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perlhack - How to hack at the Perl internals
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perlhack - How to hack at the Perl internals
This document attempts to explain how Perl development takes place,
and ends with some suggestions for people wanting to become bona fide
porters.
The perl5-porters mailing list is where the Perl standard distribution
is maintained and developed. The list can get anywhere from 10 to 150
messages a day, depending on the heatedness of the debate. Most days
there are two or three patches, extensions, features, or bugs being
discussed at a time.
A searchable archive of the list is at:
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 Christiansen is the documentation pumpking.
Larry sees Perl development along the lines of the US government:
there's the Legislature (the porters), the Executive branch (the
pumpkings), and the Supreme Court (Larry). The legislature can
discuss and submit patches to the executive branch all they like, but
the executive branch is free to veto them. Rarely, the Supreme Court
will side with the executive branch over the legislature, or the
legislature over the executive branch. Mostly, however, the
legislature and the executive branch are supposed to get along and
work out their differences without impeachment or court cases.
You might sometimes see reference to Rule 1 and Rule 2. Larry's power
as Supreme Court is expressed in The Rules:
-
Larry is always by definition right about how Perl should behave.
This means he has final veto power on the core functionality.
-
Larry is allowed to change his mind about any matter at a later date,
regardless of whether he previously invoked Rule 1.
Got that? Larry is always right, even when he was wrong. It's rare
to see either Rule exercised, but they are often alluded to.
New features and extensions to the language are contentious, because
the criteria used by the pumpkings, Larry, and other porters to decide
which features should be implemented and incorporated are not codified
in a few small design goals as with some other languages. Instead,
the heuristics are flexible and often difficult to fathom. Here is
one person's list, roughly in decreasing order of importance, of
heuristics that new features have to be weighed against:
- Does concept match the general goals of Perl?
-
These haven't been written anywhere in stone, but one approximation
is:
1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate Perl everywhere.
5. Either assimilate new technologies, or build bridges to them.
- Where is the implementation?
-
All the talk in the world is useless without an implementation. In
almost every case, the person or people who argue for a new feature
will be expected to be the ones who implement it. Porters capable
of coding new features have their own agendas, and are not available
to implement your (possibly good) idea.
- Backwards compatibility
-
It's a cardinal sin to break existing Perl programs. New warnings are
contentious--some say that a program that emits warnings is not
broken, while others say it is. Adding keywords has the potential to
break programs, changing the meaning of existing token sequences or
functions might break programs.
- Could it be a module instead?
-
Perl 5 has extension mechanisms, modules and XS, specifically to avoid
the need to keep changing the Perl interpreter. You can write modules
that export functions, you can give those functions prototypes so they
can be called like built-in functions, you can even write XS code to
mess with the runtime data structures of the Perl interpreter if you
want to implement really complicated things. If it can be done in a
module instead of in the core, it's highly unlikely to be added.
- Is the feature generic enough?
-
Is this something that only the submitter wants added to the language,
or would it be broadly useful? Sometimes, instead of adding a feature
with a tight focus, the porters might decide to wait until someone
implements the more generalized feature. For instance, instead of
implementing a ``delayed evaluation'' feature, the porters are waiting
for a macro system that would permit delayed evaluation and much more.
- Does it potentially introduce new bugs?
-
Radical rewrites of large chunks of the Perl interpreter have the
potential to introduce new bugs. The smaller and more localized the
change, the better.
- Does it preclude other desirable features?
-
A patch is likely to be rejected if it closes off future avenues of
development. For instance, a patch that placed a true and final
interpretation on prototypes is likely to be rejected because there
are still options for the future of prototypes that haven't been
addressed.
- Is the implementation robust?
-
Good patches (tight code, complete, correct) stand more chance of
going in. Sloppy or incorrect patches might be placed on the back
burner until the pumpking has time to fix, or might be discarded
altogether without further notice.
- Is the implementation generic enough to be portable?
-
The worst patches make use of a system-specific features. It's highly
unlikely that nonportable additions to the Perl language will be
accepted.
- Is there enough documentation?
-
Patches without documentation are probably ill-thought out or
incomplete. Nothing can be added without documentation, so submitting
a patch for the appropriate manpages as well as the source code is
always a good idea. 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 navigate, though--one man's essential addition is
another man's pointless cruft.
