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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
b3b6085d 7This document attempts to describe how to use the Perl API, as well as
06f6df17 8to provide some info on the basic workings of the Perl core. It is far
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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
954c1994 31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
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32as well.) They will usually be exactly 32 and 16 bits long, but on Crays
33they will both be 64 bits.
a0d0e21e 34
54310121 35=head2 Working with SVs
a0d0e21e 36
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37An SV can be created and loaded with one command. There are five types of
38values that can be loaded: an integer value (IV), an unsigned integer
39value (UV), a double (NV), a string (PV), and another scalar (SV).
a0d0e21e 40
20dbd849 41The seven routines are:
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42
43 SV* newSViv(IV);
20dbd849 44 SV* newSVuv(UV);
a0d0e21e 45 SV* newSVnv(double);
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46 SV* newSVpv(const char*, STRLEN);
47 SV* newSVpvn(const char*, STRLEN);
46fc3d4c 48 SV* newSVpvf(const char*, ...);
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49 SV* newSVsv(SV*);
50
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51C<STRLEN> is an integer type (Size_t, usually defined as size_t in
52F<config.h>) guaranteed to be large enough to represent the size of
53any string that perl can handle.
54
55In the unlikely case of a SV requiring more complex initialisation, you
56can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
57type NULL is returned, else an SV of type PV is returned with len + 1 (for
58the NUL) bytes of storage allocated, accessible via SvPVX. In both cases
59the SV has value undef.
20dbd849 60
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61 SV *sv = newSV(0); /* no storage allocated */
62 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
20dbd849 63
06f6df17 64To change the value of an I<already-existing> SV, there are eight routines:
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65
66 void sv_setiv(SV*, IV);
deb3007b 67 void sv_setuv(SV*, UV);
a0d0e21e 68 void sv_setnv(SV*, double);
08105a92 69 void sv_setpv(SV*, const char*);
06f6df17 70 void sv_setpvn(SV*, const char*, STRLEN)
46fc3d4c 71 void sv_setpvf(SV*, const char*, ...);
5b3218b9 72 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
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73 void sv_setsv(SV*, SV*);
74
75Notice that you can choose to specify the length of the string to be
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76assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
77allow Perl to calculate the length by using C<sv_setpv> or by specifying
780 as the second argument to C<newSVpv>. Be warned, though, that Perl will
79determine the string's length by using C<strlen>, which depends on the
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80string terminating with a NUL character.
81
82The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
83formatted output becomes the value.
84
328bf373 85C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
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86either a pointer to a variable argument list or the address and length of
87an array of SVs. The last argument points to a boolean; on return, if that
88boolean is true, then locale-specific information has been used to format
c2611fb3 89the string, and the string's contents are therefore untrustworthy (see
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90L<perlsec>). This pointer may be NULL if that information is not
91important. Note that this function requires you to specify the length of
92the format.
93
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94The C<sv_set*()> functions are not generic enough to operate on values
95that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 96
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97All SVs that contain strings should be terminated with a NUL character.
98If it is not NUL-terminated there is a risk of
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99core dumps and corruptions from code which passes the string to C
100functions or system calls which expect a NUL-terminated string.
101Perl's own functions typically add a trailing NUL for this reason.
102Nevertheless, you should be very careful when you pass a string stored
103in an SV to a C function or system call.
104
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105To access the actual value that an SV points to, you can use the macros:
106
107 SvIV(SV*)
954c1994 108 SvUV(SV*)
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109 SvNV(SV*)
110 SvPV(SV*, STRLEN len)
1fa8b10d 111 SvPV_nolen(SV*)
a0d0e21e 112
954c1994 113which will automatically coerce the actual scalar type into an IV, UV, double,
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114or string.
115
116In the C<SvPV> macro, the length of the string returned is placed into the
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117variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
118not care what the length of the data is, use the C<SvPV_nolen> macro.
119Historically the C<SvPV> macro with the global variable C<PL_na> has been
120used in this case. But that can be quite inefficient because C<PL_na> must
121be accessed in thread-local storage in threaded Perl. In any case, remember
122that Perl allows arbitrary strings of data that may both contain NULs and
123might not be terminated by a NUL.
a0d0e21e 124
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125Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
126len);>. It might work with your compiler, but it won't work for everyone.
