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



NAME
       perlhack - How to hack at the Perl internals

DESCRIPTION
       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 time.

       A searchable archive of the list is at either:

           http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/

       or

           http://archive.develooper.com/perl5-porters@perl.org/

       List subscribers (the porters themselves) come in(1,8) several flavours.
       Some are quiet curious lurkers, who rarely pitch in(1,8) and instead watch
       the ongoing development to ensure they're forewarned of new changes or
       features in(1,8) Perl.  Some are representatives of vendors, who are there
       to make sure that Perl continues to compile and work on their plat-
       forms.  Some patch any reported bug that they know how to fix, some are
       actively patching their pet area (threads, Win32, the regexp(3,n) engine),
       while others seem to do nothing but complain.  In other words, it's
       your usual mix of technical people.

       Over this group of porters presides Larry Wall.  He has the final word
       in(1,8) what does and does not change in(1,8) 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 is the pumpking for the 5.8 release, and Hugo van der
       Sanden will be the pumpking for the 5.10 release.

       In addition, various people are pumpkings for different things.  For
       instance, Andy Dougherty and Jarkko Hietaniemi share the Configure
       pumpkin.

       Larry sees Perl development along the lines of the US government:
       there's the Legislature (the porters), the Executive branch (the pump-
       kings), and the Supreme Court (Larry).  The legislature can discuss and
       submit patches to the executive branch all they like, but the executive
       branch is free to veto them.  Rarely, the Supreme Court will side with
       the executive branch over the legislature, or the legislature over the
       executive branch.  Mostly, however, the legislature and the executive
       branch are supposed to get along and work out their differences without
       impeachment or court cases.

       You might sometimes see reference to Rule 1 and Rule 2.  Larry's power
       as Supreme Court is expressed in(1,8) 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(1,8) 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(1,8) 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(1,8) 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(2,3,n) 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(1,8) the world is useless without an implementation.  In
           almost every case, the person or people who argue for a new feature
           will be expected to be the ones who implement it.  Porters capable
           of coding new features have their own agendas, and are not avail-
           able 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 programs, 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, specifically to
           avoid the need to keep changing the Perl interpreter.  You can
           write(1,2) modules that export functions, you can give those functions
           prototypes so they can be called like built-in functions, you can
           even write(1,2) XS code to mess with the runtime data structures of the
           Perl interpreter if(3,n) you want to implement really complicated
           things.  If it can be done in(1,8) a module instead of in(1,8) the core, it's
           highly unlikely to be added.

       Is the feature generic enough?
           Is this something that only the submitter wants added to the lan-
           guage, 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(3,n) it closes off future avenues of
           development.  For instance, a patch that placed a true and final
           interpretation on prototypes is likely to be rejected because there
           are still options for the future of prototypes that haven't been
           addressed.

       Is the implementation robust?
           Good patches (tight code, complete, correct) stand more chance of
           going in.  Sloppy or incorrect patches might be placed on the back
           burner until the pumpking has time(1,2,n) to fix, or might be discarded
           altogether without further notice.

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

       Is the implementation tested?
           Patches which change behaviour (fixing bugs or introducing new fea-
           tures) must include regression tests to verify(1,8) that everything
           works as expected.  Without tests provided by the original author,
           how can anyone else changing perl in(1,8) the future be sure that they
           haven't unwittingly broken the behaviour the patch implements? And
           without tests, how can the patch's author be confident that his/her
           hard work put into the patch won't be accidentally thrown away by
           someone in(1,8) the future?

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

       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(1,5,7)'s essential addition
           is another man(1,5,7)'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(3,4,6) feature request, no matter how fer-
           vently argued the request might be.  This ties into ``Will it be
           useful?'', as the fact that someone took the time(1,2,n) to make the patch
           demonstrates a strong desire for the feature.

       If you're on the list, you might hear the word ``core'' bandied around.
       It refers to the standard distribution.  ``Hacking on the core'' means
       you're changing the C source code to the Perl interpreter.  ``A core
       module'' is one that ships with Perl.

       Keeping in(1,8) sync(1,2,8)

       The source code to the Perl interpreter, in(1,8) its different versions, is
       kept in(1,8) a repository managed by a revision control system ( which is
       currently the Perforce program, see http://perforce.com/ ).  The pump-
       kings and a few others have access(2,5) to the repository to check in(1,8)
       changes.  Periodically the pumpking for the development version(1,3,5) of Perl
       will release a new version(1,3,5), 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 available at this location:

           http://public.activestate.com/gsar/APC/
           ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/

       If you're looking for a particular change, or a change that affected a
       particular set(7,n,1 builtins) of files, you may find the Perl Repository Browser use-
       ful:

           http://public.activestate.com/cgi-bin/perlbrowse

       You may also want to subscribe to the perl5-changes mailing list to
       receive a copy of each patch that gets(3,n) submitted to the maintenance and
       development "branches" of the perl repository.  See
       http://lists.perl.org/ for subscription information.

       If you are a member of the perl5-porters mailing list, it is a good
       thing to keep in(1,8) touch with the most recent changes. If not only to
       verify(1,8) if(3,n) what you would have posted as a bug report isn't already
       solved in(1,8) the most recent available perl development branch, also known
       as perl-current, bleading edge perl, bleedperl or bleadperl.

       Needless to say, the source code in(1,8) perl-current is usually in(1,8) a per-
       petual 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(1,8) sync(1,2,8) with the most recent branch can be done in(1,8) several
       ways, but the most convenient and reliable way is using rsync, avail-
       able at ftp://rsync.samba.org/pub/rsync/ .  (You can also get the most
       recent branch by FTP.)

       If you choose to keep in(1,8) sync(1,2,8) using rsync, there are two approaches to
       doing so:

       rsync'ing the source tree
           Presuming you are in(1,8) the directory where your perl source resides
           and you have rsync installed and available, you can `upgrade' to
           the bleadperl using:

            # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .

           This takes care of updating every single item in(1,8) the source tree to
           the latest applied patch level, creating files that are new (to
           your distribution) and setting date/time(1,2,n) stamps of existing files
           to reflect the bleadperl status.

           Note that this will not delete any files that were in(1,8) '.' 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://ftp.linux.activestate.com/perl-current/ .

           This will simulate an rsync run that also deletes files not present
           in(1,8) the bleadperl master(5,8) 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(1,8) the file(1,n) .patch, which will show the number of the lat-
           est patch.

