PERLHACK(1) Perl Programmers Reference Guide PERLHACK(1)
NAME
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 theyre forewarned of new changes or
features in Perl. Some are representatives of vendors, who are there
to make sure that Perl continues to compile and work on their plat
forms. Some patch any reported bug that they know how to fix, some are
actively patching their pet area (threads, Win32, the regexp engine),
while others seem to do nothing but complain. In other words, its
your usual mix of technical people.
Over this group of porters presides Larry Wall. He has the final word
in what does and does not change in the Perl language. Various
releases of Perl are shepherded by a "pumpking", a porter responsible
for gathering patches, deciding on a patch-by-patch, feature-by-feature
basis what will and will not go into the release. For instance,
Gurusamy Sarathy was the pumpking for the 5.6 release of Perl, and
Jarkko Hietaniemi was the pumpking for the 5.8 release, and Rafael Gar
cia-Suarez holds the pumpking crown for the 5.10 release.
In addition, various people are pumpkings for different things. For
instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the
Configure pumpkin up till the 5.8 release. For the 5.10 release H.Mer
ijn Brand took over.
Larry sees Perl development along the lines of the US government:
theres the Legislature (the porters), the Executive branch (the pump
kings), and the Supreme Court (Larry). The legislature can discuss and
submit patches to the executive branch all they like, but the executive
branch is free to veto them. Rarely, the Supreme Court will side with
the executive branch over the legislature, or the legislature over the
executive branch. Mostly, however, the legislature and the executive
branch are supposed to get along and work out their differences without
impeachment or court cases.
You might sometimes see reference to Rule 1 and Rule 2. Larrys power
as Supreme Court is expressed in The Rules:
1 Larry is always by definition right about how Perl should behave.
This means he has final veto power on the core functionality.
2 Larry is allowed to change his mind about any matter at a later
date, regardless of whether he previously invoked Rule 1.
Got that? Larry is always right, even when he was wrong. Its rare to
see either Rule exercised, but they are often alluded to.
New features and extensions to the language are contentious, because
the criteria used by the pumpkings, Larry, and other porters to decide
which features should be implemented and incorporated are not codified
in a few small design goals as with some other languages. Instead, the
heuristics are flexible and often difficult to fathom. Here is one
persons list, roughly in decreasing order of importance, of heuristics
that new features have to be weighed against:
Does concept match the general goals of Perl?
These havent been written anywhere in stone, but one approximation
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 avail
able to implement your (possibly good) idea.
Backwards compatibility
Its a cardinal sin to break existing Perl programs. New warnings
are contentious--some say that a program that emits warnings is not
broken, while others say it is. Adding keywords has the potential
to break programs, changing the meaning of existing token sequences
or functions might break programs.
Could it be a module instead?
Perl 5 has extension mechanisms, modules and XS, specifically to
avoid the need to keep changing the Perl interpreter. You can
write modules that export functions, you can give those functions
prototypes so they can be called like built-in functions, you can
even write XS code to mess with the runtime data structures of the
Perl interpreter if you want to implement really complicated
things. If it can be done in a module instead of in the core, its
highly unlikely to be added.
Is the feature generic enough?
Is this something that only the submitter wants added to the lan
guage, or would it be broadly useful? Sometimes, instead of adding
a feature with a tight focus, the porters might decide to wait
until someone implements the more generalized feature. For
instance, instead of implementing a "delayed evaluation" feature,
the porters are waiting for a macro system that would permit
delayed evaluation and much more.
Does it potentially introduce new bugs?
Radical rewrites of large chunks of the Perl interpreter have the
potential to introduce new bugs. The smaller and more localized
the change, the better.
Does it preclude other desirable features?
A patch is likely to be rejected if it closes off future avenues of
development. For instance, a patch that placed a true and final
interpretation on prototypes is likely to be rejected because there
are still options for the future of prototypes that havent been
addressed.
Is the implementation robust?
Good patches (tight code, complete, correct) stand more chance of
going in. Sloppy or incorrect patches might be placed on the back
burner until the pumpking has time to fix, or might be discarded
altogether without further notice.
Is the implementation generic enough to be portable?
The worst patches make use of a system-specific features. Its
highly unlikely that nonportable additions to the Perl language
will be accepted.
Is the implementation tested?
Patches which change behaviour (fixing bugs or introducing new fea
tures) must include regression tests to verify that everything
works as expected. Without tests provided by the original author,
how can anyone else changing perl in the future be sure that they
havent unwittingly broken the behaviour the patch implements? And
without tests, how can the patchs author be confident that his/her
hard work put into the patch wont be accidentally thrown away by
someone in the future?
Is there enough documentation?
Patches without documentation are probably ill-thought out or
incomplete. Nothing can be added without documentation, so submit
ting a patch for the appropriate manpages as well as the source
code is always a good idea.
Is there another way to do it?
Larry said "Although the Perl Slogan is Theres More Than One Way
to Do It, I hesitate to make 10 ways to do something". This is a
tricky heuristic to navigate, though--one mans essential addition
is another mans pointless cruft.
Does it create too much work?
Work for the pumpking, work for Perl programmers, work for module
authors, ... Perl is supposed to be easy.
Patches speak louder than words
Working code is always preferred to pie-in-the-sky ideas. A patch
to add a feature stands a much higher chance of making it to the
language than does a random feature request, no matter how fer
vently argued the request might be. This ties into "Will it be
useful?", as the fact that someone took the time to make the patch
demonstrates a strong desire for the feature.
If youre on the list, you might hear the word "core" bandied around.
It refers to the standard distribution. "Hacking on the core" means
youre changing the C source code to the Perl interpreter. "A core
module" is one that ships with Perl.
Keeping in sync
The source code to the Perl interpreter, in its different versions, is
kept in a repository managed by a revision control system ( which is
currently the Perforce program, see http://perforce.com/ ). The pump
kings and a few others have access to the repository to check in
changes. Periodically the pumpking for the development version of Perl
will release a new version, so the rest of the porters can see whats
changed. The current state of the main trunk of repository, and
patches that describe the individual changes that have happened since
the last public release are available at this location:
http://public.activestate.com/pub/apc/
ftp://public.activestate.com/pub/apc/
If youre looking for a particular change, or a change that affected a
particular set of files, you may find the Perl Repository Browser use
ful:
http://public.activestate.com/cgi-bin/perlbrowse
You may also want to subscribe to the perl5-changes mailing list to
receive a copy of each patch that gets submitted to the maintenance and
development "branches" of the perl repository. See
http://lists.perl.org/ for subscription information.
If you are a member of the perl5-porters mailing list, it is a good
thing to keep in touch with the most recent changes. If not only to
verify if what you would have posted as a bug report isnt already
solved in the most recent available perl development branch, also known
as perl-current, bleading edge perl, bleedperl or bleadperl.
Needless to say, the source code in perl-current is usually in a per
petual state of evolution. You should expect it to be very buggy. Do
not use it for any purpose other than testing and development.
Keeping in sync with the most recent branch can be done in several
ways, but the most convenient and reliable way is using rsync, avail
able at ftp://rsync.samba.org/pub/rsync/ . (You can also get the most
recent branch by FTP.)
If you choose to keep in sync using rsync, there are two approaches to
doing so:
rsyncing the source tree
Presuming you are in the directory where your perl source resides
and you have rsync installed and available, you can "upgrade" to
the bleadperl using:
# rsync -avz rsync://public.activestate.com/perl-current/ .
This takes care of updating every single item in the source tree to
the latest applied patch level, creating files that are new (to
your distribution) and setting date/time stamps of existing files
to reflect the bleadperl status.
