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NAME    [Toc]    [Back]

       perlhack - How to hack at the Perl internals

DESCRIPTION    [Toc]    [Back]

       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:




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

       Over this group of porters presides Larry Wall.  He has
       the final word in what does and does not change in the
       Perl language.  Various releases of Perl are shepherded by
       a ``pumpking'', a porter responsible for gathering
       patches, deciding on a patch-by-patch feature-by-feature
       basis what will and will not go into the release.  For
       instance, Gurusamy Sarathy was the pumpking for the 5.6
       release of Perl, and Jarkko Hietaniemi 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 pumpkings), and the Supreme Court
       (Larry).  The legislature can discuss and submit patches
       to the executive branch all they like, but the executive
       branch is free to veto them.  Rarely, the Supreme Court
       will side with the executive branch over the legislature,
       or the legislature over the executive branch.  Mostly,
       however, the legislature and the executive branch are supposed
 to get along and work out their differences without
       impeachment or court cases.

       You might sometimes see reference to Rule 1 and Rule 2.
       Larry's power as Supreme Court is expressed in The Rules:

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

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

       Got that?  Larry is always right, even when he was  wrong.
       It's rare to see either Rule exercised, but they are often
       alluded to.

       New features and extensions to the language are contentious,
 because the criteria used by the pumpkings,
       Larry, and other porters to decide which features should
       be implemented and incorporated are not codified in a few
       small design goals as with some other languages.  Instead,
       the heuristics are flexible and often difficult to fathom.
       Here is one person's list, roughly in decreasing order of
       importance, of heuristics that new features have to be
       weighed against:

       Does concept match the general goals of Perl?
           These haven't been written anywhere in stone, but one
           approximation is:

            1. Keep it fast, simple, and useful.
            2. Keep features/concepts as orthogonal as  possible.
            3.  No  arbitrary limits (platforms, data sizes, cultures).
            4. Keep it open and  exciting  to  use/patch/advocate
Perl everywhere.
            5.  Either  assimilate  new  technologies,  or  build
bridges to them.

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

       Backwards compatibility
           It's a cardinal sin to break existing Perl programs.
           New warnings 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 modules that export
           functions, you can give those functions prototypes so
           they can be called like built-in functions, you can
           even write XS code to mess with the runtime data
           structures of the Perl interpreter if you want to
           implement really complicated things.  If it can be
           done in a module instead of in the core, it's highly
           unlikely to be added.

       Is the feature generic enough?
           Is this something that only the submitter wants added
           to the language, or would it be broadly useful?  Sometimes,
 instead of adding a feature with a tight focus,
           the porters might decide to wait until someone implements
 the more generalized feature.  For instance,
           instead of implementing a ``delayed evaluation'' feature,
 the porters are waiting for a macro system that
           would permit delayed evaluation and much more.

       Does it potentially introduce new bugs?
           Radical rewrites of large chunks of the Perl interpreter
 have the potential to introduce new bugs.  The
           smaller and more localized the change, the better.

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

       Is the implementation robust?
           Good patches (tight code, complete, correct) stand
           more chance of going in.  Sloppy or incorrect patches
           might be placed on the back burner until the pumpking
           has time to fix, or might be discarded altogether
           without further 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 features) must include regression
           tests to verify that everything works as expected.
           Without tests provided by the original author, how can
           anyone else changing perl in the future be sure that
           they haven't unwittingly broken the behaviour the
           patch implements? And without tests, how can the
           patch's author be confident that his/her hard work put
           into the patch won't be accidentally thrown away by
           someone in the future?

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

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

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

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

       If you're on the list, you might hear the word ``core''
       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 sync    [Toc]    [Back]

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


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


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

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

       Needless to say, the source code in perl-current is usually
 in a perpetual state of evolution.  You should expect
       it to be very buggy.  Do not use it for any purpose other
       than testing and development.

