3 perlguts - Introduction to the Perl API
7 This document attempts to describe how to use the Perl API, as well as
8 to provide some info on the basic workings of the Perl core. It is far
9 from complete and probably contains many errors. Please refer any
10 questions or comments to the author below.
16 Perl has three typedefs that handle Perl's three main data types:
22 Each typedef has specific routines that manipulate the various data types.
24 =head2 What is an "IV"?
26 Perl uses a special typedef IV which is a simple signed integer type that is
27 guaranteed to be large enough to hold a pointer (as well as an integer).
28 Additionally, there is the UV, which is simply an unsigned IV.
30 Perl also uses two special typedefs, I32 and I16, which will always be at
31 least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
32 as well.) They will usually be exactly 32 and 16 bits long, but on Crays
33 they will both be 64 bits.
35 =head2 Working with SVs
37 An SV can be created and loaded with one command. There are five types of
38 values that can be loaded: an integer value (IV), an unsigned integer
39 value (UV), a double (NV), a string (PV), and another scalar (SV).
41 The seven routines are:
46 SV* newSVpv(const char*, STRLEN);
47 SV* newSVpvn(const char*, STRLEN);
48 SV* newSVpvf(const char*, ...);
51 C<STRLEN> is an integer type (Size_t, usually defined as size_t in
52 F<config.h>) guaranteed to be large enough to represent the size of
53 any string that perl can handle.
55 In the unlikely case of a SV requiring more complex initialisation, you
56 can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
57 type NULL is returned, else an SV of type PV is returned with len + 1 (for
58 the NUL) bytes of storage allocated, accessible via SvPVX. In both cases
59 the SV has value undef.
61 SV *sv = newSV(0); /* no storage allocated */
62 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
64 To change the value of an I<already-existing> SV, there are eight routines:
66 void sv_setiv(SV*, IV);
67 void sv_setuv(SV*, UV);
68 void sv_setnv(SV*, double);
69 void sv_setpv(SV*, const char*);
70 void sv_setpvn(SV*, const char*, STRLEN)
71 void sv_setpvf(SV*, const char*, ...);
72 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
73 void sv_setsv(SV*, SV*);
75 Notice that you can choose to specify the length of the string to be
76 assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
77 allow Perl to calculate the length by using C<sv_setpv> or by specifying
78 0 as the second argument to C<newSVpv>. Be warned, though, that Perl will
79 determine the string's length by using C<strlen>, which depends on the
80 string terminating with a NUL character.
82 The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
83 formatted output becomes the value.
85 C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
86 either a pointer to a variable argument list or the address and length of
87 an array of SVs. The last argument points to a boolean; on return, if that
88 boolean is true, then locale-specific information has been used to format
89 the string, and the string's contents are therefore untrustworthy (see
90 L<perlsec>). This pointer may be NULL if that information is not
91 important. Note that this function requires you to specify the length of
94 The C<sv_set*()> functions are not generic enough to operate on values
95 that have "magic". See L<Magic Virtual Tables> later in this document.
97 All SVs that contain strings should be terminated with a NUL character.
98 If it is not NUL-terminated there is a risk of
99 core dumps and corruptions from code which passes the string to C
100 functions or system calls which expect a NUL-terminated string.
101 Perl's own functions typically add a trailing NUL for this reason.
102 Nevertheless, you should be very careful when you pass a string stored
103 in an SV to a C function or system call.
105 To access the actual value that an SV points to, you can use the macros:
110 SvPV(SV*, STRLEN len)
113 which will automatically coerce the actual scalar type into an IV, UV, double,
116 In the C<SvPV> macro, the length of the string returned is placed into the
117 variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
118 not care what the length of the data is, use the C<SvPV_nolen> macro.
119 Historically the C<SvPV> macro with the global variable C<PL_na> has been
120 used in this case. But that can be quite inefficient because C<PL_na> must
121 be accessed in thread-local storage in threaded Perl. In any case, remember
122 that Perl allows arbitrary strings of data that may both contain NULs and
123 might not be terminated by a NUL.
125 Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
126 len);>. It might work with your compiler, but it won't work for everyone.
127 Break this sort of statement up into separate assignments:
135 If you want to know if the scalar value is TRUE, you can use:
139 Although Perl will automatically grow strings for you, if you need to force
140 Perl to allocate more memory for your SV, you can use the macro
142 SvGROW(SV*, STRLEN newlen)
144 which will determine if more memory needs to be allocated. If so, it will
145 call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
146 decrease, the allocated memory of an SV and that it does not automatically
147 add a byte for the a trailing NUL (perl's own string functions typically do
148 C<SvGROW(sv, len + 1)>).
150 If you have an SV and want to know what kind of data Perl thinks is stored
151 in it, you can use the following macros to check the type of SV you have.
157 You can get and set the current length of the string stored in an SV with
158 the following macros:
161 SvCUR_set(SV*, I32 val)
163 You can also get a pointer to the end of the string stored in the SV
168 But note that these last three macros are valid only if C<SvPOK()> is true.
170 If you want to append something to the end of string stored in an C<SV*>,
171 you can use the following functions:
173 void sv_catpv(SV*, const char*);
174 void sv_catpvn(SV*, const char*, STRLEN);
175 void sv_catpvf(SV*, const char*, ...);
176 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
177 void sv_catsv(SV*, SV*);
179 The first function calculates the length of the string to be appended by
180 using C<strlen>. In the second, you specify the length of the string
181 yourself. The third function processes its arguments like C<sprintf> and
182 appends the formatted output. The fourth function works like C<vsprintf>.
183 You can specify the address and length of an array of SVs instead of the
184 va_list argument. The fifth function extends the string stored in the first
185 SV with the string stored in the second SV. It also forces the second SV
186 to be interpreted as a string.
188 The C<sv_cat*()> functions are not generic enough to operate on values that
189 have "magic". See L<Magic Virtual Tables> later in this document.
191 If you know the name of a scalar variable, you can get a pointer to its SV
192 by using the following:
194 SV* get_sv("package::varname", FALSE);
196 This returns NULL if the variable does not exist.
198 If you want to know if this variable (or any other SV) is actually C<defined>,
203 The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
205 Its address can be used whenever an C<SV*> is needed. Make sure that
206 you don't try to compare a random sv with C<&PL_sv_undef>. For example
207 when interfacing Perl code, it'll work correctly for:
211 But won't work when called as:
216 So to repeat always use SvOK() to check whether an sv is defined.
218 Also you have to be careful when using C<&PL_sv_undef> as a value in
219 AVs or HVs (see L<AVs, HVs and undefined values>).
221 There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
222 boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
223 addresses can be used whenever an C<SV*> is needed.
225 Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
229 if (I-am-to-return-a-real-value) {
230 sv = sv_2mortal(newSViv(42));
234 This code tries to return a new SV (which contains the value 42) if it should
235 return a real value, or undef otherwise. Instead it has returned a NULL
236 pointer which, somewhere down the line, will cause a segmentation violation,
237 bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
238 first line and all will be well.
240 To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
241 call is not necessary (see L<Reference Counts and Mortality>).
245 Perl provides the function C<sv_chop> to efficiently remove characters
246 from the beginning of a string; you give it an SV and a pointer to
247 somewhere inside the PV, and it discards everything before the
248 pointer. The efficiency comes by means of a little hack: instead of
249 actually removing the characters, C<sv_chop> sets the flag C<OOK>
250 (offset OK) to signal to other functions that the offset hack is in
251 effect, and it puts the number of bytes chopped off into the IV field
252 of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
253 many bytes, and adjusts C<SvCUR> and C<SvLEN>.
255 Hence, at this point, the start of the buffer that we allocated lives
256 at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
257 into the middle of this allocated storage.
259 This is best demonstrated by example:
261 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
262 SV = PVIV(0x8128450) at 0x81340f0
264 FLAGS = (POK,OOK,pPOK)
266 PV = 0x8135781 ( "1" . ) "2345"\0
270 Here the number of bytes chopped off (1) is put into IV, and
271 C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
272 portion of the string between the "real" and the "fake" beginnings is
273 shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
274 the fake beginning, not the real one.
276 Something similar to the offset hack is performed on AVs to enable
277 efficient shifting and splicing off the beginning of the array; while
278 C<AvARRAY> points to the first element in the array that is visible from
279 Perl, C<AvALLOC> points to the real start of the C array. These are
280 usually the same, but a C<shift> operation can be carried out by
281 increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
282 Again, the location of the real start of the C array only comes into
283 play when freeing the array. See C<av_shift> in F<av.c>.
285 =head2 What's Really Stored in an SV?
287 Recall that the usual method of determining the type of scalar you have is
288 to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
289 usually these macros will always return TRUE and calling the C<Sv*V>
290 macros will do the appropriate conversion of string to integer/double or
291 integer/double to string.
293 If you I<really> need to know if you have an integer, double, or string
294 pointer in an SV, you can use the following three macros instead:
300 These will tell you if you truly have an integer, double, or string pointer
301 stored in your SV. The "p" stands for private.
303 The are various ways in which the private and public flags may differ.
304 For example, a tied SV may have a valid underlying value in the IV slot
305 (so SvIOKp is true), but the data should be accessed via the FETCH
306 routine rather than directly, so SvIOK is false. Another is when
307 numeric conversion has occurred and precision has been lost: only the
308 private flag is set on 'lossy' values. So when an NV is converted to an
309 IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
311 In general, though, it's best to use the C<Sv*V> macros.
313 =head2 Working with AVs
315 There are two ways to create and load an AV. The first method creates an
320 The second method both creates the AV and initially populates it with SVs:
322 AV* av_make(I32 num, SV **ptr);
324 The second argument points to an array containing C<num> C<SV*>'s. Once the
325 AV has been created, the SVs can be destroyed, if so desired.
327 Once the AV has been created, the following operations are possible on AVs:
329 void av_push(AV*, SV*);
332 void av_unshift(AV*, I32 num);
334 These should be familiar operations, with the exception of C<av_unshift>.
335 This routine adds C<num> elements at the front of the array with the C<undef>
336 value. You must then use C<av_store> (described below) to assign values
337 to these new elements.