Note that this will not delete any files that were in . before
the rsync. Once you are sure that the rsync is running correctly,
run it with the --delete and the --dry-run options like this:
# rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .
This will simulate an rsync run that also deletes files not present
in the bleadperl master copy. Observe the results from this run
closely. If you are sure that the actual run would delete no files
precious to you, you could remove the --dry-run option.
You can than check what patch was the latest that was applied by
looking in the file .patch, which will show the number of the lat
est patch.
If you have more than one machine to keep in sync, and not all of
them have access to the WAN (so you are not able to rsync all the
source trees to the real source), there are some ways to get around
this problem.
Using rsync over the LAN
Set up a local rsync server which makes the rsynced source tree
available to the LAN and sync the other machines against this
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).
rsyncing the patches
The source tree is maintained by the pumpking who applies patches
to the files in the tree. These patches are either created by the
pumpking himself using "diff -c" after updating the file manually
or by applying patches sent in by posters on the perl5-porters
list. These patches are also saved and rsyncable, so you can
apply them yourself to the source files.
Presuming you are in a directory where your patches reside, you can
get them in sync with
# rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
This makes sure the latest available patch is downloaded to your
patch directory.
Its then up to you to apply these patches, using something like
# last=ls -t *.gz | sed q
# rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
# find . -name *.gz -newer $last -exec gzcat {} \; >blead.patch
# cd ../perl-current
# patch -p1 -N <../perl-current-diffs/blead.patch
or, since this is only a hint towards how it works, use CPAN-
patchaperl from Andreas Knig to have better control over the
patching process.
Why rsync the source tree
Its easier to rsync the source tree
Since you dont have to apply the patches yourself, you are sure
all files in the source tree are in the right state.
Its more reliable
While both the rsync-able source and patch areas are automatically
updated every few minutes, keep in mind that applying patches may
sometimes mean careful hand-holding, especially if your version of
the "patch" program does not understand how to deal with new files,
files with 8-bit characters, or files without trailing newlines.
Why rsync the patches
Its easier to rsync the patches
If you have more than one machine that you want to keep in track
with bleadperl, its easier to rsync the patches only once and then
apply them to all the source trees on the different machines.
In case you try to keep in pace on 5 different machines, for which
only one of them has access to the WAN, rsyncing all the source
trees should than be done 5 times over the NFS. Having rsynced the
patches only once, I can apply them to all the source trees auto
matically. Need you say more ;-)
Its a good reference
If you do not only like to have the most recent development branch,
but also like to fix bugs, or extend features, you want to dive
into the sources. If you are a seasoned perl core diver, you dont
need no manuals, tips, roadmaps, perlguts.pod or other aids to find
your way around. But if you are a starter, the patches may help you
in finding where you should start and how to change the bits that
bug you.
The file Changes is updated on occasions the pumpking sees as his
own little sync points. On those occasions, he releases a tar-ball
of the current source tree (i.e. perl@7582.tar.gz), which will be
an excellent point to start with when choosing to use the rsync
the patches scheme. Starting with perl@7582, which means a set of
source files on which the latest applied patch is number 7582, you
apply all succeeding patches available from then on (7583, 7584,
...).
You can use the patches later as a kind of search archive.
Finding a start point
If you want to fix/change the behaviour of function/feature
Foo, just scan the patches for patches that mention Foo either
in the subject, the comments, or the body of the fix. A good
chance the patch shows you the files that are affected by that
patch which are very likely to be the starting point of your
journey into the guts of perl.
Finding how to fix a bug
If youve found where the function/feature Foo misbehaves, but
you dont know how to fix it (but you do know the change you
want to make), you can, again, peruse the patches for similar
changes and look how others apply the fix.
Finding the source of misbehaviour
When you keep in sync with bleadperl, the pumpking would love
to see that the community efforts really work. So after each of
his sync points, you are to make test to check if everything
is still in working order. If it is, you do make ok, which
will send an OK report to perlbug@perl.org. (If you do not have
access to a mailer from the system you just finished success
fully make test, you can do make okfile, which creates the
file "perl.ok", which you can than take to your favourite
mailer and mail yourself).
But of course, as always, things will not always lead to a suc
cess path, and one or more test do not pass the make test.
Before sending in a bug report (using make nok or make nok
file), check the mailing list if someone else has reported the
bug already and if so, confirm it by replying to that message.
If not, you might want to trace the source of that misbehaviour
before sending in the bug, which will help all the other
porters in finding the solution.
Here the saved patches come in very handy. You can check the
list of patches to see which patch changed what file and what
change caused the misbehaviour. If you note that in the bug
report, it saves the one trying to solve it, looking for that
point.
If searching the patches is too bothersome, you might consider
using perls bugtron to find more information about discussions and
ramblings on posted bugs.
If you want to get the best of both worlds, rsync both the source
tree for convenience, reliability and ease and rsync the patches
for reference.
Working with the source
Because you cannot use the Perforce client, you cannot easily generate
diffs against the repository, nor will merges occur when you update via
rsync. If you edit a file locally and then rsync against the latest
source, changes made in the remote copy will overwrite your local ver
sions!
The best way to deal with this is to maintain a tree of symlinks to the
rsyncd source. Then, when you want to edit a file, you remove the
symlink, copy the real file into the other tree, and edit it. You can
then diff your edited file against the original to generate a patch,
and you can safely update the original tree.
Perls Configure script can generate this tree of symlinks for you.
The following example assumes that you have used rsync to pull a copy
of the Perl source into the perl-rsync directory. In the directory
above that one, you can execute the following commands:
mkdir perl-dev
cd perl-dev
../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g"
This will start the Perl configuration process. After a few prompts,
you should see something like this:
Symbolic links are supported.
Checking how to test for symbolic links...
Your builtin test -h may be broken.
Trying external /usr/bin/test -h.
You can test for symbolic links with /usr/bin/test -h.
Creating the symbolic links...
(First creating the subdirectories...)
(Then creating the symlinks...)
The specifics may vary based on your operating system, of course.
After you see this, you can abort the Configure script, and you will
see that the directory you are in has a tree of symlinks to the perl-
rsync directories and files.
If you plan to do a lot of work with the Perl source, here are some
Bourne shell script functions that can make your life easier:
function edit {
if [ -L $1 ]; then
mv $1 $1.orig
cp $1.orig $1
vi $1
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 theyre more likely to get applied.
This function assumed a GNU diff, and may require some tweaking for
other diff variants.
Perlbug administration
There is a single remote administrative interface for modifying bug
status, category, open issues etc. using the RT bugtracker system,
maintained by Robert Spier. Become an administrator, and close any
bugs you can get your sticky mitts on:
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"
Submitting patches
Always submit patches to perl5-porters@perl.org. If youre patching a
core module and theres an author listed, send the author a copy (see
"Patching a core module"). This lets other porters review your patch,
which catches a surprising number of errors in patches. Either use the
diff program (available in source code form from
ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans makepatch (available
from CPAN/authors/id/JV/). Unified diffs are preferred, but context
diffs are accepted. Do not send RCS-style diffs or diffs without con
text lines. More information is given in the Porting/patching.pod file
in the Perl source distribution. Please patch against the latest
development version (e.g., if youre fixing a bug in the 5.005 track,
patch against the latest 5.005_5x version). Only patches that survive
the heat of the development branch get applied to maintenance versions.
Your patch should update the documentation and test suite. See "Writ
ing a test".
