MirBSD manpage: perlhack(1)

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


     perlhack - How to hack at the Perl internals


     This document attempts to explain how Perl development takes
     place, and ends with some suggestions for people wanting to
     become bona fide 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

     A searchable archive of the list is at either:




     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 com-
     plain.  In other words, it's your usual mix of technical

     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 Configure pumpkin up till the 5.8
     release. For the 5.10 release H.Merijn Brand took over.

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     Larry sees Perl development along the lines of the US
     government: there's the Legislature (the porters), the Exe-
     cutive 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 leg-
     islature 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:

     1   Larry is always by definition right about how Perl
         should behave. This means he has final veto power on the
         core functionality.

     2   Larry is allowed to change his mind about any matter at
         a later date, regardless of whether he previously
         invoked Rule 1.

     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.

     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:

     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.

     Where is the implementation?
         All the talk in the world is useless without an imple-
         mentation.  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

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         to implement your (possibly good) idea.

     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 pro-
         grams, changing the meaning of existing token sequences
         or functions might break programs.

     Could it be a module instead?
         Perl 5 has extension mechanisms, modules and XS, specif-
         ically to avoid the need to keep changing the Perl
         interpreter.  You can write modules that export func-
         tions, 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

     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.

     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.

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

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

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     Is the implementation generic enough to be portable?
         The worst patches make use of a system-specific
         features.  It's highly unlikely that nonportable addi-
         tions to the Perl language will be accepted.

     Is the implementation tested?
         Patches which change behaviour (fixing bugs or introduc-
         ing new features) must include regression tests to ver-
         ify 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 confi-
         dent that his/her hard work put into the patch won't be
         accidentally thrown away by someone in the future?

     Is there enough documentation?
         Patches without documentation are probably ill-thought
         out or incomplete.  Nothing can be added without docu-
         mentation, so submitting a patch for the appropriate
         manpages as well as the source code is always a good

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

     Does it create too much work?
         Work for the pumpking, work for Perl programmers, work
         for module authors, ...  Perl is supposed to be easy.

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

     If you're on the list, you might hear the word "core" ban-
     died 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.

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     Keeping in sync

     The source code to the Perl interpreter, in its different
     versions, is kept in a repository managed by a revision con-
     trol system ( which is currently the Perforce program, see
     http://perforce.com/ ).  The pumpkings and a few others have
     access to the repository to check in changes.  Periodically
     the pumpking for the development version of Perl will
     release a new version, so the rest of the porters can see
     what's changed.  The current state of the main trunk of
     repository, and patches that describe the individual changes
     that have happened since the last public release are avail-
     able at this location:


     If you're looking for a particular change, or a change that
     affected a particular set of files, you may find the Perl
     Repository Browser useful:


     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 repo-
     sitory.  See http://lists.perl.org/ for subscription infor-

     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, blead-
     ing 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 not use it for any purpose other than
     testing and development.

     Keeping in sync with the most recent branch can be done in
     several ways, but the most convenient and reliable way is
     using rsync, available at ftp://rsync.samba.org/pub/rsync/ .
     (You can also get the most recent branch by FTP.)

     If you choose to keep in sync using rsync, there are two
     approaches to doing so:

     rsync'ing the source tree
         Presuming you are in the directory where your perl
         source resides and you have rsync installed and

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         available, you can "upgrade" to the bleadperl using:

          # rsync -avz rsync://public.activestate.com/perl-current/ .

         This takes care of updating every single item in the
         source tree to the latest applied patch level, creating
         files that are new (to your distribution) and setting
         date/time stamps of existing files to reflect the blead-
         perl status.

         Note that this will not delete any files that were in
         '.' before the rsync. Once you are sure that the rsync
         is running correctly, run it with the --delete and the
         --dry-run options like this:

          # rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .

         This will simulate an rsync run that also deletes files
         not present in the bleadperl master copy. Observe the
         results from this run closely. If you are sure that the
         actual run would delete no files precious to you, you
         could remove the '--dry-run' option.

         You can than check what patch was the latest that was
         applied by looking in the file .patch, which will show
         the number of the latest patch.

         If you have more than one machine to keep in sync, and
         not all of them have access to the WAN (so you are not
         able to rsync all the source trees to the real source),
         there are some ways to get around this problem.

         Using rsync over the LAN
             Set up a local rsync server which makes the rsynced
             source tree available to the LAN and sync the other
             machines against this directory.

             From http://rsync.samba.org/README.html :

                "Rsync uses rsh or ssh for communication. It does not need to be
                 setuid and requires no special privileges for installation.  It
                 does not require an inetd entry or a daemon.  You must, however,
                 have a working rsh or ssh system.  Using ssh is recommended for
                 its security features."

         Using pushing over the NFS
             Having the other systems mounted over the NFS, you
             can take an active pushing approach by checking the
             just updated tree against the other not-yet synced
             trees. An example would be

               #!/usr/bin/perl -w

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               use strict;
               use File::Copy;

               my %MF = map {
                   $1 => [ (stat $1)[2, 7, 9] ];     # mode, size, mtime
                   } `cat MANIFEST`;

               my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);

               foreach my $host (keys %remote) {
                   unless (-d $remote{$host}) {
                       print STDERR "Cannot Xsync for host $host\n";
                   foreach my $file (keys %MF) {
                       my $rfile = "$remote{$host}/$file";
                       my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
                       defined $size or ($mode, $size, $mtime) = (0, 0, 0);
                       $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
                       printf "%4s %-34s %8d %9d  %8d %9d\n",
                           $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
                       unlink $rfile;
                       copy ($file, $rfile);
                       utime time, $MF{$file}[2], $rfile;
                       chmod $MF{$file}[0], $rfile;

             though this is not perfect. It could be improved
             with checking file checksums before updating. Not
             all NFS systems support reliable utime support (when
             used over the NFS).

     rsync'ing the patches
         The source tree is maintained by the pumpking who
         applies patches to the files in the tree. These patches
         are either created by the pumpking himself using "diff
         -c" after updating the file manually or by applying
         patches sent in by posters on the perl5-porters list.
         These patches are also saved and rsync'able, so you can
         apply them yourself to the source files.