127Break this sort of statement up into separate assignments:
128
b2f5ed49 129 SV *s;
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130 STRLEN len;
131 char * ptr;
b2f5ed49 132 ptr = SvPV(s, len);
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133 foo(ptr, len);
134
07fa94a1 135If you want to know if the scalar value is TRUE, you can use:
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136
137 SvTRUE(SV*)
138
139Although Perl will automatically grow strings for you, if you need to force
140Perl to allocate more memory for your SV, you can use the macro
141
142 SvGROW(SV*, STRLEN newlen)
143
144which will determine if more memory needs to be allocated. If so, it will
145call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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146decrease, the allocated memory of an SV and that it does not automatically
147add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 148C<SvGROW(sv, len + 1)>).
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149
150If you have an SV and want to know what kind of data Perl thinks is stored
151in it, you can use the following macros to check the type of SV you have.
152
153 SvIOK(SV*)
154 SvNOK(SV*)
155 SvPOK(SV*)
156
157You can get and set the current length of the string stored in an SV with
158the following macros:
159
160 SvCUR(SV*)
161 SvCUR_set(SV*, I32 val)
162
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163You can also get a pointer to the end of the string stored in the SV
164with the macro:
165
166 SvEND(SV*)
167
168But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 169
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170If you want to append something to the end of string stored in an C<SV*>,
171you can use the following functions:
172
08105a92 173 void sv_catpv(SV*, const char*);
e65f3abd 174 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 175 void sv_catpvf(SV*, const char*, ...);
328bf373 176 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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177 void sv_catsv(SV*, SV*);
178
179The first function calculates the length of the string to be appended by
180using C<strlen>. In the second, you specify the length of the string
46fc3d4c 181yourself. The third function processes its arguments like C<sprintf> and
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182appends the formatted output. The fourth function works like C<vsprintf>.
183You can specify the address and length of an array of SVs instead of the
184va_list argument. The fifth function extends the string stored in the first
185SV with the string stored in the second SV. It also forces the second SV
186to be interpreted as a string.
187
188The C<sv_cat*()> functions are not generic enough to operate on values that
189have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 190
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191If you know the name of a scalar variable, you can get a pointer to its SV
192by using the following:
193
4929bf7b 194 SV* get_sv("package::varname", FALSE);
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195
196This returns NULL if the variable does not exist.
197
d1b91892 198If you want to know if this variable (or any other SV) is actually C<defined>,
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199you can call:
200
201 SvOK(SV*)
202
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203The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
204Its address can be used whenever an C<SV*> is needed.
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205However, you have to be careful when using C<&PL_sv_undef> as a value in AVs
206or HVs (see L<AVs, HVs and undefined values>).
a0d0e21e 207
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208There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
209boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
210addresses can be used whenever an C<SV*> is needed.
a0d0e21e 211
9cde0e7f 212Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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213Take this code:
214
215 SV* sv = (SV*) 0;
216 if (I-am-to-return-a-real-value) {
217 sv = sv_2mortal(newSViv(42));
218 }
219 sv_setsv(ST(0), sv);
220
221This code tries to return a new SV (which contains the value 42) if it should
04343c6d 222return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 223pointer which, somewhere down the line, will cause a segmentation violation,
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224bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
225first line and all will be well.
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226
227To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 228call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 229
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230=head2 Offsets
231
232Perl provides the function C<sv_chop> to efficiently remove characters
233from the beginning of a string; you give it an SV and a pointer to
da75cd15 234somewhere inside the PV, and it discards everything before the
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235pointer. The efficiency comes by means of a little hack: instead of
236actually removing the characters, C<sv_chop> sets the flag C<OOK>
237(offset OK) to signal to other functions that the offset hack is in
238effect, and it puts the number of bytes chopped off into the IV field
239of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
00aadd71 240many bytes, and adjusts C<SvCUR> and C<SvLEN>.
