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

       perlhack - How to hack at the Perl internals

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

       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:




       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 theyre 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 plat
       forms.  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, its
       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 Gar
       cia-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.Mer
       ijn Brand took over.

       Larry sees Perl development along the lines of the US government:
       theres the Legislature (the porters), the Executive branch (the pump
       kings), 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.	Larrys 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.  Its 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
       persons 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 havent been written anywhere in stone, but one approximation

	    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 avail
	   able to implement your (possibly good) idea.

       Backwards compatibility
	   Its 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, its
	   highly unlikely to be added.

       Is the feature generic enough?
	   Is this something that only the submitter wants added to the lan
	   guage, 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 havent been

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

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

       Is the implementation tested?
	   Patches which change behaviour (fixing bugs or introducing new fea
	   tures) 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
	   havent unwittingly broken the behaviour the patch implements? And
	   without tests, how can the patchs author be confident that his/her
	   hard work put into the patch wont 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 submit
	   ting 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 Theres 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 mans essential addition
	   is another mans 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 fer
	   vently 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 youre on the list, you might hear the word "core" bandied around.
       It refers to the standard distribution.	"Hacking on the core" means
       youre changing the C source code to the Perl interpreter.  "A core
       module" is one that ships with Perl.

       Keeping in sync

       The source code to the Perl interpreter, in its different versions, is
       kept in a repository managed by a revision control system ( which is
       currently the Perforce program, see http://perforce.com/ ).  The pump
       kings and a few others have access to the repository to check in
       changes.  Periodically the pumpking for the development version of Perl
       will release a new version, so the rest of the porters can see whats
       changed.  The current state of the main trunk of repository, and
       patches that describe the individual changes that have happened since
       the last public release are available at this location:


       If youre looking for a particular change, or a change that affected a
       particular set of files, you may find the Perl Repository Browser use


       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 isnt 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 per
       petual state of evolution.  You should expect it to be very buggy.  Do
       not use it for any purpose other than testing and development.

       Keeping in sync with the most recent branch can be done in several
       ways, but the most convenient and reliable way is using rsync, avail
       able at ftp://rsync.samba.org/pub/rsync/ .  (You can also get the most
       recent branch by FTP.)

       If you choose to keep in sync using rsync, there are two approaches to
       doing so:

       rsyncing the source tree
	   Presuming you are in the directory where your perl source resides
	   and you have rsync installed and available, you can "upgrade" to
	   the bleadperl using:

	    # rsync -avz rsync://public.activestate.com/perl-current/ .

	   This takes care of updating every single item in the source tree to
	   the latest applied patch level, creating files that are new (to
	   your distribution) and setting date/time stamps of existing files
	   to reflect the bleadperl status.

	   Note that this will not delete any files that were in . before
	   the rsync. Once you are sure that the rsync is running correctly,
	   run it with the --delete and the --dry-run options like this:

	    # rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .

	   This will simulate an rsync run that also deletes files not present
	   in the bleadperl master copy. Observe the results from this run
	   closely. If you are sure that the actual run would delete no files
	   precious to you, you could remove the --dry-run option.

	   You can than check what patch was the latest that was applied by
	   looking in the file .patch, which will show the number of the lat
	   est patch.

	   If you have more than one machine to keep in sync, and not all of
	   them have access to the WAN (so you are not able to rsync all the
	   source trees to the real source), there are some ways to get around
	   this problem.

	   Using rsync over the LAN
	       Set up a local rsync server which makes the rsynced source tree
	       available to the LAN and sync the other machines against this

	       From http://rsync.samba.org/README.html :

		  "Rsync uses rsh or ssh for communication. It does not need to be
		   setuid and requires no special privileges for installation.	It
		   does not require an inetd entry or a daemon.  You must, however,
		   have a working rsh or ssh system.  Using ssh is recommended for
		   its security features."

