perlhack - How to hack at the Perl internals

- NAME
- DESCRIPTION

- Keeping in sync
- Why rsync the source tree
- Why rsync the patches
- Submitting patches
- Finding Your Way Around
- Elements of the interpreter
- Internal Variable Types
- Op Trees
- Stacks
- Millions of Macros
- Poking at Perl
- Using a source-level debugger
- Dumping Perl Data Structures
- Patching

- EXTERNAL TOOLS FOR DEBUGGING PERL

- Rational Software's Purify
- Purify on Unix
- Purify on NT
- CONCLUSION

- AUTHOR

# NAME

perlhack - How to hack at the Perl internals

# DESCRIPTION

This document attempts to explain how Perl development takes place, and ends with some suggestions for people wanting to become bona fide porters.

The perl5-porters mailing list is where the Perl standard distribution is maintained and developed. The list can get anywhere from 10 to 150 messages a day, depending on the heatedness of the debate. Most days there are two or three patches, extensions, features, or bugs being discussed at a time.

A searchable archive of the list is at:

        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:

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

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

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

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

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

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

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

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

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

Is the feature generic enough?
  Is this something that only the submitter wants added to the language, or would it be broadly useful? Sometimes, instead of adding a feature with a tight focus, the porters might decide to wait until someone implements the more generalized feature. For instance, instead of implementing a ``delayed evaluation'' feature, the porters are waiting for a macro system that would permit delayed evaluation and much more.

Does it potentially introduce new bugs?
  Radical rewrites of large chunks of the Perl interpreter have the potential to introduce new bugs. The smaller and more localized the change, the better.

Does it preclude other desirable features?
  A patch is likely to be rejected if it closes off future avenues of development. For instance, a patch that placed a true and final interpretation on prototypes is likely to be rejected because there are still options for the future of prototypes that haven't been addressed.

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

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

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

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

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

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

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

## Keeping in sync

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

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

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

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

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

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

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

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

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

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

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

  Using rsync over the LAN
    Set up a local rsync server which makes the rsynced source tree available to the LAN and sync the other machines against this directory.

    From http://rsync.samba.org/README.html:

           "Rsync uses rsh or ssh for communication. It does not need to be
            setuid and requires no special privileges for installation.  It
            does not require a inetd entry or a deamon.  You must, however,
            have a working rsh or ssh system.  Using ssh is recommended for
            its security features."

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

          #!/usr/bin/perl -w

          use strict;
          use File::Copy;

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

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

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

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

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

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

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

  This makes sure the latest available patch is downloaded to your patch directory.

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

       # last=`ls -rt1 *.gz | tail -1`
       # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ .
       # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
       # cd ../perl-current
       # patch -p1 -N <../perl-current-diffs/blead.patch

  or, since this is only a hint towards how it works, use CPAN-patchaperl from Andreas König to have better control over the patching process.

## Why rsync the source tree

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

It's more recent
  According to Gurusamy Sarathy:

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

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

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

## Why rsync the patches

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

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

It's a good reference
  If you do not only like to have the most recent development branch, but also like to fix bugs, or extend features, you want to dive into the sources. If you are a seasoned perl core diver, you don't need no manuals, tips, roadmaps, perlguts.pod or other aids to find your way around. But if you are a starter, the patches may help you in finding where you should start and how to change the bits that bug you.

  The file Changes is updated on occasions the pumpking sees as his own little sync points. On those occasions, he releases a tar-ball of the current source tree (i.e. perl@7582.tar.gz), which will be an excellent point to start with when choosing to use the 'rsync the patches' scheme. Starting with perl@7582, which means a set of source files on which the latest applied patch is number 7582, you apply all succeeding patches available from then on (7583, 7584, ...).

  You can use the patches later as a kind of search archive.

  Finding a start point
    If you want to fix/change the behaviour of function/feature Foo, just scan the patches for patches that mention Foo either in the subject, the comments, or the body of the fix. A good chance the patch shows you the files that are affected by that patch which are very likely to be the starting point of your journey into the guts of perl.

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

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

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

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

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

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

## Submitting patches

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

Your patch should update the documentation and test suite.