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241
242Hence, at this point, the start of the buffer that we allocated lives
243at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
244into the middle of this allocated storage.
245
246This is best demonstrated by example:
247
248 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
249 SV = PVIV(0x8128450) at 0x81340f0
250 REFCNT = 1
251 FLAGS = (POK,OOK,pPOK)
252 IV = 1 (OFFSET)
253 PV = 0x8135781 ( "1" . ) "2345"\0
254 CUR = 4
255 LEN = 5
256
257Here the number of bytes chopped off (1) is put into IV, and
258C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
259portion of the string between the "real" and the "fake" beginnings is
260shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
261the fake beginning, not the real one.
262
fe854a6f 263Something similar to the offset hack is performed on AVs to enable
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264efficient shifting and splicing off the beginning of the array; while
265C<AvARRAY> points to the first element in the array that is visible from
266Perl, C<AvALLOC> points to the real start of the C array. These are
267usually the same, but a C<shift> operation can be carried out by
268increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
269Again, the location of the real start of the C array only comes into
270play when freeing the array. See C<av_shift> in F<av.c>.
271
d1b91892 272=head2 What's Really Stored in an SV?
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273
274Recall that the usual method of determining the type of scalar you have is
5f05dabc 275to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 276usually these macros will always return TRUE and calling the C<Sv*V>
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277macros will do the appropriate conversion of string to integer/double or
278integer/double to string.
279
280If you I<really> need to know if you have an integer, double, or string
281pointer in an SV, you can use the following three macros instead:
282
283 SvIOKp(SV*)
284 SvNOKp(SV*)
285 SvPOKp(SV*)
286
287These will tell you if you truly have an integer, double, or string pointer
d1b91892 288stored in your SV. The "p" stands for private.
a0d0e21e 289
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290The are various ways in which the private and public flags may differ.
291For example, a tied SV may have a valid underlying value in the IV slot
292(so SvIOKp is true), but the data should be accessed via the FETCH
293routine rather than directly, so SvIOK is false. Another is when
294numeric conversion has occured and precision has been lost: only the
295private flag is set on 'lossy' values. So when an NV is converted to an
296IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
297
07fa94a1 298In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 299
54310121 300=head2 Working with AVs
a0d0e21e 301
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302There are two ways to create and load an AV. The first method creates an
303empty AV:
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304
305 AV* newAV();
306
54310121 307The second method both creates the AV and initially populates it with SVs:
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308
309 AV* av_make(I32 num, SV **ptr);
310
5f05dabc 311The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 312AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 313
54310121 314Once the AV has been created, the following operations are possible on AVs:
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315
316 void av_push(AV*, SV*);
317 SV* av_pop(AV*);
318 SV* av_shift(AV*);
319 void av_unshift(AV*, I32 num);
320
321These should be familiar operations, with the exception of C<av_unshift>.
322This routine adds C<num> elements at the front of the array with the C<undef>
323value. You must then use C<av_store> (described below) to assign values
324to these new elements.
325
326Here are some other functions:
327
5f05dabc 328 I32 av_len(AV*);
a0d0e21e 329 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 330 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 331
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332The C<av_len> function returns the highest index value in array (just
333like $#array in Perl). If the array is empty, -1 is returned. The
334C<av_fetch> function returns the value at index C<key>, but if C<lval>
335is non-zero, then C<av_fetch> will store an undef value at that index.
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336The C<av_store> function stores the value C<val> at index C<key>, and does
337not increment the reference count of C<val>. Thus the caller is responsible
338for taking care of that, and if C<av_store> returns NULL, the caller will
339have to decrement the reference count to avoid a memory leak. Note that
340C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
341return value.
d1b91892 342
a0d0e21e 343 void av_clear(AV*);
a0d0e21e 344 void av_undef(AV*);
cb1a09d0 345 void av_extend(AV*, I32 key);
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346
347The C<av_clear> function deletes all the elements in the AV* array, but
348does not actually delete the array itself. The C<av_undef> function will
349delete all the elements in the array plus the array itself. The
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350C<av_extend> function extends the array so that it contains at least C<key+1>
351elements. If C<key+1> is less than the currently allocated length of the array,
352then nothing is done.
