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       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 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 pump­
       king 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 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
       to the executive branch all they like, but the executive
       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.
           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

       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

       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.

       Is the implementation tested?
           Patches which change behaviour (fixing bugs or intro­
           ducing 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 docu­
           mentation, so submitting a patch for the appropriate
           manpages as well as the source code is always a good

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

       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 oth­
       ers 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 pub­
       lic 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 mail­
       ing list to receive a copy of each patch that gets submit­
       ted to the maintenance and development "branches" of the
       perl repository.  See http://lists.perl.org/ for subscrip­
       tion information.

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

           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 pre­
           cious 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 prob­

           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 $1)[2, 7, 9] ];     # mode, size, mtime
                         my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
                         defined $size or ($mode, $size, $mtime) = (0, 0, 0);
                         $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
                         printf "%4s %-34s %8d %9d  %8d %9d\n",
                             $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
                         unlink $rfile;
                         copy ($file, $rfile);
                         utime time, $MF{$file}[2], $rfile;
                         chmod $MF{$file}[0], $rfile;

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

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

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

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

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

       Why rsync the source tree

       It's easier to rsync the source tree

       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 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

           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,
               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 mis­
               behaviour. 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.

       Perlbug administration

       There is a single remote administrative interface for mod­
       ifying 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

       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 mod­
       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 plat­
       forms.  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 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:

          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/ )

       perlxstut and perlxs
          A working knowledge of XSUB programming is incredibly
          useful for core hacking; XSUBs use techniques drawn
          about Perl development.

       The perl5-porters FAQ
          This should be available from http://simon-coz­
          ens.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

       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 every­
          thing 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 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.)

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

       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 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
          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);

          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 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 compo­
          nent, the optimizer fiddles the op tree to use a func­
          tion 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" 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))) {

          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

       Internal Variable Types

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

       These variables are used not only to represent Perl-space
       variables, but also any constants in the code, as well as
       some structures completely internal to Perl. The symbol
       table, for instance, is an ordinary Perl hash. Your code
       is represented by an SV as it's read into the parser; any
       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 struc­
       tures, but they contain a pointer to a more complex struc­
       ture. In this case, it's a PV, a structure which holds a
       string value, at location 0xa041450.  Line 2 is the refer­
       ence count; there are no other references to this data, so
       it's 1.

       Line 3 are the flags for this SV - it's OK to use it as a
       PV, it's a read-only SV (because it's a constant) and the
       data is a PV internally.  Next we've got the contents of
       the string, starting at location 0xa0484e0.
       on.  More macros to manipulate these properties can be
       found in perlguts.

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

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

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

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

       In line 7, we make sure that the SV will have enough room
       to accommodate the old string, the new string and the null
       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 UTF-8-aware version of
       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) *a

       Let's start in the middle, at line 4. This is a BINOP, a
       binary operator, which is at location 0x8179828. The spe­
       cific operator in question is "sassign" - scalar assign­
       ment - and you can find the code which implements it in
       the function "pp_sassign" in pp_hot.c. As a binary opera­
       tor, 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 some­
       times 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 point­
       ers, it's easier just to replace the redundant operation
       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:

                                 / \
                                /   \
                               $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
       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
       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
       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 "new­
       BINOP", a function in op.c, is the op type. It's an addi­
       tion 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.


       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.

          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

          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 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;

               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

               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 perlcall. We call the
          "PUSH" method in scalar context, and we're going to
          discard its return value.

               9  POPSTACK;

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

       Save stack
          C doesn't have a concept of local scope, so perl 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 "Localising Changes" in perlguts for how to use the
          save stack.
          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 perlapi, and some of
       the more important ones are explained in perlxs as well.
       Pay special attention to "Background and
       PERL_IMPLICIT_CONTEXT" in perlguts for information on the
       "[pad]THX_?" macros.

       The .i Targets

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

           make foo.i

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

       Poking at Perl

       To really poke around with Perl, you'll probably want to
       build Perl for debugging, like this:

           ./Configure -d -D optimize=-g

       "-g" is a flag to the C compiler to have it produce debug­
       ging information which will allow us to step through a
       running program.  Configure will also turn on the "DEBUG­
       GING" compilation symbol which enables all the internal
       debugging code in Perl. There are a whole bunch of things

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

       Using a source-level debugger

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

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

       To fire up the debugger, type

           gdb ./perl

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


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

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

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

          Steps through the program a line at a time.

