From Linux Man Pages
perlhack - How to hack at the Perl internals
DESCRIPTION
This document attempts to explain how Perl development takes place, and ends with some suggestions for people
wanting to become bona fide porters.
The perl5-porters mailing list is where the Perl standard distribution is maintained and developed. The list can
get anywhere from 10 to 150 messages a day, depending on the heatedness of the debate. Most days there are two
or three patches, extensions, features, or bugs being discussed at a time.
A searchable archive of the list is at either:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
or
http://archive.develooper.com/perl5-porters@perl.org/
List subscribers (the porters themselves) come in several flavours. Some are quiet curious lurkers, who rarely
pitch in and instead watch the ongoing development to ensure they're forewarned of new changes or features in
Perl. Some are representatives of vendors, who are there to make sure that Perl continues to compile and work on
their platforms. Some patch any reported bug that they know how to fix, some are actively patching their pet
area (threads, Win32, the regexp engine), while others seem to do nothing but complain. In other words, it's
your usual mix of technical people.
Over this group of porters presides Larry Wall. He has the final word in what does and does not change in the
Perl language. Various releases of Perl are shepherded by a "pumpking", a porter responsible for gathering
patches, deciding on a patch-by-patch, feature-by-feature basis what will and will not go into the release. For
instance, Gurusamy Sarathy was the pumpking for the 5.6 release of Perl, and Jarkko Hietaniemi was the pumpking
for the 5.8 release, and Rafael Garcia-Suarez holds the pumpking crown for the 5.10 release.
In addition, various people are pumpkings for different things. For instance, Andy Dougherty and Jarkko
Hietaniemi did a grand job as the Configure pumpkin up till the 5.8 release. For the 5.10 release H.Merijn Brand
took over.
Larry sees Perl development along the lines of the US government: there's the Legislature (the porters), the
Executive branch (the pumpkings), and the Supreme Court (Larry). The legislature can discuss and submit patches
to the executive branch all they like, but the executive branch is free to veto them. Rarely, the Supreme Court
will side with the executive branch over the legislature, or the legislature over the executive branch. Mostly,
however, the legislature and the executive branch are supposed to get along and work out their differences with-
out 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 pro-
grams, 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? Some-
times, instead of adding a feature with a tight focus, the porters might decide to wait until someone imple-
ments 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 with-
out 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 docu-
mentation, so submitting a patch for the appropriate manpages as well as the source code is always a good
idea.
Is there another way to do it?
Larry said "Although the Perl Slogan is There's More Than One Way to Do It, I hesitate to make 10 ways to do
something". This is a tricky heuristic to navigate, though--one man's essential addition is another man's
pointless cruft.
Does it create too much work?
Work for the pumpking, work for Perl programmers, work for module authors, ... Perl is supposed to be easy.
Patches speak louder than words
Working code is always preferred to pie-in-the-sky ideas. A patch to add a feature stands a much higher
chance of making it to the language than does a random feature request, no matter how fervently argued the
request might be. This ties into "Will it be useful?", as the fact that someone took the time to make the
patch demonstrates a strong desire for the feature.
If you're on the list, you might hear the word "core" bandied around. It refers to the standard distribution.
"Hacking on the core" means you're changing the C source code to the Perl interpreter. "A core module" is one
that ships with Perl.
Keeping in sync
The source code to the Perl interpreter, in its different versions, is kept in a repository managed by 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 pub-
lic 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
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 sub-
mitted to the maintenance and development "branches" of the perl repository. See http://lists.perl.org/ for sub-
scription 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://public.activestate.com/perl-current/ .
This takes care of updating every single item in the source tree to the latest applied patch level, creating
files that are new (to your distribution) and setting date/time stamps of existing files to reflect the
bleadperl status.
Note that this will not delete any files that were in '.' before the rsync. Once you are sure that the rsync
is running correctly, run it with the --delete and the --dry-run options like this:
# rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .
This will simulate an rsync run that also deletes files not present in the bleadperl master copy. Observe the
results from this run closely. If you are sure that the actual run would delete no files precious to you, you
could remove the '--dry-run' option.
You can than check what patch was the latest that was applied by looking in the file .patch, which will show
the number of the 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/(\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).
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://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 Knig to have better con-
trol 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" pro-
gram 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 fea-
tures, 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
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 Config-
ure 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
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
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.develooper.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 contri-
bution 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 abso-
lutely 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
Perl maintenance can be split into a number of areas, and certain people (pumpkins) will have responsibility for
each area. These areas sometimes correspond to files or directories in the source kit. Among the areas are:
Core modules
Modules shipped as part of the Perl core live in the lib/ and ext/ subdirectories: lib/ is for the pure-Perl
modules, and ext/ contains the core XS modules.
Tests
There are tests for nearly all the modules, built-ins and major bits of functionality. Test files all have a
.t suffix. Module tests live in the lib/ and ext/ directories next to the module being tested. Others live
in t/. See "Writing a test"
Documentation
Documentation maintenance includes looking after everything in the pod/ directory, (as well as contributing
new documentation) and the documentation to the modules in core.
Configure
The configure process is the way we make Perl portable across the myriad of operating systems it supports.
Responsibility for the configure, build and installation process, as well as the overall portability of the
core code rests with the configure pumpkin - others help out with individual operating systems.
The files involved are the operating system directories, (win32/, os2/, vms/ and so on) the shell scripts
which generate config.h and Makefile, as well as the metaconfig files which generate Configure. (metaconfig
isn't included in the core distribution.)
Interpreter
And of course, there's the core of the Perl interpreter itself. Let's have a look at that in a little more
detail.
