1PERLGUTS(1)            Perl Programmers Reference Guide            PERLGUTS(1)
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NAME

6       perlguts - Introduction to the Perl API
7

DESCRIPTION

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

Variables

15   Datatypes
16       Perl has three typedefs that handle Perl's three main data types:
17
18           SV  Scalar Value
19           AV  Array Value
20           HV  Hash Value
21
22       Each typedef has specific routines that manipulate the various data
23       types.
24
25   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
31       Perl also uses two special typedefs, I32 and I16, which will always be
32       at least 32-bits and 16-bits long, respectively. (Again, there are U32
33       and U16, as well.)  They will usually be exactly 32 and 16 bits long,
34       but on Crays they will both be 64 bits.
35
36   Working with SVs
37       An SV can be created and loaded with one command.  There are five types
38       of values that can be loaded: an integer value (IV), an unsigned
39       integer value (UV), a double (NV), a string (PV), and another scalar
40       (SV).
41
42       The seven routines are:
43
44           SV*  newSViv(IV);
45           SV*  newSVuv(UV);
46           SV*  newSVnv(double);
47           SV*  newSVpv(const char*, STRLEN);
48           SV*  newSVpvn(const char*, STRLEN);
49           SV*  newSVpvf(const char*, ...);
50           SV*  newSVsv(SV*);
51
52       "STRLEN" is an integer type (Size_t, usually defined as size_t in
53       config.h) guaranteed to be large enough to represent the size of any
54       string that perl can handle.
55
56       In the unlikely case of a SV requiring more complex initialisation, you
57       can create an empty SV with newSV(len).  If "len" is 0 an empty SV of
58       type NULL is returned, else an SV of type PV is returned with len + 1
59       (for the NUL) bytes of storage allocated, accessible via SvPVX.  In
60       both cases the SV has value undef.
61
62           SV *sv = newSV(0);   /* no storage allocated  */
63           SV *sv = newSV(10);  /* 10 (+1) bytes of uninitialised storage allocated  */
64
65       To change the value of an already-existing SV, there are eight
66       routines:
67
68           void  sv_setiv(SV*, IV);
69           void  sv_setuv(SV*, UV);
70           void  sv_setnv(SV*, double);
71           void  sv_setpv(SV*, const char*);
72           void  sv_setpvn(SV*, const char*, STRLEN)
73           void  sv_setpvf(SV*, const char*, ...);
74           void  sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
75           void  sv_setsv(SV*, SV*);
76
77       Notice that you can choose to specify the length of the string to be
78       assigned by using "sv_setpvn", "newSVpvn", or "newSVpv", or you may
79       allow Perl to calculate the length by using "sv_setpv" or by specifying
80       0 as the second argument to "newSVpv".  Be warned, though, that Perl
81       will determine the string's length by using "strlen", which depends on
82       the string terminating with a NUL character.
83
84       The arguments of "sv_setpvf" are processed like "sprintf", and the
85       formatted output becomes the value.
86
87       "sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to
88       specify either a pointer to a variable argument list or the address and
89       length of an array of SVs.  The last argument points to a boolean; on
90       return, if that boolean is true, then locale-specific information has
91       been used to format the string, and the string's contents are therefore
92       untrustworthy (see perlsec).  This pointer may be NULL if that
93       information is not important.  Note that this function requires you to
94       specify the length of the format.
95
96       The "sv_set*()" functions are not generic enough to operate on values
97       that have "magic".  See "Magic Virtual Tables" later in this document.
98
99       All SVs that contain strings should be terminated with a NUL character.
100       If it is not NUL-terminated there is a risk of core dumps and
101       corruptions from code which passes the string to C functions or system
102       calls which expect a NUL-terminated string.  Perl's own functions
103       typically add a trailing NUL for this reason.  Nevertheless, you should
104       be very careful when you pass a string stored in an SV to a C function
105       or system call.
106
107       To access the actual value that an SV points to, you can use the
108       macros:
109
110           SvIV(SV*)
111           SvUV(SV*)
112           SvNV(SV*)
113           SvPV(SV*, STRLEN len)
114           SvPV_nolen(SV*)
115
116       which will automatically coerce the actual scalar type into an IV, UV,
117       double, or string.
118
119       In the "SvPV" macro, the length of the string returned is placed into
120       the variable "len" (this is a macro, so you do not use &len).  If you
121       do not care what the length of the data is, use the "SvPV_nolen" macro.
122       Historically the "SvPV" macro with the global variable "PL_na" has been
123       used in this case.  But that can be quite inefficient because "PL_na"
124       must be accessed in thread-local storage in threaded Perl.  In any
125       case, remember that Perl allows arbitrary strings of data that may both
126       contain NULs and might not be terminated by a NUL.
127
128       Also remember that C doesn't allow you to safely say "foo(SvPV(s, len),
129       len);". It might work with your compiler, but it won't work for
130       everyone.  Break this sort of statement up into separate assignments:
131
132           SV *s;
133           STRLEN len;
134           char * ptr;
135           ptr = SvPV(s, len);
136           foo(ptr, len);
137
138       If you want to know if the scalar value is TRUE, you can use:
139
140           SvTRUE(SV*)
141
142       Although Perl will automatically grow strings for you, if you need to
143       force Perl to allocate more memory for your SV, you can use the macro
144
145           SvGROW(SV*, STRLEN newlen)
146
147       which will determine if more memory needs to be allocated.  If so, it
148       will call the function "sv_grow".  Note that "SvGROW" can only
149       increase, not decrease, the allocated memory of an SV and that it does
150       not automatically add a byte for the a trailing NUL (perl's own string
151       functions typically do "SvGROW(sv, len + 1)").
152
153       If you have an SV and want to know what kind of data Perl thinks is
154       stored in it, you can use the following macros to check the type of SV
155       you have.
156
157           SvIOK(SV*)
158           SvNOK(SV*)
159           SvPOK(SV*)
160
161       You can get and set the current length of the string stored in an SV
162       with the following macros:
163
164           SvCUR(SV*)
165           SvCUR_set(SV*, I32 val)
166
167       You can also get a pointer to the end of the string stored in the SV
168       with the macro:
169
170           SvEND(SV*)
171
172       But note that these last three macros are valid only if "SvPOK()" is
173       true.
174
175       If you want to append something to the end of string stored in an
176       "SV*", you can use the following functions:
177
178           void  sv_catpv(SV*, const char*);
179           void  sv_catpvn(SV*, const char*, STRLEN);
180           void  sv_catpvf(SV*, const char*, ...);
181           void  sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
182           void  sv_catsv(SV*, SV*);
183
184       The first function calculates the length of the string to be appended
185       by using "strlen".  In the second, you specify the length of the string
186       yourself.  The third function processes its arguments like "sprintf"
187       and appends the formatted output.  The fourth function works like
188       "vsprintf".  You can specify the address and length of an array of SVs
189       instead of the va_list argument. The fifth function extends the string
190       stored in the first SV with the string stored in the second SV.  It
191       also forces the second SV to be interpreted as a string.
192
193       The "sv_cat*()" functions are not generic enough to operate on values
194       that have "magic".  See "Magic Virtual Tables" later in this document.
195
196       If you know the name of a scalar variable, you can get a pointer to its
197       SV by using the following:
198
199           SV*  get_sv("package::varname", 0);
200
201       This returns NULL if the variable does not exist.
202
203       If you want to know if this variable (or any other SV) is actually
204       "defined", you can call:
205
206           SvOK(SV*)
207
208       The scalar "undef" value is stored in an SV instance called
209       "PL_sv_undef".
210
211       Its address can be used whenever an "SV*" is needed. Make sure that you
212       don't try to compare a random sv with &PL_sv_undef. For example when
213       interfacing Perl code, it'll work correctly for:
214
215         foo(undef);
216
217       But won't work when called as:
218
219         $x = undef;
220         foo($x);
221
222       So to repeat always use SvOK() to check whether an sv is defined.
223
224       Also you have to be careful when using &PL_sv_undef as a value in AVs
225       or HVs (see "AVs, HVs and undefined values").
226
227       There are also the two values "PL_sv_yes" and "PL_sv_no", which contain
228       boolean TRUE and FALSE values, respectively.  Like "PL_sv_undef", their
229       addresses can be used whenever an "SV*" is needed.
230
231       Do not be fooled into thinking that "(SV *) 0" is the same as
232       &PL_sv_undef.  Take this code:
233
234           SV* sv = (SV*) 0;
235           if (I-am-to-return-a-real-value) {
236                   sv = sv_2mortal(newSViv(42));
237           }
238           sv_setsv(ST(0), sv);
239
240       This code tries to return a new SV (which contains the value 42) if it
241       should return a real value, or undef otherwise.  Instead it has
242       returned a NULL pointer which, somewhere down the line, will cause a
243       segmentation violation, bus error, or just weird results.  Change the
244       zero to &PL_sv_undef in the first line and all will be well.
245
246       To free an SV that you've created, call "SvREFCNT_dec(SV*)".  Normally
247       this call is not necessary (see "Reference Counts and Mortality").
248
249   Offsets
250       Perl provides the function "sv_chop" to efficiently remove characters
251       from the beginning of a string; you give it an SV and a pointer to
252       somewhere inside the PV, and it discards everything before the pointer.
253       The efficiency comes by means of a little hack: instead of actually
254       removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
255       signal to other functions that the offset hack is in effect, and it
256       puts the number of bytes chopped off into the IV field of the SV. It
257       then moves the PV pointer (called "SvPVX") forward that many bytes, and
258       adjusts "SvCUR" and "SvLEN".
259
260       Hence, at this point, the start of the buffer that we allocated lives
261       at "SvPVX(sv) - SvIV(sv)" in memory and the PV pointer is pointing into
262       the middle of this allocated storage.
263
264       This is best demonstrated by example:
265
266         % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
267         SV = PVIV(0x8128450) at 0x81340f0
268           REFCNT = 1
269           FLAGS = (POK,OOK,pPOK)
270           IV = 1  (OFFSET)
271           PV = 0x8135781 ( "1" . ) "2345"\0
272           CUR = 4
273           LEN = 5
274
275       Here the number of bytes chopped off (1) is put into IV, and
276       "Devel::Peek::Dump" helpfully reminds us that this is an offset. The
277       portion of the string between the "real" and the "fake" beginnings is
278       shown in parentheses, and the values of "SvCUR" and "SvLEN" reflect the
279       fake beginning, not the real one.
280
281       Something similar to the offset hack is performed on AVs to enable
282       efficient shifting and splicing off the beginning of the array; while
283       "AvARRAY" points to the first element in the array that is visible from
284       Perl, "AvALLOC" points to the real start of the C array. These are
285       usually the same, but a "shift" operation can be carried out by
286       increasing "AvARRAY" by one and decreasing "AvFILL" and "AvLEN".
287       Again, the location of the real start of the C array only comes into
288       play when freeing the array. See "av_shift" in av.c.
289
290   What's Really Stored in an SV?
291       Recall that the usual method of determining the type of scalar you have
292       is to use "Sv*OK" macros.  Because a scalar can be both a number and a
293       string, usually these macros will always return TRUE and calling the
294       "Sv*V" macros will do the appropriate conversion of string to
295       integer/double or integer/double to string.
296
297       If you really need to know if you have an integer, double, or string
298       pointer in an SV, you can use the following three macros instead:
299
300           SvIOKp(SV*)
301           SvNOKp(SV*)
302           SvPOKp(SV*)
303
304       These will tell you if you truly have an integer, double, or string
305       pointer stored in your SV.  The "p" stands for private.
306
307       The are various ways in which the private and public flags may differ.
308       For example, a tied SV may have a valid underlying value in the IV slot
309       (so SvIOKp is true), but the data should be accessed via the FETCH
310       routine rather than directly, so SvIOK is false. Another is when
311       numeric conversion has occurred and precision has been lost: only the
312       private flag is set on 'lossy' values. So when an NV is converted to an
313       IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont
314       be.
315
316       In general, though, it's best to use the "Sv*V" macros.
317
318   Working with AVs
319       There are two ways to create and load an AV.  The first method creates
320       an empty AV:
321
322           AV*  newAV();
323
324       The second method both creates the AV and initially populates it with
325       SVs:
326
327           AV*  av_make(I32 num, SV **ptr);
328
329       The second argument points to an array containing "num" "SV*"'s.  Once
330       the AV has been created, the SVs can be destroyed, if so desired.
331
332       Once the AV has been created, the following operations are possible on
333       AVs:
334
335           void  av_push(AV*, SV*);
336           SV*   av_pop(AV*);
337           SV*   av_shift(AV*);
338           void  av_unshift(AV*, I32 num);
339
340       These should be familiar operations, with the exception of
341       "av_unshift".  This routine adds "num" elements at the front of the
342       array with the "undef" value.  You must then use "av_store" (described
343       below) to assign values to these new elements.
344
345       Here are some other functions:
346
347           I32   av_len(AV*);
348           SV**  av_fetch(AV*, I32 key, I32 lval);
349           SV**  av_store(AV*, I32 key, SV* val);
350
351       The "av_len" function returns the highest index value in array (just
352       like $#array in Perl).  If the array is empty, -1 is returned.  The
353       "av_fetch" function returns the value at index "key", but if "lval" is
354       non-zero, then "av_fetch" will store an undef value at that index.  The
355       "av_store" function stores the value "val" at index "key", and does not
356       increment the reference count of "val".  Thus the caller is responsible
357       for taking care of that, and if "av_store" returns NULL, the caller
358       will have to decrement the reference count to avoid a memory leak.
359       Note that "av_fetch" and "av_store" both return "SV**"'s, not "SV*"'s
360       as their return value.
361
362           void  av_clear(AV*);
363           void  av_undef(AV*);
364           void  av_extend(AV*, I32 key);
365
366       The "av_clear" function deletes all the elements in the AV* array, but
367       does not actually delete the array itself.  The "av_undef" function
368       will delete all the elements in the array plus the array itself.  The
369       "av_extend" function extends the array so that it contains at least