	   Using pushing over the NFS
	       Having the other systems mounted over the NFS, you can take an
	       active pushing approach by checking the just updated tree
	       against the other not-yet synced trees. An example would be

		 #!/usr/bin/perl -w

		 use strict;
		 use File::Copy;

		 my %MF = map {
		     $1 => [ (stat $1)[2, 7, 9] ];     # mode, size, mtime
		     } cat MANIFEST;

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

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

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

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

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

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

	   This makes sure the latest available patch is downloaded to your
	   patch directory.

	   Its then up to you to apply these patches, using something like

	    # last=ls -t *.gz | sed q
	    # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
	    # find . -name *.gz -newer $last -exec gzcat {} \; >blead.patch
	    # cd ../perl-current
	    # patch -p1 -N <../perl-current-diffs/blead.patch

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

       Why rsync the source tree

       Its easier to rsync the source tree
	   Since you dont have to apply the patches yourself, you are sure
	   all files in the source tree are in the right state.

       Its more reliable
	   While both the rsync-able source and patch areas are automatically
	   updated every few minutes, keep in mind that applying patches may
	   sometimes mean careful hand-holding, especially if your version of
	   the "patch" program does not understand how to deal with new files,
	   files with 8-bit characters, or files without trailing newlines.

       Why rsync the patches

       Its easier to rsync the patches
	   If you have more than one machine that you want to keep in track
	   with bleadperl, its easier to rsync the patches only once and then
	   apply them to all the source trees on the different machines.

	   In case you try to keep in pace on 5 different machines, for which
	   only one of them has access to the WAN, rsyncing all the source
	   trees should than be done 5 times over the NFS. Having rsynced the
	   patches only once, I can apply them to all the source trees auto
	   matically. Need you say more ;-)

       Its a good reference
	   If you do not only like to have the most recent development branch,
	   but also like to fix bugs, or extend features, you want to dive
	   into the sources. If you are a seasoned perl core diver, you dont
	   need no manuals, tips, roadmaps, perlguts.pod or other aids to find
	   your way around. But if you are a starter, the patches may help you
	   in finding where you should start and how to change the bits that
	   bug you.

	   The file Changes is updated on occasions the pumpking sees as his
	   own little sync points. On those occasions, he releases a tar-ball
	   of the current source tree (i.e. perl@7582.tar.gz), which will be
	   an excellent point to start with when choosing to use the rsync
	   the patches scheme. Starting with perl@7582, which means a set of
	   source files on which the latest applied patch is number 7582, you
	   apply all succeeding patches available from then on (7583, 7584,

	   You can use the patches later as a kind of search archive.

	   Finding a start point
	       If you want to fix/change the behaviour of function/feature
	       Foo, just scan the patches for patches that mention Foo either
	       in the subject, the comments, or the body of the fix. A good
	       chance the patch shows you the files that are affected by that
	       patch which are very likely to be the starting point of your
	       journey into the guts of perl.

	   Finding how to fix a bug
	       If youve found where the function/feature Foo misbehaves, but
	       you dont know how to fix it (but you do know the change you
	       want to make), you can, again, peruse the patches for similar
	       changes and look how others apply the fix.

	   Finding the source of misbehaviour
	       When you keep in sync with bleadperl, the pumpking would love
	       to see that the community efforts really work. So after each of
	       his sync points, you are to make test to check if everything
	       is still in working order. If it is, you do make ok, which
	       will send an OK report to perlbug@perl.org. (If you do not have
	       access to a mailer from the system you just finished success
	       fully make test, you can do make okfile, which creates the
	       file "perl.ok", which you can than take to your favourite
	       mailer and mail yourself).

	       But of course, as always, things will not always lead to a suc
	       cess path, and one or more test do not pass the make test.
	       Before sending in a bug report (using make nok or make nok
	       file), check the mailing list if someone else has reported the
	       bug already and if so, confirm it by replying to that message.
	       If not, you might want to trace the source of that misbehaviour
	       before sending in the bug, which will help all the other
	       porters in finding the solution.

