perlhack

PERLHACK(K) Perl Programmers Reference Guide PERLHACK(K)

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 dis-
cussed at a time.

A searchable archive of the list is at:

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

The list is also archived under the usenet group name
"perl.porters-gw" at:

http://www.deja.com/

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

Over this group of porters presides Larry Wall. He has
the final word in what does and does not change in the
Perl language. Various releases of Perl are shepherded by
a ``pumpking'', a porter responsible for gathering
patches, deciding on a patch-by-patch feature-by-feature
basis what will and will not go into the release. For
instance, Gurusamy Sarathy is the pumpking for the 5.6
release of Perl.

In addition, various people are pumpkings for different
things. For instance, Andy Dougherty and Jarkko
Hietaniemi share the Configure pumpkin, and Tom Chris-
tiansen is the documentation pumpking.

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

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

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

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

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

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

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

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

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

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

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

Is the feature generic enough?
Is this something that only the submitter wants added
to the language, or would it be broadly useful? Some-
times, instead of adding a feature with a tight focus,
the porters might decide to wait until someone imple-
ments the more generalized feature. For instance,
instead of implementing a ``delayed evaluation'' fea-
ture, 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 inter-
preter 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 proto-
types is likely to be rejected because there are still
options for the future of prototypes that haven't been
addressed.

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

Is the implementation generic enough to be portable?
The worst patches make use of a system-specific fea-
tures. It's highly unlikely that nonportable addi-
tions to the Perl language will be accepted.

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

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

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

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

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

Keeping in sync

The source code to the Perl interpreter, in its different
versions, is kept in a repository managed by a revision
control system (which is currently the Perforce program,
see http://perforce.com/). The pumpkings and a few others
have access to the repository to check in changes. Peri-
odically the pumpking for the development version of Perl
will release a new version, so the rest of the porters can
see what's changed. The current state of the main trunk
of repository, and patches that describe the individual
changes that have happened since the last public release
are available at this location:

ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/

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

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

Keeping in sync with the most recent branch can be done in
several ways, but the most convenient and reliable way is
using rsync, available at ftp://rsync.samba.org/pub/rsync/
. (You can also get the most recent branch by FTP.)

If you choose to keep in sync using rsync, there are two
approaches to doing so:

rsync'ing the source tree
Presuming you are in the directory where your perl
source resides and you have rsync installed and avail-
able, you can `upgrade' to the bleadperl using:

# rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .

This takes care of updating every single item in the
source tree to the latest applied patch level, creat-
ing files that are new (to your distribution) and set-
ting date/time stamps of existing files to reflect the
bleadperl status.

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

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

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 a inetd entry or a deamon. You must, however,
have a working rsh or ssh system. Using ssh is recommended for
its security features."

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

#!/usr/bin/perl -w

use strict;
use File::Copy;

my %MF = map {
m/(\S+)/;
$1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
} `cat MANIFEST`;

my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);

foreach my $host (keys %remote) {
unless (-d $remote{$host}) {
print STDERR "Cannot Xsync for host $host\n";
next;
}
foreach my $file (keys %MF) {
my $rfile = "$remote{$host}/$file";
my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
defined $size or ($mode, $size, $mtime) = (0, 0, 0);
$size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
printf "%4s %-34s %8d %9d %8d %9d\n",
$host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
unlink $rfile;
copy ($file, $rfile);
utime time, $MF{$file}[2], $rfile;
chmod $MF{$file}[0], $rfile;
}
}

though this is not perfect. It could be improved
with checking file checksums before updating. Not
all NFS systems support reliable utime support
(when used over the NFS).

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

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

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

This makes sure the latest available patch is down-
loaded to your patch directory.

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

# last=`ls -rt1 *.gz | tail -1`
# rsync -avz rsync://ftp.linux.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

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

It's more recent
According to Gurusamy Sarathy:

"... The rsync mirror is automatic and syncs with the repository
every five minutes.

"Updating the patch area still requires manual intervention
(with all the goofiness that implies, which you've noted) and
is typically on a daily cycle. Making this process automatic
is on my tuit list, but don't ask me when."

