1PERLGUTS(1) Perl Programmers Reference Guide PERLGUTS(1)
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6 perlguts - Introduction to the Perl API
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9 This document attempts to describe how to use the Perl API, as well as
10 to provide some info on the basic workings of the Perl core. It is far
11 from complete and probably contains many errors. Please refer any
12 questions or comments to the author below.
13
15 Datatypes
16 Perl has three typedefs that handle Perl's three main data types:
17 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21 Each typedef has specific routines that manipulate the various data
23 types.
24 What is an "IV"?
26 Perl uses a special typedef IV which is a simple signed integer type
27 that is guaranteed to be large enough to hold a pointer (as well as an
28 integer). Additionally, there is the UV, which is simply an unsigned
29 IV.
30 Perl also uses several special typedefs to declare variables to hold
32 integers of (at least) a given size. Use I8, I16, I32, and I64 to
33 declare a signed integer variable which has at least as many bits as
34 the number in its name. These all evaluate to the native C type that
35 is closest to the given number of bits, but no smaller than that
36 number. For example, on many platforms, a "short" is 16 bits long, and
37 if so, I16 will evaluate to a "short". But on platforms where a
38 "short" isn't exactly 16 bits, Perl will use the smallest type that
39 contains 16 bits or more.
40 U8, U16, U32, and U64 are to declare the corresponding unsigned integer
42 types.
43 If the platform doesn't support 64-bit integers, both I64 and U64 will
45 be undefined. Use IV and UV to declare the largest practicable, and
46 ""WIDEST_UTYPE" in perlapi" for the absolute maximum unsigned, but
47 which may not be usable in all circumstances.
48 A numeric constant can be specified with ""INT16_C"" in perlapi,
50 ""UINTMAX_C"" in perlapi, and similar.
51 Working with SVs
53 An SV can be created and loaded with one command. There are five types
54 of values that can be loaded: an integer value (IV), an unsigned
55 integer value (UV), a double (NV), a string (PV), and another scalar
56 (SV). ("PV" stands for "Pointer Value". You might think that it is
57 misnamed because it is described as pointing only to strings. However,
58 it is possible to have it point to other things. For example, it could
59 point to an array of UVs. But, using it for non-strings requires care,
60 as the underlying assumption of much of the internals is that PVs are
61 just for strings. Often, for example, a trailing "NUL" is tacked on
62 automatically. The non-string use is documented only in this
63 paragraph.)
64 The seven routines are:
66 SV* newSViv(IV);
68 SV* newSVuv(UV);
69 SV* newSVnv(double);
70 SV* newSVpv(const char*, STRLEN);
71 SV* newSVpvn(const char*, STRLEN);
72 SV* newSVpvf(const char*, ...);
73 SV* newSVsv(SV*);
74 "STRLEN" is an integer type ("Size_t", usually defined as "size_t" in
76 config.h) guaranteed to be large enough to represent the size of any
77 string that perl can handle.
78 In the unlikely case of a SV requiring more complex initialization, you
80 can create an empty SV with newSV(len). If "len" is 0 an empty SV of
81 type NULL is returned, else an SV of type PV is returned with len + 1
82 (for the "NUL") bytes of storage allocated, accessible via SvPVX. In
83 both cases the SV has the undef value.
84 SV *sv = newSV(0); /* no storage allocated */
86 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
87 * allocated */
88 To change the value of an already-existing SV, there are eight
90 routines:
91 void sv_setiv(SV*, IV);
93 void sv_setuv(SV*, UV);
94 void sv_setnv(SV*, double);
95 void sv_setpv(SV*, const char*);
96 void sv_setpvn(SV*, const char*, STRLEN)
97 void sv_setpvf(SV*, const char*, ...);
98 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
99 SV **, Size_t, bool *);
100 void sv_setsv(SV*, SV*);
101 Notice that you can choose to specify the length of the string to be
103 assigned by using "sv_setpvn", "newSVpvn", or "newSVpv", or you may
104 allow Perl to calculate the length by using "sv_setpv" or by specifying
105 0 as the second argument to "newSVpv". Be warned, though, that Perl
106 will determine the string's length by using "strlen", which depends on
107 the string terminating with a "NUL" character, and not otherwise
108 containing NULs.
109 The arguments of "sv_setpvf" are processed like "sprintf", and the
111 formatted output becomes the value.
112 "sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to
114 specify either a pointer to a variable argument list or the address and
115 length of an array of SVs. The last argument points to a boolean; on
116 return, if that boolean is true, then locale-specific information has
117 been used to format the string, and the string's contents are therefore
118 untrustworthy (see perlsec). This pointer may be NULL if that
119 information is not important. Note that this function requires you to
120 specify the length of the format.
121 The "sv_set*()" functions are not generic enough to operate on values
123 that have "magic". See "Magic Virtual Tables" later in this document.
124 All SVs that contain strings should be terminated with a "NUL"
126 character. If it is not "NUL"-terminated there is a risk of core dumps
127 and corruptions from code which passes the string to C functions or
128 system calls which expect a "NUL"-terminated string. Perl's own
129 functions typically add a trailing "NUL" for this reason.
130 Nevertheless, you should be very careful when you pass a string stored
131 in an SV to a C function or system call.
132 To access the actual value that an SV points to, Perl's API exposes
134 several macros that coerce the actual scalar type into an IV, UV,
135 double, or string:
136 • "SvIV(SV*)" ("IV") and "SvUV(SV*)" ("UV")
138 • "SvNV(SV*)" ("double")
140 • Strings are a bit complicated:
142 • Byte string: "SvPVbyte(SV*, STRLEN len)" or
144 "SvPVbyte_nolen(SV*)"
145 If the Perl string is "\xff\xff", then this returns a 2-byte
147 "char*".
148 This is suitable for Perl strings that represent bytes.
150 • UTF-8 string: "SvPVutf8(SV*, STRLEN len)" or
152 "SvPVutf8_nolen(SV*)"
153 If the Perl string is "\xff\xff", then this returns a 4-byte
155 "char*".
156 This is suitable for Perl strings that represent characters.
158 CAVEAT: That "char*" will be encoded via Perl's internal UTF-8
160 variant, which means that if the SV contains non-Unicode code
161 points (e.g., 0x110000), then the result may contain extensions
162 over valid UTF-8. See "is_strict_utf8_string" in perlapi for
163 some methods Perl gives you to check the UTF-8 validity of
164 these macros' returns.
165 • You can also use "SvPV(SV*, STRLEN len)" or "SvPV_nolen(SV*)"
167 to fetch the SV's raw internal buffer. This is tricky, though;
168 if your Perl string is "\xff\xff", then depending on the SV's
169 internal encoding you might get back a 2-byte OR a 4-byte
170 "char*". Moreover, if it's the 4-byte string, that could come
171 from either Perl "\xff\xff" stored UTF-8 encoded, or Perl
172 "\xc3\xbf\xc3\xbf" stored as raw octets. To differentiate
173 between these you MUST look up the SV's UTF8 bit (cf. "SvUTF8")
174 to know whether the source Perl string is 2 characters
175 ("SvUTF8" would be on) or 4 characters ("SvUTF8" would be off).
176 IMPORTANT: Use of "SvPV", "SvPV_nolen", or similarly-named
178 macros without looking up the SV's UTF8 bit is almost certainly
179 a bug if non-ASCII input is allowed.
180 When the UTF8 bit is on, the same CAVEAT about UTF-8 validity
182 applies here as for "SvPVutf8".
183 (See "How do I pass a Perl string to a C library?" for more
185 details.)
186 In "SvPVbyte", "SvPVutf8", and "SvPV", the length of the "char*"
188 returned is placed into the variable "len" (these are macros, so
189 you do not use &len). If you do not care what the length of the
190 data is, use "SvPVbyte_nolen", "SvPVutf8_nolen", or "SvPV_nolen"
191 instead. The global variable "PL_na" can also be given to
192 "SvPVbyte"/"SvPVutf8"/"SvPV" in this case. But that can be quite
193 inefficient because "PL_na" must be accessed in thread-local
194 storage in threaded Perl. In any case, remember that Perl allows
195 arbitrary strings of data that may both contain NULs and might not
196 be terminated by a "NUL".
197 Also remember that C doesn't allow you to safely say
199 "foo(SvPVbyte(s, len), len);". It might work with your compiler,
200 but it won't work for everyone. Break this sort of statement up
201 into separate assignments:
202 SV *s;
204 STRLEN len;
205 char *ptr;
206 ptr = SvPVbyte(s, len);
207 foo(ptr, len);
208 If you want to know if the scalar value is TRUE, you can use:
210 SvTRUE(SV*)
212 Although Perl will automatically grow strings for you, if you need to
214 force Perl to allocate more memory for your SV, you can use the macro
215 SvGROW(SV*, STRLEN newlen)
217 which will determine if more memory needs to be allocated. If so, it
219 will call the function "sv_grow". Note that "SvGROW" can only
220 increase, not decrease, the allocated memory of an SV and that it does
221 not automatically add space for the trailing "NUL" byte (perl's own
222 string functions typically do "SvGROW(sv, len + 1)").
223 If you want to write to an existing SV's buffer and set its value to a
225 string, use SvPVbyte_force() or one of its variants to force the SV to
226 be a PV. This will remove any of various types of non-stringness from
227 the SV while preserving the content of the SV in the PV. This can be
228 used, for example, to append data from an API function to a buffer
229 without extra copying:
230 (void)SvPVbyte_force(sv, len);
232 s = SvGROW(sv, len + needlen + 1);
233 /* something that modifies up to needlen bytes at s+len, but
234 modifies newlen bytes
235 eg. newlen = read(fd, s + len, needlen);
236 ignoring errors for these examples
237 */
238 s[len + newlen] = '\0';
239 SvCUR_set(sv, len + newlen);
240 SvUTF8_off(sv);
241 SvSETMAGIC(sv);
242 If you already have the data in memory or if you want to keep your code
244 simple, you can use one of the sv_cat*() variants, such as sv_catpvn().
245 If you want to insert anywhere in the string you can use sv_insert() or
246 sv_insert_flags().
247 If you don't need the existing content of the SV, you can avoid some
249 copying with:
250 SvPVCLEAR(sv);
252 s = SvGROW(sv, needlen + 1);
253 /* something that modifies up to needlen bytes at s, but modifies
254 newlen bytes
255 eg. newlen = read(fd, s, needlen);
256 */
257 s[newlen] = '\0';
258 SvCUR_set(sv, newlen);
259 SvPOK_only(sv); /* also clears SVf_UTF8 */
260 SvSETMAGIC(sv);
261 Again, if you already have the data in memory or want to avoid the
263 complexity of the above, you can use sv_setpvn().
264 If you have a buffer allocated with Newx() and want to set that as the
266 SV's value, you can use sv_usepvn_flags(). That has some requirements
267 if you want to avoid perl re-allocating the buffer to fit the trailing
268 NUL:
269 Newx(buf, somesize+1, char);
271 /* ... fill in buf ... */
272 buf[somesize] = '\0';
273 sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
274 /* buf now belongs to perl, don't release it */
275 If you have an SV and want to know what kind of data Perl thinks is
277 stored in it, you can use the following macros to check the type of SV
278 you have.
279 SvIOK(SV*)
281 SvNOK(SV*)
282 SvPOK(SV*)
283 You can get and set the current length of the string stored in an SV
285 with the following macros:
286 SvCUR(SV*)
288 SvCUR_set(SV*, I32 val)
289 You can also get a pointer to the end of the string stored in the SV
291 with the macro:
292 SvEND(SV*)
294 But note that these last three macros are valid only if "SvPOK()" is
296 true.
297 If you want to append something to the end of string stored in an
299 "SV*", you can use the following functions:
300 void sv_catpv(SV*, const char*);
302 void sv_catpvn(SV*, const char*, STRLEN);
303 void sv_catpvf(SV*, const char*, ...);
304 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
305 I32, bool);
306 void sv_catsv(SV*, SV*);
307 The first function calculates the length of the string to be appended
309 by using "strlen". In the second, you specify the length of the string
310 yourself. The third function processes its arguments like "sprintf"
311 and appends the formatted output. The fourth function works like
312 "vsprintf". You can specify the address and length of an array of SVs
313 instead of the va_list argument. The fifth function extends the string
314 stored in the first SV with the string stored in the second SV. It
315 also forces the second SV to be interpreted as a string.
316 The "sv_cat*()" functions are not generic enough to operate on values
318 that have "magic". See "Magic Virtual Tables" later in this document.
319 If you know the name of a scalar variable, you can get a pointer to its
321 SV by using the following:
322 SV* get_sv("package::varname", 0);
324 This returns NULL if the variable does not exist.
326 If you want to know if this variable (or any other SV) is actually
328 "defined", you can call:
329 SvOK(SV*)
331 The scalar "undef" value is stored in an SV instance called
333 "PL_sv_undef".
334 Its address can be used whenever an "SV*" is needed. Make sure that
336 you don't try to compare a random sv with &PL_sv_undef. For example
337 when interfacing Perl code, it'll work correctly for:
338 foo(undef);
340 But won't work when called as:
342 $x = undef;
344 foo($x);
345 So to repeat always use SvOK() to check whether an sv is defined.
347 Also you have to be careful when using &PL_sv_undef as a value in AVs
349 or HVs (see "AVs, HVs and undefined values").
350 There are also the two values "PL_sv_yes" and "PL_sv_no", which contain
352 boolean TRUE and FALSE values, respectively. Like "PL_sv_undef", their
353 addresses can be used whenever an "SV*" is needed.
354 Do not be fooled into thinking that "(SV *) 0" is the same as
356 &PL_sv_undef. Take this code:
357 SV* sv = (SV*) 0;
359 if (I-am-to-return-a-real-value) {
360 sv = sv_2mortal(newSViv(42));
361 }
362 sv_setsv(ST(0), sv);
363 This code tries to return a new SV (which contains the value 42) if it
365 should return a real value, or undef otherwise. Instead it has
366 returned a NULL pointer which, somewhere down the line, will cause a
367 segmentation violation, bus error, or just weird results. Change the
368 zero to &PL_sv_undef in the first line and all will be well.
369 To free an SV that you've created, call "SvREFCNT_dec(SV*)". Normally
371 this call is not necessary (see "Reference Counts and Mortality").
372 Offsets
374 Perl provides the function "sv_chop" to efficiently remove characters
375 from the beginning of a string; you give it an SV and a pointer to
376 somewhere inside the PV, and it discards everything before the pointer.
377 The efficiency comes by means of a little hack: instead of actually
378 removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
379 signal to other functions that the offset hack is in effect, and it
380 moves the PV pointer (called "SvPVX") forward by the number of bytes
381 chopped off, and adjusts "SvCUR" and "SvLEN" accordingly. (A portion
382 of the space between the old and new PV pointers is used to store the
383 count of chopped bytes.)
384 Hence, at this point, the start of the buffer that we allocated lives
386 at "SvPVX(sv) - SvIV(sv)" in memory and the PV pointer is pointing into
387 the middle of this allocated storage.
388 This is best demonstrated by example. Normally copy-on-write will
390 prevent the substitution from operator from using this hack, but if you
391 can craft a string for which copy-on-write is not possible, you can see
392 it in play. In the current implementation, the final byte of a string
393 buffer is used as a copy-on-write reference count. If the buffer is
394 not big enough, then copy-on-write is skipped. First have a look at an
395 empty string:
396 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
398 SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
399 REFCNT = 1
400 FLAGS = (POK,pPOK)
401 PV = 0x7ffb7bc05b50 ""\0
402 CUR = 0
403 LEN = 10
404 Notice here the LEN is 10. (It may differ on your platform.) Extend
406 the length of the string to one less than 10, and do a substitution:
407 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
409 Dump($a)'
410 SV = PV(0x7ffa04008a70) at 0x7ffa04030390
411 REFCNT = 1
412 FLAGS = (POK,OOK,pPOK)
413 OFFSET = 1
414 PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
415 CUR = 8
416 LEN = 9
417 Here the number of bytes chopped off (1) is shown next as the OFFSET.
419 The portion of the string between the "real" and the "fake" beginnings
420 is shown in parentheses, and the values of "SvCUR" and "SvLEN" reflect
421 the fake beginning, not the real one. (The first character of the
422 string buffer happens to have changed to "\1" here, not "1", because
423 the current implementation stores the offset count in the string
424 buffer. This is subject to change.)
425 Something similar to the offset hack is performed on AVs to enable
427 efficient shifting and splicing off the beginning of the array; while
428 "AvARRAY" points to the first element in the array that is visible from
429 Perl, "AvALLOC" points to the real start of the C array. These are
430 usually the same, but a "shift" operation can be carried out by
431 increasing "AvARRAY" by one and decreasing "AvFILL" and "AvMAX".
432 Again, the location of the real start of the C array only comes into
433 play when freeing the array. See "av_shift" in av.c.
434 What's Really Stored in an SV?
436 Recall that the usual method of determining the type of scalar you have
437 is to use "Sv*OK" macros. Because a scalar can be both a number and a
438 string, usually these macros will always return TRUE and calling the
439 "Sv*V" macros will do the appropriate conversion of string to
440 integer/double or integer/double to string.
441 If you really need to know if you have an integer, double, or string
443 pointer in an SV, you can use the following three macros instead:
444 SvIOKp(SV*)
446 SvNOKp(SV*)
447 SvPOKp(SV*)
448 These will tell you if you truly have an integer, double, or string
450 pointer stored in your SV. The "p" stands for private.
451 There are various ways in which the private and public flags may
453 differ. For example, in perl 5.16 and earlier a tied SV may have a
454 valid underlying value in the IV slot (so SvIOKp is true), but the data
455 should be accessed via the FETCH routine rather than directly, so SvIOK
456 is false. (In perl 5.18 onwards, tied scalars use the flags the same
457 way as untied scalars.) Another is when numeric conversion has
458 occurred and precision has been lost: only the private flag is set on
459 'lossy' values. So when an NV is converted to an IV with loss, SvIOKp,
460 SvNOKp and SvNOK will be set, while SvIOK wont be.
461 In general, though, it's best to use the "Sv*V" macros.
463 Working with AVs
465 There are two ways to create and load an AV. The first method creates
466 an empty AV:
467 AV* newAV();
469 The second method both creates the AV and initially populates it with
471 SVs:
472 AV* av_make(SSize_t num, SV **ptr);
474 The second argument points to an array containing "num" "SV*"'s. Once
476 the AV has been created, the SVs can be destroyed, if so desired.
477 Once the AV has been created, the following operations are possible on
479 it:
480 void av_push(AV*, SV*);
482 SV* av_pop(AV*);
483 SV* av_shift(AV*);
484 void av_unshift(AV*, SSize_t num);
485 These should be familiar operations, with the exception of
487 "av_unshift". This routine adds "num" elements at the front of the
488 array with the "undef" value. You must then use "av_store" (described
489 below) to assign values to these new elements.
490 Here are some other functions:
492 SSize_t av_top_index(AV*);
494 SV** av_fetch(AV*, SSize_t key, I32 lval);
495 SV** av_store(AV*, SSize_t key, SV* val);
496 The "av_top_index" function returns the highest index value in an array
498 (just like $#array in Perl). If the array is empty, -1 is returned.
499 The "av_fetch" function returns the value at index "key", but if "lval"
500 is non-zero, then "av_fetch" will store an undef value at that index.
