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).
40 ("PV" stands for "Pointer Value". You might think that it is misnamed
41 because it is described as pointing only to strings. However, it is
42 possible to have it point to other things For example, it could point
43 to an array of UVs. But,
44 using it for non-strings requires care, as the underlying assumption of
45 much of the internals is that PVs are just for strings. Often, for
46 example, a trailing C<NUL> is tacked on automatically. The non-string use
47 is documented only in this paragraph.)
49 The seven routines are:
54 SV* newSVpv(const char*, STRLEN);
55 SV* newSVpvn(const char*, STRLEN);
56 SV* newSVpvf(const char*, ...);
59 C<STRLEN> is an integer type (Size_t, usually defined as size_t in
60 F<config.h>) guaranteed to be large enough to represent the size of
61 any string that perl can handle.
63 In the unlikely case of a SV requiring more complex initialization, you
64 can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
65 type NULL is returned, else an SV of type PV is returned with len + 1 (for
66 the C<NUL>) bytes of storage allocated, accessible via SvPVX. In both cases
67 the SV has the undef value.
69 SV *sv = newSV(0); /* no storage allocated */
70 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
73 To change the value of an I<already-existing> SV, there are eight routines:
75 void sv_setiv(SV*, IV);
76 void sv_setuv(SV*, UV);
77 void sv_setnv(SV*, double);
78 void sv_setpv(SV*, const char*);
79 void sv_setpvn(SV*, const char*, STRLEN)
80 void sv_setpvf(SV*, const char*, ...);
81 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
83 void sv_setsv(SV*, SV*);
85 Notice that you can choose to specify the length of the string to be
86 assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
87 allow Perl to calculate the length by using C<sv_setpv> or by specifying
88 0 as the second argument to C<newSVpv>. Be warned, though, that Perl will
89 determine the string's length by using C<strlen>, which depends on the
90 string terminating with a C<NUL> character, and not otherwise containing
93 The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
94 formatted output becomes the value.
96 C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
97 either a pointer to a variable argument list or the address and length of
98 an array of SVs. The last argument points to a boolean; on return, if that
99 boolean is true, then locale-specific information has been used to format
100 the string, and the string's contents are therefore untrustworthy (see
101 L<perlsec>). This pointer may be NULL if that information is not
102 important. Note that this function requires you to specify the length of
105 The C<sv_set*()> functions are not generic enough to operate on values
106 that have "magic". See L<Magic Virtual Tables> later in this document.
108 All SVs that contain strings should be terminated with a C<NUL> character.
109 If it is not C<NUL>-terminated there is a risk of
110 core dumps and corruptions from code which passes the string to C
111 functions or system calls which expect a C<NUL>-terminated string.
112 Perl's own functions typically add a trailing C<NUL> for this reason.
113 Nevertheless, you should be very careful when you pass a string stored
114 in an SV to a C function or system call.
116 To access the actual value that an SV points to, you can use the macros:
121 SvPV(SV*, STRLEN len)
124 which will automatically coerce the actual scalar type into an IV, UV, double,
127 In the C<SvPV> macro, the length of the string returned is placed into the
128 variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
129 not care what the length of the data is, use the C<SvPV_nolen> macro.
130 Historically the C<SvPV> macro with the global variable C<PL_na> has been
131 used in this case. But that can be quite inefficient because C<PL_na> must
132 be accessed in thread-local storage in threaded Perl. In any case, remember
133 that Perl allows arbitrary strings of data that may both contain NULs and
134 might not be terminated by a C<NUL>.
136 Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
137 len);>. It might work with your
138 compiler, but it won't work for everyone.
139 Break this sort of statement up into separate assignments:
147 If you want to know if the scalar value is TRUE, you can use:
151 Although Perl will automatically grow strings for you, if you need to force
152 Perl to allocate more memory for your SV, you can use the macro
154 SvGROW(SV*, STRLEN newlen)
156 which will determine if more memory needs to be allocated. If so, it will
157 call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
158 decrease, the allocated memory of an SV and that it does not automatically
159 add space for the trailing C<NUL> byte (perl's own string functions typically do
160 C<SvGROW(sv, len + 1)>).
162 If you want to write to an existing SV's buffer and set its value to a
163 string, use SvPV_force() or one of its variants to force the SV to be
164 a PV. This will remove any of various types of non-stringness from
165 the SV while preserving the content of the SV in the PV. This can be
166 used, for example, to append data from an API function to a buffer
167 without extra copying:
169 (void)SvPVbyte_force(sv, len);
170 s = SvGROW(sv, len + needlen + 1);
171 /* something that modifies up to needlen bytes at s+len, but
172 modifies newlen bytes
173 eg. newlen = read(fd, s + len, needlen);
174 ignoring errors for these examples
176 s[len + newlen] = '\0';
177 SvCUR_set(sv, len + newlen);
181 If you already have the data in memory or if you want to keep your
182 code simple, you can use one of the sv_cat*() variants, such as
183 sv_catpvn(). If you want to insert anywhere in the string you can use
184 sv_insert() or sv_insert_flags().
186 If you don't need the existing content of the SV, you can avoid some
189 sv_setpvn(sv, "", 0);
190 s = SvGROW(sv, needlen + 1);
191 /* something that modifies up to needlen bytes at s, but modifies
193 eg. newlen = read(fd, s. needlen);
196 SvCUR_set(sv, newlen);
197 SvPOK_only(sv); /* also clears SVf_UTF8 */
200 Again, if you already have the data in memory or want to avoid the
201 complexity of the above, you can use sv_setpvn().
203 If you have a buffer allocated with Newx() and want to set that as the
204 SV's value, you can use sv_usepvn_flags(). That has some requirements
205 if you want to avoid perl re-allocating the buffer to fit the trailing
208 Newx(buf, somesize+1, char);
209 /* ... fill in buf ... */
210 buf[somesize] = '\0';
211 sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
212 /* buf now belongs to perl, don't release it */
214 If you have an SV and want to know what kind of data Perl thinks is stored
215 in it, you can use the following macros to check the type of SV you have.
221 You can get and set the current length of the string stored in an SV with
222 the following macros:
225 SvCUR_set(SV*, I32 val)
227 You can also get a pointer to the end of the string stored in the SV
232 But note that these last three macros are valid only if C<SvPOK()> is true.
234 If you want to append something to the end of string stored in an C<SV*>,
235 you can use the following functions:
237 void sv_catpv(SV*, const char*);
238 void sv_catpvn(SV*, const char*, STRLEN);
239 void sv_catpvf(SV*, const char*, ...);
240 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
242 void sv_catsv(SV*, SV*);
244 The first function calculates the length of the string to be appended by
245 using C<strlen>. In the second, you specify the length of the string
246 yourself. The third function processes its arguments like C<sprintf> and
247 appends the formatted output. The fourth function works like C<vsprintf>.
248 You can specify the address and length of an array of SVs instead of the
249 va_list argument. The fifth function
250 extends the string stored in the first
251 SV with the string stored in the second SV. It also forces the second SV
252 to be interpreted as a string.
254 The C<sv_cat*()> functions are not generic enough to operate on values that
255 have "magic". See L<Magic Virtual Tables> later in this document.
257 If you know the name of a scalar variable, you can get a pointer to its SV
258 by using the following:
260 SV* get_sv("package::varname", 0);
262 This returns NULL if the variable does not exist.
264 If you want to know if this variable (or any other SV) is actually C<defined>,
269 The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
271 Its address can be used whenever an C<SV*> is needed. Make sure that
272 you don't try to compare a random sv with C<&PL_sv_undef>. For example
273 when interfacing Perl code, it'll work correctly for:
277 But won't work when called as:
282 So to repeat always use SvOK() to check whether an sv is defined.
284 Also you have to be careful when using C<&PL_sv_undef> as a value in
285 AVs or HVs (see L<AVs, HVs and undefined values>).
287 There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
288 boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
289 addresses can be used whenever an C<SV*> is needed.
291 Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
295 if (I-am-to-return-a-real-value) {
296 sv = sv_2mortal(newSViv(42));
300 This code tries to return a new SV (which contains the value 42) if it should
301 return a real value, or undef otherwise. Instead it has returned a NULL
302 pointer which, somewhere down the line, will cause a segmentation violation,
303 bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
304 first line and all will be well.
306 To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
307 call is not necessary (see L<Reference Counts and Mortality>).
311 Perl provides the function C<sv_chop> to efficiently remove characters
312 from the beginning of a string; you give it an SV and a pointer to
313 somewhere inside the PV, and it discards everything before the
314 pointer. The efficiency comes by means of a little hack: instead of
315 actually removing the characters, C<sv_chop> sets the flag C<OOK>
316 (offset OK) to signal to other functions that the offset hack is in
317 effect, and it moves the PV pointer (called C<SvPVX>) forward
318 by the number of bytes chopped off, and adjusts C<SvCUR> and C<SvLEN>
319 accordingly. (A portion of the space between the old and new PV
320 pointers is used to store the count of chopped bytes.)
322 Hence, at this point, the start of the buffer that we allocated lives
323 at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
324 into the middle of this allocated storage.
326 This is best demonstrated by example:
328 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
329 SV = PVIV(0x8128450) at 0x81340f0
331 FLAGS = (POK,OOK,pPOK)
333 PV = 0x8135781 ( "1" . ) "2345"\0
337 Here the number of bytes chopped off (1) is put into IV, and
338 C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
339 portion of the string between the "real" and the "fake" beginnings is
340 shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
341 the fake beginning, not the real one.
343 Something similar to the offset hack is performed on AVs to enable
344 efficient shifting and splicing off the beginning of the array; while
345 C<AvARRAY> points to the first element in the array that is visible from
346 Perl, C<AvALLOC> points to the real start of the C array. These are
347 usually the same, but a C<shift> operation can be carried out by
348 increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
349 Again, the location of the real start of the C array only comes into
350 play when freeing the array. See C<av_shift> in F<av.c>.
352 =head2 What's Really Stored in an SV?
354 Recall that the usual method of determining the type of scalar you have is
355 to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
356 usually these macros will always return TRUE and calling the C<Sv*V>
357 macros will do the appropriate conversion of string to integer/double or
358 integer/double to string.
360 If you I<really> need to know if you have an integer, double, or string
361 pointer in an SV, you can use the following three macros instead:
367 These will tell you if you truly have an integer, double, or string pointer
368 stored in your SV. The "p" stands for private.
370 There are various ways in which the private and public flags may differ.
371 For example, in perl 5.16 and earlier a tied SV may have a valid
372 underlying value in the IV slot (so SvIOKp is true), but the data
373 should be accessed via the FETCH routine rather than directly,
374 so SvIOK is false. (In perl 5.18 onwards, tied scalars use
375 the flags the same way as untied scalars.) Another is when
376 numeric conversion has occurred and precision has been lost: only the
377 private flag is set on 'lossy' values. So when an NV is converted to an
378 IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
380 In general, though, it's best to use the C<Sv*V> macros.
382 =head2 Working with AVs
384 There are two ways to create and load an AV. The first method creates an
389 The second method both creates the AV and initially populates it with SVs:
391 AV* av_make(SSize_t num, SV **ptr);
393 The second argument points to an array containing C<num> C<SV*>'s. Once the
394 AV has been created, the SVs can be destroyed, if so desired.
396 Once the AV has been created, the following operations are possible on it:
398 void av_push(AV*, SV*);
401 void av_unshift(AV*, SSize_t num);
403 These should be familiar operations, with the exception of C<av_unshift>.
404 This routine adds C<num> elements at the front of the array with the C<undef>
405 value. You must then use C<av_store> (described below) to assign values
406 to these new elements.