           If you have more than one machine to keep in(1,8) sync(1,2,8), and not all of
           them have access(2,5) 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(1,2,8) 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

                 use strict;
                 use File::Copy;

                 my %MF = map {
                     m/(\S+)/;
                     $1 => [ (stat(1,2) $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(1,5) (keys %remote) {
                     unless (-d $remote{$host(1,5)}) {
                         print STDERR "Cannot Xsync for host(1,5) $host(1,5)\n";
                         next;
                         }
                     foreach my $file(1,n) (keys %MF) {
                         my $rfile = "$remote{$host(1,5)}/$file(1,n)";
                         my ($mode, $size, $mtime) = (stat(1,2) $rfile)[2, 7, 9];
                         defined $size or ($mode, $size, $mtime) = (0, 0, 0);
                         $size == $MF{$file(1,n)}[1] && $mtime == $MF{$file(1,n)}[2] and next;
                         printf(1,3,1 builtins) "%4s %-34s %8d %9d  %8d %9d\n",
                             $host(1,5), $file(1,n), $MF{$file(1,n)}[1], $MF{$file(1,n)}[2], $size, $mtime;
                         unlink(1,2) $rfile;
                         copy ($file(1,n), $rfile);
                         utime time(1,2,n), $MF{$file(1,n)}[2], $rfile;
                         chmod(1,2) $MF{$file(1,n)}[0], $rfile;
                         }
                     }

               though this is not perfect. It could be improved with checking
               file(1,n) 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(1,8) the tree. These patches are either created by the
           pumpking himself using "diff -c" after updating the file(1,n) manually
           or by applying patches sent in(1,8) 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(1,8) a directory where your patches reside, you can
           get them in(1,8) sync(1,2,8) with

            # rsync -avz rsync://ftp.linux.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 something like

            # last=`ls -t *.gz | sed q`
            # rsync -avz rsync://ftp.linux.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 Knig to have better control 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(1,8) the source tree are in(1,8) the right state.

       It's more reliable
           While both the rsync-able source and patch areas are automatically
           updated every few minutes, keep in(1,8) mind that applying patches may
           sometimes mean careful hand-holding, especially if(3,n) your version(1,3,5) of
           the "patch" program does not understand how to deal with new files,
           files with 8-bit characters, or files without trailing newlines.

       Why rsync the patches


       It's easier to rsync the patches
           If you have more than one machine that you want to keep in(1,8) 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(1,8) pace on 5 different machines, for which
           only one of them has access(2,5) 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 auto-
           matically. Need you say more ;-)

       It's a good reference
           If you do not only like to have the most recent development 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(3,n) you are a starter, the patches may help you
           in(1,8) finding where you should start and how to change the bits that
           bug you.

           The file(1,n) Changes is updated on occasions the pumpking sees as his
           own little sync(1,2,8) 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(7,n,1 builtins) 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 archive.

           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(1,8) 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 misbehaves, 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(1,8,3 Search::Dict) how others apply the fix.

           Finding the source of misbehaviour
               When you keep in(1,8) sync(1,2,8) with bleadperl, the pumpking would love
               to see that the community efforts really work. So after each of
               his sync(1,2,8) points, you are to 'make test' to check if(3,n) everything
               is still in(1,8) working order. If it is, you do 'make ok', which
               will send(2,n) an OK report to perlbug@perl.org. (If you do not have
               access(2,5) to a mailer from the system you just finished success-
               fully 'make test', you can do 'make okfile', which creates the
               file(1,n) "perl.ok", which you can than take to your favourite
               mailer and mail(1,8) yourself).

               But of course, as always, things will not always lead to a suc-
               cess path, and one or more test do not pass the 'make test'.
               Before sending in(1,8) a bug report (using 'make nok' or 'make nok-
               file(1,n)'), check the mailing list if(3,n) someone else has reported the
               bug already and if(3,n) so, confirm it by replying to that message.
               If not, you might want to trace(3x,n,3x _nc_tracebits) the source of that misbehaviour
               before sending in(1,8) the bug, which will help all the other
               porters in(1,8) finding the solution.

               Here the saved patches come in(1,8) very handy. You can check the
               list of patches to see which patch changed what file(1,n) and what
               change caused the misbehaviour. If you note that in(1,8) 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(7,n) via
       rsync.  If you edit a file(1,n) locally and then rsync against the latest
       source, changes made in(1,8) the remote copy will overwrite your local ver-
       sions!

       The best way to deal with this is to maintain a tree of symlinks to the
       rsync'd source.  Then, when you want to edit a file(1,n), you remove the
       symlink, copy the real file(1,n) into the other tree, and edit it.  You can
       then diff your edited file(1,n) against the original to generate a patch,
       and you can safely update(7,n) 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(1,2) 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...)

       The specifics may vary based on your operating system, of course.
       After you see this, you can abort(3,7) the Configure script, and you will
       see that the directory you are in(1,8) 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(3,n) [ -L $1 ]; then
                   mv $1 $1.orig
                       cp $1.orig $1
                       vi $1
               else
                   /bin/vi $1
                       fi
           }

           function unedit {
               if(3,n) [ -L $1.orig ]; then
                   rm $1
                       mv $1.orig $1
                       fi
           }

       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(1,8) your symlink tree:

           mkpatchorig() {
               local diffopts
                   for f in(1,8) `find . -name '*.orig' | sed s,^\./,,`
                       do
                           case `echo(1,3x,1 builtins) $f | sed 's,.orig$,,;s,.*\.,,'` in(1,8)
                               c)   diffopts=-p ;;
                       pod) diffopts='-F^=' ;;
                       *)   diffopts= ;;
                       esac
                           diff -du $diffopts $f `echo(1,3x,1 builtins) $f | sed 's,.orig$,,'`
                           done
           }

       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(2,n) 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.

       Perlbug administration

       There is a single remote administrative interface for modifying bug
       status, category, open(2,3,n) issues etc. using the RT bugtracker system,
       maintained by Robert Spier.  Become an administrator, and close(2,7,n) any
       bugs you can get your sticky mitts on:

               http://rt.perl.org

       The bugtracker mechanism for perl5 bugs in(1,8) particular is at:

               http://bugs6.perl.org/perlbug

       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(2,n) the author a copy (see
       "Patching a core module").  This lets other porters review your patch,
       which catches a surprising number of errors in(1,8) patches.  Either use the
       diff program (available in(1,8) 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(2,n) RCS-style diffs or diffs without con-
       text lines.  More information is given in(1,8) the Porting/patching.pod file(1,n)
       in(1,8) the Perl source distribution.  Please patch against the latest
       development version(1,3,5) (e.g., if(3,n) you're fixing a bug in(1,8) the 5.005 track,
       patch against the latest 5.005_5x version(1,3,5)).  Only patches that survive
       the heat of the development branch get applied to maintenance versions.

       Your patch should update(7,n) the documentation and test suite.  See "Writ-
       ing a test".

       To report a bug in(1,8) Perl, use the program perlbug which comes with Perl
       (if(3,n) you can't get Perl to work, send(2,n) mail(1,8) to the address perl-
       bug@perl.org or perlbug@perl.com).  Reporting bugs through perlbug
       feeds into the automated bug-tracking system, access(2,5) to which is pro-
       vided 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(3,n) so whether it was
       considered a bug.  See above for the location of the searchable ar-
       chives.

