


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 dis-
       cussed at a time.

       A searchable archive of the list is at:

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

       The list is also archived under the usenet group name
       "perl.porters-gw" at:

           http://www.deja.com/

       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 is the pumpking for the 5.6
       release of Perl.

       In addition, various people are pumpkings for different
       things.  For instance, Andy Dougherty and Jarkko
       Hietaniemi share the Configure pumpkin, and Tom Chris-
       tiansen is the documentation pumpking.

       Larry sees Perl development along the lines of the US gov-
       ernment: there's the Legislature (the porters), the Execu-
       tive branch (the pumpkings), and the Supreme Court
       (Larry).  The legislature can discuss and submit patches



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

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

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


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

       Backwards compatibility
           It's a cardinal sin to break existing Perl programs.



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           New warnings are contentious--some say that a program
           that emits warnings is not broken, while others say it
           is.  Adding keywords has the potential to break pro-
           grams, changing the meaning of existing token
           sequences or functions might break programs.

       Could it be a module instead?
           Perl 5 has extension mechanisms, modules and XS,
           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?  Some-
           times, instead of adding a feature with a tight focus,
           the porters might decide to wait until someone imple-
           ments the more generalized feature.  For instance,
           instead of implementing a ``delayed evaluation'' fea-
           ture, 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 inter-
           preter 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 proto-
           types is likely to be rejected because there are still
           options for the future of prototypes that haven't been
           addressed.

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

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





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

       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 navi-
           gate, 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 use-
           ful?'', as the fact that someone took the time to make
           the patch demonstrates a strong desire for the fea-
           ture.

       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

       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.  Peri-
       odically 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:

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

       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



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       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 usu-
       ally 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 avail-
           able, 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 the
           source tree to the latest applied patch level, creat-
           ing files that are new (to your distribution) and set-
           ting date/time stamps of existing files to reflect the
           bleadperl status.

           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 prob-
           lem.

           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 a inetd entry or a deamon.  You must, however,
                   have a working rsh or ssh system.  Using ssh is recommended for
                   its security features."





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           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, 7, 9] ];     # mode, size, mtime
                     } `cat MANIFEST`;

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

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

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

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

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



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

           This makes sure the latest available patch is down-
           loaded to your patch directory.

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

            # last=`ls -rt1 *.gz | tail -1`
            # 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 Konig to have better
           control over the patching process.

       Why rsync the source tree


       It's easier
           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 recent
           According to Gurusamy Sarathy:

              "... The rsync mirror is automatic and syncs with the repository
               every five minutes.

              "Updating the patch  area  still  requires  manual  intervention
               (with all the goofiness that implies,  which you've noted)  and
               is typically on a daily cycle.   Making this process  automatic
               is on my tuit list, but don't ask me when."


       It's more reliable
           Well, since the patches are updated by hand, I don't
           have to say any more ... (see Sarathy's remark).

       Why rsync the patches


       It's easier
           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



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           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 devel-
           opment 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 man-
           uals, 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 occa-
           sions, he releases a tar-ball of the current source
           tree (i.e. perl@7582.tar.gz), which will be an excel-
           lent 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
           archive.

           Finding a start point
               If you want to fix/change the behaviour of func-
               tion/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 realy
               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



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               "perl.ok", which you can than take to your
               favourite mailer and mail yourself).

               But of course, as always, things will not allways
               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 mis-
               behaviour. If you note that in the bug report, it
               saves the one trying to solve it, looking for that
               point.

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

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

       Submitting patches

       Always submit patches to perl5-porters@perl.org.  This
       lets other porters review your patch, which catches a sur-
       prising 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 ver-
       sion).  Only patches that survive the heat of the develop-
       ment branch get applied to maintenance versions.

       Your patch should update the documentation and test suite.

       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



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       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 plat-
       forms.  Perl Labs (http://labs.perl.org/) automatically
       tests Perl source releases on platforms and gives feedback
       to the CPAN testers mailing list.  Both efforts welcome
       volunteers.

       It's a good idea to read and lurk for a while before chip-
       ping in.  That way you'll get to see the dynamic of the
       conversations, learn the personalities of the players, and
       hopefully be better prepared to make a useful contribution
       when do you speak up.

       If after all this you still think you want to join the
       perl5-porters mailing list, send mail to perl5-porters-
       subscribe@perl.org.  To unsubscribe, send mail to
       perl5-porters-unsubscribe@perl.org.

       To hack on the Perl guts, you'll need to read the follow-
       ing things:

       the perlguts manpage
          This is of paramount importance, since it's the docu-
          mentation 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 pok-
          ing at Perl source, and we'll do that later on.