Before we leave looking at the layout, though, don't forget that MANIFEST contains not only the file names in the
Perl distribution, but short descriptions of what's in them, too. For an overview of the important files, try
this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
Elements of the interpreter
The work of the interpreter has two main stages: compiling the code into the internal representation, or byte-
code, 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 actu-
ally 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 func-
tions, pp_ctl.c contains the functions which implement control structures ("if", "while" and the like) and
pp.c contains everything else. These are, if you like, the C code for Perl's built-in functions and operators.
Note that each "pp_" function is expected to return a pointer to the next op. Calls to perl subs (and eval
blocks) are handled within the same runops loop, and do not consume extra space on the C stack. For example,
"pp_entersub" and "pp_entertry" just push a "CxSUB" or "CxEVAL" block struct onto the context stack which con-
tain the address of the op following the sub call or eval. They then return the first op of that sub or eval
block, and so execution continues of that sub or block. Later, a "pp_leavesub" or "pp_leavetry" op pops the
"CxSUB" or "CxEVAL", retrieves the return op from it, and returns it.
Exception handing
Perl's exception handing (i.e. "die" etc) is built on top of the low-level "setjmp()"/"longjmp()" C-library
functions. These basically provide a way to capture the current PC and SP registers and later restore them;
i.e. a "longjmp()" continues at the point in code where a previous "setjmp()" was done, with anything further
up on the C stack being lost. This is why code should always save values using "SAVE_FOO" rather than in auto
variables.
The perl core wraps "setjmp()" etc in the macros "JMPENV_PUSH" and "JMPENV_JUMP". The basic rule of perl
exceptions is that "exit", and "die" (in the absence of "eval") perform a JMPENV_JUMP(2), while "die" within
"eval" does a JMPENV_JUMP(3).
At entry points to perl, such as "perl_parse()", "perl_run()" and "call_sv(cv, G_EVAL)" each does a
"JMPENV_PUSH", then enter a runops loop or whatever, and handle possible exception returns. For a 2 return,
final cleanup is performed, such as popping stacks and calling "CHECK" or "END" blocks. Amongst other things,
this is how scope cleanup still occurs during an "exit".
If a "die" can find a "CxEVAL" block on the context stack, then the stack is popped to that level and the
return op in that block is assigned to "PL_restartop"; then a JMPENV_JUMP(3) is performed. This normally
passes control back to the guard. In the case of "perl_run" and "call_sv", a non-null "PL_restartop" triggers
re-entry to the runops loop. The is the normal way that "die" or "croak" is handled within an "eval".
Sometimes ops are executed within an inner runops loop, such as tie, sort or overload code. In this case,
something like
sub FETCH { eval { die } }
would cause a longjmp right back to the guard in "perl_run", popping both runops loops, which is clearly
incorrect. One way to avoid this is for the tie code to do a "JMPENV_PUSH" before executing "FETCH" in the
inner runops loop, but for efficiency reasons, perl in fact just sets a flag, using "CATCH_SET(TRUE)". The
"pp_require", "pp_entereval" and "pp_entertry" ops check this flag, and if true, they call "docatch", which
does a "JMPENV_PUSH" and starts a new runops level to execute the code, rather than doing it on the current
loop.
As a further optimisation, on exit from the eval block in the "FETCH", execution of the code following the
block is still carried on in the inner loop. When an exception is raised, "docatch" compares the "JMPENV"
level of the "CxEVAL" with "PL_top_env" and if they differ, just re-throws the exception. In this way any
inner loops get popped.
Here's an example.
1: eval { tie @a, 'A' };
2: sub A::TIEARRAY {
3: eval { die };
4: die;
5: }
To run this code, "perl_run" is called, which does a "JMPENV_PUSH" then enters a runops loop. This loop exe-
cutes the eval and tie ops on line 1, with the eval pushing a "CxEVAL" onto the context stack.
The "pp_tie" does a "CATCH_SET(TRUE)", then starts a second runops loop to execute the body of "TIEARRAY".
When it executes the entertry op on line 3, "CATCH_GET" is true, so "pp_entertry" calls "docatch" which does a
"JMPENV_PUSH" and starts a third runops loop, which then executes the die op. At this point the C call stack
looks like this:
Perl_pp_die
Perl_runops # third loop
S_docatch_body
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
and the context and data stacks, as shown by "-Dstv", look like:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
CX 1: EVAL => *
retop=nextstate
The die pops the first "CxEVAL" off the context stack, sets "PL_restartop" from it, does a JMPENV_JUMP(3), and
control returns to the top "docatch". This then starts another third-level runops level, which executes the
nextstate, pushmark and die ops on line 4. At the point that the second "pp_die" is called, the C call stack
looks exactly like that above, even though we are no longer within an inner eval; this is because of the opti-
mization mentioned earlier. However, the context stack now looks like this, ie with the top CxEVAL popped:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
The die on line 4 pops the context stack back down to the CxEVAL, leaving it as:
STACK 0: MAIN
CX 0: BLOCK =>
As usual, "PL_restartop" is extracted from the "CxEVAL", and a JMPENV_JUMP(3) done, which pops the C stack
back to the docatch:
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
In this case, because the "JMPENV" level recorded in the "CxEVAL" differs from the current one, "docatch"
just does a JMPENV_JUMP(3) and the C stack unwinds to:
perl_run
main
Because "PL_restartop" is non-null, "run_body" starts a new runops loop and execution continues.
Internal Variable Types
You should by now have had a look at perlguts, which tells you about Perl's internal variable types: SVs, HVs,
AVs and the rest. If not, do that now.
These variables are used not only to represent Perl-space variables, but also any constants in the code, as well
as some structures completely internal to Perl. The symbol table, for instance, is an ordinary Perl hash. Your
code is represented by an SV as it's read into the parser; any program files you call are opened via ordinary
Perl filehandles, and so on.
The core Devel::Peek module lets us examine SVs from a Perl program. Let's see, for instance, how Perl treats the
constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
Reading "Devel::Peek" output takes a bit of practise, so let's go through it line by line.