       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(2,n,1 builtins) and lurk for a while before chipping in.  That
       way you'll get to see the dynamic of the conversations, learn the per-
       sonalities 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(1,n) the perl5-porters
       mailing list, send(2,n) mail(1,8) to perl5-porters-subscribe@perl.org.  To unsub-
       scribe, send(2,n) mail(1,8) to perl5-porters-unsubscribe@perl.org.

       To hack on the Perl guts, you'll need to read(2,n,1 builtins) the following things:

       perlguts
          This is of paramount importance, since it's the documentation of
          what goes where in(1,8) the Perl source. Read it over a couple of times
          and it might start to make sense - don't worry if(3,n) it doesn't yet,
          because the best way to study it is to read(2,n,1 builtins) it in(1,8) conjunction with
          poking at Perl source, and we'll do that later on.

          You might also want to look(1,8,3 Search::Dict) at Gisle Aas's illustrated perlguts -
          there's no guarantee that this will be absolutely up-to-date with
          the latest documentation in(1,8) 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 por-
          tion of the guts that actually executes a Perl program. It's a lot
          gentler to learn those techniques from simple examples and explana-
          tion than from the core itself.

       perlapi
          The documentation for the Perl API explains what some of the inter-
          nal functions do, as well as the many macros used in(1,8) the source.

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

       The perl5-porters FAQ
          This should be available from http://simon-cozens.org/writ-
          ings/p5p-faq ; alternatively, you can get the FAQ emailed to you by
          sending mail(1,8) to "perl5-porters-faq@perl.org". It contains hints on
          reading perl5-porters, information on how perl5-porters works and
          how Perl development in(1,8) general works.

       Finding Your Way Around

       Perl maintenance can be split(1,n) into a number of areas, and certain peo-
       ple (pumpkins) will have responsibility for each area. These areas
       sometimes correspond to files or directories in(1,8) the source kit. Among
       the areas are:

       Core modules
          Modules shipped as part of the Perl core live in(1,8) the lib/ and ext/
          subdirectories: lib/ is for the pure-Perl modules, and ext/ contains
          the core XS modules.

       Tests
          There are tests for nearly all the modules, built-ins and major bits
          of functionality.  Test files all have a .t suffix.  Module tests
          live in(1,8) the lib/ and ext/ directories next to the module being
          tested.  Others live in(1,8) t/.  See "Writing a test"

       Documentation
          Documentation maintenance includes looking after everything in(1,8) the
          pod/ directory, (as well as contributing new documentation) and the
          documentation to the modules in(1,8) core.

       Configure
          The configure process is the way we make Perl portable across the
          myriad of operating systems it supports. Responsibility for the con-
          figure, build and installation process, as well as the overall
          portability of the core code rests with the configure pumpkin - oth-
          ers 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(1,8) the core distribution.)

       Interpreter
          And of course, there's the core of the Perl interpreter itself.
          Let's have a look(1,8,3 Search::Dict) at that in(1,8) a little more detail.

       Before we leave looking at the layout, though, don't forget that MANI-
       FEST contains not only the file(1,n) names in(1,8) the Perl distribution, but
       short descriptions of what's in(1,8) them, too. For an overview of the
       important files, try this:

           perl -lne 'print if(3,n) /^[^\/]+\.[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(1,8) perlguts explains exactly how the compilation stage
       happens.

       Here is a short breakdown of perl's operation:

       Startup
          The action begins in(1,8) 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(1,8) perlembed; most of the real action
          takes place in(1,8) perl.c

          First, perlmain.c allocates some memory and constructs a Perl inter-
          preter:

              1 PERL_SYS_INIT3(&argc,&argv,&env(1,3));
              2
              3 if(3,n) (!PL_do_undump) {
              4     my_perl = perl_alloc();
              5     if(3,n) (!my_perl)
              6         exit(3,n,1 builtins)(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(1,8) 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(1,8) any sane con-
          text.

          Line 4 calls a function in(1,8) perl.c to allocate memory for a Perl
          interpreter. It's quite a simple function, and the guts of it looks
          like this:

              my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));

          Here you see an example of Perl's system abstraction, which we'll
          see later: "PerlMem_malloc" is either your system's "malloc", or
          Perl's own "malloc" as defined in(1,8) malloc.c if(3,n) you selected that
          option at configure time.

          Next, in(1,8) 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 go:

              exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
              if(3,n) (!exitstatus) {
                  exitstatus = perl_run(my_perl);
              }

          "perl_parse" is actually a wrapper around "S_parse_body", as defined
          in(1,8) perl.c, which processes the command line options, sets up any
          statically linked XS modules, opens the program and calls "yyparse"
          to parse it.

       Parsing
          The aim of this stage is to take the Perl source, and turn it into
          an op tree. We'll see what one of those looks like later. Strictly
          speaking, there's three things going on here.

          "yyparse", the parser, lives in(1,8) perly.c, although you're better off
          reading the original YACC input in(1,8) 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 op-
          erands 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 func-
          tion, and so on.  The main point of entry to the lexer is "yylex",
          and that and its associated routines can be found in(1,8) 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(1,3) of token some-
          thing is, or where a token ends. As such, there's a lot of interplay
          between the tokeniser and the parser, which can get pretty frighten-
          ing if(3,n) 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 rou-
          tines which construct and link(1,2) together the various operations are
          to be found in(1,8) op.c, and will be examined later.

       Optimization
          Now the parsing stage is complete, and the finished tree represents
          the operations that the Perl interpreter needs to perform to execute
          our program. Next, Perl does a dry run over the tree looking for
          optimisations: constant expressions such as "3 + 4" will be computed
          now, and the optimizer will also see if(3,n) any multiple operations can
          be replaced with a single one. For instance, to fetch the variable
          $foo, instead of grabbing the glob(1,3,7,n) *foo and looking at the scalar
          component, the optimizer fiddles the op tree to use a function which
          directly looks up the scalar in(1,8) question. The main optimizer is
          "peep" in(1,8) op.c, and many ops have their own optimizing functions.

       Running
          Now we're finally ready to go: we have compiled Perl byte code, and
          all that's left to do is run it. The actual execution is done by the
          "runops_standard" function in(1,8) run.c; more specifically, it's done by
          these three innocent looking lines:

              while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
                  PERL_ASYNC_CHECK();
              }

          You may be more comfortable with the Perl version(1,3,5) of that:

              PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};

          Well, maybe not. Anyway, each op contains a function pointer, which
          stipulates the function which will actually carry out the operation.
          This function will return the next op in(1,8) the sequence - this allows
          for things like "if(3,n)" which choose the next op dynamically at run
          time.  The "PERL_ASYNC_CHECK" makes sure that things like signals
          interrupt execution if(3,n) 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 optimized, pp_sys.c contains all the system-
          specific functions, pp_ctl.c contains the functions which implement
          control structures ("if(3,n)", "while" and the like) and pp.c contains
          everything else. These are, if(3,n) you like, the C code for Perl's
          built-in functions and operators.

