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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
954c1994 | 3 | perlguts - Introduction to the Perl API |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
b3b6085d | 7 | This document attempts to describe how to use the Perl API, as well as |
9b8ea7f3 | 8 | to provide some info on the basic workings of the Perl core. It is far |
b3b6085d PP |
9 | from complete and probably contains many errors. Please refer any |
10 | questions or comments to the author below. | |
a0d0e21e | 11 | |
0a753a76 | 12 | =head1 Variables |
13 | ||
5f05dabc | 14 | =head2 Datatypes |
a0d0e21e LW |
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 | ||
d1b91892 | 22 | Each typedef has specific routines that manipulate the various data types. |
a0d0e21e LW |
23 | |
24 | =head2 What is an "IV"? | |
25 | ||
954c1994 | 26 | Perl uses a special typedef IV which is a simple signed integer type that is |
5f05dabc | 27 | guaranteed to be large enough to hold a pointer (as well as an integer). |
954c1994 | 28 | Additionally, there is the UV, which is simply an unsigned IV. |
a0d0e21e | 29 | |
d1b91892 | 30 | Perl also uses two special typedefs, I32 and I16, which will always be at |
954c1994 | 31 | least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16, |
20dbd849 NC |
32 | as well.) They will usually be exactly 32 and 16 bits long, but on Crays |
33 | they will both be 64 bits. | |
a0d0e21e | 34 | |
54310121 | 35 | =head2 Working with SVs |
a0d0e21e | 36 | |
20dbd849 NC |
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). | |
a0d0e21e | 40 | |
20dbd849 | 41 | The seven routines are: |
a0d0e21e LW |
42 | |
43 | SV* newSViv(IV); | |
20dbd849 | 44 | SV* newSVuv(UV); |
a0d0e21e | 45 | SV* newSVnv(double); |
9b8ea7f3 JH |
46 | SV* newSVpv(const char*, STRLEN); |
47 | SV* newSVpvn(const char*, STRLEN); | |
46fc3d4c | 48 | SV* newSVpvf(const char*, ...); |
a0d0e21e LW |
49 | SV* newSVsv(SV*); |
50 | ||
9b8ea7f3 JH |
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. | |
20dbd849 | 60 | |
9b8ea7f3 JH |
61 | SV *sv = newSV(0); /* no storage allocated */ |
62 | SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */ | |
20dbd849 | 63 | |
9b8ea7f3 | 64 | To change the value of an I<already-existing> SV, there are eight routines: |
a0d0e21e LW |
65 | |
66 | void sv_setiv(SV*, IV); | |
deb3007b | 67 | void sv_setuv(SV*, UV); |
a0d0e21e | 68 | void sv_setnv(SV*, double); |
08105a92 | 69 | void sv_setpv(SV*, const char*); |
9b8ea7f3 | 70 | void sv_setpvn(SV*, const char*, STRLEN) |
46fc3d4c | 71 | void sv_setpvf(SV*, const char*, ...); |
5b3218b9 | 72 | void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *); |
a0d0e21e LW |
73 | void sv_setsv(SV*, SV*); |
74 | ||
75 | Notice that you can choose to specify the length of the string to be | |
9da1e3b5 MUN |
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 | |
9abd00ed GS |
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 | ||
328bf373 | 85 | C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify |
9abd00ed GS |
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 | |
c2611fb3 | 89 | the string, and the string's contents are therefore untrustworthy (see |
9abd00ed GS |
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 | ||
9da1e3b5 MUN |
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. | |
a0d0e21e | 96 | |
a3cb178b GS |
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 | |
5f05dabc | 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 | ||
a0d0e21e LW |
105 | To access the actual value that an SV points to, you can use the macros: |
106 | ||
107 | SvIV(SV*) | |
954c1994 | 108 | SvUV(SV*) |
a0d0e21e LW |
109 | SvNV(SV*) |
110 | SvPV(SV*, STRLEN len) | |
1fa8b10d | 111 | SvPV_nolen(SV*) |
a0d0e21e | 112 | |
954c1994 | 113 | which will automatically coerce the actual scalar type into an IV, UV, double, |
a0d0e21e LW |
114 | or string. |
115 | ||
116 | In the C<SvPV> macro, the length of the string returned is placed into the | |
1fa8b10d JD |
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. | |
a0d0e21e | 124 | |
ce2f5d8f KA |
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 | ||
7dbaea58 NC |
129 | SV *s; |
130 | STRLEN len; | |
131 | char * ptr; | |
132 | ptr = SvPV(s, len); | |
133 | foo(ptr, len); | |
ce2f5d8f | 134 | |
07fa94a1 | 135 | If you want to know if the scalar value is TRUE, you can use: |
a0d0e21e LW |
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 | |
5f05dabc | 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 | |
8ebc5c01 | 148 | C<SvGROW(sv, len + 1)>). |
a0d0e21e LW |
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 | ||
cb1a09d0 AD |
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. | |
a0d0e21e | 169 | |
d1b91892 AD |
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 | ||
08105a92 | 173 | void sv_catpv(SV*, const char*); |
e65f3abd | 174 | void sv_catpvn(SV*, const char*, STRLEN); |
46fc3d4c | 175 | void sv_catpvf(SV*, const char*, ...); |
328bf373 | 176 | void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool); |
d1b91892 AD |
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 | |
46fc3d4c | 181 | yourself. The third function processes its arguments like C<sprintf> and |
9abd00ed GS |
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. | |
d1b91892 | 190 | |
a0d0e21e LW |
191 | If you know the name of a scalar variable, you can get a pointer to its SV |
192 | by using the following: | |
193 | ||
4929bf7b | 194 | SV* get_sv("package::varname", FALSE); |
a0d0e21e LW |
195 | |
196 | This returns NULL if the variable does not exist. | |
197 | ||
d1b91892 | 198 | If you want to know if this variable (or any other SV) is actually C<defined>, |
a0d0e21e LW |
199 | you can call: |
200 | ||
201 | SvOK(SV*) | |
202 | ||
9b8ea7f3 | 203 | The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. |
a1e5c4b6 NC |
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>). | |
a0d0e21e | 220 | |
9b8ea7f3 JH |
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. | |
a0d0e21e | 224 | |
9cde0e7f | 225 | Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>. |
a0d0e21e LW |
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 | |
04343c6d | 235 | return a real value, or undef otherwise. Instead it has returned a NULL |
a0d0e21e | 236 | pointer which, somewhere down the line, will cause a segmentation violation, |
9b8ea7f3 JH |
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. | |
a0d0e21e LW |
239 | |
240 | To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this | |
3fe9a6f1 | 241 | call is not necessary (see L<Reference Counts and Mortality>). |
a0d0e21e | 242 | |
94dde4fb SC |
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 | |
da75cd15 | 247 | somewhere inside the PV, and it discards everything before the |
94dde4fb SC |
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 | |
00aadd71 | 253 | many bytes, and adjusts C<SvCUR> and C<SvLEN>. |
94dde4fb SC |
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 | ||
fe854a6f | 276 | Something similar to the offset hack is performed on AVs to enable |
319cef53 SC |
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 | ||
d1b91892 | 285 | =head2 What's Really Stored in an SV? |
a0d0e21e LW |
286 | |
287 | Recall that the usual method of determining the type of scalar you have is | |
5f05dabc | 288 | to use C<Sv*OK> macros. Because a scalar can be both a number and a string, |
d1b91892 | 289 | usually these macros will always return TRUE and calling the C<Sv*V> |
a0d0e21e LW |
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 | |
d1b91892 | 301 | stored in your SV. The "p" stands for private. |
a0d0e21e | 302 | |
9e9796d6 JH |
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 | |
de7b736d | 307 | numeric conversion has occurred and precision has been lost: only the |
9e9796d6 JH |
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 | ||
07fa94a1 | 311 | In general, though, it's best to use the C<Sv*V> macros. |
a0d0e21e | 312 | |
54310121 | 313 | =head2 Working with AVs |
a0d0e21e | 314 | |
07fa94a1 JO |
315 | There are two ways to create and load an AV. The first method creates an |
316 | empty AV: | |
a0d0e21e LW |
317 | |
318 | AV* newAV(); | |
319 | ||
54310121 | 320 | The second method both creates the AV and initially populates it with SVs: |
a0d0e21e LW |
321 | |
322 | AV* av_make(I32 num, SV **ptr); | |
323 | ||
5f05dabc | 324 | The second argument points to an array containing C<num> C<SV*>'s. Once the |
54310121 | 325 | AV has been created, the SVs can be destroyed, if so desired. |
a0d0e21e | 326 | |
54310121 | 327 | Once the AV has been created, the following operations are possible on AVs: |
a0d0e21e LW |
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 | ||
5f05dabc | 341 | I32 av_len(AV*); |
a0d0e21e | 342 | SV** av_fetch(AV*, I32 key, I32 lval); |
a0d0e21e | 343 | SV** av_store(AV*, I32 key, SV* val); |
a0d0e21e | 344 | |
5f05dabc | 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. | |
04343c6d GS |
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. | |
d1b91892 | 355 | |
a0d0e21e | 356 | void av_clear(AV*); |
a0d0e21e | 357 | void av_undef(AV*); |
cb1a09d0 | 358 | void av_extend(AV*, I32 key); |
5f05dabc | 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 | |
adc882cf GS |
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. | |
a0d0e21e LW |
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 | ||
4929bf7b | 370 | AV* get_av("package::varname", FALSE); |
a0d0e21e LW |
371 | |
372 | This returns NULL if the variable does not exist. | |
373 | ||
04343c6d GS |
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 | ||
54310121 | 377 | =head2 Working with HVs |
a0d0e21e LW |
378 | |
379 | To create an HV, you use the following routine: | |
380 | ||
381 | HV* newHV(); | |
382 | ||
54310121 | 383 | Once the HV has been created, the following operations are possible on HVs: |
a0d0e21e | 384 | |
08105a92 GS |
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); | |
a0d0e21e | 387 | |
5f05dabc | 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 | |
54310121 | 391 | scalar being stored, and C<hash> is the precomputed hash value (zero if |
5f05dabc | 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. | |
a0d0e21e | 396 | |
5f05dabc | 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. | |
a0d0e21e LW |
401 | |
402 | These two functions check if a hash table entry exists, and deletes it. | |
403 | ||
08105a92 GS |
404 | bool hv_exists(HV*, const char* key, U32 klen); |
405 | SV* hv_delete(HV*, const char* key, U32 klen, I32 flags); | |
a0d0e21e | 406 | |
5f05dabc | 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 | ||
a0d0e21e LW |
410 | And more miscellaneous functions: |
411 | ||
412 | void hv_clear(HV*); | |
a0d0e21e | 413 | void hv_undef(HV*); |
5f05dabc | 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. | |
a0d0e21e | 418 | |
d1b91892 AD |
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 | ||
a0d0e21e LW |
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 */ | |
cb1a09d0 | 433 | SV* hv_iterval(HV*, HE* entry); |
d1be9408 | 434 | /* Return an SV pointer to the value of the HE |
a0d0e21e | 435 | structure */ |
cb1a09d0 | 436 | SV* hv_iternextsv(HV*, char** key, I32* retlen); |
d1b91892 AD |
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 */ | |
a0d0e21e LW |
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 | ||
4929bf7b | 445 | HV* get_hv("package::varname", FALSE); |
a0d0e21e LW |
446 | |
447 | This returns NULL if the variable does not exist. | |
448 | ||
8ebc5c01 | 449 | The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro: |
a0d0e21e | 450 | |
a0d0e21e | 451 | hash = 0; |
ab192400 GS |
452 | while (klen--) |
453 | hash = (hash * 33) + *key++; | |
87275199 | 454 | hash = hash + (hash >> 5); /* after 5.6 */ |
ab192400 | 455 | |
87275199 | 456 | The last step was added in version 5.6 to improve distribution of |
ab192400 | 457 | lower bits in the resulting hash value. |
a0d0e21e | 458 | |
04343c6d GS |
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 | ||
1e422769 | 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); | |
c47ff5f1 | 468 | |
1e422769 | 469 | bool hv_exists_ent (HV* tb, SV* key, U32 hash); |
470 | SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash); | |
c47ff5f1 | 471 | |
1e422769 | 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 | |
4a4eefd0 GS |
481 | doesn't have to be recomputed every time). See L<perlapi> for detailed |
482 | descriptions. | |
1e422769 | 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 | |
4a4eefd0 | 487 | L<perlapi> for detailed descriptions of these macros. |
1e422769 | 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 | ||
04343c6d GS |
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. | |
1e422769 | 506 | |
b0a5877c JH |
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 | ||
a0d0e21e LW |
559 | =head2 References |
560 | ||
d1b91892 AD |
561 | References are a special type of scalar that point to other data types |
