source: trunk/essentials/dev-lang/perl/pod/perlhack.pod@ 3397

=head1 NAME

perlhack - How to hack at the Perl internals

=head1 DESCRIPTION

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

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

A searchable archive of the list is at either:

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

or

    http://archive.develooper.com/[email protected]/

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

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

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

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

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

=over 4

=item 1

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

=item 2

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

=back

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:

=over 4

=item 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.

=item 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.

=item Backwards compatibility

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

=item Could it be a module instead?

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

=item 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.

=item Does it potentially introduce new bugs?

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

=item 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.

=item 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.

=item Is the implementation generic enough to be portable?

The worst patches make use of a system-specific features.  It's highly
unlikely that nonportable additions to the Perl language will be
accepted.

=item 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?

=item 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.

=item Is there another way to do it?

Larry said "Although the Perl Slogan is I<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.

=item Does it create too much work?

Work for the pumpking, work for Perl programmers, work for module
authors, ...  Perl is supposed to be easy.

=item 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.

=back

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.

=head2 Keeping in sync

The source code to the Perl interpreter, in its different versions, is
kept in a repository managed by a revision control system ( which is
currently the Perforce program, see http://perforce.com/ ).  The
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:

    http://public.activestate.com/pub/apc/
    ftp://public.activestate.com/pub/apc/

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

    http://public.activestate.com/cgi-bin/perlbrowse

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

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

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

Keeping in sync with the most recent branch can be done in several ways,
but the most convenient and reliable way is using B<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:

=over 4

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

You can than check what patch was the latest that was applied by
looking in the file B<.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.

=over 4

=item 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."

=item 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 {
      m/(\S+)/;
      $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";
          next;
          }
      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).

=back

=item 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 C<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 with

 # 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=`ls -t *.gz | sed q`
 # rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
 # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
 # cd ../perl-current
 # patch -p1 -N <../perl-current-diffs/blead.patch

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

=back

=head2 Why rsync the source tree

=over 4

=item 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.

=item 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 C<patch> program does not understand how to deal with new files,
files with 8-bit characters, or files without trailing newlines.

=back

=head2 Why rsync the patches

=over 4

=item 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 ;-)

=item It's a good reference

If you do not only like to have the most recent development branch,
but also like to B<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 B<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. [email protected]), 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.

=over 4

=item 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.

=item Finding how to fix a bug

If you've found I<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.

=item Finding the source of misbehaviour

When you keep in sync with bleadperl, the pumpking would love to
I<see> that the community efforts really work. So after each of his
sync points, you are to 'make test' to check if everything is still
in working order. If it is, you do 'make ok', which will send an OK
report to [email protected]. (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 C<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 B<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 point.

=back

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 convenience, reliability and ease and rsync the patches
for reference.

=back

=head2 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 I<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 generate a
patch, and you can safely update the original tree.

Perl's F<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 F<perl-rsync> directory.  In the directory
above that one, you can execute the following commands:

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

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

  Symbolic links are supported.

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

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

The specifics may vary based on your operating system, of course.
After you see this, you can abort the F<Configure> script, and you
will see that the directory you are in has a tree of symlinks to the
F<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
        else
            /bin/vi $1
                fi
    }

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

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,^\./,,`
                do
                    case `echo $f | sed 's,.orig$,,;s,.*\.,,'` in
                        c)   diffopts=-p ;;
                pod) diffopts='-F^=' ;;
                *)   diffopts= ;;
                esac
                    diff -du $diffopts $f `echo $f | sed 's,.orig$,,'`
                    done
    }

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.

=head2 Perlbug administration

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

        http://rt.perl.org

The bugtracker mechanism for B<perl5> bugs in particular is at:

        http://bugs6.perl.org/perlbug

To email the bug system administrators:

        "perlbug-admin" <[email protected]>


=head2 Submitting patches

Always submit patches to I<[email protected]>.  If you're
patching a core module and there's an author listed, send the author a
copy (see L<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' I<makepatch>
(available from I<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
I<Porting/patching.pod> file in the Perl source distribution.  Please
patch against the latest B<development> version (e.g., if you're
fixing a bug in the 5.005 track, patch against the latest 5.005_5x
version).  Only patches that survive the heat of the development
branch get applied to maintenance versions.

Your patch should update the documentation and test suite.  See
L<Writing a test>.

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

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

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

If after all this you still think you want to join the perl5-porters
mailing list, send mail to I<[email protected]>.  To
unsubscribe, send mail to I<[email protected]>.

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

=over 3

=item L<perlguts>

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

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/ )

=item L<perlxstut> and L<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.

=item L<perlapi>

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

=item F<Porting/pumpkin.pod>

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

=item The perl5-porters FAQ

This should be available from http://simon-cozens.org/writings/p5p-faq ;
alternatively, you can get the FAQ emailed to you by sending mail to
C<[email protected]>. It contains hints on reading perl5-porters,
information on how perl5-porters works and how Perl development in general
works.

=back

=head2 Finding Your Way Around

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

=over 3

=item Core modules

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

=item Tests

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

=item Documentation

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

=item Configure

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

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

=item Interpreter

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

=back

Before we leave looking at the layout, though, don't forget that
F<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

=head2 Elements of the interpreter

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

Here is a short breakdown of perl's operation:

=over 3

=item Startup

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

First, F<perlmain.c> allocates some memory and constructs a Perl
interpreter:

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

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

Line 4 calls a function in F<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: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
own C<malloc> as defined in F<malloc.c> if you selected that option at
configure time.

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

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

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


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

=item Parsing

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

C<yyparse>, the parser, lives in F<perly.c>, although you're better off
reading the original YACC input in F<perly.y>. (Yes, Virginia, there
B<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 C<yylex>, and that and its
associated routines can be found in F<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 F<op.c>, and will be examined later.

=item Optimization

Now the parsing stage is complete, and the finished tree represents
the operations that the Perl interpreter needs to perform to execute our
program. Next, Perl does a dry run over the tree looking for
optimisations: constant expressions such as C<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 C<$foo>,
instead of grabbing the glob C<*foo> and looking at the scalar
component, the optimizer fiddles the op tree to use a function which
directly looks up the scalar in question. The main optimizer is C<peep>
in F<op.c>, and many ops have their own optimizing functions.

=item Running

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

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

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

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

Well, maybe not. Anyway, each op contains a function pointer, which
stipulates the function which will actually carry out the operation.
This function will return the next op in the sequence - this allows for
things like C<if> which choose the next op dynamically at run time.
The C<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: F<pp_hot.c> contains the "hot" code, which is most
often used and highly optimized, F<pp_sys.c> contains all the
system-specific functions, F<pp_ctl.c> contains the functions which
implement control structures (C<if>, C<while> and the like) and F<pp.c>
contains everything else. These are, if you like, the C code for Perl's
built-in functions and operators.

Note that each C<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,
C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or C<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
C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB> or C<CxEVAL>,
retrieves the return op from it, and returns it.