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PERLHACK(1)	       Perl Programmers Reference Guide		   PERLHACK(1)

NAME
       perlhack - How to hack at the Perl internals

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

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

       A searchable archive of the list is at either:

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

       or

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

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

       Over this group of porters presides Larry Wall.	He has the final word
       in what does and does not change in the Perl language.  Various
       releases of Perl are shepherded by a "pumpking", a porter responsible
       for gathering patches, deciding on a patch-by-patch, feature-by-feature
       basis what will and will not go into the release.  For instance,
       Gurusamy Sarathy was the pumpking for the 5.6 release of Perl, and
       Jarkko Hietaniemi was the pumpking for the 5.8 release, and Rafael
       Garcia-Suarez holds the pumpking crown for the 5.10 release.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Is the implementation generic enough to be portable?
	   The worst patches make use of a system-specific features.  It's
	   highly unlikely that non-portable additions to the Perl language
	   will be accepted.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	       http://bugs.perl.org/

       To email the bug system administrators:

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

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

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

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

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

       To report a bug in Perl, use the program perlbug which comes with Perl
       (if you can't get Perl to work, send mail to the address
       perlbug@perl.org or perlbug@perl.com).  Reporting bugs through perlbug
       feeds into the automated bug-tracking system, access to which is
       provided through the web at http://rt.perl.org/rt3/ .  It often pays to
       check the archives of the perl5-porters mailing list to see whether the
       bug you're reporting has been reported before, and if so whether it was
       considered a bug.  See above for the location of the searchable
       archives.

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

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

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

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

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

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

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

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

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

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

       The perl5-porters FAQ
	  This should be available from
	  http://dev.perl.org/perl5/docs/p5p-faq.html .	 It contains hints on
	  reading perl5-porters, information on how perl5-porters works and
	  how Perl development in general works.

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

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

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

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

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

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

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

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

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

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

       Here is a short breakdown of perl's operation:

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

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

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

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

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

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

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

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

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

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

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

	      exitstatus = perl_destruct(my_perl);

	      perl_free(my_perl);

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

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

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

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

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

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

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

	      while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
		  PERL_ASYNC_CHECK();
	      }

	  You may be more comfortable with the Perl version of that:

	      PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};

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

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

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

       Exception handing
	  Perl's exception handing (i.e. "die" etc.) is built on top of the
	  low-level "setjmp()"/"longjmp()" C-library functions. These
	  basically provide a way to capture the current PC and SP registers
	  and later restore them; i.e.	a "longjmp()" continues at the point
	  in code where a previous "setjmp()" was done, with anything further
	  up on the C stack being lost. This is why code should always save
	  values using "SAVE_FOO" rather than in auto variables.

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

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

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

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

	      sub FETCH { eval { die } }

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

	  As a further optimisation, on exit from the eval block in the
	  "FETCH", execution of the code following the block is still carried
	  on in the inner loop.	 When an exception is raised, "docatch"
	  compares the "JMPENV" level of the "CxEVAL" with "PL_top_env" and if
	  they differ, just re-throws the exception. In this way any inner
	  loops get popped.

	  Here's an example.

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

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

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

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

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

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

	  The die pops the first "CxEVAL" off the context stack, sets
	  "PL_restartop" from it, does a JMPENV_JUMP(3), and control returns
	  to the top "docatch". This then starts another third-level runops
	  level, which executes the nextstate, pushmark and die ops on line 4.
	  At the point that the second "pp_die" is called, the C call stack
	  looks exactly like that above, even though we are no longer within
	  an inner eval; this is because of the optimization mentioned
	  earlier. However, the context stack now looks like this, ie with the
	  top CxEVAL popped:

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

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

	      STACK 0: MAIN
		CX 0: BLOCK  =>

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

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

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

	      perl_run
	      main

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

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

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

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

	     % perl -MDevel::Peek -e 'Dump("hello")'
	   1 SV = PV(0xa041450) at 0xa04ecbc
	   2   REFCNT = 1
	   3   FLAGS = (POK,READONLY,pPOK)
	   4   PV = 0xa0484e0 "hello"\0
	   5   CUR = 5
	   6   LEN = 6

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

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

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

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

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

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

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

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

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

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

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

       Line 10 does the actual catenation: the "Move" macro moves a chunk of
       memory around: we move the string "ptr" to the end of the PV - that's
       the start of the PV plus its current length. We're moving "len" bytes
       of type "char". After doing so, we need to tell Perl we've extended the
       string, by altering "CUR" to reflect the new length. "SvEND" is a macro
       which gives us the end of the string, so that needs to be a "\0".