370       "key+1" elements.  If "key+1" is less than the currently allocated
371       length of the array, then nothing is done.
372
373       If you know the name of an array variable, you can get a pointer to its
374       AV by using the following:
375
376           AV*  get_av("package::varname", 0);
377
378       This returns NULL if the variable does not exist.
379
380       See "Understanding the Magic of Tied Hashes and Arrays" for more
381       information on how to use the array access functions on tied arrays.
382
383   Working with HVs
384       To create an HV, you use the following routine:
385
386           HV*  newHV();
387
388       Once the HV has been created, the following operations are possible on
389       HVs:
390
391           SV**  hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
392           SV**  hv_fetch(HV*, const char* key, U32 klen, I32 lval);
393
394       The "klen" parameter is the length of the key being passed in (Note
395       that you cannot pass 0 in as a value of "klen" to tell Perl to measure
396       the length of the key).  The "val" argument contains the SV pointer to
397       the scalar being stored, and "hash" is the precomputed hash value (zero
398       if you want "hv_store" to calculate it for you).  The "lval" parameter
399       indicates whether this fetch is actually a part of a store operation,
400       in which case a new undefined value will be added to the HV with the
401       supplied key and "hv_fetch" will return as if the value had already
402       existed.
403
404       Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just
405       "SV*".  To access the scalar value, you must first dereference the
406       return value.  However, you should check to make sure that the return
407       value is not NULL before dereferencing it.
408
409       These two functions check if a hash table entry exists, and deletes it.
410
411           bool  hv_exists(HV*, const char* key, U32 klen);
412           SV*   hv_delete(HV*, const char* key, U32 klen, I32 flags);
413
414       If "flags" does not include the "G_DISCARD" flag then "hv_delete" will
415       create and return a mortal copy of the deleted value.
416
417       And more miscellaneous functions:
418
419           void   hv_clear(HV*);
420           void   hv_undef(HV*);
421
422       Like their AV counterparts, "hv_clear" deletes all the entries in the
423       hash table but does not actually delete the hash table.  The "hv_undef"
424       deletes both the entries and the hash table itself.
425
426       Perl keeps the actual data in linked list of structures with a typedef
427       of HE.  These contain the actual key and value pointers (plus extra
428       administrative overhead).  The key is a string pointer; the value is an
429       "SV*".  However, once you have an "HE*", to get the actual key and
430       value, use the routines specified below.
431
432           I32    hv_iterinit(HV*);
433                   /* Prepares starting point to traverse hash table */
434           HE*    hv_iternext(HV*);
435                   /* Get the next entry, and return a pointer to a
436                      structure that has both the key and value */
437           char*  hv_iterkey(HE* entry, I32* retlen);
438                   /* Get the key from an HE structure and also return
439                      the length of the key string */
440           SV*    hv_iterval(HV*, HE* entry);
441                   /* Return an SV pointer to the value of the HE
442                      structure */
443           SV*    hv_iternextsv(HV*, char** key, I32* retlen);
444                   /* This convenience routine combines hv_iternext,
445                      hv_iterkey, and hv_iterval.  The key and retlen
446                      arguments are return values for the key and its
447                      length.  The value is returned in the SV* argument */
448
449       If you know the name of a hash variable, you can get a pointer to its
450       HV by using the following:
451
452           HV*  get_hv("package::varname", 0);
453
454       This returns NULL if the variable does not exist.
455
456       The hash algorithm is defined in the "PERL_HASH(hash, key, klen)"
457       macro:
458
459           hash = 0;
460           while (klen--)
461               hash = (hash * 33) + *key++;
462           hash = hash + (hash >> 5);                  /* after 5.6 */
463
464       The last step was added in version 5.6 to improve distribution of lower
465       bits in the resulting hash value.
466
467       See "Understanding the Magic of Tied Hashes and Arrays" for more
468       information on how to use the hash access functions on tied hashes.
469
470   Hash API Extensions
471       Beginning with version 5.004, the following functions are also
472       supported:
473
474           HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
475           HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);
476
477           bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
478           SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
479
480           SV*     hv_iterkeysv  (HE* entry);
481
482       Note that these functions take "SV*" keys, which simplifies writing of
483       extension code that deals with hash structures.  These functions also
484       allow passing of "SV*" keys to "tie" functions without forcing you to
485       stringify the keys (unlike the previous set of functions).
486
487       They also return and accept whole hash entries ("HE*"), making their
488       use more efficient (since the hash number for a particular string
489       doesn't have to be recomputed every time).  See perlapi for detailed
490       descriptions.
491
492       The following macros must always be used to access the contents of hash
493       entries.  Note that the arguments to these macros must be simple
494       variables, since they may get evaluated more than once.  See perlapi
495       for detailed descriptions of these macros.
496
497           HePV(HE* he, STRLEN len)
498           HeVAL(HE* he)
499           HeHASH(HE* he)
500           HeSVKEY(HE* he)
501           HeSVKEY_force(HE* he)
502           HeSVKEY_set(HE* he, SV* sv)
503
504       These two lower level macros are defined, but must only be used when
505       dealing with keys that are not "SV*"s:
506
507           HeKEY(HE* he)
508           HeKLEN(HE* he)
509
510       Note that both "hv_store" and "hv_store_ent" do not increment the
511       reference count of the stored "val", which is the caller's
512       responsibility.  If these functions return a NULL value, the caller
513       will usually have to decrement the reference count of "val" to avoid a
514       memory leak.
515
516   AVs, HVs and undefined values
517       Sometimes you have to store undefined values in AVs or HVs. Although
518       this may be a rare case, it can be tricky. That's because you're used
519       to using &PL_sv_undef if you need an undefined SV.
520
521       For example, intuition tells you that this XS code:
522
523           AV *av = newAV();
524           av_store( av, 0, &PL_sv_undef );
525
526       is equivalent to this Perl code:
527
528           my @av;
529           $av[0] = undef;
530
531       Unfortunately, this isn't true. AVs use &PL_sv_undef as a marker for
532       indicating that an array element has not yet been initialized.  Thus,
533       "exists $av[0]" would be true for the above Perl code, but false for
534       the array generated by the XS code.
535
536       Other problems can occur when storing &PL_sv_undef in HVs:
537
538           hv_store( hv, "key", 3, &PL_sv_undef, 0 );
539
540       This will indeed make the value "undef", but if you try to modify the
541       value of "key", you'll get the following error:
542
543           Modification of non-creatable hash value attempted
544
545       In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in
546       restricted hashes. This caused such hash entries not to appear when
547       iterating over the hash or when checking for the keys with the
548       "hv_exists" function.
549
550       You can run into similar problems when you store &PL_sv_true or
551       &PL_sv_false into AVs or HVs. Trying to modify such elements will give
552       you the following error:
553
554           Modification of a read-only value attempted
555
556       To make a long story short, you can use the special variables
557       &PL_sv_undef, &PL_sv_true and &PL_sv_false with AVs and HVs, but you
558       have to make sure you know what you're doing.
559
560       Generally, if you want to store an undefined value in an AV or HV, you
561       should not use &PL_sv_undef, but rather create a new undefined value
562       using the "newSV" function, for example:
563
564           av_store( av, 42, newSV(0) );
565           hv_store( hv, "foo", 3, newSV(0), 0 );
566
567   References
568       References are a special type of scalar that point to other data types
569       (including references).
570
571       To create a reference, use either of the following functions:
572
573           SV* newRV_inc((SV*) thing);
574           SV* newRV_noinc((SV*) thing);
575
576       The "thing" argument can be any of an "SV*", "AV*", or "HV*".  The
577       functions are identical except that "newRV_inc" increments the
578       reference count of the "thing", while "newRV_noinc" does not.  For
579       historical reasons, "newRV" is a synonym for "newRV_inc".
580
581       Once you have a reference, you can use the following macro to
582       dereference the reference:
583
584           SvRV(SV*)
585
586       then call the appropriate routines, casting the returned "SV*" to
587       either an "AV*" or "HV*", if required.
588
589       To determine if an SV is a reference, you can use the following macro:
590
591           SvROK(SV*)
592
593       To discover what type of value the reference refers to, use the
594       following macro and then check the return value.
595
596           SvTYPE(SvRV(SV*))
597
598       The most useful types that will be returned are:
599
600           SVt_IV    Scalar
601           SVt_NV    Scalar
602           SVt_PV    Scalar
603           SVt_RV    Scalar
604           SVt_PVAV  Array
605           SVt_PVHV  Hash
606           SVt_PVCV  Code
607           SVt_PVGV  Glob (possible a file handle)
608           SVt_PVMG  Blessed or Magical Scalar
609
610       See the sv.h header file for more details.
611
612   Blessed References and Class Objects
613       References are also used to support object-oriented programming.  In
614       perl's OO lexicon, an object is simply a reference that has been
615       blessed into a package (or class).  Once blessed, the programmer may
616       now use the reference to access the various methods in the class.
617
618       A reference can be blessed into a package with the following function:
619
620           SV* sv_bless(SV* sv, HV* stash);
621
622       The "sv" argument must be a reference value.  The "stash" argument
623       specifies which class the reference will belong to.  See "Stashes and
624       Globs" for information on converting class names into stashes.
625
626       /* Still under construction */
627
628       Upgrades rv to reference if not already one.  Creates new SV for rv to
629       point to.  If "classname" is non-null, the SV is blessed into the
630       specified class.  SV is returned.
631
632               SV* newSVrv(SV* rv, const char* classname);
633
634       Copies integer, unsigned integer or double into an SV whose reference
635       is "rv".  SV is blessed if "classname" is non-null.
636
637               SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
638               SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
639               SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
640
641       Copies the pointer value (the address, not the string!) into an SV
642       whose reference is rv.  SV is blessed if "classname" is non-null.
643
644               SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
645
646       Copies string into an SV whose reference is "rv".  Set length to 0 to
647       let Perl calculate the string length.  SV is blessed if "classname" is
648       non-null.
649
650               SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
651
652       Tests whether the SV is blessed into the specified class.  It does not
653       check inheritance relationships.
654
655               int  sv_isa(SV* sv, const char* name);
656
657       Tests whether the SV is a reference to a blessed object.
658
659               int  sv_isobject(SV* sv);
660
661       Tests whether the SV is derived from the specified class. SV can be
662       either a reference to a blessed object or a string containing a class
663       name. This is the function implementing the "UNIVERSAL::isa"
664       functionality.
665
666               bool sv_derived_from(SV* sv, const char* name);
667
668       To check if you've got an object derived from a specific class you have
669       to write:
670
671               if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
672
673   Creating New Variables
674       To create a new Perl variable with an undef value which can be accessed
675       from your Perl script, use the following routines, depending on the
676       variable type.
677
678           SV*  get_sv("package::varname", GV_ADD);
679           AV*  get_av("package::varname", GV_ADD);
680           HV*  get_hv("package::varname", GV_ADD);
681
682       Notice the use of TRUE as the second parameter.  The new variable can
683       now be set, using the routines appropriate to the data type.
684
685       There are additional macros whose values may be bitwise OR'ed with the
686       "TRUE" argument to enable certain extra features.  Those bits are:
687
688       GV_ADDMULTI
689           Marks the variable as multiply defined, thus preventing the:
690
691             Name <varname> used only once: possible typo
692
693           warning.
694
695       GV_ADDWARN
696           Issues the warning:
697
698             Had to create <varname> unexpectedly
699
700           if the variable did not exist before the function was called.
701
702       If you do not specify a package name, the variable is created in the
703       current package.
704
705   Reference Counts and Mortality
706       Perl uses a reference count-driven garbage collection mechanism. SVs,
707       AVs, or HVs (xV for short in the following) start their life with a
708       reference count of 1.  If the reference count of an xV ever drops to 0,
709       then it will be destroyed and its memory made available for reuse.
710
711       This normally doesn't happen at the Perl level unless a variable is
712       undef'ed or the last variable holding a reference to it is changed or
713       overwritten.  At the internal level, however, reference counts can be
714       manipulated with the following macros:
715
716           int SvREFCNT(SV* sv);
717           SV* SvREFCNT_inc(SV* sv);
718           void SvREFCNT_dec(SV* sv);
719
720       However, there is one other function which manipulates the reference
721       count of its argument.  The "newRV_inc" function, you will recall,
722       creates a reference to the specified argument.  As a side effect, it
723       increments the argument's reference count.  If this is not what you
724       want, use "newRV_noinc" instead.
725
726       For example, imagine you want to return a reference from an XSUB
727       function.  Inside the XSUB routine, you create an SV which initially
728       has a reference count of one.  Then you call "newRV_inc", passing it
729       the just-created SV.  This returns the reference as a new SV, but the
730       reference count of the SV you passed to "newRV_inc" has been
731       incremented to two.  Now you return the reference from the XSUB routine
732       and forget about the SV.  But Perl hasn't!  Whenever the returned
733       reference is destroyed, the reference count of the original SV is
734       decreased to one and nothing happens.  The SV will hang around without
735       any way to access it until Perl itself terminates.  This is a memory
736       leak.
737
738       The correct procedure, then, is to use "newRV_noinc" instead of
739       "newRV_inc".  Then, if and when the last reference is destroyed, the
740       reference count of the SV will go to zero and it will be destroyed,