- Does it create too much work?
-
Work for the pumpking, work for Perl programmers, work for module
authors, ... Perl is supposed to be easy.
- Patches speak louder than words
-
Working code is always preferred to pie-in-the-sky ideas. A patch to
add a feature stands a much higher chance of making it to the language
than does a random feature request, no matter how fervently argued the
request might be. This ties into ``Will it be useful?'', as the fact
that someone took the time to make the patch demonstrates a strong
desire for the feature.
If you're on the list, you might hear the word ``core'' bandied
around. It refers to the standard distribution. ``Hacking on the
core'' means you're changing the C source code to the Perl
interpreter. ``A core module'' is one that ships with Perl.
The source code to the Perl interpreter, in its different versions, is
kept in a repository managed by a revision control system (which is
currently the Perforce program, see http://perforce.com/). The
pumpkings and a few others have access to the repository to check in
changes. Periodically the pumpking for the development version of Perl
will release a new version, so the rest of the porters can see what's
changed. The current state of the main trunk of repository, and patches
that describe the individual changes that have happened since the last
public release are available at this location:
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 usually in a perpetual
state of evolution. You should expect it to be very buggy. Do not use
it for any purpose other than testing and development.
Keeping in sync with the most recent branch can be done in several ways,
but the most convenient and reliable way is using rsync, available at
ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent
branch by FTP.)
If you choose to keep in sync using rsync, there are two approaches
to doing so:
- rsync'ing the source tree
-
Presuming you are in the directory where your perl source resides
and you have rsync installed and available, you can `upgrade' to
the bleadperl using:
# rsync -avz rsync://ftp.linux.activestate.com/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.
You can than check what patch was the latest that was applied by
looking in the file .patch, which will show the number of the
latest patch.
If you have more than one machine to keep in sync, and not all of
them have access to the WAN (so you are not able to rsync all the
source trees to the real source), there are some ways to get around
this problem.
- Using rsync over the LAN
-
Set up a local rsync server which makes the rsynced source tree
available to the LAN and sync the other machines against this
directory.
From http://rsync.samba.org/README.html:
"Rsync uses rsh or ssh for communication. It does not need to be
setuid and requires no special privileges for installation. It
does not require 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 downloaded 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 König to have better control over the patching process.
- 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).
- 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 development branch,
but also like to fix bugs, or extend features, you want to dive
into the sources. If you are a seasoned perl core diver, you don't
need no manuals, tips, roadmaps, perlguts.pod or other aids to find
your way around. But if you are a starter, the patches may help you
in finding where you should start and how to change the bits that
bug you.
The file Changes is updated on occasions the pumpking sees as his
own little sync points. On those occasions, he releases a tar-ball of
the current source tree (i.e. perl@7582.tar.gz), which will be an
excellent point to start with when choosing to use the 'rsync the
patches' scheme. Starting with perl@7582, which means a set of source
files on which the latest applied patch is number 7582, you apply all
succeeding patches available from then on (7583, 7584, ...).
You can use the patches later as a kind of search archive.
- Finding a start point
-
If you want to fix/change the behaviour of function/feature Foo, just
scan the patches for patches that mention Foo either in the subject,
the comments, or the body of the fix. A good chance the patch shows
you the files that are affected by that patch which are very likely
to be the starting point of your journey into the guts of perl.
- Finding how to fix a bug
-
If you've found where the function/feature Foo misbehaves, but you
don't know how to fix it (but you do know the change you want to
make), you can, again, peruse the patches for similar changes and
look how others apply the fix.
- Finding the source of misbehaviour
-
When you keep in sync with bleadperl, the pumpking would love to
see that the community efforts 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 misbehaviour. If you note that in the bug report, it saves the
one trying to solve it, looking for that point.
If searching the patches is too bothersome, you might consider using
perl's bugtron to find more information about discussions and
ramblings on posted bugs.
If you want to get the best of both worlds, rsync both the source
tree for convenience, reliability and ease and rsync the patches
for reference.
Always submit patches to perl5-porters@perl.org. This lets other
porters review your patch, which catches a surprising number of errors
in patches. Either use the diff program (available in source code
form from ftp://ftp.gnu.org/pub/gnu/), or use Johan Vromans'
makepatch (available from CPAN/authors/id/JV/). Unified diffs
are preferred, but context diffs are accepted. Do not send RCS-style
diffs or diffs without context lines. More information is given in
the Porting/patching.pod file in the Perl source distribution.