         Presuming you are in a directory where your patches
         reside, you can get them in sync with

          # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .

         This makes sure the latest available patch is downloaded
         to your patch directory.

         It's then up to you to apply these patches, using

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         something like

          # last=`ls -t *.gz | sed q`
          # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
          # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
          # cd ../perl-current
          # patch -p1 -N <../perl-current-diffs/blead.patch

         or, since this is only a hint towards how it works, use
         CPAN-patchaperl from Andreas Kvnig to have better con-
         trol over the patching process.

     Why rsync the source tree

     It's easier to rsync the source tree
         Since you don't have to apply the patches yourself, you
         are sure all files in the source tree are in the right

     It's more reliable
         While both the rsync-able source and patch areas are
         automatically updated every few minutes, keep in mind
         that applying patches may sometimes mean careful
         hand-holding, especially if your version of the "patch"
         program does not understand how to deal with new files,
         files with 8-bit characters, or files without trailing

     Why rsync the patches

     It's easier to rsync the patches
         If you have more than one machine that you want to keep
         in track with bleadperl, it's easier to rsync the
         patches only once and then apply them to all the source
         trees on the different machines.

         In case you try to keep in pace on 5 different machines,
         for which only one of them has access to the WAN,
         rsync'ing all the source trees should than be done 5
         times over the NFS. Having rsync'ed the patches only
         once, I can apply them to all the source trees automati-
         cally. Need you say more ;-)

     It's a good reference
         If you do not only like to have the most recent develop-
         ment branch, but also like to fix bugs, or extend
         features, you want to dive into the sources. If you are
         a seasoned perl core diver, you don't need no manuals,
         tips, roadmaps, perlguts.pod or other aids to find your
         way around. But if you are a starter, the patches may
         help you in finding where you should start and how to
         change the bits that bug you.

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         The file Changes is updated on occasions the pumpking
         sees as his own little sync points. On those occasions,
         he releases a tar-ball of the current source tree (i.e.
         perl@7582.tar.gz), which will be an excellent point to
         start with when choosing to use the 'rsync the patches'
         scheme. Starting with perl@7582, which means a set of
         source files on which the latest applied patch is number
         7582, you apply all succeeding patches available from
         then on (7583, 7584, ...).

         You can use the patches later as a kind of search

         Finding a start point
             If you want to fix/change the behaviour of
             function/feature Foo, just scan the patches for
             patches that mention Foo either in the subject, the
             comments, or the body of the fix. A good chance the
             patch shows you the files that are affected by that
             patch which are very likely to be the starting point
             of your journey into the guts of perl.

         Finding how to fix a bug
             If you've found where the function/feature Foo mis-
             behaves, but you don't know how to fix it (but you
             do know the change you want to make), you can,
             again, peruse the patches for similar changes and
             look how others apply the fix.

         Finding the source of misbehaviour
             When you keep in sync with bleadperl, the pumpking
             would love to see that the community efforts really
             work. So after each of his sync points, you are to
             'make test' to check if everything is still in work-
             ing order. If it is, you do 'make ok', which will
             send an OK report to perlbug@perl.org. (If you do
             not have access to a mailer from the system you just
             finished successfully 'make test', you can do 'make
             okfile', which creates the file "perl.ok", which you
             can than take to your favourite mailer and mail

             But of course, as always, things will not always
             lead to a success path, and one or more test do not
             pass the 'make test'. Before sending in a bug report
             (using 'make nok' or 'make nokfile'), check the
             mailing list if someone else has reported the bug
             already and if so, confirm it by replying to that
             message. If not, you might want to trace the source
             of that misbehaviour before sending in the bug,
             which will help all the other porters in finding the

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             Here the saved patches come in very handy. You can
             check the list of patches to see which patch changed
             what file and what change caused the misbehaviour.
             If you note that in the bug report, it saves the one
             trying to solve it, looking for that point.

         If searching the patches is too bothersome, you might
         consider using perl's bugtron to find more information
         about discussions and ramblings on posted bugs.

         If you want to get the best of both worlds, rsync both
         the source tree for convenience, reliability and ease
         and rsync the patches for reference.

     Working with the source

     Because you cannot use the Perforce client, you cannot
     easily generate diffs against the repository, nor will
     merges occur when you update via rsync.  If you edit a file
     locally and then rsync against the latest source, changes
     made in the remote copy will overwrite your local versions!

     The best way to deal with this is to maintain a tree of sym-
     links to the rsync'd source.  Then, when you want to edit a
     file, you remove the symlink, copy the real file into the
     other tree, and edit it.  You can then diff your edited file
     against the original to generate a patch, and you can safely
     update the original tree.

     Perl's Configure script can generate this tree of symlinks
     for you. The following example assumes that you have used
     rsync to pull a copy of the Perl source into the perl-rsync
     directory.  In the directory above that one, you can execute
     the following commands:

       mkdir perl-dev
       cd perl-dev
       ../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g"

     This will start the Perl configuration process.  After a few
     prompts, you should see something like this:

       Symbolic links are supported.

       Checking how to test for symbolic links...
       Your builtin 'test -h' may be broken.
       Trying external '/usr/bin/test -h'.
       You can test for symbolic links with '/usr/bin/test -h'.

       Creating the symbolic links...
       (First creating the subdirectories...)
       (Then creating the symlinks...)

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     The specifics may vary based on your operating system, of
     course. After you see this, you can abort the Configure
     script, and you will see that the directory you are in has a
     tree of symlinks to the perl-rsync directories and files.

     If you plan to do a lot of work with the Perl source, here
     are some Bourne shell script functions that can make your
     life easier:

         function edit {
             if [ -L $1 ]; then
                 mv $1 $1.orig
                     cp $1.orig $1
                     vi $1
                 /bin/vi $1

         function unedit {
             if [ -L $1.orig ]; then
                 rm $1
                     mv $1.orig $1

     Replace "vi" with your favorite flavor of editor.

