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1 =head1 NAME 2 3 perlguts - Introduction to the Perl API 4 5 =head1 DESCRIPTION 6 7 This document attempts to describe how to use the Perl API, as well as 8 to provide some info on the basic workings of the Perl core. It is far 9 from complete and probably contains many errors. Please refer any 10 questions or comments to the author below. 11 12 =head1 Variables 13 14 =head2 Datatypes 15 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 types. 23 24 =head2 What is an "IV"? 25 26 Perl uses a special typedef IV which is a simple signed integer type that is 27 guaranteed to be large enough to hold a pointer (as well as an integer). 28 Additionally, there is the UV, which is simply an unsigned IV. 29 30 Perl also uses two special typedefs, I32 and I16, which will always be at 31 least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16, 32 as well.) They will usually be exactly 32 and 16 bits long, but on Crays 33 they will both be 64 bits. 34 35 =head2 Working with SVs 36 37 An SV can be created and loaded with one command. There are five types of 38 values that can be loaded: an integer value (IV), an unsigned integer 39 value (UV), a double (NV), a string (PV), and another scalar (SV). 40 41 The seven routines are: 42 43 SV* newSViv(IV); 44 SV* newSVuv(UV); 45 SV* newSVnv(double); 46 SV* newSVpv(const char*, STRLEN); 47 SV* newSVpvn(const char*, STRLEN); 48 SV* newSVpvf(const char*, ...); 49 SV* newSVsv(SV*); 50 51 C<STRLEN> is an integer type (Size_t, usually defined as size_t in 52 F<config.h>) guaranteed to be large enough to represent the size of 53 any string that perl can handle. 54 55 In the unlikely case of a SV requiring more complex initialisation, you 56 can create an empty SV with newSV(len). If C<len> is 0 an empty SV of 57 type NULL is returned, else an SV of type PV is returned with len + 1 (for 58 the NUL) bytes of storage allocated, accessible via SvPVX. In both cases 59 the SV has value undef. 60 61 SV *sv = newSV(0); /* no storage allocated */ 62 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */ 63 64 To change the value of an I<already-existing> SV, there are eight routines: 65 66 void sv_setiv(SV*, IV); 67 void sv_setuv(SV*, UV); 68 void sv_setnv(SV*, double); 69 void sv_setpv(SV*, const char*); 70 void sv_setpvn(SV*, const char*, STRLEN) 71 void sv_setpvf(SV*, const char*, ...); 72 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *); 73 void sv_setsv(SV*, SV*); 74 75 Notice that you can choose to specify the length of the string to be 76 assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may 77 allow Perl to calculate the length by using C<sv_setpv> or by specifying 78 0 as the second argument to C<newSVpv>. Be warned, though, that Perl will 79 determine the string's length by using C<strlen>, which depends on the 80 string terminating with a NUL character. 81 82 The arguments of C<sv_setpvf> are processed like C<sprintf>, and the 83 formatted output becomes the value. 84 85 C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify 86 either a pointer to a variable argument list or the address and length of 87 an array of SVs. The last argument points to a boolean; on return, if that 88 boolean is true, then locale-specific information has been used to format 89 the string, and the string's contents are therefore untrustworthy (see 90 L<perlsec>). This pointer may be NULL if that information is not 91 important. Note that this function requires you to specify the length of 92 the format. 93 94 The C<sv_set*()> functions are not generic enough to operate on values 95 that have "magic". See L<Magic Virtual Tables> later in this document. 96 97 All SVs that contain strings should be terminated with a NUL character. 98 If it is not NUL-terminated there is a risk of 99 core dumps and corruptions from code which passes the string to C 100 functions or system calls which expect a NUL-terminated string. 101 Perl's own functions typically add a trailing NUL for this reason. 102 Nevertheless, you should be very careful when you pass a string stored 103 in an SV to a C function or system call. 104 105 To access the actual value that an SV points to, you can use the macros: 106 107 SvIV(SV*) 108 SvUV(SV*) 109 SvNV(SV*) 110 SvPV(SV*, STRLEN len) 111 SvPV_nolen(SV*) 112 113 which will automatically coerce the actual scalar type into an IV, UV, double, 114 or string. 115 116 In the C<SvPV> macro, the length of the string returned is placed into the 117 variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do 118 not care what the length of the data is, use the C<SvPV_nolen> macro. 119 Historically the C<SvPV> macro with the global variable C<PL_na> has been 120 used in this case. But that can be quite inefficient because C<PL_na> must 121 be accessed in thread-local storage in threaded Perl. In any case, remember 122 that Perl allows arbitrary strings of data that may both contain NULs and 123 might not be terminated by a NUL. 124 125 Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len), 126 len);>. It might work with your compiler, but it won't work for everyone. 127 Break this sort of statement up into separate assignments: 128 129 SV *s; 130 STRLEN len; 131 char * ptr; 132 ptr = SvPV(s, len); 133 foo(ptr, len); 134 135 If you want to know if the scalar value is TRUE, you can use: 136 137 SvTRUE(SV*) 138 139 Although Perl will automatically grow strings for you, if you need to force 140 Perl to allocate more memory for your SV, you can use the macro 141 142 SvGROW(SV*, STRLEN newlen) 143 144 which will determine if more memory needs to be allocated. If so, it will 145 call the function C<sv_grow>. Note that C<SvGROW> can only increase, not 146 decrease, the allocated memory of an SV and that it does not automatically 147 add a byte for the a trailing NUL (perl's own string functions typically do 148 C<SvGROW(sv, len + 1)>). 149 150 If you have an SV and want to know what kind of data Perl thinks is stored 151 in it, you can use the following macros to check the type of SV you have. 152 153 SvIOK(SV*) 154 SvNOK(SV*) 155 SvPOK(SV*) 156 157 You can get and set the current length of the string stored in an SV with 158 the following macros: 159 160 SvCUR(SV*) 161 SvCUR_set(SV*, I32 val) 162 163 You can also get a pointer to the end of the string stored in the SV 164 with the macro: 165 166 SvEND(SV*) 167 168 But note that these last three macros are valid only if C<SvPOK()> is true. 169 170 If you want to append something to the end of string stored in an C<SV*>, 171 you can use the following functions: 172 173 void sv_catpv(SV*, const char*); 174 void sv_catpvn(SV*, const char*, STRLEN); 175 void sv_catpvf(SV*, const char*, ...); 176 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool); 177 void sv_catsv(SV*, SV*); 178 179 The first function calculates the length of the string to be appended by 180 using C<strlen>. In the second, you specify the length of the string 181 yourself. The third function processes its arguments like C<sprintf> and 182 appends the formatted output. The fourth function works like C<vsprintf>. 183 You can specify the address and length of an array of SVs instead of the 184 va_list argument. The fifth function extends the string stored in the first 185 SV with the string stored in the second SV. It also forces the second SV 186 to be interpreted as a string. 187 188 The C<sv_cat*()> functions are not generic enough to operate on values that 189 have "magic". See L<Magic Virtual Tables> later in this document. 190 191 If you know the name of a scalar variable, you can get a pointer to its SV 192 by using the following: 193 194 SV* get_sv("package::varname", FALSE); 195 196 This returns NULL if the variable does not exist. 197 198 If you want to know if this variable (or any other SV) is actually C<defined>, 199 you can call: 200 201 SvOK(SV*) 202 203 The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. 204 205 Its address can be used whenever an C<SV*> is needed. Make sure that 206 you don't try to compare a random sv with C<&PL_sv_undef>. For example 207 when interfacing Perl code, it'll work correctly for: 208 209 foo(undef); 210 211 But won't work when called as: 212 213 $x = undef; 214 foo($x); 215 216 So to repeat always use SvOK() to check whether an sv is defined. 217 218 Also you have to be careful when using C<&PL_sv_undef> as a value in 219 AVs or HVs (see L<AVs, HVs and undefined values>). 220 221 There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain 222 boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their 223 addresses can be used whenever an C<SV*> is needed. 224 225 Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>. 226 Take this code: 227 228 SV* sv = (SV*) 0; 229 if (I-am-to-return-a-real-value) { 230 sv = sv_2mortal(newSViv(42)); 231 } 232 sv_setsv(ST(0), sv); 233 234 This code tries to return a new SV (which contains the value 42) if it should 235 return a real value, or undef otherwise. Instead it has returned a NULL 236 pointer which, somewhere down the line, will cause a segmentation violation, 237 bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the 238 first line and all will be well. 239 240 To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this 241 call is not necessary (see L<Reference Counts and Mortality>). 242 243 =head2 Offsets 244 245 Perl provides the function C<sv_chop> to efficiently remove characters 246 from the beginning of a string; you give it an SV and a pointer to 247 somewhere inside the PV, and it discards everything before the 248 pointer. The efficiency comes by means of a little hack: instead of 249 actually removing the characters, C<sv_chop> sets the flag C<OOK> 250 (offset OK) to signal to other functions that the offset hack is in 251 effect, and it puts the number of bytes chopped off into the IV field 252 of the SV. It then moves the PV pointer (called C<SvPVX>) forward that 253 many bytes, and adjusts C<SvCUR> and C<SvLEN>. 254 255 Hence, at this point, the start of the buffer that we allocated lives 256 at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing 257 into the middle of this allocated storage. 258 259 This is best demonstrated by example: 260 261 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)' 262 SV = PVIV(0x8128450) at 0x81340f0 263 REFCNT = 1 264 FLAGS = (POK,OOK,pPOK) 265 IV = 1 (OFFSET) 266 PV = 0x8135781 ( "1" . ) "2345"\0 267 CUR = 4 268 LEN = 5 269 270 Here the number of bytes chopped off (1) is put into IV, and 271 C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The 272 portion of the string between the "real" and the "fake" beginnings is 273 shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect 274 the fake beginning, not the real one. 275 276 Something similar to the offset hack is performed on AVs to enable 277 efficient shifting and splicing off the beginning of the array; while 278 C<AvARRAY> points to the first element in the array that is visible from 279 Perl, C<AvALLOC> points to the real start of the C array. These are 280 usually the same, but a C<shift> operation can be carried out by 281 increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>. 282 Again, the location of the real start of the C array only comes into 283 play when freeing the array. See C<av_shift> in F<av.c>. 284 285 =head2 What's Really Stored in an SV? 286 287 Recall that the usual method of determining the type of scalar you have is 288 to use C<Sv*OK> macros. Because a scalar can be both a number and a string, 289 usually these macros will always return TRUE and calling the C<Sv*V> 290 macros will do the appropriate conversion of string to integer/double or 291 integer/double to string. 292 293 If you I<really> need to know if you have an integer, double, or string 294 pointer in an SV, you can use the following three macros instead: 295 296 SvIOKp(SV*) 297 SvNOKp(SV*) 298 SvPOKp(SV*) 299 300 These will tell you if you truly have an integer, double, or string pointer 301 stored in your SV. The "p" stands for private. 302 303 The are various ways in which the private and public flags may differ. 304 For example, a tied SV may have a valid underlying value in the IV slot 305 (so SvIOKp is true), but the data should be accessed via the FETCH 306 routine rather than directly, so SvIOK is false. Another is when 307 numeric conversion has occurred and precision has been lost: only the 308 private flag is set on 'lossy' values. So when an NV is converted to an 309 IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be. 310 311 In general, though, it's best to use the C<Sv*V> macros. 312 313 =head2 Working with AVs 314 315 There are two ways to create and load an AV. The first method creates an 316 empty AV: 317 318 AV* newAV(); 319 320 The second method both creates the AV and initially populates it with SVs: 321 322 AV* av_make(I32 num, SV **ptr); 323 324 The second argument points to an array containing C<num> C<SV*>'s. Once the 325 AV has been created, the SVs can be destroyed, if so desired. 326 327 Once the AV has been created, the following operations are possible on AVs: 328 329 void av_push(AV*, SV*); 330 SV* av_pop(AV*); 331 SV* av_shift(AV*); 332 void av_unshift(AV*, I32 num); 333 334 These should be familiar operations, with the exception of C<av_unshift>. 335 This routine adds C<num> elements at the front of the array with the C<undef> 336 value. You must then use C<av_store> (described below) to assign values 337 to these new elements. 