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perlhack (1)


perlhack - How to hack at the Perl internals


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Perl Programmers Reference Guide                      PERLHACK(1)

     perlhack - How to hack at the Perl internals

     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

     A searchable archive of the list is at either:


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

     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.

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

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

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

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

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

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

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

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

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

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         available to implement your (possibly good) idea.

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

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

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

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

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

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

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     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
         additions to the Perl language will be accepted.

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

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

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

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

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

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

  Keeping in sync
     The source code to the Perl interpreter, in its different

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     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.

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

     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:// .
         $ rsync -avz rsync:// .
         $ rsync -avz rsync:// .
         $ rsync -avz rsync:// .
         $ rsync -avz rsync:// .
         $ rsync -avz rsync:// .

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

     To get a full list of the available sync points:

         $ rsync

     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 for subscription

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

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

  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:


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     To email the bug system administrators:

             "perlbug-admin" <>

  Submitting patches
     Always submit patches to  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 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 or  Reporting
     bugs through perlbug feeds into the automated bug-tracking
     system, access to which is provided through the web at .  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 ( ) are a group of
     volunteers who test CPAN modules on a variety of platforms.
     Perl Smokers ( and )
     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

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     <>. In order to start
     smoke testing CPAN modules visit
     <> or
     <> or

     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  To unsubscribe, send mail

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

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

        Gisle Aas's "illustrated perlguts", also known as
        illguts, has very helpful pictures:


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

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

        This is a collection of words of wisdom for a Perl
        porter; some of it is only useful to the pumpkin holder,

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        but most of it applies to anyone wanting to go about Perl

     The perl5-porters FAQ
        This should be available from .  It
        contains hints on reading perl5-porters, information on
        how perl5-porters works and how Perl development in
        general works.

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

     Core modules
        Modules shipped as part of the Perl core live in various
        subdirectories, where two are dedicated to core-only
        modules, and two are for the dual-life modules which live
        on CPAN and may be maintained separately with respect to
        the Perl core:

            lib/  is for pure-Perl modules, which exist in the core only.

            ext/  is for XS extensions, and modules with special Makefile.PL
                  requirements, which exist in the core only.

            cpan/ is for dual-life modules, where the CPAN module is
                  canonical (should be patched first).

            dist/ is for dual-life modules, where the blead source is

        For some dual-life modules it has not been discussed if
        the CPAN version or the blead source is canonical. Until
        that is done, those modules should be in cpan/.

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

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


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        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 three files that fall under his/her responsibility
        are Configure, config_h.SH, and Porting/Glossary (and a
        whole bunch of small related files that are less
        important here). The Configure pumpkin decides how
        patches to these are dealt with. Currently, the Configure
        pumpkin will accept patches in most common formats, even
        directly to these files.  Other committers are allowed to
        commit to these files under the strict condition that
        they will inform the Configure pumpkin, either on IRC (if
        he/she happens to be around) or through (personal)

        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.)

        for a description of the full process involved.

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

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

        The action begins in perlmain.c. (or miniperlmain.c for
        miniperl) This is very high-level code, enough to fit on

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        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

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

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

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

        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

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

         exitstatus = perl_destruct(my_perl);

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

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

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

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

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

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

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

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

        You may be more comfortable with the Perl version of

            PERL_ASYNC_CHECK() while $Perl::op = &{$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

        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
        functions. These basically provide a way to capture the
        current PC and SP registers and later restore them; i.e.

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        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

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

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        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_runops      # third loop
            Perl_runops      # second loop
            Perl_runops      # first loop

        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)
            STACK 1: MAGIC
              CX 0: SUB    =>
              CX 1: EVAL   => *

        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)
            STACK 1: MAGIC
              CX 0: SUB    =>

        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:

            Perl_runops      # second loop
            Perl_runops      # first loop

        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:


        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

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

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

          6      junk = SvPV_force(sv, tlen);
          7      SvGROW(sv, tlen + len + 1);
          8      if (ptr == junk)
          9          ptr = SvPVX(sv);
         10      Move(ptr,SvPVX(sv)+tlen,len,char);
         11      SvCUR(sv) += len;
         12      *SvEND(sv) = '\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

     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.

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

  Op Trees
     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

     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.