Line 1 tells us we're looking at an SV which lives at 0xa04ecbc in memory. SVs themselves are very simple struc-
tures, 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 auto-
matically extends the available storage via a routine called "SvGROW".
You can get at any of these quantities from C very easily; just add "Sv" to the name of the field shown in the
snippet, and you've got a macro which will return the value: "SvCUR(sv)" returns the current length of the
string, "SvREFCOUNT(sv)" returns the reference count, "SvPV(sv, len)" returns the string itself with its length,
and so on. More macros to manipulate these properties can be found in perlguts.
Let's take an example of manipulating a PV, from "sv_catpvn", in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\0';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length "len" onto the end of the PV stored in "sv". The first
thing we do in line 6 is make sure that the SV has a valid PV, by calling the "SvPV_force" macro to force a PV.
As a side effect, "tlen" gets set to the current value of the PV, and the PV itself is returned to "junk".
In line 7, we make sure that the SV will have enough room to accommodate the old string, the new string and the
null terminator. If "LEN" isn't big enough, "SvGROW" will reallocate space for us.
Now, if "junk" is the same as the string we're trying to add, we can grab the string directly from the SV;
"SvPVX" is the address of the PV in the SV.
Line 10 does the actual catenation: the "Move" macro moves a chunk of memory around: we move the string "ptr" to
the end of the PV - that's the start of the PV plus its current length. We're moving "len" bytes of type "char".
After doing so, we need to tell Perl we've extended the string, by altering "CUR" to reflect the new length.
"SvEND" is a macro which gives us the end of the string, so that needs to be a "\0".
Line 13 manipulates the flags; since we've changed the PV, any IV or NV values will no longer be valid: if we
have "$a=10; $a.="6";" we don't want to use the old IV of 10. "SvPOK_only_utf8" is a special UTF-8-aware version
of "SvPOK_only", a macro which turns off the IOK and NOK flags and turns on POK. The final "SvTAINT" is a macro
which launders tainted data if taint mode is turned on.
AVs and HVs are more complicated, but SVs are by far the most common variable type being thrown around. Having
seen something of how we manipulate these, let's go on and look at how the op tree is constructed.
Op Trees
First, what is the op tree, anyway? The op tree is the parsed representation of your program, as we saw in our
section on parsing, and it's the sequence of operations that Perl goes through to execute your program, as we saw
in "Running".
An op is a fundamental operation that Perl can perform: all the built-in functions and operators are ops, and
there are a series of ops which deal with concepts the interpreter needs internally - entering and leaving a
block, ending a statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that there are two "routes" through it, two orders in which
you can traverse the tree. First, parse order reflects how the parser understood the code, and secondly, execu-
tion order tells perl what order to perform the operations in.
The easiest way to examine the op tree is to stop Perl after it has finished parsing, and get it to dump out the
tree. This is exactly what the compiler backends B::Terse, B::Concise and B::Debug do.
Let's have a look at how Perl sees "$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 com-
plex forms, "non-terminal symbols" are generally placed in lower case. "term" here is a non-terminal symbol, rep-
resenting a single expression.
The grammar gives you the following rule: you can make the thing on the left of the colon if you see all the
things on the right in sequence. This is called a "reduction", and the aim of parsing is to completely reduce
the input. There are several different ways you can perform a reduction, separated by vertical bars: so, "term"
followed by "=" followed by "term" makes a "term", and "term" followed by "+" followed by "term" can also make a
"term".
So, if you see two terms with an "=" or "+", between them, you can turn them into a single expression. When you
do this, you execute the code in the block on the next line: if you see "=", you'll do the code in line 2. If you
see "+", you'll do the code in line 4. It's this code which contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a number of variables. The variables refer to the tokens:
$1 is the first token in the input, $2 the second, and so on - think regular expression backreferences. $$ is the
op returned from this reduction. So, we call "newBINOP" to create a new binary operator. The first parameter to
"newBINOP", a function in op.c, is the op type. It's an addition operator, so we want the type to be "ADDOP". We
could specify this directly, but it's right there as the second token in the input, so we use $2. The second
parameter is the op's flags: 0 means "nothing special". Then the things to add: the left and right hand side of
our expression, in scalar context.
Stacks
When perl executes something like "addop", how does it pass on its results to the next op? The answer is, through
the use of stacks. Perl has a number of stacks to store things it's currently working on, and we'll look at the
three most important ones here.
Argument stack
Arguments are passed to PP code and returned from PP code using the argument stack, "ST". The typical way to
handle arguments is to pop them off the stack, deal with them how you wish, and then push the result back onto
the stack. This is how, for instance, the cosine operator works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
We'll see a more tricky example of this when we consider Perl's macros below. "POPn" gives you the NV (float-
ing 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 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 make the change to the global stack pointer: "dSP" only gave us a local copy, not a ref-
erence 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.
9 POPSTACK;
Finally, we remove the value we placed on the mark stack, since we don't need it any more.
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 declara-
tion as Perl expects for PP code; line 3 sets up variable declarations for the argument stack and the target, the
return value of the operation. Finally, it tries to see if the addition operation is overloaded; if so, the
appropriate subroutine is called.
Line 5 is another variable declaration - all variable declarations start with "d" - which pops from the top of
the argument stack two NVs (hence "nn") and puts them into the variables "right" and "left", hence the "rl".
These are the two operands to the addition operator. Next, we call "SETn" to set the NV of the return value to
the result of adding the two values. This done, we return - the "RETURN" macro makes sure that our return value
is properly handled, and we pass the next operator to run back to the main run loop.
Most of these macros are explained in perlapi, and some of the more important ones are explained in perlxs as
well. Pay special attention to "Background and PERL_IMPLICIT_CONTEXT" in perlguts for information on the
"[pad]THX_?" macros.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by the results.