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

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





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       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 is posted to perl5-porters at the beginning on
          every month, and should be available from http://perl-
          hacker.org/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

       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.

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

       Configure
          The configure process is the way we make Perl portable
          across the myriad of operating systems it supports.
          Responsibility for the configure, build and installa-
          tion 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 directo-
          ries, (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.)

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

       Before we leave looking at the layout, though, don't for-
       get that MANIFEST contains not only the file names in the



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       Perl distribution, but short descriptions of what's in
       them, too. For an overview of the important files, try
       this:

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


       Elements of the interpreter

       The work of the interpreter has two main stages: compiling
       the code into the internal representation, or bytecode,
       and then executing it.  the Compiled code entry in the
       perlguts manpage explains exactly how the compilation
       stage happens.

       Here is a short breakdown of perl's operation:

       Startup
          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
          the perlembed manpage; most of the real action takes
          place in perl.c

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

              1 PERL_SYS_INIT3(&argc,&argv,&env);
              2
              3 if (!PL_do_undump) {
              4     my_perl = perl_alloc();
              5     if (!my_perl)
              6         exit(1);
              7     perl_construct(my_perl);
              8     PL_perl_destruct_level = 0;
              9 }

          Line 1 is a macro, and its definition is dependent on
          your operating system. Line 3 references
          "PL_do_undump", a global variable - all global vari-
          ables 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



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          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.

       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 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.

       Optimization
          Now the parsing stage is complete, and the finished



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          tree represents the operations that the Perl inter-
          preter needs to perform to execute our program. Next,
          Perl does a dry run over the tree looking for optimisa-
          tions: 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 com-
          ponent, the optimizer fiddles the op tree to use a
          function which directly looks up the scalar in ques-
          tion. The main optimizer is "peep" in 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" func-
          tion in 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 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 actu-
          ally carry out the operation.  This function will
          return the next op in the sequence - this allows for
          things like "if" which choose the next op dynamically
          at run time.  The "PERL_ASYNC_CHECK" makes sure that
          things like 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 imple-
          ment control structures ("if", "while" and the like)
          and pp.c contains everything else. These are, if you
          like, the C code for Perl's built-in functions and
          operators.

       Internal Variable Types

       You should by now have had a look at the perlguts manpage,
       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



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       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 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 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 cur-
       rently allocated buffer; as the string grows, Perl auto-
       matically 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 snip-
       pet, 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 the perlguts manpage.

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




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            1  void
            2  Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
            3  {
            4      STRLEN tlen;
            5      char *junk;

            6      junk = SvPV_force(sv, tlen);
            7      SvGROW(sv, tlen + len + 1);
            8      if (ptr == junk)
            9          ptr = SvPVX(sv);
           10      Move(ptr,SvPVX(sv)+tlen,len,char);
           11      SvCUR(sv) += len;
           12      *SvEND(sv) = '\0';
           13      (void)SvPOK_only_UTF8(sv);          /* validate pointer */
           14      SvTAINT(sv);
           15  }

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

       In line 7, we make sure that the SV will have enough room
       to accommodate the old string, the new string and the null
       terminator. If "LEN" isn't big enough, "SvGROW" will real-
       locate 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 cur-
       rent 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 ""\0"".

       Line 13 manipulates the flags; since we've changed the PV,
       any IV or NV values will no longer be valid: if we have
       "$a=10; $a.="6";" we don't want to use the old IV of 10.
       "SvPOK_only_utf8" is a special UTF8-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.



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       Op Trees

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

       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
       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) *a

       Let's start in the middle, at line 4. This is a BINOP, a
       binary operator, which is at location "0x8179828". The
       specific operator in question is "sassign" - scalar
       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 the Optimization entry elsewhere in this doc-
       ument, the optimization stage sometimes converts two



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       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 vari-
       ables are destroyed, and so on. Every program will have
       those first three lines: "leave" is a list, and its chil-
       dren 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:

                               Program
                                  |
                              Statement
                                  |
                                  =
                                 / \
                                /   \
                               $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 exe-
       cutes the code. We can traverse the tree in this order



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       using the "exec" option to "B::Terse":

            % perl -MO=Terse,exec -e '$a=$b+$c'
            1  OP (0x8179928) enter
            2  COP (0x81798c8) nextstate
            3  SVOP (0x81796c8) gvsv  GV (0x80fa4d4) *b
            4  SVOP (0x8179798) gvsv  GV (0x80efeb0) *c
            5  BINOP (0x8179878) add [1]
            6  SVOP (0x816dd38) gvsv  GV (0x80fa468) *a
            7  BINOP (0x81798a0) sassign
            8  LISTOP (0x8179900) leave