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

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

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

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

     Now, what's this about?

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

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

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

                               / \
                              /   \
                             $a   +
                                 / \
                               $b   $c

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     However, it's impossible to perform the operations in this
     order: you have to find the values of $b and $c before you
     add them together, for instance. So, the other thread that
     runs through the op tree is the execution order: each op has
     a field "op_next" which points to the next op to be run, so
     following these pointers tells us how perl executes the
     code. We can traverse the tree in this order using the
     "exec" option to "B::Terse":

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

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

     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 "="

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     followed by "term" makes a "term", and "term" followed by
     "+" followed by "term" can also make a "term".

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

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

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

     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

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

        We'll see a more tricky example of this when we consider
        Perl's macros below. "POPn" gives you the NV (floating
        point value) of the top SV on the stack: the $x in
        "cos($x)". Then we compute the cosine, and push the
        result back as an NV. The "X" in "XPUSHn" means that the

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        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


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

        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

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        stack. This is so that when we've finished adding items
        to the argument stack, Perl knows how many things we've
        added recently.

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

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

             5  PUTBACK;

        Next we tell Perl to 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.

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

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

     Every line here (apart from the braces, of course) 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.

     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

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     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.

  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 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 ( 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 ( is part of the pmd
     project (  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 () 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

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     "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.

     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

     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.

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     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 or unexpected

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

         ./Configure -d -D optimize=-g

     "-g" is a flag to the C compiler to have it produce
     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

     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

     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

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     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
     copyright message, followed by the prompt.


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

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

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

        Steps through the program a line at a time.

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

        Run until the next breakpoint.

        Run until the end of the current function, then stop

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

        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

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        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.

  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;

     We looked at this bit of code before, and we said that
     "dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into

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     "left" and "right" - let's slightly expand it:

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

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

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

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

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

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

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

         (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

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                     FLAGS = (SCALAR)
                     GV = main::b

     # finish this later #

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

     If you are working with a git clone of the Perl repository,
     you will want to create a branch for your changes. This will
     make creating a proper patch much simpler. See the
     perlrepository for details on how to do this.

     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,

     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;

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     We'll have another string pointer in there:

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

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

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

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

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

     Remember that it has to be "patcopy+1" because the first
     character of the string is the "U" which has been 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))

     needs to become

         if (isSPACE(datumtype)) {

     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

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     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/

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

     so instead of this:

      print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
      print "ok $test\n"; $test++;

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

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

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

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

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

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

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

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     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

  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/

  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 as

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     possibly lots of include files.  This is strictly pumpking

     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 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  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 ;).  Also see
     perlrepository 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

     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/  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.

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

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

        Tests basic issues of how Perl parses and compiles

        Tests for built-in IO functions, including command line

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

        Tests for perl's method resolution order implementations
        (see mro).

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

        Tests for regex related functions or behaviour. (These
        used to live in t/op).

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

        Tests for the core support of Unicode.

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        Windows-specific tests.

        A test suite for the s2p converter.

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

     There are three ways to write a test in the core.
     Test::More, t/ 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
     breaking) from causing basic functionality tests to fail.
     If you write your own test, use the Test Anything Protocol.

     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/ 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

  Special Make Test Targets
     There are various special make targets that can be used to

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     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
     are not available on certain operating systems.

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

         (Not available on Win32)

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

         (Not available on Win32)

         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)

         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,

         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

         (Not available on Win32)

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

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

         (Not available on Win32)

         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

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

     Parallel tests
         The core distribution can now run its regression tests
         in parallel on Unix-like platforms. Instead of running
         "make test", set "TEST_JOBS" in your environment to the
         number of tests to run in parallel, and run "make
         test_harness". On a Bourne-like shell, this can be done

             TEST_JOBS=3 make test_harness  # Run 3 tests in parallel

         An environment variable is used, rather than parallel
         make itself, because TAP::Harness needs to be able to
         schedule individual non-conflicting test scripts itself,
         and there is no standard interface to "make" utilities
         to interact with their job schedulers.

         Note that currently some test scripts may fail when run
         in parallel (most notably "ext/IO/t/io_dir.t"). If
         necessary run just the failing scripts again
         sequentially and see if the failures go away.  =item
         test-notty test_notty

         Sets PERL_SKIP_TTY_TEST to true before running normal

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

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


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

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

     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

     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.