     Here is another function which will quickly generate a patch
     for the files which have been edited in your symlink tree:

         mkpatchorig() {
             local diffopts
                 for f in `find . -name '*.orig' | sed s,^\./,,`
                         case `echo $f | sed 's,.orig$,,;s,.*\.,,'` in
                             c)   diffopts=-p ;;
                     pod) diffopts='-F^=' ;;
                     *)   diffopts= ;;
                         diff -du $diffopts $f `echo $f | sed 's,.orig$,,'`

     This function produces patches which include enough context
     to make your changes obvious.  This makes it easier for the
     Perl pumpking(s) to review them when you send them to the
     perl5-porters list, and that means they're more likely to
     get applied.

     This function assumed a GNU diff, and may require some
     tweaking for other diff variants.

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     Perlbug administration

     There is a single remote administrative interface for modi-
     fying bug status, category, open issues etc. using the RT
     bugtracker system, maintained by Robert Spier.  Become an
     administrator, and close any bugs you can get your sticky
     mitts on:


     The bugtracker mechanism for perl5 bugs in particular is at:


     To email the bug system administrators:

             "perlbug-admin" <perlbug-admin@perl.org>

     Submitting patches

     Always submit patches to perl5-porters@perl.org.  If you're
     patching a core module and there's an author listed, send
     the author a copy (see "Patching a core module").  This lets
     other porters review your patch, which catches a surprising
     number of errors in patches. Either use the diff program
     (available in source code form from
     ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' makepatch
     (available from CPAN/authors/id/JV/).  Unified diffs are
     preferred, but context diffs are accepted.  Do not send RCS-
     style diffs or diffs without context lines.  More informa-
     tion is given in the Porting/patching.pod file in the Perl
     source distribution.  Please patch against the latest
     development version (e.g., if you're fixing a bug in the
     5.005 track, patch against the latest 5.005_5x version).
     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 "Writing a test".

     To report a bug in Perl, use the program perlbug which comes
     with Perl (if you can't get Perl to work, send mail to the
     address perlbug@perl.org or perlbug@perl.com).  Reporting
     bugs through perlbug feeds into the automated bug-tracking
     system, access to which is provided through the web at
     http://bugs.perl.org/ .  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.

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     The CPAN testers ( http://testers.cpan.org/ ) are a group of
     volunteers who test CPAN modules on a variety of platforms.
     Perl Smokers (
     http://archives.develooper.com/daily-build@perl.org/ )
     automatically tests Perl source releases on platforms with
     various configurations.  Both efforts welcome volunteers.

     It's a good idea to read and lurk for a while before chip-
     ping 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
     perl5-porters-subscribe@perl.org.  To unsubscribe, send mail
     to perl5-porters-unsubscribe@perl.org.

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

        This is of paramount importance, since it's the documen-
        tation 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.

        You might also want to look at Gisle Aas's illustrated
        perlguts - there's no guarantee that this will be abso-
        lutely up-to-date with the latest documentation in the
        Perl core, but the fundamentals will be right. (
        http://gisle.aas.no/perl/illguts/ )

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

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

        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

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     The perl5-porters FAQ
        This should be available from
        http://simon-cozens.org/writings/p5p-faq ; alternatively,
        you can get the FAQ emailed to you by sending mail to
        "perl5-porters-faq@perl.org". It contains hints on read-
        ing perl5-porters, information on how perl5-porters works
        and how Perl development in general works.

     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 direc-
     tories in the source kit. Among the areas are:

     Core modules
        Modules shipped as part of the Perl core live in the lib/
        and ext/ subdirectories: lib/ is for the pure-Perl
        modules, and ext/ contains the core XS modules.

        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 lib/ and ext/ direc-
        tories next to the module being tested.  Others live in
        t/.  See "Writing a test"

        Documentation maintenance includes looking after every-
        thing in the pod/ directory, (as well as contributing new
        documentation) and the documentation to the modules in

        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 files involved are the operating system directories,
        (win32/, os2/, vms/ and so on) the shell scripts which
        generate config.h and Makefile, as well as the metaconfig
        files which generate Configure. (metaconfig isn't
        included in the core distribution.)

        And of course, there's the core of the Perl interpreter
        itself. Let's have a look at that in a little more

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     Before we leave looking at the layout, though, don't forget
     that 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

     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. "Compiled code" in perlguts explains
     exactly how the compilation stage happens.

     Here is a short breakdown of perl's operation:

        The action begins in perlmain.c. (or miniperlmain.c for
        miniperl) This is very high-level code, enough to fit on
        a single screen, and it resembles the code found in per-
        lembed; most of the real action takes place in perl.c

        First, perlmain.c allocates some memory and constructs a
        Perl interpreter:

            1 PERL_SYS_INIT3(&argc,&argv,&env);
            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 "PL_do_undump",
        a global variable - all global variables in Perl start
        with "PL_". This tells you whether the current running
        program was created with the "-u" flag to perl and then
        undump, which means it's going to be false in any sane

        Line 4 calls a function in 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,

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        which we'll see later: "PerlMem_malloc" is either your
        system's "malloc", or Perl's own "malloc" as defined in
        malloc.c if you selected that option at configure time.

        Next, in line 7, we construct the interpreter; 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

            exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
            if (!exitstatus) {
                exitstatus = perl_run(my_perl);

        "perl_parse" is actually a wrapper around "S_parse_body",
        as defined in perl.c, which processes the command line
        options, sets up any statically linked XS modules, opens
        the program and calls "yyparse" to parse it.

        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.

        "yyparse", the parser, lives in perly.c, although you're
        better off reading the original YACC input in perly.y.
        (Yes, Virginia, there 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 "yylex", and that and its asso-
        ciated routines can be found in toke.c. Perl isn't much
        like other computer languages; it's highly context sensi-
        tive 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 op.c, and will
        be examined later.

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        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 "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 $foo, instead of grabbing the glob *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 "peep" in op.c,
        and many ops have their own optimizing functions.

        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 "runops_standard" function in
        run.c; more specifically, it's done by these three inno-
        cent looking lines:

            while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {

        You may be more comfortable with the Perl version of

            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 actu-
        ally carry out the operation. This function will return
        the next op in the sequence - this allows for things like
        "if" which choose the next op dynamically at run time.
        The "PERL_ASYNC_CHECK" makes sure that things like sig-
        nals interrupt execution if required.

