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X11R7.4 - man page for perlhack (x11r4 section 1)

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 porters.

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

       A searchable archive of the list is at either:




       List subscribers (the porters themselves) come in several flavours.  Some are quiet curi-
       ous lurkers, who rarely pitch in and instead watch the ongoing development to ensure
       they're forewarned of new changes or features in Perl.  Some are representatives of ven-
       dors, 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, it's your usual mix of technical people.

       Over this group of porters presides Larry Wall.	He has the final word in what does and
       does not change in the Perl language.  Various releases of Perl are shepherded by a "pump-
       king", 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 pump-
       king for the 5.8 release, and Rafael Garcia-Suarez holds the pumpking crown for the 5.10

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

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

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

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

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

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

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

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

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

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

       Backwards compatibility
	   It's a cardinal sin to break existing Perl programs.  New warnings are con-
	   tentious--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 inter-
	   preter if you want to implement really complicated things.  If it can be done in a
	   module instead of in the core, it's highly unlikely to be added.

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

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

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

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

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

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

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

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

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

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

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

       Keeping in sync

       The source code to the Perl interpreter, in its different versions, is kept in a reposi-
       tory managed by a revision control system ( which is currently the Perforce program, see
       http://perforce.com/ ).	The pumpkings 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 what's changed.  The current
       state of the main trunk of repository, and patches that describe the individual changes
       that have happened since the last public release are available at this location:


       If you're looking for a particular change, or a change that affected a particular set of
       files, you may find the Perl Repository Browser useful:


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

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

       Needless to say, the source code in perl-current is usually in a perpetual state of evolu-
       tion.  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 con-
       venient and reliable way is using rsync, available 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:

       rsync'ing 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'

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

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

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

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

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

		 #!/usr/bin/perl -w

		 use strict;
		 use File::Copy;

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

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

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

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

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

	   It's then up to you to apply these patches, using something like

	    # last="`cat ../perl-current/.patch`.gz"
	    # 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
	   Konig to have better control over the patching process.

       Why rsync the source tree

       It's easier to rsync the source tree
	   Since you don't have to apply the patches yourself, you are sure all files in the
	   source tree are in the right state.

       It's 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

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

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

       It's a good reference
	   If you do not only like to have the most recent 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 don't 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 you've found where the function/feature Foo misbehaves, but you don't know how
	       to fix it (but you do know the change you want to make), you can, 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 com-
	       munity 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 successfully '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 success path, and one
	       or more test do not pass the 'make test'. Before sending in a bug report (using
	       'make nok' or 'make nokfile'), check the mailing list if someone else has reported
	       the bug already and if so, confirm it by replying to that message. If not, you
	       might want to trace the source of that misbehaviour before sending in the bug,
	       which will help all the other porters in finding the solution.

	       Here the saved patches come in very handy. You can check the list of patches to
	       see which patch changed what file and what change caused the 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 perl's bugtron to
	   find more information about discussions and ramblings on posted bugs.

	   If you want to get the best of both worlds, rsync both the source tree for conve-
	   nience, 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 versions!

       The best way to deal with this is to maintain a tree of symlinks to the rsync'd 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 gen-
       erate a patch, and you can safely update the original tree.

       Perl's 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 some-
       thing 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
		   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 they're 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 admin-
       istrator, and close any bugs you can get your sticky mitts on:


       To email the bug system administrators:

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

       Submitting patches

       Always submit patches to perl5-porters@perl.org.  If you're patching a core module and
       there's an author listed, send the author a copy (see "Patching a core module").  This
       lets other porters review your patch, which catches a surprising number of errors in
       patches.  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 context lines.  More information is given in the
       Porting/patching.pod file in the Perl source distribution.  Please patch against the lat-
       est development version. (e.g., even if you're fixing a bug in the 5.8 track, patch
       against the latest development version rsynced from rsync://public.actives-
       tate.com/perl-current/ )

       If changes are accepted, they are applied to the development branch. Then the 5.8 pumpking
       decides which of those patches is to be backported to the maint branch.	Only patches that
       survive the heat of the development branch get applied to maintenance versions.

