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:
http://www.xray.mpe.mpg.de/mailing-
lists/perl5-porters/
or
http://archive.develooper.com/perl5-porters@perl.org/
List subscribers (the porters themselves) come in several
flavours. Some are quiet curious lurkers, who rarely
pitch in and instead watch the ongoing development to
ensure they're forewarned of new changes or features in
Perl. Some are representatives of vendors, who are there
to make sure that Perl continues to compile and work on
their platforms. Some patch any reported bug that they
know how to fix, some are actively patching their pet area
(threads, Win32, the regexp engine), while others seem to
do nothing but complain. In other words, it's your usual
mix of technical people.
Over this group of porters presides Larry Wall. He has
the final word in what does and does not change in the
Perl language. Various releases of Perl are shepherded by
a ``pumpking'', a porter responsible for gathering
patches, deciding on a patch-by-patch feature-by-feature
basis what will and will not go into the release. For
instance, Gurusamy Sarathy was the pumpking for the 5.6
release of Perl, and Jarkko Hietaniemi is the pumpking for
the 5.8 release, and Hugo van der Sanden will be the pumpking
for the 5.10 release.
In addition, various people are pumpkings for different
things. For instance, Andy Dougherty and Jarkko
Hietaniemi share the Configure pumpkin.
Larry sees Perl development along the lines of the US government:
there's the Legislature (the porters), the Executive
branch (the pumpkings), and the Supreme Court
(Larry). The legislature can discuss and submit patches
to the executive branch all they like, but the executive
branch is free to veto them. Rarely, the Supreme Court
will side with the executive branch over the legislature,
or the legislature over the executive branch. Mostly,
however, the legislature and the executive branch are supposed
to get along and work out their differences without
impeachment or court cases.
You might sometimes see reference to Rule 1 and Rule 2.
Larry's power as Supreme Court is expressed in The Rules:
1 Larry is always by definition right about how Perl
should behave. This means he has final veto power on
the core functionality.
2 Larry is allowed to change his mind about any matter
at a later date, regardless of whether he previously
invoked Rule 1.
Got that? Larry is always right, even when he was wrong.
It's rare to see either Rule exercised, but they are often
alluded to.
New features and extensions to the language are contentious,
because the criteria used by the pumpkings,
Larry, and other porters to decide which features should
be implemented and incorporated are not codified in a few
small design goals as with some other languages. Instead,
the heuristics are flexible and often difficult to fathom.
Here is one person's list, roughly in decreasing order of
importance, of heuristics that new features have to be
weighed against:
Does concept match the general goals of Perl?
These haven't been written anywhere in stone, but one
approximation is:
1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate
Perl everywhere.
5. Either assimilate new technologies, or build
bridges to them.
Where is the implementation?
All the talk in the world is useless without an implementation.
In almost every case, the person or people
who argue for a new feature will be expected to be the
ones who implement it. Porters capable of coding new
features have their own agendas, and are not available
to implement your (possibly good) idea.
Backwards compatibility
It's a cardinal sin to break existing Perl programs.
New warnings are contentious--some say that a program
that emits warnings is not broken, while others say it
is. Adding keywords has the potential to break programs,
changing the meaning of existing token
sequences or functions might break programs.
Could it be a module instead?
Perl 5 has extension mechanisms, modules and XS,
specifically to avoid the need to keep changing the
Perl interpreter. You can write modules that export
functions, you can give those functions prototypes so
they can be called like built-in functions, you can
even write XS code to mess with the runtime data
structures of the Perl interpreter if you want to
implement really complicated things. If it can be
done in a module instead of in the core, it's highly
unlikely to be added.
Is the feature generic enough?
Is this something that only the submitter wants added
to the language, or would it be broadly useful? Sometimes,
instead of adding a feature with a tight focus,
the porters might decide to wait until someone implements
the more generalized feature. For instance,
instead of implementing a ``delayed evaluation'' feature,
the porters are waiting for a macro system that
would permit delayed evaluation and much more.
Does it potentially introduce new bugs?
