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This document attempts to explain how Perl development takes place, and ends with some
suggestions for people wanting to become bona fide porters.
The perl5-porters mailing list is where the Perl standard distribution is maintained and
developed. The list can get anywhere from 10 to 150 messages a day, depending on the
heatedness of the debate. Most days there are two or three patches, extensions, features, or
bugs being discussed at a time.
A searchable archive of the list is at either:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
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or
http://archive.develooper.com/perl5-porters@perl.org/
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List subscribers (the porters themselves) come in several flavours. Some are quiet curious
lurkers, who rarely pitch in and instead watch the ongoing development to ensure they're
forewarned of new changes or features in Perl. Some are representatives of vendors, who are
there to make sure that Perl continues to compile and work on their platforms. Some patch any
reported bug that they know how to fix, some are actively patching their pet area (threads,
Win32, the regexp engine), while others seem to do nothing but complain. In other words, it's
your usual mix of technical people.
Over this group of porters presides Larry Wall. He has the final word in what does and does
not change in the Perl language. Various releases of Perl are shepherded by a ``pumpking'', a
porter responsible for gathering patches, deciding on a patch-by-patch feature-by-feature
basis what will and will not go into the release. For instance, Gurusamy Sarathy was the
pumpking for the 5.6 release of Perl, and Jarkko Hietaniemi is the pumpking for the 5.8
release, and Hugo van der Sanden will be the pumpking for the 5.10 release.
In addition, various people are pumpkings for different things. For instance, Andy
Dougherty and Jarkko Hietaniemi share the Configure pumpkin.
Larry sees Perl development along the lines of the US government: there's the Legislature
(the porters), the Executive branch (the pumpkings), and the Supreme Court (Larry). The
legislature can discuss and submit patches to the executive branch all they like, but the
executive branch is free to veto them. Rarely, the Supreme Court will side with the executive
branch over the legislature, or the legislature over the executive branch. Mostly, however,
the legislature and the executive branch are supposed to get along and work out their
differences without impeachment or court cases.
You might sometimes see reference to Rule 1 and Rule 2. Larry's power as Supreme Court is
expressed in The Rules:
- 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.
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- Where is the implementation?
- All the talk in the world is useless without an implementation. In almost every case,
the person or people who argue for a new feature will be expected to be the ones who
implement it. Porters capable of coding new features have their own agendas, and are not
available to implement your (possibly good) idea.
- Backwards compatibility
- It's a cardinal sin to break existing Perl programs. New warnings are contentious--some
say that a program that emits warnings is not broken, while others say it is. Adding
keywords has the potential to break programs, changing the meaning of existing token
sequences or functions might break programs.
- Could it be a module instead?
- Perl 5 has extension mechanisms, modules and XS, specifically to avoid the need to keep
changing the Perl interpreter. You can write modules that export functions, you can give
those functions prototypes so they can be called like built-in functions, you can even
write XS code to mess with the runtime data structures of the Perl interpreter if you want
to implement really complicated things. If it can be done in a module instead of in the
core, it's highly unlikely to be added.
- Is the feature generic enough?
- Is this something that only the submitter wants added to the language, or would it be
broadly useful? Sometimes, instead of adding a feature with a tight focus, the porters
might decide to wait until someone implements the more generalized feature. For instance,
instead of implementing a ``delayed evaluation'' feature, the porters are waiting for a
macro system that would permit delayed evaluation and much more.
- Does it potentially introduce new
bugs?
- Radical rewrites of large chunks of the Perl interpreter have the potential to introduce
new bugs. The smaller and more localized the change, the better.
- Does it preclude other desirable
features?
- A patch is likely to be rejected if it closes off future avenues of development. For
instance, a patch that placed a true and final interpretation on prototypes is likely to
be rejected because there are still options for the future of prototypes that haven't been
addressed.
- Is the implementation robust?
- Good patches (tight code, complete, correct) stand more chance of going in. Sloppy or
incorrect patches might be placed on the back burner until the pumpking has time to fix,
or might be discarded altogether without further notice.
- Is the implementation
generic enough to be portable?
- The worst patches make use of a system-specific features. It's highly unlikely that
nonportable additions to the Perl language will be accepted.
- Is the implementation tested?
- Patches which change behaviour (fixing bugs or introducing new features) must include
regression tests to verify that everything works as expected. Without tests provided by
the original author, how can anyone else changing perl in the future be sure that they
haven't unwittingly broken the behaviour the patch implements? And without tests, how can
the patch's author be confident that his/her hard work put into the patch won't be
accidentally thrown away by someone in the future?
- Is there enough documentation?