408 Here are some other functions:
410 SSize_t av_top_index(AV*);
411 SV** av_fetch(AV*, SSize_t key, I32 lval);
412 SV** av_store(AV*, SSize_t key, SV* val);
414 The C<av_top_index> function returns the highest index value in an array (just
415 like $#array in Perl). If the array is empty, -1 is returned. The
416 C<av_fetch> function returns the value at index C<key>, but if C<lval>
417 is non-zero, then C<av_fetch> will store an undef value at that index.
418 The C<av_store> function stores the value C<val> at index C<key>, and does
419 not increment the reference count of C<val>. Thus the caller is responsible
420 for taking care of that, and if C<av_store> returns NULL, the caller will
421 have to decrement the reference count to avoid a memory leak. Note that
422 C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
429 void av_extend(AV*, SSize_t key);
431 The C<av_clear> function deletes all the elements in the AV* array, but
432 does not actually delete the array itself. The C<av_undef> function will
433 delete all the elements in the array plus the array itself. The
434 C<av_extend> function extends the array so that it contains at least C<key+1>
435 elements. If C<key+1> is less than the currently allocated length of the array,
436 then nothing is done.
438 If you know the name of an array variable, you can get a pointer to its AV
439 by using the following:
441 AV* get_av("package::varname", 0);
443 This returns NULL if the variable does not exist.
445 See L<Understanding the Magic of Tied Hashes and Arrays> for more
446 information on how to use the array access functions on tied arrays.
448 =head2 Working with HVs
450 To create an HV, you use the following routine:
454 Once the HV has been created, the following operations are possible on it:
456 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
457 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
459 The C<klen> parameter is the length of the key being passed in (Note that
460 you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
461 length of the key). The C<val> argument contains the SV pointer to the
462 scalar being stored, and C<hash> is the precomputed hash value (zero if
463 you want C<hv_store> to calculate it for you). The C<lval> parameter
464 indicates whether this fetch is actually a part of a store operation, in
465 which case a new undefined value will be added to the HV with the supplied
466 key and C<hv_fetch> will return as if the value had already existed.
468 Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
469 C<SV*>. To access the scalar value, you must first dereference the return
470 value. However, you should check to make sure that the return value is
471 not NULL before dereferencing it.
473 The first of these two functions checks if a hash table entry exists, and the
476 bool hv_exists(HV*, const char* key, U32 klen);
477 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
479 If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
480 create and return a mortal copy of the deleted value.
482 And more miscellaneous functions:
487 Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
488 table but does not actually delete the hash table. The C<hv_undef> deletes
489 both the entries and the hash table itself.
491 Perl keeps the actual data in a linked list of structures with a typedef of HE.
492 These contain the actual key and value pointers (plus extra administrative
493 overhead). The key is a string pointer; the value is an C<SV*>. However,
494 once you have an C<HE*>, to get the actual key and value, use the routines
497 I32 hv_iterinit(HV*);
498 /* Prepares starting point to traverse hash table */
499 HE* hv_iternext(HV*);
500 /* Get the next entry, and return a pointer to a
501 structure that has both the key and value */
502 char* hv_iterkey(HE* entry, I32* retlen);
503 /* Get the key from an HE structure and also return
504 the length of the key string */
505 SV* hv_iterval(HV*, HE* entry);
506 /* Return an SV pointer to the value of the HE
508 SV* hv_iternextsv(HV*, char** key, I32* retlen);
509 /* This convenience routine combines hv_iternext,
510 hv_iterkey, and hv_iterval. The key and retlen
511 arguments are return values for the key and its
512 length. The value is returned in the SV* argument */
514 If you know the name of a hash variable, you can get a pointer to its HV
515 by using the following:
517 HV* get_hv("package::varname", 0);
519 This returns NULL if the variable does not exist.
521 The hash algorithm is defined in the C<PERL_HASH> macro:
523 PERL_HASH(hash, key, klen)
525 The exact implementation of this macro varies by architecture and version
526 of perl, and the return value may change per invocation, so the value
527 is only valid for the duration of a single perl process.
529 See L<Understanding the Magic of Tied Hashes and Arrays> for more
530 information on how to use the hash access functions on tied hashes.
532 =head2 Hash API Extensions
534 Beginning with version 5.004, the following functions are also supported:
536 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
537 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
539 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
540 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
542 SV* hv_iterkeysv (HE* entry);
544 Note that these functions take C<SV*> keys, which simplifies writing
545 of extension code that deals with hash structures. These functions
546 also allow passing of C<SV*> keys to C<tie> functions without forcing
547 you to stringify the keys (unlike the previous set of functions).
549 They also return and accept whole hash entries (C<HE*>), making their
550 use more efficient (since the hash number for a particular string
551 doesn't have to be recomputed every time). See L<perlapi> for detailed
554 The following macros must always be used to access the contents of hash
555 entries. Note that the arguments to these macros must be simple
556 variables, since they may get evaluated more than once. See
557 L<perlapi> for detailed descriptions of these macros.
559 HePV(HE* he, STRLEN len)
563 HeSVKEY_force(HE* he)
564 HeSVKEY_set(HE* he, SV* sv)
566 These two lower level macros are defined, but must only be used when
567 dealing with keys that are not C<SV*>s:
572 Note that both C<hv_store> and C<hv_store_ent> do not increment the
573 reference count of the stored C<val>, which is the caller's responsibility.
574 If these functions return a NULL value, the caller will usually have to
575 decrement the reference count of C<val> to avoid a memory leak.
577 =head2 AVs, HVs and undefined values
579 Sometimes you have to store undefined values in AVs or HVs. Although
580 this may be a rare case, it can be tricky. That's because you're
581 used to using C<&PL_sv_undef> if you need an undefined SV.
583 For example, intuition tells you that this XS code:
586 av_store( av, 0, &PL_sv_undef );
588 is equivalent to this Perl code:
593 Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
594 for indicating that an array element has not yet been initialized.
595 Thus, C<exists $av[0]> would be true for the above Perl code, but
596 false for the array generated by the XS code. In perl 5.20, storing
597 &PL_sv_undef will create a read-only element, because the scalar
598 &PL_sv_undef itself is stored, not a copy.
600 Similar problems can occur when storing C<&PL_sv_undef> in HVs:
602 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
604 This will indeed make the value C<undef>, but if you try to modify
605 the value of C<key>, you'll get the following error:
607 Modification of non-creatable hash value attempted
609 In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
610 in restricted hashes. This caused such hash entries not to appear
611 when iterating over the hash or when checking for the keys
612 with the C<hv_exists> function.
614 You can run into similar problems when you store C<&PL_sv_yes> or
615 C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
616 will give you the following error:
618 Modification of a read-only value attempted
620 To make a long story short, you can use the special variables
621 C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
622 HVs, but you have to make sure you know what you're doing.
624 Generally, if you want to store an undefined value in an AV
625 or HV, you should not use C<&PL_sv_undef>, but rather create a
626 new undefined value using the C<newSV> function, for example:
628 av_store( av, 42, newSV(0) );
629 hv_store( hv, "foo", 3, newSV(0), 0 );
633 References are a special type of scalar that point to other data types
634 (including other references).
636 To create a reference, use either of the following functions:
638 SV* newRV_inc((SV*) thing);
639 SV* newRV_noinc((SV*) thing);
641 The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
642 functions are identical except that C<newRV_inc> increments the reference
643 count of the C<thing>, while C<newRV_noinc> does not. For historical
644 reasons, C<newRV> is a synonym for C<newRV_inc>.
646 Once you have a reference, you can use the following macro to dereference
651 then call the appropriate routines, casting the returned C<SV*> to either an
652 C<AV*> or C<HV*>, if required.
654 To determine if an SV is a reference, you can use the following macro:
658 To discover what type of value the reference refers to, use the following
659 macro and then check the return value.
663 The most useful types that will be returned are:
669 SVt_PVGV Glob (possibly a file handle)
671 See L<perlapi/svtype> for more details.
673 =head2 Blessed References and Class Objects
675 References are also used to support object-oriented programming. In perl's
676 OO lexicon, an object is simply a reference that has been blessed into a
677 package (or class). Once blessed, the programmer may now use the reference
678 to access the various methods in the class.
680 A reference can be blessed into a package with the following function:
682 SV* sv_bless(SV* sv, HV* stash);
684 The C<sv> argument must be a reference value. The C<stash> argument
685 specifies which class the reference will belong to. See
686 L<Stashes and Globs> for information on converting class names into stashes.
688 /* Still under construction */
690 The following function upgrades rv to reference if not already one.
691 Creates a new SV for rv to point to. If C<classname> is non-null, the SV
692 is blessed into the specified class. SV is returned.
694 SV* newSVrv(SV* rv, const char* classname);
696 The following three functions copy integer, unsigned integer or double
697 into an SV whose reference is C<rv>. SV is blessed if C<classname> is
700 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
701 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
702 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
704 The following function copies the pointer value (I<the address, not the
705 string!>) into an SV whose reference is rv. SV is blessed if C<classname>
708 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
710 The following function copies a string into an SV whose reference is C<rv>.
711 Set length to 0 to let Perl calculate the string length. SV is blessed if
712 C<classname> is non-null.
714 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
717 The following function tests whether the SV is blessed into the specified
718 class. It does not check inheritance relationships.
720 int sv_isa(SV* sv, const char* name);
722 The following function tests whether the SV is a reference to a blessed object.
724 int sv_isobject(SV* sv);
726 The following function tests whether the SV is derived from the specified
727 class. SV can be either a reference to a blessed object or a string
728 containing a class name. This is the function implementing the
729 C<UNIVERSAL::isa> functionality.
731 bool sv_derived_from(SV* sv, const char* name);
733 To check if you've got an object derived from a specific class you have
736 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
738 =head2 Creating New Variables
740 To create a new Perl variable with an undef value which can be accessed from
741 your Perl script, use the following routines, depending on the variable type.
743 SV* get_sv("package::varname", GV_ADD);
744 AV* get_av("package::varname", GV_ADD);
745 HV* get_hv("package::varname", GV_ADD);
747 Notice the use of GV_ADD as the second parameter. The new variable can now
748 be set, using the routines appropriate to the data type.
750 There are additional macros whose values may be bitwise OR'ed with the
751 C<GV_ADD> argument to enable certain extra features. Those bits are:
757 Marks the variable as multiply defined, thus preventing the:
759 Name <varname> used only once: possible typo
767 Had to create <varname> unexpectedly
769 if the variable did not exist before the function was called.
773 If you do not specify a package name, the variable is created in the current
776 =head2 Reference Counts and Mortality
778 Perl uses a reference count-driven garbage collection mechanism. SVs,
779 AVs, or HVs (xV for short in the following) start their life with a
780 reference count of 1. If the reference count of an xV ever drops to 0,
781 then it will be destroyed and its memory made available for reuse.
783 This normally doesn't happen at the Perl level unless a variable is
784 undef'ed or the last variable holding a reference to it is changed or
785 overwritten. At the internal level, however, reference counts can be
786 manipulated with the following macros:
788 int SvREFCNT(SV* sv);
789 SV* SvREFCNT_inc(SV* sv);
790 void SvREFCNT_dec(SV* sv);
792 However, there is one other function which manipulates the reference
793 count of its argument. The C<newRV_inc> function, you will recall,
794 creates a reference to the specified argument. As a side effect,
795 it increments the argument's reference count. If this is not what
796 you want, use C<newRV_noinc> instead.