741       stopping any memory leak.
742
743       There are some convenience functions available that can help with the
744       destruction of xVs.  These functions introduce the concept of
745       "mortality".  An xV that is mortal has had its reference count marked
746       to be decremented, but not actually decremented, until "a short time
747       later".  Generally the term "short time later" means a single Perl
748       statement, such as a call to an XSUB function.  The actual determinant
749       for when mortal xVs have their reference count decremented depends on
750       two macros, SAVETMPS and FREETMPS.  See perlcall and perlxs for more
751       details on these macros.
752
753       "Mortalization" then is at its simplest a deferred "SvREFCNT_dec".
754       However, if you mortalize a variable twice, the reference count will
755       later be decremented twice.
756
757       "Mortal" SVs are mainly used for SVs that are placed on perl's stack.
758       For example an SV which is created just to pass a number to a called
759       sub is made mortal to have it cleaned up automatically when it's popped
760       off the stack. Similarly, results returned by XSUBs (which are pushed
761       on the stack) are often made mortal.
762
763       To create a mortal variable, use the functions:
764
765           SV*  sv_newmortal()
766           SV*  sv_2mortal(SV*)
767           SV*  sv_mortalcopy(SV*)
768
769       The first call creates a mortal SV (with no value), the second converts
770       an existing SV to a mortal SV (and thus defers a call to
771       "SvREFCNT_dec"), and the third creates a mortal copy of an existing SV.
772       Because "sv_newmortal" gives the new SV no value,it must normally be
773       given one via "sv_setpv", "sv_setiv", etc. :
774
775           SV *tmp = sv_newmortal();
776           sv_setiv(tmp, an_integer);
777
778       As that is multiple C statements it is quite common so see this idiom
779       instead:
780
781           SV *tmp = sv_2mortal(newSViv(an_integer));
782
783       You should be careful about creating mortal variables.  Strange things
784       can happen if you make the same value mortal within multiple contexts,
785       or if you make a variable mortal multiple times. Thinking of
786       "Mortalization" as deferred "SvREFCNT_dec" should help to minimize such
787       problems.  For example if you are passing an SV which you know has high
788       enough REFCNT to survive its use on the stack you need not do any
789       mortalization.  If you are not sure then doing an "SvREFCNT_inc" and
790       "sv_2mortal", or making a "sv_mortalcopy" is safer.
791
792       The mortal routines are not just for SVs -- AVs and HVs can be made
793       mortal by passing their address (type-casted to "SV*") to the
794       "sv_2mortal" or "sv_mortalcopy" routines.
795
796   Stashes and Globs
797       A stash is a hash that contains all variables that are defined within a
798       package.  Each key of the stash is a symbol name (shared by all the
799       different types of objects that have the same name), and each value in
800       the hash table is a GV (Glob Value).  This GV in turn contains
801       references to the various objects of that name, including (but not
802       limited to) the following:
803
804           Scalar Value
805           Array Value
806           Hash Value
807           I/O Handle
808           Format
809           Subroutine
810
811       There is a single stash called "PL_defstash" that holds the items that
812       exist in the "main" package.  To get at the items in other packages,
813       append the string "::" to the package name.  The items in the "Foo"
814       package are in the stash "Foo::" in PL_defstash.  The items in the
815       "Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash.
816
817       To get the stash pointer for a particular package, use the function:
818
819           HV*  gv_stashpv(const char* name, I32 flags)
820           HV*  gv_stashsv(SV*, I32 flags)
821
822       The first function takes a literal string, the second uses the string
823       stored in the SV.  Remember that a stash is just a hash table, so you
824       get back an "HV*".  The "flags" flag will create a new package if it is
825       set to GV_ADD.
826
827       The name that "gv_stash*v" wants is the name of the package whose
828       symbol table you want.  The default package is called "main".  If you
829       have multiply nested packages, pass their names to "gv_stash*v",
830       separated by "::" as in the Perl language itself.
831
832       Alternately, if you have an SV that is a blessed reference, you can
833       find out the stash pointer by using:
834
835           HV*  SvSTASH(SvRV(SV*));
836
837       then use the following to get the package name itself:
838
839           char*  HvNAME(HV* stash);
840
841       If you need to bless or re-bless an object you can use the following
842       function:
843
844           SV*  sv_bless(SV*, HV* stash)
845
846       where the first argument, an "SV*", must be a reference, and the second
847       argument is a stash.  The returned "SV*" can now be used in the same
848       way as any other SV.
849
850       For more information on references and blessings, consult perlref.
851
852   Double-Typed SVs
853       Scalar variables normally contain only one type of value, an integer,
854       double, pointer, or reference.  Perl will automatically convert the
855       actual scalar data from the stored type into the requested type.
856
857       Some scalar variables contain more than one type of scalar data.  For
858       example, the variable $! contains either the numeric value of "errno"
859       or its string equivalent from either "strerror" or "sys_errlist[]".
860
861       To force multiple data values into an SV, you must do two things: use
862       the "sv_set*v" routines to add the additional scalar type, then set a
863       flag so that Perl will believe it contains more than one type of data.
864       The four macros to set the flags are:
865
866               SvIOK_on
867               SvNOK_on
868               SvPOK_on
869               SvROK_on
870
871       The particular macro you must use depends on which "sv_set*v" routine
872       you called first.  This is because every "sv_set*v" routine turns on
873       only the bit for the particular type of data being set, and turns off
874       all the rest.
875
876       For example, to create a new Perl variable called "dberror" that
877       contains both the numeric and descriptive string error values, you
878       could use the following code:
879
880           extern int  dberror;
881           extern char *dberror_list;
882
883           SV* sv = get_sv("dberror", GV_ADD);
884           sv_setiv(sv, (IV) dberror);
885           sv_setpv(sv, dberror_list[dberror]);
886           SvIOK_on(sv);
887
888       If the order of "sv_setiv" and "sv_setpv" had been reversed, then the
889       macro "SvPOK_on" would need to be called instead of "SvIOK_on".
890
891   Magic Variables
892       [This section still under construction.  Ignore everything here.  Post
893       no bills.  Everything not permitted is forbidden.]
894
895       Any SV may be magical, that is, it has special features that a normal
896       SV does not have.  These features are stored in the SV structure in a
897       linked list of "struct magic"'s, typedef'ed to "MAGIC".
898
899           struct magic {
900               MAGIC*      mg_moremagic;
901               MGVTBL*     mg_virtual;
902               U16         mg_private;
903               char        mg_type;
904               U8          mg_flags;
905               I32         mg_len;
906               SV*         mg_obj;
907               char*       mg_ptr;
908           };
909
910       Note this is current as of patchlevel 0, and could change at any time.
911
912   Assigning Magic
913       Perl adds magic to an SV using the sv_magic function:
914
915           void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
916
917       The "sv" argument is a pointer to the SV that is to acquire a new
918       magical feature.
919
920       If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to
921       convert "sv" to type "SVt_PVMG". Perl then continues by adding new
922       magic to the beginning of the linked list of magical features.  Any
923       prior entry of the same type of magic is deleted.  Note that this can
924       be overridden, and multiple instances of the same type of magic can be
925       associated with an SV.
926
927       The "name" and "namlen" arguments are used to associate a string with
928       the magic, typically the name of a variable. "namlen" is stored in the
929       "mg_len" field and if "name" is non-null then either a "savepvn" copy
930       of "name" or "name" itself is stored in the "mg_ptr" field, depending
931       on whether "namlen" is greater than zero or equal to zero respectively.
932       As a special case, if "(name && namlen == HEf_SVKEY)" then "name" is
933       assumed to contain an "SV*" and is stored as-is with its REFCNT
934       incremented.
935
936       The sv_magic function uses "how" to determine which, if any, predefined
937       "Magic Virtual Table" should be assigned to the "mg_virtual" field.
938       See the "Magic Virtual Tables" section below.  The "how" argument is
939       also stored in the "mg_type" field. The value of "how" should be chosen
940       from the set of macros "PERL_MAGIC_foo" found in perl.h. Note that
941       before these macros were added, Perl internals used to directly use
942       character literals, so you may occasionally come across old code or
943       documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for
944       example.
945
946       The "obj" argument is stored in the "mg_obj" field of the "MAGIC"
947       structure.  If it is not the same as the "sv" argument, the reference
948       count of the "obj" object is incremented.  If it is the same, or if the
949       "how" argument is "PERL_MAGIC_arylen", or if it is a NULL pointer, then
950       "obj" is merely stored, without the reference count being incremented.
951
952       See also "sv_magicext" in perlapi for a more flexible way to add magic
953       to an SV.
954
955       There is also a function to add magic to an "HV":
956
957           void hv_magic(HV *hv, GV *gv, int how);
958
959       This simply calls "sv_magic" and coerces the "gv" argument into an
960       "SV".
961
962       To remove the magic from an SV, call the function sv_unmagic:
963
964           void sv_unmagic(SV *sv, int type);
965
966       The "type" argument should be equal to the "how" value when the "SV"
967       was initially made magical.
968
969   Magic Virtual Tables
970       The "mg_virtual" field in the "MAGIC" structure is a pointer to an
971       "MGVTBL", which is a structure of function pointers and stands for
972       "Magic Virtual Table" to handle the various operations that might be
973       applied to that variable.
974
975       The "MGVTBL" has five (or sometimes eight) pointers to the following
976       routine types:
977
978           int  (*svt_get)(SV* sv, MAGIC* mg);
979           int  (*svt_set)(SV* sv, MAGIC* mg);
980           U32  (*svt_len)(SV* sv, MAGIC* mg);
981           int  (*svt_clear)(SV* sv, MAGIC* mg);
982           int  (*svt_free)(SV* sv, MAGIC* mg);
983
984           int  (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen);
985           int  (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
986           int  (*svt_local)(SV *nsv, MAGIC *mg);
987
988       This MGVTBL structure is set at compile-time in perl.h and there are
989       currently 32 types.  These different structures contain pointers to
990       various routines that perform additional actions depending on which
991       function is being called.
992
993           Function pointer    Action taken
994           ----------------    ------------
995           svt_get             Do something before the value of the SV is retrieved.
996           svt_set             Do something after the SV is assigned a value.
997           svt_len             Report on the SV's length.
998           svt_clear           Clear something the SV represents.
999           svt_free            Free any extra storage associated with the SV.
1000
1001           svt_copy            copy tied variable magic to a tied element
1002           svt_dup             duplicate a magic structure during thread cloning
1003           svt_local           copy magic to local value during 'local'
1004
1005       For instance, the MGVTBL structure called "vtbl_sv" (which corresponds
1006       to an "mg_type" of "PERL_MAGIC_sv") contains:
1007
1008           { magic_get, magic_set, magic_len, 0, 0 }
1009
1010       Thus, when an SV is determined to be magical and of type
1011       "PERL_MAGIC_sv", if a get operation is being performed, the routine
1012       "magic_get" is called.  All the various routines for the various
1013       magical types begin with "magic_".  NOTE: the magic routines are not
1014       considered part of the Perl API, and may not be exported by the Perl
1015       library.
1016
1017       The last three slots are a recent addition, and for source code
1018       compatibility they are only checked for if one of the three flags
1019       MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
1020       code can continue declaring a vtable as a 5-element value. These three
1021       are currently used exclusively by the threading code, and are highly
1022       subject to change.
1023
1024       The current kinds of Magic Virtual Tables are:
1025
1026           mg_type
1027           (old-style char and macro)   MGVTBL          Type of magic
1028           --------------------------   ------          -------------
1029           \0 PERL_MAGIC_sv             vtbl_sv         Special scalar variable
1030           A  PERL_MAGIC_overload       vtbl_amagic     %OVERLOAD hash
1031           a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
1032           c  PERL_MAGIC_overload_table (none)          Holds overload table (AMT)
1033                                                        on stash
1034           B  PERL_MAGIC_bm             vtbl_bm         Boyer-Moore (fast string search)
1035           D  PERL_MAGIC_regdata        vtbl_regdata    Regex match position data
1036                                                        (@+ and @- vars)
1037           d  PERL_MAGIC_regdatum       vtbl_regdatum   Regex match position data
1038                                                        element
1039           E  PERL_MAGIC_env            vtbl_env        %ENV hash
1040           e  PERL_MAGIC_envelem        vtbl_envelem    %ENV hash element
1041           f  PERL_MAGIC_fm             vtbl_fm         Formline ('compiled' format)
1042           g  PERL_MAGIC_regex_global   vtbl_mglob      m//g target / study()ed string
1043           H  PERL_MAGIC_hints          vtbl_sig        %^H hash
1044           h  PERL_MAGIC_hintselem      vtbl_hintselem  %^H hash element
1045           I  PERL_MAGIC_isa            vtbl_isa        @ISA array
1046           i  PERL_MAGIC_isaelem        vtbl_isaelem    @ISA array element
1047           k  PERL_MAGIC_nkeys          vtbl_nkeys      scalar(keys()) lvalue
1048           L  PERL_MAGIC_dbfile         (none)          Debugger %_<filename
1049           l  PERL_MAGIC_dbline         vtbl_dbline     Debugger %_<filename element
1050           o  PERL_MAGIC_collxfrm       vtbl_collxfrm   Locale collate transformation
1051           P  PERL_MAGIC_tied           vtbl_pack       Tied array or hash
1052           p  PERL_MAGIC_tiedelem       vtbl_packelem   Tied array or hash element
1053           q  PERL_MAGIC_tiedscalar     vtbl_packelem   Tied scalar or handle
1054           r  PERL_MAGIC_qr             vtbl_qr         precompiled qr// regex
1055           S  PERL_MAGIC_sig            vtbl_sig        %SIG hash
1056           s  PERL_MAGIC_sigelem        vtbl_sigelem    %SIG hash element
1057           t  PERL_MAGIC_taint          vtbl_taint      Taintedness
1058           U  PERL_MAGIC_uvar           vtbl_uvar       Available for use by extensions
1059           v  PERL_MAGIC_vec            vtbl_vec        vec() lvalue
1060           V  PERL_MAGIC_vstring        (none)          v-string scalars