562 | (including references). | |
a0d0e21e | 563 | |
07fa94a1 | 564 | To create a reference, use either of the following functions: |
a0d0e21e | 565 | |
5f05dabc | 566 | SV* newRV_inc((SV*) thing); |
567 | SV* newRV_noinc((SV*) thing); | |
a0d0e21e | 568 | |
5f05dabc | 569 | The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The |
07fa94a1 JO |
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: | |
a0d0e21e LW |
576 | |
577 | SvRV(SV*) | |
578 | ||
579 | then call the appropriate routines, casting the returned C<SV*> to either an | |
d1b91892 | 580 | C<AV*> or C<HV*>, if required. |
a0d0e21e | 581 | |
d1b91892 | 582 | To determine if an SV is a reference, you can use the following macro: |
a0d0e21e LW |
583 | |
584 | SvROK(SV*) | |
585 | ||
07fa94a1 JO |
586 | To discover what type of value the reference refers to, use the following |
587 | macro and then check the return value. | |
d1b91892 AD |
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 | |
5f05dabc | 596 | SVt_RV Scalar |
d1b91892 AD |
597 | SVt_PVAV Array |
598 | SVt_PVHV Hash | |
599 | SVt_PVCV Code | |
5f05dabc | 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. | |
d1b91892 | 604 | |
cb1a09d0 AD |
605 | =head2 Blessed References and Class Objects |
606 | ||
9b8ea7f3 | 607 | References are also used to support object-oriented programming. In perl's |
cb1a09d0 AD |
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 | ||
9b8ea7f3 JH |
616 | The C<sv> argument must be a reference value. The C<stash> argument |
617 | specifies which class the reference will belong to. See | |
2ae324a7 | 618 | L<Stashes and Globs> for information on converting class names into stashes. |
cb1a09d0 AD |
619 | |
620 | /* Still under construction */ | |
621 | ||
622 | Upgrades rv to reference if not already one. Creates new SV for rv to | |
8ebc5c01 | 623 | point to. If C<classname> is non-null, the SV is blessed into the specified |
624 | class. SV is returned. | |
cb1a09d0 | 625 | |
08105a92 | 626 | SV* newSVrv(SV* rv, const char* classname); |
cb1a09d0 | 627 | |
e1c57cef | 628 | Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed |
8ebc5c01 | 629 | if C<classname> is non-null. |
cb1a09d0 | 630 | |
08105a92 | 631 | SV* sv_setref_iv(SV* rv, const char* classname, IV iv); |
e1c57cef | 632 | SV* sv_setref_uv(SV* rv, const char* classname, UV uv); |
08105a92 | 633 | SV* sv_setref_nv(SV* rv, const char* classname, NV iv); |
cb1a09d0 | 634 | |
5f05dabc | 635 | Copies the pointer value (I<the address, not the string!>) into an SV whose |
8ebc5c01 | 636 | reference is rv. SV is blessed if C<classname> is non-null. |
cb1a09d0 | 637 | |
08105a92 | 638 | SV* sv_setref_pv(SV* rv, const char* classname, PV iv); |
cb1a09d0 | 639 | |
8ebc5c01 | 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. | |
cb1a09d0 | 642 | |
e65f3abd | 643 | SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); |
cb1a09d0 | 644 | |
9abd00ed GS |
645 | Tests whether the SV is blessed into the specified class. It does not |
646 | check inheritance relationships. | |
647 | ||
08105a92 | 648 | int sv_isa(SV* sv, const char* name); |
9abd00ed GS |
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 | ||
08105a92 | 658 | bool sv_derived_from(SV* sv, const char* name); |
9abd00ed | 659 | |
00aadd71 | 660 | To check if you've got an object derived from a specific class you have |
9abd00ed GS |
661 | to write: |
662 | ||
663 | if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... } | |
cb1a09d0 | 664 | |
5f05dabc | 665 | =head2 Creating New Variables |
cb1a09d0 | 666 | |
5f05dabc | 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. | |
cb1a09d0 | 669 | |
4929bf7b GS |
670 | SV* get_sv("package::varname", TRUE); |
671 | AV* get_av("package::varname", TRUE); | |
672 | HV* get_hv("package::varname", TRUE); | |
cb1a09d0 AD |
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 | ||
5f05dabc | 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: | |
cb1a09d0 | 679 | |
9a68f1db SB |
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 | ||
9a68f1db SB |
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 | |
cb1a09d0 | 699 | |
07fa94a1 JO |
700 | If you do not specify a package name, the variable is created in the current |
701 | package. | |
cb1a09d0 | 702 | |
5f05dabc | 703 | =head2 Reference Counts and Mortality |
a0d0e21e | 704 | |
d1be9408 | 705 | Perl uses a reference count-driven garbage collection mechanism. SVs, |
54310121 | 706 | AVs, or HVs (xV for short in the following) start their life with a |
55497cff | 707 | reference count of 1. If the reference count of an xV ever drops to 0, |
07fa94a1 | 708 | then it will be destroyed and its memory made available for reuse. |
55497cff | 709 | |
710 | This normally doesn't happen at the Perl level unless a variable is | |
5f05dabc | 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 | |
55497cff | 713 | manipulated with the following macros: |
714 | ||
715 | int SvREFCNT(SV* sv); | |
5f05dabc | 716 | SV* SvREFCNT_inc(SV* sv); |
55497cff | 717 | void SvREFCNT_dec(SV* sv); |
718 | ||
719 | However, there is one other function which manipulates the reference | |
07fa94a1 JO |
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. | |
5f05dabc | 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 | |
07fa94a1 JO |
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. | |
5f05dabc | 735 | |
736 | The correct procedure, then, is to use C<newRV_noinc> instead of | |
faed5253 JO |
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, | |
07fa94a1 | 739 | stopping any memory leak. |
55497cff | 740 | |
5f05dabc | 741 | There are some convenience functions available that can help with the |
54310121 | 742 | destruction of xVs. These functions introduce the concept of "mortality". |
07fa94a1 JO |
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 | |
54310121 | 746 | an XSUB function. The actual determinant for when mortal xVs have their |
07fa94a1 JO |
747 | reference count decremented depends on two macros, SAVETMPS and FREETMPS. |
748 | See L<perlcall> and L<perlxs> for more details on these macros. | |
55497cff | 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 | ||
00aadd71 NIS |
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 | |
9b8ea7f3 JH |
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. | |
a0d0e21e LW |
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 | ||
00aadd71 | 766 | The first call creates a mortal SV (with no value), the second converts an existing |
5f05dabc | 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. | |
00aadd71 | 769 | Because C<sv_newmortal> gives the new SV no value,it must normally be given one |
9a68f1db | 770 | via C<sv_setpv>, C<sv_setiv>, etc. : |
00aadd71 NIS |
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. | |
a0d0e21e | 788 | |
54310121 | 789 | The mortal routines are not just for SVs -- AVs and HVs can be |
faed5253 | 790 | made mortal by passing their address (type-casted to C<SV*>) to the |
07fa94a1 | 791 | C<sv_2mortal> or C<sv_mortalcopy> routines. |
a0d0e21e | 792 | |
5f05dabc | 793 | =head2 Stashes and Globs |
a0d0e21e | 794 | |
9b8ea7f3 JH |
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 | |
aa689395 | 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: | |
cb1a09d0 | 801 | |
a0d0e21e LW |
802 | Scalar Value |
803 | Array Value | |
804 | Hash Value | |
a3cb178b | 805 | I/O Handle |
a0d0e21e LW |
806 | Format |
807 | Subroutine | |
808 | ||
9b8ea7f3 JH |
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. | |
a0d0e21e | 814 | |
d1b91892 | 815 | To get the stash pointer for a particular package, use the function: |
a0d0e21e | 816 | |
08105a92 | 817 | HV* gv_stashpv(const char* name, I32 create) |
a0d0e21e LW |
818 | HV* gv_stashsv(SV*, I32 create) |
819 | ||
820 | The first function takes a literal string, the second uses the string stored | |
d1b91892 | 821 | in the SV. Remember that a stash is just a hash table, so you get back an |
cb1a09d0 | 822 | C<HV*>. The C<create> flag will create a new package if it is set. |
a0d0e21e LW |
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 | |
d1b91892 AD |
826 | packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl |
827 | language itself. | |
a0d0e21e LW |
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 | ||
5f05dabc | 838 | If you need to bless or re-bless an object you can use the following |
839 | function: | |
a0d0e21e LW |
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 | ||
d1b91892 AD |
847 | For more information on references and blessings, consult L<perlref>. |
848 | ||
54310121 | 849 | =head2 Double-Typed SVs |
0a753a76 | 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 | ||
4929bf7b | 881 | SV* sv = get_sv("dberror", TRUE); |
0a753a76 | 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 | |
a0d0e21e | 890 | |
d1b91892 AD |
891 | [This section still under construction. Ignore everything here. Post no |
892 | bills. Everything not permitted is forbidden.] | |
893 | ||
d1b91892 AD |
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 | |
5f05dabc | 896 | linked list of C<struct magic>'s, typedef'ed to C<MAGIC>. |
d1b91892 AD |
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 | SV* mg_obj; | |
905 | char* mg_ptr; | |
906 | I32 mg_len; | |
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 | ||
08105a92 | 915 | void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen); |
d1b91892 AD |
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 | |
645c22ef DM |
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. | |
d1b91892 | 926 | |
54310121 | 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 | |
af70ddd4 NC |
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. | |
d1b91892 AD |
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. | |
9b8ea7f3 | 937 | See the L<Magic Virtual Tables> section below. The C<how> argument is also |
14befaf4 | 938 | stored in the C<mg_type> field. The value of C<how> should be chosen |
9b8ea7f3 | 939 | from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before |
645c22ef | 940 | these macros were added, Perl internals used to directly use character |
14befaf4 | 941 | literals, so you may occasionally come across old code or documentation |
75d0f26d | 942 | referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example. |
d1b91892 AD |
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 | |
645c22ef | 947 | the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer, |
14befaf4 | 948 | then C<obj> is merely stored, without the reference count being incremented. |
d1b91892 | 949 | |
af70ddd4 NC |
950 | See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic |
951 | to an SV. | |
952 | ||
cb1a09d0 AD |
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 | ||
d1b91892 AD |
966 | =head2 Magic Virtual Tables |
967 | ||
d1be9408 | 968 | The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an |
d1b91892 AD |
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 pointers to the following routine types: | |
974 | ||
975 | int (*svt_get)(SV* sv, MAGIC* mg); | |
976 | int (*svt_set)(SV* sv, MAGIC* mg); | |
977 | U32 (*svt_len)(SV* sv, MAGIC* mg); | |
978 | int (*svt_clear)(SV* sv, MAGIC* mg); | |
979 | int (*svt_free)(SV* sv, MAGIC* mg); | |
980 | ||
9b8ea7f3 | 981 | This MGVTBL structure is set at compile-time in F<perl.h> and there are |
d1b91892 AD |
982 | currently 19 types (or 21 with overloading turned on). These different |
983 | structures contain pointers to various routines that perform additional | |
984 | actions depending on which function is being called. | |
985 | ||
986 | Function pointer Action taken | |
987 | ---------------- ------------ | |
8b0711c3 | 988 | svt_get Do something before the value of the SV is retrieved. |
d1b91892 AD |
989 | svt_set Do something after the SV is assigned a value. |
990 | svt_len Report on the SV's length. | |
991 | svt_clear Clear something the SV represents. | |
992 | svt_free Free any extra storage associated with the SV. | |
993 | ||
994 | For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds | |
14befaf4 | 995 | to an C<mg_type> of C<PERL_MAGIC_sv>) contains: |
d1b91892 AD |
996 | |
997 | { magic_get, magic_set, magic_len, 0, 0 } | |
998 | ||
14befaf4 DM |
999 | Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>, |
1000 | if a get operation is being performed, the routine C<magic_get> is | |
1001 | called. All the various routines for the various magical types begin | |
1002 | with C<magic_>. NOTE: the magic routines are not considered part of | |
1003 | the Perl API, and may not be exported by the Perl library. | |
d1b91892 AD |
1004 | |
1005 | The current kinds of Magic Virtual Tables are: | |
1006 | ||
14befaf4 DM |
1007 | mg_type |
1008 | (old-style char and macro) MGVTBL Type of magic | |
1009 | -------------------------- ------ ---------------------------- | |
1010 | \0 PERL_MAGIC_sv vtbl_sv Special scalar variable | |
1011 | A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash | |
1012 | a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element | |
1013 | c PERL_MAGIC_overload_table (none) Holds overload table (AMT) | |
1014 | on stash | |
1015 | B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search) | |
1016 | D PERL_MAGIC_regdata vtbl_regdata Regex match position data | |
1017 | (@+ and @- vars) | |
1018 | d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data | |
1019 | element | |
1020 | E PERL_MAGIC_env vtbl_env %ENV hash | |
1021 | e PERL_MAGIC_envelem vtbl_envelem %ENV hash element | |
1022 | f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format) | |
1023 | g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string | |
1024 | I PERL_MAGIC_isa vtbl_isa @ISA array | |
1025 | i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element | |
1026 | k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue | |
1027 | L PERL_MAGIC_dbfile (none) Debugger %_<filename | |
1028 | l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element | |
1029 | m PERL_MAGIC_mutex vtbl_mutex ??? | |
645c22ef | 1030 | o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation |
14befaf4 DM |
1031 | P PERL_MAGIC_tied vtbl_pack Tied array or hash |
1032 | p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element | |
1033 | q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle | |
1034 | r PERL_MAGIC_qr vtbl_qr precompiled qr// regex | |
1035 | S PERL_MAGIC_sig vtbl_sig %SIG hash | |
1036 | s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element | |
1037 | t PERL_MAGIC_taint vtbl_taint Taintedness | |
1038 | U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions | |
1039 | v PERL_MAGIC_vec vtbl_vec vec() lvalue | |
7d7ce6cc | 1040 | V PERL_MAGIC_vstring (none) v-string scalars |
b95e49bb | 1041 | w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache |
14befaf4 DM |
1042 | x PERL_MAGIC_substr vtbl_substr substr() lvalue |
1043 | y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator | |
1044 | variable / smart parameter | |
1045 | vivification | |
1046 | * PERL_MAGIC_glob vtbl_glob GV (typeglob) | |
1047 | # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary) | |
1048 | . PERL_MAGIC_pos vtbl_pos pos() lvalue | |
1049 | < PERL_MAGIC_backref vtbl_backref ??? | |
1050 | ~ PERL_MAGIC_ext (none) Available for use by extensions | |
d1b91892 | 1051 | |
68dc0745 | 1052 | When an uppercase and lowercase letter both exist in the table, then the |
7d7ce6cc JH |
1053 | uppercase letter is typically used to represent some kind of composite type |
1054 | (a list or a hash), and the lowercase letter is used to represent an element | |
1055 | of that composite type. Some internals code makes use of this case | |
1056 | relationship. However, 'v' and 'V' (vec and v-string) are in no way related. | |
14befaf4 DM |
1057 | |
1058 | The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined | |
1059 | specifically for use by extensions and will not be used by perl itself. | |
1060 | Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information | |
1061 | to variables (typically objects). This is especially useful because | |
1062 | there is no way for normal perl code to corrupt this private information | |
1063 | (unlike using extra elements of a hash object). | |
1064 | ||
1065 | Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a | |
1066 | C function any time a scalar's value is used or changed. The C<MAGIC>'s | |
bdbeb323 SM |
1067 | C<mg_ptr> field points to a C<ufuncs> structure: |
1068 | ||
1069 | struct ufuncs { | |
a9402793 AB |
1070 | I32 (*uf_val)(pTHX_ IV, SV*); |
1071 | I32 (*uf_set)(pTHX_ IV, SV*); | |
bdbeb323 SM |
1072 | IV uf_index; |
1073 | }; | |
1074 | ||
1075 | When the SV is read from or written to, the C<uf_val> or C<uf_set> | |
14befaf4 DM |
1076 | function will be called with C<uf_index> as the first arg and a pointer to |
1077 | the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar> | |
1526ead6 AB |
1078 | magic is shown below. Note that the ufuncs structure is copied by |
1079 | sv_magic, so you can safely allocate it on the stack. | |
1080 | ||
1081 | void | |
1082 | Umagic(sv) | |
1083 | SV *sv; | |
1084 | PREINIT: | |
1085 | struct ufuncs uf; | |
1086 | CODE: | |
1087 | uf.uf_val = &my_get_fn; | |
1088 | uf.uf_set = &my_set_fn; | |
1089 | uf.uf_index = 0; | |
14befaf4 | 1090 | sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf)); |
5f05dabc | 1091 | |
14befaf4 DM |
1092 | Note that because multiple extensions may be using C<PERL_MAGIC_ext> |
1093 | or C<PERL_MAGIC_uvar> magic, it is important for extensions to take | |
1094 | extra care to avoid conflict. Typically only using the magic on | |
1095 | objects blessed into the same class as the extension is sufficient. | |
1096 | For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32 | |
1097 | 'signature' at the top of the private data area and check that. | |
5f05dabc | 1098 | |
ef50df4b GS |
1099 | Also note that the C<sv_set*()> and C<sv_cat*()> functions described |
1100 | earlier do B<not> invoke 'set' magic on their targets. This must | |
1101 | be done by the user either by calling the C<SvSETMAGIC()> macro after | |
1102 | calling these functions, or by using one of the C<sv_set*_mg()> or | |
1103 | C<sv_cat*_mg()> functions. Similarly, generic C code must call the | |
1104 | C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV | |
1105 | obtained from external sources in functions that don't handle magic. | |
4a4eefd0 | 1106 | See L<perlapi> for a description of these functions. |
189b2af5 GS |
1107 | For example, calls to the C<sv_cat*()> functions typically need to be |
1108 | followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()> | |
1109 | since their implementation handles 'get' magic. | |
1110 | ||
d1b91892 AD |
1111 | =head2 Finding Magic |
1112 | ||
1113 | MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ | |
1114 | ||
1115 | This routine returns a pointer to the C<MAGIC> structure stored in the SV. | |
1116 | If the SV does not have that magical feature, C<NULL> is returned. Also, | |
54310121 | 1117 | if the SV is not of type SVt_PVMG, Perl may core dump. |
d1b91892 | 1118 | |
08105a92 | 1119 | int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen); |
d1b91892 AD |
1120 | |
1121 | This routine checks to see what types of magic C<sv> has. If the mg_type | |
68dc0745 | 1122 | field is an uppercase letter, then the mg_obj is copied to C<nsv>, but |
1123 | the mg_type field is changed to be the lowercase letter. | |
a0d0e21e | 1124 | |
04343c6d GS |
1125 | =head2 Understanding the Magic of Tied Hashes and Arrays |
1126 | ||
14befaf4 DM |
1127 | Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied> |
1128 | magic type. | |
9edb2b46 GS |
1129 | |
1130 | WARNING: As of the 5.004 release, proper usage of the array and hash | |
1131 | access functions requires understanding a few caveats. Some | |
1132 | of these caveats are actually considered bugs in the API, to be fixed | |
1133 | in later releases, and are bracketed with [MAYCHANGE] below. If | |
1134 | you find yourself actually applying such information in this section, be | |
1135 | aware that the behavior may change in the future, umm, without warning. | |
04343c6d | 1136 | |
1526ead6 | 1137 | The perl tie function associates a variable with an object that implements |
9a68f1db | 1138 | the various GET, SET, etc methods. To perform the equivalent of the perl |
1526ead6 AB |
1139 | tie function from an XSUB, you must mimic this behaviour. The code below |
1140 | carries out the necessary steps - firstly it creates a new hash, and then | |
1141 | creates a second hash which it blesses into the class which will implement | |
1142 | the tie methods. Lastly it ties the two hashes together, and returns a | |
1143 | reference to the new tied hash. Note that the code below does NOT call the | |
1144 | TIEHASH method in the MyTie class - | |
1145 | see L<Calling Perl Routines from within C Programs> for details on how | |
1146 | to do this. | |
1147 | ||
1148 | SV* | |
1149 | mytie() | |
1150 | PREINIT: | |
1151 | HV *hash; | |
1152 | HV *stash; | |
1153 | SV *tie; | |
1154 | CODE: | |
1155 | hash = newHV(); | |
1156 | tie = newRV_noinc((SV*)newHV()); | |
1157 | stash = gv_stashpv("MyTie", TRUE); | |
1158 | sv_bless(tie, stash); | |
899e16d0 | 1159 | hv_magic(hash, (GV*)tie, PERL_MAGIC_tied); |
1526ead6 AB |
1160 | RETVAL = newRV_noinc(hash); |
1161 | OUTPUT: | |
1162 | RETVAL | |
1163 | ||
04343c6d GS |
1164 | The C<av_store> function, when given a tied array argument, merely |
1165 | copies the magic of the array onto the value to be "stored", using | |
1166 | C<mg_copy>. It may also return NULL, indicating that the value did not | |
9edb2b46 GS |
1167 | actually need to be stored in the array. [MAYCHANGE] After a call to |
1168 | C<av_store> on a tied array, the caller will usually need to call | |
1169 | C<mg_set(val)> to actually invoke the perl level "STORE" method on the | |
1170 | TIEARRAY object. If C<av_store> did return NULL, a call to | |
1171 | C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory | |
1172 | leak. [/MAYCHANGE] | |
04343c6d GS |
1173 | |
1174 | The previous paragraph is applicable verbatim to tied hash access using the | |
1175 | C<hv_store> and C<hv_store_ent> functions as well. | |
1176 | ||
1177 | C<av_fetch> and the corresponding hash functions C<hv_fetch> and | |
1178 | C<hv_fetch_ent> actually return an undefined mortal value whose magic | |
1179 | has been initialized using C<mg_copy>. Note the value so returned does not | |
9edb2b46 GS |
1180 | need to be deallocated, as it is already mortal. [MAYCHANGE] But you will |
1181 | need to call C<mg_get()> on the returned value in order to actually invoke | |
1182 | the perl level "FETCH" method on the underlying TIE object. Similarly, | |
04343c6d GS |
1183 | you may also call C<mg_set()> on the return value after possibly assigning |
1184 | a suitable value to it using C<sv_setsv>, which will invoke the "STORE" | |
9edb2b46 | 1185 | method on the TIE object. [/MAYCHANGE] |
04343c6d | 1186 | |
9edb2b46 | 1187 | [MAYCHANGE] |
04343c6d GS |
1188 | In other words, the array or hash fetch/store functions don't really |
1189 | fetch and store actual values in the case of tied arrays and hashes. They | |
1190 | merely call C<mg_copy> to attach magic to the values that were meant to be | |
1191 | "stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually | |
1192 | do the job of invoking the TIE methods on the underlying objects. Thus | |
9edb2b46 | 1193 | the magic mechanism currently implements a kind of lazy access to arrays |
04343c6d GS |
1194 | and hashes. |
1195 | ||
1196 | Currently (as of perl version 5.004), use of the hash and array access | |
1197 | functions requires the user to be aware of whether they are operating on | |
9edb2b46 GS |
1198 | "normal" hashes and arrays, or on their tied variants. The API may be |
1199 | changed to provide more transparent access to both tied and normal data | |
1200 | types in future versions. | |
1201 | [/MAYCHANGE] | |
04343c6d GS |
1202 | |
1203 | You would do well to understand that the TIEARRAY and TIEHASH interfaces | |
1204 | are mere sugar to invoke some perl method calls while using the uniform hash | |
1205 | and array syntax. The use of this sugar imposes some overhead (typically | |
1206 | about two to four extra opcodes per FETCH/STORE operation, in addition to | |
1207 | the creation of all the mortal variables required to invoke the methods). | |
1208 | This overhead will be comparatively small if the TIE methods are themselves | |
1209 | substantial, but if they are only a few statements long, the overhead | |
1210 | will not be insignificant. | |
1211 | ||
d1c897a1 IZ |
1212 | =head2 Localizing changes |
1213 | ||
1214 | Perl has a very handy construction | |
1215 | ||
1216 | { | |
1217 | local $var = 2; | |
1218 | ... | |
1219 | } | |
1220 | ||
1221 | This construction is I<approximately> equivalent to | |
1222 | ||
1223 | { | |
1224 | my $oldvar = $var; | |
1225 | $var = 2; | |
1226 | ... | |
1227 | $var = $oldvar; | |
1228 | } | |
1229 | ||
1230 | The biggest difference is that the first construction would | |
1231 | reinstate the initial value of $var, irrespective of how control exits | |
9a68f1db | 1232 | the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit |
d1c897a1 IZ |
1233 | more efficient as well. |
1234 | ||
1235 | There is a way to achieve a similar task from C via Perl API: create a | |
1236 | I<pseudo-block>, and arrange for some changes to be automatically | |
1237 | undone at the end of it, either explicit, or via a non-local exit (via | |
1238 | die()). A I<block>-like construct is created by a pair of | |
b687b08b TC |
1239 | C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">). |
1240 | Such a construct may be created specially for some important localized | |
1241 | task, or an existing one (like boundaries of enclosing Perl | |
1242 | subroutine/block, or an existing pair for freeing TMPs) may be | |