338 339 Here are some other functions: 340 341 I32 av_len(AV*); 342 SV** av_fetch(AV*, I32 key, I32 lval); 343 SV** av_store(AV*, I32 key, SV* val); 344 345 The C<av_len> function returns the highest index value in array (just 346 like $#array in Perl). If the array is empty, -1 is returned. The 347 C<av_fetch> function returns the value at index C<key>, but if C<lval> 348 is non-zero, then C<av_fetch> will store an undef value at that index. 349 The C<av_store> function stores the value C<val> at index C<key>, and does 350 not increment the reference count of C<val>. Thus the caller is responsible 351 for taking care of that, and if C<av_store> returns NULL, the caller will 352 have to decrement the reference count to avoid a memory leak. Note that 353 C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their 354 return value. 355 356 void av_clear(AV*); 357 void av_undef(AV*); 358 void av_extend(AV*, I32 key); 359 360 The C<av_clear> function deletes all the elements in the AV* array, but 361 does not actually delete the array itself. The C<av_undef> function will 362 delete all the elements in the array plus the array itself. The 363 C<av_extend> function extends the array so that it contains at least C<key+1> 364 elements. If C<key+1> is less than the currently allocated length of the array, 365 then nothing is done. 366 367 If you know the name of an array variable, you can get a pointer to its AV 368 by using the following: 369 370 AV* get_av("package::varname", FALSE); 371 372 This returns NULL if the variable does not exist. 373 374 See L<Understanding the Magic of Tied Hashes and Arrays> for more 375 information on how to use the array access functions on tied arrays. 376 377 =head2 Working with HVs 378 379 To create an HV, you use the following routine: 380 381 HV* newHV(); 382 383 Once the HV has been created, the following operations are possible on HVs: 384 385 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash); 386 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval); 387 388 The C<klen> parameter is the length of the key being passed in (Note that 389 you cannot pass 0 in as a value of C<klen> to tell Perl to measure the 390 length of the key). The C<val> argument contains the SV pointer to the 391 scalar being stored, and C<hash> is the precomputed hash value (zero if 392 you want C<hv_store> to calculate it for you). The C<lval> parameter 393 indicates whether this fetch is actually a part of a store operation, in 394 which case a new undefined value will be added to the HV with the supplied 395 key and C<hv_fetch> will return as if the value had already existed. 396 397 Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just 398 C<SV*>. To access the scalar value, you must first dereference the return 399 value. However, you should check to make sure that the return value is 400 not NULL before dereferencing it. 401 402 These two functions check if a hash table entry exists, and deletes it. 403 404 bool hv_exists(HV*, const char* key, U32 klen); 405 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags); 406 407 If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will 408 create and return a mortal copy of the deleted value. 409 410 And more miscellaneous functions: 411 412 void hv_clear(HV*); 413 void hv_undef(HV*); 414 415 Like their AV counterparts, C<hv_clear> deletes all the entries in the hash 416 table but does not actually delete the hash table. The C<hv_undef> deletes 417 both the entries and the hash table itself. 418 419 Perl keeps the actual data in linked list of structures with a typedef of HE. 420 These contain the actual key and value pointers (plus extra administrative 421 overhead). The key is a string pointer; the value is an C<SV*>. However, 422 once you have an C<HE*>, to get the actual key and value, use the routines 423 specified below. 424 425 I32 hv_iterinit(HV*); 426 /* Prepares starting point to traverse hash table */ 427 HE* hv_iternext(HV*); 428 /* Get the next entry, and return a pointer to a 429 structure that has both the key and value */ 430 char* hv_iterkey(HE* entry, I32* retlen); 431 /* Get the key from an HE structure and also return 432 the length of the key string */ 433 SV* hv_iterval(HV*, HE* entry); 434 /* Return an SV pointer to the value of the HE 435 structure */ 436 SV* hv_iternextsv(HV*, char** key, I32* retlen); 437 /* This convenience routine combines hv_iternext, 438 hv_iterkey, and hv_iterval. The key and retlen 439 arguments are return values for the key and its 440 length. The value is returned in the SV* argument */ 441 442 If you know the name of a hash variable, you can get a pointer to its HV 443 by using the following: 444 445 HV* get_hv("package::varname", FALSE); 446 447 This returns NULL if the variable does not exist. 448 449 The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro: 450 451 hash = 0; 452 while (klen--) 453 hash = (hash * 33) + *key++; 454 hash = hash + (hash >> 5); /* after 5.6 */ 455 456 The last step was added in version 5.6 to improve distribution of 457 lower bits in the resulting hash value. 458 459 See L<Understanding the Magic of Tied Hashes and Arrays> for more 460 information on how to use the hash access functions on tied hashes. 461 462 =head2 Hash API Extensions 463 464 Beginning with version 5.004, the following functions are also supported: 465 466 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash); 467 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash); 468 469 bool hv_exists_ent (HV* tb, SV* key, U32 hash); 470 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash); 471 472 SV* hv_iterkeysv (HE* entry); 473 474 Note that these functions take C<SV*> keys, which simplifies writing 475 of extension code that deals with hash structures. These functions 476 also allow passing of C<SV*> keys to C<tie> functions without forcing 477 you to stringify the keys (unlike the previous set of functions). 478 479 They also return and accept whole hash entries (C<HE*>), making their 480 use more efficient (since the hash number for a particular string 481 doesn't have to be recomputed every time). See L<perlapi> for detailed 482 descriptions. 483 484 The following macros must always be used to access the contents of hash 485 entries. Note that the arguments to these macros must be simple 486 variables, since they may get evaluated more than once. See 487 L<perlapi> for detailed descriptions of these macros. 488 489 HePV(HE* he, STRLEN len) 490 HeVAL(HE* he) 491 HeHASH(HE* he) 492 HeSVKEY(HE* he) 493 HeSVKEY_force(HE* he) 494 HeSVKEY_set(HE* he, SV* sv) 495 496 These two lower level macros are defined, but must only be used when 497 dealing with keys that are not C<SV*>s: 498 499 HeKEY(HE* he) 500 HeKLEN(HE* he) 501 502 Note that both C<hv_store> and C<hv_store_ent> do not increment the 503 reference count of the stored C<val>, which is the caller's responsibility. 504 If these functions return a NULL value, the caller will usually have to 505 decrement the reference count of C<val> to avoid a memory leak. 506 507 =head2 AVs, HVs and undefined values 508 509 Sometimes you have to store undefined values in AVs or HVs. Although 510 this may be a rare case, it can be tricky. That's because you're 511 used to using C<&PL_sv_undef> if you need an undefined SV. 512 513 For example, intuition tells you that this XS code: 514 515 AV *av = newAV(); 516 av_store( av, 0, &PL_sv_undef ); 517 518 is equivalent to this Perl code: 519 520 my @av; 521 $av[0] = undef; 522 523 Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker 524 for indicating that an array element has not yet been initialized. 525 Thus, C<exists $av[0]> would be true for the above Perl code, but 526 false for the array generated by the XS code. 527 528 Other problems can occur when storing C<&PL_sv_undef> in HVs: 529 530 hv_store( hv, "key", 3, &PL_sv_undef, 0 ); 531 532 This will indeed make the value C<undef>, but if you try to modify 533 the value of C<key>, you'll get the following error: 534 535 Modification of non-creatable hash value attempted 536 537 In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders 538 in restricted hashes. This caused such hash entries not to appear 539 when iterating over the hash or when checking for the keys 540 with the C<hv_exists> function. 541 542 You can run into similar problems when you store C<&PL_sv_true> or 543 C<&PL_sv_false> into AVs or HVs. Trying to modify such elements 544 will give you the following error: 545 546 Modification of a read-only value attempted 547 548 To make a long story short, you can use the special variables 549 C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and 550 HVs, but you have to make sure you know what you're doing. 551 552 Generally, if you want to store an undefined value in an AV 553 or HV, you should not use C<&PL_sv_undef>, but rather create a 554 new undefined value using the C<newSV> function, for example: 555 556 av_store( av, 42, newSV(0) ); 557 hv_store( hv, "foo", 3, newSV(0), 0 ); 558 559 =head2 References 560 561 References are a special type of scalar that point to other data types 562 (including references). 563 564 To create a reference, use either of the following functions: 565 566 SV* newRV_inc((SV*) thing); 567 SV* newRV_noinc((SV*) thing); 568 569 The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The 570 functions are identical except that C<newRV_inc> increments the reference 571 count of the C<thing>, while C<newRV_noinc> does not. For historical 572 reasons, C<newRV> is a synonym for C<newRV_inc>. 573 574 Once you have a reference, you can use the following macro to dereference 575 the reference: 576 577 SvRV(SV*) 578 579 then call the appropriate routines, casting the returned C<SV*> to either an 580 C<AV*> or C<HV*>, if required. 581 582 To determine if an SV is a reference, you can use the following macro: 583 584 SvROK(SV*) 585 586 To discover what type of value the reference refers to, use the following 587 macro and then check the return value. 588 589 SvTYPE(SvRV(SV*)) 590 591 The most useful types that will be returned are: 592 593 SVt_IV Scalar 594 SVt_NV Scalar 595 SVt_PV Scalar 596 SVt_RV Scalar 597 SVt_PVAV Array 598 SVt_PVHV Hash 599 SVt_PVCV Code 600 SVt_PVGV Glob (possible a file handle) 601 SVt_PVMG Blessed or Magical Scalar 602 603 See the sv.h header file for more details. 604 605 =head2 Blessed References and Class Objects 606 607 References are also used to support object-oriented programming. In perl's 608 OO lexicon, an object is simply a reference that has been blessed into a 609 package (or class). Once blessed, the programmer may now use the reference 610 to access the various methods in the class. 611 612 A reference can be blessed into a package with the following function: 613 614 SV* sv_bless(SV* sv, HV* stash); 615 616 The C<sv> argument must be a reference value. The C<stash> argument 617 specifies which class the reference will belong to. See 618 L<Stashes and Globs> for information on converting class names into stashes. 619 620 /* Still under construction */ 621 622 Upgrades rv to reference if not already one. Creates new SV for rv to 623 point to. If C<classname> is non-null, the SV is blessed into the specified 624 class. SV is returned. 625 626 SV* newSVrv(SV* rv, const char* classname); 627 628 Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed 629 if C<classname> is non-null. 630 631 SV* sv_setref_iv(SV* rv, const char* classname, IV iv); 632 SV* sv_setref_uv(SV* rv, const char* classname, UV uv); 633 SV* sv_setref_nv(SV* rv, const char* classname, NV iv); 634 635 Copies the pointer value (I<the address, not the string!>) into an SV whose 636 reference is rv. SV is blessed if C<classname> is non-null. 637 638 SV* sv_setref_pv(SV* rv, const char* classname, PV iv); 639 640 Copies string into an SV whose reference is C<rv>. Set length to 0 to let 641 Perl calculate the string length. SV is blessed if C<classname> is non-null. 642 643 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); 644 645 Tests whether the SV is blessed into the specified class. It does not 646 check inheritance relationships. 647 648 int sv_isa(SV* sv, const char* name); 649 650 Tests whether the SV is a reference to a blessed object. 651 652 int sv_isobject(SV* sv); 653 654 Tests whether the SV is derived from the specified class. SV can be either 655 a reference to a blessed object or a string containing a class name. This 656 is the function implementing the C<UNIVERSAL::isa> functionality. 657 658 bool sv_derived_from(SV* sv, const char* name); 659 660 To check if you've got an object derived from a specific class you have 661 to write: 662 663 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... } 664 665 =head2 Creating New Variables 666 667 To create a new Perl variable with an undef value which can be accessed from 668 your Perl script, use the following routines, depending on the variable type. 669 670 SV* get_sv("package::varname", TRUE); 671 AV* get_av("package::varname", TRUE); 672 HV* get_hv("package::varname", TRUE); 673 674 Notice the use of TRUE as the second parameter. The new variable can now 675 be set, using the routines appropriate to the data type. 676 677 There are additional macros whose values may be bitwise OR'ed with the 678 C<TRUE> argument to enable certain extra features. Those bits are: 679 680 =over 681 682 =item GV_ADDMULTI 683 684 Marks the variable as multiply defined, thus preventing the: 685 686 Name <varname> used only once: possible typo 687 688 warning. 689 690 =item GV_ADDWARN 691 692 Issues the warning: 693 694 Had to create <varname> unexpectedly 695 696 if the variable did not exist before the function was called. 697 698 =back 699 700 If you do not specify a package name, the variable is created in the current 701 package. 