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

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

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

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

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

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

       Let's have a look at how Perl sees "$a = $b + $c":

	    % perl -MO=Terse -e '$a=$b+$c'
	    1  LISTOP (0x8179888) leave
	    2	   OP (0x81798b0) enter
	    3	   COP (0x8179850) nextstate
	    4	   BINOP (0x8179828) sassign
	    5	       BINOP (0x8179800) add [1]
	    6		   UNOP (0x81796e0) null [15]
	    7		       SVOP (0x80fafe0) gvsv  GV (0x80fa4cc) *b
	    8		   UNOP (0x81797e0) null [15]
	    9		       SVOP (0x8179700) gvsv  GV (0x80efeb0) *c
	   10	       UNOP (0x816b4f0) null [15]
	   11		   SVOP (0x816dcf0) gvsv  GV (0x80fa460) *a

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

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

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

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

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

       Now, what's this about?

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

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

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

			       Program
				  |
			      Statement
				  |
				  =
				 / \
				/   \
			       $a   +
				   / \
				 $b   $c

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	       SETi(-TOPi);

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

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

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

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

	  Let's examine the whole implementation, for practice:

	       1  PUSHMARK(SP);

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

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

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

	       5  PUTBACK;

	  Next we tell Perl to update the global stack pointer from our
	  internal variable: "dSP" only gave us a local copy, not a reference
	  to the global.

	       6  ENTER;
	       7  call_method("PUSH", G_SCALAR|G_DISCARD);
	       8  LEAVE;

	  "ENTER" and "LEAVE" localise a block of code - they make sure that
	  all variables are tidied up, everything that has been localised gets
	  its previous value returned, and so on. Think of them as the "{" and
	  "}" of a Perl block.

	  To actually do the magic method call, we have to call a subroutine
	  in Perl space: "call_method" takes care of that, and it's described
	  in perlcall. We call the "PUSH" method in scalar context, and we're
	  going to discard its return value.  The call_method() function
	  removes the top element of the mark stack, so there is nothing for
	  the caller to clean up.

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

	      {
		  local $foo = 42;
		  ...
	      }

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

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

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

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

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

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

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

	   make foo.i

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

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

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

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

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

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

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

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

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

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

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

	 java -Xmx512M ...

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

       The "-Wall" is by default on.

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

       Starting from Perl 5.9.4 the following extra flags are added:

       ·   "-Wendif-labels"

       ·   "-Wextra"

       ·   "-Wdeclaration-after-statement"

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

       ·   "-Wpointer-arith"

       ·   "-Wshadow"

       ·   "-Wstrict-prototypes"

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

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

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

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

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

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

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

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

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

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

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

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

       To fire up the debugger, type

	   gdb ./perl

       Or if you have a core dump:

	   gdb ./perl core

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

	   (gdb)

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

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

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

       step
	  Steps through the program a line at a time.

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

       continue
	  Run until the next breakpoint.

       finish
	  Run until the end of the current function, then stop again.

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

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

	      print SvPV_nolen(sv)

	  but you have to say

	      print Perl_sv_2pv_nolen(sv)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	   (gdb) finish
	   Run till exit from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
	   0x462669 in Perl_pp_add () at pp_hot.c:311
	   311		 dPOPTOPnnrl_ul;

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

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

       # finish this later #

   Patching
       All right, we've now had a look at how to navigate the Perl sources and
       some things you'll need to know when fiddling with them. Let's now get
       on and create a simple patch. Here's something Larry suggested: if a
       "U" is the first active format during a "pack", (for example, "pack
       "U3C8", @stuff") then the resulting string should be treated as UTF-8
       encoded.

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

       [Well, it was in pp.c when this tutorial was written. It has now been
       split off with "pp_unpack" to its own file, pp_pack.c]

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

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

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

       We'll have another string pointer in there:

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

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

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

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

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

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

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

	   if (isSPACE(datumtype))
	       continue;

       needs to become

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

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

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

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

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

       so instead of this:

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

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

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

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

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

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

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

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

	print "1..156\n";

       or this:

	plan( tests => 156 );

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

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

	=item *

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

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

	   diff -ruN old new > patch

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       t/win32/
	  Windows-specific tests.

       t/x2p
	  A test suite for the s2p converter.

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

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

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

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

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

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

       When you say "make test" Perl uses the t/TEST program to run the test
       suite (except under Win32 where it uses t/harness instead.)  All tests
       are run from the t/ directory, not the directory which contains the
       test.  This causes some problems with the tests in lib/, so here's some
       opportunity for some patching.

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

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

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

	   (Not available on Win32)

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

	   (Not available on Win32)

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

	   (Not available on Win32)

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

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

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

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

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

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

	   (Not available on Win32)

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

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

	   (Not available on Win32)

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

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

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

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

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

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

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

       or

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

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

       Using t/harness for testing

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

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

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

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

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

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

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

       You can run an individual test by a command similar to

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

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

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

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

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

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

       Other environment variables that may influence tests

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

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

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

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

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

   Perl environment problems
       ·   Not compiling with threading

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

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

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

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

       ·   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.

       ·   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
	   compatibility).

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

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

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

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

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

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

       ·   Not exporting your new function

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

       ·   Exporting your new function

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

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

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

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

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

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

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

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

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

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

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

       Do not assume an operating system indicates a certain compiler.