798 For example, imagine you want to return a reference from an XSUB function.
799 Inside the XSUB routine, you create an SV which initially has a reference
800 count of one. Then you call C<newRV_inc>, passing it the just-created SV.
801 This returns the reference as a new SV, but the reference count of the
802 SV you passed to C<newRV_inc> has been incremented to two. Now you
803 return the reference from the XSUB routine and forget about the SV.
804 But Perl hasn't! Whenever the returned reference is destroyed, the
805 reference count of the original SV is decreased to one and nothing happens.
806 The SV will hang around without any way to access it until Perl itself
807 terminates. This is a memory leak.
809 The correct procedure, then, is to use C<newRV_noinc> instead of
810 C<newRV_inc>. Then, if and when the last reference is destroyed,
811 the reference count of the SV will go to zero and it will be destroyed,
812 stopping any memory leak.
814 There are some convenience functions available that can help with the
815 destruction of xVs. These functions introduce the concept of "mortality".
816 An xV that is mortal has had its reference count marked to be decremented,
817 but not actually decremented, until "a short time later". Generally the
818 term "short time later" means a single Perl statement, such as a call to
819 an XSUB function. The actual determinant for when mortal xVs have their
820 reference count decremented depends on two macros, SAVETMPS and FREETMPS.
821 See L<perlcall> and L<perlxs> for more details on these macros.
823 "Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
824 However, if you mortalize a variable twice, the reference count will
825 later be decremented twice.
827 "Mortal" SVs are mainly used for SVs that are placed on perl's stack.
828 For example an SV which is created just to pass a number to a called sub
829 is made mortal to have it cleaned up automatically when it's popped off
830 the stack. Similarly, results returned by XSUBs (which are pushed on the
831 stack) are often made mortal.
833 To create a mortal variable, use the functions:
837 SV* sv_mortalcopy(SV*)
839 The first call creates a mortal SV (with no value), the second converts an existing
840 SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
841 third creates a mortal copy of an existing SV.
842 Because C<sv_newmortal> gives the new SV no value, it must normally be given one
843 via C<sv_setpv>, C<sv_setiv>, etc. :
845 SV *tmp = sv_newmortal();
846 sv_setiv(tmp, an_integer);
848 As that is multiple C statements it is quite common so see this idiom instead:
850 SV *tmp = sv_2mortal(newSViv(an_integer));
853 You should be careful about creating mortal variables. Strange things
854 can happen if you make the same value mortal within multiple contexts,
855 or if you make a variable mortal multiple
856 times. Thinking of "Mortalization"
857 as deferred C<SvREFCNT_dec> should help to minimize such problems.
858 For example if you are passing an SV which you I<know> has a high enough REFCNT
859 to survive its use on the stack you need not do any mortalization.
860 If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
861 making a C<sv_mortalcopy> is safer.
863 The mortal routines are not just for SVs; AVs and HVs can be
864 made mortal by passing their address (type-casted to C<SV*>) to the
865 C<sv_2mortal> or C<sv_mortalcopy> routines.
867 =head2 Stashes and Globs
869 A B<stash> is a hash that contains all variables that are defined
870 within a package. Each key of the stash is a symbol
871 name (shared by all the different types of objects that have the same
872 name), and each value in the hash table is a GV (Glob Value). This GV
873 in turn contains references to the various objects of that name,
874 including (but not limited to) the following:
883 There is a single stash called C<PL_defstash> that holds the items that exist
884 in the C<main> package. To get at the items in other packages, append the
885 string "::" to the package name. The items in the C<Foo> package are in
886 the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
887 in the stash C<Baz::> in C<Bar::>'s stash.
889 To get the stash pointer for a particular package, use the function:
891 HV* gv_stashpv(const char* name, I32 flags)
892 HV* gv_stashsv(SV*, I32 flags)
894 The first function takes a literal string, the second uses the string stored
895 in the SV. Remember that a stash is just a hash table, so you get back an
896 C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
898 The name that C<gv_stash*v> wants is the name of the package whose symbol table
899 you want. The default package is called C<main>. If you have multiply nested
900 packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
903 Alternately, if you have an SV that is a blessed reference, you can find
904 out the stash pointer by using:
906 HV* SvSTASH(SvRV(SV*));
908 then use the following to get the package name itself:
910 char* HvNAME(HV* stash);
912 If you need to bless or re-bless an object you can use the following
915 SV* sv_bless(SV*, HV* stash)
917 where the first argument, an C<SV*>, must be a reference, and the second
918 argument is a stash. The returned C<SV*> can now be used in the same way
921 For more information on references and blessings, consult L<perlref>.
923 =head2 Double-Typed SVs
925 Scalar variables normally contain only one type of value, an integer,
926 double, pointer, or reference. Perl will automatically convert the
927 actual scalar data from the stored type into the requested type.
929 Some scalar variables contain more than one type of scalar data. For
930 example, the variable C<$!> contains either the numeric value of C<errno>
931 or its string equivalent from either C<strerror> or C<sys_errlist[]>.
933 To force multiple data values into an SV, you must do two things: use the
934 C<sv_set*v> routines to add the additional scalar type, then set a flag
935 so that Perl will believe it contains more than one type of data. The
936 four macros to set the flags are:
943 The particular macro you must use depends on which C<sv_set*v> routine
944 you called first. This is because every C<sv_set*v> routine turns on
945 only the bit for the particular type of data being set, and turns off
948 For example, to create a new Perl variable called "dberror" that contains
949 both the numeric and descriptive string error values, you could use the
953 extern char *dberror_list;
955 SV* sv = get_sv("dberror", GV_ADD);
956 sv_setiv(sv, (IV) dberror);
957 sv_setpv(sv, dberror_list[dberror]);
960 If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
961 macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
963 =head2 Read-Only Values
965 In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
966 flag bit with read-only scalars. So the only way to test whether
967 C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
968 in those versions is:
970 SvREADONLY(sv) && !SvIsCOW(sv)
972 Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
973 and, under 5.20, copy-on-write scalars can also be read-only, so the above
974 check is incorrect. You just want:
978 If you need to do this check often, define your own macro like this:
980 #if PERL_VERSION >= 18
981 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
983 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
988 Perl implements a copy-on-write (COW) mechanism for scalars, in which
989 string copies are not immediately made when requested, but are deferred
990 until made necessary by one or the other scalar changing. This is mostly
991 transparent, but one must take care not to modify string buffers that are
992 shared by multiple SVs.
994 You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
996 You can force an SV to make its own copy of its string buffer by calling C<sv_force_normal(sv)> or SvPV_force_nolen(sv).
998 If you want to make the SV drop its string buffer, use
999 C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
1000 C<sv_setsv(sv, NULL)>.
1002 All of these functions will croak on read-only scalars (see the previous
1003 section for more on those).
1005 To test that your code is behaving correctly and not modifying COW buffers,
1006 on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
1007 C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
1008 into crashes. You will find it to be marvellously slow, so you may want to
1009 skip perl's own tests.
1011 =head2 Magic Variables
1013 [This section still under construction. Ignore everything here. Post no
1014 bills. Everything not permitted is forbidden.]
1016 Any SV may be magical, that is, it has special features that a normal
1017 SV does not have. These features are stored in the SV structure in a
1018 linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
1021 MAGIC* mg_moremagic;
1031 Note this is current as of patchlevel 0, and could change at any time.
1033 =head2 Assigning Magic
1035 Perl adds magic to an SV using the sv_magic function:
1037 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
1039 The C<sv> argument is a pointer to the SV that is to acquire a new magical
1042 If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
1043 convert C<sv> to type C<SVt_PVMG>.
1044 Perl then continues by adding new magic
1045 to the beginning of the linked list of magical features. Any prior entry
1046 of the same type of magic is deleted. Note that this can be overridden,
1047 and multiple instances of the same type of magic can be associated with an
1050 The C<name> and C<namlen> arguments are used to associate a string with
1051 the magic, typically the name of a variable. C<namlen> is stored in the
1052 C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
1053 C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
1054 whether C<namlen> is greater than zero or equal to zero respectively. As a
1055 special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
1056 to contain an C<SV*> and is stored as-is with its REFCNT incremented.
1058 The sv_magic function uses C<how> to determine which, if any, predefined
1059 "Magic Virtual Table" should be assigned to the C<mg_virtual> field.
1060 See the L<Magic Virtual Tables> section below. The C<how> argument is also
1061 stored in the C<mg_type> field. The value of
1062 C<how> should be chosen from the set of macros
1063 C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
1064 these macros were added, Perl internals used to directly use character
1065 literals, so you may occasionally come across old code or documentation
1066 referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
1068 The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
1069 structure. If it is not the same as the C<sv> argument, the reference
1070 count of the C<obj> object is incremented. If it is the same, or if
1071 the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
1072 then C<obj> is merely stored, without the reference count being incremented.
1074 See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
1077 There is also a function to add magic to an C<HV>:
1079 void hv_magic(HV *hv, GV *gv, int how);
1081 This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
1083 To remove the magic from an SV, call the function sv_unmagic:
1085 int sv_unmagic(SV *sv, int type);
1087 The C<type> argument should be equal to the C<how> value when the C<SV>
1088 was initially made magical.
1090 However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
1091 C<SV>. If you want to remove only certain
1092 magic of a C<type> based on the magic
1093 virtual table, use C<sv_unmagicext> instead:
1095 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1097 =head2 Magic Virtual Tables
1099 The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
1100 C<MGVTBL>, which is a structure of function pointers and stands for
1101 "Magic Virtual Table" to handle the various operations that might be
1102 applied to that variable.
1104 The C<MGVTBL> has five (or sometimes eight) pointers to the following
1107 int (*svt_get)(SV* sv, MAGIC* mg);
1108 int (*svt_set)(SV* sv, MAGIC* mg);
1109 U32 (*svt_len)(SV* sv, MAGIC* mg);
1110 int (*svt_clear)(SV* sv, MAGIC* mg);
1111 int (*svt_free)(SV* sv, MAGIC* mg);
1113 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
1114 const char *name, I32 namlen);
1115 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
1116 int (*svt_local)(SV *nsv, MAGIC *mg);
1119 This MGVTBL structure is set at compile-time in F<perl.h> and there are
1120 currently 32 types. These different structures contain pointers to various
1121 routines that perform additional actions depending on which function is
1124 Function pointer Action taken
1125 ---------------- ------------
1126 svt_get Do something before the value of the SV is
1128 svt_set Do something after the SV is assigned a value.
1129 svt_len Report on the SV's length.
1130 svt_clear Clear something the SV represents.
1131 svt_free Free any extra storage associated with the SV.
1133 svt_copy copy tied variable magic to a tied element
1134 svt_dup duplicate a magic structure during thread cloning
1135 svt_local copy magic to local value during 'local'
1137 For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
1138 to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
1140 { magic_get, magic_set, magic_len, 0, 0 }
1142 Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1143 if a get operation is being performed, the routine C<magic_get> is
1144 called. All the various routines for the various magical types begin
1145 with C<magic_>. NOTE: the magic routines are not considered part of
1146 the Perl API, and may not be exported by the Perl library.
