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NAME
       perlhack - How to hack at the Perl internals

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

       The perl5-porters mailing list is where the Perl standard
       distribution is maintained and developed.  The list can
       get anywhere from 10 to 150 messages a day, depending on
       the heatedness of the debate.  Most days there are two or
       three patches, extensions, features, or bugs being
       discussed at a time.

       A searchable archive of the list is at either:

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

       or

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

       List subscribers (the porters themselves) come in 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 was the pumpking for the 5.8
       release, and Rafael Garcia-Suarez holds the pumpking crown
       for the 5.10 release.

       In addition, various people are pumpkings for different
       things.  For instance, Andy Dougherty and Jarkko
       Hietaniemi did a grand job as the Configure pumpkin up
       till the 5.8 release. For the 5.10 release H.Merijn Brand
       took over.

       Larry sees Perl development along the lines of the US
       government: there's the Legislature (the porters), the



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       Executive branch (the pumpkings), and the Supreme Court
       (Larry).  The legislature can discuss and submit patches
       to the executive branch all they like, but the executive
       branch is free to veto them.  Rarely, the Supreme Court
       will side with the executive branch over the legislature,
       or the legislature over the executive branch.  Mostly,
       however, the legislature and the executive branch are
       supposed to get along and work out their differences
       without impeachment or court cases.

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

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

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

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

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

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

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

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





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       Backwards compatibility
           It's a cardinal sin to break existing Perl programs.
           New warnings are contentious--some say that a program
           that emits warnings is not broken, while others say it
           is.  Adding keywords has the potential to break
           programs, changing the meaning of existing token
           sequences or functions might break programs.

       Could it be a module instead?
           Perl 5 has extension mechanisms, modules and XS,
           specifically to avoid the need to keep changing the
           Perl interpreter.  You can write modules that export
           functions, you can give those functions prototypes so
           they can be called like built-in functions, you can
           even write XS code to mess with the runtime data
           structures of the Perl interpreter if you want to
           implement really complicated things.  If it can be
           done in a module instead of in the core, it's highly
           unlikely to be added.

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

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

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



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           additions to the Perl language will be accepted.

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

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

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

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

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

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

   Keeping in sync
       The source code to the Perl interpreter, in its different
       versions, is kept in a repository managed by the git
       revision control system. The pumpkings and a few others
       have write access to the repository to check in changes.




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       How to clone and use the git perl repository is described
       in perlrepository.

       You can also choose to use rsync to get a copy of the
       current source tree for the bleadperl branch and all
       maintenance branches :

           $ rsync -avz rsync://perl5.git.perl.org/APC/perl-current .
           $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.10.x .
           $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.8.x .
           $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.6.x .
           $ rsync -avz rsync://perl5.git.perl.org/APC/perl-5.005xx .

       (Add the "--delete" option to remove leftover files)

       You may also want to subscribe to the perl5-changes
       mailing list to receive a copy of each patch that gets
       submitted to the maintenance and development "branches" of
       the perl repository.  See http://lists.perl.org/ for
       subscription 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
       usually in a perpetual state of evolution.  You should
       expect it to be very buggy.  Do not use it for any purpose
       other than testing and development.

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

               http://bugs.perl.org/

       To email the bug system administrators:

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

   Submitting patches
       Always submit patches to perl5-porters@perl.org.  If
       you're patching a core module and there's an author
       listed, send the author a copy (see "Patching a core
       module").  This lets other porters review your patch,
       which catches a surprising number of errors in patches.
       Please patch against the latest development version.
       (e.g., even if you're fixing a bug in the 5.8 track, patch



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       against the "blead" branch in the git repository.)

       If changes are accepted, they are applied to the
       development branch. Then the maintenance pumpking decides
       which of those patches is to be backported to the maint
       branch.  Only patches that survive the heat of the
       development branch get applied to maintenance versions.

       Your patch should update the documentation and test suite.
       See "Writing a test".  If you have added or removed files
       in the distribution, edit the MANIFEST file accordingly,
       sort the MANIFEST file using "make manisort", and include
       those changes as part of your patch.

       Patching documentation also follows the same order: if
       accepted, a patch is first applied to development, and if
       relevant then it's backported to maintenance. (With an
       exception for some patches that document behaviour that
       only appears in the maintenance branch, but which has
       changed in the development version.)

       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://rt.perl.org/rt3/ .  It often pays to
       check the archives of the perl5-porters mailing list to
       see whether the bug you're reporting has been reported
       before, and if so whether it was considered a bug.  See
       above for the location of the searchable archives.

       The CPAN testers ( http://testers.cpan.org/ ) are a group
       of volunteers who test CPAN modules on a variety of
       platforms.  Perl Smokers (
       http://www.nntp.perl.org/group/perl.daily-build and
       http://www.nntp.perl.org/group/perl.daily-build.reports/ )
       automatically test Perl source releases on platforms with
       various configurations.  Both efforts welcome volunteers.
       In order to get involved in smoke testing of the perl
       itself visit <http://search.cpan.org/dist/Test-Smoke>. In
       order to start smoke testing CPAN modules visit
       <http://search.cpan.org/dist/CPAN-YACSmoke/> or
       <http://search.cpan.org/dist/POE-Component-CPAN-YACSmoke/>
       or <http://search.cpan.org/dist/CPAN-Reporter/>.

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

       If after all this you still think you want to join the
       perl5-porters mailing list, send mail to



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       perl5-porters-subscribe@perl.org.  To unsubscribe, send
       mail to perl5-porters-unsubscribe@perl.org.

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

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

          Gisle Aas's illustrated perlguts (aka: illguts) is
          wonderful, although a little out of date wrt some size
          details; the various SV structures have since been
          reworked for smaller memory footprint.  The
          fundamentals are right however, and the pictures are
          very helpful.

          http://www.perl.org/tpc/1998/Perl_Language_and_Modules/Perl%20Illustrated/

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

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

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

       The perl5-porters FAQ
          This should be available from
          http://dev.perl.org/perl5/docs/p5p-faq.html .  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:



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       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.

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

       Documentation
          Documentation maintenance includes looking after
          everything 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
          installation process, as well as the overall
          portability of the core code rests with the configure
          pumpkin - others help out with individual operating
          systems.

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

       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
       forget that MANIFEST contains not only the file names in
       the Perl distribution, but short descriptions of what's in
       them, too. For an overview of the important files, try
       this:

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

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

       Here is a short breakdown of perl's operation:




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

          perlmain.c is generated by writemain from
          miniperlmain.c at make time, so you should make perl to
          follow this along.

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

              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
          variables in Perl start with "PL_". This tells you
          whether the current running program was created with
          the "-u" flag to perl and then undump, which means it's
          going to be false in any sane context.