       ·   Casting pointers to integers or casting integers to pointers

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

	   or

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

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

       ·   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.

       ·   Assuming sizeof(int) == sizeof(long)

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

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

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

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

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

       ·   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.

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

	   ·	   That a certain field exists in a struct

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

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

	   ·	   That the fields are in a certain order

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

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

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

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

       ·   Assuming the character set is ASCIIish

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

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

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

	   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.

       ·   Assuming the character set is just ASCII

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

       ·   Mixing #define and #ifdef

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

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

       ·   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).

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

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

	   is not portable.  Leave out the last comma.

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

       ·   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.

       ·   Mixing declarations and code

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

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

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

       ·   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.

       ·   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.

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

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

	   Pre-ANSI semantics for that was equivalent to

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

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

       ·   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);

	   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.

       ·   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.

       ·   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.

       ·   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.

       ·   Binding together several statements in a macro

	   Use the macros STMT_START and STMT_END.

	      STMT_START {
		 ...
	      } STMT_END

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

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

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

	     #ifdef HAS_QUUX
	     foo = quux();
	     #endif

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

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

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

	     ...

	     #ifdef HAS_QUUX
	     foo = quux();
	     #endif

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

   Problematic System Interfaces
       ·   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.)

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

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

       ·   Do not use gets()

	   Or we will publicly ridicule you.  Seriously.

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

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

       ·   Do not use sprintf() or vsprintf()

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

EXTERNAL TOOLS FOR DEBUGGING PERL
       Sometimes it helps to use external tools while debugging and testing
       Perl.  This section tries to guide you through using some common
       testing and debugging tools with Perl.  This is meant as a guide to
       interfacing these tools with Perl, not as any kind of guide to the use
       of the tools themselves.

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

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

	   setenv PERL_DESTRUCT_LEVEL 2

       and for Bourne-type shells:

	   PERL_DESTRUCT_LEVEL=2
	   export PERL_DESTRUCT_LEVEL

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

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

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

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

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

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

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

       where these arguments mean:

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

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

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

       -Dusemultiplicity
	   Enabling the multiplicity option allows perl to clean up thoroughly
	   when the interpreter shuts down, which reduces the number of bogus
	   leak reports from Purify.

       Once you've compiled a perl suitable for Purify'ing, then you can just:

	   make pureperl

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

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

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

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

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

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

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

	   setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:

	   PURIFYOPTIONS="..."
	   export PURIFYOPTIONS

       or if you have the "env" utility:

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

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

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

	       DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1

	   to disable Perl's arena memory allocation functions, as well as to
	   force use of memory allocation functions derived from the system
	   malloc.

       USE_MULTI = define
	   Enabling the multiplicity option allows perl to clean up thoroughly
	   when the interpreter shuts down, which reduces the number of bogus
	   leak reports from Purify.

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

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

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

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

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

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

       Valgrind also provides a cachegrind tool, invoked on perl as:

	   VG_OPTS=--tool=cachegrind make test.valgrind

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

       To get valgrind and for more information see

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

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

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

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

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

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

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

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

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

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

       If, at the end of a run you get the message N scalars leaked, you can
       recompile with "-DDEBUG_LEAKING_SCALARS", which will cause the
       addresses of all those leaked SVs to be dumped along with details as to
       where each SV was originally allocated. This information is also
       displayed by Devel::Peek. Note that the extra details recorded with
       each SV increases memory usage, so it shouldn't be used in production
       environments. It also converts "new_SV()" from a macro into a real
       function, so you can use your favourite debugger to discover where
       those pesky SVs were allocated.

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

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

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

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

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

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

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

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

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

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

   Gprof Profiling
       gprof is a profiling tool available in many UNIX platforms, it uses
       statistical time-sampling.

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

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

       -a  Suppress statically defined functions from the profile.

       -b  Suppress the verbose descriptions in the profile.

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

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

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

       -z  Display routines that have zero usage.

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

       quick hint:

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

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

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

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

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

	   gcov sv.c

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

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

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

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

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

       quick hint:

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

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

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

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

       To display the results you use the prof utility.	 The exact incantation
       depends on your operating system, "prof perl.Counts" in IRIX, and "prof
       -pixie -all -L. perl" in Tru64.

       In IRIX the following prof options are available:

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

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

       In Tru64 the following options are available:

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

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

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

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

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

       -z[ero]
	   Unexecuted procedures.

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

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

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

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

	   the following two lines:

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

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

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

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

	   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.

       ·   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.

       ·   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 decreased.

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

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

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

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

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

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

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

       ·  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.

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

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

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

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

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

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

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

       ·   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.

       ·   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.

       ·   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 Tolkien, retaining the original
       spelling and punctuation and using the same format the rest of the
       quotes are in.  Indirect and oblique is just fine; remember, it's a
       metaphor, so being meta is, after all, what it's for.

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

SEE ALSO
       perlrepository

perl v5.10.1			  2009-08-03			   PERLHACK(1)
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