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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
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7This document attempts to describe how to use the Perl API, as well as
8containing some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl 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 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
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31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
32as well.)
a0d0e21e 33
54310121 34=head2 Working with SVs
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35
36An SV can be created and loaded with one command. There are four types of
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37values that can be loaded: an integer value (IV), a double (NV),
38a string (PV), and another scalar (SV).
a0d0e21e 39
9da1e3b5 40The six routines are:
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41
42 SV* newSViv(IV);
43 SV* newSVnv(double);
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44 SV* newSVpv(const char*, int);
45 SV* newSVpvn(const char*, int);
46fc3d4c 46 SV* newSVpvf(const char*, ...);
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47 SV* newSVsv(SV*);
48
deb3007b 49To change the value of an *already-existing* SV, there are seven routines:
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50
51 void sv_setiv(SV*, IV);
deb3007b 52 void sv_setuv(SV*, UV);
a0d0e21e 53 void sv_setnv(SV*, double);
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54 void sv_setpv(SV*, const char*);
55 void sv_setpvn(SV*, const char*, int)
46fc3d4c 56 void sv_setpvf(SV*, const char*, ...);
9abd00ed 57 void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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58 void sv_setsv(SV*, SV*);
59
60Notice that you can choose to specify the length of the string to be
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61assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
62allow Perl to calculate the length by using C<sv_setpv> or by specifying
630 as the second argument to C<newSVpv>. Be warned, though, that Perl will
64determine the string's length by using C<strlen>, which depends on the
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65string terminating with a NUL character.
66
67The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
68formatted output becomes the value.
69
70C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify
71either a pointer to a variable argument list or the address and length of
72an array of SVs. The last argument points to a boolean; on return, if that
73boolean is true, then locale-specific information has been used to format
c2611fb3 74the string, and the string's contents are therefore untrustworthy (see
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75L<perlsec>). This pointer may be NULL if that information is not
76important. Note that this function requires you to specify the length of
77the format.
78
7c906a97 79STRLEN is an integer type (Size_t, usually defined as size_t in
00aadd71 80config.h) guaranteed to be large enough to represent the size of
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81any string that perl can handle.
82
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83The C<sv_set*()> functions are not generic enough to operate on values
84that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 85
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86All SVs that contain strings should be terminated with a NUL character.
87If it is not NUL-terminated there is a risk of
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88core dumps and corruptions from code which passes the string to C
89functions or system calls which expect a NUL-terminated string.
90Perl's own functions typically add a trailing NUL for this reason.
91Nevertheless, you should be very careful when you pass a string stored
92in an SV to a C function or system call.
93
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94To access the actual value that an SV points to, you can use the macros:
95
96 SvIV(SV*)
954c1994 97 SvUV(SV*)
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98 SvNV(SV*)
99 SvPV(SV*, STRLEN len)
1fa8b10d 100 SvPV_nolen(SV*)
a0d0e21e 101
954c1994 102which will automatically coerce the actual scalar type into an IV, UV, double,
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103or string.
104
105In the C<SvPV> macro, the length of the string returned is placed into the
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106variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
107not care what the length of the data is, use the C<SvPV_nolen> macro.
108Historically the C<SvPV> macro with the global variable C<PL_na> has been
109used in this case. But that can be quite inefficient because C<PL_na> must
110be accessed in thread-local storage in threaded Perl. In any case, remember
111that Perl allows arbitrary strings of data that may both contain NULs and
112might not be terminated by a NUL.
a0d0e21e 113
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114Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
115len);>. It might work with your compiler, but it won't work for everyone.
116Break this sort of statement up into separate assignments:
117
b2f5ed49 118 SV *s;
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119 STRLEN len;
120 char * ptr;
b2f5ed49 121 ptr = SvPV(s, len);
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122 foo(ptr, len);
123
07fa94a1 124If you want to know if the scalar value is TRUE, you can use:
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125
126 SvTRUE(SV*)
127
128Although Perl will automatically grow strings for you, if you need to force
129Perl to allocate more memory for your SV, you can use the macro
130
131 SvGROW(SV*, STRLEN newlen)
132
133which will determine if more memory needs to be allocated. If so, it will
134call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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135decrease, the allocated memory of an SV and that it does not automatically
136add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 137C<SvGROW(sv, len + 1)>).
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138
139If you have an SV and want to know what kind of data Perl thinks is stored
140in it, you can use the following macros to check the type of SV you have.
141
142 SvIOK(SV*)
143 SvNOK(SV*)
144 SvPOK(SV*)
145
146You can get and set the current length of the string stored in an SV with
147the following macros:
148
149 SvCUR(SV*)
150 SvCUR_set(SV*, I32 val)
151
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152You can also get a pointer to the end of the string stored in the SV
153with the macro:
154
155 SvEND(SV*)
156
157But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 158
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159If you want to append something to the end of string stored in an C<SV*>,
160you can use the following functions:
161
08105a92 162 void sv_catpv(SV*, const char*);
e65f3abd 163 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 164 void sv_catpvf(SV*, const char*, ...);
9abd00ed 165 void sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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166 void sv_catsv(SV*, SV*);
167
168The first function calculates the length of the string to be appended by
169using C<strlen>. In the second, you specify the length of the string
46fc3d4c 170yourself. The third function processes its arguments like C<sprintf> and
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171appends the formatted output. The fourth function works like C<vsprintf>.
172You can specify the address and length of an array of SVs instead of the
173va_list argument. The fifth function extends the string stored in the first
174SV with the string stored in the second SV. It also forces the second SV
175to be interpreted as a string.
176
177The C<sv_cat*()> functions are not generic enough to operate on values that
178have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 179
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180If you know the name of a scalar variable, you can get a pointer to its SV
181by using the following:
182
4929bf7b 183 SV* get_sv("package::varname", FALSE);
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184
185This returns NULL if the variable does not exist.
186
d1b91892 187If you want to know if this variable (or any other SV) is actually C<defined>,
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188you can call:
189
190 SvOK(SV*)
191
9cde0e7f 192The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its
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193address can be used whenever an C<SV*> is needed.
194
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195There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean
196TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can
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197be used whenever an C<SV*> is needed.
198
9cde0e7f 199Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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200Take this code:
201
202 SV* sv = (SV*) 0;
203 if (I-am-to-return-a-real-value) {
204 sv = sv_2mortal(newSViv(42));
205 }
206 sv_setsv(ST(0), sv);
207
208This code tries to return a new SV (which contains the value 42) if it should
04343c6d 209return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 210pointer which, somewhere down the line, will cause a segmentation violation,
9cde0e7f 211bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first
5f05dabc 212line and all will be well.
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213
214To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 215call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 216
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217=head2 Offsets
218
219Perl provides the function C<sv_chop> to efficiently remove characters
220from the beginning of a string; you give it an SV and a pointer to
221somewhere inside the the PV, and it discards everything before the
222pointer. The efficiency comes by means of a little hack: instead of
223actually removing the characters, C<sv_chop> sets the flag C<OOK>
224(offset OK) to signal to other functions that the offset hack is in
225effect, and it puts the number of bytes chopped off into the IV field
226of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
00aadd71 227many bytes, and adjusts C<SvCUR> and C<SvLEN>.
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228
229Hence, at this point, the start of the buffer that we allocated lives
230at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
231into the middle of this allocated storage.
232
233This is best demonstrated by example:
234
235 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
236 SV = PVIV(0x8128450) at 0x81340f0
237 REFCNT = 1
238 FLAGS = (POK,OOK,pPOK)
239 IV = 1 (OFFSET)
240 PV = 0x8135781 ( "1" . ) "2345"\0
241 CUR = 4
242 LEN = 5
243
244Here the number of bytes chopped off (1) is put into IV, and
245C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
246portion of the string between the "real" and the "fake" beginnings is
247shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
248the fake beginning, not the real one.
249
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250Something similar to the offset hack is perfomed on AVs to enable
251efficient shifting and splicing off the beginning of the array; while
252C<AvARRAY> points to the first element in the array that is visible from
253Perl, C<AvALLOC> points to the real start of the C array. These are
254usually the same, but a C<shift> operation can be carried out by
255increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
256Again, the location of the real start of the C array only comes into
257play when freeing the array. See C<av_shift> in F<av.c>.
258
d1b91892 259=head2 What's Really Stored in an SV?
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260
261Recall that the usual method of determining the type of scalar you have is
5f05dabc 262to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 263usually these macros will always return TRUE and calling the C<Sv*V>
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264macros will do the appropriate conversion of string to integer/double or
265integer/double to string.
266
267If you I<really> need to know if you have an integer, double, or string
268pointer in an SV, you can use the following three macros instead:
269
270 SvIOKp(SV*)
271 SvNOKp(SV*)
272 SvPOKp(SV*)
273
274These will tell you if you truly have an integer, double, or string pointer
d1b91892 275stored in your SV. The "p" stands for private.
a0d0e21e 276
07fa94a1 277In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 278
54310121 279=head2 Working with AVs
a0d0e21e 280
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281There are two ways to create and load an AV. The first method creates an
282empty AV:
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283
284 AV* newAV();
285
54310121 286The second method both creates the AV and initially populates it with SVs:
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287
288 AV* av_make(I32 num, SV **ptr);
289
5f05dabc 290The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 291AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 292
54310121 293Once the AV has been created, the following operations are possible on AVs:
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294
295 void av_push(AV*, SV*);
296 SV* av_pop(AV*);
297 SV* av_shift(AV*);
298 void av_unshift(AV*, I32 num);
299
300These should be familiar operations, with the exception of C<av_unshift>.
301This routine adds C<num> elements at the front of the array with the C<undef>
302value. You must then use C<av_store> (described below) to assign values
303to these new elements.
304
305Here are some other functions:
306
5f05dabc 307 I32 av_len(AV*);
a0d0e21e 308 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 309 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 310
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311The C<av_len> function returns the highest index value in array (just
312like $#array in Perl). If the array is empty, -1 is returned. The
313C<av_fetch> function returns the value at index C<key>, but if C<lval>
314is non-zero, then C<av_fetch> will store an undef value at that index.