702 703 =head2 Reference Counts and Mortality 704 705 Perl uses a reference count-driven garbage collection mechanism. SVs, 706 AVs, or HVs (xV for short in the following) start their life with a 707 reference count of 1. If the reference count of an xV ever drops to 0, 708 then it will be destroyed and its memory made available for reuse. 709 710 This normally doesn't happen at the Perl level unless a variable is 711 undef'ed or the last variable holding a reference to it is changed or 712 overwritten. At the internal level, however, reference counts can be 713 manipulated with the following macros: 714 715 int SvREFCNT(SV* sv); 716 SV* SvREFCNT_inc(SV* sv); 717 void SvREFCNT_dec(SV* sv); 718 719 However, there is one other function which manipulates the reference 720 count of its argument. The C<newRV_inc> function, you will recall, 721 creates a reference to the specified argument. As a side effect, 722 it increments the argument's reference count. If this is not what 723 you want, use C<newRV_noinc> instead. 724 725 For example, imagine you want to return a reference from an XSUB function. 726 Inside the XSUB routine, you create an SV which initially has a reference 727 count of one. Then you call C<newRV_inc>, passing it the just-created SV. 728 This returns the reference as a new SV, but the reference count of the 729 SV you passed to C<newRV_inc> has been incremented to two. Now you 730 return the reference from the XSUB routine and forget about the SV. 731 But Perl hasn't! Whenever the returned reference is destroyed, the 732 reference count of the original SV is decreased to one and nothing happens. 733 The SV will hang around without any way to access it until Perl itself 734 terminates. This is a memory leak. 735 736 The correct procedure, then, is to use C<newRV_noinc> instead of 737 C<newRV_inc>. Then, if and when the last reference is destroyed, 738 the reference count of the SV will go to zero and it will be destroyed, 739 stopping any memory leak. 740 741 There are some convenience functions available that can help with the 742 destruction of xVs. These functions introduce the concept of "mortality". 743 An xV that is mortal has had its reference count marked to be decremented, 744 but not actually decremented, until "a short time later". Generally the 745 term "short time later" means a single Perl statement, such as a call to 746 an XSUB function. The actual determinant for when mortal xVs have their 747 reference count decremented depends on two macros, SAVETMPS and FREETMPS. 748 See L<perlcall> and L<perlxs> for more details on these macros. 749 750 "Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>. 751 However, if you mortalize a variable twice, the reference count will 752 later be decremented twice. 753 754 "Mortal" SVs are mainly used for SVs that are placed on perl's stack. 755 For example an SV which is created just to pass a number to a called sub 756 is made mortal to have it cleaned up automatically when it's popped off 757 the stack. Similarly, results returned by XSUBs (which are pushed on the 758 stack) are often made mortal. 759 760 To create a mortal variable, use the functions: 761 762 SV* sv_newmortal() 763 SV* sv_2mortal(SV*) 764 SV* sv_mortalcopy(SV*) 765 766 The first call creates a mortal SV (with no value), the second converts an existing 767 SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the 768 third creates a mortal copy of an existing SV. 769 Because C<sv_newmortal> gives the new SV no value,it must normally be given one 770 via C<sv_setpv>, C<sv_setiv>, etc. : 771 772 SV *tmp = sv_newmortal(); 773 sv_setiv(tmp, an_integer); 774 775 As that is multiple C statements it is quite common so see this idiom instead: 776 777 SV *tmp = sv_2mortal(newSViv(an_integer)); 778 779 780 You should be careful about creating mortal variables. Strange things 781 can happen if you make the same value mortal within multiple contexts, 782 or if you make a variable mortal multiple times. Thinking of "Mortalization" 783 as deferred C<SvREFCNT_dec> should help to minimize such problems. 784 For example if you are passing an SV which you I<know> has high enough REFCNT 785 to survive its use on the stack you need not do any mortalization. 786 If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or 787 making a C<sv_mortalcopy> is safer. 788 789 The mortal routines are not just for SVs -- AVs and HVs can be 790 made mortal by passing their address (type-casted to C<SV*>) to the 791 C<sv_2mortal> or C<sv_mortalcopy> routines. 792 793 =head2 Stashes and Globs 794 795 A B<stash> is a hash that contains all variables that are defined 796 within a package. Each key of the stash is a symbol 797 name (shared by all the different types of objects that have the same 798 name), and each value in the hash table is a GV (Glob Value). This GV 799 in turn contains references to the various objects of that name, 800 including (but not limited to) the following: 801 802 Scalar Value 803 Array Value 804 Hash Value 805 I/O Handle 806 Format 807 Subroutine 808 809 There is a single stash called C<PL_defstash> that holds the items that exist 810 in the C<main> package. To get at the items in other packages, append the 811 string "::" to the package name. The items in the C<Foo> package are in 812 the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are 813 in the stash C<Baz::> in C<Bar::>'s stash. 814 815 To get the stash pointer for a particular package, use the function: 816 817 HV* gv_stashpv(const char* name, I32 flags) 818 HV* gv_stashsv(SV*, I32 flags) 819 820 The first function takes a literal string, the second uses the string stored 821 in the SV. Remember that a stash is just a hash table, so you get back an 822 C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD. 823 824 The name that C<gv_stash*v> wants is the name of the package whose symbol table 825 you want. The default package is called C<main>. If you have multiply nested 826 packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl 827 language itself. 828 829 Alternately, if you have an SV that is a blessed reference, you can find 830 out the stash pointer by using: 831 832 HV* SvSTASH(SvRV(SV*)); 833 834 then use the following to get the package name itself: 835 836 char* HvNAME(HV* stash); 837 838 If you need to bless or re-bless an object you can use the following 839 function: 840 841 SV* sv_bless(SV*, HV* stash) 842 843 where the first argument, an C<SV*>, must be a reference, and the second 844 argument is a stash. The returned C<SV*> can now be used in the same way 845 as any other SV. 846 847 For more information on references and blessings, consult L<perlref>. 848 849 =head2 Double-Typed SVs 850 851 Scalar variables normally contain only one type of value, an integer, 852 double, pointer, or reference. Perl will automatically convert the 853 actual scalar data from the stored type into the requested type. 854 855 Some scalar variables contain more than one type of scalar data. For 856 example, the variable C<$!> contains either the numeric value of C<errno> 857 or its string equivalent from either C<strerror> or C<sys_errlist[]>. 858 859 To force multiple data values into an SV, you must do two things: use the 860 C<sv_set*v> routines to add the additional scalar type, then set a flag 861 so that Perl will believe it contains more than one type of data. The 862 four macros to set the flags are: 863 864 SvIOK_on 865 SvNOK_on 866 SvPOK_on 867 SvROK_on 868 869 The particular macro you must use depends on which C<sv_set*v> routine 870 you called first. This is because every C<sv_set*v> routine turns on 871 only the bit for the particular type of data being set, and turns off 872 all the rest. 873 874 For example, to create a new Perl variable called "dberror" that contains 875 both the numeric and descriptive string error values, you could use the 876 following code: 877 878 extern int dberror; 879 extern char *dberror_list; 880 881 SV* sv = get_sv("dberror", TRUE); 882 sv_setiv(sv, (IV) dberror); 883 sv_setpv(sv, dberror_list[dberror]); 884 SvIOK_on(sv); 885 886 If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the 887 macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>. 888 889 =head2 Magic Variables 890 891 [This section still under construction. Ignore everything here. Post no 892 bills. Everything not permitted is forbidden.] 893 894 Any SV may be magical, that is, it has special features that a normal 895 SV does not have. These features are stored in the SV structure in a 896 linked list of C<struct magic>'s, typedef'ed to C<MAGIC>. 897 898 struct magic { 899 MAGIC* mg_moremagic; 900 MGVTBL* mg_virtual; 901 U16 mg_private; 902 char mg_type; 903 U8 mg_flags; 904 I32 mg_len; 905 SV* mg_obj; 906 char* mg_ptr; 907 }; 908 909 Note this is current as of patchlevel 0, and could change at any time. 910 911 =head2 Assigning Magic 912 913 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 C<sv> argument is a pointer to the SV that is to acquire a new magical 918 feature. 919 920 If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to 921 convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic 922 to the beginning of the linked list of magical features. Any prior entry 923 of the same type of magic is deleted. Note that this can be overridden, 924 and multiple instances of the same type of magic can be associated with an 925 SV. 926 927 The C<name> and C<namlen> arguments are used to associate a string with 928 the magic, typically the name of a variable. C<namlen> is stored in the 929 C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of 930 C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on 931 whether C<namlen> is greater than zero or equal to zero respectively. As a 932 special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed 933 to contain an C<SV*> and is stored as-is with its REFCNT incremented. 934 935 The sv_magic function uses C<how> to determine which, if any, predefined 936 "Magic Virtual Table" should be assigned to the C<mg_virtual> field. 937 See the L<Magic Virtual Tables> section below. The C<how> argument is also 938 stored in the C<mg_type> field. The value of C<how> should be chosen 939 from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before 940 these macros were added, Perl internals used to directly use character 941 literals, so you may occasionally come across old code or documentation 942 referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example. 943 944 The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC> 945 structure. If it is not the same as the C<sv> argument, the reference 946 count of the C<obj> object is incremented. If it is the same, or if 947 the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer, 948 then C<obj> is merely stored, without the reference count being incremented. 949 950 See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic 951 to an SV. 952 953 There is also a function to add magic to an C<HV>: 954 955 void hv_magic(HV *hv, GV *gv, int how); 956 957 This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>. 958 959 To remove the magic from an SV, call the function sv_unmagic: 960 961 void sv_unmagic(SV *sv, int type); 962 963 The C<type> argument should be equal to the C<how> value when the C<SV> 964 was initially made magical. 965 966 =head2 Magic Virtual Tables 967 968 The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an 969 C<MGVTBL>, which is a structure of function pointers and stands for 970 "Magic Virtual Table" to handle the various operations that might be 971 applied to that variable. 972 973 The C<MGVTBL> has five (or sometimes eight) pointers to the following 974 routine types: 975 976 int (*svt_get)(SV* sv, MAGIC* mg); 977 int (*svt_set)(SV* sv, MAGIC* mg); 978 U32 (*svt_len)(SV* sv, MAGIC* mg); 979 int (*svt_clear)(SV* sv, MAGIC* mg); 980 int (*svt_free)(SV* sv, MAGIC* mg); 981 982 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen); 983 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param); 984 int (*svt_local)(SV *nsv, MAGIC *mg); 985 986 987 This MGVTBL structure is set at compile-time in F<perl.h> and there are 988 currently 19 types (or 21 with overloading turned on). These different 989 structures contain pointers to various routines that perform additional 990 actions depending on which function is being called. 991 992 Function pointer Action taken 993 ---------------- ------------ 994 svt_get Do something before the value of the SV is retrieved. 995 svt_set Do something after the SV is assigned a value. 996 svt_len Report on the SV's length. 997 svt_clear Clear something the SV represents. 998 svt_free Free any extra storage associated with the SV. 999 1000 svt_copy copy tied variable magic to a tied element 1001 svt_dup duplicate a magic structure during thread cloning 1002 svt_local copy magic to local value during 'local' 1003 1004 For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds 1005 to an C<mg_type> of C<PERL_MAGIC_sv>) contains: 1006 1007 { magic_get, magic_set, magic_len, 0, 0 } 1008 1009 Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>, 1010 if a get operation is being performed, the routine C<magic_get> is 1011 called. All the various routines for the various magical types begin 1012 with C<magic_>. NOTE: the magic routines are not considered part of 1013 the Perl API, and may not be exported by the Perl library. 1014 1015 The last three slots are a recent addition, and for source code 1016 compatibility they are only checked for if one of the three flags 1017 MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most 1018 code can continue declaring a vtable as a 5-element value. These three are 1019 currently used exclusively by the threading code, and are highly subject 1020 to change. 