	       Here the saved patches come in very handy. You can check the
	       list of patches to see which patch changed what file and what
	       change caused the misbehaviour. If you note that in the bug
	       report, it saves the one trying to solve it, looking for that

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

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

       Working with the source

       Because you cannot use the Perforce client, you cannot easily generate
       diffs against the repository, nor will merges occur when you update via
       rsync.  If you edit a file locally and then rsync against the latest
       source, changes made in the remote copy will overwrite your local ver

       The best way to deal with this is to maintain a tree of symlinks to the
       rsyncd source.  Then, when you want to edit a file, you remove the
       symlink, copy the real file into the other tree, and edit it.  You can
       then diff your edited file against the original to generate a patch,
       and you can safely update the original tree.

       Perls Configure script can generate this tree of symlinks for you.
       The following example assumes that you have used rsync to pull a copy
       of the Perl source into the perl-rsync directory.  In the directory
       above that one, you can execute the following commands:

	 mkdir perl-dev
	 cd perl-dev
	 ../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g"

       This will start the Perl configuration process.	After a few prompts,
       you should see something like this:

	 Symbolic links are supported.

	 Checking how to test for symbolic links...
	 Your builtin test -h may be broken.
	 Trying external /usr/bin/test -h.
	 You can test for symbolic links with /usr/bin/test -h.

	 Creating the symbolic links...
	 (First creating the subdirectories...)
	 (Then creating the symlinks...)

       The specifics may vary based on your operating system, of course.
       After you see this, you can abort the Configure script, and you will
       see that the directory you are in has a tree of symlinks to the perl-
       rsync directories and files.

       If you plan to do a lot of work with the Perl source, here are some
       Bourne shell script functions that can make your life easier:

	   function edit {
	       if [ -L $1 ]; then
		   mv $1 $1.orig
		       cp $1.orig $1
		       vi $1
		   /bin/vi $1

	   function unedit {
	       if [ -L $1.orig ]; then
		   rm $1
		       mv $1.orig $1

       Replace "vi" with your favorite flavor of editor.

       Here is another function which will quickly generate a patch for the
       files which have been edited in your symlink tree:

	   mkpatchorig() {
	       local diffopts
		   for f in find . -name *.orig | sed s,^\./,,
			   case echo $f | sed s,.orig$,,;s,.*\.,, in
			       c)   diffopts=-p ;;
		       pod) diffopts=-F^= ;;
		       *)   diffopts= ;;
			   diff -du $diffopts $f echo $f | sed s,.orig$,,

       This function produces patches which include enough context to make
       your changes obvious.  This makes it easier for the Perl pumpking(s) to
       review them when you send them to the perl5-porters list, and that
       means theyre more likely to get applied.

       This function assumed a GNU diff, and may require some tweaking for
       other diff variants.

       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:


       The bugtracker mechanism for perl5 bugs in particular is at:


       To email the bug system administrators:


       Submitting patches

       Always submit patches to perl5-porters@perl.org.  If youre patching a
       core module and theres 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.	Either use the
       diff program (available in source code form from
       ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans makepatch (available
       from CPAN/authors/id/JV/).  Unified diffs are preferred, but context
       diffs are accepted.  Do not send RCS-style diffs or diffs without con
       text lines.  More information is given in the Porting/patching.pod file
       in the Perl source distribution.  Please patch against the latest
       development version (e.g., if youre fixing a bug in the 5.005 track,
       patch against the latest 5.005_5x version).  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 "Writ
       ing a test".

       To report a bug in Perl, use the program perlbug which comes with Perl
       (if you cant get Perl to work, send mail to the address perl
       bug@perl.org or perlbug@perl.com).  Reporting bugs through perlbug
       feeds into the automated bug-tracking system, access to which is pro
       vided through the web at http://bugs.perl.org/ .  It often pays to
       check the archives of the perl5-porters mailing list to see whether the
       bug youre reporting has been reported before, and if so whether it was
       considered a bug.  See above for the location of the searchable

       The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers
       who test CPAN modules on a variety of platforms.  Perl Smokers (
       http://archives.develooper.com/daily-build@perl.org/ ) automatically
       tests Perl source releases on platforms with various configurations.
       Both efforts welcome volunteers.