To report a bug in Perl, use the program perlbug which comes with Perl
(if you cant get Perl to work, send mail to the address perl
bug@perl.org or perlbug@perl.com). Reporting bugs through perlbug
feeds into the automated bug-tracking system, access to which is pro
vided through the web at http://bugs.perl.org/ . It often pays to
check the archives of the perl5-porters mailing list to see whether the
bug youre reporting has been reported before, and if so whether it was
considered a bug. See above for the location of the searchable
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.
Its a good idea to read and lurk for a while before chipping in. That
way youll get to see the dynamic of the conversations, learn the per
sonalities of the players, and hopefully be better prepared to make a
useful contribution when do you speak up.
If after all this you still think you want to join the perl5-porters
mailing list, send mail to perl5-porters-subscribe@perl.org. To unsub
scribe, send mail to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, youll need to read the following things:
perlguts
This is of paramount importance, since its the documentation of
what goes where in the Perl source. Read it over a couple of times
and it might start to make sense - dont worry if it doesnt yet,
because the best way to study it is to read it in conjunction with
poking at Perl source, and well do that later on.
You might also want to look at Gisle Aass illustrated perlguts -
theres no guarantee that this will be absolutely up-to-date with
the latest documentation in the Perl core, but the fundamentals will
be right. ( http://gisle.aas.no/perl/illguts/ )
perlxstut and perlxs
A working knowledge of XSUB programming is incredibly useful for
core hacking; XSUBs use techniques drawn from the PP code, the por
tion of the guts that actually executes a Perl program. Its a lot
gentler to learn those techniques from simple examples and explana
tion than from the core itself.
perlapi
The documentation for the Perl API explains what some of the inter
nal functions do, as well as the many macros used in the source.
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/writ
ings/p5p-faq ; alternatively, you can get the FAQ emailed to you by
sending mail to "perl5-porters-faq@perl.org". It contains hints on
reading perl5-porters, information on how perl5-porters works and
how Perl development in general works.
Finding Your Way Around
Perl maintenance can be split into a number of areas, and certain peo
ple (pumpkins) will have responsibility for each area. These areas
sometimes correspond to files or directories in the source kit. Among
the areas are:
Core modules
Modules shipped as part of the Perl core live in the lib/ and ext/
subdirectories: lib/ is for the pure-Perl modules, and ext/ contains
the core XS modules.
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 con
figure, build and installation process, as well as the overall
portability of the core code rests with the configure pumpkin - oth
ers help out with individual operating systems.
The files involved are the operating system directories, (win32/,
os2/, vms/ and so on) the shell scripts which generate config.h and
Makefile, as well as the metaconfig files which generate Configure.
(metaconfig isnt included in the core distribution.)
Interpreter
And of course, theres the core of the Perl interpreter itself.
Lets have a look at that in a little more detail.
Before we leave looking at the layout, though, dont forget that MANI
FEST contains not only the file names in the Perl distribution, but
short descriptions of whats in them, too. For an overview of the
important files, try this:
perl -lne print if /^[^\/]+\.[ch]\s+/ MANIFEST
Elements of the interpreter
The work of the interpreter has two main stages: compiling the code
into the internal representation, or bytecode, and then executing it.
"Compiled code" in perlguts explains exactly how the compilation stage
happens.
Here is a short breakdown of perls 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 inter
preter:
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 its going to be false in any sane con
text.
Line 4 calls a function in perl.c to allocate memory for a Perl
interpreter. Its quite a simple function, and the guts of it looks
like this:
my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
Here you see an example of Perls system abstraction, which well
see later: "PerlMem_malloc" is either your systems "malloc", or
Perls own "malloc" as defined in malloc.c if you selected that
option at configure time.
Next, in line 7, we construct the interpreter; this sets up all the
special variables that Perl needs, the stacks, and so on.
Now we pass Perl the command line options, and tell it to go:
exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
if (!exitstatus) {
exitstatus = perl_run(my_perl);
}
"perl_parse" is actually a wrapper around "S_parse_body", as defined
in perl.c, which processes the command line options, sets up any
statically linked XS modules, opens the program and calls "yyparse"
to parse it.
Parsing
The aim of this stage is to take the Perl source, and turn it into
an op tree. Well see what one of those looks like later. Strictly
speaking, theres three things going on here.
"yyparse", the parser, lives in perly.c, although youre better off
reading the original YACC input in perly.y. (Yes, Virginia, there is
a YACC grammar for Perl!) The job of the parser is to take your code
and "understand" it, splitting it into sentences, deciding which
operands go with which operators and so on.
The parser is nobly assisted by the lexer, which chunks up your
input into tokens, and decides what type of thing each token is: a
variable name, an operator, a bareword, a subroutine, a core func
tion, and so on. The main point of entry to the lexer is "yylex",
and that and its associated routines can be found in toke.c. Perl
isnt much like other computer languages; its highly context sensi
tive at times, it can be tricky to work out what sort of token some
thing is, or where a token ends. As such, theres a lot of interplay
between the tokeniser and the parser, which can get pretty frighten
ing if youre not used to it.
As the parser understands a Perl program, it builds up a tree of
operations for the interpreter to perform during execution. The rou
tines which construct and link together the various operations are
to be found in op.c, and will be examined later.
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 were finally ready to go: we have compiled Perl byte code, and
all thats left to do is run it. The actual execution is done by the
"runops_standard" function in run.c; more specifically, its done by
these three innocent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of that:
PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function pointer, which
stipulates the function which will actually carry out the operation.
This function will return the next op in the sequence - this allows
for things like "if" which choose the next op dynamically at run
time. The "PERL_ASYNC_CHECK" makes sure that things like signals
interrupt execution if required.
The actual functions called are known as PP code, and theyre spread
between four files: pp_hot.c contains the "hot" code, which is most
often used and highly optimized, pp_sys.c contains all the system-
specific functions, pp_ctl.c contains the functions which implement
control structures ("if", "while" and the like) and pp.c contains
everything else. These are, if you like, the C code for Perls
built-in functions and operators.
Note that each "pp_" function is expected to return a pointer to the
next op. Calls to perl subs (and eval blocks) are handled within the
same runops loop, and do not consume extra space on the C stack. For
example, "pp_entersub" and "pp_entertry" just push a "CxSUB" or
"CxEVAL" block struct onto the context stack which contain the
address of the op following the sub call or eval. They then return
the first op of that sub or eval block, and so execution continues
of that sub or block. Later, a "pp_leavesub" or "pp_leavetry" op
pops the "CxSUB" or "CxEVAL", retrieves the return op from it, and
returns it.
Exception handing
Perls exception handing (i.e. "die" etc) is built on top of the
low-level "setjmp()"/"longjmp()" C-library functions. These basi
cally provide a way to capture the current PC and SP registers and
later restore them; i.e. a "longjmp()" continues at the point in
code where a previous "setjmp()" was done, with anything further up
on the C stack being lost. This is why code should always save val
ues using "SAVE_FOO" rather than in auto variables.
The perl core wraps "setjmp()" etc in the macros "JMPENV_PUSH" and
"JMPENV_JUMP". The basic rule of perl exceptions is that "exit", and
"die" (in the absence of "eval") perform a JMPENV_JUMP(2), while
"die" within "eval" does a JMPENV_JUMP(3).
At entry points to perl, such as "perl_parse()", "perl_run()" and
"call_sv(cv, G_EVAL)" each does a "JMPENV_PUSH", then enter a runops
loop or whatever, and handle possible exception returns. For a 2
return, final cleanup is performed, such as popping stacks and call
ing "CHECK" or "END" blocks. Amongst other things, this is how scope
cleanup still occurs during an "exit".
If a "die" can find a "CxEVAL" block on the context stack, then the
stack is popped to that level and the return op in that block is
assigned to "PL_restartop"; then a JMPENV_JUMP(3) is performed.