- Patches without documentation are probably ill-thought out or incomplete. Nothing can be
added without documentation, so submitting a patch for the appropriate manpages as well as
the source code is always a good idea.
- Is there another way to do it?
- Larry said ``Although the Perl Slogan is There's More Than One Way to Do It, I
hesitate to make 10 ways to do something''. This is a tricky heuristic to navigate,
though--one man's essential addition is another man's pointless cruft.
- Does it create too much work?
- Work for the pumpking, work for Perl programmers, work for module authors, ... Perl is
supposed to be easy.
- Patches speak louder than words
- Working code is always preferred to pie-in-the-sky ideas. A patch to add a feature
stands a much higher chance of making it to the language than does a random feature
request, no matter how fervently argued the request might be. This ties into ``Will it be
useful?'', as the fact that someone took the time to make the patch demonstrates a strong
desire for the feature.
If you're on the list, you might hear the word ``core'' bandied around. It refers to the
standard distribution. ``Hacking on the core'' means you're changing the C source code to the
Perl interpreter. ``A core module'' is one that ships with Perl.
The source code to the Perl interpreter, in its different versions, is kept in a repository
managed by a revision control system ( which is currently the Perforce program, see http://perforce.com/
). The pumpkings and a few others have access to the repository to check in changes.
Periodically the pumpking for the development version of Perl will release a new version, so
the rest of the porters can see what's changed. The current state of the main trunk of
repository, and patches that describe the individual changes that have happened since the last
public release are available at this location:
http://public.activestate.com/gsar/APC/
ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/
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If you're looking for a particular change, or a change that affected a particular set of
files, you may find the Perl Repository Browser useful:
http://public.activestate.com/cgi-bin/perlbrowse
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You may also want to subscribe to the perl5-changes mailing list to receive a copy of each
patch that gets submitted to the maintenance and development "branches" of the perl
repository. See http://lists.perl.org/ for subscription information.
If you are a member of the perl5-porters mailing list, it is a good thing to keep in touch
with the most recent changes. If not only to verify if what you would have posted as a bug
report isn't already solved in the most recent available perl development branch, also known
as perl-current, bleading edge perl, bleedperl or bleadperl.
Needless to say, the source code in perl-current is usually in a perpetual state of
evolution. You should expect it to be very buggy. Do not use it for any purpose other
than testing and development.
Keeping in sync with the most recent branch can be done in several ways, but the most
convenient and reliable way is using rsync, available at ftp://rsync.samba.org/pub/rsync/
. (You can also get the most recent branch by FTP.)
If you choose to keep in sync using rsync, there are two approaches to doing so:
- rsync'ing the source tree
-
Presuming you are in the directory where your perl source resides and you have rsync
installed and available, you can `upgrade' to the bleadperl using:
# rsync -avz rsync://ftp.linux.activestate.com/perl-current/ .
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This takes care of updating every single item in the source tree to the latest applied
patch level, creating files that are new (to your distribution) and setting date/time
stamps of existing files to reflect the bleadperl status.
Note that this will not delete any files that were in '.' before the rsync. Once you
are sure that the rsync is running correctly, run it with the --delete and the --dry-run
options like this:
# rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ .
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This will simulate an rsync run that also deletes files not present in the
bleadperl master copy. Observe the results from this run closely. If you are sure that the
actual run would delete no files precious to you, you could remove the '--dry-run' option.
You can than check what patch was the latest that was applied by looking in the file .patch,
which will show the number of the latest patch.
If you have more than one machine to keep in sync, and not all of them have access to
the WAN (so you are not able to rsync all the source trees to the real source), there are
some ways to get around this problem.
- Using rsync over the LAN
-
Set up a local rsync server which makes the rsynced source tree available to the
LAN and sync the other machines against this directory.
From http://rsync.samba.org/README.html :
"Rsync uses rsh or ssh for communication. It does not need to be
setuid and requires no special privileges for installation. It
does not require an inetd entry or a daemon. You must, however,
have a working rsh or ssh system. Using ssh is recommended for
its security features."
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- 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;
}
}
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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/ .
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This makes sure the latest available patch is downloaded to your patch directory.
It's then up to you to apply these patches, using something like
# last=`ls -t *.gz | sed q`
# rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
# find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
# cd ../perl-current
# patch -p1 -N <../perl-current-diffs/blead.patch
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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 to rsync the source tree
- Since you don't have to apply the patches yourself, you are sure all files in the source
tree are in the right state.
- It's more reliable
- While both the rsync-able source and patch areas are automatically updated every few
minutes, keep in mind that applying patches may sometimes mean careful hand-holding,
especially if your version of the
patch program does not understand how to
deal with new files, files with 8-bit characters, or files without trailing newlines.