Poking at Perl
To really poke around with Perl, you'll probably want to build Perl for debugging, like this:
./Configure -d -D optimize=-g
make
"-g" is a flag to the C compiler to have it produce debugging information which will allow us to step through a
running program. Configure will also turn on the "DEBUGGING" compilation symbol which enables all the internal
debugging code in Perl. There are a whole bunch of things you can debug with this: perlrun lists them all, and
the best way to find out about them is to play about with them. The most useful options are probably
l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be achieved using XS modules.
-Dr => use re 'debug'
-Dx => use O 'Debug'
Using a source-level debugger
If the debugging output of "-D" doesn't help you, it's time to step through perl's execution with a source-level
debugger.
� We'll use "gdb" for our examples here; the principles will apply to any debugger, but check the manual of the
one you're using.
To fire up the debugger, type
gdb ./perl
You'll want to do that in your Perl source tree so the debugger can read the source code. You should see the
copyright message, followed by the prompt.
(gdb)
"help" will get you into the documentation, but here are the most useful commands:
run [args]
Run the program with the given arguments.
break function_name
break source.c:xxx
Tells the debugger that we'll want to pause execution when we reach either the named function (but see "Inter-
nal Functions" in perlguts!) or the given line in the named source file.
step
Steps through the program a line at a time.
next
Steps through the program a line at a time, without descending into functions.
continue
Run until the next breakpoint.
finish
Run until the end of the current function, then stop again.
'enter'
Just pressing Enter will do the most recent operation again - it's a blessing when stepping through miles of
source code.
print
Execute the given C code and print its results. WARNING: Perl makes heavy use of macros, and gdb does not nec-
essarily support macros (see later "gdb macro support"). You'll have to substitute them yourself, or to
invoke cpp on the source code files (see "The .i Targets") So, for instance, you can't say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a "macro dictionary", which you can produce by saying "cpp -dM perl.c | sort".
Even then, cpp won't recursively apply those macros for you.
gdb macro support
Recent versions of gdb have fairly good macro support, but in order to use it you'll need to compile perl with
macro definitions included in the debugging information. Using gcc version 3.1, this means configuring with
"-Doptimize=-g3". Other compilers might use a different switch (if they support debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping functions in dump.c; these work a little like an
internal Devel::Peek, but they also cover OPs and other structures that you can't get at from Perl. Let's take an
example. We'll use the "$a = $b + $c" we used before, but give it a bit of context: "$b = "6XXXX"; $c = 2.3;".
Where's a good place to stop and poke around?
What about "pp_add", the function we examined earlier to implement the "+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions" in perlguts. With the breakpoint in
place, we can run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
Lots of junk will go past as gdb reads in the relevant source files and libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(gdb) step
311 dPOPTOPnnrl_ul;
(gdb)
We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into
"left" and "right" - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
"POPn" takes the SV from the top of the stack and obtains its NV either directly (if "SvNOK" is set) or by call-
ing 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 our-
selves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
We know we're going to get 6 from this, so let's finish the subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always stored in "PL_op", and we can dump it with "Perl_op_dump".
This'll give us similar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (12)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
11 TYPE = gvsv ===> 12
FLAGS = (SCALAR)
GV = main::b
}
}
# finish this later #
Patching
All right, we've now had a look at how to navigate the Perl sources and some things you'll need to know when fid-
dling with them. Let's now get on and create a simple patch. Here's something Larry suggested: if a "U" is the
first active format during a "pack", (for example, "pack "U3C8", @stuff") then the resulting string should be
treated as UTF-8 encoded.
How do we prepare to fix this up? First we locate the code in question - the "pack" happens at runtime, so it's
going to be in one of the pp files. Sure enough, "pp_pack" is in pp.c. Since we're going to be altering this
file, let's copy it to pp.c~.
[Well, it was in pp.c when this tutorial was written. It has now been split off with "pp_unpack" to its own file,
pp_pack.c]
Now let's look over "pp_pack": we take a pattern into "pat", and then loop over the pattern, taking each format
character in turn into "datum_type". Then for each possible format character, we swallow up the other arguments
in the pattern (a field width, an asterisk, and so on) and convert the next chunk input into the specified for-
mat, adding it onto the output SV "cat".
How do we know if the "U" is the first format in the "pat"? Well, if we have a pointer to the start of "pat"
then, if we see a "U" we can test whether we're still at the start of the string. So, here's where "pat" is set
up:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
SV *fromstr;
We'll have another string pointer in there:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
+ char *patcopy;
register I32 len;
I32 datumtype;
SV *fromstr;
And just before we start the loop, we'll set "patcopy" to be the start of "pat":
items = SP - MARK;
MARK++;
sv_setpvn(cat, "", 0);
+ patcopy = pat;
while (pat < patend) {
Now if we see a "U" which was at the start of the string, we turn on the "UTF8" flag for the output SV, "cat":
+ if (datumtype == 'U' && pat==patcopy+1)
+ SvUTF8_on(cat);
if (datumtype == '#') {
while (pat < patend && *pat != '\n')
pat++;
Remember that it has to be "patcopy+1" because the first character of the string is the "U" which has been swal-
lowed into "datumtype!"
Oops, we forgot one thing: what if there are spaces at the start of the pattern? "pack(" U*", @stuff)" will have
"U" as the first active character, even though it's not the first thing in the pattern. In this case, we have to
advance "patcopy" along with "pat" when we see spaces:
if (isSPACE(datumtype))
continue;
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
OK. That's the C part done. Now we must do two additional things before this patch is ready to go: we've changed
the behaviour of Perl, and so we must document that change. We must also provide some more regression tests to
make sure our patch works and doesn't create a bug somewhere else along the line.