          Steps through the program a line at a time, without
          descending into functions.

          Run until the next breakpoint.

          Run until the end of the current function, then stop

          but you have to say

              print Perl_sv_2pv_nolen(sv)

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

       gdb macro support

       Recent versions of gdb have fairly good macro support, but
       in order to use it you'll need to compile perl with macro
       definitions included in the debugging information.  Using
       gcc version 3.1, this means configuring with "-Dopti­
       mize=-g3".  Other compilers might use a different switch
       (if they support debugging macros at all).

       Dumping Perl Data Structures

       One way to get around this macro hell is to use the 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 "Inter­
       nal Functions" in perlguts.  With the breakpoint in place,
       we can run our program:

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

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

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

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

           Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
           1669        if (!sv)

       We can now use "Perl_sv_dump" to investigate the SV:

           SV = PV(0xa057cc0) at 0xa0675d0
           REFCNT = 1
           FLAGS = (POK,pPOK)
           PV = 0xa06a510 "6XXXX"\0
           CUR = 5
           LEN = 6
           $1 = void

       We know we're going to get 6 from this, so let's finish
       the 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.

           13  TYPE = add  ===> 14
               TARG = 1
               FLAGS = (SCALAR,KIDS)
                   TYPE = null  ===> (12)
                     (was rv2sv)
                   FLAGS = (SCALAR,KIDS)
           11          TYPE = gvsv  ===> 12
                       FLAGS = (SCALAR)
                       GV = main::b

       # finish this later #


       All right, we've now had a look at how to navigate the
       Perl sources and some things you'll need to know when 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
       "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:

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

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

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

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

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

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

       Oops, we forgot one thing: what if there are spaces at the

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

       The regression tests for each operator live in t/op/, and
       so we make a copy of t/op/pack.t to t/op/pack.t~. Now we
       can add our tests to the end. First, we'll test that the
       "U" does indeed create Unicode strings.

       t/op/pack.t has a sensible ok() function, but if it didn't
       we could use the one from t/test.pl.

        require './test.pl';
        plan( tests => 159 );

       so instead of this:

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

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

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

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

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

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

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

       Mustn't forget to change the number of tests which appears
       at the top, or else the automated tester will get con­
       fused.  This will either look like this:

        print "1..156\n";

       or this:

        plan( tests => 156 );

        Unicode characters. If you don't want this to happen, you can begin your
        pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
        string, and then follow this with a C<U*> somewhere in your pattern.

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

           diff -ruN old new > patch

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

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

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

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

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

       Patching a core module

       This works just like patching anything else, with an extra
       consideration.  Many core modules also live on CPAN.  If
       this is so, patch the CPAN version instead of the core and
       send the patch off to the module maintainer (with a copy
       to p5p).  This will help the module maintainer keep the
       CPAN version in sync with the core version without con­
       stantly scanning p5p.

       Adding a new function to the core

       If, as part of a patch to fix a bug, or just because you
       have an especially good idea, you decide to add a new
       function to the core, discuss your ideas on p5p well
       before you start work.  It may be that someone else has

       The location of any new code is also an important consid­
       eration.  Don't just create a new top level .c file and
       put your code there; you would have to make changes to
       Configure (so the Makefile is created properly), as well
       as possibly lots of include files.  This is strictly pump­
       king business.

       It is better to add your function to one of the existing
       top level source code files, but your choice is compli­
       cated by the nature of the Perl distribution.  Only the
       files that are marked as compiled static are located in
       the perl executable.  Everything else is located in the
       shared library (or DLL if you are running under WIN32).
       So, for example, if a function was only used by functions
       located in toke.c, then your code can go in toke.c.  If,
       however, you want to call the function from universal.c,
       then you should put your code in another location, for
       example util.c.