This normally passes control back to the guard. In the case of
"perl_run" and "call_sv", a non-null "PL_restartop" triggers re-
entry to the runops loop. The is the normal way that "die" or
"croak" is handled within an "eval".
Sometimes ops are executed within an inner runops loop, such as tie,
sort or overload code. In this case, something like
sub FETCH { eval { die } }
would cause a longjmp right back to the guard in "perl_run", popping
both runops loops, which is clearly incorrect. One way to avoid this
is for the tie code to do a "JMPENV_PUSH" before executing "FETCH"
in the inner runops loop, but for efficiency reasons, perl in fact
just sets a flag, using "CATCH_SET(TRUE)". The "pp_require",
"pp_entereval" and "pp_entertry" ops check this flag, and if true,
they call "docatch", which does a "JMPENV_PUSH" and starts a new
runops level to execute the code, rather than doing it on the cur
rent loop.
As a further optimisation, on exit from the eval block in the
"FETCH", execution of the code following the block is still carried
on in the inner loop. When an exception is raised, "docatch" com
pares the "JMPENV" level of the "CxEVAL" with "PL_top_env" and if
they differ, just re-throws the exception. In this way any inner
loops get popped.
Heres an example.
1: eval { tie @a, A };
2: sub A::TIEARRAY {
3: eval { die };
4: die;
5: }
To run this code, "perl_run" is called, which does a "JMPENV_PUSH"
then enters a runops loop. This loop executes the eval and tie ops
on line 1, with the eval pushing a "CxEVAL" onto the context stack.
The "pp_tie" does a "CATCH_SET(TRUE)", then starts a second runops
loop to execute the body of "TIEARRAY". When it executes the
entertry op on line 3, "CATCH_GET" is true, so "pp_entertry" calls
"docatch" which does a "JMPENV_PUSH" and starts a third runops loop,
which then executes the die op. At this point the C call stack looks
like this:
Perl_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 optimization mentioned ear
lier. However, the context stack now looks like this, ie with the
top CxEVAL popped:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
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
Perls internal variable types: SVs, HVs, AVs and the rest. If not, do
that now.
These variables are used not only to represent Perl-space variables,
but also any constants in the code, as well as some structures com
pletely internal to Perl. The symbol table, for instance, is an ordi
nary Perl hash. Your code is represented by an SV as its read into the
parser; any program files you call are opened via ordinary Perl file
handles, and so on.
The core Devel::Peek module lets us examine SVs from a Perl program.
Lets see, for instance, how Perl treats the constant "hello".
% perl -MDevel::Peek -e Dump("hello")
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
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 lets go
through it line by line.
Line 1 tells us were looking at an SV which lives at 0xa04ecbc in mem
ory. SVs themselves are very simple structures, but they contain a
pointer to a more complex structure. In this case, its a PV, a struc
ture which holds a string value, at location 0xa041450. Line 2 is the
reference count; there are no other references to this data, so its 1.
Line 3 are the flags for this SV - its OK to use it as a PV, its a
read-only SV (because its a constant) and the data is a PV internally.
Next weve got the contents of the string, starting at location
0xa0484e0.
Line 5 gives us the current length of the string - note that this does
not include the null terminator. Line 6 is not the length of the
string, but the length of the currently allocated buffer; as the string
grows, Perl automatically extends the available storage via a routine
called "SvGROW".
You can get at any of these quantities from C very easily; just add
"Sv" to the name of the field shown in the snippet, and youve got a
macro which will return the value: "SvCUR(sv)" returns the current
length of the string, "SvREFCOUNT(sv)" returns the reference count,
"SvPV(sv, len)" returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in perlguts.
Lets take an example of manipulating a PV, from "sv_catpvn", in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = \0;
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length "len" onto the
end of the PV stored in "sv". The first thing we do in line 6 is make
sure that the SV has a valid PV, by calling the "SvPV_force" macro to
force a PV. As a side effect, "tlen" gets set to the current value of
the PV, and the PV itself is returned to "junk".
In line 7, we make sure that the SV will have enough room to accommo
date the old string, the new string and the null terminator. If "LEN"
isnt big enough, "SvGROW" will reallocate space for us.
Now, if "junk" is the same as the string were trying to add, we can
grab the string directly from the SV; "SvPVX" is the address of the PV
in the SV.
Line 10 does the actual catenation: the "Move" macro moves a chunk of
memory around: we move the string "ptr" to the end of the PV - thats
the start of the PV plus its current length. Were moving "len" bytes
of type "char". After doing so, we need to tell Perl weve extended the
string, by altering "CUR" to reflect the new length. "SvEND" is a macro
which gives us the end of the string, so that needs to be a "\0".
Line 13 manipulates the flags; since weve changed the PV, any IV or NV
values will no longer be valid: if we have "$a=10; $a.="6";" we dont
want to use the old IV of 10. "SvPOK_only_utf8" is a special
UTF-8-aware version of "SvPOK_only", a macro which turns off the IOK
and NOK flags and turns on POK. The final "SvTAINT" is a macro which
launders tainted data if taint mode is turned on.
AVs and HVs are more complicated, but SVs are by far the most common
variable type being thrown around. Having seen something of how we
manipulate these, lets go on and look at how the op tree is con
structed.
Op Trees
First, what is the op tree, anyway? The op tree is the parsed represen
tation of your program, as we saw in our section on parsing, and its
the sequence of operations that Perl goes through to execute your pro
gram, as we saw in "Running".
An op is a fundamental operation that Perl can perform: all the built-
in functions and operators are ops, and there are a series of ops which
deal with concepts the interpreter needs internally - entering and
leaving a block, ending a statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that there are
two "routes" through it, two orders in which you can traverse the tree.
First, parse order reflects how the parser understood the code, and
secondly, execution order tells perl what order to perform the opera
tions in.
The easiest way to examine the op tree is to stop Perl after it has
finished parsing, and get it to dump out the tree. This is exactly what
the compiler backends B::Terse, B::Concise and B::Debug do.
Lets have a look at how Perl sees "$a = $b + $c":
% perl -MO=Terse -e $a=$b+$c
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
Lets start in the middle, at line 4. This is a BINOP, a binary opera
tor, which is at location 0x8179828. The specific operator in question
is "sassign" - scalar assignment - and you can find the code which
implements it in the function "pp_sassign" in pp_hot.c. As a binary
operator, it has two children: the add operator, providing the result
of "$b+$c", is uppermost on line 5, and the left hand side is on line
10.
Line 10 is the null op: this does exactly nothing. What is that doing
there? If you see the null op, its a sign that something has been
optimized away after parsing. As we mentioned in "Optimization", the
optimization stage sometimes converts two operations into one, for
example when fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers, its eas
ier just to replace the redundant operation with the null op. Origi
nally, the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the "a" entry from the main symbol table, and then look
at the scalar component of it: "gvsv" ("pp_gvsv" into pp_hot.c) happens
to do both these things.
The right hand side, starting at line 5 is similar to what weve just
seen: we have the "add" op ("pp_add" also in pp_hot.c) add together two
"gvsv"s.
Now, whats this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
"enter" and "leave" are scoping ops, and their job is to perform any
housekeeping every time you enter and leave a block: lexical variables
are tidied up, unreferenced variables are destroyed, and so on. Every
program will have those first three lines: "leave" is a list, and its
children are all the statements in the block. Statements are delimited
by "nextstate", so a block is a collection of "nextstate" ops, with the
ops to be performed for each statement being the children of
"nextstate". "enter" is a single op which functions as a marker.