It's more reliable
Well, since the patches are updated by hand, I don't
have to say any more ... (see Sarathy's remark).

Why rsync the patches

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

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

It's a good reference
If you do not only like to have the most recent devel-
opment branch, but also like to fix bugs, or extend
features, you want to dive into the sources. If you
are a seasoned perl core diver, you don't need no man-
uals, 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 occa-
sions, he releases a tar-ball of the current source
tree (i.e. perl@7582.tar.gz), which will be an excel-
lent 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 func-
tion/feature Foo, just scan the patches for
patches that mention Foo either in the subject,
the comments, or the body of the fix. A good
chance the patch shows you the files that are
affected by that patch which are very likely to be
the starting point of your journey into the guts
of perl.

Finding how to fix a bug
If you've found where the function/feature Foo
misbehaves, but you don't know how to fix it (but
you do know the change you want to make), you can,
again, peruse the patches for similar changes and
look how others apply the fix.

Finding the source of misbehaviour
When you keep in sync with bleadperl, the pumpking
would love to see that the community efforts realy
work. So after each of his sync points, you are to
'make test' to check if everything is still in
working order. If it is, you do 'make ok', which
will send an OK report to perlbug@perl.org. (If
you do not have access to a mailer from the system
you just finished successfully 'make test', you
can do 'make okfile', which creates the file
"perl.ok", which you can than take to your
favourite mailer and mail yourself).

But of course, as always, things will not allways
lead to a success path, and one or more test do
not pass the 'make test'. Before sending in a bug
report (using 'make nok' or 'make nokfile'), check
the mailing list if someone else has reported the
bug already and if so, confirm it by replying to
that message. If not, you might want to trace the
source of that misbehaviour before sending in the
bug, which will help all the other porters in
finding the solution.

Here the saved patches come in very handy. You can
check the list of patches to see which patch
changed what file and what change caused the mis-
behaviour. If you note that in the bug report, it
saves the one trying to solve it, looking for that
point.

If searching the patches is too bothersome, you might
consider using perl's bugtron to find more information
about discussions and ramblings on posted bugs.

If you want to get the best of both worlds, rsync both the
source tree for convenience, reliability and ease and
rsync the patches for reference.

Submitting patches

Always submit patches to perl5-porters@perl.org. This
lets other porters review your patch, which catches a sur-
prising number of errors in patches. Either use the diff
program (available in source code form from
ftp://ftp.gnu.org/pub/gnu/), or use Johan Vromans'
makepatch (available from CPAN/authors/id/JV/). Unified
diffs are preferred, but context diffs are accepted. Do
not send RCS-style diffs or diffs without context lines.
More information is given in the Porting/patching.pod file
in the Perl source distribution. Please patch against the
latest development version (e.g., if you're fixing a bug
in the 5.005 track, patch against the latest 5.005_5x ver-
sion). Only patches that survive the heat of the develop-
ment branch get applied to maintenance versions.

Your patch should update the documentation and test suite.

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

The CPAN testers (http://testers.cpan.org/) are a group of
volunteers who test CPAN modules on a variety of plat-
forms. Perl Labs (http://labs.perl.org/) automatically
tests Perl source releases on platforms and gives feedback
to the CPAN testers mailing list. Both efforts welcome
volunteers.

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

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

To hack on the Perl guts, you'll need to read the follow-
ing things:

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

You might also want to look at Gisle Aas's illustrated
perlguts - there's no guarantee that this will be abso-
lutely up-to-date with the latest documentation in the
Perl core, but the fundamentals will be right.
(http://gisle.aas.no/perl/illguts/)

perlxstut and perlxs
A working knowledge of XSUB programming is incredibly
useful for core hacking; XSUBs use techniques drawn
from the PP code, the portion of the guts that actually
executes a Perl program. It's a lot gentler to learn
those techniques from simple examples and explanation
than from the core itself.

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

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

The perl5-porters FAQ
This is posted to perl5-porters at the beginning on
every month, and should be available from http://perl-
hacker.org/p5p-faq; alternatively, you can get the FAQ
emailed to you by sending mail to
"perl5-porters-faq@perl.org". It contains hints on
reading perl5-porters, information on how perl5-porters
works and how Perl development in general works.