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

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

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

            #  rsync -avz rsync://ftp.linux.activestate.com/perlcurrent/ .

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

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

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

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

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

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

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

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

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

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

                 #!/usr/bin/perl -w

                 use strict;
                 use File::Copy;

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

                 my     %remote     =     map     {     $_     =>
"/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
                 foreach my $host (keys %remote) {
                     unless (-d $remote{$host}) {
                         print  STDERR  "Cannot  Xsync  for  host
                     foreach my $file (keys %MF) {
                         my $rfile = "$remote{$host}/$file";
                         my   ($mode,   $size,  $mtime)  =  (stat
$rfile)[2, 7, 9];
                         defined $size or ($mode, $size,  $mtime)
= (0, 0, 0);
                         $size  ==  $MF{$file}[1]  &&  $mtime  ==
$MF{$file}[2] and next;
                         printf "%4s %-34s %8d %9d  %8d %9d0,
                             $host,     $file,     $MF{$file}[1],
$MF{$file}[2], $size, $mtime;
                         unlink $rfile;
                         copy ($file, $rfile);
                         utime time, $MF{$file}[2], $rfile;
                         chmod $MF{$file}[0], $rfile;

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

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

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

            # rsync -avz  rsync://ftp.linux.activestate.com/perlcurrent-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/perlcurrent-diffs/ .
            # find . -name '*.gz' -newer $last -exec gzcat  {}  ;
            # 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 Konig 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 the source tree are in the
           right state.

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

       Why rsync the patches    [Toc]    [Back]

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

           In case you try to keep in pace on 5 different
           machines, for which only one of them has access to the
           WAN, rsync'ing all the source trees should than be
           done 5 times over the NFS. Having rsync'ed the patches
           only once, I can apply them to all the source trees
           automatically. 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 you are a starter, the
           patches may help you in finding where you should start
           and how to change the bits that bug you.

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

           You can use the patches later as a kind of search

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

           Finding how to fix a bug
               If you've found where the function/feature Foo
               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 how others apply the fix.

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

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

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

           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    [Toc]    [Back]

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

       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, you remove the symlink, copy the real file
       into the other tree, and edit it.  You can then diff your
       edited file against the original to generate a patch, and
       you can safely update the original tree.

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

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

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

         Symbolic links are supported.

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

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

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

       If you plan to do a lot of work with the Perl source, here
       are some Bourne shell script functions that can make your
       life easier:
           function edit {
               if [ -L $1 ]; then
                   mv $1 $1.orig
                       cp $1.orig $1
                       vi $1
                   /bin/vi $1

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

       Replace "vi" with your favorite flavor of editor.

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

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

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

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

       Perlbug administration    [Toc]    [Back]

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

       The bugtracker mechanism for perl5 bugs in particular is


       To email the bug system administrators:

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

       Submitting patches    [Toc]    [Back]

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

       Your patch should update the documentation and test suite.
       See "Writing a test".

       To report a bug in Perl, use the program perlbug which
       comes with Perl (if you can't get Perl to work, send mail
       to the address perlbug@perl.org or perlbug@perl.com).
       Reporting bugs through perlbug feeds into the automated
       bug-tracking system, access to which is provided through
       the web at http://bugs.perl.org/ .  It often pays to check
       the archives of the perl5-porters mailing list to see
       whether the bug you're reporting has been reported before,
       and if so whether it was considered a bug.  See above for
       the location of the searchable archives.

       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.devel-
       ooper.com/daily-build@perl.org/ ) automatically tests Perl
       source  releases on platforms with various configurations.
       Both efforts welcome volunteers.

       It's a good idea to read and lurk for a while before chipping
 in.  That way you'll get to see the dynamic of the
       conversations, learn the personalities of the players, and
       hopefully be better prepared to make a useful contribution
       when do you speak up.
       If after all this you still think you want to join the
       perl5-porters mailing list, send mail to
       perl5-porters-subscribe@perl.org.  To unsubscribe, send
       mail to perl5-porters-unsubscribe@perl.org.

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

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

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

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

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

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

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

       Finding Your Way Around    [Toc]    [Back]

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

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

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

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

          The configure process is the way we make Perl portable
          across the myriad of operating systems it supports.
          Responsibility for the configure, build and installation
 process, as well as the overall portability of the
          core code rests with the configure pumpkin - others
          help out with individual operating systems.

