Startup

perlmain.c is generated by "ExtUtils::Miniperl" from miniperlmain.c at make time, so you should make perl to follow this along.

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

    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 using perl_construct, also in perl.c; 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:

 if (!perl_parse(my_perl, xs_init, argc, argv, (char **)NULL))
     perl_run(my_perl);

 exitstatus = perl_destruct(my_perl);

 perl_free(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

"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

Running

    while ((PL_op = 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.

Note that each "pp_" function is expected to return a pointer to the next op. Calls to perl subs (and eval blocks) are handled within the same runops loop, and do not consume extra space on the C stack. For example, "pp_entersub" and "pp_entertry" just push a "CxSUB" or "CxEVAL" block struct onto the context stack which contain the address of the op following the sub call or eval. They then return the first op of that sub or eval block, and so execution continues of that sub or block. Later, a "pp_leavesub" or "pp_leavetry" op pops the "CxSUB" or "CxEVAL", retrieves the return op from it, and returns it.

Exception handing

The perl core wraps "setjmp()" etc in the macros "JMPENV_PUSH" and "JMPENV_JUMP". The basic rule of perl exceptions is that "exit", and "die" (in the absence of "eval") perform a JMPENV_JUMP(2), while "die" within "eval" does a JMPENV_JUMP(3).

At entry points to perl, such as "perl_parse()", "perl_run()" and "call_sv(cv, G_EVAL)" each does a "JMPENV_PUSH", then enter a runops loop or whatever, and handle possible exception returns. For a 2 return, final cleanup is performed, such as popping stacks and calling "CHECK" or "END" blocks. Amongst other things, this is how scope cleanup still occurs during an "exit".

If a "die" can find a "CxEVAL" block on the context stack, then the stack is popped to that level and the return op in that block is assigned to "PL_restartop"; then a JMPENV_JUMP(3) is performed. This normally passes control back to the guard. In the case of "perl_run" and "call_sv", a non-null "PL_restartop" triggers re-entry to the runops loop. The is the normal way that "die" or "croak" is handled within an "eval".

Sometimes ops are executed within an inner runops loop, such as tie, sort or overload code. In this case, something like

    sub FETCH { eval { die } }

would cause a longjmp right back to the guard in "perl_run", popping both runops loops, which is clearly incorrect. One way to avoid this is for the tie code to do a "JMPENV_PUSH" before executing "FETCH" in the inner runops loop, but for efficiency reasons, perl in fact just sets a flag, using "CATCH_SET(TRUE)". The "pp_require", "pp_entereval" and "pp_entertry" ops check this flag, and if true, they call "docatch", which does a "JMPENV_PUSH" and starts a new runops level to execute the code, rather than doing it on the current loop.

As a further optimisation, on exit from the eval block in the "FETCH", execution of the code following the block is still carried on in the inner loop. When an exception is raised, "docatch" compares the "JMPENV" level of the "CxEVAL" with "PL_top_env" and if they differ, just re-throws the exception. In this way any inner loops get popped.

Here's an example.

    1: eval { tie @a, 'A' };
    2: sub A::TIEARRAY {
    3:     eval { die };
    4:     die;
    5: }

To run this code, "perl_run" is called, which does a "JMPENV_PUSH" then enters a runops loop. This loop executes the eval and tie ops on line 1, with the eval pushing a "CxEVAL" onto the context stack.

The "pp_tie" does a "CATCH_SET(TRUE)", then starts a second runops loop to execute the body of "TIEARRAY". When it executes the entertry op on line 3, "CATCH_GET" is true, so "pp_entertry" calls "docatch" which does a "JMPENV_PUSH" and starts a third runops loop, which then executes the die op. At this point the C call stack looks like this:

    Perl_pp_die
    Perl_runops      # third loop
    S_docatch_body
    S_docatch
    Perl_pp_entertry
    Perl_runops      # second loop
    S_call_body
    Perl_call_sv
    Perl_pp_tie
    Perl_runops      # first loop
    S_run_body
    perl_run
    main

and the context and data stacks, as shown by "-Dstv", look like:

    STACK 0: MAIN
      CX 0: BLOCK  =>
      CX 1: EVAL   => AV()  PV("A"\0)
      retop=leave
    STACK 1: MAGIC
      CX 0: SUB    =>
      retop=(null)
      CX 1: EVAL   => *
    retop=nextstate

The die pops the first "CxEVAL" off the context stack, sets "PL_restartop" from it, does a JMPENV_JUMP(3), and control returns to the top "docatch". This then starts another third-level runops level, which executes the nextstate, pushmark and die ops on line 4. At the point that the second "pp_die" is called, the C call stack looks exactly like that above, even though we are no longer within an inner eval; this is because of the optimization mentioned earlier. However, the context stack now looks like this, ie with the top CxEVAL popped:

    STACK 0: MAIN
      CX 0: BLOCK  =>
      CX 1: EVAL   => AV()  PV("A"\0)
      retop=leave
    STACK 1: MAGIC
      CX 0: SUB    =>
      retop=(null)

The die on line 4 pops the context stack back down to the CxEVAL, leaving it as:

    STACK 0: MAIN
      CX 0: BLOCK  =>

As usual, "PL_restartop" is extracted from the "CxEVAL", and a JMPENV_JUMP(3) done, which pops the C stack back to the docatch:

    S_docatch
    Perl_pp_entertry
    Perl_runops      # second loop
    S_call_body
    Perl_call_sv
    Perl_pp_tie
    Perl_runops      # first loop
    S_run_body
    perl_run
    main

In this case, because the "JMPENV" level recorded in the "CxEVAL" differs from the current one, "docatch" just does a JMPENV_JUMP(3) and the C stack unwinds to:

    perl_run
    main

Because "PL_restartop" is non-null, "run_body" starts a new runops loop and execution continues.

INTERNAL VARIABLE TYPES

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_ SV *sv, const char *ptr, 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 UTF-8-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.

Argument stack

      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 perlxstut, perlxs and perlguts for a longer description of the macros used in stack manipulation.

Mark stack

     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;

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 update the global stack pointer from our internal variable: "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. The call_method() function removes the top element of the mark stack, so there is nothing for the caller to clean up.

Save stack

    {
        local $foo = 42;
        ...
    }

See "Localizing changes" in perlguts for how to use the save stack.