339 Here are some other functions:
342 SV** av_fetch(AV*, I32 key, I32 lval);
343 SV** av_store(AV*, I32 key, SV* val);
345 The C<av_len> function returns the highest index value in array (just
346 like $#array in Perl). If the array is empty, -1 is returned. The
347 C<av_fetch> function returns the value at index C<key>, but if C<lval>
348 is non-zero, then C<av_fetch> will store an undef value at that index.
349 The C<av_store> function stores the value C<val> at index C<key>, and does
350 not increment the reference count of C<val>. Thus the caller is responsible
351 for taking care of that, and if C<av_store> returns NULL, the caller will
352 have to decrement the reference count to avoid a memory leak. Note that
353 C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
358 void av_extend(AV*, I32 key);
360 The C<av_clear> function deletes all the elements in the AV* array, but
361 does not actually delete the array itself. The C<av_undef> function will
362 delete all the elements in the array plus the array itself. The
363 C<av_extend> function extends the array so that it contains at least C<key+1>
364 elements. If C<key+1> is less than the currently allocated length of the array,
365 then nothing is done.
367 If you know the name of an array variable, you can get a pointer to its AV
368 by using the following:
370 AV* get_av("package::varname", FALSE);
372 This returns NULL if the variable does not exist.
374 See L<Understanding the Magic of Tied Hashes and Arrays> for more
375 information on how to use the array access functions on tied arrays.
377 =head2 Working with HVs
379 To create an HV, you use the following routine:
383 Once the HV has been created, the following operations are possible on HVs:
385 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
386 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
388 The C<klen> parameter is the length of the key being passed in (Note that
389 you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
390 length of the key). The C<val> argument contains the SV pointer to the
391 scalar being stored, and C<hash> is the precomputed hash value (zero if
392 you want C<hv_store> to calculate it for you). The C<lval> parameter
393 indicates whether this fetch is actually a part of a store operation, in
394 which case a new undefined value will be added to the HV with the supplied
395 key and C<hv_fetch> will return as if the value had already existed.
397 Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
398 C<SV*>. To access the scalar value, you must first dereference the return
399 value. However, you should check to make sure that the return value is
400 not NULL before dereferencing it.
402 These two functions check if a hash table entry exists, and deletes it.
404 bool hv_exists(HV*, const char* key, U32 klen);
405 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
407 If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
408 create and return a mortal copy of the deleted value.
410 And more miscellaneous functions:
415 Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
416 table but does not actually delete the hash table. The C<hv_undef> deletes
417 both the entries and the hash table itself.
419 Perl keeps the actual data in linked list of structures with a typedef of HE.
420 These contain the actual key and value pointers (plus extra administrative
421 overhead). The key is a string pointer; the value is an C<SV*>. However,
422 once you have an C<HE*>, to get the actual key and value, use the routines
425 I32 hv_iterinit(HV*);
426 /* Prepares starting point to traverse hash table */
427 HE* hv_iternext(HV*);
428 /* Get the next entry, and return a pointer to a
429 structure that has both the key and value */
430 char* hv_iterkey(HE* entry, I32* retlen);
431 /* Get the key from an HE structure and also return
432 the length of the key string */
433 SV* hv_iterval(HV*, HE* entry);
434 /* Return an SV pointer to the value of the HE
436 SV* hv_iternextsv(HV*, char** key, I32* retlen);
437 /* This convenience routine combines hv_iternext,
438 hv_iterkey, and hv_iterval. The key and retlen
439 arguments are return values for the key and its
440 length. The value is returned in the SV* argument */
442 If you know the name of a hash variable, you can get a pointer to its HV
443 by using the following:
445 HV* get_hv("package::varname", FALSE);
447 This returns NULL if the variable does not exist.
449 The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
453 hash = (hash * 33) + *key++;
454 hash = hash + (hash >> 5); /* after 5.6 */
456 The last step was added in version 5.6 to improve distribution of
457 lower bits in the resulting hash value.
459 See L<Understanding the Magic of Tied Hashes and Arrays> for more
460 information on how to use the hash access functions on tied hashes.
462 =head2 Hash API Extensions
464 Beginning with version 5.004, the following functions are also supported:
466 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
467 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
469 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
470 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
472 SV* hv_iterkeysv (HE* entry);
474 Note that these functions take C<SV*> keys, which simplifies writing
475 of extension code that deals with hash structures. These functions
476 also allow passing of C<SV*> keys to C<tie> functions without forcing
477 you to stringify the keys (unlike the previous set of functions).
479 They also return and accept whole hash entries (C<HE*>), making their
480 use more efficient (since the hash number for a particular string
481 doesn't have to be recomputed every time). See L<perlapi> for detailed
484 The following macros must always be used to access the contents of hash
485 entries. Note that the arguments to these macros must be simple
486 variables, since they may get evaluated more than once. See
487 L<perlapi> for detailed descriptions of these macros.
489 HePV(HE* he, STRLEN len)
493 HeSVKEY_force(HE* he)
494 HeSVKEY_set(HE* he, SV* sv)
496 These two lower level macros are defined, but must only be used when
497 dealing with keys that are not C<SV*>s:
502 Note that both C<hv_store> and C<hv_store_ent> do not increment the
503 reference count of the stored C<val>, which is the caller's responsibility.
504 If these functions return a NULL value, the caller will usually have to
505 decrement the reference count of C<val> to avoid a memory leak.
507 =head2 AVs, HVs and undefined values
509 Sometimes you have to store undefined values in AVs or HVs. Although
510 this may be a rare case, it can be tricky. That's because you're
511 used to using C<&PL_sv_undef> if you need an undefined SV.
513 For example, intuition tells you that this XS code:
516 av_store( av, 0, &PL_sv_undef );
518 is equivalent to this Perl code:
523 Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
524 for indicating that an array element has not yet been initialized.
525 Thus, C<exists $av[0]> would be true for the above Perl code, but
526 false for the array generated by the XS code.
528 Other problems can occur when storing C<&PL_sv_undef> in HVs:
530 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
532 This will indeed make the value C<undef>, but if you try to modify
533 the value of C<key>, you'll get the following error:
535 Modification of non-creatable hash value attempted
537 In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
538 in restricted hashes. This caused such hash entries not to appear
539 when iterating over the hash or when checking for the keys
540 with the C<hv_exists> function.
542 You can run into similar problems when you store C<&PL_sv_true> or
543 C<&PL_sv_false> into AVs or HVs. Trying to modify such elements
544 will give you the following error:
546 Modification of a read-only value attempted
548 To make a long story short, you can use the special variables
549 C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and
550 HVs, but you have to make sure you know what you're doing.
552 Generally, if you want to store an undefined value in an AV
553 or HV, you should not use C<&PL_sv_undef>, but rather create a
554 new undefined value using the C<newSV> function, for example:
556 av_store( av, 42, newSV(0) );
557 hv_store( hv, "foo", 3, newSV(0), 0 );
561 References are a special type of scalar that point to other data types
562 (including references).
564 To create a reference, use either of the following functions:
566 SV* newRV_inc((SV*) thing);
567 SV* newRV_noinc((SV*) thing);
569 The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
570 functions are identical except that C<newRV_inc> increments the reference
571 count of the C<thing>, while C<newRV_noinc> does not. For historical
572 reasons, C<newRV> is a synonym for C<newRV_inc>.
574 Once you have a reference, you can use the following macro to dereference
579 then call the appropriate routines, casting the returned C<SV*> to either an
580 C<AV*> or C<HV*>, if required.
582 To determine if an SV is a reference, you can use the following macro:
586 To discover what type of value the reference refers to, use the following
587 macro and then check the return value.
591 The most useful types that will be returned are:
600 SVt_PVGV Glob (possible a file handle)
601 SVt_PVMG Blessed or Magical Scalar
603 See the sv.h header file for more details.
605 =head2 Blessed References and Class Objects
607 References are also used to support object-oriented programming. In perl's
608 OO lexicon, an object is simply a reference that has been blessed into a
609 package (or class). Once blessed, the programmer may now use the reference
610 to access the various methods in the class.
612 A reference can be blessed into a package with the following function:
614 SV* sv_bless(SV* sv, HV* stash);
616 The C<sv> argument must be a reference value. The C<stash> argument
617 specifies which class the reference will belong to. See
618 L<Stashes and Globs> for information on converting class names into stashes.
620 /* Still under construction */
622 Upgrades rv to reference if not already one. Creates new SV for rv to
623 point to. If C<classname> is non-null, the SV is blessed into the specified
624 class. SV is returned.
626 SV* newSVrv(SV* rv, const char* classname);
628 Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
629 if C<classname> is non-null.
631 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
632 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
633 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
635 Copies the pointer value (I<the address, not the string!>) into an SV whose
636 reference is rv. SV is blessed if C<classname> is non-null.
638 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
640 Copies string into an SV whose reference is C<rv>. Set length to 0 to let
641 Perl calculate the string length. SV is blessed if C<classname> is non-null.
643 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
645 Tests whether the SV is blessed into the specified class. It does not
646 check inheritance relationships.
648 int sv_isa(SV* sv, const char* name);
650 Tests whether the SV is a reference to a blessed object.
652 int sv_isobject(SV* sv);
654 Tests whether the SV is derived from the specified class. SV can be either
655 a reference to a blessed object or a string containing a class name. This
656 is the function implementing the C<UNIVERSAL::isa> functionality.
658 bool sv_derived_from(SV* sv, const char* name);
660 To check if you've got an object derived from a specific class you have
663 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
665 =head2 Creating New Variables
667 To create a new Perl variable with an undef value which can be accessed from
668 your Perl script, use the following routines, depending on the variable type.
670 SV* get_sv("package::varname", TRUE);
671 AV* get_av("package::varname", TRUE);
672 HV* get_hv("package::varname", TRUE);
674 Notice the use of TRUE as the second parameter. The new variable can now
675 be set, using the routines appropriate to the data type.
