
-should...). If you add or change functionality, you have to write a
-test. If you fix a bug, you have to write a test so that bug never
-comes back. If you alter the docs, it would be nice to test what the
-new documentation says.
-
-In short, if you submit a patch you probably also have to patch the
-tests.
-
-=head2 Where to find test files
-
-For modules, the test file is right next to the module itself.
-F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation,
-so there are some snags (and it would be wonderful for you to brush
-them out), but it basically works that way. Everything else lives in
-F<t/>.
-
-Testing of warning messages is often separately done by using expect scripts in
-F<t/lib/warnings>. This is because much of the setup for them is already done
-for you.
-
-If you add a new test directory under F<t/>, it is imperative that you
-add that directory to F<t/HARNESS> and F<t/TEST>.
-
-=over 3
-
-=item F<t/base/>
-
-Testing of the absolute basic functionality of Perl. Things like
-C<if>, basic file reads and writes, simple regexes, etc. These are
-run first in the test suite and if any of them fail, something is
-I<really> broken.
-
-=item F<t/cmd/>
-
-These test the basic control structures, C<if/else>, C<while>,
-subroutines, etc.
-
-=item F<t/comp/>
-
-Tests basic issues of how Perl parses and compiles itself.
-
-=item F<t/io/>
-
-Tests for built-in IO functions, including command line arguments.
-
-=item F<t/lib/>
-
-The old home for the module tests, you shouldn't put anything new in
-here. There are still some bits and pieces hanging around in here
-that need to be moved. Perhaps you could move them? Thanks!
-
-=item F<t/mro/>
-
-Tests for perl's method resolution order implementations
-(see L<mro>).
-
-=item F<t/op/>
-
-Tests for perl's built in functions that don't fit into any of the
-other directories.
-
-=item F<t/re/>
-
-Tests for regex related functions or behaviour. (These used to live
-in t/op).
-
-=item F<t/run/>
-
-Testing features of how perl actually runs, including exit codes and
-handling of PERL* environment variables.
-
-=item F<t/uni/>
-
-Tests for the core support of Unicode.
-
-=item F<t/win32/>
-
-Windows-specific tests.
-
-=item F<t/x2p>
-
-A test suite for the s2p converter.
-
-=back
-
-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 C<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.
-If you write your own test, use the L<Test Anything Protocol|TAP>.
-
-=over 4
-
-=item t/base 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.
-
-=item t/cmd t/run t/io t/op
-
-Now that basic require() and subroutines are tested, you can use the
-t/test.pl library which emulates the important features of Test::More
-while using a minimum of core features.
-
-You can also conditionally use certain libraries like Config, but be
-sure to skip the test gracefully if it's not there.
-
-=item t/lib ext lib
-
-Now that the core of Perl is tested, Test::More can be used. You can
-also use the full suite of core modules in the tests.
-
-=back
-
-When you say "make test" Perl uses the F<t/TEST> program to run the
-test suite (except under Win32 where it uses F<t/harness> instead.)
-All tests are run from the F<t/> directory, B<not> the directory
-which contains the test. This causes some problems with the tests
-in F<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 C<fork()> and
-C<system()> unless absolutely necessary.
-
-=head2 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.
-
-=over 4
-
-=item coretest
-
-Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests).
-
-(Not available on Win32)
-
-=item test.deparse
-
-Run all the tests through B::Deparse. Not all tests will succeed.
-
-(Not available on Win32)
-
-=item test.taintwarn
-
-Run all tests with the B<-t> command-line switch. Not all tests
-are expected to succeed (until they're specifically fixed, of course).
-
-(Not available on Win32)
-
-=item minitest
-
-Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
-F<t/op>, F<t/uni> and F<t/mro> tests.
-
-=item 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
-F<testname.valgrind>.
-
-=item test.third check.third utest.third ucheck.third
-
-(Only in Tru64) Run all the tests using the memory leak + naughty
-memory access tool "Third Degree". The log files will be named
-F<perl.3log.testname>.
-
-=item 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, F<t/japh/abigail.t>.
-
-You can also run the torture test with F<t/harness> by giving
-C<-torture> argument to F<t/harness>.
-
-=item utest ucheck test.utf8 check.utf8
-
-Run all the tests with -Mutf8. Not all tests will succeed.
-
-(Not available on Win32)
-
-=item minitest.utf16 test.utf16
-
-Runs the tests with UTF-16 encoded scripts, encoded with different
-versions of this encoding.
-
-C<make utest.utf16> runs the test suite with a combination of C<-utf8> and
-C<-utf16> arguments to F<t/TEST>.
-
-(Not available on Win32)
-
-=item test_harness
-
-Run the test suite with the F<t/harness> controlling program, instead of
-F<t/TEST>. F<t/harness> is more sophisticated, and uses the
-L<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 F<t/TEST>, it
-doesn't redirect stderr to stdout.
-
-Note that under Win32 F<t/harness> is always used instead of F<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 F<t/harness>. This means
-you can say
-
- nmake test TEST_FILES="op/*.t"
- nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
-
-=item Parallel tests
-
-The core distribution can now run its regression tests in parallel on
-Unix-like platforms. Instead of running C<make test>, set C<TEST_JOBS> in
-your environment to the number of tests to run in parallel, and run
-C<make test_harness>. On a Bourne-like shell, this can be done as
-
- TEST_JOBS=3 make test_harness # Run 3 tests in parallel
-
-An environment variable is used, rather than parallel make itself, because
-L<TAP::Harness> needs to be able to schedule individual non-conflicting test
-scripts itself, and there is no standard interface to C<make> utilities to
-interact with their job schedulers.
-
-Note that currently some test scripts may fail when run in parallel (most
-notably C<ext/IO/t/io_dir.t>). If necessary run just the failing scripts
-again sequentially and see if the failures go away.
-=item test-notty test_notty
-
-Sets PERL_SKIP_TTY_TEST to true before running normal test.
-
-=back
-
-=head2 Running tests by hand
-
-You can run part of the test suite by hand by using one the following
-commands from the F<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.)
-
-=head3 Using t/harness for testing
-
-If you use C<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 C<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.
-
-=over 4
-
-=item -v
-
-Run the tests under verbose mode so you can see what tests were run,
-and debug output.
-
-=item -torture
-
-Run the torture tests as well as the normal set.
-
-=item -re=PATTERN
-
-Filter the file list so that all the test files run match PATTERN.
-Note that this form is distinct from the B<-re LIST OF PATTERNS> form below
-in that it allows the file list to be provided as well.
-
-=item -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.
-
-=back
-
-You can run an individual test by a command similar to
-
- ./perl -I../lib patho/to/foo.t
-
-except that the harnesses set up some environment variables that may
-affect the execution of the test :
-
-=over 4
-
-=item PERL_CORE=1
-
-indicates that we're running this test part of the perl core test suite.
-This is useful for modules that have a dual life on CPAN.
-
-=item PERL_DESTRUCT_LEVEL=2
-
-is set to 2 if it isn't set already (see L</PERL_DESTRUCT_LEVEL>)
-
-=item PERL
-
-(used only by F<t/TEST>) if set, overrides the path to the perl executable
-that should be used to run the tests (the default being F<./perl>).
-
-=item PERL_SKIP_TTY_TEST
-
-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'.
-
-=back
-
-=head3 Other environment variables that may influence tests
-
-=over 4
-
-=item 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 L<perl58delta>.
-
-=item PERL_TEST_NOVREXX
-
-Setting this variable skips the vrexx.t tests for OS2::REXX.
-
-=item PERL_TEST_NUMCONVERTS
-
-This sets a variable in op/numconvert.t.
-
-=back
-
-See also the documentation for the Test and Test::Harness modules,
-for more environment variables that affect testing.
-
-=head1 EXAMPLE OF A SIMPLE PATCH
-
-All right, we've now had a look at how to navigate the Perl sources and
-some things you'll need to know when fiddling with them. Let's now get
-on and create a simple patch. Here's something Larry suggested: if a
-C<U> is the first active format during a C<pack>, (for example,
-C<pack "U3C8", @stuff>) then the resulting string should be treated as
-UTF-8 encoded.
-
-If you are working with a git clone of the Perl repository, you will want to
-create a branch for your changes. This will make creating a proper patch much
-simpler. See the L<perlrepository> for details on how to do this.
-
-=head2 Writing the patch
-
-How do we prepare to fix this up? First we locate the code in question -
-the C<pack> happens at runtime, so it's going to be in one of the F<pp>
-files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
-altering this file, let's copy it to F<pp.c~>.
-
-[Well, it was in F<pp.c> when this tutorial was written. It has now been
-split off with C<pp_unpack> to its own file, F<pp_pack.c>]
-
-Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
-loop over the pattern, taking each format character in turn into
-C<datum_type>. Then for each possible format character, we swallow up
-the other arguments in the pattern (a field width, an asterisk, and so
-on) and convert the next chunk input into the specified format, adding
-it onto the output SV C<cat>.
-
-How do we know if the C<U> is the first format in the C<pat>? Well, if
-we have a pointer to the start of C<pat> then, if we see a C<U> we can
-test whether we're still at the start of the string. So, here's where
-C<pat> is set up:
-
- STRLEN fromlen;
- register char *pat = SvPVx(*++MARK, fromlen);
- register char *patend = pat + fromlen;
- register I32 len;
- I32 datumtype;
- SV *fromstr;
-
-We'll have another string pointer in there:
-
- STRLEN fromlen;
- register char *pat = SvPVx(*++MARK, fromlen);
- register char *patend = pat + fromlen;
- + char *patcopy;
- register I32 len;
- I32 datumtype;
- SV *fromstr;
-
-And just before we start the loop, we'll set C<patcopy> to be the start
-of C<pat>:
-
- items = SP - MARK;
- MARK++;
- sv_setpvn(cat, "", 0);
- + patcopy = pat;
- while (pat < patend) {
-
-Now if we see a C<U> which was at the start of the string, we turn on
-the C<UTF8> flag for the output SV, C<cat>:
-
- + if (datumtype == 'U' && pat==patcopy+1)
- + SvUTF8_on(cat);
- if (datumtype == '#') {
- while (pat < patend && *pat != '\n')
- pat++;
-
-Remember that it has to be C<patcopy+1> because the first character of
-the string is the C<U> which has been swallowed into C<datumtype!>
-
-Oops, we forgot one thing: what if there are spaces at the start of the
-pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
-character, even though it's not the first thing in the pattern. In this
-case, we have to advance C<patcopy> along with C<pat> when we see spaces:
-
- if (isSPACE(datumtype))
- continue;
-
-needs to become
-
- if (isSPACE(datumtype)) {
- patcopy++;
- continue;
- }
-
-OK. That's the C part done. Now we must do two additional things before
-this patch is ready to go: we've changed the behaviour of Perl, and so
-we must document that change. We must also provide some more regression
-tests to make sure our patch works and doesn't create a bug somewhere
-else along the line.
-
-=head2 Testing the patch
-
-The regression tests for each operator live in F<t/op/>, and so we
-make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our
-tests to the end. First, we'll test that the C<U> does indeed create
-Unicode strings.
-
-t/op/pack.t has a sensible ok() function, but if it didn't we could
-use the one from t/test.pl.
-
- require './test.pl';
- plan( tests => 159 );
-
-so instead of this:
-
- print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
- pack("U*",1,20,300,4000);
- print "ok $test\n"; $test++;
-
-we can write the more sensible (see L<Test::More> for a full
-explanation of is() and other testing functions).
-
- is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
- "U* produces Unicode" );
-
-Now we'll test that we got that space-at-the-beginning business right:
-
- is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
- " with spaces at the beginning" );
-
-And finally we'll test that we don't make Unicode strings if C<U> is B<not>
-the first active format:
-
- isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
- "U* not first isn't Unicode" );
-
-Mustn't forget to change the number of tests which appears at the top,
-or else the automated tester will get confused. This will either look
-like this:
-
- print "1..156\n";
-
-or this:
-
- plan( tests => 156 );
-
-We now compile up Perl, and run it through the test suite. Our new
-tests pass, hooray!
-
-=head2 Documenting the patch
-
-Finally, the documentation. The job is never done until the paperwork is
-over, so let's describe the change we've just made. The relevant place
-is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
-this text in the description of C<pack>:
-
- =item *
-
- If the pattern begins with a C<U>, the resulting string will be treated
- as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
- with an initial C<U0>, and the bytes that follow will be interpreted as
- Unicode characters. If you don't want this to happen, you can begin
- your pattern with C<C0> (or anything else) to force Perl not to UTF-8
- encode your string, and then follow this with a C<U*> somewhere in your
- pattern.
-
-=head1 COMMON PROBLEMS
-
-Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
-some cases we have to take pre-ANSI requirements into consideration.
-You don't care about some particular platform having broken Perl?
-I hear there is still a strong demand for J2EE programmers.
-
-=head2 Perl environment problems
-
-=over 4
-
-=item *
-
-Not compiling with threading
-
-Compiling with threading (-Duseithreads) completely rewrites
-the function prototypes of Perl. You better try your changes
-with that. Related to this is the difference between "Perl_-less"
-and "Perl_-ly" APIs, for example:
-
- Perl_sv_setiv(aTHX_ ...);
- sv_setiv(...);
-
-The first one explicitly passes in the context, which is needed for e.g.
-threaded builds. The second one does that implicitly; do not get them
-mixed. If you are not passing in a aTHX_, you will need to do a dTHX
-(or a dVAR) as the first thing in the function.
-
-See L<perlguts/"How multiple interpreters and concurrency are supported">
-for further discussion about context.
-
-=item *
-
-Not compiling with -DDEBUGGING
-
-The DEBUGGING define exposes more code to the compiler,
-therefore more ways for things to go wrong. You should try it.
-
-=item *
-
-Introducing (non-read-only) globals
-
-Do not introduce any modifiable globals, truly global or file static.
-They are bad form and complicate multithreading and other forms of
-concurrency. The right way is to introduce them as new interpreter
-variables, see F<intrpvar.h> (at the very end for binary compatibility).
-
-Introducing read-only (const) globals is okay, as long as you verify
-with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
-BSD-style output) that the data you added really is read-only.
-(If it is, it shouldn't show up in the output of that command.)
-
-If you want to have static strings, make them constant:
-
- static const char etc[] = "...";
-
-If you want to have arrays of constant strings, note carefully
-the right combination of C<const>s:
-
- static const char * const yippee[] =
- {"hi", "ho", "silver"};
-
-There is a way to completely hide any modifiable globals (they are all
-moved to heap), the compilation setting C<-DPERL_GLOBAL_STRUCT_PRIVATE>.
-It is not normally used, but can be used for testing, read more
-about it in L<perlguts/"Background and PERL_IMPLICIT_CONTEXT">.
-
-=item *
-
-Not exporting your new function
-
-Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
-function that is part of the public API (the shared Perl library)
-to be explicitly marked as exported. See the discussion about
-F<embed.pl> in L<perlguts>.
-
-=item *
-
-Exporting your new function
-
-The new shiny result of either genuine new functionality or your
-arduous refactoring is now ready and correctly exported. So what
-could possibly go wrong?
-
-Maybe simply that your function did not need to be exported in the
-first place. Perl has a long and not so glorious history of exporting
-functions that it should not have.
-
-If the function is used only inside one source code file, make it
-static. See the discussion about F<embed.pl> in L<perlguts>.
-
-If the function is used across several files, but intended only for
-Perl's internal use (and this should be the common case), do not
-export it to the public API. See the discussion about F<embed.pl>
-in L<perlguts>.
-
-=back
-
-=head2 Portability problems
-
-The following are common causes of compilation and/or execution
-failures, not common to Perl as such. The C FAQ is good bedtime
-reading. Please test your changes with as many C compilers and
-platforms as possible; we will, anyway, and it's nice to save
-oneself from public embarrassment.
-
-If using gcc, you can add the C<-std=c89> option which will hopefully
-catch most of these unportabilities. (However it might also catch
-incompatibilities in your system's header files.)
-
-Use the Configure C<-Dgccansipedantic> flag to enable the gcc
-C<-ansi -pedantic> flags which enforce stricter ANSI rules.
-
-If using the C<gcc -Wall> note that not all the possible warnings
-(like C<-Wunitialized>) are given unless you also compile with C<-O>.
-
-Note that if using gcc, starting from Perl 5.9.5 the Perl core source
-code files (the ones at the top level of the source code distribution,
-but not e.g. the extensions under ext/) are automatically compiled
-with as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>,
-and a selection of C<-W> flags (see cflags.SH).
-
-Also study L<perlport> carefully to avoid any bad assumptions
-about the operating system, filesystems, and so forth.
-
-You may once in a while try a "make microperl" to see whether we
-can still compile Perl with just the bare minimum of interfaces.
-(See README.micro.)
-
-Do not assume an operating system indicates a certain compiler.
-
-=over 4
-
-=item *
-
-Casting pointers to integers or casting integers to pointers
-
- void castaway(U8* p)
- {
- IV i = p;
-
-or
+=encoding utf8
- void castaway(U8* p)
- {
- IV i = (IV)p;
+=for comment
+Consistent formatting of this file is achieved with:
+ perl ./Porting/podtidy pod/perlhack.pod
-Both are bad, and broken, and unportable. Use the PTR2IV()
-macro that does it right. (Likewise, there are PTR2UV(), PTR2NV(),
-INT2PTR(), and NUM2PTR().)
-
-=item *
-
-Casting between data function pointers and data pointers
-
-Technically speaking casting between function pointers and data
-pointers is unportable and undefined, but practically speaking
-it seems to work, but you should use the FPTR2DPTR() and DPTR2FPTR()
-macros. Sometimes you can also play games with unions.
+=head1 NAME
-=item *
+perlhack - How to hack on Perl
-Assuming sizeof(int) == sizeof(long)
+=head1 DESCRIPTION
-There are platforms where longs are 64 bits, and platforms where ints
-are 64 bits, and while we are out to shock you, even platforms where
-shorts are 64 bits. This is all legal according to the C standard.
-(In other words, "long long" is not a portable way to specify 64 bits,
-and "long long" is not even guaranteed to be any wider than "long".)
+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.
-Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
-Avoid things like I32 because they are B<not> guaranteed to be
-I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
-guaranteed to be B<int> or B<long>. If you really explicitly need
-64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
+=head1 SUPER QUICK PATCH GUIDE
-=item *
+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:
-Assuming one can dereference any type of pointer for any type of data
+=over 4
- char *p = ...;
- long pony = *p; /* BAD */
+=item * Check out the source repository
-Many platforms, quite rightly so, will give you a core dump instead
-of a pony if the p happens not be correctly aligned.
+The perl source is in a git repository. You can clone the repository
+with the following command:
-=item *
+ % git clone git://perl5.git.perl.org/perl.git perl
-Lvalue casts
+=item * Make your change
- (int)*p = ...; /* BAD */
+Hack, hack, hack.
-Simply not portable. Get your lvalue to be of the right type,
-or maybe use temporary variables, or dirty tricks with unions.
+=item * Test your change
-=item *
+You can run all the tests with the following commands:
-Assume B<anything> about structs (especially the ones you
-don't control, like the ones coming from the system headers)
+ % ./Configure -des -Dusedevel
+ % make test
-=over 8
+Keep hacking until the tests pass.
-=item *
+=item * Commit your change
-That a certain field exists in a struct
+Commiting your work will save the change I<on your local system>:
-=item *
+ % git commit -a -m 'Commit message goes here'
-That no other fields exist besides the ones you know of
+Make sure the commit message describes your change in a single
+sentence. For example, "Fixed spelling errors in perlhack.pod".
-=item *
+=item * Send your change to perlbug
-That a field is of certain signedness, sizeof, or type
+The next step is to submit your patch to the Perl core ticket system
+via email.
-=item *
+Assuming your patch consists of a single git commit, you can send it to
+perlbug with this command line:
-That the fields are in a certain order
+ % git format-patch HEAD^1..HEAD
+ % perlbug -s '[PATCH] `git log --pretty=format:%s HEAD^1..HEAD`' -f 0001-*.patch
-=over 8
+The perlbug program will ask you a few questions about your email
+address and the patch you're submitting. Once you've answered them you
+can submit your patch.
-=item *
+=item * Thank you
-While C guarantees the ordering specified in the struct definition,
-between different platforms the definitions might differ
+The porters appreciate the time you spent helping to make Perl better.
+Thank you!
=back
-=item *
-
-That the sizeof(struct) or the alignments are the same everywhere
-
-=over 8
-
-=item *
-
-There might be padding bytes between the fields to align the fields -
-the bytes can be anything
+=head1 BUG REPORTING
-=item *
+If you want to report a bug in Perl you must use the F<perlbug> command
+line tool. This tool will ensure that your bug report includes all the
+relevant system and configuration information.
-Structs are required to be aligned to the maximum alignment required
-by the fields - which for native types is for usually equivalent to
-sizeof() of the field
+To browse existing Perl bugs and patches, you can use the web interface
+at L<http://rt.perl.org>.
-=back
+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 already.
-=back
+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.
-=item *
+=head1 PERL 5 PORTERS
-Assuming the character set is ASCIIish
-
-Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
-This is transparent for the most part, but because the character sets
-differ, you shouldn't use numeric (decimal, octal, nor hex) constants
-to refer to characters. You can safely say 'A', but not 0x41.
-You can safely say '\n', but not \012.
-If a character doesn't have a trivial input form, you can
-create a #define for it in both C<utfebcdic.h> and C<utf8.h>, so that
-it resolves to different values depending on the character set being used.
-(There are three different EBCDIC character sets defined in C<utfebcdic.h>,
-so it might be best to insert the #define three times in that file.)
-
-Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper case
-alphabetic characters. That is not true in EBCDIC. Nor for 'a' to 'z'.
-But '0' - '9' is an unbroken range in both systems. Don't assume anything
-about other ranges.
-
-Many of the comments in the existing code ignore the possibility of EBCDIC,
-and may be wrong therefore, even if the code works.
-This is actually a tribute to the successful transparent insertion of being
-able to handle EBCDIC without having to change pre-existing code.
-
-UTF-8 and UTF-EBCDIC are two different encodings used to represent Unicode
-code points as sequences of bytes. Macros
-with the same names (but different definitions)
-in C<utf8.h> and C<utfebcdic.h>
-are used to allow the calling code to think that there is only one such
-encoding.
-This is almost always referred to as C<utf8>, but it means the EBCDIC version
-as well. Again, comments in the code may well be wrong even if the code itself
-is right.
-For example, the concept of C<invariant characters> differs between ASCII and
-EBCDIC.
-On ASCII platforms, only characters that do not have the high-order
-bit set (i.e. whose ordinals are strict ASCII, 0 - 127)
-are invariant, and the documentation and comments in the code
-may assume that,
-often referring to something like, say, C<hibit>.
-The situation differs and is not so simple on EBCDIC machines, but as long as
-the code itself uses the C<NATIVE_IS_INVARIANT()> macro appropriately, it
-works, even if the comments are wrong.
+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", or just the "porters".
-=item *
+A searchable archive of the list is available at
+L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/>. There is
+also another archive at
+L<http://archive.develooper.com/perl5-porters@perl.org/>.
-Assuming the character set is just ASCII
+=head2 perl-changes mailing list
-ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
-characters have different meanings depending on the locale. Absent a locale,
-currently these extra characters are generally considered to be unassigned,
-and this has presented some problems.
-This is being changed starting in 5.12 so that these characters will
-be considered to be Latin-1 (ISO-8859-1).
+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 L<http://lists.perl.org/list/perl5-changes.html> for
+subscription and archive information.
-=item *
+=head1 GETTING THE PERL SOURCE
-Mixing #define and #ifdef
+All of Perl's source code is kept centrally in a Git repository at
+I<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.
- #define BURGLE(x) ... \
- #ifdef BURGLE_OLD_STYLE /* BAD */
- ... do it the old way ... \
- #else
- ... do it the new way ... \
- #endif
+For much more detail on using git with the Perl repository, please see
+L<perlgit>.
-You cannot portably "stack" cpp directives. For example in the above
-you need two separate BURGLE() #defines, one for each #ifdef branch.
+=head2 Read access via Git
-=item *
+You will need a copy of Git for your computer. You can fetch a copy of
+the repository using the git protocol:
-Adding non-comment stuff after #endif or #else
+ % git clone git://perl5.git.perl.org/perl.git perl
- #ifdef SNOSH
- ...
- #else !SNOSH /* BAD */
- ...
- #endif SNOSH /* BAD */
+This clones the repository and makes a local copy in the F<perl>
+directory.
-The #endif and #else cannot portably have anything non-comment after
-them. If you want to document what is going (which is a good idea
-especially if the branches are long), use (C) comments:
+If you cannot use the git protocol for firewall reasons, you can also
+clone via http, though this is much slower:
- #ifdef SNOSH
- ...
- #else /* !SNOSH */
- ...
- #endif /* SNOSH */
+ % git clone http://perl5.git.perl.org/perl.git perl
-The gcc option C<-Wendif-labels> warns about the bad variant
-(by default on starting from Perl 5.9.4).
+=head2 Read access via the web
-=item *
+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
+L<http://perl5.git.perl.org/perl.git>. A mirror of the repository is
+found at L<http://github.com/mirrors/perl>
-Having a comma after the last element of an enum list
+=head2 Read access via rsync
- enum color {
- CERULEAN,
- CHARTREUSE,
- CINNABAR, /* BAD */
- };
+You can also choose to use rsync to get a copy of the current source
+tree for the bleadperl branch and all maintenance branches:
-is not portable. Leave out the last comma.
+ $ 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 .
-Also note that whether enums are implicitly morphable to ints
-varies between compilers, you might need to (int).
+(Add the C<--delete> option to remove leftover files)
-=item *
+To get a full list of the available sync points:
-Using //-comments
+ $ rsync perl5.git.perl.org::
- // This function bamfoodles the zorklator. /* BAD */
+=head2 Write access via git
-That is C99 or C++. Perl is C89. Using the //-comments is silently
-allowed by many C compilers but cranking up the ANSI C89 strictness
-(which we like to do) causes the compilation to fail.
+If you have a commit bit, please see L<perlgit> for more details on
+using git.
-=item *
+=head1 PATCHING PERL
-Mixing declarations and code
+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.
- void zorklator()
- {
- int n = 3;
- set_zorkmids(n); /* BAD */
- int q = 4;
+=head2 Submitting patches
-That is C99 or C++. Some C compilers allow that, but you shouldn't.
+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.
-The gcc option C<-Wdeclaration-after-statements> scans for such problems
-(by default on starting from Perl 5.9.4).
+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.
-=item *
+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.
-Introducing variables inside for()
+In other cases, the patch will need more work or discussion. That will
+happen on the p5p list.
- for(int i = ...; ...; ...) { /* BAD */
+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.
-That is C99 or C++. While it would indeed be awfully nice to have that
-also in C89, to limit the scope of the loop variable, alas, we cannot.
+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.
-=item *
+=head2 Getting your patch accepted
-Mixing signed char pointers with unsigned char pointers
+If you are submitting a code patch there are several things that you
+can do to help the Perl 5 Porters accept your patch.
- int foo(char *s) { ... }
- ...
- unsigned char *t = ...; /* Or U8* t = ... */
- foo(t); /* BAD */
+=head3 Commit message
-While this is legal practice, it is certainly dubious, and downright
-fatal in at least one platform: for example VMS cc considers this a
-fatal error. One cause for people often making this mistake is that a
-"naked char" and therefore dereferencing a "naked char pointer" have
-an undefined signedness: it depends on the compiler and the flags of
-the compiler and the underlying platform whether the result is signed
-or unsigned. For this very same reason using a 'char' as an array
-index is bad.
+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.
-=item *
+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
+E-Mail, 50 characters being a good rule of thumb.
-Macros that have string constants and their arguments as substrings of
-the string constants
+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.
- #define FOO(n) printf("number = %d\n", n) /* BAD */
- FOO(10);
+The commit message should include a description of the problem that the
+patch corrects or new functionality that the patch adds.
-Pre-ANSI semantics for that was equivalent to
+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.
- printf("10umber = %d\10");
+=over 4
-which is probably not what you were expecting. Unfortunately at least
-one reasonably common and modern C compiler does "real backward
-compatibility" here, in AIX that is what still happens even though the
-rest of the AIX compiler is very happily C89.
+=item * Why
-=item *
+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.
-Using printf formats for non-basic C types
+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.
- IV i = ...;
- printf("i = %d\n", i); /* BAD */
+=item * What
-While this might by accident work in some platform (where IV happens
-to be an C<int>), in general it cannot. IV might be something larger.
-Even worse the situation is with more specific types (defined by Perl's
-configuration step in F<config.h>):
+Your commit message should describe what part of the Perl core you're
+changing and what you expect your patch to do.
- Uid_t who = ...;
- printf("who = %d\n", who); /* BAD */
+=item * How
-The problem here is that Uid_t might be not only not C<int>-wide
-but it might also be unsigned, in which case large uids would be
-printed as negative values.
+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.
-There is no simple solution to this because of printf()'s limited
-intelligence, but for many types the right format is available as
-with either 'f' or '_f' suffix, for example:
+=back
- IVdf /* IV in decimal */
- UVxf /* UV is hexadecimal */
+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.
- printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
+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.
- Uid_t_f /* Uid_t in decimal */
+=head3 Comments, Comments, Comments
- printf("who = %"Uid_t_f"\n", who);
+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.
-Or you can try casting to a "wide enough" type:
+The best comments explain I<why> the code does what it does, not I<what
+it does>.
- printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
+=head3 Style
-Also remember that the C<%p> format really does require a void pointer:
+In general, please follow the particular style of the code you are
+patching.
- U8* p = ...;
- printf("p = %p\n", (void*)p);
+In particular, follow these general guidelines for patching Perl
+sources:
-The gcc option C<-Wformat> scans for such problems.
+=over 4
=item *
-Blindly using variadic macros
-
-gcc has had them for a while with its own syntax, and C99 brought
-them with a standardized syntax. Don't use the former, and use
-the latter only if the HAS_C99_VARIADIC_MACROS is defined.
+8-wide tabs (no exceptions!)
=item *
-Blindly passing va_list
-
-Not all platforms support passing va_list to further varargs (stdarg)
-functions. The right thing to do is to copy the va_list using the
-Perl_va_copy() if the NEED_VA_COPY is defined.
+4-wide indents for code, 2-wide indents for nested CPP #defines
=item *
-Using gcc statement expressions
-
- val = ({...;...;...}); /* BAD */
-
-While a nice extension, it's not portable. The Perl code does
-admittedly use them if available to gain some extra speed
-(essentially as a funky form of inlining), but you shouldn't.
+Try hard not to exceed 79-columns
=item *
-Binding together several statements in a macro
-
-Use the macros STMT_START and STMT_END.
-
- STMT_START {
- ...
- } STMT_END
+ANSI C prototypes
=item *
-Testing for operating systems or versions when should be testing for features
-
- #ifdef __FOONIX__ /* BAD */
- foo = quux();
- #endif
-
-Unless you know with 100% certainty that quux() is only ever available
-for the "Foonix" operating system B<and> that is available B<and>
-correctly working for B<all> past, present, B<and> future versions of
-"Foonix", the above is very wrong. This is more correct (though still
-not perfect, because the below is a compile-time check):
-
- #ifdef HAS_QUUX
- foo = quux();
- #endif
-
-How does the HAS_QUUX become defined where it needs to be? Well, if
-Foonix happens to be Unixy enough to be able to run the Configure
-script, and Configure has been taught about detecting and testing
-quux(), the HAS_QUUX will be correctly defined. In other platforms,
-the corresponding configuration step will hopefully do the same.
-
-In a pinch, if you cannot wait for Configure to be educated,
-or if you have a good hunch of where quux() might be available,
-you can temporarily try the following:
-
- #if (defined(__FOONIX__) || defined(__BARNIX__))
- # define HAS_QUUX
- #endif
-
- ...
-
- #ifdef HAS_QUUX
- foo = quux();
- #endif
-
-But in any case, try to keep the features and operating systems separate.
+Uncuddled elses and "K&R" style for indenting control constructs
-=back
-
-=head2 Problematic System Interfaces
+=item *
-=over 4
+No C++ style (//) comments
=item *
-malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
-allocate at least one byte. (In general you should rarely need to
-work at this low level, but instead use the various malloc wrappers.)
+Mark places that need to be revisited with XXX (and revisit often!)
=item *
-snprintf() - the return type is unportable. Use my_snprintf() instead.
+Opening brace lines up with "if" when conditional spans multiple lines;
+should be at end-of-line otherwise
-=back
+=item *
-=head2 Security problems
+In function definitions, name starts in column 0 (return value is on
+previous line)
-Last but not least, here are various tips for safer coding.
+=item *
-=over 4
+Single space after keywords that are followed by parens, no space
+between function name and following paren
=item *
-Do not use gets()
-
-Or we will publicly ridicule you. Seriously.
+Avoid assignments in conditionals, but if they're unavoidable, use
+extra paren, e.g. "if (a && (b = c)) ..."
=item *
-Do not use strcpy() or strcat() or strncpy() or strncat()
-
-Use my_strlcpy() and my_strlcat() instead: they either use the native
-implementation, or Perl's own implementation (borrowed from the public
-domain implementation of INN).
+"return foo;" rather than "return(foo);"
=item *
-Do not use sprintf() or vsprintf()
-
-If you really want just plain byte strings, use my_snprintf()
-and my_vsnprintf() instead, which will try to use snprintf() and
-vsnprintf() if those safer APIs are available. If you want something
-fancier than a plain byte string, use SVs and Perl_sv_catpvf().
+"if (!foo) ..." rather than "if (foo == FALSE) ..." etc.
=back
+=head3 Test suite
-=head1 DEBUGGING
-
-You can compile a special debugging version of Perl, which allows you
-to use the C<-D> option of Perl to tell more about what Perl is doing.
-But sometimes there is no alternative than to dive in with a debugger,
-either to see the stack trace of a core dump (very useful in a bug
-report), or trying to figure out what went wrong before the core dump
-happened, or how did we end up having wrong or unexpected results.
-
-=head2 Poking at Perl
-
-To really poke around with Perl, you'll probably want to build Perl for
-debugging, like this:
-
- ./Configure -d -D optimize=-g
- make
-
-C<-g> is a flag to the C compiler to have it produce debugging
-information which will allow us to step through a running program,
-and to see in which C function we are at (without the debugging
-information we might see only the numerical addresses of the functions,
-which is not very helpful).
+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.
-F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
-enables all the internal debugging code in Perl. There are a whole bunch
-of things you can debug with this: L<perlrun> lists them all, and the
-best way to find out about them is to play about with them. The most
-useful options are probably
+Your test suite additions should generally follow these guidelines
+(courtesy of Gurusamy Sarathy <gsar@activestate.com>):
- l Context (loop) stack processing
- t Trace execution
- o Method and overloading resolution
- c String/numeric conversions
-
-Some of the functionality of the debugging code can be achieved using XS
-modules.
-
- -Dr => use re 'debug'
- -Dx => use O 'Debug'
-
-=head2 Using a source-level debugger
+=over 4
-If the debugging output of C<-D> doesn't help you, it's time to step
-through perl's execution with a source-level debugger.
+=item *
-=over 3
+Know what you're testing. Read the docs, and the source.
=item *
-We'll use C<gdb> for our examples here; the principles will apply to
-any debugger (many vendors call their debugger C<dbx>), but check the
-manual of the one you're using.
-
-=back
+Tend to fail, not succeed.
-To fire up the debugger, type
+=item *
- gdb ./perl
+Interpret results strictly.
-Or if you have a core dump:
+=item *
- gdb ./perl core
+Use unrelated features (this will flush out bizarre interactions).
-You'll want to do that in your Perl source tree so the debugger can read
-the source code. You should see the copyright message, followed by the
-prompt.
+=item *
- (gdb)
+Use non-standard idioms (otherwise you are not testing TIMTOWTDI).
-C<help> will get you into the documentation, but here are the most
-useful commands:
+=item *
-=over 3
+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).
-=item run [args]
+=item *
-Run the program with the given arguments.
+Give meaningful error messages when a test fails.
-=item break function_name
+=item *
-=item break source.c:xxx
+Avoid using qx// and system() unless you are testing for them. If you
+do use them, make sure that you cover _all_ perl platforms.
-Tells the debugger that we'll want to pause execution when we reach
-either the named function (but see L<perlguts/Internal Functions>!) or the given
-line in the named source file.