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353
354If you know the name of an array variable, you can get a pointer to its AV
355by using the following:
356
4929bf7b 357 AV* get_av("package::varname", FALSE);
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358
359This returns NULL if the variable does not exist.
360
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361See L<Understanding the Magic of Tied Hashes and Arrays> for more
362information on how to use the array access functions on tied arrays.
363
54310121 364=head2 Working with HVs
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365
366To create an HV, you use the following routine:
367
368 HV* newHV();
369
54310121 370Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 371
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372 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
373 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 374
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375The C<klen> parameter is the length of the key being passed in (Note that
376you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
377length of the key). The C<val> argument contains the SV pointer to the
54310121 378scalar being stored, and C<hash> is the precomputed hash value (zero if
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379you want C<hv_store> to calculate it for you). The C<lval> parameter
380indicates whether this fetch is actually a part of a store operation, in
381which case a new undefined value will be added to the HV with the supplied
382key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 383
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384Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
385C<SV*>. To access the scalar value, you must first dereference the return
386value. However, you should check to make sure that the return value is
387not NULL before dereferencing it.
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388
389These two functions check if a hash table entry exists, and deletes it.
390
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391 bool hv_exists(HV*, const char* key, U32 klen);
392 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 393
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394If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
395create and return a mortal copy of the deleted value.
396
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397And more miscellaneous functions:
398
399 void hv_clear(HV*);
a0d0e21e 400 void hv_undef(HV*);
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401
402Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
403table but does not actually delete the hash table. The C<hv_undef> deletes
404both the entries and the hash table itself.
a0d0e21e 405
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406Perl keeps the actual data in linked list of structures with a typedef of HE.
407These contain the actual key and value pointers (plus extra administrative
408overhead). The key is a string pointer; the value is an C<SV*>. However,
409once you have an C<HE*>, to get the actual key and value, use the routines
410specified below.
411
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412 I32 hv_iterinit(HV*);
413 /* Prepares starting point to traverse hash table */
414 HE* hv_iternext(HV*);
415 /* Get the next entry, and return a pointer to a
416 structure that has both the key and value */
417 char* hv_iterkey(HE* entry, I32* retlen);
418 /* Get the key from an HE structure and also return
419 the length of the key string */
cb1a09d0 420 SV* hv_iterval(HV*, HE* entry);
d1be9408 421 /* Return an SV pointer to the value of the HE
a0d0e21e 422 structure */
cb1a09d0 423 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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424 /* This convenience routine combines hv_iternext,
425 hv_iterkey, and hv_iterval. The key and retlen
426 arguments are return values for the key and its
427 length. The value is returned in the SV* argument */
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428
429If you know the name of a hash variable, you can get a pointer to its HV
430by using the following:
431
4929bf7b 432 HV* get_hv("package::varname", FALSE);
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433
434This returns NULL if the variable does not exist.
435
8ebc5c01 436The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 437
a0d0e21e 438 hash = 0;
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439 while (klen--)
440 hash = (hash * 33) + *key++;
87275199 441 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 442
87275199 443The last step was added in version 5.6 to improve distribution of
ab192400 444lower bits in the resulting hash value.
a0d0e21e 445
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446See L<Understanding the Magic of Tied Hashes and Arrays> for more
447information on how to use the hash access functions on tied hashes.
448
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449=head2 Hash API Extensions
450
451Beginning with version 5.004, the following functions are also supported:
452
453 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
454 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 455
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456 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
457 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 458
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459 SV* hv_iterkeysv (HE* entry);
460
461Note that these functions take C<SV*> keys, which simplifies writing
462of extension code that deals with hash structures. These functions
463also allow passing of C<SV*> keys to C<tie> functions without forcing
464you to stringify the keys (unlike the previous set of functions).
465
466They also return and accept whole hash entries (C<HE*>), making their
467use more efficient (since the hash number for a particular string
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468doesn't have to be recomputed every time). See L<perlapi> for detailed
469descriptions.