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

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

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

          Next, in line 7, we construct the interpreter using
          perl_construct, also in perl.c; 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)
                  perl_run(my_perl);



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              exitstatus = perl_destruct(my_perl);

              perl_free(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
          tree represents the operations that the Perl
          interpreter needs to perform to execute our program.
          Next, Perl does a dry run over the tree looking for
          optimisations: constant expressions such as "3 + 4"
          will be computed now, and the optimizer will also see
          if any multiple operations can be replaced with a
          single one. For instance, to fetch the variable $foo,
          instead of grabbing the glob *foo and looking at the
          scalar component, the optimizer fiddles the op tree to



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          use a function which directly looks up the scalar in
          question. The main optimizer is "peep" in op.c, and
          many ops have their own optimizing functions.

       Running
          Now we're finally ready to go: we have compiled Perl
          byte code, and all that's left to do is run it. The
          actual execution is done by the "runops_standard"
          function in 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
          actually carry out the operation.  This function will
          return the next op in the sequence - this allows for
          things like "if" which choose the next op dynamically
          at run time.  The "PERL_ASYNC_CHECK" makes sure that
          things like signals interrupt execution if required.

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

          Note that each "pp_" function is expected to return a
          pointer to the next op. Calls to perl subs (and eval
          blocks) are handled within the same runops loop, and do
          not consume extra space on the C stack. For example,
          "pp_entersub" and "pp_entertry" just push a "CxSUB" or
          "CxEVAL" block struct onto the context stack which
          contain the address of the op following the sub call or
          eval. They then return the first op of that sub or eval
          block, and so execution continues of that sub or block.
          Later, a "pp_leavesub" or "pp_leavetry" op pops the
          "CxSUB" or "CxEVAL", retrieves the return op from it,
          and returns it.

       Exception handing
          Perl's exception handing (i.e. "die" etc.) is built on
          top of the low-level "setjmp()"/"longjmp()" C-library



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          functions. These basically provide a way to capture the
          current PC and SP registers and later restore them;
          i.e.  a "longjmp()" continues at the point in code
          where a previous "setjmp()" was done, with anything
          further up on the C stack being lost. This is why code
          should always save values using "SAVE_FOO" rather than
          in auto variables.

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

          At entry points to perl, such as "perl_parse()",
          "perl_run()" and "call_sv(cv, G_EVAL)" each does a
          "JMPENV_PUSH", then enter a runops loop or whatever,
          and handle possible exception returns. For a 2 return,
          final cleanup is performed, such as popping stacks and
          calling "CHECK" or "END" blocks. Amongst other things,
          this is how scope cleanup still occurs during an
          "exit".

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

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

              sub FETCH { eval { die } }

          would cause a longjmp right back to the guard in
          "perl_run", popping both runops loops, which is clearly
          incorrect. One way to avoid this is for the tie code to
          do a "JMPENV_PUSH" before executing "FETCH" in the
          inner runops loop, but for efficiency reasons, perl in
          fact just sets a flag, using "CATCH_SET(TRUE)". The
          "pp_require", "pp_entereval" and "pp_entertry" ops
          check this flag, and if true, they call "docatch",
          which does a "JMPENV_PUSH" and starts a new runops
          level to execute the code, rather than doing it on the
          current loop.

          As a further optimisation, on exit from the eval block
          in the "FETCH", execution of the code following the
          block is still carried on in the inner loop.  When an
          exception is raised, "docatch" compares the "JMPENV"



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          level of the "CxEVAL" with "PL_top_env" and if they
          differ, just re-throws the exception. In this way any
          inner loops get popped.

          Here's an example.

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

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

          The "pp_tie" does a "CATCH_SET(TRUE)", then starts a
          second runops loop to execute the body of "TIEARRAY".
          When it executes the entertry op on line 3, "CATCH_GET"
          is true, so "pp_entertry" calls "docatch" which does a
          "JMPENV_PUSH" and starts a third runops loop, which
          then executes the die op. At this point the C call
          stack looks like this:

              Perl_pp_die
              Perl_runops      # third loop
              S_docatch_body
              S_docatch
              Perl_pp_entertry
              Perl_runops      # second loop
              S_call_body
              Perl_call_sv
              Perl_pp_tie
              Perl_runops      # first loop
              S_run_body
              perl_run
              main

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

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

          The die pops the first "CxEVAL" off the context stack,
          sets "PL_restartop" from it, does a JMPENV_JUMP(3), and



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          control returns to the top "docatch". This then starts
          another third-level runops level, which executes the
          nextstate, pushmark and die ops on line 4. At the point
          that the second "pp_die" is called, the C call stack
          looks exactly like that above, even though we are no
          longer within an inner eval; this is because of the
          optimization mentioned earlier. However, the context
          stack now looks like this, ie with the top CxEVAL
          popped:

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

          The die on line 4 pops the context stack back down to
          the CxEVAL, leaving it as:

              STACK 0: MAIN
                CX 0: BLOCK  =>

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

              S_docatch
              Perl_pp_entertry
              Perl_runops      # second loop
              S_call_body
              Perl_call_sv
              Perl_pp_tie
              Perl_runops      # first loop
              S_run_body
              perl_run
              main

          In  this case, because the "JMPENV" level recorded in
          the "CxEVAL" differs from the current one, "docatch"
          just does a JMPENV_JUMP(3) and the C stack unwinds to:

              perl_run
              main

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

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




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

       The core Devel::Peek module lets us examine SVs from a
       Perl program. Let's see, for instance, how Perl treats the
       constant "hello".

             % perl -MDevel::Peek -e 'Dump("hello")'
           1 SV = PV(0xa041450) at 0xa04ecbc
           2   REFCNT = 1
           3   FLAGS = (POK,READONLY,pPOK)
           4   PV = 0xa0484e0 "hello"\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
       currently allocated buffer; as the string grows, Perl
       automatically extends the available storage via a routine
       called "SvGROW".

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

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



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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
       reallocate space for us.

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

       Line 10 does the actual catenation: the "Move" macro moves
       a chunk of memory around: we move the string "ptr" to the
       end of the PV - that's the start of the PV plus its
       current length. We're moving "len" bytes of type "char".
       After doing so, we need to tell Perl we've extended the
       string, by altering "CUR" to reflect the new length.
       "SvEND" is a macro which gives us the end of the string,
       so that needs to be a "\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
       "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 "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
       specific operator in question is "sassign" - scalar
       assignment - and you can find the code which implements it
       in the function "pp_sassign" in pp_hot.c. As a binary
       operator, it has two children: the add operator, providing
       the result of "$b+$c", is uppermost on line 5, and the
       left hand side is on line 10.

       Line 10 is the null op: this does exactly nothing. What is
       that doing there? If you see the null op, it's a sign that
       something has been optimized away after parsing. As we
       mentioned in "Optimization", the optimization stage
       sometimes converts two operations into one, for example
       when fetching a scalar variable. When this happens,



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       instead of rewriting the op tree and cleaning up the
       dangling pointers, it's easier just to replace the
       redundant operation with the null op. Originally, the tree
       would have looked like this:

           10          SVOP (0x816b4f0) rv2sv [15]
           11              SVOP (0x816dcf0) gv  GV (0x80fa460) *a

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

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

       Now, what's this about?