- It's easier to rsync the patches
-
If you have more than one machine that you want to keep in track with bleadperl, it's
easier to rsync the patches only once and then apply them to all the source trees on the
different machines.
In case you try to keep in pace on 5 different machines, for which only one of them has
access to the WAN, rsync'ing all the source trees should than be done 5 times over the NFS.
Having rsync'ed the patches only once, I can apply them to all the source trees
automatically. Need you say more ;-)
- It's a good reference
-
If you do not only like to have the most recent development branch, but also like to fix
bugs, or extend features, you want to dive into the sources. If you are a seasoned perl
core diver, you don't need no manuals, tips, roadmaps, perlguts.pod or other aids to find
your way around. But if you are a starter, the patches may help you in finding where you
should start and how to change the bits that bug you.
The file Changes is updated on occasions the pumpking sees as his own little
sync points. On those occasions, he releases a tar-ball of the current source tree (i.e.
perl@7582.tar.gz), which will be an excellent point to start with when choosing to use the
'rsync the patches' scheme. Starting with perl@7582, which means a set of source files on
which the latest applied patch is number 7582, you apply all succeeding patches available
from then on (7583, 7584, ...).
You can use the patches later as a kind of search archive.
- Finding a start point
- If you want to fix/change the behaviour of function/feature Foo, just scan the
patches for patches that mention Foo either in the subject, the comments, or the body
of the fix. A good chance the patch shows you the files that are affected by that
patch which are very likely to be the starting point of your journey into the guts of
perl.
- Finding how to fix a bug
- If you've found where the function/feature Foo misbehaves, but you don't know
how to fix it (but you do know the change you want to make), you can, again, peruse
the patches for similar changes and look how others apply the fix.
- Finding the source of misbehaviour
-
When you keep in sync with bleadperl, the pumpking would love to see that
the community efforts really work. So after each of his sync points, you are to 'make
test' to check if everything is still in working order. If it is, you do 'make ok',
which will send an OK report to perlbug@perl.org. (If you do not have access to a
mailer from the system you just finished successfully 'make test', you can do 'make
okfile', which creates the file perl.ok, which you can than take to your
favourite mailer and mail yourself).
But of course, as always, things will not always lead to a success path, and one or
more test do not pass the 'make test'. Before sending in a bug report (using 'make nok'
or 'make nokfile'), check the mailing list if someone else has reported the bug
already and if so, confirm it by replying to that message. If not, you might want to
trace the source of that misbehaviour before sending in the bug, which will
help all the other porters in finding the solution.
Here the saved patches come in very handy. You can check the list of patches to see
which patch changed what file and what change caused the misbehaviour. If you note
that in the bug report, it saves the one trying to solve it, looking for that point.
If searching the patches is too bothersome, you might consider using perl's bugtron to
find more information about discussions and ramblings on posted bugs.
If you want to get the best of both worlds, rsync both the source tree for convenience,
reliability and ease and rsync the patches for reference.
There are three (3) remote administrative interfaces for modifying bug status, category,
etc. In all cases an admin must be first registered with the Perlbug database by sending an
email request to richard@perl.org or bugmongers@perl.org.
The main requirement is the willingness to classify, (with the emphasis on closing where
possible :), outstanding bugs. Further explanation can be garnered from the web at http://bugs.perl.org/
, or by asking on the admin mailing list at: bugmongers@perl.org
For more info on the web see
http://bugs.perl.org/perlbug.cgi?req=spec
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- 1 http://bugs.perl.org
-
Login via the web, (remove admin/ if only browsing), where interested Cc's,
tests, patches and change-ids, etc. may be assigned.
http://bugs.perl.org/admin/index.html
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- 2 bugdb@perl.org
-
Where the subject line is used for commands:
To: bugdb@perl.org
Subject: -a close bugid1 bugid2 aix install
To: bugdb@perl.org
Subject: -h
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- 3 commands_and_bugdids@bugs.perl.org
-
Where the address itself is the source for the commands:
To: close_bugid1_bugid2_aix@bugs.perl.org
To: help@bugs.perl.org
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- notes, patches, tests
-
For patches and tests, the message body is assigned to the appropriate bugs and
forwarded to p5p for their attention.
To: test_<bugid1>_aix_close@bugs.perl.org
Subject: this is a test for the (now closed) aix bug
Test is the body of the mail
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Always submit patches to perl5-porters@perl.org. If you're patching a core module
and there's an author listed, send the author a copy (see Patching
a core module). This lets other porters review your patch, which catches a surprising
number of errors in patches. Either use the diff program (available in source code form from
ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' makepatch (available from CPAN/authors/id/JV/).