Thats how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c
However, its impossible to perform the operations in this order: you
have to find the values of $b and $c before you add them together, for
instance. So, the other thread that runs through the op tree is the
execution order: each op has a field "op_next" which points to the next
op to be run, so following these pointers tells us how perl executes
the code. We can traverse the tree in this order using the "exec"
option to "B::Terse":
% perl -MO=Terse,exec -e $a=$b+$c
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block, start a
statement. Get the values of $b and $c, and add them together. Find
$a, and assign one to the other. Then leave.
The way Perl builds up these op trees in the parsing process can be
unravelled by examining perly.y, the YACC grammar. Lets take the piece
we need to construct the tree for "$a = $b + $c"
1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
If youre not used to reading BNF grammars, this is how it works:
Youre fed certain things by the tokeniser, which generally end up in
upper case. Here, "ADDOP", is provided when the tokeniser sees "+" in
your code. "ASSIGNOP" is provided when "=" is used for assigning. These
are "terminal symbols", because you cant get any simpler than them.
The grammar, lines one and three of the snippet above, tells you how to
build up more complex forms. These complex forms, "non-terminal sym
bols" are generally placed in lower case. "term" here is a non-terminal
symbol, representing a single expression.
The grammar gives you the following rule: you can make the thing on the
left of the colon if you see all the things on the right in sequence.
This is called a "reduction", and the aim of parsing is to completely
reduce the input. There are several different ways you can perform a
reduction, separated by vertical bars: so, "term" followed by "=" fol
lowed by "term" makes a "term", and "term" followed by "+" followed by
"term" can also make a "term".
So, if you see two terms with an "=" or "+", between them, you can turn
them into a single expression. When you do this, you execute the code
in the block on the next line: if you see "=", youll do the code in
line 2. If you see "+", youll do the code in line 4. Its this code
which contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a number of
variables. The variables refer to the tokens: $1 is the first token in
the input, $2 the second, and so on - think regular expression backref
erences. $$ is the op returned from this reduction. So, we call "new
BINOP" to create a new binary operator. The first parameter to "new
BINOP", a function in op.c, is the op type. Its an addition operator,
so we want the type to be "ADDOP". We could specify this directly, but
its right there as the second token in the input, so we use $2. The
second parameter is the ops flags: 0 means "nothing special". Then the
things to add: the left and right hand side of our expression, in
scalar context.
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 its currently working on, and
well look at the three most important ones here.
Argument stack
Arguments are passed to PP code and returned from PP code using the
argument stack, "ST". The typical way to handle arguments is to pop
them off the stack, deal with them how you wish, and then push the
result back onto the stack. This is how, for instance, the cosine
operator works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
Well see a more tricky example of this when we consider Perls
macros below. "POPn" gives you the NV (floating point value) of the
top SV on the stack: the $x in "cos($x)". Then we compute the
cosine, and push the result back as an NV. The "X" in "XPUSHn" means
that the stack should be extended if necessary - it cant be neces
sary here, because we know theres room for one more item on the
stack, since weve just removed one! The "XPUSH*" macros at least
guarantee safety.
Alternatively, you can fiddle with the stack directly: "SP" gives
you the first element in your portion of the stack, and "TOP*" gives
you the top SV/IV/NV/etc. on the stack. So, for instance, to do
unary negation of an integer:
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 descrip
tion of the macros used in stack manipulation.
Mark stack
I say "your portion of the stack" above because PP code doesnt nec
essarily get the whole stack to itself: if your function calls
another function, youll only want to expose the arguments aimed for
the called function, and not (necessarily) let it get at your own
data. The way we do this is to have a "virtual" bottom-of-stack,
exposed to each function. The mark stack keeps bookmarks to loca
tions in the argument stack usable by each function. For instance,
when dealing with a tied variable, (internally, something with "P"
magic) Perl has to call methods for accesses to the tied variables.
However, we need to separate the arguments exposed to the method to
the argument exposed to the original function - the store or fetch
or whatever it may be. Heres how the tied "push" is implemented;
see "av_push" in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
9 POPSTACK;
The lines which concern the mark stack are the first, fifth and last
lines: they save away, restore and remove the current position of
the argument stack.
Lets examine the whole implementation, for practice:
1 PUSHMARK(SP);
Push the current state of the stack pointer onto the mark stack.
This is so that when weve finished adding items to the argument
stack, Perl knows how many things weve added recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
Were going to add two more items onto the argument stack: when you
have a tied array, the "PUSH" subroutine receives the object and the
value to be pushed, and thats exactly what we have here - the tied
object, retrieved with "SvTIED_obj", and the value, the SV "val".
5 PUTBACK;
Next we tell Perl to make the change to the global stack pointer:
"dSP" only gave us a local copy, not a reference to the global.
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
"ENTER" and "LEAVE" localise a block of code - they make sure that
all variables are tidied up, everything that has been localised gets
its previous value returned, and so on. Think of them as the "{" and
"}" of a Perl block.
To actually do the magic method call, we have to call a subroutine
in Perl space: "call_method" takes care of that, and its described
in perlcall. We call the "PUSH" method in scalar context, and were
going to discard its return value.
9 POPSTACK;
Finally, we remove the value we placed on the mark stack, since we
dont need it any more.
Save stack
C doesnt have a concept of local scope, so perl provides one. Weve
seen that "ENTER" and "LEAVE" are used as scoping braces; the save
stack implements the C equivalent of, for example:
{
local $foo = 42;
...
}
See "Localising Changes" in perlguts for how to use the save stack.
Millions of Macros
One thing youll notice about the Perl source is that its full of
macros. Some have called the pervasive use of macros the hardest thing
to understand, others find it adds to clarity. Lets take an example,
the code which implements the addition operator:
1 PP(pp_add)
2 {
3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
4 {
5 dPOPTOPnnrl_ul;
6 SETn( left + right );
7 RETURN;
8 }
9 }
Every line here (apart from the braces, of course) contains a macro.
The first line sets up the function declaration as Perl expects for PP
code; line 3 sets up variable declarations for the argument stack and
the target, the return value of the operation. Finally, it tries to see
if the addition operation is overloaded; if so, the appropriate subrou
tine is called.
Line 5 is another variable declaration - all variable declarations
start with "d" - which pops from the top of the argument stack two NVs
(hence "nn") and puts them into the variables "right" and "left", hence
the "rl". These are the two operands to the addition operator. Next, we
call "SETn" to set the NV of the return value to the result of adding
the two values. This done, we return - the "RETURN" macro makes sure
that our return value is properly handled, and we pass the next opera
tor to run back to the main run loop.
Most of these macros are explained in perlapi, and some of the more
important ones are explained in perlxs as well. Pay special attention
to "Background and PERL_IMPLICIT_CONTEXT" in perlguts for information
on the "[pad]THX_?" macros.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Dont be scared by the
results.
Poking at Perl
To really poke around with Perl, youll 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 informa
tion which will allow us to step through a running program. Configure
will also turn on the "DEBUGGING" compilation symbol which enables all
the internal debugging code in Perl. There are a whole bunch of things
you can debug with this: perlrun lists them all, and the best way to
find out about them is to play about with them. The most useful options
are probably
l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be achieved using
XS modules.
-Dr => use re debug
-Dx => use O Debug
Using a source-level debugger
If the debugging output of "-D" doesnt help you, its time to step
through perls execution with a source-level debugger.
Well use "gdb" for our examples here; the principles will apply to
any debugger, but check the manual of the one youre using.
To fire up the debugger, type
gdb ./perl
Youll want to do that in your Perl source tree so the debugger can
read the source code. You should see the copyright message, followed by
the prompt.