Please patch against the latest development version (e.g., if
you're fixing a bug in the 5.005 track, patch against the latest
5.005_5x version). Only patches that survive the heat of the
development branch get applied to maintenance versions.
Your patch should update the documentation and test suite.
To report a bug in Perl, use the program perlbug which comes with
Perl (if you can't get Perl to work, send mail to the address
perlbug@perl.org or perlbug@perl.com). Reporting bugs through
perlbug feeds into the automated bug-tracking system, access to
which is provided through the web at http://bugs.perl.org/. It
often pays to check the archives of the perl5-porters mailing list to
see whether the bug you're reporting has been reported before, and if
so whether it was considered a bug. See above for the location of
the searchable archives.
The CPAN testers (http://testers.cpan.org/) are a group of
volunteers who test CPAN modules on a variety of platforms. Perl 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 chipping in.
That way you'll get to see the dynamic of the conversations, learn the
personalities of the players, and hopefully be better prepared to make
a useful contribution when do you speak up.
If after all this you still think you want to join the perl5-porters
mailing list, send mail to perl5-porters-subscribe@perl.org. To
unsubscribe, send mail to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following things:
- the perlguts manpage
-
This is of paramount importance, since it's the documentation of what
goes where in the Perl source. Read it over a couple of times and it
might start to make sense - don't worry if it doesn't yet, because the
best way to study it is to read it in conjunction with poking at Perl
source, and we'll do that later on.
You might also want to look at Gisle Aas's illustrated perlguts -
there's no guarantee that this will be absolutely up-to-date with the
latest documentation in the Perl core, but the fundamentals will be
right. (http://gisle.aas.no/perl/illguts/)
- the perlxstut manpage and the perlxs manpage
-
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.
- the perlapi manpage
-
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://perlhacker.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.
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 everything in the
pod/ directory, (as well as contributing new documentation) and
the documentation to the modules in core.
- Configure
-
The configure process is the way we make Perl portable across the
myriad of operating systems it supports. Responsibility for the
configure, build and installation process, as well as the overall
portability of the core code rests with the configure pumpkin - others
help out with individual operating systems.
The files involved are the operating system directories, (win32/,
os2/, vms/ and so on) the shell scripts which generate config.h
and Makefile, as well as the metaconfig files which generate
Configure. (metaconfig isn't included in the core distribution.)
- Interpreter
-
And of course, there's the core of the Perl interpreter itself. Let's
have a look at that in a little more detail.
Before we leave looking at the layout, though, don't forget that
MANIFEST contains not only the file names in the Perl distribution,
but short descriptions of what's in them, too. For an overview of the
important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
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 the perlguts manpage 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 the perlembed manpage; most of the real action takes
place in perl.c
First, perlmain.c allocates some memory and constructs a Perl
interpreter:
1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(1);
7 perl_construct(my_perl);
8 PL_perl_destruct_level = 0;
9 }
Line 1 is a macro, and its definition is dependent on your operating
system. Line 3 references PL_do_undump, a global variable - all
global variables in Perl start with PL_. This tells you whether the
current running program was created with the -u flag to perl and then
undump, which means it's going to be false in any sane context.
Line 4 calls a function in perl.c to allocate memory for a Perl
interpreter. It's quite a simple function, and the guts of it looks like
this:
my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
Here you see an example of Perl's system abstraction, which we'll see
later: PerlMem_malloc is either your system's malloc, or Perl's
own malloc as defined in malloc.c if you selected that option at
configure time.
Next, in line 7, we construct the interpreter; this sets up all the
special variables that Perl needs, the stacks, and so on.
Now we pass Perl the command line options, and tell it to go:
exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
if (!exitstatus) {
exitstatus = perl_run(my_perl);
}
perl_parse is actually a wrapper around S_parse_body, as defined
in perl.c, which processes the command line options, sets up any
statically linked XS modules, opens the program and calls yyparse to
parse it.
