4 Consistent formatting of this file is achieved with:
5 perl ./Porting/podtidy pod/perlinterp.pod
9 perlinterp - An overview of the Perl interpreter
13 This document provides an overview of how the Perl interpreter works at
14 the level of C code, along with pointers to the relevant C source code
17 =head1 ELEMENTS OF THE INTERPRETER
19 The work of the interpreter has two main stages: compiling the code
20 into the internal representation, or bytecode, and then executing it.
21 L<perlguts/Compiled code> explains exactly how the compilation stage
24 Here is a short breakdown of perl's operation:
28 The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
29 This is very high-level code, enough to fit on a single screen, and it
30 resembles the code found in L<perlembed>; most of the real action takes
33 F<perlmain.c> is generated by C<ExtUtils::Miniperl> from
34 F<miniperlmain.c> at make time, so you should make perl to follow this
37 First, F<perlmain.c> allocates some memory and constructs a Perl
38 interpreter, along these lines:
40 1 PERL_SYS_INIT3(&argc,&argv,&env);
42 3 if (!PL_do_undump) {
43 4 my_perl = perl_alloc();
46 7 perl_construct(my_perl);
47 8 PL_perl_destruct_level = 0;
50 Line 1 is a macro, and its definition is dependent on your operating
51 system. Line 3 references C<PL_do_undump>, a global variable - all
52 global variables in Perl start with C<PL_>. This tells you whether the
53 current running program was created with the C<-u> flag to perl and
54 then F<undump>, which means it's going to be false in any sane context.
56 Line 4 calls a function in F<perl.c> to allocate memory for a Perl
57 interpreter. It's quite a simple function, and the guts of it looks
60 my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
62 Here you see an example of Perl's system abstraction, which we'll see
63 later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
64 own C<malloc> as defined in F<malloc.c> if you selected that option at
67 Next, in line 7, we construct the interpreter using perl_construct,
68 also in F<perl.c>; this sets up all the special variables that Perl
69 needs, the stacks, and so on.
71 Now we pass Perl the command line options, and tell it to go:
73 exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
77 exitstatus = perl_destruct(my_perl);
81 C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
82 in F<perl.c>, which processes the command line options, sets up any
83 statically linked XS modules, opens the program and calls C<yyparse> to
88 The aim of this stage is to take the Perl source, and turn it into an
89 op tree. We'll see what one of those looks like later. Strictly
90 speaking, there's three things going on here.
92 C<yyparse>, the parser, lives in F<perly.c>, although you're better off
93 reading the original YACC input in F<perly.y>. (Yes, Virginia, there
94 B<is> a YACC grammar for Perl!) The job of the parser is to take your
95 code and "understand" it, splitting it into sentences, deciding which
96 operands go with which operators and so on.
98 The parser is nobly assisted by the lexer, which chunks up your input
99 into tokens, and decides what type of thing each token is: a variable
100 name, an operator, a bareword, a subroutine, a core function, and so
101 on. The main point of entry to the lexer is C<yylex>, and that and its
102 associated routines can be found in F<toke.c>. Perl isn't much like
103 other computer languages; it's highly context sensitive at times, it
104 can be tricky to work out what sort of token something is, or where a
105 token ends. As such, there's a lot of interplay between the tokeniser
106 and the parser, which can get pretty frightening if you're not used to
109 As the parser understands a Perl program, it builds up a tree of
110 operations for the interpreter to perform during execution. The
111 routines which construct and link together the various operations are
112 to be found in F<op.c>, and will be examined later.
116 Now the parsing stage is complete, and the finished tree represents the
117 operations that the Perl interpreter needs to perform to execute our
118 program. Next, Perl does a dry run over the tree looking for
119 optimisations: constant expressions such as C<3 + 4> will be computed
120 now, and the optimizer will also see if any multiple operations can be
121 replaced with a single one. For instance, to fetch the variable
122 C<$foo>, instead of grabbing the glob C<*foo> and looking at the scalar
123 component, the optimizer fiddles the op tree to use a function which
124 directly looks up the scalar in question. The main optimizer is C<peep>
125 in F<op.c>, and many ops have their own optimizing functions.