+=item *
-=item step
+Unlink any temporary files you create.
-Steps through the program a line at a time.
+=item *
-=item next
+Promote unforeseen warnings to errors with $SIG{__WARN__}.
-Steps through the program a line at a time, without descending into
-functions.
+=item *
-=item continue
+Be sure to use the libraries and modules shipped with the version being
+tested, not those that were already installed.
-Run until the next breakpoint.
+=item *
-=item finish
+Add comments to the code explaining what you are testing for.
-Run until the end of the current function, then stop again.
+=item *
-=item 'enter'
+Make updating the '1..42' string unnecessary. Or make sure that you
+update it.
-Just pressing Enter will do the most recent operation again - it's a
-blessing when stepping through miles of source code.
+=item *
-=item print
+Test _all_ behaviors of a given operator, library, or function.
-Execute the given C code and print its results. B<WARNING>: Perl makes
-heavy use of macros, and F<gdb> does not necessarily support macros
-(see later L</"gdb macro support">). You'll have to substitute them
-yourself, or to invoke cpp on the source code files
-(see L</"The .i Targets">)
-So, for instance, you can't say
+Test all optional arguments.
- print SvPV_nolen(sv)
+Test return values in various contexts (boolean, scalar, list, lvalue).
-but you have to say
+Use both global and lexical variables.
- print Perl_sv_2pv_nolen(sv)
+Don't forget the exceptional, pathological cases.
=back
-You may find it helpful to have a "macro dictionary", which you can
-produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
-recursively apply those macros for you.
-
-=head2 gdb macro support
-
-Recent versions of F<gdb> have fairly good macro support, but
-in order to use it you'll need to compile perl with macro definitions
-included in the debugging information. Using F<gcc> version 3.1, this
-means configuring with C<-Doptimize=-g3>. Other compilers might use a
-different switch (if they support debugging macros at all).
-
-=head2 Dumping Perl Data Structures
-
-One way to get around this macro hell is to use the dumping functions in
-F<dump.c>; these work a little like an internal
-L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures
-that you can't get at from Perl. Let's take an example. We'll use the
-C<$a = $b + $c> we used before, but give it a bit of context:
-C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
-
-What about C<pp_add>, the function we examined earlier to implement the
-C<+> operator:
-
- (gdb) break Perl_pp_add
- Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
-
-Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>.
-With the breakpoint in place, we can run our program:
-
- (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
-
-Lots of junk will go past as gdb reads in the relevant source files and
-libraries, and then:
-
- Breakpoint 1, Perl_pp_add () at pp_hot.c:309
- 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
- (gdb) step
- 311 dPOPTOPnnrl_ul;
- (gdb)
-
-We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
-arranges for two C<NV>s to be placed into C<left> and C<right> - let's
-slightly expand it:
-
- #define dPOPTOPnnrl_ul NV right = POPn; \
- SV *leftsv = TOPs; \
- NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
-
-C<POPn> takes the SV from the top of the stack and obtains its NV either
-directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
-C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
-C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
-C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
-
-Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
-convert it. If we step again, we'll find ourselves there:
-
- Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
- 1669 if (!sv)
- (gdb)
-
-We can now use C<Perl_sv_dump> to investigate the SV:
-
- SV = PV(0xa057cc0) at 0xa0675d0
- REFCNT = 1
- FLAGS = (POK,pPOK)
- PV = 0xa06a510 "6XXXX"\0
- CUR = 5
- LEN = 6
- $1 = void
-
-We know we're going to get C<6> from this, so let's finish the
-subroutine:
-
- (gdb) finish
- Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
- 0x462669 in Perl_pp_add () at pp_hot.c:311
- 311 dPOPTOPnnrl_ul;
-
-We can also dump out this op: the current op is always stored in
-C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
-similar output to L<B::Debug|B::Debug>.
-
- {
- 13 TYPE = add ===> 14
- TARG = 1
- FLAGS = (SCALAR,KIDS)
- {
- TYPE = null ===> (12)
- (was rv2sv)
- FLAGS = (SCALAR,KIDS)
- {
- 11 TYPE = gvsv ===> 12
- FLAGS = (SCALAR)
- GV = main::b
- }
- }
+=head2 Patching a core module
-# finish this later #
+This works just like patching anything else, with one extra
+consideration.
-=head1 SOURCE CODE STATIC ANALYSIS
+Some core modules also live on CPAN and are maintained outside of the
+Perl core. When the author updates the module, the updates are simply
+copied into the core.
-Various tools exist for analysing C source code B<statically>, as
-opposed to B<dynamically>, that is, without executing the code.
-It is possible to detect resource leaks, undefined behaviour, type
-mismatches, portability problems, code paths that would cause illegal
-memory accesses, and other similar problems by just parsing the C code
-and looking at the resulting graph, what does it tell about the
-execution and data flows. As a matter of fact, this is exactly
-how C compilers know to give warnings about dubious code.
+Module in the F<cpan/> directory of the source tree are maintained
+outside of the Perl core. See that module's listing on documentation or
+its listing on L<http://search.cpan.org/> for more information on
+reporting bugs and submitting patches.
-=head2 lint, splint
+In contrast, modules in the F<dist/> directory are maintained in the
+core.
-The good old C code quality inspector, C<lint>, is available in
-several platforms, but please be aware that there are several
-different implementations of it by different vendors, which means that
-the flags are not identical across different platforms.
+=head2 Updating perldelta
-There is a lint variant called C<splint> (Secure Programming Lint)
-available from http://www.splint.org/ that should compile on any
-Unix-like platform.
+For changes significant enough to warrant a F<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:
-There are C<lint> and <splint> targets in Makefile, but you may have
-to diddle with the flags (see above).
+=over 4
-=head2 Coverity
+=item *
-Coverity (http://www.coverity.com/) is a product similar to lint and
-as a testbed for their product they periodically check several open
-source projects, and they give out accounts to open source developers
-to the defect databases.
-
-=head2 cpd (cut-and-paste detector)
-
-The cpd tool detects cut-and-paste coding. If one instance of the
-cut-and-pasted code changes, all the other spots should probably be
-changed, too. Therefore such code should probably be turned into a
-subroutine or a macro.
+Adding, deprecating, or removing core features
-cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
-(http://pmd.sourceforge.net/). pmd was originally written for static
-analysis of Java code, but later the cpd part of it was extended to
-parse also C and C++.
+=item *
-Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
-pmd-X.Y.jar from it, and then run that on source code thusly:
+Adding, deprecating, removing, or upgrading core or dual-life modules
- java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
-
-You may run into memory limits, in which case you should use the -Xmx option:
+=item *
- java -Xmx512M ...
+Adding new core tests
-=head2 gcc warnings
+=item *
-Though much can be written about the inconsistency and coverage
-problems of gcc warnings (like C<-Wall> not meaning "all the
-warnings", or some common portability problems not being covered by
-C<-Wall>, or C<-ansi> and C<-pedantic> both being a poorly defined
-collection of warnings, and so forth), gcc is still a useful tool in
-keeping our coding nose clean.
-
-The C<-Wall> is by default on.
-
-The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
-always, but unfortunately they are not safe on all platforms, they can
-for example cause fatal conflicts with the system headers (Solaris
-being a prime example). If Configure C<-Dgccansipedantic> is used,
-the C<cflags> frontend selects C<-ansi -pedantic> for the platforms
-where they are known to be safe.
+Fixing security issues and user-visible bugs in the core
-Starting from Perl 5.9.4 the following extra flags are added:
+=item *
-=over 4
+Changes that might break existing code, either on the perl or C level
=item *
-C<-Wendif-labels>
+Significant performance improvements
=item *
-C<-Wextra>
+Adding, removing, or significantly changing documentation in the
+F<pod/> directory
=item *
-C<-Wdeclaration-after-statement>
+Important platform-specific changes
=back
-The following flags would be nice to have but they would first need
-their own Augean stablemaster:
-
-=over 4
-
-=item *
+Please make sure you add the perldelta entry to the right section
+within F<pod/perldelta.pod>. More information on how to write good
+perldelta entries is available in the C<Style> section of
+F<Porting/how_to_write_a_perldelta.pod>.
-C<-Wpointer-arith>
+=head2 What makes for a good patch?
-=item *
+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:
-C<-Wshadow>
+=head3 Does the concept match the general goals of Perl?
-=item *
+Our goals include, but are not limited to:
-C<-Wstrict-prototypes>
+=over 4
-=back
+=item 1.
-The C<-Wtraditional> is another example of the annoying tendency of
-gcc to bundle a lot of warnings under one switch (it would be
-impossible to deploy in practice because it would complain a lot) but
-it does contain some warnings that would be beneficial to have available
-on their own, such as the warning about string constants inside macros
-containing the macro arguments: this behaved differently pre-ANSI
-than it does in ANSI, and some C compilers are still in transition,
-AIX being an example.
+Keep it fast, simple, and useful.
-=head2 Warnings of other C compilers
+=item 2.
-Other C compilers (yes, there B<are> other C compilers than gcc) often
-have their "strict ANSI" or "strict ANSI with some portability extensions"
-modes on, like for example the Sun Workshop has its C<-Xa> mode on
-(though implicitly), or the DEC (these days, HP...) has its C<-std1>
-mode on.
+Keep features/concepts as orthogonal as possible.
-=head1 MEMORY DEBUGGERS
+=item 3.
-B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or
-Third Degree greatly slows down the execution: seconds become minutes,
-minutes become hours. For example as of Perl 5.8.1, the
-ext/Encode/t/Unicode.t takes extraordinarily long to complete under
-e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
-than six hours, even on a snappy computer. The said test must be
-doing something that is quite unfriendly for memory debuggers. If you
-don't feel like waiting, that you can simply kill away the perl
-process.
+No arbitrary limits (platforms, data sizes, cultures).
-B<NOTE 2>: To minimize the number of memory leak false alarms (see
-L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
-environment variable PERL_DESTRUCT_LEVEL to 2.
+=item 4.
-For csh-like shells:
+Keep it open and exciting to use/patch/advocate Perl everywhere.
- setenv PERL_DESTRUCT_LEVEL 2
+=item 5.
-For Bourne-type shells:
+Either assimilate new technologies, or build bridges to them.
- PERL_DESTRUCT_LEVEL=2
- export PERL_DESTRUCT_LEVEL
+=back
-In Unixy environments you can also use the C<env> command:
+=head3 Where is the implementation?
- env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
+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.
-B<NOTE 3>: There are known memory leaks when there are compile-time
-errors within eval or require, seeing C<S_doeval> in the call stack
-is a good sign of these. Fixing these leaks is non-trivial,
-unfortunately, but they must be fixed eventually.
+=head3 Backwards compatibility
-B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
-unless Perl is built with the Configure option
-C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
+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.
-=head2 Rational Software's Purify
+The Perl 5 core includes mechanisms to help porters make backwards
+incompatible changes more compatible such as the L<feature> and
+L<deprecate> modules. Please use them when appropriate.
-Purify is a commercial tool that is helpful in identifying
-memory overruns, wild pointers, memory leaks and other such
-badness. Perl must be compiled in a specific way for
-optimal testing with Purify. Purify is available under
-Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
+=head3 Could it be a module instead?
-=head3 Purify on Unix
+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.
-On Unix, Purify creates a new Perl binary. To get the most
-benefit out of Purify, you should create the perl to Purify
-using:
+Whenever possible, new features should be prototyped in a CPAN module
+before they will be considered for the core.
- sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
- -Uusemymalloc -Dusemultiplicity
+=head3 Is the feature generic enough?
-where these arguments mean:
+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.
-=over 4
+=head3 Does it potentially introduce new bugs?
-=item -Accflags=-DPURIFY
+Radical rewrites of large chunks of the Perl interpreter have the
+potential to introduce new bugs.
-Disables Perl's arena memory allocation functions, as well as
-forcing use of memory allocation functions derived from the
-system malloc.
+=head3 How big is it?
-=item -Doptimize='-g'
+The smaller and more localized the change, the better. Similarly, a
+series of small patches is greatly preferred over a single large patch.
-Adds debugging information so that you see the exact source
-statements where the problem occurs. Without this flag, all
-you will see is the source filename of where the error occurred.
+=head3 Does it preclude other desirable features?
-=item -Uusemymalloc
+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.
-Disable Perl's malloc so that Purify can more closely monitor
-allocations and leaks. Using Perl's malloc will make Purify
-report most leaks in the "potential" leaks category.
+=head3 Is the implementation robust?
-=item -Dusemultiplicity
+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.
-Enabling the multiplicity option allows perl to clean up
-thoroughly when the interpreter shuts down, which reduces the
-number of bogus leak reports from Purify.
+=head3 Is the implementation generic enough to be portable?
-=back
+The worst patches make use of a system-specific features. It's highly
+unlikely that non-portable additions to the Perl language will be
+accepted.
-Once you've compiled a perl suitable for Purify'ing, then you
-can just:
+=head3 Is the implementation tested?
- make pureperl
+Patches which change behaviour (fixing bugs or introducing new
+features) must include regression tests to verify that everything works
+as expected.
-which creates a binary named 'pureperl' that has been Purify'ed.
-This binary is used in place of the standard 'perl' binary
-when you want to debug Perl memory problems.
+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?
-As an example, to show any memory leaks produced during the
-standard Perl testset you would create and run the Purify'ed
-perl as:
+=head3 Is there enough documentation?
- make pureperl
- cd t
- ../pureperl -I../lib harness
+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.
-which would run Perl on test.pl and report any memory problems.
+=head3 Is there another way to do it?
-Purify outputs messages in "Viewer" windows by default. If
-you don't have a windowing environment or if you simply
-want the Purify output to unobtrusively go to a log file
-instead of to the interactive window, use these following
-options to output to the log file "perl.log":
+Larry said "Although the Perl Slogan is I<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.
- setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
- -log-file=perl.log -append-logfile=yes"
+=head3 Does it create too much work?
-If you plan to use the "Viewer" windows, then you only need this option:
+Work for the pumpking, work for Perl programmers, work for module
+authors, ... Perl is supposed to be easy.
- setenv PURIFYOPTIONS "-chain-length=25"
+=head3 Patches speak louder than words
-In Bourne-type shells:
+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 feature.
- PURIFYOPTIONS="..."
- export PURIFYOPTIONS
+=head1 TESTING
-or if you have the "env" utility:
+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.
- env PURIFYOPTIONS="..." ../pureperl ...
+There are three ways to write a test in the core. L<Test::More>,
+F<t/test.pl> and ad hoc C<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.
-=head3 Purify on NT
+The F<t/test.pl> library provides some of the features of
+L<Test::More>, but avoids loading most modules and uses as few core
+features as possible.
-Purify on Windows NT instruments the Perl binary 'perl.exe'
-on the fly. There are several options in the makefile you
-should change to get the most use out of Purify:
+If you write your own test, use the L<Test Anything Protocol|TAP>.
=over 4
-=item DEFINES
-
-You should add -DPURIFY to the DEFINES line so the DEFINES
-line looks something like:
-
- DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
-
-to disable Perl's arena memory allocation functions, as
-well as to force use of memory allocation functions derived
-from the system malloc.
+=item * F<t/base> and F<t/comp>
-=item USE_MULTI = define
+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.
-Enabling the multiplicity option allows perl to clean up
-thoroughly when the interpreter shuts down, which reduces the
-number of bogus leak reports from Purify.
+=item * F<t/cmd>, F<t/run> F<t/io>, and F<t/op>
-=item #PERL_MALLOC = define
+Now that basic require() and subroutines are tested, you can use the
+F<t/test.pl> library.
-Disable Perl's malloc so that Purify can more closely monitor
-allocations and leaks. Using Perl's malloc will make Purify
-report most leaks in the "potential" leaks category.
+You can also use certain libraries like Config conditionally , but be
+sure to skip the test gracefully if it's not there.
-=item CFG = Debug
+=item * Everything else
-Adds debugging information so that you see the exact source
-statements where the problem occurs. Without this flag, all
-you will see is the source filename of where the error occurred.
+Now that the core of Perl is tested, L<Test::More> can and should be
+used. You can also use the full suite of core modules in the tests.
=back
-As an example, to show any memory leaks produced during the
-standard Perl testset you would create and run Purify as:
-
- cd win32
- make
- cd ../t
- purify ../perl -I../lib harness
-
-which would instrument Perl in memory, run Perl on test.pl,
-then finally report any memory problems.
+When you say "make test" Perl uses the F<t/TEST> program to run the
+test suite (except under Win32 where it uses F<t/harness> instead.) All
+tests are run from the F<t/> directory, B<not> the directory which
+contains the test. This causes some problems with the tests in F<lib/>,
+so here's some opportunity for some patching.
-=head2 valgrind
+You must be triply conscious of cross-platform concerns. This usually
+boils down to using L<File::Spec> and avoiding things like C<fork()>
+and C<system()> unless absolutely necessary.
-The excellent valgrind tool can be used to find out both memory leaks
-and illegal memory accesses. As of version 3.3.0, Valgrind only
-supports Linux on x86, x86-64 and PowerPC. The special "test.valgrind"
-target can be used to run the tests under valgrind. Found errors
-and memory leaks are logged in files named F<testfile.valgrind>.
+=head2 Special C<make test> targets
-Valgrind also provides a cachegrind tool, invoked on perl as:
+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.
- VG_OPTS=--tool=cachegrind make test.valgrind
+=over 4
-As system libraries (most notably glibc) are also triggering errors,
-valgrind allows to suppress such errors using suppression files. The
-default suppression file that comes with valgrind already catches a lot
-of them. Some additional suppressions are defined in F<t/perl.supp>.
+=item * test_porting
-To get valgrind and for more information see
+This runs some basic sanity tests on the source tree and helps catch
+basic errors before you submit a patch.
- http://developer.kde.org/~sewardj/
+=item * coretest
-=head2 Compaq's/Digital's/HP's Third Degree
+Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests).
-Third Degree is a tool for memory leak detection and memory access checks.
-It is one of the many tools in the ATOM toolkit. The toolkit is only
-available on Tru64 (formerly known as Digital UNIX formerly known as
-DEC OSF/1).
+(Not available on Win32)
-When building Perl, you must first run Configure with -Doptimize=-g
-and -Uusemymalloc flags, after that you can use the make targets
-"perl.third" and "test.third". (What is required is that Perl must be
-compiled using the C<-g> flag, you may need to re-Configure.)