1148 The last three slots are a recent addition, and for source code
1149 compatibility they are only checked for if one of the three flags
1150 MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
1151 This means that most code can continue declaring
1152 a vtable as a 5-element value. These three are
1153 currently used exclusively by the threading code, and are highly subject
1156 The current kinds of Magic Virtual Tables are:
1159 This table is generated by regen/mg_vtable.pl. Any changes made here
1162 =for mg_vtable.pl begin
1165 (old-style char and macro) MGVTBL Type of magic
1166 -------------------------- ------ -------------
1167 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1168 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1169 % PERL_MAGIC_rhash (none) Extra data for restricted
1171 & PERL_MAGIC_proto (none) my sub prototype CV
1172 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1174 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1175 : PERL_MAGIC_symtab (none) Extra data for symbol
1177 < PERL_MAGIC_backref vtbl_backref For weak ref data
1178 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
1179 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1180 (fast string search)
1181 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1183 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1185 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1187 E PERL_MAGIC_env vtbl_env %ENV hash
1188 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1189 f PERL_MAGIC_fm vtbl_regexp Formline
1191 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1192 H PERL_MAGIC_hints vtbl_hints %^H hash
1193 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1194 I PERL_MAGIC_isa vtbl_isa @ISA array
1195 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1196 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1197 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1198 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1200 N PERL_MAGIC_shared (none) Shared between threads
1201 n PERL_MAGIC_shared_scalar (none) Shared between threads
1202 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1203 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1204 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1205 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1206 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
1207 S PERL_MAGIC_sig (none) %SIG hash
1208 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1209 t PERL_MAGIC_taint vtbl_taint Taintedness
1210 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1212 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1214 V PERL_MAGIC_vstring (none) SV was vstring literal
1215 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1216 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1217 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1218 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1219 variable / smart parameter
1221 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference in list
1223 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
1225 ~ PERL_MAGIC_ext (none) Available for use by
1228 =for mg_vtable.pl end
1230 When an uppercase and lowercase letter both exist in the table, then the
1231 uppercase letter is typically used to represent some kind of composite type
1232 (a list or a hash), and the lowercase letter is used to represent an element
1233 of that composite type. Some internals code makes use of this case
1234 relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
1236 The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1237 specifically for use by extensions and will not be used by perl itself.
1238 Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1239 to variables (typically objects). This is especially useful because
1240 there is no way for normal perl code to corrupt this private information
1241 (unlike using extra elements of a hash object).
1243 Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1244 C function any time a scalar's value is used or changed. The C<MAGIC>'s
1245 C<mg_ptr> field points to a C<ufuncs> structure:
1248 I32 (*uf_val)(pTHX_ IV, SV*);
1249 I32 (*uf_set)(pTHX_ IV, SV*);
1253 When the SV is read from or written to, the C<uf_val> or C<uf_set>
1254 function will be called with C<uf_index> as the first arg and a pointer to
1255 the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1256 magic is shown below. Note that the ufuncs structure is copied by
1257 sv_magic, so you can safely allocate it on the stack.
1265 uf.uf_val = &my_get_fn;
1266 uf.uf_set = &my_set_fn;
1268 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1270 Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1272 For hashes there is a specialized hook that gives control over hash
1273 keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1274 if the "set" function in the C<ufuncs> structure is NULL. The hook
1275 is activated whenever the hash is accessed with a key specified as
1276 an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1277 C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1278 through the functions without the C<..._ent> suffix circumvents the
1279 hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1281 Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1282 or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1283 extra care to avoid conflict. Typically only using the magic on
1284 objects blessed into the same class as the extension is sufficient.
1285 For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1286 C<MGVTBL>, even if all its fields will be C<0>, so that individual
1287 C<MAGIC> pointers can be identified as a particular kind of magic
1288 using their magic virtual table. C<mg_findext> provides an easy way
1291 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1294 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1295 /* this is really ours, not another module's PERL_MAGIC_ext */
1296 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1300 Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1301 earlier do B<not> invoke 'set' magic on their targets. This must
1302 be done by the user either by calling the C<SvSETMAGIC()> macro after
1303 calling these functions, or by using one of the C<sv_set*_mg()> or
1304 C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1305 C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1306 obtained from external sources in functions that don't handle magic.
1307 See L<perlapi> for a description of these functions.
1308 For example, calls to the C<sv_cat*()> functions typically need to be
1309 followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1310 since their implementation handles 'get' magic.
1312 =head2 Finding Magic
1314 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1317 This routine returns a pointer to a C<MAGIC> structure stored in the SV.
1318 If the SV does not have that magical
1319 feature, C<NULL> is returned. If the
1320 SV has multiple instances of that magical feature, the first one will be
1321 returned. C<mg_findext> can be used
1322 to find a C<MAGIC> structure of an SV
1323 based on both its magic type and its magic virtual table:
1325 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
1327 Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1328 SVt_PVMG, Perl may core dump.
1330 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1332 This routine checks to see what types of magic C<sv> has. If the mg_type
1333 field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1334 the mg_type field is changed to be the lowercase letter.
1336 =head2 Understanding the Magic of Tied Hashes and Arrays
1338 Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1341 WARNING: As of the 5.004 release, proper usage of the array and hash
1342 access functions requires understanding a few caveats. Some
1343 of these caveats are actually considered bugs in the API, to be fixed
1344 in later releases, and are bracketed with [MAYCHANGE] below. If
1345 you find yourself actually applying such information in this section, be
1346 aware that the behavior may change in the future, umm, without warning.
1348 The perl tie function associates a variable with an object that implements
1349 the various GET, SET, etc methods. To perform the equivalent of the perl
1350 tie function from an XSUB, you must mimic this behaviour. The code below
1351 carries out the necessary steps - firstly it creates a new hash, and then
1352 creates a second hash which it blesses into the class which will implement
1353 the tie methods. Lastly it ties the two hashes together, and returns a
1354 reference to the new tied hash. Note that the code below does NOT call the
1355 TIEHASH method in the MyTie class -
1356 see L<Calling Perl Routines from within C Programs> for details on how
1367 tie = newRV_noinc((SV*)newHV());
1368 stash = gv_stashpv("MyTie", GV_ADD);
1369 sv_bless(tie, stash);
1370 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1371 RETVAL = newRV_noinc(hash);
1375 The C<av_store> function, when given a tied array argument, merely
1376 copies the magic of the array onto the value to be "stored", using
1377 C<mg_copy>. It may also return NULL, indicating that the value did not
1378 actually need to be stored in the array. [MAYCHANGE] After a call to
1379 C<av_store> on a tied array, the caller will usually need to call
1380 C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1381 TIEARRAY object. If C<av_store> did return NULL, a call to
1382 C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1385 The previous paragraph is applicable verbatim to tied hash access using the
1386 C<hv_store> and C<hv_store_ent> functions as well.
1388 C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1389 C<hv_fetch_ent> actually return an undefined mortal value whose magic
1390 has been initialized using C<mg_copy>. Note the value so returned does not
1391 need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1392 need to call C<mg_get()> on the returned value in order to actually invoke
1393 the perl level "FETCH" method on the underlying TIE object. Similarly,
1394 you may also call C<mg_set()> on the return value after possibly assigning
1395 a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
1396 method on the TIE object. [/MAYCHANGE]
1399 In other words, the array or hash fetch/store functions don't really
1400 fetch and store actual values in the case of tied arrays and hashes. They
1401 merely call C<mg_copy> to attach magic to the values that were meant to be
1402 "stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1403 do the job of invoking the TIE methods on the underlying objects. Thus
1404 the magic mechanism currently implements a kind of lazy access to arrays
1407 Currently (as of perl version 5.004), use of the hash and array access
1408 functions requires the user to be aware of whether they are operating on
1409 "normal" hashes and arrays, or on their tied variants. The API may be
1410 changed to provide more transparent access to both tied and normal data
1411 types in future versions.
1414 You would do well to understand that the TIEARRAY and TIEHASH interfaces
1415 are mere sugar to invoke some perl method calls while using the uniform hash
1416 and array syntax. The use of this sugar imposes some overhead (typically
1417 about two to four extra opcodes per FETCH/STORE operation, in addition to
1418 the creation of all the mortal variables required to invoke the methods).
1419 This overhead will be comparatively small if the TIE methods are themselves
1420 substantial, but if they are only a few statements long, the overhead
1421 will not be insignificant.
1423 =head2 Localizing changes
1425 Perl has a very handy construction
1432 This construction is I<approximately> equivalent to
1441 The biggest difference is that the first construction would
1442 reinstate the initial value of $var, irrespective of how control exits
1443 the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
1444 more efficient as well.
1446 There is a way to achieve a similar task from C via Perl API: create a
1447 I<pseudo-block>, and arrange for some changes to be automatically
1448 undone at the end of it, either explicit, or via a non-local exit (via
1449 die()). A I<block>-like construct is created by a pair of
1450 C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1451 Such a construct may be created specially for some important localized
1452 task, or an existing one (like boundaries of enclosing Perl
1453 subroutine/block, or an existing pair for freeing TMPs) may be
1454 used. (In the second case the overhead of additional localization must
1455 be almost negligible.) Note that any XSUB is automatically enclosed in
1456 an C<ENTER>/C<LEAVE> pair.
1458 Inside such a I<pseudo-block> the following service is available:
1462 =item C<SAVEINT(int i)>
1464 =item C<SAVEIV(IV i)>
1466 =item C<SAVEI32(I32 i)>
1468 =item C<SAVELONG(long i)>
1470 These macros arrange things to restore the value of integer variable
1471 C<i> at the end of enclosing I<pseudo-block>.
1473 =item C<SAVESPTR(s)>
1475 =item C<SAVEPPTR(p)>
1477 These macros arrange things to restore the value of pointers C<s> and
1478 C<p>. C<s> must be a pointer of a type which survives conversion to
1479 C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1482 =item C<SAVEFREESV(SV *sv)>
1484 The refcount of C<sv> would be decremented at the end of
1485 I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1486 mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1487 extends the lifetime of C<sv> until the beginning of the next statement,
1488 C<SAVEFREESV> extends it until the end of the enclosing scope. These
1489 lifetimes can be wildly different.
1491 Also compare C<SAVEMORTALIZESV>.
1493 =item C<SAVEMORTALIZESV(SV *sv)>
1495 Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1496 scope instead of decrementing its reference count. This usually has the
1497 effect of keeping C<sv> alive until the statement that called the currently
1498 live scope has finished executing.
1500 =item C<SAVEFREEOP(OP *op)>
1502 The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1504 =item C<SAVEFREEPV(p)>
1506 The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1507 end of I<pseudo-block>.
1509 =item C<SAVECLEARSV(SV *sv)>
1511 Clears a slot in the current scratchpad which corresponds to C<sv> at
1512 the end of I<pseudo-block>.
1514 =item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1516 The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1517 string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1518 short-lived storage, the corresponding string may be reallocated like
1521 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1523 =item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
1525 At the end of I<pseudo-block> the function C<f> is called with the
1528 =item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1530 At the end of I<pseudo-block> the function C<f> is called with the
1531 implicit context argument (if any), and C<p>.
1533 =item C<SAVESTACK_POS()>
1535 The current offset on the Perl internal stack (cf. C<SP>) is restored
1536 at the end of I<pseudo-block>.
1540 The following API list contains functions, thus one needs to
1541 provide pointers to the modifiable data explicitly (either C pointers,
1542 or Perlish C<GV *>s). Where the above macros take C<int>, a similar
1543 function takes C<int *>.