1243 | used. (In the second case the overhead of additional localization must | |
1244 | be almost negligible.) Note that any XSUB is automatically enclosed in | |
1245 | an C<ENTER>/C<LEAVE> pair. | |
d1c897a1 IZ |
1246 | |
1247 | Inside such a I<pseudo-block> the following service is available: | |
1248 | ||
13a2d996 | 1249 | =over 4 |
d1c897a1 IZ |
1250 | |
1251 | =item C<SAVEINT(int i)> | |
1252 | ||
1253 | =item C<SAVEIV(IV i)> | |
1254 | ||
1255 | =item C<SAVEI32(I32 i)> | |
1256 | ||
1257 | =item C<SAVELONG(long i)> | |
1258 | ||
1259 | These macros arrange things to restore the value of integer variable | |
1260 | C<i> at the end of enclosing I<pseudo-block>. | |
1261 | ||
1262 | =item C<SAVESPTR(s)> | |
1263 | ||
1264 | =item C<SAVEPPTR(p)> | |
1265 | ||
1266 | These macros arrange things to restore the value of pointers C<s> and | |
1267 | C<p>. C<s> must be a pointer of a type which survives conversion to | |
1268 | C<SV*> and back, C<p> should be able to survive conversion to C<char*> | |
1269 | and back. | |
1270 | ||
1271 | =item C<SAVEFREESV(SV *sv)> | |
1272 | ||
1273 | The refcount of C<sv> would be decremented at the end of | |
26d9b02f JH |
1274 | I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a |
1275 | mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal> | |
1276 | extends the lifetime of C<sv> until the beginning of the next statement, | |
1277 | C<SAVEFREESV> extends it until the end of the enclosing scope. These | |
1278 | lifetimes can be wildly different. | |
1279 | ||
1280 | Also compare C<SAVEMORTALIZESV>. | |
1281 | ||
1282 | =item C<SAVEMORTALIZESV(SV *sv)> | |
1283 | ||
1284 | Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current | |
1285 | scope instead of decrementing its reference count. This usually has the | |
1286 | effect of keeping C<sv> alive until the statement that called the currently | |
1287 | live scope has finished executing. | |
d1c897a1 IZ |
1288 | |
1289 | =item C<SAVEFREEOP(OP *op)> | |
1290 | ||
1291 | The C<OP *> is op_free()ed at the end of I<pseudo-block>. | |
1292 | ||
1293 | =item C<SAVEFREEPV(p)> | |
1294 | ||
1295 | The chunk of memory which is pointed to by C<p> is Safefree()ed at the | |
1296 | end of I<pseudo-block>. | |
1297 | ||
1298 | =item C<SAVECLEARSV(SV *sv)> | |
1299 | ||
1300 | Clears a slot in the current scratchpad which corresponds to C<sv> at | |
1301 | the end of I<pseudo-block>. | |
1302 | ||
1303 | =item C<SAVEDELETE(HV *hv, char *key, I32 length)> | |
1304 | ||
1305 | The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The | |
1306 | string pointed to by C<key> is Safefree()ed. If one has a I<key> in | |
1307 | short-lived storage, the corresponding string may be reallocated like | |
1308 | this: | |
1309 | ||
9cde0e7f | 1310 | SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf)); |
d1c897a1 | 1311 | |
c76ac1ee | 1312 | =item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)> |
d1c897a1 IZ |
1313 | |
1314 | At the end of I<pseudo-block> the function C<f> is called with the | |
c76ac1ee GS |
1315 | only argument C<p>. |
1316 | ||
1317 | =item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)> | |
1318 | ||
1319 | At the end of I<pseudo-block> the function C<f> is called with the | |
1320 | implicit context argument (if any), and C<p>. | |
d1c897a1 IZ |
1321 | |
1322 | =item C<SAVESTACK_POS()> | |
1323 | ||
1324 | The current offset on the Perl internal stack (cf. C<SP>) is restored | |
1325 | at the end of I<pseudo-block>. | |
1326 | ||
1327 | =back | |
1328 | ||
1329 | The following API list contains functions, thus one needs to | |
1330 | provide pointers to the modifiable data explicitly (either C pointers, | |
00aadd71 | 1331 | or Perlish C<GV *>s). Where the above macros take C<int>, a similar |
d1c897a1 IZ |
1332 | function takes C<int *>. |
1333 | ||
13a2d996 | 1334 | =over 4 |
d1c897a1 IZ |
1335 | |
1336 | =item C<SV* save_scalar(GV *gv)> | |
1337 | ||
1338 | Equivalent to Perl code C<local $gv>. | |
1339 | ||
1340 | =item C<AV* save_ary(GV *gv)> | |
1341 | ||
1342 | =item C<HV* save_hash(GV *gv)> | |
1343 | ||
1344 | Similar to C<save_scalar>, but localize C<@gv> and C<%gv>. | |
1345 | ||
1346 | =item C<void save_item(SV *item)> | |
1347 | ||
1348 | Duplicates the current value of C<SV>, on the exit from the current | |
1349 | C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV> | |
1350 | using the stored value. | |
1351 | ||
1352 | =item C<void save_list(SV **sarg, I32 maxsarg)> | |
1353 | ||
1354 | A variant of C<save_item> which takes multiple arguments via an array | |
1355 | C<sarg> of C<SV*> of length C<maxsarg>. | |
1356 | ||
1357 | =item C<SV* save_svref(SV **sptr)> | |
1358 | ||
d1be9408 | 1359 | Similar to C<save_scalar>, but will reinstate an C<SV *>. |
d1c897a1 IZ |
1360 | |
1361 | =item C<void save_aptr(AV **aptr)> | |
1362 | ||
1363 | =item C<void save_hptr(HV **hptr)> | |
1364 | ||
1365 | Similar to C<save_svref>, but localize C<AV *> and C<HV *>. | |
1366 | ||
1367 | =back | |
1368 | ||
1369 | The C<Alias> module implements localization of the basic types within the | |
1370 | I<caller's scope>. People who are interested in how to localize things in | |
1371 | the containing scope should take a look there too. | |
1372 | ||
0a753a76 | 1373 | =head1 Subroutines |
a0d0e21e | 1374 | |
68dc0745 | 1375 | =head2 XSUBs and the Argument Stack |
5f05dabc | 1376 | |
1377 | The XSUB mechanism is a simple way for Perl programs to access C subroutines. | |
1378 | An XSUB routine will have a stack that contains the arguments from the Perl | |
1379 | program, and a way to map from the Perl data structures to a C equivalent. | |
1380 | ||
1381 | The stack arguments are accessible through the C<ST(n)> macro, which returns | |
1382 | the C<n>'th stack argument. Argument 0 is the first argument passed in the | |
1383 | Perl subroutine call. These arguments are C<SV*>, and can be used anywhere | |
1384 | an C<SV*> is used. | |
1385 | ||
1386 | Most of the time, output from the C routine can be handled through use of | |
1387 | the RETVAL and OUTPUT directives. However, there are some cases where the | |
1388 | argument stack is not already long enough to handle all the return values. | |
1389 | An example is the POSIX tzname() call, which takes no arguments, but returns | |
1390 | two, the local time zone's standard and summer time abbreviations. | |
1391 | ||
1392 | To handle this situation, the PPCODE directive is used and the stack is | |
1393 | extended using the macro: | |
1394 | ||
924508f0 | 1395 | EXTEND(SP, num); |
5f05dabc | 1396 | |
924508f0 GS |
1397 | where C<SP> is the macro that represents the local copy of the stack pointer, |
1398 | and C<num> is the number of elements the stack should be extended by. | |
5f05dabc | 1399 | |
00aadd71 | 1400 | Now that there is room on the stack, values can be pushed on it using C<PUSHs> |
9b8ea7f3 | 1401 | macro. The pushed values will often need to be "mortal" (See |
fb7377f8 | 1402 | L</Reference Counts and Mortality>): |
5f05dabc | 1403 | |
00aadd71 | 1404 | PUSHs(sv_2mortal(newSViv(an_integer))) |
fb7377f8 NC |
1405 | PUSHs(sv_2mortal(newSVuv(an_unsigned_integer))) |
1406 | PUSHs(sv_2mortal(newSVnv(a_double))) | |
00aadd71 | 1407 | PUSHs(sv_2mortal(newSVpv("Some String",0))) |
5f05dabc | 1408 | |
1409 | And now the Perl program calling C<tzname>, the two values will be assigned | |
1410 | as in: | |
1411 | ||
1412 | ($standard_abbrev, $summer_abbrev) = POSIX::tzname; | |
1413 | ||
1414 | An alternate (and possibly simpler) method to pushing values on the stack is | |
00aadd71 | 1415 | to use the macro: |
5f05dabc | 1416 | |
5f05dabc | 1417 | XPUSHs(SV*) |
1418 | ||
00aadd71 | 1419 | This macro automatically adjust the stack for you, if needed. Thus, you |
5f05dabc | 1420 | do not need to call C<EXTEND> to extend the stack. |
00aadd71 NIS |
1421 | |
1422 | Despite their suggestions in earlier versions of this document the macros | |
fb7377f8 NC |
1423 | C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results. |
1424 | For that, either stick to the C<(X)PUSHs> macros shown above, or use the new | |
1425 | C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>. | |
5f05dabc | 1426 | |
1427 | For more information, consult L<perlxs> and L<perlxstut>. | |
1428 | ||
1429 | =head2 Calling Perl Routines from within C Programs | |
a0d0e21e LW |
1430 | |
1431 | There are four routines that can be used to call a Perl subroutine from | |
1432 | within a C program. These four are: | |
1433 | ||
954c1994 GS |
1434 | I32 call_sv(SV*, I32); |
1435 | I32 call_pv(const char*, I32); | |
1436 | I32 call_method(const char*, I32); | |
1437 | I32 call_argv(const char*, I32, register char**); | |
a0d0e21e | 1438 | |
954c1994 | 1439 | The routine most often used is C<call_sv>. The C<SV*> argument |
d1b91892 AD |
1440 | contains either the name of the Perl subroutine to be called, or a |
1441 | reference to the subroutine. The second argument consists of flags | |
1442 | that control the context in which the subroutine is called, whether | |
1443 | or not the subroutine is being passed arguments, how errors should be | |
1444 | trapped, and how to treat return values. | |
a0d0e21e LW |
1445 | |
1446 | All four routines return the number of arguments that the subroutine returned | |
1447 | on the Perl stack. | |
1448 | ||
9a68f1db | 1449 | These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0, |
954c1994 GS |
1450 | but those names are now deprecated; macros of the same name are provided for |
1451 | compatibility. | |
1452 | ||
1453 | When using any of these routines (except C<call_argv>), the programmer | |
d1b91892 AD |
1454 | must manipulate the Perl stack. These include the following macros and |
1455 | functions: | |
a0d0e21e LW |
1456 | |
1457 | dSP | |
924508f0 | 1458 | SP |
a0d0e21e LW |
1459 | PUSHMARK() |
1460 | PUTBACK | |
1461 | SPAGAIN | |
1462 | ENTER | |
1463 | SAVETMPS | |
1464 | FREETMPS | |
1465 | LEAVE | |
1466 | XPUSH*() | |
cb1a09d0 | 1467 | POP*() |
a0d0e21e | 1468 | |
5f05dabc | 1469 | For a detailed description of calling conventions from C to Perl, |
1470 | consult L<perlcall>. | |
a0d0e21e | 1471 | |
5f05dabc | 1472 | =head2 Memory Allocation |
a0d0e21e | 1473 | |
9b8ea7f3 JH |
1474 | =head3 Allocation |
1475 | ||
86058a2d GS |
1476 | All memory meant to be used with the Perl API functions should be manipulated |
1477 | using the macros described in this section. The macros provide the necessary | |
1478 | transparency between differences in the actual malloc implementation that is | |
1479 | used within perl. | |
1480 | ||
1481 | It is suggested that you enable the version of malloc that is distributed | |
5f05dabc | 1482 | with Perl. It keeps pools of various sizes of unallocated memory in |
07fa94a1 JO |
1483 | order to satisfy allocation requests more quickly. However, on some |
1484 | platforms, it may cause spurious malloc or free errors. | |
d1b91892 | 1485 | |
9b8ea7f3 JH |
1486 | The following three macros are used to initially allocate memory : |
1487 | ||
cd7a8267 JC |
1488 | Newx(pointer, number, type); |
1489 | Newxc(pointer, number, type, cast); | |
1490 | Newxz(pointer, number, type); | |
d1b91892 | 1491 | |
cd7a8267 | 1492 | The first argument C<pointer> should be the name of a variable that will |
5f05dabc | 1493 | point to the newly allocated memory. |
d1b91892 | 1494 | |
cd7a8267 | 1495 | The second and third arguments C<number> and C<type> specify how many of |
d1b91892 | 1496 | the specified type of data structure should be allocated. The argument |
cd7a8267 | 1497 | C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>, |
d1b91892 AD |
1498 | should be used if the C<pointer> argument is different from the C<type> |
1499 | argument. | |
1500 | ||
cd7a8267 | 1501 | Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero> |
d1b91892 AD |
1502 | to zero out all the newly allocated memory. |
1503 | ||
9b8ea7f3 JH |
1504 | =head3 Reallocation |
1505 | ||
d1b91892 AD |
1506 | Renew(pointer, number, type); |
1507 | Renewc(pointer, number, type, cast); | |
1508 | Safefree(pointer) | |
1509 | ||
1510 | These three macros are used to change a memory buffer size or to free a | |
1511 | piece of memory no longer needed. The arguments to C<Renew> and C<Renewc> | |
1512 | match those of C<New> and C<Newc> with the exception of not needing the | |
1513 | "magic cookie" argument. | |
1514 | ||
9b8ea7f3 JH |
1515 | =head3 Moving |
1516 | ||
d1b91892 AD |
1517 | Move(source, dest, number, type); |
1518 | Copy(source, dest, number, type); | |
1519 | Zero(dest, number, type); | |
1520 | ||
1521 | These three macros are used to move, copy, or zero out previously allocated | |
1522 | memory. The C<source> and C<dest> arguments point to the source and | |
1523 | destination starting points. Perl will move, copy, or zero out C<number> | |
1524 | instances of the size of the C<type> data structure (using the C<sizeof> | |
1525 | function). | |
a0d0e21e | 1526 | |
5f05dabc | 1527 | =head2 PerlIO |
ce3d39e2 | 1528 | |