Unified diffs are preferred, but context diffs are accepted. Do not send RCS-style diffs or
diffs without context lines. More information is given in the Porting/patching.pod file
in the Perl source distribution. Please patch against the latest development version
(e.g., if you're fixing a bug in the 5.005 track, patch against the latest 5.005_5x version).
Only patches that survive the heat of the development branch get applied to maintenance
versions.
Your patch should update the documentation and test suite. See Writing
a test.
To report a bug in Perl, use the program perlbug which comes with Perl (if you can't
get Perl to work, send mail to the address perlbug@perl.org or perlbug@perl.com).
Reporting bugs through perlbug feeds into the automated bug-tracking system, access to
which is provided through the web at http://bugs.perl.org/ . It often pays to check the
archives of the perl5-porters mailing list to see whether the bug you're reporting has been
reported before, and if so whether it was considered a bug. See above for the location of the
searchable archives.
The CPAN testers ( http://testers.cpan.org/ ) are a group of volunteers who test CPAN
modules on a variety of platforms. Perl Smokers ( http://archives.develooper.com/daily-build@perl.org/
) automatically tests Perl source releases on platforms with various configurations. Both
efforts welcome volunteers.
It's a good idea to read and lurk for a while before chipping in. That way you'll get to
see the dynamic of the conversations, learn the personalities of the players, and hopefully be
better prepared to make a useful contribution when do you speak up.
If after all this you still think you want to join the perl5-porters mailing list, send
mail to perl5-porters-subscribe@perl.org. To unsubscribe, send mail to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following things:
- perlguts
-
This is of paramount importance, since it's the documentation of what goes where in the
Perl source. Read it over a couple of times and it might start to make sense - don't worry
if it doesn't yet, because the best way to study it is to read it in conjunction with
poking at Perl source, and we'll do that later on.
You might also want to look at Gisle Aas's illustrated perlguts - there's no guarantee
that this will be absolutely up-to-date with the latest documentation in the Perl core,
but the fundamentals will be right. ( http://gisle.aas.no/perl/illguts/ )
- perlxstut and perlxs
- A working knowledge of XSUB programming is incredibly useful for core hacking; XSUBs use
techniques drawn from the PP code, the portion of the guts that actually executes a Perl
program. It's a lot gentler to learn those techniques from simple examples and explanation
than from the core itself.
- perlapi
- The documentation for the Perl API explains what some of the internal functions do, as
well as the many macros used in the source.
- Porting/pumpkin.pod
- This is a collection of words of wisdom for a Perl porter; some of it is only useful to
the pumpkin holder, but most of it applies to anyone wanting to go about Perl development.
- The perl5-porters FAQ
- This should be available from http://simon-cozens.org/writings/p5p-faq ; alternatively,
you can get the FAQ emailed to you by sending mail to
perl5-porters-faq@perl.org.
It contains hints on reading perl5-porters, information on how perl5-porters works and how
Perl development in general works.
Perl maintenance can be split into a number of areas, and certain people (pumpkins) will
have responsibility for each area. These areas sometimes correspond to files or directories in
the source kit. Among the areas are:
- Core modules
- Modules shipped as part of the Perl core live in the lib/ and ext/
subdirectories: lib/ is for the pure-Perl modules, and ext/ contains the
core XS modules.
- Tests
- There are tests for nearly all the modules, built-ins and major bits of functionality.
Test files all have a .t suffix. Module tests live in the lib/ and ext/
directories next to the module being tested. Others live in t/. See Writing a test
- Documentation
- Documentation maintenance includes looking after everything in the pod/
directory, (as well as contributing new documentation) and the documentation to the
modules in core.
- Configure
-
The configure process is the way we make Perl portable across the myriad of operating
systems it supports. Responsibility for the configure, build and installation process, as
well as the overall portability of the core code rests with the configure pumpkin - others
help out with individual operating systems.
The files involved are the operating system directories, (win32/, os2/, vms/
and so on) the shell scripts which generate config.h and Makefile, as well
as the metaconfig files which generate Configure. (metaconfig isn't included in the
core distribution.)
- Interpreter
- And of course, there's the core of the Perl interpreter itself. Let's have a look at
that in a little more detail.
Before we leave looking at the layout, though, don't forget that MANIFEST contains
not only the file names in the Perl distribution, but short descriptions of what's in them,
too. For an overview of the important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
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The work of the interpreter has two main stages: compiling the code into the internal
representation, or bytecode, and then executing it. perlguts/Compiled code
explains exactly how the compilation stage happens.