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

       "enter" and "leave" are scoping ops, and their job is to
       perform any housekeeping every time you enter and leave a
       block: lexical variables are tidied up, unreferenced
       variables are destroyed, and so on. Every program will
       have those first three lines: "leave" is a list, and its
       children are all the statements in the block. Statements
       are delimited by "nextstate", so a block is a collection
       of "nextstate" ops, with the ops to be performed for each
       statement being the children of "nextstate". "enter" is a
       single op which functions as a marker.

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

                               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
       executes the code. We can traverse the tree in this order
       using the "exec" option to "B::Terse":



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

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

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

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

       If you're not used to reading BNF grammars, this is how it
       works: You're fed certain things by the tokeniser, which
       generally end up in upper case. Here, "ADDOP", is provided
       when the tokeniser sees "+" in your code. "ASSIGNOP" is
       provided when "=" is used for assigning. These are
       "terminal symbols", because you can't get any simpler than
       them.

       The grammar, lines one and three of the snippet above,
       tells you how to build up more complex forms. These
       complex forms, "non-terminal symbols" are generally placed
       in lower case. "term" here is a non-terminal symbol,
       representing a single expression.

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

       So, if you see two terms with an "=" or "+", between them,
       you can turn them into a single expression. When you do
       this, you execute the code in the block on the next line:
       if you see "=", you'll do the code in line 2. If you see
       "+", you'll do the code in line 4. It's this code which
       contributes to the op tree.



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                   |   term ADDOP term
                   { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }

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

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

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




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               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 perlxstut, perlxs and
          perlguts for a longer description of the macros used in
          stack manipulation.

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

          Let's examine the whole implementation, for practice:

               1  PUSHMARK(SP);

          Push the current state of the stack pointer onto the
          mark stack. This is so that when we've finished adding
          items to the argument stack, Perl knows how many things
          we've added recently.

               2  EXTEND(SP,2);
               3  PUSHs(SvTIED_obj((SV*)av, mg));
               4  PUSHs(val);

          We're going to add two more items onto the argument
          stack: when you have a tied array, the "PUSH"
          subroutine receives the object and the value to be
          pushed, and that's exactly what we have here - the tied



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          object, retrieved with "SvTIED_obj", and the value, the
          SV "val".

               5  PUTBACK;

          Next we tell Perl to update the global stack pointer
          from our internal variable: "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.  The call_method() function
          removes the top element of the mark stack, so there is
          nothing for the caller to clean up.

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

              {
                  local $foo = 42;
                  ...
              }

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

   Millions of Macros
       One thing you'll notice about the Perl source is that it's
       full of macros. Some have called the pervasive use of
       macros the hardest thing to understand, others find it
       adds to clarity. Let's take an example, the code which
       implements the addition operator:










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          1  PP(pp_add)
          2  {
          3      dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
          4      {
          5        dPOPTOPnnrl_ul;
          6        SETn( left + right );
          7        RETURN;
          8      }
          9  }

       Every line here (apart from the braces, of course)
       contains a macro. The first line sets up the function
       declaration as Perl expects for PP code; line 3 sets up
       variable declarations for the argument stack and the
       target, the return value of the operation. Finally, it
       tries to see if the addition operation is overloaded; if
       so, the appropriate subroutine is called.

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

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

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

           make foo.i

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

SOURCE CODE STATIC ANALYSIS
       Various tools exist for analysing C source code
       statically, as opposed to dynamically, that is, without
       executing the code.  It is possible to detect resource
       leaks, undefined behaviour, type mismatches, portability
       problems, code paths that would cause illegal memory
       accesses, and other similar problems by just parsing the C
       code and looking at the resulting graph, what does it tell
       about the execution and data flows.  As a matter of fact,
       this is exactly how C compilers know to give warnings
       about dubious code.



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   lint, splint
       The good old C code quality inspector, "lint", is
       available in several platforms, but please be aware that
       there are several different implementations of it by
       different vendors, which means that the flags are not
       identical across different platforms.

       There is a lint variant called "splint" (Secure
       Programming Lint) available from http://www.splint.org/
       that should compile on any Unix-like platform.

       There are "lint" and <splint> targets in Makefile, but you
       may have to diddle with the flags (see above).

   Coverity
       Coverity (http://www.coverity.com/) is a product similar
       to lint and as a testbed for their product they
       periodically check several open source projects, and they
       give out accounts to open source developers to the defect
       databases.

   cpd (cut-and-paste detector)
       The cpd tool detects cut-and-paste coding.  If one
       instance of the cut-and-pasted code changes, all the other
       spots should probably be changed, too.  Therefore such
       code should probably be turned into a subroutine or a
       macro.

       cpd (http://pmd.sourceforge.net/cpd.html) is part of the
       pmd project (http://pmd.sourceforge.net/).  pmd was
       originally written for static analysis of Java code, but
       later the cpd part of it was extended to parse also C and
       C++.

       Download the pmd-bin-X.Y.zip () from the SourceForge site,
       extract the pmd-X.Y.jar from it, and then run that on
       source code thusly:

         java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt

       You may run into memory limits, in which case you should
       use the -Xmx option:

         java -Xmx512M ...

   gcc warnings
       Though much can be written about the inconsistency and
       coverage problems of gcc warnings (like "-Wall" not
       meaning "all the warnings", or some common portability
       problems not being covered by "-Wall", or "-ansi" and
       "-pedantic" both being a poorly defined collection of
       warnings, and so forth), gcc is still a useful tool in
       keeping our coding nose clean.




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       The "-Wall" is by default on.

       The "-ansi" (and its sidekick, "-pedantic") would be nice
       to be on always, but unfortunately they are not safe on
       all platforms, they can for example cause fatal conflicts
       with the system headers (Solaris being a prime example).
       If Configure "-Dgccansipedantic" is used, the "cflags"
       frontend selects "-ansi -pedantic" for the platforms where
       they are known to be safe.

       Starting from Perl 5.9.4 the following extra flags are
       added:

       o   "-Wendif-labels"

       o   "-Wextra"

       o   "-Wdeclaration-after-statement"

       The following flags would be nice to have but they would
       first need their own Augean stablemaster:

       o   "-Wpointer-arith"

       o   "-Wshadow"

       o   "-Wstrict-prototypes"

       The "-Wtraditional" is another example of the annoying
       tendency of gcc to bundle a lot of warnings under one
       switch -- it would be impossible to deploy in practice
       because it would complain a lot -- but it does contain
       some warnings that would be beneficial to have available
       on their own, such as the warning about string constants
       inside macros containing the macro arguments: this behaved
       differently pre-ANSI than it does in ANSI, and some C
       compilers are still in transition, AIX being an example.

   Warnings of other C compilers
       Other C compilers (yes, there are other C compilers than
       gcc) often have their "strict ANSI" or "strict ANSI with
       some portability extensions" modes on, like for example
       the Sun Workshop has its "-Xa" mode on (though
       implicitly), or the DEC (these days, HP...) has its
       "-std1" mode on.