677 There are additional macros whose values may be bitwise OR'ed with the
678 C<TRUE> argument to enable certain extra features. Those bits are:
684 Marks the variable as multiply defined, thus preventing the:
686 Name <varname> used only once: possible typo
694 Had to create <varname> unexpectedly
696 if the variable did not exist before the function was called.
700 If you do not specify a package name, the variable is created in the current
703 =head2 Reference Counts and Mortality
705 Perl uses a reference count-driven garbage collection mechanism. SVs,
706 AVs, or HVs (xV for short in the following) start their life with a
707 reference count of 1. If the reference count of an xV ever drops to 0,
708 then it will be destroyed and its memory made available for reuse.
710 This normally doesn't happen at the Perl level unless a variable is
711 undef'ed or the last variable holding a reference to it is changed or
712 overwritten. At the internal level, however, reference counts can be
713 manipulated with the following macros:
715 int SvREFCNT(SV* sv);
716 SV* SvREFCNT_inc(SV* sv);
717 void SvREFCNT_dec(SV* sv);
719 However, there is one other function which manipulates the reference
720 count of its argument. The C<newRV_inc> function, you will recall,
721 creates a reference to the specified argument. As a side effect,
722 it increments the argument's reference count. If this is not what
723 you want, use C<newRV_noinc> instead.
725 For example, imagine you want to return a reference from an XSUB function.
726 Inside the XSUB routine, you create an SV which initially has a reference
727 count of one. Then you call C<newRV_inc>, passing it the just-created SV.
728 This returns the reference as a new SV, but the reference count of the
729 SV you passed to C<newRV_inc> has been incremented to two. Now you
730 return the reference from the XSUB routine and forget about the SV.
731 But Perl hasn't! Whenever the returned reference is destroyed, the
732 reference count of the original SV is decreased to one and nothing happens.
733 The SV will hang around without any way to access it until Perl itself
734 terminates. This is a memory leak.
736 The correct procedure, then, is to use C<newRV_noinc> instead of
737 C<newRV_inc>. Then, if and when the last reference is destroyed,
738 the reference count of the SV will go to zero and it will be destroyed,
739 stopping any memory leak.
741 There are some convenience functions available that can help with the
742 destruction of xVs. These functions introduce the concept of "mortality".
743 An xV that is mortal has had its reference count marked to be decremented,
744 but not actually decremented, until "a short time later". Generally the
745 term "short time later" means a single Perl statement, such as a call to
746 an XSUB function. The actual determinant for when mortal xVs have their
747 reference count decremented depends on two macros, SAVETMPS and FREETMPS.
748 See L<perlcall> and L<perlxs> for more details on these macros.
750 "Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
751 However, if you mortalize a variable twice, the reference count will
752 later be decremented twice.
754 "Mortal" SVs are mainly used for SVs that are placed on perl's stack.
755 For example an SV which is created just to pass a number to a called sub
756 is made mortal to have it cleaned up automatically when it's popped off
757 the stack. Similarly, results returned by XSUBs (which are pushed on the
758 stack) are often made mortal.
760 To create a mortal variable, use the functions:
764 SV* sv_mortalcopy(SV*)
766 The first call creates a mortal SV (with no value), the second converts an existing
767 SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
768 third creates a mortal copy of an existing SV.
769 Because C<sv_newmortal> gives the new SV no value,it must normally be given one
770 via C<sv_setpv>, C<sv_setiv>, etc. :
772 SV *tmp = sv_newmortal();
773 sv_setiv(tmp, an_integer);
775 As that is multiple C statements it is quite common so see this idiom instead:
777 SV *tmp = sv_2mortal(newSViv(an_integer));
780 You should be careful about creating mortal variables. Strange things
781 can happen if you make the same value mortal within multiple contexts,
782 or if you make a variable mortal multiple times. Thinking of "Mortalization"
783 as deferred C<SvREFCNT_dec> should help to minimize such problems.
784 For example if you are passing an SV which you I<know> has high enough REFCNT
785 to survive its use on the stack you need not do any mortalization.
786 If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
787 making a C<sv_mortalcopy> is safer.
789 The mortal routines are not just for SVs -- AVs and HVs can be
790 made mortal by passing their address (type-casted to C<SV*>) to the
791 C<sv_2mortal> or C<sv_mortalcopy> routines.
793 =head2 Stashes and Globs
795 A B<stash> is a hash that contains all variables that are defined
796 within a package. Each key of the stash is a symbol
797 name (shared by all the different types of objects that have the same
798 name), and each value in the hash table is a GV (Glob Value). This GV
799 in turn contains references to the various objects of that name,
800 including (but not limited to) the following:
809 There is a single stash called C<PL_defstash> that holds the items that exist
810 in the C<main> package. To get at the items in other packages, append the
811 string "::" to the package name. The items in the C<Foo> package are in
812 the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
813 in the stash C<Baz::> in C<Bar::>'s stash.
815 To get the stash pointer for a particular package, use the function:
817 HV* gv_stashpv(const char* name, I32 create)
818 HV* gv_stashsv(SV*, I32 create)
820 The first function takes a literal string, the second uses the string stored
821 in the SV. Remember that a stash is just a hash table, so you get back an
822 C<HV*>. The C<create> flag will create a new package if it is set.
824 The name that C<gv_stash*v> wants is the name of the package whose symbol table
825 you want. The default package is called C<main>. If you have multiply nested
826 packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
829 Alternately, if you have an SV that is a blessed reference, you can find
830 out the stash pointer by using:
832 HV* SvSTASH(SvRV(SV*));
834 then use the following to get the package name itself:
836 char* HvNAME(HV* stash);
838 If you need to bless or re-bless an object you can use the following
841 SV* sv_bless(SV*, HV* stash)
843 where the first argument, an C<SV*>, must be a reference, and the second
844 argument is a stash. The returned C<SV*> can now be used in the same way
847 For more information on references and blessings, consult L<perlref>.
849 =head2 Double-Typed SVs
851 Scalar variables normally contain only one type of value, an integer,
852 double, pointer, or reference. Perl will automatically convert the
853 actual scalar data from the stored type into the requested type.
855 Some scalar variables contain more than one type of scalar data. For
856 example, the variable C<$!> contains either the numeric value of C<errno>
857 or its string equivalent from either C<strerror> or C<sys_errlist[]>.
859 To force multiple data values into an SV, you must do two things: use the
860 C<sv_set*v> routines to add the additional scalar type, then set a flag
861 so that Perl will believe it contains more than one type of data. The
862 four macros to set the flags are:
869 The particular macro you must use depends on which C<sv_set*v> routine
870 you called first. This is because every C<sv_set*v> routine turns on
871 only the bit for the particular type of data being set, and turns off
874 For example, to create a new Perl variable called "dberror" that contains
875 both the numeric and descriptive string error values, you could use the
879 extern char *dberror_list;
881 SV* sv = get_sv("dberror", TRUE);
882 sv_setiv(sv, (IV) dberror);
883 sv_setpv(sv, dberror_list[dberror]);
886 If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
887 macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
889 =head2 Magic Variables
891 [This section still under construction. Ignore everything here. Post no
892 bills. Everything not permitted is forbidden.]
894 Any SV may be magical, that is, it has special features that a normal
895 SV does not have. These features are stored in the SV structure in a
896 linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
909 Note this is current as of patchlevel 0, and could change at any time.
911 =head2 Assigning Magic
913 Perl adds magic to an SV using the sv_magic function:
915 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
917 The C<sv> argument is a pointer to the SV that is to acquire a new magical
920 If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
921 convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
922 to the beginning of the linked list of magical features. Any prior entry
923 of the same type of magic is deleted. Note that this can be overridden,
924 and multiple instances of the same type of magic can be associated with an
927 The C<name> and C<namlen> arguments are used to associate a string with
928 the magic, typically the name of a variable. C<namlen> is stored in the
929 C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
930 C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
931 whether C<namlen> is greater than zero or equal to zero respectively. As a
932 special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
933 to contain an C<SV*> and is stored as-is with its REFCNT incremented.
935 The sv_magic function uses C<how> to determine which, if any, predefined
936 "Magic Virtual Table" should be assigned to the C<mg_virtual> field.
937 See the L<Magic Virtual Tables> section below. The C<how> argument is also
938 stored in the C<mg_type> field. The value of C<how> should be chosen
939 from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
940 these macros were added, Perl internals used to directly use character
941 literals, so you may occasionally come across old code or documentation
942 referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
944 The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
945 structure. If it is not the same as the C<sv> argument, the reference
946 count of the C<obj> object is incremented. If it is the same, or if
947 the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
948 then C<obj> is merely stored, without the reference count being incremented.
950 See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
953 There is also a function to add magic to an C<HV>:
955 void hv_magic(HV *hv, GV *gv, int how);
957 This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
959 To remove the magic from an SV, call the function sv_unmagic:
961 void sv_unmagic(SV *sv, int type);
963 The C<type> argument should be equal to the C<how> value when the C<SV>
964 was initially made magical.
966 =head2 Magic Virtual Tables
968 The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
969 C<MGVTBL>, which is a structure of function pointers and stands for
970 "Magic Virtual Table" to handle the various operations that might be
971 applied to that variable.
973 The C<MGVTBL> has five (or sometimes eight) pointers to the following
976 int (*svt_get)(SV* sv, MAGIC* mg);
977 int (*svt_set)(SV* sv, MAGIC* mg);
978 U32 (*svt_len)(SV* sv, MAGIC* mg);
979 int (*svt_clear)(SV* sv, MAGIC* mg);
980 int (*svt_free)(SV* sv, MAGIC* mg);
982 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen);
983 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
984 int (*svt_local)(SV *nsv, MAGIC *mg);
987 This MGVTBL structure is set at compile-time in F<perl.h> and there are
988 currently 19 types (or 21 with overloading turned on). These different
989 structures contain pointers to various routines that perform additional
990 actions depending on which function is being called.