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315The C<av_store> function stores the value C<val> at index C<key>, and does
316not increment the reference count of C<val>. Thus the caller is responsible
317for taking care of that, and if C<av_store> returns NULL, the caller will
318have to decrement the reference count to avoid a memory leak. Note that
319C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
320return value.
d1b91892 321
a0d0e21e 322 void av_clear(AV*);
a0d0e21e 323 void av_undef(AV*);
cb1a09d0 324 void av_extend(AV*, I32 key);
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325
326The C<av_clear> function deletes all the elements in the AV* array, but
327does not actually delete the array itself. The C<av_undef> function will
328delete all the elements in the array plus the array itself. The
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329C<av_extend> function extends the array so that it contains at least C<key+1>
330elements. If C<key+1> is less than the currently allocated length of the array,
331then nothing is done.
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332
333If you know the name of an array variable, you can get a pointer to its AV
334by using the following:
335
4929bf7b 336 AV* get_av("package::varname", FALSE);
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337
338This returns NULL if the variable does not exist.
339
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340See L<Understanding the Magic of Tied Hashes and Arrays> for more
341information on how to use the array access functions on tied arrays.
342
54310121 343=head2 Working with HVs
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344
345To create an HV, you use the following routine:
346
347 HV* newHV();
348
54310121 349Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 350
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351 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
352 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 353
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354The C<klen> parameter is the length of the key being passed in (Note that
355you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
356length of the key). The C<val> argument contains the SV pointer to the
54310121 357scalar being stored, and C<hash> is the precomputed hash value (zero if
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358you want C<hv_store> to calculate it for you). The C<lval> parameter
359indicates whether this fetch is actually a part of a store operation, in
360which case a new undefined value will be added to the HV with the supplied
361key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 362
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363Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
364C<SV*>. To access the scalar value, you must first dereference the return
365value. However, you should check to make sure that the return value is
366not NULL before dereferencing it.
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367
368These two functions check if a hash table entry exists, and deletes it.
369
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370 bool hv_exists(HV*, const char* key, U32 klen);
371 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 372
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373If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
374create and return a mortal copy of the deleted value.
375
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376And more miscellaneous functions:
377
378 void hv_clear(HV*);
a0d0e21e 379 void hv_undef(HV*);
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380
381Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
382table but does not actually delete the hash table. The C<hv_undef> deletes
383both the entries and the hash table itself.
a0d0e21e 384
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385Perl keeps the actual data in linked list of structures with a typedef of HE.
386These contain the actual key and value pointers (plus extra administrative
387overhead). The key is a string pointer; the value is an C<SV*>. However,
388once you have an C<HE*>, to get the actual key and value, use the routines
389specified below.
390
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391 I32 hv_iterinit(HV*);
392 /* Prepares starting point to traverse hash table */
393 HE* hv_iternext(HV*);
394 /* Get the next entry, and return a pointer to a
395 structure that has both the key and value */
396 char* hv_iterkey(HE* entry, I32* retlen);
397 /* Get the key from an HE structure and also return
398 the length of the key string */
cb1a09d0 399 SV* hv_iterval(HV*, HE* entry);
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400 /* Return a SV pointer to the value of the HE
401 structure */
cb1a09d0 402 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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403 /* This convenience routine combines hv_iternext,
404 hv_iterkey, and hv_iterval. The key and retlen
405 arguments are return values for the key and its
406 length. The value is returned in the SV* argument */
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407
408If you know the name of a hash variable, you can get a pointer to its HV
409by using the following:
410
4929bf7b 411 HV* get_hv("package::varname", FALSE);
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412
413This returns NULL if the variable does not exist.
414
8ebc5c01 415The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 416
a0d0e21e 417 hash = 0;
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418 while (klen--)
419 hash = (hash * 33) + *key++;
87275199 420 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 421
87275199 422The last step was added in version 5.6 to improve distribution of
ab192400 423lower bits in the resulting hash value.
a0d0e21e 424
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425See L<Understanding the Magic of Tied Hashes and Arrays> for more
426information on how to use the hash access functions on tied hashes.
427
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428=head2 Hash API Extensions
429
430Beginning with version 5.004, the following functions are also supported:
431
432 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
433 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 434
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435 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
436 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 437
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438 SV* hv_iterkeysv (HE* entry);
439
440Note that these functions take C<SV*> keys, which simplifies writing
441of extension code that deals with hash structures. These functions
442also allow passing of C<SV*> keys to C<tie> functions without forcing
443you to stringify the keys (unlike the previous set of functions).
444
445They also return and accept whole hash entries (C<HE*>), making their
446use more efficient (since the hash number for a particular string
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447doesn't have to be recomputed every time). See L<perlapi> for detailed
448descriptions.
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449
450The following macros must always be used to access the contents of hash
451entries. Note that the arguments to these macros must be simple
452variables, since they may get evaluated more than once. See
4a4eefd0 453L<perlapi> for detailed descriptions of these macros.
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454
455 HePV(HE* he, STRLEN len)
456 HeVAL(HE* he)
457 HeHASH(HE* he)
458 HeSVKEY(HE* he)
459 HeSVKEY_force(HE* he)
460 HeSVKEY_set(HE* he, SV* sv)
461
462These two lower level macros are defined, but must only be used when
463dealing with keys that are not C<SV*>s:
464
465 HeKEY(HE* he)
466 HeKLEN(HE* he)
467
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468Note that both C<hv_store> and C<hv_store_ent> do not increment the
469reference count of the stored C<val>, which is the caller's responsibility.
470If these functions return a NULL value, the caller will usually have to
471decrement the reference count of C<val> to avoid a memory leak.
1e422769 472
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473=head2 References
474
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475References are a special type of scalar that point to other data types
476(including references).
a0d0e21e 477
07fa94a1 478To create a reference, use either of the following functions:
a0d0e21e 479
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480 SV* newRV_inc((SV*) thing);
481 SV* newRV_noinc((SV*) thing);
a0d0e21e 482
5f05dabc 483The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
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484functions are identical except that C<newRV_inc> increments the reference
485count of the C<thing>, while C<newRV_noinc> does not. For historical
486reasons, C<newRV> is a synonym for C<newRV_inc>.
487
488Once you have a reference, you can use the following macro to dereference
489the reference:
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490
491 SvRV(SV*)
492
493then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 494C<AV*> or C<HV*>, if required.
a0d0e21e 495
d1b91892 496To determine if an SV is a reference, you can use the following macro:
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497
498 SvROK(SV*)
499
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500To discover what type of value the reference refers to, use the following
501macro and then check the return value.
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502
503 SvTYPE(SvRV(SV*))
504
505The most useful types that will be returned are:
506
507 SVt_IV Scalar
508 SVt_NV Scalar
509 SVt_PV Scalar
5f05dabc 510 SVt_RV Scalar
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511 SVt_PVAV Array
512 SVt_PVHV Hash
513 SVt_PVCV Code
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514 SVt_PVGV Glob (possible a file handle)
515 SVt_PVMG Blessed or Magical Scalar
516
517 See the sv.h header file for more details.
d1b91892 518
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519=head2 Blessed References and Class Objects
520
521References are also used to support object-oriented programming. In the
522OO lexicon, an object is simply a reference that has been blessed into a
523package (or class). Once blessed, the programmer may now use the reference
524to access the various methods in the class.
525
526A reference can be blessed into a package with the following function:
527
528 SV* sv_bless(SV* sv, HV* stash);
529
530The C<sv> argument must be a reference. The C<stash> argument specifies
3fe9a6f1 531which class the reference will belong to. See
2ae324a7 532L<Stashes and Globs> for information on converting class names into stashes.
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533
534/* Still under construction */
535
536Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
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537point to. If C<classname> is non-null, the SV is blessed into the specified
538class. SV is returned.
cb1a09d0 539
08105a92 540 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 541
e1c57cef 542Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 543if C<classname> is non-null.
cb1a09d0 544
08105a92 545 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 546 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 547 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 548
5f05dabc 549Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 550reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 551
08105a92 552 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 553
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554Copies string into an SV whose reference is C<rv>. Set length to 0 to let
555Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 556
e65f3abd 557 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 558
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559Tests whether the SV is blessed into the specified class. It does not
560check inheritance relationships.
561
08105a92 562 int sv_isa(SV* sv, const char* name);
9abd00ed
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563
564Tests whether the SV is a reference to a blessed object.
565
566 int sv_isobject(SV* sv);
567
568Tests whether the SV is derived from the specified class. SV can be either
569a reference to a blessed object or a string containing a class name. This
570is the function implementing the C<UNIVERSAL::isa> functionality.
571
08105a92 572 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 573
00aadd71 574To check if you've got an object derived from a specific class you have
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575to write:
576
577 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 578
5f05dabc 579=head2 Creating New Variables
cb1a09d0 580
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581To create a new Perl variable with an undef value which can be accessed from
582your Perl script, use the following routines, depending on the variable type.
cb1a09d0 583
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584 SV* get_sv("package::varname", TRUE);
585 AV* get_av("package::varname", TRUE);
586 HV* get_hv("package::varname", TRUE);
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587
588Notice the use of TRUE as the second parameter. The new variable can now
589be set, using the routines appropriate to the data type.
590
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591There are additional macros whose values may be bitwise OR'ed with the
592C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 593
5f05dabc 594 GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
54310121 595 "Name <varname> used only once: possible typo" warning.
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596 GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
597 the variable did not exist before the function was called.
cb1a09d0 598
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599If you do not specify a package name, the variable is created in the current
600package.
cb1a09d0 601
5f05dabc 602=head2 Reference Counts and Mortality
a0d0e21e 603
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604Perl uses an reference count-driven garbage collection mechanism. SVs,
605AVs, or HVs (xV for short in the following) start their life with a
55497cff 606reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 607then it will be destroyed and its memory made available for reuse.
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608
609This normally doesn't happen at the Perl level unless a variable is
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610undef'ed or the last variable holding a reference to it is changed or
611overwritten. At the internal level, however, reference counts can be
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612manipulated with the following macros:
613
614 int SvREFCNT(SV* sv);
5f05dabc 615 SV* SvREFCNT_inc(SV* sv);
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616 void SvREFCNT_dec(SV* sv);
617
618However, there is one other function which manipulates the reference
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619count of its argument. The C<newRV_inc> function, you will recall,
620creates a reference to the specified argument. As a side effect,
621it increments the argument's reference count. If this is not what
622you want, use C<newRV_noinc> instead.
623
624For example, imagine you want to return a reference from an XSUB function.
625Inside the XSUB routine, you create an SV which initially has a reference
626count of one. Then you call C<newRV_inc>, passing it the just-created SV.