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

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

       Before we leave looking at the layout, though, don't forget
 that MANIFEST contains not only the file names in the
       Perl distribution, but short descriptions of what's in
       them, too. For an overview of the important files, try

           perl -lne 'print if /^[^]+.[ch]' MANIFEST

       Elements of the interpreter    [Toc]    [Back]

       The work of the interpreter has two main stages: compiling
       the code into the internal representation, or bytecode,
       and then executing it.  "Compiled code" in perlguts
       explains exactly how the compilation stage happens.
       Here is a short breakdown of perl's operation:

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

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

              1 PERL_SYS_INIT3(&argc,&argv,&env);
              3 if (!PL_do_undump) {
              4     my_perl = perl_alloc();
              5     if (!my_perl)
              6         exit(1);
              7     perl_construct(my_perl);
              8     PL_perl_destruct_level = 0;
              9 }

          Line 1 is a macro, and its definition is dependent on
          your operating system. Line 3 references
          "PL_do_undump", a global variable - all global variables
 in Perl start with "PL_". This tells you whether
          the current running program was created with the "-u"
          flag to perl and then undump, which means it's going to
          be false in any sane context.

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

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

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

          Next, in line 7, we construct the interpreter; this
          sets up all the special variables that Perl needs, the
          stacks, and so on.

          Now we pass Perl the command line options, and tell it
          to go:

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

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

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

          "yyparse", the parser, lives in perly.c, although
          you're better off reading the original YACC input in
          perly.y. (Yes, Virginia, there is a YACC grammar for
          Perl!) The job of the parser is to take your code and
          `understand' it, splitting it into sentences, deciding
          which operands go with which operators and so on.

          The parser is nobly assisted by the lexer, which chunks
          up your input into tokens, and decides what type of
          thing each token is: a variable name, an operator, a
          bareword, a subroutine, a core function, and so on.
          The main point of entry to the lexer is "yylex", and
          that and its associated routines can be found in
          toke.c. Perl isn't much like other computer languages;
          it's highly context sensitive at times, it can be
          tricky to work out what sort of token something is, or
          where a token ends. As such, there's a lot of interplay
          between the tokeniser and the parser, which can get
          pretty frightening if you're not used to it.

          As the parser understands a Perl program, it builds up
          a tree of operations for the interpreter to perform
          during execution. The routines which construct and link
          together the various operations are to be found in
          op.c, and will be examined later.

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

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

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

          You may be more comfortable with the Perl version of

              PERL_ASYNC_CHECK()      while      $Perl::op      =

          Well, maybe not. Anyway, each op contains a function
          pointer, which stipulates the function which will actually
 carry out the operation.  This function will
          return the next op in the sequence - this allows for
          things like "if" which choose the next op dynamically
          at run time.  The "PERL_ASYNC_CHECK" makes sure that
          things like signals interrupt execution if required.

          The actual functions called are known as PP code, and
          they're spread between four files: 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", "while" and the like)
          and pp.c contains everything else. These are, if you
          like, the C code for Perl's built-in functions and

       Internal Variable Types    [Toc]    [Back]

       You should by now have had a look at perlguts, which tells
       you about Perl's internal variable types: SVs, HVs, AVs
       and the rest. If not, do that now.

       These variables are used not only to represent Perl-space
       variables, but also any constants in the code, as well as
       some structures completely internal to Perl. The symbol
       table, for instance, is an ordinary Perl hash. Your code
       is represented by an SV as it's read into the parser; any
       program files you call are opened via ordinary Perl filehandles,
 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"
           5   CUR = 5
           6   LEN = 6

       Reading "Devel::Peek" output takes a bit of practise, so
       let's go through it line by line.

       Line 1 tells us we're looking at an SV which lives at
       0xa04ecbc in memory. SVs themselves are very simple structures,
 but they contain a pointer to a more complex structure.
 In this case, it's a PV, a structure which holds a
       string value, at location 0xa041450.  Line 2 is the reference
 count; there are no other references to this data, so
       it's 1.

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

       Line 5 gives us the current length of the string - note
       that this does not include the null terminator. Line 6 is
       not the length of the string, but the length of the currently
 allocated buffer; as the string grows, Perl automatically
 extends the available storage via a routine
       called "SvGROW".