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

       The way Perl builds up these op trees in the parsing pro-
       cess 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 `termi-
       nal 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 com-
       plex forms, `non-terminal symbols' are generally placed in
       lower case. "term" here is a non-terminal symbol, repre-
       senting 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 reduc-
       tion, 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



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       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 "new-
       BINOP" to create a new binary operator. The first parame-
       ter 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

       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);
                XPUSHn(value);

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

          Alternatively, you can fiddle with the stack directly:
          "SP" gives you the first element in your portion of the
          stack, and "TOP*" gives you the top SV/IV/NV/etc. on



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          the stack. So, for instance, to do unary negation of an
          integer:

               SETi(-TOPi);

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

          Argument stack manipulation in the core is exactly the
          same as it is in XSUBs - see the perlxstut manpage, the
          perlxs manpage and the perlguts manpage 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 book-
          marks to locations in the argument stack usable by each
          function. For instance, when dealing with a tied vari-
          able, (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 when we've finished adding
          items to the argument stack, Perl knows how many things
          we've added recently.




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               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" subrou-
          tine 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 its previous value
          returned, and so on. Think of them as the "{" and "}"
          of a Perl block.

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

               9  POPSTACK;

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

       Save stack
          C doesn't have a concept of local scope, so perl pro-
          vides 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 the Localising Changes entry in the perlguts man-
          page for how to use the save stack.






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       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) con-
       tains a macro. The first line sets up the function decla-
       ration as Perl expects for PP code; line 3 sets up vari-
       able declarations for the argument stack and the target,
       the return value of the operation. Finally, it tries to
       see if the addition operation is overloaded; if so, the
       appropriate subroutine is called.

       Line 5 is another variable declaration - all variable dec-
       larations start with "d" - which pops from the top of the
       argument stack two NVs (hence "nn") and puts them into the
       variables "right" and "left", hence the "rl". These are
       the two operands to the addition operator. Next, we call
       "SETn" to set the NV of the return value to the result of
       adding the two values. This done, we return - the "RETURN"
       macro makes sure that our return value is properly han-
       dled, and we pass the next operator to run back to the
       main run loop.

       Most of these macros are explained in the perlapi manpage,
       and some of the more important ones are explained in the
       perlxs manpage as well. Pay special attention to the Back-
       ground and PERL_IMPLICIT_CONTEXT entry in the perlguts
       manpage for information on the "[pad]THX_?" macros.

       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 debug-
       ging information which will allow us to step through a
       running program.  Configure will also turn on the



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       "DEBUGGING" compilation symbol which enables all the
       internal debugging code in Perl. There are a whole bunch
       of things you can debug with this: the perlrun manpage
       lists them all, and the best way to find out about them is
       to play about with them. The most useful options are prob-
       ably

           l  Context (loop) stack processing
           t  Trace execution
           o  Method and overloading resolution
           c  String/numeric conversions

       Some of the functionality of the debugging code can be
       achieved using XS modules.

           -Dr => use re 'debug'
           -Dx => use O 'Debug'


       Using a source-level debugger

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

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

       To fire up the debugger, type

           gdb ./perl

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

           (gdb)

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

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

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

       step
          Steps through the program a line at a time.



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       next
          Steps through the program a line at a time, 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. WARN-
          ING: Perl makes heavy use of macros, and gdb is not
          aware of macros. You'll have to substitute them your-
          self. So, for instance, you can't say

              print SvPV_nolen(sv)

          but you have to say

              print Perl_sv_2pv_nolen(sv)

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

       Dumping Perl Data Structures

       One way to get around this macro hell is to use the dump-
       ing functions in dump.c; these work a little like an
       internal Devel::Peek, but they also cover OPs and other
       structures that you can't get at from Perl. Let's take an
       example. We'll use the "$a = $b + $c" we used before, but
       give it a bit of context: "$b = "6XXXX"; $c = 2.3;".
       Where's a good place to stop and poke around?

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

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

       Notice we use "Perl_pp_add" and not "pp_add" - see the
       Internal Functions entry in the perlguts manpage.  With
       the breakpoint in place, we can run our program:

           (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'



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       Lots of junk will go past as gdb reads in the relevant
       source files and libraries, and then:

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

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

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

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

       Since we don't have an NV for "$b", we'll have to use
       "sv_2nv" to convert it. If we step again, we'll find our-
       selves there:

           Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
           1669        if (!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 from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
           0x462669 in Perl_pp_add () at pp_hot.c:311
           311           dPOPTOPnnrl_ul;

       We can also dump out this op: the current op is always
       stored in "PL_op", and we can dump it with "Perl_op_dump".
       This'll give us similar output to B::Debug.