501 The "av_store" function stores the value "val" at index "key", and does
502 not increment the reference count of "val". Thus the caller is
503 responsible for taking care of that, and if "av_store" returns NULL,
504 the caller will have to decrement the reference count to avoid a memory
505 leak. Note that "av_fetch" and "av_store" both return "SV**"'s, not
506 "SV*"'s as their return value.
507 A few more:
509 void av_clear(AV*);
511 void av_undef(AV*);
512 void av_extend(AV*, SSize_t key);
513 The "av_clear" function deletes all the elements in the AV* array, but
515 does not actually delete the array itself. The "av_undef" function
516 will delete all the elements in the array plus the array itself. The
517 "av_extend" function extends the array so that it contains at least
518 "key+1" elements. If "key+1" is less than the currently allocated
519 length of the array, then nothing is done.
520 If you know the name of an array variable, you can get a pointer to its
522 AV by using the following:
523 AV* get_av("package::varname", 0);
525 This returns NULL if the variable does not exist.
527 See "Understanding the Magic of Tied Hashes and Arrays" for more
529 information on how to use the array access functions on tied arrays.
530 Working with HVs
532 To create an HV, you use the following routine:
533 HV* newHV();
535 Once the HV has been created, the following operations are possible on
537 it:
538 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
540 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
541 The "klen" parameter is the length of the key being passed in (Note
543 that you cannot pass 0 in as a value of "klen" to tell Perl to measure
544 the length of the key). The "val" argument contains the SV pointer to
545 the scalar being stored, and "hash" is the precomputed hash value (zero
546 if you want "hv_store" to calculate it for you). The "lval" parameter
547 indicates whether this fetch is actually a part of a store operation,
548 in which case a new undefined value will be added to the HV with the
549 supplied key and "hv_fetch" will return as if the value had already
550 existed.
551 Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just
553 "SV*". To access the scalar value, you must first dereference the
554 return value. However, you should check to make sure that the return
555 value is not NULL before dereferencing it.
556 The first of these two functions checks if a hash table entry exists,
558 and the second deletes it.
559 bool hv_exists(HV*, const char* key, U32 klen);
561 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
562 If "flags" does not include the "G_DISCARD" flag then "hv_delete" will
564 create and return a mortal copy of the deleted value.
565 And more miscellaneous functions:
567 void hv_clear(HV*);
569 void hv_undef(HV*);
570 Like their AV counterparts, "hv_clear" deletes all the entries in the
572 hash table but does not actually delete the hash table. The "hv_undef"
573 deletes both the entries and the hash table itself.
574 Perl keeps the actual data in a linked list of structures with a
576 typedef of HE. These contain the actual key and value pointers (plus
577 extra administrative overhead). The key is a string pointer; the value
578 is an "SV*". However, once you have an "HE*", to get the actual key
579 and value, use the routines specified below.
580 I32 hv_iterinit(HV*);
582 /* Prepares starting point to traverse hash table */
583 HE* hv_iternext(HV*);
584 /* Get the next entry, and return a pointer to a
585 structure that has both the key and value */
586 char* hv_iterkey(HE* entry, I32* retlen);
587 /* Get the key from an HE structure and also return
588 the length of the key string */
589 SV* hv_iterval(HV*, HE* entry);
590 /* Return an SV pointer to the value of the HE
591 structure */
592 SV* hv_iternextsv(HV*, char** key, I32* retlen);
593 /* This convenience routine combines hv_iternext,
594 hv_iterkey, and hv_iterval. The key and retlen
595 arguments are return values for the key and its
596 length. The value is returned in the SV* argument */
597 If you know the name of a hash variable, you can get a pointer to its
599 HV by using the following:
600 HV* get_hv("package::varname", 0);
602 This returns NULL if the variable does not exist.
604 The hash algorithm is defined in the "PERL_HASH" macro:
606 PERL_HASH(hash, key, klen)
608 The exact implementation of this macro varies by architecture and
610 version of perl, and the return value may change per invocation, so the
611 value is only valid for the duration of a single perl process.
612 See "Understanding the Magic of Tied Hashes and Arrays" for more
614 information on how to use the hash access functions on tied hashes.
615 Hash API Extensions
617 Beginning with version 5.004, the following functions are also
618 supported:
619 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
621 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
622 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
624 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
625 SV* hv_iterkeysv (HE* entry);
627 Note that these functions take "SV*" keys, which simplifies writing of
629 extension code that deals with hash structures. These functions also
630 allow passing of "SV*" keys to "tie" functions without forcing you to
631 stringify the keys (unlike the previous set of functions).
632 They also return and accept whole hash entries ("HE*"), making their
634 use more efficient (since the hash number for a particular string
635 doesn't have to be recomputed every time). See perlapi for detailed
636 descriptions.
637 The following macros must always be used to access the contents of hash
639 entries. Note that the arguments to these macros must be simple
640 variables, since they may get evaluated more than once. See perlapi
641 for detailed descriptions of these macros.
642 HePV(HE* he, STRLEN len)
644 HeVAL(HE* he)
645 HeHASH(HE* he)
646 HeSVKEY(HE* he)
647 HeSVKEY_force(HE* he)
648 HeSVKEY_set(HE* he, SV* sv)
649 These two lower level macros are defined, but must only be used when
651 dealing with keys that are not "SV*"s:
652 HeKEY(HE* he)
654 HeKLEN(HE* he)
655 Note that both "hv_store" and "hv_store_ent" do not increment the
657 reference count of the stored "val", which is the caller's
658 responsibility. If these functions return a NULL value, the caller
659 will usually have to decrement the reference count of "val" to avoid a
660 memory leak.
661 AVs, HVs and undefined values
663 Sometimes you have to store undefined values in AVs or HVs. Although
664 this may be a rare case, it can be tricky. That's because you're used
665 to using &PL_sv_undef if you need an undefined SV.
666 For example, intuition tells you that this XS code:
668 AV *av = newAV();
670 av_store( av, 0, &PL_sv_undef );
671 is equivalent to this Perl code:
673 my @av;
675 $av[0] = undef;
676 Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use
678 &PL_sv_undef as a marker for indicating that an array element has not
679 yet been initialized. Thus, "exists $av[0]" would be true for the
680 above Perl code, but false for the array generated by the XS code. In
681 perl 5.20, storing &PL_sv_undef will create a read-only element,
682 because the scalar &PL_sv_undef itself is stored, not a copy.
683 Similar problems can occur when storing &PL_sv_undef in HVs:
685 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
687 This will indeed make the value "undef", but if you try to modify the
689 value of "key", you'll get the following error:
690 Modification of non-creatable hash value attempted
692 In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in
694 restricted hashes. This caused such hash entries not to appear when
695 iterating over the hash or when checking for the keys with the
696 "hv_exists" function.
697 You can run into similar problems when you store &PL_sv_yes or
699 &PL_sv_no into AVs or HVs. Trying to modify such elements will give
700 you the following error:
701 Modification of a read-only value attempted
703 To make a long story short, you can use the special variables
705 &PL_sv_undef, &PL_sv_yes and &PL_sv_no with AVs and HVs, but you have
706 to make sure you know what you're doing.
707 Generally, if you want to store an undefined value in an AV or HV, you
709 should not use &PL_sv_undef, but rather create a new undefined value
710 using the "newSV" function, for example:
711 av_store( av, 42, newSV(0) );
713 hv_store( hv, "foo", 3, newSV(0), 0 );
714 References
716 References are a special type of scalar that point to other data types
717 (including other references).
718 To create a reference, use either of the following functions:
720 SV* newRV_inc((SV*) thing);
722 SV* newRV_noinc((SV*) thing);
723 The "thing" argument can be any of an "SV*", "AV*", or "HV*". The
725 functions are identical except that "newRV_inc" increments the
726 reference count of the "thing", while "newRV_noinc" does not. For
727 historical reasons, "newRV" is a synonym for "newRV_inc".
728 Once you have a reference, you can use the following macro to
730 dereference the reference:
731 SvRV(SV*)
733 then call the appropriate routines, casting the returned "SV*" to
735 either an "AV*" or "HV*", if required.
736 To determine if an SV is a reference, you can use the following macro:
738 SvROK(SV*)
740 To discover what type of value the reference refers to, use the
742 following macro and then check the return value.
743 SvTYPE(SvRV(SV*))
745 The most useful types that will be returned are:
747 SVt_PVAV Array
749 SVt_PVHV Hash
750 SVt_PVCV Code
751 SVt_PVGV Glob (possibly a file handle)
752 Any numerical value returned which is less than SVt_PVAV will be a
754 scalar of some form.
755 See "svtype" in perlapi for more details.
757 Blessed References and Class Objects
759 References are also used to support object-oriented programming. In
760 perl's OO lexicon, an object is simply a reference that has been
761 blessed into a package (or class). Once blessed, the programmer may
762 now use the reference to access the various methods in the class.
763 A reference can be blessed into a package with the following function:
765 SV* sv_bless(SV* sv, HV* stash);
767 The "sv" argument must be a reference value. The "stash" argument
769 specifies which class the reference will belong to. See "Stashes and
770 Globs" for information on converting class names into stashes.
771 /* Still under construction */
773 The following function upgrades rv to reference if not already one.
775 Creates a new SV for rv to point to. If "classname" is non-null, the
776 SV is blessed into the specified class. SV is returned.
777 SV* newSVrv(SV* rv, const char* classname);
779 The following three functions copy integer, unsigned integer or double
781 into an SV whose reference is "rv". SV is blessed if "classname" is
782 non-null.
783 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
785 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
786 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
787 The following function copies the pointer value (the address, not the
789 string!) into an SV whose reference is rv. SV is blessed if
790 "classname" is non-null.
791 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
793 The following function copies a string into an SV whose reference is
795 "rv". Set length to 0 to let Perl calculate the string length. SV is
796 blessed if "classname" is non-null.
797 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
799 STRLEN length);
800 The following function tests whether the SV is blessed into the
802 specified class. It does not check inheritance relationships.
803 int sv_isa(SV* sv, const char* name);
805 The following function tests whether the SV is a reference to a blessed
807 object.
808 int sv_isobject(SV* sv);
810 The following function tests whether the SV is derived from the
812 specified class. SV can be either a reference to a blessed object or a
813 string containing a class name. This is the function implementing the
814 "UNIVERSAL::isa" functionality.
815 bool sv_derived_from(SV* sv, const char* name);
817 To check if you've got an object derived from a specific class you have
819 to write:
820 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
822 Creating New Variables
824 To create a new Perl variable with an undef value which can be accessed
825 from your Perl script, use the following routines, depending on the
826 variable type.
827 SV* get_sv("package::varname", GV_ADD);
829 AV* get_av("package::varname", GV_ADD);
830 HV* get_hv("package::varname", GV_ADD);
831 Notice the use of GV_ADD as the second parameter. The new variable can
833 now be set, using the routines appropriate to the data type.
834 There are additional macros whose values may be bitwise OR'ed with the
836 "GV_ADD" argument to enable certain extra features. Those bits are:
837 GV_ADDMULTI
839 Marks the variable as multiply defined, thus preventing the:
840 Name <varname> used only once: possible typo
842 warning.
844 GV_ADDWARN
846 Issues the warning:
847 Had to create <varname> unexpectedly
849 if the variable did not exist before the function was called.
851 If you do not specify a package name, the variable is created in the
853 current package.
854 Reference Counts and Mortality
856 Perl uses a reference count-driven garbage collection mechanism. SVs,
857 AVs, or HVs (xV for short in the following) start their life with a
858 reference count of 1. If the reference count of an xV ever drops to 0,
859 then it will be destroyed and its memory made available for reuse. At
860 the most basic internal level, reference counts can be manipulated with
861 the following macros:
862 int SvREFCNT(SV* sv);
864 SV* SvREFCNT_inc(SV* sv);
865 void SvREFCNT_dec(SV* sv);
866 (There are also suffixed versions of the increment and decrement
868 macros, for situations where the full generality of these basic macros
869 can be exchanged for some performance.)
870 However, the way a programmer should think about references is not so
872 much in terms of the bare reference count, but in terms of ownership of
873 references. A reference to an xV can be owned by any of a variety of
874 entities: another xV, the Perl interpreter, an XS data structure, a
875 piece of running code, or a dynamic scope. An xV generally does not
876 know what entities own the references to it; it only knows how many
877 references there are, which is the reference count.
878 To correctly maintain reference counts, it is essential to keep track
880 of what references the XS code is manipulating. The programmer should
881 always know where a reference has come from and who owns it, and be
882 aware of any creation or destruction of references, and any transfers
883 of ownership. Because ownership isn't represented explicitly in the xV
884 data structures, only the reference count need be actually maintained
885 by the code, and that means that this understanding of ownership is not
886 actually evident in the code. For example, transferring ownership of a
887 reference from one owner to another doesn't change the reference count
888 at all, so may be achieved with no actual code. (The transferring code
889 doesn't touch the referenced object, but does need to ensure that the
890 former owner knows that it no longer owns the reference, and that the
891 new owner knows that it now does.)
892 An xV that is visible at the Perl level should not become unreferenced
894 and thus be destroyed. Normally, an object will only become
895 unreferenced when it is no longer visible, often by the same means that
896 makes it invisible. For example, a Perl reference value (RV) owns a
897 reference to its referent, so if the RV is overwritten that reference
898 gets destroyed, and the no-longer-reachable referent may be destroyed
899 as a result.
900 Many functions have some kind of reference manipulation as part of
902 their purpose. Sometimes this is documented in terms of ownership of
903 references, and sometimes it is (less helpfully) documented in terms of
904 changes to reference counts. For example, the newRV_inc() function is
905 documented to create a new RV (with reference count 1) and increment
906 the reference count of the referent that was supplied by the caller.
907 This is best understood as creating a new reference to the referent,
908 which is owned by the created RV, and returning to the caller ownership
909 of the sole reference to the RV. The newRV_noinc() function instead
910 does not increment the reference count of the referent, but the RV
911 nevertheless ends up owning a reference to the referent. It is
912 therefore implied that the caller of "newRV_noinc()" is relinquishing a
913 reference to the referent, making this conceptually a more complicated
914 operation even though it does less to the data structures.
915 For example, imagine you want to return a reference from an XSUB
917 function. Inside the XSUB routine, you create an SV which initially
918 has just a single reference, owned by the XSUB routine. This reference
919 needs to be disposed of before the routine is complete, otherwise it
920 will leak, preventing the SV from ever being destroyed. So to create
921 an RV referencing the SV, it is most convenient to pass the SV to
922 "newRV_noinc()", which consumes that reference. Now the XSUB routine
923 no longer owns a reference to the SV, but does own a reference to the
924 RV, which in turn owns a reference to the SV. The ownership of the
925 reference to the RV is then transferred by the process of returning the
926 RV from the XSUB.
927 There are some convenience functions available that can help with the
929 destruction of xVs. These functions introduce the concept of
930 "mortality". Much documentation speaks of an xV itself being mortal,
931 but this is misleading. It is really a reference to an xV that is
932 mortal, and it is possible for there to be more than one mortal
933 reference to a single xV. For a reference to be mortal means that it
934 is owned by the temps stack, one of perl's many internal stacks, which
935 will destroy that reference "a short time later". Usually the "short
936 time later" is the end of the current Perl statement. However, it gets
937 more complicated around dynamic scopes: there can be multiple sets of
938 mortal references hanging around at the same time, with different death
939 dates. Internally, the actual determinant for when mortal xV
940 references are destroyed depends on two macros, SAVETMPS and FREETMPS.
941 See perlcall and perlxs and "Temporaries Stack" below for more details
942 on these macros.
943 Mortal references are mainly used for xVs that are placed on perl's
945 main stack. The stack is problematic for reference tracking, because
946 it contains a lot of xV references, but doesn't own those references:
947 they are not counted. Currently, there are many bugs resulting from
948 xVs being destroyed while referenced by the stack, because the stack's
949 uncounted references aren't enough to keep the xVs alive. So when
950 putting an (uncounted) reference on the stack, it is vitally important
951 to ensure that there will be a counted reference to the same xV that
952 will last at least as long as the uncounted reference. But it's also
953 important that that counted reference be cleaned up at an appropriate
954 time, and not unduly prolong the xV's life. For there to be a mortal
955 reference is often the best way to satisfy this requirement, especially
956 if the xV was created especially to be put on the stack and would
957 otherwise be unreferenced.
958 To create a mortal reference, use the functions:
960 SV* sv_newmortal()
962 SV* sv_mortalcopy(SV*)
963 SV* sv_2mortal(SV*)
964 "sv_newmortal()" creates an SV (with the undefined value) whose sole
966 reference is mortal. "sv_mortalcopy()" creates an xV whose value is a
967 copy of a supplied xV and whose sole reference is mortal.
968 "sv_2mortal()" mortalises an existing xV reference: it transfers
969 ownership of a reference from the caller to the temps stack. Because
970 "sv_newmortal" gives the new SV no value, it must normally be given one
971 via "sv_setpv", "sv_setiv", etc. :
972 SV *tmp = sv_newmortal();
974 sv_setiv(tmp, an_integer);
975 As that is multiple C statements it is quite common so see this idiom
977 instead:
978 SV *tmp = sv_2mortal(newSViv(an_integer));
980 The mortal routines are not just for SVs; AVs and HVs can be made
982 mortal by passing their address (type-casted to "SV*") to the
983 "sv_2mortal" or "sv_mortalcopy" routines.
984 Stashes and Globs
986 A stash is a hash that contains all variables that are defined within a
987 package. Each key of the stash is a symbol name (shared by all the
988 different types of objects that have the same name), and each value in
989 the hash table is a GV (Glob Value). This GV in turn contains
990 references to the various objects of that name, including (but not
991 limited to) the following:
992 Scalar Value
994 Array Value
995 Hash Value
996 I/O Handle
997 Format
998 Subroutine
999 There is a single stash called "PL_defstash" that holds the items that
1001 exist in the "main" package. To get at the items in other packages,
1002 append the string "::" to the package name. The items in the "Foo"
1003 package are in the stash "Foo::" in PL_defstash. The items in the
1004 "Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash.
1005 To get the stash pointer for a particular package, use the function:
1007 HV* gv_stashpv(const char* name, I32 flags)
1009 HV* gv_stashsv(SV*, I32 flags)
1010 The first function takes a literal string, the second uses the string
1012 stored in the SV. Remember that a stash is just a hash table, so you
1013 get back an "HV*". The "flags" flag will create a new package if it is
1014 set to GV_ADD.
1015 The name that "gv_stash*v" wants is the name of the package whose
1017 symbol table you want. The default package is called "main". If you
1018 have multiply nested packages, pass their names to "gv_stash*v",
1019 separated by "::" as in the Perl language itself.
1020 Alternately, if you have an SV that is a blessed reference, you can
1022 find out the stash pointer by using:
1023 HV* SvSTASH(SvRV(SV*));
1025 then use the following to get the package name itself:
1027 char* HvNAME(HV* stash);
1029 If you need to bless or re-bless an object you can use the following
1031 function:
1032 SV* sv_bless(SV*, HV* stash)
1034 where the first argument, an "SV*", must be a reference, and the second
1036 argument is a stash. The returned "SV*" can now be used in the same
1037 way as any other SV.