1547 =item C<SV* save_scalar(GV *gv)>
1549 Equivalent to Perl code C<local $gv>.
1551 =item C<AV* save_ary(GV *gv)>
1553 =item C<HV* save_hash(GV *gv)>
1555 Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1557 =item C<void save_item(SV *item)>
1559 Duplicates the current value of C<SV>, on the exit from the current
1560 C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1561 using the stored value. It doesn't handle magic. Use C<save_scalar> if
1564 =item C<void save_list(SV **sarg, I32 maxsarg)>
1566 A variant of C<save_item> which takes multiple arguments via an array
1567 C<sarg> of C<SV*> of length C<maxsarg>.
1569 =item C<SV* save_svref(SV **sptr)>
1571 Similar to C<save_scalar>, but will reinstate an C<SV *>.
1573 =item C<void save_aptr(AV **aptr)>
1575 =item C<void save_hptr(HV **hptr)>
1577 Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1581 The C<Alias> module implements localization of the basic types within the
1582 I<caller's scope>. People who are interested in how to localize things in
1583 the containing scope should take a look there too.
1587 =head2 XSUBs and the Argument Stack
1589 The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1590 An XSUB routine will have a stack that contains the arguments from the Perl
1591 program, and a way to map from the Perl data structures to a C equivalent.
1593 The stack arguments are accessible through the C<ST(n)> macro, which returns
1594 the C<n>'th stack argument. Argument 0 is the first argument passed in the
1595 Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1598 Most of the time, output from the C routine can be handled through use of
1599 the RETVAL and OUTPUT directives. However, there are some cases where the
1600 argument stack is not already long enough to handle all the return values.
1601 An example is the POSIX tzname() call, which takes no arguments, but returns
1602 two, the local time zone's standard and summer time abbreviations.
1604 To handle this situation, the PPCODE directive is used and the stack is
1605 extended using the macro:
1609 where C<SP> is the macro that represents the local copy of the stack pointer,
1610 and C<num> is the number of elements the stack should be extended by.
1612 Now that there is room on the stack, values can be pushed on it using C<PUSHs>
1613 macro. The pushed values will often need to be "mortal" (See
1614 L</Reference Counts and Mortality>):
1616 PUSHs(sv_2mortal(newSViv(an_integer)))
1617 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1618 PUSHs(sv_2mortal(newSVnv(a_double)))
1619 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1620 /* Although the last example is better written as the more
1622 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
1624 And now the Perl program calling C<tzname>, the two values will be assigned
1627 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1629 An alternate (and possibly simpler) method to pushing values on the stack is
1634 This macro automatically adjusts the stack for you, if needed. Thus, you
1635 do not need to call C<EXTEND> to extend the stack.
1637 Despite their suggestions in earlier versions of this document the macros
1638 C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1639 For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1640 C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
1642 For more information, consult L<perlxs> and L<perlxstut>.
1644 =head2 Autoloading with XSUBs
1646 If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1647 fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1648 of the XSUB's package.
1650 But it also puts the same information in certain fields of the XSUB itself:
1652 HV *stash = CvSTASH(cv);
1653 const char *subname = SvPVX(cv);
1654 STRLEN name_length = SvCUR(cv); /* in bytes */
1655 U32 is_utf8 = SvUTF8(cv);
1657 C<SvPVX(cv)> contains just the sub name itself, not including the package.
1658 For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1659 C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
1661 B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1662 XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1663 XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1664 to support 5.8-5.14, use the XSUB's fields.
1666 =head2 Calling Perl Routines from within C Programs
1668 There are four routines that can be used to call a Perl subroutine from
1669 within a C program. These four are:
1671 I32 call_sv(SV*, I32);
1672 I32 call_pv(const char*, I32);
1673 I32 call_method(const char*, I32);
1674 I32 call_argv(const char*, I32, char**);
1676 The routine most often used is C<call_sv>. The C<SV*> argument
1677 contains either the name of the Perl subroutine to be called, or a
1678 reference to the subroutine. The second argument consists of flags
1679 that control the context in which the subroutine is called, whether
1680 or not the subroutine is being passed arguments, how errors should be
1681 trapped, and how to treat return values.
1683 All four routines return the number of arguments that the subroutine returned
1686 These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
1687 but those names are now deprecated; macros of the same name are provided for
1690 When using any of these routines (except C<call_argv>), the programmer
1691 must manipulate the Perl stack. These include the following macros and
1706 For a detailed description of calling conventions from C to Perl,
1707 consult L<perlcall>.
1709 =head2 Putting a C value on Perl stack
1711 A lot of opcodes (this is an elementary operation in the internal perl
1712 stack machine) put an SV* on the stack. However, as an optimization
1713 the corresponding SV is (usually) not recreated each time. The opcodes
1714 reuse specially assigned SVs (I<target>s) which are (as a corollary)
1715 not constantly freed/created.
1717 Each of the targets is created only once (but see
1718 L<Scratchpads and recursion> below), and when an opcode needs to put
1719 an integer, a double, or a string on stack, it just sets the
1720 corresponding parts of its I<target> and puts the I<target> on stack.
1722 The macro to put this target on stack is C<PUSHTARG>, and it is
1723 directly used in some opcodes, as well as indirectly in zillions of
1724 others, which use it via C<(X)PUSH[iunp]>.
1726 Because the target is reused, you must be careful when pushing multiple
1727 values on the stack. The following code will not do what you think:
1732 This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1733 the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1734 At the end of the operation, the stack does not contain the values 10
1735 and 20, but actually contains two pointers to C<TARG>, which we have set
1738 If you need to push multiple different values then you should either use
1739 the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1740 none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1741 SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1742 will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1743 this a little easier to achieve by creating a new mortal for you (via
1744 C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1745 in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1746 Thus, instead of writing this to "fix" the example above:
1748 XPUSHs(sv_2mortal(newSViv(10)))
1749 XPUSHs(sv_2mortal(newSViv(20)))
1751 you can simply write:
1756 On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1757 need a C<dTARG> in your variable declarations so that the C<*PUSH*>
1758 macros can make use of the local variable C<TARG>. See also C<dTARGET>
1763 The question remains on when the SVs which are I<target>s for opcodes
1764 are created. The answer is that they are created when the current
1765 unit--a subroutine or a file (for opcodes for statements outside of
1766 subroutines)--is compiled. During this time a special anonymous Perl
1767 array is created, which is called a scratchpad for the current unit.
1769 A scratchpad keeps SVs which are lexicals for the current unit and are
1770 targets for opcodes. A previous version of this document
1771 stated that one can deduce that an SV lives on a scratchpad
1772 by looking on its flags: lexicals have C<SVs_PADMY> set, and
1773 I<target>s have C<SVs_PADTMP> set. But this have never been fully true.
1774 C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1775 While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
1776 that have never resided in a pad, but nonetheless act like I<target>s.
1778 The correspondence between OPs and I<target>s is not 1-to-1. Different
1779 OPs in the compile tree of the unit can use the same target, if this
1780 would not conflict with the expected life of the temporary.
1782 =head2 Scratchpads and recursion
1784 In fact it is not 100% true that a compiled unit contains a pointer to
1785 the scratchpad AV. In fact it contains a pointer to an AV of
1786 (initially) one element, and this element is the scratchpad AV. Why do
1787 we need an extra level of indirection?
1789 The answer is B<recursion>, and maybe B<threads>. Both
1790 these can create several execution pointers going into the same
1791 subroutine. For the subroutine-child not write over the temporaries
1792 for the subroutine-parent (lifespan of which covers the call to the
1793 child), the parent and the child should have different
1794 scratchpads. (I<And> the lexicals should be separate anyway!)
1796 So each subroutine is born with an array of scratchpads (of length 1).
1797 On each entry to the subroutine it is checked that the current
1798 depth of the recursion is not more than the length of this array, and
1799 if it is, new scratchpad is created and pushed into the array.
1801 The I<target>s on this scratchpad are C<undef>s, but they are already
1802 marked with correct flags.
1804 =head1 Memory Allocation
1808 All memory meant to be used with the Perl API functions should be manipulated
1809 using the macros described in this section. The macros provide the necessary
1810 transparency between differences in the actual malloc implementation that is
1813 It is suggested that you enable the version of malloc that is distributed
1814 with Perl. It keeps pools of various sizes of unallocated memory in
1815 order to satisfy allocation requests more quickly. However, on some
1816 platforms, it may cause spurious malloc or free errors.
1818 The following three macros are used to initially allocate memory :
1820 Newx(pointer, number, type);
1821 Newxc(pointer, number, type, cast);
1822 Newxz(pointer, number, type);
1824 The first argument C<pointer> should be the name of a variable that will
1825 point to the newly allocated memory.
1827 The second and third arguments C<number> and C<type> specify how many of
1828 the specified type of data structure should be allocated. The argument
1829 C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1830 should be used if the C<pointer> argument is different from the C<type>
1833 Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1834 to zero out all the newly allocated memory.
1838 Renew(pointer, number, type);
1839 Renewc(pointer, number, type, cast);
1842 These three macros are used to change a memory buffer size or to free a
1843 piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1844 match those of C<New> and C<Newc> with the exception of not needing the
1845 "magic cookie" argument.
1849 Move(source, dest, number, type);
1850 Copy(source, dest, number, type);
1851 Zero(dest, number, type);
1853 These three macros are used to move, copy, or zero out previously allocated
1854 memory. The C<source> and C<dest> arguments point to the source and
1855 destination starting points. Perl will move, copy, or zero out C<number>
1856 instances of the size of the C<type> data structure (using the C<sizeof>
1861 The most recent development releases of Perl have been experimenting with
1862 removing Perl's dependency on the "normal" standard I/O suite and allowing
1863 other stdio implementations to be used. This involves creating a new
1864 abstraction layer that then calls whichever implementation of stdio Perl
1865 was compiled with. All XSUBs should now use the functions in the PerlIO
1866 abstraction layer and not make any assumptions about what kind of stdio
1869 For a complete description of the PerlIO abstraction, consult L<perlapio>.
1871 =head1 Compiled code
1875 Here we describe the internal form your code is converted to by
1876 Perl. Start with a simple example:
1880 This is converted to a tree similar to this one:
1888 (but slightly more complicated). This tree reflects the way Perl
1889 parsed your code, but has nothing to do with the execution order.
1890 There is an additional "thread" going through the nodes of the tree
1891 which shows the order of execution of the nodes. In our simplified
1892 example above it looks like:
1894 $b ---> $c ---> + ---> $a ---> assign-to
1896 But with the actual compile tree for C<$a = $b + $c> it is different:
1897 some nodes I<optimized away>. As a corollary, though the actual tree
1898 contains more nodes than our simplified example, the execution order
1899 is the same as in our example.
1901 =head2 Examining the tree
1903 If you have your perl compiled for debugging (usually done with
1904 C<-DDEBUGGING> on the C<Configure> command line), you may examine the
1905 compiled tree by specifying C<-Dx> on the Perl command line. The
1906 output takes several lines per node, and for C<$b+$c> it looks like
1911 FLAGS = (SCALAR,KIDS)
1913 TYPE = null ===> (4)
1915 FLAGS = (SCALAR,KIDS)
1917 3 TYPE = gvsv ===> 4
1923 TYPE = null ===> (5)
1925 FLAGS = (SCALAR,KIDS)
1927 4 TYPE = gvsv ===> 5
1933 This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1934 not optimized away (one per number in the left column). The immediate
1935 children of the given node correspond to C<{}> pairs on the same level
1936 of indentation, thus this listing corresponds to the tree:
1944 The execution order is indicated by C<===E<gt>> marks, thus it is C<3
1945 4 5 6> (node C<6> is not included into above listing), i.e.,
1946 C<gvsv gvsv add whatever>.
1948 Each of these nodes represents an op, a fundamental operation inside the
1949 Perl core. The code which implements each operation can be found in the
1950 F<pp*.c> files; the function which implements the op with type C<gvsv>
1951 is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1952 different numbers of children: C<add> is a binary operator, as one would
1953 expect, and so has two children. To accommodate the various different
1954 numbers of children, there are various types of op data structure, and
1955 they link together in different ways.