1021 1022 The current kinds of Magic Virtual Tables are: 1023 1024 mg_type 1025 (old-style char and macro) MGVTBL Type of magic 1026 -------------------------- ------ ------------- 1027 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable 1028 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash 1029 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element 1030 c PERL_MAGIC_overload_table (none) Holds overload table (AMT) 1031 on stash 1032 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search) 1033 D PERL_MAGIC_regdata vtbl_regdata Regex match position data 1034 (@+ and @- vars) 1035 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data 1036 element 1037 E PERL_MAGIC_env vtbl_env %ENV hash 1038 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element 1039 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format) 1040 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string 1041 H PERL_MAGIC_hints vtbl_sig %^H hash 1042 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element 1043 I PERL_MAGIC_isa vtbl_isa @ISA array 1044 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element 1045 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue 1046 L PERL_MAGIC_dbfile (none) Debugger %_<filename 1047 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element 1048 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation 1049 P PERL_MAGIC_tied vtbl_pack Tied array or hash 1050 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element 1051 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle 1052 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex 1053 S PERL_MAGIC_sig vtbl_sig %SIG hash 1054 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element 1055 t PERL_MAGIC_taint vtbl_taint Taintedness 1056 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions 1057 v PERL_MAGIC_vec vtbl_vec vec() lvalue 1058 V PERL_MAGIC_vstring (none) v-string scalars 1059 w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache 1060 x PERL_MAGIC_substr vtbl_substr substr() lvalue 1061 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator 1062 variable / smart parameter 1063 vivification 1064 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary) 1065 . PERL_MAGIC_pos vtbl_pos pos() lvalue 1066 < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref 1067 ~ PERL_MAGIC_ext (none) Available for use by extensions 1068 : PERL_MAGIC_symtab (none) hash used as symbol table 1069 % PERL_MAGIC_rhash (none) hash used as restricted hash 1070 @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a 1071 1072 1073 When an uppercase and lowercase letter both exist in the table, then the 1074 uppercase letter is typically used to represent some kind of composite type 1075 (a list or a hash), and the lowercase letter is used to represent an element 1076 of that composite type. Some internals code makes use of this case 1077 relationship. However, 'v' and 'V' (vec and v-string) are in no way related. 1078 1079 The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined 1080 specifically for use by extensions and will not be used by perl itself. 1081 Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information 1082 to variables (typically objects). This is especially useful because 1083 there is no way for normal perl code to corrupt this private information 1084 (unlike using extra elements of a hash object). 1085 1086 Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a 1087 C function any time a scalar's value is used or changed. The C<MAGIC>'s 1088 C<mg_ptr> field points to a C<ufuncs> structure: 1089 1090 struct ufuncs { 1091 I32 (*uf_val)(pTHX_ IV, SV*); 1092 I32 (*uf_set)(pTHX_ IV, SV*); 1093 IV uf_index; 1094 }; 1095 1096 When the SV is read from or written to, the C<uf_val> or C<uf_set> 1097 function will be called with C<uf_index> as the first arg and a pointer to 1098 the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar> 1099 magic is shown below. Note that the ufuncs structure is copied by 1100 sv_magic, so you can safely allocate it on the stack. 1101 1102 void 1103 Umagic(sv) 1104 SV *sv; 1105 PREINIT: 1106 struct ufuncs uf; 1107 CODE: 1108 uf.uf_val = &my_get_fn; 1109 uf.uf_set = &my_set_fn; 1110 uf.uf_index = 0; 1111 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf)); 1112 1113 Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect. 1114 1115 For hashes there is a specialized hook that gives control over hash 1116 keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic 1117 if the "set" function in the C<ufuncs> structure is NULL. The hook 1118 is activated whenever the hash is accessed with a key specified as 1119 an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>, 1120 C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string 1121 through the functions without the C<..._ent> suffix circumvents the 1122 hook. See L<Hash::Util::Fieldhash/Guts> for a detailed description. 1123 1124 Note that because multiple extensions may be using C<PERL_MAGIC_ext> 1125 or C<PERL_MAGIC_uvar> magic, it is important for extensions to take 1126 extra care to avoid conflict. Typically only using the magic on 1127 objects blessed into the same class as the extension is sufficient. 1128 For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32 1129 'signature' at the top of the private data area and check that. 1130 1131 Also note that the C<sv_set*()> and C<sv_cat*()> functions described 1132 earlier do B<not> invoke 'set' magic on their targets. This must 1133 be done by the user either by calling the C<SvSETMAGIC()> macro after 1134 calling these functions, or by using one of the C<sv_set*_mg()> or 1135 C<sv_cat*_mg()> functions. Similarly, generic C code must call the 1136 C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV 1137 obtained from external sources in functions that don't handle magic. 1138 See L<perlapi> for a description of these functions. 1139 For example, calls to the C<sv_cat*()> functions typically need to be 1140 followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()> 1141 since their implementation handles 'get' magic. 1142 1143 =head2 Finding Magic 1144 1145 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ 1146 1147 This routine returns a pointer to the C<MAGIC> structure stored in the SV. 1148 If the SV does not have that magical feature, C<NULL> is returned. Also, 1149 if the SV is not of type SVt_PVMG, Perl may core dump. 1150 1151 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen); 1152 1153 This routine checks to see what types of magic C<sv> has. If the mg_type 1154 field is an uppercase letter, then the mg_obj is copied to C<nsv>, but 1155 the mg_type field is changed to be the lowercase letter. 1156 1157 =head2 Understanding the Magic of Tied Hashes and Arrays 1158 1159 Tied hashes and arrays are magical beasts of the C<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 1164 of these caveats are actually considered bugs in the API, to be fixed 1165 in later releases, and are bracketed with [MAYCHANGE] below. If 1166 you find yourself actually applying such information in this section, be 1167 aware that the behavior may change in the future, umm, without warning. 1168 1169 The perl tie function associates a variable with an object that implements 1170 the various GET, SET, etc methods. To perform the equivalent of the perl 1171 tie function from an XSUB, you must mimic this behaviour. The code below 1172 carries out the necessary steps - firstly it creates a new hash, and then 1173 creates a second hash which it blesses into the class which will implement 1174 the tie methods. Lastly it ties the two hashes together, and returns a 1175 reference to the new tied hash. Note that the code below does NOT call the 1176 TIEHASH method in the MyTie class - 1177 see L<Calling Perl Routines from within C Programs> for details on how 1178 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 C<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 C<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 C<av_store> on a tied array, the caller will usually need to call 1201 C<mg_set(val)> to actually invoke the perl level "STORE" method on the 1202 TIEARRAY object. If C<av_store> did return NULL, a call to 1203 C<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 the 1207 C<hv_store> and C<hv_store_ent> functions as well. 1208 1209 C<av_fetch> and the corresponding hash functions C<hv_fetch> and 1210 C<hv_fetch_ent> actually return an undefined mortal value whose magic 1211 has been initialized using C<mg_copy>. Note the value so returned does not 1212 need to be deallocated, as it is already mortal. [MAYCHANGE] But you will 1213 need to call C<mg_get()> on the returned value in order to actually invoke 1214 the perl level "FETCH" method on the underlying TIE object. Similarly, 1215 you may also call C<mg_set()> on the return value after possibly assigning 1216 a suitable value to it using C<sv_setsv>, which will invoke the "STORE" 1217 method on the TIE object. [/MAYCHANGE] 1218 1219 [MAYCHANGE] 1220 In other words, the array or hash fetch/store functions don't really 1221 fetch and store actual values in the case of tied arrays and hashes. They 1222 merely call C<mg_copy> to attach magic to the values that were meant to be 1223 "stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually 1224 do the job of invoking the TIE methods on the underlying objects. Thus 1225 the magic mechanism currently implements a kind of lazy access to arrays 1226 and hashes. 1227 1228 Currently (as of perl version 5.004), use of the hash and array access 1229 functions requires the user to be aware of whether they are operating on 1230 "normal" hashes and arrays, or on their tied variants. The API may be 1231 changed to provide more transparent access to both tied and normal data 1232 types in future versions. 1233 [/MAYCHANGE] 1234 1235 You would do well to understand that the TIEARRAY and TIEHASH interfaces 1236 are mere sugar to invoke some perl method calls while using the uniform hash 1237 and array syntax. The use of this sugar imposes some overhead (typically 1238 about two to four extra opcodes per FETCH/STORE operation, in addition to 1239 the creation of all the mortal variables required to invoke the methods). 1240 This overhead will be comparatively small if the TIE methods are themselves 1241 substantial, but if they are only a few statements long, the overhead 1242 will not be insignificant. 1243 1244 =head2 Localizing changes 1245 1246 Perl has a very handy construction 1247 1248 { 1249 local $var = 2; 1250 ... 1251 } 1252 1253 This construction is I<approximately> equivalent to 1254 1255 { 1256 my $oldvar = $var; 1257 $var = 2; 1258 ... 1259 $var = $oldvar; 1260 } 1261 1262 The biggest difference is that the first construction would 1263 reinstate the initial value of $var, irrespective of how control exits 1264 the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit 1265 more efficient as well. 1266 1267 There is a way to achieve a similar task from C via Perl API: create a 1268 I<pseudo-block>, and arrange for some changes to be automatically 1269 undone at the end of it, either explicit, or via a non-local exit (via 1270 die()). A I<block>-like construct is created by a pair of 1271 C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">). 1272 Such a construct may be created specially for some important localized 1273 task, or an existing one (like boundaries of enclosing Perl 1274 subroutine/block, or an existing pair for freeing TMPs) may be 1275 used. (In the second case the overhead of additional localization must 1276 be almost negligible.) Note that any XSUB is automatically enclosed in 1277 an C<ENTER>/C<LEAVE> pair. 1278 1279 Inside such a I<pseudo-block> the following service is available: 1280 1281 =over 4 1282 1283 =item C<SAVEINT(int i)> 1284 1285 =item C<SAVEIV(IV i)> 1286 1287 =item C<SAVEI32(I32 i)> 1288 1289 =item C<SAVELONG(long i)> 1290 1291 These macros arrange things to restore the value of integer variable 1292 C<i> at the end of enclosing I<pseudo-block>. 1293 1294 =item C<SAVESPTR(s)> 1295 1296 =item C<SAVEPPTR(p)> 1297 1298 These macros arrange things to restore the value of pointers C<s> and 1299 C<p>. C<s> must be a pointer of a type which survives conversion to 1300 C<SV*> and back, C<p> should be able to survive conversion to C<char*> 1301 and back. 1302 1303 =item C<SAVEFREESV(SV *sv)> 1304 1305 The refcount of C<sv> would be decremented at the end of 1306 I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a 1307 mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal> 1308 extends the lifetime of C<sv> until the beginning of the next statement, 1309 C<SAVEFREESV> extends it until the end of the enclosing scope. These 1310 lifetimes can be wildly different. 1311 1312 Also compare C<SAVEMORTALIZESV>. 1313 1314 =item C<SAVEMORTALIZESV(SV *sv)> 1315 1316 Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current 1317 scope instead of decrementing its reference count. This usually has the 1318 effect of keeping C<sv> alive until the statement that called the currently 1319 live scope has finished executing. 1320 1321 =item C<SAVEFREEOP(OP *op)> 1322 1323 The C<OP *> is op_free()ed at the end of I<pseudo-block>. 1324 1325 =item C<SAVEFREEPV(p)> 1326 1327 The chunk of memory which is pointed to by C<p> is Safefree()ed at the 1328 end of I<pseudo-block>. 1329 1330 =item C<SAVECLEARSV(SV *sv)> 1331 1332 Clears a slot in the current scratchpad which corresponds to C<sv> at 1333 the end of I<pseudo-block>. 