992 Function pointer Action taken
993 ---------------- ------------
994 svt_get Do something before the value of the SV is retrieved.
995 svt_set Do something after the SV is assigned a value.
996 svt_len Report on the SV's length.
997 svt_clear Clear something the SV represents.
998 svt_free Free any extra storage associated with the SV.
1000 svt_copy copy tied variable magic to a tied element
1001 svt_dup duplicate a magic structure during thread cloning
1002 svt_local copy magic to local value during 'local'
1004 For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
1005 to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
1007 { magic_get, magic_set, magic_len, 0, 0 }
1009 Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1010 if a get operation is being performed, the routine C<magic_get> is
1011 called. All the various routines for the various magical types begin
1012 with C<magic_>. NOTE: the magic routines are not considered part of
1013 the Perl API, and may not be exported by the Perl library.
1015 The last three slots are a recent addition, and for source code
1016 compatibility they are only checked for if one of the three flags
1017 MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
1018 code can continue declaring a vtable as a 5-element value. These three are
1019 currently used exclusively by the threading code, and are highly subject
1022 The current kinds of Magic Virtual Tables are:
1025 (old-style char and macro) MGVTBL Type of magic
1026 -------------------------- ------ ----------------------------
1027 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1028 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
1029 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
1030 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
1032 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
1033 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1035 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1037 E PERL_MAGIC_env vtbl_env %ENV hash
1038 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1039 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
1040 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
1041 H PERL_MAGIC_hints vtbl_sig %^H hash
1042 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1043 I PERL_MAGIC_isa vtbl_isa @ISA array
1044 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1045 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1046 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1047 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
1048 m PERL_MAGIC_mutex vtbl_mutex ???
1049 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
1050 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1051 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1052 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1053 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
1054 S PERL_MAGIC_sig vtbl_sig %SIG hash
1055 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1056 t PERL_MAGIC_taint vtbl_taint Taintedness
1057 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
1058 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1059 V PERL_MAGIC_vstring (none) v-string scalars
1060 w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
1061 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1062 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1063 variable / smart parameter
1065 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1066 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1067 < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref
1068 ~ PERL_MAGIC_ext (none) Available for use by extensions
1069 : PERL_MAGIC_symtab (none) hash used as symbol table
1070 % PERL_MAGIC_rhash (none) hash used as restricted hash
1071 @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a
1074 When an uppercase and lowercase letter both exist in the table, then the
1075 uppercase letter is typically used to represent some kind of composite type
1076 (a list or a hash), and the lowercase letter is used to represent an element
1077 of that composite type. Some internals code makes use of this case
1078 relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
1080 The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1081 specifically for use by extensions and will not be used by perl itself.
1082 Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1083 to variables (typically objects). This is especially useful because
1084 there is no way for normal perl code to corrupt this private information
1085 (unlike using extra elements of a hash object).
1087 Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1088 C function any time a scalar's value is used or changed. The C<MAGIC>'s
1089 C<mg_ptr> field points to a C<ufuncs> structure:
1092 I32 (*uf_val)(pTHX_ IV, SV*);
1093 I32 (*uf_set)(pTHX_ IV, SV*);
1097 When the SV is read from or written to, the C<uf_val> or C<uf_set>
1098 function will be called with C<uf_index> as the first arg and a pointer to
1099 the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1100 magic is shown below. Note that the ufuncs structure is copied by
1101 sv_magic, so you can safely allocate it on the stack.
1109 uf.uf_val = &my_get_fn;
1110 uf.uf_set = &my_set_fn;
1112 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1114 Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1115 or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1116 extra care to avoid conflict. Typically only using the magic on
1117 objects blessed into the same class as the extension is sufficient.
1118 For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1119 'signature' at the top of the private data area and check that.
1121 Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1122 earlier do B<not> invoke 'set' magic on their targets. This must
1123 be done by the user either by calling the C<SvSETMAGIC()> macro after
1124 calling these functions, or by using one of the C<sv_set*_mg()> or
1125 C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1126 C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1127 obtained from external sources in functions that don't handle magic.
1128 See L<perlapi> for a description of these functions.
1129 For example, calls to the C<sv_cat*()> functions typically need to be
1130 followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1131 since their implementation handles 'get' magic.
1133 =head2 Finding Magic
1135 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1137 This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1138 If the SV does not have that magical feature, C<NULL> is returned. Also,
1139 if the SV is not of type SVt_PVMG, Perl may core dump.
1141 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1143 This routine checks to see what types of magic C<sv> has. If the mg_type
1144 field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1145 the mg_type field is changed to be the lowercase letter.
1147 =head2 Understanding the Magic of Tied Hashes and Arrays
1149 Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1152 WARNING: As of the 5.004 release, proper usage of the array and hash
1153 access functions requires understanding a few caveats. Some
1154 of these caveats are actually considered bugs in the API, to be fixed
1155 in later releases, and are bracketed with [MAYCHANGE] below. If
1156 you find yourself actually applying such information in this section, be
1157 aware that the behavior may change in the future, umm, without warning.
1159 The perl tie function associates a variable with an object that implements
1160 the various GET, SET, etc methods. To perform the equivalent of the perl
1161 tie function from an XSUB, you must mimic this behaviour. The code below
1162 carries out the necessary steps - firstly it creates a new hash, and then
1163 creates a second hash which it blesses into the class which will implement
1164 the tie methods. Lastly it ties the two hashes together, and returns a
1165 reference to the new tied hash. Note that the code below does NOT call the
1166 TIEHASH method in the MyTie class -
1167 see L<Calling Perl Routines from within C Programs> for details on how
1178 tie = newRV_noinc((SV*)newHV());
1179 stash = gv_stashpv("MyTie", TRUE);
1180 sv_bless(tie, stash);
1181 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1182 RETVAL = newRV_noinc(hash);
1186 The C<av_store> function, when given a tied array argument, merely
1187 copies the magic of the array onto the value to be "stored", using
1188 C<mg_copy>. It may also return NULL, indicating that the value did not
1189 actually need to be stored in the array. [MAYCHANGE] After a call to
1190 C<av_store> on a tied array, the caller will usually need to call
1191 C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1192 TIEARRAY object. If C<av_store> did return NULL, a call to
1193 C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1196 The previous paragraph is applicable verbatim to tied hash access using the
1197 C<hv_store> and C<hv_store_ent> functions as well.
1199 C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1200 C<hv_fetch_ent> actually return an undefined mortal value whose magic
1201 has been initialized using C<mg_copy>. Note the value so returned does not
1202 need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1203 need to call C<mg_get()> on the returned value in order to actually invoke
1204 the perl level "FETCH" method on the underlying TIE object. Similarly,
1205 you may also call C<mg_set()> on the return value after possibly assigning
1206 a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
1207 method on the TIE object. [/MAYCHANGE]
1210 In other words, the array or hash fetch/store functions don't really
1211 fetch and store actual values in the case of tied arrays and hashes. They
1212 merely call C<mg_copy> to attach magic to the values that were meant to be
1213 "stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1214 do the job of invoking the TIE methods on the underlying objects. Thus
1215 the magic mechanism currently implements a kind of lazy access to arrays
1218 Currently (as of perl version 5.004), use of the hash and array access
1219 functions requires the user to be aware of whether they are operating on
1220 "normal" hashes and arrays, or on their tied variants. The API may be
1221 changed to provide more transparent access to both tied and normal data
1222 types in future versions.
1225 You would do well to understand that the TIEARRAY and TIEHASH interfaces
1226 are mere sugar to invoke some perl method calls while using the uniform hash
1227 and array syntax. The use of this sugar imposes some overhead (typically
1228 about two to four extra opcodes per FETCH/STORE operation, in addition to
1229 the creation of all the mortal variables required to invoke the methods).
1230 This overhead will be comparatively small if the TIE methods are themselves
1231 substantial, but if they are only a few statements long, the overhead
1232 will not be insignificant.
1234 =head2 Localizing changes
1236 Perl has a very handy construction
1243 This construction is I<approximately> equivalent to
1252 The biggest difference is that the first construction would
1253 reinstate the initial value of $var, irrespective of how control exits
1254 the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
1255 more efficient as well.
1257 There is a way to achieve a similar task from C via Perl API: create a
1258 I<pseudo-block>, and arrange for some changes to be automatically
1259 undone at the end of it, either explicit, or via a non-local exit (via
1260 die()). A I<block>-like construct is created by a pair of
1261 C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1262 Such a construct may be created specially for some important localized
1263 task, or an existing one (like boundaries of enclosing Perl
1264 subroutine/block, or an existing pair for freeing TMPs) may be
1265 used. (In the second case the overhead of additional localization must
1266 be almost negligible.) Note that any XSUB is automatically enclosed in
1267 an C<ENTER>/C<LEAVE> pair.
1269 Inside such a I<pseudo-block> the following service is available:
1273 =item C<SAVEINT(int i)>
1275 =item C<SAVEIV(IV i)>
1277 =item C<SAVEI32(I32 i)>
1279 =item C<SAVELONG(long i)>
1281 These macros arrange things to restore the value of integer variable
1282 C<i> at the end of enclosing I<pseudo-block>.
1284 =item C<SAVESPTR(s)>
1286 =item C<SAVEPPTR(p)>
1288 These macros arrange things to restore the value of pointers C<s> and
1289 C<p>. C<s> must be a pointer of a type which survives conversion to
1290 C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1293 =item C<SAVEFREESV(SV *sv)>
1295 The refcount of C<sv> would be decremented at the end of
1296 I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1297 mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1298 extends the lifetime of C<sv> until the beginning of the next statement,
1299 C<SAVEFREESV> extends it until the end of the enclosing scope. These
1300 lifetimes can be wildly different.
1302 Also compare C<SAVEMORTALIZESV>.
1304 =item C<SAVEMORTALIZESV(SV *sv)>
1306 Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1307 scope instead of decrementing its reference count. This usually has the
1308 effect of keeping C<sv> alive until the statement that called the currently
1309 live scope has finished executing.