1957 The simplest type of op structure is C<OP>: this has no children. Unary
1958 operators, C<UNOP>s, have one child, and this is pointed to by the
1959 C<op_first> field. Binary operators (C<BINOP>s) have not only an
1960 C<op_first> field but also an C<op_last> field. The most complex type of
1961 op is a C<LISTOP>, which has any number of children. In this case, the
1962 first child is pointed to by C<op_first> and the last child by
1963 C<op_last>. The children in between can be found by iteratively
1964 following the C<op_sibling> pointer from the first child to the last 9but
1967 There are also some other op types: a C<PMOP> holds a regular expression,
1968 and has no children, and a C<LOOP> may or may not have children. If the
1969 C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1970 complicate matters, if a C<UNOP> is actually a C<null> op after
1971 optimization (see L</Compile pass 2: context propagation>) it will still
1972 have children in accordance with its former type.
1974 Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
1975 or more children, but it doesn't have an C<op_last> field: so you have to
1976 follow C<op_first> and then the C<op_sibling> chain itself to find the
1977 last child. Instead it has an C<op_other> field, which is comparable to
1978 the C<op_next> field described below, and represents an alternate
1979 execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
1980 that in general, C<op_other> may not point to any of the direct children
1983 Starting in version 5.21.2, perls built with the experimental
1984 define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
1985 C<op_lastsib>. When set, this indicates that this is the last op in an
1986 C<op_sibling> chain. This frees up the C<op_sibling> field on the last
1987 sibling to point back to the parent op. The macro C<OP_SIBLING(o)> wraps
1988 this special behaviour, and always returns NULL on the last sibling.
1989 With this build the C<op_parent(o)> function can be used to find the
1992 Another way to examine the tree is to use a compiler back-end module, such
1995 =head2 Compile pass 1: check routines
1997 The tree is created by the compiler while I<yacc> code feeds it
1998 the constructions it recognizes. Since I<yacc> works bottom-up, so does
1999 the first pass of perl compilation.
2001 What makes this pass interesting for perl developers is that some
2002 optimization may be performed on this pass. This is optimization by
2003 so-called "check routines". The correspondence between node names
2004 and corresponding check routines is described in F<opcode.pl> (do not
2005 forget to run C<make regen_headers> if you modify this file).
2007 A check routine is called when the node is fully constructed except
2008 for the execution-order thread. Since at this time there are no
2009 back-links to the currently constructed node, one can do most any
2010 operation to the top-level node, including freeing it and/or creating
2011 new nodes above/below it.
2013 The check routine returns the node which should be inserted into the
2014 tree (if the top-level node was not modified, check routine returns
2017 By convention, check routines have names C<ck_*>. They are usually
2018 called from C<new*OP> subroutines (or C<convert>) (which in turn are
2019 called from F<perly.y>).
2021 =head2 Compile pass 1a: constant folding
2023 Immediately after the check routine is called the returned node is
2024 checked for being compile-time executable. If it is (the value is
2025 judged to be constant) it is immediately executed, and a I<constant>
2026 node with the "return value" of the corresponding subtree is
2027 substituted instead. The subtree is deleted.
2029 If constant folding was not performed, the execution-order thread is
2032 =head2 Compile pass 2: context propagation
2034 When a context for a part of compile tree is known, it is propagated
2035 down through the tree. At this time the context can have 5 values
2036 (instead of 2 for runtime context): void, boolean, scalar, list, and
2037 lvalue. In contrast with the pass 1 this pass is processed from top
2038 to bottom: a node's context determines the context for its children.
2040 Additional context-dependent optimizations are performed at this time.
2041 Since at this moment the compile tree contains back-references (via
2042 "thread" pointers), nodes cannot be free()d now. To allow
2043 optimized-away nodes at this stage, such nodes are null()ified instead
2044 of free()ing (i.e. their type is changed to OP_NULL).
2046 =head2 Compile pass 3: peephole optimization
2048 After the compile tree for a subroutine (or for an C<eval> or a file)
2049 is created, an additional pass over the code is performed. This pass
2050 is neither top-down or bottom-up, but in the execution order (with
2051 additional complications for conditionals). Optimizations performed
2052 at this stage are subject to the same restrictions as in the pass 2.
2054 Peephole optimizations are done by calling the function pointed to
2055 by the global variable C<PL_peepp>. By default, C<PL_peepp> just
2056 calls the function pointed to by the global variable C<PL_rpeepp>.
2057 By default, that performs some basic op fixups and optimisations along
2058 the execution-order op chain, and recursively calls C<PL_rpeepp> for
2059 each side chain of ops (resulting from conditionals). Extensions may
2060 provide additional optimisations or fixups, hooking into either the
2061 per-subroutine or recursive stage, like this:
2063 static peep_t prev_peepp;
2064 static void my_peep(pTHX_ OP *o)
2066 /* custom per-subroutine optimisation goes here */
2067 prev_peepp(aTHX_ o);
2068 /* custom per-subroutine optimisation may also go here */
2071 prev_peepp = PL_peepp;
2074 static peep_t prev_rpeepp;
2075 static void my_rpeep(pTHX_ OP *o)
2078 for(; o; o = o->op_next) {
2079 /* custom per-op optimisation goes here */
2081 prev_rpeepp(aTHX_ orig_o);
2084 prev_rpeepp = PL_rpeepp;
2085 PL_rpeepp = my_rpeep;
2087 =head2 Pluggable runops
2089 The compile tree is executed in a runops function. There are two runops
2090 functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2091 with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2092 control over the execution of the compile tree it is possible to provide
2093 your own runops function.
2095 It's probably best to copy one of the existing runops functions and
2096 change it to suit your needs. Then, in the BOOT section of your XS
2099 PL_runops = my_runops;
2101 This function should be as efficient as possible to keep your programs
2102 running as fast as possible.
2104 =head2 Compile-time scope hooks
2106 As of perl 5.14 it is possible to hook into the compile-time lexical
2107 scope mechanism using C<Perl_blockhook_register>. This is used like
2110 STATIC void my_start_hook(pTHX_ int full);
2111 STATIC BHK my_hooks;
2114 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
2115 Perl_blockhook_register(aTHX_ &my_hooks);
2117 This will arrange to have C<my_start_hook> called at the start of
2118 compiling every lexical scope. The available hooks are:
2122 =item C<void bhk_start(pTHX_ int full)>
2124 This is called just after starting a new lexical scope. Note that Perl
2129 creates two scopes: the first starts at the C<(> and has C<full == 1>,
2130 the second starts at the C<{> and has C<full == 0>. Both end at the
2131 C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
2132 pushed onto the save stack by this hook will be popped just before the
2133 scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2135 =item C<void bhk_pre_end(pTHX_ OP **o)>
2137 This is called at the end of a lexical scope, just before unwinding the
2138 stack. I<o> is the root of the optree representing the scope; it is a
2139 double pointer so you can replace the OP if you need to.
2141 =item C<void bhk_post_end(pTHX_ OP **o)>
2143 This is called at the end of a lexical scope, just after unwinding the
2144 stack. I<o> is as above. Note that it is possible for calls to C<pre_>
2145 and C<post_end> to nest, if there is something on the save stack that
2148 =item C<void bhk_eval(pTHX_ OP *const o)>
2150 This is called just before starting to compile an C<eval STRING>, C<do
2151 FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
2152 OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2153 C<OP_DOFILE> or C<OP_REQUIRE>.
2157 Once you have your hook functions, you need a C<BHK> structure to put
2158 them in. It's best to allocate it statically, since there is no way to
2159 free it once it's registered. The function pointers should be inserted
2160 into this structure using the C<BhkENTRY_set> macro, which will also set
2161 flags indicating which entries are valid. If you do need to allocate
2162 your C<BHK> dynamically for some reason, be sure to zero it before you
2165 Once registered, there is no mechanism to switch these hooks off, so if
2166 that is necessary you will need to do this yourself. An entry in C<%^H>
2167 is probably the best way, so the effect is lexically scoped; however it
2168 is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
2169 temporarily switch entries on and off. You should also be aware that
2170 generally speaking at least one scope will have opened before your
2171 extension is loaded, so you will see some C<pre/post_end> pairs that
2172 didn't have a matching C<start>.
2174 =head1 Examining internal data structures with the C<dump> functions
2176 To aid debugging, the source file F<dump.c> contains a number of
2177 functions which produce formatted output of internal data structures.
2179 The most commonly used of these functions is C<Perl_sv_dump>; it's used
2180 for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
2181 C<sv_dump> to produce debugging output from Perl-space, so users of that
2182 module should already be familiar with its format.
2184 C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
2185 derivatives, and produces output similar to C<perl -Dx>; in fact,
2186 C<Perl_dump_eval> will dump the main root of the code being evaluated,
2187 exactly like C<-Dx>.
2189 Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2190 op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2191 subroutines in a package like so: (Thankfully, these are all xsubs, so
2192 there is no op tree)
2194 (gdb) print Perl_dump_packsubs(PL_defstash)
2196 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2198 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2200 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2202 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2204 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2206 and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2207 the op tree of the main root.
2209 =head1 How multiple interpreters and concurrency are supported
2211 =head2 Background and PERL_IMPLICIT_CONTEXT
2213 The Perl interpreter can be regarded as a closed box: it has an API
2214 for feeding it code or otherwise making it do things, but it also has
2215 functions for its own use. This smells a lot like an object, and
2216 there are ways for you to build Perl so that you can have multiple
2217 interpreters, with one interpreter represented either as a C structure,
2218 or inside a thread-specific structure. These structures contain all
2219 the context, the state of that interpreter.
2221 One macro controls the major Perl build flavor: MULTIPLICITY. The
2222 MULTIPLICITY build has a C structure that packages all the interpreter
2223 state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
2224 normally defined, and enables the support for passing in a "hidden" first
2225 argument that represents all three data structures. MULTIPLICITY makes
2226 multi-threaded perls possible (with the ithreads threading model, related
2227 to the macro USE_ITHREADS.)
2229 Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2230 PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2231 former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2232 internal variables of Perl to be wrapped inside a single global struct,
2233 struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2234 the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2235 one step further, there is still a single struct (allocated in main()
2236 either from heap or from stack) but there are no global data symbols
2237 pointing to it. In either case the global struct should be initialized
2238 as the very first thing in main() using Perl_init_global_struct() and
2239 correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2240 please see F<miniperlmain.c> for usage details. You may also need
2241 to use C<dVAR> in your coding to "declare the global variables"
2242 when you are using them. dTHX does this for you automatically.
2244 To see whether you have non-const data you can use a BSD (or GNU)
2247 nm libperl.a | grep -v ' [TURtr] '
2249 If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
2250 you have non-const data. The symbols the C<grep> removed are as follows:
2251 C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
2252 and the C<U> is <undefined>, external symbols referred to.
2254 The test F<t/porting/libperl.t> does this kind of symbol sanity
2255 checking on C<libperl.a>.