1334 1335 =item C<SAVEDELETE(HV *hv, char *key, I32 length)> 1336 1337 The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The 1338 string pointed to by C<key> is Safefree()ed. If one has a I<key> in 1339 short-lived storage, the corresponding string may be reallocated like 1340 this: 1341 1342 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf)); 1343 1344 =item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)> 1345 1346 At the end of I<pseudo-block> the function C<f> is called with the 1347 only argument C<p>. 1348 1349 =item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)> 1350 1351 At the end of I<pseudo-block> the function C<f> is called with the 1352 implicit context argument (if any), and C<p>. 1353 1354 =item C<SAVESTACK_POS()> 1355 1356 The current offset on the Perl internal stack (cf. C<SP>) is restored 1357 at the end of I<pseudo-block>. 1358 1359 =back 1360 1361 The following API list contains functions, thus one needs to 1362 provide pointers to the modifiable data explicitly (either C pointers, 1363 or Perlish C<GV *>s). Where the above macros take C<int>, a similar 1364 function takes C<int *>. 1365 1366 =over 4 1367 1368 =item C<SV* save_scalar(GV *gv)> 1369 1370 Equivalent to Perl code C<local $gv>. 1371 1372 =item C<AV* save_ary(GV *gv)> 1373 1374 =item C<HV* save_hash(GV *gv)> 1375 1376 Similar to C<save_scalar>, but localize C<@gv> and C<%gv>. 1377 1378 =item C<void save_item(SV *item)> 1379 1380 Duplicates the current value of C<SV>, on the exit from the current 1381 C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV> 1382 using the stored value. It doesn't handle magic. Use C<save_scalar> if 1383 magic is affected. 1384 1385 =item C<void save_list(SV **sarg, I32 maxsarg)> 1386 1387 A variant of C<save_item> which takes multiple arguments via an array 1388 C<sarg> of C<SV*> of length C<maxsarg>. 1389 1390 =item C<SV* save_svref(SV **sptr)> 1391 1392 Similar to C<save_scalar>, but will reinstate an C<SV *>. 1393 1394 =item C<void save_aptr(AV **aptr)> 1395 1396 =item C<void save_hptr(HV **hptr)> 1397 1398 Similar to C<save_svref>, but localize C<AV *> and C<HV *>. 1399 1400 =back 1401 1402 The C<Alias> module implements localization of the basic types within the 1403 I<caller's scope>. People who are interested in how to localize things in 1404 the containing scope should take a look there too. 1405 1406 =head1 Subroutines 1407 1408 =head2 XSUBs and the Argument Stack 1409 1410 The XSUB mechanism is a simple way for Perl programs to access C subroutines. 1411 An XSUB routine will have a stack that contains the arguments from the Perl 1412 program, and a way to map from the Perl data structures to a C equivalent. 1413 1414 The stack arguments are accessible through the C<ST(n)> macro, which returns 1415 the C<n>'th stack argument. Argument 0 is the first argument passed in the 1416 Perl subroutine call. These arguments are C<SV*>, and can be used anywhere 1417 an C<SV*> is used. 1418 1419 Most of the time, output from the C routine can be handled through use of 1420 the RETVAL and OUTPUT directives. However, there are some cases where the 1421 argument stack is not already long enough to handle all the return values. 1422 An example is the POSIX tzname() call, which takes no arguments, but returns 1423 two, the local time zone's standard and summer time abbreviations. 1424 1425 To handle this situation, the PPCODE directive is used and the stack is 1426 extended using the macro: 1427 1428 EXTEND(SP, num); 1429 1430 where C<SP> is the macro that represents the local copy of the stack pointer, 1431 and C<num> is the number of elements the stack should be extended by. 1432 1433 Now that there is room on the stack, values can be pushed on it using C<PUSHs> 1434 macro. The pushed values will often need to be "mortal" (See 1435 L</Reference Counts and Mortality>): 1436 1437 PUSHs(sv_2mortal(newSViv(an_integer))) 1438 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer))) 1439 PUSHs(sv_2mortal(newSVnv(a_double))) 1440 PUSHs(sv_2mortal(newSVpv("Some String",0))) 1441 1442 And now the Perl program calling C<tzname>, the two values will be assigned 1443 as in: 1444 1445 ($standard_abbrev, $summer_abbrev) = POSIX::tzname; 1446 1447 An alternate (and possibly simpler) method to pushing values on the stack is 1448 to use the macro: 1449 1450 XPUSHs(SV*) 1451 1452 This macro automatically adjust the stack for you, if needed. Thus, you 1453 do not need to call C<EXTEND> to extend the stack. 1454 1455 Despite their suggestions in earlier versions of this document the macros 1456 C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results. 1457 For that, either stick to the C<(X)PUSHs> macros shown above, or use the new 1458 C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>. 1459 1460 For more information, consult L<perlxs> and L<perlxstut>. 1461 1462 =head2 Calling Perl Routines from within C Programs 1463 1464 There are four routines that can be used to call a Perl subroutine from 1465 within a C program. These four are: 1466 1467 I32 call_sv(SV*, I32); 1468 I32 call_pv(const char*, I32); 1469 I32 call_method(const char*, I32); 1470 I32 call_argv(const char*, I32, register char**); 1471 1472 The routine most often used is C<call_sv>. The C<SV*> argument 1473 contains either the name of the Perl subroutine to be called, or a 1474 reference to the subroutine. The second argument consists of flags 1475 that control the context in which the subroutine is called, whether 1476 or not the subroutine is being passed arguments, how errors should be 1477 trapped, and how to treat return values. 1478 1479 All four routines return the number of arguments that the subroutine returned 1480 on the Perl stack. 1481 1482 These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0, 1483 but those names are now deprecated; macros of the same name are provided for 1484 compatibility. 1485 1486 When using any of these routines (except C<call_argv>), the programmer 1487 must manipulate the Perl stack. These include the following macros and 1488 functions: 1489 1490 dSP 1491 SP 1492 PUSHMARK() 1493 PUTBACK 1494 SPAGAIN 1495 ENTER 1496 SAVETMPS 1497 FREETMPS 1498 LEAVE 1499 XPUSH*() 1500 POP*() 1501 1502 For a detailed description of calling conventions from C to Perl, 1503 consult L<perlcall>. 1504 1505 =head2 Memory Allocation 1506 1507 =head3 Allocation 1508 1509 All memory meant to be used with the Perl API functions should be manipulated 1510 using the macros described in this section. The macros provide the necessary 1511 transparency between differences in the actual malloc implementation that is 1512 used within perl. 1513 1514 It is suggested that you enable the version of malloc that is distributed 1515 with Perl. It keeps pools of various sizes of unallocated memory in 1516 order to satisfy allocation requests more quickly. However, on some 1517 platforms, it may cause spurious malloc or free errors. 1518 1519 The following three macros are used to initially allocate memory : 1520 1521 Newx(pointer, number, type); 1522 Newxc(pointer, number, type, cast); 1523 Newxz(pointer, number, type); 1524 1525 The first argument C<pointer> should be the name of a variable that will 1526 point to the newly allocated memory. 1527 1528 The second and third arguments C<number> and C<type> specify how many of 1529 the specified type of data structure should be allocated. The argument 1530 C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>, 1531 should be used if the C<pointer> argument is different from the C<type> 1532 argument. 1533 1534 Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero> 1535 to zero out all the newly allocated memory. 1536 1537 =head3 Reallocation 1538 1539 Renew(pointer, number, type); 1540 Renewc(pointer, number, type, cast); 1541 Safefree(pointer) 1542 1543 These three macros are used to change a memory buffer size or to free a 1544 piece of memory no longer needed. The arguments to C<Renew> and C<Renewc> 1545 match those of C<New> and C<Newc> with the exception of not needing the 1546 "magic cookie" argument. 1547 1548 =head3 Moving 1549 1550 Move(source, dest, number, type); 1551 Copy(source, dest, number, type); 1552 Zero(dest, number, type); 1553 1554 These three macros are used to move, copy, or zero out previously allocated 1555 memory. The C<source> and C<dest> arguments point to the source and 1556 destination starting points. Perl will move, copy, or zero out C<number> 1557 instances of the size of the C<type> data structure (using the C<sizeof> 1558 function). 1559 1560 =head2 PerlIO 1561 1562 The most recent development releases of Perl has been experimenting with 1563 removing Perl's dependency on the "normal" standard I/O suite and allowing 1564 other stdio implementations to be used. This involves creating a new 1565 abstraction layer that then calls whichever implementation of stdio Perl 1566 was compiled with. All XSUBs should now use the functions in the PerlIO 1567 abstraction layer and not make any assumptions about what kind of stdio 1568 is being used. 1569 1570 For a complete description of the PerlIO abstraction, consult L<perlapio>. 1571 1572 =head2 Putting a C value on Perl stack 1573 1574 A lot of opcodes (this is an elementary operation in the internal perl 1575 stack machine) put an SV* on the stack. However, as an optimization 1576 the corresponding SV is (usually) not recreated each time. The opcodes 1577 reuse specially assigned SVs (I<target>s) which are (as a corollary) 1578 not constantly freed/created. 1579 1580 Each of the targets is created only once (but see 1581 L<Scratchpads and recursion> below), and when an opcode needs to put 1582 an integer, a double, or a string on stack, it just sets the 1583 corresponding parts of its I<target> and puts the I<target> on stack. 1584 1585 The macro to put this target on stack is C<PUSHTARG>, and it is 1586 directly used in some opcodes, as well as indirectly in zillions of 1587 others, which use it via C<(X)PUSH[iunp]>. 1588 1589 Because the target is reused, you must be careful when pushing multiple 1590 values on the stack. The following code will not do what you think: 1591 1592 XPUSHi(10); 1593 XPUSHi(20); 1594 1595 This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto 1596 the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack". 1597 At the end of the operation, the stack does not contain the values 10 1598 and 20, but actually contains two pointers to C<TARG>, which we have set 1599 to 20. 1600 1601 If you need to push multiple different values then you should either use 1602 the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros, 1603 none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an 1604 SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>, 1605 will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make 1606 this a little easier to achieve by creating a new mortal for you (via 1607 C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary 1608 in the case of the C<mXPUSH[iunp]> macros), and then setting its value. 1609 Thus, instead of writing this to "fix" the example above: 1610 1611 XPUSHs(sv_2mortal(newSViv(10))) 1612 XPUSHs(sv_2mortal(newSViv(20))) 1613 1614 you can simply write: 1615 1616 mXPUSHi(10) 1617 mXPUSHi(20) 1618 1619 On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to 1620 need a C<dTARG> in your variable declarations so that the C<*PUSH*> 1621 macros can make use of the local variable C<TARG>. See also C<dTARGET> 1622 and C<dXSTARG>. 1623 1624 =head2 Scratchpads 1625 1626 The question remains on when the SVs which are I<target>s for opcodes 1627 are created. The answer is that they are created when the current unit -- 1628 a subroutine or a file (for opcodes for statements outside of 1629 subroutines) -- is compiled. During this time a special anonymous Perl 1630 array is created, which is called a scratchpad for the current 1631 unit. 1632 1633 A scratchpad keeps SVs which are lexicals for the current unit and are 1634 targets for opcodes. One can deduce that an SV lives on a scratchpad 1635 by looking on its flags: lexicals have C<SVs_PADMY> set, and 1636 I<target>s have C<SVs_PADTMP> set. 1637 1638 The correspondence between OPs and I<target>s is not 1-to-1. Different 1639 OPs in the compile tree of the unit can use the same target, if this 1640 would not conflict with the expected life of the temporary. 1641 1642 =head2 Scratchpads and recursion 1643 1644 In fact it is not 100% true that a compiled unit contains a pointer to 1645 the scratchpad AV. In fact it contains a pointer to an AV of 1646 (initially) one element, and this element is the scratchpad AV. Why do 1647 we need an extra level of indirection? 1648 1649 The answer is B<recursion>, and maybe B<threads>. Both 1650 these can create several execution pointers going into the same 1651 subroutine. For the subroutine-child not write over the temporaries 1652 for the subroutine-parent (lifespan of which covers the call to the 1653 child), the parent and the child should have different 1654 scratchpads. (I<And> the lexicals should be separate anyway!) 1655 1656 So each subroutine is born with an array of scratchpads (of length 1). 1657 On each entry to the subroutine it is checked that the current 1658 depth of the recursion is not more than the length of this array, and 1659 if it is, new scratchpad is created and pushed into the array. 1660 1661 The I<target>s on this scratchpad are C<undef>s, but they are already 1662 marked with correct flags. 1663 1664 =head1 Compiled code 1665 1666 =head2 Code tree 1667 1668 Here we describe the internal form your code is converted to by 1669 Perl. Start with a simple example: 1670 1671 $a = $b + $c; 1672 1673 This is converted to a tree similar to this one: 1674 1675 assign-to 1676 / \ 1677 + $a 1678 / \ 1679 $b $c 1680 1681 (but slightly more complicated). This tree reflects the way Perl 1682 parsed your code, but has nothing to do with the execution order. 