Radical rewrites of large chunks of the Perl interpreter
have the potential to introduce new bugs. The
smaller and more localized the change, the better.
Does it preclude other desirable features?
A patch is likely to be rejected if it closes off
future avenues of development. For instance, a patch
that placed a true and final interpretation on prototypes
is likely to be rejected because there are still
options for the future of prototypes that haven't been
addressed.
Is the implementation robust?
Good patches (tight code, complete, correct) stand
more chance of going in. Sloppy or incorrect patches
might be placed on the back burner until the pumpking
has time to fix, or might be discarded altogether
without further notice.
Is the implementation generic enough to be portable?
The worst patches make use of a system-specific features.
It's highly unlikely that nonportable 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 [Toc] [Back]
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/gsar/APC/
ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
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:
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 not use it for any purpose other
than testing and development.
Keeping in sync with the most recent branch can be done in
several ways, but the most convenient and reliable way is
using rsync, 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://ftp.linux.activestate.com/perlcurrent/ .
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://ftp.linux.activestate.com/perl-current/ .
This will simulate an rsync run that also deletes
files not present in the bleadperl master copy.
Observe the results from this run closely. If you are
sure that the actual run would delete no files precious
to you, you could remove the '--dry-run' option.
You can than check what patch was the latest that was
applied by looking in the file .patch, which will show
the number of the 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 {
m/()/;
$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
$host0;
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 %9d0,
$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 with
# rsync -avz rsync://ftp.linux.activestate.com/perlcurrent-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://ftp.linux.activestate.com/perlcurrent-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 [Toc] [Back]
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 community efforts
really work. So after each of his sync points, you
are to 'make test' to check if everything is still
in working order. If it is, you do 'make ok',
which will send an OK report to perlbug@perl.org.
(If you do not have access to a mailer from the
system you just finished 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
point.
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.
Working with the source [Toc] [Back]
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 generate 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 something like this:
Symbolic links are supported.
Checking how to test for symbolic links...
Your builtin 'test -h' may be broken.
Trying external '/usr/bin/test -h'.
You can test for symbolic links with '/usr/bin/test -h'.
Creating the symbolic links...
(First creating the subdirectories...)
(Then creating the symlinks...)
The specifics may vary based on your operating system, of
course. After you see this, you can abort the Configure
script, and you will see that the directory you are in has
a tree of symlinks to the perl-rsync directories and
files.
If you plan to do a lot of work with the Perl source, here
are some Bourne shell script functions that can make your
life easier:
function edit {
if [ -L $1 ]; then
mv $1 $1.orig
cp $1.orig $1
vi $1
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.
Perlbug administration [Toc] [Back]
There is a single remote administrative interface for modifying
bug status, category, open issues etc. using the RT
bugtracker system, maintained by Robert Spier. Become an
administrator, and close any bugs you can get your sticky
mitts on:
http://rt.perl.org
The bugtracker mechanism for perl5 bugs in particular is
at:
http://bugs6.perl.org/perlbug
To email the bug system administrators:
"perlbug-admin" <perlbug-admin@perl.org>
Submitting patches [Toc] [Back]
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
latest 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 "Writing a test".
To report a bug in Perl, use the program perlbug which
comes with Perl (if you can't get Perl to work, send mail
to the address perlbug@perl.org or perlbug@perl.com).
Reporting bugs through perlbug feeds into the automated
bug-tracking system, access to which is provided through
the web at http://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.devel-
ooper.com/daily-build@perl.org/ ) 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
perl5-porters-subscribe@perl.org. To unsubscribe, send
mail to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following
things:
perlguts
This is of paramount importance, since it's the documentation
of what goes where in the Perl source. Read
it over a couple of times and it might start to make
sense - don't worry if it doesn't yet, because the best
way to study it is to read it in conjunction with poking
at Perl source, and we'll do that later on.
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.
perlapi
The documentation for the Perl API explains what some
of the internal functions do, as well as the many
macros used in the source.
Porting/pumpkin.pod
This is a collection of words of wisdom for a Perl
porter; some of it is only useful to the pumpkin
holder, but most of it applies to anyone wanting to go
about Perl development.