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

       Let's take an example of manipulating a PV, from "sv_catpvn",
 in sv.c

            1  void
            2  Perl_sv_catpvn(pTHX_  register  SV  *sv,  register
const char *ptr, register STRLEN len)
            3  {
            4      STRLEN tlen;
            5      char *junk;
            6      junk = SvPV_force(sv, tlen);
            7      SvGROW(sv, tlen + len + 1);
            8      if (ptr == junk)
            9          ptr = SvPVX(sv);
           10      Move(ptr,SvPVX(sv)+tlen,len,char);
           11      SvCUR(sv) += len;
           12      *SvEND(sv) = ' ';
           13       (void)SvPOK_only_UTF8(sv);           /* validate pointer */
           14      SvTAINT(sv);
           15  }

       This is a function which adds a string, "ptr", of length
       "len" onto the end of the PV stored in "sv". The first
       thing we do in line 6 is make sure that the SV has a valid
       PV, by calling the "SvPV_force" macro to force a PV. As a
       side effect, "tlen" gets set to the current value of the
       PV, and the PV itself is returned to "junk".

       In line 7, we make sure that the SV will have enough room
       to accommodate the old string, the new string and the null
       terminator. If "LEN" isn't big enough, "SvGROW" will reallocate
 space for us.

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

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

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

       AVs and HVs are more complicated, but SVs are by far the
       most common variable type being thrown around. Having seen
       something of how we manipulate these, let's go on and look
       at how the op tree is constructed.

       Op Trees    [Toc]    [Back]

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

       An op is a fundamental operation that Perl can perform:
       all the built-in functions and operators are ops, and
       there are a series of ops which deal with concepts the
       interpreter needs internally - entering and leaving a
       block, ending a statement, fetching a variable, and so on.

       The op tree is connected in two ways: you can imagine that
       there are two "routes" through it, two orders in which you
       can traverse the tree.  First, parse order reflects how
       the parser understood the code, and secondly, execution
       order tells perl what order to perform the operations  in.

       The easiest way to examine the op tree is to stop Perl
       after it has finished parsing, and get it to dump out the
       tree. This is exactly what the compiler backends B::Terse,
       B::Concise and B::Debug do.

       Let's have a look 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)

       Let's start in the middle, at line 4. This is a BINOP, a
       binary operator, which is at location 0x8179828. The specific
 operator in question is "sassign" - scalar assignment
 - and you can find the code which implements it in
       the function "pp_sassign" in pp_hot.c. As a binary operator,
 it has two children: the add operator, providing the
       result of "$b+$c", is uppermost on line 5, and the left
       hand side is on line 10.

       Line 10 is the null op: this does exactly nothing. What is
       that doing there? If you see the null op, it's a sign that
       something has been optimized away after parsing. As we
       mentioned in "Optimization", the optimization stage sometimes
 converts two operations into one, for example when
       fetching a scalar variable. When this happens, instead of
       rewriting the op tree and cleaning up the dangling pointers,
 it's easier just to replace the redundant operation
       with the null op. Originally, the tree would have looked
       like this:
           10          SVOP (0x816b4f0) rv2sv [15]
           11              SVOP (0x816dcf0) gv  GV (0x80fa460) *a

       That is, fetch the "a" entry from the main symbol table,
       and then look at the scalar component of it: "gvsv"
       ("pp_gvsv" into pp_hot.c) happens to do both these things.

       The right hand side, starting at line 5 is similar to what
       we've just seen: we have the "add" op ("pp_add" also in
       pp_hot.c) add together two "gvsv"s.

       Now, what's this about?

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

       "enter" and "leave" are scoping ops, and their job is to
       perform any housekeeping every time you enter and leave a
       block: lexical variables are tidied up, unreferenced variables
 are destroyed, and so on. Every program will have
       those first three lines: "leave" is a list, and its children
 are all the statements in the block. Statements are
       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:

/                                  $a   +
$b   $c

       However, it's impossible to perform the operations in this
       order: you have to find the values of $b and $c before you
       add them together, for instance. So, the other thread that
       runs through the op tree is the execution order: each op
       has a field "op_next" which points to the next op to be
       run, so following these pointers tells us how perl executes
 the code. We can traverse the tree in this order
       using the "exec" option to "B::Terse":
            % perl -MO=Terse,exec -e '$a=$b+$c'
            1  OP (0x8179928) enter
            2  COP (0x81798c8) nextstate
            3  SVOP (0x81796c8) gvsv  GV (0x80fa4d4) *b
            4  SVOP (0x8179798) gvsv  GV (0x80efeb0) *c
            5  BINOP (0x8179878) add [1]
            6  SVOP (0x816dd38) gvsv  GV (0x80fa468) *a
            7  BINOP (0x81798a0) sassign
            8  LISTOP (0x8179900) leave