       Internal Variable Types

       You should by now have had a look(1,8,3 Search::Dict) 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(1,8) the code, as well as some structures com-
       pletely internal to Perl. The symbol table, for instance, is an ordi-
       nary Perl hash. Your code is represented by an SV as it's read(2,n,1 builtins) into the
       parser; any program files you call are opened via ordinary Perl file-
       handles, 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 constant "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(1,8) mem-
       ory. SVs themselves are very simple structures, but they contain a
       pointer to a more complex structure. In this case, it's a PV, a struc-
       ture which holds a string(3,n) value, at location 0xa041450.  Line 2 is the
       reference count; there are no other references to this data, so it's 1.

       Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
       read-only SV (because it's a constant) and the data is a PV internally.
       Next we've got the contents of the string(3,n), starting at location
       0xa0484e0.

       Line 5 gives us the current length of the string(3,n) - note that this does
       not include the null terminator. Line 6 is not the length of the
       string(3,n), but the length of the currently allocated buffer; as the string(3,n)
       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(1,8) the snippet, and you've got a
       macro which will return the value: "SvCUR(sv)" returns the current
       length of the string(3,n), "SvREFCOUNT(sv)" returns the reference count,
       "SvPV(sv, len)" returns the string(3,n) itself with its length, and so on.
       More macros to manipulate these properties can be found in(1,8) perlguts.

       Let's take an example of manipulating a PV, from "sv_catpvn", in(1,8) 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(3,n) (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(3,n), "ptr", of length "len" onto the
       end of the PV stored in(1,8) "sv". The first thing we do in(1,8) 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(3,n) set(7,n,1 builtins) 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 accommo-
       date the old string(3,n), the new string(3,n) and the null terminator. If "LEN"
       isn't big enough, "SvGROW" will reallocate space for us.

       Now, if(3,n) "junk" is the same as the string(3,n) we're trying to add, we can
       grab the string(3,n) directly from the SV; "SvPVX" is the address of the PV
       in(1,8) the SV.

       Line 10 does the actual catenation: the "Move" macro moves a chunk of
       memory around: we move(3x,7,3x curs_move) the string(3,n) "ptr" to the end of the PV - that's
       the start of the PV plus its current length. We're moving "len" bytes
       of type "char". After doing so, we need to tell Perl we've extended the
       string(3,n), by altering "CUR" to reflect the new length. "SvEND" is a macro
       which gives us the end of the string(3,n), so that needs to be a "\0".

       Line 13 manipulates the flags; since we've changed the PV, any IV or NV
       values will no longer be valid: if(3,n) 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(1,3,5) 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(3,n) 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(1,8,3 Search::Dict) at how the op tree is con-
       structed.

       Op Trees

       First, what is the op tree, anyway? The op tree is the parsed represen-
       tation of your program, as we saw in(1,8) our section on parsing, and it's
       the sequence of operations that Perl goes through to execute your pro-
       gram, as we saw in(1,8) "Running".

       An op is a fundamental operation that Perl can perform: all the built-
       in(1,8) 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(1,8) two ways: you can imagine that there are
       two "routes" through it, two orders in(1,8) 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 opera-
       tions 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(1,8,3 Search::Dict) at how Perl sees "$a = $b + $c":

            % perl -MO=Terse -e '$a=$b+$c'
            1  LISTOP (0x8179888) leave
            2      OP (0x81798b0) enter
            3      COP (0x8179850) nextstate
            4      BINOP (0x8179828) sassign
            5          BINOP (0x8179800) add [1]
            6              UNOP (0x81796e0) null [15]
            7                  SVOP (0x80fafe0) gvsv  GV (0x80fa4cc) *b
            8              UNOP (0x81797e0) null [15]
            9                  SVOP (0x8179700) gvsv  GV (0x80efeb0) *c
           10          UNOP (0x816b4f0) null [15]
           11              SVOP (0x816dcf0) gvsv  GV (0x80fa460) *a

       Let's start in(1,8) the middle, at line 4. This is a BINOP, a binary opera-
       tor, which is at location 0x8179828. The specific operator in(1,8) question
       is "sassign" - scalar assignment - and you can find the code which
       implements it in(1,8) the function "pp_sassign" in(1,8) 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 mentioned in(1,8) "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 eas-
       ier just to replace the redundant operation with the null op. Origi-
       nally, 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(1,8,3 Search::Dict)
       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(1,8) 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(1,2,n) you enter and leave a block: lexical variables
       are tidied up, unreferenced variables are destroyed, and so on. Every
       program will have those first three lines: "leave" is a list, and its
       children are all the statements in(1,8) the block. Statements are delimited
       by "nextstate", so a block is a collection of "nextstate" 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:

                               Program
                                  |
                              Statement
                                  |
                                  =
                                 / \
                                /   \
                               $a   +
                                   / \
                                 $b   $c

       However, it's impossible to perform the operations in(1,8) 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(1,8) this order using the "exec(3,n,1 builtins)"
       option to "B::Terse":

            % perl -MO=Terse,exec(3,n,1 builtins) -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(1,8) 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"

           1 term(5,7)    :   term(5,7) ASSIGNOP term(5,7)
           2                { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
           3         |   term(5,7) ADDOP term(5,7)
           4                { $$ = 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(1,8)
       upper case. Here, "ADDOP", is provided when the tokeniser sees "+" in(1,8)
       your code. "ASSIGNOP" is provided 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 sym-
       bols' are generally placed in(1,8) lower case. "term(5,7)" here is a non-terminal
       symbol, representing a single expression.

       The grammar gives you the following rule: you can make the thing on the
       left of the colon if(3,n) you see all the things on the right in(1,8) 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(5,7)" followed by "=" fol-
       lowed by "term(5,7)" makes a "term(5,7)", and "term(5,7)" followed by "+" followed by
       "term(5,7)" can also make a "term(5,7)".

       So, if(3,n) 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(1,8) the block on the next line: if(3,n) you see "=", you'll do the code in(1,8)
       line 2. If you see "+", you'll do the code in(1,8) line 4. It's this code
       which contributes to the op tree.

                   |   term(5,7) ADDOP term(5,7)
                   { $$ = 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(1,8)
       the input, $2 the second, and so on - think regular expression backref-
       erences. $$ is the op returned from this reduction. So, we call "new-
       BINOP" to create a new binary operator. The first parameter to "new-
       BINOP", a function in(1,8) 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(1,8) 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(1,8)
       scalar context.