1038 For more information on references and blessings, consult perlref.
1040 I/O Handles
1042 Like AVs and HVs, IO objects are another type of non-scalar SV which
1043 may contain input and output PerlIO objects or a "DIR *" from
1044 opendir().
1045 You can create a new IO object:
1047 IO* newIO();
1049 Unlike other SVs, a new IO object is automatically blessed into the
1051 IO::File class.
1052 The IO object contains an input and output PerlIO handle:
1054 PerlIO *IoIFP(IO *io);
1056 PerlIO *IoOFP(IO *io);
1057 Typically if the IO object has been opened on a file, the input handle
1059 is always present, but the output handle is only present if the file is
1060 open for output. For a file, if both are present they will be the same
1061 PerlIO object.
1062 Distinct input and output PerlIO objects are created for sockets and
1064 character devices.
1065 The IO object also contains other data associated with Perl I/O
1067 handles:
1068 IV IoLINES(io); /* $. */
1070 IV IoPAGE(io); /* $% */
1071 IV IoPAGE_LEN(io); /* $= */
1072 IV IoLINES_LEFT(io); /* $- */
1073 char *IoTOP_NAME(io); /* $^ */
1074 GV *IoTOP_GV(io); /* $^ */
1075 char *IoFMT_NAME(io); /* $~ */
1076 GV *IoFMT_GV(io); /* $~ */
1077 char *IoBOTTOM_NAME(io);
1078 GV *IoBOTTOM_GV(io);
1079 char IoTYPE(io);
1080 U8 IoFLAGS(io);
1081 Most of these are involved with formats.
1083 IoFLAGs() may contain a combination of flags, the most interesting of
1085 which are "IOf_FLUSH" ($|) for autoflush and "IOf_UNTAINT", settable
1086 with IO::Handle's untaint() method.
1087 The IO object may also contains a directory handle:
1089 DIR *IoDIRP(io);
1091 suitable for use with PerlDir_read() etc.
1093 All of these accessors macros are lvalues, there are no distinct
1095 "_set()" macros to modify the members of the IO object.
1096 Double-Typed SVs
1098 Scalar variables normally contain only one type of value, an integer,
1099 double, pointer, or reference. Perl will automatically convert the
1100 actual scalar data from the stored type into the requested type.
1101 Some scalar variables contain more than one type of scalar data. For
1103 example, the variable $! contains either the numeric value of "errno"
1104 or its string equivalent from either "strerror" or "sys_errlist[]".
1105 To force multiple data values into an SV, you must do two things: use
1107 the "sv_set*v" routines to add the additional scalar type, then set a
1108 flag so that Perl will believe it contains more than one type of data.
1109 The four macros to set the flags are:
1110 SvIOK_on
1112 SvNOK_on
1113 SvPOK_on
1114 SvROK_on
1115 The particular macro you must use depends on which "sv_set*v" routine
1117 you called first. This is because every "sv_set*v" routine turns on
1118 only the bit for the particular type of data being set, and turns off
1119 all the rest.
1120 For example, to create a new Perl variable called "dberror" that
1122 contains both the numeric and descriptive string error values, you
1123 could use the following code:
1124 extern int dberror;
1126 extern char *dberror_list;
1127 SV* sv = get_sv("dberror", GV_ADD);
1129 sv_setiv(sv, (IV) dberror);
1130 sv_setpv(sv, dberror_list[dberror]);
1131 SvIOK_on(sv);
1132 If the order of "sv_setiv" and "sv_setpv" had been reversed, then the
1134 macro "SvPOK_on" would need to be called instead of "SvIOK_on".
1135 Read-Only Values
1137 In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
1138 flag bit with read-only scalars. So the only way to test whether
1139 "sv_setsv", etc., will raise a "Modification of a read-only value"
1140 error in those versions is:
1141 SvREADONLY(sv) && !SvIsCOW(sv)
1143 Under Perl 5.18 and later, SvREADONLY only applies to read-only
1145 variables, and, under 5.20, copy-on-write scalars can also be read-
1146 only, so the above check is incorrect. You just want:
1147 SvREADONLY(sv)
1149 If you need to do this check often, define your own macro like this:
1151 #if PERL_VERSION >= 18
1153 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
1154 #else
1155 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
1156 #endif
1157 Copy on Write
1159 Perl implements a copy-on-write (COW) mechanism for scalars, in which
1160 string copies are not immediately made when requested, but are deferred
1161 until made necessary by one or the other scalar changing. This is
1162 mostly transparent, but one must take care not to modify string buffers
1163 that are shared by multiple SVs.
1164 You can test whether an SV is using copy-on-write with "SvIsCOW(sv)".
1166 You can force an SV to make its own copy of its string buffer by
1168 calling "sv_force_normal(sv)" or SvPV_force_nolen(sv).
1169 If you want to make the SV drop its string buffer, use
1171 "sv_force_normal_flags(sv, SV_COW_DROP_PV)" or simply "sv_setsv(sv,
1172 NULL)".
1173 All of these functions will croak on read-only scalars (see the
1175 previous section for more on those).
1176 To test that your code is behaving correctly and not modifying COW
1178 buffers, on systems that support mmap(2) (i.e., Unix) you can configure
1179 perl with "-Accflags=-DPERL_DEBUG_READONLY_COW" and it will turn buffer
1180 violations into crashes. You will find it to be marvellously slow, so
1181 you may want to skip perl's own tests.
1182 Magic Variables
1184 [This section still under construction. Ignore everything here. Post
1185 no bills. Everything not permitted is forbidden.]
1186 Any SV may be magical, that is, it has special features that a normal
1188 SV does not have. These features are stored in the SV structure in a
1189 linked list of "struct magic"'s, typedef'ed to "MAGIC".
1190 struct magic {
1192 MAGIC* mg_moremagic;
1193 MGVTBL* mg_virtual;
1194 U16 mg_private;
1195 char mg_type;
1196 U8 mg_flags;
1197 I32 mg_len;
1198 SV* mg_obj;
1199 char* mg_ptr;
1200 };
1201 Note this is current as of patchlevel 0, and could change at any time.
1203 Assigning Magic
1205 Perl adds magic to an SV using the sv_magic function:
1206 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
1208 The "sv" argument is a pointer to the SV that is to acquire a new
1210 magical feature.
1211 If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to
1213 convert "sv" to type "SVt_PVMG". Perl then continues by adding new
1214 magic to the beginning of the linked list of magical features. Any
1215 prior entry of the same type of magic is deleted. Note that this can
1216 be overridden, and multiple instances of the same type of magic can be
1217 associated with an SV.
1218 The "name" and "namlen" arguments are used to associate a string with
1220 the magic, typically the name of a variable. "namlen" is stored in the
1221 "mg_len" field and if "name" is non-null then either a "savepvn" copy
1222 of "name" or "name" itself is stored in the "mg_ptr" field, depending
1223 on whether "namlen" is greater than zero or equal to zero respectively.
1224 As a special case, if "(name && namlen == HEf_SVKEY)" then "name" is
1225 assumed to contain an "SV*" and is stored as-is with its REFCNT
1226 incremented.
1227 The sv_magic function uses "how" to determine which, if any, predefined
1229 "Magic Virtual Table" should be assigned to the "mg_virtual" field.
1230 See the "Magic Virtual Tables" section below. The "how" argument is
1231 also stored in the "mg_type" field. The value of "how" should be
1232 chosen from the set of macros "PERL_MAGIC_foo" found in perl.h. Note
1233 that before these macros were added, Perl internals used to directly
1234 use character literals, so you may occasionally come across old code or
1235 documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for
1236 example.
1237 The "obj" argument is stored in the "mg_obj" field of the "MAGIC"
1239 structure. If it is not the same as the "sv" argument, the reference
1240 count of the "obj" object is incremented. If it is the same, or if the
1241 "how" argument is "PERL_MAGIC_arylen", "PERL_MAGIC_regdatum",
1242 "PERL_MAGIC_regdata", or if it is a NULL pointer, then "obj" is merely
1243 stored, without the reference count being incremented.
1244 See also "sv_magicext" in perlapi for a more flexible way to add magic
1246 to an SV.
1247 There is also a function to add magic to an "HV":
1249 void hv_magic(HV *hv, GV *gv, int how);
1251 This simply calls "sv_magic" and coerces the "gv" argument into an
1253 "SV".
1254 To remove the magic from an SV, call the function sv_unmagic:
1256 int sv_unmagic(SV *sv, int type);
1258 The "type" argument should be equal to the "how" value when the "SV"
1260 was initially made magical.
1261 However, note that "sv_unmagic" removes all magic of a certain "type"
1263 from the "SV". If you want to remove only certain magic of a "type"
1264 based on the magic virtual table, use "sv_unmagicext" instead:
1265 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1267 Magic Virtual Tables
1269 The "mg_virtual" field in the "MAGIC" structure is a pointer to an
1270 "MGVTBL", which is a structure of function pointers and stands for
1271 "Magic Virtual Table" to handle the various operations that might be
1272 applied to that variable.
1273 The "MGVTBL" has five (or sometimes eight) pointers to the following
1275 routine types:
1276 int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
1278 int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
1279 U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
1280 int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
1281 int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
1282 int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
1284 const char *name, I32 namlen);
1285 int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
1286 int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
1287 This MGVTBL structure is set at compile-time in perl.h and there are
1289 currently 32 types. These different structures contain pointers to
1290 various routines that perform additional actions depending on which
1291 function is being called.
1292 Function pointer Action taken
1294 ---------------- ------------
1295 svt_get Do something before the value of the SV is
1296 retrieved.
1297 svt_set Do something after the SV is assigned a value.
1298 svt_len Report on the SV's length.
1299 svt_clear Clear something the SV represents.
1300 svt_free Free any extra storage associated with the SV.
1301 svt_copy copy tied variable magic to a tied element
1303 svt_dup duplicate a magic structure during thread cloning
1304 svt_local copy magic to local value during 'local'
1305 For instance, the MGVTBL structure called "vtbl_sv" (which corresponds
1307 to an "mg_type" of "PERL_MAGIC_sv") contains:
1308 { magic_get, magic_set, magic_len, 0, 0 }
1310 Thus, when an SV is determined to be magical and of type
1312 "PERL_MAGIC_sv", if a get operation is being performed, the routine
1313 "magic_get" is called. All the various routines for the various
1314 magical types begin with "magic_". NOTE: the magic routines are not
1315 considered part of the Perl API, and may not be exported by the Perl
1316 library.
1317 The last three slots are a recent addition, and for source code
1319 compatibility they are only checked for if one of the three flags
1320 MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that
1321 most code can continue declaring a vtable as a 5-element value. These
1322 three are currently used exclusively by the threading code, and are
1323 highly subject to change.
1324 The current kinds of Magic Virtual Tables are:
1326 mg_type
1328 (old-style char and macro) MGVTBL Type of magic
1329 -------------------------- ------ -------------
1330 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1331 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1332 % PERL_MAGIC_rhash (none) Extra data for restricted
1333 hashes
1334 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1335 vars
1336 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1337 : PERL_MAGIC_symtab (none) Extra data for symbol
1338 tables
1339 < PERL_MAGIC_backref vtbl_backref For weak ref data
1340 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
1341 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1342 (fast string search)
1343 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1344 (AMT) on stash
1345 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1346 (@+ and @- vars)
1347 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1348 element
1349 E PERL_MAGIC_env vtbl_env %ENV hash
1350 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1351 f PERL_MAGIC_fm vtbl_regexp Formline
1352 ('compiled' format)
1353 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1354 H PERL_MAGIC_hints vtbl_hints %^H hash
1355 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1356 I PERL_MAGIC_isa vtbl_isa @ISA array
1357 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1358 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1359 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1360 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1361 element
1362 N PERL_MAGIC_shared (none) Shared between threads
1363 n PERL_MAGIC_shared_scalar (none) Shared between threads
1364 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1365 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1366 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1367 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1368 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
1369 S PERL_MAGIC_sig (none) %SIG hash
1370 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1371 t PERL_MAGIC_taint vtbl_taint Taintedness
1372 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1373 extensions
1374 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1375 extensions
1376 V PERL_MAGIC_vstring (none) SV was vstring literal
1377 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1378 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1379 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1380 Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
1381 exist
1382 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1383 variable / smart parameter
1384 vivification
1385 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
1386 constructor
1387 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
1388 to this CV
1389 ~ PERL_MAGIC_ext (none) Available for use by
1390 extensions
1391 When an uppercase and lowercase letter both exist in the table, then
1393 the uppercase letter is typically used to represent some kind of
1394 composite type (a list or a hash), and the lowercase letter is used to
1395 represent an element of that composite type. Some internals code makes
1396 use of this case relationship. However, 'v' and 'V' (vec and v-string)
1397 are in no way related.
1398 The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined
1400 specifically for use by extensions and will not be used by perl itself.
1401 Extensions can use "PERL_MAGIC_ext" magic to 'attach' private
1402 information to variables (typically objects). This is especially
1403 useful because there is no way for normal perl code to corrupt this
1404 private information (unlike using extra elements of a hash object).
1405 Similarly, "PERL_MAGIC_uvar" magic can be used much like tie() to call
1407 a C function any time a scalar's value is used or changed. The
1408 "MAGIC"'s "mg_ptr" field points to a "ufuncs" structure:
1409 struct ufuncs {
1411 I32 (*uf_val)(pTHX_ IV, SV*);
1412 I32 (*uf_set)(pTHX_ IV, SV*);
1413 IV uf_index;
1414 };
1415 When the SV is read from or written to, the "uf_val" or "uf_set"
1417 function will be called with "uf_index" as the first arg and a pointer
1418 to the SV as the second. A simple example of how to add
1419 "PERL_MAGIC_uvar" magic is shown below. Note that the ufuncs structure
1420 is copied by sv_magic, so you can safely allocate it on the stack.
1421 void
1423 Umagic(sv)
1424 SV *sv;
1425 PREINIT:
1426 struct ufuncs uf;
1427 CODE:
1428 uf.uf_val = &my_get_fn;
1429 uf.uf_set = &my_set_fn;
1430 uf.uf_index = 0;
1431 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1432 Attaching "PERL_MAGIC_uvar" to arrays is permissible but has no effect.
1434 For hashes there is a specialized hook that gives control over hash
1436 keys (but not values). This hook calls "PERL_MAGIC_uvar" 'get' magic
1437 if the "set" function in the "ufuncs" structure is NULL. The hook is
1438 activated whenever the hash is accessed with a key specified as an "SV"
1439 through the functions "hv_store_ent", "hv_fetch_ent", "hv_delete_ent",
1440 and "hv_exists_ent". Accessing the key as a string through the
1441 functions without the "..._ent" suffix circumvents the hook. See
1442 "GUTS" in Hash::Util::FieldHash for a detailed description.
1443 Note that because multiple extensions may be using "PERL_MAGIC_ext" or
1445 "PERL_MAGIC_uvar" magic, it is important for extensions to take extra
1446 care to avoid conflict. Typically only using the magic on objects
1447 blessed into the same class as the extension is sufficient. For
1448 "PERL_MAGIC_ext" magic, it is usually a good idea to define an
1449 "MGVTBL", even if all its fields will be 0, so that individual "MAGIC"
1450 pointers can be identified as a particular kind of magic using their
1451 magic virtual table. "mg_findext" provides an easy way to do that:
1452 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1454 MAGIC *mg;
1456 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1457 /* this is really ours, not another module's PERL_MAGIC_ext */
1458 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1459 ...
1460 }
1461 Also note that the "sv_set*()" and "sv_cat*()" functions described
1463 earlier do not invoke 'set' magic on their targets. This must be done
1464 by the user either by calling the "SvSETMAGIC()" macro after calling
1465 these functions, or by using one of the "sv_set*_mg()" or
1466 "sv_cat*_mg()" functions. Similarly, generic C code must call the
1467 "SvGETMAGIC()" macro to invoke any 'get' magic if they use an SV
1468 obtained from external sources in functions that don't handle magic.
1469 See perlapi for a description of these functions. For example, calls
1470 to the "sv_cat*()" functions typically need to be followed by
1471 "SvSETMAGIC()", but they don't need a prior "SvGETMAGIC()" since their
1472 implementation handles 'get' magic.
1473 Finding Magic
1475 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1476 * type */
1477 This routine returns a pointer to a "MAGIC" structure stored in the SV.
1479 If the SV does not have that magical feature, "NULL" is returned. If
1480 the SV has multiple instances of that magical feature, the first one
1481 will be returned. "mg_findext" can be used to find a "MAGIC" structure
1482 of an SV based on both its magic type and its magic virtual table:
1483 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
1485 Also, if the SV passed to "mg_find" or "mg_findext" is not of type
1487 SVt_PVMG, Perl may core dump.
1488 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1490 This routine checks to see what types of magic "sv" has. If the
1492 mg_type field is an uppercase letter, then the mg_obj is copied to
1493 "nsv", but the mg_type field is changed to be the lowercase letter.
1494 Understanding the Magic of Tied Hashes and Arrays
1496 Tied hashes and arrays are magical beasts of the "PERL_MAGIC_tied"
1497 magic type.
1498 WARNING: As of the 5.004 release, proper usage of the array and hash
1500 access functions requires understanding a few caveats. Some of these
1501 caveats are actually considered bugs in the API, to be fixed in later
1502 releases, and are bracketed with [MAYCHANGE] below. If you find
1503 yourself actually applying such information in this section, be aware
1504 that the behavior may change in the future, umm, without warning.
1505 The perl tie function associates a variable with an object that
1507 implements the various GET, SET, etc methods. To perform the
1508 equivalent of the perl tie function from an XSUB, you must mimic this
1509 behaviour. The code below carries out the necessary steps -- firstly
1510 it creates a new hash, and then creates a second hash which it blesses
1511 into the class which will implement the tie methods. Lastly it ties
1512 the two hashes together, and returns a reference to the new tied hash.
1513 Note that the code below does NOT call the TIEHASH method in the MyTie
1514 class - see "Calling Perl Routines from within C Programs" for details
1515 on how to do this.
1516 SV*
1518 mytie()
1519 PREINIT:
1520 HV *hash;
1521 HV *stash;
1522 SV *tie;
1523 CODE:
1524 hash = newHV();
1525 tie = newRV_noinc((SV*)newHV());
1526 stash = gv_stashpv("MyTie", GV_ADD);
1527 sv_bless(tie, stash);
1528 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1529 RETVAL = newRV_noinc(hash);
1530 OUTPUT:
1531 RETVAL
1532 The "av_store" function, when given a tied array argument, merely
1534 copies the magic of the array onto the value to be "stored", using
1535 "mg_copy". It may also return NULL, indicating that the value did not
1536 actually need to be stored in the array. [MAYCHANGE] After a call to
1537 "av_store" on a tied array, the caller will usually need to call
1538 "mg_set(val)" to actually invoke the perl level "STORE" method on the
1539 TIEARRAY object. If "av_store" did return NULL, a call to
1540 "SvREFCNT_dec(val)" will also be usually necessary to avoid a memory
1541 leak. [/MAYCHANGE]
1542 The previous paragraph is applicable verbatim to tied hash access using
1544 the "hv_store" and "hv_store_ent" functions as well.