+=item * test.deparse
-The short story is that with "atom" you can instrument the Perl
-executable to create a new executable called F<perl.third>. When the
-instrumented executable is run, it creates a log of dubious memory
-traffic in file called F<perl.3log>. See the manual pages of atom and
-third for more information. The most extensive Third Degree
-documentation is available in the Compaq "Tru64 UNIX Programmer's
-Guide", chapter "Debugging Programs with Third Degree".
+Run all the tests through L<B::Deparse>. Not all tests will succeed.
-The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/
-subdirectory. There is a problem with these files: Third Degree is so
-effective that it finds problems also in the system libraries.
-Therefore you should used the Porting/thirdclean script to cleanup
-the F<*.3log> files.
+(Not available on Win32)
-There are also leaks that for given certain definition of a leak,
-aren't. See L</PERL_DESTRUCT_LEVEL> for more information.
+=item * test.taintwarn
-=head1 PROFILING
+Run all tests with the B<-t> command-line switch. Not all tests are
+expected to succeed (until they're specifically fixed, of course).
-Depending on your platform there are various ways of profiling Perl.
+(Not available on Win32)
-There are two commonly used techniques of profiling executables:
-I<statistical time-sampling> and I<basic-block counting>.
+=item * minitest
-The first method takes periodically samples of the CPU program
-counter, and since the program counter can be correlated with the code
-generated for functions, we get a statistical view of in which
-functions the program is spending its time. The caveats are that very
-small/fast functions have lower probability of showing up in the
-profile, and that periodically interrupting the program (this is
-usually done rather frequently, in the scale of milliseconds) imposes
-an additional overhead that may skew the results. The first problem
-can be alleviated by running the code for longer (in general this is a
-good idea for profiling), the second problem is usually kept in guard
-by the profiling tools themselves.
+Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>,
+F<t/op>, F<t/uni> and F<t/mro> tests.
-The second method divides up the generated code into I<basic blocks>.
-Basic blocks are sections of code that are entered only in the
-beginning and exited only at the end. For example, a conditional jump
-starts a basic block. Basic block profiling usually works by
-I<instrumenting> the code by adding I<enter basic block #nnnn>
-book-keeping code to the generated code. During the execution of the
-code the basic block counters are then updated appropriately. The
-caveat is that the added extra code can skew the results: again, the
-profiling tools usually try to factor their own effects out of the
-results.
+=item * test.valgrind check.valgrind utest.valgrind ucheck.valgrind
-=head2 Gprof Profiling
+(Only in Linux) Run all the tests using the memory leak + naughty
+memory access tool "valgrind". The log files will be named
+F<testname.valgrind>.
-gprof is a profiling tool available in many Unix platforms,
-it uses F<statistical time-sampling>.
+=item * test.torture torturetest
-You can build a profiled version of perl called "perl.gprof" by
-invoking the make target "perl.gprof" (What is required is that Perl
-must be compiled using the C<-pg> flag, you may need to re-Configure).
-Running the profiled version of Perl will create an output file called
-F<gmon.out> is created which contains the profiling data collected
-during the execution.
-
-The gprof tool can then display the collected data in various ways.
-Usually gprof understands the following options:
+Run all the usual tests and some extra tests. As of Perl 5.8.0 the only
+extra tests are Abigail's JAPHs, F<t/japh/abigail.t>.
-=over 4
+You can also run the torture test with F<t/harness> by giving
+C<-torture> argument to F<t/harness>.
-=item -a
+=item * utest ucheck test.utf8 check.utf8
-Suppress statically defined functions from the profile.
+Run all the tests with -Mutf8. Not all tests will succeed.
-=item -b
+(Not available on Win32)
-Suppress the verbose descriptions in the profile.
+=item * minitest.utf16 test.utf16
-=item -e routine
+Runs the tests with UTF-16 encoded scripts, encoded with different
+versions of this encoding.
-Exclude the given routine and its descendants from the profile.
+C<make utest.utf16> runs the test suite with a combination of C<-utf8>
+and C<-utf16> arguments to F<t/TEST>.
-=item -f routine
+(Not available on Win32)
-Display only the given routine and its descendants in the profile.
+=item * test_harness
-=item -s
+Run the test suite with the F<t/harness> controlling program, instead
+of F<t/TEST>. F<t/harness> is more sophisticated, and uses the
+L<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 F<t/TEST>, it
+doesn't redirect stderr to stdout.
-Generate a summary file called F<gmon.sum> which then may be given
-to subsequent gprof runs to accumulate data over several runs.
+Note that under Win32 F<t/harness> is always used instead of F<t/TEST>,
+so there is no special "test_harness" target.
-=item -z
+Under Win32's "test" target you may use the TEST_SWITCHES and
+TEST_FILES environment variables to control the behaviour of
+F<t/harness>. This means you can say
-Display routines that have zero usage.
+ nmake test TEST_FILES="op/*.t"
+ nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
=back
-For more detailed explanation of the available commands and output
-formats, see your own local documentation of gprof.
+=head2 Parallel tests
-quick hint:
+The core distribution can now run its regression tests in parallel on
+Unix-like platforms. Instead of running C<make test>, set C<TEST_JOBS>
+in your environment to the number of tests to run in parallel, and run
+C<make test_harness>. On a Bourne-like shell, this can be done as
- $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
- $ ./perl.gprof someprog # creates gmon.out in current directory
- $ gprof ./perl.gprof > out
- $ view out
+ TEST_JOBS=3 make test_harness # Run 3 tests in parallel
-=head2 GCC gcov Profiling
+An environment variable is used, rather than parallel make itself,
+because L<TAP::Harness> needs to be able to schedule individual
+non-conflicting test scripts itself, and there is no standard interface
+to C<make> utilities to interact with their job schedulers.
-Starting from GCC 3.0 I<basic block profiling> is officially available
-for the GNU CC.
+Note that currently some test scripts may fail when run in parallel (most
+notably C<ext/IO/t/io_dir.t>). If necessary run just the failing scripts
+again sequentially and see if the failures go away.
+=item * test-notty test_notty
-You can build a profiled version of perl called F<perl.gcov> by
-invoking the make target "perl.gcov" (what is required that Perl must
-be compiled using gcc with the flags C<-fprofile-arcs
--ftest-coverage>, you may need to re-Configure).
+Sets PERL_SKIP_TTY_TEST to true before running normal test.
-Running the profiled version of Perl will cause profile output to be
-generated. For each source file an accompanying ".da" file will be
-created.
+=head2 Running tests by hand
-To display the results you use the "gcov" utility (which should
-be installed if you have gcc 3.0 or newer installed). F<gcov> is
-run on source code files, like this
+You can run part of the test suite by hand by using one the following
+commands from the F<t/> directory :
- gcov sv.c
+ ./perl -I../lib TEST list-of-.t-files
-which will cause F<sv.c.gcov> to be created. The F<.gcov> files
-contain the source code annotated with relative frequencies of
-execution indicated by "#" markers.
+or
-Useful options of F<gcov> include C<-b> which will summarise the
-basic block, branch, and function call coverage, and C<-c> which
-instead of relative frequencies will use the actual counts. For
-more information on the use of F<gcov> and basic block profiling
-with gcc, see the latest GNU CC manual, as of GCC 3.0 see
+ ./perl -I../lib harness list-of-.t-files
- http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
+(if you don't specify test scripts, the whole test suite will be run.)
-and its section titled "8. gcov: a Test Coverage Program"
+=head2 Using F<t/harness> for testing
- http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
+If you use C<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.
-quick hint:
+ harness -v -torture -re=pattern LIST OF FILES TO TEST
+ harness -v -torture -re LIST OF PATTERNS TO MATCH
- $ sh Configure -des -Dusedevel -Doptimize='-g' \
- -Accflags='-fprofile-arcs -ftest-coverage' \
- -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
- $ rm -f regexec.c.gcov regexec.gcda
- $ ./perl.gcov
- $ gcov regexec.c
- $ view regexec.c.gcov
+If C<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.
-=head2 Pixie Profiling
+=over 4
-Pixie is a profiling tool available on IRIX and Tru64 (aka Digital
-UNIX aka DEC OSF/1) platforms. Pixie does its profiling using
-I<basic-block counting>.
+=item * -v
-You can build a profiled version of perl called F<perl.pixie> by
-invoking the make target "perl.pixie" (what is required is that Perl
-must be compiled using the C<-g> flag, you may need to re-Configure).
+Run the tests under verbose mode so you can see what tests were run,
+and debug output.
-In Tru64 a file called F<perl.Addrs> will also be silently created,
-this file contains the addresses of the basic blocks. Running the
-profiled version of Perl will create a new file called "perl.Counts"
-which contains the counts for the basic block for that particular
-program execution.
+=item * -torture
-To display the results you use the F<prof> utility. The exact
-incantation depends on your operating system, "prof perl.Counts" in
-IRIX, and "prof -pixie -all -L. perl" in Tru64.
+Run the torture tests as well as the normal set.
-In IRIX the following prof options are available:
+=item * -re=PATTERN
-=over 4
+Filter the file list so that all the test files run match PATTERN. Note
+that this form is distinct from the B<-re LIST OF PATTERNS> form below
+in that it allows the file list to be provided as well.
-=item -h
+=item * -re LIST OF PATTERNS
-Reports the most heavily used lines in descending order of use.
-Useful for finding the hotspot lines.
+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.
-=item -l
+=back
-Groups lines by procedure, with procedures sorted in descending order of use.
-Within a procedure, lines are listed in source order.
-Useful for finding the hotspots of procedures.
+You can run an individual test by a command similar to
-=back
+ ./perl -I../lib patho/to/foo.t
-In Tru64 the following options are available:
+except that the harnesses set up some environment variables that may
+affect the execution of the test:
=over 4
-=item -p[rocedures]
+=item * PERL_CORE=1
-Procedures sorted in descending order by the number of cycles executed
-in each procedure. Useful for finding the hotspot procedures.
-(This is the default option.)
+indicates that we're running this test part of the perl core test
+suite. This is useful for modules that have a dual life on CPAN.
-=item -h[eavy]
+=item * PERL_DESTRUCT_LEVEL=2
-Lines sorted in descending order by the number of cycles executed in
-each line. Useful for finding the hotspot lines.
+is set to 2 if it isn't set already (see
+L<perlhacktips/PERL_DESTRUCT_LEVEL>)
-=item -i[nvocations]
+=item * PERL
-The called procedures are sorted in descending order by number of calls
-made to the procedures. Useful for finding the most used procedures.
+(used only by F<t/TEST>) if set, overrides the path to the perl
+executable that should be used to run the tests (the default being
+F<./perl>).
-=item -l[ines]
+=item * PERL_SKIP_TTY_TEST
-Grouped by procedure, sorted by cycles executed per procedure.
-Useful for finding the hotspots of procedures.
+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'.
-=item -testcoverage
+=back
-The compiler emitted code for these lines, but the code was unexecuted.
+=head3 Other environment variables that may influence tests
-=item -z[ero]
+=over 4
-Unexecuted procedures.
+=item * PERL_TEST_Net_Ping
-=back
+Setting this variable runs all the Net::Ping modules tests, otherwise
+some tests that interact with the outside world are skipped. See
+L<perl58delta>.
-For further information, see your system's manual pages for pixie and prof.
-
-=head1 MISCELLANEOUS TRICKS
-
-=head2 PERL_DESTRUCT_LEVEL
-
-If you want to run any of the tests yourself manually using e.g.
-valgrind, or the pureperl or perl.third executables, please note that
-by default perl B<does not> explicitly cleanup all the memory it has
-allocated (such as global memory arenas) but instead lets the exit()
-of the whole program "take care" of such allocations, also known as
-"global destruction of objects".
-
-There is a way to tell perl to do complete cleanup: set the
-environment variable PERL_DESTRUCT_LEVEL to a non-zero value.
-The t/TEST wrapper does set this to 2, and this is what you
-need to do too, if you don't want to see the "global leaks":
-For example, for "third-degreed" Perl:
-
- env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
-
-(Note: the mod_perl apache module uses also this environment variable
-for its own purposes and extended its semantics. Refer to the mod_perl
-documentation for more information. Also, spawned threads do the
-equivalent of setting this variable to the value 1.)
+=item * PERL_TEST_NOVREXX
-If, at the end of a run you get the message I<N scalars leaked>, you can
-recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the addresses
-of all those leaked SVs to be dumped along with details as to where each
-SV was originally allocated. This information is also displayed by
-Devel::Peek. Note that the extra details recorded with each SV increases
-memory usage, so it shouldn't be used in production environments. It also
-converts C<new_SV()> from a macro into a real function, so you can use
-your favourite debugger to discover where those pesky SVs were allocated.
-
-If you see that you're leaking memory at runtime, but neither valgrind
-nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
-leaking SVs that are still reachable and will be properly cleaned up
-during destruction of the interpreter. In such cases, using the C<-Dm>
-switch can point you to the source of the leak. If the executable was
-built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV allocations
-in addition to memory allocations. Each SV allocation has a distinct
-serial number that will be written on creation and destruction of the SV.
-So if you're executing the leaking code in a loop, you need to look for
-SVs that are created, but never destroyed between each cycle. If such an
-SV is found, set a conditional breakpoint within C<new_SV()> and make it
-break only when C<PL_sv_serial> is equal to the serial number of the
-leaking SV. Then you will catch the interpreter in exactly the state
-where the leaking SV is allocated, which is sufficient in many cases to
-find the source of the leak.
+Setting this variable skips the vrexx.t tests for OS2::REXX.
-As C<-Dm> is using the PerlIO layer for output, it will by itself
-allocate quite a bunch of SVs, which are hidden to avoid recursion.
-You can bypass the PerlIO layer if you use the SV logging provided
-by C<-DPERL_MEM_LOG> instead.
+=item * PERL_TEST_NUMCONVERTS
-=head2 PERL_MEM_LOG
+This sets a variable in op/numconvert.t.
-If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
-through logging functions, which is handy for breakpoint setting.
+=back
-Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging
-functions read $ENV{PERL_MEM_LOG} to determine whether to log the
-event, and if so how:
+See also the documentation for the Test and Test::Harness modules, for
+more environment variables that affect testing.
- $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
- $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
- $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
- $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
+=head1 MORE READING FOR GUTS HACKERS
-Memory logging is somewhat similar to C<-Dm> but is independent of
-C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(),
-and Safefree() are logged with the caller's source code file and line
-number (and C function name, if supported by the C compiler). In
-contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging
-is similar.
+To hack on the Perl guts, you'll need to read the following things:
-Since the logging doesn't use PerlIO, all SV allocations are logged
-and no extra SV allocations are introduced by enabling the logging.
-If compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for
-each SV allocation is also logged.
+=over 4
-=head2 DDD over gdb
+=item * L<perlsource>
-Those debugging perl with the DDD frontend over gdb may find the
-following useful:
+An overview of the Perl source tree. This will help you find the files
+you're looking for.
-You can extend the data conversion shortcuts menu, so for example you
-can display an SV's IV value with one click, without doing any typing.
-To do that simply edit ~/.ddd/init file and add after:
+=item * L<perlinterp>
- ! Display shortcuts.
- Ddd*gdbDisplayShortcuts: \
- /t () // Convert to Bin\n\
- /d () // Convert to Dec\n\
- /x () // Convert to Hex\n\
- /o () // Convert to Oct(\n\
+An overview of the Perl interpreter source code and some details on how
+Perl does what it does.
-the following two lines:
+=item * L<perlhacktut>
- ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
- ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
+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.
-so now you can do ivx and pvx lookups or you can plug there the
-sv_peek "conversion":
+=item * L<perlhacktips>
- Perl_sv_peek(my_perl, (SV*)()) // sv_peek
+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.
-(The my_perl is for threaded builds.)
-Just remember that every line, but the last one, should end with \n\
+If you plan on doing serious C hacking, make sure to read this.
-Alternatively edit the init file interactively via:
-3rd mouse button -> New Display -> Edit Menu
+=item * L<perlguts>
-Note: you can define up to 20 conversion shortcuts in the gdb
-section.
+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.
-=head2 Poison
+Gisle Aas's "illustrated perlguts", also known as I<illguts>, has very
+helpful pictures:
-If you see in a debugger a memory area mysteriously full of 0xABABABAB
-or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros,
-see L<perlclib>.
+L<http://search.cpan.org/dist/illguts/>
-=head2 Read-only optrees
+=item * L<perlxstut> and L<perlxs>
-Under ithreads the optree is read only. If you want to enforce this, to check
-for write accesses from buggy code, compile with C<-DPL_OP_SLAB_ALLOC> to
-enable the OP slab allocator and C<-DPERL_DEBUG_READONLY_OPS> to enable code
-that allocates op memory via C<mmap>, and sets it read-only at run time.
-Any write access to an op results in a C<SIGBUS> and abort.
+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.
-This code is intended for development only, and may not be portable even to
-all Unix variants. Also, it is an 80% solution, in that it isn't able to make
-all ops read only. Specifically it
+=item * L<perlapi>
-=over
+The documentation for the Perl API explains what some of the internal
+functions do, as well as the many macros used in the source.
-=item 1
+=item * F<Porting/pumpkin.pod>
-Only sets read-only on all slabs of ops at C<CHECK> time, hence ops allocated
-later via C<require> or C<eval> will be re-write
+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 development.
-=item 2
+=item * The perl5-porters FAQ
-Turns an entire slab of ops read-write if the refcount of any op in the slab
-needs to be decreased.
+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.
-=item 3
+=back
-Turns an entire slab of ops read-write if any op from the slab is freed.
+=head1 CPAN TESTERS AND PERL SMOKERS
-=back
+The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers
+who test CPAN modules on a variety of platforms.
-It's not possible to turn the slabs to read-only after an action requiring
-read-write access, as either can happen during op tree building time, so
-there may still be legitimate write access.
+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.
-However, as an 80% solution it is still effective, as currently it catches
-a write access during the generation of F<Config.pm>, which means that we
-can't yet build F<perl> with this enabled.
+Both efforts welcome volunteers. In order to get involved in smoke
+testing of the perl itself visit
+L<http://search.cpan.org/dist/Test-Smoke>. In order to start smoke
+testing CPAN modules visit
+L<http://search.cpan.org/dist/CPANPLUS-YACSmoke/> or
+L<http://search.cpan.org/dist/minismokebox/> or
+L<http://search.cpan.org/dist/CPAN-Reporter/>.