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627This returns the reference as a new SV, but the reference count of the
628SV you passed to C<newRV_inc> has been incremented to two. Now you
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629return the reference from the XSUB routine and forget about the SV.
630But Perl hasn't! Whenever the returned reference is destroyed, the
631reference count of the original SV is decreased to one and nothing happens.
632The SV will hang around without any way to access it until Perl itself
633terminates. This is a memory leak.
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634
635The correct procedure, then, is to use C<newRV_noinc> instead of
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636C<newRV_inc>. Then, if and when the last reference is destroyed,
637the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 638stopping any memory leak.
55497cff 639
5f05dabc 640There are some convenience functions available that can help with the
54310121 641destruction of xVs. These functions introduce the concept of "mortality".
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642An xV that is mortal has had its reference count marked to be decremented,
643but not actually decremented, until "a short time later". Generally the
644term "short time later" means a single Perl statement, such as a call to
54310121 645an XSUB function. The actual determinant for when mortal xVs have their
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646reference count decremented depends on two macros, SAVETMPS and FREETMPS.
647See L<perlcall> and L<perlxs> for more details on these macros.
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648
649"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
650However, if you mortalize a variable twice, the reference count will
651later be decremented twice.
652
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653"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
654For example an SV which is created just to pass a number to a called sub
655is made mortal to have it cleaned up automatically when stack is popped.
656Similarly results returned by XSUBs (which go in the stack) are often
657made mortal.
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658
659To create a mortal variable, use the functions:
660
661 SV* sv_newmortal()
662 SV* sv_2mortal(SV*)
663 SV* sv_mortalcopy(SV*)
664
00aadd71 665The first call creates a mortal SV (with no value), the second converts an existing
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666SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
667third creates a mortal copy of an existing SV.
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668Because C<sv_newmortal> gives the new SV no value,it must normally be given one
669via C<sv_setpv>, C<sv_setiv> etc. :
670
671 SV *tmp = sv_newmortal();
672 sv_setiv(tmp, an_integer);
673
674As that is multiple C statements it is quite common so see this idiom instead:
675
676 SV *tmp = sv_2mortal(newSViv(an_integer));
677
678
679You should be careful about creating mortal variables. Strange things
680can happen if you make the same value mortal within multiple contexts,
681or if you make a variable mortal multiple times. Thinking of "Mortalization"
682as deferred C<SvREFCNT_dec> should help to minimize such problems.
683For example if you are passing an SV which you I<know> has high enough REFCNT
684to survive its use on the stack you need not do any mortalization.
685If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
686making a C<sv_mortalcopy> is safer.
a0d0e21e 687
54310121 688The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 689made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 690C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 691
5f05dabc 692=head2 Stashes and Globs
a0d0e21e 693
aa689395
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694A "stash" is a hash that contains all of the different objects that
695are contained within a package. Each key of the stash is a symbol
696name (shared by all the different types of objects that have the same
697name), and each value in the hash table is a GV (Glob Value). This GV
698in turn contains references to the various objects of that name,
699including (but not limited to) the following:
cb1a09d0 700
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701 Scalar Value
702 Array Value
703 Hash Value
a3cb178b 704 I/O Handle
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705 Format
706 Subroutine
707
9cde0e7f 708There is a single stash called "PL_defstash" that holds the items that exist
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709in the "main" package. To get at the items in other packages, append the
710string "::" to the package name. The items in the "Foo" package are in
9cde0e7f 711the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are
5f05dabc 712in the stash "Baz::" in "Bar::"'s stash.
a0d0e21e 713
d1b91892 714To get the stash pointer for a particular package, use the function:
a0d0e21e 715
08105a92 716 HV* gv_stashpv(const char* name, I32 create)
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717 HV* gv_stashsv(SV*, I32 create)
718
719The first function takes a literal string, the second uses the string stored
d1b91892 720in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 721C<HV*>. The C<create> flag will create a new package if it is set.
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722
723The name that C<gv_stash*v> wants is the name of the package whose symbol table
724you want. The default package is called C<main>. If you have multiply nested
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AD
725packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
726language itself.
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727
728Alternately, if you have an SV that is a blessed reference, you can find
729out the stash pointer by using:
730
731 HV* SvSTASH(SvRV(SV*));
732
733then use the following to get the package name itself:
734
735 char* HvNAME(HV* stash);
736
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737If you need to bless or re-bless an object you can use the following
738function:
a0d0e21e
LW
739
740 SV* sv_bless(SV*, HV* stash)
741
742where the first argument, an C<SV*>, must be a reference, and the second
743argument is a stash. The returned C<SV*> can now be used in the same way
744as any other SV.
745
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746For more information on references and blessings, consult L<perlref>.
747
54310121 748=head2 Double-Typed SVs
0a753a76
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749
750Scalar variables normally contain only one type of value, an integer,
751double, pointer, or reference. Perl will automatically convert the
752actual scalar data from the stored type into the requested type.
753
754Some scalar variables contain more than one type of scalar data. For
755example, the variable C<$!> contains either the numeric value of C<errno>
756or its string equivalent from either C<strerror> or C<sys_errlist[]>.
757
758To force multiple data values into an SV, you must do two things: use the
759C<sv_set*v> routines to add the additional scalar type, then set a flag
760so that Perl will believe it contains more than one type of data. The
761four macros to set the flags are:
762
763 SvIOK_on
764 SvNOK_on
765 SvPOK_on
766 SvROK_on
767
768The particular macro you must use depends on which C<sv_set*v> routine
769you called first. This is because every C<sv_set*v> routine turns on
770only the bit for the particular type of data being set, and turns off
771all the rest.
772
773For example, to create a new Perl variable called "dberror" that contains
774both the numeric and descriptive string error values, you could use the
775following code:
776
777 extern int dberror;
778 extern char *dberror_list;
779
4929bf7b 780 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
781 sv_setiv(sv, (IV) dberror);
782 sv_setpv(sv, dberror_list[dberror]);
783 SvIOK_on(sv);
784
785If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
786macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
787
788=head2 Magic Variables
a0d0e21e 789
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790[This section still under construction. Ignore everything here. Post no
791bills. Everything not permitted is forbidden.]
792
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793Any SV may be magical, that is, it has special features that a normal
794SV does not have. These features are stored in the SV structure in a
5f05dabc 795linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
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796
797 struct magic {
798 MAGIC* mg_moremagic;
799 MGVTBL* mg_virtual;
800 U16 mg_private;
801 char mg_type;
802 U8 mg_flags;
803 SV* mg_obj;
804 char* mg_ptr;
805 I32 mg_len;
806 };
807
808Note this is current as of patchlevel 0, and could change at any time.
809
810=head2 Assigning Magic
811
812Perl adds magic to an SV using the sv_magic function:
813
08105a92 814 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
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815
816The C<sv> argument is a pointer to the SV that is to acquire a new magical
817feature.
818
819If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
820set the C<SVt_PVMG> flag for the C<sv>. Perl then continues by adding
821it to the beginning of the linked list of magical features. Any prior
822entry of the same type of magic is deleted. Note that this can be
5fb8527f 823overridden, and multiple instances of the same type of magic can be
d1b91892
AD
824associated with an SV.
825
54310121
PP
826The C<name> and C<namlen> arguments are used to associate a string with
827the magic, typically the name of a variable. C<namlen> is stored in the
828C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd
d1b91892
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829copy of the name is stored in C<mg_ptr> field.
830
831The sv_magic function uses C<how> to determine which, if any, predefined
832"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
cb1a09d0 833See the "Magic Virtual Table" section below. The C<how> argument is also
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DM
834stored in the C<mg_type> field. The value of C<how> should be chosen
835from the set of macros C<PERL_MAGIC_foo> found perl.h. Note that before
836these macros were added, perl internals used to directly use character
837literals, so you may occasionally come across old code or documentation
838referrring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
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839
840The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
841structure. If it is not the same as the C<sv> argument, the reference
842count of the C<obj> object is incremented. If it is the same, or if
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843the C<how> argument is C<PERL_MAGIC_arylen>", or if it is a NULL pointer,
844then C<obj> is merely stored, without the reference count being incremented.
d1b91892 845
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AD
846There is also a function to add magic to an C<HV>:
847
848 void hv_magic(HV *hv, GV *gv, int how);
849
850This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
851
852To remove the magic from an SV, call the function sv_unmagic:
853
854 void sv_unmagic(SV *sv, int type);
855
856The C<type> argument should be equal to the C<how> value when the C<SV>
857was initially made magical.
858
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859=head2 Magic Virtual Tables
860
861The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a
862C<MGVTBL>, which is a structure of function pointers and stands for
863"Magic Virtual Table" to handle the various operations that might be
864applied to that variable.
865
866The C<MGVTBL> has five pointers to the following routine types:
867
868 int (*svt_get)(SV* sv, MAGIC* mg);
869 int (*svt_set)(SV* sv, MAGIC* mg);
870 U32 (*svt_len)(SV* sv, MAGIC* mg);
871 int (*svt_clear)(SV* sv, MAGIC* mg);
872 int (*svt_free)(SV* sv, MAGIC* mg);
873
874This MGVTBL structure is set at compile-time in C<perl.h> and there are
875currently 19 types (or 21 with overloading turned on). These different
876structures contain pointers to various routines that perform additional
877actions depending on which function is being called.
878
879 Function pointer Action taken
880 ---------------- ------------
881 svt_get Do something after the value of the SV is retrieved.
882 svt_set Do something after the SV is assigned a value.
883 svt_len Report on the SV's length.
884 svt_clear Clear something the SV represents.
885 svt_free Free any extra storage associated with the SV.
886
887For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 888to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
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889
890 { magic_get, magic_set, magic_len, 0, 0 }
891
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892Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
893if a get operation is being performed, the routine C<magic_get> is
894called. All the various routines for the various magical types begin
895with C<magic_>. NOTE: the magic routines are not considered part of
896the Perl API, and may not be exported by the Perl library.
d1b91892
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897
898The current kinds of Magic Virtual Tables are:
899
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900 mg_type
901 (old-style char and macro) MGVTBL Type of magic
902 -------------------------- ------ ----------------------------
903 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
904 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
905 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
906 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
907 on stash
908 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
909 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
910 (@+ and @- vars)
911 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
912 element
913 E PERL_MAGIC_env vtbl_env %ENV hash
914 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
915 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
916 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
917 I PERL_MAGIC_isa vtbl_isa @ISA array
918 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
919 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
920 L PERL_MAGIC_dbfile (none) Debugger %_<filename
921 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
922 m PERL_MAGIC_mutex vtbl_mutex ???