       Its a good idea to read and lurk for a while before chipping in.  That
       way youll get to see the dynamic of the conversations, learn the per
       sonalities 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 unsub
       scribe, send mail to perl5-porters-unsubscribe@perl.org.

       To hack on the Perl guts, youll need to read the following things:

	  This is of paramount importance, since its the documentation of
	  what goes where in the Perl source. Read it over a couple of times
	  and it might start to make sense - dont worry if it doesnt yet,
	  because the best way to study it is to read it in conjunction with
	  poking at Perl source, and well do that later on.

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

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

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

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

       The perl5-porters FAQ
	  This should be available from http://simon-cozens.org/writ
	  ings/p5p-faq ; alternatively, you can get the FAQ emailed to you by
	  sending mail to "perl5-porters-faq@perl.org". It contains hints on
	  reading perl5-porters, information on how perl5-porters works and
	  how Perl development in general works.

       Finding Your Way Around

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

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

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

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

	  The configure process is the way we make Perl portable across the
	  myriad of operating systems it supports. Responsibility for the con
	  figure, build and installation process, as well as the overall
	  portability of the core code rests with the configure pumpkin - oth
	  ers 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 isnt included in the core distribution.)

	  And of course, theres the core of the Perl interpreter itself.
	  Lets have a look at that in a little more detail.

       Before we leave looking at the layout, though, dont forget that MANI
       FEST contains not only the file names in the Perl distribution, but
       short descriptions of whats 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

       Here is a short breakdown of perls operation:

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

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

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

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

	  Line 4 calls a function in perl.c to allocate memory for a Perl
	  interpreter. Its 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 Perls system abstraction, which well
	  see later: "PerlMem_malloc" is either your systems "malloc", or
	  Perls own "malloc" as defined in malloc.c if you selected that
	  option at configure time.

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

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

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

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

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

	  "yyparse", the parser, lives in perly.c, although youre 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 func
	  tion, 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
	  isnt much like other computer languages; its highly context sensi
	  tive at times, it can be tricky to work out what sort of token some
	  thing is, or where a token ends. As such, theres a lot of interplay
	  between the tokeniser and the parser, which can get pretty frighten
	  ing if youre 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 rou
	  tines which construct and link together the various operations are
	  to be found in op.c, and will be examined later.

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

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

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

	  You may be more comfortable with the Perl version of 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 theyre 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 Perls
	  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
	  Perls exception handing (i.e. "die" etc) is built on top of the
	  low-level "setjmp()"/"longjmp()" C-library functions. These basi
	  cally 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 val
	  ues 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 call
	  ing "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 cur
	  rent 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" com
	  pares 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.

	  Heres an example.

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

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

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

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

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

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

	  The die pops the first "CxEVAL" off the context stack, sets
	  "PL_restartop" from it, does a JMPENV_JUMP(3), and 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 ear
	  lier. However, the context stack now looks like this, ie with the
	  top CxEVAL popped:

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

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

	      STACK 0: MAIN
		CX 0: BLOCK  =>

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

	      Perl_runops      # second loop
	      Perl_runops      # first loop

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


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

       Internal Variable Types

       You should by now have had a look at perlguts, which tells you about
       Perls 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 com
       pletely internal to Perl. The symbol table, for instance, is an ordi
       nary Perl hash. Your code is represented by an SV as its read into the
       parser; any program files you call are opened via ordinary Perl file
       handles, and so on.

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

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

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

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

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

       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 youve 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.

       Lets 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 accommo
       date the old string, the new string and the null terminator. If "LEN"
       isnt big enough, "SvGROW" will reallocate space for us.

       Now, if "junk" is the same as the string were 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 - thats
       the start of the PV plus its current length. Were moving "len" bytes
       of type "char". After doing so, we need to tell Perl weve 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 weve changed the PV, any IV or NV
       values will no longer be valid: if we have "$a=10; $a.="6";" we dont
       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, lets go on and look at how the op tree is con

       Op Trees

       First, what is the op tree, anyway? The op tree is the parsed represen
       tation of your program, as we saw in our section on parsing, and its
       the sequence of operations that Perl goes through to execute your pro
       gram, 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 opera
       tions 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.