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           {
           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 at how to navigate the
       Perl sources and some things you'll need to know when fid-
       dling with them. Let's now get on and create a simple
       patch. Here's something Larry suggested: if a "U" is the
       first active format during a "pack", (for example, "pack
       "U3C8", @stuff") then the resulting string should be
       treated as UTF8 encoded.

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

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

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

           STRLEN fromlen;
           register char *pat = SvPVx(*++MARK, fromlen);
           register char *patend = pat + fromlen;
           register I32 len;
           I32 datumtype;
           SV *fromstr;

       We'll have another string pointer in there:



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           STRLEN fromlen;
           register char *pat = SvPVx(*++MARK, fromlen);
           register char *patend = pat + fromlen;
        +  char *patcopy;
           register I32 len;
           I32 datumtype;
           SV *fromstr;

       And just before we start the loop, we'll set "patcopy" to
       be the start of "pat":

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

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

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

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

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

           if (isSPACE(datumtype))
               continue;

       needs to become

           if (isSPACE(datumtype)) {
               patcopy++;
               continue;
           }

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

       The regression tests for each operator live in t/op/, and



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       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:

        print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
        print "ok $test\n"; $test++;

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

        print 'not ' unless "1.20.300.4000" eq
                            sprintf "%vd", pack("  U*",1,20,300,4000);
        print "ok $test\n"; $test++;

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

        print 'not ' unless v1.20.300.4000 ne
                            sprintf "%vd", pack("C0U*",1,20,300,4000);
        print "ok $test\n"; $test++;

       Mustn't forget to change the number of tests which appears
       at the top, or else the automated tester will get con-
       fused:

        -print "1..156\n";
        +print "1..159\n";

       We now compile up Perl, and run it through the test suite.
       Our new tests pass, hooray!

       Finally, the documentation. The job is never done until
       the paperwork is over, so let's describe the change we've
       just made. The relevant place is pod/perlfunc.pod; again,
       we make a copy, and then we'll insert this text in the
       description of "pack":

        =item *

        If the pattern begins with a C<U>, the resulting string will be treated
        as Unicode-encoded. You can force UTF8 encoding on in a string with an
        initial C<U0>, and the bytes that follow will be interpreted as Unicode
        characters. If you don't want this to happen, you can begin your pattern
        with C<C0> (or anything else) to force Perl not to UTF8 encode your
        string, and then follow this with a C<U*> somewhere in your pattern.

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

           diff -ruN old new > patch

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



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       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!

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 testing and debugging tools with Perl.
       This is meant as a guide to interfacing these tools with
       Perl, not as any kind of guide to the use of the tools
       themselves.

       Rational Software's Purify

       Purify is a commercial tool that is helpful in identifying
       memory overruns, wild pointers, memory leaks and other
       such badness.  Perl must be compiled in a specific way for
       optimal testing with Purify.  Purify is available under
       Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.

       The only currently known leaks happen when there are com-
       pile-time errors within eval or require.  (Fixing these is
       non-trivial, unfortunately, but they must be fixed eventu-
       ally.)

       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




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       where these arguments mean:

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

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

       -Uusemymalloc
           Disable Perl's malloc so that Purify can more closely
           monitor allocations and leaks.  Using Perl's malloc
           will make Purify report most leaks in the "potential"
           leaks category.

       -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 'pureperl' that has been
       Purify'ed.  This binary is used in place of the standard
       'perl' binary when you want to debug Perl memory problems.

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

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

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

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

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

       If you plan to use the "Viewer" windows, then you only



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       need this option:

           setenv PURIFYOPTIONS "-chain-length=25"


       Purify on NT

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

       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 allocations and leaks.  Using Perl's malloc
           will make Purify report most leaks in the "potential"
           leaks category.

       CFG = Debug
           Adds debugging information so that you see the exact
           source statements where the problem occurs.  Without
           this flag, all you will see is the source filename of
           where the error occurred.

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

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

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







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       CONCLUSION

       We've had a brief look around the Perl source, an overview
       of the stages perl goes through when it's running your
       code, and how to use a debugger to poke at the Perl guts.
       We took a very simple problem and demonstrated how to
       solve it fully - with documentation, regression tests, and
       finally a patch for submission to p5p.  Finally, we talked
       about how to use external tools to debug and test Perl.

       I'd now suggest you read over those references again, and
       then, as soon as possible, get your hands dirty. The best
       way to learn is by doing, so:

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

       o  Keep up to date with the bleeding edge Perl distribu-
          tions and get familiar with the changes. Try and get an
          idea of what areas people are working on and the
          changes they're making.

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

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

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

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

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










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