1545 "av_fetch" and the corresponding hash functions "hv_fetch" and
1547 "hv_fetch_ent" actually return an undefined mortal value whose magic
1548 has been initialized using "mg_copy". Note the value so returned does
1549 not need to be deallocated, as it is already mortal. [MAYCHANGE] But
1550 you will need to call "mg_get()" on the returned value in order to
1551 actually invoke the perl level "FETCH" method on the underlying TIE
1552 object. Similarly, you may also call "mg_set()" on the return value
1553 after possibly assigning a suitable value to it using "sv_setsv",
1554 which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
1555 [MAYCHANGE] In other words, the array or hash fetch/store functions
1557 don't really fetch and store actual values in the case of tied arrays
1558 and hashes. They merely call "mg_copy" to attach magic to the values
1559 that were meant to be "stored" or "fetched". Later calls to "mg_get"
1560 and "mg_set" actually do the job of invoking the TIE methods on the
1561 underlying objects. Thus the magic mechanism currently implements a
1562 kind of lazy access to arrays and hashes.
1563 Currently (as of perl version 5.004), use of the hash and array access
1565 functions requires the user to be aware of whether they are operating
1566 on "normal" hashes and arrays, or on their tied variants. The API may
1567 be changed to provide more transparent access to both tied and normal
1568 data types in future versions. [/MAYCHANGE]
1569 You would do well to understand that the TIEARRAY and TIEHASH
1571 interfaces are mere sugar to invoke some perl method calls while using
1572 the uniform hash and array syntax. The use of this sugar imposes some
1573 overhead (typically about two to four extra opcodes per FETCH/STORE
1574 operation, in addition to the creation of all the mortal variables
1575 required to invoke the methods). This overhead will be comparatively
1576 small if the TIE methods are themselves substantial, but if they are
1577 only a few statements long, the overhead will not be insignificant.
1578 Localizing changes
1580 Perl has a very handy construction
1581 {
1583 local $var = 2;
1584 ...
1585 }
1586 This construction is approximately equivalent to
1588 {
1590 my $oldvar = $var;
1591 $var = 2;
1592 ...
1593 $var = $oldvar;
1594 }
1595 The biggest difference is that the first construction would reinstate
1597 the initial value of $var, irrespective of how control exits the block:
1598 "goto", "return", "die"/"eval", etc. It is a little bit more efficient
1599 as well.
1600 There is a way to achieve a similar task from C via Perl API: create a
1602 pseudo-block, and arrange for some changes to be automatically undone
1603 at the end of it, either explicit, or via a non-local exit (via die()).
1604 A block-like construct is created by a pair of "ENTER"/"LEAVE" macros
1605 (see "Returning a Scalar" in perlcall). Such a construct may be
1606 created specially for some important localized task, or an existing one
1607 (like boundaries of enclosing Perl subroutine/block, or an existing
1608 pair for freeing TMPs) may be used. (In the second case the overhead
1609 of additional localization must be almost negligible.) Note that any
1610 XSUB is automatically enclosed in an "ENTER"/"LEAVE" pair.
1611 Inside such a pseudo-block the following service is available:
1613 "SAVEINT(int i)"
1615 "SAVEIV(IV i)"
1616 "SAVEI32(I32 i)"
1617 "SAVELONG(long i)"
1618 "SAVEI8(I8 i)"
1619 "SAVEI16(I16 i)"
1620 "SAVEBOOL(int i)"
1621 These macros arrange things to restore the value of integer
1622 variable "i" at the end of the enclosing pseudo-block.
1623 SAVESPTR(s)
1625 SAVEPPTR(p)
1626 These macros arrange things to restore the value of pointers "s"
1627 and "p". "s" must be a pointer of a type which survives conversion
1628 to "SV*" and back, "p" should be able to survive conversion to
1629 "char*" and back.
1630 "SAVEFREESV(SV *sv)"
1632 The refcount of "sv" will be decremented at the end of pseudo-
1633 block. This is similar to "sv_2mortal" in that it is also a
1634 mechanism for doing a delayed "SvREFCNT_dec". However, while
1635 "sv_2mortal" extends the lifetime of "sv" until the beginning of
1636 the next statement, "SAVEFREESV" extends it until the end of the
1637 enclosing scope. These lifetimes can be wildly different.
1638 Also compare "SAVEMORTALIZESV".
1640 "SAVEMORTALIZESV(SV *sv)"
1642 Just like "SAVEFREESV", but mortalizes "sv" at the end of the
1643 current scope instead of decrementing its reference count. This
1644 usually has the effect of keeping "sv" alive until the statement
1645 that called the currently live scope has finished executing.
1646 "SAVEFREEOP(OP *op)"
1648 The "OP *" is op_free()ed at the end of pseudo-block.
1649 SAVEFREEPV(p)
1651 The chunk of memory which is pointed to by "p" is Safefree()ed at
1652 the end of pseudo-block.
1653 "SAVECLEARSV(SV *sv)"
1655 Clears a slot in the current scratchpad which corresponds to "sv"
1656 at the end of pseudo-block.
1657 "SAVEDELETE(HV *hv, char *key, I32 length)"
1659 The key "key" of "hv" is deleted at the end of pseudo-block. The
1660 string pointed to by "key" is Safefree()ed. If one has a key in
1661 short-lived storage, the corresponding string may be reallocated
1662 like this:
1663 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1665 "SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
1667 At the end of pseudo-block the function "f" is called with the only
1668 argument "p".
1669 "SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
1671 At the end of pseudo-block the function "f" is called with the
1672 implicit context argument (if any), and "p".
1673 "SAVESTACK_POS()"
1675 The current offset on the Perl internal stack (cf. "SP") is
1676 restored at the end of pseudo-block.
1677 The following API list contains functions, thus one needs to provide
1679 pointers to the modifiable data explicitly (either C pointers, or
1680 Perlish "GV *"s). Where the above macros take "int", a similar
1681 function takes "int *".
1682 Other macros above have functions implementing them, but its probably
1684 best to just use the macro, and not those or the ones below.
1685 "SV* save_scalar(GV *gv)"
1687 Equivalent to Perl code "local $gv".
1688 "AV* save_ary(GV *gv)"
1690 "HV* save_hash(GV *gv)"
1691 Similar to "save_scalar", but localize @gv and %gv.
1692 "void save_item(SV *item)"
1694 Duplicates the current value of "SV". On the exit from the current
1695 "ENTER"/"LEAVE" pseudo-block the value of "SV" will be restored
1696 using the stored value. It doesn't handle magic. Use
1697 "save_scalar" if magic is affected.
1698 "void save_list(SV **sarg, I32 maxsarg)"
1700 A variant of "save_item" which takes multiple arguments via an
1701 array "sarg" of "SV*" of length "maxsarg".
1702 "SV* save_svref(SV **sptr)"
1704 Similar to "save_scalar", but will reinstate an "SV *".
1705 "void save_aptr(AV **aptr)"
1707 "void save_hptr(HV **hptr)"
1708 Similar to "save_svref", but localize "AV *" and "HV *".
1709 The "Alias" module implements localization of the basic types within
1711 the caller's scope. People who are interested in how to localize
1712 things in the containing scope should take a look there too.
1713
1715 XSUBs and the Argument Stack
1716 The XSUB mechanism is a simple way for Perl programs to access C
1717 subroutines. An XSUB routine will have a stack that contains the
1718 arguments from the Perl program, and a way to map from the Perl data
1719 structures to a C equivalent.
1720 The stack arguments are accessible through the ST(n) macro, which
1722 returns the "n"'th stack argument. Argument 0 is the first argument
1723 passed in the Perl subroutine call. These arguments are "SV*", and can
1724 be used anywhere an "SV*" is used.
1725 Most of the time, output from the C routine can be handled through use
1727 of the RETVAL and OUTPUT directives. However, there are some cases
1728 where the argument stack is not already long enough to handle all the
1729 return values. An example is the POSIX tzname() call, which takes no
1730 arguments, but returns two, the local time zone's standard and summer
1731 time abbreviations.
1732 To handle this situation, the PPCODE directive is used and the stack is
1734 extended using the macro:
1735 EXTEND(SP, num);
1737 where "SP" is the macro that represents the local copy of the stack
1739 pointer, and "num" is the number of elements the stack should be
1740 extended by.
1741 Now that there is room on the stack, values can be pushed on it using
1743 "PUSHs" macro. The pushed values will often need to be "mortal" (See
1744 "Reference Counts and Mortality"):
1745 PUSHs(sv_2mortal(newSViv(an_integer)))
1747 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1748 PUSHs(sv_2mortal(newSVnv(a_double)))
1749 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1750 /* Although the last example is better written as the more
1751 * efficient: */
1752 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
1753 And now the Perl program calling "tzname", the two values will be
1755 assigned as in:
1756 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1758 An alternate (and possibly simpler) method to pushing values on the
1760 stack is to use the macro:
1761 XPUSHs(SV*)
1763 This macro automatically adjusts the stack for you, if needed. Thus,
1765 you do not need to call "EXTEND" to extend the stack.
1766 Despite their suggestions in earlier versions of this document the
1768 macros "(X)PUSH[iunp]" are not suited to XSUBs which return multiple
1769 results. For that, either stick to the "(X)PUSHs" macros shown above,
1770 or use the new "m(X)PUSH[iunp]" macros instead; see "Putting a C value
1771 on Perl stack".
1772 For more information, consult perlxs and perlxstut.
1774 Autoloading with XSUBs
1776 If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts
1777 the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD
1778 variable of the XSUB's package.
1779 But it also puts the same information in certain fields of the XSUB
1781 itself:
1782 HV *stash = CvSTASH(cv);
1784 const char *subname = SvPVX(cv);
1785 STRLEN name_length = SvCUR(cv); /* in bytes */
1786 U32 is_utf8 = SvUTF8(cv);
1787 "SvPVX(cv)" contains just the sub name itself, not including the
1789 package. For an AUTOLOAD routine in UNIVERSAL or one of its
1790 superclasses, "CvSTASH(cv)" returns NULL during a method call on a
1791 nonexistent package.
1792 Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1794 XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in
1795 the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If
1796 you need to support 5.8-5.14, use the XSUB's fields.
1797 Calling Perl Routines from within C Programs
1799 There are four routines that can be used to call a Perl subroutine from
1800 within a C program. These four are:
1801 I32 call_sv(SV*, I32);
1803 I32 call_pv(const char*, I32);
1804 I32 call_method(const char*, I32);
1805 I32 call_argv(const char*, I32, char**);
1806 The routine most often used is "call_sv". The "SV*" argument contains
1808 either the name of the Perl subroutine to be called, or a reference to
1809 the subroutine. The second argument consists of flags that control the
1810 context in which the subroutine is called, whether or not the
1811 subroutine is being passed arguments, how errors should be trapped, and
1812 how to treat return values.
1813 All four routines return the number of arguments that the subroutine
1815 returned on the Perl stack.
1816 These routines used to be called "perl_call_sv", etc., before Perl
1818 v5.6.0, but those names are now deprecated; macros of the same name are
1819 provided for compatibility.
1820 When using any of these routines (except "call_argv"), the programmer
1822 must manipulate the Perl stack. These include the following macros and
1823 functions:
1824 dSP
1826 SP
1827 PUSHMARK()
1828 PUTBACK
1829 SPAGAIN
1830 ENTER
1831 SAVETMPS
1832 FREETMPS
1833 LEAVE
1834 XPUSH*()
1835 POP*()
1836 For a detailed description of calling conventions from C to Perl,
1838 consult perlcall.
1839 Putting a C value on Perl stack
1841 A lot of opcodes (this is an elementary operation in the internal perl
1842 stack machine) put an SV* on the stack. However, as an optimization
1843 the corresponding SV is (usually) not recreated each time. The opcodes
1844 reuse specially assigned SVs (targets) which are (as a corollary) not
1845 constantly freed/created.
1846 Each of the targets is created only once (but see "Scratchpads and
1848 recursion" below), and when an opcode needs to put an integer, a
1849 double, or a string on stack, it just sets the corresponding parts of
1850 its target and puts the target on stack.
1851 The macro to put this target on stack is "PUSHTARG", and it is directly
1853 used in some opcodes, as well as indirectly in zillions of others,
1854 which use it via "(X)PUSH[iunp]".
1855 Because the target is reused, you must be careful when pushing multiple
1857 values on the stack. The following code will not do what you think:
1858 XPUSHi(10);
1860 XPUSHi(20);
1861 This translates as "set "TARG" to 10, push a pointer to "TARG" onto the
1863 stack; set "TARG" to 20, push a pointer to "TARG" onto the stack". At
1864 the end of the operation, the stack does not contain the values 10 and
1865 20, but actually contains two pointers to "TARG", which we have set to
1866 20.
1867 If you need to push multiple different values then you should either
1869 use the "(X)PUSHs" macros, or else use the new "m(X)PUSH[iunp]" macros,
1870 none of which make use of "TARG". The "(X)PUSHs" macros simply push an
1871 SV* on the stack, which, as noted under "XSUBs and the Argument Stack",
1872 will often need to be "mortal". The new "m(X)PUSH[iunp]" macros make
1873 this a little easier to achieve by creating a new mortal for you (via
1874 "(X)PUSHmortal"), pushing that onto the stack (extending it if
1875 necessary in the case of the "mXPUSH[iunp]" macros), and then setting
1876 its value. Thus, instead of writing this to "fix" the example above:
1877 XPUSHs(sv_2mortal(newSViv(10)))
1879 XPUSHs(sv_2mortal(newSViv(20)))
1880 you can simply write:
1882 mXPUSHi(10)
1884 mXPUSHi(20)
1885 On a related note, if you do use "(X)PUSH[iunp]", then you're going to
1887 need a "dTARG" in your variable declarations so that the "*PUSH*"
1888 macros can make use of the local variable "TARG". See also "dTARGET"
1889 and "dXSTARG".
1890 Scratchpads
1892 The question remains on when the SVs which are targets for opcodes are
1893 created. The answer is that they are created when the current unit--a
1894 subroutine or a file (for opcodes for statements outside of
1895 subroutines)--is compiled. During this time a special anonymous Perl
1896 array is created, which is called a scratchpad for the current unit.
1897 A scratchpad keeps SVs which are lexicals for the current unit and are
1899 targets for opcodes. A previous version of this document stated that
1900 one can deduce that an SV lives on a scratchpad by looking on its
1901 flags: lexicals have "SVs_PADMY" set, and targets have "SVs_PADTMP"
1902 set. But this has never been fully true. "SVs_PADMY" could be set on
1903 a variable that no longer resides in any pad. While targets do have
1904 "SVs_PADTMP" set, it can also be set on variables that have never
1905 resided in a pad, but nonetheless act like targets. As of perl 5.21.5,
1906 the "SVs_PADMY" flag is no longer used and is defined as 0.
1907 "SvPADMY()" now returns true for anything without "SVs_PADTMP".
1908 The correspondence between OPs and targets is not 1-to-1. Different
1910 OPs in the compile tree of the unit can use the same target, if this
1911 would not conflict with the expected life of the temporary.
1912 Scratchpads and recursion
1914 In fact it is not 100% true that a compiled unit contains a pointer to
1915 the scratchpad AV. In fact it contains a pointer to an AV of
1916 (initially) one element, and this element is the scratchpad AV. Why do
1917 we need an extra level of indirection?
1918 The answer is recursion, and maybe threads. Both these can create
1920 several execution pointers going into the same subroutine. For the
1921 subroutine-child not write over the temporaries for the subroutine-
1922 parent (lifespan of which covers the call to the child), the parent and
1923 the child should have different scratchpads. (And the lexicals should
1924 be separate anyway!)
1925 So each subroutine is born with an array of scratchpads (of length 1).
1927 On each entry to the subroutine it is checked that the current depth of
1928 the recursion is not more than the length of this array, and if it is,
1929 new scratchpad is created and pushed into the array.
1930 The targets on this scratchpad are "undef"s, but they are already
1932 marked with correct flags.
1933
1935 Allocation
1936 All memory meant to be used with the Perl API functions should be
1937 manipulated using the macros described in this section. The macros
1938 provide the necessary transparency between differences in the actual
1939 malloc implementation that is used within perl.
1940 The following three macros are used to initially allocate memory :
1942 Newx(pointer, number, type);
1944 Newxc(pointer, number, type, cast);
1945 Newxz(pointer, number, type);
1946 The first argument "pointer" should be the name of a variable that will
1948 point to the newly allocated memory.
1949 The second and third arguments "number" and "type" specify how many of
1951 the specified type of data structure should be allocated. The argument
1952 "type" is passed to "sizeof". The final argument to "Newxc", "cast",
1953 should be used if the "pointer" argument is different from the "type"
1954 argument.
1955 Unlike the "Newx" and "Newxc" macros, the "Newxz" macro calls "memzero"
1957 to zero out all the newly allocated memory.
1958 Reallocation
1960 Renew(pointer, number, type);
1961 Renewc(pointer, number, type, cast);
1962 Safefree(pointer)
1963 These three macros are used to change a memory buffer size or to free a
1965 piece of memory no longer needed. The arguments to "Renew" and
1966 "Renewc" match those of "New" and "Newc" with the exception of not
1967 needing the "magic cookie" argument.
1968 Moving
1970 Move(source, dest, number, type);
1971 Copy(source, dest, number, type);
1972 Zero(dest, number, type);
1973 These three macros are used to move, copy, or zero out previously
1975 allocated memory. The "source" and "dest" arguments point to the
1976 source and destination starting points. Perl will move, copy, or zero
1977 out "number" instances of the size of the "type" data structure (using
1978 the "sizeof" function).
1979
1981 The most recent development releases of Perl have been experimenting
1982 with removing Perl's dependency on the "normal" standard I/O suite and
1983 allowing other stdio implementations to be used. This involves
1984 creating a new abstraction layer that then calls whichever
1985 implementation of stdio Perl was compiled with. All XSUBs should now
1986 use the functions in the PerlIO abstraction layer and not make any
1987 assumptions about what kind of stdio is being used.
1988 For a complete description of the PerlIO abstraction, consult perlapio.
1990
1992 Code tree
1993 Here we describe the internal form your code is converted to by Perl.
1994 Start with a simple example:
1995 $a = $b + $c;
1997 This is converted to a tree similar to this one:
1999 assign-to
2001 / \
2002 + $a
2003 / \
2004 $b $c
2005 (but slightly more complicated). This tree reflects the way Perl
2007 parsed your code, but has nothing to do with the execution order.
2008 There is an additional "thread" going through the nodes of the tree
2009 which shows the order of execution of the nodes. In our simplified
2010 example above it looks like:
2011 $b ---> $c ---> + ---> $a ---> assign-to
2013 But with the actual compile tree for "$a = $b + $c" it is different:
2015 some nodes optimized away. As a corollary, though the actual tree
2016 contains more nodes than our simplified example, the execution order is
2017 the same as in our example.