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

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

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

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

To hack on the Perl guts, you'll need to read the following things:

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

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

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

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

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

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

## Finding Your Way Around

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

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

Documentation
  Documentation maintenance includes looking after 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

## Elements of the interpreter

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

Here is a short breakdown of perl's operation:

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

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

          1 PERL_SYS_INIT3(&argc,&argv,&env);
          2
          3 if (!PL_do_undump) {
          4     my_perl = perl_alloc();
          5     if (!my_perl)
          6         exit(1);
          7     perl_construct(my_perl);
          8     PL_perl_destruct_level = 0;
          9 }

  Line 1 is a macro, and its definition is dependent on your operating system. Line 3 references `PL_do_undump`, a global variable - all global variables in Perl start with `PL_`. This tells you whether the current running program was created with the `-u` flag to perl and then undump, which means it's going to be false in any sane context.

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

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

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

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

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

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

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

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

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

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

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

Optimization
  Now the parsing stage is complete, and the finished tree represents the operations that the Perl interpreter needs to perform to execute our program. Next, Perl does a dry run over the tree looking for optimisations: constant expressions such as `3 + 4` will be computed now, and the optimizer will also see if any multiple operations can be replaced with a single one. For instance, to fetch the variable `$foo`, instead of grabbing the glob `*foo` and looking at the scalar component, the optimizer fiddles the op tree to use a function which directly looks up the scalar in question. The main optimizer is `peep` in op.c, and many ops have their own optimizing functions.

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

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

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

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

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

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

## Internal Variable Types

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.

## Op Trees

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

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

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

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

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

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

Let's start in the middle, at line 4. This is a BINOP, a binary operator, which is at location `0x8179828`. The 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 `gvsv`s.

Now, what's this about?

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

`enter` and `leave` are scoping ops, and their job is to perform any housekeeping every time you enter and leave a block: lexical variables are tidied up, unreferenced 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.

## Stacks

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

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

            NV value;
            value = POPn;
            value = Perl_cos(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.

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:

           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.

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:

          {
              local $foo = 42;
              ...
          }

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

## Millions of Macros

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

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

Every line here (apart from the braces, of course) 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.

## Poking at Perl

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

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

`-g` is a flag to the C compiler to have it produce 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'

## Using a source-level debugger

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

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

To fire up the debugger, type

        gdb ./perl

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

        (gdb)

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

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

break function_name
break source.c:xxx
  Tells the debugger that we'll want to pause execution when we reach either the named function (but see Internal Functions in the perlguts manpage!) 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

          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.

## Dumping Perl Data Structures

One way to get around this macro hell is to use the dumping functions in dump.c; these work a little like an internal Devel::Peek, but they also cover OPs and other structures that you 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 `NV`s to be placed into `left` and `right` - let's slightly expand it:

        #define dPOPTOPnnrl_ul  NV right = POPn; \
                                SV *leftsv = TOPs; \
                                NV left = USE_LEFT(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 >

## Patching

All right, we've now had a look at how to navigate the Perl sources and some things you'll need to know when 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!

# EXTERNAL TOOLS FOR DEBUGGING PERL

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

## Rational Software's Purify

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

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

## Purify on Unix

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

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

where these arguments mean:

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

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

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

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

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

        make pureperl

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

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

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

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

Purify outputs messages in ``Viewer'' windows by default. If you don't have a windowing environment or if you simply want the Purify output to unobtrusively go to a log file instead of to the interactive window, use these following options to output to the log file ``perl.log'':

        setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
         -log-file=perl.log -append-logfile=yes"

If you plan to use the ``Viewer'' windows, then you only need this option:

        setenv PURIFYOPTIONS "-chain-length=25"

## Purify on NT

Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly. There are several options in the makefile you should change to get the most use out of Purify:

DEFINES
  You should add -DPURIFY to the DEFINES line so the DEFINES line looks something like:

          DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1

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

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

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

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

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

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

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

## CONCLUSION

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

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

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

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

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

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

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

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

# AUTHOR

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

perlhack - How to hack at the Perl internals