Finding Your Way Around

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

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

Documentation
Documentation maintenance includes looking after every-
thing in the pod/ directory, (as well as contributing
new documentation) and the documentation to the modules
in core.

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

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

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

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

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

Elements of the interpreter

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

Here is a short breakdown of perl's operation:

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

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

1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(t);
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 vari-
ables in Perl start with "PL_". This tells you whether
the current running program was created with the "-u"
flag to perl and then undump, which means it's going to
be false in any sane context.

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

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

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

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

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

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

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

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

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

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

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

Optimization
Now the parsing stage is complete, and the finished
tree represents the operations that the Perl inter-
preter needs to perform to execute our program. Next,
Perl does a dry run over the tree looking for optimisa-
tions: 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 compo-
nent, the optimizer fiddles the op tree to use a func-
tion which directly looks up the scalar in question.
The main optimizer is "peep" in op.c, and many ops have
their own optimizing functions.

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

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

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

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

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

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

Internal Variable Types

You should by now have had a look at perlguts, which tells
you about Perl's internal variable types: SVs, HVs, AVs
and the rest. If not, do that now.

These variables are used not only to represent Perl-space
variables, but also any constants in the code, as well as
some structures completely internal to Perl. The symbol
table, for instance, is an ordinary Perl hash. Your code
is represented by an SV as it's read into the parser; any
program files you call are opened via ordinary Perl file-
handles, and so on.

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

% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(V) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6

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

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

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

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

You can get at any of these quantities from C very easily;
just add "Sv" to the name of the field shown in the snip-
pet, and you've got a macro which will return the value:
"SvCUR(R)" returns the current length of the string,
"SvREFCOUNT(T)" returns the reference count, "SvPV(sv,
len)" returns the string itself with its length, and so
on. More macros to manipulate these properties can be
found in perlguts.

Let's take an example of manipulating a PV, from "sv_cat-
pvn", 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(X);
10 Move(ptr,SvPVX(X)+tlen,len,char);
11 SvCUR(R) += len;
12 *SvEND(D) = '\0';
13 (void)SvPOK_only_UTF8(8); /* validate pointer */
14 SvTAINT(T);
15 }

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

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

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

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

Line 13 manipulates the flags; since we've changed the PV,
any IV or NV values will no longer be valid: if we have
"$a=10; $a.="6";" we don't want to use the old IV of 10.
"SvPOK_only_utf8" is a special UTF8-aware version of
"SvPOK_only", a macro which turns off the IOK and NOK
flags and turns on POK. The final "SvTAINT" is a macro
which launders tainted data if taint mode is turned on.

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

Op Trees

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

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

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

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

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

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

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

Line 10 is the null op: this does exactly nothing. What is
that doing there? If you see the null op, it's a sign that
something has been optimized away after parsing. As we
mentioned in "Optimization", the optimization stage some-
times 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 point-
ers, it's easier just to replace the redundant operation
with the null op. Originally, the tree would have looked
like this:

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

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

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

Now, what's this about?

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

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

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

Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c

However, it's impossible to perform the operations in this
order: you have to find the values of $b and $c before you
add them together, for instance. So, the other thread that
runs through the op tree is the execution order: each op
has a field "op_next" which points to the next op to be
run, so following these pointers tells us how perl exe-
cutes 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 pro-
cess can be unravelled by examining perly.y, the YACC
grammar. Let's take the piece we need to construct the
tree for "$a = $b + $c"

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

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

The grammar, lines one and three of the snippet above,
tells you how to build up more complex forms. These com-
plex forms, `non-terminal symbols' are generally placed in
lower case. "term" here is a non-terminal symbol, repre-
senting 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 reduc-
tion, separated by vertical bars: so, "term" followed by
"=" followed by "term" makes a "term", and "term" followed
by "+" followed by "term" can also make a "term".

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

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

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

Stacks

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

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

NV value;
value = POPn;
value = Perl_cos(s);
XPUSHn(n);

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

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

SETi(-TOPi);

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

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

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

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

The lines which concern the mark stack are the first,
fifth and last lines: they save away, restore and
remove the current position of the argument stack.