2018 Examining the tree
2020 If you have your perl compiled for debugging (usually done with
2021 "-DDEBUGGING" on the "Configure" command line), you may examine the
2022 compiled tree by specifying "-Dx" on the Perl command line. The output
2023 takes several lines per node, and for "$b+$c" it looks like this:
2024 5 TYPE = add ===> 6
2026 TARG = 1
2027 FLAGS = (SCALAR,KIDS)
2028 {
2029 TYPE = null ===> (4)
2030 (was rv2sv)
2031 FLAGS = (SCALAR,KIDS)
2032 {
2033 3 TYPE = gvsv ===> 4
2034 FLAGS = (SCALAR)
2035 GV = main::b
2036 }
2037 }
2038 {
2039 TYPE = null ===> (5)
2040 (was rv2sv)
2041 FLAGS = (SCALAR,KIDS)
2042 {
2043 4 TYPE = gvsv ===> 5
2044 FLAGS = (SCALAR)
2045 GV = main::c
2046 }
2047 }
2048 This tree has 5 nodes (one per "TYPE" specifier), only 3 of them are
2050 not optimized away (one per number in the left column). The immediate
2051 children of the given node correspond to "{}" pairs on the same level
2052 of indentation, thus this listing corresponds to the tree:
2053 add
2055 / \
2056 null null
2057 | |
2058 gvsv gvsv
2059 The execution order is indicated by "===>" marks, thus it is "3 4 5 6"
2061 (node 6 is not included into above listing), i.e., "gvsv gvsv add
2062 whatever".
2063 Each of these nodes represents an op, a fundamental operation inside
2065 the Perl core. The code which implements each operation can be found
2066 in the pp*.c files; the function which implements the op with type
2067 "gvsv" is "pp_gvsv", and so on. As the tree above shows, different ops
2068 have different numbers of children: "add" is a binary operator, as one
2069 would expect, and so has two children. To accommodate the various
2070 different numbers of children, there are various types of op data
2071 structure, and they link together in different ways.
2072 The simplest type of op structure is "OP": this has no children. Unary
2074 operators, "UNOP"s, have one child, and this is pointed to by the
2075 "op_first" field. Binary operators ("BINOP"s) have not only an
2076 "op_first" field but also an "op_last" field. The most complex type of
2077 op is a "LISTOP", which has any number of children. In this case, the
2078 first child is pointed to by "op_first" and the last child by
2079 "op_last". The children in between can be found by iteratively
2080 following the "OpSIBLING" pointer from the first child to the last (but
2081 see below).
2082 There are also some other op types: a "PMOP" holds a regular
2084 expression, and has no children, and a "LOOP" may or may not have
2085 children. If the "op_children" field is non-zero, it behaves like a
2086 "LISTOP". To complicate matters, if a "UNOP" is actually a "null" op
2087 after optimization (see "Compile pass 2: context propagation") it will
2088 still have children in accordance with its former type.
2089 Finally, there is a "LOGOP", or logic op. Like a "LISTOP", this has one
2091 or more children, but it doesn't have an "op_last" field: so you have
2092 to follow "op_first" and then the "OpSIBLING" chain itself to find the
2093 last child. Instead it has an "op_other" field, which is comparable to
2094 the "op_next" field described below, and represents an alternate
2095 execution path. Operators like "and", "or" and "?" are "LOGOP"s. Note
2096 that in general, "op_other" may not point to any of the direct children
2097 of the "LOGOP".
2098 Starting in version 5.21.2, perls built with the experimental define
2100 "-DPERL_OP_PARENT" add an extra boolean flag for each op, "op_moresib".
2101 When not set, this indicates that this is the last op in an "OpSIBLING"
2102 chain. This frees up the "op_sibling" field on the last sibling to
2103 point back to the parent op. Under this build, that field is also
2104 renamed "op_sibparent" to reflect its joint role. The macro
2105 OpSIBLING(o) wraps this special behaviour, and always returns NULL on
2106 the last sibling. With this build the op_parent(o) function can be
2107 used to find the parent of any op. Thus for forward compatibility, you
2108 should always use the OpSIBLING(o) macro rather than accessing
2109 "op_sibling" directly.
2110 Another way to examine the tree is to use a compiler back-end module,
2112 such as B::Concise.
2113 Compile pass 1: check routines
2115 The tree is created by the compiler while yacc code feeds it the
2116 constructions it recognizes. Since yacc works bottom-up, so does the
2117 first pass of perl compilation.
2118 What makes this pass interesting for perl developers is that some
2120 optimization may be performed on this pass. This is optimization by
2121 so-called "check routines". The correspondence between node names and
2122 corresponding check routines is described in opcode.pl (do not forget
2123 to run "make regen_headers" if you modify this file).
2124 A check routine is called when the node is fully constructed except for
2126 the execution-order thread. Since at this time there are no back-links
2127 to the currently constructed node, one can do most any operation to the
2128 top-level node, including freeing it and/or creating new nodes
2129 above/below it.
2130 The check routine returns the node which should be inserted into the
2132 tree (if the top-level node was not modified, check routine returns its
2133 argument).
2134 By convention, check routines have names "ck_*". They are usually
2136 called from "new*OP" subroutines (or "convert") (which in turn are
2137 called from perly.y).
2138 Compile pass 1a: constant folding
2140 Immediately after the check routine is called the returned node is
2141 checked for being compile-time executable. If it is (the value is
2142 judged to be constant) it is immediately executed, and a constant node
2143 with the "return value" of the corresponding subtree is substituted
2144 instead. The subtree is deleted.
2145 If constant folding was not performed, the execution-order thread is
2147 created.
2148 Compile pass 2: context propagation
2150 When a context for a part of compile tree is known, it is propagated
2151 down through the tree. At this time the context can have 5 values
2152 (instead of 2 for runtime context): void, boolean, scalar, list, and
2153 lvalue. In contrast with the pass 1 this pass is processed from top to
2154 bottom: a node's context determines the context for its children.
2155 Additional context-dependent optimizations are performed at this time.
2157 Since at this moment the compile tree contains back-references (via
2158 "thread" pointers), nodes cannot be free()d now. To allow optimized-
2159 away nodes at this stage, such nodes are null()ified instead of
2160 free()ing (i.e. their type is changed to OP_NULL).
2161 Compile pass 3: peephole optimization
2163 After the compile tree for a subroutine (or for an "eval" or a file) is
2164 created, an additional pass over the code is performed. This pass is
2165 neither top-down or bottom-up, but in the execution order (with
2166 additional complications for conditionals). Optimizations performed at
2167 this stage are subject to the same restrictions as in the pass 2.
2168 Peephole optimizations are done by calling the function pointed to by
2170 the global variable "PL_peepp". By default, "PL_peepp" just calls the
2171 function pointed to by the global variable "PL_rpeepp". By default,
2172 that performs some basic op fixups and optimisations along the
2173 execution-order op chain, and recursively calls "PL_rpeepp" for each
2174 side chain of ops (resulting from conditionals). Extensions may
2175 provide additional optimisations or fixups, hooking into either the
2176 per-subroutine or recursive stage, like this:
2177 static peep_t prev_peepp;
2179 static void my_peep(pTHX_ OP *o)
2180 {
2181 /* custom per-subroutine optimisation goes here */
2182 prev_peepp(aTHX_ o);
2183 /* custom per-subroutine optimisation may also go here */
2184 }
2185 BOOT:
2186 prev_peepp = PL_peepp;
2187 PL_peepp = my_peep;
2188 static peep_t prev_rpeepp;
2190 static void my_rpeep(pTHX_ OP *first)
2191 {
2192 OP *o = first, *t = first;
2193 for(; o = o->op_next, t = t->op_next) {
2194 /* custom per-op optimisation goes here */
2195 o = o->op_next;
2196 if (!o || o == t) break;
2197 /* custom per-op optimisation goes AND here */
2198 }
2199 prev_rpeepp(aTHX_ orig_o);
2200 }
2201 BOOT:
2202 prev_rpeepp = PL_rpeepp;
2203 PL_rpeepp = my_rpeep;
2204 Pluggable runops
2206 The compile tree is executed in a runops function. There are two
2207 runops functions, in run.c and in dump.c. "Perl_runops_debug" is used
2208 with DEBUGGING and "Perl_runops_standard" is used otherwise. For fine
2209 control over the execution of the compile tree it is possible to
2210 provide your own runops function.
2211 It's probably best to copy one of the existing runops functions and
2213 change it to suit your needs. Then, in the BOOT section of your XS
2214 file, add the line:
2215 PL_runops = my_runops;
2217 This function should be as efficient as possible to keep your programs
2219 running as fast as possible.
2220 Compile-time scope hooks
2222 As of perl 5.14 it is possible to hook into the compile-time lexical
2223 scope mechanism using "Perl_blockhook_register". This is used like
2224 this:
2225 STATIC void my_start_hook(pTHX_ int full);
2227 STATIC BHK my_hooks;
2228 BOOT:
2230 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
2231 Perl_blockhook_register(aTHX_ &my_hooks);
2232 This will arrange to have "my_start_hook" called at the start of
2234 compiling every lexical scope. The available hooks are:
2235 "void bhk_start(pTHX_ int full)"
2237 This is called just after starting a new lexical scope. Note that
2238 Perl code like
2239 if ($x) { ... }
2241 creates two scopes: the first starts at the "(" and has "full ==
2243 1", the second starts at the "{" and has "full == 0". Both end at
2244 the "}", so calls to "start" and "pre"/"post_end" will match.
2245 Anything pushed onto the save stack by this hook will be popped
2246 just before the scope ends (between the "pre_" and "post_end"
2247 hooks, in fact).
2248 "void bhk_pre_end(pTHX_ OP **o)"
2250 This is called at the end of a lexical scope, just before unwinding
2251 the stack. o is the root of the optree representing the scope; it
2252 is a double pointer so you can replace the OP if you need to.
2253 "void bhk_post_end(pTHX_ OP **o)"
2255 This is called at the end of a lexical scope, just after unwinding
2256 the stack. o is as above. Note that it is possible for calls to
2257 "pre_" and "post_end" to nest, if there is something on the save
2258 stack that calls string eval.
2259 "void bhk_eval(pTHX_ OP *const o)"
2261 This is called just before starting to compile an "eval STRING",
2262 "do FILE", "require" or "use", after the eval has been set up. o
2263 is the OP that requested the eval, and will normally be an
2264 "OP_ENTEREVAL", "OP_DOFILE" or "OP_REQUIRE".
2265 Once you have your hook functions, you need a "BHK" structure to put
2267 them in. It's best to allocate it statically, since there is no way to
2268 free it once it's registered. The function pointers should be inserted
2269 into this structure using the "BhkENTRY_set" macro, which will also set
2270 flags indicating which entries are valid. If you do need to allocate
2271 your "BHK" dynamically for some reason, be sure to zero it before you
2272 start.
2273 Once registered, there is no mechanism to switch these hooks off, so if
2275 that is necessary you will need to do this yourself. An entry in "%^H"
2276 is probably the best way, so the effect is lexically scoped; however it
2277 is also possible to use the "BhkDISABLE" and "BhkENABLE" macros to
2278 temporarily switch entries on and off. You should also be aware that
2279 generally speaking at least one scope will have opened before your
2280 extension is loaded, so you will see some "pre"/"post_end" pairs that
2281 didn't have a matching "start".
2282
2284 To aid debugging, the source file dump.c contains a number of functions
2285 which produce formatted output of internal data structures.
2286 The most commonly used of these functions is "Perl_sv_dump"; it's used
2288 for dumping SVs, AVs, HVs, and CVs. The "Devel::Peek" module calls
2289 "sv_dump" to produce debugging output from Perl-space, so users of that
2290 module should already be familiar with its format.
2291 "Perl_op_dump" can be used to dump an "OP" structure or any of its
2293 derivatives, and produces output similar to "perl -Dx"; in fact,
2294 "Perl_dump_eval" will dump the main root of the code being evaluated,
2295 exactly like "-Dx".
2296 Other useful functions are "Perl_dump_sub", which turns a "GV" into an
2298 op tree, "Perl_dump_packsubs" which calls "Perl_dump_sub" on all the
2299 subroutines in a package like so: (Thankfully, these are all xsubs, so
2300 there is no op tree)
2301 (gdb) print Perl_dump_packsubs(PL_defstash)
2303 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2305 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2307 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2309 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2311 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2313 and "Perl_dump_all", which dumps all the subroutines in the stash and
2315 the op tree of the main root.
2316
2318 Background and PERL_IMPLICIT_CONTEXT
2319 The Perl interpreter can be regarded as a closed box: it has an API for
2320 feeding it code or otherwise making it do things, but it also has
2321 functions for its own use. This smells a lot like an object, and there
2322 is a way for you to build Perl so that you can have multiple
2323 interpreters, with one interpreter represented either as a C structure,
2324 or inside a thread-specific structure. These structures contain all
2325 the context, the state of that interpreter.
2326 The macro that controls the major Perl build flavor is MULTIPLICITY.
2328 The MULTIPLICITY build has a C structure that packages all the
2329 interpreter state. With multiplicity-enabled perls,
2330 PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support
2331 for passing in a "hidden" first argument that represents all three data
2332 structures. MULTIPLICITY makes multi-threaded perls possible (with the
2333 ithreads threading model, related to the macro USE_ITHREADS.)
2334 To see whether you have non-const data you can use a BSD (or GNU)
2336 compatible "nm":
2337 nm libperl.a | grep -v ' [TURtr] '
2339 If this displays any "D" or "d" symbols (or possibly "C" or "c"), you
2341 have non-const data. The symbols the "grep" removed are as follows:
2342 "Tt" are text, or code, the "Rr" are read-only (const) data, and the
2343 "U" is <undefined>, external symbols referred to.
2344 The test t/porting/libperl.t does this kind of symbol sanity checking
2346 on "libperl.a".
2347 All this obviously requires a way for the Perl internal functions to be
2349 either subroutines taking some kind of structure as the first argument,
2350 or subroutines taking nothing as the first argument. To enable these
2351 two very different ways of building the interpreter, the Perl source
2352 (as it does in so many other situations) makes heavy use of macros and
2353 subroutine naming conventions.
2354 First problem: deciding which functions will be public API functions
2356 and which will be private. All functions whose names begin "S_" are
2357 private (think "S" for "secret" or "static"). All other functions
2358 begin with "Perl_", but just because a function begins with "Perl_"
2359 does not mean it is part of the API. (See "Internal Functions".) The
2360 easiest way to be sure a function is part of the API is to find its
2361 entry in perlapi. If it exists in perlapi, it's part of the API. If
2362 it doesn't, and you think it should be (i.e., you need it for your
2363 extension), submit an issue at <https://github.com/Perl/perl5/issues>
2364 explaining why you think it should be.
2365 Second problem: there must be a syntax so that the same subroutine
2367 declarations and calls can pass a structure as their first argument, or
2368 pass nothing. To solve this, the subroutines are named and declared in
2369 a particular way. Here's a typical start of a static function used
2370 within the Perl guts:
2371 STATIC void
2373 S_incline(pTHX_ char *s)
2374 STATIC becomes "static" in C, and may be #define'd to nothing in some
2376 configurations in the future.
2377 A public function (i.e. part of the internal API, but not necessarily
2379 sanctioned for use in extensions) begins like this:
2380 void
2382 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
2383 "pTHX_" is one of a number of macros (in perl.h) that hide the details
2385 of the interpreter's context. THX stands for "thread", "this", or
2386 "thingy", as the case may be. (And no, George Lucas is not involved.
2387 :-) The first character could be 'p' for a prototype, 'a' for argument,
2388 or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their
2389 variants.
2390 When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
2392 there is no first argument containing the interpreter's context. The
2393 trailing underscore in the pTHX_ macro indicates that the macro
2394 expansion needs a comma after the context argument because other
2395 arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_
2396 will be ignored, and the subroutine is not prototyped to take the extra
2397 argument. The form of the macro without the trailing underscore is
2398 used when there are no additional explicit arguments.
2399 When a core function calls another, it must pass the context. This is
2401 normally hidden via macros. Consider "sv_setiv". It expands into
2402 something like this:
2403 #ifdef PERL_IMPLICIT_CONTEXT
2405 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
2406 /* can't do this for vararg functions, see below */
2407 #else
2408 #define sv_setiv Perl_sv_setiv
2409 #endif
2410 This works well, and means that XS authors can gleefully write:
2412 sv_setiv(foo, bar);
2414 and still have it work under all the modes Perl could have been
2416 compiled with.
2417 This doesn't work so cleanly for varargs functions, though, as macros
2419 imply that the number of arguments is known in advance. Instead we
2420 either need to spell them out fully, passing "aTHX_" as the first
2421 argument (the Perl core tends to do this with functions like
2422 Perl_warner), or use a context-free version.
2423 The context-free version of Perl_warner is called
2425 Perl_warner_nocontext, and does not take the extra argument. Instead
2426 it does "dTHX;" to get the context from thread-local storage. We
2427 "#define warner Perl_warner_nocontext" so that extensions get source
2428 compatibility at the expense of performance. (Passing an arg is
2429 cheaper than grabbing it from thread-local storage.)
2430 You can ignore [pad]THXx when browsing the Perl headers/sources. Those
2432 are strictly for use within the core. Extensions and embedders need
2433 only be aware of [pad]THX.
2434 So what happened to dTHR?
2436 "dTHR" was introduced in perl 5.005 to support the older thread model.
2437 The older thread model now uses the "THX" mechanism to pass context
2438 pointers around, so "dTHR" is not useful any more. Perl 5.6.0 and
2439 later still have it for backward source compatibility, but it is
2440 defined to be a no-op.
2441 How do I use all this in extensions?
2443 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
2444 functions in the Perl API will need to pass the initial context
2445 argument somehow. The kicker is that you will need to write it in such
2446 a way that the extension still compiles when Perl hasn't been built
2447 with PERL_IMPLICIT_CONTEXT enabled.
2448 There are three ways to do this. First, the easy but inefficient way,
2450 which is also the default, in order to maintain source compatibility
2451 with extensions: whenever XSUB.h is #included, it redefines the aTHX
2452 and aTHX_ macros to call a function that will return the context.
2453 Thus, something like:
2454 sv_setiv(sv, num);
2456 in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
2458 in effect:
2459 Perl_sv_setiv(Perl_get_context(), sv, num);
2461 or to this otherwise:
2463 Perl_sv_setiv(sv, num);
2465 You don't have to do anything new in your extension to get this; since
2467 the Perl library provides Perl_get_context(), it will all just work.
2468 The second, more efficient way is to use the following template for
2470 your Foo.xs:
2471 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2473 #include "EXTERN.h"
2474 #include "perl.h"
2475 #include "XSUB.h"
2476 STATIC void my_private_function(int arg1, int arg2);
2478 STATIC void
2480 my_private_function(int arg1, int arg2)
2481 {
2482 dTHX; /* fetch context */
2483 ... call many Perl API functions ...
2484 }
2485 [... etc ...]
2487 MODULE = Foo PACKAGE = Foo
2489 /* typical XSUB */
2491 void
2493 my_xsub(arg)
2494 int arg
2495 CODE:
2496 my_private_function(arg, 10);
2497 Note that the only two changes from the normal way of writing an
2499 extension is the addition of a "#define PERL_NO_GET_CONTEXT" before
2500 including the Perl headers, followed by a "dTHX;" declaration at the
2501 start of every function that will call the Perl API. (You'll know
2502 which functions need this, because the C compiler will complain that
2503 there's an undeclared identifier in those functions.) No changes are
2504 needed for the XSUBs themselves, because the XS() macro is correctly
2505 defined to pass in the implicit context if needed.
2506 The third, even more efficient way is to ape how it is done within the
2508 Perl guts:
2509 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2511 #include "EXTERN.h"
2512 #include "perl.h"
2513 #include "XSUB.h"
2514 /* pTHX_ only needed for functions that call Perl API */
2516 STATIC void my_private_function(pTHX_ int arg1, int arg2);
2517 STATIC void
2519 my_private_function(pTHX_ int arg1, int arg2)
2520 {
2521 /* dTHX; not needed here, because THX is an argument */
2522 ... call Perl API functions ...