1061           w  PERL_MAGIC_utf8           vtbl_utf8       UTF-8 length+offset cache
1062           x  PERL_MAGIC_substr         vtbl_substr     substr() lvalue
1063           y  PERL_MAGIC_defelem        vtbl_defelem    Shadow "foreach" iterator
1064                                                        variable / smart parameter
1065                                                        vivification
1066           #  PERL_MAGIC_arylen         vtbl_arylen     Array length ($#ary)
1067           .  PERL_MAGIC_pos            vtbl_pos        pos() lvalue
1068           <  PERL_MAGIC_backref        vtbl_backref    back pointer to a weak ref
1069           ~  PERL_MAGIC_ext            (none)          Available for use by extensions
1070           :  PERL_MAGIC_symtab         (none)          hash used as symbol table
1071           %  PERL_MAGIC_rhash          (none)          hash used as restricted hash
1072           @  PERL_MAGIC_arylen_p       vtbl_arylen_p   pointer to $#a from @a
1073
1074       When an uppercase and lowercase letter both exist in the table, then
1075       the uppercase letter is typically used to represent some kind of
1076       composite type (a list or a hash), and the lowercase letter is used to
1077       represent an element of that composite type. Some internals code makes
1078       use of this case relationship.  However, 'v' and 'V' (vec and v-string)
1079       are in no way related.
1080
1081       The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined
1082       specifically for use by extensions and will not be used by perl itself.
1083       Extensions can use "PERL_MAGIC_ext" magic to 'attach' private
1084       information to variables (typically objects).  This is especially
1085       useful because there is no way for normal perl code to corrupt this
1086       private information (unlike using extra elements of a hash object).
1087
1088       Similarly, "PERL_MAGIC_uvar" magic can be used much like tie() to call
1089       a C function any time a scalar's value is used or changed.  The
1090       "MAGIC"'s "mg_ptr" field points to a "ufuncs" structure:
1091
1092           struct ufuncs {
1093               I32 (*uf_val)(pTHX_ IV, SV*);
1094               I32 (*uf_set)(pTHX_ IV, SV*);
1095               IV uf_index;
1096           };
1097
1098       When the SV is read from or written to, the "uf_val" or "uf_set"
1099       function will be called with "uf_index" as the first arg and a pointer
1100       to the SV as the second.  A simple example of how to add
1101       "PERL_MAGIC_uvar" magic is shown below.  Note that the ufuncs structure
1102       is copied by sv_magic, so you can safely allocate it on the stack.
1103
1104           void
1105           Umagic(sv)
1106               SV *sv;
1107           PREINIT:
1108               struct ufuncs uf;
1109           CODE:
1110               uf.uf_val   = &my_get_fn;
1111               uf.uf_set   = &my_set_fn;
1112               uf.uf_index = 0;
1113               sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
1114
1115       Attaching "PERL_MAGIC_uvar" to arrays is permissible but has no effect.
1116
1117       For hashes there is a specialized hook that gives control over hash
1118       keys (but not values).  This hook calls "PERL_MAGIC_uvar" 'get' magic
1119       if the "set" function in the "ufuncs" structure is NULL.  The hook is
1120       activated whenever the hash is accessed with a key specified as an "SV"
1121       through the functions "hv_store_ent", "hv_fetch_ent", "hv_delete_ent",
1122       and "hv_exists_ent".  Accessing the key as a string through the
1123       functions without the "..._ent" suffix circumvents the hook.  See
1124       "Guts" in Hash::Util::Fieldhash for a detailed description.
1125
1126       Note that because multiple extensions may be using "PERL_MAGIC_ext" or
1127       "PERL_MAGIC_uvar" magic, it is important for extensions to take extra
1128       care to avoid conflict.  Typically only using the magic on objects
1129       blessed into the same class as the extension is sufficient.  For
1130       "PERL_MAGIC_ext" magic, it may also be appropriate to add an I32
1131       'signature' at the top of the private data area and check that.
1132
1133       Also note that the "sv_set*()" and "sv_cat*()" functions described
1134       earlier do not invoke 'set' magic on their targets.  This must be done
1135       by the user either by calling the "SvSETMAGIC()" macro after calling
1136       these functions, or by using one of the "sv_set*_mg()" or
1137       "sv_cat*_mg()" functions.  Similarly, generic C code must call the
1138       "SvGETMAGIC()" macro to invoke any 'get' magic if they use an SV
1139       obtained from external sources in functions that don't handle magic.
1140       See perlapi for a description of these functions.  For example, calls
1141       to the "sv_cat*()" functions typically need to be followed by
1142       "SvSETMAGIC()", but they don't need a prior "SvGETMAGIC()" since their
1143       implementation handles 'get' magic.
1144
1145   Finding Magic
1146           MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1147
1148       This routine returns a pointer to the "MAGIC" structure stored in the
1149       SV.  If the SV does not have that magical feature, "NULL" is returned.
1150       Also, if the SV is not of type SVt_PVMG, Perl may core dump.
1151
1152           int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
1153
1154       This routine checks to see what types of magic "sv" has.  If the
1155       mg_type field is an uppercase letter, then the mg_obj is copied to
1156       "nsv", but the mg_type field is changed to be the lowercase letter.
1157
1158   Understanding the Magic of Tied Hashes and Arrays
1159       Tied hashes and arrays are magical beasts of the "PERL_MAGIC_tied"
1160       magic type.
1161
1162       WARNING: As of the 5.004 release, proper usage of the array and hash
1163       access functions requires understanding a few caveats.  Some of these
1164       caveats are actually considered bugs in the API, to be fixed in later
1165       releases, and are bracketed with [MAYCHANGE] below. If you find
1166       yourself actually applying such information in this section, be aware
1167       that the behavior may change in the future, umm, without warning.
1168
1169       The perl tie function associates a variable with an object that
1170       implements the various GET, SET, etc methods.  To perform the
1171       equivalent of the perl tie function from an XSUB, you must mimic this
1172       behaviour.  The code below carries out the necessary steps - firstly it
1173       creates a new hash, and then creates a second hash which it blesses
1174       into the class which will implement the tie methods. Lastly it ties the
1175       two hashes together, and returns a reference to the new tied hash.
1176       Note that the code below does NOT call the TIEHASH method in the MyTie
1177       class - see "Calling Perl Routines from within C Programs" for details
1178       on how to do this.
1179
1180           SV*
1181           mytie()
1182           PREINIT:
1183               HV *hash;
1184               HV *stash;
1185               SV *tie;
1186           CODE:
1187               hash = newHV();
1188               tie = newRV_noinc((SV*)newHV());
1189               stash = gv_stashpv("MyTie", GV_ADD);
1190               sv_bless(tie, stash);
1191               hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1192               RETVAL = newRV_noinc(hash);
1193           OUTPUT:
1194               RETVAL
1195
1196       The "av_store" function, when given a tied array argument, merely
1197       copies the magic of the array onto the value to be "stored", using
1198       "mg_copy".  It may also return NULL, indicating that the value did not
1199       actually need to be stored in the array.  [MAYCHANGE] After a call to
1200       "av_store" on a tied array, the caller will usually need to call
1201       "mg_set(val)" to actually invoke the perl level "STORE" method on the
1202       TIEARRAY object.  If "av_store" did return NULL, a call to
1203       "SvREFCNT_dec(val)" will also be usually necessary to avoid a memory
1204       leak. [/MAYCHANGE]
1205
1206       The previous paragraph is applicable verbatim to tied hash access using
1207       the "hv_store" and "hv_store_ent" functions as well.
1208
1209       "av_fetch" and the corresponding hash functions "hv_fetch" and
1210       "hv_fetch_ent" actually return an undefined mortal value whose magic
1211       has been initialized using "mg_copy".  Note the value so returned does
1212       not need to be deallocated, as it is already mortal.  [MAYCHANGE] But
1213       you will need to call "mg_get()" on the returned value in order to
1214       actually invoke the perl level "FETCH" method on the underlying TIE
1215       object.  Similarly, you may also call "mg_set()" on the return value
1216       after possibly assigning a suitable value to it using "sv_setsv",
1217       which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
1218
1219       [MAYCHANGE] In other words, the array or hash fetch/store functions
1220       don't really fetch and store actual values in the case of tied arrays
1221       and hashes.  They merely call "mg_copy" to attach magic to the values
1222       that were meant to be "stored" or "fetched".  Later calls to "mg_get"
1223       and "mg_set" actually do the job of invoking the TIE methods on the
1224       underlying objects.  Thus the magic mechanism currently implements a
1225       kind of lazy access to arrays and hashes.
1226
1227       Currently (as of perl version 5.004), use of the hash and array access
1228       functions requires the user to be aware of whether they are operating
1229       on "normal" hashes and arrays, or on their tied variants.  The API may
1230       be changed to provide more transparent access to both tied and normal
1231       data types in future versions.  [/MAYCHANGE]
1232
1233       You would do well to understand that the TIEARRAY and TIEHASH
1234       interfaces are mere sugar to invoke some perl method calls while using
1235       the uniform hash and array syntax.  The use of this sugar imposes some
1236       overhead (typically about two to four extra opcodes per FETCH/STORE
1237       operation, in addition to the creation of all the mortal variables
1238       required to invoke the methods).  This overhead will be comparatively
1239       small if the TIE methods are themselves substantial, but if they are
1240       only a few statements long, the overhead will not be insignificant.
1241
1242   Localizing changes
1243       Perl has a very handy construction
1244
1245         {
1246           local $var = 2;
1247           ...
1248         }
1249
1250       This construction is approximately equivalent to
1251
1252         {
1253           my $oldvar = $var;
1254           $var = 2;
1255           ...
1256           $var = $oldvar;
1257         }
1258
1259       The biggest difference is that the first construction would reinstate
1260       the initial value of $var, irrespective of how control exits the block:
1261       "goto", "return", "die"/"eval", etc. It is a little bit more efficient
1262       as well.
1263
1264       There is a way to achieve a similar task from C via Perl API: create a
1265       pseudo-block, and arrange for some changes to be automatically undone
1266       at the end of it, either explicit, or via a non-local exit (via die()).
1267       A block-like construct is created by a pair of "ENTER"/"LEAVE" macros
1268       (see "Returning a Scalar" in perlcall).  Such a construct may be
1269       created specially for some important localized task, or an existing one
1270       (like boundaries of enclosing Perl subroutine/block, or an existing
1271       pair for freeing TMPs) may be used. (In the second case the overhead of
1272       additional localization must be almost negligible.) Note that any XSUB
1273       is automatically enclosed in an "ENTER"/"LEAVE" pair.
1274
1275       Inside such a pseudo-block the following service is available:
1276
1277       "SAVEINT(int i)"
1278       "SAVEIV(IV i)"
1279       "SAVEI32(I32 i)"
1280       "SAVELONG(long i)"
1281           These macros arrange things to restore the value of integer
1282           variable "i" at the end of enclosing pseudo-block.
1283
1284       SAVESPTR(s)
1285       SAVEPPTR(p)
1286           These macros arrange things to restore the value of pointers "s"
1287           and "p". "s" must be a pointer of a type which survives conversion
1288           to "SV*" and back, "p" should be able to survive conversion to
1289           "char*" and back.
1290
1291       "SAVEFREESV(SV *sv)"
1292           The refcount of "sv" would be decremented at the end of pseudo-
1293           block.  This is similar to "sv_2mortal" in that it is also a
1294           mechanism for doing a delayed "SvREFCNT_dec".  However, while
1295           "sv_2mortal" extends the lifetime of "sv" until the beginning of
1296           the next statement, "SAVEFREESV" extends it until the end of the
1297           enclosing scope.  These lifetimes can be wildly different.
1298
1299           Also compare "SAVEMORTALIZESV".
1300
1301       "SAVEMORTALIZESV(SV *sv)"
1302           Just like "SAVEFREESV", but mortalizes "sv" at the end of the
1303           current scope instead of decrementing its reference count.  This
1304           usually has the effect of keeping "sv" alive until the statement
1305           that called the currently live scope has finished executing.
1306
1307       "SAVEFREEOP(OP *op)"
1308           The "OP *" is op_free()ed at the end of pseudo-block.
1309
1310       SAVEFREEPV(p)
1311           The chunk of memory which is pointed to by "p" is Safefree()ed at
1312           the end of pseudo-block.
1313
1314       "SAVECLEARSV(SV *sv)"
1315           Clears a slot in the current scratchpad which corresponds to "sv"
1316           at the end of pseudo-block.
1317
1318       "SAVEDELETE(HV *hv, char *key, I32 length)"
1319           The key "key" of "hv" is deleted at the end of pseudo-block. The
1320           string pointed to by "key" is Safefree()ed.  If one has a key in
1321           short-lived storage, the corresponding string may be reallocated
1322           like this:
1323
1324             SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
1325
1326       "SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
1327           At the end of pseudo-block the function "f" is called with the only
1328           argument "p".
1329
1330       "SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
1331           At the end of pseudo-block the function "f" is called with the
1332           implicit context argument (if any), and "p".
1333
1334       "SAVESTACK_POS()"
1335           The current offset on the Perl internal stack (cf. "SP") is
1336           restored at the end of pseudo-block.
1337
1338       The following API list contains functions, thus one needs to provide
1339       pointers to the modifiable data explicitly (either C pointers, or
1340       Perlish "GV *"s).  Where the above macros take "int", a similar
1341       function takes "int *".
1342
1343       "SV* save_scalar(GV *gv)"
1344           Equivalent to Perl code "local $gv".
1345
1346       "AV* save_ary(GV *gv)"
1347       "HV* save_hash(GV *gv)"
1348           Similar to "save_scalar", but localize @gv and %gv.
1349
1350       "void save_item(SV *item)"
1351           Duplicates the current value of "SV", on the exit from the current
1352           "ENTER"/"LEAVE" pseudo-block will restore the value of "SV" using
1353           the stored value. It doesn't handle magic. Use "save_scalar" if
1354           magic is affected.
1355
1356       "void save_list(SV **sarg, I32 maxsarg)"
1357           A variant of "save_item" which takes multiple arguments via an
1358           array "sarg" of "SV*" of length "maxsarg".
1359
1360       "SV* save_svref(SV **sptr)"
1361           Similar to "save_scalar", but will reinstate an "SV *".
1362
1363       "void save_aptr(AV **aptr)"
1364       "void save_hptr(HV **hptr)"
1365           Similar to "save_svref", but localize "AV *" and "HV *".
1366
1367       The "Alias" module implements localization of the basic types within
1368       the caller's scope.  People who are interested in how to localize
1369       things in the containing scope should take a look there too.
1370