-=head1 CONCLUSION
+=head1 WHAT NEXT?
-We've had a brief look around the Perl source, how to maintain quality
-of the source code, an overview of the stages F<perl> goes through
-when it's running your code, how to use debuggers to poke at the Perl
-guts, and finally how to analyse the execution of Perl. We took a very
-simple problem and demonstrated how to solve it fully - with
-documentation, regression tests, and finally a patch for submission to
-p5p. Finally, we talked about how to use external tools to debug and
-test Perl.
+If you've read all the documentation in the document and the ones
+listed above, you're more than ready to hack on Perl.
-I'd now suggest you read over those references again, and then, as soon
-as possible, get your hands dirty. The best way to learn is by doing,
-so:
+Here's some more recommendations
-=over 3
+=over 4
=item *
=item *
-Keep up to date with the bleeding edge Perl distributions and get
-familiar with the changes. Try and get an idea of what areas people are
-working on and the changes they're making.
-
-=item *
-
-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.
+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.
=item *
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 F<perl>'s
-activity as well, and probably sooner than you'd think.
+understand not just your chosen area but a much wider range of
+F<perl>'s activity as well, and probably sooner than you'd think.
=back
-=over 3
-
-=item I<The Road goes ever on and on, down from the door where it began.>
+=head2 "The Road goes ever on and on, down from the door where it began."
-=back
-
-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!
+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!
=head2 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.
+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 epigramic inscription that alludes,
-indirectly and metaphorically, to the material you're about to read.
+Like chapters in many books, all top-level Perl source files (along
+with a few others here and there) begin with an epigramic 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 I<The Lord of the Rings>. Chapters and
+Legendarium, almost always from I<The Lord of the Rings>. Chapters and
page numbers are given using the following editions:
=over 4
-=item *
+=item *
-I<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.
+I<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.
=item *
-I<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.
+I<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.
=item *
-I<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 I<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.
+I<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 I<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.
=back
-Other JRRT books fair game for quotes would thus include I<The Adventures of
-Tom Bombadil>, I<The Silmarillion>, I<Unfinished Tales>, and I<The Tale of
-the Children of Hurin>, all but the first posthumously assembled by CJRT.
-But I<The Lord of the Rings> itself is perfectly fine and probably best to
-quote from, provided you can find a suitable quote there.
+Other JRRT books fair game for quotes would thus include I<The
+Adventures of Tom Bombadil>, I<The Silmarillion>, I<Unfinished Tales>,
+and I<The Tale of the Children of Hurin>, all but the first
+posthumously assembled by CJRT. But I<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.
+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.
=head1 AUTHOR
-This document was written by Nathan Torkington, and is maintained by
-the perl5-porters mailing list.
-
-=head1 SEE ALSO
+This document was originally written by Nathan Torkington, and is
+maintained by the perl5-porters mailing list.
-L<perlrepository>
--- /dev/null
+=encoding utf8
+
+=for comment
+Consistent formatting of this file is achieved with:
+ perl ./Porting/podtidy pod/perlhacktips.pod
+
+=head1 NAME
+
+perlhacktips - Tips for Perl core C code hacking
+
+=head1 DESCRIPTION
+
+This document will help you learn the best way to go about hacking on
+the Perl core C code. It covers common problems, debugging, profiling,
+and more.
+
+If you haven't read L<perlhack> and L<perlhacktut> yet, you might want
+to do that first.
+
+=head1 COMMON PROBLEMS
+
+Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
+some cases we have to take pre-ANSI requirements into consideration.
+You don't care about some particular platform having broken Perl? I
+hear there is still a strong demand for J2EE programmers.
+
+=head2 Perl environment problems
+
+=over 4
+
+=item *
+
+Not compiling with threading
+
+Compiling with threading (-Duseithreads) completely rewrites the
+function prototypes of Perl. You better try your changes with that.
+Related to this is the difference between "Perl_-less" and "Perl_-ly"
+APIs, for example:
+
+ Perl_sv_setiv(aTHX_ ...);
+ sv_setiv(...);
+
+The first one explicitly passes in the context, which is needed for
+e.g. threaded builds. The second one does that implicitly; do not get
+them mixed. If you are not passing in a aTHX_, you will need to do a
+dTHX (or a dVAR) as the first thing in the function.
+
+See L<perlguts/"How multiple interpreters and concurrency are
+supported"> for further discussion about context.
+
+=item *
+
+Not compiling with -DDEBUGGING
+
+The DEBUGGING define exposes more code to the compiler, therefore more
+ways for things to go wrong. You should try it.
+
+=item *
+
+Introducing (non-read-only) globals
+
+Do not introduce any modifiable globals, truly global or file static.
+They are bad form and complicate multithreading and other forms of
+concurrency. The right way is to introduce them as new interpreter
+variables, see F<intrpvar.h> (at the very end for binary
+compatibility).
+
+Introducing read-only (const) globals is okay, as long as you verify
+with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
+BSD-style output) that the data you added really is read-only. (If it
+is, it shouldn't show up in the output of that command.)
+
+If you want to have static strings, make them constant:
+
+ static const char etc[] = "...";
+
+If you want to have arrays of constant strings, note carefully the
+right combination of C<const>s:
+
+ static const char * const yippee[] =
+ {"hi", "ho", "silver"};
+
+There is a way to completely hide any modifiable globals (they are all
+moved to heap), the compilation setting
+C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be
+used for testing, read more about it in L<perlguts/"Background and
+PERL_IMPLICIT_CONTEXT">.
+
+=item *
+
+Not exporting your new function
+
+Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
+function that is part of the public API (the shared Perl library) to be
+explicitly marked as exported. See the discussion about F<embed.pl> in
+L<perlguts>.
+
+=item *
+
+Exporting your new function
+
+The new shiny result of either genuine new functionality or your
+arduous refactoring is now ready and correctly exported. So what could
+possibly go wrong?
+
+Maybe simply that your function did not need to be exported in the
+first place. Perl has a long and not so glorious history of exporting
+functions that it should not have.
+
+If the function is used only inside one source code file, make it
+static. See the discussion about F<embed.pl> in L<perlguts>.
+
+If the function is used across several files, but intended only for
+Perl's internal use (and this should be the common case), do not export
+it to the public API. See the discussion about F<embed.pl> in
+L<perlguts>.
+
+=back
+
+=head2 Portability problems
+
+The following are common causes of compilation and/or execution
+failures, not common to Perl as such. The C FAQ is good bedtime
+reading. Please test your changes with as many C compilers and
+platforms as possible; we will, anyway, and it's nice to save oneself
+from public embarrassment.
+
+If using gcc, you can add the C<-std=c89> option which will hopefully
+catch most of these unportabilities. (However it might also catch
+incompatibilities in your system's header files.)
+
+Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi
+-pedantic> flags which enforce stricter ANSI rules.
+
+If using the C<gcc -Wall> note that not all the possible warnings (like
+C<-Wunitialized>) are given unless you also compile with C<-O>.
+
+Note that if using gcc, starting from Perl 5.9.5 the Perl core source
+code files (the ones at the top level of the source code distribution,
+but not e.g. the extensions under ext/) are automatically compiled with
+as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a
+selection of C<-W> flags (see cflags.SH).
+
+Also study L<perlport> carefully to avoid any bad assumptions about the
+operating system, filesystems, and so forth.
+
+You may once in a while try a "make microperl" to see whether we can
+still compile Perl with just the bare minimum of interfaces. (See
+README.micro.)
+
+Do not assume an operating system indicates a certain compiler.
+
+=over 4
+
+=item *
+
+Casting pointers to integers or casting integers to pointers
+
+ void castaway(U8* p)
+ {
+ IV i = p;
+
+or
+
+ void castaway(U8* p)
+ {
+ IV i = (IV)p;
+
+Both are bad, and broken, and unportable. Use the PTR2IV() macro that
+does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and
+NUM2PTR().)
+
+=item *
+
+Casting between data function pointers and data pointers
+
+Technically speaking casting between function pointers and data
+pointers is unportable and undefined, but practically speaking it seems
+to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros.
+Sometimes you can also play games with unions.
+
+=item *
+
+Assuming sizeof(int) == sizeof(long)
+
+There are platforms where longs are 64 bits, and platforms where ints
+are 64 bits, and while we are out to shock you, even platforms where
+shorts are 64 bits. This is all legal according to the C standard. (In
+other words, "long long" is not a portable way to specify 64 bits, and
+"long long" is not even guaranteed to be any wider than "long".)
+
+Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
+Avoid things like I32 because they are B<not> guaranteed to be
+I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
+guaranteed to be B<int> or B<long>. If you really explicitly need
+64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
+
+=item *
+
+Assuming one can dereference any type of pointer for any type of data
+
+ char *p = ...;
+ long pony = *p; /* BAD */
+
+Many platforms, quite rightly so, will give you a core dump instead of
+a pony if the p happens not be correctly aligned.
+
+=item *
+
+Lvalue casts
+
+ (int)*p = ...; /* BAD */
+
+Simply not portable. Get your lvalue to be of the right type, or maybe
+use temporary variables, or dirty tricks with unions.
+
+=item *
+
+Assume B<anything> about structs (especially the ones you don't
+control, like the ones coming from the system headers)
+
+=over 8
+
+=item *
+
+That a certain field exists in a struct
+
+=item *
+
+That no other fields exist besides the ones you know of
+
+=item *
+
+That a field is of certain signedness, sizeof, or type
+
+=item *
+
+That the fields are in a certain order
+
+=over 8
+
+=item *
+
+While C guarantees the ordering specified in the struct definition,
+between different platforms the definitions might differ
+
+=back
+
+=item *
+
+That the sizeof(struct) or the alignments are the same everywhere
+
+=over 8
+
+=item *
+
+There might be padding bytes between the fields to align the fields -
+the bytes can be anything
+
+=item *
+
+Structs are required to be aligned to the maximum alignment required by
+the fields - which for native types is for usually equivalent to
+sizeof() of the field
+
+=back
+
+=back
+
+=item *
+
+Assuming the character set is ASCIIish
+
+Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
+This is transparent for the most part, but because the character sets
+differ, you shouldn't use numeric (decimal, octal, nor hex) constants
+to refer to characters. You can safely say 'A', but not 0x41. You can
+safely say '\n', but not \012. If a character doesn't have a trivial
+input form, you can create a #define for it in both C<utfebcdic.h> and
+C<utf8.h>, so that it resolves to different values depending on the
+character set being used. (There are three different EBCDIC character
+sets defined in C<utfebcdic.h>, so it might be best to insert the
+#define three times in that file.)
+
+Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
+case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to
+'z'. But '0' - '9' is an unbroken range in both systems. Don't assume
+anything about other ranges.
+
+Many of the comments in the existing code ignore the possibility of
+EBCDIC, and may be wrong therefore, even if the code works. This is
+actually a tribute to the successful transparent insertion of being
+able to handle EBCDIC without having to change pre-existing code.
+
+UTF-8 and UTF-EBCDIC are two different encodings used to represent
+Unicode code points as sequences of bytes. Macros with the same names
+(but different definitions) in C<utf8.h> and C<utfebcdic.h> are used to
+allow the calling code to think that there is only one such encoding.
+This is almost always referred to as C<utf8>, but it means the EBCDIC
+version as well. Again, comments in the code may well be wrong even if
+the code itself is right. For example, the concept of C<invariant
+characters> differs between ASCII and EBCDIC. On ASCII platforms, only
+characters that do not have the high-order bit set (i.e. whose ordinals
+are strict ASCII, 0 - 127) are invariant, and the documentation and
+comments in the code may assume that, often referring to something
+like, say, C<hibit>. The situation differs and is not so simple on
+EBCDIC machines, but as long as the code itself uses the
+C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
+comments are wrong.
+
+=item *
+
+Assuming the character set is just ASCII
+
+ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
+characters have different meanings depending on the locale. Absent a
+locale, currently these extra characters are generally considered to be
+unassigned, and this has presented some problems. This is being changed
+starting in 5.12 so that these characters will be considered to be
+Latin-1 (ISO-8859-1).
+
+=item *
+
+Mixing #define and #ifdef
+
+ #define BURGLE(x) ... \
+ #ifdef BURGLE_OLD_STYLE /* BAD */
+ ... do it the old way ... \
+ #else
+ ... do it the new way ... \
+ #endif
+
+You cannot portably "stack" cpp directives. For example in the above
+you need two separate BURGLE() #defines, one for each #ifdef branch.
+
+=item *
+
+Adding non-comment stuff after #endif or #else
+
+ #ifdef SNOSH
+ ...
+ #else !SNOSH /* BAD */
+ ...
+ #endif SNOSH /* BAD */
+
+The #endif and #else cannot portably have anything non-comment after
+them. If you want to document what is going (which is a good idea
+especially if the branches are long), use (C) comments:
+
+ #ifdef SNOSH
+ ...
+ #else /* !SNOSH */
+ ...
+ #endif /* SNOSH */
+
+The gcc option C<-Wendif-labels> warns about the bad variant (by
+default on starting from Perl 5.9.4).
+
+=item *
+
+Having a comma after the last element of an enum list
+
+ enum color {
+ CERULEAN,
+ CHARTREUSE,
+ CINNABAR, /* BAD */
+ };
+
+is not portable. Leave out the last comma.
+
+Also note that whether enums are implicitly morphable to ints varies
+between compilers, you might need to (int).
+
+=item *
+
+Using //-comments
+
+ // This function bamfoodles the zorklator. /* BAD */
+
+That is C99 or C++. Perl is C89. Using the //-comments is silently
+allowed by many C compilers but cranking up the ANSI C89 strictness
+(which we like to do) causes the compilation to fail.
+
+=item *
+
+Mixing declarations and code
+
+ void zorklator()
+ {
+ int n = 3;
+ set_zorkmids(n); /* BAD */
+ int q = 4;
+
+That is C99 or C++. Some C compilers allow that, but you shouldn't.
+
+The gcc option C<-Wdeclaration-after-statements> scans for such
+problems (by default on starting from Perl 5.9.4).
+
+=item *
+
+Introducing variables inside for()
+
+ for(int i = ...; ...; ...) { /* BAD */
+
+That is C99 or C++. While it would indeed be awfully nice to have that
+also in C89, to limit the scope of the loop variable, alas, we cannot.
+
+=item *
+
+Mixing signed char pointers with unsigned char pointers
+
+ int foo(char *s) { ... }
+ ...
+ unsigned char *t = ...; /* Or U8* t = ... */
+ foo(t); /* BAD */
+
+While this is legal practice, it is certainly dubious, and downright
+fatal in at least one platform: for example VMS cc considers this a
+fatal error. One cause for people often making this mistake is that a
+"naked char" and therefore dereferencing a "naked char pointer" have an
+undefined signedness: it depends on the compiler and the flags of the
+compiler and the underlying platform whether the result is signed or
+unsigned. For this very same reason using a 'char' as an array index is
+bad.
+
+=item *
+
+Macros that have string constants and their arguments as substrings of
+the string constants
+
+ #define FOO(n) printf("number = %d\n", n) /* BAD */
+ FOO(10);
+
+Pre-ANSI semantics for that was equivalent to
+
+ printf("10umber = %d\10");
+
+which is probably not what you were expecting. Unfortunately at least
+one reasonably common and modern C compiler does "real backward
+compatibility" here, in AIX that is what still happens even though the
+rest of the AIX compiler is very happily C89.
+
+=item *
+
+Using printf formats for non-basic C types
+
+ IV i = ...;
+ printf("i = %d\n", i); /* BAD */
+
+While this might by accident work in some platform (where IV happens to
+be an C<int>), in general it cannot. IV might be something larger. Even
+worse the situation is with more specific types (defined by Perl's
+configuration step in F<config.h>):
+
+ Uid_t who = ...;
+ printf("who = %d\n", who); /* BAD */
+
+The problem here is that Uid_t might be not only not C<int>-wide but it
+might also be unsigned, in which case large uids would be printed as
+negative values.
+
+There is no simple solution to this because of printf()'s limited
+intelligence, but for many types the right format is available as with
+either 'f' or '_f' suffix, for example:
+
+ IVdf /* IV in decimal */
+ UVxf /* UV is hexadecimal */
+
+ printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
+
+ Uid_t_f /* Uid_t in decimal */
+
+ printf("who = %"Uid_t_f"\n", who);
+
+Or you can try casting to a "wide enough" type:
+
+ printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
+
+Also remember that the C<%p> format really does require a void pointer:
+
+ U8* p = ...;
+ printf("p = %p\n", (void*)p);
+
+The gcc option C<-Wformat> scans for such problems.
+
+=item *
+
+Blindly using variadic macros
+
+gcc has had them for a while with its own syntax, and C99 brought them
+with a standardized syntax. Don't use the former, and use the latter
+only if the HAS_C99_VARIADIC_MACROS is defined.
+
+=item *
+
+Blindly passing va_list
+
+Not all platforms support passing va_list to further varargs (stdarg)
+functions. The right thing to do is to copy the va_list using the
+Perl_va_copy() if the NEED_VA_COPY is defined.
+
+=item *
+
+Using gcc statement expressions
+
+ val = ({...;...;...}); /* BAD */
+
+While a nice extension, it's not portable. The Perl code does
+admittedly use them if available to gain some extra speed (essentially
+as a funky form of inlining), but you shouldn't.
+
+=item *
+
+Binding together several statements in a macro
+
+Use the macros STMT_START and STMT_END.
+
+ STMT_START {
+ ...
+ } STMT_END
+
+=item *
+
+Testing for operating systems or versions when should be testing for
+features
+
+ #ifdef __FOONIX__ /* BAD */
+ foo = quux();
+ #endif
+
+Unless you know with 100% certainty that quux() is only ever available
+for the "Foonix" operating system B<and> that is available B<and>
+correctly working for B<all> past, present, B<and> future versions of
+"Foonix", the above is very wrong. This is more correct (though still
+not perfect, because the below is a compile-time check):
+
+ #ifdef HAS_QUUX
+ foo = quux();
+ #endif
+
+How does the HAS_QUUX become defined where it needs to be? Well, if
+Foonix happens to be Unixy enough to be able to run the Configure
+script, and Configure has been taught about detecting and testing
+quux(), the HAS_QUUX will be correctly defined. In other platforms, the
+corresponding configuration step will hopefully do the same.