The perl5-porters FAQ
This should be available from http://simon-cozens.org/writings/p5p-faq
; alternatively, you can get
the FAQ emailed to you by sending mail to
"perl5-porters-faq@perl.org". It contains hints on
reading perl5-porters, information on how perl5-porters
works and how Perl development in general works.
Finding Your Way Around [Toc] [Back]
Perl maintenance can be split into a number of areas, and
certain people (pumpkins) will have responsibility for
each area. These areas sometimes correspond to files or
directories in the source kit. Among the areas are:
Core modules
Modules shipped as part of the Perl core live in the
lib/ and ext/ subdirectories: lib/ is for the pure-Perl
modules, and ext/ contains the core XS modules.
Tests
There are tests for nearly all the modules, built-ins
and major bits of functionality. Test files all have a
.t suffix. Module tests live in the lib/ and ext/
directories next to the module being tested. Others
live in t/. See "Writing a test"
Documentation
Documentation maintenance includes looking after everything
in the pod/ directory, (as well as contributing
new documentation) and the documentation to the modules
in core.
Configure
The configure process is the way we make Perl portable
across the myriad of operating systems it supports.
Responsibility for the configure, build and installation
process, as well as the overall portability of the
core code rests with the configure pumpkin - others
help out with individual operating systems.
The files involved are the operating system directories,
(win32/, os2/, vms/ and so on) the shell scripts
which generate config.h and Makefile, as well as the
metaconfig files which generate Configure. (metaconfig
isn't included in the core distribution.)
Interpreter
And of course, there's the core of the Perl interpreter
itself. Let's have a look at that in a little more
detail.
Before we leave looking at the layout, though, don't forget
that MANIFEST contains not only the file names in the
Perl distribution, but short descriptions of what's in
them, too. For an overview of the important files, try
this:
perl -lne 'print if /^[^]+.[ch]' MANIFEST
Elements of the interpreter [Toc] [Back]
The work of the interpreter has two main stages: compiling
the code into the internal representation, or bytecode,
and then executing it. "Compiled code" in perlguts
explains exactly how the compilation stage happens.
Here is a short breakdown of perl's operation:
Startup
The action begins in perlmain.c. (or miniperlmain.c for
miniperl) This is very high-level code, enough to fit
on a single screen, and it resembles the code found in
perlembed; most of the real action takes place in
perl.c
First, 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
"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; this
sets up all the special variables that Perl needs, the
stacks, and so on.
Now we pass Perl the command line options, and tell it
to go:
exitstatus = perl_parse(my_perl, xs_init, argc,
argv, (char **)NULL);
if (!exitstatus) {
exitstatus = perl_run(my_perl);
}
"perl_parse" is actually a wrapper around
"S_parse_body", as defined in perl.c, which processes
the command line options, sets up any statically linked
XS modules, opens the program and calls "yyparse" to
parse it.
Parsing
The aim of this stage is to take the Perl source, and
turn it into an op tree. We'll see what one of those
looks like later. Strictly speaking, there's three
things going on here.
"yyparse", the parser, lives in perly.c, although
you're better off reading the original YACC input in
perly.y. (Yes, Virginia, there is a YACC grammar for
Perl!) The job of the parser is to take your code and
`understand' it, splitting it into sentences, deciding
which operands go with which operators and so on.
The parser is nobly assisted by the lexer, which chunks
up your input into tokens, and decides what type of
thing each token is: a variable name, an operator, a
bareword, a subroutine, a core function, and so on.
The main point of entry to the lexer is "yylex", and
that and its associated routines can be found in
toke.c. Perl isn't much like other computer languages;
it's highly context sensitive at times, it can be
tricky to work out what sort of token something is, or
where a token ends. As such, there's a lot of interplay
between the tokeniser and the parser, which can get
pretty frightening if you're not used to it.
As the parser understands a Perl program, it builds up
a tree of operations for the interpreter to perform
during execution. The routines which construct and link
together the various operations are to be found in
op.c, and will be examined later.