   DEBUGGING
       You can compile a special debugging version of Perl, which
       allows you to use the "-D" option of Perl to tell more
       about what Perl is doing.  But sometimes there is no
       alternative than to dive in with a debugger, either to see
       the stack trace of a core dump (very useful in a bug
       report), or trying to figure out what went wrong before
       the core dump happened, or how did we end up having wrong



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       or unexpected results.

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

           ./Configure -d -D optimize=-g
           make

       "-g" is a flag to the C compiler to have it produce
       debugging information which will allow us to step through
       a running program, and to see in which C function we are
       at (without the debugging information we might see only
       the numerical addresses of the functions, which is not
       very helpful).

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

           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 (many vendors call their
          debugger "dbx"), but check the manual of the one you're
          using.

       To fire up the debugger, type

           gdb ./perl

       Or if you have a core dump:

           gdb ./perl core

       You'll want to do that in your Perl source tree so the
       debugger can read the source code. You should see the



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       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
          "Internal Functions" in perlguts!) or the given line in
          the named source file.

       step
          Steps through the program a line at a time.

       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.
          WARNING: Perl makes heavy use of macros, and gdb does
          not necessarily support macros (see later "gdb macro
          support").  You'll have to substitute them yourself, or
          to invoke cpp on the source code files (see "The .i
          Targets") So, for instance, you can't say

              print SvPV_nolen(sv)

          but you have to say

              print Perl_sv_2pv_nolen(sv)

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



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

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

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

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

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

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

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

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



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       "sv_2nv" to convert it. If we step again, we'll find
       ourselves there:

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

           {
           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
       fiddling with them. Let's now get on and create a simple
       patch. Here's something Larry suggested: if a "U" is the
       first active format during a "pack", (for example, "pack
       "U3C8", @stuff") then the resulting string should be
       treated as UTF-8 encoded.



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

       [Well, it was in pp.c when this tutorial was written. It
       has now been split off with "pp_unpack" to its own file,
       pp_pack.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
       possible format character, we swallow up the other
       arguments in the pattern (a field width, an asterisk, and
       so on) and convert the next chunk input into the specified
       format, adding it onto the 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;
           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":






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        +  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
       swallowed into "datumtype!"

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

           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
       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" );



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       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
       confused.  This will either look like this:

        print "1..156\n";

       or this:

        plan( tests => 156 );

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

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

        =item *

        If the pattern begins with a C<U>, the resulting string will be treated
        as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
        with an initial C<U0>, and the bytes that follow will be interpreted as
        Unicode characters. If you don't want this to happen, you can begin your
        pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
        string, and then follow this with a C<U*> somewhere in your pattern.

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




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       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
       constantly scanning p5p.

       The list of maintainers of core modules is usefully
       documented in Porting/Maintainers.pl.

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

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

       The location of any new code is also an important
       consideration.  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



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       as possibly lots of include files.  This is strictly
       pumpking business.

       It is better to add your function to one of the existing
       top level source code files, but your choice is
       complicated by the nature of the Perl distribution.  Only
       the files that are marked as compiled static are located
       in the perl 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
       create an appropriate entry in embed.pl describing your
       function, then run 'make regen_headers' to create the
       entries in the numerous header files that perl needs to
       compile correctly.  See "Internal Functions" in perlguts
       for information on the various options that you can set in
       embed.pl.  You will 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
       guidelines discussed seem to focus on Perl code, rather
       than c, they all apply (except when they don't ;).  See
       also Porting/patching.pod file in the Perl source
       distribution for lots of details about both formatting and
       submitting patches of your changes.

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

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

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



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

       If you add a new test directory under t/, it is imperative
       that you add that directory to t/HARNESS and t/TEST.

       t/base/
          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.

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

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

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

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

       t/mro/
          Tests for perl's method resolution order
          implementations (see mro).

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

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

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

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





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       t/win32/
          Windows-specific tests.

       t/x2p
          A test suite for the s2p converter.

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

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

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

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

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

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

       When you say "make test" Perl uses the t/TEST program to
       run the test suite (except under Win32 where it uses
       t/harness instead.)  All tests are run from the t/
       directory, not the directory which contains the test.
       This causes some 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
       necessary.

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



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       are not available on certain operating systems.

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

           (Not available on Win32)

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

           (Not available on Win32)

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

           (Not available on Win32)

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

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

       test.third check.third utest.third ucheck.third
           (Only in Tru64)  Run all the tests using the memory
           leak + naughty memory access tool "Third Degree".  The
           log files will be named perl.3log.testname.

       test.torture torturetest
           Run all the usual tests and some extra tests.  As of
           Perl 5.8.0 the only extra tests are Abigail's JAPHs,
           t/japh/abigail.t.

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

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

           (Not available on Win32)

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




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

           (Not available on Win32)

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

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

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

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

       test-notty test_notty
           Sets PERL_SKIP_TTY_TEST to true before running normal
           test.

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

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

       or

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

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

       Using t/harness for testing

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

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



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

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

       -torture
           Run the torture tests as well as the normal set.

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

       -re LIST OF PATTERNS
           Filter the file list so that all the test files run
           match /(LIST|OF|PATTERNS)/. Note that with this form
           the patterns are joined by '|' and you cannot supply a
           list of files, instead the test files are obtained
           from the MANIFEST.

       You can run an individual test by a command similar to

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

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

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

       PERL_DESTRUCT_LEVEL=2
           is set to 2 if it isn't set already (see
           "PERL_DESTRUCT_LEVEL")

       PERL
           (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).

       PERL_SKIP_TTY_TEST
           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
           test_notty'.

       Other environment variables that may influence tests

       PERL_TEST_Net_Ping
           Setting this variable runs all the Net::Ping modules



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           tests, otherwise some tests that interact with the
           outside world are skipped.  See perl58delta.

       PERL_TEST_NOVREXX
           Setting this variable skips the vrexx.t tests for
           OS2::REXX.

       PERL_TEST_NUMCONVERTS
           This sets a variable in op/numconvert.t.

       See also the documentation for the Test and Test::Harness
       modules, for more environment variables that affect
       testing.

   Common problems when patching Perl source code
       Perl source plays by ANSI C89 rules: no C99 (or C++)
       extensions.  In some cases we have to take pre-ANSI
       requirements into consideration.  You don't care about
       some particular platform having broken Perl?  I hear there
       is still a strong demand for J2EE programmers.

   Perl environment problems
       o   Not compiling with threading

           Compiling with threading (-Duseithreads) completely
           rewrites the function prototypes of Perl.  You better
           try your changes with that.  Related to this is the
           difference between "Perl_-less" and "Perl_-ly" APIs,
           for example:

             Perl_sv_setiv(aTHX_ ...);
             sv_setiv(...);

           The first one explicitly passes in the context, which
           is needed for e.g.  threaded builds.  The second one
           does that implicitly; do not get them mixed.  If you
           are not passing in a aTHX_, you will need to do a dTHX
           (or a dVAR) as the first thing in the function.