Subroutines

1372   XSUBs and the Argument Stack
1373       The XSUB mechanism is a simple way for Perl programs to access C
1374       subroutines.  An XSUB routine will have a stack that contains the
1375       arguments from the Perl program, and a way to map from the Perl data
1376       structures to a C equivalent.
1377
1378       The stack arguments are accessible through the ST(n) macro, which
1379       returns the "n"'th stack argument.  Argument 0 is the first argument
1380       passed in the Perl subroutine call.  These arguments are "SV*", and can
1381       be used anywhere an "SV*" is used.
1382
1383       Most of the time, output from the C routine can be handled through use
1384       of the RETVAL and OUTPUT directives.  However, there are some cases
1385       where the argument stack is not already long enough to handle all the
1386       return values.  An example is the POSIX tzname() call, which takes no
1387       arguments, but returns two, the local time zone's standard and summer
1388       time abbreviations.
1389
1390       To handle this situation, the PPCODE directive is used and the stack is
1391       extended using the macro:
1392
1393           EXTEND(SP, num);
1394
1395       where "SP" is the macro that represents the local copy of the stack
1396       pointer, and "num" is the number of elements the stack should be
1397       extended by.
1398
1399       Now that there is room on the stack, values can be pushed on it using
1400       "PUSHs" macro. The pushed values will often need to be "mortal" (See
1401       "Reference Counts and Mortality"):
1402
1403           PUSHs(sv_2mortal(newSViv(an_integer)))
1404           PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1405           PUSHs(sv_2mortal(newSVnv(a_double)))
1406           PUSHs(sv_2mortal(newSVpv("Some String",0)))
1407
1408       And now the Perl program calling "tzname", the two values will be
1409       assigned as in:
1410
1411           ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1412
1413       An alternate (and possibly simpler) method to pushing values on the
1414       stack is to use the macro:
1415
1416           XPUSHs(SV*)
1417
1418       This macro automatically adjust the stack for you, if needed.  Thus,
1419       you do not need to call "EXTEND" to extend the stack.
1420
1421       Despite their suggestions in earlier versions of this document the
1422       macros "(X)PUSH[iunp]" are not suited to XSUBs which return multiple
1423       results.  For that, either stick to the "(X)PUSHs" macros shown above,
1424       or use the new "m(X)PUSH[iunp]" macros instead; see "Putting a C value
1425       on Perl stack".
1426
1427       For more information, consult perlxs and perlxstut.
1428
1429   Calling Perl Routines from within C Programs
1430       There are four routines that can be used to call a Perl subroutine from
1431       within a C program.  These four are:
1432
1433           I32  call_sv(SV*, I32);
1434           I32  call_pv(const char*, I32);
1435           I32  call_method(const char*, I32);
1436           I32  call_argv(const char*, I32, register char**);
1437
1438       The routine most often used is "call_sv".  The "SV*" argument contains
1439       either the name of the Perl subroutine to be called, or a reference to
1440       the subroutine.  The second argument consists of flags that control the
1441       context in which the subroutine is called, whether or not the
1442       subroutine is being passed arguments, how errors should be trapped, and
1443       how to treat return values.
1444
1445       All four routines return the number of arguments that the subroutine
1446       returned on the Perl stack.
1447
1448       These routines used to be called "perl_call_sv", etc., before Perl
1449       v5.6.0, but those names are now deprecated; macros of the same name are
1450       provided for compatibility.
1451
1452       When using any of these routines (except "call_argv"), the programmer
1453       must manipulate the Perl stack.  These include the following macros and
1454       functions:
1455
1456           dSP
1457           SP
1458           PUSHMARK()
1459           PUTBACK
1460           SPAGAIN
1461           ENTER
1462           SAVETMPS
1463           FREETMPS
1464           LEAVE
1465           XPUSH*()
1466           POP*()
1467
1468       For a detailed description of calling conventions from C to Perl,
1469       consult perlcall.
1470
1471   Memory Allocation
1472       Allocation
1473
1474       All memory meant to be used with the Perl API functions should be
1475       manipulated using the macros described in this section.  The macros
1476       provide the necessary transparency between differences in the actual
1477       malloc implementation that is used within perl.
1478
1479       It is suggested that you enable the version of malloc that is
1480       distributed with Perl.  It keeps pools of various sizes of unallocated
1481       memory in order to satisfy allocation requests more quickly.  However,
1482       on some platforms, it may cause spurious malloc or free errors.
1483
1484       The following three macros are used to initially allocate memory :
1485
1486           Newx(pointer, number, type);
1487           Newxc(pointer, number, type, cast);
1488           Newxz(pointer, number, type);
1489
1490       The first argument "pointer" should be the name of a variable that will
1491       point to the newly allocated memory.
1492
1493       The second and third arguments "number" and "type" specify how many of
1494       the specified type of data structure should be allocated.  The argument
1495       "type" is passed to "sizeof".  The final argument to "Newxc", "cast",
1496       should be used if the "pointer" argument is different from the "type"
1497       argument.
1498
1499       Unlike the "Newx" and "Newxc" macros, the "Newxz" macro calls "memzero"
1500       to zero out all the newly allocated memory.
1501
1502       Reallocation
1503
1504           Renew(pointer, number, type);
1505           Renewc(pointer, number, type, cast);
1506           Safefree(pointer)
1507
1508       These three macros are used to change a memory buffer size or to free a
1509       piece of memory no longer needed.  The arguments to "Renew" and
1510       "Renewc" match those of "New" and "Newc" with the exception of not
1511       needing the "magic cookie" argument.
1512
1513       Moving
1514
1515           Move(source, dest, number, type);
1516           Copy(source, dest, number, type);
1517           Zero(dest, number, type);
1518
1519       These three macros are used to move, copy, or zero out previously
1520       allocated memory.  The "source" and "dest" arguments point to the
1521       source and destination starting points.  Perl will move, copy, or zero
1522       out "number" instances of the size of the "type" data structure (using
1523       the "sizeof" function).
1524
1525   PerlIO
1526       The most recent development releases of Perl has been experimenting
1527       with removing Perl's dependency on the "normal" standard I/O suite and
1528       allowing other stdio implementations to be used.  This involves
1529       creating a new abstraction layer that then calls whichever
1530       implementation of stdio Perl was compiled with.  All XSUBs should now
1531       use the functions in the PerlIO abstraction layer and not make any
1532       assumptions about what kind of stdio is being used.
1533
1534       For a complete description of the PerlIO abstraction, consult perlapio.
1535
1536   Putting a C value on Perl stack
1537       A lot of opcodes (this is an elementary operation in the internal perl
1538       stack machine) put an SV* on the stack. However, as an optimization the
1539       corresponding SV is (usually) not recreated each time. The opcodes
1540       reuse specially assigned SVs (targets) which are (as a corollary) not
1541       constantly freed/created.
1542
1543       Each of the targets is created only once (but see "Scratchpads and
1544       recursion" below), and when an opcode needs to put an integer, a
1545       double, or a string on stack, it just sets the corresponding parts of
1546       its target and puts the target on stack.
1547
1548       The macro to put this target on stack is "PUSHTARG", and it is directly
1549       used in some opcodes, as well as indirectly in zillions of others,
1550       which use it via "(X)PUSH[iunp]".
1551
1552       Because the target is reused, you must be careful when pushing multiple
1553       values on the stack. The following code will not do what you think:
1554
1555           XPUSHi(10);
1556           XPUSHi(20);
1557
1558       This translates as "set "TARG" to 10, push a pointer to "TARG" onto the
1559       stack; set "TARG" to 20, push a pointer to "TARG" onto the stack".  At
1560       the end of the operation, the stack does not contain the values 10 and
1561       20, but actually contains two pointers to "TARG", which we have set to
1562       20.
1563
1564       If you need to push multiple different values then you should either
1565       use the "(X)PUSHs" macros, or else use the new "m(X)PUSH[iunp]" macros,
1566       none of which make use of "TARG".  The "(X)PUSHs" macros simply push an
1567       SV* on the stack, which, as noted under "XSUBs and the Argument Stack",
1568       will often need to be "mortal".  The new "m(X)PUSH[iunp]" macros make
1569       this a little easier to achieve by creating a new mortal for you (via
1570       "(X)PUSHmortal"), pushing that onto the stack (extending it if
1571       necessary in the case of the "mXPUSH[iunp]" macros), and then setting
1572       its value.  Thus, instead of writing this to "fix" the example above:
1573
1574           XPUSHs(sv_2mortal(newSViv(10)))
1575           XPUSHs(sv_2mortal(newSViv(20)))
1576
1577       you can simply write:
1578
1579           mXPUSHi(10)
1580           mXPUSHi(20)
1581
1582       On a related note, if you do use "(X)PUSH[iunp]", then you're going to
1583       need a "dTARG" in your variable declarations so that the "*PUSH*"
1584       macros can make use of the local variable "TARG".  See also "dTARGET"
1585       and "dXSTARG".
1586
1587   Scratchpads
1588       The question remains on when the SVs which are targets for opcodes are
1589       created. The answer is that they are created when the current unit -- a
1590       subroutine or a file (for opcodes for statements outside of
1591       subroutines) -- is compiled. During this time a special anonymous Perl
1592       array is created, which is called a scratchpad for the current unit.
1593
1594       A scratchpad keeps SVs which are lexicals for the current unit and are
1595       targets for opcodes. One can deduce that an SV lives on a scratchpad by
1596       looking on its flags: lexicals have "SVs_PADMY" set, and targets have
1597       "SVs_PADTMP" set.
1598
1599       The correspondence between OPs and targets is not 1-to-1. Different OPs
1600       in the compile tree of the unit can use the same target, if this would
1601       not conflict with the expected life of the temporary.
1602
1603   Scratchpads and recursion
1604       In fact it is not 100% true that a compiled unit contains a pointer to
1605       the scratchpad AV. In fact it contains a pointer to an AV of
1606       (initially) one element, and this element is the scratchpad AV. Why do
1607       we need an extra level of indirection?
1608
1609       The answer is recursion, and maybe threads. Both these can create
1610       several execution pointers going into the same subroutine. For the
1611       subroutine-child not write over the temporaries for the subroutine-
1612       parent (lifespan of which covers the call to the child), the parent and
1613       the child should have different scratchpads. (And the lexicals should
1614       be separate anyway!)
1615
1616       So each subroutine is born with an array of scratchpads (of length 1).
1617       On each entry to the subroutine it is checked that the current depth of
1618       the recursion is not more than the length of this array, and if it is,
1619       new scratchpad is created and pushed into the array.
1620
1621       The targets on this scratchpad are "undef"s, but they are already
1622       marked with correct flags.
1623