1683 There is an additional "thread" going through the nodes of the tree 1684 which shows the order of execution of the nodes. In our simplified 1685 example above it looks like: 1686 1687 $b ---> $c ---> + ---> $a ---> assign-to 1688 1689 But with the actual compile tree for C<$a = $b + $c> it is different: 1690 some nodes I<optimized away>. As a corollary, though the actual tree 1691 contains more nodes than our simplified example, the execution order 1692 is the same as in our example. 1693 1694 =head2 Examining the tree 1695 1696 If you have your perl compiled for debugging (usually done with 1697 C<-DDEBUGGING> on the C<Configure> command line), you may examine the 1698 compiled tree by specifying C<-Dx> on the Perl command line. The 1699 output takes several lines per node, and for C<$b+$c> it looks like 1700 this: 1701 1702 5 TYPE = add ===> 6 1703 TARG = 1 1704 FLAGS = (SCALAR,KIDS) 1705 { 1706 TYPE = null ===> (4) 1707 (was rv2sv) 1708 FLAGS = (SCALAR,KIDS) 1709 { 1710 3 TYPE = gvsv ===> 4 1711 FLAGS = (SCALAR) 1712 GV = main::b 1713 } 1714 } 1715 { 1716 TYPE = null ===> (5) 1717 (was rv2sv) 1718 FLAGS = (SCALAR,KIDS) 1719 { 1720 4 TYPE = gvsv ===> 5 1721 FLAGS = (SCALAR) 1722 GV = main::c 1723 } 1724 } 1725 1726 This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are 1727 not optimized away (one per number in the left column). The immediate 1728 children of the given node correspond to C<{}> pairs on the same level 1729 of indentation, thus this listing corresponds to the tree: 1730 1731 add 1732 / \ 1733 null null 1734 | | 1735 gvsv gvsv 1736 1737 The execution order is indicated by C<===E<gt>> marks, thus it is C<3 1738 4 5 6> (node C<6> is not included into above listing), i.e., 1739 C<gvsv gvsv add whatever>. 1740 1741 Each of these nodes represents an op, a fundamental operation inside the 1742 Perl core. The code which implements each operation can be found in the 1743 F<pp*.c> files; the function which implements the op with type C<gvsv> 1744 is C<pp_gvsv>, and so on. As the tree above shows, different ops have 1745 different numbers of children: C<add> is a binary operator, as one would 1746 expect, and so has two children. To accommodate the various different 1747 numbers of children, there are various types of op data structure, and 1748 they link together in different ways. 1749 1750 The simplest type of op structure is C<OP>: this has no children. Unary 1751 operators, C<UNOP>s, have one child, and this is pointed to by the 1752 C<op_first> field. Binary operators (C<BINOP>s) have not only an 1753 C<op_first> field but also an C<op_last> field. The most complex type of 1754 op is a C<LISTOP>, which has any number of children. In this case, the 1755 first child is pointed to by C<op_first> and the last child by 1756 C<op_last>. The children in between can be found by iteratively 1757 following the C<op_sibling> pointer from the first child to the last. 1758 1759 There are also two other op types: a C<PMOP> holds a regular expression, 1760 and has no children, and a C<LOOP> may or may not have children. If the 1761 C<op_children> field is non-zero, it behaves like a C<LISTOP>. To 1762 complicate matters, if a C<UNOP> is actually a C<null> op after 1763 optimization (see L</Compile pass 2: context propagation>) it will still 1764 have children in accordance with its former type. 1765 1766 Another way to examine the tree is to use a compiler back-end module, such 1767 as L<B::Concise>. 1768 1769 =head2 Compile pass 1: check routines 1770 1771 The tree is created by the compiler while I<yacc> code feeds it 1772 the constructions it recognizes. Since I<yacc> works bottom-up, so does 1773 the first pass of perl compilation. 1774 1775 What makes this pass interesting for perl developers is that some 1776 optimization may be performed on this pass. This is optimization by 1777 so-called "check routines". The correspondence between node names 1778 and corresponding check routines is described in F<opcode.pl> (do not 1779 forget to run C<make regen_headers> if you modify this file). 1780 1781 A check routine is called when the node is fully constructed except 1782 for the execution-order thread. Since at this time there are no 1783 back-links to the currently constructed node, one can do most any 1784 operation to the top-level node, including freeing it and/or creating 1785 new nodes above/below it. 1786 1787 The check routine returns the node which should be inserted into the 1788 tree (if the top-level node was not modified, check routine returns 1789 its argument). 1790 1791 By convention, check routines have names C<ck_*>. They are usually 1792 called from C<new*OP> subroutines (or C<convert>) (which in turn are 1793 called from F<perly.y>). 1794 1795 =head2 Compile pass 1a: constant folding 1796 1797 Immediately after the check routine is called the returned node is 1798 checked for being compile-time executable. If it is (the value is 1799 judged to be constant) it is immediately executed, and a I<constant> 1800 node with the "return value" of the corresponding subtree is 1801 substituted instead. The subtree is deleted. 1802 1803 If constant folding was not performed, the execution-order thread is 1804 created. 1805 1806 =head2 Compile pass 2: context propagation 1807 1808 When a context for a part of compile tree is known, it is propagated 1809 down through the tree. At this time the context can have 5 values 1810 (instead of 2 for runtime context): void, boolean, scalar, list, and 1811 lvalue. In contrast with the pass 1 this pass is processed from top 1812 to bottom: a node's context determines the context for its children. 1813 1814 Additional context-dependent optimizations are performed at this time. 1815 Since at this moment the compile tree contains back-references (via 1816 "thread" pointers), nodes cannot be free()d now. To allow 1817 optimized-away nodes at this stage, such nodes are null()ified instead 1818 of free()ing (i.e. their type is changed to OP_NULL). 1819 1820 =head2 Compile pass 3: peephole optimization 1821 1822 After the compile tree for a subroutine (or for an C<eval> or a file) 1823 is created, an additional pass over the code is performed. This pass 1824 is neither top-down or bottom-up, but in the execution order (with 1825 additional complications for conditionals). These optimizations are 1826 done in the subroutine peep(). Optimizations performed at this stage 1827 are subject to the same restrictions as in the pass 2. 1828 1829 =head2 Pluggable runops 1830 1831 The compile tree is executed in a runops function. There are two runops 1832 functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used 1833 with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine 1834 control over the execution of the compile tree it is possible to provide 1835 your own runops function. 1836 1837 It's probably best to copy one of the existing runops functions and 1838 change it to suit your needs. Then, in the BOOT section of your XS 1839 file, add the line: 1840 1841 PL_runops = my_runops; 1842 1843 This function should be as efficient as possible to keep your programs 1844 running as fast as possible. 1845 1846 =head1 Examining internal data structures with the C<dump> functions 1847 1848 To aid debugging, the source file F<dump.c> contains a number of 1849 functions which produce formatted output of internal data structures. 1850 1851 The most commonly used of these functions is C<Perl_sv_dump>; it's used 1852 for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls 1853 C<sv_dump> to produce debugging output from Perl-space, so users of that 1854 module should already be familiar with its format. 1855 1856 C<Perl_op_dump> can be used to dump an C<OP> structure or any of its 1857 derivatives, and produces output similar to C<perl -Dx>; in fact, 1858 C<Perl_dump_eval> will dump the main root of the code being evaluated, 1859 exactly like C<-Dx>. 1860 1861 Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an 1862 op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the 1863 subroutines in a package like so: (Thankfully, these are all xsubs, so 1864 there is no op tree) 1865 1866 (gdb) print Perl_dump_packsubs(PL_defstash) 1867 1868 SUB attributes::bootstrap = (xsub 0x811fedc 0) 1869 1870 SUB UNIVERSAL::can = (xsub 0x811f50c 0) 1871 1872 SUB UNIVERSAL::isa = (xsub 0x811f304 0) 1873 1874 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0) 1875 1876 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0) 1877 1878 and C<Perl_dump_all>, which dumps all the subroutines in the stash and 1879 the op tree of the main root. 1880 1881 =head1 How multiple interpreters and concurrency are supported 1882 1883 =head2 Background and PERL_IMPLICIT_CONTEXT 1884 1885 The Perl interpreter can be regarded as a closed box: it has an API 1886 for feeding it code or otherwise making it do things, but it also has 1887 functions for its own use. This smells a lot like an object, and 1888 there are ways for you to build Perl so that you can have multiple 1889 interpreters, with one interpreter represented either as a C structure, 1890 or inside a thread-specific structure. These structures contain all 1891 the context, the state of that interpreter. 1892 1893 One macro controls the major Perl build flavor: MULTIPLICITY. The 1894 MULTIPLICITY build has a C structure that packages all the interpreter 1895 state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also 1896 normally defined, and enables the support for passing in a "hidden" first 1897 argument that represents all three data structures. MULTIPLICITY makes 1898 mutli-threaded perls possible (with the ithreads threading model, related 1899 to the macro USE_ITHREADS.) 1900 1901 Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and 1902 PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the 1903 former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the 1904 internal variables of Perl to be wrapped inside a single global struct, 1905 struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or 1906 the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes 1907 one step further, there is still a single struct (allocated in main() 1908 either from heap or from stack) but there are no global data symbols 1909 pointing to it. In either case the global struct should be initialised 1910 as the very first thing in main() using Perl_init_global_struct() and 1911 correspondingly tear it down after perl_free() using Perl_free_global_struct(), 1912 please see F<miniperlmain.c> for usage details. You may also need 1913 to use C<dVAR> in your coding to "declare the global variables" 1914 when you are using them. dTHX does this for you automatically. 1915 1916 To see whether you have non-const data you can use a BSD-compatible C<nm>: 1917 1918 nm libperl.a | grep -v ' [TURtr] ' 1919 1920 If this displays any C<D> or C<d> symbols, you have non-const data. 1921 1922 For backward compatibility reasons defining just PERL_GLOBAL_STRUCT 1923 doesn't actually hide all symbols inside a big global struct: some 1924 PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE 1925 then hides everything (see how the PERLIO_FUNCS_DECL is used). 1926 1927 All this obviously requires a way for the Perl internal functions to be 1928 either subroutines taking some kind of structure as the first 1929 argument, or subroutines taking nothing as the first argument. To 1930 enable these two very different ways of building the interpreter, 1931 the Perl source (as it does in so many other situations) makes heavy 1932 use of macros and subroutine naming conventions. 1933 1934 First problem: deciding which functions will be public API functions and 1935 which will be private. All functions whose names begin C<S_> are private 1936 (think "S" for "secret" or "static"). All other functions begin with 1937 "Perl_", but just because a function begins with "Perl_" does not mean it is 1938 part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a 1939 function is part of the API is to find its entry in L<perlapi>. 1940 If it exists in L<perlapi>, it's part of the API. If it doesn't, and you 1941 think it should be (i.e., you need it for your extension), send mail via 1942 L<perlbug> explaining why you think it should be. 1943 1944 Second problem: there must be a syntax so that the same subroutine 1945 declarations and calls can pass a structure as their first argument, 1946 or pass nothing. To solve this, the subroutines are named and 1947 declared in a particular way. Here's a typical start of a static 1948 function used within the Perl guts: 1949 1950 STATIC void 1951 S_incline(pTHX_ char *s) 1952 1953 STATIC becomes "static" in C, and may be #define'd to nothing in some 1954 configurations in future. 1955 1956 A public function (i.e. part of the internal API, but not necessarily 1957 sanctioned for use in extensions) begins like this: 1958 1959 void 1960 Perl_sv_setiv(pTHX_ SV* dsv, IV num) 1961 1962 C<pTHX_> is one of a number of macros (in perl.h) that hide the 1963 details of the interpreter's context. THX stands for "thread", "this", 1964 or "thingy", as the case may be. (And no, George Lucas is not involved. :-) 1965 The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument, 1966 or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and 1967 their variants. 1968 1969 When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no 1970 first argument containing the interpreter's context. The trailing underscore 1971 in the pTHX_ macro indicates that the macro expansion needs a comma 1972 after the context argument because other arguments follow it. If 1973 PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the 1974 subroutine is not prototyped to take the extra argument. The form of the 1975 macro without the trailing underscore is used when there are no additional 1976 explicit arguments. 