       Stacks

       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(1,8,3 Search::Dict) 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 works:

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

          We'll see a more tricky example of this when we consider Perl's
          macros below. "POPn" gives you the NV (floating point value) of the
          top SV on the stack: the $x in(1,8) "cos($x)". Then we compute the
          cosine, and push the result back as an NV. The "X" in(1,8) "XPUSHn" means
          that the stack should be extended if(3,n) necessary - it can't be neces-
          sary 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(1,8) 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 integer:

               SETi(-TOPi);

          Just set(7,n,1 builtins) the integer value of the top stack entry to its negation.

          Argument stack manipulation in(1,8) the core is exactly the same as it is
          in(1,8) XSUBs - see perlxstut, perlxs and perlguts for a longer descrip-
          tion of the macros used in(1,8) stack manipulation.

       Mark stack
          I say `your portion of the stack' above because PP code doesn't nec-
          essarily get the whole stack to itself: if(3,n) your function calls
          another function, you'll only want to expose the arguments aimed for
          the called function, and not (necessarily) let it get at your own
          data. The way we do this is to have a `virtual(5,8)' bottom-of-stack,
          exposed to each function. The mark stack keeps bookmarks to loca-
          tions in(1,8) the argument stack usable by each function. For instance,
          when dealing with a tied variable, (internally, something with `P'
          magic(4,5)) Perl has to call methods for accesses to the tied variables.
          However, we need to separate the arguments exposed to the method to
          the argument exposed to the original function - the store or fetch
          or whatever it may be. Here's how the tied "push" is implemented;
          see "av_push" in(1,8) 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.

               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(3,n)
          its previous value returned, and so on. Think of them as the "{" and
          "}" of a Perl block.

          To actually do the magic(4,5) method call, we have to call a subroutine
          in(1,8) Perl space: "call_method" takes care of that, and it's described
          in(1,8) perlcall. We call the "PUSH" method in(1,8) 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(1,8) 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  }

       Every line here (apart from the braces, of course) contains a macro.
       The first line sets up the function declaration as Perl expects for PP
       code; line 3 sets up variable declarations for the argument stack and
       the target, the return value of the operation. Finally, it tries to see
       if(3,n) the addition operation is overloaded; if(3,n) so, the appropriate subrou-
       tine 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(3,n) 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(7,n,1 builtins) 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 opera-
       tor to run back to the main run loop.

       Most of these macros are explained in(1,8) perlapi, and some of the more
       important ones are explained in(1,8) perlxs as well. Pay special attention
       to "Background and PERL_IMPLICIT_CONTEXT" in(1,8) perlguts for information
       on the "[pad]THX_?" macros.

       The .i Targets

       You can expand the macros in(1,8) a foo.c file(1,n) 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
           make

       "-g" is a flag to the C compiler to have it produce debugging informa-
       tion which will allow us to step through a running program.  Configure
       will also turn on the "DEBUGGING" compilation symbol which enables all
       the internal debugging code in(1,8) 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

           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(1,2,n) to step
       through perl's execution with a source-level debugger.

         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(1,8) your Perl source tree so the debugger can
       read(2,n,1 builtins) the source code. You should see the copyright message, followed by
       the prompt.

           (gdb)

       "help" will get you into the documentation, but here are the most use-
       ful 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(5,8) function (but see "Internal Functions" in(1,8)
          perlguts!) or the given line in(1,8) the named(5,8) source file.

       step
          Steps through the program a line at a time.

       next
          Steps through the program a line at a time(1,2,n), without descending into
          functions.

       continue
          Run until the next breakpoint.

       finish
          Run until the end of the current function, then stop again.

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

       print
          Execute the given C code and print its results. WARNING: Perl makes
          heavy use of macros, and gdb does not necessarily 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(1,3)". Even then, cpp won't recur-
       sively apply those macros for you.

       gdb macro support

       Recent versions of gdb have fairly good macro support, but in(1,8) order to
       use it you'll need to compile perl with macro definitions included in(1,8)
       the debugging information.  Using gcc version(1,3,5) 3.1, this means configur-
       ing with "-Doptimize=-g3".  Other compilers might use a different
       switch(1,n) (if(3,n) 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(1,8) 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:

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

       Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
       in(1,8) perlguts.  With the breakpoint in(1,8) 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(1,8) the relevant source files and
       libraries, and then:

           Breakpoint 1, Perl_pp_add () at pp_hot.c:309
           309         dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
           (gdb) step
           311           dPOPTOPnnrl_ul;
           (gdb)

       We looked at this bit of code before, and we said that "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(3,n) "SvNOK" is set(7,n,1 builtins)) 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(1,8) 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(3,n) (!sv)
           (gdb)

       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 subroutine:

           (gdb) finish
           Run till exit(3,n,1 builtins) from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
           0x462669 in(1,8) 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(1,8)
       "PL_op", and we can dump it with "Perl_op_dump". This'll give us simi-
       lar 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 #

       Patching

       All right, we've now had a look(1,8,3 Search::Dict) 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(3,n) a
       "U" is the first active format during a "pack(3,n,n pack-old)", (for example, "pack(3,n,n pack-old)
       "U3C8", @stuff") then the resulting string(3,n) should be treated as UTF-8
       encoded.

       How do we prepare to fix this up? First we locate the code in(1,8) question
       - the "pack(3,n,n pack-old)" happens at runtime, so it's going to be in(1,8) one of the pp
       files. Sure enough, "pp_pack" is in(1,8) pp.c. Since we're going to be
       altering this file(1,n), let's copy it to pp.c~.

       [Well, it was in(1,8) pp.c when this tutorial was written. It has now been
       split(1,n) off with "pp_unpack" to its own file(1,n), pp_pack.c]

       Now let's look(1,8,3 Search::Dict) over "pp_pack": we take a pattern into "pat", and then
       loop over the pattern, taking each format character in(1,8) turn into
       "datum_type". Then for each possible format character, we swallow up
       the other arguments in(1,8) the pattern (a field width, an asterisk, and so
       on) and convert the next chunk input into the specified format, adding
       it onto the output SV "cat".

       How do we know if(3,n) the "U" is the first format in(1,8) the "pat"? Well, if(3,n) we
       have a pointer to the start of "pat" then, if(3,n) we see a "U" we can test
       whether we're still at the start of the string. So, here's where "pat"
       is set(7,n,1 builtins) 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(3,n) pointer in(1,8) 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(7,n,1 builtins) "patcopy" to be the start
       of "pat":

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

       Now if(3,n) we see a "U" which was at the start of the string(3,n), we turn on
       the "UTF8" flag for the output SV, "cat":

        +  if(3,n) (datumtype == 'U' && pat==patcopy+1)
        +      SvUTF8_on(cat);
           if(3,n) (datumtype == '#') {
               while (pat < patend && *pat != '\n')
                   pat++;

       Remember that it has to be "patcopy+1" because the first character of
       the string(3,n) is the "U" which has been swallowed into "datumtype!"