           See "How multiple interpreters and concurrency are
           supported" in perlguts for further discussion about
           context.

       o   Not compiling with -DDEBUGGING

           The DEBUGGING define exposes more code to the
           compiler, therefore more ways for things to go wrong.
           You should try it.

       o   Introducing (non-read-only) globals

           Do not introduce any modifiable globals, truly global
           or file static.  They are bad form and complicate
           multithreading and other forms of concurrency.  The



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           right way is to introduce them as new interpreter
           variables, see intrpvar.h (at the very end for binary
           compatibility).

           Introducing read-only (const) globals is okay, as long
           as you verify with e.g. "nm libperl.a|egrep -v '
           [TURtr] '" (if your "nm" has BSD-style output) that
           the data you added really is read-only.  (If it is, it
           shouldn't show up in the output of that command.)

           If you want to have static strings, make them
           constant:

             static const char etc[] = "...";

           If you want to have arrays of constant strings, note
           carefully the right combination of "const"s:

               static const char * const yippee[] =
                   {"hi", "ho", "silver"};

           There is a way to completely hide any modifiable
           globals (they are all moved to heap), the compilation
           setting "-DPERL_GLOBAL_STRUCT_PRIVATE".  It is not
           normally used, but can be used for testing, read more
           about it in "Background and PERL_IMPLICIT_CONTEXT" in
           perlguts.

       o   Not exporting your new function

           Some platforms (Win32, AIX, VMS, OS/2, to name a few)
           require any function that is part of the public API
           (the shared Perl library) to be explicitly marked as
           exported.  See the discussion about embed.pl in
           perlguts.

       o   Exporting your new function

           The new shiny result of either genuine new
           functionality or your arduous refactoring is now ready
           and correctly exported.  So what could possibly go
           wrong?

           Maybe simply that your function did not need to be
           exported in the first place.  Perl has a long and not
           so glorious history of exporting functions that it
           should not have.

           If the function is used only inside one source code
           file, make it static.  See the discussion about
           embed.pl in perlguts.

           If the function is used across several files, but
           intended only for Perl's internal use (and this should



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           be the common case), do not export it to the public
           API.  See the discussion about embed.pl in perlguts.

   Portability problems
       The following are common causes of compilation and/or
       execution failures, not common to Perl as such.  The C FAQ
       is good bedtime reading.  Please test your changes with as
       many C compilers and platforms as possible -- we will,
       anyway, and it's nice to save oneself from public
       embarrassment.

       If using gcc, you can add the "-std=c89" option which will
       hopefully catch most of these unportabilities. (However it
       might also catch incompatibilities in your system's header
       files.)

       Use the Configure "-Dgccansipedantic" flag to enable the
       gcc "-ansi -pedantic" flags which enforce stricter ANSI
       rules.

       If using the "gcc -Wall" note that not all the possible
       warnings (like "-Wunitialized") are given unless you also
       compile with "-O".

       Note that if using gcc, starting from Perl 5.9.5 the Perl
       core source code files (the ones at the top level of the
       source code distribution, but not e.g. the extensions
       under ext/) are automatically compiled with as many as
       possible of the "-std=c89", "-ansi", "-pedantic", and a
       selection of "-W" flags (see cflags.SH).

       Also study perlport carefully to avoid any bad assumptions
       about the operating system, filesystems, and so forth.

       You may once in a while try a "make microperl" to see
       whether we can still compile Perl with just the bare
       minimum of interfaces.  (See README.micro.)

       Do not assume an operating system indicates a certain
       compiler.

       o   Casting pointers to integers or casting integers to
           pointers

               void castaway(U8* p)
               {
                 IV i = p;

           or

               void castaway(U8* p)
               {
                 IV i = (IV)p;




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           Both are bad, and broken, and unportable.  Use the
           PTR2IV() macro that does it right.  (Likewise, there
           are PTR2UV(), PTR2NV(), INT2PTR(), and NUM2PTR().)

       o   Casting between data function pointers and data
           pointers

           Technically speaking casting between function pointers
           and data pointers is unportable and undefined, but
           practically speaking it seems to work, but you should
           use the FPTR2DPTR() and DPTR2FPTR() macros.  Sometimes
           you can also play games with unions.

       o   Assuming sizeof(int) == sizeof(long)

           There are platforms where longs are 64 bits, and
           platforms where ints are 64 bits, and while we are out
           to shock you, even platforms where shorts are 64 bits.
           This is all legal according to the C standard.  (In
           other words, "long long" is not a portable way to
           specify 64 bits, and "long long" is not even
           guaranteed to be any wider than "long".)

           Instead, use the definitions IV, UV, IVSIZE, I32SIZE,
           and so forth.  Avoid things like I32 because they are
           not guaranteed to be exactly 32 bits, they are at
           least 32 bits, nor are they guaranteed to be int or
           long.  If you really explicitly need 64-bit variables,
           use I64 and U64, but only if guarded by HAS_QUAD.

       o   Assuming one can dereference any type of pointer for
           any type of data

             char *p = ...;
             long pony = *p;    /* BAD */

           Many platforms, quite rightly so, will give you a core
           dump instead of a pony if the p happens not be
           correctly aligned.

       o   Lvalue casts

             (int)*p = ...;    /* BAD */

           Simply not portable.  Get your lvalue to be of the
           right type, or maybe use temporary variables, or dirty
           tricks with unions.

       o   Assume anything about structs (especially the ones you
           don't control, like the ones coming from the system
           headers)

           o       That a certain field exists in a struct




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           o       That no other fields exist besides the ones
                   you know of

           o       That a field is of certain signedness, sizeof,
                   or type

           o       That the fields are in a certain order

                   o       While C guarantees the ordering
                           specified in the struct definition,
                           between different platforms the
                           definitions might differ

           o       That the sizeof(struct) or the alignments are
                   the same everywhere

                   o       There might be padding bytes between
                           the fields to align the fields - the
                           bytes can be anything

                   o       Structs are required to be aligned to
                           the maximum alignment required by the
                           fields - which for native types is for
                           usually equivalent to sizeof() of the
                           field

       o   Assuming the character set is ASCIIish

           Perl can compile and run under EBCDIC platforms.  See
           perlebcdic.  This is transparent for the most part,
           but because the character sets differ, you shouldn't
           use numeric (decimal, octal, nor hex) constants to
           refer to characters.  You can safely say 'A', but not
           0x41.  You can safely say '\n', but not \012.  If a
           character doesn't have a trivial input form, you can
           create a #define for it in both "utfebcdic.h" and
           "utf8.h", so that it resolves to different values
           depending on the character set being used.  (There are
           three different EBCDIC character sets defined in
           "utfebcdic.h", so it might be best to insert the
           #define three times in that file.)

           Also, the range 'A' - 'Z' in ASCII is an unbroken
           sequence of 26 upper case alphabetic characters.  That
           is not true in EBCDIC.  Nor for 'a' to 'z'.  But '0' -
           '9' is an unbroken range in both systems.  Don't
           assume anything about other ranges.