The regression tests for each operator live in t/op/, and so we make a copy of t/op/pack.t to t/op/pack.t~. Now
we can add our tests to the end. First, we'll test that the "U" does indeed create Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didn't we could use the one from t/test.pl.
require './test.pl';
plan( tests => 159 );
so instead of this:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
we can write the more sensible (see Test::More for a full explanation of is() and other testing functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces unicode" );
Now we'll test that we got that space-at-the-beginning business right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
And finally we'll test that we don't make Unicode strings if "U" is not the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isn't unicode" );
Mustn't forget to change the number of tests which appears at the top, or else the automated tester will get con-
fused. This will either look like this:
print "1..156\n";
or this:
plan( tests => 156 );
We now compile up Perl, and run it through the test suite. Our new tests pass, hooray!
Finally, the documentation. The job is never done until the paperwork is over, so let's describe the change we've
just made. The relevant place is pod/perlfunc.pod; again, we make a copy, and then we'll insert this text in the
description of "pack":
=item *
If the pattern begins with a C<U>, the resulting string will be treated
as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
with an initial C<U0>, and the bytes that follow will be interpreted as
Unicode characters. If you don't want this to happen, you can begin your
pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
string, and then follow this with a C<U*> somewhere in your pattern.
All done. Now let's create the patch. Porting/patching.pod tells us that if we're making major changes, we should
copy the entire directory to somewhere safe before we begin fiddling, and then do
diff -ruN old new > patch
However, we know which files we've changed, and we can simply do this:
diff -u pp.c~ pp.c > patch
diff -u t/op/pack.t~ t/op/pack.t >> patch
diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
We end up with a patch looking a little like this:
--- pp.c~ Fri Jun 02 04:34:10 2000
+++ pp.c Fri Jun 16 11:37:25 2000
@@ -4375,6 +4375,7 @@
register I32 items;
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
+ char *patcopy;
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
@@ -4405,6 +4406,7 @@
...
And finally, we submit it, with our rationale, to perl5-porters. Job done!
Patching a core module
This works just like patching anything else, with an extra consideration. Many core modules also live on CPAN.
If this is so, patch the CPAN version instead of the core and send the patch off to the module maintainer (with a
copy to p5p). This will help the module maintainer keep the CPAN version in sync with the core version without
constantly scanning p5p.
The list of maintainers of core modules is usefully documented in Porting/Maintainers.pl.
Adding a new function to the core
If, as part of a patch to fix a bug, or just because you have an especially good idea, you decide to add a new
function to the core, discuss your ideas on p5p well before you start work. It may be that someone else has
already attempted to do what you are considering and can give lots of good advice or even provide you with bits
of code that they already started (but never finished).
You have to follow all of the advice given above for patching. It is extremely important to test any addition
thoroughly and add new tests to explore all boundary conditions that your new function is expected to handle. If
your new function is used only by one module (e.g. toke), then it should probably be named S_your_function (for
static); on the other hand, if you expect it to accessible from other functions in Perl, you should name it
Perl_your_function. See "Internal Functions" in perlguts for more details.
The location of any new code is also an important consideration. Don't just create a new top level .c file and
put your code there; you would have to make changes to Configure (so the Makefile is created properly), as well
as possibly lots of include files. This is strictly pumpking business.
It is better to add your function to one of the existing top level source code files, but your choice is compli-
cated by the nature of the Perl distribution. Only the files that are marked as compiled static are located in
the perl executable. Everything else is located in the shared library (or DLL if you are running under WIN32).
So, for example, if a function was only used by functions located in toke.c, then your code can go in toke.c.
If, however, you want to call the function from universal.c, then you should put your code in another location,
for example util.c.
In addition to writing your c-code, you will need to create an appropriate entry in embed.pl describing your
function, then run 'make regen_headers' to create the entries in the numerous header files that perl needs to
compile correctly. See "Internal Functions" in perlguts for information on the various options that you can set
in embed.pl. You will forget to do this a few (or many) times and you will get warnings during the compilation
phase. Make sure that you mention this when you post your patch to P5P; the pumpking needs to know this.
When you write your new code, please be conscious of existing code conventions used in the perl source files.
See perlstyle for details. Although most of the guidelines discussed seem to focus on Perl code, rather than c,
they all apply (except when they don't ;). See also Porting/patching.pod file in the Perl source distribution
for lots of details about both formatting and submitting patches of your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to p5p. Test on as many platforms as you can find.
Test as many perl Configure options as you can (e.g. MULTIPLICITY). If you have profiling or memory tools, see
"EXTERNAL TOOLS FOR DEBUGGING PERL" below for how to use them to further test your code. Remember that most of
the people on P5P are doing this on their own time and don't have the time to debug your code.
Writing a test
Every module and built-in function has an associated test file (or should...). If you add or change functional-
ity, you have to write a test. If you fix a bug, you have to write a test so that bug never comes back. If you
alter the docs, it would be nice to test what the new documentation says.
In short, if you submit a patch you probably also have to patch the tests.
For modules, the test file is right next to the module itself. lib/strict.t tests lib/strict.pm. This is a
recent innovation, so there are some snags (and it would be wonderful for you to brush them out), but it basi-
cally works that way. Everything else lives in t/.
t/base/
Testing of the absolute basic functionality of Perl. Things like "if", basic file reads and writes, simple
regexes, etc. These are run first in the test suite and if any of them fail, something is really broken.
t/cmd/
These test the basic control structures, "if/else", "while", subroutines, etc.
t/comp/
Tests basic issues of how Perl parses and compiles itself.
t/io/
Tests for built-in IO functions, including command line arguments.
t/lib/
The old home for the module tests, you shouldn't put anything new in here. There are still some bits and
pieces hanging around in here that need to be moved. Perhaps you could move them? Thanks!
t/op/
Tests for perl's built in functions that don't fit into any of the other directories.
t/pod/
Tests for POD directives. There are still some tests for the Pod modules hanging around in here that need to
be moved out into lib/.
t/run/
Testing features of how perl actually runs, including exit codes and handling of PERL* environment variables.
t/uni/
Tests for the core support of Unicode.
t/win32/
Windows-specific tests.
t/x2p
A test suite for the s2p converter.