       Lets 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

       Lets start in the middle, at line 4. This is a BINOP, a binary opera
       tor, 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

       Line 10 is the null op: this does exactly nothing. What is that doing
       there? If you see the null op, its 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, its eas
       ier just to replace the redundant operation with the null op. Origi
       nally, 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 weve just
       seen: we have the "add" op ("pp_add" also in pp_hot.c) add together two

       Now, whats 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.

       Thats how Perl parsed the program, from top to bottom:

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

       However, its 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. Lets 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 youre not used to reading BNF grammars, this is how it works:
       Youre 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 cant 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 sym
       bols" 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 "=" fol
       lowed 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 "=", youll do the code in
       line 2. If you see "+", youll do the code in line 4. Its 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 backref
       erences. $$ is the op returned from this reduction. So, we call "new
       BINOP" to create a new binary operator. The first parameter to "new
       BINOP", a function in op.c, is the op type. Its an addition operator,
       so we want the type to be "ADDOP". We could specify this directly, but
       its right there as the second token in the input, so we use $2. The
       second parameter is the ops flags: 0 means "nothing special". Then the
       things to add: the left and right hand side of our expression, in
       scalar context.


       When perl executes something like "addop", how does it pass on its
       results to the next op? The answer is, through the use of stacks. Perl
       has a number of stacks to store things its currently working on, and
       well 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);

	  Well see a more tricky example of this when we consider Perls
	  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 cant be neces
	  sary here, because we know theres room for one more item on the
	  stack, since weve 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:


	  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 descrip
	  tion of the macros used in stack manipulation.

       Mark stack
	  I say "your portion of the stack" above because PP code doesnt nec
	  essarily get the whole stack to itself: if your function calls
	  another function, youll 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 loca
	  tions 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. Heres how the tied "push" is implemented;
	  see "av_push" in av.c:

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

	  The lines which concern the mark stack are the first, fifth and last
	  lines: they save away, restore and remove the current position of
	  the argument stack.

	  Lets 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 weve finished adding items to the argument
	  stack, Perl knows how many things weve added recently.

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

	  Were 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 thats exactly what we have here - the tied
	  object, retrieved with "SvTIED_obj", and the value, the SV "val".

	       5  PUTBACK;

	  Next we tell Perl to make the change to the global stack pointer:
	  "dSP" only gave us a local copy, not a reference to the global.

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

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

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

	       9  POPSTACK;

	  Finally, we remove the value we placed on the mark stack, since we
	  dont need it any more.

       Save stack
	  C doesnt have a concept of local scope, so perl provides one. Weve
	  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 youll notice about the Perl source is that its full of
       macros. Some have called the pervasive use of macros the hardest thing
       to understand, others find it adds to clarity. Lets 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 subrou
       tine 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 opera
       tor 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.	Dont be scared by the

       Poking at Perl

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

	   ./Configure -d -D optimize=-g

       "-g" is a flag to the C compiler to have it produce debugging informa
       tion which will allow us to step through a running program.  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" doesnt help you, its time to step
       through perls execution with a source-level debugger.

	 Well use "gdb" for our examples here; the principles will apply to
	  any debugger, but check the manual of the one youre using.

       To fire up the debugger, type

	   gdb ./perl

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


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

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

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

	  Steps through the program a line at a time.

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

	  Run until the next breakpoint.

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

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

	  Execute the given C code and print its results. WARNING: Perl makes
	  heavy use of macros, and gdb does not necessarily support macros
	  (see later "gdb macro support").  Youll have to substitute them
	  yourself, or to invoke cpp on the source code files (see "The .i
	  Targets") So, for instance, you cant 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 wont recur
       sively apply those macros for you.

       gdb macro support

       Recent versions of gdb have fairly good macro support, but in order to
       use it youll need to compile perl with macro definitions included in
       the debugging information.  Using gcc version 3.1, this means configur
       ing 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 cant get at from Perl.
       Lets take an example. Well use the "$a = $b + $c" we used before, but
       give it a bit of context: "$b = "6XXXX"; $c = 2.3;". Wheres a good
       place to stop and poke around?