2523 }
2524 [... etc ...]
2526 MODULE = Foo PACKAGE = Foo
2528 /* typical XSUB */
2530 void
2532 my_xsub(arg)
2533 int arg
2534 CODE:
2535 my_private_function(aTHX_ arg, 10);
2536 This implementation never has to fetch the context using a function
2538 call, since it is always passed as an extra argument. Depending on
2539 your needs for simplicity or efficiency, you may mix the previous two
2540 approaches freely.
2541 Never add a comma after "pTHX" yourself--always use the form of the
2543 macro with the underscore for functions that take explicit arguments,
2544 or the form without the argument for functions with no explicit
2545 arguments.
2546 Should I do anything special if I call perl from multiple threads?
2548 If you create interpreters in one thread and then proceed to call them
2549 in another, you need to make sure perl's own Thread Local Storage (TLS)
2550 slot is initialized correctly in each of those threads.
2551 The "perl_alloc" and "perl_clone" API functions will automatically set
2553 the TLS slot to the interpreter they created, so that there is no need
2554 to do anything special if the interpreter is always accessed in the
2555 same thread that created it, and that thread did not create or call any
2556 other interpreters afterwards. If that is not the case, you have to
2557 set the TLS slot of the thread before calling any functions in the Perl
2558 API on that particular interpreter. This is done by calling the
2559 "PERL_SET_CONTEXT" macro in that thread as the first thing you do:
2560 /* do this before doing anything else with some_perl */
2562 PERL_SET_CONTEXT(some_perl);
2563 ... other Perl API calls on some_perl go here ...
2565 Future Plans and PERL_IMPLICIT_SYS
2567 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2568 that the interpreter knows about itself and pass it around, so too are
2569 there plans to allow the interpreter to bundle up everything it knows
2570 about the environment it's running on. This is enabled with the
2571 PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2572 Windows.
2573 This allows the ability to provide an extra pointer (called the "host"
2575 environment) for all the system calls. This makes it possible for all
2576 the system stuff to maintain their own state, broken down into seven C
2577 structures. These are thin wrappers around the usual system calls (see
2578 win32/perllib.c) for the default perl executable, but for a more
2579 ambitious host (like the one that would do fork() emulation) all the
2580 extra work needed to pretend that different interpreters are actually
2581 different "processes", would be done here.
2582 The Perl engine/interpreter and the host are orthogonal entities.
2584 There could be one or more interpreters in a process, and one or more
2585 "hosts", with free association between them.
2586
2588 All of Perl's internal functions which will be exposed to the outside
2589 world are prefixed by "Perl_" so that they will not conflict with XS
2590 functions or functions used in a program in which Perl is embedded.
2591 Similarly, all global variables begin with "PL_". (By convention,
2592 static functions start with "S_".)
2593 Inside the Perl core ("PERL_CORE" defined), you can get at the
2595 functions either with or without the "Perl_" prefix, thanks to a bunch
2596 of defines that live in embed.h. Note that extension code should not
2597 set "PERL_CORE"; this exposes the full perl internals, and is likely to
2598 cause breakage of the XS in each new perl release.
2599 The file embed.h is generated automatically from embed.pl and
2601 embed.fnc. embed.pl also creates the prototyping header files for the
2602 internal functions, generates the documentation and a lot of other bits
2603 and pieces. It's important that when you add a new function to the
2604 core or change an existing one, you change the data in the table in
2605 embed.fnc as well. Here's a sample entry from that table:
2606 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2608 The first column is a set of flags, the second column the return type,
2610 the third column the name. Columns after that are the arguments. The
2611 flags are documented at the top of embed.fnc.
2612 If you edit embed.pl or embed.fnc, you will need to run "make
2614 regen_headers" to force a rebuild of embed.h and other auto-generated
2615 files.
2616 Formatted Printing of IVs, UVs, and NVs
2618 If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2619 formatting codes like %d, %ld, %f, you should use the following macros
2620 for portability
2621 IVdf IV in decimal
2623 UVuf UV in decimal
2624 UVof UV in octal
2625 UVxf UV in hexadecimal
2626 NVef NV %e-like
2627 NVff NV %f-like
2628 NVgf NV %g-like
2629 These will take care of 64-bit integers and long doubles. For example:
2631 printf("IV is %" IVdf "\n", iv);
2633 The "IVdf" will expand to whatever is the correct format for the IVs.
2635 Note that the spaces are required around the format in case the code is
2636 compiled with C++, to maintain compliance with its standard.
2637 Note that there are different "long doubles": Perl will use whatever
2639 the compiler has.
2640 If you are printing addresses of pointers, use %p or UVxf combined with
2642 PTR2UV().
2643 Formatted Printing of SVs
2645 The contents of SVs may be printed using the "SVf" format, like so:
2646 Perl_croak(aTHX_ "This croaked because: %" SVf "\n", SVfARG(err_msg))
2648 where "err_msg" is an SV.
2650 Not all scalar types are printable. Simple values certainly are: one
2652 of IV, UV, NV, or PV. Also, if the SV is a reference to some value,
2653 either it will be dereferenced and the value printed, or information
2654 about the type of that value and its address are displayed. The
2655 results of printing any other type of SV are undefined and likely to
2656 lead to an interpreter crash. NVs are printed using a %g-ish format.
2657 Note that the spaces are required around the "SVf" in case the code is
2659 compiled with C++, to maintain compliance with its standard.
2660 Note that any filehandle being printed to under UTF-8 must be expecting
2662 UTF-8 in order to get good results and avoid Wide-character warnings.
2663 One way to do this for typical filehandles is to invoke perl with the
2664 "-C"> parameter. (See "-C [number/list]" in perlrun.
2665 You can use this to concatenate two scalars:
2667 SV *var1 = get_sv("var1", GV_ADD);
2669 SV *var2 = get_sv("var2", GV_ADD);
2670 SV *var3 = newSVpvf("var1=%" SVf " and var2=%" SVf,
2671 SVfARG(var1), SVfARG(var2));
2672 Formatted Printing of Strings
2674 If you just want the bytes printed in a 7bit NUL-terminated string, you
2675 can just use %s (assuming they are all really only 7bit). But if there
2676 is a possibility the value will be encoded as UTF-8 or contains bytes
2677 above 0x7F (and therefore 8bit), you should instead use the "UTF8f"
2678 format. And as its parameter, use the "UTF8fARG()" macro:
2679 chr * msg;
2681 /* U+2018: \xE2\x80\x98 LEFT SINGLE QUOTATION MARK
2683 U+2019: \xE2\x80\x99 RIGHT SINGLE QUOTATION MARK */
2684 if (can_utf8)
2685 msg = "\xE2\x80\x98Uses fancy quotes\xE2\x80\x99";
2686 else
2687 msg = "'Uses simple quotes'";
2688 Perl_croak(aTHX_ "The message is: %" UTF8f "\n",
2690 UTF8fARG(can_utf8, strlen(msg), msg));
2691 The first parameter to "UTF8fARG" is a boolean: 1 if the string is in
2693 UTF-8; 0 if string is in native byte encoding (Latin1). The second
2694 parameter is the number of bytes in the string to print. And the third
2695 and final parameter is a pointer to the first byte in the string.
2696 Note that any filehandle being printed to under UTF-8 must be expecting
2698 UTF-8 in order to get good results and avoid Wide-character warnings.
2699 One way to do this for typical filehandles is to invoke perl with the
2700 "-C"> parameter. (See "-C [number/list]" in perlrun.
2701 Formatted Printing of "Size_t" and "SSize_t"
2703 The most general way to do this is to cast them to a UV or IV, and
2704 print as in the previous section.
2705 But if you're using "PerlIO_printf()", it's less typing and visual
2707 clutter to use the %z length modifier (for siZe):
2708 PerlIO_printf("STRLEN is %zu\n", len);
2710 This modifier is not portable, so its use should be restricted to
2712 "PerlIO_printf()".
2713 Formatted Printing of "Ptrdiff_t", "intmax_t", "short" and other special
2715 sizes
2716 There are modifiers for these special situations if you are using
2717 "PerlIO_printf()". See "size" in perlfunc.
2718 Pointer-To-Integer and Integer-To-Pointer
2720 Because pointer size does not necessarily equal integer size, use the
2721 follow macros to do it right.
2722 PTR2UV(pointer)
2724 PTR2IV(pointer)
2725 PTR2NV(pointer)
2726 INT2PTR(pointertotype, integer)
2727 For example:
2729 IV iv = ...;
2731 SV *sv = INT2PTR(SV*, iv);
2732 and
2734 AV *av = ...;
2736 UV uv = PTR2UV(av);
2737 There are also
2739 PTR2nat(pointer) /* pointer to integer of PTRSIZE */
2741 PTR2ul(pointer) /* pointer to unsigned long */
2742 And "PTRV" which gives the native type for an integer the same size as
2744 pointers, such as "unsigned" or "unsigned long".
2745 Exception Handling
2747 There are a couple of macros to do very basic exception handling in XS
2748 modules. You have to define "NO_XSLOCKS" before including XSUB.h to be
2749 able to use these macros:
2750 #define NO_XSLOCKS
2752 #include "XSUB.h"
2753 You can use these macros if you call code that may croak, but you need
2755 to do some cleanup before giving control back to Perl. For example:
2756 dXCPT; /* set up necessary variables */
2758 XCPT_TRY_START {
2760 code_that_may_croak();
2761 } XCPT_TRY_END
2762 XCPT_CATCH
2764 {
2765 /* do cleanup here */
2766 XCPT_RETHROW;
2767 }
2768 Note that you always have to rethrow an exception that has been caught.
2770 Using these macros, it is not possible to just catch the exception and
2771 ignore it. If you have to ignore the exception, you have to use the
2772 "call_*" function.
2773 The advantage of using the above macros is that you don't have to setup
2775 an extra function for "call_*", and that using these macros is faster
2776 than using "call_*".
2777 Source Documentation
2779 There's an effort going on to document the internal functions and
2780 automatically produce reference manuals from them -- perlapi is one
2781 such manual which details all the functions which are available to XS
2782 writers. perlintern is the autogenerated manual for the functions
2783 which are not part of the API and are supposedly for internal use only.
2784 Source documentation is created by putting POD comments into the C
2786 source, like this:
2787 /*
2789 =for apidoc sv_setiv
2790 Copies an integer into the given SV. Does not handle 'set' magic. See
2792 L<perlapi/sv_setiv_mg>.
2793 =cut
2795 */
2796 Please try and supply some documentation if you add functions to the
2798 Perl core.
2799 Backwards compatibility
2801 The Perl API changes over time. New functions are added or the
2802 interfaces of existing functions are changed. The "Devel::PPPort"
2803 module tries to provide compatibility code for some of these changes,
2804 so XS writers don't have to code it themselves when supporting multiple
2805 versions of Perl.
2806 "Devel::PPPort" generates a C header file ppport.h that can also be run
2808 as a Perl script. To generate ppport.h, run:
2809 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2811 Besides checking existing XS code, the script can also be used to
2813 retrieve compatibility information for various API calls using the
2814 "--api-info" command line switch. For example:
2815 % perl ppport.h --api-info=sv_magicext
2817 For details, see "perldoc ppport.h".
2819
2821 Perl 5.6.0 introduced Unicode support. It's important for porters and
2822 XS writers to understand this support and make sure that the code they
2823 write does not corrupt Unicode data.
2824 What is Unicode, anyway?
2826 In the olden, less enlightened times, we all used to use ASCII. Most
2827 of us did, anyway. The big problem with ASCII is that it's American.
2828 Well, no, that's not actually the problem; the problem is that it's not
2829 particularly useful for people who don't use the Roman alphabet. What
2830 used to happen was that particular languages would stick their own
2831 alphabet in the upper range of the sequence, between 128 and 255. Of
2832 course, we then ended up with plenty of variants that weren't quite
2833 ASCII, and the whole point of it being a standard was lost.
2834 Worse still, if you've got a language like Chinese or Japanese that has
2836 hundreds or thousands of characters, then you really can't fit them
2837 into a mere 256, so they had to forget about ASCII altogether, and
2838 build their own systems using pairs of numbers to refer to one
2839 character.
2840 To fix this, some people formed Unicode, Inc. and produced a new
2842 character set containing all the characters you can possibly think of
2843 and more. There are several ways of representing these characters, and
2844 the one Perl uses is called UTF-8. UTF-8 uses a variable number of
2845 bytes to represent a character. You can learn more about Unicode and
2846 Perl's Unicode model in perlunicode.
2847 (On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
2849 UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
2850 UTF-EBCDIC is like UTF-8, but the details are different. The macros
2851 hide the differences from you, just remember that the particular
2852 numbers and bit patterns presented below will differ in UTF-EBCDIC.)
2853 How can I recognise a UTF-8 string?
2855 You can't. This is because UTF-8 data is stored in bytes just like
2856 non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
2857 capital E with a grave accent, is represented by the two bytes
2858 "v196.172". Unfortunately, the non-Unicode string "chr(196).chr(172)"
2859 has that byte sequence as well. So you can't tell just by looking --
2860 this is what makes Unicode input an interesting problem.
2861 In general, you either have to know what you're dealing with, or you
2863 have to guess. The API function "is_utf8_string" can help; it'll tell
2864 you if a string contains only valid UTF-8 characters, and the chances
2865 of a non-UTF-8 string looking like valid UTF-8 become very small very
2866 quickly with increasing string length. On a character-by-character
2867 basis, "isUTF8_CHAR" will tell you whether the current character in a
2868 string is valid UTF-8.
2869 How does UTF-8 represent Unicode characters?
2871 As mentioned above, UTF-8 uses a variable number of bytes to store a
2872 character. Characters with values 0...127 are stored in one byte, just
2873 like good ol' ASCII. Character 128 is stored as "v194.128"; this
2874 continues up to character 191, which is "v194.191". Now we've run out
2875 of bits (191 is binary 10111111) so we move on; character 192 is
2876 "v195.128". And so it goes on, moving to three bytes at character
2877 2048. "Unicode Encodings" in perlunicode has pictures of how this
2878 works.
2879 Assuming you know you're dealing with a UTF-8 string, you can find out
2881 how long the first character in it is with the "UTF8SKIP" macro:
2882 char *utf = "\305\233\340\240\201";
2884 I32 len;
2885 len = UTF8SKIP(utf); /* len is 2 here */
2887 utf += len;
2888 len = UTF8SKIP(utf); /* len is 3 here */
2889 Another way to skip over characters in a UTF-8 string is to use
2891 "utf8_hop", which takes a string and a number of characters to skip
2892 over. You're on your own about bounds checking, though, so don't use
2893 it lightly.
2894 All bytes in a multi-byte UTF-8 character will have the high bit set,
2896 so you can test if you need to do something special with this character
2897 like this (the "UTF8_IS_INVARIANT()" is a macro that tests whether the
2898 byte is encoded as a single byte even in UTF-8):
2899 U8 *utf; /* Initialize this to point to the beginning of the
2901 sequence to convert */
2902 U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
2903 pointed to by 'utf' */
2904 UV uv; /* Returned code point; note: a UV, not a U8, not a
2905 char */
2906 STRLEN len; /* Returned length of character in bytes */
2907 if (!UTF8_IS_INVARIANT(*utf))
2909 /* Must treat this as UTF-8 */
2910 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
2911 else
2912 /* OK to treat this character as a byte */
2913 uv = *utf;
2914 You can also see in that example that we use "utf8_to_uvchr_buf" to get
2916 the value of the character; the inverse function "uvchr_to_utf8" is
2917 available for putting a UV into UTF-8:
2918 if (!UVCHR_IS_INVARIANT(uv))
2920 /* Must treat this as UTF8 */
2921 utf8 = uvchr_to_utf8(utf8, uv);
2922 else
2923 /* OK to treat this character as a byte */
2924 *utf8++ = uv;
2925 You must convert characters to UVs using the above functions if you're
2927 ever in a situation where you have to match UTF-8 and non-UTF-8
2928 characters. You may not skip over UTF-8 characters in this case. If
2929 you do this, you'll lose the ability to match hi-bit non-UTF-8
2930 characters; for instance, if your UTF-8 string contains "v196.172", and
2931 you skip that character, you can never match a "chr(200)" in a
2932 non-UTF-8 string. So don't do that!
2933 (Note that we don't have to test for invariant characters in the
2935 examples above. The functions work on any well-formed UTF-8 input.
2936 It's just that its faster to avoid the function overhead when it's not
2937 needed.)
2938 How does Perl store UTF-8 strings?
2940 Currently, Perl deals with UTF-8 strings and non-UTF-8 strings slightly
2941 differently. A flag in the SV, "SVf_UTF8", indicates that the string
2942 is internally encoded as UTF-8. Without it, the byte value is the
2943 codepoint number and vice versa. This flag is only meaningful if the
2944 SV is "SvPOK" or immediately after stringification via "SvPV" or a
2945 similar macro. You can check and manipulate this flag with the
2946 following macros:
2947 SvUTF8(sv)
2949 SvUTF8_on(sv)
2950 SvUTF8_off(sv)
2951 This flag has an important effect on Perl's treatment of the string: if
2953 UTF-8 data is not properly distinguished, regular expressions,
2954 "length", "substr" and other string handling operations will have
2955 undesirable (wrong) results.
2956 The problem comes when you have, for instance, a string that isn't
2958 flagged as UTF-8, and contains a byte sequence that could be UTF-8 --
2959 especially when combining non-UTF-8 and UTF-8 strings.
2960 Never forget that the "SVf_UTF8" flag is separate from the PV value;
2962 you need to be sure you don't accidentally knock it off while you're
2963 manipulating SVs. More specifically, you cannot expect to do this:
2964 SV *sv;
2966 SV *nsv;
2967 STRLEN len;
2968 char *p;
2969 p = SvPV(sv, len);
2971 frobnicate(p);
2972 nsv = newSVpvn(p, len);
2973 The "char*" string does not tell you the whole story, and you can't
2975 copy or reconstruct an SV just by copying the string value. Check if
2976 the old SV has the UTF8 flag set (after the "SvPV" call), and act
2977 accordingly:
2978 p = SvPV(sv, len);
2980 is_utf8 = SvUTF8(sv);
2981 frobnicate(p, is_utf8);
2982 nsv = newSVpvn(p, len);
2983 if (is_utf8)
2984 SvUTF8_on(nsv);
2985 In the above, your "frobnicate" function has been changed to be made
2987 aware of whether or not it's dealing with UTF-8 data, so that it can
2988 handle the string appropriately.
2989 Since just passing an SV to an XS function and copying the data of the
2991 SV is not enough to copy the UTF8 flags, even less right is just
2992 passing a "char *" to an XS function.
2993 For full generality, use the "DO_UTF8" macro to see if the string in an
2995 SV is to be treated as UTF-8. This takes into account if the call to
2996 the XS function is being made from within the scope of "use bytes". If
2997 so, the underlying bytes that comprise the UTF-8 string are to be
2998 exposed, rather than the character they represent. But this pragma
2999 should only really be used for debugging and perhaps low-level testing
3000 at the byte level. Hence most XS code need not concern itself with
3001 this, but various areas of the perl core do need to support it.