1311 =item C<SAVEFREEOP(OP *op)>
1313 The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1315 =item C<SAVEFREEPV(p)>
1317 The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1318 end of I<pseudo-block>.
1320 =item C<SAVECLEARSV(SV *sv)>
1322 Clears a slot in the current scratchpad which corresponds to C<sv> at
1323 the end of I<pseudo-block>.
1325 =item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1327 The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1328 string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1329 short-lived storage, the corresponding string may be reallocated like
1332 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1334 =item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
1336 At the end of I<pseudo-block> the function C<f> is called with the
1339 =item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1341 At the end of I<pseudo-block> the function C<f> is called with the
1342 implicit context argument (if any), and C<p>.
1344 =item C<SAVESTACK_POS()>
1346 The current offset on the Perl internal stack (cf. C<SP>) is restored
1347 at the end of I<pseudo-block>.
1351 The following API list contains functions, thus one needs to
1352 provide pointers to the modifiable data explicitly (either C pointers,
1353 or Perlish C<GV *>s). Where the above macros take C<int>, a similar
1354 function takes C<int *>.
1358 =item C<SV* save_scalar(GV *gv)>
1360 Equivalent to Perl code C<local $gv>.
1362 =item C<AV* save_ary(GV *gv)>
1364 =item C<HV* save_hash(GV *gv)>
1366 Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1368 =item C<void save_item(SV *item)>
1370 Duplicates the current value of C<SV>, on the exit from the current
1371 C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1372 using the stored value. It doesn't handle magic. Use C<save_scalar> if
1375 =item C<void save_list(SV **sarg, I32 maxsarg)>
1377 A variant of C<save_item> which takes multiple arguments via an array
1378 C<sarg> of C<SV*> of length C<maxsarg>.
1380 =item C<SV* save_svref(SV **sptr)>
1382 Similar to C<save_scalar>, but will reinstate an C<SV *>.
1384 =item C<void save_aptr(AV **aptr)>
1386 =item C<void save_hptr(HV **hptr)>
1388 Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1392 The C<Alias> module implements localization of the basic types within the
1393 I<caller's scope>. People who are interested in how to localize things in
1394 the containing scope should take a look there too.
1398 =head2 XSUBs and the Argument Stack
1400 The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1401 An XSUB routine will have a stack that contains the arguments from the Perl
1402 program, and a way to map from the Perl data structures to a C equivalent.
1404 The stack arguments are accessible through the C<ST(n)> macro, which returns
1405 the C<n>'th stack argument. Argument 0 is the first argument passed in the
1406 Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1409 Most of the time, output from the C routine can be handled through use of
1410 the RETVAL and OUTPUT directives. However, there are some cases where the
1411 argument stack is not already long enough to handle all the return values.
1412 An example is the POSIX tzname() call, which takes no arguments, but returns
1413 two, the local time zone's standard and summer time abbreviations.
1415 To handle this situation, the PPCODE directive is used and the stack is
1416 extended using the macro:
1420 where C<SP> is the macro that represents the local copy of the stack pointer,
1421 and C<num> is the number of elements the stack should be extended by.
1423 Now that there is room on the stack, values can be pushed on it using C<PUSHs>
1424 macro. The pushed values will often need to be "mortal" (See
1425 L</Reference Counts and Mortality>):
1427 PUSHs(sv_2mortal(newSViv(an_integer)))
1428 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1429 PUSHs(sv_2mortal(newSVnv(a_double)))
1430 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1432 And now the Perl program calling C<tzname>, the two values will be assigned
1435 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1437 An alternate (and possibly simpler) method to pushing values on the stack is
1442 This macro automatically adjust the stack for you, if needed. Thus, you
1443 do not need to call C<EXTEND> to extend the stack.
1445 Despite their suggestions in earlier versions of this document the macros
1446 C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1447 For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1448 C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
1450 For more information, consult L<perlxs> and L<perlxstut>.
1452 =head2 Calling Perl Routines from within C Programs
1454 There are four routines that can be used to call a Perl subroutine from
1455 within a C program. These four are:
1457 I32 call_sv(SV*, I32);
1458 I32 call_pv(const char*, I32);
1459 I32 call_method(const char*, I32);
1460 I32 call_argv(const char*, I32, register char**);
1462 The routine most often used is C<call_sv>. The C<SV*> argument
1463 contains either the name of the Perl subroutine to be called, or a
1464 reference to the subroutine. The second argument consists of flags
1465 that control the context in which the subroutine is called, whether
1466 or not the subroutine is being passed arguments, how errors should be
1467 trapped, and how to treat return values.
1469 All four routines return the number of arguments that the subroutine returned
1472 These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
1473 but those names are now deprecated; macros of the same name are provided for
1476 When using any of these routines (except C<call_argv>), the programmer
1477 must manipulate the Perl stack. These include the following macros and
1492 For a detailed description of calling conventions from C to Perl,
1493 consult L<perlcall>.
1495 =head2 Memory Allocation
1499 All memory meant to be used with the Perl API functions should be manipulated
1500 using the macros described in this section. The macros provide the necessary
1501 transparency between differences in the actual malloc implementation that is
1504 It is suggested that you enable the version of malloc that is distributed
1505 with Perl. It keeps pools of various sizes of unallocated memory in
1506 order to satisfy allocation requests more quickly. However, on some
1507 platforms, it may cause spurious malloc or free errors.
1509 The following three macros are used to initially allocate memory :
1511 Newx(pointer, number, type);
1512 Newxc(pointer, number, type, cast);
1513 Newxz(pointer, number, type);
1515 The first argument C<pointer> should be the name of a variable that will
1516 point to the newly allocated memory.
1518 The second and third arguments C<number> and C<type> specify how many of
1519 the specified type of data structure should be allocated. The argument
1520 C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1521 should be used if the C<pointer> argument is different from the C<type>
1524 Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1525 to zero out all the newly allocated memory.
1529 Renew(pointer, number, type);
1530 Renewc(pointer, number, type, cast);
1533 These three macros are used to change a memory buffer size or to free a
1534 piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1535 match those of C<New> and C<Newc> with the exception of not needing the
1536 "magic cookie" argument.
1540 Move(source, dest, number, type);
1541 Copy(source, dest, number, type);
1542 Zero(dest, number, type);
1544 These three macros are used to move, copy, or zero out previously allocated
1545 memory. The C<source> and C<dest> arguments point to the source and
1546 destination starting points. Perl will move, copy, or zero out C<number>
1547 instances of the size of the C<type> data structure (using the C<sizeof>
1552 The most recent development releases of Perl has been experimenting with
1553 removing Perl's dependency on the "normal" standard I/O suite and allowing
1554 other stdio implementations to be used. This involves creating a new
1555 abstraction layer that then calls whichever implementation of stdio Perl
1556 was compiled with. All XSUBs should now use the functions in the PerlIO
1557 abstraction layer and not make any assumptions about what kind of stdio
1560 For a complete description of the PerlIO abstraction, consult L<perlapio>.
1562 =head2 Putting a C value on Perl stack
1564 A lot of opcodes (this is an elementary operation in the internal perl
1565 stack machine) put an SV* on the stack. However, as an optimization
1566 the corresponding SV is (usually) not recreated each time. The opcodes
1567 reuse specially assigned SVs (I<target>s) which are (as a corollary)
1568 not constantly freed/created.
1570 Each of the targets is created only once (but see
1571 L<Scratchpads and recursion> below), and when an opcode needs to put
1572 an integer, a double, or a string on stack, it just sets the
1573 corresponding parts of its I<target> and puts the I<target> on stack.
1575 The macro to put this target on stack is C<PUSHTARG>, and it is
1576 directly used in some opcodes, as well as indirectly in zillions of
1577 others, which use it via C<(X)PUSH[iunp]>.
1579 Because the target is reused, you must be careful when pushing multiple
1580 values on the stack. The following code will not do what you think:
1585 This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1586 the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1587 At the end of the operation, the stack does not contain the values 10
1588 and 20, but actually contains two pointers to C<TARG>, which we have set
1591 If you need to push multiple different values then you should either use
1592 the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1593 none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1594 SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1595 will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1596 this a little easier to achieve by creating a new mortal for you (via
1597 C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1598 in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1599 Thus, instead of writing this to "fix" the example above:
1601 XPUSHs(sv_2mortal(newSViv(10)))
1602 XPUSHs(sv_2mortal(newSViv(20)))
1604 you can simply write:
1609 On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1610 need a C<dTARG> in your variable declarations so that the C<*PUSH*>
1611 macros can make use of the local variable C<TARG>. See also C<dTARGET>
1616 The question remains on when the SVs which are I<target>s for opcodes
1617 are created. The answer is that they are created when the current unit --
1618 a subroutine or a file (for opcodes for statements outside of
1619 subroutines) -- is compiled. During this time a special anonymous Perl
1620 array is created, which is called a scratchpad for the current
1623 A scratchpad keeps SVs which are lexicals for the current unit and are
1624 targets for opcodes. One can deduce that an SV lives on a scratchpad
1625 by looking on its flags: lexicals have C<SVs_PADMY> set, and
1626 I<target>s have C<SVs_PADTMP> set.
1628 The correspondence between OPs and I<target>s is not 1-to-1. Different
1629 OPs in the compile tree of the unit can use the same target, if this
1630 would not conflict with the expected life of the temporary.
1632 =head2 Scratchpads and recursion
1634 In fact it is not 100% true that a compiled unit contains a pointer to
1635 the scratchpad AV. In fact it contains a pointer to an AV of
1636 (initially) one element, and this element is the scratchpad AV. Why do
1637 we need an extra level of indirection?
1639 The answer is B<recursion>, and maybe B<threads>. Both
1640 these can create several execution pointers going into the same
1641 subroutine. For the subroutine-child not write over the temporaries
1642 for the subroutine-parent (lifespan of which covers the call to the
1643 child), the parent and the child should have different
1644 scratchpads. (I<And> the lexicals should be separate anyway!)