2257 For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2258 doesn't actually hide all symbols inside a big global struct: some
2259 PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2260 then hides everything (see how the PERLIO_FUNCS_DECL is used).
2262 All this obviously requires a way for the Perl internal functions to be
2263 either subroutines taking some kind of structure as the first
2264 argument, or subroutines taking nothing as the first argument. To
2265 enable these two very different ways of building the interpreter,
2266 the Perl source (as it does in so many other situations) makes heavy
2267 use of macros and subroutine naming conventions.
2269 First problem: deciding which functions will be public API functions and
2270 which will be private. All functions whose names begin C<S_> are private
2271 (think "S" for "secret" or "static"). All other functions begin with
2272 "Perl_", but just because a function begins with "Perl_" does not mean it is
2273 part of the API. (See L</Internal
2274 Functions>.) The easiest way to be B<sure> a
2275 function is part of the API is to find its entry in L<perlapi>.
2276 If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2277 think it should be (i.e., you need it for your extension), send mail via
2278 L<perlbug> explaining why you think it should be.
2280 Second problem: there must be a syntax so that the same subroutine
2281 declarations and calls can pass a structure as their first argument,
2282 or pass nothing. To solve this, the subroutines are named and
2283 declared in a particular way. Here's a typical start of a static
2284 function used within the Perl guts:
2287 S_incline(pTHX_ char *s)
2289 STATIC becomes "static" in C, and may be #define'd to nothing in some
2290 configurations in the future.
2292 A public function (i.e. part of the internal API, but not necessarily
2293 sanctioned for use in extensions) begins like this:
2296 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
2298 C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
2299 details of the interpreter's context. THX stands for "thread", "this",
2300 or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2301 The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
2302 or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2305 When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2306 first argument containing the interpreter's context. The trailing underscore
2307 in the pTHX_ macro indicates that the macro expansion needs a comma
2308 after the context argument because other arguments follow it. If
2309 PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
2310 subroutine is not prototyped to take the extra argument. The form of the
2311 macro without the trailing underscore is used when there are no additional
2314 When a core function calls another, it must pass the context. This
2315 is normally hidden via macros. Consider C<sv_setiv>. It expands into
2316 something like this:
2318 #ifdef PERL_IMPLICIT_CONTEXT
2319 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
2320 /* can't do this for vararg functions, see below */
2322 #define sv_setiv Perl_sv_setiv
2325 This works well, and means that XS authors can gleefully write:
2329 and still have it work under all the modes Perl could have been
2332 This doesn't work so cleanly for varargs functions, though, as macros
2333 imply that the number of arguments is known in advance. Instead we
2334 either need to spell them out fully, passing C<aTHX_> as the first
2335 argument (the Perl core tends to do this with functions like
2336 Perl_warner), or use a context-free version.
2338 The context-free version of Perl_warner is called
2339 Perl_warner_nocontext, and does not take the extra argument. Instead
2340 it does dTHX; to get the context from thread-local storage. We
2341 C<#define warner Perl_warner_nocontext> so that extensions get source
2342 compatibility at the expense of performance. (Passing an arg is
2343 cheaper than grabbing it from thread-local storage.)
2345 You can ignore [pad]THXx when browsing the Perl headers/sources.
2346 Those are strictly for use within the core. Extensions and embedders
2347 need only be aware of [pad]THX.
2349 =head2 So what happened to dTHR?
2351 C<dTHR> was introduced in perl 5.005 to support the older thread model.
2352 The older thread model now uses the C<THX> mechanism to pass context
2353 pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2354 later still have it for backward source compatibility, but it is defined
2357 =head2 How do I use all this in extensions?
2359 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2360 any functions in the Perl API will need to pass the initial context
2361 argument somehow. The kicker is that you will need to write it in
2362 such a way that the extension still compiles when Perl hasn't been
2363 built with PERL_IMPLICIT_CONTEXT enabled.
2365 There are three ways to do this. First, the easy but inefficient way,
2366 which is also the default, in order to maintain source compatibility
2367 with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
2368 and aTHX_ macros to call a function that will return the context.
2369 Thus, something like:
2373 in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
2376 Perl_sv_setiv(Perl_get_context(), sv, num);
2378 or to this otherwise:
2380 Perl_sv_setiv(sv, num);
2382 You don't have to do anything new in your extension to get this; since
2383 the Perl library provides Perl_get_context(), it will all just
2386 The second, more efficient way is to use the following template for
2389 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2394 STATIC void my_private_function(int arg1, int arg2);
2397 my_private_function(int arg1, int arg2)
2399 dTHX; /* fetch context */
2400 ... call many Perl API functions ...
2405 MODULE = Foo PACKAGE = Foo
2413 my_private_function(arg, 10);
2415 Note that the only two changes from the normal way of writing an
2416 extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2417 including the Perl headers, followed by a C<dTHX;> declaration at
2418 the start of every function that will call the Perl API. (You'll
2419 know which functions need this, because the C compiler will complain
2420 that there's an undeclared identifier in those functions.) No changes
2421 are needed for the XSUBs themselves, because the XS() macro is
2422 correctly defined to pass in the implicit context if needed.
2424 The third, even more efficient way is to ape how it is done within
2428 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2433 /* pTHX_ only needed for functions that call Perl API */
2434 STATIC void my_private_function(pTHX_ int arg1, int arg2);
2437 my_private_function(pTHX_ int arg1, int arg2)
2439 /* dTHX; not needed here, because THX is an argument */
2440 ... call Perl API functions ...
2445 MODULE = Foo PACKAGE = Foo
2453 my_private_function(aTHX_ arg, 10);
2455 This implementation never has to fetch the context using a function
2456 call, since it is always passed as an extra argument. Depending on
2457 your needs for simplicity or efficiency, you may mix the previous
2458 two approaches freely.
2460 Never add a comma after C<pTHX> yourself--always use the form of the
2461 macro with the underscore for functions that take explicit arguments,
2462 or the form without the argument for functions with no explicit arguments.
2464 If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2465 definition is needed if the Perl global variables (see F<perlvars.h>
2466 or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2467 used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2468 the need for C<dVAR> only with the said compile-time define, because
2469 otherwise the Perl global variables are visible as-is.
2471 =head2 Should I do anything special if I call perl from multiple threads?
2473 If you create interpreters in one thread and then proceed to call them in
2474 another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2475 initialized correctly in each of those threads.
2477 The C<perl_alloc> and C<perl_clone> API functions will automatically set
2478 the TLS slot to the interpreter they created, so that there is no need to do
2479 anything special if the interpreter is always accessed in the same thread that
2480 created it, and that thread did not create or call any other interpreters
2481 afterwards. If that is not the case, you have to set the TLS slot of the
2482 thread before calling any functions in the Perl API on that particular
2483 interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2484 thread as the first thing you do:
2486 /* do this before doing anything else with some_perl */
2487 PERL_SET_CONTEXT(some_perl);
2489 ... other Perl API calls on some_perl go here ...
2491 =head2 Future Plans and PERL_IMPLICIT_SYS
2493 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2494 that the interpreter knows about itself and pass it around, so too are
2495 there plans to allow the interpreter to bundle up everything it knows
2496 about the environment it's running on. This is enabled with the
2497 PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2500 This allows the ability to provide an extra pointer (called the "host"
2501 environment) for all the system calls. This makes it possible for
2502 all the system stuff to maintain their own state, broken down into
2503 seven C structures. These are thin wrappers around the usual system
2504 calls (see F<win32/perllib.c>) for the default perl executable, but for a
2505 more ambitious host (like the one that would do fork() emulation) all
2506 the extra work needed to pretend that different interpreters are
2507 actually different "processes", would be done here.
2509 The Perl engine/interpreter and the host are orthogonal entities.
2510 There could be one or more interpreters in a process, and one or
2511 more "hosts", with free association between them.
2513 =head1 Internal Functions
2515 All of Perl's internal functions which will be exposed to the outside
2516 world are prefixed by C<Perl_> so that they will not conflict with XS
2517 functions or functions used in a program in which Perl is embedded.
2518 Similarly, all global variables begin with C<PL_>. (By convention,
2519 static functions start with C<S_>.)
2521 Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2522 either with or without the C<Perl_> prefix, thanks to a bunch of defines
2523 that live in F<embed.h>. Note that extension code should I<not> set
2524 C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2525 breakage of the XS in each new perl release.
2527 The file F<embed.h> is generated automatically from
2528 F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2529 header files for the internal functions, generates the documentation
2530 and a lot of other bits and pieces. It's important that when you add
2531 a new function to the core or change an existing one, you change the
2532 data in the table in F<embed.fnc> as well. Here's a sample entry from
2535 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2537 The second column is the return type, the third column the name. Columns
2538 after that are the arguments. The first column is a set of flags:
2544 This function is a part of the public
2545 API. All such functions should also
2546 have 'd', very few do not.
2550 This function has a C<Perl_> prefix; i.e. it is defined as
2555 This function has documentation using the C<apidoc> feature which we'll
2556 look at in a second. Some functions have 'd' but not 'A'; docs are good.
2560 Other available flags are:
2566 This is a static function and is defined as C<STATIC S_whatever>, and
2567 usually called within the sources as C<whatever(...)>.
2571 This does not need an interpreter context, so the definition has no
2572 C<pTHX>, and it follows that callers don't use C<aTHX>. (See
2573 L</Background and PERL_IMPLICIT_CONTEXT>.)
2577 This function never returns; C<croak>, C<exit> and friends.
2581 This function takes a variable number of arguments, C<printf> style.
2582 The argument list should end with C<...>, like this:
2584 Afprd |void |croak |const char* pat|...
2588 This function is part of the experimental development API, and may change
2589 or disappear without notice.
2593 This function should not have a compatibility macro to define, say,
2594 C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2598 This function isn't exported out of the Perl core.
2602 This is implemented as a macro.
2606 This function is explicitly exported.
2610 This function is visible to extensions included in the Perl core.
2614 Binary backward compatibility; this function is a macro but also has
2615 a C<Perl_> implementation (which is exported).
2619 See the comments at the top of C<embed.fnc> for others.
2623 If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2624 C<make regen_headers> to force a rebuild of F<embed.h> and other
2625 auto-generated files.
2627 =head2 Formatted Printing of IVs, UVs, and NVs
2629 If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2630 formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2631 following macros for portability
2636 UVxf UV in hexadecimal
2641 These will take care of 64-bit integers and long doubles.
2644 printf("IV is %"IVdf"\n", iv);
2646 The IVdf will expand to whatever is the correct format for the IVs.
2648 Note that there are different "long doubles": Perl will use
2649 whatever the compiler has.
2651 If you are printing addresses of pointers, use UVxf combined
2652 with PTR2UV(), do not use %lx or %p.
2654 =head2 Pointer-To-Integer and Integer-To-Pointer
2656 Because pointer size does not necessarily equal integer size,
2657 use the follow macros to do it right.
2662 INT2PTR(pointertotype, integer)
2667 SV *sv = INT2PTR(SV*, iv);
2674 =head2 Exception Handling
2676 There are a couple of macros to do very basic exception handling in XS
2677 modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2678 be able to use these macros:
2683 You can use these macros if you call code that may croak, but you need
2684 to do some cleanup before giving control back to Perl. For example:
2686 dXCPT; /* set up necessary variables */
2689 code_that_may_croak();
2694 /* do cleanup here */
2698 Note that you always have to rethrow an exception that has been
2699 caught. Using these macros, it is not possible to just catch the
2700 exception and ignore it. If you have to ignore the exception, you
2701 have to use the C<call_*> function.