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

       When you write your new code, please be conscious of
       existing code conventions used in the perl source files.
       See perlstyle for details.  Although most of the guide­
       lines discussed seem to focus on Perl code, rather than c,
       they all apply (except when they don't ;).  See also Port­
       ing/patching.pod file in the Perl source distribution for
       lots of details about both formatting and submitting
       patches of your changes.

       Lastly, TEST TEST TEST TEST TEST any code before posting
       to p5p.  Test on as many platforms as you can find.  Test
       as many perl Configure options as you can (e.g. MULTIPLIC­
       ITY).  If you have profiling or memory tools, see "EXTER­
       NAL TOOLS FOR DEBUGGING PERL" below for how to use them to
       further test your code.  Remember that most of the people
       on P5P are doing this on their own time and don't have the
       time to debug your code.

       Writing a test

       Every module and built-in function has an associated test

          Testing of the absolute basic functionality of Perl.
          Things like "if", basic file reads and writes, simple
          regexes, etc.  These are run first in the test suite
          and if any of them fail, something is really broken.

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

          Tests basic issues of how Perl parses and compiles

          Tests for built-in IO functions, including command line

          The old home for the module tests, you shouldn't put
          anything new in here.  There are still some bits and
          pieces hanging around in here that need to be moved.
          Perhaps you could move them?  Thanks!

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

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

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

          Tests for the core support of Unicode.

          Windows-specific tests.

          A test suite for the s2p converter.

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

       There are three ways to write a test in the core.
           important features of Test::More while using a minimum
           of core features.

           You can also conditionally use certain libraries like
           Config, but be sure to skip the test gracefully if
           it's not there.

       t/lib ext lib
           Now that the core of Perl is tested, Test::More can be
           used.  You can also use the full suite of core modules
           in the tests.

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

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

       Special Make Test Targets

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

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

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

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

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

       test.third check.third utest.third ucheck.third
           (Only in Tru64)  Run all the tests using the memory
           leak + naughty memory access tool "Third Degree".  The
           log files will be named perl3.log.testname.
           Run the test suite with the t/harness controlling pro­
           gram, instead of t/TEST. t/harness is more sophisti­
           cated, and uses the Test::Harness module, thus using
           this test target supposes that perl mostly works. The
           main advantage for our purposes is that it prints a
           detailed summary of failed tests at the end. Also,
           unlike t/TEST, it doesn't redirect stderr to stdout.

       Running tests by hand

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

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


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

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

       You can run an individual test by a command similar to

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

       except that the harnesses set up some environment vari­
       ables that may affect the execution of the test :

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

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

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

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


       Sometimes it helps to use external tools while debugging
       and testing Perl.  This section tries to guide you through
       can simply kill away the perl process.

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

           setenv PERL_DESTRUCT_LEVEL 2

       and for Bourne-type shells:

           export PERL_DESTRUCT_LEVEL

       or in UNIXy environments you can also use the "env" com­

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

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

       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.

       Purify on Unix

       On Unix, Purify creates a new Perl binary.  To get the
       most benefit out of Purify, you should create the perl to
       Purify using:

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

       where these arguments mean:

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

           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

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

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

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

           setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:

           export PURIFYOPTIONS

       or if you have the "env" utility:

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

       Purify on NT

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

       #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

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

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


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

       As system libraries (most notably glibc) are also trigger­
       ing errors, valgrind allows to suppress such errors using
       suppression files. The default suppression file that comes
       with valgrind already catches a lot of them. Some addi­
       tional suppressions are defined in t/perl.supp.

       To get valgrind and for more information see


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

       Third Degree is a tool for memory leak detection and mem­
       ory access checks.  It is one of the many tools in the
       ATOM toolkit.  The toolkit is only available on Tru64
       (formerly known as Digital UNIX formerly known as DEC


       The "test.third" leaves a lot of files named foo_bar.3log
       in the t/ subdirectory.  There is a problem with these
       files: Third Degree is so effective that it finds problems
       also in the system libraries.  Therefore you should used
       the Porting/thirdclean script to cleanup the *.3log files.

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


       If you want to run any of the tests yourself manually
       using e.g.  valgrind, or the pureperl or perl.third exe­
       cutables, please note that by default perl does not
       explicitly cleanup all the memory it has allocated (such
       as global memory arenas) but instead lets the exit() of
       the whole program "take care" of such allocations, also
       known as "global destruction of objects".