129 Now we're finally ready to go: we have compiled Perl byte code, and all
130 that's left to do is run it. The actual execution is done by the
131 C<runops_standard> function in F<run.c>; more specifically, it's done
132 by these three innocent looking lines:
134 while ((PL_op = PL_op->op_ppaddr(aTHX))) {
138 You may be more comfortable with the Perl version of that:
140 PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
142 Well, maybe not. Anyway, each op contains a function pointer, which
143 stipulates the function which will actually carry out the operation.
144 This function will return the next op in the sequence - this allows for
145 things like C<if> which choose the next op dynamically at run time. The
146 C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
147 execution if required.
149 The actual functions called are known as PP code, and they're spread
150 between four files: F<pp_hot.c> contains the "hot" code, which is most
151 often used and highly optimized, F<pp_sys.c> contains all the
152 system-specific functions, F<pp_ctl.c> contains the functions which
153 implement control structures (C<if>, C<while> and the like) and F<pp.c>
154 contains everything else. These are, if you like, the C code for Perl's
155 built-in functions and operators.
157 Note that each C<pp_> function is expected to return a pointer to the
158 next op. Calls to perl subs (and eval blocks) are handled within the
159 same runops loop, and do not consume extra space on the C stack. For
160 example, C<pp_entersub> and C<pp_entertry> just push a C<CxSUB> or
161 C<CxEVAL> block struct onto the context stack which contain the address
162 of the op following the sub call or eval. They then return the first op
163 of that sub or eval block, and so execution continues of that sub or
164 block. Later, a C<pp_leavesub> or C<pp_leavetry> op pops the C<CxSUB>
165 or C<CxEVAL>, retrieves the return op from it, and returns it.
167 =head2 Exception handing
169 Perl's exception handing (i.e. C<die> etc.) is built on top of the
170 low-level C<setjmp()>/C<longjmp()> C-library functions. These basically
171 provide a way to capture the current PC and SP registers and later
172 restore them; i.e. a C<longjmp()> continues at the point in code where
173 a previous C<setjmp()> was done, with anything further up on the C
174 stack being lost. This is why code should always save values using
175 C<SAVE_FOO> rather than in auto variables.
177 The perl core wraps C<setjmp()> etc in the macros C<JMPENV_PUSH> and
178 C<JMPENV_JUMP>. The basic rule of perl exceptions is that C<exit>, and
179 C<die> (in the absence of C<eval>) perform a C<JMPENV_JUMP(2)>, while
180 C<die> within C<eval> does a C<JMPENV_JUMP(3)>.
182 At entry points to perl, such as C<perl_parse()>, C<perl_run()> and
183 C<call_sv(cv, G_EVAL)> each does a C<JMPENV_PUSH>, then enter a runops
184 loop or whatever, and handle possible exception returns. For a 2
185 return, final cleanup is performed, such as popping stacks and calling
186 C<CHECK> or C<END> blocks. Amongst other things, this is how scope
187 cleanup still occurs during an C<exit>.
189 If a C<die> can find a C<CxEVAL> block on the context stack, then the
190 stack is popped to that level and the return op in that block is
191 assigned to C<PL_restartop>; then a C<JMPENV_JUMP(3)> is performed.
192 This normally passes control back to the guard. In the case of
193 C<perl_run> and C<call_sv>, a non-null C<PL_restartop> triggers
194 re-entry to the runops loop. The is the normal way that C<die> or
195 C<croak> is handled within an C<eval>.
197 Sometimes ops are executed within an inner runops loop, such as tie,
198 sort or overload code. In this case, something like
200 sub FETCH { eval { die } }
202 would cause a longjmp right back to the guard in C<perl_run>, popping
203 both runops loops, which is clearly incorrect. One way to avoid this is
204 for the tie code to do a C<JMPENV_PUSH> before executing C<FETCH> in
205 the inner runops loop, but for efficiency reasons, perl in fact just
206 sets a flag, using C<CATCH_SET(TRUE)>. The C<pp_require>,
207 C<pp_entereval> and C<pp_entertry> ops check this flag, and if true,
208 they call C<docatch>, which does a C<JMPENV_PUSH> and starts a new
209 runops level to execute the code, rather than doing it on the current
212 As a further optimisation, on exit from the eval block in the C<FETCH>,
213 execution of the code following the block is still carried on in the
214 inner loop. When an exception is raised, C<docatch> compares the
215 C<JMPENV> level of the C<CxEVAL> with C<PL_top_env> and if they differ,
216 just re-throws the exception. In this way any inner loops get popped.