        The actual functions called are known as PP code, and
        they're spread between four files: pp_hot.c contains the
        "hot" code, which is most often used and highly optim-
        ized, pp_sys.c contains all the system-specific func-
        tions, pp_ctl.c contains the functions which implement
        control structures ("if", "while" and the like) and pp.c
        contains everything else. These are, if you like, the C
        code for Perl's built-in functions and operators.

        Note that each "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,
        "pp_entersub" and "pp_entertry" just push a "CxSUB" or

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        "CxEVAL" block struct onto the context stack which con-
        tain 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 "pp_leavesub" or "pp_leavetry" op pops the
        "CxSUB" or "CxEVAL", retrieves the return op from it, and
        returns it.

     Exception handing
        Perl's exception handing (i.e. "die" etc) is built on top
        of the low-level "setjmp()"/"longjmp()" C-library func-
        tions. These basically provide a way to capture the
        current PC and SP registers and later restore them; i.e.
        a "longjmp()" continues at the point in code where a pre-
        vious "setjmp()" was done, with anything further up on
        the C stack being lost. This is why code should always
        save values using "SAVE_FOO" rather than in auto vari-

        The perl core wraps "setjmp()" etc in the macros
        "JMPENV_PUSH" and "JMPENV_JUMP". The basic rule of perl
        exceptions is that "exit", and "die" (in the absence of
        "eval") perform a JMPENV_JUMP(2), while "die" within
        "eval" does a JMPENV_JUMP(3).

        At entry points to perl, such as "perl_parse()",
        "perl_run()" and "call_sv(cv, G_EVAL)" each does a
        "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
        "CHECK" or "END" blocks. Amongst other things, this is
        how scope cleanup still occurs during an "exit".

        If a "die" can find a "CxEVAL" block on the context
        stack, then the stack is popped to that level and the
        return op in that block is assigned to "PL_restartop";
        then a JMPENV_JUMP(3) is performed.  This normally passes
        control back to the guard. In the case of "perl_run" and
        "call_sv", a non-null "PL_restartop" triggers re-entry to
        the runops loop. The is the normal way that "die" or
        "croak" is handled within an "eval".

        Sometimes ops are executed within an inner runops loop,
        such as tie, sort or overload code. In this case, some-
        thing like

            sub FETCH { eval { die } }

        would cause a longjmp right back to the guard in
        "perl_run", popping both runops loops, which is clearly
        incorrect. One way to avoid this is for the tie code to
        do a "JMPENV_PUSH" before executing "FETCH" in the inner

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        runops loop, but for efficiency reasons, perl in fact
        just sets a flag, using "CATCH_SET(TRUE)". The
        "pp_require", "pp_entereval" and "pp_entertry" ops check
        this flag, and if true, they call "docatch", which does a
        "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 "FETCH", execution of the code following the block is
        still carried on in the inner loop.  When an exception is
        raised, "docatch" compares the "JMPENV" level of the
        "CxEVAL" with "PL_top_env" and if they differ, just re-
        throws the exception. In this way any inner loops get

        Here's an example.

            1: eval { tie @a, 'A' };
            2: sub A::TIEARRAY {
            3:     eval { die };
            4:     die;
            5: }

        To run this code, "perl_run" is called, which does a
        "JMPENV_PUSH" then enters a runops loop. This loop exe-
        cutes the eval and tie ops on line 1, with the eval push-
        ing a "CxEVAL" onto the context stack.

        The "pp_tie" does a "CATCH_SET(TRUE)", then starts a
        second runops loop to execute the body of "TIEARRAY".
        When it executes the entertry op on line 3, "CATCH_GET"
        is true, so "pp_entertry" calls "docatch" which does a
        "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_runops      # third loop
            Perl_runops      # second loop
            Perl_runops      # first loop

        and the context and data stacks, as shown by "-Dstv",
        look like:

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            STACK 0: MAIN
              CX 0: BLOCK  =>
              CX 1: EVAL   => AV()  PV("A"\0)
            STACK 1: MAGIC
              CX 0: SUB    =>
              CX 1: EVAL   => *

        The die pops the first "CxEVAL" off the context stack,
        sets "PL_restartop" from it, does a JMPENV_JUMP(3), and
        control returns to the top "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 "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)
            STACK 1: MAGIC
              CX 0: SUB    =>

        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, "PL_restartop" is extracted from the "CxEVAL",
        and a JMPENV_JUMP(3) done, which pops the C stack back to
        the docatch:

            Perl_runops      # second loop
            Perl_runops      # first loop

        In  this case, because the "JMPENV" level recorded in the
        "CxEVAL" differs from the current one, "docatch" just

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        does a JMPENV_JUMP(3) and the C stack unwinds to:


        Because "PL_restartop" is non-null, "run_body" starts a
        new runops loop and execution continues.

     Internal Variable Types

     You should by now have had a look at 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 pro-
     gram files you call are opened via ordinary Perl filehan-
     dles, and so on.

     The core Devel::Peek module lets us examine SVs from a Perl
     program. Let's see, for instance, how Perl treats the con-
     stant "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 "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
     0xa04ecbc in memory. SVs themselves are very simple struc-
     tures, but they contain a pointer to a more complex struc-
     ture. In this case, it's a PV, a structure which holds a
     string value, at location 0xa041450.  Line 2 is the refer-
     ence 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 0xa0484e0.

     Line 5 gives us the current length of the string - note that
     this does not include the null terminator. Line 6 is not the

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     length of the string, but the length of the currently allo-
     cated buffer; as the string grows, Perl automatically
     extends the available storage via a routine called "SvGROW".

     You can get at any of these quantities from C very easily;
     just add "Sv" to the name of the field shown in the snippet,
     and you've got a macro which will return the value:
     "SvCUR(sv)" returns the current length of the string,
     "SvREFCOUNT(sv)" returns the reference count, "SvPV(sv,
     len)" returns the string itself with its length, and so on.
     More macros to manipulate these properties can be found in

     Let's take an example of manipulating a PV, from
     "sv_catpvn", in 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, "ptr", of length
     "len" onto the end of the PV stored in "sv". The first thing
     we do in line 6 is make sure that the SV has a valid PV, by
     calling the "SvPV_force" macro to force a PV. As a side
     effect, "tlen" gets set to the current value of the PV, and
     the PV itself is returned to "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 ter-
     minator. If "LEN" isn't big enough, "SvGROW" will reallocate
     space for us.