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

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

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

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

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

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

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

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

	  You might also want to look at Gisle Aas's illustrated perlguts - there's 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 portion of the guts that actually executes a
	  Perl program. It's a lot gentler to learn those techniques from simple examples and
	  explanation than from the core itself.

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

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

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

       Finding Your Way Around

       Perl maintenance can be split into a number of areas, and certain people (pumpkins) will
       have responsibility for each area. These areas sometimes correspond to files or directo-
       ries 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 configure, build and installation process,
	  as well as the overall portability of the core code rests with the configure pumpkin -
	  others help out with individual operating systems.

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

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

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

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

       Elements of the interpreter

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

       Here is a short breakdown of perl's operation:

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

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

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

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

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

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

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

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

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

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

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

	      exitstatus = perl_destruct(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. We'll
	  see what one of those looks like later. Strictly speaking, there's three things going
	  on here.

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

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

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

	  Now the parsing stage is complete, and the finished tree represents the operations that
	  the Perl interpreter needs to perform to execute our program. Next, Perl does a dry run
	  over the tree looking for optimisations: constant expressions such as "3 + 4" will be
	  computed now, and the optimizer will also see if any multiple operations can be
	  replaced with a single one. For instance, to fetch the variable $foo, instead of grab-
	  bing 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 opti-
	  mizer is "peep" in op.c, and many ops have their own optimizing functions.

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

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

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

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

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

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

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

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

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

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

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

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

	      sub FETCH { eval { die } }

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

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

	  Here's an example.

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

	  To run this code, "perl_run" is called, which does a "JMPENV_PUSH" then enters a runops
	  loop. This loop executes the eval and tie ops on line 1, with the eval pushing a "CxE-
	  VAL" 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 earlier. However, the context stack now looks like this, ie
	  with the top CxEVAL popped:

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

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

	      STACK 0: MAIN
		CX 0: BLOCK  =>

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

	      Perl_runops      # second loop
	      Perl_runops      # first loop

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


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

       Internal Variable Types

       You should by now have had a look at perlguts, which tells you about Perl's internal vari-
       able 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 con-
       stants in the code, as well as some structures completely internal to Perl. The symbol ta-
       ble, for instance, is an ordinary Perl hash. Your code is represented by an SV as it's
       read into the parser; any program files you call are opened via ordinary Perl filehandles,
       and so on.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Op Trees

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

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

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

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

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

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

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

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

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

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

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

       Now, what's this about?

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

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

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

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

       However, it's impossible to perform the operations in this order: you have to find the
       values of $b and $c before you add them together, for instance. So, the other thread that
       runs through the op tree is the execution order: each op has a field "op_next" which
       points to the next op to be run, so following these pointers tells us how perl 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 val-
       ues of $b and $c, and add them together.  Find $a, and assign one to the other. Then

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

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

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

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

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

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

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

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


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

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

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

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

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


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

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

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

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

	  Let's examine the whole implementation, for practice:

	       1  PUSHMARK(SP);

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

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

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

	       5  PUTBACK;

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

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

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

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

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

		  local $foo = 42;

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

       Millions of Macros

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

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

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

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

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

       The .i Targets

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

	   make foo.i

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

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

       lint, splint

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

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

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


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

       cpd (cut-and-paste detector)

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

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

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

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

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

	 java -Xmx512M ...

       gcc warnings

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

       The "-Wall" is by default on.

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

       Starting from Perl 5.9.4 the following extra flags are added:

       o   "-Wendif-labels"

       o   "-Wextra"

       o   "-Wdeclaration-after-statement"

       The following flags would be nice to have but they would first need their own Augean sta-

       o   "-Wpointer-arith"

       o   "-Wshadow"

       o   "-Wstrict-prototypes"

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

       Warnings of other C compilers

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


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

       Poking at Perl

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

	   ./Configure -d -D optimize=-g

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

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

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

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

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

       Using a source-level debugger

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

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

       To fire up the debugger, type

	   gdb ./perl

       Or if you have a core dump:

	   gdb ./perl core

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


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

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

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

	  Steps through the program a line at a time.