1977 1978 When a core function calls another, it must pass the context. This 1979 is normally hidden via macros. Consider C<sv_setiv>. It expands into 1980 something like this: 1981 1982 #ifdef PERL_IMPLICIT_CONTEXT 1983 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b) 1984 /* can't do this for vararg functions, see below */ 1985 #else 1986 #define sv_setiv Perl_sv_setiv 1987 #endif 1988 1989 This works well, and means that XS authors can gleefully write: 1990 1991 sv_setiv(foo, bar); 1992 1993 and still have it work under all the modes Perl could have been 1994 compiled with. 1995 1996 This doesn't work so cleanly for varargs functions, though, as macros 1997 imply that the number of arguments is known in advance. Instead we 1998 either need to spell them out fully, passing C<aTHX_> as the first 1999 argument (the Perl core tends to do this with functions like 2000 Perl_warner), or use a context-free version. 2001 2002 The context-free version of Perl_warner is called 2003 Perl_warner_nocontext, and does not take the extra argument. Instead 2004 it does dTHX; to get the context from thread-local storage. We 2005 C<#define warner Perl_warner_nocontext> so that extensions get source 2006 compatibility at the expense of performance. (Passing an arg is 2007 cheaper than grabbing it from thread-local storage.) 2008 2009 You can ignore [pad]THXx when browsing the Perl headers/sources. 2010 Those are strictly for use within the core. Extensions and embedders 2011 need only be aware of [pad]THX. 2012 2013 =head2 So what happened to dTHR? 2014 2015 C<dTHR> was introduced in perl 5.005 to support the older thread model. 2016 The older thread model now uses the C<THX> mechanism to pass context 2017 pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and 2018 later still have it for backward source compatibility, but it is defined 2019 to be a no-op. 2020 2021 =head2 How do I use all this in extensions? 2022 2023 When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call 2024 any functions in the Perl API will need to pass the initial context 2025 argument somehow. The kicker is that you will need to write it in 2026 such a way that the extension still compiles when Perl hasn't been 2027 built with PERL_IMPLICIT_CONTEXT enabled. 2028 2029 There are three ways to do this. First, the easy but inefficient way, 2030 which is also the default, in order to maintain source compatibility 2031 with extensions: whenever XSUB.h is #included, it redefines the aTHX 2032 and aTHX_ macros to call a function that will return the context. 2033 Thus, something like: 2034 2035 sv_setiv(sv, num); 2036 2037 in your extension will translate to this when PERL_IMPLICIT_CONTEXT is 2038 in effect: 2039 2040 Perl_sv_setiv(Perl_get_context(), sv, num); 2041 2042 or to this otherwise: 2043 2044 Perl_sv_setiv(sv, num); 2045 2046 You have to do nothing new in your extension to get this; since 2047 the Perl library provides Perl_get_context(), it will all just 2048 work. 2049 2050 The second, more efficient way is to use the following template for 2051 your Foo.xs: 2052 2053 #define PERL_NO_GET_CONTEXT /* we want efficiency */ 2054 #include "EXTERN.h" 2055 #include "perl.h" 2056 #include "XSUB.h" 2057 2058 STATIC void my_private_function(int arg1, int arg2); 2059 2060 STATIC void 2061 my_private_function(int arg1, int arg2) 2062 { 2063 dTHX; /* fetch context */ 2064 ... call many Perl API functions ... 2065 } 2066 2067 [... etc ...] 2068 2069 MODULE = Foo PACKAGE = Foo 2070 2071 /* typical XSUB */ 2072 2073 void 2074 my_xsub(arg) 2075 int arg 2076 CODE: 2077 my_private_function(arg, 10); 2078 2079 Note that the only two changes from the normal way of writing an 2080 extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before 2081 including the Perl headers, followed by a C<dTHX;> declaration at 2082 the start of every function that will call the Perl API. (You'll 2083 know which functions need this, because the C compiler will complain 2084 that there's an undeclared identifier in those functions.) No changes 2085 are needed for the XSUBs themselves, because the XS() macro is 2086 correctly defined to pass in the implicit context if needed. 2087 2088 The third, even more efficient way is to ape how it is done within 2089 the Perl guts: 2090 2091 2092 #define PERL_NO_GET_CONTEXT /* we want efficiency */ 2093 #include "EXTERN.h" 2094 #include "perl.h" 2095 #include "XSUB.h" 2096 2097 /* pTHX_ only needed for functions that call Perl API */ 2098 STATIC void my_private_function(pTHX_ int arg1, int arg2); 2099 2100 STATIC void 2101 my_private_function(pTHX_ int arg1, int arg2) 2102 { 2103 /* dTHX; not needed here, because THX is an argument */ 2104 ... call Perl API functions ... 2105 } 2106 2107 [... etc ...] 2108 2109 MODULE = Foo PACKAGE = Foo 2110 2111 /* typical XSUB */ 2112 2113 void 2114 my_xsub(arg) 2115 int arg 2116 CODE: 2117 my_private_function(aTHX_ arg, 10); 2118 2119 This implementation never has to fetch the context using a function 2120 call, since it is always passed as an extra argument. Depending on 2121 your needs for simplicity or efficiency, you may mix the previous 2122 two approaches freely. 2123 2124 Never add a comma after C<pTHX> yourself--always use the form of the 2125 macro with the underscore for functions that take explicit arguments, 2126 or the form without the argument for functions with no explicit arguments. 2127 2128 If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR> 2129 definition is needed if the Perl global variables (see F<perlvars.h> 2130 or F<globvar.sym>) are accessed in the function and C<dTHX> is not 2131 used (the C<dTHX> includes the C<dVAR> if necessary). One notices 2132 the need for C<dVAR> only with the said compile-time define, because 2133 otherwise the Perl global variables are visible as-is. 2134 2135 =head2 Should I do anything special if I call perl from multiple threads? 2136 2137 If you create interpreters in one thread and then proceed to call them in 2138 another, you need to make sure perl's own Thread Local Storage (TLS) slot is 2139 initialized correctly in each of those threads. 2140 2141 The C<perl_alloc> and C<perl_clone> API functions will automatically set 2142 the TLS slot to the interpreter they created, so that there is no need to do 2143 anything special if the interpreter is always accessed in the same thread that 2144 created it, and that thread did not create or call any other interpreters 2145 afterwards. If that is not the case, you have to set the TLS slot of the 2146 thread before calling any functions in the Perl API on that particular 2147 interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that 2148 thread as the first thing you do: 2149 2150 /* do this before doing anything else with some_perl */ 2151 PERL_SET_CONTEXT(some_perl); 2152 2153 ... other Perl API calls on some_perl go here ... 2154 2155 =head2 Future Plans and PERL_IMPLICIT_SYS 2156 2157 Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything 2158 that the interpreter knows about itself and pass it around, so too are 2159 there plans to allow the interpreter to bundle up everything it knows 2160 about the environment it's running on. This is enabled with the 2161 PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on 2162 Windows. 2163 2164 This allows the ability to provide an extra pointer (called the "host" 2165 environment) for all the system calls. This makes it possible for 2166 all the system stuff to maintain their own state, broken down into 2167 seven C structures. These are thin wrappers around the usual system 2168 calls (see win32/perllib.c) for the default perl executable, but for a 2169 more ambitious host (like the one that would do fork() emulation) all 2170 the extra work needed to pretend that different interpreters are 2171 actually different "processes", would be done here. 2172 2173 The Perl engine/interpreter and the host are orthogonal entities. 2174 There could be one or more interpreters in a process, and one or 2175 more "hosts", with free association between them. 2176 2177 =head1 Internal Functions 2178 2179 All of Perl's internal functions which will be exposed to the outside 2180 world are prefixed by C<Perl_> so that they will not conflict with XS 2181 functions or functions used in a program in which Perl is embedded. 2182 Similarly, all global variables begin with C<PL_>. (By convention, 2183 static functions start with C<S_>.) 2184 2185 Inside the Perl core, you can get at the functions either with or 2186 without the C<Perl_> prefix, thanks to a bunch of defines that live in 2187 F<embed.h>. This header file is generated automatically from 2188 F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping 2189 header files for the internal functions, generates the documentation 2190 and a lot of other bits and pieces. It's important that when you add 2191 a new function to the core or change an existing one, you change the 2192 data in the table in F<embed.fnc> as well. Here's a sample entry from 2193 that table: 2194 2195 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval 2196 2197 The second column is the return type, the third column the name. Columns 2198 after that are the arguments. The first column is a set of flags: 2199 2200 =over 3 2201 2202 =item A 2203 2204 This function is a part of the public API. All such functions should also 2205 have 'd', very few do not. 2206 2207 =item p 2208 2209 This function has a C<Perl_> prefix; i.e. it is defined as 2210 C<Perl_av_fetch>. 2211 2212 =item d 2213 2214 This function has documentation using the C<apidoc> feature which we'll 2215 look at in a second. Some functions have 'd' but not 'A'; docs are good. 2216 2217 =back 2218 2219 Other available flags are: 2220 2221 =over 3 2222 2223 =item s 2224 2225 This is a static function and is defined as C<STATIC S_whatever>, and 2226 usually called within the sources as C<whatever(...)>. 2227 2228 =item n 2229 2230 This does not need a interpreter context, so the definition has no 2231 C<pTHX>, and it follows that callers don't use C<aTHX>. (See 2232 L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.) 2233 2234 =item r 2235 2236 This function never returns; C<croak>, C<exit> and friends. 2237 2238 =item f 2239 2240 This function takes a variable number of arguments, C<printf> style. 2241 The argument list should end with C<...>, like this: 2242 2243 Afprd |void |croak |const char* pat|... 2244 2245 =item M 2246 2247 This function is part of the experimental development API, and may change 2248 or disappear without notice. 2249 2250 =item o 2251 2252 This function should not have a compatibility macro to define, say, 2253 C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>. 2254 2255 =item x 2256 2257 This function isn't exported out of the Perl core. 2258 2259 =item m 2260 2261 This is implemented as a macro. 2262 2263 =item X 2264 2265 This function is explicitly exported. 2266 2267 =item E 2268 2269 This function is visible to extensions included in the Perl core. 2270 2271 =item b 2272 2273 Binary backward compatibility; this function is a macro but also has 2274 a C<Perl_> implementation (which is exported). 2275 2276 =item others 2277 2278 See the comments at the top of C<embed.fnc> for others. 2279 2280 =back 2281 2282 If you edit F<embed.pl> or F<embed.fnc>, you will need to run 2283 C<make regen_headers> to force a rebuild of F<embed.h> and other 2284 auto-generated files. 2285 2286 =head2 Formatted Printing of IVs, UVs, and NVs 2287 2288 If you are printing IVs, UVs, or NVS instead of the stdio(3) style 2289 formatting codes like C<%d>, C<%ld>, C<%f>, you should use the 2290 following macros for portability 2291 2292 IVdf IV in decimal 2293 UVuf UV in decimal 2294 UVof UV in octal 2295 UVxf UV in hexadecimal 2296 NVef NV %e-like 2297 NVff NV %f-like 2298 NVgf NV %g-like 2299 2300 These will take care of 64-bit integers and long doubles. 2301 For example: 2302 2303 printf("IV is %"IVdf"\n", iv); 2304 2305 The IVdf will expand to whatever is the correct format for the IVs. 2306 2307 If you are printing addresses of pointers, use UVxf combined 2308 with PTR2UV(), do not use %lx or %p. 2309 2310 =head2 Pointer-To-Integer and Integer-To-Pointer 2311 2312 Because pointer size does not necessarily equal integer size, 2313 use the follow macros to do it right. 2314 2315 PTR2UV(pointer) 2316 PTR2IV(pointer) 2317 PTR2NV(pointer) 2318 INT2PTR(pointertotype, integer) 2319 2320 For example: 2321 2322 IV iv = ...; 2323 SV *sv = INT2PTR(SV*, iv); 2324 2325 and 2326 2327 AV *av = ...; 2328 UV uv = PTR2UV(av); 2329 2330 =head2 Exception Handling 2331 2332 There are a couple of macros to do very basic exception handling in XS 2333 modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to 2334 be able to use these macros: 2335 2336 #define NO_XSLOCKS 2337 #include "XSUB.h" 2338 2339 You can use these macros if you call code that may croak, but you need 2340 to do some cleanup before giving control back to Perl. For example: 2341 2342 dXCPT; /* set up necessary variables */ 2343 2344 XCPT_TRY_START { 2345 code_that_may_croak(); 2346 } XCPT_TRY_END 2347 2348 XCPT_CATCH 2349 { 2350 /* do cleanup here */ 2351 XCPT_RETHROW; 2352 } 2353 2354 Note that you always have to rethrow an exception that has been 2355 caught. Using these macros, it is not possible to just catch the 2356 exception and ignore it. If you have to ignore the exception, you 2357 have to use the C<call_*> function. 