923 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
924 P PERL_MAGIC_tied vtbl_pack Tied array or hash
925 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
926 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
927 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
928 S PERL_MAGIC_sig vtbl_sig %SIG hash
929 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
930 t PERL_MAGIC_taint vtbl_taint Taintedness
931 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
932 v PERL_MAGIC_vec vtbl_vec vec() lvalue
933 x PERL_MAGIC_substr vtbl_substr substr() lvalue
934 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
935 variable / smart parameter
936 vivification
937 * PERL_MAGIC_glob vtbl_glob GV (typeglob)
938 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
939 . PERL_MAGIC_pos vtbl_pos pos() lvalue
940 < PERL_MAGIC_backref vtbl_backref ???
941 ~ PERL_MAGIC_ext (none) Available for use by extensions
d1b91892 942
68dc0745
PP
943When an uppercase and lowercase letter both exist in the table, then the
944uppercase letter is used to represent some kind of composite type (a list
945or a hash), and the lowercase letter is used to represent an element of
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946that composite type. Some internals code makes use of this case
947relationship.
948
949The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
950specifically for use by extensions and will not be used by perl itself.
951Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
952to variables (typically objects). This is especially useful because
953there is no way for normal perl code to corrupt this private information
954(unlike using extra elements of a hash object).
955
956Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
957C function any time a scalar's value is used or changed. The C<MAGIC>'s
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958C<mg_ptr> field points to a C<ufuncs> structure:
959
960 struct ufuncs {
961 I32 (*uf_val)(IV, SV*);
962 I32 (*uf_set)(IV, SV*);
963 IV uf_index;
964 };
965
966When the SV is read from or written to, the C<uf_val> or C<uf_set>
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967function will be called with C<uf_index> as the first arg and a pointer to
968the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
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AB
969magic is shown below. Note that the ufuncs structure is copied by
970sv_magic, so you can safely allocate it on the stack.
971
972 void
973 Umagic(sv)
974 SV *sv;
975 PREINIT:
976 struct ufuncs uf;
977 CODE:
978 uf.uf_val = &my_get_fn;
979 uf.uf_set = &my_set_fn;
980 uf.uf_index = 0;
14befaf4 981 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 982
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983Note that because multiple extensions may be using C<PERL_MAGIC_ext>
984or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
985extra care to avoid conflict. Typically only using the magic on
986objects blessed into the same class as the extension is sufficient.
987For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
988'signature' at the top of the private data area and check that.
5f05dabc 989
ef50df4b
GS
990Also note that the C<sv_set*()> and C<sv_cat*()> functions described
991earlier do B<not> invoke 'set' magic on their targets. This must
992be done by the user either by calling the C<SvSETMAGIC()> macro after
993calling these functions, or by using one of the C<sv_set*_mg()> or
994C<sv_cat*_mg()> functions. Similarly, generic C code must call the
995C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
996obtained from external sources in functions that don't handle magic.
4a4eefd0 997See L<perlapi> for a description of these functions.
189b2af5
GS
998For example, calls to the C<sv_cat*()> functions typically need to be
999followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1000since their implementation handles 'get' magic.
1001
d1b91892
AD
1002=head2 Finding Magic
1003
1004 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1005
1006This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1007If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1008if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1009
08105a92 1010 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1011
1012This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1013field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1014the mg_type field is changed to be the lowercase letter.
a0d0e21e 1015
04343c6d
GS
1016=head2 Understanding the Magic of Tied Hashes and Arrays
1017
14befaf4
DM
1018Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1019magic type.
9edb2b46
GS
1020
1021WARNING: As of the 5.004 release, proper usage of the array and hash
1022access functions requires understanding a few caveats. Some
1023of these caveats are actually considered bugs in the API, to be fixed
1024in later releases, and are bracketed with [MAYCHANGE] below. If
1025you find yourself actually applying such information in this section, be
1026aware that the behavior may change in the future, umm, without warning.
04343c6d 1027
1526ead6
AB
1028The perl tie function associates a variable with an object that implements
1029the various GET, SET etc methods. To perform the equivalent of the perl
1030tie function from an XSUB, you must mimic this behaviour. The code below
1031carries out the necessary steps - firstly it creates a new hash, and then
1032creates a second hash which it blesses into the class which will implement
1033the tie methods. Lastly it ties the two hashes together, and returns a
1034reference to the new tied hash. Note that the code below does NOT call the
1035TIEHASH method in the MyTie class -
1036see L<Calling Perl Routines from within C Programs> for details on how
1037to do this.
1038
1039 SV*
1040 mytie()
1041 PREINIT:
1042 HV *hash;
1043 HV *stash;
1044 SV *tie;
1045 CODE:
1046 hash = newHV();
1047 tie = newRV_noinc((SV*)newHV());
1048 stash = gv_stashpv("MyTie", TRUE);
1049 sv_bless(tie, stash);
899e16d0 1050 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1051 RETVAL = newRV_noinc(hash);
1052 OUTPUT:
1053 RETVAL
1054
04343c6d
GS
1055The C<av_store> function, when given a tied array argument, merely
1056copies the magic of the array onto the value to be "stored", using
1057C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1058actually need to be stored in the array. [MAYCHANGE] After a call to
1059C<av_store> on a tied array, the caller will usually need to call
1060C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1061TIEARRAY object. If C<av_store> did return NULL, a call to
1062C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1063leak. [/MAYCHANGE]
04343c6d
GS
1064
1065The previous paragraph is applicable verbatim to tied hash access using the
1066C<hv_store> and C<hv_store_ent> functions as well.
1067
1068C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1069C<hv_fetch_ent> actually return an undefined mortal value whose magic
1070has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1071need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1072need to call C<mg_get()> on the returned value in order to actually invoke
1073the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1074you may also call C<mg_set()> on the return value after possibly assigning
1075a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1076method on the TIE object. [/MAYCHANGE]
04343c6d 1077
9edb2b46 1078[MAYCHANGE]
04343c6d
GS
1079In other words, the array or hash fetch/store functions don't really
1080fetch and store actual values in the case of tied arrays and hashes. They
1081merely call C<mg_copy> to attach magic to the values that were meant to be
1082"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1083do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1084the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1085and hashes.
1086
1087Currently (as of perl version 5.004), use of the hash and array access
1088functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1089"normal" hashes and arrays, or on their tied variants. The API may be
1090changed to provide more transparent access to both tied and normal data
1091types in future versions.
1092[/MAYCHANGE]
04343c6d
GS
1093
1094You would do well to understand that the TIEARRAY and TIEHASH interfaces
1095are mere sugar to invoke some perl method calls while using the uniform hash
1096and array syntax. The use of this sugar imposes some overhead (typically
1097about two to four extra opcodes per FETCH/STORE operation, in addition to
1098the creation of all the mortal variables required to invoke the methods).
1099This overhead will be comparatively small if the TIE methods are themselves
1100substantial, but if they are only a few statements long, the overhead
1101will not be insignificant.
1102
d1c897a1
IZ
1103=head2 Localizing changes
1104
1105Perl has a very handy construction
1106
1107 {
1108 local $var = 2;
1109 ...
1110 }
1111
1112This construction is I<approximately> equivalent to
1113
1114 {
1115 my $oldvar = $var;
1116 $var = 2;
1117 ...
1118 $var = $oldvar;
1119 }
1120
1121The biggest difference is that the first construction would
1122reinstate the initial value of $var, irrespective of how control exits
1123the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit
1124more efficient as well.
1125
1126There is a way to achieve a similar task from C via Perl API: create a
1127I<pseudo-block>, and arrange for some changes to be automatically
1128undone at the end of it, either explicit, or via a non-local exit (via
1129die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1130C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1131Such a construct may be created specially for some important localized
1132task, or an existing one (like boundaries of enclosing Perl
1133subroutine/block, or an existing pair for freeing TMPs) may be
1134used. (In the second case the overhead of additional localization must
1135be almost negligible.) Note that any XSUB is automatically enclosed in
1136an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1137
1138Inside such a I<pseudo-block> the following service is available:
1139
13a2d996 1140=over 4
d1c897a1
IZ
1141
1142=item C<SAVEINT(int i)>
1143
1144=item C<SAVEIV(IV i)>
1145
1146=item C<SAVEI32(I32 i)>
1147
1148=item C<SAVELONG(long i)>
1149
1150These macros arrange things to restore the value of integer variable
1151C<i> at the end of enclosing I<pseudo-block>.
1152
1153=item C<SAVESPTR(s)>
1154
1155=item C<SAVEPPTR(p)>
1156
1157These macros arrange things to restore the value of pointers C<s> and
1158C<p>. C<s> must be a pointer of a type which survives conversion to
1159C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1160and back.
1161
1162=item C<SAVEFREESV(SV *sv)>
1163
1164The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1165I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1166mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1167extends the lifetime of C<sv> until the beginning of the next statement,
1168C<SAVEFREESV> extends it until the end of the enclosing scope. These
1169lifetimes can be wildly different.
1170
1171Also compare C<SAVEMORTALIZESV>.
1172
1173=item C<SAVEMORTALIZESV(SV *sv)>
1174
1175Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1176scope instead of decrementing its reference count. This usually has the
1177effect of keeping C<sv> alive until the statement that called the currently
1178live scope has finished executing.
d1c897a1
IZ
1179
1180=item C<SAVEFREEOP(OP *op)>
1181
1182The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1183
1184=item C<SAVEFREEPV(p)>
1185
1186The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1187end of I<pseudo-block>.
1188
1189=item C<SAVECLEARSV(SV *sv)>
1190
1191Clears a slot in the current scratchpad which corresponds to C<sv> at
1192the end of I<pseudo-block>.
1193
1194=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1195
1196The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1197string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1198short-lived storage, the corresponding string may be reallocated like
1199this:
1200
9cde0e7f 1201 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1202
c76ac1ee 1203=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1204
1205At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1206only argument C<p>.
1207
1208=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1209
1210At the end of I<pseudo-block> the function C<f> is called with the
1211implicit context argument (if any), and C<p>.
d1c897a1
IZ
1212
1213=item C<SAVESTACK_POS()>
1214
1215The current offset on the Perl internal stack (cf. C<SP>) is restored
1216at the end of I<pseudo-block>.