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470
471The following macros must always be used to access the contents of hash
472entries. Note that the arguments to these macros must be simple
473variables, since they may get evaluated more than once. See
4a4eefd0 474L<perlapi> for detailed descriptions of these macros.
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475
476 HePV(HE* he, STRLEN len)
477 HeVAL(HE* he)
478 HeHASH(HE* he)
479 HeSVKEY(HE* he)
480 HeSVKEY_force(HE* he)
481 HeSVKEY_set(HE* he, SV* sv)
482
483These two lower level macros are defined, but must only be used when
484dealing with keys that are not C<SV*>s:
485
486 HeKEY(HE* he)
487 HeKLEN(HE* he)
488
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489Note that both C<hv_store> and C<hv_store_ent> do not increment the
490reference count of the stored C<val>, which is the caller's responsibility.
491If these functions return a NULL value, the caller will usually have to
492decrement the reference count of C<val> to avoid a memory leak.
1e422769 493
a9381218
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494=head2 AVs, HVs and undefined values
495
496Sometimes you have to store undefined values in AVs or HVs. Although
497this may be a rare case, it can be tricky. That's because you're
498used to using C<&PL_sv_undef> if you need an undefined SV.
499
500For example, intuition tells you that this XS code:
501
502 AV *av = newAV();
503 av_store( av, 0, &PL_sv_undef );
504
505is equivalent to this Perl code:
506
507 my @av;
508 $av[0] = undef;
509
510Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
511for indicating that an array element has not yet been initialized.
512Thus, C<exists $av[0]> would be true for the above Perl code, but
513false for the array generated by the XS code.
514
515Other problems can occur when storing C<&PL_sv_undef> in HVs:
516
517 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
518
519This will indeed make the value C<undef>, but if you try to modify
520the value of C<key>, you'll get the following error:
521
522 Modification of non-creatable hash value attempted
523
524In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
525in restricted hashes. This caused such hash entries not to appear
526when iterating over the hash or when checking for the keys
527with the C<hv_exists> function.
528
529You can run into similar problems when you store C<&PL_sv_true> or
530C<&PL_sv_false> into AVs or HVs. Trying to modify such elements
531will give you the following error:
532
533 Modification of a read-only value attempted
534
535To make a long story short, you can use the special variables
536C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and
537HVs, but you have to make sure you know what you're doing.
538
539Generally, if you want to store an undefined value in an AV
540or HV, you should not use C<&PL_sv_undef>, but rather create a
541new undefined value using the C<newSV> function, for example:
542
543 av_store( av, 42, newSV(0) );
544 hv_store( hv, "foo", 3, newSV(0), 0 );
545
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546=head2 References
547
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AD
548References are a special type of scalar that point to other data types
549(including references).
a0d0e21e 550
07fa94a1 551To create a reference, use either of the following functions:
a0d0e21e 552
5f05dabc
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553 SV* newRV_inc((SV*) thing);
554 SV* newRV_noinc((SV*) thing);
a0d0e21e 555
5f05dabc 556The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
557functions are identical except that C<newRV_inc> increments the reference
558count of the C<thing>, while C<newRV_noinc> does not. For historical
559reasons, C<newRV> is a synonym for C<newRV_inc>.
560
561Once you have a reference, you can use the following macro to dereference
562the reference:
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563
564 SvRV(SV*)
565
566then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 567C<AV*> or C<HV*>, if required.
a0d0e21e 568
d1b91892 569To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
570
571 SvROK(SV*)
572
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573To discover what type of value the reference refers to, use the following
574macro and then check the return value.
d1b91892
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575
576 SvTYPE(SvRV(SV*))
577
578The most useful types that will be returned are:
579
580 SVt_IV Scalar
581 SVt_NV Scalar
582 SVt_PV Scalar
5f05dabc 583 SVt_RV Scalar
d1b91892
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584 SVt_PVAV Array
585 SVt_PVHV Hash
586 SVt_PVCV Code
5f05dabc
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587 SVt_PVGV Glob (possible a file handle)
588 SVt_PVMG Blessed or Magical Scalar
589
590 See the sv.h header file for more details.
d1b91892 591
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592=head2 Blessed References and Class Objects
593
06f6df17 594References are also used to support object-oriented programming. In perl's
cb1a09d0
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595OO lexicon, an object is simply a reference that has been blessed into a
596package (or class). Once blessed, the programmer may now use the reference
597to access the various methods in the class.