Here is a short breakdown of perl's operation:
- Startup
-
The action begins in perlmain.c. (or miniperlmain.c for miniperl) This is
very high-level code, enough to fit on a single screen, and it resembles the code found in
perlembed; most of the
real action takes place in perl.c
First, perlmain.c allocates some memory and constructs a Perl interpreter:
1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(1);
7 perl_construct(my_perl);
8 PL_perl_destruct_level = 0;
9 }
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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));
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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);
}
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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 perlguts, which tells you about
Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do that now.
These variables are used not only to represent Perl-space variables, but also any constants
in the code, as well as some structures completely internal to Perl. The symbol table, for
instance, is an ordinary Perl hash. Your code is represented by an SV as it's read into the
parser; any program files you call are opened via ordinary Perl filehandles, and so on.
The core Devel::Peek
module lets us examine SVs from a Perl program. Let's see, for instance, how Perl treats the
constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
|
|
Reading Devel::Peek output takes a bit of practise, so let's go through it
line by line.
Line 1 tells us we're looking at an SV which lives at 0xa04ecbc in memory. SVs
themselves are very simple structures, but they contain a pointer to a more complex structure.
In this case, it's a PV, a structure which holds a string value, at location 0xa041450.
Line 2 is the reference count; there are no other references to this data, so it's 1.
Line 3 are the flags for this SV - it's OK to use it as a PV, it's a read-only SV (because
it's a constant) and the data is a PV internally. Next we've got the contents of the string,
starting at location 0xa0484e0.
Line 5 gives us the current length of the string - note that this does not include
the null terminator. Line 6 is not the length of the string, but the length of the currently
allocated buffer; as the string grows, Perl automatically extends the available storage via a
routine called SvGROW.
You can get at any of these quantities from C very easily; just add Sv to the
name of the field shown in the snippet, and you've got a macro which will return the value: SvCUR(sv)
returns the current length of the string, SvREFCOUNT(sv) returns the reference
count, SvPV(sv, len) returns the string itself with its length, and so on. More
macros to manipulate these properties can be found in perlguts.
Let's take an example of manipulating a PV, from sv_catpvn, in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\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, B::Concise and B::Debug do.
Let's have a look at how Perl sees $a = $b + $c:
% perl -MO=Terse -e '$a=$b+$c'
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
|
|
Let's start in the middle, at line 4. This is a BINOP, a binary operator, which is at
location 0x8179828. The specific operator in question is sassign -
scalar assignment - and you can find the code which implements it in the function pp_sassign
in pp_hot.c. As a binary operator, it has two children: the add operator, providing the
result of $b+$c, is uppermost on line 5, and the left hand side is on line 10.
Line 10 is the null op: this does exactly nothing. What is that doing there? If you see the
null op, it's a sign that something has been optimized away after parsing. As we mentioned in /Optimization, the optimization stage sometimes converts two
operations into one, for example when fetching a scalar variable. When this happens, instead
of rewriting the op tree and cleaning up the dangling pointers, it's easier just to replace
the redundant operation with the null op. Originally, the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
|
|
That is, fetch the a entry from the main symbol table, and then look at the
scalar component of it: gvsv (pp_gvsv into pp_hot.c) happens
to do both these things.
The right hand side, starting at line 5 is similar to what we've just seen: we have the add
op (pp_add also in pp_hot.c) add together two 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 here.
- Argument stack
-
Arguments are passed to PP code and returned from PP code using the argument stack, ST.
The typical way to handle arguments is to pop them off the stack, deal with them how you
wish, and then push the result back onto the stack. This is how, for instance, the cosine
operator works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
|
|
We'll see a more tricky example of this when we consider Perl's macros below. POPn
gives you the NV (floating point value) of the top SV on the stack: the $x in
cos($x). Then we compute the cosine, and push the result back as an NV. The X
in XPUSHn means that the stack should be extended if necessary - it can't be
necessary here, because we know there's room for one more item on the stack, since we've
just removed one! The XPUSH* macros at least guarantee safety.
Alternatively, you can fiddle with the stack directly: SP gives you the
first element in your portion of the stack, and TOP* gives you the top SV/IV/NV/etc.
on the stack. So, for instance, to do unary negation of an integer:
Just set the integer value of the top stack entry to its negation.
Argument stack manipulation in the core is exactly the same as it is in XSUBs - see perlxstut, perlxs and perlguts for a longer
description of the macros used in stack manipulation.