Compiled code

1625   Code tree
1626       Here we describe the internal form your code is converted to by Perl.
1627       Start with a simple example:
1628
1629         $a = $b + $c;
1630
1631       This is converted to a tree similar to this one:
1632
1633                    assign-to
1634                  /           \
1635                 +             $a
1636               /   \
1637             $b     $c
1638
1639       (but slightly more complicated).  This tree reflects the way Perl
1640       parsed your code, but has nothing to do with the execution order.
1641       There is an additional "thread" going through the nodes of the tree
1642       which shows the order of execution of the nodes.  In our simplified
1643       example above it looks like:
1644
1645            $b ---> $c ---> + ---> $a ---> assign-to
1646
1647       But with the actual compile tree for "$a = $b + $c" it is different:
1648       some nodes optimized away.  As a corollary, though the actual tree
1649       contains more nodes than our simplified example, the execution order is
1650       the same as in our example.
1651
1652   Examining the tree
1653       If you have your perl compiled for debugging (usually done with
1654       "-DDEBUGGING" on the "Configure" command line), you may examine the
1655       compiled tree by specifying "-Dx" on the Perl command line.  The output
1656       takes several lines per node, and for "$b+$c" it looks like this:
1657
1658           5           TYPE = add  ===> 6
1659                       TARG = 1
1660                       FLAGS = (SCALAR,KIDS)
1661                       {
1662                           TYPE = null  ===> (4)
1663                             (was rv2sv)
1664                           FLAGS = (SCALAR,KIDS)
1665                           {
1666           3                   TYPE = gvsv  ===> 4
1667                               FLAGS = (SCALAR)
1668                               GV = main::b
1669                           }
1670                       }
1671                       {
1672                           TYPE = null  ===> (5)
1673                             (was rv2sv)
1674                           FLAGS = (SCALAR,KIDS)
1675                           {
1676           4                   TYPE = gvsv  ===> 5
1677                               FLAGS = (SCALAR)
1678                               GV = main::c
1679                           }
1680                       }
1681
1682       This tree has 5 nodes (one per "TYPE" specifier), only 3 of them are
1683       not optimized away (one per number in the left column).  The immediate
1684       children of the given node correspond to "{}" pairs on the same level
1685       of indentation, thus this listing corresponds to the tree:
1686
1687                          add
1688                        /     \
1689                      null    null
1690                       |       |
1691                      gvsv    gvsv
1692
1693       The execution order is indicated by "===>" marks, thus it is "3 4 5 6"
1694       (node 6 is not included into above listing), i.e., "gvsv gvsv add
1695       whatever".
1696
1697       Each of these nodes represents an op, a fundamental operation inside
1698       the Perl core. The code which implements each operation can be found in
1699       the pp*.c files; the function which implements the op with type "gvsv"
1700       is "pp_gvsv", and so on. As the tree above shows, different ops have
1701       different numbers of children: "add" is a binary operator, as one would
1702       expect, and so has two children. To accommodate the various different
1703       numbers of children, there are various types of op data structure, and
1704       they link together in different ways.
1705
1706       The simplest type of op structure is "OP": this has no children. Unary
1707       operators, "UNOP"s, have one child, and this is pointed to by the
1708       "op_first" field. Binary operators ("BINOP"s) have not only an
1709       "op_first" field but also an "op_last" field. The most complex type of
1710       op is a "LISTOP", which has any number of children. In this case, the
1711       first child is pointed to by "op_first" and the last child by
1712       "op_last". The children in between can be found by iteratively
1713       following the "op_sibling" pointer from the first child to the last.
1714
1715       There are also two other op types: a "PMOP" holds a regular expression,
1716       and has no children, and a "LOOP" may or may not have children. If the
1717       "op_children" field is non-zero, it behaves like a "LISTOP". To
1718       complicate matters, if a "UNOP" is actually a "null" op after
1719       optimization (see "Compile pass 2: context propagation") it will still
1720       have children in accordance with its former type.
1721
1722       Another way to examine the tree is to use a compiler back-end module,
1723       such as B::Concise.
1724
1725   Compile pass 1: check routines
1726       The tree is created by the compiler while yacc code feeds it the
1727       constructions it recognizes. Since yacc works bottom-up, so does the
1728       first pass of perl compilation.
1729
1730       What makes this pass interesting for perl developers is that some
1731       optimization may be performed on this pass.  This is optimization by
1732       so-called "check routines".  The correspondence between node names and
1733       corresponding check routines is described in opcode.pl (do not forget
1734       to run "make regen_headers" if you modify this file).
1735
1736       A check routine is called when the node is fully constructed except for
1737       the execution-order thread.  Since at this time there are no back-links
1738       to the currently constructed node, one can do most any operation to the
1739       top-level node, including freeing it and/or creating new nodes
1740       above/below it.
1741
1742       The check routine returns the node which should be inserted into the
1743       tree (if the top-level node was not modified, check routine returns its
1744       argument).
1745
1746       By convention, check routines have names "ck_*". They are usually
1747       called from "new*OP" subroutines (or "convert") (which in turn are
1748       called from perly.y).
1749
1750   Compile pass 1a: constant folding
1751       Immediately after the check routine is called the returned node is
1752       checked for being compile-time executable.  If it is (the value is
1753       judged to be constant) it is immediately executed, and a constant node
1754       with the "return value" of the corresponding subtree is substituted
1755       instead.  The subtree is deleted.
1756
1757       If constant folding was not performed, the execution-order thread is
1758       created.
1759
1760   Compile pass 2: context propagation
1761       When a context for a part of compile tree is known, it is propagated
1762       down through the tree.  At this time the context can have 5 values
1763       (instead of 2 for runtime context): void, boolean, scalar, list, and
1764       lvalue.  In contrast with the pass 1 this pass is processed from top to
1765       bottom: a node's context determines the context for its children.
1766
1767       Additional context-dependent optimizations are performed at this time.
1768       Since at this moment the compile tree contains back-references (via
1769       "thread" pointers), nodes cannot be free()d now.  To allow optimized-
1770       away nodes at this stage, such nodes are null()ified instead of
1771       free()ing (i.e. their type is changed to OP_NULL).
1772
1773   Compile pass 3: peephole optimization
1774       After the compile tree for a subroutine (or for an "eval" or a file) is
1775       created, an additional pass over the code is performed. This pass is
1776       neither top-down or bottom-up, but in the execution order (with
1777       additional complications for conditionals).  These optimizations are
1778       done in the subroutine peep().  Optimizations performed at this stage
1779       are subject to the same restrictions as in the pass 2.
1780
1781   Pluggable runops
1782       The compile tree is executed in a runops function.  There are two
1783       runops functions, in run.c and in dump.c.  "Perl_runops_debug" is used
1784       with DEBUGGING and "Perl_runops_standard" is used otherwise.  For fine
1785       control over the execution of the compile tree it is possible to
1786       provide your own runops function.
1787
1788       It's probably best to copy one of the existing runops functions and
1789       change it to suit your needs.  Then, in the BOOT section of your XS
1790       file, add the line:
1791
1792         PL_runops = my_runops;
1793
1794       This function should be as efficient as possible to keep your programs
1795       running as fast as possible.
1796

Examining internal data structures with the "dump" functions

1798       To aid debugging, the source file dump.c contains a number of functions
1799       which produce formatted output of internal data structures.
1800
1801       The most commonly used of these functions is "Perl_sv_dump"; it's used
1802       for dumping SVs, AVs, HVs, and CVs. The "Devel::Peek" module calls
1803       "sv_dump" to produce debugging output from Perl-space, so users of that
1804       module should already be familiar with its format.
1805
1806       "Perl_op_dump" can be used to dump an "OP" structure or any of its
1807       derivatives, and produces output similar to "perl -Dx"; in fact,
1808       "Perl_dump_eval" will dump the main root of the code being evaluated,
1809       exactly like "-Dx".
1810
1811       Other useful functions are "Perl_dump_sub", which turns a "GV" into an
1812       op tree, "Perl_dump_packsubs" which calls "Perl_dump_sub" on all the
1813       subroutines in a package like so: (Thankfully, these are all xsubs, so
1814       there is no op tree)
1815
1816           (gdb) print Perl_dump_packsubs(PL_defstash)
1817
1818           SUB attributes::bootstrap = (xsub 0x811fedc 0)
1819
1820           SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1821
1822           SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1823
1824           SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1825
1826           SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1827
1828       and "Perl_dump_all", which dumps all the subroutines in the stash and
1829       the op tree of the main root.
1830

