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

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

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 = 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 "CXt_SUB" or "CXt_EVAL" 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 "CXt_SUB" or "CXt_EVAL", retrieves the return op from it, and returns it.

Exception handing

Perl's exception handing (i.e. "die" etc.) is built on top of the low-level setjmp()/longjmp() C-library functions. These basically provide a way to capture the current PC and SP registers of the CPU and later restore them: i.e. a longjmp() continues at the point in code where a previous setjmp() was done, with anything further up on the C stack being lost. (This is why code should always save values using "SAVE_FOO" rather than in auto variables.)

The perl core wraps setjmp() and longjmp() in the macros "JMPENV_PUSH" and "JMPENV_JUMP". The push operation, as well as setting a setjump(), stores some temporary state in a struct local to the current function (allocated by "dJMPENV"). In particular, it stores a pointer to the previous "JMPENV" struct, and updates "PL_top_env" to point to the newest one, forming a chain of "JMPENV" states. Both the push and jump can output debugging information under "perl -Dl".

A basic rule of the perl internals is that all interpreter exits are achieved via a JMPENV_JUMP(). In particular:

• level 2: perl-level exit() and internals my_exit()

  These unwind all stacks, then perform a JMPENV_JUMP(2).

• level 3: perl-level die() and internals croak()

  If currently within an eval, these pop the context stack back to the nearest "CXt_EVAL" frame, set $@ as appropriate, set "PL_restartop" to the op which follows the eval associated with that frame, then perform a JMPENV_JUMP(3).

  Otherwise, the error message is printed to "STDERR", then it is treated as an exit: unwind all stacks and perform a JMPENV_JUMP(2).

• level 1: unused

  JMPENV_JUMP(1) is currently unused except in perl_run().

• level 0: normal return.

  The zero value is for a normal return from JMPENV_PUSH()

So the perl interpreter expects that, at all times, there is a suitable "JMPENV_PUSH" set up (and at a suitable location within the CPU call stack) that can catch and process a 2- or 3-valued jump; and in the case of a 3, start a new runops loop to execute "PL_restartop" and all remaining ops (as will be explained shortly).

The entry points to the perl interpreter all provide such a facility. For example, perl_parse(), perl_run() and "call_sv(cv, G_EVAL)" all contain something similar in outline to:

{
    dJMPENV;
    JMPENV_PUSH(ret);
    switch (ret) {
    case 0:                     /* normal return from JMPENV_PUSH() */
      redo_body:
        CALLRUNOPS(aTHX);
        break;
    case 2:                     /* caught longjmp(2) - exit / die */
        break;
    case 3:                     /* caught longjmp(3) - eval { die } */
        PL_op = PL_restartop;
        goto redo_body;
    }
    JMPENV_POP;
}

A runops loop such as Perl_runops_standard() (as set up by CALLRUNOPS()) is, at its heart, just a simple:

while ((PL_op = PL_op->op_ppaddr(aTHX))) { 1; }

which calls the pp() function associated with each op, relying on that to return a pointer to the next op to be executed.

As well as setting catches at the entry points to the perl interpreter, you might expect perl to also do a JMPENV_PUSH() in places like pp_entertry(), just before some trappable ops are executed. In fact perl doesn't normally do this. The drawback with doing it is that with nested or recursive code such as:

sub foo { my ($i) = @_; return if $i < 0; eval { foo(--$i) } }

Then the C stack would quickly overflow with pairs of entries like

...
#N+3 Perl_runops()
#N+2 Perl_pp_entertry()
#N+1 Perl_runops()
#N   Perl_pp_entertry()
...

Instead, perl puts its guards at the callers of runops loops. Then as many nested subroutine calls and evals may be called as you like, all within the one runops loop. If an exception occurs, control passes back to the caller of the loop, which just immediately restarts a new loop with "PL_restartop" being the next op to call.

So in normal operation where there are several nested evals, there will be multiple "CXt_EVAL" context stack entries, but only a single runops loop, guarded by a single "JMPENV_PUSH". Each caught eval will pop the next "CXt_EVAL" off the stack, set "PL_restartop", then longjmp() back to perl_run() and continue.

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

sub FETCH { eval { die }; .... }

would, unless handled specially, cause a longjmp() right back to the guard in perl_run(), popping both the 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 temporarily sets a flag using CATCH_SET(TRUE). This flag warns any subsequent "require", "entereval" or "entertry" ops that the caller is no longer promising to catch any raised exceptions on their behalf.

These ops check this flag, and if true, they (via docatch()) do a "JMPENV_PUSH" and start a new runops loop to execute the code, rather than doing it with the current loop.

As a consequence, on exit from the eval block in the "FETCH" above, execution of the code following the block is still carried on in the inner loop (i.e. the one established by the pp_entertry()). To avoid confusion, if a further exception is then raised, docatch() compares the "JMPENV" level of the "CXt_EVAL" with "PL_top_env" and if they differ, just re-throws the exception. In this way any inner loops get popped, and the exception will be dealt with properly by the level which is expecting it.

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 "entereval" and "tie" ops on line 1, with the "entereval" pushing a "CXt_EVAL" 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 the loop 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 restarts the pp_entertry(), then executes the "die" op. At this point the C call stack looks like this:

#10 Perl_pp_die()
#9  Perl_runops()      # runops loop 3
#8  S_docatch()        # JMPENV level 2
#7  Perl_pp_entertry()
#6  Perl_runops()      # runops loop 2
#5  Perl_call_sv()
#4  Perl_pp_tie()
#3  Perl_runops()      # runops loop 1
#2  S_run_body()
#1  perl_run()         # JMPENV level 1
#0  main()

and the context and data stacks, as shown by "perl -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 "CXt_EVAL" off the context stack, sets "PL_restartop" from it, does a JMPENV_JUMP(3), and control returns to the "JMPENV" level set in docatch(). This then starts another third-level runops level, which executes the "nextstate", "pushmark" and "die" ops from 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. However, the context stack now looks like this, i.e. with the top CXt_EVAL 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 "CXt_EVAL", leaving it as:

STACK 0: MAIN
  CX 0: BLOCK  =>

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

#8  S_docatch()        # JMPENV level 2
#7  Perl_pp_entertry()
#6  Perl_runops()      # runops loop 2
#5  Perl_call_sv()
#4  Perl_pp_tie()
#3  Perl_runops()      # runops loop 1
#2  S_run_body()
#1  perl_run()         # JMPENV level 1
#0  main()

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

#1  perl_run()         # JMPENV level 1
#0  main()

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

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_ 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 "$x=10; $x.="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

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 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 roughly 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;

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

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