- Mark stack
-
I say `your portion of the stack' above because PP code doesn't necessarily get the
whole stack to itself: if your function calls another function, you'll only want to expose
the arguments aimed for the called function, and not (necessarily) let it get at your own
data. The way we do this is to have a `virtual' bottom-of-stack, exposed to each function.
The mark stack keeps bookmarks to locations in the argument stack usable by each function.
For instance, when dealing with a tied variable, (internally, something with `P' magic)
Perl has to call methods for accesses to the tied variables. However, we need to separate
the arguments exposed to the method to the argument exposed to the original function - the
store or fetch or whatever it may be. Here's how the tied push is
implemented; see av_push in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
9 POPSTACK;
|
|
The lines which concern the mark stack are the first, fifth and last lines: they save
away, restore and remove the current position of the argument stack.
Let's examine the whole implementation, for practice:
Push the current state of the stack pointer onto the mark stack. This is so that when
we've finished adding items to the argument stack, Perl knows how many things we've added
recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
|
|
We're going to add two more items onto the argument stack: when you have a tied array,
the PUSH subroutine receives the object and the value to be pushed, and
that's exactly what we have here - the tied object, retrieved with SvTIED_obj,
and the value, the SV val.
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.
Finally, we remove the value we placed on the mark stack, since we don't need it any
more.
- Save stack
-
C doesn't have a concept of local scope, so perl provides one. We've seen that ENTER
and LEAVE are used as scoping braces; the save stack implements the C
equivalent of, for example:
See perlguts/Localising
Changes for how to use the save stack.
One thing you'll notice about the Perl source is that it's full of macros. Some have called
the pervasive use of macros the hardest thing to understand, others find it adds to clarity.
Let's take an example, the code which implements the addition operator:
1 PP(pp_add)
2 {
3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
4 {
5 dPOPTOPnnrl_ul;
6 SETn( left + right );
7 RETURN;
8 }
9 }
|
|
Every line here (apart from the braces, of course) contains a macro. The first line sets up
the function declaration as Perl expects for PP code; line 3 sets up variable declarations for
the argument stack and the target, the return value of the operation. Finally, it tries to see
if the addition operation is overloaded; if so, the appropriate subroutine is called.
Line 5 is another variable declaration - all variable declarations start with d
- which pops from the top of the argument stack two NVs (hence nn) and puts them
into the variables right and left, hence the rl. These
are the two operands to the addition operator. Next, we call SETn to set the NV
of the return value to the result of adding the two values. This done, we return - the RETURN
macro makes sure that our return value is properly handled, and we pass the next operator to
run back to the main run loop.
Most of these macros are explained in perlapi, and some of the more
important ones are explained in perlxs
as well. Pay special attention to perlguts/Background
and PERL_IMPLICIT_CONTEXT for information on the [pad]THX_? macros.
To really poke around with Perl, you'll probably want to build Perl for debugging, like
this:
./Configure -d -D optimize=-g
make
|
|
-g is a flag to the C compiler to have it produce debugging information which
will allow us to step through a running program. Configure will also turn on the DEBUGGING
compilation symbol which enables all the internal debugging code in Perl. There are a whole
bunch of things you can debug with this: perlrun lists them all, and the
best way to find out about them is to play about with them. The most useful options are
probably
l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
|
|
Some of the functionality of the debugging code can be achieved using XS modules.
-Dr => use re 'debug'
-Dx => use O 'Debug'
|
|
If the debugging output of -D doesn't help you, it's time to step through
perl's execution with a source-level debugger.
- We'll use
gdb for our examples here; the principles will apply to any
debugger, but check the manual of the one you're using.
To fire up the debugger, type
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.
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 perlguts/Internal
Functions!) or the given line in the named source file.
- step
- Steps through the program a line at a time.
- next
- Steps through the program a line at a time, without descending into functions.
- continue
- Run until the next breakpoint.
- finish
- Run until the end of the current function, then stop again.
- 'enter'
- Just pressing Enter will do the most recent operation again - it's a blessing when
stepping through miles of source code.
- print
-
Execute the given C code and print its results. WARNING: Perl makes heavy use of
macros, and gdb is not aware of macros. You'll have to substitute them yourself.
So, for instance, you can't say
but you have to say
print Perl_sv_2pv_nolen(sv)
|
|
You may find it helpful to have a "macro dictionary", which you can produce
by saying cpp -dM perl.c | sort. Even then, cpp won't recursively
apply the macros for you.
One way to get around this macro hell is to use the dumping functions in dump.c;
these work a little like an internal Devel::Peek, but they also
cover OPs and other structures that you can't get at from Perl. Let's take an example. We'll
use the $a = $b + $c we used before, but give it a bit of context: $b =
"6XXXX"; $c = 2.3;. Where's a good place to stop and poke around?