5f05dabc | 1529 | The most recent development releases of Perl has been experimenting with |
1530 | removing Perl's dependency on the "normal" standard I/O suite and allowing | |
1531 | other stdio implementations to be used. This involves creating a new | |
1532 | abstraction layer that then calls whichever implementation of stdio Perl | |
68dc0745 | 1533 | was compiled with. All XSUBs should now use the functions in the PerlIO |
5f05dabc | 1534 | abstraction layer and not make any assumptions about what kind of stdio |
1535 | is being used. | |
1536 | ||
1537 | For a complete description of the PerlIO abstraction, consult L<perlapio>. | |
1538 | ||
8ebc5c01 | 1539 | =head2 Putting a C value on Perl stack |
ce3d39e2 IZ |
1540 | |
1541 | A lot of opcodes (this is an elementary operation in the internal perl | |
1542 | stack machine) put an SV* on the stack. However, as an optimization | |
1543 | the corresponding SV is (usually) not recreated each time. The opcodes | |
1544 | reuse specially assigned SVs (I<target>s) which are (as a corollary) | |
1545 | not constantly freed/created. | |
1546 | ||
0a753a76 | 1547 | Each of the targets is created only once (but see |
ce3d39e2 IZ |
1548 | L<Scratchpads and recursion> below), and when an opcode needs to put |
1549 | an integer, a double, or a string on stack, it just sets the | |
1550 | corresponding parts of its I<target> and puts the I<target> on stack. | |
1551 | ||
1552 | The macro to put this target on stack is C<PUSHTARG>, and it is | |
1553 | directly used in some opcodes, as well as indirectly in zillions of | |
fb7377f8 | 1554 | others, which use it via C<(X)PUSH[iunp]>. |
ce3d39e2 | 1555 | |
1bd1c0d5 SC |
1556 | Because the target is reused, you must be careful when pushing multiple |
1557 | values on the stack. The following code will not do what you think: | |
1558 | ||
1559 | XPUSHi(10); | |
1560 | XPUSHi(20); | |
1561 | ||
1562 | This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto | |
1563 | the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack". | |
1564 | At the end of the operation, the stack does not contain the values 10 | |
1565 | and 20, but actually contains two pointers to C<TARG>, which we have set | |
fb7377f8 | 1566 | to 20. |
1bd1c0d5 | 1567 | |
fb7377f8 NC |
1568 | If you need to push multiple different values then you should either use |
1569 | the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros, | |
1570 | none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an | |
1571 | SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>, | |
1572 | will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make | |
1573 | this a little easier to achieve by creating a new mortal for you (via | |
1574 | C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary | |
1575 | in the case of the C<mXPUSH[iunp]> macros), and then setting its value. | |
1576 | Thus, instead of writing this to "fix" the example above: | |
1577 | ||
1578 | XPUSHs(sv_2mortal(newSViv(10))) | |
1579 | XPUSHs(sv_2mortal(newSViv(20))) | |
1580 | ||
1581 | you can simply write: | |
1582 | ||
1583 | mXPUSHi(10) | |
1584 | mXPUSHi(20) | |
1585 | ||
1586 | On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to | |
1bd1c0d5 | 1587 | need a C<dTARG> in your variable declarations so that the C<*PUSH*> |
fb7377f8 NC |
1588 | macros can make use of the local variable C<TARG>. See also C<dTARGET> |
1589 | and C<dXSTARG>. | |
1bd1c0d5 | 1590 | |
8ebc5c01 | 1591 | =head2 Scratchpads |
ce3d39e2 | 1592 | |
54310121 | 1593 | The question remains on when the SVs which are I<target>s for opcodes |
5f05dabc | 1594 | are created. The answer is that they are created when the current unit -- |
1595 | a subroutine or a file (for opcodes for statements outside of | |
1596 | subroutines) -- is compiled. During this time a special anonymous Perl | |
ce3d39e2 IZ |
1597 | array is created, which is called a scratchpad for the current |
1598 | unit. | |
1599 | ||
54310121 | 1600 | A scratchpad keeps SVs which are lexicals for the current unit and are |
ce3d39e2 IZ |
1601 | targets for opcodes. One can deduce that an SV lives on a scratchpad |
1602 | by looking on its flags: lexicals have C<SVs_PADMY> set, and | |
1603 | I<target>s have C<SVs_PADTMP> set. | |
1604 | ||
54310121 | 1605 | The correspondence between OPs and I<target>s is not 1-to-1. Different |
1606 | OPs in the compile tree of the unit can use the same target, if this | |
ce3d39e2 IZ |
1607 | would not conflict with the expected life of the temporary. |
1608 | ||
2ae324a7 | 1609 | =head2 Scratchpads and recursion |
ce3d39e2 IZ |
1610 | |
1611 | In fact it is not 100% true that a compiled unit contains a pointer to | |
1612 | the scratchpad AV. In fact it contains a pointer to an AV of | |
1613 | (initially) one element, and this element is the scratchpad AV. Why do | |
1614 | we need an extra level of indirection? | |
1615 | ||
9a68f1db | 1616 | The answer is B<recursion>, and maybe B<threads>. Both |
ce3d39e2 IZ |
1617 | these can create several execution pointers going into the same |
1618 | subroutine. For the subroutine-child not write over the temporaries | |
1619 | for the subroutine-parent (lifespan of which covers the call to the | |
1620 | child), the parent and the child should have different | |
1621 | scratchpads. (I<And> the lexicals should be separate anyway!) | |
1622 | ||
5f05dabc | 1623 | So each subroutine is born with an array of scratchpads (of length 1). |
1624 | On each entry to the subroutine it is checked that the current | |
ce3d39e2 IZ |
1625 | depth of the recursion is not more than the length of this array, and |
1626 | if it is, new scratchpad is created and pushed into the array. | |
1627 | ||
1628 | The I<target>s on this scratchpad are C<undef>s, but they are already | |
1629 | marked with correct flags. | |
1630 | ||
0a753a76 | 1631 | =head1 Compiled code |
1632 | ||
1633 | =head2 Code tree | |
1634 | ||
1635 | Here we describe the internal form your code is converted to by | |
1636 | Perl. Start with a simple example: | |
1637 | ||
1638 | $a = $b + $c; | |
1639 | ||
1640 | This is converted to a tree similar to this one: | |
1641 | ||
1642 | assign-to | |
1643 | / \ | |
1644 | + $a | |
1645 | / \ | |
1646 | $b $c | |
1647 | ||
7b8d334a | 1648 | (but slightly more complicated). This tree reflects the way Perl |
0a753a76 | 1649 | parsed your code, but has nothing to do with the execution order. |
1650 | There is an additional "thread" going through the nodes of the tree | |
1651 | which shows the order of execution of the nodes. In our simplified | |
1652 | example above it looks like: | |
1653 | ||
1654 | $b ---> $c ---> + ---> $a ---> assign-to | |
1655 | ||
1656 | But with the actual compile tree for C<$a = $b + $c> it is different: | |
1657 | some nodes I<optimized away>. As a corollary, though the actual tree | |
1658 | contains more nodes than our simplified example, the execution order | |
1659 | is the same as in our example. | |
1660 | ||
1661 | =head2 Examining the tree | |
1662 | ||
9b8ea7f3 JH |
1663 | If you have your perl compiled for debugging (usually done with |
1664 | C<-DDEBUGGING> on the C<Configure> command line), you may examine the | |
0a753a76 | 1665 | compiled tree by specifying C<-Dx> on the Perl command line. The |
1666 | output takes several lines per node, and for C<$b+$c> it looks like | |
1667 | this: | |
1668 | ||
1669 | 5 TYPE = add ===> 6 | |
1670 | TARG = 1 | |
1671 | FLAGS = (SCALAR,KIDS) | |
1672 | { | |
1673 | TYPE = null ===> (4) | |
1674 | (was rv2sv) | |
1675 | FLAGS = (SCALAR,KIDS) | |
1676 | { | |
1677 | 3 TYPE = gvsv ===> 4 | |
1678 | FLAGS = (SCALAR) | |
1679 | GV = main::b | |
1680 | } | |
1681 | } | |
1682 | { | |
1683 | TYPE = null ===> (5) | |
1684 | (was rv2sv) | |
1685 | FLAGS = (SCALAR,KIDS) | |
1686 | { | |
1687 | 4 TYPE = gvsv ===> 5 | |
1688 | FLAGS = (SCALAR) | |
1689 | GV = main::c | |
1690 | } | |
1691 | } | |
1692 | ||
1693 | This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are | |
1694 | not optimized away (one per number in the left column). The immediate | |
1695 | children of the given node correspond to C<{}> pairs on the same level | |
1696 | of indentation, thus this listing corresponds to the tree: | |
1697 | ||
1698 | add | |
1699 | / \ | |
1700 | null null | |
1701 | | | | |
1702 | gvsv gvsv | |
1703 | ||
1704 | The execution order is indicated by C<===E<gt>> marks, thus it is C<3 | |
1705 | 4 5 6> (node C<6> is not included into above listing), i.e., | |
1706 | C<gvsv gvsv add whatever>. | |
1707 | ||
9afa14e3 SC |
1708 | Each of these nodes represents an op, a fundamental operation inside the |
1709 | Perl core. The code which implements each operation can be found in the | |
1710 | F<pp*.c> files; the function which implements the op with type C<gvsv> | |
1711 | is C<pp_gvsv>, and so on. As the tree above shows, different ops have | |
1712 | different numbers of children: C<add> is a binary operator, as one would | |
1713 | expect, and so has two children. To accommodate the various different | |
1714 | numbers of children, there are various types of op data structure, and | |
1715 | they link together in different ways. | |
1716 | ||
1717 | The simplest type of op structure is C<OP>: this has no children. Unary | |
1718 | operators, C<UNOP>s, have one child, and this is pointed to by the | |
1719 | C<op_first> field. Binary operators (C<BINOP>s) have not only an | |
1720 | C<op_first> field but also an C<op_last> field. The most complex type of | |
1721 | op is a C<LISTOP>, which has any number of children. In this case, the | |
1722 | first child is pointed to by C<op_first> and the last child by | |
1723 | C<op_last>. The children in between can be found by iteratively | |
1724 | following the C<op_sibling> pointer from the first child to the last. | |
1725 | ||
1726 | There are also two other op types: a C<PMOP> holds a regular expression, | |
1727 | and has no children, and a C<LOOP> may or may not have children. If the | |
1728 | C<op_children> field is non-zero, it behaves like a C<LISTOP>. To | |
1729 | complicate matters, if a C<UNOP> is actually a C<null> op after | |
1730 | optimization (see L</Compile pass 2: context propagation>) it will still | |
1731 | have children in accordance with its former type. | |
1732 | ||
9b8ea7f3 JH |
1733 | Another way to examine the tree is to use a compiler back-end module, such |
1734 | as L<B::Concise>. | |
1735 | ||
0a753a76 | 1736 | =head2 Compile pass 1: check routines |
1737 | ||
8870b5c7 GS |
1738 | The tree is created by the compiler while I<yacc> code feeds it |
1739 | the constructions it recognizes. Since I<yacc> works bottom-up, so does | |
0a753a76 | 1740 | the first pass of perl compilation. |
1741 | ||
1742 | What makes this pass interesting for perl developers is that some | |
1743 | optimization may be performed on this pass. This is optimization by | |
8870b5c7 | 1744 | so-called "check routines". The correspondence between node names |
0a753a76 | 1745 | and corresponding check routines is described in F<opcode.pl> (do not |
1746 | forget to run C<make regen_headers> if you modify this file). | |
1747 | ||
1748 | A check routine is called when the node is fully constructed except | |
7b8d334a | 1749 | for the execution-order thread. Since at this time there are no |
0a753a76 | 1750 | back-links to the currently constructed node, one can do most any |
1751 | operation to the top-level node, including freeing it and/or creating | |
1752 | new nodes above/below it. | |
1753 | ||
1754 | The check routine returns the node which should be inserted into the | |
1755 | tree (if the top-level node was not modified, check routine returns | |
1756 | its argument). | |
1757 | ||
1758 | By convention, check routines have names C<ck_*>. They are usually | |
1759 | called from C<new*OP> subroutines (or C<convert>) (which in turn are | |
1760 | called from F<perly.y>). | |
1761 | ||
1762 | =head2 Compile pass 1a: constant folding | |
1763 | ||
1764 | Immediately after the check routine is called the returned node is | |
1765 | checked for being compile-time executable. If it is (the value is | |
1766 | judged to be constant) it is immediately executed, and a I<constant> | |
1767 | node with the "return value" of the corresponding subtree is | |
1768 | substituted instead. The subtree is deleted. | |
1769 | ||
1770 | If constant folding was not performed, the execution-order thread is | |
1771 | created. | |
1772 | ||
1773 | =head2 Compile pass 2: context propagation | |
1774 | ||
1775 | When a context for a part of compile tree is known, it is propagated | |
a3cb178b | 1776 | down through the tree. At this time the context can have 5 values |
0a753a76 | 1777 | (instead of 2 for runtime context): void, boolean, scalar, list, and |
1778 | lvalue. In contrast with the pass 1 this pass is processed from top | |
1779 | to bottom: a node's context determines the context for its children. | |
1780 | ||
1781 | Additional context-dependent optimizations are performed at this time. | |