       Oops, we forgot one thing: what if(3,n) there are spaces at the start of the
       pattern? "pack(3,n,n pack-old)("  U*", @stuff)" will have "U" as the first active char-
       acter, even though it's not the first thing in(1,8) the pattern. In this
       case, we have to advance "patcopy" along with "pat" when we see spaces:

           if(3,n) (isSPACE(datumtype))
               continue;

       needs to become

           if(3,n) (isSPACE(datumtype)) {
               patcopy++;
               continue;
           }

       OK. That's the C part done. Now we must do two additional things before
       this patch is ready to go: we've changed the behaviour of Perl, and so
       we must document that change. We must also provide some more regression
       tests to make sure our patch works and doesn't create a bug somewhere
       else along the line.

       The regression tests for each operator live in(1,8) 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(3,n) 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(3,n,n pack-old)("U*",1,20,300,4000);
        print "ok $test\n"; $test++;

       we can write(1,2) the more sensible (see Test::More for a full explanation
       of is() and other testing functions).

        is( "1.20.300.4000", sprintf "%vd", pack(3,n,n pack-old)("U*",1,20,300,4000),
                                              "U* produces unicode" );

       Now we'll test that we got that space-at-the-beginning business right:

        is( "1.20.300.4000", sprintf "%vd", pack(3,n,n pack-old)("  U*",1,20,300,4000),
                                              "  with spaces at the beginning" );

       And finally we'll test that we don't make Unicode strings if(3,n) "U" is not
       the first active format:

        isnt( v1.20.300.4000, sprintf "%vd", pack(3,n,n pack-old)("C0U*",1,20,300,4000),
                                              "U* not first isn't unicode" );

       Mustn't forget to change the number of tests which appears at the top,
       or else the automated tester will get confused.  This will either look(1,8,3 Search::Dict)
       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(1,8) the description of "pack(3,n,n pack-old)":

        =item *

        If the pattern begins with a C<U>, the resulting string(3,n) will be treated
        as UTF-8-encoded Unicode. You can force UTF-8 encoding(3,n) on in(1,8) a string(3,n)
        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(3,n), and then follow this with a C<U*> somewhere in(1,8) your pattern.

       All done. Now let's create the patch. Porting/patching.pod tells us
       that if(3,n) we're making major changes, we should copy the entire directory
       to somewhere safe before we begin fiddling, and then do

           diff -ruN old new > patch

       However, we know which files we've changed, and we can simply 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:

           --- 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 considera-
       tion.  Many core modules also live on CPAN.  If this is so, patch the
       CPAN version(1,3,5) instead of the core and send(2,n) the patch off to the module
       maintainer (with a copy to p5p).  This will help the module maintainer
       keep the CPAN version(1,3,5) in(1,8) sync(1,2,8) with the core version(1,3,5) without constantly
       scanning p5p.

       Adding a new function to the core

       If, as part of a patch to fix a bug, or just because you have an espe-
       cially good idea, you decide to add a new function to the core, discuss
       your ideas on p5p well before you start work.  It may be that someone
       else has already attempted to do what you are considering and can give
       lots of good advice or even provide you with bits of code that they
       already started (but never finished).

       You have to follow all of the advice given above for patching.  It is
       extremely important to test any addition thoroughly and add new tests
       to explore all boundary conditions that your new function is expected
       to handle.  If your new function is used only by one module (e.g.
       toke), then it should probably be named(5,8) S_your_function (for static);
       on the other hand, if(3,n) you expect it to accessible from other functions
       in(1,8) Perl, you should name it Perl_your_function.  See "Internal Func-
       tions" in(1,8) perlguts for more details.

       The location of any new code is also an important consideration.  Don't
       just create a new top level .c file(1,n) and put your code there; you would
       have to make changes to Configure (so the Makefile is created prop-
       erly), 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(1,8) the perl executable.  Everything else is located in(1,8) the
       shared library (or DLL if(3,n) you are running under WIN32).  So, for exam-
       ple, if(3,n) a function was only used by functions located in(1,8) toke.c, then
       your code can go in(1,8) toke.c.  If, however, you want to call the function
       from universal.c, then you should put your code in(1,8) another location,
       for example util.c.

       In addition to writing your c-code, you will need to create an appro-
       priate entry in(1,8) embed.pl describing your function, then run 'make
       regen_headers' to create the entries in(1,8) the numerous header files that
       perl needs to compile correctly.  See "Internal Functions" in(1,8) perlguts
       for information on the various options that you can set(7,n,1 builtins) in(1,8) embed.pl.
       You will forget to do this a few (or many) times and you will get warn-
       ings during the compilation phase.  Make sure that you mention this
       when you post your patch to P5P; the pumpking needs to know this.

       When you write(1,2) your new code, please be conscious of existing code con-
       ventions used in(1,8) the perl source files.  See perlstyle for details.
       Although most of the guidelines discussed seem to focus on Perl code,
       rather than c, they all apply (except when they don't ;).  See also
       Porting/patching.pod file(1,n) in(1,8) 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 mem-
       ory 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(1,2,n) and don't have the time(1,2,n) to debug
       your code.

       Writing a test

       Every module and built-in function has an associated test file(1,n) (or
       should...).  If you add or change functionality, you have to write(1,2) a
       test.  If you fix a bug, you have to write(1,2) a test so that bug never
       comes back.  If you alter the docs, it would be nice(1,2) to test what the
       new documentation says.

       In short, if(3,n) you submit a patch you probably also have to patch the
       tests.

       For modules, the test file(1,n) 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 wonderful for you to brush them
       out), but it basically works that way.  Everything else lives in(1,8) t/.

       t/base/
          Testing of the absolute basic functionality of Perl.  Things like
          "if(3,n)", basic file(1,n) reads and writes, simple regexes, etc.  These are
          run first in(1,8) the test suite and if(3,n) any of them fail, something is
          really broken.

       t/cmd/
          These test the basic control structures, "if(3,n)/else", "while", subrou-
          tines, etc.

       t/comp/
          Tests basic issues of how Perl parses and compiles itself.

       t/io/
          Tests for built-in IO functions, including command line arguments.

       t/lib/
          The old home for the module tests, you shouldn't put anything new in(1,8)
          here.  There are still some bits and pieces hanging around in(1,8) here
          that need to be moved.  Perhaps you could move(3x,7,3x curs_move) them?  Thanks!

       t/op/
          Tests for perl's built in(1,8) functions that don't fit into any of the
          other directories.

       t/pod/
          Tests for POD directives.  There are still some tests for the Pod
          modules hanging around in(1,8) here that need to be moved out into lib/.

       t/run/
          Testing features of how perl actually runs, including exit(3,n,1 builtins) codes and
          handling of PERL* environment variables.

       t/uni/
          Tests for the core support of Unicode.

       t/win32/
          Windows-specific tests.

       t/x2p
          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 con-
       siderations.