What about pp_add, the function we examined earlier to implement the +
operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
|
|
Notice we use Perl_pp_add and not pp_add - see perlguts/Internal
Functions. With the breakpoint in place, we can run our program:
(gdb) run -e '$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 NVs to be placed into left and right - let's
slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
|
|
POPn takes the SV from the top of the stack and obtains its NV either directly
(if SvNOK is set) or by 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(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
|
|
We know we're going to get 6 from this, so let's finish the subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
|
|
We can also dump out this op: the current op is always stored in PL_op, and we
can dump it with Perl_op_dump. This'll give us similar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (12)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
11 TYPE = gvsv ===> 12
FLAGS = (SCALAR)
GV = main::b
}
}
|
|
# finish this later #
All right, we've now had a look at how to navigate the Perl sources and some things you'll
need to know when fiddling 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~.
[Well, it was in pp.c when this tutorial was written. It has now been split off with
pp_unpack to its own file, pp_pack.c]
Now let's look over pp_pack: we take a pattern into pat, and then
loop over the pattern, taking each format character in turn into datum_type. Then
for each possible format character, we swallow up the other arguments in the pattern (a field
width, an asterisk, and so on) and convert the next chunk input into the specified 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(cat);
if (datumtype == '#') {
while (pat < patend && *pat != '\n')
pat++;
|
|
Remember that it has to be patcopy+1 because the first character of the string
is the U which has been swallowed into datumtype!
Oops, we forgot one thing: what if there are spaces at the start of the pattern? pack("
U*", @stuff) will have U as the first active character, even though
it's not the first thing in the pattern. In this case, we have to advance patcopy
along with pat when we see spaces:
if (isSPACE(datumtype))
continue;
|
|
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
|
|
OK. That's the C part done. Now we must do two additional things before this patch is ready
to go: we've changed the behaviour of Perl, and so we must document that change. We must also
provide some more regression tests to make sure our patch works and doesn't create a bug
somewhere else along the line.
The regression tests for each operator live in t/op/, and so we make a copy of t/op/pack.t
to t/op/pack.t~. Now we can add our tests to the end. First, we'll test that the U
does indeed create Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didn't we could use the one from t/test.pl.
require './test.pl';
plan( tests => 159 );
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so instead of this:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
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we can write the more sensible (see Test::More for a full
explanation of is() and other testing functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces unicode" );
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Now we'll test that we got that space-at-the-beginning business right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
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And finally we'll test that we don't make Unicode strings if U is not
the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isn't unicode" );
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Mustn't forget to change the number of tests which appears at the top, or else the
automated tester will get confused. This will either look like this:
or this:
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.
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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
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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
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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 @@
...
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And finally, we submit it, with our rationale, to perl5-porters. Job done!
This works just like patching anything else, with an extra consideration. Many core modules
also live on CPAN. If this is so, patch the CPAN version instead of the core and send the
patch off to the module maintainer (with a copy to p5p). This will help the module maintainer
keep the CPAN version in sync with the core version without constantly scanning p5p.
If, as part of a patch to fix a bug, or just because you have an especially good idea, you
decide to add a new function to the core, discuss your ideas on p5p well before you start
work. It may be that someone else has already attempted to do what you are considering and can
give lots of good advice or even provide you with bits of code that they already started (but
never finished).
You have to follow all of the advice given above for patching. It is extremely important to
test any addition thoroughly and add new tests to explore all boundary conditions that your
new function is expected to handle. If your new function is used only by one module (e.g.
toke), then it should probably be named S_your_function (for static); on the other hand, if
you expect it to accessible from other functions in Perl, you should name it
Perl_your_function. See perlguts/Internal
Functions for more details.
The location of any new code is also an important consideration. Don't just create a new
top level .c file and put your code there; you would have to make changes to Configure (so the
Makefile is created properly), as well as possibly lots of include files. This is strictly
pumpking business.
It is better to add your function to one of the existing top level source code files, but
your choice is complicated by the nature of the Perl distribution. Only the files that are
marked as compiled static are located in the perl executable. Everything else is located in
the shared library (or DLL if you are running under WIN32). So, for example, if a function was
only used by functions located in toke.c, then your code can go in toke.c. If, however, you
want to call the function from universal.c, then you should put your code in another location,
for example util.c.