+
+In a pinch, if you cannot wait for Configure to be educated, or if you
+have a good hunch of where quux() might be available, you can
+temporarily try the following:
+
+ #if (defined(__FOONIX__) || defined(__BARNIX__))
+ # define HAS_QUUX
+ #endif
+
+ ...
+
+ #ifdef HAS_QUUX
+ foo = quux();
+ #endif
+
+But in any case, try to keep the features and operating systems
+separate.
+
+=back
+
+=head2 Problematic System Interfaces
+
+=over 4
+
+=item *
+
+malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
+allocate at least one byte. (In general you should rarely need to work
+at this low level, but instead use the various malloc wrappers.)
+
+=item *
+
+snprintf() - the return type is unportable. Use my_snprintf() instead.
+
+=back
+
+=head2 Security problems
+
+Last but not least, here are various tips for safer coding.
+
+=over 4
+
+=item *
+
+Do not use gets()
+
+Or we will publicly ridicule you. Seriously.
+
+=item *
+
+Do not use strcpy() or strcat() or strncpy() or strncat()
+
+Use my_strlcpy() and my_strlcat() instead: they either use the native
+implementation, or Perl's own implementation (borrowed from the public
+domain implementation of INN).
+
+=item *
+
+Do not use sprintf() or vsprintf()
+
+If you really want just plain byte strings, use my_snprintf() and
+my_vsnprintf() instead, which will try to use snprintf() and
+vsnprintf() if those safer APIs are available. If you want something
+fancier than a plain byte string, use SVs and Perl_sv_catpvf().
+
+=back
+
+=head1 DEBUGGING
+
+You can compile a special debugging version of Perl, which allows you
+to use the C<-D> option of Perl to tell more about what Perl is doing.
+But sometimes there is no alternative than to dive in with a debugger,
+either to see the stack trace of a core dump (very useful in a bug
+report), or trying to figure out what went wrong before the core dump
+happened, or how did we end up having wrong or unexpected results.
+
+=head2 Poking at Perl
+
+To really poke around with Perl, you'll probably want to build Perl for
+debugging, like this:
+
+ ./Configure -d -D optimize=-g
+ make
+
+C<-g> is a flag to the C compiler to have it produce debugging
+information which will allow us to step through a running program, and
+to see in which C function we are at (without the debugging information
+we might see only the numerical addresses of the functions, which is
+not very helpful).
+
+F<Configure> will also turn on the C<DEBUGGING> compilation symbol
+which enables all the internal debugging code in Perl. There are a
+whole bunch of things you can debug with this: L<perlrun> lists them
+all, and the best way to find out about them is to play about with
+them. The most useful options are probably
+
+ l Context (loop) stack processing
+ t Trace execution
+ o Method and overloading resolution
+ c String/numeric conversions
+
+Some of the functionality of the debugging code can be achieved using
+XS modules.
+
+ -Dr => use re 'debug'
+ -Dx => use O 'Debug'
+
+=head2 Using a source-level debugger
+
+If the debugging output of C<-D> doesn't help you, it's time to step
+through perl's execution with a source-level debugger.
+
+=over 3
+
+=item *
+
+We'll use C<gdb> for our examples here; the principles will apply to
+any debugger (many vendors call their debugger C<dbx>), but check the
+manual of the one you're using.
+
+=back
+
+To fire up the debugger, type
+
+ gdb ./perl
+
+Or if you have a core dump:
+
+ gdb ./perl core
+
+You'll want to do that in your Perl source tree so the debugger can
+read the source code. You should see the copyright message, followed by
+the prompt.
+
+ (gdb)
+
+C<help> will get you into the documentation, but here are the most
+useful commands:
+
+=over 3
+
+=item * run [args]
+
+Run the program with the given arguments.
+
+=item * break function_name
+
+=item * break source.c:xxx
+
+Tells the debugger that we'll want to pause execution when we reach
+either the named function (but see L<perlguts/Internal Functions>!) or
+the given line in the named source file.
+
+=item * step
+
+Steps through the program a line at a time.
+
+=item * next
+
+Steps through the program a line at a time, without descending into
+functions.
+
+=item * continue
+
+Run until the next breakpoint.
+
+=item * finish
+
+Run until the end of the current function, then stop again.
+
+=item * 'enter'
+
+Just pressing Enter will do the most recent operation again - it's a
+blessing when stepping through miles of source code.
+
+=item * print
+
+Execute the given C code and print its results. B<WARNING>: Perl makes
+heavy use of macros, and F<gdb> does not necessarily support macros
+(see later L</"gdb macro support">). You'll have to substitute them
+yourself, or to invoke cpp on the source code files (see L</"The .i
+Targets">) So, for instance, you can't say
+
+ print SvPV_nolen(sv)
+
+but you have to say
+
+ print Perl_sv_2pv_nolen(sv)
+
+=back
+
+You may find it helpful to have a "macro dictionary", which you can
+produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
+recursively apply those macros for you.
+
+=head2 gdb macro support
+
+Recent versions of F<gdb> have fairly good macro support, but in order
+to use it you'll need to compile perl with macro definitions included
+in the debugging information. Using F<gcc> version 3.1, this means
+configuring with C<-Doptimize=-g3>. Other compilers might use a
+different switch (if they support debugging macros at all).
+
+=head2 Dumping Perl Data Structures
+
+One way to get around this macro hell is to use the dumping functions
+in F<dump.c>; these work a little like an internal
+L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
+structures that you can't get at from Perl. Let's take an example.
+We'll use the C<$a = $b + $c> we used before, but give it a bit of
+context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
+poke around?
+
+What about C<pp_add>, the function we examined earlier to implement the
+C<+> operator:
+
+ (gdb) break Perl_pp_add
+ Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
+
+Notice we use C<Perl_pp_add> and not C<pp_add> - see
+L<perlguts/Internal Functions>. With the breakpoint in place, we can
+run our program:
+
+ (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
+
+Lots of junk will go past as gdb reads in the relevant source files and
+libraries, and then:
+
+ Breakpoint 1, Perl_pp_add () at pp_hot.c:309
+ 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
+ (gdb) step
+ 311 dPOPTOPnnrl_ul;
+ (gdb)
+
+We looked at this bit of code before, and we said that
+C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
+C<right> - let's slightly expand it:
+
+ #define dPOPTOPnnrl_ul NV right = POPn; \
+ SV *leftsv = TOPs; \
+ NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
+
+C<POPn> takes the SV from the top of the stack and obtains its NV
+either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
+function. C<TOPs> takes the next SV from the top of the stack - yes,
+C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
+get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
+C<SvNV>.
+
+Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
+convert it. If we step again, we'll find ourselves there:
+
+ Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
+ 1669 if (!sv)
+ (gdb)
+
+We can now use C<Perl_sv_dump> to investigate the SV:
+
+ SV = PV(0xa057cc0) at 0xa0675d0
+ REFCNT = 1
+ FLAGS = (POK,pPOK)
+ PV = 0xa06a510 "6XXXX"\0
+ CUR = 5
+ LEN = 6
+ $1 = void
+
+We know we're going to get C<6> from this, so let's finish the
+subroutine:
+
+ (gdb) finish
+ Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
+ 0x462669 in Perl_pp_add () at pp_hot.c:311
+ 311 dPOPTOPnnrl_ul;
+
+We can also dump out this op: the current op is always stored in
+C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
+similar output to L<B::Debug|B::Debug>.
+
+ {
+ 13 TYPE = add ===> 14
+ TARG = 1
+ FLAGS = (SCALAR,KIDS)
+ {
+ TYPE = null ===> (12)
+ (was rv2sv)
+ FLAGS = (SCALAR,KIDS)
+ {
+ 11 TYPE = gvsv ===> 12
+ FLAGS = (SCALAR)
+ GV = main::b
+ }
+ }
+
+# finish this later #
+
+=head1 SOURCE CODE STATIC ANALYSIS
+
+Various tools exist for analysing C source code B<statically>, as
+opposed to B<dynamically>, that is, without executing the code. It is
+possible to detect resource leaks, undefined behaviour, type
+mismatches, portability problems, code paths that would cause illegal
+memory accesses, and other similar problems by just parsing the C code
+and looking at the resulting graph, what does it tell about the
+execution and data flows. As a matter of fact, this is exactly how C
+compilers know to give warnings about dubious code.
+
+=head2 lint, splint
+
+The good old C code quality inspector, C<lint>, is available in several
+platforms, but please be aware that there are several different
+implementations of it by different vendors, which means that the flags
+are not identical across different platforms.
+
+There is a lint variant called C<splint> (Secure Programming Lint)
+available from http://www.splint.org/ that should compile on any
+Unix-like platform.
+
+There are C<lint> and <splint> targets in Makefile, but you may have to
+diddle with the flags (see above).
+
+=head2 Coverity
+
+Coverity (http://www.coverity.com/) is a product similar to lint and as
+a testbed for their product they periodically check several open source
+projects, and they give out accounts to open source developers to the
+defect databases.
+
+=head2 cpd (cut-and-paste detector)
+
+The cpd tool detects cut-and-paste coding. If one instance of the
+cut-and-pasted code changes, all the other spots should probably be
+changed, too. Therefore such code should probably be turned into a
+subroutine or a macro.
+
+cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
+(http://pmd.sourceforge.net/). pmd was originally written for static
+analysis of Java code, but later the cpd part of it was extended to
+parse also C and C++.
+
+Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
+pmd-X.Y.jar from it, and then run that on source code thusly:
+
+ java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
+
+You may run into memory limits, in which case you should use the -Xmx
+option:
+
+ java -Xmx512M ...
+
+=head2 gcc warnings
+
+Though much can be written about the inconsistency and coverage
+problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
+or some common portability problems not being covered by C<-Wall>, or
+C<-ansi> and C<-pedantic> both being a poorly defined collection of
+warnings, and so forth), gcc is still a useful tool in keeping our
+coding nose clean.
+
+The C<-Wall> is by default on.
+
+The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
+always, but unfortunately they are not safe on all platforms, they can
+for example cause fatal conflicts with the system headers (Solaris
+being a prime example). If Configure C<-Dgccansipedantic> is used, the
+C<cflags> frontend selects C<-ansi -pedantic> for the platforms where
+they are known to be safe.
+
+Starting from Perl 5.9.4 the following extra flags are added:
+
+=over 4
+
+=item *
+
+C<-Wendif-labels>
+
+=item *
+
+C<-Wextra>
+
+=item *
+
+C<-Wdeclaration-after-statement>
+
+=back
+
+The following flags would be nice to have but they would first need
+their own Augean stablemaster:
+
+=over 4
+
+=item *
+
+C<-Wpointer-arith>
+
+=item *
+
+C<-Wshadow>
+
+=item *
+
+C<-Wstrict-prototypes>
+
+=back
+
+The C<-Wtraditional> is another example of the annoying tendency of gcc
+to bundle a lot of warnings under one switch (it would be impossible to
+deploy in practice because it would complain a lot) but it does contain
+some warnings that would be beneficial to have available on their own,
+such as the warning about string constants inside macros containing the
+macro arguments: this behaved differently pre-ANSI than it does in
+ANSI, and some C compilers are still in transition, AIX being an
+example.
+
+=head2 Warnings of other C compilers
+
+Other C compilers (yes, there B<are> other C compilers than gcc) often
+have their "strict ANSI" or "strict ANSI with some portability
+extensions" modes on, like for example the Sun Workshop has its C<-Xa>
+mode on (though implicitly), or the DEC (these days, HP...) has its
+C<-std1> mode on.
+
+=head1 MEMORY DEBUGGERS
+
+B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or
+Third Degree greatly slows down the execution: seconds become minutes,
+minutes become hours. For example as of Perl 5.8.1, the
+ext/Encode/t/Unicode.t takes extraordinarily long to complete under
+e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
+than six hours, even on a snappy computer. The said test must be doing
+something that is quite unfriendly for memory debuggers. If you don't
+feel like waiting, that you can simply kill away the perl process.
+
+B<NOTE 2>: To minimize the number of memory leak false alarms (see
+L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
+environment variable PERL_DESTRUCT_LEVEL to 2.
+
+For csh-like shells:
+
+ setenv PERL_DESTRUCT_LEVEL 2
+
+For Bourne-type shells:
+
+ PERL_DESTRUCT_LEVEL=2
+ export PERL_DESTRUCT_LEVEL
+
+In Unixy environments you can also use the C<env> command:
+
+ env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
+
+B<NOTE 3>: There are known memory leaks when there are compile-time
+errors within eval or require, seeing C<S_doeval> in the call stack is
+a good sign of these. Fixing these leaks is non-trivial, unfortunately,
+but they must be fixed eventually.
+
+B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
+unless Perl is built with the Configure option
+C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
+
+=head2 Rational Software's Purify
+
+Purify is a commercial tool that is helpful in identifying memory
+overruns, wild pointers, memory leaks and other such badness. Perl must
+be compiled in a specific way for optimal testing with Purify. Purify
+is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
+
+=head3 Purify on Unix
+
+On Unix, Purify creates a new Perl binary. To get the most benefit out
+of Purify, you should create the perl to Purify using:
+
+ sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
+ -Uusemymalloc -Dusemultiplicity
+
+where these arguments mean:
+
+=over 4
+
+=item * -Accflags=-DPURIFY
+
+Disables Perl's arena memory allocation functions, as well as forcing
+use of memory allocation functions derived from the system malloc.
+
+=item * -Doptimize='-g'
+
+Adds debugging information so that you see the exact source statements
+where the problem occurs. Without this flag, all you will see is the
+source filename of where the error occurred.
+
+=item * -Uusemymalloc
+
+Disable Perl's malloc so that Purify can more closely monitor
+allocations and leaks. Using Perl's malloc will make Purify report most
+leaks in the "potential" leaks category.
+
+=item * -Dusemultiplicity
+
+Enabling the multiplicity option allows perl to clean up thoroughly
+when the interpreter shuts down, which reduces the number of bogus leak
+reports from Purify.
+
+=back
+
+Once you've compiled a perl suitable for Purify'ing, then you can just:
+
+ make pureperl
+
+which creates a binary named 'pureperl' that has been Purify'ed. This
+binary is used in place of the standard 'perl' binary when you want to
+debug Perl memory problems.
+
+As an example, to show any memory leaks produced during the standard
+Perl testset you would create and run the Purify'ed perl as:
+
+ make pureperl
+ cd t
+ ../pureperl -I../lib harness
+
+which would run Perl on test.pl and report any memory problems.
+
+Purify outputs messages in "Viewer" windows by default. If you don't
+have a windowing environment or if you simply want the Purify output to
+unobtrusively go to a log file instead of to the interactive window,
+use these following options to output to the log file "perl.log":
+
+ setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
+ -log-file=perl.log -append-logfile=yes"
+
+If you plan to use the "Viewer" windows, then you only need this
+option:
+
+ setenv PURIFYOPTIONS "-chain-length=25"
+
+In Bourne-type shells:
+
+ PURIFYOPTIONS="..."
+ export PURIFYOPTIONS
+
+or if you have the "env" utility:
+
+ env PURIFYOPTIONS="..." ../pureperl ...
+
+=head3 Purify on NT
+
+Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly.
+ There are several options in the makefile you should change to get the
+most use out of Purify:
+
+=over 4
+
+=item * DEFINES
+
+You should add -DPURIFY to the DEFINES line so the DEFINES line looks
+something like:
+
+ DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
+
+to disable Perl's arena memory allocation functions, as well as to
+force use of memory allocation functions derived from the system
+malloc.
+
+=item * USE_MULTI = define
+
+Enabling the multiplicity option allows perl to clean up thoroughly
+when the interpreter shuts down, which reduces the number of bogus leak
+reports from Purify.
+
+=item * #PERL_MALLOC = define
+
+Disable Perl's malloc so that Purify can more closely monitor
+allocations and leaks. Using Perl's malloc will make Purify report most
+leaks in the "potential" leaks category.
+
+=item * CFG = Debug
+
+Adds debugging information so that you see the exact source statements
+where the problem occurs. Without this flag, all you will see is the
+source filename of where the error occurred.
+
+=back
+
+As an example, to show any memory leaks produced during the standard
+Perl testset you would create and run Purify as:
+
+ cd win32
+ make
+ cd ../t
+ purify ../perl -I../lib harness
+
+which would instrument Perl in memory, run Perl on test.pl, then
+finally report any memory problems.
+
+=head2 valgrind
+
+The excellent valgrind tool can be used to find out both memory leaks
+and illegal memory accesses. As of version 3.3.0, Valgrind only
+supports Linux on x86, x86-64 and PowerPC. The special "test.valgrind"
+target can be used to run the tests under valgrind. Found errors and
+memory leaks are logged in files named F<testfile.valgrind>.
+
+Valgrind also provides a cachegrind tool, invoked on perl as:
+
+ VG_OPTS=--tool=cachegrind make test.valgrind
+
+As system libraries (most notably glibc) are also triggering errors,
+valgrind allows to suppress such errors using suppression files. The
+default suppression file that comes with valgrind already catches a lot
+of them. Some additional suppressions are defined in F<t/perl.supp>.
+
+To get valgrind and for more information see
+
+ http://developer.kde.org/~sewardj/
+
+=head1 PROFILING
+
+Depending on your platform there are various ways of profiling Perl.
+
+There are two commonly used techniques of profiling executables:
+I<statistical time-sampling> and I<basic-block counting>.
+
+The first method takes periodically samples of the CPU program counter,
+and since the program counter can be correlated with the code generated
+for functions, we get a statistical view of in which functions the
+program is spending its time. The caveats are that very small/fast
+functions have lower probability of showing up in the profile, and that
+periodically interrupting the program (this is usually done rather
+frequently, in the scale of milliseconds) imposes an additional
+overhead that may skew the results. The first problem can be alleviated
+by running the code for longer (in general this is a good idea for
+profiling), the second problem is usually kept in guard by the
+profiling tools themselves.
+
+The second method divides up the generated code into I<basic blocks>.
+Basic blocks are sections of code that are entered only in the
+beginning and exited only at the end. For example, a conditional jump
+starts a basic block. Basic block profiling usually works by
+I<instrumenting> the code by adding I<enter basic block #nnnn>
+book-keeping code to the generated code. During the execution of the
+code the basic block counters are then updated appropriately. The
+caveat is that the added extra code can skew the results: again, the
+profiling tools usually try to factor their own effects out of the
+results.