1782 | Since at this moment the compile tree contains back-references (via | |
1783 | "thread" pointers), nodes cannot be free()d now. To allow | |
1784 | optimized-away nodes at this stage, such nodes are null()ified instead | |
1785 | of free()ing (i.e. their type is changed to OP_NULL). | |
1786 | ||
1787 | =head2 Compile pass 3: peephole optimization | |
1788 | ||
1789 | After the compile tree for a subroutine (or for an C<eval> or a file) | |
1790 | is created, an additional pass over the code is performed. This pass | |
1791 | is neither top-down or bottom-up, but in the execution order (with | |
7b8d334a | 1792 | additional complications for conditionals). These optimizations are |
0a753a76 | 1793 | done in the subroutine peep(). Optimizations performed at this stage |
1794 | are subject to the same restrictions as in the pass 2. | |
1795 | ||
1ba7f851 PJ |
1796 | =head2 Pluggable runops |
1797 | ||
1798 | The compile tree is executed in a runops function. There are two runops | |
d36fa37b NC |
1799 | functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used |
1800 | with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine | |
1801 | control over the execution of the compile tree it is possible to provide | |
1802 | your own runops function. | |
1ba7f851 PJ |
1803 | |
1804 | It's probably best to copy one of the existing runops functions and | |
1805 | change it to suit your needs. Then, in the BOOT section of your XS | |
1806 | file, add the line: | |
1807 | ||
1808 | PL_runops = my_runops; | |
1809 | ||
1810 | This function should be as efficient as possible to keep your programs | |
1811 | running as fast as possible. | |
1812 | ||
9afa14e3 SC |
1813 | =head1 Examining internal data structures with the C<dump> functions |
1814 | ||
1815 | To aid debugging, the source file F<dump.c> contains a number of | |
1816 | functions which produce formatted output of internal data structures. | |
1817 | ||
1818 | The most commonly used of these functions is C<Perl_sv_dump>; it's used | |
1819 | for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls | |
1820 | C<sv_dump> to produce debugging output from Perl-space, so users of that | |
00aadd71 | 1821 | module should already be familiar with its format. |
9afa14e3 SC |
1822 | |
1823 | C<Perl_op_dump> can be used to dump an C<OP> structure or any of its | |
210b36aa | 1824 | derivatives, and produces output similar to C<perl -Dx>; in fact, |
9afa14e3 SC |
1825 | C<Perl_dump_eval> will dump the main root of the code being evaluated, |
1826 | exactly like C<-Dx>. | |
1827 | ||
1828 | Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an | |
1829 | op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the | |
1830 | subroutines in a package like so: (Thankfully, these are all xsubs, so | |
1831 | there is no op tree) | |
1832 | ||
1833 | (gdb) print Perl_dump_packsubs(PL_defstash) | |
1834 | ||
1835 | SUB attributes::bootstrap = (xsub 0x811fedc 0) | |
1836 | ||
1837 | SUB UNIVERSAL::can = (xsub 0x811f50c 0) | |
1838 | ||
1839 | SUB UNIVERSAL::isa = (xsub 0x811f304 0) | |
1840 | ||
1841 | SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0) | |
1842 | ||
1843 | SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0) | |
1844 | ||
1845 | and C<Perl_dump_all>, which dumps all the subroutines in the stash and | |
1846 | the op tree of the main root. | |
1847 | ||
954c1994 | 1848 | =head1 How multiple interpreters and concurrency are supported |
ee072b34 | 1849 | |
ee072b34 GS |
1850 | =head2 Background and PERL_IMPLICIT_CONTEXT |
1851 | ||
1852 | The Perl interpreter can be regarded as a closed box: it has an API | |
1853 | for feeding it code or otherwise making it do things, but it also has | |
1854 | functions for its own use. This smells a lot like an object, and | |
1855 | there are ways for you to build Perl so that you can have multiple | |
acfe0abc GS |
1856 | interpreters, with one interpreter represented either as a C structure, |
1857 | or inside a thread-specific structure. These structures contain all | |
1858 | the context, the state of that interpreter. | |
1859 | ||
9a68f1db | 1860 | Two macros control the major Perl build flavors: MULTIPLICITY and |
acfe0abc GS |
1861 | USE_5005THREADS. The MULTIPLICITY build has a C structure |
1862 | that packages all the interpreter state, and there is a similar thread-specific | |
1863 | data structure under USE_5005THREADS. In both cases, | |
54aff467 GS |
1864 | PERL_IMPLICIT_CONTEXT is also normally defined, and enables the |
1865 | support for passing in a "hidden" first argument that represents all three | |
651a3225 | 1866 | data structures. |
54aff467 GS |
1867 | |
1868 | All this obviously requires a way for the Perl internal functions to be | |
acfe0abc | 1869 | either subroutines taking some kind of structure as the first |
ee072b34 | 1870 | argument, or subroutines taking nothing as the first argument. To |
acfe0abc | 1871 | enable these two very different ways of building the interpreter, |
ee072b34 GS |
1872 | the Perl source (as it does in so many other situations) makes heavy |
1873 | use of macros and subroutine naming conventions. | |
1874 | ||
54aff467 | 1875 | First problem: deciding which functions will be public API functions and |
00aadd71 | 1876 | which will be private. All functions whose names begin C<S_> are private |
954c1994 GS |
1877 | (think "S" for "secret" or "static"). All other functions begin with |
1878 | "Perl_", but just because a function begins with "Perl_" does not mean it is | |
00aadd71 NIS |
1879 | part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a |
1880 | function is part of the API is to find its entry in L<perlapi>. | |
1881 | If it exists in L<perlapi>, it's part of the API. If it doesn't, and you | |
1882 | think it should be (i.e., you need it for your extension), send mail via | |
a422fd2d | 1883 | L<perlbug> explaining why you think it should be. |
ee072b34 GS |
1884 | |
1885 | Second problem: there must be a syntax so that the same subroutine | |
1886 | declarations and calls can pass a structure as their first argument, | |
1887 | or pass nothing. To solve this, the subroutines are named and | |
1888 | declared in a particular way. Here's a typical start of a static | |
1889 | function used within the Perl guts: | |
1890 | ||
1891 | STATIC void | |
1892 | S_incline(pTHX_ char *s) | |
1893 | ||
acfe0abc GS |
1894 | STATIC becomes "static" in C, and may be #define'd to nothing in some |
1895 | configurations in future. | |
ee072b34 | 1896 | |
651a3225 GS |
1897 | A public function (i.e. part of the internal API, but not necessarily |
1898 | sanctioned for use in extensions) begins like this: | |
ee072b34 GS |
1899 | |
1900 | void | |
68da2b4b | 1901 | Perl_sv_setiv(pTHX_ SV* dsv, IV num) |
ee072b34 GS |
1902 | |
1903 | C<pTHX_> is one of a number of macros (in perl.h) that hide the | |
1904 | details of the interpreter's context. THX stands for "thread", "this", | |
1905 | or "thingy", as the case may be. (And no, George Lucas is not involved. :-) | |
1906 | The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument, | |
a7486cbb JH |
1907 | or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and |
1908 | their variants. | |
ee072b34 | 1909 | |
a7486cbb JH |
1910 | When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no |
1911 | first argument containing the interpreter's context. The trailing underscore | |
ee072b34 GS |
1912 | in the pTHX_ macro indicates that the macro expansion needs a comma |
1913 | after the context argument because other arguments follow it. If | |
1914 | PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the | |
54aff467 GS |
1915 | subroutine is not prototyped to take the extra argument. The form of the |
1916 | macro without the trailing underscore is used when there are no additional | |
ee072b34 GS |
1917 | explicit arguments. |
1918 | ||
54aff467 | 1919 | When a core function calls another, it must pass the context. This |
68da2b4b | 1920 | is normally hidden via macros. Consider C<sv_setiv>. It expands into |
ee072b34 GS |
1921 | something like this: |
1922 | ||
68da2b4b JH |
1923 | #ifdef PERL_IMPLICIT_CONTEXT |
1924 | #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b) | |
ee072b34 | 1925 | /* can't do this for vararg functions, see below */ |
68da2b4b JH |
1926 | #else |
1927 | #define sv_setiv Perl_sv_setiv | |
1928 | #endif | |
ee072b34 GS |
1929 | |
1930 | This works well, and means that XS authors can gleefully write: | |
1931 | ||
68da2b4b | 1932 | sv_setiv(foo, bar); |
ee072b34 GS |
1933 | |
1934 | and still have it work under all the modes Perl could have been | |
1935 | compiled with. | |
1936 | ||
ee072b34 GS |
1937 | This doesn't work so cleanly for varargs functions, though, as macros |
1938 | imply that the number of arguments is known in advance. Instead we | |
1939 | either need to spell them out fully, passing C<aTHX_> as the first | |
1940 | argument (the Perl core tends to do this with functions like | |
1941 | Perl_warner), or use a context-free version. | |
1942 | ||
1943 | The context-free version of Perl_warner is called | |
1944 | Perl_warner_nocontext, and does not take the extra argument. Instead | |
1945 | it does dTHX; to get the context from thread-local storage. We | |
1946 | C<#define warner Perl_warner_nocontext> so that extensions get source | |
1947 | compatibility at the expense of performance. (Passing an arg is | |
1948 | cheaper than grabbing it from thread-local storage.) | |
1949 | ||
acfe0abc | 1950 | You can ignore [pad]THXx when browsing the Perl headers/sources. |
ee072b34 GS |
1951 | Those are strictly for use within the core. Extensions and embedders |
1952 | need only be aware of [pad]THX. | |
1953 | ||
a7486cbb JH |
1954 | =head2 So what happened to dTHR? |
1955 | ||
1956 | C<dTHR> was introduced in perl 5.005 to support the older thread model. | |
1957 | The older thread model now uses the C<THX> mechanism to pass context | |
1958 | pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and | |
1959 | later still have it for backward source compatibility, but it is defined | |
1960 | to be a no-op. | |
1961 | ||
ee072b34 GS |
1962 | =head2 How do I use all this in extensions? |
1963 | ||
1964 | When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call | |
1965 | any functions in the Perl API will need to pass the initial context | |
1966 | argument somehow. The kicker is that you will need to write it in | |
1967 | such a way that the extension still compiles when Perl hasn't been | |
1968 | built with PERL_IMPLICIT_CONTEXT enabled. | |
1969 | ||
1970 | There are three ways to do this. First, the easy but inefficient way, | |
1971 | which is also the default, in order to maintain source compatibility | |
1972 | with extensions: whenever XSUB.h is #included, it redefines the aTHX | |
1973 | and aTHX_ macros to call a function that will return the context. | |
1974 | Thus, something like: | |
1975 | ||
68da2b4b | 1976 | sv_setiv(sv, num); |
ee072b34 | 1977 | |
4375e838 | 1978 | in your extension will translate to this when PERL_IMPLICIT_CONTEXT is |
54aff467 | 1979 | in effect: |
ee072b34 | 1980 | |
68da2b4b | 1981 | Perl_sv_setiv(Perl_get_context(), sv, num); |
ee072b34 | 1982 | |
54aff467 | 1983 | or to this otherwise: |
ee072b34 | 1984 | |
68da2b4b | 1985 | Perl_sv_setiv(sv, num); |
ee072b34 GS |
1986 | |
1987 | You have to do nothing new in your extension to get this; since | |
2fa86c13 | 1988 | the Perl library provides Perl_get_context(), it will all just |
ee072b34 GS |
1989 | work. |
1990 | ||
1991 | The second, more efficient way is to use the following template for | |
1992 | your Foo.xs: | |
1993 | ||
c52f9dcd JH |
1994 | #define PERL_NO_GET_CONTEXT /* we want efficiency */ |
1995 | #include "EXTERN.h" | |
1996 | #include "perl.h" | |
1997 | #include "XSUB.h" | |
ee072b34 GS |
1998 | |
1999 | static my_private_function(int arg1, int arg2); | |
2000 | ||
c52f9dcd JH |
2001 | static SV * |
2002 | my_private_function(int arg1, int arg2) | |
2003 | { | |
2004 | dTHX; /* fetch context */ | |
2005 | ... call many Perl API functions ... | |
2006 | } | |
ee072b34 GS |
2007 | |
2008 | [... etc ...] | |
2009 | ||
c52f9dcd | 2010 | MODULE = Foo PACKAGE = Foo |
ee072b34 | 2011 | |
c52f9dcd | 2012 | /* typical XSUB */ |
ee072b34 | 2013 | |
c52f9dcd JH |
2014 | void |
2015 | my_xsub(arg) | |
2016 | int arg | |
2017 | CODE: | |
2018 | my_private_function(arg, 10); | |
ee072b34 GS |
2019 | |
2020 | Note that the only two changes from the normal way of writing an | |
2021 | extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before | |
2022 | including the Perl headers, followed by a C<dTHX;> declaration at | |
2023 | the start of every function that will call the Perl API. (You'll | |
2024 | know which functions need this, because the C compiler will complain | |
2025 | that there's an undeclared identifier in those functions.) No changes | |
2026 | are needed for the XSUBs themselves, because the XS() macro is | |