In addition to writing your c-code, you will need to create an appropriate entry in
embed.pl describing your function, then run 'make regen_headers' to create the entries in the
numerous header files that perl needs to compile correctly. See perlguts/Internal
Functions for information on the various options that you can set in embed.pl. You will
forget to do this a few (or many) times and you will get warnings during the compilation
phase. Make sure that you mention this when you post your patch to P5P; the pumpking needs to
know this.
When you write your new code, please be conscious of existing code conventions used in the
perl source files. See perlstyle
for details. Although most of the guidelines discussed seem to focus on Perl code, rather than
c, they all apply (except when they don't ;). See also Porting/patching.pod file in the
Perl source distribution for lots of details about both formatting and submitting patches of
your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to p5p. Test on as many platforms
as you can find. Test as many perl Configure options as you can (e.g. MULTIPLICITY). If you
have profiling or memory tools, see EXTERNAL
TOOLS FOR DEBUGGING PERL below for how to use them to further test your code. Remember
that most of the people on P5P are doing this on their own time and don't have the time to
debug your code.
Every module and built-in function has an associated test file (or should...). If you add
or change functionality, you have to write a test. If you fix a bug, you have to write a test
so that bug never comes back. If you alter the docs, it would be nice to test what the new
documentation says.
In short, if you submit a patch you probably also have to patch the tests.
For modules, the test file is right next to the module itself. lib/strict.t tests lib/strict.pm.
This is a recent innovation, so there are some snags (and it would be wonderful for you to
brush them out), but it basically works that way. Everything else lives in t/.
- t/base/
- Testing of the absolute basic functionality of Perl. Things like
if, basic
file reads and writes, simple regexes, etc. These are run first in the test suite and if
any of them fail, something is really broken.
- t/cmd/
- These test the basic control structures,
if/else, while,
subroutines, etc.
- t/comp/
- Tests basic issues of how Perl parses and compiles itself.
- t/io/
- Tests for built-in IO functions, including command line arguments.
- t/lib/
- The old home for the module tests, you shouldn't put anything new in here. There are
still some bits and pieces hanging around in here that need to be moved. Perhaps you could
move them? Thanks!
- t/op/
- Tests for perl's built in functions that don't fit into any of the other directories.
- t/pod/
- Tests for POD directives. There are still some tests for the Pod modules hanging around
in here that need to be moved out into lib/.
- t/run/
- Testing features of how perl actually runs, including exit codes and handling of PERL*
environment variables.
The core uses the same testing style as the rest of Perl, a simple "ok/not ok"
run through Test::Harness, but there are a few special considerations.
There are three ways to write a test in the core. Test::More, t/test.pl and ad hoc print
$test ? "ok 42\n" : "not ok 42\n". The decision of which to use
depends on what part of the test suite you're working on. This is a measure to prevent a
high-level failure (such as Config.pm breaking) from causing basic functionality tests to
fail.
- t/base t/comp
- Since we don't know if require works, or even subroutines, use ad hoc tests for these
two. Step carefully to avoid using the feature being tested.
- t/cmd t/run t/io t/op
-
Now that basic require() and subroutines are tested, you can use the t/test.pl library
which emulates the important features of Test::More while using a minimum of core
features.
You can also conditionally use certain libraries like Config, but be sure to skip the
test gracefully if it's not there.
- t/lib ext lib
- Now that the core of Perl is tested, Test::More can be used. You can also use the full
suite of core modules in the tests.
When you say "make test" Perl uses the t/TEST program to run the test
suite. All tests are run from the t/ directory, not the directory which contains
the test. This causes some problems with the tests in lib/, so here's some opportunity
for some patching.
You must be triply conscious of cross-platform concerns. This usually boils down to using
File::Spec and avoiding things like fork() and system() unless
absolutely necessary.
There are various special make targets that can be used to test Perl slightly differently
than the standard "test" target. Not all them are expected to give a 100% success
rate. Many of them have several aliases.
- coretest
- Run perl on all core tests (t/* and lib/[a-z]* pragma tests).
- test.deparse
- Run all the tests through the B::Deparse. Not all tests will succeed.
- minitest
- Run miniperl on t/base, t/comp, t/cmd, t/run, t/io,
t/op, and t/uni tests.
- test.third check.third
utest.third ucheck.third
- (Only in Tru64) Run all the tests using the memory leak + naughty memory access tool
"Third Degree". The log files will be named perl3.log.testname.
- test.torture torturetest
-
Run all the usual tests and some extra tests. As of Perl 5.8.0 the only extra tests are
Abigail's JAPHs, t/japh/abigail.t.
You can also run the torture test with t/harness by giving -torture
argument to t/harness.
- utest ucheck test.utf8 check.utf8
- Run all the tests with -Mutf8. Not all tests will succeed.
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