       This probably makes more sense for a human: enter a block,
       start a statement. Get the values of $b and $c, and add
       them together.  Find $a, and assign one to the other. Then

       The way Perl builds up these op trees in the parsing process
 can be unravelled by examining perly.y, the YACC
       grammar. Let's take the piece we need to construct the
       tree for "$a = $b + $c"

           1 term    :   term ASSIGNOP term
           2                { $$ =  newASSIGNOP(OPf_STACKED,  $1,
$2, $3); }
           3         |   term ADDOP term
           4                 {  $$  = newBINOP($2, 0, scalar($1),
scalar($3)); }

       If you're not used to reading BNF grammars, this is how it
       works: You're fed certain things by the tokeniser, which
       generally end up in upper case. Here, "ADDOP", is provided
       when the tokeniser sees "+" in your code. "ASSIGNOP" is
       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 symbols' are generally placed in
       lower case. "term" here is a non-terminal symbol, representing
 a single expression.

       The grammar gives you the following rule: you can make the
       thing on the left of the colon if you see all the things
       on the right in sequence.  This is called a "reduction",
       and the aim of parsing is to completely reduce the  input.
       There are several different ways you can perform a reduction,
 separated by vertical bars: so, "term" followed by
       "=" followed by "term" makes a "term", and "term" followed
       by "+" followed by "term" can also make a "term".

       So, if you see two terms with an "=" or "+", between them,
       you can turn them into a single expression. When you do
       this, you execute the code in the block on the next line:
       if you see "=", you'll do the code in line 2. If you see
       "+", you'll do the code in line 4. It's this code which
       contributes to the op tree.
                   |   term ADDOP term
                   {    $$    =   newBINOP($2,   0,   scalar($1),
scalar($3)); }

       What this does is creates a new binary op, and feeds it a
       number of variables. The variables refer to the tokens: $1
       is the first token in the input, $2 the second, and so on
       - think regular expression backreferences. $$ is the op
       returned from this reduction. So, we call "newBINOP" to
       create a new binary operator. The first parameter to "newBINOP",
 a function in op.c, is the op type. It's an addition
 operator, so we want the type to be "ADDOP". We could
       specify this directly, but it's right there as the second
       token in the input, so we use $2. The second parameter is
       the op's flags: 0 means `nothing special'. Then the things
       to add: the left and right hand side of our expression, in
       scalar context.

       Stacks    [Toc]    [Back]

       When perl executes something like "addop", how does it
       pass on its results to the next op? The answer is, through
       the use of stacks. Perl has a number of stacks to store
       things it's currently working on, and we'll look at the
       three most important ones here.

       Argument stack
          Arguments are passed to PP code and returned from PP
          code using the argument stack, "ST". The typical way to
          handle arguments is to pop them off the stack, deal
          with them how you wish, and then push the result back
          onto the stack. This is how, for instance, the cosine
          operator works:

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

          We'll see a more tricky example of this when we consider
 Perl's macros below. "POPn" gives you the NV
          (floating point value) of the top SV on the stack: the
          $x in "cos($x)". Then we compute the cosine, and push
          the result back as an NV. The "X" in "XPUSHn" means
          that the stack should be extended if necessary - it
          can't be necessary here, because we know there's room
          for one more item on the stack, since we've just
          removed one! The "XPUSH*" macros at least guarantee

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

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

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

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

               1  PUSHMARK(SP);
               2  EXTEND(SP,2);
               3  PUSHs(SvTIED_obj((SV*)av, mg));
               4  PUSHs(val);
               5  PUTBACK;
               6  ENTER;
               7  call_method("PUSH", G_SCALAR|G_DISCARD);
               8  LEAVE;
               9  POPSTACK;

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

          Let's examine the whole implementation, for practice:

               1  PUSHMARK(SP);

          Push the current state of the stack pointer onto the
          mark stack. This is so that w

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