(gdb)
"help" will get you into the documentation, but here are the most use
ful commands:
run [args]
Run the program with the given arguments.
break function_name
break source.c:xxx
Tells the debugger that well want to pause execution when we reach
either the named function (but see "Internal Functions" in
perlguts!) or the given line in the named source file.
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 - its 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 necessarily support macros
(see later "gdb macro support"). Youll have to substitute them
yourself, or to invoke cpp on the source code files (see "The .i
Targets") So, for instance, you cant say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a "macro dictionary", which you can
produce by saying "cpp -dM perl.c | sort". Even then, cpp wont recur
sively apply those macros for you.
gdb macro support
Recent versions of gdb have fairly good macro support, but in order to
use it youll need to compile perl with macro definitions included in
the debugging information. Using gcc version 3.1, this means configur
ing with "-Doptimize=-g3". Other compilers might use a different
switch (if they support debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping functions
in dump.c; these work a little like an internal Devel::Peek, but they
also cover OPs and other structures that you cant get at from Perl.
Lets take an example. Well use the "$a = $b + $c" we used before, but
give it a bit of context: "$b = "6XXXX"; $c = 2.3;". Wheres a good
place to stop and poke around?
What about "pp_add", the function we examined earlier to implement the
"+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
in perlguts. With the breakpoint in place, we can run our program:
(gdb) run -e $b = "6XXXX"; $c = 2.3; $a = $b + $c
Lots of junk will go past as gdb reads in the relevant source files and
libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(gdb) step
311 dPOPTOPnnrl_ul;
(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" - lets
slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
"POPn" takes the SV from the top of the stack and obtains its NV either
directly (if "SvNOK" is set) or by calling the "sv_2nv" function.
"TOPs" takes the next SV from the top of the stack - yes, "POPn" uses
"TOPs" - but doesnt remove it. We then use "SvNV" to get the NV from
"leftsv" in the same way as before - yes, "POPn" uses "SvNV".
Since we dont have an NV for $b, well have to use "sv_2nv" to convert
it. If we step again, well find ourselves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(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 were going to get 6 from this, so lets finish the subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always stored in
"PL_op", and we can dump it with "Perl_op_dump". Thisll give us simi
lar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
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, weve now had a look at how to navigate the Perl sources and
some things youll need to know when fiddling with them. Lets now get
on and create a simple patch. Heres something Larry suggested: if a
"U" is the first active format during a "pack", (for example, "pack
"U3C8", @stuff") then the resulting string should be treated as UTF-8
encoded.
How do we prepare to fix this up? First we locate the code in question
- the "pack" happens at runtime, so its going to be in one of the pp
files. Sure enough, "pp_pack" is in pp.c. Since were going to be
altering this file, lets copy it to pp.c~.
[Well, it was in pp.c when this tutorial was written. It has now been
split off with "pp_unpack" to its own file, pp_pack.c]
Now lets look over "pp_pack": we take a pattern into "pat", and then
loop over the pattern, taking each format character in turn into
"datum_type". Then for each possible format character, we swallow up
the other arguments in the pattern (a field width, an asterisk, and so
on) and convert the next chunk input into the specified format, adding
it onto the output SV "cat".
How do we know if the "U" is the first format in the "pat"? Well, if we
have a pointer to the start of "pat" then, if we see a "U" we can test
whether were still at the start of the string. So, heres where "pat"
is set up:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
SV *fromstr;
Well have another string pointer in there:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
+ char *patcopy;
register I32 len;
I32 datumtype;
SV *fromstr;
And just before we start the loop, well set "patcopy" to be the start
of "pat":
items = SP - MARK;
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 swallowed into "datumtype!"
Oops, we forgot one thing: what if there are spaces at the start of the
pattern? "pack(" U*", @stuff)" will have "U" as the first active char
acter, even though its not the first thing in the pattern. In this
case, we have to advance "patcopy" along with "pat" when we see spaces:
if (isSPACE(datumtype))
continue;
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
OK. Thats the C part done. Now we must do two additional things before
this patch is ready to go: weve changed the behaviour of Perl, and so
we must document that change. We must also provide some more regression
tests to make sure our patch works and doesnt create a bug somewhere
else along the line.
The regression tests for each operator live in t/op/, and so we make a
copy of t/op/pack.t to t/op/pack.t~. Now we can add our tests to the
end. First, well test that the "U" does indeed create Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didnt we could use
the one from t/test.pl.
require ./test.pl;
plan( tests => 159 );
so instead of this:
print not unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
we can write the more sensible (see Test::More for a full explanation
of is() and other testing functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces unicode" );
Now well test that we got that space-at-the-beginning business right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
And finally well test that we dont make Unicode strings if "U" is not
the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isnt unicode" );
Mustnt forget to change the number of tests which appears at the top,
or else the automated tester will get confused. This will either look
like this:
print "1..156\n";
or this:
plan( tests => 156 );
We now compile up Perl, and run it through the test suite. Our new
tests pass, hooray!
Finally, the documentation. The job is never done until the paperwork
is over, so lets describe the change weve just made. The relevant
place is pod/perlfunc.pod; again, we make a copy, and then well insert
this text in the description of "pack":
=item *
If the pattern begins with a C, the resulting string will be treated
as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
with an initial C, and the bytes that follow will be interpreted as
Unicode characters. If you dont want this to happen, you can begin your
pattern with C (or anything else) to force Perl not to UTF-8 encode your
string, and then follow this with a C somewhere in your pattern.
All done. Now lets create the patch. Porting/patching.pod tells us
that if were making major changes, we should copy the entire directory
to somewhere safe before we begin fiddling, and then do
diff -ruN old new > patch
However, we know which files weve changed, and we can simply do this:
diff -u pp.c~ pp.c > patch
diff -u t/op/pack.t~ t/op/pack.t >> patch
diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
We end up with a patch looking a little like this:
--- pp.c~ Fri Jun 02 04:34:10 2000
+++ pp.c Fri Jun 16 11:37:25 2000
@@ -4375,6 +4375,7 @@
register I32 items;
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
+ char *patcopy;
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
@@ -4405,6 +4406,7 @@
...
And finally, we submit it, with our rationale, to perl5-porters. Job
done!
Patching a core module
This works just like patching anything else, with an extra considera
tion. Many core modules also live on CPAN. If this is so, patch the
CPAN version instead of the core and send the patch off to the module
maintainer (with a copy to p5p). This will help the module maintainer
keep the CPAN version in sync with the core version without constantly
scanning p5p.
The list of maintainers of core modules is usefully documented in Port
ing/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 espe
cially good idea, you decide to add a new function to the core, discuss
your ideas on p5p well before you start work. It may be that someone
else has already attempted to do what you are considering and can give
lots of good advice or even provide you with bits of code that they
already started (but never finished).
You have to follow all of the advice given above for patching. It is
extremely important to test any addition thoroughly and add new tests
to explore all boundary conditions that your new function is expected
to handle. If your new function is used only by one module (e.g.
toke), then it should probably be named S_your_function (for static);
on the other hand, if you expect it to accessible from other functions
in Perl, you should name it Perl_your_function. See "Internal Func
tions" in perlguts for more details.
The location of any new code is also an important consideration. Dont
just create a new top level .c file and put your code there; you would
have to make changes to Configure (so the Makefile is created prop
erly), as well as possibly lots of include files. This is strictly
pumpking business.
It is better to add your function to one of the existing top level
source code files, but your choice is complicated by the nature of the
Perl distribution. Only the files that are marked as compiled static
are located in the perl executable. Everything else is located in the
shared library (or DLL if you are running under WIN32). So, for exam
ple, if a function was only used by functions located in toke.c, then
your code can go in toke.c. If, however, you want to call the function
from universal.c, then you should put your code in another location,
for example util.c.