1646 So each subroutine is born with an array of scratchpads (of length 1).
1647 On each entry to the subroutine it is checked that the current
1648 depth of the recursion is not more than the length of this array, and
1649 if it is, new scratchpad is created and pushed into the array.
1651 The I<target>s on this scratchpad are C<undef>s, but they are already
1652 marked with correct flags.
1654 =head1 Compiled code
1658 Here we describe the internal form your code is converted to by
1659 Perl. Start with a simple example:
1663 This is converted to a tree similar to this one:
1671 (but slightly more complicated). This tree reflects the way Perl
1672 parsed your code, but has nothing to do with the execution order.
1673 There is an additional "thread" going through the nodes of the tree
1674 which shows the order of execution of the nodes. In our simplified
1675 example above it looks like:
1677 $b ---> $c ---> + ---> $a ---> assign-to
1679 But with the actual compile tree for C<$a = $b + $c> it is different:
1680 some nodes I<optimized away>. As a corollary, though the actual tree
1681 contains more nodes than our simplified example, the execution order
1682 is the same as in our example.
1684 =head2 Examining the tree
1686 If you have your perl compiled for debugging (usually done with
1687 C<-DDEBUGGING> on the C<Configure> command line), you may examine the
1688 compiled tree by specifying C<-Dx> on the Perl command line. The
1689 output takes several lines per node, and for C<$b+$c> it looks like
1694 FLAGS = (SCALAR,KIDS)
1696 TYPE = null ===> (4)
1698 FLAGS = (SCALAR,KIDS)
1700 3 TYPE = gvsv ===> 4
1706 TYPE = null ===> (5)
1708 FLAGS = (SCALAR,KIDS)
1710 4 TYPE = gvsv ===> 5
1716 This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1717 not optimized away (one per number in the left column). The immediate
1718 children of the given node correspond to C<{}> pairs on the same level
1719 of indentation, thus this listing corresponds to the tree:
1727 The execution order is indicated by C<===E<gt>> marks, thus it is C<3
1728 4 5 6> (node C<6> is not included into above listing), i.e.,
1729 C<gvsv gvsv add whatever>.
1731 Each of these nodes represents an op, a fundamental operation inside the
1732 Perl core. The code which implements each operation can be found in the
1733 F<pp*.c> files; the function which implements the op with type C<gvsv>
1734 is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1735 different numbers of children: C<add> is a binary operator, as one would
1736 expect, and so has two children. To accommodate the various different
1737 numbers of children, there are various types of op data structure, and
1738 they link together in different ways.
1740 The simplest type of op structure is C<OP>: this has no children. Unary
1741 operators, C<UNOP>s, have one child, and this is pointed to by the
1742 C<op_first> field. Binary operators (C<BINOP>s) have not only an
1743 C<op_first> field but also an C<op_last> field. The most complex type of
1744 op is a C<LISTOP>, which has any number of children. In this case, the
1745 first child is pointed to by C<op_first> and the last child by
1746 C<op_last>. The children in between can be found by iteratively
1747 following the C<op_sibling> pointer from the first child to the last.
1749 There are also two other op types: a C<PMOP> holds a regular expression,
1750 and has no children, and a C<LOOP> may or may not have children. If the
1751 C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1752 complicate matters, if a C<UNOP> is actually a C<null> op after
1753 optimization (see L</Compile pass 2: context propagation>) it will still
1754 have children in accordance with its former type.
1756 Another way to examine the tree is to use a compiler back-end module, such
1759 =head2 Compile pass 1: check routines
1761 The tree is created by the compiler while I<yacc> code feeds it
1762 the constructions it recognizes. Since I<yacc> works bottom-up, so does
1763 the first pass of perl compilation.
1765 What makes this pass interesting for perl developers is that some
1766 optimization may be performed on this pass. This is optimization by
1767 so-called "check routines". The correspondence between node names
1768 and corresponding check routines is described in F<opcode.pl> (do not
1769 forget to run C<make regen_headers> if you modify this file).
1771 A check routine is called when the node is fully constructed except
1772 for the execution-order thread. Since at this time there are no
1773 back-links to the currently constructed node, one can do most any
1774 operation to the top-level node, including freeing it and/or creating
1775 new nodes above/below it.
1777 The check routine returns the node which should be inserted into the
1778 tree (if the top-level node was not modified, check routine returns
1781 By convention, check routines have names C<ck_*>. They are usually
1782 called from C<new*OP> subroutines (or C<convert>) (which in turn are
1783 called from F<perly.y>).
1785 =head2 Compile pass 1a: constant folding
1787 Immediately after the check routine is called the returned node is
1788 checked for being compile-time executable. If it is (the value is
1789 judged to be constant) it is immediately executed, and a I<constant>
1790 node with the "return value" of the corresponding subtree is
1791 substituted instead. The subtree is deleted.
1793 If constant folding was not performed, the execution-order thread is
1796 =head2 Compile pass 2: context propagation
1798 When a context for a part of compile tree is known, it is propagated
1799 down through the tree. At this time the context can have 5 values
1800 (instead of 2 for runtime context): void, boolean, scalar, list, and
1801 lvalue. In contrast with the pass 1 this pass is processed from top
1802 to bottom: a node's context determines the context for its children.
1804 Additional context-dependent optimizations are performed at this time.
1805 Since at this moment the compile tree contains back-references (via
1806 "thread" pointers), nodes cannot be free()d now. To allow
1807 optimized-away nodes at this stage, such nodes are null()ified instead
1808 of free()ing (i.e. their type is changed to OP_NULL).
1810 =head2 Compile pass 3: peephole optimization
1812 After the compile tree for a subroutine (or for an C<eval> or a file)
1813 is created, an additional pass over the code is performed. This pass
1814 is neither top-down or bottom-up, but in the execution order (with
1815 additional complications for conditionals). These optimizations are
1816 done in the subroutine peep(). Optimizations performed at this stage
1817 are subject to the same restrictions as in the pass 2.
1819 =head2 Pluggable runops
1821 The compile tree is executed in a runops function. There are two runops
1822 functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
1823 with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
1824 control over the execution of the compile tree it is possible to provide
1825 your own runops function.
1827 It's probably best to copy one of the existing runops functions and
1828 change it to suit your needs. Then, in the BOOT section of your XS
1831 PL_runops = my_runops;
1833 This function should be as efficient as possible to keep your programs
1834 running as fast as possible.
1836 =head1 Examining internal data structures with the C<dump> functions
1838 To aid debugging, the source file F<dump.c> contains a number of
1839 functions which produce formatted output of internal data structures.
1841 The most commonly used of these functions is C<Perl_sv_dump>; it's used
1842 for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1843 C<sv_dump> to produce debugging output from Perl-space, so users of that
1844 module should already be familiar with its format.
1846 C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
1847 derivatives, and produces output similar to C<perl -Dx>; in fact,
1848 C<Perl_dump_eval> will dump the main root of the code being evaluated,
1849 exactly like C<-Dx>.
1851 Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1852 op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1853 subroutines in a package like so: (Thankfully, these are all xsubs, so
1854 there is no op tree)
1856 (gdb) print Perl_dump_packsubs(PL_defstash)
1858 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1860 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1862 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1864 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1866 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1868 and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1869 the op tree of the main root.
1871 =head1 How multiple interpreters and concurrency are supported
1873 =head2 Background and PERL_IMPLICIT_CONTEXT
1875 The Perl interpreter can be regarded as a closed box: it has an API
1876 for feeding it code or otherwise making it do things, but it also has
1877 functions for its own use. This smells a lot like an object, and
1878 there are ways for you to build Perl so that you can have multiple
1879 interpreters, with one interpreter represented either as a C structure,
1880 or inside a thread-specific structure. These structures contain all
1881 the context, the state of that interpreter.
1883 Two macros control the major Perl build flavors: MULTIPLICITY and
1884 USE_5005THREADS. The MULTIPLICITY build has a C structure
1885 that packages all the interpreter state, and there is a similar thread-specific
1886 data structure under USE_5005THREADS. In both cases,
1887 PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1888 support for passing in a "hidden" first argument that represents all three
1891 Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
1892 PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
1893 former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
1894 internal variables of Perl to be wrapped inside a single global struct,
1895 struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
1896 the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
1897 one step further, there is still a single struct (allocated in main()
1898 either from heap or from stack) but there are no global data symbols
1899 pointing to it. In either case the global struct should be initialised
1900 as the very first thing in main() using Perl_init_global_struct() and
1901 correspondingly tear it down after perl_free() using Perl_free_global_struct(),
1902 please see F<miniperlmain.c> for usage details. You may also need
1903 to use C<dVAR> in your coding to "declare the global variables"
1904 when you are using them. dTHX does this for you automatically.
1906 For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
1907 doesn't actually hide all symbols inside a big global struct: some
1908 PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
1909 then hides everything (see how the PERLIO_FUNCS_DECL is used).
1911 All this obviously requires a way for the Perl internal functions to be
1912 either subroutines taking some kind of structure as the first
1913 argument, or subroutines taking nothing as the first argument. To
1914 enable these two very different ways of building the interpreter,
1915 the Perl source (as it does in so many other situations) makes heavy
1916 use of macros and subroutine naming conventions.
1918 First problem: deciding which functions will be public API functions and
1919 which will be private. All functions whose names begin C<S_> are private
1920 (think "S" for "secret" or "static"). All other functions begin with
1921 "Perl_", but just because a function begins with "Perl_" does not mean it is
1922 part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1923 function is part of the API is to find its entry in L<perlapi>.
1924 If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1925 think it should be (i.e., you need it for your extension), send mail via
1926 L<perlbug> explaining why you think it should be.
1928 Second problem: there must be a syntax so that the same subroutine
1929 declarations and calls can pass a structure as their first argument,
1930 or pass nothing. To solve this, the subroutines are named and
1931 declared in a particular way. Here's a typical start of a static
1932 function used within the Perl guts:
1935 S_incline(pTHX_ char *s)
1937 STATIC becomes "static" in C, and may be #define'd to nothing in some
1938 configurations in future.