How multiple interpreters and concurrency are supported

1832   Background and PERL_IMPLICIT_CONTEXT
1833       The Perl interpreter can be regarded as a closed box: it has an API for
1834       feeding it code or otherwise making it do things, but it also has
1835       functions for its own use.  This smells a lot like an object, and there
1836       are ways for you to build Perl so that you can have multiple
1837       interpreters, with one interpreter represented either as a C structure,
1838       or inside a thread-specific structure.  These structures contain all
1839       the context, the state of that interpreter.
1840
1841       One macro controls the major Perl build flavor: MULTIPLICITY. The
1842       MULTIPLICITY build has a C structure that packages all the interpreter
1843       state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
1844       normally defined, and enables the support for passing in a "hidden"
1845       first argument that represents all three data structures. MULTIPLICITY
1846       makes mutli-threaded perls possible (with the ithreads threading model,
1847       related to the macro USE_ITHREADS.)
1848
1849       Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
1850       PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
1851       former turns on MULTIPLICITY.)  The PERL_GLOBAL_STRUCT causes all the
1852       internal variables of Perl to be wrapped inside a single global struct,
1853       struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the
1854       function  Perl_GetVars().  The PERL_GLOBAL_STRUCT_PRIVATE goes one step
1855       further, there is still a single struct (allocated in main() either
1856       from heap or from stack) but there are no global data symbols pointing
1857       to it.  In either case the global struct should be initialised as the
1858       very first thing in main() using Perl_init_global_struct() and
1859       correspondingly tear it down after perl_free() using
1860       Perl_free_global_struct(), please see miniperlmain.c for usage details.
1861       You may also need to use "dVAR" in your coding to "declare the global
1862       variables" when you are using them.  dTHX does this for you
1863       automatically.
1864
1865       To see whether you have non-const data you can use a BSD-compatible
1866       "nm":
1867
1868         nm libperl.a | grep -v ' [TURtr] '
1869
1870       If this displays any "D" or "d" symbols, you have non-const data.
1871
1872       For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
1873       doesn't actually hide all symbols inside a big global struct: some
1874       PerlIO_xxx vtables are left visible.  The PERL_GLOBAL_STRUCT_PRIVATE
1875       then hides everything (see how the PERLIO_FUNCS_DECL is used).
1876
1877       All this obviously requires a way for the Perl internal functions to be
1878       either subroutines taking some kind of structure as the first argument,
1879       or subroutines taking nothing as the first argument.  To enable these
1880       two very different ways of building the interpreter, the Perl source
1881       (as it does in so many other situations) makes heavy use of macros and
1882       subroutine naming conventions.
1883
1884       First problem: deciding which functions will be public API functions
1885       and which will be private.  All functions whose names begin "S_" are
1886       private (think "S" for "secret" or "static").  All other functions
1887       begin with "Perl_", but just because a function begins with "Perl_"
1888       does not mean it is part of the API. (See "Internal Functions".) The
1889       easiest way to be sure a function is part of the API is to find its
1890       entry in perlapi.  If it exists in perlapi, it's part of the API.  If
1891       it doesn't, and you think it should be (i.e., you need it for your
1892       extension), send mail via perlbug explaining why you think it should
1893       be.
1894
1895       Second problem: there must be a syntax so that the same subroutine
1896       declarations and calls can pass a structure as their first argument, or
1897       pass nothing.  To solve this, the subroutines are named and declared in
1898       a particular way.  Here's a typical start of a static function used
1899       within the Perl guts:
1900
1901         STATIC void
1902         S_incline(pTHX_ char *s)
1903
1904       STATIC becomes "static" in C, and may be #define'd to nothing in some
1905       configurations in future.
1906
1907       A public function (i.e. part of the internal API, but not necessarily
1908       sanctioned for use in extensions) begins like this:
1909
1910         void
1911         Perl_sv_setiv(pTHX_ SV* dsv, IV num)
1912
1913       "pTHX_" is one of a number of macros (in perl.h) that hide the details
1914       of the interpreter's context.  THX stands for "thread", "this", or
1915       "thingy", as the case may be.  (And no, George Lucas is not involved.
1916       :-) The first character could be 'p' for a prototype, 'a' for argument,
1917       or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their
1918       variants.
1919
1920       When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
1921       there is no first argument containing the interpreter's context.  The
1922       trailing underscore in the pTHX_ macro indicates that the macro
1923       expansion needs a comma after the context argument because other
1924       arguments follow it.  If PERL_IMPLICIT_CONTEXT is not defined, pTHX_
1925       will be ignored, and the subroutine is not prototyped to take the extra
1926       argument.  The form of the macro without the trailing underscore is
1927       used when there are no additional explicit arguments.
1928
1929       When a core function calls another, it must pass the context.  This is
1930       normally hidden via macros.  Consider "sv_setiv".  It expands into
1931       something like this:
1932
1933           #ifdef PERL_IMPLICIT_CONTEXT
1934             #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
1935             /* can't do this for vararg functions, see below */
1936           #else
1937             #define sv_setiv           Perl_sv_setiv
1938           #endif
1939
1940       This works well, and means that XS authors can gleefully write:
1941
1942           sv_setiv(foo, bar);
1943
1944       and still have it work under all the modes Perl could have been
1945       compiled with.
1946
1947       This doesn't work so cleanly for varargs functions, though, as macros
1948       imply that the number of arguments is known in advance.  Instead we
1949       either need to spell them out fully, passing "aTHX_" as the first
1950       argument (the Perl core tends to do this with functions like
1951       Perl_warner), or use a context-free version.
1952
1953       The context-free version of Perl_warner is called
1954       Perl_warner_nocontext, and does not take the extra argument.  Instead
1955       it does dTHX; to get the context from thread-local storage.  We
1956       "#define warner Perl_warner_nocontext" so that extensions get source
1957       compatibility at the expense of performance.  (Passing an arg is
1958       cheaper than grabbing it from thread-local storage.)
1959
1960       You can ignore [pad]THXx when browsing the Perl headers/sources.  Those
1961       are strictly for use within the core.  Extensions and embedders need
1962       only be aware of [pad]THX.
1963
1964   So what happened to dTHR?
1965       "dTHR" was introduced in perl 5.005 to support the older thread model.
1966       The older thread model now uses the "THX" mechanism to pass context
1967       pointers around, so "dTHR" is not useful any more.  Perl 5.6.0 and
1968       later still have it for backward source compatibility, but it is
1969       defined to be a no-op.
1970
1971   How do I use all this in extensions?
1972       When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
1973       functions in the Perl API will need to pass the initial context
1974       argument somehow.  The kicker is that you will need to write it in such
1975       a way that the extension still compiles when Perl hasn't been built
1976       with PERL_IMPLICIT_CONTEXT enabled.
1977
1978       There are three ways to do this.  First, the easy but inefficient way,
1979       which is also the default, in order to maintain source compatibility
1980       with extensions: whenever XSUB.h is #included, it redefines the aTHX
1981       and aTHX_ macros to call a function that will return the context.
1982       Thus, something like:
1983
1984               sv_setiv(sv, num);
1985
1986       in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
1987       in effect:
1988
1989               Perl_sv_setiv(Perl_get_context(), sv, num);
1990
1991       or to this otherwise:
1992
1993               Perl_sv_setiv(sv, num);
1994
1995       You have to do nothing new in your extension to get this; since the
1996       Perl library provides Perl_get_context(), it will all just work.
1997
1998       The second, more efficient way is to use the following template for
1999       your Foo.xs:
2000
2001               #define PERL_NO_GET_CONTEXT     /* we want efficiency */
2002               #include "EXTERN.h"
2003               #include "perl.h"
2004               #include "XSUB.h"
2005
2006               STATIC void my_private_function(int arg1, int arg2);
2007
2008               STATIC void
2009               my_private_function(int arg1, int arg2)
2010               {
2011                   dTHX;       /* fetch context */
2012                   ... call many Perl API functions ...
2013               }
2014
2015               [... etc ...]
2016
2017               MODULE = Foo            PACKAGE = Foo
2018
2019               /* typical XSUB */
2020
2021               void
2022               my_xsub(arg)
2023                       int arg
2024                   CODE:
2025                       my_private_function(arg, 10);
2026
2027       Note that the only two changes from the normal way of writing an
2028       extension is the addition of a "#define PERL_NO_GET_CONTEXT" before
2029       including the Perl headers, followed by a "dTHX;" declaration at the
2030       start of every function that will call the Perl API.  (You'll know
2031       which functions need this, because the C compiler will complain that
2032       there's an undeclared identifier in those functions.)  No changes are
2033       needed for the XSUBs themselves, because the XS() macro is correctly
2034       defined to pass in the implicit context if needed.
2035
2036       The third, even more efficient way is to ape how it is done within the
2037       Perl guts:
2038
2039               #define PERL_NO_GET_CONTEXT     /* we want efficiency */
2040               #include "EXTERN.h"
2041               #include "perl.h"
2042               #include "XSUB.h"
2043
2044               /* pTHX_ only needed for functions that call Perl API */
2045               STATIC void my_private_function(pTHX_ int arg1, int arg2);
2046
2047               STATIC void
2048               my_private_function(pTHX_ int arg1, int arg2)
2049               {
2050                   /* dTHX; not needed here, because THX is an argument */
2051                   ... call Perl API functions ...
2052               }
2053
2054               [... etc ...]
2055
2056               MODULE = Foo            PACKAGE = Foo
2057
2058               /* typical XSUB */
2059
2060               void
2061               my_xsub(arg)
2062                       int arg
2063                   CODE:
2064                       my_private_function(aTHX_ arg, 10);
2065
2066       This implementation never has to fetch the context using a function
2067       call, since it is always passed as an extra argument.  Depending on
2068       your needs for simplicity or efficiency, you may mix the previous two
2069       approaches freely.
2070
2071       Never add a comma after "pTHX" yourself--always use the form of the
2072       macro with the underscore for functions that take explicit arguments,
2073       or the form without the argument for functions with no explicit
2074       arguments.
2075
2076       If one is compiling Perl with the "-DPERL_GLOBAL_STRUCT" the "dVAR"
2077       definition is needed if the Perl global variables (see perlvars.h or
2078       globvar.sym) are accessed in the function and "dTHX" is not used (the
2079       "dTHX" includes the "dVAR" if necessary).  One notices the need for
2080       "dVAR" only with the said compile-time define, because otherwise the
2081       Perl global variables are visible as-is.
2082
2083   Should I do anything special if I call perl from multiple threads?
2084       If you create interpreters in one thread and then proceed to call them
2085       in another, you need to make sure perl's own Thread Local Storage (TLS)
2086       slot is initialized correctly in each of those threads.
2087
2088       The "perl_alloc" and "perl_clone" API functions will automatically set
2089       the TLS slot to the interpreter they created, so that there is no need
2090       to do anything special if the interpreter is always accessed in the
2091       same thread that created it, and that thread did not create or call any
2092       other interpreters afterwards.  If that is not the case, you have to
2093       set the TLS slot of the thread before calling any functions in the Perl
2094       API on that particular interpreter.  This is done by calling the
2095       "PERL_SET_CONTEXT" macro in that thread as the first thing you do:
2096
2097               /* do this before doing anything else with some_perl */
2098               PERL_SET_CONTEXT(some_perl);
2099
2100               ... other Perl API calls on some_perl go here ...
2101
2102   Future Plans and PERL_IMPLICIT_SYS
2103       Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2104       that the interpreter knows about itself and pass it around, so too are
2105       there plans to allow the interpreter to bundle up everything it knows
2106       about the environment it's running on.  This is enabled with the
2107       PERL_IMPLICIT_SYS macro.  Currently it only works with USE_ITHREADS on
2108       Windows.
2109
2110       This allows the ability to provide an extra pointer (called the "host"
2111       environment) for all the system calls.  This makes it possible for all
2112       the system stuff to maintain their own state, broken down into seven C
2113       structures.  These are thin wrappers around the usual system calls (see
2114       win32/perllib.c) for the default perl executable, but for a more
2115       ambitious host (like the one that would do fork() emulation) all the
2116       extra work needed to pretend that different interpreters are actually
2117       different "processes", would be done here.
2118
2119       The Perl engine/interpreter and the host are orthogonal entities.
2120       There could be one or more interpreters in a process, and one or more
2121       "hosts", with free association between them.
2122

Internal Functions

2124       All of Perl's internal functions which will be exposed to the outside
2125       world are prefixed by "Perl_" so that they will not conflict with XS
2126       functions or functions used in a program in which Perl is embedded.
2127       Similarly, all global variables begin with "PL_". (By convention,
2128       static functions start with "S_".)
2129
2130       Inside the Perl core, you can get at the functions either with or
2131       without the "Perl_" prefix, thanks to a bunch of defines that live in
2132       embed.h. This header file is generated automatically from embed.pl and
2133       embed.fnc. embed.pl also creates the prototyping header files for the
2134       internal functions, generates the documentation and a lot of other bits
2135       and pieces. It's important that when you add a new function to the core
2136       or change an existing one, you change the data in the table in
2137       embed.fnc as well. Here's a sample entry from that table:
2138
2139           Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 lval
2140
2141       The second column is the return type, the third column the name.
2142       Columns after that are the arguments. The first column is a set of
2143       flags:
2144
2145       A  This function is a part of the public API. All such functions should
2146          also have 'd', very few do not.
2147
2148       p  This function has a "Perl_" prefix; i.e. it is defined as
2149          "Perl_av_fetch".
2150
2151       d  This function has documentation using the "apidoc" feature which
2152          we'll look at in a second.  Some functions have 'd' but not 'A';
2153          docs are good.
2154
2155       Other available flags are:
2156
2157       s  This is a static function and is defined as "STATIC S_whatever", and
2158          usually called within the sources as "whatever(...)".
2159
2160       n  This does not need a interpreter context, so the definition has no
2161          "pTHX", and it follows that callers don't use "aTHX".  (See
2162          "Background and PERL_IMPLICIT_CONTEXT" in perlguts.)
2163
2164       r  This function never returns; "croak", "exit" and friends.
2165
2166       f  This function takes a variable number of arguments, "printf" style.
2167          The argument list should end with "...", like this:
2168
2169              Afprd   |void   |croak          |const char* pat|...
2170
2171       M  This function is part of the experimental development API, and may
2172          change or disappear without notice.
2173
2174       o  This function should not have a compatibility macro to define, say,
2175          "Perl_parse" to "parse". It must be called as "Perl_parse".
2176
2177       x  This function isn't exported out of the Perl core.
2178
2179       m  This is implemented as a macro.
2180
2181       X  This function is explicitly exported.
2182
2183       E  This function is visible to extensions included in the Perl core.
2184
2185       b  Binary backward compatibility; this function is a macro but also has
2186          a "Perl_" implementation (which is exported).
2187
2188       others
2189          See the comments at the top of "embed.fnc" for others.
2190
2191       If you edit embed.pl or embed.fnc, you will need to run "make
2192       regen_headers" to force a rebuild of embed.h and other auto-generated
2193       files.
2194
2195   Formatted Printing of IVs, UVs, and NVs
2196       If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2197       formatting codes like %d, %ld, %f, you should use the following macros
2198       for portability
2199
2200               IVdf            IV in decimal
2201               UVuf            UV in decimal
2202               UVof            UV in octal
2203               UVxf            UV in hexadecimal
2204               NVef            NV %e-like
2205               NVff            NV %f-like
2206               NVgf            NV %g-like
2207
2208       These will take care of 64-bit integers and long doubles.  For example:
2209
2210               printf("IV is %"IVdf"\n", iv);
2211
2212       The IVdf will expand to whatever is the correct format for the IVs.
2213
2214       If you are printing addresses of pointers, use UVxf combined with
2215       PTR2UV(), do not use %lx or %p.
2216
2217   Pointer-To-Integer and Integer-To-Pointer
2218       Because pointer size does not necessarily equal integer size, use the
2219       follow macros to do it right.
2220
2221               PTR2UV(pointer)
2222               PTR2IV(pointer)
2223               PTR2NV(pointer)
2224               INT2PTR(pointertotype, integer)
2225
2226       For example:
2227
2228               IV  iv = ...;
2229               SV *sv = INT2PTR(SV*, iv);
2230
2231       and
2232
2233               AV *av = ...;
2234               UV  uv = PTR2UV(av);
2235
2236   Exception Handling
2237       There are a couple of macros to do very basic exception handling in XS
2238       modules. You have to define "NO_XSLOCKS" before including XSUB.h to be
2239       able to use these macros:
2240
2241               #define NO_XSLOCKS
2242               #include "XSUB.h"
2243
2244       You can use these macros if you call code that may croak, but you need
2245       to do some cleanup before giving control back to Perl. For example:
2246
2247               dXCPT;    /* set up necessary variables */
2248
2249               XCPT_TRY_START {
2250                 code_that_may_croak();
2251               } XCPT_TRY_END
2252
2253               XCPT_CATCH
2254               {
2255                 /* do cleanup here */
2256                 XCPT_RETHROW;
2257               }
2258
2259       Note that you always have to rethrow an exception that has been caught.
2260       Using these macros, it is not possible to just catch the exception and
2261       ignore it. If you have to ignore the exception, you have to use the
2262       "call_*" function.
2263
2264       The advantage of using the above macros is that you don't have to setup
2265       an extra function for "call_*", and that using these macros is faster
2266       than using "call_*".
2267
2268   Source Documentation
2269       There's an effort going on to document the internal functions and
2270       automatically produce reference manuals from them - perlapi is one such
2271       manual which details all the functions which are available to XS
2272       writers. perlintern is the autogenerated manual for the functions which
2273       are not part of the API and are supposedly for internal use only.
2274
2275       Source documentation is created by putting POD comments into the C
2276       source, like this:
2277
2278        /*
2279        =for apidoc sv_setiv
2280
2281        Copies an integer into the given SV.  Does not handle 'set' magic.  See
2282        C<sv_setiv_mg>.
2283
2284        =cut
2285        */
2286
2287       Please try and supply some documentation if you add functions to the
2288       Perl core.
2289
2290   Backwards compatibility
2291       The Perl API changes over time. New functions are added or the
2292       interfaces of existing functions are changed. The "Devel::PPPort"
2293       module tries to provide compatibility code for some of these changes,
2294       so XS writers don't have to code it themselves when supporting multiple
2295       versions of Perl.
2296
2297       "Devel::PPPort" generates a C header file ppport.h that can also be run
2298       as a Perl script. To generate ppport.h, run:
2299
2300           perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2301
2302       Besides checking existing XS code, the script can also be used to
2303       retrieve compatibility information for various API calls using the
2304       "--api-info" command line switch. For example:
2305
2306         % perl ppport.h --api-info=sv_magicext
2307
2308       For details, see "perldoc ppport.h".
2309