3002 And this isn't the whole story. Starting in Perl v5.12, strings that
3004 aren't encoded in UTF-8 may also be treated as Unicode under various
3005 conditions (see "ASCII Rules versus Unicode Rules" in perlunicode).
3006 This is only really a problem for characters whose ordinals are between
3007 128 and 255, and their behavior varies under ASCII versus Unicode rules
3008 in ways that your code cares about (see "The "Unicode Bug"" in
3009 perlunicode). There is no published API for dealing with this, as it
3010 is subject to change, but you can look at the code for "pp_lc" in pp.c
3011 for an example as to how it's currently done.
3012 How do I pass a Perl string to a C library?
3014 A Perl string, conceptually, is an opaque sequence of code points.
3015 Many C libraries expect their inputs to be "classical" C strings, which
3016 are arrays of octets 1-255, terminated with a NUL byte. Your job when
3017 writing an interface between Perl and a C library is to define the
3018 mapping between Perl and that library.
3019 Generally speaking, "SvPVbyte" and related macros suit this task well.
3021 These assume that your Perl string is a "byte string", i.e., is either
3022 raw, undecoded input into Perl or is pre-encoded to, e.g., UTF-8.
3023 Alternatively, if your C library expects UTF-8 text, you can use
3025 "SvPVutf8" and related macros. This has the same effect as encoding to
3026 UTF-8 then calling the corresponding "SvPVbyte"-related macro.
3027 Some C libraries may expect other encodings (e.g., UTF-16LE). To give
3029 Perl strings to such libraries you must either do that encoding in Perl
3030 then use "SvPVbyte", or use an intermediary C library to convert from
3031 however Perl stores the string to the desired encoding.
3032 Take care also that NULs in your Perl string don't confuse the C
3034 library. If possible, give the string's length to the C library; if
3035 that's not possible, consider rejecting strings that contain NUL bytes.
3036 What about "SvPV", "SvPV_nolen", etc.?
3038 Consider a 3-character Perl string "$foo = "\x64\x78\x8c"". Perl can
3040 store these 3 characters either of two ways:
3041 • bytes: 0x64 0x78 0x8c
3043 • UTF-8: 0x64 0x78 0xc2 0x8c
3045 Now let's say you convert $foo to a C string thus:
3047 STRLEN strlen;
3049 char *str = SvPV(foo_sv, strlen);
3050 At this point "str" could point to a 3-byte C string or a 4-byte one.
3052 Generally speaking, we want "str" to be the same regardless of how Perl
3054 stores $foo, so the ambiguity here is undesirable. "SvPVbyte" and
3055 "SvPVutf8" solve that by giving predictable output: use "SvPVbyte" if
3056 your C library expects byte strings, or "SvPVutf8" if it expects UTF-8.
3057 If your C library happens to support both encodings, then
3059 "SvPV"--always in tandem with lookups to "SvUTF8"!--may be safe and
3060 (slightly) more efficient.
3061 TESTING TIP: Use utf8's "upgrade" and "downgrade" functions in your
3063 tests to ensure consistent handling regardless of Perl's internal
3064 encoding.
3065 How do I convert a string to UTF-8?
3067 If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to
3068 upgrade the non-UTF-8 strings to UTF-8. If you've got an SV, the
3069 easiest way to do this is:
3070 sv_utf8_upgrade(sv);
3072 However, you must not do this, for example:
3074 if (!SvUTF8(left))
3076 sv_utf8_upgrade(left);
3077 If you do this in a binary operator, you will actually change one of
3079 the strings that came into the operator, and, while it shouldn't be
3080 noticeable by the end user, it can cause problems in deficient code.
3081 Instead, "bytes_to_utf8" will give you a UTF-8-encoded copy of its
3083 string argument. This is useful for having the data available for
3084 comparisons and so on, without harming the original SV. There's also
3085 "utf8_to_bytes" to go the other way, but naturally, this will fail if
3086 the string contains any characters above 255 that can't be represented
3087 in a single byte.
3088 How do I compare strings?
3090 "sv_cmp" in perlapi and "sv_cmp_flags" in perlapi do a lexigraphic
3091 comparison of two SV's, and handle UTF-8ness properly. Note, however,
3092 that Unicode specifies a much fancier mechanism for collation,
3093 available via the Unicode::Collate module.
3094 To just compare two strings for equality/non-equality, you can just use
3096 "memEQ()" and "memNE()" as usual, except the strings must be both UTF-8
3097 or not UTF-8 encoded.
3098 To compare two strings case-insensitively, use "foldEQ_utf8()" (the
3100 strings don't have to have the same UTF-8ness).
3101 Is there anything else I need to know?
3103 Not really. Just remember these things:
3104 • There's no way to tell if a "char *" or "U8 *" string is UTF-8 or
3106 not. But you can tell if an SV is to be treated as UTF-8 by calling
3107 "DO_UTF8" on it, after stringifying it with "SvPV" or a similar
3108 macro. And, you can tell if SV is actually UTF-8 (even if it is not
3109 to be treated as such) by looking at its "SvUTF8" flag (again after
3110 stringifying it). Don't forget to set the flag if something should
3111 be UTF-8. Treat the flag as part of the PV, even though it's not --
3112 if you pass on the PV to somewhere, pass on the flag too.
3113 • If a string is UTF-8, always use "utf8_to_uvchr_buf" to get at the
3115 value, unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s.
3116 • When writing a character UV to a UTF-8 string, always use
3118 "uvchr_to_utf8", unless "UVCHR_IS_INVARIANT(uv))" in which case you
3119 can use "*s = uv".
3120 • Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8"
3122 to get a new string which is UTF-8 encoded, and then combine them.
3123
3125 Custom operator support is an experimental feature that allows you to
3126 define your own ops. This is primarily to allow the building of
3127 interpreters for other languages in the Perl core, but it also allows
3128 optimizations through the creation of "macro-ops" (ops which perform
3129 the functions of multiple ops which are usually executed together, such
3130 as "gvsv, gvsv, add".)
3131 This feature is implemented as a new op type, "OP_CUSTOM". The Perl
3133 core does not "know" anything special about this op type, and so it
3134 will not be involved in any optimizations. This also means that you
3135 can define your custom ops to be any op structure -- unary, binary,
3136 list and so on -- you like.
3137 It's important to know what custom operators won't do for you. They
3139 won't let you add new syntax to Perl, directly. They won't even let
3140 you add new keywords, directly. In fact, they won't change the way
3141 Perl compiles a program at all. You have to do those changes yourself,
3142 after Perl has compiled the program. You do this either by
3143 manipulating the op tree using a "CHECK" block and the "B::Generate"
3144 module, or by adding a custom peephole optimizer with the "optimize"
3145 module.
3146 When you do this, you replace ordinary Perl ops with custom ops by
3148 creating ops with the type "OP_CUSTOM" and the "op_ppaddr" of your own
3149 PP function. This should be defined in XS code, and should look like
3150 the PP ops in "pp_*.c". You are responsible for ensuring that your op
3151 takes the appropriate number of values from the stack, and you are
3152 responsible for adding stack marks if necessary.
3153 You should also "register" your op with the Perl interpreter so that it
3155 can produce sensible error and warning messages. Since it is possible
3156 to have multiple custom ops within the one "logical" op type
3157 "OP_CUSTOM", Perl uses the value of "o->op_ppaddr" to determine which
3158 custom op it is dealing with. You should create an "XOP" structure for
3159 each ppaddr you use, set the properties of the custom op with
3160 "XopENTRY_set", and register the structure against the ppaddr using
3161 "Perl_custom_op_register". A trivial example might look like:
3162 static XOP my_xop;
3164 static OP *my_pp(pTHX);
3165 BOOT:
3167 XopENTRY_set(&my_xop, xop_name, "myxop");
3168 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3169 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3170 The available fields in the structure are:
3172 xop_name
3174 A short name for your op. This will be included in some error
3175 messages, and will also be returned as "$op->name" by the B module,
3176 so it will appear in the output of module like B::Concise.
3177 xop_desc
3179 A short description of the function of the op.
3180 xop_class
3182 Which of the various *OP structures this op uses. This should be
3183 one of the "OA_*" constants from op.h, namely
3184 OA_BASEOP
3186 OA_UNOP
3187 OA_BINOP
3188 OA_LOGOP
3189 OA_LISTOP
3190 OA_PMOP
3191 OA_SVOP
3192 OA_PADOP
3193 OA_PVOP_OR_SVOP
3194 This should be interpreted as '"PVOP"' only. The "_OR_SVOP" is
3195 because the only core "PVOP", "OP_TRANS", can sometimes be a
3196 "SVOP" instead.
3197 OA_LOOP
3199 OA_COP
3200 The other "OA_*" constants should not be used.
3202 xop_peep
3204 This member is of type "Perl_cpeep_t", which expands to "void
3205 (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)". If it is set, this
3206 function will be called from "Perl_rpeep" when ops of this type are
3207 encountered by the peephole optimizer. o is the OP that needs
3208 optimizing; oldop is the previous OP optimized, whose "op_next"
3209 points to o.
3210 "B::Generate" directly supports the creation of custom ops by name.
3212
3214 Descriptions above occasionally refer to "the stack", but there are in
3215 fact many stack-like data structures within the perl interpreter. When
3216 otherwise unqualified, "the stack" usually refers to the value stack.
3217 The various stacks have different purposes, and operate in slightly
3219 different ways. Their differences are noted below.
3220 Value Stack
3222 This stack stores the values that regular perl code is operating on,
3223 usually intermediate values of expressions within a statement. The
3224 stack itself is formed of an array of SV pointers.
3225 The base of this stack is pointed to by the interpreter variable
3227 "PL_stack_base", of type "SV **".
3228 The head of the stack is "PL_stack_sp", and points to the most
3230 recently-pushed item.
3231 Items are pushed to the stack by using the "PUSHs()" macro or its
3233 variants described above; "XPUSHs()", "mPUSHs()", "mXPUSHs()" and the
3234 typed versions. Note carefully that the non-"X" versions of these
3235 macros do not check the size of the stack and assume it to be big
3236 enough. These must be paired with a suitable check of the stack's size,
3237 such as the "EXTEND" macro to ensure it is large enough. For example
3238 EXTEND(SP, 4);
3240 mPUSHi(10);
3241 mPUSHi(20);
3242 mPUSHi(30);
3243 mPUSHi(40);
3244 This is slightly more performant than making four separate checks in
3246 four separate "mXPUSHi()" calls.
3247 As a further performance optimisation, the various "PUSH" macros all
3249 operate using a local variable "SP", rather than the interpreter-global
3250 variable "PL_stack_sp". This variable is declared by the "dSP" macro -
3251 though it is normally implied by XSUBs and similar so it is rare you
3252 have to consider it directly. Once declared, the "PUSH" macros will
3253 operate only on this local variable, so before invoking any other perl
3254 core functions you must use the "PUTBACK" macro to return the value
3255 from the local "SP" variable back to the interpreter variable.
3256 Similarly, after calling a perl core function which may have had reason
3257 to move the stack or push/pop values to it, you must use the "SPAGAIN"
3258 macro which refreshes the local "SP" value back from the interpreter
3259 one.
3260 Items are popped from the stack by using the "POPs" macro or its typed
3262 versions, There is also a macro "TOPs" that inspects the topmost item
3263 without removing it.
3264 Note specifically that SV pointers on the value stack do not contribute
3266 to the overall reference count of the xVs being referred to. If newly-
3267 created xVs are being pushed to the stack you must arrange for them to
3268 be destroyed at a suitable time; usually by using one of the "mPUSH*"
3269 macros or "sv_2mortal()" to mortalise the xV.
3270 Mark Stack
3272 The value stack stores individual perl scalar values as temporaries
3273 between expressions. Some perl expressions operate on entire lists; for
3274 that purpose we need to know where on the stack each list begins. This
3275 is the purpose of the mark stack.
3276 The mark stack stores integers as I32 values, which are the height of
3278 the value stack at the time before the list began; thus the mark itself
3279 actually points to the value stack entry one before the list. The list
3280 itself starts at "mark + 1".
3281 The base of this stack is pointed to by the interpreter variable
3283 "PL_markstack", of type "I32 *".
3284 The head of the stack is "PL_markstack_ptr", and points to the most
3286 recently-pushed item.
3287 Items are pushed to the stack by using the "PUSHMARK()" macro. Even
3289 though the stack itself stores (value) stack indices as integers, the
3290 "PUSHMARK" macro should be given a stack pointer directly; it will
3291 calculate the index offset by comparing to the "PL_stack_sp" variable.
3292 Thus almost always the code to perform this is
3293 PUSHMARK(SP);
3295 Items are popped from the stack by the "POPMARK" macro. There is also a
3297 macro "TOPMARK" that inspects the topmost item without removing it.
3298 These macros return I32 index values directly. There is also the
3299 "dMARK" macro which declares a new SV double-pointer variable, called
3300 "mark", which points at the marked stack slot; this is the usual macro
3301 that C code will use when operating on lists given on the stack.
3302 As noted above, the "mark" variable itself will point at the most
3304 recently pushed value on the value stack before the list begins, and so
3305 the list itself starts at "mark + 1". The values of the list may be
3306 iterated by code such as
3307 for(SV **svp = mark + 1; svp <= PL_stack_sp; svp++) {
3309 SV *item = *svp;
3310 ...
3311 }
3312 Note specifically in the case that the list is already empty, "mark"
3314 will equal "PL_stack_sp".
3315 Because the "mark" variable is converted to a pointer on the value
3317 stack, extra care must be taken if "EXTEND" or any of the "XPUSH"
3318 macros are invoked within the function, because the stack may need to
3319 be moved to extend it and so the existing pointer will now be invalid.
3320 If this may be a problem, a possible solution is to track the mark
3321 offset as an integer and track the mark itself later on after the stack
3322 had been moved.
3323 I32 markoff = POPMARK;
3325 ...
3327 SP **mark = PL_stack_base + markoff;
3329 Temporaries Stack
3331 As noted above, xV references on the main value stack do not contribute
3332 to the reference count of an xV, and so another mechanism is used to
3333 track when temporary values which live on the stack must be released.
3334 This is the job of the temporaries stack.
3335 The temporaries stack stores pointers to xVs whose reference counts
3337 will be decremented soon.
3338 The base of this stack is pointed to by the interpreter variable
3340 "PL_tmps_stack", of type "SV **".
3341 The head of the stack is indexed by "PL_tmps_ix", an integer which
3343 stores the index in the array of the most recently-pushed item.
3344 There is no public API to directly push items to the temporaries stack.
3346 Instead, the API function "sv_2mortal()" is used to mortalize an xV,
3347 adding its address to the temporaries stack.
3348 Likewise, there is no public API to read values from the temporaries
3350 stack. Instead, the macros "SAVETMPS" and "FREETMPS" are used. The
3351 "SAVETMPS" macro establishes the base levels of the temporaries stack,
3352 by capturing the current value of "PL_tmps_ix" into "PL_tmps_floor" and
3353 saving the previous value to the save stack. Thereafter, whenever
3354 "FREETMPS" is invoked all of the temporaries that have been pushed
3355 since that level are reclaimed.
3356 While it is common to see these two macros in pairs within an "ENTER"/
3358 "LEAVE" pair, it is not necessary to match them. It is permitted to
3359 invoke "FREETMPS" multiple times since the most recent "SAVETMPS"; for
3360 example in a loop iterating over elements of a list. While you can
3361 invoke "SAVETMPS" multiple times within a scope pair, it is unlikely to
3362 be useful. Subsequent invocations will move the temporaries floor
3363 further up, thus effectively trapping the existing temporaries to only
3364 be released at the end of the scope.
3365 Save Stack
3367 The save stack is used by perl to implement the "local" keyword and
3368 other similar behaviours; any cleanup operations that need to be
3369 performed when leaving the current scope. Items pushed to this stack
3370 generally capture the current value of some internal variable or state,
3371 which will be restored when the scope is unwound due to leaving,
3372 "return", "die", "goto" or other reasons.
3373 Whereas other perl internal stacks store individual items all of the
3375 same type (usually SV pointers or integers), the items pushed to the
3376 save stack are formed of many different types, having multiple fields
3377 to them. For example, the "SAVEt_INT" type needs to store both the
3378 address of the "int" variable to restore, and the value to restore it
3379 to. This information could have been stored using fields of a "struct",
3380 but would have to be large enough to store three pointers in the
3381 largest case, which would waste a lot of space in most of the smaller
3382 cases.
3383 Instead, the stack stores information in a variable-length encoding of
3385 "ANY" structures. The final value pushed is stored in the "UV" field
3386 which encodes the kind of item held by the preceeding items; the count
3387 and types of which will depend on what kind of item is being stored.
3388 The kind field is pushed last because that will be the first field to
3389 be popped when unwinding items from the stack.
3390 The base of this stack is pointed to by the interpreter variable
3392 "PL_savestack", of type "ANY *".
3393 The head of the stack is indexed by "PL_savestack_ix", an integer which
3395 stores the index in the array at which the next item should be pushed.
3396 (Note that this is different to most other stacks, which reference the
3397 most recently-pushed item).
3398 Items are pushed to the save stack by using the various "SAVE...()"
3400 macros. Many of these macros take a variable and store both its
3401 address and current value on the save stack, ensuring that value gets
3402 restored on scope exit.
3403 SAVEI8(i8)
3405 SAVEI16(i16)
3406 SAVEI32(i32)
3407 SAVEINT(i)
3408 ...
3409 There are also a variety of other special-purpose macros which save
3411 particular types or values of interest. "SAVETMPS" has already been
3412 mentioned above. Others include "SAVEFREEPV" which arranges for a PV
3413 (i.e. a string buffer) to be freed, or "SAVEDESTRUCTOR" which arranges
3414 for a given function pointer to be invoked on scope exit. A full list
3415 of such macros can be found in scope.h.
3416 There is no public API for popping individual values or items from the
3418 save stack. Instead, via the scope stack, the "ENTER" and "LEAVE" pair
3419 form a way to start and stop nested scopes. Leaving a nested scope via
3420 "LEAVE" will restore all of the saved values that had been pushed since
3421 the most recent "ENTER".
3422 Scope Stack
3424 As with the mark stack to the value stack, the scope stack forms a pair
3425 with the save stack. The scope stack stores the height of the save
3426 stack at which nested scopes begin, and allows the save stack to be
3427 unwound back to that point when the scope is left.
3428 When perl is built with debugging enabled, there is a second part to
3430 this stack storing human-readable string names describing the type of
3431 stack context. Each push operation saves the name as well as the height
3432 of the save stack, and each pop operation checks the topmost name with
3433 what is expected, causing an assertion failure if the name does not
3434 match.
3435 The base of this stack is pointed to by the interpreter variable
3437 "PL_scopestack", of type "I32 *". If enabled, the scope stack names are
3438 stored in a separate array pointed to by "PL_scopestack_name", of type
3439 "const char **".
3440 The head of the stack is indexed by "PL_scopestack_ix", an integer
3442 which stores the index of the array or arrays at which the next item
3443 should be pushed. (Note that this is different to most other stacks,
3444 which reference the most recently-pushed item).
3445 Values are pushed to the scope stack using the "ENTER" macro, which
3447 begins a new nested scope. Any items pushed to the save stack are then
3448 restored at the next nested invocation of the "LEAVE" macro.