     Now, if "junk" is the same as the string we're trying to
     add, we can grab the string directly from the SV; "SvPVX" is
     the address of the PV in the SV.

     Line 10 does the actual catenation: the "Move" macro moves a
     chunk of memory around: we move the string "ptr" to the end
     of the PV - that's the start of the PV plus its current

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     length. We're moving "len" bytes of type "char". After doing
     so, we need to tell Perl we've extended the string, by
     altering "CUR" to reflect the new length. "SvEND" is a macro
     which gives us the end of the string, so that needs to be a

     Line 13 manipulates the flags; since we've changed the PV,
     any IV or NV values will no longer be valid: if we have
     "$a=10; $a.="6";" we don't want to use the old IV of 10.
     "SvPOK_only_utf8" is a special UTF-8-aware version of
     "SvPOK_only", a macro which turns off the IOK and NOK flags
     and turns on POK. The final "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.

     Op Trees

     First, what is the op tree, anyway? The op tree is the
     parsed representation of your program, as we saw in our sec-
     tion on parsing, and it's the sequence of operations that
     Perl goes through to execute your program, as we saw in

     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 B::Terse,
     B::Concise and B::Debug do.

     Let's have a look at how Perl sees "$a = $b + $c":

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          % 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 0x8179828. The
     specific operator in question is "sassign" - scalar assign-
     ment - and you can find the code which implements it in the
     function "pp_sassign" in pp_hot.c. As a binary operator, it
     has two children: the add operator, providing the result of
     "$b+$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 men-
     tioned in "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 "a" entry from the main symbol table, and
     then look at the scalar component of it: "gvsv" ("pp_gvsv"
     into 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 "add" op ("pp_add" also in
     pp_hot.c) add together two "gvsv"s.

     Now, what's this about?

          1  LISTOP (0x8179888) leave
          2      OP (0x81798b0) enter
          3      COP (0x8179850) nextstate

     "enter" and "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

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     variables are destroyed, and so on. Every program will have
     those first three lines: "leave" is a list, and its children
     are all the statements in the block. Statements are delim-
     ited by "nextstate", so a block is a collection of "next-
     state" ops, with the ops to be performed for each statement
     being the children of "nextstate". "enter" is a single op
     which functions as a marker.

     That's how Perl parsed the program, from top to bottom:

                               / \
                              /   \
                             $a   +
                                 / \
                               $b   $c

     However, it's impossible to perform the operations in this
     order: you have to find the values of $b and $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 "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
     "exec" option to "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 $b and $c, and add them
     together. Find $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 perly.y, the YACC grammar.
     Let's take the piece we need to construct the tree for "$a =
     $b + $c"

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         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, "ADDOP", is provided
     when the tokeniser sees "+" in your code. "ASSIGNOP" is pro-
     vided when "=" 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.
     "term" here is a non-terminal symbol, representing a single

     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, "term" followed by "=" fol-
     lowed by "term" makes a "term", and "term" followed by "+"
     followed by "term" can also make a "term".

     So, if you see two terms with an "=" or "+", 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 "=", you'll do the code in line 2. If you see "+",
     you'll do the code in line 4. It's this code which contri-
     butes 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: $1
     is the first token in the input, $2 the second, and so on -
     think regular expression backreferences. $$ is the op
     returned from this reduction. So, we call "newBINOP" to
     create a new binary operator. The first parameter to "newBI-
     NOP", a function in op.c, is the op type. It's an addition
     operator, so we want the type to be "ADDOP". We could
     specify this directly, but it's right there as the second
     token in the input, so we use $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.

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     When perl executes something like "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.

     Argument stack
        Arguments are passed to PP code and returned from PP code
        using the argument stack, "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

              NV value;
              value = POPn;
              value = Perl_cos(value);

        We'll see a more tricky example of this when we consider
        Perl's macros below. "POPn" gives you the NV (floating
        point value) of the top SV on the stack: the $x in
        "cos($x)". Then we compute the cosine, and push the
        result back as an NV. The "X" in "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
        "XPUSH*" macros at least guarantee safety.

        Alternatively, you can fiddle with the stack directly:
        "SP" gives you the first element in your portion of the
        stack, and "TOP*" gives you the top SV/IV/NV/etc. on the
        stack. So, for instance, to do unary negation of an


        Just set the integer value of the top stack entry to its

        Argument stack manipulation in the core is exactly the
        same as it is in XSUBs - see perlxstut, perlxs and perl-
        guts for a longer description of the macros used in stack

     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

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        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 loca-
        tions 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 argu-
        ment exposed to the original function - the store or
        fetch or whatever it may be. Here's how the tied "push"
        is implemented; see "av_push" in 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;
             9  POPSTACK;

        The lines which concern the mark stack are the first,
        fifth and last lines: they save away, restore and remove
        the current position of the argument stack.

        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 "PUSH" subroutine
        receives the object and the value to be pushed, and
        that's exactly what we have here - the tied object,
        retrieved with "SvTIED_obj", and the value, the SV "val".

             5  PUTBACK;

        Next we tell Perl to make the change to the global stack
        pointer: "dSP" only gave us a local copy, not a reference
        to the global.

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             6  ENTER;
             7  call_method("PUSH", G_SCALAR|G_DISCARD);
             8  LEAVE;

        "ENTER" and "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 "{" and "}" of a Perl block.

        To actually do the magic method call, we have to call a
        subroutine in Perl space: "call_method" takes care of
        that, and it's described in perlcall. We call the "PUSH"
        method in scalar context, and we're going to discard its
        return value.

             9  POPSTACK;

        Finally, we remove the value we placed on the mark stack,
        since we don't need it any more.