In addition to writing your c-code, you will need to create an appro
priate entry in embed.pl describing your function, then run make
regen_headers to create the entries in the numerous header files that
perl needs to compile correctly. See "Internal Functions" in perlguts
for information on the various options that you can set in embed.pl.
You will forget to do this a few (or many) times and you will get warn
ings during the compilation phase. Make sure that you mention this
when you post your patch to P5P; the pumpking needs to know this.
When you write your new code, please be conscious of existing code con
ventions used in the perl source files. See perlstyle for details.
Although most of the guidelines discussed seem to focus on Perl code,
rather than c, they all apply (except when they dont ;). See also
Porting/patching.pod file in the Perl source distribution for lots of
details about both formatting and submitting patches of your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to p5p. Test
on as many platforms as you can find. Test as many perl Configure
options as you can (e.g. MULTIPLICITY). If you have profiling or mem
ory tools, see "EXTERNAL TOOLS FOR DEBUGGING PERL" below for how to use
them to further test your code. Remember that most of the people on
P5P are doing this on their own time and dont have the time to debug
your code.
Writing a test
Every module and built-in function has an associated test file (or
should...). If you add or change functionality, you have to write a
test. If you fix a bug, you have to write a test so that bug never
comes back. If you alter the docs, it would be nice to test what the
new documentation says.
In short, if you submit a patch you probably also have to patch the
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 basically 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", subrou
tines, 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 shouldnt put anything new in
here. There are still some bits and pieces hanging around in here
that need to be moved. Perhaps you could move them? Thanks!
t/op/
Tests for perls built in functions that dont 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 con
siderations.
There are three ways to write a test in the core. Test::More,
t/test.pl and ad hoc "print $test ? "ok 42\n" : "not ok 42\n"". The
decision of which to use depends on what part of the test suite youre
working on. This is a measure to prevent a high-level failure (such as
Config.pm breaking) from causing basic functionality tests to fail.
t/base t/comp
Since we dont know if require works, or even subroutines, use ad
hoc tests for these two. Step carefully to avoid using the feature
being tested.
t/cmd t/run t/io t/op
Now that basic require() and subroutines are tested, you can use
the t/test.pl library which emulates the important features of
Test::More while using a minimum of core features.
You can also conditionally use certain libraries like Config, but
be sure to skip the test gracefully if its not there.
t/lib ext lib
Now that the core of Perl is tested, Test::More can be used. You
can also use the full suite of core modules in the tests.
When you say "make test" Perl uses the t/TEST program to run the test
suite (except under Win32 where it uses t/harness instead.) All tests
are run from the t/ directory, not the directory which contains the
test. This causes some problems with the tests in lib/, so heres some
opportunity for some patching.
You must be triply conscious of cross-platform concerns. This usually
boils down to using File::Spec and avoiding things like "fork()" and
"system()" unless absolutely necessary.
Special Make Test Targets
There are various special make targets that can be used to test Perl
slightly differently than the standard "test" target. Not all them are
expected to give a 100% success rate. Many of them have several
aliases, and many of them are not available on certain operating sys
tems.
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 theyre specifically 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 test
name.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 Abigails JAPHs, t/japh/abigail.t.
You can also run the torture test with t/harness by giving "-tor
ture" argument to t/harness.
utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will succeed.
(Not available on Win32)
minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with different
versions of this encoding.
"make utest.utf16" runs the test suite with a combination of
"-utf8" and "-utf16" arguments to t/TEST.
(Not available on Win32)
test_harness
Run the test suite with the t/harness controlling program, instead
of t/TEST. t/harness is more sophisticated, and uses the Test::Har
ness module, thus using this test target supposes that perl mostly
works. The main advantage for our purposes is that it prints a
detailed summary of failed tests at the end. Also, unlike t/TEST,
it doesnt redirect stderr to stdout.
Note that under Win32 t/harness is always used instead of t/TEST,
so there is no special "test_harness" target.
Under Win32s "test" target you may use the TEST_SWITCHES and
TEST_FILES environment variables to control the behaviour of t/har
ness. This means you can say
nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal test.
Running tests by hand
You can run part of the test suite by hand by using one the following
commands from the t/ directory :
./perl -I../lib TEST list-of-.t-files
or
./perl -I../lib harness list-of-.t-files
(if you dont specify test scripts, the whole test suite will be run.)
Using t/harness for testing
If you use "harness" for testing you have several command line options
available to you. The arguments are as follows, and are in the order
that they must appear if used together.
harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH
If "LIST OF FILES TO TEST" is omitted the file list is obtained from
the manifest. The file list may include shell wildcards which will be
expanded out.
-v Run the tests under verbose mode so you can see what tests were
run, and debug outbut.
-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 were 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 isnt set already (see "PERL_DESTRUCT_LEVEL")
PERL
(used only by t/TEST) if set, overrides the path to the perl exe
cutable that should be used to run the tests (the default being
./perl).
PERL_SKIP_TTY_TEST
if set, tells to skip the tests that need a terminal. Its actually
set automatically by the Makefile, but can also be forced artifi
cially by running make test_notty.
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 test
ing and debugging tools with Perl. This is meant as a guide to inter
facing these tools with Perl, not as any kind of guide to the use of
the tools themselves.
NOTE 1: Running under memory debuggers such as Purify, valgrind, or
Third Degree greatly slows down the execution: seconds become minutes,
minutes become hours. For example as of Perl 5.8.1, the
ext/Encode/t/Unicode.t takes extraordinarily long to complete under
e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
than six hours, even on a snappy computer-- the said test must be doing
something that is quite unfriendly for memory debuggers. If you dont
feel like waiting, that you can simply kill away the perl process.
NOTE 2: To minimize the number of memory leak false alarms (see
"PERL_DESTRUCT_LEVEL" for more information), you have to have environ
ment variable PERL_DESTRUCT_LEVEL set to 2. The TEST and harness
scripts do that automatically. But if you are running some of the
tests manually-- for csh-like shells:
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 Softwares Purify
Purify is a commercial tool that is helpful in identifying memory over
runs, wild pointers, memory leaks and other such badness. Perl must be
compiled in a specific way for optimal testing with Purify. Purify is
available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most benefit out
of Purify, you should create the perl to Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize=-g \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
-Accflags=-DPURIFY
Disables Perls arena memory allocation functions, as well as forc
ing use of memory allocation functions derived from the system mal
loc.
-Doptimize=-g
Adds debugging information so that you see the exact source state
ments where the problem occurs. Without this flag, all you will
see is the source filename of where the error occurred.
-Uusemymalloc
Disable Perls malloc so that Purify can more closely monitor allo
cations and leaks. Using Perls malloc will make Purify report
most leaks in the "potential" leaks category.
-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 youve compiled a perl suitable for Purifying, then you can just:
make pureperl
which creates a binary named pureperl that has been Purifyed. This
binary is used in place of the standard perl binary when you want to
debug Perl memory problems.
As an example, to show any memory leaks produced during the standard
Perl testset you would create and run the Purifyed perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory problems.
Purify outputs messages in "Viewer" windows by default. If you dont
have a windowing environment or if you simply want the Purify output to
unobtrusively go to a log file instead of to the interactive window,
use these following options to output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the "Viewer" windows, then you only need this
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 makefile 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 Perls 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 Perls malloc so that Purify can more closely monitor allo
cations and leaks. Using Perls malloc will make Purify report
most leaks in the "potential" leaks category.