Optimization
Now the parsing stage is complete, and the finished
tree represents the operations that the Perl interpreter
needs to perform to execute our program. Next,
Perl does a dry run over the tree looking for optimisations:
constant expressions such as "3 + 4" will be
computed now, and the optimizer will also see if any
multiple operations can be replaced with a single one.
For instance, to fetch the variable $foo, instead of
grabbing the glob *foo and looking at the scalar component,
the optimizer fiddles the op tree to use a function
which directly looks up the scalar in question.
The main optimizer is "peep" in op.c, and many ops have
their own optimizing functions.
Running
Now we're finally ready to go: we have compiled Perl
byte code, and all that's left to do is run it. The
actual execution is done by the "runops_standard" function
in run.c; more specifically, it's done by these
three innocent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX)))
{
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of
that:
PERL_ASYNC_CHECK() while $Perl::op =
&{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function
pointer, which stipulates the function which will actually
carry out the operation. This function will
return the next op in the sequence - this allows for
things like "if" which choose the next op dynamically
at run time. The "PERL_ASYNC_CHECK" makes sure that
things like signals interrupt execution if required.
The actual functions called are known as PP code, and
they're spread between four files: pp_hot.c contains
the `hot' code, which is most often used and highly
optimized, pp_sys.c contains all the system-specific
functions, pp_ctl.c contains the functions which implement
control structures ("if", "while" and the like)
and pp.c contains everything else. These are, if you
like, the C code for Perl's built-in functions and
operators.
Internal Variable Types [Toc] [Back]
You should by now have had a look at perlguts, which tells
you about Perl's internal variable types: SVs, HVs, AVs
and the rest. If not, do that now.
These variables are used not only to represent Perl-space
variables, but also any constants in the code, as well as
some structures completely internal to Perl. The symbol
table, for instance, is an ordinary Perl hash. Your code
is represented by an SV as it's read into the parser; any
program files you call are opened via ordinary Perl filehandles,
and so on.
The core Devel::Peek module lets us examine SVs from a
Perl program. Let's see, for instance, how Perl treats the
constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"
5 CUR = 5
6 LEN = 6
Reading "Devel::Peek" output takes a bit of practise, so
let's go through it line by line.
Line 1 tells us we're looking at an SV which lives at
0xa04ecbc in memory. SVs themselves are very simple structures,
but they contain a pointer to a more complex structure.
In this case, it's a PV, a structure which holds a
string value, at location 0xa041450. Line 2 is the reference
count; there are no other references to this data, so
it's 1.
Line 3 are the flags for this SV - it's OK to use it as a
PV, it's a read-only SV (because it's a constant) and the
data is a PV internally. Next we've got the contents of
the string, starting at location 0xa0484e0.
Line 5 gives us the current length of the string - note
that this does not include the null terminator. Line 6 is
not the length of the string, but the length of the currently
allocated buffer; as the string grows, Perl automatically
extends the available storage via a routine
called "SvGROW".
You can get at any of these quantities from C very easily;
just add "Sv" to the name of the field shown in the snippet,
and you've got a macro which will return the value:
"SvCUR(sv)" returns the current length of the string,
"SvREFCOUNT(sv)" returns the reference count, "SvPV(sv,
len)" returns the string itself with its length, and so
on. More macros to manipulate these properties can be
found in perlguts.
Let's take an example of manipulating a PV, from "sv_catpvn",
in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register
const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = ' ';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length
"len" onto the end of the PV stored in "sv". The first
thing we do in line 6 is make sure that the SV has a valid
PV, by calling the "SvPV_force" macro to force a PV. As a
side effect, "tlen" gets set to the current value of the
PV, and the PV itself is returned to "junk".
In line 7, we make sure that the SV will have enough room
to accommodate the old string, the new string and the null
terminator. If "LEN" isn't big enough, "SvGROW" will reallocate
space for us.
Now, if "junk" is the same as the string we're trying to
add, we can grab the string directly from the SV; "SvPVX"
is the address of the PV in the SV.