The core uses the same testing style as the rest of Perl, a simple "ok/not ok" run through Test::Harness, but
there are a few special considerations.
There are three ways to write a test in the core. Test::More, t/test.pl and ad hoc "print $test ? "ok 42\n" :
"not ok 42\n"". The decision of which to use depends on what part of the test suite you're working on. This is
a measure to prevent a high-level failure (such as Config.pm breaking) from causing basic functionality tests to
fail.
t/base t/comp
Since we don't know if require works, or even subroutines, use ad hoc tests for these two. Step carefully to
avoid using the feature being tested.
t/cmd t/run t/io t/op
Now that basic require() and subroutines are tested, you can use the t/test.pl library which emulates the
important features of Test::More while using a minimum of core features.
You can also conditionally use certain libraries like Config, but be sure to skip the test gracefully if it's
not there.
t/lib ext lib
Now that the core of Perl is tested, Test::More can be used. You can also use the full suite of core modules
in the tests.
When you say "make test" Perl uses the t/TEST program to run the test suite (except under Win32 where it uses
t/harness instead.) All tests are run from the t/ directory, not the directory which contains the test. This
causes some problems with the tests in lib/, so here's some opportunity for some patching.
You must be triply conscious of cross-platform concerns. This usually boils down to using File::Spec and avoid-
ing things like "fork()" and "system()" unless absolutely necessary.
Special Make Test Targets
There are various special make targets that can be used to test Perl slightly differently than the standard
"test" target. Not all them are expected to give a 100% success rate. Many of them have several aliases, and
many of them are not available on certain operating systems.
coretest
Run perl on all core tests (t/* and lib/[a-z]* pragma tests).
(Not available on Win32)
test.deparse
Run all the tests through B::Deparse. Not all tests will succeed.
(Not available on Win32)
test.taintwarn
Run all tests with the -t command-line switch. Not all tests are expected to succeed (until they're specifi-
cally fixed, of course).
(Not available on Win32)
minitest
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 t/harness.
utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will succeed.
(Not available on Win32)
minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with different versions of this encoding.
"make utest.utf16" runs the test suite with a combination of "-utf8" and "-utf16" arguments to t/TEST.
(Not available on Win32)
test_harness
Run the test suite with the t/harness controlling program, instead of t/TEST. t/harness is more sophisti-
cated, and uses the Test::Harness module, thus using this test target supposes that perl mostly works. The
main advantage for our purposes is that it prints a detailed summary of failed tests at the end. Also, unlike
t/TEST, it doesn't redirect stderr to stdout.
Note that under Win32 t/harness is always used instead of t/TEST, so there is no special "test_harness" tar-
get.
Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES environment variables to control the
behaviour of t/harness. This means you can say
nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal test.
Running tests by hand
You can run part of the test suite by hand by using one the following commands from the t/ directory :
./perl -I../lib TEST list-of-.t-files
or
./perl -I../lib harness list-of-.t-files
(if you don't specify test scripts, the whole test suite will be run.)
Using t/harness for testing
If you use "harness" for testing you have several command line options available to you. The arguments are as
follows, and are in the order that they must appear if used together.
harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH
If "LIST OF FILES TO TEST" is omitted the file list is obtained from the manifest. The file list may include
shell wildcards which will be expanded out.
-v Run the tests under verbose mode so you can see what tests were run, and debug outbut.
-torture
Run the torture tests as well as the normal set.
-re=PATTERN
Filter the file list so that all the test files run match PATTERN. Note that this form is distinct from the
-re LIST OF PATTERNS form below in that it allows the file list to be provided as well.
-re LIST OF PATTERNS
Filter the file list so that all the test files run match /(LIST|OF|PATTERNS)/. Note that with this form the
patterns are joined by '|' and you cannot supply a list of files, instead the test files are obtained from
the MANIFEST.
You can run an individual test by a command similar to
./perl -I../lib patho/to/foo.t
except that the harnesses set up some environment variables that may affect the execution of the test :
PERL_CORE=1
indicates that we're running this test part of the perl core test suite. This is useful for modules that
have a dual life on CPAN.
PERL_DESTRUCT_LEVEL=2
is set to 2 if it isn't set already (see "PERL_DESTRUCT_LEVEL")
PERL
(used only by t/TEST) if set, overrides the path to the perl executable that should be used to run the tests
(the default being ./perl).
PERL_SKIP_TTY_TEST
if set, tells to skip the tests that need a terminal. It's actually set automatically by the Makefile, but
can also be forced artificially by running 'make test_notty'.
EXTERNAL TOOLS FOR DEBUGGING PERL
Sometimes it helps to use external tools while debugging and testing Perl. This section tries to guide you
through using some common testing and debugging tools with Perl. This is meant as a guide to interfacing these
tools with Perl, not as any kind of guide to the use of the tools themselves.
NOTE 1: Running under memory debuggers such as Purify, valgrind, or Third Degree greatly slows down the execu-
tion: seconds become minutes, minutes become hours. For example as of Perl 5.8.1, the ext/Encode/t/Unicode.t
takes extraordinarily long to complete under e.g. Purify, Third Degree, and valgrind. Under valgrind it takes
more than six hours, even on a snappy computer-- the said test must be doing something that is quite unfriendly
for memory debuggers. If you don't feel like waiting, that you can simply kill away the perl process.