1217
1218=back
1219
1220The following API list contains functions, thus one needs to
1221provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1222or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1223function takes C<int *>.
1224
13a2d996 1225=over 4
d1c897a1
IZ
1226
1227=item C<SV* save_scalar(GV *gv)>
1228
1229Equivalent to Perl code C<local $gv>.
1230
1231=item C<AV* save_ary(GV *gv)>
1232
1233=item C<HV* save_hash(GV *gv)>
1234
1235Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1236
1237=item C<void save_item(SV *item)>
1238
1239Duplicates the current value of C<SV>, on the exit from the current
1240C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1241using the stored value.
1242
1243=item C<void save_list(SV **sarg, I32 maxsarg)>
1244
1245A variant of C<save_item> which takes multiple arguments via an array
1246C<sarg> of C<SV*> of length C<maxsarg>.
1247
1248=item C<SV* save_svref(SV **sptr)>
1249
1250Similar to C<save_scalar>, but will reinstate a C<SV *>.
1251
1252=item C<void save_aptr(AV **aptr)>
1253
1254=item C<void save_hptr(HV **hptr)>
1255
1256Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1257
1258=back
1259
1260The C<Alias> module implements localization of the basic types within the
1261I<caller's scope>. People who are interested in how to localize things in
1262the containing scope should take a look there too.
1263
0a753a76 1264=head1 Subroutines
a0d0e21e 1265
68dc0745 1266=head2 XSUBs and the Argument Stack
5f05dabc
PP
1267
1268The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1269An XSUB routine will have a stack that contains the arguments from the Perl
1270program, and a way to map from the Perl data structures to a C equivalent.
1271
1272The stack arguments are accessible through the C<ST(n)> macro, which returns
1273the C<n>'th stack argument. Argument 0 is the first argument passed in the
1274Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1275an C<SV*> is used.
1276
1277Most of the time, output from the C routine can be handled through use of
1278the RETVAL and OUTPUT directives. However, there are some cases where the
1279argument stack is not already long enough to handle all the return values.
1280An example is the POSIX tzname() call, which takes no arguments, but returns
1281two, the local time zone's standard and summer time abbreviations.
1282
1283To handle this situation, the PPCODE directive is used and the stack is
1284extended using the macro:
1285
924508f0 1286 EXTEND(SP, num);
5f05dabc 1287
924508f0
GS
1288where C<SP> is the macro that represents the local copy of the stack pointer,
1289and C<num> is the number of elements the stack should be extended by.
5f05dabc 1290
00aadd71 1291Now that there is room on the stack, values can be pushed on it using C<PUSHs>
484ce0c5 1292macro. The values pushed will often need to be "mortal" (See L</Reference Counts and Mortality>).
5f05dabc 1293
00aadd71
NIS
1294 PUSHs(sv_2mortal(newSViv(an_integer)))
1295 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1296 PUSHs(sv_2mortal(newSVnv(3.141592)))
5f05dabc
PP
1297
1298And now the Perl program calling C<tzname>, the two values will be assigned
1299as in:
1300
1301 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1302
1303An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1304to use the macro:
5f05dabc 1305
5f05dabc
PP
1306 XPUSHs(SV*)
1307
00aadd71 1308This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1309do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1310
1311Despite their suggestions in earlier versions of this document the macros
1312C<PUSHi>, C<PUSHn> and C<PUSHp> are I<not> suited to XSUBs which return
1313multiple results, see L</Putting a C value on Perl stack>.
5f05dabc
PP
1314
1315For more information, consult L<perlxs> and L<perlxstut>.
1316
1317=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1318
1319There are four routines that can be used to call a Perl subroutine from
1320within a C program. These four are:
1321
954c1994
GS
1322 I32 call_sv(SV*, I32);
1323 I32 call_pv(const char*, I32);
1324 I32 call_method(const char*, I32);
1325 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1326
954c1994 1327The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1328contains either the name of the Perl subroutine to be called, or a
1329reference to the subroutine. The second argument consists of flags
1330that control the context in which the subroutine is called, whether
1331or not the subroutine is being passed arguments, how errors should be
1332trapped, and how to treat return values.
a0d0e21e
LW
1333
1334All four routines return the number of arguments that the subroutine returned
1335on the Perl stack.
1336
954c1994
GS
1337These routines used to be called C<perl_call_sv> etc., before Perl v5.6.0,
1338but those names are now deprecated; macros of the same name are provided for
1339compatibility.
1340
1341When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1342must manipulate the Perl stack. These include the following macros and
1343functions:
a0d0e21e
LW
1344
1345 dSP
924508f0 1346 SP
a0d0e21e
LW
1347 PUSHMARK()
1348 PUTBACK
1349 SPAGAIN
1350 ENTER
1351 SAVETMPS
1352 FREETMPS
1353 LEAVE
1354 XPUSH*()
cb1a09d0 1355 POP*()
a0d0e21e 1356
5f05dabc
PP
1357For a detailed description of calling conventions from C to Perl,
1358consult L<perlcall>.
a0d0e21e 1359
5f05dabc 1360=head2 Memory Allocation
a0d0e21e 1361
86058a2d
GS
1362All memory meant to be used with the Perl API functions should be manipulated
1363using the macros described in this section. The macros provide the necessary
1364transparency between differences in the actual malloc implementation that is
1365used within perl.
1366
1367It is suggested that you enable the version of malloc that is distributed
5f05dabc 1368with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1369order to satisfy allocation requests more quickly. However, on some
1370platforms, it may cause spurious malloc or free errors.
d1b91892
AD
1371
1372 New(x, pointer, number, type);
1373 Newc(x, pointer, number, type, cast);
1374 Newz(x, pointer, number, type);
1375
07fa94a1 1376These three macros are used to initially allocate memory.
5f05dabc
PP
1377
1378The first argument C<x> was a "magic cookie" that was used to keep track
1379of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1380the current code makes no use of this feature (most Perl developers now
1381use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1382
1383The second argument C<pointer> should be the name of a variable that will
1384point to the newly allocated memory.
d1b91892 1385
d1b91892
AD
1386The third and fourth arguments C<number> and C<type> specify how many of
1387the specified type of data structure should be allocated. The argument
1388C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1389should be used if the C<pointer> argument is different from the C<type>
1390argument.
1391
1392Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1393to zero out all the newly allocated memory.
1394
1395 Renew(pointer, number, type);
1396 Renewc(pointer, number, type, cast);
1397 Safefree(pointer)
1398
1399These three macros are used to change a memory buffer size or to free a
1400piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1401match those of C<New> and C<Newc> with the exception of not needing the
1402"magic cookie" argument.
1403
1404 Move(source, dest, number, type);
1405 Copy(source, dest, number, type);
1406 Zero(dest, number, type);
1407
1408These three macros are used to move, copy, or zero out previously allocated
1409memory. The C<source> and C<dest> arguments point to the source and
1410destination starting points. Perl will move, copy, or zero out C<number>
1411instances of the size of the C<type> data structure (using the C<sizeof>
1412function).
a0d0e21e 1413
5f05dabc 1414=head2 PerlIO
ce3d39e2 1415
5f05dabc
PP
1416The most recent development releases of Perl has been experimenting with
1417removing Perl's dependency on the "normal" standard I/O suite and allowing
1418other stdio implementations to be used. This involves creating a new
1419abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1420was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1421abstraction layer and not make any assumptions about what kind of stdio
1422is being used.
1423
1424For a complete description of the PerlIO abstraction, consult L<perlapio>.
1425
8ebc5c01 1426=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1427
1428A lot of opcodes (this is an elementary operation in the internal perl
1429stack machine) put an SV* on the stack. However, as an optimization
1430the corresponding SV is (usually) not recreated each time. The opcodes
1431reuse specially assigned SVs (I<target>s) which are (as a corollary)
1432not constantly freed/created.
1433
0a753a76 1434Each of the targets is created only once (but see
ce3d39e2
IZ
1435L<Scratchpads and recursion> below), and when an opcode needs to put
1436an integer, a double, or a string on stack, it just sets the
1437corresponding parts of its I<target> and puts the I<target> on stack.
1438
1439The macro to put this target on stack is C<PUSHTARG>, and it is
1440directly used in some opcodes, as well as indirectly in zillions of
1441others, which use it via C<(X)PUSH[pni]>.
1442
1bd1c0d5
SC
1443Because the target is reused, you must be careful when pushing multiple
1444values on the stack. The following code will not do what you think:
1445
1446 XPUSHi(10);
1447 XPUSHi(20);
1448
1449This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1450the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1451At the end of the operation, the stack does not contain the values 10
1452and 20, but actually contains two pointers to C<TARG>, which we have set
1453to 20. If you need to push multiple different values, use C<XPUSHs>,
1454which bypasses C<TARG>.
1455
1456On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
1457need a C<dTARG> in your variable declarations so that the C<*PUSH*>
00aadd71 1458macros can make use of the local variable C<TARG>.
1bd1c0d5 1459
8ebc5c01 1460=head2 Scratchpads
ce3d39e2 1461
54310121 1462The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1463are created. The answer is that they are created when the current unit --
1464a subroutine or a file (for opcodes for statements outside of
1465subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1466array is created, which is called a scratchpad for the current
1467unit.
1468
54310121 1469A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1470targets for opcodes. One can deduce that an SV lives on a scratchpad
1471by looking on its flags: lexicals have C<SVs_PADMY> set, and
1472I<target>s have C<SVs_PADTMP> set.
1473
54310121
PP
1474The correspondence between OPs and I<target>s is not 1-to-1. Different
1475OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1476would not conflict with the expected life of the temporary.
1477
2ae324a7 1478=head2 Scratchpads and recursion
ce3d39e2
IZ
1479
1480In fact it is not 100% true that a compiled unit contains a pointer to
1481the scratchpad AV. In fact it contains a pointer to an AV of
1482(initially) one element, and this element is the scratchpad AV. Why do
1483we need an extra level of indirection?
1484
1485The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both
1486these can create several execution pointers going into the same
1487subroutine. For the subroutine-child not write over the temporaries
1488for the subroutine-parent (lifespan of which covers the call to the
1489child), the parent and the child should have different
1490scratchpads. (I<And> the lexicals should be separate anyway!)