         Run the torture tests as well as the normal set.

         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.

         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

     You can run an individual test by a command similar to

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

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

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

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

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

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

     Other environment variables that may influence tests

         Setting this variable runs all the Net::Ping modules
         tests, otherwise some tests that interact with the
         outside world are skipped.  See perl58delta.

         Setting this variable skips the vrexx.t tests for

         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

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         try your changes with that.  Related to this is the
         difference between "Perl_-less" and "Perl_-ly" APIs, for

           Perl_sv_setiv(aTHX_ ...);

         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

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

         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

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         "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 in

     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
         in perlguts.

         If the function is used across several files, but
         intended only for Perl's internal use (and this should
         be the common case), do not export it to the public API.
         See the discussion about 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

     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

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     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

     o   Casting pointers to integers or casting integers to

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


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

         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

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         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

     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

         o       That a certain field exists in a struct

         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

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     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

         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.

         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

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         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 being changed starting in 5.12 so that these
         characters will be considered to be Latin-1

     o   Mixing #define and #ifdef

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

         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 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 {
             CINNABAR,     /* BAD */

         is not portable.  Leave out the last comma.

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         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

     o   Introducing variables inside for()

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

         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.

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

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

         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
         "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);

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         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

         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();

         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):

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           #ifdef HAS_QUUX
           foo = quux();

         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

         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

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


           #ifdef HAS_QUUX
           foo = quux();

         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 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

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

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         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().

     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 set the environment variable PERL_DESTRUCT_LEVEL to 2.

     For csh-like shells:

         setenv PERL_DESTRUCT_LEVEL 2

     For Bourne-type shells:

         export PERL_DESTRUCT_LEVEL

     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

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     NOTE 4: DynaLoader will not clean up after itself completely
     unless Perl is built with the Configure option

  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

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

     where these arguments mean:

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

         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.

         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

         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.

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     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 and report any memory

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

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

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

         setenv PURIFYOPTIONS "-chain-length=25"

     In Bourne-type shells:

         export PURIFYOPTIONS

     or if you have the "env" utility:

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

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

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


         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

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         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

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

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

     The excellent valgrind tool can be used to find out both
     memory leaks and illegal memory accesses.  As of 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

         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

  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

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     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

     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 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",

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     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.

     If compiled with "-DPERL_MEM_LOG", both memory and SV
     allocations go through logging functions, which is handy for
     breakpoint setting.

     Unless "-DPERL_MEM_LOG_NOIMPL" is also compiled, the logging
     functions read $ENV{PERL_MEM_LOG} to determine whether to
     log the event, and if so how:

         $ENV{PERL_MEM_LOG} =~ /m/           Log all memory ops
         $ENV{PERL_MEM_LOG} =~ /s/           Log all SV ops
         $ENV{PERL_MEM_LOG} =~ /t/           include timestamp in Log
         $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)

     Memory logging is somewhat similar to "-Dm" but is
     independent of "-DDEBUGGING", and at a higher level; all
     uses of Newx(), Renew(), and Safefree() are logged with the
     caller's source code file and line number (and C function
     name, if supported by the C compiler).  In contrast, "-Dm"

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     is directly at the point of "malloc()".  SV logging is

     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.

     Depending on your platform there are various of profiling

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

     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.  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

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     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

     -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

     -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='-pg' && make perl.gprof
         $ ./perl.gprof someprog # creates gmon.out in current directory
         $ gprof ./perl.gprof > out
         $ view out

  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-

     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

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         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

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

     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
     need to re-Configure).

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

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

     In IRIX the following prof options are available:

     -h  Reports the most heavily used lines in descending order

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         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:

         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.)

         Lines sorted in descending order by the number of cycles
         executed in each line.  Useful for finding the hotspot

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

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

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

         Unexecuted procedures.

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

  Miscellaneous tricks
     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:

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           ! 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
             "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

         3.  Turns an entire slab of ops read-write if any op

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             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, which means that we can't yet
         build perl with this enabled.

     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

     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
        perl's activity as well, and probably sooner than you'd

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

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     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

     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 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

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     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.

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

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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