+
+=head2 Gprof Profiling
+
+gprof is a profiling tool available in many Unix platforms, it uses
+F<statistical time-sampling>.
+
+You can build a profiled version of perl called "perl.gprof" by
+invoking the make target "perl.gprof" (What is required is that Perl
+must be compiled using the C<-pg> flag, you may need to re-Configure).
+Running the profiled version of Perl will create an output file called
+F<gmon.out> is created which contains the profiling data collected
+during the execution.
+
+The gprof tool can then display the collected data in various ways.
+Usually gprof understands the following options:
+
+=over 4
+
+=item * -a
+
+Suppress statically defined functions from the profile.
+
+=item * -b
+
+Suppress the verbose descriptions in the profile.
+
+=item * -e routine
+
+Exclude the given routine and its descendants from the profile.
+
+=item * -f routine
+
+Display only the given routine and its descendants in the profile.
+
+=item * -s
+
+Generate a summary file called F<gmon.sum> which then may be given to
+subsequent gprof runs to accumulate data over several runs.
+
+=item * -z
+
+Display routines that have zero usage.
+
+=back
+
+For more detailed explanation of the available commands and output
+formats, see your own local documentation of gprof.
+
+quick hint:
+
+ $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
+ $ ./perl.gprof someprog # creates gmon.out in current directory
+ $ gprof ./perl.gprof > out
+ $ view out
+
+=head2 GCC gcov Profiling
+
+Starting from GCC 3.0 I<basic block profiling> is officially available
+for the GNU CC.
+
+You can build a profiled version of perl called F<perl.gcov> by
+invoking the make target "perl.gcov" (what is required that Perl must
+be compiled using gcc with the flags C<-fprofile-arcs -ftest-coverage>,
+you may need to re-Configure).
+
+Running the profiled version of Perl will cause profile output to be
+generated. For each source file an accompanying ".da" file will be
+created.
+
+To display the results you use the "gcov" utility (which should be
+installed if you have gcc 3.0 or newer installed). F<gcov> is run on
+source code files, like this
+
+ gcov sv.c
+
+which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain
+the source code annotated with relative frequencies of execution
+indicated by "#" markers.
+
+Useful options of F<gcov> include C<-b> which will summarise the basic
+block, branch, and function call coverage, and C<-c> which instead of
+relative frequencies will use the actual counts. For more information
+on the use of F<gcov> and basic block profiling with gcc, see the
+latest GNU CC manual, as of GCC 3.0 see
+
+ http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
+
+and its section titled "8. gcov: a Test Coverage Program"
+
+ http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
+
+quick hint:
+
+ $ sh Configure -des -Dusedevel -Doptimize='-g' \
+ -Accflags='-fprofile-arcs -ftest-coverage' \
+ -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
+ $ rm -f regexec.c.gcov regexec.gcda
+ $ ./perl.gcov
+ $ gcov regexec.c
+ $ view regexec.c.gcov
+
+=head1 MISCELLANEOUS TRICKS
+
+=head2 PERL_DESTRUCT_LEVEL
+
+If you want to run any of the tests yourself manually using e.g.
+valgrind, or the pureperl or perl.third executables, please note that
+by default perl B<does not> explicitly cleanup all the memory it has
+allocated (such as global memory arenas) but instead lets the exit() of
+the whole program "take care" of such allocations, also known as
+"global destruction of objects".
+
+There is a way to tell perl to do complete cleanup: set the environment
+variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
+does set this to 2, and this is what you need to do too, if you don't
+want to see the "global leaks": For example, for "third-degreed" Perl:
+
+ env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
+
+(Note: the mod_perl apache module uses also this environment variable
+for its own purposes and extended its semantics. Refer to the mod_perl
+documentation for more information. Also, spawned threads do the
+equivalent of setting this variable to the value 1.)
+
+If, at the end of a run you get the message I<N scalars leaked>, you
+can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the
+addresses of all those leaked SVs to be dumped along with details as to
+where each SV was originally allocated. This information is also
+displayed by Devel::Peek. Note that the extra details recorded with
+each SV increases memory usage, so it shouldn't be used in production
+environments. It also converts C<new_SV()> from a macro into a real
+function, so you can use your favourite debugger to discover where
+those pesky SVs were allocated.
+
+If you see that you're leaking memory at runtime, but neither valgrind
+nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
+leaking SVs that are still reachable and will be properly cleaned up
+during destruction of the interpreter. In such cases, using the C<-Dm>
+switch can point you to the source of the leak. If the executable was
+built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
+allocations in addition to memory allocations. Each SV allocation has a
+distinct serial number that will be written on creation and destruction
+of the SV. So if you're executing the leaking code in a loop, you need
+to look for SVs that are created, but never destroyed between each
+cycle. If such an SV is found, set a conditional breakpoint within
+C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the
+serial number of the leaking SV. Then you will catch the interpreter in
+exactly the state where the leaking SV is allocated, which is
+sufficient in many cases to find the source of the leak.
+
+As C<-Dm> is using the PerlIO layer for output, it will by itself
+allocate quite a bunch of SVs, which are hidden to avoid recursion. You
+can bypass the PerlIO layer if you use the SV logging provided by
+C<-DPERL_MEM_LOG> instead.
+
+=head2 PERL_MEM_LOG
+
+If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
+through logging functions, which is handy for breakpoint setting.
+
+Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions
+read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
+so how:
+
+ $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
+ $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
+ $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
+ $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
+
+Memory logging is somewhat similar to C<-Dm> but is independent of
+C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
+Safefree() are logged with the caller's source code file and line
+number (and C function name, if supported by the C compiler). In
+contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is
+similar.
+
+Since the logging doesn't use PerlIO, all SV allocations are logged and
+no extra SV allocations are introduced by enabling the logging. If
+compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
+allocation is also logged.
+
+=head2 DDD over gdb
+
+Those debugging perl with the DDD frontend over gdb may find the
+following useful:
+
+You can extend the data conversion shortcuts menu, so for example you
+can display an SV's IV value with one click, without doing any typing.
+To do that simply edit ~/.ddd/init file and add after:
+
+ ! Display shortcuts.
+ Ddd*gdbDisplayShortcuts: \
+ /t () // Convert to Bin\n\
+ /d () // Convert to Dec\n\
+ /x () // Convert to Hex\n\
+ /o () // Convert to Oct(\n\
+
+the following two lines:
+
+ ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
+ ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
+
+so now you can do ivx and pvx lookups or you can plug there the sv_peek
+"conversion":
+
+ Perl_sv_peek(my_perl, (SV*)()) // sv_peek
+
+(The my_perl is for threaded builds.) Just remember that every line,
+but the last one, should end with \n\
+
+Alternatively edit the init file interactively via: 3rd mouse button ->
+New Display -> Edit Menu
+
+Note: you can define up to 20 conversion shortcuts in the gdb section.
+
+=head2 Poison
+
+If you see in a debugger a memory area mysteriously full of 0xABABABAB
+or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
+L<perlclib>.
+
+=head2 Read-only optrees
+
+Under ithreads the optree is read only. If you want to enforce this, to
+check for write accesses from buggy code, compile with
+C<-DPL_OP_SLAB_ALLOC> to enable the OP slab allocator and
+C<-DPERL_DEBUG_READONLY_OPS> to enable code that allocates op memory
+via C<mmap>, and sets it read-only at run time. Any write access to an
+op results in a C<SIGBUS> and abort.
+
+This code is intended for development only, and may not be portable
+even to all Unix variants. Also, it is an 80% solution, in that it
+isn't able to make all ops read only. Specifically it
+
+=over
+
+=item * 1
+
+Only sets read-only on all slabs of ops at C<CHECK> time, hence ops
+allocated later via C<require> or C<eval> will be re-write
+
+=item * 2
+
+Turns an entire slab of ops read-write if the refcount of any op in the
+slab needs to be decreased.
+
+=item * 3
+
+Turns an entire slab of ops read-write if any op from the slab is
+freed.
+
+=back
+
+It's not possible to turn the slabs to read-only after an action
+requiring read-write access, as either can happen during op tree
+building time, so there may still be legitimate write access.
+
+However, as an 80% solution it is still effective, as currently it
+catches a write access during the generation of F<Config.pm>, which
+means that we can't yet build F<perl> with this enabled.
+
+=head2 The .i Targets
+
+You can expand the macros in a F<foo.c> file by saying
+
+ make foo.i
+
+which will expand the macros using cpp. Don't be scared by the results.
+
+=head1 AUTHOR
+
+This document was originally written by Nathan Torkington, and is
+maintained by the perl5-porters mailing list.
--- /dev/null
+=encoding utf8
+
+=for comment
+Consistent formatting of this file is achieved with:
+ perl ./Porting/podtidy pod/perlinterp.pod
+
+=head1 NAME
+
+perlinterp - An overview of the Perl interpreter
+
+=head1 DESCRIPTION
+
+This document provides an overview of how the Perl interpreter works at
+the level of C code, along with pointers to the relevant C source code
+files.
+
+=head1 ELEMENTS OF THE INTERPRETER
+
+The work of the interpreter has two main stages: compiling the code
+into the internal representation, or bytecode, and then executing it.
+L<perlguts/Compiled code> explains exactly how the compilation stage
+happens.
+
+Here is a short breakdown of perl's operation:
+
+=head2 Startup
+
+The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
+This is very high-level code, enough to fit on a single screen, and it
+resembles the code found in L<perlembed>; most of the real action takes
+place in F<perl.c>
+
+F<perlmain.c> is generated by C<ExtUtils::Miniperl> from
+F<miniperlmain.c> at make time, so you should make perl to follow this
+along.
+
+First, F<perlmain.c> allocates some memory and constructs a Perl
+interpreter, along these lines:
+
+ 1 PERL_SYS_INIT3(&argc,&argv,&env);
+ 2
+ 3 if (!PL_do_undump) {
+ 4 my_perl = perl_alloc();
+ 5 if (!my_perl)
+ 6 exit(1);
+ 7 perl_construct(my_perl);
+ 8 PL_perl_destruct_level = 0;
+ 9 }
+
+Line 1 is a macro, and its definition is dependent on your operating
+system. Line 3 references C<PL_do_undump>, a global variable - all
+global variables in Perl start with C<PL_>. This tells you whether the
+current running program was created with the C<-u> flag to perl and
+then F<undump>, which means it's going to be false in any sane context.
+
+Line 4 calls a function in F<perl.c> to allocate memory for a Perl
+interpreter. It's quite a simple function, and the guts of it looks
+like this:
+
+ my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
+
+Here you see an example of Perl's system abstraction, which we'll see
+later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
+own C<malloc> as defined in F<malloc.c> if you selected that option at
+configure time.
+
+Next, in line 7, we construct the interpreter using perl_construct,
+also in F<perl.c>; this sets up all the special variables that Perl
+needs, the stacks, and so on.
+
+Now we pass Perl the command line options, and tell it to go:
+
+ exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
+ if (!exitstatus)
+ perl_run(my_perl);
+
+ exitstatus = perl_destruct(my_perl);
+
+ perl_free(my_perl);
+
+C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
+in F<perl.c>, which processes the command line options, sets up any
+statically linked XS modules, opens the program and calls C<yyparse> to
+parse it.
+
+=head2 Parsing
+
+The aim of this stage is to take the Perl source, and turn it into an
+op tree. We'll see what one of those looks like later. Strictly
+speaking, there's three things going on here.
+
+C<yyparse>, the parser, lives in F<perly.c>, although you're better off
+reading the original YACC input in F<perly.y>. (Yes, Virginia, there
+B<is> a YACC grammar for Perl!) The job of the parser is to take your
+code and "understand" it, splitting it into sentences, deciding which
+operands go with which operators and so on.
+
+The parser is nobly assisted by the lexer, which chunks up your input
+into tokens, and decides what type of thing each token is: a variable
+name, an operator, a bareword, a subroutine, a core function, and so
+on. The main point of entry to the lexer is C<yylex>, and that and its
+associated routines can be found in F<toke.c>. Perl isn't much like
+other computer languages; it's highly context sensitive at times, it
+can be tricky to work out what sort of token something is, or where a
+token ends. As such, there's a lot of interplay between the tokeniser
+and the parser, which can get pretty frightening if you're not used to
+it.
+
+As the parser understands a Perl program, it builds up a tree of
+operations for the interpreter to perform during execution. The
+routines which construct and link together the various operations are
+to be found in F<op.c>, and will be examined later.
+
+=head2 Optimization
+
+Now the parsing stage is complete, and the finished tree represents the
+operations that the Perl interpreter needs to perform to execute our
+program. Next, Perl does a dry run over the tree looking for
+optimisations: constant expressions such as C<3 + 4> will be computed
+now, and the optimizer will also see if any multiple operations can be
+replaced with a single one. For instance, to fetch the variable
+C<$foo>, instead of grabbing the glob C<*foo> and looking at the scalar
+component, the optimizer fiddles the op tree to use a function which
+directly looks up the scalar in question. The main optimizer is C<peep>
+in F<op.c>, and many ops have their own optimizing functions.
+
+=head2 Running
+
+Now we're finally ready to go: we have compiled Perl byte code, and all
+that's left to do is run it. The actual execution is done by the
+C<runops_standard> function in F<run.c>; more specifically, it's done
+by these three innocent looking lines:
+
+ while ((PL_op = PL_op->op_ppaddr(aTHX))) {
+ PERL_ASYNC_CHECK();
+ }
+
+You may be more comfortable with the Perl version of that:
+
+ PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
+
+Well, maybe not. Anyway, each op contains a function pointer, which
+stipulates the function which will actually carry out the operation.
+This function will return the next op in the sequence - this allows for
+things like C<if> which choose the next op dynamically at run time. The
+C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
+execution if required.
+
+The actual functions called are known as PP code, and they're spread
+between four files: F<pp_hot.c> contains the "hot" code, which is most
+often used and highly optimized, F<pp_sys.c> contains all the
+system-specific functions, F<pp_ctl.c> contains the functions which
+implement control structures (C<if>, C<while> and the like) and F<pp.c>
+contains everything else. These are, if you like, the C code for Perl's
+built-in functions and operators.
+
+Note that each C<pp_> function is expected to return a pointer to the
+next op. Calls to perl subs (and eval blocks) are handled within the
+same runops loop, and do not consume extra space on the C stack. For
+example, C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or
+C<CxEVAL> block struct onto the context stack which contain the address
+of the op following the sub call or eval. They then return the first op
+of that sub or eval block, and so execution continues of that sub or
+block. Later, a C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB>
+or C<CxEVAL>, retrieves the return op from it, and returns it.
+
+=head2 Exception handing
+
+Perl's exception handing (i.e. C<die> etc.) is built on top of the
+low-level C<setjmp()>/C<longjmp()> C-library functions. These basically
+provide a way to capture the current PC and SP registers and later
+restore them; i.e. a C<longjmp()> continues at the point in code where
+a previous C<setjmp()> was done, with anything further up on the C
+stack being lost. This is why code should always save values using
+C<SAVE_FOO> rather than in auto variables.
+
+The perl core wraps C<setjmp()> etc in the macros C<JMPENV_PUSH> and
+C<JMPENV_JUMP>. The basic rule of perl exceptions is that C<exit>, and
+C<die> (in the absence of C<eval>) perform a C<JMPENV_JUMP(2)>, while
+C<die> within C<eval> does a C<JMPENV_JUMP(3)>.
+
+At entry points to perl, such as C<perl_parse()>, C<perl_run()> and
+C<call_sv(cv, G_EVAL)> each does a C<JMPENV_PUSH>, then enter a runops
+loop or whatever, and handle possible exception returns. For a 2
+return, final cleanup is performed, such as popping stacks and calling
+C<CHECK> or C<END> blocks. Amongst other things, this is how scope
+cleanup still occurs during an C<exit>.
+
+If a C<die> can find a C<CxEVAL> block on the context stack, then the
+stack is popped to that level and the return op in that block is
+assigned to C<PL_restartop>; then a C<JMPENV_JUMP(3)> is performed.
+This normally passes control back to the guard. In the case of
+C<perl_run> and C<call_sv>, a non-null C<PL_restartop> triggers
+re-entry to the runops loop. The is the normal way that C<die> or
+C<croak> is handled within an C<eval>.
+
+Sometimes ops are executed within an inner runops loop, such as tie,
+sort or overload code. In this case, something like
+
+ sub FETCH { eval { die } }
+
+would cause a longjmp right back to the guard in C<perl_run>, popping
+both runops loops, which is clearly incorrect. One way to avoid this is
+for the tie code to do a C<JMPENV_PUSH> before executing C<FETCH> in
+the inner runops loop, but for efficiency reasons, perl in fact just
+sets a flag, using C<CATCH_SET(TRUE)>. The C<pp_require>,
+C<pp_entereval> and C<pp_entertry> ops check this flag, and if true,
+they call C<docatch>, which does a C<JMPENV_PUSH> and starts a new
+runops level to execute the code, rather than doing it on the current
+loop.
+
+As a further optimisation, on exit from the eval block in the C<FETCH>,
+execution of the code following the block is still carried on in the
+inner loop. When an exception is raised, C<docatch> compares the
+C<JMPENV> level of the C<CxEVAL> with C<PL_top_env> and if they differ,
+just re-throws the exception. In this way any inner loops get popped.
+
+Here's an example.
+
+ 1: eval { tie @a, 'A' };
+ 2: sub A::TIEARRAY {
+ 3: eval { die };
+ 4: die;
+ 5: }
+
+To run this code, C<perl_run> is called, which does a C<JMPENV_PUSH>
+then enters a runops loop. This loop executes the eval and tie ops on
+line 1, with the eval pushing a C<CxEVAL> onto the context stack.