1491
5f05dabc
PP
1492So each subroutine is born with an array of scratchpads (of length 1).
1493On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1494depth of the recursion is not more than the length of this array, and
1495if it is, new scratchpad is created and pushed into the array.
1496
1497The I<target>s on this scratchpad are C<undef>s, but they are already
1498marked with correct flags.
1499
0a753a76
PP
1500=head1 Compiled code
1501
1502=head2 Code tree
1503
1504Here we describe the internal form your code is converted to by
1505Perl. Start with a simple example:
1506
1507 $a = $b + $c;
1508
1509This is converted to a tree similar to this one:
1510
1511 assign-to
1512 / \
1513 + $a
1514 / \
1515 $b $c
1516
7b8d334a 1517(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1518parsed your code, but has nothing to do with the execution order.
1519There is an additional "thread" going through the nodes of the tree
1520which shows the order of execution of the nodes. In our simplified
1521example above it looks like:
1522
1523 $b ---> $c ---> + ---> $a ---> assign-to
1524
1525But with the actual compile tree for C<$a = $b + $c> it is different:
1526some nodes I<optimized away>. As a corollary, though the actual tree
1527contains more nodes than our simplified example, the execution order
1528is the same as in our example.
1529
1530=head2 Examining the tree
1531
1532If you have your perl compiled for debugging (usually done with C<-D
1533optimize=-g> on C<Configure> command line), you may examine the
1534compiled tree by specifying C<-Dx> on the Perl command line. The
1535output takes several lines per node, and for C<$b+$c> it looks like
1536this:
1537
1538 5 TYPE = add ===> 6
1539 TARG = 1
1540 FLAGS = (SCALAR,KIDS)
1541 {
1542 TYPE = null ===> (4)
1543 (was rv2sv)
1544 FLAGS = (SCALAR,KIDS)
1545 {
1546 3 TYPE = gvsv ===> 4
1547 FLAGS = (SCALAR)
1548 GV = main::b
1549 }
1550 }
1551 {
1552 TYPE = null ===> (5)
1553 (was rv2sv)
1554 FLAGS = (SCALAR,KIDS)
1555 {
1556 4 TYPE = gvsv ===> 5
1557 FLAGS = (SCALAR)
1558 GV = main::c
1559 }
1560 }
1561
1562This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1563not optimized away (one per number in the left column). The immediate
1564children of the given node correspond to C<{}> pairs on the same level
1565of indentation, thus this listing corresponds to the tree:
1566
1567 add
1568 / \
1569 null null
1570 | |
1571 gvsv gvsv
1572
1573The execution order is indicated by C<===E<gt>> marks, thus it is C<3
15744 5 6> (node C<6> is not included into above listing), i.e.,
1575C<gvsv gvsv add whatever>.
1576
9afa14e3
SC
1577Each of these nodes represents an op, a fundamental operation inside the
1578Perl core. The code which implements each operation can be found in the
1579F<pp*.c> files; the function which implements the op with type C<gvsv>
1580is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1581different numbers of children: C<add> is a binary operator, as one would
1582expect, and so has two children. To accommodate the various different
1583numbers of children, there are various types of op data structure, and
1584they link together in different ways.
1585
1586The simplest type of op structure is C<OP>: this has no children. Unary
1587operators, C<UNOP>s, have one child, and this is pointed to by the
1588C<op_first> field. Binary operators (C<BINOP>s) have not only an
1589C<op_first> field but also an C<op_last> field. The most complex type of
1590op is a C<LISTOP>, which has any number of children. In this case, the
1591first child is pointed to by C<op_first> and the last child by
1592C<op_last>. The children in between can be found by iteratively
1593following the C<op_sibling> pointer from the first child to the last.
1594
1595There are also two other op types: a C<PMOP> holds a regular expression,
1596and has no children, and a C<LOOP> may or may not have children. If the
1597C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1598complicate matters, if a C<UNOP> is actually a C<null> op after
1599optimization (see L</Compile pass 2: context propagation>) it will still
1600have children in accordance with its former type.
1601
0a753a76
PP
1602=head2 Compile pass 1: check routines
1603
8870b5c7
GS
1604The tree is created by the compiler while I<yacc> code feeds it
1605the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1606the first pass of perl compilation.
1607
1608What makes this pass interesting for perl developers is that some
1609optimization may be performed on this pass. This is optimization by
8870b5c7 1610so-called "check routines". The correspondence between node names
0a753a76
PP
1611and corresponding check routines is described in F<opcode.pl> (do not
1612forget to run C<make regen_headers> if you modify this file).
1613
1614A check routine is called when the node is fully constructed except
7b8d334a 1615for the execution-order thread. Since at this time there are no
0a753a76
PP
1616back-links to the currently constructed node, one can do most any
1617operation to the top-level node, including freeing it and/or creating
1618new nodes above/below it.
1619
1620The check routine returns the node which should be inserted into the
1621tree (if the top-level node was not modified, check routine returns
1622its argument).
1623
1624By convention, check routines have names C<ck_*>. They are usually
1625called from C<new*OP> subroutines (or C<convert>) (which in turn are
1626called from F<perly.y>).
1627
1628=head2 Compile pass 1a: constant folding
1629
1630Immediately after the check routine is called the returned node is
1631checked for being compile-time executable. If it is (the value is
1632judged to be constant) it is immediately executed, and a I<constant>
1633node with the "return value" of the corresponding subtree is
1634substituted instead. The subtree is deleted.
1635
1636If constant folding was not performed, the execution-order thread is
1637created.
1638
1639=head2 Compile pass 2: context propagation
1640
1641When a context for a part of compile tree is known, it is propagated
a3cb178b 1642down through the tree. At this time the context can have 5 values
0a753a76
PP
1643(instead of 2 for runtime context): void, boolean, scalar, list, and
1644lvalue. In contrast with the pass 1 this pass is processed from top
1645to bottom: a node's context determines the context for its children.
1646
1647Additional context-dependent optimizations are performed at this time.
1648Since at this moment the compile tree contains back-references (via
1649"thread" pointers), nodes cannot be free()d now. To allow
1650optimized-away nodes at this stage, such nodes are null()ified instead
1651of free()ing (i.e. their type is changed to OP_NULL).
1652
1653=head2 Compile pass 3: peephole optimization
1654
1655After the compile tree for a subroutine (or for an C<eval> or a file)
1656is created, an additional pass over the code is performed. This pass
1657is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1658additional complications for conditionals). These optimizations are
0a753a76
PP
1659done in the subroutine peep(). Optimizations performed at this stage
1660are subject to the same restrictions as in the pass 2.
1661
9afa14e3
SC
1662=head1 Examining internal data structures with the C<dump> functions
1663
1664To aid debugging, the source file F<dump.c> contains a number of
1665functions which produce formatted output of internal data structures.
1666
1667The most commonly used of these functions is C<Perl_sv_dump>; it's used
1668for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1669C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1670module should already be familiar with its format.
9afa14e3
SC
1671
1672C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
1673derivatives, and produces output similiar to C<perl -Dx>; in fact,
1674C<Perl_dump_eval> will dump the main root of the code being evaluated,
1675exactly like C<-Dx>.
1676
1677Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1678op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1679subroutines in a package like so: (Thankfully, these are all xsubs, so
1680there is no op tree)
1681
1682 (gdb) print Perl_dump_packsubs(PL_defstash)
1683
1684 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1685
1686 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1687
1688 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1689
1690 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1691
1692 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1693
1694and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1695the op tree of the main root.
1696
954c1994 1697=head1 How multiple interpreters and concurrency are supported
ee072b34 1698
ee072b34
GS
1699=head2 Background and PERL_IMPLICIT_CONTEXT
1700
1701The Perl interpreter can be regarded as a closed box: it has an API
1702for feeding it code or otherwise making it do things, but it also has
1703functions for its own use. This smells a lot like an object, and
1704there are ways for you to build Perl so that you can have multiple
1705interpreters, with one interpreter represented either as a C++ object,
1706a C structure, or inside a thread. The thread, the C structure, or
1707the C++ object will contain all the context, the state of that
1708interpreter.
1709
54aff467
GS
1710Three macros control the major Perl build flavors: MULTIPLICITY,
1711USE_THREADS and PERL_OBJECT. The MULTIPLICITY build has a C structure
1712that packages all the interpreter state, there is a similar thread-specific
a7486cbb
JH
1713data structure under USE_THREADS, and the (now deprecated) PERL_OBJECT
1714build has a C++ class to maintain interpreter state. In all three cases,
54aff467
GS
1715PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1716support for passing in a "hidden" first argument that represents all three
651a3225 1717data structures.
54aff467
GS
1718
1719All this obviously requires a way for the Perl internal functions to be
ee072b34
GS
1720C++ methods, subroutines taking some kind of structure as the first
1721argument, or subroutines taking nothing as the first argument. To
1722enable these three very different ways of building the interpreter,
1723the Perl source (as it does in so many other situations) makes heavy
1724use of macros and subroutine naming conventions.
1725
54aff467 1726First problem: deciding which functions will be public API functions and
00aadd71 1727which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1728(think "S" for "secret" or "static"). All other functions begin with
1729"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1730part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1731function is part of the API is to find its entry in L<perlapi>.
1732If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1733think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1734L<perlbug> explaining why you think it should be.
ee072b34
GS
1735
1736Second problem: there must be a syntax so that the same subroutine
1737declarations and calls can pass a structure as their first argument,
1738or pass nothing. To solve this, the subroutines are named and
1739declared in a particular way. Here's a typical start of a static
1740function used within the Perl guts:
1741
1742 STATIC void
1743 S_incline(pTHX_ char *s)
1744
1745STATIC becomes "static" in C, and is #define'd to nothing in C++.
1746
651a3225
GS
1747A public function (i.e. part of the internal API, but not necessarily
1748sanctioned for use in extensions) begins like this:
ee072b34
GS
1749
1750 void
1751 Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)
1752
1753C<pTHX_> is one of a number of macros (in perl.h) that hide the
1754details of the interpreter's context. THX stands for "thread", "this",
1755or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1756The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1757or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1758their variants.
ee072b34 1759
a7486cbb
JH
1760When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1761first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1762in the pTHX_ macro indicates that the macro expansion needs a comma
1763after the context argument because other arguments follow it. If
1764PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1765subroutine is not prototyped to take the extra argument. The form of the
1766macro without the trailing underscore is used when there are no additional
ee072b34
GS
1767explicit arguments.