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

	  Run until the next breakpoint.

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

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

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

	      print SvPV_nolen(sv)

	  but you have to say

	      print Perl_sv_2pv_nolen(sv)

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

       gdb macro support

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

       Dumping Perl Data Structures

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       # finish this later #


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

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

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

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

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

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

       We'll have another string pointer in there:

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

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

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

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

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

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

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

	   if (isSPACE(datumtype))

       needs to become

	   if (isSPACE(datumtype)) {

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

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

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

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

       so instead of this:

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

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

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

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

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

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

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

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

	print "1..156\n";

       or this:

	plan( tests => 156 );

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

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

	=item *

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

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

	   diff -ruN old new > patch

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

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

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

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

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

       Patching a core module

       This works just like patching anything else, with an extra consideration.  Many core mod-
       ules 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 mod-
       ule maintainer keep the CPAN version in sync with the core version without constantly
       scanning p5p.

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

       Adding a new function to the core

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

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

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

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

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

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

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

       Writing a test

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

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

       For modules, the test file is right next to the module itself.  lib/strict.t tests
       lib/strict.pm.  This is a recent innovation, so there are some snags (and it would be won-
       derful 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", subroutines, 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 shouldn't put anything new in here.  There are
	  still some bits and pieces hanging around in here that need to be moved.  Perhaps you
	  could move them?  Thanks!

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

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

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

	  Tests for the core support of Unicode.

	  Windows-specific tests.

	  A test suite for the s2p converter.

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

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

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

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

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

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

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

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

       Special Make Test Targets

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

	   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 they're specifically fixed, of course).

	   (Not available on Win32)

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

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

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

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

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

       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

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

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

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

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

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

       Running tests by hand

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

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


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

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

       Using t/harness for testing

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

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

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

       -v  Run the tests under verbose mode so you can see what tests were run, and debug 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 we're running this test part of the perl core test suite.  This is use-
	   ful for modules that have a dual life on CPAN.

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

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

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

       Other environment variables that may influence tests

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

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

	   This sets a variable in op/numconvert.t.

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

       Common problems when patching Perl source code

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

       Perl environment problems

       o   Not compiling with threading

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

	     Perl_sv_setiv(aTHX_ ...);

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

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

       o   Not compiling with -DDEBUGGING

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

       o   Introducing (non-read-only) globals

	   Do not introduce any modifiable globals, truly global or file static.  They are bad
	   form and complicate multithreading and other forms of concurrency.  The right way is
	   to introduce them as new interpreter variables, see intrpvar.h (at the very end for
	   binary compatibility).

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

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

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

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

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

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

       o   Not exporting your new function

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

       o   Exporting your new function

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

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

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

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

       Portability problems

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

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

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

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

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

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

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

       Do not assume an operating system indicates a certain compiler.

       o   Casting pointers to integers or casting integers to pointers

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


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

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

       o   Casting between data function pointers and data pointers

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

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

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

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

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

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

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

       o   Lvalue casts

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

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

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

	   o	   That a certain field exists in a struct

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

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

	   o	   That the fields are in a certain order

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

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

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

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

       o   Mixing #define and #ifdef

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

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

       o   Adding stuff after #endif or #else

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

	   The #endif and #else cannot portably have anything non-comment after them.  If you
	   want to document what is going (which is a good idea especially if the branches are
	   long), use (C) comments:

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

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

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

	     enum color {
	       CINNABAR,     /* BAD */

	   is not portable.  Leave out the last comma.