Unicode Support

2311       Perl 5.6.0 introduced Unicode support. It's important for porters and
2312       XS writers to understand this support and make sure that the code they
2313       write does not corrupt Unicode data.
2314
2315   What is Unicode, anyway?
2316       In the olden, less enlightened times, we all used to use ASCII. Most of
2317       us did, anyway. The big problem with ASCII is that it's American. Well,
2318       no, that's not actually the problem; the problem is that it's not
2319       particularly useful for people who don't use the Roman alphabet. What
2320       used to happen was that particular languages would stick their own
2321       alphabet in the upper range of the sequence, between 128 and 255. Of
2322       course, we then ended up with plenty of variants that weren't quite
2323       ASCII, and the whole point of it being a standard was lost.
2324
2325       Worse still, if you've got a language like Chinese or Japanese that has
2326       hundreds or thousands of characters, then you really can't fit them
2327       into a mere 256, so they had to forget about ASCII altogether, and
2328       build their own systems using pairs of numbers to refer to one
2329       character.
2330
2331       To fix this, some people formed Unicode, Inc. and produced a new
2332       character set containing all the characters you can possibly think of
2333       and more. There are several ways of representing these characters, and
2334       the one Perl uses is called UTF-8. UTF-8 uses a variable number of
2335       bytes to represent a character. You can learn more about Unicode and
2336       Perl's Unicode model in perlunicode.
2337
2338   How can I recognise a UTF-8 string?
2339       You can't. This is because UTF-8 data is stored in bytes just like
2340       non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
2341       capital E with a grave accent, is represented by the two bytes
2342       "v196.172". Unfortunately, the non-Unicode string "chr(196).chr(172)"
2343       has that byte sequence as well. So you can't tell just by looking -
2344       this is what makes Unicode input an interesting problem.
2345
2346       In general, you either have to know what you're dealing with, or you
2347       have to guess.  The API function "is_utf8_string" can help; it'll tell
2348       you if a string contains only valid UTF-8 characters. However, it can't
2349       do the work for you. On a character-by-character basis, "is_utf8_char"
2350       will tell you whether the current character in a string is valid UTF-8.
2351
2352   How does UTF-8 represent Unicode characters?
2353       As mentioned above, UTF-8 uses a variable number of bytes to store a
2354       character. Characters with values 0...127 are stored in one byte, just
2355       like good ol' ASCII. Character 128 is stored as "v194.128"; this
2356       continues up to character 191, which is "v194.191". Now we've run out
2357       of bits (191 is binary 10111111) so we move on; 192 is "v195.128". And
2358       so it goes on, moving to three bytes at character 2048.
2359
2360       Assuming you know you're dealing with a UTF-8 string, you can find out
2361       how long the first character in it is with the "UTF8SKIP" macro:
2362
2363           char *utf = "\305\233\340\240\201";
2364           I32 len;
2365
2366           len = UTF8SKIP(utf); /* len is 2 here */
2367           utf += len;
2368           len = UTF8SKIP(utf); /* len is 3 here */
2369
2370       Another way to skip over characters in a UTF-8 string is to use
2371       "utf8_hop", which takes a string and a number of characters to skip
2372       over. You're on your own about bounds checking, though, so don't use it
2373       lightly.
2374
2375       All bytes in a multi-byte UTF-8 character will have the high bit set,
2376       so you can test if you need to do something special with this character
2377       like this (the UTF8_IS_INVARIANT() is a macro that tests whether the
2378       byte can be encoded as a single byte even in UTF-8):
2379
2380           U8 *utf;
2381           UV uv;      /* Note: a UV, not a U8, not a char */
2382
2383           if (!UTF8_IS_INVARIANT(*utf))
2384               /* Must treat this as UTF-8 */
2385               uv = utf8_to_uv(utf);
2386           else
2387               /* OK to treat this character as a byte */
2388               uv = *utf;
2389
2390       You can also see in that example that we use "utf8_to_uv" to get the
2391       value of the character; the inverse function "uv_to_utf8" is available
2392       for putting a UV into UTF-8:
2393
2394           if (!UTF8_IS_INVARIANT(uv))
2395               /* Must treat this as UTF8 */
2396               utf8 = uv_to_utf8(utf8, uv);
2397           else
2398               /* OK to treat this character as a byte */
2399               *utf8++ = uv;
2400
2401       You must convert characters to UVs using the above functions if you're
2402       ever in a situation where you have to match UTF-8 and non-UTF-8
2403       characters. You may not skip over UTF-8 characters in this case. If you
2404       do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2405       for instance, if your UTF-8 string contains "v196.172", and you skip
2406       that character, you can never match a "chr(200)" in a non-UTF-8 string.
2407       So don't do that!
2408
2409   How does Perl store UTF-8 strings?
2410       Currently, Perl deals with Unicode strings and non-Unicode strings
2411       slightly differently. A flag in the SV, "SVf_UTF8", indicates that the
2412       string is internally encoded as UTF-8. Without it, the byte value is
2413       the codepoint number and vice versa (in other words, the string is
2414       encoded as iso-8859-1). You can check and manipulate this flag with the
2415       following macros:
2416
2417           SvUTF8(sv)
2418           SvUTF8_on(sv)
2419           SvUTF8_off(sv)
2420
2421       This flag has an important effect on Perl's treatment of the string: if
2422       Unicode data is not properly distinguished, regular expressions,
2423       "length", "substr" and other string handling operations will have
2424       undesirable results.
2425
2426       The problem comes when you have, for instance, a string that isn't
2427       flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
2428       especially when combining non-UTF-8 and UTF-8 strings.
2429
2430       Never forget that the "SVf_UTF8" flag is separate to the PV value; you
2431       need be sure you don't accidentally knock it off while you're
2432       manipulating SVs. More specifically, you cannot expect to do this:
2433
2434           SV *sv;
2435           SV *nsv;
2436           STRLEN len;
2437           char *p;
2438
2439           p = SvPV(sv, len);
2440           frobnicate(p);
2441           nsv = newSVpvn(p, len);
2442
2443       The "char*" string does not tell you the whole story, and you can't
2444       copy or reconstruct an SV just by copying the string value. Check if
2445       the old SV has the UTF8 flag set, and act accordingly:
2446
2447           p = SvPV(sv, len);
2448           frobnicate(p);
2449           nsv = newSVpvn(p, len);
2450           if (SvUTF8(sv))
2451               SvUTF8_on(nsv);
2452
2453       In fact, your "frobnicate" function should be made aware of whether or
2454       not it's dealing with UTF-8 data, so that it can handle the string
2455       appropriately.
2456
2457       Since just passing an SV to an XS function and copying the data of the
2458       SV is not enough to copy the UTF8 flags, even less right is just
2459       passing a "char *" to an XS function.
2460
2461   How do I convert a string to UTF-8?
2462       If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to
2463       upgrade one of the strings to UTF-8. If you've got an SV, the easiest
2464       way to do this is:
2465
2466           sv_utf8_upgrade(sv);
2467
2468       However, you must not do this, for example:
2469
2470           if (!SvUTF8(left))
2471               sv_utf8_upgrade(left);
2472
2473       If you do this in a binary operator, you will actually change one of
2474       the strings that came into the operator, and, while it shouldn't be
2475       noticeable by the end user, it can cause problems in deficient code.
2476
2477       Instead, "bytes_to_utf8" will give you a UTF-8-encoded copy of its
2478       string argument. This is useful for having the data available for
2479       comparisons and so on, without harming the original SV. There's also
2480       "utf8_to_bytes" to go the other way, but naturally, this will fail if
2481       the string contains any characters above 255 that can't be represented
2482       in a single byte.
2483
2484   Is there anything else I need to know?
2485       Not really. Just remember these things:
2486
2487       ·  There's no way to tell if a string is UTF-8 or not. You can tell if
2488          an SV is UTF-8 by looking at is "SvUTF8" flag. Don't forget to set
2489          the flag if something should be UTF-8. Treat the flag as part of the
2490          PV, even though it's not - if you pass on the PV to somewhere, pass
2491          on the flag too.
2492
2493       ·  If a string is UTF-8, always use "utf8_to_uv" to get at the value,
2494          unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s.
2495
2496       ·  When writing a character "uv" to a UTF-8 string, always use
2497          "uv_to_utf8", unless "UTF8_IS_INVARIANT(uv))" in which case you can
2498          use "*s = uv".
2499
2500       ·  Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8" to
2501          get a new string which is UTF-8 encoded, and then combine them.
2502

Custom Operators

2504       Custom operator support is a new experimental feature that allows you
2505       to define your own ops. This is primarily to allow the building of
2506       interpreters for other languages in the Perl core, but it also allows
2507       optimizations through the creation of "macro-ops" (ops which perform
2508       the functions of multiple ops which are usually executed together, such
2509       as "gvsv, gvsv, add".)
2510
2511       This feature is implemented as a new op type, "OP_CUSTOM". The Perl
2512       core does not "know" anything special about this op type, and so it
2513       will not be involved in any optimizations. This also means that you can
2514       define your custom ops to be any op structure - unary, binary, list and
2515       so on - you like.
2516
2517       It's important to know what custom operators won't do for you. They
2518       won't let you add new syntax to Perl, directly. They won't even let you
2519       add new keywords, directly. In fact, they won't change the way Perl
2520       compiles a program at all. You have to do those changes yourself, after
2521       Perl has compiled the program. You do this either by manipulating the
2522       op tree using a "CHECK" block and the "B::Generate" module, or by
2523       adding a custom peephole optimizer with the "optimize" module.
2524
2525       When you do this, you replace ordinary Perl ops with custom ops by
2526       creating ops with the type "OP_CUSTOM" and the "pp_addr" of your own PP
2527       function. This should be defined in XS code, and should look like the
2528       PP ops in "pp_*.c". You are responsible for ensuring that your op takes
2529       the appropriate number of values from the stack, and you are
2530       responsible for adding stack marks if necessary.
2531
2532       You should also "register" your op with the Perl interpreter so that it
2533       can produce sensible error and warning messages. Since it is possible
2534       to have multiple custom ops within the one "logical" op type
2535       "OP_CUSTOM", Perl uses the value of "o->op_ppaddr" as a key into the
2536       "PL_custom_op_descs" and "PL_custom_op_names" hashes. This means you
2537       need to enter a name and description for your op at the appropriate
2538       place in the "PL_custom_op_names" and "PL_custom_op_descs" hashes.
2539
2540       Forthcoming versions of "B::Generate" (version 1.0 and above) should
2541       directly support the creation of custom ops by name.
2542

AUTHORS

2544       Until May 1997, this document was maintained by Jeff Okamoto
2545       <okamoto@corp.hp.com>.  It is now maintained as part of Perl itself by
2546       the Perl 5 Porters <perl5-porters@perl.org>.
2547
2548       With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2549       Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2550       Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2551       Stephen McCamant, and Gurusamy Sarathy.
2552

SEE ALSO

2554       perlapi(1), perlintern(1), perlxs(1), perlembed(1)
2555
2556
2557
2558perl v5.10.1                      2009-05-10                       PERLGUTS(1)
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