Let's examine the whole implementation, for practice:

1 PUSHMARK(K);

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

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

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

5 PUTBACK;

Next we tell Perl to make the change to the global
stack pointer: "dSP" only gave us a local copy, not a
reference to the global.

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

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

To actually do the magic method call, we have to call a
subroutine in Perl space: "call_method" takes care of
that, and it's described in perlcall. We call the
"PUSH" method in scalar context, and we're going to
discard its return value.

9 POPSTACK;

Finally, we remove the value we placed on the mark
stack, since we don't need it any more.

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

{
local $foo = 42;
...
}

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

Millions of Macros

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

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

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

Line 5 is another variable declaration - all variable dec-
larations 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 han-
dled, and we pass the next operator to run back to the
main run loop.

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

Poking at Perl

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

./Configure -d -D optimize=-g
make

"-g" is a flag to the C compiler to have it produce debug-
ging information which will allow us to step through a
running program. Configure will also turn on the "DEBUG-
GING" compilation symbol which enables all the internal
debugging code in Perl. There are a whole bunch of things
you can debug with this: perlrun lists them all, and the
best way to find out about them is to play about with
them. The most useful options are probably

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

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

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

Using a source-level debugger

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

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

To fire up the debugger, type

gdb ./perl

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

(gdb)

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

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

break function_name
break source.c:xxx
Tells the debugger that we'll want to pause execution
when we reach either the named function (but see
"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 - it's a blessing when stepping through miles of
source code.

print
Execute the given C code and print its results. WARN-
ING: Perl makes heavy use of macros, and gdb is not
aware of macros. You'll have to substitute them your-
self. So, for instance, you can't say

print SvPV_nolen(n)

but you have to say

print Perl_sv_2pv_nolen(n)

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

Dumping Perl Data Structures

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

< finish this later >

Patching

All right, we've now had a look at how to navigate the
Perl sources and some things you'll need to know when fid-
dling with them. Let's now get on and create a simple
patch. Here's something Larry suggested: if a "U" is the
first active format during a "pack", (for example, "pack
"U3C8", @stuff") then the resulting string should be
treated as UTF8 encoded.

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

Now let's look over "pp_pack": we take a pattern into
"pat", and then loop over the pattern, taking each format
character in turn into "datum_type". Then for each possi-
ble 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 we're still at
the start of the string. So, here's where "pat" is set up:

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

We'll have another string pointer in there:

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

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

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

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

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

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

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

if (isSPACE(E))
continue;

needs to become

if (isSPACE(E)) {
patcopy++;
continue;
}

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

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

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

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

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

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

print 'not ' unless v1.20.300.4000 ne
sprintf "%vd", pack("C0U*",1,20,300,4000);
print "ok $test\n"; $test++;

Mustn't forget to change the number of tests which appears
at the top, or else the automated tester will get con-
fused:

-print "1..156\n";
+print "1..159\n";

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

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

=item *

If the pattern begins with a C<U>, the resulting string will be treated
as Unicode-encoded. You can force UTF8 encoding on in a string with an
initial C<U0>, and the bytes that follow will be interpreted as Unicode
characters. If you don't want this to happen, you can begin your pattern
with C<C0> (or anything else) to force Perl not to UTF8 encode your
string, and then follow this with a C<U*> somewhere in your pattern.

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

diff -ruN old new > patch

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

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

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

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

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

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

Rational Software's Purify

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

The only currently known leaks happen when there are com-
pile-time errors within eval or require. (Fixing these is
non-trivial, unfortunately, but they must be fixed eventu-
ally.)

Purify on Unix

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

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

where these arguments mean:

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

-Doptimize='-g'
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.

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

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

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

make pureperl

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

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

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

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

Purify outputs messages in "Viewer" windows by default.
If you don't have a windowing environment or if you 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"

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

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

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

CFG = Debug
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.

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

cd win32
make
cd ../t
purify ../perl -I../lib harness

which would instrument Perl in memory, run Perl on
test.pl, then finally report any memory problems.

CONCLUSION

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

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

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

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

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

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

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

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

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

perl v5.6.1 2002-11-30 PERLHACK(K)

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