2027 | correctly defined to pass in the implicit context if needed. | |
2028 | ||
2029 | The third, even more efficient way is to ape how it is done within | |
2030 | the Perl guts: | |
2031 | ||
2032 | ||
c52f9dcd JH |
2033 | #define PERL_NO_GET_CONTEXT /* we want efficiency */ |
2034 | #include "EXTERN.h" | |
2035 | #include "perl.h" | |
2036 | #include "XSUB.h" | |
ee072b34 GS |
2037 | |
2038 | /* pTHX_ only needed for functions that call Perl API */ | |
2039 | static my_private_function(pTHX_ int arg1, int arg2); | |
2040 | ||
c52f9dcd JH |
2041 | static SV * |
2042 | my_private_function(pTHX_ int arg1, int arg2) | |
2043 | { | |
2044 | /* dTHX; not needed here, because THX is an argument */ | |
2045 | ... call Perl API functions ... | |
2046 | } | |
ee072b34 GS |
2047 | |
2048 | [... etc ...] | |
2049 | ||
c52f9dcd | 2050 | MODULE = Foo PACKAGE = Foo |
ee072b34 | 2051 | |
c52f9dcd | 2052 | /* typical XSUB */ |
ee072b34 | 2053 | |
c52f9dcd JH |
2054 | void |
2055 | my_xsub(arg) | |
2056 | int arg | |
2057 | CODE: | |
2058 | my_private_function(aTHX_ arg, 10); | |
ee072b34 GS |
2059 | |
2060 | This implementation never has to fetch the context using a function | |
2061 | call, since it is always passed as an extra argument. Depending on | |
2062 | your needs for simplicity or efficiency, you may mix the previous | |
2063 | two approaches freely. | |
2064 | ||
651a3225 GS |
2065 | Never add a comma after C<pTHX> yourself--always use the form of the |
2066 | macro with the underscore for functions that take explicit arguments, | |
2067 | or the form without the argument for functions with no explicit arguments. | |
ee072b34 | 2068 | |
a7486cbb JH |
2069 | =head2 Should I do anything special if I call perl from multiple threads? |
2070 | ||
2071 | If you create interpreters in one thread and then proceed to call them in | |
2072 | another, you need to make sure perl's own Thread Local Storage (TLS) slot is | |
2073 | initialized correctly in each of those threads. | |
2074 | ||
2075 | The C<perl_alloc> and C<perl_clone> API functions will automatically set | |
2076 | the TLS slot to the interpreter they created, so that there is no need to do | |
2077 | anything special if the interpreter is always accessed in the same thread that | |
2078 | created it, and that thread did not create or call any other interpreters | |
2079 | afterwards. If that is not the case, you have to set the TLS slot of the | |
2080 | thread before calling any functions in the Perl API on that particular | |
2081 | interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that | |
2082 | thread as the first thing you do: | |
2083 | ||
2084 | /* do this before doing anything else with some_perl */ | |
2085 | PERL_SET_CONTEXT(some_perl); | |
2086 | ||
2087 | ... other Perl API calls on some_perl go here ... | |
2088 | ||
ee072b34 GS |
2089 | =head2 Future Plans and PERL_IMPLICIT_SYS |
2090 | ||
2091 | Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything | |
2092 | that the interpreter knows about itself and pass it around, so too are | |
2093 | there plans to allow the interpreter to bundle up everything it knows | |
2094 | about the environment it's running on. This is enabled with the | |
acfe0abc | 2095 | PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS |
4d1ff10f | 2096 | and USE_5005THREADS on Windows (see inside iperlsys.h). |
ee072b34 GS |
2097 | |
2098 | This allows the ability to provide an extra pointer (called the "host" | |
2099 | environment) for all the system calls. This makes it possible for | |
2100 | all the system stuff to maintain their own state, broken down into | |
2101 | seven C structures. These are thin wrappers around the usual system | |
2102 | calls (see win32/perllib.c) for the default perl executable, but for a | |
2103 | more ambitious host (like the one that would do fork() emulation) all | |
2104 | the extra work needed to pretend that different interpreters are | |
2105 | actually different "processes", would be done here. | |
2106 | ||
2107 | The Perl engine/interpreter and the host are orthogonal entities. | |
2108 | There could be one or more interpreters in a process, and one or | |
2109 | more "hosts", with free association between them. | |
2110 | ||
a422fd2d SC |
2111 | =head1 Internal Functions |
2112 | ||
2113 | All of Perl's internal functions which will be exposed to the outside | |
9b8ea7f3 | 2114 | world are prefixed by C<Perl_> so that they will not conflict with XS |
a422fd2d SC |
2115 | functions or functions used in a program in which Perl is embedded. |
2116 | Similarly, all global variables begin with C<PL_>. (By convention, | |
9b8ea7f3 | 2117 | static functions start with C<S_>.) |
a422fd2d SC |
2118 | |
2119 | Inside the Perl core, you can get at the functions either with or | |
2120 | without the C<Perl_> prefix, thanks to a bunch of defines that live in | |
2121 | F<embed.h>. This header file is generated automatically from | |
57701cb8 NC |
2122 | F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping |
2123 | header files for the internal functions, generates the documentation | |
2124 | and a lot of other bits and pieces. It's important that when you add | |
2125 | a new function to the core or change an existing one, you change the | |
2126 | data in the table in F<embed.fnc> as well. Here's a sample entry from | |
2127 | that table: | |
a422fd2d SC |
2128 | |
2129 | Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval | |
2130 | ||
2131 | The second column is the return type, the third column the name. Columns | |
2132 | after that are the arguments. The first column is a set of flags: | |
2133 | ||
2134 | =over 3 | |
2135 | ||
2136 | =item A | |
2137 | ||
7dbaea58 NC |
2138 | This function is a part of the public API. All such functions should also |
2139 | have 'd', very few do not. | |
a422fd2d SC |
2140 | |
2141 | =item p | |
2142 | ||
7dbaea58 NC |
2143 | This function has a C<Perl_> prefix; i.e. it is defined as |
2144 | C<Perl_av_fetch>. | |
a422fd2d SC |
2145 | |
2146 | =item d | |
2147 | ||
2148 | This function has documentation using the C<apidoc> feature which we'll | |
7dbaea58 | 2149 | look at in a second. Some functions have 'd' but not 'A'; docs are good. |
a422fd2d SC |
2150 | |
2151 | =back | |
2152 | ||
2153 | Other available flags are: | |
2154 | ||
2155 | =over 3 | |
2156 | ||
2157 | =item s | |
2158 | ||
7dbaea58 NC |
2159 | This is a static function and is defined as C<STATIC S_whatever>, and |
2160 | usually called within the sources as C<whatever(...)>. | |
a422fd2d SC |
2161 | |
2162 | =item n | |
2163 | ||
7dbaea58 NC |
2164 | This does not need a interpreter context, so the definition has no |
2165 | C<pTHX>, and it follows that callers don't use C<aTHX>. (See | |
a422fd2d SC |
2166 | L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.) |
2167 | ||
2168 | =item r | |
2169 | ||
2170 | This function never returns; C<croak>, C<exit> and friends. | |
2171 | ||
2172 | =item f | |
2173 | ||
2174 | This function takes a variable number of arguments, C<printf> style. | |
2175 | The argument list should end with C<...>, like this: | |
2176 | ||
2177 | Afprd |void |croak |const char* pat|... | |
2178 | ||
a7486cbb | 2179 | =item M |
a422fd2d | 2180 | |
00aadd71 | 2181 | This function is part of the experimental development API, and may change |
a422fd2d SC |
2182 | or disappear without notice. |
2183 | ||
2184 | =item o | |
2185 | ||
2186 | This function should not have a compatibility macro to define, say, | |
2187 | C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>. | |
2188 | ||
a422fd2d SC |
2189 | =item x |
2190 | ||
2191 | This function isn't exported out of the Perl core. | |
2192 | ||
57701cb8 NC |
2193 | =item m |
2194 | ||
2195 | This is implemented as a macro. | |
2196 | ||
2197 | =item X | |
2198 | ||
2199 | This function is explicitly exported. | |
2200 | ||
2201 | =item E | |
2202 | ||
2203 | This function is visible to extensions included in the Perl core. | |
2204 | ||
2205 | =item b | |
2206 | ||
2207 | Binary backward compatibility; this function is a macro but also has | |
2208 | a C<Perl_> implementation (which is exported). | |
2209 | ||
7dbaea58 NC |
2210 | =item others |
2211 | ||
2212 | See the comments at the top of C<embed.fnc> for others. | |
2213 | ||
a422fd2d SC |
2214 | =back |
2215 | ||
57701cb8 NC |
2216 | If you edit F<embed.pl> or F<embed.fnc>, you will need to run |
2217 | C<make regen_headers> to force a rebuild of F<embed.h> and other | |
2218 | auto-generated files. | |
a422fd2d | 2219 | |
6b4667fc | 2220 | =head2 Formatted Printing of IVs, UVs, and NVs |
9dd9db0b | 2221 | |
6b4667fc A |
2222 | If you are printing IVs, UVs, or NVS instead of the stdio(3) style |
2223 | formatting codes like C<%d>, C<%ld>, C<%f>, you should use the | |
2224 | following macros for portability | |
9dd9db0b | 2225 | |
c52f9dcd JH |
2226 | IVdf IV in decimal |
2227 | UVuf UV in decimal | |
2228 | UVof UV in octal | |
2229 | UVxf UV in hexadecimal | |
2230 | NVef NV %e-like | |
2231 | NVff NV %f-like | |
2232 | NVgf NV %g-like | |
9dd9db0b | 2233 | |
6b4667fc A |
2234 | These will take care of 64-bit integers and long doubles. |
2235 | For example: | |
2236 | ||
c52f9dcd | 2237 | printf("IV is %"IVdf"\n", iv); |
6b4667fc A |
2238 | |
2239 | The IVdf will expand to whatever is the correct format for the IVs. | |
9dd9db0b | 2240 | |
8908e76d JH |
2241 | If you are printing addresses of pointers, use UVxf combined |
2242 | with PTR2UV(), do not use %lx or %p. | |
2243 | ||
2244 | =head2 Pointer-To-Integer and Integer-To-Pointer | |
2245 | ||
2246 | Because pointer size does not necessarily equal integer size, | |
2247 | use the follow macros to do it right. | |
2248 | ||
c52f9dcd JH |
2249 | PTR2UV(pointer) |
2250 | PTR2IV(pointer) | |
2251 | PTR2NV(pointer) | |
2252 | INT2PTR(pointertotype, integer) | |
8908e76d JH |
2253 | |
2254 | For example: | |
2255 | ||
c52f9dcd JH |
2256 | IV iv = ...; |
2257 | SV *sv = INT2PTR(SV*, iv); | |
8908e76d JH |
2258 | |
2259 | and | |
2260 | ||
c52f9dcd JH |
2261 | AV *av = ...; |
2262 | UV uv = PTR2UV(av); | |
8908e76d | 2263 | |
a422fd2d SC |
2264 | =head2 Source Documentation |
2265 | ||
2266 | There's an effort going on to document the internal functions and | |
2267 | automatically produce reference manuals from them - L<perlapi> is one | |
2268 | such manual which details all the functions which are available to XS | |
2269 | writers. L<perlintern> is the autogenerated manual for the functions | |
2270 | which are not part of the API and are supposedly for internal use only. | |
2271 | ||
2272 | Source documentation is created by putting POD comments into the C | |
2273 | source, like this: | |
2274 | ||
2275 | /* | |
2276 | =for apidoc sv_setiv | |
2277 | ||
2278 | Copies an integer into the given SV. Does not handle 'set' magic. See | |
2279 | C<sv_setiv_mg>. | |
2280 | ||
2281 | =cut | |
2282 | */ | |
2283 | ||
2284 | Please try and supply some documentation if you add functions to the | |
2285 | Perl core. | |
2286 | ||
4aeeda6f BD |
2287 | =head2 Backwards compatibility |
2288 | ||
2289 | The Perl API changes over time. New functions are added or the interfaces | |
2290 | of existing functions are changed. The C<Devel::PPPort> module tries to | |
2291 | provide compatibility code for some of these changes, so XS writers don't | |
2292 | have to code it themselves when supporting multiple versions of Perl. | |
2293 | ||
2294 | C<Devel::PPPort> generates a C header file F<ppport.h> that can also | |
2295 | be run as a Perl script. To generate F<ppport.h>, run: | |
2296 | ||
2297 | perl -MDevel::PPPort -eDevel::PPPort::WriteFile | |
2298 | ||
2299 | Besides checking existing XS code, the script can also be used to retrieve | |
2300 | compatibility information for various API calls using the C<--api-info> | |
2301 | command line switch. For example: | |
2302 | ||
2303 | % perl ppport.h --api-info=sv_magicext | |
2304 | ||
2305 | For details, see C<perldoc ppport.h>. | |
2306 | ||
a422fd2d SC |
2307 | =head1 Unicode Support |
2308 | ||
2309 | Perl 5.6.0 introduced Unicode support. It's important for porters and XS | |
2310 | writers to understand this support and make sure that the code they | |
2311 | write does not corrupt Unicode data. | |
2312 | ||
2313 | =head2 What B<is> Unicode, anyway? | |
2314 | ||
2315 | In the olden, less enlightened times, we all used to use ASCII. Most of | |
2316 | us did, anyway. The big problem with ASCII is that it's American. Well, | |
2317 | no, that's not actually the problem; the problem is that it's not | |
2318 | particularly useful for people who don't use the Roman alphabet. What | |
2319 | used to happen was that particular languages would stick their own | |
2320 | alphabet in the upper range of the sequence, between 128 and 255. Of | |
2321 | course, we then ended up with plenty of variants that weren't quite | |
2322 | ASCII, and the whole point of it being a standard was lost. | |
2323 | ||
2324 | Worse still, if you've got a language like Chinese or | |