2703 The advantage of using the above macros is that you don't have
2704 to setup an extra function for C<call_*>, and that using these
2705 macros is faster than using C<call_*>.
2707 =head2 Source Documentation
2709 There's an effort going on to document the internal functions and
2710 automatically produce reference manuals from them - L<perlapi> is one
2711 such manual which details all the functions which are available to XS
2712 writers. L<perlintern> is the autogenerated manual for the functions
2713 which are not part of the API and are supposedly for internal use only.
2715 Source documentation is created by putting POD comments into the C
2719 =for apidoc sv_setiv
2721 Copies an integer into the given SV. Does not handle 'set' magic. See
2727 Please try and supply some documentation if you add functions to the
2730 =head2 Backwards compatibility
2732 The Perl API changes over time. New functions are
2733 added or the interfaces of existing functions are
2734 changed. The C<Devel::PPPort> module tries to
2735 provide compatibility code for some of these changes, so XS writers don't
2736 have to code it themselves when supporting multiple versions of Perl.
2738 C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2739 be run as a Perl script. To generate F<ppport.h>, run:
2741 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2743 Besides checking existing XS code, the script can also be used to retrieve
2744 compatibility information for various API calls using the C<--api-info>
2745 command line switch. For example:
2747 % perl ppport.h --api-info=sv_magicext
2749 For details, see C<perldoc ppport.h>.
2751 =head1 Unicode Support
2753 Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2754 writers to understand this support and make sure that the code they
2755 write does not corrupt Unicode data.
2757 =head2 What B<is> Unicode, anyway?
2759 In the olden, less enlightened times, we all used to use ASCII. Most of
2760 us did, anyway. The big problem with ASCII is that it's American. Well,
2761 no, that's not actually the problem; the problem is that it's not
2762 particularly useful for people who don't use the Roman alphabet. What
2763 used to happen was that particular languages would stick their own
2764 alphabet in the upper range of the sequence, between 128 and 255. Of
2765 course, we then ended up with plenty of variants that weren't quite
2766 ASCII, and the whole point of it being a standard was lost.
2768 Worse still, if you've got a language like Chinese or
2769 Japanese that has hundreds or thousands of characters, then you really
2770 can't fit them into a mere 256, so they had to forget about ASCII
2771 altogether, and build their own systems using pairs of numbers to refer
2774 To fix this, some people formed Unicode, Inc. and
2775 produced a new character set containing all the characters you can
2776 possibly think of and more. There are several ways of representing these
2777 characters, and the one Perl uses is called UTF-8. UTF-8 uses
2778 a variable number of bytes to represent a character. You can learn more
2779 about Unicode and Perl's Unicode model in L<perlunicode>.
2781 =head2 How can I recognise a UTF-8 string?
2783 You can't. This is because UTF-8 data is stored in bytes just like
2784 non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
2785 capital E with a grave accent, is represented by the two bytes
2786 C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2787 has that byte sequence as well. So you can't tell just by looking - this
2788 is what makes Unicode input an interesting problem.
2790 In general, you either have to know what you're dealing with, or you
2791 have to guess. The API function C<is_utf8_string> can help; it'll tell
2792 you if a string contains only valid UTF-8 characters. However, it can't
2793 do the work for you. On a character-by-character basis,
2795 will tell you whether the current character in a string is valid UTF-8.
2797 =head2 How does UTF-8 represent Unicode characters?
2799 As mentioned above, UTF-8 uses a variable number of bytes to store a
2800 character. Characters with values 0...127 are stored in one
2801 byte, just like good ol' ASCII. Character 128 is stored as
2802 C<v194.128>; this continues up to character 191, which is
2803 C<v194.191>. Now we've run out of bits (191 is binary
2804 C<10111111>) so we move on; 192 is C<v195.128>. And
2805 so it goes on, moving to three bytes at character 2048.
2807 Assuming you know you're dealing with a UTF-8 string, you can find out
2808 how long the first character in it is with the C<UTF8SKIP> macro:
2810 char *utf = "\305\233\340\240\201";
2813 len = UTF8SKIP(utf); /* len is 2 here */
2815 len = UTF8SKIP(utf); /* len is 3 here */
2817 Another way to skip over characters in a UTF-8 string is to use
2818 C<utf8_hop>, which takes a string and a number of characters to skip
2819 over. You're on your own about bounds checking, though, so don't use it
2822 All bytes in a multi-byte UTF-8 character will have the high bit set,
2823 so you can test if you need to do something special with this
2824 character like this (the UTF8_IS_INVARIANT() is a macro that tests
2825 whether the byte is encoded as a single byte even in UTF-8):
2828 U8 *utf_end; /* 1 beyond buffer pointed to by utf */
2829 UV uv; /* Note: a UV, not a U8, not a char */
2830 STRLEN len; /* length of character in bytes */
2832 if (!UTF8_IS_INVARIANT(*utf))
2833 /* Must treat this as UTF-8 */
2834 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
2836 /* OK to treat this character as a byte */
2839 You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
2840 value of the character; the inverse function C<uvchr_to_utf8> is available
2841 for putting a UV into UTF-8:
2843 if (!UTF8_IS_INVARIANT(uv))
2844 /* Must treat this as UTF8 */
2845 utf8 = uvchr_to_utf8(utf8, uv);
2847 /* OK to treat this character as a byte */
2850 You B<must> convert characters to UVs using the above functions if
2851 you're ever in a situation where you have to match UTF-8 and non-UTF-8
2852 characters. You may not skip over UTF-8 characters in this case. If you
2853 do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2854 for instance, if your UTF-8 string contains C<v196.172>, and you skip
2855 that character, you can never match a C<chr(200)> in a non-UTF-8 string.
2858 =head2 How does Perl store UTF-8 strings?
2860 Currently, Perl deals with Unicode strings and non-Unicode strings
2861 slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2862 string is internally encoded as UTF-8. Without it, the byte value is the
2863 codepoint number and vice versa (in other words, the string is encoded
2864 as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
2865 semantics). This flag is only meaningful if the SV is C<SvPOK>
2866 or immediately after stringification via C<SvPV> or a similar
2867 macro. You can check and manipulate this flag with the
2874 This flag has an important effect on Perl's treatment of the string: if
2875 Unicode data is not properly distinguished, regular expressions,
2876 C<length>, C<substr> and other string handling operations will have
2877 undesirable results.
2879 The problem comes when you have, for instance, a string that isn't
2880 flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
2881 especially when combining non-UTF-8 and UTF-8 strings.
2883 Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2884 need be sure you don't accidentally knock it off while you're
2885 manipulating SVs. More specifically, you cannot expect to do this:
2894 nsv = newSVpvn(p, len);
2896 The C<char*> string does not tell you the whole story, and you can't
2897 copy or reconstruct an SV just by copying the string value. Check if the
2898 old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2903 nsv = newSVpvn(p, len);
2907 In fact, your C<frobnicate> function should be made aware of whether or
2908 not it's dealing with UTF-8 data, so that it can handle the string
2911 Since just passing an SV to an XS function and copying the data of
2912 the SV is not enough to copy the UTF8 flags, even less right is just
2913 passing a C<char *> to an XS function.
2915 =head2 How do I convert a string to UTF-8?
2917 If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
2918 one of the strings to UTF-8. If you've got an SV, the easiest way to do
2921 sv_utf8_upgrade(sv);
2923 However, you must not do this, for example:
2926 sv_utf8_upgrade(left);
2928 If you do this in a binary operator, you will actually change one of the
2929 strings that came into the operator, and, while it shouldn't be noticeable
2930 by the end user, it can cause problems in deficient code.
2932 Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
2933 string argument. This is useful for having the data available for
2934 comparisons and so on, without harming the original SV. There's also
2935 C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2936 the string contains any characters above 255 that can't be represented
2939 =head2 Is there anything else I need to know?
2941 Not really. Just remember these things:
2947 There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2948 is UTF-8 by looking at its C<SvUTF8> flag after stringifying it
2949 with C<SvPV> or a similar macro. Don't forget to set the flag if
2950 something should be UTF-8. Treat the flag as part of the PV, even though
2951 it's not - if you pass on the PV to somewhere, pass on the flag too.
2955 If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
2956 unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
2960 When writing a character C<uv> to a UTF-8 string, B<always> use
2961 C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2962 you can use C<*s = uv>.
2966 Mixing UTF-8 and non-UTF-8 strings is
2967 tricky. Use C<bytes_to_utf8> to get
2968 a new string which is UTF-8 encoded, and then combine them.
2972 =head1 Custom Operators
2974 Custom operator support is an experimental feature that allows you to
2975 define your own ops. This is primarily to allow the building of
2976 interpreters for other languages in the Perl core, but it also allows
2977 optimizations through the creation of "macro-ops" (ops which perform the
2978 functions of multiple ops which are usually executed together, such as
2979 C<gvsv, gvsv, add>.)
2981 This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
2982 core does not "know" anything special about this op type, and so it will
2983 not be involved in any optimizations. This also means that you can
2984 define your custom ops to be any op structure - unary, binary, list and
2987 It's important to know what custom operators won't do for you. They
2988 won't let you add new syntax to Perl, directly. They won't even let you
2989 add new keywords, directly. In fact, they won't change the way Perl
2990 compiles a program at all. You have to do those changes yourself, after
2991 Perl has compiled the program. You do this either by manipulating the op
2992 tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2993 a custom peephole optimizer with the C<optimize> module.
2995 When you do this, you replace ordinary Perl ops with custom ops by
2996 creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
2997 PP function. This should be defined in XS code, and should look like
2998 the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2999 takes the appropriate number of values from the stack, and you are
3000 responsible for adding stack marks if necessary.
3002 You should also "register" your op with the Perl interpreter so that it
3003 can produce sensible error and warning messages. Since it is possible to
3004 have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
3005 Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
3006 it is dealing with. You should create an C<XOP> structure for each
3007 ppaddr you use, set the properties of the custom op with
3008 C<XopENTRY_set>, and register the structure against the ppaddr using
3009 C<Perl_custom_op_register>. A trivial example might look like:
3012 static OP *my_pp(pTHX);
3015 XopENTRY_set(&my_xop, xop_name, "myxop");
3016 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3017 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3019 The available fields in the structure are:
3025 A short name for your op. This will be included in some error messages,
3026 and will also be returned as C<< $op->name >> by the L<B|B> module, so
3027 it will appear in the output of module like L<B::Concise|B::Concise>.
3031 A short description of the function of the op.
3035 Which of the various C<*OP> structures this op uses. This should be one of
3036 the C<OA_*> constants from F<op.h>, namely
3056 =item OA_PVOP_OR_SVOP
3058 This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
3059 the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
3067 The other C<OA_*> constants should not be used.
3071 This member is of type C<Perl_cpeep_t>, which expands to C<void
3072 (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
3073 will be called from C<Perl_rpeep> when ops of this type are encountered
3074 by the peephole optimizer. I<o> is the OP that needs optimizing;
3075 I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
3079 C<B::Generate> directly supports the creation of custom ops by name.
3083 Until May 1997, this document was maintained by Jeff Okamoto
3084 E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
3085 itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
3087 With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3088 Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3089 Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3090 Stephen McCamant, and Gurusamy Sarathy.
3094 L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>
https://perl5.git.perl.org / perl5.git / blob