     Save stack
        C doesn't have a concept of local scope, so perl provides
        one. We've seen that "ENTER" and "LEAVE" are used as
        scoping braces; the save stack implements the C
        equivalent of, for example:

                local $foo = 42;

        See "Localising Changes" in perlguts for how to use the
        save stack.

     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  }

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     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 declara-
     tions 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 "d" - which pops from the top of the
     argument stack two NVs (hence "nn") and puts them into the
     variables "right" and "left", hence the "rl". These are the
     two operands to the addition operator. Next, we call "SETn"
     to set the NV of the return value to the result of adding
     the two values. This done, we return - the "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 perlapi, and some of
     the more important ones are explained in perlxs as well. Pay
     special attention to "Background and PERL_IMPLICIT_CONTEXT"
     in perlguts for information on the "[pad]THX_?" macros.

     The .i Targets

     You can expand the macros in a foo.c file by saying

         make foo.i

     which will expand the macros using cpp.  Don't be scared by
     the results.

     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

     "-g" is a flag to the C compiler to have it produce debug-
     ging information which will allow us to step through a run-
     ning program. Configure will also turn on the "DEBUGGING"
     compilation symbol which enables all the internal debugging
     code in Perl. There are a whole bunch of things you can
     debug with this: 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

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         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'

     Using a source-level debugger

     If the debugging output of "-D" doesn't help you, it's time
     to step through perl's execution with a source-level

     +  We'll use "gdb" for our examples here; the principles
        will apply to any debugger, but check the manual of the
        one you're using.

     To fire up the debugger, type

         gdb ./perl

     You'll want to do that in your Perl source tree so the
     debugger can read the source code. You should see the copy-
     right message, followed by the prompt.


     "help" will get you into the documentation, but here are the
     most useful commands:

     run [args]
        Run the program with the given arguments.

     break function_name
     break source.c:xxx
        Tells the debugger that we'll want to pause execution
        when we reach either the named function (but see "Inter-
        nal Functions" in perlguts!) or the given line in the
        named source file.

        Steps through the program a line at a time.

        Steps through the program a line at a time, without des-
        cending into functions.


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        Run until the next breakpoint.

        Run until the end of the current function, then stop

        Just pressing Enter will do the most recent operation
        again - it's a blessing when stepping through miles of
        source code.

        Execute the given C code and print its results. WARNING:
        Perl makes heavy use of macros, and gdb does not neces-
        sarily support macros (see later "gdb macro support").
        You'll have to substitute them yourself, or to invoke cpp
        on the source code files (see "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)

     You may find it helpful to have a "macro dictionary", which
     you can produce by saying "cpp -dM perl.c | sort". Even
     then, cpp won't recursively apply those macros for you.

     gdb macro support

     Recent versions of 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
     gcc version 3.1, this means configuring with
     "-Doptimize=-g3".  Other compilers might use a different
     switch (if they support debugging macros at all).

     Dumping Perl Data Structures

     One way to get around this macro hell is to use the dumping
     functions in dump.c; these work a little like an internal
     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 "$a = $b + $c" we used before, but give it a
     bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good
     place to stop and poke around?

     What about "pp_add", the function we examined earlier to
     implement the "+" operator:

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         (gdb) break Perl_pp_add
         Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.

     Notice we use "Perl_pp_add" and not "pp_add" - see "Internal
     Functions" in perlguts. 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;

     We looked at this bit of code before, and we said that
     "dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into
     "left" and "right" - let's slightly expand it:

         #define dPOPTOPnnrl_ul  NV right = POPn; \
                                 SV *leftsv = TOPs; \
                                 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0

     "POPn" takes the SV from the top of the stack and obtains
     its NV either directly (if "SvNOK" is set) or by calling the
     "sv_2nv" function. "TOPs" takes the next SV from the top of
     the stack - yes, "POPn" uses "TOPs" - but doesn't remove it.
     We then use "SvNV" to get the NV from "leftsv" in the same
     way as before - yes, "POPn" uses "SvNV".

     Since we don't have an NV for $b, we'll have to use "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)

     We can now use "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 6 from this, so let's finish the

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         (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 "PL_op", and we can dump it with "Perl_op_dump".
     This'll give us similar output to 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 #


     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 "U" is the first
     active format during a "pack", (for example, "pack "U3C8",
     @stuff") then the resulting string should be treated as
     UTF-8 encoded.

     How do we prepare to fix this up? First we locate the code
     in question - the "pack" happens at runtime, so it's going
     to be in one of the pp files. Sure enough, "pp_pack" is in
     pp.c. Since we're going to be altering this file, let's copy
     it to pp.c~.

     [Well, it was in pp.c when this tutorial was written. It has
     now been split off with "pp_unpack" to its own file,

     Now let's look over "pp_pack": we take a pattern into "pat",
     and then loop over the pattern, taking each format character
     in turn into "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

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     output SV "cat".

     How do we know if the "U" is the first format in the "pat"?
     Well, if we have a pointer to the start of "pat" then, if we
     see a "U" we can test whether we're still at the start of
     the string. So, here's where "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 "patcopy" to be
     the start of "pat":

         items = SP - MARK;
         sv_setpvn(cat, "", 0);
      +  patcopy = pat;
         while (pat < patend) {

     Now if we see a "U" which was at the start of the string, we
     turn on the "UTF8" flag for the output SV, "cat":

      +  if (datumtype == 'U' && pat==patcopy+1)
      +      SvUTF8_on(cat);
         if (datumtype == '#') {
             while (pat < patend && *pat != '\n')

     Remember that it has to be "patcopy+1" because the first
     character of the string is the "U" which has been swallowed
     into "datumtype!"

     Oops, we forgot one thing: what if there are spaces at the
     start of the pattern? "pack("  U*", @stuff)" will have "U"
     as the first active character, even though it's not the
     first thing in the pattern. In this case, we have to advance
     "patcopy" along with "pat" when we see spaces:

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         if (isSPACE(datumtype))

     needs to become

         if (isSPACE(datumtype)) {

     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.

     The regression tests for each operator live in t/op/, and so
     we make a copy of t/op/pack.t to t/op/pack.t~. Now we can
     add our tests to the end. First, we'll test that the "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 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 busi-
     ness 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
     "U" is not the first active format:

      isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
                                            "U* not first isn't unicode" );

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     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!