2325 | Japanese that has hundreds or thousands of characters, then you really | |
2326 | can't fit them into a mere 256, so they had to forget about ASCII | |
2327 | altogether, and build their own systems using pairs of numbers to refer | |
2328 | to one character. | |
2329 | ||
2330 | To fix this, some people formed Unicode, Inc. and | |
2331 | produced a new character set containing all the characters you can | |
2332 | possibly think of and more. There are several ways of representing these | |
cd458e05 | 2333 | characters, and the one Perl uses is called UTF-8. UTF-8 uses |
a422fd2d | 2334 | a variable number of bytes to represent a character, instead of just |
b3b6085d | 2335 | one. You can learn more about Unicode at http://www.unicode.org/ |
a422fd2d | 2336 | |
cd458e05 | 2337 | =head2 How can I recognise a UTF-8 string? |
a422fd2d | 2338 | |
cd458e05 JH |
2339 | You can't. This is because UTF-8 data is stored in bytes just like |
2340 | non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types) | |
a422fd2d SC |
2341 | capital E with a grave accent, is represented by the two bytes |
2342 | C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)> | |
2343 | has that byte sequence as well. So you can't tell just by looking - this | |
2344 | is what makes Unicode input an interesting problem. | |
2345 | ||
2346 | The API function C<is_utf8_string> can help; it'll tell you if a string | |
cd458e05 | 2347 | contains only valid UTF-8 characters. However, it can't do the work for |
a422fd2d | 2348 | you. On a character-by-character basis, C<is_utf8_char> will tell you |
cd458e05 | 2349 | whether the current character in a string is valid UTF-8. |
a422fd2d | 2350 | |
cd458e05 | 2351 | =head2 How does UTF-8 represent Unicode characters? |
a422fd2d | 2352 | |
cd458e05 | 2353 | As mentioned above, UTF-8 uses a variable number of bytes to store a |
a422fd2d SC |
2354 | character. Characters with values 1...128 are stored in one byte, just |
2355 | like good ol' ASCII. Character 129 is stored as C<v194.129>; this | |
a31a806a | 2356 | continues up to character 191, which is C<v194.191>. Now we've run out of |
a422fd2d SC |
2357 | bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And |
2358 | so it goes on, moving to three bytes at character 2048. | |
2359 | ||
cd458e05 | 2360 | Assuming you know you're dealing with a UTF-8 string, you can find out |
a422fd2d SC |
2361 | how long the first character in it is with the C<UTF8SKIP> macro: |
2362 | ||
2363 | char *utf = "\305\233\340\240\201"; | |
2364 | I32 len; | |
2365 | ||
2366 | len = UTF8SKIP(utf); /* len is 2 here */ | |
2367 | utf += len; | |
2368 | len = UTF8SKIP(utf); /* len is 3 here */ | |
2369 | ||
cd458e05 | 2370 | Another way to skip over characters in a UTF-8 string is to use |
a422fd2d SC |
2371 | C<utf8_hop>, which takes a string and a number of characters to skip |
2372 | over. You're on your own about bounds checking, though, so don't use it | |
2373 | lightly. | |
2374 | ||
cd458e05 | 2375 | All bytes in a multi-byte UTF-8 character will have the high bit set, |
7801ec30 | 2376 | so you can test if you need to do something special with this |
c06c0673 JH |
2377 | character like this (the UTF8_IS_INVARIANT() is a macro that tests |
2378 | whether the byte can be encoded as a single byte even in UTF-8): | |
a422fd2d | 2379 | |
7801ec30 JH |
2380 | U8 *utf; |
2381 | UV uv; /* Note: a UV, not a U8, not a char */ | |
a422fd2d | 2382 | |
c06c0673 | 2383 | if (!UTF8_IS_INVARIANT(*utf)) |
cd458e05 | 2384 | /* Must treat this as UTF-8 */ |
a422fd2d SC |
2385 | uv = utf8_to_uv(utf); |
2386 | else | |
2387 | /* OK to treat this character as a byte */ | |
2388 | uv = *utf; | |
2389 | ||
2390 | You can also see in that example that we use C<utf8_to_uv> to get the | |
2391 | value of the character; the inverse function C<uv_to_utf8> is available | |
cd458e05 | 2392 | for putting a UV into UTF-8: |
a422fd2d | 2393 | |
c06c0673 | 2394 | if (!UTF8_IS_INVARIANT(uv)) |
a422fd2d SC |
2395 | /* Must treat this as UTF8 */ |
2396 | utf8 = uv_to_utf8(utf8, uv); | |
2397 | else | |
2398 | /* OK to treat this character as a byte */ | |
2399 | *utf8++ = uv; | |
2400 | ||
2401 | You B<must> convert characters to UVs using the above functions if | |
cd458e05 JH |
2402 | you're ever in a situation where you have to match UTF-8 and non-UTF-8 |
2403 | characters. You may not skip over UTF-8 characters in this case. If you | |
2404 | do this, you'll lose the ability to match hi-bit non-UTF-8 characters; | |
2405 | for instance, if your UTF-8 string contains C<v196.172>, and you skip | |
2406 | that character, you can never match a C<chr(200)> in a non-UTF-8 string. | |
a422fd2d SC |
2407 | So don't do that! |
2408 | ||
cd458e05 | 2409 | =head2 How does Perl store UTF-8 strings? |
a422fd2d SC |
2410 | |
2411 | Currently, Perl deals with Unicode strings and non-Unicode strings | |
2412 | slightly differently. If a string has been identified as being UTF-8 | |
2413 | encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and | |
2414 | manipulate this flag with the following macros: | |
2415 | ||
2416 | SvUTF8(sv) | |
2417 | SvUTF8_on(sv) | |
2418 | SvUTF8_off(sv) | |
2419 | ||
2420 | This flag has an important effect on Perl's treatment of the string: if | |
2421 | Unicode data is not properly distinguished, regular expressions, | |
2422 | C<length>, C<substr> and other string handling operations will have | |
2423 | undesirable results. | |
2424 | ||
2425 | The problem comes when you have, for instance, a string that isn't | |
cd458e05 JH |
2426 | flagged is UTF-8, and contains a byte sequence that could be UTF-8 - |
2427 | especially when combining non-UTF-8 and UTF-8 strings. | |
a422fd2d SC |
2428 | |
2429 | Never forget that the C<SVf_UTF8> flag is separate to the PV value; you | |
2430 | need be sure you don't accidentally knock it off while you're | |
2431 | manipulating SVs. More specifically, you cannot expect to do this: | |
2432 | ||
2433 | SV *sv; | |
2434 | SV *nsv; | |
2435 | STRLEN len; | |
2436 | char *p; | |
2437 | ||
2438 | p = SvPV(sv, len); | |
2439 | frobnicate(p); | |
2440 | nsv = newSVpvn(p, len); | |
2441 | ||
2442 | The C<char*> string does not tell you the whole story, and you can't | |
2443 | copy or reconstruct an SV just by copying the string value. Check if the | |
cd458e05 | 2444 | old SV has the UTF-8 flag set, and act accordingly: |
a422fd2d SC |
2445 | |
2446 | p = SvPV(sv, len); | |
2447 | frobnicate(p); | |
2448 | nsv = newSVpvn(p, len); | |
2449 | if (SvUTF8(sv)) | |
2450 | SvUTF8_on(nsv); | |
2451 | ||
2452 | In fact, your C<frobnicate> function should be made aware of whether or | |
cd458e05 | 2453 | not it's dealing with UTF-8 data, so that it can handle the string |
a422fd2d SC |
2454 | appropriately. |
2455 | ||
7801ec30 | 2456 | Since just passing an SV to an XS function and copying the data of |
cd458e05 | 2457 | the SV is not enough to copy the UTF-8 flags, even less right is just |
7801ec30 JH |
2458 | passing a C<char *> to an XS function. |
2459 | ||
cd458e05 | 2460 | =head2 How do I convert a string to UTF-8? |
a422fd2d | 2461 | |
cd458e05 JH |
2462 | If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary |
2463 | to upgrade one of the strings to UTF-8. If you've got an SV, the easiest | |
a422fd2d SC |
2464 | way to do this is: |
2465 | ||
2466 | sv_utf8_upgrade(sv); | |
2467 | ||
2468 | However, you must not do this, for example: | |
2469 | ||
2470 | if (!SvUTF8(left)) | |
2471 | sv_utf8_upgrade(left); | |
2472 | ||
2473 | If you do this in a binary operator, you will actually change one of the | |
b1866b2d | 2474 | strings that came into the operator, and, while it shouldn't be noticeable |
a422fd2d SC |
2475 | by the end user, it can cause problems. |
2476 | ||
cd458e05 | 2477 | Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its |
a422fd2d | 2478 | string argument. This is useful for having the data available for |
b1866b2d | 2479 | comparisons and so on, without harming the original SV. There's also |
a422fd2d SC |
2480 | C<utf8_to_bytes> to go the other way, but naturally, this will fail if |
2481 | the string contains any characters above 255 that can't be represented | |
2482 | in a single byte. | |
2483 | ||
2484 | =head2 Is there anything else I need to know? | |
2485 | ||
2486 | Not really. Just remember these things: | |
2487 | ||
2488 | =over 3 | |
2489 | ||
2490 | =item * | |
2491 | ||
cd458e05 JH |
2492 | There's no way to tell if a string is UTF-8 or not. You can tell if an SV |
2493 | is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if | |
2494 | something should be UTF-8. Treat the flag as part of the PV, even though | |
a422fd2d SC |
2495 | it's not - if you pass on the PV to somewhere, pass on the flag too. |
2496 | ||
2497 | =item * | |
2498 | ||
cd458e05 | 2499 | If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value, |
c06c0673 | 2500 | unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>. |
a422fd2d SC |
2501 | |
2502 | =item * | |
2503 | ||
cd458e05 | 2504 | When writing a character C<uv> to a UTF-8 string, B<always> use |
c06c0673 | 2505 | C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case |
7801ec30 | 2506 | you can use C<*s = uv>. |
a422fd2d SC |
2507 | |
2508 | =item * | |
2509 | ||
cd458e05 JH |
2510 | Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get |
2511 | a new string which is UTF-8 encoded. There are tricks you can use to | |
2512 | delay deciding whether you need to use a UTF-8 string until you get to a | |
a422fd2d SC |
2513 | high character - C<HALF_UPGRADE> is one of those. |
2514 | ||
2515 | =back | |
2516 | ||
53e06cf0 SC |
2517 | =head1 Custom Operators |
2518 | ||
9a68f1db | 2519 | Custom operator support is a new experimental feature that allows you to |
53e06cf0 SC |
2520 | define your own ops. This is primarily to allow the building of |
2521 | interpreters for other languages in the Perl core, but it also allows | |
2522 | optimizations through the creation of "macro-ops" (ops which perform the | |
2523 | functions of multiple ops which are usually executed together, such as | |
7dbaea58 | 2524 | C<gvsv, gvsv, add>.) |
53e06cf0 | 2525 | |
b455bf3f | 2526 | This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl |
53e06cf0 SC |
2527 | core does not "know" anything special about this op type, and so it will |
2528 | not be involved in any optimizations. This also means that you can | |
2529 | define your custom ops to be any op structure - unary, binary, list and | |
2530 | so on - you like. | |
2531 | ||
2532 | It's important to know what custom operators won't do for you. They | |
2533 | won't let you add new syntax to Perl, directly. They won't even let you | |
2534 | add new keywords, directly. In fact, they won't change the way Perl | |
2535 | compiles a program at all. You have to do those changes yourself, after | |
2536 | Perl has compiled the program. You do this either by manipulating the op | |
2537 | tree using a C<CHECK> block and the C<B::Generate> module, or by adding | |
2538 | a custom peephole optimizer with the C<optimize> module. | |
2539 | ||
2540 | When you do this, you replace ordinary Perl ops with custom ops by | |
2541 | creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own | |
2542 | PP function. This should be defined in XS code, and should look like | |
2543 | the PP ops in C<pp_*.c>. You are responsible for ensuring that your op | |
2544 | takes the appropriate number of values from the stack, and you are | |
2545 | responsible for adding stack marks if necessary. | |
2546 | ||
2547 | You should also "register" your op with the Perl interpreter so that it | |
2548 | can produce sensible error and warning messages. Since it is possible to | |
2549 | have multiple custom ops within the one "logical" op type C<OP_CUSTOM>, | |
2550 | Perl uses the value of C<< o->op_ppaddr >> as a key into the | |
2551 | C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you | |
2552 | need to enter a name and description for your op at the appropriate | |
2553 | place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes. | |
2554 | ||
2555 | Forthcoming versions of C<B::Generate> (version 1.0 and above) should | |
ba5c064d | 2556 | directly support the creation of custom ops by name. |
53e06cf0 | 2557 | |
954c1994 | 2558 | =head1 AUTHORS |
e89caa19 | 2559 | |
954c1994 | 2560 | Until May 1997, this document was maintained by Jeff Okamoto |
9b5bb84f SB |
2561 | E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl |
2562 | itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>. | |
cb1a09d0 | 2563 | |
954c1994 GS |
2564 | With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, |
2565 | Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil | |
2566 | Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, | |
2567 | Stephen McCamant, and Gurusamy Sarathy. | |
cb1a09d0 | 2568 | |
954c1994 | 2569 | =head1 SEE ALSO |
cb1a09d0 | 2570 | |
954c1994 | 2571 | perlapi(1), perlintern(1), perlxs(1), perlembed(1) |