       There are three ways to write(1,2) a test in(1,8) 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(3,n) require works, or even subroutines, use ad
           hoc tests for these two.  Step carefully to avoid using the feature
           being tested.

       t/cmd t/run t/io t/op
           Now that basic require() and subroutines are tested, you can use
           the t/test.pl library which emulates the important features of
           Test::More while using a minimum of core features.

           You can also conditionally use certain libraries like Config, but
           be sure to skip the test gracefully if(3,n) 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(1,8) the tests.

       When you say "make test" Perl uses the t/TEST program to run the test
       suite.  All tests are run from the t/ directory, not the directory
       which contains the test.  This causes some problems with the tests in(1,8)
       lib/, so here's some opportunity for some patching.

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

       Special Make Test Targets

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

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

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

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

       minitest
           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(1,8) Linux) Run all the tests using the memory leak + naughty
           memory access(2,5) tool "valgrind".  The log files will be named(5,8) test-
           name.valgrind.

       test.third check.third utest.third ucheck.third
           (Only in(1,8) Tru64)  Run all the tests using the memory leak + naughty
           memory access(2,5) tool "Third Degree".  The log files will be named(5,8)
           perl3.log.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, t/japh/abigail.t.

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

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

       test_harness
           Run the test suite with the t/harness controlling program, instead
           of t/TEST. t/harness is more sophisticated, and uses the Test::Har-
           ness 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.

       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

       or

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

       (if(3,n) you don't specify test scripts, the whole test suite will be run.)

       You can run an individual test by a command similar to

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

       except that the harnesses set(7,n,1 builtins) up some environment variables that may
       affect the execution of the test :

       PERL_CORE=1
           indicates that we're running this test part of the perl core test
           suite.  This is useful for modules that have a dual life on CPAN.

       PERL_DESTRUCT_LEVEL=2
           is set(7,n,1 builtins) to 2 if(3,n) it isn't set(7,n,1 builtins) already (see "PERL_DESTRUCT_LEVEL")

       PERL
           (used only by t/TEST) if(3,n) set(7,n,1 builtins), overrides the path to the perl exe-
           cutable that should be used to run the tests (the default being
           ./perl).

       PERL_SKIP_TTY_TEST
           if(3,n) set(7,n,1 builtins), tells to skip the tests that need a terminal. It's actually
           set(7,n,1 builtins) automatically by the Makefile, but can also be forced artifi-
           cially by running 'make test_notty'.

EXTERNAL TOOLS FOR DEBUGGING PERL
       Sometimes it helps to use external tools while debugging and testing
       Perl.  This section tries to guide you through using some common test-
       ing and debugging tools with Perl.  This is meant as a guide to inter-
       facing 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, valgrind, or
       Third Degree greatly slows down the execution: seconds become minutes,
       minutes become hours.  For example as of Perl 5.8.1, the
       ext/Encode/t/Unicode.t takes extraordinarily long to complete under
       e.g. Purify, Third Degree, and valgrind.  Under valgrind it takes more
       than six hours, even on a snappy computer-- the said test must be doing
       something that is quite unfriendly for memory debuggers.  If you don't
       feel like waiting, that you can simply kill(1,2,1 builtins) 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 environ-
       ment variable PERL_DESTRUCT_LEVEL set(7,n,1 builtins) to 2.  The TEST and harness
       scripts do that automatically.  But if(3,n) you are running some of the
       tests manually-- for csh-like shells:

           setenv PERL_DESTRUCT_LEVEL 2

       and for Bourne-type shells:

           PERL_DESTRUCT_LEVEL=2
           export PERL_DESTRUCT_LEVEL

       or in(1,8) UNIXy environments you can also use the "env(1,3)" command:

           env(1,3) 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(1,8) the call stack is a good
       sign of these.  Fixing these leaks is non-trivial, unfortunately, but
       they must be fixed eventually.

       Rational Software's Purify

       Purify is a commercial tool that is helpful in(1,8) identifying memory over-
       runs, wild pointers, memory leaks and other such badness.  Perl must be
       compiled in(1,8) a specific way for optimal testing with Purify.  Purify is
       available under Windows 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 using:

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

       where these arguments mean:

       -Accflags=-DPURIFY
           Disables Perl's arena memory allocation functions, as well as forc-
           ing use of memory allocation functions derived from the system mal-
           loc.

       -Doptimize='-g'
           Adds debugging information so that you see the exact source state-
           ments where the problem occurs.  Without this flag, all you will
           see is the source filename of where the error(8,n) occurred.

       -Uusemymalloc
           Disable Perl's malloc so that Purify can more closely monitor allo-
           cations and leaks.  Using Perl's malloc will make Purify report
           most leaks in(1,8) the "potential" leaks category.

       -Dusemultiplicity
           Enabling the multiplicity option allows perl to clean up thoroughly
           when the interpreter shuts down, which reduces the number of bogus
           leak reports from Purify.

       Once you've compiled a perl suitable for Purify'ing, then you can just:

           make pureperl

       which creates a binary named(5,8) 'pureperl' that has been Purify'ed.  This
       binary is used in(1,8) place of the standard 'perl' binary when you want to
       debug Perl memory problems.

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

           make pureperl
           cd t
           ../pureperl -I../lib harness

       which would run Perl on test.pl and report any memory problems.

       Purify outputs messages in(1,8) "Viewer" windows by default.  If you don't
       have a windowing environment or if(3,n) you simply want the Purify output to
       unobtrusively go to a log file(1,n) instead of to the interactive window,
       use these following options to output to the log file(1,n) "perl.log":

           setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
            -log-file=perl.log -append-logfile=yes"

       If you plan to use the "Viewer" windows, then you only need this
       option:

           setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:

           PURIFYOPTIONS="..."
           export PURIFYOPTIONS

       or if(3,n) you have the "env(1,3)" utility:

           env(1,3) PURIFYOPTIONS="..." ../pureperl ...

       Purify on NT

       Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly.
       There are several options in(1,8) the makefile you should change to get the
       most use out of Purify:

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

               DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1

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

       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 allo-
           cations and leaks.  Using Perl's malloc will make Purify report
           most leaks in(1,8) the "potential" leaks category.

       CFG = Debug
           Adds debugging information so that you see the exact source state-
           ments where the problem occurs.  Without this flag, all you will
           see is the source filename of where the error(8,n) occurred.

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

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

       which would instrument Perl in(1,8) memory, run Perl on test.pl, then
       finally report any memory problems.

       valgrind

       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(1,8) files named(5,8) 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(1,n) that comes with valgrind already catches a lot
       of them. Some additional suppressions are defined in(1,8) t/perl.supp.

       To get valgrind and for more information see

           http://developer.kde.org/~sewardj/

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

       Third Degree is a tool for memory leak detection and memory access(2,5)
       checks.  It is one of the many tools in(1,8) 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 exe-
       cutable to create a new executable called perl.third.  When the instru-
       mented executable is run, it creates a log of dubious memory traffic in(1,8)
       file(1,n) called perl.3log.  See the manual pages of atom and third for more
       information.  The most extensive Third Degree documentation is avail-
       able in(1,8) the Compaq "Tru64 UNIX Programmer's Guide", chapter "Debugging
       Programs with Third Degree".