     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 pod/perlfunc.pod; again, we make
     a copy, and then we'll insert this text in the description
     of "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.

     All done. Now let's create the patch. Porting/patching.pod
     tells us that if we're making major changes, we should copy
     the entire directory to somewhere safe before we begin fid-
     dling, and then do

         diff -ruN old new > patch

     However, we know which files we've changed, and we can sim-
     ply do this:

         diff -u pp.c~             pp.c             >  patch
         diff -u t/op/pack.t~      t/op/pack.t      >> patch
         diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch

     We end up with a patch looking a little like this:

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         --- pp.c~       Fri Jun 02 04:34:10 2000
         +++ pp.c        Fri Jun 16 11:37:25 2000
         @@ -4375,6 +4375,7 @@
              register I32 items;
              STRLEN fromlen;
              register char *pat = SvPVx(*++MARK, fromlen);
         +    char *patcopy;
              register char *patend = pat + fromlen;
              register I32 len;
              I32 datumtype;
         @@ -4405,6 +4406,7 @@

     And finally, we submit it, with our rationale, to
     perl5-porters. Job done!

     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 docu-
     mented in Porting/Maintainers.pl.

     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 func-
     tion 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 patch-
     ing.  It is extremely important to test any addition
     thoroughly and add new tests to explore all boundary condi-
     tions 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 "Internal Functions" in perlguts for more details.

     The location of any new code is also an important considera-
     tion.  Don't just create a new top level .c file and put
     your code there; you would have to make changes to Configure

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     (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 exe-
     cutable.  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 "Internal Functions" in perlguts for information on the
     various options that you can set in embed.pl. You will for-
     get to do this a few (or many) times and you will get warn-
     ings during the compilation phase.  Make sure that you men-
     tion this when you post your patch to P5P; the pumpking
     needs to know this.

     When you write your new code, please be conscious of exist-
     ing code conventions used in the perl source files.  See
     perlstyle for details.  Although most of the guidelines dis-
     cussed seem to focus on Perl code, rather than c, they all
     apply (except when they don't ;). See also
     Porting/patching.pod file in the Perl source distribution
     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 "EXTERNAL TOOLS
     FOR DEBUGGING PERL" 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.

     Writing a test

     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

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     In short, if you submit a patch you probably also have to
     patch the tests.

     For modules, the test file is right next to the module
     itself. lib/strict.t tests lib/strict.pm.  This is a recent
     innovation, so there are some snags (and it would be wonder-
     ful for you to brush them out), but it basically works that
     way.  Everything else lives in t/.

        Testing of the absolute basic functionality of Perl.
        Things like "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 really broken.

        These test the basic control structures, "if/else",
        "while", subroutines, etc.

        Tests basic issues of how Perl parses and compiles

        Tests for built-in IO functions, including command line

        The old home for the module tests, you shouldn't put any-
        thing 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!

        Tests for perl's built in functions that don't fit into
        any of the other directories.

        Tests for POD directives.  There are still some tests for
        the Pod modules hanging around in here that need to be
        moved out into lib/.

        Testing features of how perl actually runs, including
        exit codes and handling of PERL* environment variables.

        Tests for the core support of Unicode.

        Windows-specific tests.

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        A test suite for the s2p converter.

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

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

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

     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 impor-
         tant 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.

     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.

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

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

     Special Make Test Targets

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

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     available on certain operating systems.

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

         (Not available on Win32)

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

         (Not available on Win32)

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

         (Not available on Win32)

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

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

     test.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 perl.3log.testname.

     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,

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

     utest ucheck test.utf8 check.utf8
         Run all the tests with -Mutf8.  Not all tests will

         (Not available on Win32)

     minitest.utf16 test.utf16
         Runs the tests with UTF-16 encoded scripts, encoded with
         different versions of this encoding.

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

         (Not available on Win32)

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

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

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

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

     test-notty test_notty
         Sets PERL_SKIP_TTY_TEST to true before running normal

     Running tests by hand

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

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


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

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

     Using t/harness for testing

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

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

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

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

         Run the torture tests as well as the normal set.

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

         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

     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 :

         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.

         is set to 2 if it isn't set already (see

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

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


     Sometimes it helps to use external tools while debugging and

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     testing Perl.  This section tries to guide you through using
     some common testing and debugging tools with Perl.  This is
     meant as a guide to interfacing these tools with Perl, not
     as any kind of guide to the use of the tools themselves.

     NOTE 1: Running under memory debuggers such as Purify, val-
     grind, 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 extraor-
     dinarily 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.

     NOTE 2: To minimize the number of memory leak false alarms
     (see "PERL_DESTRUCT_LEVEL" for more information), you have
     to have environment variable PERL_DESTRUCT_LEVEL set to 2.
     The TEST and harness scripts do that automatically.  But if
     you are running some of the tests manually-- for csh-like

         setenv PERL_DESTRUCT_LEVEL 2

     and for Bourne-type shells:

         export PERL_DESTRUCT_LEVEL

     or in UNIXy environments you can also use the "env" command:

         env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...

     NOTE 3: There are known memory leaks when there are compile-
     time errors within eval or require, seeing "S_doeval" in the
     call stack is a good sign of these.  Fixing these leaks is
     non-trivial, unfortunately, but they must be fixed eventu-

     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 Win-
     dows NT, Solaris, HP-UX, SGI, and Siemens Unix.

     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

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         sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
          -Uusemymalloc -Dusemultiplicity

     where these arguments mean:

         Disables Perl's arena memory allocation functions, as
         well as forcing use of memory allocation functions
         derived from the system malloc.

         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.

         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

         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.

     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 prob-

     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

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     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:

         export PURIFYOPTIONS

     or if you have the "env" utility:

         env PURIFYOPTIONS="..." ../pureperl ...

     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:

         You should add -DPURIFY to the DEFINES line so the
         DEFINES line looks something like:


         to disable Perl's arena memory allocation functions, as
         well as to force use of memory allocation functions
         derived from the system malloc.

     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.

     #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

     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.