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

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

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

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

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

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

       We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul"
       arranges for two "NV"s to be placed into "left" and "right" - lets
       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 doesnt remove it. We then use "SvNV" to get the NV from
       "leftsv" in the same way as before - yes, "POPn" uses "SvNV".

       Since we dont have an NV for $b, well have to use "sv_2nv" to convert
       it. If we step again, well find ourselves there:

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

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

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

       We know were going to get 6 from this, so lets 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". Thisll give us simi
       lar output to B::Debug.

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

       # finish this later #


       All right, weve now had a look at how to navigate the Perl sources and
       some things youll need to know when fiddling with them. Lets now get
       on and create a simple patch. Heres 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

       How do we prepare to fix this up? First we locate the code in question
       - the "pack" happens at runtime, so its going to be in one of the pp
       files. Sure enough, "pp_pack" is in pp.c. Since were going to be
       altering this file, lets 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 lets 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 were still at the start of the string. So, heres 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;

       Well 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, well set "patcopy" to be the start
       of "pat":

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

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

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

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

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

	   if (isSPACE(datumtype))

       needs to become

	   if (isSPACE(datumtype)) {

       OK. Thats the C part done. Now we must do two additional things before
       this patch is ready to go: weve 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 doesnt 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, well test that the "U" does indeed create Unicode strings.

       t/op/pack.t has a sensible ok() function, but if it didnt 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 well 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 well test that we dont 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 isnt unicode" );

       Mustnt 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 lets describe the change weve just made. The relevant
       place is pod/perlfunc.pod; again, we make a copy, and then well insert
       this text in the description of "pack":

	=item *

	If the pattern begins with a C, 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, and the bytes that follow will be interpreted as
	Unicode characters. If you dont want this to happen, you can begin your
	pattern with C (or anything else) to force Perl not to UTF-8 encode your
	string, and then follow this with a C somewhere in your pattern.

       All done. Now lets create the patch. Porting/patching.pod tells us
       that if were 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 weve 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

       Patching a core module

       This works just like patching anything else, with an extra considera
       tion.  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 Port

       Adding a new function to the core

       If, as part of a patch to fix a bug, or just because you have an espe
       cially 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 Func
       tions" in perlguts for more details.

       The location of any new code is also an important consideration.  Dont
       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 prop
       erly), 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 exam
       ple, 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 appro
       priate 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 warn
       ings 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 con
       ventions 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 dont ;).  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 mem
       ory 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 dont 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

       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/.

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

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

	  Tests basic issues of how Perl parses and compiles itself.

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

	  The old home for the module tests, you shouldnt put anything new in
	  here.  There are still some bits and pieces hanging around in here
	  that need to be moved.  Perhaps you could move them?	Thanks!

	  Tests for perls built in functions that dont fit into any of the
	  other directories.

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

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

	  Tests for the core support of Unicode.

	  Windows-specific tests.

	  A test suite for the s2p converter.

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

       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 youre
       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 dont 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 its 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 heres 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 sys

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

	   (Not available on Win32)

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

	   (Not available on Win32)

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

	   (Not available on Win32)

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

       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 test

       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

       test.torture torturetest
	   Run all the usual tests and some extra tests.  As of Perl 5.8.0 the
	   only extra tests are Abigails JAPHs, t/japh/abigail.t.

	   You can also run the torture test with t/harness by giving "-tor
	   ture" 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)

	   Run the test suite with the t/harness controlling program, instead
	   of t/TEST. t/harness is more sophisticated, and uses the Test::Har
	   ness 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 doesnt 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 Win32s "test" target you may use the TEST_SWITCHES and
	   TEST_FILES environment variables to control the behaviour of t/har
	   ness.  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


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

       (if you dont 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 outbut.

	   Run the torture tests as well as the normal set.