       The "test.third" leaves a lot of files named(5,8) foo_bar.3log in(1,8) the t/
       subdirectory.  There is a problem with these files: Third Degree is so
       effective that it finds problems also in(1,8) the system libraries.  There-
       fore 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 information.

       PERL_DESTRUCT_LEVEL

       If you want to run any of the tests yourself manually using e.g.  val-
       grind, or the pureperl or perl.third executables, please note that by
       default perl does not explicitly cleanup all the memory it has allo-
       cated (such as global memory arenas) but instead lets the exit(3,n,1 builtins)() 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(7,n,1 builtins) the environment
       variable PERL_DESTRUCT_LEVEL to a non-zero value.  The t/TEST wrapper
       does set(7,n,1 builtins) this to 2, and this is what you need to do too, if(3,n) you don't
       want to see the "global leaks": For example, for "third-degreed" Perl:

               env(1,3) 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 equiv-
       alent 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(1,3,5) where those pesky SVs were allocated.

       Profiling

       Depending on your platform there are various of profiling Perl.

       There are two commonly used techniques of profiling executables: sta-
       tistical time-sampling and basic-block counting.

       The first method takes periodically samples of the CPU program counter,
       and since the program counter can be correlated with the code generated
       for functions, we get a statistical view of in(1,8) which functions the pro-
       gram is spending its time.  The caveats are that very small/fast func-
       tions have lower probability of showing up in(1,8) the profile, and that
       periodically interrupting the program (this is usually done rather fre-
       quently, in(1,8) the scale of milliseconds) imposes an additional overhead
       that may skew the results.  The first problem can be alleviated by run-
       ning the code for longer (in(1,8) general this is a good idea for profil-
       ing), the second problem is usually kept in(1,8) 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(1,8) the begin-
       ning and exited only at the end.  For example, a conditional jump
       starts a basic block.  Basic block profiling usually works by instru-
       menting 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 profiling tools usually try
       to factor(1,6) their own effects out of the results.

       Gprof Profiling

       gprof is a profiling tool available in(1,8) many UNIX platforms, it uses
       statistical time-sampling.

       You can build a profiled version(1,3,5) of perl called "perl.gprof" by invok-
       ing the make target "perl.gprof"  (What is required is that Perl must
       be compiled using the "-pg" flag, you may need to re-Configure).  Run-
       ning the profiled version(1,3,5) of Perl will create an output file(1,n) called
       gmon.out is created which contains the profiling data collected during
       the execution.

       The gprof tool can then display the collected data in(1,8) various ways.
       Usually gprof understands the following options:

       -a  Suppress statically defined functions from the profile.

       -b  Suppress the verbose descriptions in(1,8) the profile.

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

       -f routine
           Display only the given routine and its descendants in(1,8) the profile.

       -s  Generate a summary file(1,n) called gmon.sum which then may be given to
           subsequent gprof runs to accumulate data over several runs.

       -z  Display routines that have zero usage.

       For more detailed explanation of the available commands and output for-
       mats, 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(1,3,5) of perl called perl.gcov by invoking
       the make target "perl.gcov" (what is required that Perl must be com-
       piled using gcc with the flags "-fprofile-arcs -ftest-coverage", you
       may need to re-Configure).

       Running the profiled version(1,3,5) of Perl will cause profile output to be
       generated.  For each source file(1,n) an accompanying ".da" file(1,n) will be
       created.

       To display the results you use the "gcov" utility (which should be
       installed if(3,n) 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

           http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html

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

           http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132

       Pixie Profiling

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

       You can build a profiled version(1,3,5) 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(1,n) called perl.Addrs will also be silently created, this
       file(1,n) contains the addresses of the basic blocks.  Running the profiled
       version(1,3,5) of Perl will create a new file(1,n) called "perl.Counts" which con-
       tains the counts for the basic block for that particular program execu-
       tion.

       To display the results you use the prof utility.  The exact incantation
       depends on your operating system, "prof perl.Counts" in(1,8) IRIX, and "prof
       -pixie -all -L. perl" in(1,8) Tru64.

       In IRIX the following prof options are available:

       -h  Reports the most heavily used lines in(1,8) descending order of use.
           Useful for finding the hotspot lines.

       -l  Groups lines by procedure, with procedures sorted in(1,8) descending
           order of use.  Within a procedure, lines are listed in(1,8) source
           order.  Useful for finding the hotspots of procedures.

       In Tru64 the following options are available:

       -p[rocedures]
           Procedures sorted in(1,8) descending order by the number of cycles exe-
           cuted in(1,8) each procedure.  Useful for finding the hotspot proce-
           dures.  (This is the default option.)

       -h[eavy]
           Lines sorted in(1,8) descending order by the number of cycles executed
           in(1,8) each line.  Useful for finding the hotspot lines.

       -i[nvocations]
           The called procedures are sorted in(1,8) descending order by number of
           calls made to the procedures.  Useful for finding the most used
           procedures.

       -l[ines]
           Grouped by procedure, sorted by cycles executed per procedure.
           Useful for finding the hotspots of procedures.

       -testcoverage
           The compiler emitted code for these lines, but the code was unexe-
           cuted.

       -z[ero]
           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(3x,n,n tk_menuSetFocus), 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(1,n) 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(1,n) interactively via: 3rd mouse but-
           ton -> New Display -> Edit Menu

           Note: you can define up to 20 conversion shortcuts in(1,8) the gdb sec-
           tion.

          If you see in(1,8) a debugger a memory area mysteriously full of
           0xabababab, you may be seeing the effect of the Poison() macro, see
           perlclib.

       CONCLUSION

       We've had a brief look(1,8,3 Search::Dict) 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 for submission to p5p.  Finally, we talked
       about how to use external tools to debug and test Perl.

       I'd now suggest you read(2,n,1 builtins) 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 under-
          stand them; don't be afraid to ask if(3,n) there's a portion you're not
          clear(1,3x,3x clrtobot) on - who knows, you may unearth a bug in(1,8) 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 making.

         Do read(2,n,1 builtins) the README associated with your operating system, e.g.
          README.aix on the IBM AIX OS. Don't hesitate to supply patches to
          that README if(3,n) you find anything missing or changed over a new OS
          release.

         Find an area of Perl that seems interesting to you, and see if(3,n) you
          can work out how it works. Scan through the source, and step over it
          in(1,8) the debugger. Play, poke, investigate, fiddle! You'll probably
          get to understand not just your chosen area but a much wider range
          of perl's activity as well, and probably sooner than you'd think.

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

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

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



perl v5.8.5                       2004-04-23                       PERLHACK(1)

References for this manual (incoming links)