1768
54aff467 1769When a core function calls another, it must pass the context. This
a7486cbb 1770is normally hidden via macros. Consider C<sv_setsv>. It expands into
ee072b34
GS
1771something like this:
1772
1773 ifdef PERL_IMPLICIT_CONTEXT
c52f9dcd 1774 define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
ee072b34
GS
1775 /* can't do this for vararg functions, see below */
1776 else
c52f9dcd 1777 define sv_setsv Perl_sv_setsv
ee072b34
GS
1778 endif
1779
1780This works well, and means that XS authors can gleefully write:
1781
1782 sv_setsv(foo, bar);
1783
1784and still have it work under all the modes Perl could have been
1785compiled with.
1786
1787Under PERL_OBJECT in the core, that will translate to either:
1788
1789 CPerlObj::Perl_sv_setsv(foo,bar); # in CPerlObj functions,
1790 # C++ takes care of 'this'
1791 or
1792
1793 pPerl->Perl_sv_setsv(foo,bar); # in truly static functions,
1794 # see objXSUB.h
1795
1796Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
a7486cbb
JH
1797MULTIPLICITY/USE_THREADS with PERL_IMPLICIT_CONTEXT in both core
1798and extensions, it will become:
ee072b34
GS
1799
1800 Perl_sv_setsv(aTHX_ foo, bar); # the canonical Perl "API"
1801 # for all build flavors
1802
1803This doesn't work so cleanly for varargs functions, though, as macros
1804imply that the number of arguments is known in advance. Instead we
1805either need to spell them out fully, passing C<aTHX_> as the first
1806argument (the Perl core tends to do this with functions like
1807Perl_warner), or use a context-free version.
1808
1809The context-free version of Perl_warner is called
1810Perl_warner_nocontext, and does not take the extra argument. Instead
1811it does dTHX; to get the context from thread-local storage. We
1812C<#define warner Perl_warner_nocontext> so that extensions get source
1813compatibility at the expense of performance. (Passing an arg is
1814cheaper than grabbing it from thread-local storage.)
1815
1816You can ignore [pad]THX[xo] when browsing the Perl headers/sources.
1817Those are strictly for use within the core. Extensions and embedders
1818need only be aware of [pad]THX.
1819
a7486cbb
JH
1820=head2 So what happened to dTHR?
1821
1822C<dTHR> was introduced in perl 5.005 to support the older thread model.
1823The older thread model now uses the C<THX> mechanism to pass context
1824pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
1825later still have it for backward source compatibility, but it is defined
1826to be a no-op.
1827
ee072b34
GS
1828=head2 How do I use all this in extensions?
1829
1830When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1831any functions in the Perl API will need to pass the initial context
1832argument somehow. The kicker is that you will need to write it in
1833such a way that the extension still compiles when Perl hasn't been
1834built with PERL_IMPLICIT_CONTEXT enabled.
1835
1836There are three ways to do this. First, the easy but inefficient way,
1837which is also the default, in order to maintain source compatibility
1838with extensions: whenever XSUB.h is #included, it redefines the aTHX
1839and aTHX_ macros to call a function that will return the context.
1840Thus, something like:
1841
1842 sv_setsv(asv, bsv);
1843
4375e838 1844in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1845in effect:
ee072b34 1846
2fa86c13 1847 Perl_sv_setsv(Perl_get_context(), asv, bsv);
ee072b34 1848
54aff467 1849or to this otherwise:
ee072b34
GS
1850
1851 Perl_sv_setsv(asv, bsv);
1852
1853You have to do nothing new in your extension to get this; since
2fa86c13 1854the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1855work.
1856
1857The second, more efficient way is to use the following template for
1858your Foo.xs:
1859
c52f9dcd
JH
1860 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1861 #include "EXTERN.h"
1862 #include "perl.h"
1863 #include "XSUB.h"
ee072b34
GS
1864
1865 static my_private_function(int arg1, int arg2);
1866
c52f9dcd
JH
1867 static SV *
1868 my_private_function(int arg1, int arg2)
1869 {
1870 dTHX; /* fetch context */
1871 ... call many Perl API functions ...
1872 }
ee072b34
GS
1873
1874 [... etc ...]
1875
c52f9dcd 1876 MODULE = Foo PACKAGE = Foo
ee072b34 1877
c52f9dcd 1878 /* typical XSUB */
ee072b34 1879
c52f9dcd
JH
1880 void
1881 my_xsub(arg)
1882 int arg
1883 CODE:
1884 my_private_function(arg, 10);
ee072b34
GS
1885
1886Note that the only two changes from the normal way of writing an
1887extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
1888including the Perl headers, followed by a C<dTHX;> declaration at
1889the start of every function that will call the Perl API. (You'll
1890know which functions need this, because the C compiler will complain
1891that there's an undeclared identifier in those functions.) No changes
1892are needed for the XSUBs themselves, because the XS() macro is
1893correctly defined to pass in the implicit context if needed.
1894
1895The third, even more efficient way is to ape how it is done within
1896the Perl guts:
1897
1898
c52f9dcd
JH
1899 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1900 #include "EXTERN.h"
1901 #include "perl.h"
1902 #include "XSUB.h"
ee072b34
GS
1903
1904 /* pTHX_ only needed for functions that call Perl API */
1905 static my_private_function(pTHX_ int arg1, int arg2);
1906
c52f9dcd
JH
1907 static SV *
1908 my_private_function(pTHX_ int arg1, int arg2)
1909 {
1910 /* dTHX; not needed here, because THX is an argument */
1911 ... call Perl API functions ...
1912 }
ee072b34
GS
1913
1914 [... etc ...]
1915
c52f9dcd 1916 MODULE = Foo PACKAGE = Foo
ee072b34 1917
c52f9dcd 1918 /* typical XSUB */
ee072b34 1919
c52f9dcd
JH
1920 void
1921 my_xsub(arg)
1922 int arg
1923 CODE:
1924 my_private_function(aTHX_ arg, 10);
ee072b34
GS
1925
1926This implementation never has to fetch the context using a function
1927call, since it is always passed as an extra argument. Depending on
1928your needs for simplicity or efficiency, you may mix the previous
1929two approaches freely.
1930
651a3225
GS
1931Never add a comma after C<pTHX> yourself--always use the form of the
1932macro with the underscore for functions that take explicit arguments,
1933or the form without the argument for functions with no explicit arguments.
ee072b34 1934
a7486cbb
JH
1935=head2 Should I do anything special if I call perl from multiple threads?
1936
1937If you create interpreters in one thread and then proceed to call them in
1938another, you need to make sure perl's own Thread Local Storage (TLS) slot is
1939initialized correctly in each of those threads.
1940
1941The C<perl_alloc> and C<perl_clone> API functions will automatically set
1942the TLS slot to the interpreter they created, so that there is no need to do
1943anything special if the interpreter is always accessed in the same thread that
1944created it, and that thread did not create or call any other interpreters
1945afterwards. If that is not the case, you have to set the TLS slot of the
1946thread before calling any functions in the Perl API on that particular
1947interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
1948thread as the first thing you do:
1949
1950 /* do this before doing anything else with some_perl */
1951 PERL_SET_CONTEXT(some_perl);
1952
1953 ... other Perl API calls on some_perl go here ...
1954
ee072b34
GS
1955=head2 Future Plans and PERL_IMPLICIT_SYS
1956
1957Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
1958that the interpreter knows about itself and pass it around, so too are
1959there plans to allow the interpreter to bundle up everything it knows
1960about the environment it's running on. This is enabled with the
a7486cbb
JH
1961PERL_IMPLICIT_SYS macro. Currently it only works with PERL_OBJECT
1962and USE_THREADS on Windows (see inside iperlsys.h).
ee072b34
GS
1963
1964This allows the ability to provide an extra pointer (called the "host"
1965environment) for all the system calls. This makes it possible for
1966all the system stuff to maintain their own state, broken down into
1967seven C structures. These are thin wrappers around the usual system
1968calls (see win32/perllib.c) for the default perl executable, but for a
1969more ambitious host (like the one that would do fork() emulation) all
1970the extra work needed to pretend that different interpreters are
1971actually different "processes", would be done here.
1972
1973The Perl engine/interpreter and the host are orthogonal entities.
1974There could be one or more interpreters in a process, and one or
1975more "hosts", with free association between them.
1976
a422fd2d
SC
1977=head1 Internal Functions
1978
1979All of Perl's internal functions which will be exposed to the outside
1980world are be prefixed by C<Perl_> so that they will not conflict with XS
1981functions or functions used in a program in which Perl is embedded.
1982Similarly, all global variables begin with C<PL_>. (By convention,
1983static functions start with C<S_>)
1984
1985Inside the Perl core, you can get at the functions either with or
1986without the C<Perl_> prefix, thanks to a bunch of defines that live in
1987F<embed.h>. This header file is generated automatically from
1988F<embed.pl>. F<embed.pl> also creates the prototyping header files for
1989the internal functions, generates the documentation and a lot of other
1990bits and pieces. It's important that when you add a new function to the
1991core or change an existing one, you change the data in the table at the
1992end of F<embed.pl> as well. Here's a sample entry from that table:
1993
1994 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
1995
1996The second column is the return type, the third column the name. Columns
1997after that are the arguments. The first column is a set of flags:
1998
1999=over 3
2000
2001=item A
2002
2003This function is a part of the public API.
2004
2005=item p
2006
2007This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
2008
2009=item d
2010
2011This function has documentation using the C<apidoc> feature which we'll
2012look at in a second.
2013
2014=back
2015
2016Other available flags are:
2017
2018=over 3
2019
2020=item s
2021
a7486cbb
JH
2022This is a static function and is defined as C<S_whatever>, and usually
2023called within the sources as C<whatever(...)>.
a422fd2d
SC
2024
2025=item n
2026
2027This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
2028L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2029
2030=item r
2031
2032This function never returns; C<croak>, C<exit> and friends.
2033
2034=item f
2035
2036This function takes a variable number of arguments, C<printf> style.
2037The argument list should end with C<...>, like this:
2038
2039 Afprd |void |croak |const char* pat|...
2040
a7486cbb 2041=item M
a422fd2d 2042
00aadd71 2043This function is part of the experimental development API, and may change
a422fd2d
SC
2044or disappear without notice.