NOTE 2: To minimize the number of memory leak false alarms (see "PERL_DESTRUCT_LEVEL" for more information), you
have to have environment variable PERL_DESTRUCT_LEVEL set to 2. The TEST and harness scripts do that automati-
cally. But if you are running some of the tests manually-- for csh-like shells:
setenv PERL_DESTRUCT_LEVEL 2
and for Bourne-type shells:
PERL_DESTRUCT_LEVEL=2
export PERL_DESTRUCT_LEVEL
or in UNIXy environments you can also use the "env" command:
env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
NOTE 3: There are known memory leaks when there are compile-time errors within eval or require, seeing "S_doeval"
in the call stack is a good sign of these. Fixing these leaks is non-trivial, unfortunately, but they must be
fixed eventually.
Rational Software's Purify
Purify is a commercial tool that is helpful in identifying memory overruns, wild pointers, memory leaks and other
such badness. Perl must be compiled in a specific way for optimal testing with Purify. Purify is available
under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most benefit out of Purify, you should create the perl to
Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
-Accflags=-DPURIFY
Disables Perl's arena memory allocation functions, as well as forcing use of memory allocation functions
derived from the system malloc.
-Doptimize='-g'
Adds debugging information so that you see the exact source statements where the problem occurs. Without
this flag, all you will see is the source filename of where the error occurred.
-Uusemymalloc
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using Perl's malloc
will make Purify report most leaks in the "potential" leaks category.
-Dusemultiplicity
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
Once you've compiled a perl suitable for Purify'ing, then you can just:
make pureperl
which creates a binary named 'pureperl' that has been Purify'ed. This binary is used in place of the standard
'perl' binary when you want to debug Perl memory problems.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run the
Purify'ed perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory problems.
Purify outputs messages in "Viewer" windows by default. If you don't have a windowing environment or if you sim-
ply want the Purify output to unobtrusively go to a log file instead of to the interactive window, use these fol-
lowing options to output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the "Viewer" windows, then you only need this option:
setenv PURIFYOPTIONS "-chain-length=25"
In Bourne-type shells:
PURIFYOPTIONS="..."
export PURIFYOPTIONS
or if you have the "env" utility:
env PURIFYOPTIONS="..." ../pureperl ...
Purify on NT
Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly. There are several options in the make-
file you should change to get the most use out of Purify:
DEFINES
You should add -DPURIFY to the DEFINES line so the DEFINES line looks something like:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as well as to force use of memory allocation functions
derived from the system malloc.
USE_MULTI = define
Enabling the multiplicity option allows perl to clean up thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
#PERL_MALLOC = define
Disable Perl's malloc so that Purify can more closely monitor allocations and leaks. Using Perl's malloc
will make Purify report most leaks in the "potential" leaks category.
CFG = Debug
Adds debugging information so that you see the exact source statements where the problem occurs. Without
this flag, all you will see is the source filename of where the error occurred.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run Purify
as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl, then finally report any memory problems.
valgrind
The excellent valgrind tool can be used to find out both memory leaks and illegal memory accesses. As of August
2003 it unfortunately works only on x86 (ELF) Linux. The special "test.valgrind" target can be used to run the
tests under valgrind. Found errors and memory leaks are logged in files named test.valgrind.
As system libraries (most notably glibc) are also triggering errors, valgrind allows to suppress such errors
using suppression files. The default suppression file that comes with valgrind already catches a lot of them.
Some additional suppressions are defined in t/perl.supp.
To get valgrind and for more information see
http://developer.kde.org/~sewardj/
Compaq's/Digital's/HP's Third Degree
Third Degree is a tool for memory leak detection and memory access checks. It is one of the many tools in the
ATOM toolkit. The toolkit is only available on Tru64 (formerly known as Digital UNIX formerly known as DEC
OSF/1).
When building Perl, you must first run Configure with -Doptimize=-g and -Uusemymalloc flags, after that you can
use the make targets "perl.third" and "test.third". (What is required is that Perl must be compiled using the
"-g" flag, you may need to re-Configure.)
The short story is that with "atom" you can instrument the Perl executable to create a new executable called
perl.third. When the instrumented executable is run, it creates a log of dubious memory traffic in file called
perl.3log. See the manual pages of atom and third for more information. The most extensive Third Degree docu-
mentation is available in the Compaq "Tru64 UNIX Programmer's Guide", chapter "Debugging Programs with Third
Degree".
The "test.third" leaves a lot of files named foo_bar.3log in the t/ subdirectory. There is a problem with these
files: Third Degree is so effective that it finds problems also in the system libraries. Therefore you should
used the Porting/thirdclean script to cleanup the *.3log files.
There are also leaks that for given certain definition of a leak, aren't. See "PERL_DESTRUCT_LEVEL" for more
information.
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using e.g. valgrind, or the pureperl or perl.third exe-
cutables, 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.
Profiling
Depending on your platform there are various of profiling Perl.
There are two commonly used techniques of profiling executables: statistical time-sampling and basic-block count-
ing.
The first method takes periodically samples of the CPU program counter, and since the program counter can be cor-
related with the code generated for functions, we get a statistical view of in which functions the program is
spending its time. The caveats are that very small/fast functions have lower probability of showing up in the
profile, and that periodically interrupting the program (this is usually done rather frequently, in the scale of
milliseconds) imposes an additional overhead that may skew the results. The first problem can be alleviated by
running the code for longer (in general this is a good idea for profiling), the second problem is usually kept in
guard by the profiling tools themselves.
The second method divides up the generated code into basic blocks. Basic blocks are sections of code that are
entered only in the beginning and exited only at the end. For example, a conditional jump starts a basic block.
Basic block profiling usually works by instrumenting the code by adding enter basic block #nnnn book-keeping code
to the generated code. During the execution of the code the basic block counters are then updated appropriately.
The caveat is that the added extra code can skew the results: again, the profiling tools usually try to factor
their own effects out of the results.
Gprof Profiling
gprof is a profiling tool available in many UNIX platforms, it uses statistical time-sampling.
You can build a profiled version of perl called "perl.gprof" by invoking the make target "perl.gprof" (What is
required is that Perl must be compiled using the "-pg" flag, you may need to re-Configure). Running the profiled
version of Perl will create an output file called gmon.out is created which contains the profiling data collected
during the execution.