           Many of the comments in the existing code ignore the
           possibility of EBCDIC, and may be wrong therefore,
           even if the code works.  This is actually a tribute to
           the successful transparent insertion of being able to
           handle EBCDIC without having to change pre-existing
           code.



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           UTF-8 and UTF-EBCDIC are two different encodings used
           to represent Unicode code points as sequences of
           bytes.  Macros with the same names (but different
           definitions) in "utf8.h" and "utfebcdic.h" are used to
           allow the calling code to think that there is only one
           such encoding.  This is almost always referred to as
           "utf8", but it means the EBCDIC version as well.
           Again, comments in the code may well be wrong even if
           the code itself is right.  For example, the concept of
           "invariant characters" differs between ASCII and
           EBCDIC.  On ASCII platforms, only characters that do
           not have the high-order bit set (i.e. whose ordinals
           are strict ASCII, 0 - 127) are invariant, and the
           documentation and comments in the code may assume
           that, often referring to something like, say, "hibit".
           The situation differs and is not so simple on EBCDIC
           machines, but as long as the code itself uses the
           "NATIVE_IS_INVARIANT()" macro appropriately, it works,
           even if the comments are wrong.

       o   Assuming the character set is just ASCII

           ASCII is a 7 bit encoding, but bytes have 8 bits in
           them.  The 128 extra characters have different
           meanings depending on the locale.  Absent a locale,
           currently these extra characters are generally
           considered to be unassigned, and this has presented
           some problems.  This is scheduled to be changed in
           5.12 so that these characters will be considered to be
           Latin-1 (ISO-8859-1).

       o   Mixing #define and #ifdef

             #define BURGLE(x) ... \
             #ifdef BURGLE_OLD_STYLE        /* BAD */
             ... do it the old way ... \
             #else
             ... do it the new way ... \
             #endif

           You cannot portably "stack" cpp directives.  For
           example in the above you need two separate BURGLE()
           #defines, one for each #ifdef branch.

       o   Adding non-comment stuff after #endif or #else

             #ifdef SNOSH
             ...
             #else !SNOSH    /* BAD */
             ...
             #endif SNOSH    /* BAD */

           The #endif and #else cannot portably have anything
           non-comment after them.  If you want to document what



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           is going (which is a good idea especially if the
           branches are long), use (C) comments:

             #ifdef SNOSH
             ...
             #else /* !SNOSH */
             ...
             #endif /* SNOSH */

           The gcc option "-Wendif-labels" warns about the bad
           variant (by default on starting from Perl 5.9.4).

       o   Having a comma after the last element of an enum list

             enum color {
               CERULEAN,
               CHARTREUSE,
               CINNABAR,     /* BAD */
             };

           is not portable.  Leave out the last comma.

           Also note that whether enums are implicitly morphable
           to ints varies between compilers, you might need to
           (int).

       o   Using //-comments

             // This function bamfoodles the zorklator.    /* BAD */

           That is C99 or C++.  Perl is C89.  Using the
           //-comments is silently allowed by many C compilers
           but cranking up the ANSI C89 strictness (which we like
           to do) causes the compilation to fail.

       o   Mixing declarations and code

             void zorklator()
             {
               int n = 3;
               set_zorkmids(n);    /* BAD */
               int q = 4;

           That is C99 or C++.  Some C compilers allow that, but
           you shouldn't.

           The gcc option "-Wdeclaration-after-statements" scans
           for such problems (by default on starting from Perl
           5.9.4).

       o   Introducing variables inside for()

             for(int i = ...; ...; ...) {    /* BAD */




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           That is C99 or C++.  While it would indeed be awfully
           nice to have that also in C89, to limit the scope of
           the loop variable, alas, we cannot.

       o   Mixing signed char pointers with unsigned char
           pointers

             int foo(char *s) { ... }
             ...
             unsigned char *t = ...; /* Or U8* t = ... */
             foo(t);   /* BAD */

           While this is legal practice, it is certainly dubious,
           and downright fatal in at least one platform: for
           example VMS cc considers this a fatal error.  One
           cause for people often making this mistake is that a
           "naked char" and therefore dereferencing a "naked char
           pointer" have an undefined signedness: it depends on
           the compiler and the flags of the compiler and the
           underlying platform whether the result is signed or
           unsigned.  For this very same reason using a 'char' as
           an array index is bad.

       o   Macros that have string constants and their arguments
           as substrings of the string constants

             #define FOO(n) printf("number = %d\n", n)    /* BAD */
             FOO(10);

           Pre-ANSI semantics for that was equivalent to

             printf("10umber = %d\10");

           which is probably not what you were expecting.
           Unfortunately at least one reasonably common and
           modern C compiler does "real backward compatibility"
           here, in AIX that is what still happens even though
           the rest of the AIX compiler is very happily C89.

       o   Using printf formats for non-basic C types

              IV i = ...;
              printf("i = %d\n", i);    /* BAD */

           While this might by accident work in some platform
           (where IV happens to be an "int"), in general it
           cannot.  IV might be something larger.  Even worse the
           situation is with more specific types (defined by
           Perl's configuration step in config.h):

              Uid_t who = ...;
              printf("who = %d\n", who);    /* BAD */

           The problem here is that Uid_t might be not only not



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           "int"-wide but it might also be unsigned, in which
           case large uids would be printed as negative values.

           There is no simple solution to this because of
           printf()'s limited intelligence, but for many types
           the right format is available as with either 'f' or
           '_f' suffix, for example:

              IVdf /* IV in decimal */
              UVxf /* UV is hexadecimal */

              printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */

              Uid_t_f /* Uid_t in decimal */

              printf("who = %"Uid_t_f"\n", who);

           Or you can try casting to a "wide enough" type:

              printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);

           Also remember that the %p format really does require a
           void pointer:

              U8* p = ...;
              printf("p = %p\n", (void*)p);

           The gcc option "-Wformat" scans for such problems.

       o   Blindly using variadic macros

           gcc has had them for a while with its own syntax, and
           C99 brought them with a standardized syntax.  Don't
           use the former, and use the latter only if the
           HAS_C99_VARIADIC_MACROS is defined.

       o   Blindly passing va_list

           Not all platforms support passing va_list to further
           varargs (stdarg) functions.  The right thing to do is
           to copy the va_list using the Perl_va_copy() if the
           NEED_VA_COPY is defined.

       o   Using gcc statement expressions

              val = ({...;...;...});    /* BAD */

           While a nice extension, it's not portable.  The Perl
           code does admittedly use them if available to gain
           some extra speed (essentially as a funky form of
           inlining), but you shouldn't.

       o   Binding together several statements in a macro




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           Use the macros STMT_START and STMT_END.