2045
2046=item o
2047
2048This function should not have a compatibility macro to define, say,
2049C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2050
2051=item j
2052
2053This function is not a member of C<CPerlObj>. If you don't know
2054what this means, don't use it.
2055
2056=item x
2057
2058This function isn't exported out of the Perl core.
2059
2060=back
2061
2062If you edit F<embed.pl>, you will need to run C<make regen_headers> to
2063force a rebuild of F<embed.h> and other auto-generated files.
2064
6b4667fc 2065=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2066
6b4667fc
A
2067If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2068formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2069following macros for portability
9dd9db0b 2070
c52f9dcd
JH
2071 IVdf IV in decimal
2072 UVuf UV in decimal
2073 UVof UV in octal
2074 UVxf UV in hexadecimal
2075 NVef NV %e-like
2076 NVff NV %f-like
2077 NVgf NV %g-like
9dd9db0b 2078
6b4667fc
A
2079These will take care of 64-bit integers and long doubles.
2080For example:
2081
c52f9dcd 2082 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2083
2084The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2085
8908e76d
JH
2086If you are printing addresses of pointers, use UVxf combined
2087with PTR2UV(), do not use %lx or %p.
2088
2089=head2 Pointer-To-Integer and Integer-To-Pointer
2090
2091Because pointer size does not necessarily equal integer size,
2092use the follow macros to do it right.
2093
c52f9dcd
JH
2094 PTR2UV(pointer)
2095 PTR2IV(pointer)
2096 PTR2NV(pointer)
2097 INT2PTR(pointertotype, integer)
8908e76d
JH
2098
2099For example:
2100
c52f9dcd
JH
2101 IV iv = ...;
2102 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2103
2104and
2105
c52f9dcd
JH
2106 AV *av = ...;
2107 UV uv = PTR2UV(av);
8908e76d 2108
a422fd2d
SC
2109=head2 Source Documentation
2110
2111There's an effort going on to document the internal functions and
2112automatically produce reference manuals from them - L<perlapi> is one
2113such manual which details all the functions which are available to XS
2114writers. L<perlintern> is the autogenerated manual for the functions
2115which are not part of the API and are supposedly for internal use only.
2116
2117Source documentation is created by putting POD comments into the C
2118source, like this:
2119
2120 /*
2121 =for apidoc sv_setiv
2122
2123 Copies an integer into the given SV. Does not handle 'set' magic. See
2124 C<sv_setiv_mg>.
2125
2126 =cut
2127 */
2128
2129Please try and supply some documentation if you add functions to the
2130Perl core.
2131
2132=head1 Unicode Support
2133
2134Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2135writers to understand this support and make sure that the code they
2136write does not corrupt Unicode data.
2137
2138=head2 What B<is> Unicode, anyway?
2139
2140In the olden, less enlightened times, we all used to use ASCII. Most of
2141us did, anyway. The big problem with ASCII is that it's American. Well,
2142no, that's not actually the problem; the problem is that it's not
2143particularly useful for people who don't use the Roman alphabet. What
2144used to happen was that particular languages would stick their own
2145alphabet in the upper range of the sequence, between 128 and 255. Of
2146course, we then ended up with plenty of variants that weren't quite
2147ASCII, and the whole point of it being a standard was lost.
2148
2149Worse still, if you've got a language like Chinese or
2150Japanese that has hundreds or thousands of characters, then you really
2151can't fit them into a mere 256, so they had to forget about ASCII
2152altogether, and build their own systems using pairs of numbers to refer
2153to one character.
2154
2155To fix this, some people formed Unicode, Inc. and
2156produced a new character set containing all the characters you can
2157possibly think of and more. There are several ways of representing these
2158characters, and the one Perl uses is called UTF8. UTF8 uses
2159a variable number of bytes to represent a character, instead of just
b3b6085d 2160one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d
SC
2161
2162=head2 How can I recognise a UTF8 string?
2163
2164You can't. This is because UTF8 data is stored in bytes just like
2165non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
2166capital E with a grave accent, is represented by the two bytes
2167C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2168has that byte sequence as well. So you can't tell just by looking - this
2169is what makes Unicode input an interesting problem.
2170
2171The API function C<is_utf8_string> can help; it'll tell you if a string
2172contains only valid UTF8 characters. However, it can't do the work for
2173you. On a character-by-character basis, C<is_utf8_char> will tell you
2174whether the current character in a string is valid UTF8.
2175
2176=head2 How does UTF8 represent Unicode characters?
2177
2178As mentioned above, UTF8 uses a variable number of bytes to store a
2179character. Characters with values 1...128 are stored in one byte, just
2180like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2181continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2182bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2183so it goes on, moving to three bytes at character 2048.
2184
2185Assuming you know you're dealing with a UTF8 string, you can find out
2186how long the first character in it is with the C<UTF8SKIP> macro:
2187
2188 char *utf = "\305\233\340\240\201";
2189 I32 len;
2190
2191 len = UTF8SKIP(utf); /* len is 2 here */
2192 utf += len;
2193 len = UTF8SKIP(utf); /* len is 3 here */
2194
2195Another way to skip over characters in a UTF8 string is to use
2196C<utf8_hop>, which takes a string and a number of characters to skip
2197over. You're on your own about bounds checking, though, so don't use it
2198lightly.
2199
2200All bytes in a multi-byte UTF8 character will have the high bit set, so
2201you can test if you need to do something special with this character
2202like this:
2203
2204 UV uv;
2205
2206 if (utf & 0x80)
2207 /* Must treat this as UTF8 */
2208 uv = utf8_to_uv(utf);
2209 else
2210 /* OK to treat this character as a byte */
2211 uv = *utf;
2212
2213You can also see in that example that we use C<utf8_to_uv> to get the
2214value of the character; the inverse function C<uv_to_utf8> is available
2215for putting a UV into UTF8:
2216
2217 if (uv > 0x80)
2218 /* Must treat this as UTF8 */
2219 utf8 = uv_to_utf8(utf8, uv);
2220 else
2221 /* OK to treat this character as a byte */
2222 *utf8++ = uv;
2223
2224You B<must> convert characters to UVs using the above functions if
2225you're ever in a situation where you have to match UTF8 and non-UTF8
2226characters. You may not skip over UTF8 characters in this case. If you
2227do this, you'll lose the ability to match hi-bit non-UTF8 characters;
2228for instance, if your UTF8 string contains C<v196.172>, and you skip
2229that character, you can never match a C<chr(200)> in a non-UTF8 string.
2230So don't do that!
2231
2232=head2 How does Perl store UTF8 strings?
2233
2234Currently, Perl deals with Unicode strings and non-Unicode strings
2235slightly differently. If a string has been identified as being UTF-8
2236encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2237manipulate this flag with the following macros:
2238
2239 SvUTF8(sv)
2240 SvUTF8_on(sv)
2241 SvUTF8_off(sv)
2242
2243This flag has an important effect on Perl's treatment of the string: if
2244Unicode data is not properly distinguished, regular expressions,
2245C<length>, C<substr> and other string handling operations will have
2246undesirable results.
2247
2248The problem comes when you have, for instance, a string that isn't
2249flagged is UTF8, and contains a byte sequence that could be UTF8 -
2250especially when combining non-UTF8 and UTF8 strings.
2251
2252Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2253need be sure you don't accidentally knock it off while you're
2254manipulating SVs. More specifically, you cannot expect to do this:
2255
2256 SV *sv;
2257 SV *nsv;
2258 STRLEN len;
2259 char *p;
2260
2261 p = SvPV(sv, len);
2262 frobnicate(p);
2263 nsv = newSVpvn(p, len);
2264
2265The C<char*> string does not tell you the whole story, and you can't
2266copy or reconstruct an SV just by copying the string value. Check if the
2267old SV has the UTF8 flag set, and act accordingly:
2268
2269 p = SvPV(sv, len);
2270 frobnicate(p);
2271 nsv = newSVpvn(p, len);
2272 if (SvUTF8(sv))
2273 SvUTF8_on(nsv);
2274
2275In fact, your C<frobnicate> function should be made aware of whether or
2276not it's dealing with UTF8 data, so that it can handle the string
2277appropriately.
2278
2279=head2 How do I convert a string to UTF8?
2280
2281If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
2282to upgrade one of the strings to UTF8. If you've got an SV, the easiest
2283way to do this is:
2284
2285 sv_utf8_upgrade(sv);
2286
2287However, you must not do this, for example:
2288
2289 if (!SvUTF8(left))
2290 sv_utf8_upgrade(left);
2291
2292If you do this in a binary operator, you will actually change one of the
b1866b2d 2293strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2294by the end user, it can cause problems.
2295
2296Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
2297string argument. This is useful for having the data available for
b1866b2d 2298comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2299C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2300the string contains any characters above 255 that can't be represented
2301in a single byte.
2302
2303=head2 Is there anything else I need to know?
2304
2305Not really. Just remember these things:
2306
2307=over 3
2308
2309=item *
2310
2311There's no way to tell if a string is UTF8 or not. You can tell if an SV
2312is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2313something should be UTF8. Treat the flag as part of the PV, even though
2314it's not - if you pass on the PV to somewhere, pass on the flag too.
2315
2316=item *
2317
2318If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
2319unless C<!(*s & 0x80)> in which case you can use C<*s>.
2320
2321=item *
2322
2323When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
2324C<uv < 0x80> in which case you can use C<*s = uv>.
2325
2326=item *
2327
2328Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
2329a new string which is UTF8 encoded. There are tricks you can use to
2330delay deciding whether you need to use a UTF8 string until you get to a
2331high character - C<HALF_UPGRADE> is one of those.
2332
2333=back
2334
954c1994 2335=head1 AUTHORS
e89caa19 2336
954c1994
GS
2337Until May 1997, this document was maintained by Jeff Okamoto
2338<okamoto@corp.hp.com>. It is now maintained as part of Perl itself
2339by the Perl 5 Porters <perl5-porters@perl.org>.
cb1a09d0 2340
954c1994
GS
2341With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2342Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2343Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2344Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2345
954c1994 2346API Listing originally by Dean Roehrich <roehrich@cray.com>.
cb1a09d0 2347
954c1994
GS
2348Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
2349Stuhl.
cb1a09d0 2350
954c1994 2351=head1 SEE ALSO
cb1a09d0 2352
954c1994 2353perlapi(1), perlintern(1), perlxs(1), perlembed(1)