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     As an example, to show any memory leaks produced during the
     standard Perl testset you would create and run Purify as:

         cd win32
         cd ../t
         purify ../perl -I../lib harness

     which would instrument Perl in memory, run Perl on test.pl,
     then finally report any memory problems.


     The excellent valgrind tool can be used to find out both
     memory leaks and illegal memory accesses.  As of August 2003
     it unfortunately works only on x86 (ELF) Linux.  The special
     "test.valgrind" target can be used to run the tests under
     valgrind.  Found errors and memory leaks are logged in files
     named 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 t/perl.supp.

     To get valgrind and for more information see


     Compaq's/Digital's/HP's Third Degree

     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).

     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 "-g"
     flag, you may need to re-Configure.)

     The short story is that with "atom" you can instrument the
     Perl executable to create a new executable called
     perl.third.  When the instrumented executable is run, it
     creates a log of dubious memory traffic in file called
     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".

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     The "test.third" leaves a lot of files named 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 *.3log files.

     There are also leaks that for given certain definition of a
     leak, aren't.  See "PERL_DESTRUCT_LEVEL" for more informa-


     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 does not explicitly cleanup
     all the memory it has allocated (such as global memory are-
     nas) 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 "glo-
     bal 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 N scalars
     leaked, you can recompile with "-DDEBUG_LEAKING_SCALARS",
     which will cause the addresses of all those leaked SVs to be
     dumped; it also converts "new_SV()" from a macro into a real
     function, so you can use your favourite debugger to discover
     where those pesky SVs were allocated.


     Depending on your platform there are various of profiling

     There are two commonly used techniques of profiling execut-
     ables: statistical time-sampling and basic-block counting.

     The first method takes periodically samples of the CPU pro-
     gram counter, and since the program counter can be corre-
     lated with the code generated for functions, we get a

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     statistical view of in which functions the program is spend-
     ing its time.  The caveats are that very small/fast func-
     tions have lower probability of showing up in the profile,
     and that periodically interrupting the program (this is usu-
     ally 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 basic
     blocks. Basic blocks are sections of code that are entered
     only in the beginning and exited only at the end.  For exam-
     ple, a conditional jump starts a basic block.  Basic block
     profiling usually works by instrumenting the code by adding
     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 pro-
     filing tools usually try to factor their own effects out of
     the results.

     Gprof Profiling

     gprof is a profiling tool available in many UNIX platforms,
     it uses 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 "-pg" flag, you may
     need to re-Configure). Running the profiled version of Perl
     will create an output file called gmon.out is created which
     contains the profiling data collected during the execution.

     The gprof tool can then display the collected data in vari-
     ous ways. Usually gprof understands the following options:

     -a  Suppress statically defined functions from the profile.

     -b  Suppress the verbose descriptions in the profile.

     -e routine
         Exclude the given routine and its descendants from the

     -f routine
         Display only the given routine and its descendants in
         the profile.

     -s  Generate a summary file called gmon.sum which then may
         be given to subsequent gprof runs to accumulate data

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         over several runs.

     -z  Display routines that have zero usage.

     For more detailed explanation of the available commands and
     output formats, see your own local documentation of gprof.

     GCC gcov Profiling

     Starting from GCC 3.0 basic block profiling is officially
     available for the GNU CC.

     You can build a profiled version of perl called perl.gcov by
     invoking the make target "perl.gcov" (what is required that
     Perl must be compiled using gcc with the flags
     "-fprofile-arcs -ftest-coverage", you may need to

     Running the profiled version of Perl will cause profile out-
     put 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).
     gcov is run on source code files, like this

         gcov sv.c

     which will cause sv.c.gcov to be created.  The .gcov files
     contain the source code annotated with relative frequencies
     of execution indicated by "#" markers.

     Useful options of gcov include "-b" which will summarise the
     basic block, branch, and function call coverage, and "-c"
     which instead of relative frequencies will use the actual
     counts.  For more information on the use of gcov and basic
     block profiling with gcc, see the latest GNU CC manual, as
     of GCC 3.0 see


     and its section titled "8. gcov: a Test Coverage Program"


     Pixie Profiling

     Pixie is a profiling tool available on IRIX and Tru64 (aka
     Digital UNIX aka DEC OSF/1) platforms.  Pixie does its pro-
     filing using basic-block counting.

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     You can build a profiled version of perl called perl.pixie
     by invoking the make target "perl.pixie" (what is required
     is that Perl must be compiled using the "-g" flag, you may
     need to re-Configure).

     In Tru64 a file called 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.

     To display the results you use the prof utility.  The exact
     incantation depends on your operating system, "prof
     perl.Counts" in IRIX, and "prof -pixie -all -L. perl" in

     In IRIX the following prof options are available:

     -h  Reports the most heavily used lines in descending order
         of use. Useful for finding the hotspot lines.

     -l  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.

     In Tru64 the following options are available:

         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.)

         Lines sorted in descending order by the number of cycles
         executed in each line.  Useful for finding the hotspot

         The called procedures are sorted in descending order by
         number of calls made to the procedures.  Useful for
         finding the most used procedures.

         Grouped by procedure, sorted by cycles executed per pro-
         cedure. Useful for finding the hotspots of procedures.

         The compiler emitted code for these lines, but the code
         was unexecuted.


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         Unexecuted procedures.

     For further information, see your system's manual pages for
     pixie and prof.

     Miscellaneous tricks

     +   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.

     +   If you see in a debugger a memory area mysteriously full
         of 0xabababab, you may be seeing the effect of the
         Poison() macro, see perlclib.


     We've had a brief look around the Perl source, an overview
     of the stages perl goes through when it's running your code,
     and how to use a debugger to poke at the Perl guts. We took
     a very simple problem and demonstrated how to solve it fully
     - with documentation, regression tests, and finally a patch

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     for submission to p5p.  Finally, we talked about how to use
     external tools to debug and test 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:

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

     +  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

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

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

     The Road goes ever on and on, down from the door where it began.

     If you can do these things, you've started on the long road
     to Perl porting. Thanks for wanting to help make Perl better
     - and happy hacking!


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

perl v5.8.8                2006-06-30                          54

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