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

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

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

	   is set to 2 if it isnt set already (see "PERL_DESTRUCT_LEVEL")

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

	   if set, tells to skip the tests that need a terminal. Its actually
	   set automatically by the Makefile, but can also be forced artifi
	   cially by running make test_notty.

       Sometimes it helps to use external tools while debugging and testing
       Perl.  This section tries to guide you through using some common test
       ing and debugging tools with Perl.  This is meant as a guide to inter
       facing 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 dont
       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 environ
       ment 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:


       and for Bourne-type shells:


       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.

       Rational Softwares Purify

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

       Purify on Unix

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

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

       where these arguments mean:

	   Disables Perls arena memory allocation functions, as well as forc
	   ing use of memory allocation functions derived from the system mal

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

	   Disable Perls malloc so that Purify can more closely monitor allo
	   cations and leaks.  Using Perls malloc will make Purify report
	   most leaks in the "potential" leaks category.

	   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 youve compiled a perl suitable for Purifying, then you can just:

	   make pureperl

       which creates a binary named pureperl that has been Purifyed.  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 Purifyed 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 dont
       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

	   setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:


       or if you have the "env" utility:

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

       Purify on NT

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

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


	   to disable Perls arena memory allocation functions, as well as to
	   force use of memory allocation functions derived from the system

       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 Perls malloc so that Purify can more closely monitor allo
	   cations and leaks.  Using Perls malloc will make Purify report
	   most leaks in the "potential" leaks category.

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

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

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

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


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

       As system libraries (most notably glibc) are also 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


       Compaqs/Digitals/HPs 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 exe
       cutable to create a new executable called perl.third.  When the instru
       mented 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 avail
       able in the Compaq "Tru64 UNIX Programmers 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.  There
       fore you should used the Porting/thirdclean script to cleanup the
       *.3log files.

       There are also leaks that for given certain definition of a leak,
       arent.  See "PERL_DESTRUCT_LEVEL" for more information.


       If you want to run any of the tests yourself manually using e.g.  val
       grind, or the pureperl or perl.third executables, please note that by
       default perl does not explicitly cleanup all the memory it has allo
       cated (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 dont
       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 equiv
       alent of setting this variable to the value 1.)

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


       Depending on your platform there are various of profiling Perl.

       There are two commonly used techniques of profiling executables: sta
       tistical 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 fre
       quently, in the scale of milliseconds) imposes an additional overhead
       that may skew the results.  The first problem can be alleviated by run
       ning the code for longer (in general this is a good idea for profil
       ing), 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 begin
       ning and exited only at the end.  For example, a conditional jump
       starts a basic block.  Basic block profiling usually works by instru
       menting 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 invok
       ing the make target "perl.gprof"  (What is required is that Perl must
       be compiled using the "-pg" flag, you may need to re-Configure).  Run
       ning 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 for
       mats, see your own local documentation of gprof.

       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 com
       piled 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

       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


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


       Pixie Profiling

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

       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 con
       tains the counts for the basic block for that particular program execu

       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:

	   Procedures sorted in descending order by the number of cycles exe
	   cuted in each procedure.  Useful for finding the hotspot proce
	   dures.  (This is the default option.)

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

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

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

	   The compiler emitted code for these lines, but the code was unexe

	   Unexecuted procedures.

       For further information, see your systems manual pages for pixie and

       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 SVs 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 but
	   ton -> New Display -> Edit Menu

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

	  If you see in a debugger a memory area mysteriously full of
	   0xabababab, you may be seeing the effect of the Poison() macro, see


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

       Id 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,

	 Subscribe to perl5-porters, follow the patches and try and under
	  stand them; dont be afraid to ask if theres a portion youre 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 theyre making.

	 Do read the README associated with your operating system, e.g.
	  README.aix on the IBM AIX OS. Dont hesitate to supply patches to
	  that README if you find anything missing or changed over a new OS

	 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! Youll probably
	  get to understand not just your chosen area but a much wider range
	  of perls activity as well, and probably sooner than youd think.

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

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

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

perl v5.8.8			  2008-04-25			   PERLHACK(1)

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