CFG = Debug
Adds debugging information so that you see the exact source state
ments where the problem occurs. Without this flag, all you will
see is the source filename of where the error occurred.
As an example, to show any memory leaks produced during the standard
Perl testset you would create and run Purify as:
cd win32
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/
Compaqs/Digitals/HPs Third Degree
Third Degree is a tool for memory leak detection and memory access
checks. It is one of the many tools in the ATOM toolkit. The toolkit
is only available on Tru64 (formerly known as Digital UNIX formerly
known as DEC OSF/1).
When building Perl, you must first run Configure with -Doptimize=-g and
-Uusemymalloc flags, after that you can use the make targets
"perl.third" and "test.third". (What is required is that Perl must be
compiled using the "-g" flag, you may need to re-Configure.)
The short story is that with "atom" you can instrument the Perl exe
cutable to create a new executable called perl.third. When the instru
mented executable is run, it creates a log of dubious memory traffic in
file called perl.3log. See the manual pages of atom and third for more
information. The most extensive Third Degree documentation is avail
able in the Compaq "Tru64 UNIX Programmers Guide", chapter "Debugging
Programs with Third Degree".
The "test.third" leaves a lot of files named foo_bar.3log in the t/
subdirectory. There is a problem with these files: Third Degree is so
effective that it finds problems also in the system libraries. There
fore you should used the Porting/thirdclean script to cleanup the
*.3log files.
There are also leaks that for given certain definition of a leak,
arent. See "PERL_DESTRUCT_LEVEL" for more information.
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using e.g. val
grind, or the pureperl or perl.third executables, please note that by
default perl does not explicitly cleanup all the memory it has allo
cated (such as global memory arenas) but instead lets the exit() of the
whole program "take care" of such allocations, also known as "global
destruction of objects".
There is a way to tell perl to do complete cleanup: set the environment
variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
does set this to 2, and this is what you need to do too, if you dont
want to see the "global leaks": For example, for "third-degreed" Perl:
env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
(Note: the mod_perl apache module uses also this environment variable
for its own purposes and extended its semantics. Refer to the mod_perl
documentation for more information. Also, spawned threads do the equiv
alent of setting this variable to the value 1.)
If, at the end of a run you get the message N scalars leaked, you can
recompile with "-DDEBUG_LEAKING_SCALARS", which will cause the
addresses of all those leaked SVs to be dumped; it also converts
"new_SV()" from a macro into a real function, so you can use your
favourite debugger to discover where those pesky SVs were allocated.
Profiling
Depending on your platform there are various of profiling Perl.
There are two commonly used techniques of profiling executables: sta
tistical time-sampling and basic-block counting.
The first method takes periodically samples of the CPU program counter,
and since the program counter can be correlated with the code generated
for functions, we get a statistical view of in which functions the
program is spending its time. The caveats are that very small/fast
functions have lower probability of showing up in the profile, and that
periodically interrupting the program (this is usually done rather fre
quently, in the scale of milliseconds) imposes an additional overhead
that may skew the results. The first problem can be alleviated by run
ning the code for longer (in general this is a good idea for profil
ing), the second problem is usually kept in guard by the profiling
tools themselves.
The second method divides up the generated code into basic blocks.
Basic blocks are sections of code that are entered only in the begin
ning and exited only at the end. For example, a conditional jump
starts a basic block. Basic block profiling usually works by instru
menting the code by adding enter basic block #nnnn book-keeping code to
the generated code. During the execution of the code the basic block
counters are then updated appropriately. The caveat is that the added
extra code can skew the results: again, the profiling tools usually try
to factor their own effects out of the results.
Gprof Profiling
gprof is a profiling tool available in many UNIX platforms, it uses
statistical time-sampling.
You can build a profiled version of perl called "perl.gprof" by invok
ing the make target "perl.gprof" (What is required is that Perl must
be compiled using the "-pg" flag, you may need to re-Configure). Run
ning the profiled version of Perl will create an output file called
gmon.out is created which contains the profiling data collected during
the execution.
The gprof tool can then display the collected data in various ways.
Usually gprof understands the following options:
-a Suppress statically defined functions from the profile.
-b Suppress the verbose descriptions in the profile.
-e routine
Exclude the given routine and its descendants from the profile.
-f routine
Display only the given routine and its descendants in the profile.
-s Generate a summary file called gmon.sum which then may be given to
subsequent gprof runs to accumulate data over several runs.
-z Display routines that have zero usage.
For more detailed explanation of the available commands and output for
mats, see your own local documentation of gprof.
GCC gcov Profiling
Starting from GCC 3.0 basic block profiling is officially available for
the GNU CC.
You can build a profiled version of perl called perl.gcov by invoking
the make target "perl.gcov" (what is required that Perl must be com
piled using gcc with the flags "-fprofile-arcs -ftest-coverage", you
may need to re-Configure).
Running the profiled version of Perl will cause profile output to be
generated. For each source file an accompanying ".da" file will be
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 frequencies of execution indicated
by "#" markers.
Useful options of gcov include "-b" which will summarise the basic
block, branch, and function call coverage, and "-c" which instead of
relative frequencies will use the actual counts. For more information
on the use of gcov and basic block profiling with gcc, see the latest
GNU CC manual, as of GCC 3.0 see
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 con
tains the counts for the basic block for that particular program execu
tion.
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 exe
cuted in each procedure. Useful for finding the hotspot proce
dures. (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 procedures.
-testcoverage
The compiler emitted code for these lines, but the code was unexe
cuted.
-z[ero]
Unexecuted procedures.
For further information, see your systems 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 SVs IV value with one click, without doing any
typing. To do that simply edit ~/.ddd/init file and add after:
! Display shortcuts.
Ddd*gdbDisplayShortcuts: \
/t () // Convert to Bin\n\
/d () // Convert to Dec\n\
/x () // Convert to Hex\n\
/o () // Convert to Oct(\n\
the following two lines:
((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
so now you can do ivx and pvx lookups or you can plug there the
sv_peek "conversion":
Perl_sv_peek(my_perl, (SV*)()) // sv_peek
(The my_perl is for threaded builds.) Just remember that every
line, but the last one, should end with \n\
Alternatively edit the init file interactively via: 3rd mouse but
ton -> New Display -> Edit Menu
Note: you can define up to 20 conversion shortcuts in the gdb sec
tion.
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
Weve had a brief look around the Perl source, an overview of the
stages perl goes through when its running your code, and how to use a
debugger to poke at the Perl guts. We took a very simple problem and
demonstrated how to solve it fully - with documentation, regression
tests, and finally a patch for submission to p5p. Finally, we talked
about how to use external tools to debug and test Perl.
Id now suggest you read over those references again, and then, as soon
as possible, get your hands dirty. The best way to learn is by doing,
so:
Subscribe to perl5-porters, follow the patches and try and under
stand them; dont be afraid to ask if theres a portion youre not
clear on - who knows, you may unearth a bug in the patch...
Keep up to date with the bleeding edge Perl distributions and get
familiar with the changes. Try and get an idea of what areas people
are working on and the changes theyre making.
Do read the README associated with your operating system, e.g.
README.aix on the IBM AIX OS. Dont hesitate to supply patches to
that README if you find anything missing or changed over a new OS
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! Youll probably
get to understand not just your chosen area but a much wider range
of perls activity as well, and probably sooner than youd think.
The Road goes ever on and on, down from the door where it began.
If you can do these things, youve started on the long road to Perl
porting. Thanks for wanting to help make Perl better - and happy hack
ing!
AUTHOR
This document was written by Nathan Torkington, and is maintained by
the perl5-porters mailing list.
perl v5.8.8 2008-04-25 PERLHACK(1)
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