- Parsing
-
The aim of this stage is to take the Perl source, and turn it into an op
tree. We'll see what one of those looks like later. Strictly speaking,
there's three things going on here.
yyparse, the parser, lives in perly.c, although you're better off
reading the original YACC input in perly.y. (Yes, Virginia, there
is a YACC grammar for Perl!) The job of the parser is to take your
code and `understand' it, splitting it into sentences, deciding which
operands go with which operators and so on.
The parser is nobly assisted by the lexer, which chunks up your input
into tokens, and decides what type of thing each token is: a variable
name, an operator, a bareword, a subroutine, a core function, and so on.
The main point of entry to the lexer is yylex, and that and its
associated routines can be found in toke.c. Perl isn't much like
other computer languages; it's highly context sensitive at times, it can
be tricky to work out what sort of token something is, or where a token
ends. As such, there's a lot of interplay between the tokeniser and the
parser, which can get pretty frightening if you're not used to it.
As the parser understands a Perl program, it builds up a tree of
operations for the interpreter to perform during execution. The routines
which construct and link together the various operations are to be found
in op.c, and will be examined later.
- Optimization
-
Now the parsing stage is complete, and the finished tree represents
the operations that the Perl interpreter needs to perform to execute our
program. Next, Perl does a dry run over the tree looking for
optimisations: constant expressions such as
3 + 4 will be computed
now, and the optimizer will also see if any multiple operations can be
replaced with a single one. For instance, to fetch the variable $foo,
instead of grabbing the glob *foo and looking at the scalar
component, the optimizer fiddles the op tree to use a function which
directly looks up the scalar in question. The main optimizer is peep
in op.c, and many ops have their own optimizing functions.
- Running
-
Now we're finally ready to go: we have compiled Perl byte code, and all
that's left to do is run it. The actual execution is done by the
runops_standard function in run.c; more specifically, it's done by
these three innocent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of that:
PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function pointer, which
stipulates the function which will actually carry out the operation.
This function will return the next op in the sequence - this allows for
things like if which choose the next op dynamically at run time.
The PERL_ASYNC_CHECK makes sure that things like signals interrupt
execution if required.
The actual functions called are known as PP code, and they're spread
between four files: pp_hot.c contains the `hot' code, which is most
often used and highly optimized, pp_sys.c contains all the
system-specific functions, pp_ctl.c contains the functions which
implement control structures (if, while and the like) and pp.c
contains everything else. These are, if you like, the C code for Perl's
built-in functions and operators.
You should by now have had a look at the perlguts manpage, which tells you about
Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
that now.
These variables are used not only to represent Perl-space variables, but
also any constants in the code, as well as some structures completely
internal to Perl. The symbol table, for instance, is an ordinary Perl
hash. Your code is represented by an SV as it's read into the parser;
any program files you call are opened via ordinary Perl filehandles, and
so on.
The core Devel::Peek module lets us examine SVs from a
Perl program. Let's see, for instance, how Perl treats the constant
"hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
Reading Devel::Peek output takes a bit of practise, so let's go
through it line by line.
Line 1 tells us we're looking at an SV which lives at 0xa04ecbc in
memory. SVs themselves are very simple structures, but they contain a
pointer to a more complex structure. In this case, it's a PV, a
structure which holds a string value, at location 0xa041450. Line 2
is the reference count; there are no other references to this data, so
it's 1.
Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
read-only SV (because it's a constant) and the data is a PV internally.
Next we've got the contents of the string, starting at location
0xa0484e0.
Line 5 gives us the current length of the string - note that this does
not include the null terminator. Line 6 is not the length of the
string, but the length of the currently allocated buffer; as the string
grows, Perl automatically extends the available storage via a routine
called SvGROW.
You can get at any of these quantities from C very easily; just add
Sv to the name of the field shown in the snippet, and you've got a
macro which will return the value: SvCUR(sv) returns the current
length of the string, SvREFCOUNT(sv) returns the reference count,
SvPV(sv, len) returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in the perlguts manpage.
Let's take an example of manipulating a PV, from sv_catpvn, in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\0';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, ptr, of length len onto
the end of the PV stored in sv. The first thing we do in line 6 is
make sure that the SV has a valid PV, by calling the SvPV_force
macro to force a PV. As a side effect, tlen gets set to the current
value of the PV, and the PV itself is returned to junk.
In line 7, we make sure that the SV will have enough room to accommodate
the old string, the new string and the null terminator. If LEN isn't
big enough, SvGROW will reallocate space for us.