              STMT_START {
                 ...
              } STMT_END

       o   Testing for operating systems or versions when should
           be testing for features

             #ifdef __FOONIX__    /* BAD */
             foo = quux();
             #endif

           Unless you know with 100% certainty that quux() is
           only ever available for the "Foonix" operating system
           and that is available and correctly working for all
           past, present, and future versions of "Foonix", the
           above is very wrong.  This is more correct (though
           still not perfect, because the below is a compile-time
           check):

             #ifdef HAS_QUUX
             foo = quux();
             #endif

           How does the HAS_QUUX become defined where it needs to
           be?  Well, if Foonix happens to be UNIXy enough to be
           able to run the Configure script, and Configure has
           been taught about detecting and testing quux(), the
           HAS_QUUX will be correctly defined.  In other
           platforms, the corresponding configuration step will
           hopefully do the same.

           In a pinch, if you cannot wait for Configure to be
           educated, or if you have a good hunch of where quux()
           might be available, you can temporarily try the
           following:

             #if (defined(__FOONIX__) || defined(__BARNIX__))
             # define HAS_QUUX
             #endif

             ...

             #ifdef HAS_QUUX
             foo = quux();
             #endif

           But in any case, try to keep the features and
           operating systems separate.

   Problematic System Interfaces
       o   malloc(0), realloc(0), calloc(0, 0) are non-portable.
           To be portable allocate at least one byte.  (In



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           general you should rarely need to work at this low
           level, but instead use the various malloc wrappers.)

       o   snprintf() - the return type is unportable.  Use
           my_snprintf() instead.

   Security problems
       Last but not least, here are various tips for safer
       coding.

       o   Do not use gets()

           Or we will publicly ridicule you.  Seriously.

       o   Do not use strcpy() or strcat() or strncpy() or
           strncat()

           Use my_strlcpy() and my_strlcat() instead: they either
           use the native implementation, or Perl's own
           implementation (borrowed from the public domain
           implementation of INN).

       o   Do not use sprintf() or vsprintf()

           If you really want just plain byte strings, use
           my_snprintf() and my_vsnprintf() instead, which will
           try to use snprintf() and vsnprintf() if those safer
           APIs are available.  If you want something fancier
           than a plain byte string, use SVs and
           Perl_sv_catpvf().

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.

       NOTE 1: Running under memory debuggers such as Purify,
       valgrind, or Third Degree greatly slows down the
       execution: seconds become minutes, minutes become hours.
       For example as of Perl 5.8.1, the ext/Encode/t/Unicode.t
       takes extraordinarily long to complete under e.g. Purify,
       Third Degree, and valgrind.  Under valgrind it takes more
       than six hours, even on a snappy computer-- the said test
       must be doing something that is quite unfriendly for
       memory debuggers.  If you don't feel like waiting, that
       you can simply kill 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



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       if you are running some of the tests manually-- for csh-
       like shells:

           setenv PERL_DESTRUCT_LEVEL 2

       and for Bourne-type shells:

           PERL_DESTRUCT_LEVEL=2
           export PERL_DESTRUCT_LEVEL

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

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

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

       NOTE 4: DynaLoader will not clean up after itself
       completely unless Perl is built with the Configure option
       "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".

   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:

       -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.





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       -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
       need this option:

           setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:

           PURIFYOPTIONS="..."
           export PURIFYOPTIONS

       or if you have the "env" utility:

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




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   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.

   valgrind
       The excellent valgrind tool can be used to find out both
       memory leaks and illegal memory accesses.  As of version
       3.3.0, Valgrind only supports Linux on x86, x86-64 and
       PowerPC.  The special "test.valgrind" target can be used
       to run the tests under valgrind.  Found errors and memory
       leaks are logged in files named testfile.valgrind.

       Valgrind also provides a cachegrind tool, invoked on perl



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       as:

           VG_OPTS=--tool=cachegrind make test.valgrind

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

       To get valgrind and for more information see

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

   Compaq's/Digital's/HP's Third Degree
       Third Degree is a tool for memory leak detection and
       memory access checks.  It is one of the many tools in the
       ATOM toolkit.  The toolkit is only available on Tru64
       (formerly known as Digital UNIX formerly known as DEC
       OSF/1).

       When building Perl, you must first run Configure with
       -Doptimize=-g and -Uusemymalloc flags, after that you can
       use the make targets "perl.third" and "test.third".  (What
       is required is that Perl must be compiled using the "-g"
       flag, you may need to re-Configure.)

       The short story is that with "atom" you can instrument the
       Perl executable to create a new executable called
       perl.third.  When the instrumented executable is run, it
       creates a log of dubious memory traffic in file called
       perl.3log.  See the manual pages of atom and third for
       more information.  The most extensive Third Degree
       documentation is available in the Compaq "Tru64 UNIX
       Programmer's Guide", chapter "Debugging Programs with
       Third Degree".

       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
       information.

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



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       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
       environment variable for its own purposes and extended its
       semantics. Refer to the mod_perl documentation for more
       information. Also, spawned threads do the equivalent of
       setting this variable to the value 1.)

       If, at the end of a run you get the message N scalars
       leaked, you can recompile with "-DDEBUG_LEAKING_SCALARS",
       which will cause the addresses of all those leaked SVs to
       be dumped along with details as to where each SV was
       originally allocated. This information is also displayed
       by Devel::Peek. Note that the extra details recorded with
       each SV increases memory usage, so it shouldn't be used in
       production environments. 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.

       If you see that you're leaking memory at runtime, but
       neither valgrind nor "-DDEBUG_LEAKING_SCALARS" will find
       anything, you're probably leaking SVs that are still
       reachable and will be properly cleaned up during
       destruction of the interpreter. In such cases, using the
       "-Dm" switch can point you to the source of the leak. If
       the executable was built with "-DDEBUG_LEAKING_SCALARS",
       "-Dm" will output SV allocations in addition to memory
       allocations. Each SV allocation has a distinct serial
       number that will be written on creation and destruction of
       the SV.  So if you're executing the leaking code in a
       loop, you need to look for SVs that are created, but never
       destroyed between each cycle. If such an SV is found, set
       a conditional breakpoint within "new_SV()" and make it
       break only when "PL_sv_serial" is equal to the serial
       number of the leaking SV. Then you will catch the
       interpreter in exactly the state where the leaking SV is
       allocated, which is sufficient in many cases to find the
       source of the leak.

       As "-Dm" is using the PerlIO layer for output, it will by
       itself allocate quite a bunch of SVs, which are hidden to
       avoid recursion.  You can bypass the PerlIO layer if you
       use the SV logging provided by "-DPERL_MEM_LOG" instead.





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   PERL_MEM_LOG
       If compiled with "-DPERL_MEM_LOG", all Newx() and Renew()
       allocations and Safefree() in the Perl core go through
       logging functions, which is handy for breakpoint setting.
       If also compiled with "-DPERL_MEM_LOG_STDERR", the
       allocations and frees are logged to STDERR (or more
       precisely, to the file descriptor 2) in these logging
       functions, with the calling source code file and line
       number (and C function name, if supported by the C
       compiler).

       This logging is somewhat similar to "-Dm" but independent
       of "-DDEBUGGING", and at a higher level (the "-Dm" is
       directly at the point of "malloc()", while the
       "PERL_MEM_LOG" is at the level of "New()").

