NAME

perlhacktut - so you want to be a Perl porter?

DESCRIPTION

Getting involved in Perl development isn't as difficult as it might seem. Most of the barriers are actually psychological - an experienced Perl programmer with a smattering of C should have no problems at all picking up enough about the internals to get involved with development. Of course, there are some things you'll need to know. The purpose of this document is to help you find them.

Getting started

Here are some things a Perl porter will need to get hold of:

The latest Perl source
It's a lot easier to hack on the Perl source if you've got a copy of it, right? So, download src/perl-devel.tar.gz from CPAN. Later on, you'll see that we sometimes refer to an even more bleeding-edge version of Perl than the development release called ``perl-current''. For the time being, though, perl-devel is fine.

A C compiler and debugger
On a Unix system, you'll probably have a C compiler installed; on Windows or similar, my personal recommendation is to install the Cygwin development environment, (http://sourceware.cygnus.org/cygwin) although Perl can built with Microsoft Visual C or Borland C. Mac people will want to get MPW.

You'll want a source-level C debugger which allows you to step through the execution of a program and evaluate expressions much like the built-in Perl debugger does for Perl programs. gdb is the standard console debugger on non-commercial Unices and also comes with Cygwin. (http://sourceware.cygnus.com/gdb/)

A subscription to perl5-porters
perl5-porters is the central development list for Perl. If you're interested in how Perl works, where it's going, or how you can contribute to it, you will want to subscribe: send mail to perl5-porters-subscribe@perl.org

You'll also need to read the following things:

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

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

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

perlapi
The documentation for the Perl API explains what some of the internal functions to, 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 vast 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. perlguts/Compiled code explains exactly how the compilation stage happens.

Here is a short breakdown of perl's operation:

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

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

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

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 that it needs to perform. 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 perlguts, which tells you about Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do that now.

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

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

      % perl -MDevel::Peek -e 'Dump("hello")'
    1 SV = PV(0xa041450) at 0xa04ecbc
    2   REFCNT = 1
    3   FLAGS = (POK,READONLY,pPOK)
    4   PV = 0xa0484e0 "hello"\0
    5   CUR = 5
    6   LEN = 6

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

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

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

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

You can get at any of these quantities from C very easily; just add Sv to the name of the field shown in the snippet, and you've got a macro which will return the value: SvCUR(sv) returns the current length of the string, SvREFCOUNT(sv) returns the reference count, SvPV(sv, len) returns the string itself with its length, and so on. More macros to manipulate these properties can be found in perlguts.

Let's take an example of manipulating a PV, from sv_catpvn, in sv.c

     1  void
     2  Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
     3  {
     4      STRLEN tlen;
     5      char *junk;
     6      junk = SvPV_force(sv, tlen);
     7      SvGROW(sv, tlen + len + 1);
     8      if (ptr == junk)
     9          ptr = SvPVX(sv);
    10      Move(ptr,SvPVX(sv)+tlen,len,char);
    11      SvCUR(sv) += len;
    12      *SvEND(sv) = '\0';
    13      (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    14      SvTAINT(sv);
    15  }

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

In line 7, we make sure that the SV will have enough room to fit 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's finished parsing, and get it to dump out the tree. This is exactly what the compiler backends B::Terse|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 C<0x8179828>. The specific operator in
question is C<sassign> - scalar assignment - and you can find the code
which implements in the function C<pp_sassign> in F<pp_hot.c>. As a
binary operator, it has two children: the add operator, providing the
result of C<$b+$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 above, 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, it's faster 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 C<$b> and C<$c>, and add them together.
Find C<$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 pack, 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 back an NV. The X in XPUSHn means that the stack should be extended if necessary - it can't be necessary here, because we know there's room for one more item on the stack, since we've just removed one! The XPUSH* macros at least guarantee safety.

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

     SETi(-TOPi);

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

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

Mark stack
I say `your portion of the stack' above because PP code doesn't necessarily get the whole stack to itself: if your function calls another function, you'll only want to expose the arguments aimed for the called function, and not (necessarily) let it get at your own data. The way we do this is to have a `virtual' bottom-of-stack, exposed to each function. The mark stack keeps 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 practise:

     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 perlcall. We call the PUSH method in scalar context, and we're going to discard its return value.

     9  POPSTACK;

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

Save stack
C doesn't have a concept of local scope, so perl 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 perlguts/Localising Changes 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      djSP; 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 together. This done, we return - the RETURN macro makes sure that our return value is properly handled, and we pass the next operator to run back to the main run loop.

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

Poking at Perl

To really poke around with Perl, you'll probably want to build Perl for debugging. Let's get hold of the latest source tree and build it.

We noted earlier that there was a version of Perl that's even more up-to-date than the latest development release. If you've got rsync installed, you can get it like this:

    rsync -auvz rsync://ftp.linux.activestate.com/perl-current/ bleadperl/

Otherwise, you'll have to download the whole of it via FTP:

    ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/perl-current/

Alternatively, you can download it patch-by-patch:
    ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/diffs/

Now type

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

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

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

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


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

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.

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 Function names!) 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.

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 Function Names. 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         djSP; 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 >

Internal functions

Earlier we remarked that pp_add was actually known as Perl_pp_add to the debugger - all of Perl's internal functions which will be exposed to the outside world are be prefixed by Perl_ so that they will not conflict with XS functions or functions used in a program in which Perl is embedded. Similarly, all global variables begin with PL_. (By convention, static functions start with S_)

Inside the Perl core, you can get at the functions either with or without the Perl_ prefix, thanks to a bunch of defines that live in embed.h. This header file is generated automatically from embed.pl. embed.pl also creates the prototyping header files for the internal functions, generates the documentation and a lot of other bits and pieces. It's important that when you add a new function to the core or change an existing one, you change the data in the table at the end of embed.pl as well. Here's a sample entry from that table:

    Apd |SV**   |av_fetch       |AV* ar|I32 key|I32 lval

The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags:

A
This function is a part of the public API.

p
This function has a Perl_ prefix; ie, it is defined as Perl_av_fetch.

d
This function has documentation using the apidoc feature which we'll look at in a second.

Other available flags are:

s
This is a static function and is defined as S_whatever.

n
This does not use aTHX_ and pTHX to pass interpreter context. (See perlguts/Background and PERL_IMPLICIT_CONTEXT.)

r
This function never returns; croak, exit and friends.

f
This function takes a variable number of arguments, printf style. The argument list should end with ..., like this:
    Afprd   |void   |croak          |const char* pat|...

o
This function should not have a compatibility macro to define, say, Perl_parse to parse. It must be called as Perl_parse.

j
This function is not a member of CPerlObj. If you don't know what this means, don't use it.

x
This function isn't exported out of the Perl core.

If you edit embed.pl, you will need to run make regen_headers to force a rebuild of embed.h and other auto-generated files.

Source Documentation

There's an effort going on to document the internal functions and automatically produce reference manuals from them - perlapi is one such manual which details all the functions which are available to XS writers. perlintern is the autogenerated manual for the functions which are not part of the API and are supposedly for internal use only.

Source documentation is created by putting POD comments into the C source, like this:

 /*
 =for apidoc sv_setiv

 Copies an integer into the given SV.  Does not handle 'set' magic.  See
 C<sv_setiv_mg>.

 =cut
 */

Please try and supply some documentation if you add functions to the Perl core.

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++;

Musn'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!

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. Finally, 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.

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:

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

Copyright (C) 2000 Simon Cozens All rights reserved.

This document may be distributed under the same terms as Perl itself.

Acknowledgments

Thanks to the Perl Porters for their comments.