3449
3451 Note: this section describes a non-public internal API that is subject
3452 to change without notice.
3453 Introduction to the context stack
3455 In Perl, dynamic scoping refers to the runtime nesting of things like
3456 subroutine calls, evals etc, as well as the entering and exiting of
3457 block scopes. For example, the restoring of a "local"ised variable is
3458 determined by the dynamic scope.
3459 Perl tracks the dynamic scope by a data structure called the context
3461 stack, which is an array of "PERL_CONTEXT" structures, and which is
3462 itself a big union for all the types of context. Whenever a new scope
3463 is entered (such as a block, a "for" loop, or a subroutine call), a new
3464 context entry is pushed onto the stack. Similarly when leaving a block
3465 or returning from a subroutine call etc. a context is popped. Since the
3466 context stack represents the current dynamic scope, it can be searched.
3467 For example, "next LABEL" searches back through the stack looking for a
3468 loop context that matches the label; "return" pops contexts until it
3469 finds a sub or eval context or similar; "caller" examines sub contexts
3470 on the stack.
3471 Each context entry is labelled with a context type, "cx_type". Typical
3473 context types are "CXt_SUB", "CXt_EVAL" etc., as well as "CXt_BLOCK"
3474 and "CXt_NULL" which represent a basic scope (as pushed by "pp_enter")
3475 and a sort block. The type determines which part of the context union
3476 are valid.
3477 The main division in the context struct is between a substitution scope
3479 ("CXt_SUBST") and block scopes, which are everything else. The former
3480 is just used while executing "s///e", and won't be discussed further
3481 here.
3482 All the block scope types share a common base, which corresponds to
3484 "CXt_BLOCK". This stores the old values of various scope-related
3485 variables like "PL_curpm", as well as information about the current
3486 scope, such as "gimme". On scope exit, the old variables are restored.
3487 Particular block scope types store extra per-type information. For
3489 example, "CXt_SUB" stores the currently executing CV, while the various
3490 for loop types might hold the original loop variable SV. On scope exit,
3491 the per-type data is processed; for example the CV has its reference
3492 count decremented, and the original loop variable is restored.
3493 The macro "cxstack" returns the base of the current context stack,
3495 while "cxstack_ix" is the index of the current frame within that stack.
3496 In fact, the context stack is actually part of a stack-of-stacks
3498 system; whenever something unusual is done such as calling a "DESTROY"
3499 or tie handler, a new stack is pushed, then popped at the end.
3500 Note that the API described here changed considerably in perl 5.24;
3502 prior to that, big macros like "PUSHBLOCK" and "POPSUB" were used; in
3503 5.24 they were replaced by the inline static functions described below.
3504 In addition, the ordering and detail of how these macros/function work
3505 changed in many ways, often subtly. In particular they didn't handle
3506 saving the savestack and temps stack positions, and required additional
3507 "ENTER", "SAVETMPS" and "LEAVE" compared to the new functions. The old-
3508 style macros will not be described further.
3509 Pushing contexts
3511 For pushing a new context, the two basic functions are "cx =
3512 cx_pushblock()", which pushes a new basic context block and returns its
3513 address, and a family of similar functions with names like
3514 "cx_pushsub(cx)" which populate the additional type-dependent fields in
3515 the "cx" struct. Note that "CXt_NULL" and "CXt_BLOCK" don't have their
3516 own push functions, as they don't store any data beyond that pushed by
3517 "cx_pushblock".
3518 The fields of the context struct and the arguments to the "cx_*"
3520 functions are subject to change between perl releases, representing
3521 whatever is convenient or efficient for that release.
3522 A typical context stack pushing can be found in "pp_entersub"; the
3524 following shows a simplified and stripped-down example of a non-XS
3525 call, along with comments showing roughly what each function does.
3526 dMARK;
3528 U8 gimme = GIMME_V;
3529 bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
3530 OP *retop = PL_op->op_next;
3531 I32 old_ss_ix = PL_savestack_ix;
3532 CV *cv = ....;
3533 /* ... make mortal copies of stack args which are PADTMPs here ... */
3535 /* ... do any additional savestack pushes here ... */
3537 /* Now push a new context entry of type 'CXt_SUB'; initially just
3539 * doing the actions common to all block types: */
3540 cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
3542 /* this does (approximately):
3544 CXINC; /* cxstack_ix++ (grow if necessary) */
3545 cx = CX_CUR(); /* and get the address of new frame */
3546 cx->cx_type = CXt_SUB;
3547 cx->blk_gimme = gimme;
3548 cx->blk_oldsp = MARK - PL_stack_base;
3549 cx->blk_oldsaveix = old_ss_ix;
3550 cx->blk_oldcop = PL_curcop;
3551 cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
3552 cx->blk_oldscopesp = PL_scopestack_ix;
3553 cx->blk_oldpm = PL_curpm;
3554 cx->blk_old_tmpsfloor = PL_tmps_floor;
3555 PL_tmps_floor = PL_tmps_ix;
3557 */
3558 /* then update the new context frame with subroutine-specific info,
3561 * such as the CV about to be executed: */
3562 cx_pushsub(cx, cv, retop, hasargs);
3564 /* this does (approximately):
3566 cx->blk_sub.cv = cv;
3567 cx->blk_sub.olddepth = CvDEPTH(cv);
3568 cx->blk_sub.prevcomppad = PL_comppad;
3569 cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
3570 cx->blk_sub.retop = retop;
3571 SvREFCNT_inc_simple_void_NN(cv);
3572 */
3573 Note that "cx_pushblock()" sets two new floors: for the args stack (to
3575 "MARK") and the temps stack (to "PL_tmps_ix"). While executing at this
3576 scope level, every "nextstate" (amongst others) will reset the args and
3577 tmps stack levels to these floors. Note that since "cx_pushblock" uses
3578 the current value of "PL_tmps_ix" rather than it being passed as an
3579 arg, this dictates at what point "cx_pushblock" should be called. In
3580 particular, any new mortals which should be freed only on scope exit
3581 (rather than at the next "nextstate") should be created first.
3582 Most callers of "cx_pushblock" simply set the new args stack floor to
3584 the top of the previous stack frame, but for "CXt_LOOP_LIST" it stores
3585 the items being iterated over on the stack, and so sets "blk_oldsp" to
3586 the top of these items instead. Note that, contrary to its name,
3587 "blk_oldsp" doesn't always represent the value to restore "PL_stack_sp"
3588 to on scope exit.
3589 Note the early capture of "PL_savestack_ix" to "old_ss_ix", which is
3591 later passed as an arg to "cx_pushblock". In the case of "pp_entersub",
3592 this is because, although most values needing saving are stored in
3593 fields of the context struct, an extra value needs saving only when the
3594 debugger is running, and it doesn't make sense to bloat the struct for
3595 this rare case. So instead it is saved on the savestack. Since this
3596 value gets calculated and saved before the context is pushed, it is
3597 necessary to pass the old value of "PL_savestack_ix" to "cx_pushblock",
3598 to ensure that the saved value gets freed during scope exit. For most
3599 users of "cx_pushblock", where nothing needs pushing on the save stack,
3600 "PL_savestack_ix" is just passed directly as an arg to "cx_pushblock".
3601 Note that where possible, values should be saved in the context struct
3603 rather than on the save stack; it's much faster that way.
3604 Normally "cx_pushblock" should be immediately followed by the
3606 appropriate "cx_pushfoo", with nothing between them; this is because if
3607 code in-between could die (e.g. a warning upgraded to fatal), then the
3608 context stack unwinding code in "dounwind" would see (in the example
3609 above) a "CXt_SUB" context frame, but without all the subroutine-
3610 specific fields set, and crashes would soon ensue.
3611 Where the two must be separate, initially set the type to "CXt_NULL" or
3613 "CXt_BLOCK", and later change it to "CXt_foo" when doing the
3614 "cx_pushfoo". This is exactly what "pp_enteriter" does, once it's
3615 determined which type of loop it's pushing.
3616 Popping contexts
3618 Contexts are popped using "cx_popsub()" etc. and "cx_popblock()". Note
3619 however, that unlike "cx_pushblock", neither of these functions
3620 actually decrement the current context stack index; this is done
3621 separately using "CX_POP()".
3622 There are two main ways that contexts are popped. During normal
3624 execution as scopes are exited, functions like "pp_leave",
3625 "pp_leaveloop" and "pp_leavesub" process and pop just one context using
3626 "cx_popfoo" and "cx_popblock". On the other hand, things like
3627 "pp_return" and "next" may have to pop back several scopes until a sub
3628 or loop context is found, and exceptions (such as "die") need to pop
3629 back contexts until an eval context is found. Both of these are
3630 accomplished by "dounwind()", which is capable of processing and
3631 popping all contexts above the target one.
3632 Here is a typical example of context popping, as found in "pp_leavesub"
3634 (simplified slightly):
3635 U8 gimme;
3637 PERL_CONTEXT *cx;
3638 SV **oldsp;
3639 OP *retop;
3640 cx = CX_CUR();
3642 gimme = cx->blk_gimme;
3644 oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
3645 if (gimme == G_VOID)
3647 PL_stack_sp = oldsp;
3648 else
3649 leave_adjust_stacks(oldsp, oldsp, gimme, 0);
3650 CX_LEAVE_SCOPE(cx);
3652 cx_popsub(cx);
3653 cx_popblock(cx);
3654 retop = cx->blk_sub.retop;
3655 CX_POP(cx);
3656 return retop;
3658 The steps above are in a very specific order, designed to be the
3660 reverse order of when the context was pushed. The first thing to do is
3661 to copy and/or protect any return arguments and free any temps in the
3662 current scope. Scope exits like an rvalue sub normally return a mortal
3663 copy of their return args (as opposed to lvalue subs). It is important
3664 to make this copy before the save stack is popped or variables are
3665 restored, or bad things like the following can happen:
3666 sub f { my $x =...; $x } # $x freed before we get to copy it
3668 sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
3669 Although we wish to free any temps at the same time, we have to be
3671 careful not to free any temps which are keeping return args alive; nor
3672 to free the temps we have just created while mortal copying return
3673 args. Fortunately, "leave_adjust_stacks()" is capable of making mortal
3674 copies of return args, shifting args down the stack, and only
3675 processing those entries on the temps stack that are safe to do so.
3676 In void context no args are returned, so it's more efficient to skip
3678 calling "leave_adjust_stacks()". Also in void context, a "nextstate" op
3679 is likely to be imminently called which will do a "FREETMPS", so
3680 there's no need to do that either.
3681 The next step is to pop savestack entries: "CX_LEAVE_SCOPE(cx)" is just
3683 defined as "LEAVE_SCOPE(cx->blk_oldsaveix)". Note that during the
3684 popping, it's possible for perl to call destructors, call "STORE" to
3685 undo localisations of tied vars, and so on. Any of these can die or
3686 call "exit()". In this case, "dounwind()" will be called, and the
3687 current context stack frame will be re-processed. Thus it is vital that
3688 all steps in popping a context are done in such a way to support
3689 reentrancy. The other alternative, of decrementing "cxstack_ix" before
3690 processing the frame, would lead to leaks and the like if something
3691 died halfway through, or overwriting of the current frame.
3692 "CX_LEAVE_SCOPE" itself is safely re-entrant: if only half the
3694 savestack items have been popped before dying and getting trapped by
3695 eval, then the "CX_LEAVE_SCOPE"s in "dounwind" or "pp_leaveeval" will
3696 continue where the first one left off.
3697 The next step is the type-specific context processing; in this case
3699 "cx_popsub". In part, this looks like:
3700 cv = cx->blk_sub.cv;
3702 CvDEPTH(cv) = cx->blk_sub.olddepth;
3703 cx->blk_sub.cv = NULL;
3704 SvREFCNT_dec(cv);
3705 where its processing the just-executed CV. Note that before it
3707 decrements the CV's reference count, it nulls the "blk_sub.cv". This
3708 means that if it re-enters, the CV won't be freed twice. It also means
3709 that you can't rely on such type-specific fields having useful values
3710 after the return from "cx_popfoo".
3711 Next, "cx_popblock" restores all the various interpreter vars to their
3713 previous values or previous high water marks; it expands to:
3714 PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
3716 PL_scopestack_ix = cx->blk_oldscopesp;
3717 PL_curpm = cx->blk_oldpm;
3718 PL_curcop = cx->blk_oldcop;
3719 PL_tmps_floor = cx->blk_old_tmpsfloor;
3720 Note that it doesn't restore "PL_stack_sp"; as mentioned earlier, which
3722 value to restore it to depends on the context type (specifically "for
3723 (list) {}"), and what args (if any) it returns; and that will already
3724 have been sorted out earlier by "leave_adjust_stacks()".
3725 Finally, the context stack pointer is actually decremented by
3727 "CX_POP(cx)". After this point, it's possible that that the current
3728 context frame could be overwritten by other contexts being pushed.
3729 Although things like ties and "DESTROY" are supposed to work within a
3730 new context stack, it's best not to assume this. Indeed on debugging
3731 builds, "CX_POP(cx)" deliberately sets "cx" to null to detect code that
3732 is still relying on the field values in that context frame. Note in the
3733 "pp_leavesub()" example above, we grab "blk_sub.retop" before calling
3734 "CX_POP".
3735 Redoing contexts
3737 Finally, there is "cx_topblock(cx)", which acts like a
3738 super-"nextstate" as regards to resetting various vars to their base
3739 values. It is used in places like "pp_next", "pp_redo" and "pp_goto"
3740 where rather than exiting a scope, we want to re-initialise the scope.
3741 As well as resetting "PL_stack_sp" like "nextstate", it also resets
3742 "PL_markstack_ptr", "PL_scopestack_ix" and "PL_curpm". Note that it
3743 doesn't do a "FREETMPS".
3744
3746 Note: this section describes a non-public internal API that is subject
3747 to change without notice.
3748 Perl's internal error-handling mechanisms implement "die" (and its
3750 internal equivalents) using longjmp. If this occurs during lexing,
3751 parsing or compilation, we must ensure that any ops allocated as part
3752 of the compilation process are freed. (Older Perl versions did not
3753 adequately handle this situation: when failing a parse, they would leak
3754 ops that were stored in C "auto" variables and not linked anywhere
3755 else.)
3756 To handle this situation, Perl uses op slabs that are attached to the
3758 currently-compiling CV. A slab is a chunk of allocated memory. New ops
3759 are allocated as regions of the slab. If the slab fills up, a new one
3760 is created (and linked from the previous one). When an error occurs and
3761 the CV is freed, any ops remaining are freed.
3762 Each op is preceded by two pointers: one points to the next op in the
3764 slab, and the other points to the slab that owns it. The next-op
3765 pointer is needed so that Perl can iterate over a slab and free all its
3766 ops. (Op structures are of different sizes, so the slab's ops can't
3767 merely be treated as a dense array.) The slab pointer is needed for
3768 accessing a reference count on the slab: when the last op on a slab is
3769 freed, the slab itself is freed.
3770 The slab allocator puts the ops at the end of the slab first. This will
3772 tend to allocate the leaves of the op tree first, and the layout will
3773 therefore hopefully be cache-friendly. In addition, this means that
3774 there's no need to store the size of the slab (see below on why slabs
3775 vary in size), because Perl can follow pointers to find the last op.
3776 It might seem possible to eliminate slab reference counts altogether,
3778 by having all ops implicitly attached to "PL_compcv" when allocated and
3779 freed when the CV is freed. That would also allow "op_free" to skip
3780 "FreeOp" altogether, and thus free ops faster. But that doesn't work in
3781 those cases where ops need to survive beyond their CVs, such as re-
3782 evals.
3783 The CV also has to have a reference count on the slab. Sometimes the
3785 first op created is immediately freed. If the reference count of the
3786 slab reaches 0, then it will be freed with the CV still pointing to it.
3787 CVs use the "CVf_SLABBED" flag to indicate that the CV has a reference
3789 count on the slab. When this flag is set, the slab is accessible via
3790 "CvSTART" when "CvROOT" is not set, or by subtracting two pointers
3791 "(2*sizeof(I32 *))" from "CvROOT" when it is set. The alternative to
3792 this approach of sneaking the slab into "CvSTART" during compilation
3793 would be to enlarge the "xpvcv" struct by another pointer. But that
3794 would make all CVs larger, even though slab-based op freeing is
3795 typically of benefit only for programs that make significant use of
3796 string eval.
3797 When the "CVf_SLABBED" flag is set, the CV takes responsibility for
3799 freeing the slab. If "CvROOT" is not set when the CV is freed or
3800 undeffed, it is assumed that a compilation error has occurred, so the
3801 op slab is traversed and all the ops are freed.
3802 Under normal circumstances, the CV forgets about its slab (decrementing
3804 the reference count) when the root is attached. So the slab reference
3805 counting that happens when ops are freed takes care of freeing the
3806 slab. In some cases, the CV is told to forget about the slab
3807 ("cv_forget_slab") precisely so that the ops can survive after the CV
3808 is done away with.
3809 Forgetting the slab when the root is attached is not strictly
3811 necessary, but avoids potential problems with "CvROOT" being written
3812 over. There is code all over the place, both in core and on CPAN, that
3813 does things with "CvROOT", so forgetting the slab makes things more
3814 robust and avoids potential problems.
3815 Since the CV takes ownership of its slab when flagged, that flag is
3817 never copied when a CV is cloned, as one CV could free a slab that
3818 another CV still points to, since forced freeing of ops ignores the
3819 reference count (but asserts that it looks right).
3820 To avoid slab fragmentation, freed ops are marked as freed and attached
3822 to the slab's freed chain (an idea stolen from DBM::Deep). Those freed
3823 ops are reused when possible. Not reusing freed ops would be simpler,
3824 but it would result in significantly higher memory usage for programs
3825 with large "if (DEBUG) {...}" blocks.
3826 "SAVEFREEOP" is slightly problematic under this scheme. Sometimes it
3828 can cause an op to be freed after its CV. If the CV has forcibly freed
3829 the ops on its slab and the slab itself, then we will be fiddling with
3830 a freed slab. Making "SAVEFREEOP" a no-op doesn't help, as sometimes an
3831 op can be savefreed when there is no compilation error, so the op would
3832 never be freed. It holds a reference count on the slab, so the whole
3833 slab would leak. So "SAVEFREEOP" now sets a special flag on the op
3834 ("->op_savefree"). The forced freeing of ops after a compilation error
3835 won't free any ops thus marked.
3836 Since many pieces of code create tiny subroutines consisting of only a
3838 few ops, and since a huge slab would be quite a bit of baggage for
3839 those to carry around, the first slab is always very small. To avoid
3840 allocating too many slabs for a single CV, each subsequent slab is
3841 twice the size of the previous.
3842 Smartmatch expects to be able to allocate an op at run time, run it,
3844 and then throw it away. For that to work the op is simply malloced when
3845 PL_compcv hasn't been set up. So all slab-allocated ops are marked as
3846 such ("->op_slabbed"), to distinguish them from malloced ops.
3847
3849 Until May 1997, this document was maintained by Jeff Okamoto
3850 <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by
3851 the Perl 5 Porters <perl5-porters@perl.org>.
3852 With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3854 Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3855 Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3856 Stephen McCamant, and Gurusamy Sarathy.
3857
3859 perlapi, perlintern, perlxs, perlembed
3860perl v5.34.1 2022-03-15 PERLGUTS(1)