       There is a way to tell perl to do complete cleanup: set
       the environment variable PERL_DESTRUCT_LEVEL to a non-zero
       value.  The t/TEST wrapper does set this to 2, and this is
       what you need to do too, if you don't want to see the
       "global leaks": For example, for "third-degreed" Perl:

               env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t

       (Note: the mod_perl apache module uses also this environ­
       ment variable for its own purposes and extended its seman­
       tics. Refer to the mod_perl documentation for more infor­
       mation. Also, spawned threads do the equivalent of setting
       this variable to the value 1.)

       If, at the end of a run you get the message N scalars
       leaked, you can recompile with "-DDEBUG_LEAKING_SCALARS",
       which will cause the addresses of all those leaked SVs to
       be dumped; it also converts "new_SV()" from a macro into a
       real function, so you can use your favourite debugger to
       discover where those pesky SVs were allocated.


       Depending on your platform there are various of profiling

       There are two commonly used techniques of profiling exe­
       cutables: statistical time-sampling and basic-block count­

       The second method divides up the generated code into basic
       blocks.  Basic blocks are sections of code that are
       entered only in the beginning and exited only at the end.
       For example, a conditional jump starts a basic block.
       Basic block profiling usually works by instrumenting the
       code by adding enter basic block #nnnn book-keeping code
       to the generated code.  During the execution of the code
       the basic block counters are then updated appropriately.
       The caveat is that the added extra code can skew the
       results: again, the profiling tools usually try to factor
       their own effects out of the results.

       Gprof Profiling

       gprof is a profiling tool available in many UNIX plat­
       forms, it uses statistical time-sampling.

       You can build a profiled version of perl called
       "perl.gprof" by invoking the make target "perl.gprof"
       (What is required is that Perl must be compiled using the
       "-pg" flag, you may need to re-Configure).  Running the
       profiled version of Perl will create an output file called
       gmon.out is created which contains the profiling data col­
       lected during the execution.

       The gprof tool can then display the collected data in var­
       ious ways.  Usually gprof understands the following

       -a  Suppress statically defined functions from the pro­

       -b  Suppress the verbose descriptions in the profile.

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

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

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

       -z  Display routines that have zero usage.

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

       To display the results you use the "gcov" utility (which
       should be installed if you have gcc 3.0 or newer
       installed).  gcov is run on source code files, like this

           gcov sv.c

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

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


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


       Pixie Profiling

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

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

       In Tru64 a file called perl.Addrs will also be silently
       created, this file contains the addresses of the basic
       blocks.  Running the profiled version of Perl will create
       a new file called "perl.Counts" which contains the counts
       for the basic block for that particular program execution.

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

       In IRIX the following prof options are available:

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

       -l  Groups lines by procedure, with procedures sorted in
           hotspot lines.

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

           Grouped by procedure, sorted by cycles executed per
           procedure.  Useful for finding the hotspots of proce­

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

           Unexecuted procedures.

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

       Miscellaneous tricks

       ·   Those debugging perl with the DDD frontend over gdb
           may find the following useful:

           You can extend the data conversion shortcuts menu, so
           for example you can display an SV's IV value with one
           click, without doing any typing.  To do that simply
           edit ~/.ddd/init file and add after:

             ! Display shortcuts.
             Ddd*gdbDisplayShortcuts: \
             /t ()   // Convert to Bin\n\
             /d ()   // Convert to Dec\n\
             /x ()   // Convert to Hex\n\
             /o ()   // Convert to Oct(\n\

           the following two lines:

             ((XPV*) (())->sv_any )->xpv_pv  // 2pvx\n\
             ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx

           so now you can do ivx and pvx lookups or you can plug
           there the sv_peek "conversion":

             Perl_sv_peek(my_perl, (SV*)()) // sv_peek

           (The my_perl is for threaded builds.)  Just remember
           that every line, but the last one, should end with \n\

       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:

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

       ·  Keep up to date with the bleeding edge Perl 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.

       ·  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.

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

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

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


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

perl v5.8.1                 2003-09-02                PERLHACK(1)



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