       In addition to memory allocations, SV allocations will be
       logged, just as with "-Dm". However, since the logging
       doesn't use PerlIO, all SV allocations are logged and no
       extra SV allocations are introduced by enabling the
       logging.  If compiled with "-DDEBUG_LEAKING_SCALARS", the
       serial number for each SV allocation is also logged.

       You can control the logging from your environment if you
       compile with "-DPERL_MEM_LOG_ENV". Then you need to
       explicitly set "PERL_MEM_LOG" and/or "PERL_SV_LOG" to a
       non-zero value to enable logging of memory and/or SV
       allocations.

   Profiling
       Depending on your platform there are various of profiling
       Perl.

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

       The first method takes periodically samples of the CPU
       program counter, and since the program counter can be
       correlated with the code generated for functions, we get a
       statistical view of in which functions the program is
       spending its time.  The caveats are that very small/fast
       functions have lower probability of showing up in the
       profile, and that periodically interrupting the program
       (this is usually done rather frequently, in the scale of
       milliseconds) imposes an additional overhead that may skew
       the results.  The first problem can be alleviated by
       running the code for longer (in general this is a good
       idea for profiling), the second problem is usually kept in
       guard by the profiling tools themselves.

       The second method divides up the generated code into basic
       blocks.  Basic blocks are sections of code that are
       entered only in the beginning and exited only at the end.



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       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
       platforms, it uses statistical time-sampling.

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

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

       -a  Suppress statically defined functions from the
           profile.

       -b  Suppress the verbose descriptions in the profile.

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

       -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
       gprof.

       quick hint:

           $ sh Configure -des -Dusedevel -Doptimize='-g' -Accflags='-pg' -Aldflags='-pg' && make
           $ ./perl someprog # creates gmon.out in current directory
           $ gprof perl > out
           $ view out



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

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

       Running the profiled version of Perl will cause profile
       output to be generated.  For each source file an
       accompanying ".da" file will be created.

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

           gcov sv.c

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

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

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

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

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

       quick hint:

           $ sh Configure -des  -Doptimize='-g' -Accflags='-fprofile-arcs -ftest-coverage' \
               -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
           $ rm -f regexec.c.gcov regexec.gcda
           $ ./perl.gcov
           $ gcov regexec.c
           $ view regexec.c.gcov

   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



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       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
       Tru64.

       In IRIX the following prof options are available:

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

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

       In Tru64 the following options are available:

       -p[rocedures]
           Procedures sorted in descending order by the number of
           cycles executed in each procedure.  Useful for finding
           the hotspot procedures.  (This is the default option.)

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

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

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

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

       -z[ero]
           Unexecuted procedures.

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



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   Miscellaneous tricks
       o   Those debugging perl with the DDD frontend over gdb
           may find the following useful:

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

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

           the following two lines:

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

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

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

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

           Alternatively edit the init file interactively via:
           3rd mouse button -> New Display -> Edit Menu

           Note: you can define up to 20 conversion shortcuts in
           the gdb section.

       o   If you see in a debugger a memory area mysteriously
           full of 0xABABABAB or 0xEFEFEFEF, you may be seeing
           the effect of the Poison() macros, see perlclib.

       o   Under ithreads the optree is read only. If you want to
           enforce this, to check for write accesses from buggy
           code, compile with "-DPL_OP_SLAB_ALLOC" to enable the
           OP slab allocator and "-DPERL_DEBUG_READONLY_OPS" to
           enable code that allocates op memory via "mmap", and
           sets it read-only at run time.  Any write access to an
           op results in a "SIGBUS" and abort.

           This code is intended for development only, and may
           not be portable even to all Unix variants. Also, it is
           an 80% solution, in that it isn't able to make all ops
           read only. Specifically it

           1.  Only sets read-only on all slabs of ops at "CHECK"
               time, hence ops allocated later via "require" or



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               "eval" will be re-write

           2.  Turns an entire slab of ops read-write if the
               refcount of any op in the slab needs to be
               decreased.

           3.  Turns an entire slab of ops read-write if any op
               from the slab is freed.

           It's not possible to turn the slabs to read-only after
           an action requiring read-write access, as either can
           happen during op tree building time, so there may
           still be legitimate write access.

           However, as an 80% solution it is still effective, as
           currently it catches a write access during the
           generation of Config.pm, which means that we can't yet
           build perl with this enabled.

CONCLUSION
       We've had a brief look around the Perl source, how to
       maintain quality of the source code, an overview of the
       stages perl goes through when it's running your code, how
       to use debuggers to poke at the Perl guts, and finally how
       to analyse the execution of Perl. 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
          distributions and get familiar with the changes. Try
          and get an idea of what areas people are working on and
          the changes they're making.

       o  Do read the README associated with your operating
          system, e.g. README.aix on the IBM AIX OS. Don't
          hesitate to supply patches to that README if 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



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          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!

   Metaphoric Quotations
       If you recognized the quote about the Road above, you're
       in luck.

       Most software projects begin each file with a literal
       description of each file's purpose.  Perl instead begins
       each with a literary allusion to that file's purpose.

       Like chapters in many books, all top-level Perl source
       files (along with a few others here and there) begin with
       an epigramic inscription that alludes, indirectly and
       metaphorically, to the material you're about to read.

       Quotations are taken from writings of J.R.R Tolkien
       pertaining to his Legendarium, almost always from The Lord
       of the Rings.  Chapters and page numbers are given using
       the following editions:

       o   The Hobbit, by J.R.R. Tolkien.  The hardcover,
           70th-anniversary edition of 2007 was used, published
           in the UK by Harper Collins Publishers and in the US
           by the Houghton Mifflin Company.

       o   The Lord of the Rings, by J.R.R. Tolkien.  The
           hardcover, 50th-anniversary edition of 2004 was used,
           published in the UK by Harper Collins Publishers and
           in the US by the Houghton Mifflin Company.

       o   The Lays of Beleriand, by J.R.R. Tolkien and published
           posthumously by his son and literary executor, C.J.R.
           Tolkien, being the 3rd of the 12 volumes in
           Christopher's mammoth History of Middle Earth.  Page
           numbers derive from the hardcover edition, first
           published in 1983 by George Allen & Unwin; no page
           numbers changed for the special 3-volume omnibus
           edition of 2002 or the various trade-paper editions,
           all again now by Harper Collins or Houghton Mifflin.

       Other JRRT books fair game for quotes would thus include
       The Adventures of Tom Bombadil, The Silmarillion,
       Unfinished Tales, and The Tale of the Children of Hurin,
       all but the first posthumously assembled by CJRT.  But The
       Lord of the Rings itself is perfectly fine and probably
       best to quote from, provided you can find a suitable quote



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       there.

       So if you were to supply a new, complete, top-level source
       file to add to Perl, you should conform to this peculiar
       practice by yourself selecting an appropriate quotation
       from Tolkien, retaining the original spelling and
       punctuation and using the same format the rest of the
       quotes are in.  Indirect and oblique is just fine;
       remember, it's a metaphor, so being meta is, after all,
       what it's for.

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

SEE ALSO
       perlrepository








































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