+
+The C<pp_tie> does a C<CATCH_SET(TRUE)>, then starts a second runops
+loop to execute the body of C<TIEARRAY>. When it executes the entertry
+op on line 3, C<CATCH_GET> is true, so C<pp_entertry> calls C<docatch>
+which does a C<JMPENV_PUSH> and starts a third runops loop, which then
+executes the die op. At this point the C call stack looks like this:
+
+ Perl_pp_die
+ Perl_runops # third loop
+ S_docatch_body
+ S_docatch
+ Perl_pp_entertry
+ Perl_runops # second loop
+ S_call_body
+ Perl_call_sv
+ Perl_pp_tie
+ Perl_runops # first loop
+ S_run_body
+ perl_run
+ main
+
+and the context and data stacks, as shown by C<-Dstv>, look like:
+
+ STACK 0: MAIN
+ CX 0: BLOCK =>
+ CX 1: EVAL => AV() PV("A"\0)
+ retop=leave
+ STACK 1: MAGIC
+ CX 0: SUB =>
+ retop=(null)
+ CX 1: EVAL => *
+ retop=nextstate
+
+The die pops the first C<CxEVAL> off the context stack, sets
+C<PL_restartop> from it, does a C<JMPENV_JUMP(3)>, and control returns
+to the top C<docatch>. This then starts another third-level runops
+level, which executes the nextstate, pushmark and die ops on line 4. At
+the point that the second C<pp_die> is called, the C call stack looks
+exactly like that above, even though we are no longer within an inner
+eval; this is because of the optimization mentioned earlier. However,
+the context stack now looks like this, ie with the top CxEVAL popped:
+
+ STACK 0: MAIN
+ CX 0: BLOCK =>
+ CX 1: EVAL => AV() PV("A"\0)
+ retop=leave
+ STACK 1: MAGIC
+ CX 0: SUB =>
+ retop=(null)
+
+The die on line 4 pops the context stack back down to the CxEVAL,
+leaving it as:
+
+ STACK 0: MAIN
+ CX 0: BLOCK =>
+
+As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
+C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:
+
+ S_docatch
+ Perl_pp_entertry
+ Perl_runops # second loop
+ S_call_body
+ Perl_call_sv
+ Perl_pp_tie
+ Perl_runops # first loop
+ S_run_body
+ perl_run
+ main
+
+In this case, because the C<JMPENV> level recorded in the C<CxEVAL>
+differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
+and the C stack unwinds to:
+
+ perl_run
+ main
+
+Because C<PL_restartop> is non-null, C<run_body> starts a new runops
+loop and execution continues.
+
+=head2 INTERNAL VARIABLE TYPES
+
+You should by now have had a look at L<perlguts>, which tells you about
+Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
+that now.
+
+These variables are used not only to represent Perl-space variables,
+but also any constants in the code, as well as some structures
+completely internal to Perl. The symbol table, for instance, is an
+ordinary Perl hash. Your code is represented by an SV as it's read into
+the parser; any program files you call are opened via ordinary Perl
+filehandles, and so on.
+
+The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
+Perl program. Let's see, for instance, how Perl treats the constant
+C<"hello">.
+
+ % perl -MDevel::Peek -e 'Dump("hello")'
+ 1 SV = PV(0xa041450) at 0xa04ecbc
+ 2 REFCNT = 1
+ 3 FLAGS = (POK,READONLY,pPOK)
+ 4 PV = 0xa0484e0 "hello"\0
+ 5 CUR = 5
+ 6 LEN = 6
+
+Reading C<Devel::Peek> output takes a bit of practise, so let's go
+through it line by line.
+
+Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
+memory. SVs themselves are very simple structures, but they contain a
+pointer to a more complex structure. In this case, it's a PV, a
+structure which holds a string value, at location C<0xa041450>. Line 2
+is the reference count; there are no other references to this data, so
+it's 1.
+
+Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
+read-only SV (because it's a constant) and the data is a PV internally.
+Next we've got the contents of the string, starting at location
+C<0xa0484e0>.
+
+Line 5 gives us the current length of the string - note that this does
+B<not> include the null terminator. Line 6 is not the length of the
+string, but the length of the currently allocated buffer; as the string
+grows, Perl automatically extends the available storage via a routine
+called C<SvGROW>.
+
+You can get at any of these quantities from C very easily; just add
+C<Sv> to the name of the field shown in the snippet, and you've got a
+macro which will return the value: C<SvCUR(sv)> returns the current
+length of the string, C<SvREFCOUNT(sv)> returns the reference count,
+C<SvPV(sv, len)> returns the string itself with its length, and so on.
+More macros to manipulate these properties can be found in L<perlguts>.
+
+Let's take an example of manipulating a PV, from C<sv_catpvn>, in
+F<sv.c>
+
+ 1 void
+ 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
+ 3 {
+ 4 STRLEN tlen;
+ 5 char *junk;
+
+ 6 junk = SvPV_force(sv, tlen);
+ 7 SvGROW(sv, tlen + len + 1);
+ 8 if (ptr == junk)
+ 9 ptr = SvPVX(sv);
+ 10 Move(ptr,SvPVX(sv)+tlen,len,char);
+ 11 SvCUR(sv) += len;
+ 12 *SvEND(sv) = '\0';
+ 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
+ 14 SvTAINT(sv);
+ 15 }
+
+This is a function which adds a string, C<ptr>, of length C<len> onto
+the end of the PV stored in C<sv>. The first thing we do in line 6 is
+make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
+macro to force a PV. As a side effect, C<tlen> gets set to the current
+value of the PV, and the PV itself is returned to C<junk>.
+
+In line 7, we make sure that the SV will have enough room to
+accommodate the old string, the new string and the null terminator. If
+C<LEN> isn't big enough, C<SvGROW> will reallocate space for us.
+
+Now, if C<junk> is the same as the string we're trying to add, we can
+grab the string directly from the SV; C<SvPVX> is the address of the PV
+in the SV.
+
+Line 10 does the actual catenation: the C<Move> macro moves a chunk of
+memory around: we move the string C<ptr> to the end of the PV - that's
+the start of the PV plus its current length. We're moving C<len> bytes
+of type C<char>. After doing so, we need to tell Perl we've extended
+the string, by altering C<CUR> to reflect the new length. C<SvEND> is a
+macro which gives us the end of the string, so that needs to be a
+C<"\0">.
+
+Line 13 manipulates the flags; since we've changed the PV, any IV or NV
+values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
+want to use the old IV of 10. C<SvPOK_only_utf8> is a special
+UTF-8-aware version of C<SvPOK_only>, a macro which turns off the IOK
+and NOK flags and turns on POK. The final C<SvTAINT> is a macro which
+launders tainted data if taint mode is turned on.
+
+AVs and HVs are more complicated, but SVs are by far the most common
+variable type being thrown around. Having seen something of how we
+manipulate these, let's go on and look at how the op tree is
+constructed.
+
+=head1 OP TREES
+
+First, what is the op tree, anyway? The op tree is the parsed
+representation of your program, as we saw in our section on parsing,
+and it's the sequence of operations that Perl goes through to execute
+your program, as we saw in L</Running>.
+
+An op is a fundamental operation that Perl can perform: all the
+built-in functions and operators are ops, and there are a series of ops
+which deal with concepts the interpreter needs internally - entering
+and leaving a block, ending a statement, fetching a variable, and so
+on.
+
+The op tree is connected in two ways: you can imagine that there are
+two "routes" through it, two orders in which you can traverse the tree.
+First, parse order reflects how the parser understood the code, and
+secondly, execution order tells perl what order to perform the
+operations in.
+
+The easiest way to examine the op tree is to stop Perl after it has
+finished parsing, and get it to dump out the tree. This is exactly what
+the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
+and L<B::Debug|B::Debug> do.
+
+Let's have a look at how Perl sees C<$a = $b + $c>:
+
+ % perl -MO=Terse -e '$a=$b+$c'
+ 1 LISTOP (0x8179888) leave
+ 2 OP (0x81798b0) enter
+ 3 COP (0x8179850) nextstate
+ 4 BINOP (0x8179828) sassign
+ 5 BINOP (0x8179800) add [1]
+ 6 UNOP (0x81796e0) null [15]
+ 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
+ 8 UNOP (0x81797e0) null [15]
+ 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
+ 10 UNOP (0x816b4f0) null [15]
+ 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
+
+Let's start in the middle, at line 4. This is a BINOP, a binary
+operator, which is at location C<0x8179828>. The specific operator in
+question is C<sassign> - scalar assignment - and you can find the code
+which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
+binary operator, it has two children: the add operator, providing the
+result of C<$b+$c>, is uppermost on line 5, and the left hand side is
+on line 10.
+
+Line 10 is the null op: this does exactly nothing. What is that doing
+there? If you see the null op, it's a sign that something has been
+optimized away after parsing. As we mentioned in L</Optimization>, the
+optimization stage sometimes converts two operations into one, for
+example when fetching a scalar variable. When this happens, instead of
+rewriting the op tree and cleaning up the dangling pointers, it's
+easier just to replace the redundant operation with the null op.
+Originally, the tree would have looked like this:
+
+ 10 SVOP (0x816b4f0) rv2sv [15]
+ 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
+
+That is, fetch the C<a> entry from the main symbol table, and then look
+at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>)
+happens to do both these things.
+
+The right hand side, starting at line 5 is similar to what we've just
+seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add
+together two C<gvsv>s.
+
+Now, what's this about?
+
+ 1 LISTOP (0x8179888) leave
+ 2 OP (0x81798b0) enter
+ 3 COP (0x8179850) nextstate
+
+C<enter> and C<leave> are scoping ops, and their job is to perform any
+housekeeping every time you enter and leave a block: lexical variables
+are tidied up, unreferenced variables are destroyed, and so on. Every
+program will have those first three lines: C<leave> is a list, and its
+children are all the statements in the block. Statements are delimited
+by C<nextstate>, so a block is a collection of C<nextstate> ops, with
+the ops to be performed for each statement being the children of
+C<nextstate>. C<enter> is a single op which functions as a marker.
+
+That's how Perl parsed the program, from top to bottom:
+
+ Program
+ |
+ Statement
+ |
+ =
+ / \
+ / \
+ $a +
+ / \
+ $b $c
+
+However, it's impossible to B<perform> the operations in this order:
+you have to find the values of C<$b> and C<$c> before you add them
+together, for instance. So, the other thread that runs through the op
+tree is the execution order: each op has a field C<op_next> which
+points to the next op to be run, so following these pointers tells us
+how perl executes the code. We can traverse the tree in this order
+using the C<exec> option to C<B::Terse>:
+
+ % perl -MO=Terse,exec -e '$a=$b+$c'
+ 1 OP (0x8179928) enter
+ 2 COP (0x81798c8) nextstate
+ 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
+ 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
+ 5 BINOP (0x8179878) add [1]
+ 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
+ 7 BINOP (0x81798a0) sassign
+ 8 LISTOP (0x8179900) leave
+
+This probably makes more sense for a human: enter a block, start a
+statement. Get the values of C<$b> and C<$c>, and add them together.
+Find C<$a>, and assign one to the other. Then leave.
+
+The way Perl builds up these op trees in the parsing process can be
+unravelled by examining F<perly.y>, the YACC grammar. Let's take the
+piece we need to construct the tree for C<$a = $b + $c>
+
+ 1 term : term ASSIGNOP term
+ 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
+ 3 | term ADDOP term
+ 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
+
+If you're not used to reading BNF grammars, this is how it works:
+You're fed certain things by the tokeniser, which generally end up in
+upper case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in
+your code. C<ASSIGNOP> is provided when C<=> is used for assigning.
+These are "terminal symbols", because you can't get any simpler than
+them.
+
+The grammar, lines one and three of the snippet above, tells you how to
+build up more complex forms. These complex forms, "non-terminal
+symbols" are generally placed in lower case. C<term> here is a
+non-terminal symbol, representing a single expression.
+
+The grammar gives you the following rule: you can make the thing on the
+left of the colon if you see all the things on the right in sequence.
+This is called a "reduction", and the aim of parsing is to completely
+reduce the input. There are several different ways you can perform a
+reduction, separated by vertical bars: so, C<term> followed by C<=>
+followed by C<term> makes a C<term>, and C<term> followed by C<+>
+followed by C<term> can also make a C<term>.
+
+So, if you see two terms with an C<=> or C<+>, between them, you can
+turn them into a single expression. When you do this, you execute the
+code in the block on the next line: if you see C<=>, you'll do the code
+in line 2. If you see C<+>, you'll do the code in line 4. It's this
+code which contributes to the op tree.
+
+ | term ADDOP term
+ { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
+
+What this does is creates a new binary op, and feeds it a number of
+variables. The variables refer to the tokens: C<$1> is the first token
+in the input, C<$2> the second, and so on - think regular expression
+backreferences. C<$$> is the op returned from this reduction. So, we
+call C<newBINOP> to create a new binary operator. The first parameter
+to C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
+operator, so we want the type to be C<ADDOP>. We could specify this
+directly, but it's right there as the second token in the input, so we
+use C<$2>. The second parameter is the op's flags: 0 means "nothing
+special". Then the things to add: the left and right hand side of our
+expression, in scalar context.
+
+=head1 STACKS
+
+When perl executes something like C<addop>, how does it pass on its
+results to the next op? The answer is, through the use of stacks. Perl
+has a number of stacks to store things it's currently working on, and
+we'll look at the three most important ones here.
+
+=head2 Argument stack
+
+Arguments are passed to PP code and returned from PP code using the
+argument stack, C<ST>. The typical way to handle arguments is to pop
+them off the stack, deal with them how you wish, and then push the
+result back onto the stack. This is how, for instance, the cosine
+operator works:
+
+ NV value;
+ value = POPn;
+ value = Perl_cos(value);
+ XPUSHn(value);
+
+We'll see a more tricky example of this when we consider Perl's macros
+below. C<POPn> gives you the NV (floating point value) of the top SV on
+the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and
+push the result back as an NV. The C<X> in C<XPUSHn> means that the
+stack should be extended if necessary - it can't be necessary here,
+because we know there's room for one more item on the stack, since
+we've just removed one! The C<XPUSH*> macros at least guarantee safety.
+
+Alternatively, you can fiddle with the stack directly: C<SP> gives you
+the first element in your portion of the stack, and C<TOP*> gives you
+the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
+negation of an integer:
+
+ SETi(-TOPi);
+
+Just set the integer value of the top stack entry to its negation.
+
+Argument stack manipulation in the core is exactly the same as it is in
+XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
+description of the macros used in stack manipulation.
+
+=head2 Mark stack
+
+I say "your portion of the stack" above because PP code doesn't
+necessarily get the whole stack to itself: if your function calls
+another function, you'll only want to expose the arguments aimed for
+the called function, and not (necessarily) let it get at your own data.
+The way we do this is to have a "virtual" bottom-of-stack, exposed to
+each function. The mark stack keeps bookmarks to locations in the
+argument stack usable by each function. For instance, when dealing with
+a tied variable, (internally, something with "P" magic) Perl has to
+call methods for accesses to the tied variables. However, we need to
+separate the arguments exposed to the method to the argument exposed to
+the original function - the store or fetch or whatever it may be.
+Here's roughly how the tied C<push> is implemented; see C<av_push> in
+F<av.c>:
+
+ 1 PUSHMARK(SP);
+ 2 EXTEND(SP,2);
+ 3 PUSHs(SvTIED_obj((SV*)av, mg));
+ 4 PUSHs(val);
+ 5 PUTBACK;
+ 6 ENTER;
+ 7 call_method("PUSH", G_SCALAR|G_DISCARD);
+ 8 LEAVE;
+
+Let's examine the whole implementation, for practice:
+
+ 1 PUSHMARK(SP);
+
+Push the current state of the stack pointer onto the mark stack. This
+is so that when we've finished adding items to the argument stack, Perl
+knows how many things we've added recently.
+
+ 2 EXTEND(SP,2);
+ 3 PUSHs(SvTIED_obj((SV*)av, mg));
+ 4 PUSHs(val);
+
+We're going to add two more items onto the argument stack: when you
+have a tied array, the C<PUSH> subroutine receives the object and the
+value to be pushed, and that's exactly what we have here - the tied
+object, retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
+
+ 5 PUTBACK;
+
+Next we tell Perl to update the global stack pointer from our internal
+variable: C<dSP> only gave us a local copy, not a reference to the
+global.
+
+ 6 ENTER;
+ 7 call_method("PUSH", G_SCALAR|G_DISCARD);
+ 8 LEAVE;
+
+C<ENTER> and C<LEAVE> localise a block of code - they make sure that
+all variables are tidied up, everything that has been localised gets
+its previous value returned, and so on. Think of them as the C<{> and
+C<}> of a Perl block.
+
+To actually do the magic method call, we have to call a subroutine in
+Perl space: C<call_method> takes care of that, and it's described in
+L<perlcall>. We call the C<PUSH> method in scalar context, and we're
+going to discard its return value. The call_method() function removes
+the top element of the mark stack, so there is nothing for the caller
+to clean up.
+
+=head2 Save stack
+
+C doesn't have a concept of local scope, so perl provides one. We've
+seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
+stack implements the C equivalent of, for example:
+
+ {
+ local $foo = 42;
+ ...
+ }
+
+See L<perlguts/Localising Changes> for how to use the save stack.
+
+=head1 MILLIONS OF MACROS
+
+One thing you'll notice about the Perl source is that it's full of
+macros. Some have called the pervasive use of macros the hardest thing
+to understand, others find it adds to clarity. Let's take an example,
+the code which implements the addition operator:
+
+ 1 PP(pp_add)
+ 2 {
+ 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
+ 4 {
+ 5 dPOPTOPnnrl_ul;
+ 6 SETn( left + right );
+ 7 RETURN;
+ 8 }
+ 9 }
+
+Every line here (apart from the braces, of course) contains a macro.
+The first line sets up the function declaration as Perl expects for PP
+code; line 3 sets up variable declarations for the argument stack and
+the target, the return value of the operation. Finally, it tries to see
+if the addition operation is overloaded; if so, the appropriate
+subroutine is called.
+
+Line 5 is another variable declaration - all variable declarations
+start with C<d> - which pops from the top of the argument stack two NVs
+(hence C<nn>) and puts them into the variables C<right> and C<left>,
+hence the C<rl>. These are the two operands to the addition operator.
+Next, we call C<SETn> to set the NV of the return value to the result
+of adding the two values. This done, we return - the C<RETURN> macro
+makes sure that our return value is properly handled, and we pass the
+next operator to run back to the main run loop.
+
+Most of these macros are explained in L<perlapi>, and some of the more
+important ones are explained in L<perlxs> as well. Pay special
+attention to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for
+information on the C<[pad]THX_?> macros.
+
+=head1 FURTHER READING
+
+For more information on the Perl internals, please see the documents
+listed at L<perl/Internals and C Language Interface>.
projects / platform / upstream / perl.git / commitdiff