2358 2359 The advantage of using the above macros is that you don't have 2360 to setup an extra function for C<call_*>, and that using these 2361 macros is faster than using C<call_*>. 2362 2363 =head2 Source Documentation 2364 2365 There's an effort going on to document the internal functions and 2366 automatically produce reference manuals from them - L<perlapi> is one 2367 such manual which details all the functions which are available to XS 2368 writers. L<perlintern> is the autogenerated manual for the functions 2369 which are not part of the API and are supposedly for internal use only. 2370 2371 Source documentation is created by putting POD comments into the C 2372 source, like this: 2373 2374 /* 2375 =for apidoc sv_setiv 2376 2377 Copies an integer into the given SV. Does not handle 'set' magic. See 2378 C<sv_setiv_mg>. 2379 2380 =cut 2381 */ 2382 2383 Please try and supply some documentation if you add functions to the 2384 Perl core. 2385 2386 =head2 Backwards compatibility 2387 2388 The Perl API changes over time. New functions are added or the interfaces 2389 of existing functions are changed. The C<Devel::PPPort> module tries to 2390 provide compatibility code for some of these changes, so XS writers don't 2391 have to code it themselves when supporting multiple versions of Perl. 2392 2393 C<Devel::PPPort> generates a C header file F<ppport.h> that can also 2394 be run as a Perl script. To generate F<ppport.h>, run: 2395 2396 perl -MDevel::PPPort -eDevel::PPPort::WriteFile 2397 2398 Besides checking existing XS code, the script can also be used to retrieve 2399 compatibility information for various API calls using the C<--api-info> 2400 command line switch. For example: 2401 2402 % perl ppport.h --api-info=sv_magicext 2403 2404 For details, see C<perldoc ppport.h>. 2405 2406 =head1 Unicode Support 2407 2408 Perl 5.6.0 introduced Unicode support. It's important for porters and XS 2409 writers to understand this support and make sure that the code they 2410 write does not corrupt Unicode data. 2411 2412 =head2 What B<is> Unicode, anyway? 2413 2414 In the olden, less enlightened times, we all used to use ASCII. Most of 2415 us did, anyway. The big problem with ASCII is that it's American. Well, 2416 no, that's not actually the problem; the problem is that it's not 2417 particularly useful for people who don't use the Roman alphabet. What 2418 used to happen was that particular languages would stick their own 2419 alphabet in the upper range of the sequence, between 128 and 255. Of 2420 course, we then ended up with plenty of variants that weren't quite 2421 ASCII, and the whole point of it being a standard was lost. 2422 2423 Worse still, if you've got a language like Chinese or 2424 Japanese that has hundreds or thousands of characters, then you really 2425 can't fit them into a mere 256, so they had to forget about ASCII 2426 altogether, and build their own systems using pairs of numbers to refer 2427 to one character. 2428 2429 To fix this, some people formed Unicode, Inc. and 2430 produced a new character set containing all the characters you can 2431 possibly think of and more. There are several ways of representing these 2432 characters, and the one Perl uses is called UTF-8. UTF-8 uses 2433 a variable number of bytes to represent a character. You can learn more 2434 about Unicode and Perl's Unicode model in L<perlunicode>. 2435 2436 =head2 How can I recognise a UTF-8 string? 2437 2438 You can't. This is because UTF-8 data is stored in bytes just like 2439 non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types) 2440 capital E with a grave accent, is represented by the two bytes 2441 C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)> 2442 has that byte sequence as well. So you can't tell just by looking - this 2443 is what makes Unicode input an interesting problem. 2444 2445 In general, you either have to know what you're dealing with, or you 2446 have to guess. The API function C<is_utf8_string> can help; it'll tell 2447 you if a string contains only valid UTF-8 characters. However, it can't 2448 do the work for you. On a character-by-character basis, C<is_utf8_char> 2449 will tell you whether the current character in a string is valid UTF-8. 2450 2451 =head2 How does UTF-8 represent Unicode characters? 2452 2453 As mentioned above, UTF-8 uses a variable number of bytes to store a 2454 character. Characters with values 0...127 are stored in one byte, just 2455 like good ol' ASCII. Character 128 is stored as C<v194.128>; this 2456 continues up to character 191, which is C<v194.191>. Now we've run out of 2457 bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And 2458 so it goes on, moving to three bytes at character 2048. 2459 2460 Assuming you know you're dealing with a UTF-8 string, you can find out 2461 how long the first character in it is with the C<UTF8SKIP> macro: 2462 2463 char *utf = "\305\233\340\240\201"; 2464 I32 len; 2465 2466 len = UTF8SKIP(utf); /* len is 2 here */ 2467 utf += len; 2468 len = UTF8SKIP(utf); /* len is 3 here */ 2469 2470 Another way to skip over characters in a UTF-8 string is to use 2471 C<utf8_hop>, which takes a string and a number of characters to skip 2472 over. You're on your own about bounds checking, though, so don't use it 2473 lightly. 2474 2475 All bytes in a multi-byte UTF-8 character will have the high bit set, 2476 so you can test if you need to do something special with this 2477 character like this (the UTF8_IS_INVARIANT() is a macro that tests 2478 whether the byte can be encoded as a single byte even in UTF-8): 2479 2480 U8 *utf; 2481 UV uv; /* Note: a UV, not a U8, not a char */ 2482 2483 if (!UTF8_IS_INVARIANT(*utf)) 2484 /* Must treat this as UTF-8 */ 2485 uv = utf8_to_uv(utf); 2486 else 2487 /* OK to treat this character as a byte */ 2488 uv = *utf; 2489 2490 You can also see in that example that we use C<utf8_to_uv> to get the 2491 value of the character; the inverse function C<uv_to_utf8> is available 2492 for putting a UV into UTF-8: 2493 2494 if (!UTF8_IS_INVARIANT(uv)) 2495 /* Must treat this as UTF8 */ 2496 utf8 = uv_to_utf8(utf8, uv); 2497 else 2498 /* OK to treat this character as a byte */ 2499 *utf8++ = uv; 2500 2501 You B<must> convert characters to UVs using the above functions if 2502 you're ever in a situation where you have to match UTF-8 and non-UTF-8 2503 characters. You may not skip over UTF-8 characters in this case. If you 2504 do this, you'll lose the ability to match hi-bit non-UTF-8 characters; 2505 for instance, if your UTF-8 string contains C<v196.172>, and you skip 2506 that character, you can never match a C<chr(200)> in a non-UTF-8 string. 2507 So don't do that! 2508 2509 =head2 How does Perl store UTF-8 strings? 2510 2511 Currently, Perl deals with Unicode strings and non-Unicode strings 2512 slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the 2513 string is internally encoded as UTF-8. Without it, the byte value is the 2514 codepoint number and vice versa (in other words, the string is encoded 2515 as iso-8859-1). You can check and manipulate this flag with the 2516 following macros: 2517 2518 SvUTF8(sv) 2519 SvUTF8_on(sv) 2520 SvUTF8_off(sv) 2521 2522 This flag has an important effect on Perl's treatment of the string: if 2523 Unicode data is not properly distinguished, regular expressions, 2524 C<length>, C<substr> and other string handling operations will have 2525 undesirable results. 2526 2527 The problem comes when you have, for instance, a string that isn't 2528 flagged as UTF-8, and contains a byte sequence that could be UTF-8 - 2529 especially when combining non-UTF-8 and UTF-8 strings. 2530 2531 Never forget that the C<SVf_UTF8> flag is separate to the PV value; you 2532 need be sure you don't accidentally knock it off while you're 2533 manipulating SVs. More specifically, you cannot expect to do this: 2534 2535 SV *sv; 2536 SV *nsv; 2537 STRLEN len; 2538 char *p; 2539 2540 p = SvPV(sv, len); 2541 frobnicate(p); 2542 nsv = newSVpvn(p, len); 2543 2544 The C<char*> string does not tell you the whole story, and you can't 2545 copy or reconstruct an SV just by copying the string value. Check if the 2546 old SV has the UTF8 flag set, and act accordingly: 2547 2548 p = SvPV(sv, len); 2549 frobnicate(p); 2550 nsv = newSVpvn(p, len); 2551 if (SvUTF8(sv)) 2552 SvUTF8_on(nsv); 2553 2554 In fact, your C<frobnicate> function should be made aware of whether or 2555 not it's dealing with UTF-8 data, so that it can handle the string 2556 appropriately. 2557 2558 Since just passing an SV to an XS function and copying the data of 2559 the SV is not enough to copy the UTF8 flags, even less right is just 2560 passing a C<char *> to an XS function. 2561 2562 =head2 How do I convert a string to UTF-8? 2563 2564 If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade 2565 one of the strings to UTF-8. If you've got an SV, the easiest way to do 2566 this is: 2567 2568 sv_utf8_upgrade(sv); 2569 2570 However, you must not do this, for example: 2571 2572 if (!SvUTF8(left)) 2573 sv_utf8_upgrade(left); 2574 2575 If you do this in a binary operator, you will actually change one of the 2576 strings that came into the operator, and, while it shouldn't be noticeable 2577 by the end user, it can cause problems in deficient code. 2578 2579 Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its 2580 string argument. This is useful for having the data available for 2581 comparisons and so on, without harming the original SV. There's also 2582 C<utf8_to_bytes> to go the other way, but naturally, this will fail if 2583 the string contains any characters above 255 that can't be represented 2584 in a single byte. 2585 2586 =head2 Is there anything else I need to know? 2587 2588 Not really. Just remember these things: 2589 2590 =over 3 2591 2592 =item * 2593 2594 There's no way to tell if a string is UTF-8 or not. You can tell if an SV 2595 is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if 2596 something should be UTF-8. Treat the flag as part of the PV, even though 2597 it's not - if you pass on the PV to somewhere, pass on the flag too. 2598 2599 =item * 2600 2601 If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value, 2602 unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>. 2603 2604 =item * 2605 2606 When writing a character C<uv> to a UTF-8 string, B<always> use 2607 C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case 2608 you can use C<*s = uv>. 2609 2610 =item * 2611 2612 Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get 2613 a new string which is UTF-8 encoded. There are tricks you can use to 2614 delay deciding whether you need to use a UTF-8 string until you get to a 2615 high character - C<HALF_UPGRADE> is one of those. 2616 2617 =back 2618 2619 =head1 Custom Operators 2620 2621 Custom operator support is a new experimental feature that allows you to 2622 define your own ops. This is primarily to allow the building of 2623 interpreters for other languages in the Perl core, but it also allows 2624 optimizations through the creation of "macro-ops" (ops which perform the 2625 functions of multiple ops which are usually executed together, such as 2626 C<gvsv, gvsv, add>.) 2627 2628 This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl 2629 core does not "know" anything special about this op type, and so it will 2630 not be involved in any optimizations. This also means that you can 2631 define your custom ops to be any op structure - unary, binary, list and 2632 so on - you like. 2633 2634 It's important to know what custom operators won't do for you. They 2635 won't let you add new syntax to Perl, directly. They won't even let you 2636 add new keywords, directly. In fact, they won't change the way Perl 2637 compiles a program at all. You have to do those changes yourself, after 2638 Perl has compiled the program. You do this either by manipulating the op 2639 tree using a C<CHECK> block and the C<B::Generate> module, or by adding 2640 a custom peephole optimizer with the C<optimize> module. 2641 2642 When you do this, you replace ordinary Perl ops with custom ops by 2643 creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own 2644 PP function. This should be defined in XS code, and should look like 2645 the PP ops in C<pp_*.c>. You are responsible for ensuring that your op 2646 takes the appropriate number of values from the stack, and you are 2647 responsible for adding stack marks if necessary. 2648 2649 You should also "register" your op with the Perl interpreter so that it 2650 can produce sensible error and warning messages. Since it is possible to 2651 have multiple custom ops within the one "logical" op type C<OP_CUSTOM>, 2652 Perl uses the value of C<< o->op_ppaddr >> as a key into the 2653 C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you 2654 need to enter a name and description for your op at the appropriate 2655 place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes. 2656 2657 Forthcoming versions of C<B::Generate> (version 1.0 and above) should 2658 directly support the creation of custom ops by name. 2659 2660 =head1 AUTHORS 2661 2662 Until May 1997, this document was maintained by Jeff Okamoto 2663 E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl 2664 itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>. 2665 2666 With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, 2667 Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil 2668 Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, 2669 Stephen McCamant, and Gurusamy Sarathy. 2670 2671 =head1 SEE ALSO 2672 2673 perlapi(1), perlintern(1), perlxs(1), perlembed(1)
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