Now, if junk is the same as the string we're trying to add, we can
grab the string directly from the SV; SvPVX is the address of the PV
in the SV.
Line 10 does the actual catenation: the Move macro moves a chunk of
memory around: we move the string ptr to the end of the PV - that's
the start of the PV plus its current length. We're moving len bytes
of type char. After doing so, we need to tell Perl we've extended the
string, by altering CUR to reflect the new length. SvEND is a
macro which gives us the end of the string, so that needs to be a
"\0".
Line 13 manipulates the flags; since we've changed the PV, any IV or NV
values will no longer be valid: if we have $a=10; $a.="6"; we don't
want to use the old IV of 10. SvPOK_only_utf8 is a special 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.
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 specific operator in
question is sassign - scalar assignment - and you can find the code
which implements it in the function pp_sassign in pp_hot.c. As a
binary operator, it has two children: the add operator, providing the
result of $b+$c, is uppermost on line 5, and the left hand side is on
line 10.
Line 10 is the null op: this does exactly nothing. What is that doing
there? If you see the null op, it's a sign that something has been
optimized away after parsing. As we mentioned in Optimization,
the optimization stage sometimes converts two operations into one, for
example when fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers, it's easier
just to replace the redundant operation with the null op. Originally,
the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the a entry from the main symbol table, and then look
at the scalar component of it: gvsv (pp_gvsv into pp_hot.c)
happens to do both these things.
The right hand side, starting at line 5 is similar to what we've just
seen: we have the add op (pp_add also in pp_hot.c) add together
two gvsvs.
Now, what's this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
enter and leave are scoping ops, and their job is to perform any
housekeeping every time you enter and leave a block: lexical variables
are tidied up, unreferenced variables are destroyed, and so on. Every
program will have those first three lines: leave is a list, and its
children are all the statements in the block. Statements are delimited
by nextstate, so a block is a collection of nextstate ops, with
the ops to be performed for each statement being the children of
nextstate. enter is a single op which functions as a marker.
That's how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c
However, it's impossible to perform the operations in this order:
you have to find the values of $b and $c before you add them
together, for instance. So, the other thread that runs through the op
tree is the execution order: each op has a field op_next which points
to the next op to be run, so following these pointers tells us how perl
executes the code. We can traverse the tree in this order using
the exec option to B::Terse:
% perl -MO=Terse,exec -e '$a=$b+$c'
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block, start a
statement. Get the values of $b and $c, and add them together.
Find $a, and assign one to the other. Then leave.
The way Perl builds up these op trees in the parsing process can be
unravelled by examining perly.y, the YACC grammar. Let's take the
piece we need to construct the tree for $a = $b + $c
1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
If you're not used to reading BNF grammars, this is how it works: You're
fed certain things by the tokeniser, which generally end up in upper
case. Here, ADDOP, is provided when the tokeniser sees + in your
code. ASSIGNOP is provided when = is used for assigning. These are
`terminal symbols', because you can't get any simpler than them.
The grammar, lines one and three of the snippet above, tells you how to
build up more complex forms. These complex forms, `non-terminal symbols'
are generally placed in lower case. term here is a non-terminal
symbol, representing a single expression.
The grammar gives you the following rule: you can make the thing on the
left of the colon if you see all the things on the right in sequence.
This is called a ``reduction'', and the aim of parsing is to completely
reduce the input. There are several different ways you can perform a
reduction, separated by vertical bars: so, term followed by =
followed by term makes a term, and term followed by +
followed by term can also make a term.
So, if you see two terms with an = or +, between them, you can
turn them into a single expression. When you do this, you execute the
code in the block on the next line: if you see =, you'll do the code
in line 2. If you see +, you'll do the code in line 4. It's this code
which contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a number of
variables. The variables refer to the tokens: $1 is the first token in
the input, $2 the second, and so on - think regular expression
backreferences. $$ is the op returned from this reduction. So, we
call newBINOP to create a new binary operator. The first parameter to
newBINOP, a function in op.c, is the op type. It's an addition
operator, so we want the type to be ADDOP. We could specify this
directly, but it's right there as the second token in the input, so we
use $2. The second parameter is the op's flags: 0 means `nothing
special'. Then the things to add: the left and right hand side of our
expression, in scalar context.
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 |