220 1: eval { tie @a, 'A' };
226 To run this code, C<perl_run> is called, which does a C<JMPENV_PUSH>
227 then enters a runops loop. This loop executes the eval and tie ops on
228 line 1, with the eval pushing a C<CxEVAL> onto the context stack.
230 The C<pp_tie> does a C<CATCH_SET(TRUE)>, then starts a second runops
231 loop to execute the body of C<TIEARRAY>. When it executes the entertry
232 op on line 3, C<CATCH_GET> is true, so C<pp_entertry> calls C<docatch>
233 which does a C<JMPENV_PUSH> and starts a third runops loop, which then
234 executes the die op. At this point the C call stack looks like this:
237 Perl_runops # third loop
241 Perl_runops # second loop
245 Perl_runops # first loop
250 and the context and data stacks, as shown by C<-Dstv>, look like:
254 CX 1: EVAL => AV() PV("A"\0)
262 The die pops the first C<CxEVAL> off the context stack, sets
263 C<PL_restartop> from it, does a C<JMPENV_JUMP(3)>, and control returns
264 to the top C<docatch>. This then starts another third-level runops
265 level, which executes the nextstate, pushmark and die ops on line 4. At
266 the point that the second C<pp_die> is called, the C call stack looks
267 exactly like that above, even though we are no longer within an inner
268 eval; this is because of the optimization mentioned earlier. However,
269 the context stack now looks like this, ie with the top CxEVAL popped:
273 CX 1: EVAL => AV() PV("A"\0)
279 The die on line 4 pops the context stack back down to the CxEVAL,
285 As usual, C<PL_restartop> is extracted from the C<CxEVAL>, and a
286 C<JMPENV_JUMP(3)> done, which pops the C stack back to the docatch:
290 Perl_runops # second loop
294 Perl_runops # first loop
299 In this case, because the C<JMPENV> level recorded in the C<CxEVAL>
300 differs from the current one, C<docatch> just does a C<JMPENV_JUMP(3)>
301 and the C stack unwinds to:
306 Because C<PL_restartop> is non-null, C<run_body> starts a new runops
307 loop and execution continues.
309 =head2 INTERNAL VARIABLE TYPES
311 You should by now have had a look at L<perlguts>, which tells you about
312 Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do
315 These variables are used not only to represent Perl-space variables,
316 but also any constants in the code, as well as some structures
317 completely internal to Perl. The symbol table, for instance, is an
318 ordinary Perl hash. Your code is represented by an SV as it's read into
319 the parser; any program files you call are opened via ordinary Perl
320 filehandles, and so on.
322 The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
323 Perl program. Let's see, for instance, how Perl treats the constant
326 % perl -MDevel::Peek -e 'Dump("hello")'
327 1 SV = PV(0xa041450) at 0xa04ecbc
329 3 FLAGS = (POK,READONLY,pPOK)
330 4 PV = 0xa0484e0 "hello"\0
334 Reading C<Devel::Peek> output takes a bit of practise, so let's go
335 through it line by line.
337 Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in
338 memory. SVs themselves are very simple structures, but they contain a
339 pointer to a more complex structure. In this case, it's a PV, a
340 structure which holds a string value, at location C<0xa041450>. Line 2
341 is the reference count; there are no other references to this data, so
344 Line 3 are the flags for this SV - it's OK to use it as a PV, it's a
345 read-only SV (because it's a constant) and the data is a PV internally.