1940 A public function (i.e. part of the internal API, but not necessarily
1941 sanctioned for use in extensions) begins like this:
1944 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
1946 C<pTHX_> is one of a number of macros (in perl.h) that hide the
1947 details of the interpreter's context. THX stands for "thread", "this",
1948 or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1949 The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
1950 or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1953 When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1954 first argument containing the interpreter's context. The trailing underscore
1955 in the pTHX_ macro indicates that the macro expansion needs a comma
1956 after the context argument because other arguments follow it. If
1957 PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
1958 subroutine is not prototyped to take the extra argument. The form of the
1959 macro without the trailing underscore is used when there are no additional
1962 When a core function calls another, it must pass the context. This
1963 is normally hidden via macros. Consider C<sv_setiv>. It expands into
1964 something like this:
1966 #ifdef PERL_IMPLICIT_CONTEXT
1967 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
1968 /* can't do this for vararg functions, see below */
1970 #define sv_setiv Perl_sv_setiv
1973 This works well, and means that XS authors can gleefully write:
1977 and still have it work under all the modes Perl could have been
1980 This doesn't work so cleanly for varargs functions, though, as macros
1981 imply that the number of arguments is known in advance. Instead we
1982 either need to spell them out fully, passing C<aTHX_> as the first
1983 argument (the Perl core tends to do this with functions like
1984 Perl_warner), or use a context-free version.
1986 The context-free version of Perl_warner is called
1987 Perl_warner_nocontext, and does not take the extra argument. Instead
1988 it does dTHX; to get the context from thread-local storage. We
1989 C<#define warner Perl_warner_nocontext> so that extensions get source
1990 compatibility at the expense of performance. (Passing an arg is
1991 cheaper than grabbing it from thread-local storage.)
1993 You can ignore [pad]THXx when browsing the Perl headers/sources.
1994 Those are strictly for use within the core. Extensions and embedders
1995 need only be aware of [pad]THX.
1997 =head2 So what happened to dTHR?
1999 C<dTHR> was introduced in perl 5.005 to support the older thread model.
2000 The older thread model now uses the C<THX> mechanism to pass context
2001 pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2002 later still have it for backward source compatibility, but it is defined
2005 =head2 How do I use all this in extensions?
2007 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2008 any functions in the Perl API will need to pass the initial context
2009 argument somehow. The kicker is that you will need to write it in
2010 such a way that the extension still compiles when Perl hasn't been
2011 built with PERL_IMPLICIT_CONTEXT enabled.
2013 There are three ways to do this. First, the easy but inefficient way,
2014 which is also the default, in order to maintain source compatibility
2015 with extensions: whenever XSUB.h is #included, it redefines the aTHX
2016 and aTHX_ macros to call a function that will return the context.
2017 Thus, something like:
2021 in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
2024 Perl_sv_setiv(Perl_get_context(), sv, num);
2026 or to this otherwise:
2028 Perl_sv_setiv(sv, num);
2030 You have to do nothing new in your extension to get this; since
2031 the Perl library provides Perl_get_context(), it will all just
2034 The second, more efficient way is to use the following template for
2037 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2042 static my_private_function(int arg1, int arg2);
2045 my_private_function(int arg1, int arg2)
2047 dTHX; /* fetch context */
2048 ... call many Perl API functions ...
2053 MODULE = Foo PACKAGE = Foo
2061 my_private_function(arg, 10);
2063 Note that the only two changes from the normal way of writing an
2064 extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2065 including the Perl headers, followed by a C<dTHX;> declaration at
2066 the start of every function that will call the Perl API. (You'll
2067 know which functions need this, because the C compiler will complain
2068 that there's an undeclared identifier in those functions.) No changes
2069 are needed for the XSUBs themselves, because the XS() macro is
2070 correctly defined to pass in the implicit context if needed.
2072 The third, even more efficient way is to ape how it is done within
2076 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2081 /* pTHX_ only needed for functions that call Perl API */
2082 static my_private_function(pTHX_ int arg1, int arg2);
2085 my_private_function(pTHX_ int arg1, int arg2)
2087 /* dTHX; not needed here, because THX is an argument */
2088 ... call Perl API functions ...
2093 MODULE = Foo PACKAGE = Foo
2101 my_private_function(aTHX_ arg, 10);
2103 This implementation never has to fetch the context using a function
2104 call, since it is always passed as an extra argument. Depending on
2105 your needs for simplicity or efficiency, you may mix the previous
2106 two approaches freely.
2108 Never add a comma after C<pTHX> yourself--always use the form of the
2109 macro with the underscore for functions that take explicit arguments,
2110 or the form without the argument for functions with no explicit arguments.
2112 If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2113 definition is needed if the Perl global variables (see F<perlvars.h>
2114 or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2115 used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2116 the need for C<dVAR> only with the said compile-time define, because
2117 otherwise the Perl global variables are visible as-is.
2119 =head2 Should I do anything special if I call perl from multiple threads?
2121 If you create interpreters in one thread and then proceed to call them in
2122 another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2123 initialized correctly in each of those threads.
2125 The C<perl_alloc> and C<perl_clone> API functions will automatically set
2126 the TLS slot to the interpreter they created, so that there is no need to do
2127 anything special if the interpreter is always accessed in the same thread that
2128 created it, and that thread did not create or call any other interpreters
2129 afterwards. If that is not the case, you have to set the TLS slot of the
2130 thread before calling any functions in the Perl API on that particular
2131 interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2132 thread as the first thing you do:
2134 /* do this before doing anything else with some_perl */
2135 PERL_SET_CONTEXT(some_perl);
2137 ... other Perl API calls on some_perl go here ...
2139 =head2 Future Plans and PERL_IMPLICIT_SYS
2141 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2142 that the interpreter knows about itself and pass it around, so too are
2143 there plans to allow the interpreter to bundle up everything it knows
2144 about the environment it's running on. This is enabled with the
2145 PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS
2146 and USE_5005THREADS on Windows (see inside iperlsys.h).
2148 This allows the ability to provide an extra pointer (called the "host"
2149 environment) for all the system calls. This makes it possible for
2150 all the system stuff to maintain their own state, broken down into
2151 seven C structures. These are thin wrappers around the usual system
2152 calls (see win32/perllib.c) for the default perl executable, but for a
2153 more ambitious host (like the one that would do fork() emulation) all
2154 the extra work needed to pretend that different interpreters are
2155 actually different "processes", would be done here.
2157 The Perl engine/interpreter and the host are orthogonal entities.
2158 There could be one or more interpreters in a process, and one or
2159 more "hosts", with free association between them.
2161 =head1 Internal Functions
2163 All of Perl's internal functions which will be exposed to the outside
2164 world are prefixed by C<Perl_> so that they will not conflict with XS
2165 functions or functions used in a program in which Perl is embedded.
2166 Similarly, all global variables begin with C<PL_>. (By convention,
2167 static functions start with C<S_>.)
2169 Inside the Perl core, you can get at the functions either with or
2170 without the C<Perl_> prefix, thanks to a bunch of defines that live in
2171 F<embed.h>. This header file is generated automatically from
2172 F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2173 header files for the internal functions, generates the documentation
2174 and a lot of other bits and pieces. It's important that when you add
2175 a new function to the core or change an existing one, you change the
2176 data in the table in F<embed.fnc> as well. Here's a sample entry from
2179 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2181 The second column is the return type, the third column the name. Columns
2182 after that are the arguments. The first column is a set of flags:
2188 This function is a part of the public API. All such functions should also
2189 have 'd', very few do not.
2193 This function has a C<Perl_> prefix; i.e. it is defined as
2198 This function has documentation using the C<apidoc> feature which we'll
2199 look at in a second. Some functions have 'd' but not 'A'; docs are good.
2203 Other available flags are:
2209 This is a static function and is defined as C<STATIC S_whatever>, and
2210 usually called within the sources as C<whatever(...)>.
2214 This does not need a interpreter context, so the definition has no
2215 C<pTHX>, and it follows that callers don't use C<aTHX>. (See
2216 L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2220 This function never returns; C<croak>, C<exit> and friends.
2224 This function takes a variable number of arguments, C<printf> style.
2225 The argument list should end with C<...>, like this:
2227 Afprd |void |croak |const char* pat|...
2231 This function is part of the experimental development API, and may change
2232 or disappear without notice.
2236 This function should not have a compatibility macro to define, say,
2237 C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2241 This function isn't exported out of the Perl core.
2245 This is implemented as a macro.
2249 This function is explicitly exported.
2253 This function is visible to extensions included in the Perl core.
2257 Binary backward compatibility; this function is a macro but also has
2258 a C<Perl_> implementation (which is exported).
2262 See the comments at the top of C<embed.fnc> for others.
2266 If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2267 C<make regen_headers> to force a rebuild of F<embed.h> and other
2268 auto-generated files.
2270 =head2 Formatted Printing of IVs, UVs, and NVs
2272 If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2273 formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2274 following macros for portability
2279 UVxf UV in hexadecimal
2284 These will take care of 64-bit integers and long doubles.
2287 printf("IV is %"IVdf"\n", iv);
2289 The IVdf will expand to whatever is the correct format for the IVs.
2291 If you are printing addresses of pointers, use UVxf combined
2292 with PTR2UV(), do not use %lx or %p.
2294 =head2 Pointer-To-Integer and Integer-To-Pointer
2296 Because pointer size does not necessarily equal integer size,
2297 use the follow macros to do it right.
2302 INT2PTR(pointertotype, integer)
2307 SV *sv = INT2PTR(SV*, iv);
2314 =head2 Exception Handling
2316 There are a couple of macros to do very basic exception handling in XS
2317 modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2318 be able to use these macros:
2323 You can use these macros if you call code that may croak, but you need
2324 to do some cleanup before giving control back to Perl. For example:
2326 dXCPT; /* set up necessary variables */
2329 code_that_may_croak();
2334 /* do cleanup here */
2338 Note that you always have to rethrow an exception that has been
2339 caught. Using these macros, it is not possible to just catch the
2340 exception and ignore it. If you have to ignore the exception, you
2341 have to use the C<call_*> function.