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

       o   Using //-comments

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

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

       o   Mixing declarations and code

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

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

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

       o   Introducing variables inside for()

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

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

       o   Mixing signed char pointers with unsigned char pointers

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

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

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

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

	   Pre-ANSI semantics for that was equivalent to

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

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

       o   Using printf formats for non-basic C types

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

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

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

	   The problem here is that Uid_t might be not only not "int"-wide but it might also be
	   unsigned, in which case large uids would be printed as negative values.

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

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

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

	      Uid_t_f /* Uid_t in decimal */

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

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

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

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

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

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

       o   Blindly using variadic macros

	   gcc has had them for a while with its own syntax, and C99 brought them with a stan-
	   dardized syntax.  Don't use the former, and use the latter only if the HAS_C99_VARI-
	   ADIC_MACROS is defined.

       o   Blindly passing va_list

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

       o   Using gcc statement expressions

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

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

       o   Binding together several statements

	   Use the macros STMT_START and STMT_END.

	      STMT_START {
	      } STMT_END

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

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

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

	     #ifdef HAS_QUUX
	     foo = quux();

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

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

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


	     #ifdef HAS_QUUX
	     foo = quux();

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

       Problematic System Interfaces

       o   malloc(0), realloc(0), calloc(0, 0) are non-portable.  To be portable allocate at
	   least one byte.  (In general you should rarely need to work at this low level, but
	   instead use the various malloc wrappers.)

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

       Security problems

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

       o   Do not use gets()

	   Or we will publicly ridicule you.  Seriously.

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

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

       o   Do not use sprintf() or vsprintf()

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

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

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

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


       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.

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

       Rational Software's Purify

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

       Purify on Unix

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

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

       where these arguments mean:

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

	   Adds debugging information so that you see the exact source statements where the prob-
	   lem occurs.	Without this flag, all you will see is the source filename of where the
	   error occurred.

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

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

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

	   make pureperl

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

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

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

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

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

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

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

	   setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:


       or if you have the "env" utility:

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

       Purify on NT

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

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


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

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

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

       CFG = Debug
	   Adds debugging information so that you see the exact source statements where the prob-
	   lem 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


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

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

	   VG_OPTS=--tool=cachegrind make test.valgrind

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

       To get valgrind and for more information see


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

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

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

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

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

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


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

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

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

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

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


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

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


       Depending on your platform there are various of profiling Perl.

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

       The first method takes periodically samples of the CPU program counter, and since the pro-
       gram 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 period-
       ically interrupting the program (this is usually done rather frequently, in the scale of
       milliseconds) imposes an additional overhead that may skew the results.	The first problem
       can be alleviated by running the code for longer (in general this is a good idea for pro-
       filing), 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 sec-
       tions of code that are entered only in the beginning and exited only at the end.  For
       example, a conditional jump starts a basic block.  Basic block profiling usually works by
       instrumenting the code by adding enter basic block #nnnn book-keeping code to the gener-
       ated 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-sam-

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

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

       -a  Suppress statically defined functions from the profile.

       -b  Suppress the verbose descriptions in the profile.

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

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

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

       -z  Display routines that have zero usage.

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

       quick hint:

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

       GCC gcov Profiling

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

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

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

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

	   gcov sv.c

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

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


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


       quick hint:

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

       Pixie Profiling

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

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

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

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

       In IRIX the following prof options are available:

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

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

       In Tru64 the following options are available:

	   Procedures sorted in descending order by the number of cycles executed in each proce-
	   dure.  Useful for finding the hotspot procedures.  (This is the default option.)

	   Lines sorted in descending order by the number of cycles executed in each line.  Use-
	   ful 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 procedures.

	   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 unexecuted.

	   Unexecuted procedures.

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

       Miscellaneous tricks

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

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

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

	   the following two lines:

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

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

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

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

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

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

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

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

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

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

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

       o  Do read the README associated with your operating system, e.g. README.aix on the IBM
	  AIX OS. Don't hesitate to supply patches to that README if you find anything missing or
	  changed over a new OS release.

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

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

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

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

perl v5.8.9				    2007-11-17				      PERLHACK(1)

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