Line 10 does the actual catenation: the "Move" macro moves
a chunk of memory around: we move the string "ptr" to the
end of the PV - that's the start of the PV plus its current
length. We're moving "len" bytes of type "char".
After doing so, we need to tell Perl we've extended the
string, by altering "CUR" to reflect the new length.
"SvEND" is a macro which gives us the end of the string,
so that needs to be a " ".
Line 13 manipulates the flags; since we've changed the PV,
any IV or NV values will no longer be valid: if we have
"$a=10; $a.="6";" we don't want to use the old IV of 10.
"SvPOK_only_utf8" is a special UTF-8-aware version of
"SvPOK_only", a macro which turns off the IOK and NOK
flags and turns on POK. The final "SvTAINT" is a macro
which launders tainted data if taint mode is turned on.
AVs and HVs are more complicated, but SVs are by far the
most common variable type being thrown around. Having seen
something of how we manipulate these, let's go on and look
at how the op tree is constructed.
Op Trees [Toc] [Back]
First, what is the op tree, anyway? The op tree is the
parsed representation of your program, as we saw in our
section on parsing, and it's the sequence of operations
that Perl goes through to execute your program, as we saw
in "Running".
An op is a fundamental operation that Perl can perform:
all the built-in functions and operators are ops, and
there are a series of ops which deal with concepts the
interpreter needs internally - entering and leaving a
block, ending a statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that
there are two "routes" through it, two orders in which you
can traverse the tree. First, parse order reflects how
the parser understood the code, and secondly, execution
order tells perl what order to perform the operations in.
The easiest way to examine the op tree is to stop Perl
after it has finished parsing, and get it to dump out the
tree. This is exactly what the compiler backends B::Terse,
B::Concise and B::Debug do.
Let's have a look at how Perl sees "$a = $b + $c":
% perl -MO=Terse -e '$a=$b+$c'
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV
(0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV
(0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460)
*a
Let's start in the middle, at line 4. This is a BINOP, a
binary operator, which is at location 0x8179828. The specific
operator in question is "sassign" - scalar assignment
- and you can find the code which implements it in
the function "pp_sassign" in pp_hot.c. As a binary operator,
it has two children: the add operator, providing the
result of "$b+$c", is uppermost on line 5, and the left
hand side is on line 10.
Line 10 is the null op: this does exactly nothing. What is
that doing there? If you see the null op, it's a sign that
something has been optimized away after parsing. As we
mentioned in "Optimization", the optimization stage sometimes
converts two operations into one, for example when
fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers,
it's easier just to replace the redundant operation
with the null op. Originally, the tree would have looked
like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the "a" entry from the main symbol table,
and then look at the scalar component of it: "gvsv"
("pp_gvsv" into pp_hot.c) happens to do both these things.
The right hand side, starting at line 5 is similar to what
we've just seen: we have the "add" op ("pp_add" also in
pp_hot.c) add together two "gvsv"s.
Now, what's this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
"enter" and "leave" are scoping ops, and their job is to
perform any housekeeping every time you enter and leave a
block: lexical variables are tidied up, unreferenced variables
are destroyed, and so on. Every program will have
those first three lines: "leave" is a list, and its children
are all the statements in the block. Statements are
delimited by "nextstate", so a block is a collection of
"nextstate" ops, with the ops to be performed for each
statement being the children of "nextstate". "enter" is a
single op which functions as a marker.
That's how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/
/ $a +
/
$b $c
However, it's impossible to perform the operations in this
order: you have to find the values of $b and $c before you
add them together, for instance. So, the other thread that
runs through the op tree is the execution order: each op
has a field "op_next" which points to the next op to be
run, so following these pointers tells us how perl executes
the code. We can traverse the tree in this order
using the "exec" option to "B::Terse":
% perl -MO=Terse,exec -e '$a=$b+$c'
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block,
start a statement. Get the values of $b and $c, and add
them together. Find $a, and assign one to the other. Then
leave.