346 Next we've got the contents of the string, starting at location
349 Line 5 gives us the current length of the string - note that this does
350 B<not> include the null terminator. Line 6 is not the length of the
351 string, but the length of the currently allocated buffer; as the string
352 grows, Perl automatically extends the available storage via a routine
355 You can get at any of these quantities from C very easily; just add
356 C<Sv> to the name of the field shown in the snippet, and you've got a
357 macro which will return the value: C<SvCUR(sv)> returns the current
358 length of the string, C<SvREFCOUNT(sv)> returns the reference count,
359 C<SvPV(sv, len)> returns the string itself with its length, and so on.
360 More macros to manipulate these properties can be found in L<perlguts>.
362 Let's take an example of manipulating a PV, from C<sv_catpvn>, in
366 2 Perl_sv_catpvn(pTHX_ SV *sv, const char *ptr, STRLEN len)
371 6 junk = SvPV_force(sv, tlen);
372 7 SvGROW(sv, tlen + len + 1);
375 10 Move(ptr,SvPVX(sv)+tlen,len,char);
377 12 *SvEND(sv) = '\0';
378 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
382 This is a function which adds a string, C<ptr>, of length C<len> onto
383 the end of the PV stored in C<sv>. The first thing we do in line 6 is
384 make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
385 macro to force a PV. As a side effect, C<tlen> gets set to the current
386 value of the PV, and the PV itself is returned to C<junk>.
388 In line 7, we make sure that the SV will have enough room to
389 accommodate the old string, the new string and the null terminator. If
390 C<LEN> isn't big enough, C<SvGROW> will reallocate space for us.
392 Now, if C<junk> is the same as the string we're trying to add, we can
393 grab the string directly from the SV; C<SvPVX> is the address of the PV
396 Line 10 does the actual catenation: the C<Move> macro moves a chunk of
397 memory around: we move the string C<ptr> to the end of the PV - that's
398 the start of the PV plus its current length. We're moving C<len> bytes
399 of type C<char>. After doing so, we need to tell Perl we've extended
400 the string, by altering C<CUR> to reflect the new length. C<SvEND> is a
401 macro which gives us the end of the string, so that needs to be a
404 Line 13 manipulates the flags; since we've changed the PV, any IV or NV
405 values will no longer be valid: if we have C<$a=10; $a.="6";> we don't
406 want to use the old IV of 10. C<SvPOK_only_utf8> is a special
407 UTF-8-aware version of C<SvPOK_only>, a macro which turns off the IOK
408 and NOK flags and turns on POK. The final C<SvTAINT> is a macro which
409 launders tainted data if taint mode is turned on.
411 AVs and HVs are more complicated, but SVs are by far the most common
412 variable type being thrown around. Having seen something of how we
413 manipulate these, let's go on and look at how the op tree is
418 First, what is the op tree, anyway? The op tree is the parsed
419 representation of your program, as we saw in our section on parsing,
420 and it's the sequence of operations that Perl goes through to execute
421 your program, as we saw in L</Running>.
423 An op is a fundamental operation that Perl can perform: all the
424 built-in functions and operators are ops, and there are a series of ops
425 which deal with concepts the interpreter needs internally - entering
426 and leaving a block, ending a statement, fetching a variable, and so
429 The op tree is connected in two ways: you can imagine that there are
430 two "routes" through it, two orders in which you can traverse the tree.
431 First, parse order reflects how the parser understood the code, and
432 secondly, execution order tells perl what order to perform the
435 The easiest way to examine the op tree is to stop Perl after it has
436 finished parsing, and get it to dump out the tree. This is exactly what
437 the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise>
438 and L<B::Debug|B::Debug> do.
440 Let's have a look at how Perl sees C<$a = $b + $c>:
442 % perl -MO=Terse -e '$a=$b+$c'
443 1 LISTOP (0x8179888) leave
444 2 OP (0x81798b0) enter
445 3 COP (0x8179850) nextstate
446 4 BINOP (0x8179828) sassign
447 5 BINOP (0x8179800) add [1]
448 6 UNOP (0x81796e0) null [15]
449 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
450 8 UNOP (0x81797e0) null [15]
451 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
452 10 UNOP (0x816b4f0) null [15]
453 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
455 Let's start in the middle, at line 4. This is a BINOP, a binary
456 operator, which is at location C<0x8179828>. The specific operator in
457 question is C<sassign> - scalar assignment - and you can find the code
458 which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
459 binary operator, it has two children: the add operator, providing the
460 result of C<$b+$c>, is uppermost on line 5, and the left hand side is
463 Line 10 is the null op: this does exactly nothing. What is that doing
464 there? If you see the null op, it's a sign that something has been
465 optimized away after parsing. As we mentioned in L</Optimization>, the
466 optimization stage sometimes converts two operations into one, for
467 example when fetching a scalar variable. When this happens, instead of
468 rewriting the op tree and cleaning up the dangling pointers, it's
469 easier just to replace the redundant operation with the null op.
