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perlhack - How to hack at the Perl internals |
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:
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:
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.
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 -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.
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."
#!/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).
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.
"... 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."
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 ;-)
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.
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:
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/)
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:
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.)
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:
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.
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.
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.
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 here.
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:
SETi(-TOPi);
Just set the integer value of the top stack entry to its negation.
Argument stack manipulation in the core is exactly the same as it is in XSUBs - see the perlxstut manpage, the perlxs manpage and the perlguts manpage for a longer description of the macros used in stack manipulation.
push is implemented; see av_push in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
9 POPSTACK;
The lines which concern the mark stack are the first, fifth and last lines: they save away, restore and remove the current position of the argument stack.
Let's examine the whole implementation, for practice:
1 PUSHMARK(SP);
Push the current state of the stack pointer onto the mark stack. This is so that when we've finished adding items to the argument stack, Perl knows how many things we've added recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
We're going to add two more items onto the argument stack: when you have
a tied array, the PUSH subroutine receives the object and the value
to be pushed, and that's exactly what we have here - the tied object,
retrieved with SvTIED_obj, and the value, the SV val.
5 PUTBACK;
Next we tell Perl to make the change to the global stack pointer: dSP
only gave us a local copy, not a 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
the perlcall manpage. We call the PUSH method in scalar context, and we're
going to discard its return value.
9 POPSTACK;
Finally, we remove the value we placed on the mark stack, since we don't need it any more.
ENTER and LEAVE are used as scoping braces; the save
stack implements the C equivalent of, for example:
{
local $foo = 42;
...
}
See Localising Changes in the perlguts manpage 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 the perlapi manpage, and some of the more
important ones are explained in the perlxs manpage as well. Pay special attention
to Background and PERL_IMPLICIT_CONTEXT in the perlguts manpage 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: the perlrun manpage 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.
gdb for our examples here; the principles will apply to any
debugger, but check the manual of the one you're using.
To fire up the debugger, type
gdb ./perl
You'll want to do that in your Perl source tree so the debugger can read the source code. You should see the copyright message, followed by the prompt.
(gdb)
help will get you into the documentation, but here are the most
useful commands:
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a ``macro dictionary'', which you can
produce by saying cpp -dM perl.c | sort. Even then, cpp won't
recursively apply 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"; $c = 2.3;. Where's a good place to stop and poke around?
What about pp_add, the function we examined earlier to implement the
+ operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use Perl_pp_add and not pp_add - see Internal Functions in the perlguts manpage.
With the breakpoint in place, we can run our program:
(gdb) run -e '$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~.
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:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
Now we'll test that we got that space-at-the-beginning business right:
print 'not ' unless "1.20.300.4000" eq
sprintf "%vd", pack(" U*",1,20,300,4000);
print "ok $test\n"; $test++;
And finally we'll test that we don't make Unicode strings if U is not
the first active format:
print 'not ' unless v1.20.300.4000 ne
sprintf "%vd", pack("C0U*",1,20,300,4000);
print "ok $test\n"; $test++;
Mustn't forget to change the number of tests which appears at the top, or else the automated tester will get confused:
-print "1..156\n"; +print "1..159\n";
We now compile up Perl, and run it through the test suite. Our new tests pass, hooray!
Finally, the documentation. The job is never done until the paperwork is
over, so let's describe the change we've just made. The relevant place
is pod/perlfunc.pod; again, we make a copy, and then we'll insert
this text in the description of pack:
=item *
If the pattern begins with a C<U>, the resulting string will be treated as Unicode-encoded. You can force UTF8 encoding on in a string with an initial C<U0>, and the bytes that follow will be interpreted as Unicode characters. If you don't want this to happen, you can begin your pattern with C<C0> (or anything else) to force Perl not to UTF8 encode your string, and then follow this with a C<U*> somewhere in your pattern.
All done. Now let's create the patch. Porting/patching.pod tells us that if we're making major changes, we should copy the entire directory to somewhere safe before we begin fiddling, and then do
diff -ruN old new > patch
However, we know which files we've changed, and we can simply do this:
diff -u pp.c~ pp.c > patch
diff -u t/op/pack.t~ t/op/pack.t >> patch
diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
We end up with a patch looking a little like this:
--- pp.c~ Fri Jun 02 04:34:10 2000
+++ pp.c Fri Jun 16 11:37:25 2000
@@ -4375,6 +4375,7 @@
register I32 items;
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
+ char *patcopy;
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
@@ -4405,6 +4406,7 @@
...
And finally, we submit it, with our rationale, to perl5-porters. Job done!
Sometimes it helps to use external tools while debugging and testing Perl. This section tries to guide you through using some common testing and debugging tools with Perl. This is meant as a guide to interfacing these tools with Perl, not as any kind of guide to the use of the tools themselves.
Purify is a commercial tool that is helpful in identifying memory overruns, wild pointers, memory leaks and other such badness. Perl must be compiled in a specific way for optimal testing with Purify. Purify is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
The only currently known leaks happen when there are compile-time errors within eval or require. (Fixing these is non-trivial, unfortunately, but they must be fixed eventually.)
On Unix, Purify creates a new Perl binary. To get the most benefit out of Purify, you should create the perl to Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
Once you've compiled a perl suitable for Purify'ing, then you can just:
make pureperl
which creates a binary named 'pureperl' that has been Purify'ed. This binary is used in place of the standard 'perl' binary when you want to debug Perl memory problems.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run the Purify'ed perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory problems.
Purify outputs messages in ``Viewer'' windows by default. If you don't have a windowing environment or if you simply want the Purify output to unobtrusively go to a log file instead of to the interactive window, use these following options to output to the log file ``perl.log'':
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the ``Viewer'' windows, then you only need this option:
setenv PURIFYOPTIONS "-chain-length=25"
Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly. There are several options in the makefile you should change to get the most use out of Purify:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as well as to force use of memory allocation functions derived from the system malloc.
As an example, to show any memory leaks produced during the standard Perl testset you would create and run Purify as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl, then finally report any memory problems.
We've had a brief look around the Perl source, an overview of the stages perl goes through when it's running your code, and how to use a debugger to poke at the Perl guts. We took a very simple problem and demonstrated how to solve it fully - with documentation, regression tests, and finally a patch for submission to p5p. Finally, we talked about how to use external tools to debug and test Perl.
I'd now suggest you read over those references again, and then, as soon as possible, get your hands dirty. The best way to learn is by doing, so:
If you can do these things, you've started on the long road to Perl porting. Thanks for wanting to help make Perl better - and happy hacking!
This document was written by Nathan Torkington, and is maintained by the perl5-porters mailing list.
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