The way Perl builds up these op trees in the parsing process
can be unravelled by examining perly.y, the YACC
grammar. Let's take the piece we need to construct the
tree for "$a = $b + $c"
1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1,
$2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1),
scalar($3)); }
If you're not used to reading BNF grammars, this is how it
works: You're fed certain things by the tokeniser, which
generally end up in upper case. Here, "ADDOP", is provided
when the tokeniser sees "+" in your code. "ASSIGNOP" is
provided when "=" is used for assigning. These are `terminal
symbols', because you can't get any simpler than them.
The grammar, lines one and three of the snippet above,
tells you how to build up more complex forms. These complex
forms, `non-terminal symbols' are generally placed in
lower case. "term" here is a non-terminal symbol, representing
a single expression.
The grammar gives you the following rule: you can make the
thing on the left of the colon if you see all the things
on the right in sequence. This is called a "reduction",
and the aim of parsing is to completely reduce the input.
There are several different ways you can perform a reduction,
separated by vertical bars: so, "term" followed by
"=" followed by "term" makes a "term", and "term" followed
by "+" followed by "term" can also make a "term".
So, if you see two terms with an "=" or "+", between them,
you can turn them into a single expression. When you do
this, you execute the code in the block on the next line:
if you see "=", you'll do the code in line 2. If you see
"+", you'll do the code in line 4. It's this code which
contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1),
scalar($3)); }
What this does is creates a new binary op, and feeds it a
number of variables. The variables refer to the tokens: $1
is the first token in the input, $2 the second, and so on
- think regular expression backreferences. $$ is the op
returned from this reduction. So, we call "newBINOP" to
create a new binary operator. The first parameter to "newBINOP",
a function in op.c, is the op type. It's an addition
operator, so we want the type to be "ADDOP". We could
specify this directly, but it's right there as the second
token in the input, so we use $2. The second parameter is
the op's flags: 0 means `nothing special'. Then the things
to add: the left and right hand side of our expression, in
scalar context.
Stacks [Toc] [Back]
When perl executes something like "addop", how does it
pass on its results to the next op? The answer is, through
the use of stacks. Perl has a number of stacks to store
things it's currently working on, and we'll look at the
three most important ones here.
Argument stack
Arguments are passed to PP code and returned from PP
code using the argument stack, "ST". The typical way to
handle arguments is to pop them off the stack, deal
with them how you wish, and then push the result back
onto the stack. This is how, for instance, the cosine
operator works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
We'll see a more tricky example of this when we consider
Perl's macros below. "POPn" gives you the NV
(floating point value) of the top SV on the stack: the
$x in "cos($x)". Then we compute the cosine, and push
the result back as an NV. The "X" in "XPUSHn" means
that the stack should be extended if necessary - it
can't be necessary here, because we know there's room
for one more item on the stack, since we've just
removed one! The "XPUSH*" macros at least guarantee
safety.
Alternatively, you can fiddle with the stack directly:
"SP" gives you the first element in your portion of the
stack, and "TOP*" gives you the top SV/IV/NV/etc. on
the stack. So, for instance, to do unary negation of an
integer:
SETi(-TOPi);
Just set the integer value of the top stack entry to
its negation.
Argument stack manipulation in the core is exactly the
same as it is in XSUBs - see perlxstut, perlxs and
perlguts for a longer description of the macros used in
stack manipulation.
Mark stack
I say `your portion of the stack' above because PP code
doesn't necessarily get the whole stack to itself: if
your function calls another function, you'll only want
to expose the arguments aimed for the called function,
and not (necessarily) let it get at your own data. The
way we do this is to have a `virtual' bottom-of-stack,
exposed to each function. The mark stack keeps bookmarks
to locations in the argument stack usable by each
function. For instance, when dealing with a tied variable,
(internally, something with `P' magic) Perl has
to call methods for accesses to the tied variables.
However, we need to separate the arguments exposed to
the method to the argument exposed to the original
function - the store or fetch or whatever it may be.
Here's how the tied "push" is implemented; see
"av_push" in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
9 POPSTACK;
The lines which concern the mark stack are the first,
fifth and last lines: they save away, restore and
remove the current position of the argument stack.
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 w
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