470 Originally, the tree would have looked like this:
472 10 SVOP (0x816b4f0) rv2sv [15]
473 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
475 That is, fetch the C<a> entry from the main symbol table, and then look
476 at the scalar component of it: C<gvsv> (C<pp_gvsv> in F<pp_hot.c>)
477 happens to do both these things.
479 The right hand side, starting at line 5 is similar to what we've just
480 seen: we have the C<add> op (C<pp_add>, also in F<pp_hot.c>) add
481 together two C<gvsv>s.
483 Now, what's this about?
485 1 LISTOP (0x8179888) leave
486 2 OP (0x81798b0) enter
487 3 COP (0x8179850) nextstate
489 C<enter> and C<leave> are scoping ops, and their job is to perform any
490 housekeeping every time you enter and leave a block: lexical variables
491 are tidied up, unreferenced variables are destroyed, and so on. Every
492 program will have those first three lines: C<leave> is a list, and its
493 children are all the statements in the block. Statements are delimited
494 by C<nextstate>, so a block is a collection of C<nextstate> ops, with
495 the ops to be performed for each statement being the children of
496 C<nextstate>. C<enter> is a single op which functions as a marker.
498 That's how Perl parsed the program, from top to bottom:
511 However, it's impossible to B<perform> the operations in this order:
512 you have to find the values of C<$b> and C<$c> before you add them
513 together, for instance. So, the other thread that runs through the op
514 tree is the execution order: each op has a field C<op_next> which
515 points to the next op to be run, so following these pointers tells us
516 how perl executes the code. We can traverse the tree in this order
517 using the C<exec> option to C<B::Terse>:
519 % perl -MO=Terse,exec -e '$a=$b+$c'
520 1 OP (0x8179928) enter
521 2 COP (0x81798c8) nextstate
522 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
523 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
524 5 BINOP (0x8179878) add [1]
525 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
526 7 BINOP (0x81798a0) sassign
527 8 LISTOP (0x8179900) leave
529 This probably makes more sense for a human: enter a block, start a
530 statement. Get the values of C<$b> and C<$c>, and add them together.
531 Find C<$a>, and assign one to the other. Then leave.
533 The way Perl builds up these op trees in the parsing process can be
534 unravelled by examining F<toke.c>, the lexer, and F<perly.y>, the YACC
535 grammar. Let's look at the code that constructs the tree for C<$a = $b +
538 First, we'll look at the C<Perl_yylex> function in the lexer. We want to
539 look for C<case 'x'>, where x is the first character of the operator.
540 (Incidentally, when looking for the code that handles a keyword, you'll
541 want to search for C<KEY_foo> where "foo" is the keyword.) Here is the code
542 that handles assignment (there are quite a few operators beginning with
543 C<=>, so most of it is omitted for brevity):
547 ... code that handles == => etc. and pod ...
548 3 pl_yylval.ival = 0;
549 4 OPERATOR(ASSIGNOP);
551 We can see on line 4 that our token type is C<ASSIGNOP> (C<OPERATOR> is a
552 macro, defined in F<toke.c>, that returns the token type, among other
557 3 const char tmp = *s++;
559 4 if (PL_expect == XOPERATOR) {
566 Line 4 checks what type of token we are expecting. C<Aop> returns a token.
567 If you search for C<Aop> elsewhere in F<toke.c>, you will see that it
568 returns an C<ADDOP> token.