The gprof tool can then display the collected data in various ways. Usually gprof understands the following
options:
-a Suppress statically defined functions from the profile.
-b Suppress the verbose descriptions in the profile.
-e routine
Exclude the given routine and its descendants from the profile.
-f routine
Display only the given routine and its descendants in the profile.
-s Generate a summary file called gmon.sum which then may be given to subsequent gprof runs to accumulate data
over several runs.
-z Display routines that have zero usage.
For more detailed explanation of the available commands and output formats, see your own local documentation of
gprof.
GCC gcov Profiling
Starting from GCC 3.0 basic block profiling is officially available for the GNU CC.
You can build a profiled version of perl called perl.gcov by invoking the make target "perl.gcov" (what is
required that Perl must be compiled using gcc with the flags "-fprofile-arcs -ftest-coverage", you may need to
re-Configure).
Running the profiled version of Perl will cause profile output to be generated. For each source file an accompa-
nying ".da" file will be created.
To display the results you use the "gcov" utility (which should be installed if you have gcc 3.0 or newer
installed). gcov is run on source code files, like this
gcov sv.c
which will cause sv.c.gcov to be created. The .gcov files contain the source code annotated with relative fre-
quencies of execution indicated by "#" markers.
Useful options of gcov include "-b" which will summarise the basic block, branch, and function call coverage, and
"-c" which instead of relative frequencies will use the actual counts. For more information on the use of gcov
and basic block profiling with gcc, see the latest GNU CC manual, as of GCC 3.0 see
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
and its section titled "8. gcov: a Test Coverage Program"
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
Pixie Profiling
Pixie is a profiling tool available on IRIX and Tru64 (aka Digital UNIX aka DEC OSF/1) platforms. Pixie does its
profiling using basic-block counting.
You can build a profiled version of perl called perl.pixie by invoking the make target "perl.pixie" (what is
required is that Perl must be compiled using the "-g" flag, you may need to re-Configure).
In Tru64 a file called perl.Addrs will also be silently created, this file contains the addresses of the basic
blocks. Running the profiled version of Perl will create a new file called "perl.Counts" which contains the
counts for the basic block for that particular program execution.
To display the results you use the prof utility. The exact incantation depends on your operating system, "prof
perl.Counts" in IRIX, and "prof -pixie -all -L. perl" in Tru64.
In IRIX the following prof options are available:
-h Reports the most heavily used lines in descending order of use. Useful for finding the hotspot lines.
-l Groups lines by procedure, with procedures sorted in descending order of use. Within a procedure, lines are
listed in source order. Useful for finding the hotspots of procedures.
In Tru64 the following options are available:
-p[rocedures]
Procedures sorted in descending order by the number of cycles executed in each procedure. Useful for finding
the hotspot procedures. (This is the default option.)
-h[eavy]
Lines sorted in descending order by the number of cycles executed in each line. Useful for finding the
hotspot lines.
-i[nvocations]
The called procedures are sorted in descending order by number of calls made to the procedures. Useful for
finding the most used procedures.
-l[ines]
Grouped by procedure, sorted by cycles executed per procedure. Useful for finding the hotspots of proce-
dures.
-testcoverage
The compiler emitted code for these lines, but the code was unexecuted.
-z[ero]
Unexecuted procedures.
For further information, see your system's manual pages for pixie and prof.
Miscellaneous tricks
� Those debugging perl with the DDD frontend over gdb may find the following useful:
You can extend the data conversion shortcuts menu, so for example you can display an SV's IV value with one
click, without doing any typing. To do that simply edit ~/.ddd/init file and add after:
! Display shortcuts.
Ddd*gdbDisplayShortcuts: \
/t () // Convert to Bin\n\
/d () // Convert to Dec\n\
/x () // Convert to Hex\n\
/o () // Convert to Oct(\n\
the following two lines:
((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
so now you can do ivx and pvx lookups or you can plug there the sv_peek "conversion":
Perl_sv_peek(my_perl, (SV*)()) // sv_peek
(The my_perl is for threaded builds.) Just remember that every line, but the last one, should end with \n\
Alternatively edit the init file interactively via: 3rd mouse button -> New Display -> Edit Menu
Note: you can define up to 20 conversion shortcuts in the gdb section.
� If you see in a debugger a memory area mysteriously full of 0xabababab, you may be seeing the effect of the
Poison() macro, see perlclib.
CONCLUSION
We've had a brief look around the Perl source, an overview of the stages perl goes through when it's running your
code, and how to use a debugger to poke at the Perl guts. We took a very simple problem and demonstrated how to
solve it fully - with documentation, regression tests, and finally a patch for submission to p5p. Finally, we
talked about how to use external tools to debug and test Perl.
I'd now suggest you read over those references again, and then, as soon as possible, get your hands dirty. The
best way to learn is by doing, so:
� Subscribe to perl5-porters, follow the patches and try and understand them; don't be afraid to ask if there's
a portion you're not clear on - who knows, you may unearth a bug in the patch...
� Keep up to date with the bleeding edge Perl distributions and get familiar with the changes. Try and get an
idea of what areas people are working on and the changes they're making.
� Do read the README associated with your operating system, e.g. README.aix on the IBM AIX OS. Don't hesitate to
supply patches to that README if you find anything missing or changed over a new OS release.
� Find an area of Perl that seems interesting to you, and see if you can work out how it works. Scan through the
source, and step over it in the debugger. Play, poke, investigate, fiddle! You'll probably get to understand
not just your chosen area but a much wider range of perl's activity as well, and probably sooner than you'd
think.
The Road goes ever on and on, down from the door where it began.
If you can do these things, you've started on the long road to Perl porting. Thanks for wanting to help make
Perl better - and happy hacking!
CATEGORY