2343 The advantage of using the above macros is that you don't have
2344 to setup an extra function for C<call_*>, and that using these
2345 macros is faster than using C<call_*>.
2347 =head2 Source Documentation
2349 There's an effort going on to document the internal functions and
2350 automatically produce reference manuals from them - L<perlapi> is one
2351 such manual which details all the functions which are available to XS
2352 writers. L<perlintern> is the autogenerated manual for the functions
2353 which are not part of the API and are supposedly for internal use only.
2355 Source documentation is created by putting POD comments into the C
2359 =for apidoc sv_setiv
2361 Copies an integer into the given SV. Does not handle 'set' magic. See
2367 Please try and supply some documentation if you add functions to the
2370 =head2 Backwards compatibility
2372 The Perl API changes over time. New functions are added or the interfaces
2373 of existing functions are changed. The C<Devel::PPPort> module tries to
2374 provide compatibility code for some of these changes, so XS writers don't
2375 have to code it themselves when supporting multiple versions of Perl.
2377 C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2378 be run as a Perl script. To generate F<ppport.h>, run:
2380 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2382 Besides checking existing XS code, the script can also be used to retrieve
2383 compatibility information for various API calls using the C<--api-info>
2384 command line switch. For example:
2386 % perl ppport.h --api-info=sv_magicext
2388 For details, see C<perldoc ppport.h>.
2390 =head1 Unicode Support
2392 Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2393 writers to understand this support and make sure that the code they
2394 write does not corrupt Unicode data.
2396 =head2 What B<is> Unicode, anyway?
2398 In the olden, less enlightened times, we all used to use ASCII. Most of
2399 us did, anyway. The big problem with ASCII is that it's American. Well,
2400 no, that's not actually the problem; the problem is that it's not
2401 particularly useful for people who don't use the Roman alphabet. What
2402 used to happen was that particular languages would stick their own
2403 alphabet in the upper range of the sequence, between 128 and 255. Of
2404 course, we then ended up with plenty of variants that weren't quite
2405 ASCII, and the whole point of it being a standard was lost.
2407 Worse still, if you've got a language like Chinese or
2408 Japanese that has hundreds or thousands of characters, then you really
2409 can't fit them into a mere 256, so they had to forget about ASCII
2410 altogether, and build their own systems using pairs of numbers to refer
2413 To fix this, some people formed Unicode, Inc. and
2414 produced a new character set containing all the characters you can
2415 possibly think of and more. There are several ways of representing these
2416 characters, and the one Perl uses is called UTF-8. UTF-8 uses
2417 a variable number of bytes to represent a character, instead of just
2418 one. You can learn more about Unicode at http://www.unicode.org/
2420 =head2 How can I recognise a UTF-8 string?
2422 You can't. This is because UTF-8 data is stored in bytes just like
2423 non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
2424 capital E with a grave accent, is represented by the two bytes
2425 C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2426 has that byte sequence as well. So you can't tell just by looking - this
2427 is what makes Unicode input an interesting problem.
2429 The API function C<is_utf8_string> can help; it'll tell you if a string
2430 contains only valid UTF-8 characters. However, it can't do the work for
2431 you. On a character-by-character basis, C<is_utf8_char> will tell you
2432 whether the current character in a string is valid UTF-8.
2434 =head2 How does UTF-8 represent Unicode characters?
2436 As mentioned above, UTF-8 uses a variable number of bytes to store a
2437 character. Characters with values 1...128 are stored in one byte, just
2438 like good ol' ASCII. Character 129 is stored as C<v194.129>; this
2439 continues up to character 191, which is C<v194.191>. Now we've run out of
2440 bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2441 so it goes on, moving to three bytes at character 2048.
2443 Assuming you know you're dealing with a UTF-8 string, you can find out
2444 how long the first character in it is with the C<UTF8SKIP> macro:
2446 char *utf = "\305\233\340\240\201";
2449 len = UTF8SKIP(utf); /* len is 2 here */
2451 len = UTF8SKIP(utf); /* len is 3 here */
2453 Another way to skip over characters in a UTF-8 string is to use
2454 C<utf8_hop>, which takes a string and a number of characters to skip
2455 over. You're on your own about bounds checking, though, so don't use it
2458 All bytes in a multi-byte UTF-8 character will have the high bit set,
2459 so you can test if you need to do something special with this
2460 character like this (the UTF8_IS_INVARIANT() is a macro that tests
2461 whether the byte can be encoded as a single byte even in UTF-8):
2464 UV uv; /* Note: a UV, not a U8, not a char */
2466 if (!UTF8_IS_INVARIANT(*utf))
2467 /* Must treat this as UTF-8 */
2468 uv = utf8_to_uv(utf);
2470 /* OK to treat this character as a byte */
2473 You can also see in that example that we use C<utf8_to_uv> to get the
2474 value of the character; the inverse function C<uv_to_utf8> is available
2475 for putting a UV into UTF-8:
2477 if (!UTF8_IS_INVARIANT(uv))
2478 /* Must treat this as UTF8 */
2479 utf8 = uv_to_utf8(utf8, uv);
2481 /* OK to treat this character as a byte */
2484 You B<must> convert characters to UVs using the above functions if
2485 you're ever in a situation where you have to match UTF-8 and non-UTF-8
2486 characters. You may not skip over UTF-8 characters in this case. If you
2487 do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2488 for instance, if your UTF-8 string contains C<v196.172>, and you skip
2489 that character, you can never match a C<chr(200)> in a non-UTF-8 string.
2492 =head2 How does Perl store UTF-8 strings?
2494 Currently, Perl deals with Unicode strings and non-Unicode strings
2495 slightly differently. If a string has been identified as being UTF-8
2496 encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2497 manipulate this flag with the following macros:
2503 This flag has an important effect on Perl's treatment of the string: if
2504 Unicode data is not properly distinguished, regular expressions,
2505 C<length>, C<substr> and other string handling operations will have
2506 undesirable results.
2508 The problem comes when you have, for instance, a string that isn't
2509 flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
2510 especially when combining non-UTF-8 and UTF-8 strings.
2512 Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2513 need be sure you don't accidentally knock it off while you're
2514 manipulating SVs. More specifically, you cannot expect to do this:
2523 nsv = newSVpvn(p, len);
2525 The C<char*> string does not tell you the whole story, and you can't
2526 copy or reconstruct an SV just by copying the string value. Check if the
2527 old SV has the UTF-8 flag set, and act accordingly:
2531 nsv = newSVpvn(p, len);
2535 In fact, your C<frobnicate> function should be made aware of whether or
2536 not it's dealing with UTF-8 data, so that it can handle the string
2539 Since just passing an SV to an XS function and copying the data of
2540 the SV is not enough to copy the UTF-8 flags, even less right is just
2541 passing a C<char *> to an XS function.
2543 =head2 How do I convert a string to UTF-8?
2545 If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary
2546 to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
2549 sv_utf8_upgrade(sv);
2551 However, you must not do this, for example:
2554 sv_utf8_upgrade(left);
2556 If you do this in a binary operator, you will actually change one of the
2557 strings that came into the operator, and, while it shouldn't be noticeable
2558 by the end user, it can cause problems.
2560 Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
2561 string argument. This is useful for having the data available for
2562 comparisons and so on, without harming the original SV. There's also
2563 C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2564 the string contains any characters above 255 that can't be represented
2567 =head2 Is there anything else I need to know?
2569 Not really. Just remember these things:
2575 There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2576 is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2577 something should be UTF-8. Treat the flag as part of the PV, even though
2578 it's not - if you pass on the PV to somewhere, pass on the flag too.
2582 If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
2583 unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
2587 When writing a character C<uv> to a UTF-8 string, B<always> use
2588 C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2589 you can use C<*s = uv>.
2593 Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2594 a new string which is UTF-8 encoded. There are tricks you can use to
2595 delay deciding whether you need to use a UTF-8 string until you get to a
2596 high character - C<HALF_UPGRADE> is one of those.
2600 =head1 Custom Operators
2602 Custom operator support is a new experimental feature that allows you to
2603 define your own ops. This is primarily to allow the building of
2604 interpreters for other languages in the Perl core, but it also allows
2605 optimizations through the creation of "macro-ops" (ops which perform the
2606 functions of multiple ops which are usually executed together, such as
2607 C<gvsv, gvsv, add>.)
2609 This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
2610 core does not "know" anything special about this op type, and so it will
2611 not be involved in any optimizations. This also means that you can
2612 define your custom ops to be any op structure - unary, binary, list and
2615 It's important to know what custom operators won't do for you. They
2616 won't let you add new syntax to Perl, directly. They won't even let you
2617 add new keywords, directly. In fact, they won't change the way Perl
2618 compiles a program at all. You have to do those changes yourself, after
2619 Perl has compiled the program. You do this either by manipulating the op
2620 tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2621 a custom peephole optimizer with the C<optimize> module.
2623 When you do this, you replace ordinary Perl ops with custom ops by
2624 creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2625 PP function. This should be defined in XS code, and should look like
2626 the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2627 takes the appropriate number of values from the stack, and you are
2628 responsible for adding stack marks if necessary.
2630 You should also "register" your op with the Perl interpreter so that it
2631 can produce sensible error and warning messages. Since it is possible to
2632 have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2633 Perl uses the value of C<< o->op_ppaddr >> as a key into the
2634 C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2635 need to enter a name and description for your op at the appropriate
2636 place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2638 Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2639 directly support the creation of custom ops by name.
2643 Until May 1997, this document was maintained by Jeff Okamoto
2644 E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2645 itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
2647 With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2648 Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2649 Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2650 Stephen McCamant, and Gurusamy Sarathy.
2654 perlapi(1), perlintern(1), perlxs(1), perlembed(1)