570 Now that we know the two token types we want to look for in the parser,
571 let's take the piece of F<perly.y> we need to construct the tree for
574 1 term : term ASSIGNOP term
575 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
577 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
579 If you're not used to reading BNF grammars, this is how it works:
580 You're fed certain things by the tokeniser, which generally end up in
581 upper case. C<ADDOP> and C<ASSIGNOP> are examples of "terminal symbols",
582 because you can't get any simpler than
585 The grammar, lines one and three of the snippet above, tells you how to
586 build up more complex forms. These complex forms, "non-terminal
587 symbols" are generally placed in lower case. C<term> here is a
588 non-terminal symbol, representing a single expression.
590 The grammar gives you the following rule: you can make the thing on the
591 left of the colon if you see all the things on the right in sequence.
592 This is called a "reduction", and the aim of parsing is to completely
593 reduce the input. There are several different ways you can perform a
594 reduction, separated by vertical bars: so, C<term> followed by C<=>
595 followed by C<term> makes a C<term>, and C<term> followed by C<+>
596 followed by C<term> can also make a C<term>.
598 So, if you see two terms with an C<=> or C<+>, between them, you can
599 turn them into a single expression. When you do this, you execute the
600 code in the block on the next line: if you see C<=>, you'll do the code
601 in line 2. If you see C<+>, you'll do the code in line 4. It's this
602 code which contributes to the op tree.
605 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
607 What this does is creates a new binary op, and feeds it a number of
608 variables. The variables refer to the tokens: C<$1> is the first token
609 in the input, C<$2> the second, and so on - think regular expression
610 backreferences. C<$$> is the op returned from this reduction. So, we
611 call C<newBINOP> to create a new binary operator. The first parameter
612 to C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
613 operator, so we want the type to be C<ADDOP>. We could specify this
614 directly, but it's right there as the second token in the input, so we
615 use C<$2>. The second parameter is the op's flags: 0 means "nothing
616 special". Then the things to add: the left and right hand side of our
617 expression, in scalar context.
619 The functions that create ops, which have names like C<newUNOP> and
620 C<newBINOP>, call a "check" function associated with each op type, before
621 returning the op. The check functions can mangle the op as they see fit,
622 and even replace it with an entirely new one. These functions are defined
623 in F<op.c>, and have a C<Perl_ck_> prefix. You can find out which
624 check function is used for a particular op type by looking in
625 F<regen/opcodes>. Take C<OP_ADD>, for example. (C<OP_ADD> is the token
626 value from the C<Aop(OP_ADD)> in F<toke.c> which the parser passes to
627 C<newBINOP> as its first argument.) Here is the relevant line:
629 add addition (+) ck_null IfsT2 S S
631 The check function in this case is C<Perl_ck_null>, which does nothing.
632 Let's look at a more interesting case:
634 readline <HANDLE> ck_readline t% F?
636 And here is the function from F<op.c>:
639 2 Perl_ck_readline(pTHX_ OP *o)
641 4 PERL_ARGS_ASSERT_CK_READLINE;
643 6 if (o->op_flags & OPf_KIDS) {
644 7 OP *kid = cLISTOPo->op_first;
645 8 if (kid->op_type == OP_RV2GV)
646 9 kid->op_private |= OPpALLOW_FAKE;
650 13 = newUNOP(OP_READLINE, 0, newGVOP(OP_GV, 0,
658 One particularly interesting aspect is that if the op has no kids (i.e.,
659 C<readline()> or C<< <> >>) the op is freed and replaced with an entirely
660 new one that references C<*ARGV> (lines 12-16).
664 When perl executes something like C<addop>, how does it pass on its
665 results to the next op? The answer is, through the use of stacks. Perl
666 has a number of stacks to store things it's currently working on, and
667 we'll look at the three most important ones here.
669 =head2 Argument stack
671 Arguments are passed to PP code and returned from PP code using the
672 argument stack, C<ST>. The typical way to handle arguments is to pop
673 them off the stack, deal with them how you wish, and then push the
674 result back onto the stack. This is how, for instance, the cosine
679 value = Perl_cos(value);
682 We'll see a more tricky example of this when we consider Perl's macros
683 below. C<POPn> gives you the NV (floating point value) of the top SV on
684 the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and
685 push the result back as an NV. The C<X> in C<XPUSHn> means that the
686 stack should be extended if necessary - it can't be necessary here,
687 because we know there's room for one more item on the stack, since
688 we've just removed one! The C<XPUSH*> macros at least guarantee safety.
690 Alternatively, you can fiddle with the stack directly: C<SP> gives you
691 the first element in your portion of the stack, and C<TOP*> gives you
692 the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
693 negation of an integer:
697 Just set the integer value of the top stack entry to its negation.
699 Argument stack manipulation in the core is exactly the same as it is in
700 XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
701 description of the macros used in stack manipulation.
705 I say "your portion of the stack" above because PP code doesn't
706 necessarily get the whole stack to itself: if your function calls
707 another function, you'll only want to expose the arguments aimed for
708 the called function, and not (necessarily) let it get at your own data.
709 The way we do this is to have a "virtual" bottom-of-stack, exposed to
710 each function. The mark stack keeps bookmarks to locations in the
711 argument stack usable by each function. For instance, when dealing with
712 a tied variable, (internally, something with "P" magic) Perl has to
713 call methods for accesses to the tied variables. However, we need to
714 separate the arguments exposed to the method to the argument exposed to
715 the original function - the store or fetch or whatever it may be.
716 Here's roughly how the tied C<push> is implemented; see C<av_push> in
721 3 PUSHs(SvTIED_obj((SV*)av, mg));
725 7 call_method("PUSH", G_SCALAR|G_DISCARD);
728 Let's examine the whole implementation, for practice:
732 Push the current state of the stack pointer onto the mark stack. This
733 is so that when we've finished adding items to the argument stack, Perl
734 knows how many things we've added recently.
737 3 PUSHs(SvTIED_obj((SV*)av, mg));
740 We're going to add two more items onto the argument stack: when you
741 have a tied array, the C<PUSH> subroutine receives the object and the
742 value to be pushed, and that's exactly what we have here - the tied
743 object, retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
747 Next we tell Perl to update the global stack pointer from our internal
748 variable: C<dSP> only gave us a local copy, not a reference to the
752 7 call_method("PUSH", G_SCALAR|G_DISCARD);
755 C<ENTER> and C<LEAVE> localise a block of code - they make sure that
756 all variables are tidied up, everything that has been localised gets
757 its previous value returned, and so on. Think of them as the C<{> and
758 C<}> of a Perl block.
760 To actually do the magic method call, we have to call a subroutine in
761 Perl space: C<call_method> takes care of that, and it's described in
762 L<perlcall>. We call the C<PUSH> method in scalar context, and we're
763 going to discard its return value. The call_method() function removes
764 the top element of the mark stack, so there is nothing for the caller
769 C doesn't have a concept of local scope, so perl provides one. We've
770 seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
771 stack implements the C equivalent of, for example:
778 See L<perlguts/"Localizing changes"> for how to use the save stack.
780 =head1 MILLIONS OF MACROS
782 One thing you'll notice about the Perl source is that it's full of
783 macros. Some have called the pervasive use of macros the hardest thing
784 to understand, others find it adds to clarity. Let's take an example,
785 the code which implements the addition operator:
789 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
792 6 SETn( left + right );
797 Every line here (apart from the braces, of course) contains a macro.
798 The first line sets up the function declaration as Perl expects for PP
799 code; line 3 sets up variable declarations for the argument stack and
800 the target, the return value of the operation. Finally, it tries to see
801 if the addition operation is overloaded; if so, the appropriate
802 subroutine is called.
804 Line 5 is another variable declaration - all variable declarations
805 start with C<d> - which pops from the top of the argument stack two NVs
806 (hence C<nn>) and puts them into the variables C<right> and C<left>,
807 hence the C<rl>. These are the two operands to the addition operator.
808 Next, we call C<SETn> to set the NV of the return value to the result
809 of adding the two values. This done, we return - the C<RETURN> macro
810 makes sure that our return value is properly handled, and we pass the
811 next operator to run back to the main run loop.
813 Most of these macros are explained in L<perlapi>, and some of the more
814 important ones are explained in L<perlxs> as well. Pay special
815 attention to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for
816 information on the C<[pad]THX_?> macros.
818 =head1 FURTHER READING
820 For more information on the Perl internals, please see the documents
821 listed at L<perl/Internals and C Language Interface>.