598
599A reference can be blessed into a package with the following function:
600
601 SV* sv_bless(SV* sv, HV* stash);
602
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RGS
603The C<sv> argument must be a reference value. The C<stash> argument
604specifies which class the reference will belong to. See
2ae324a7 605L<Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
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606
607/* Still under construction */
608
609Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
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610point to. If C<classname> is non-null, the SV is blessed into the specified
611class. SV is returned.
cb1a09d0 612
08105a92 613 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 614
e1c57cef 615Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 616if C<classname> is non-null.
cb1a09d0 617
08105a92 618 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 619 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 620 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 621
5f05dabc 622Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 623reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 624
08105a92 625 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 626
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PP
627Copies string into an SV whose reference is C<rv>. Set length to 0 to let
628Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 629
e65f3abd 630 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 631
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GS
632Tests whether the SV is blessed into the specified class. It does not
633check inheritance relationships.
634
08105a92 635 int sv_isa(SV* sv, const char* name);
9abd00ed
GS
636
637Tests whether the SV is a reference to a blessed object.
638
639 int sv_isobject(SV* sv);
640
641Tests whether the SV is derived from the specified class. SV can be either
642a reference to a blessed object or a string containing a class name. This
643is the function implementing the C<UNIVERSAL::isa> functionality.
644
08105a92 645 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 646
00aadd71 647To check if you've got an object derived from a specific class you have
9abd00ed
GS
648to write:
649
650 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 651
5f05dabc 652=head2 Creating New Variables
cb1a09d0 653
5f05dabc
PP
654To create a new Perl variable with an undef value which can be accessed from
655your Perl script, use the following routines, depending on the variable type.
cb1a09d0 656
4929bf7b
GS
657 SV* get_sv("package::varname", TRUE);
658 AV* get_av("package::varname", TRUE);
659 HV* get_hv("package::varname", TRUE);
cb1a09d0
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660
661Notice the use of TRUE as the second parameter. The new variable can now
662be set, using the routines appropriate to the data type.
663
5f05dabc
PP
664There are additional macros whose values may be bitwise OR'ed with the
665C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 666
9a68f1db
SB
667=over
668
669=item GV_ADDMULTI
670
671Marks the variable as multiply defined, thus preventing the:
672
673 Name <varname> used only once: possible typo
674
675warning.
676
9a68f1db
SB
677=item GV_ADDWARN
678
679Issues the warning:
680
681 Had to create <varname> unexpectedly
682
683if the variable did not exist before the function was called.
684
685=back
cb1a09d0 686
07fa94a1
JO
687If you do not specify a package name, the variable is created in the current
688package.
cb1a09d0 689
5f05dabc 690=head2 Reference Counts and Mortality
a0d0e21e 691
d1be9408 692Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 693AVs, or HVs (xV for short in the following) start their life with a
55497cff 694reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 695then it will be destroyed and its memory made available for reuse.
55497cff
PP
696
697This normally doesn't happen at the Perl level unless a variable is
5f05dabc
PP
698undef'ed or the last variable holding a reference to it is changed or
699overwritten. At the internal level, however, reference counts can be
55497cff
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700manipulated with the following macros:
701
702 int SvREFCNT(SV* sv);
5f05dabc 703 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
704 void SvREFCNT_dec(SV* sv);
705
706However, there is one other function which manipulates the reference
07fa94a1
JO
707count of its argument. The C<newRV_inc> function, you will recall,
708creates a reference to the specified argument. As a side effect,
709it increments the argument's reference count. If this is not what
710you want, use C<newRV_noinc> instead.
711
712For example, imagine you want to return a reference from an XSUB function.
713Inside the XSUB routine, you create an SV which initially has a reference
714count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
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715This returns the reference as a new SV, but the reference count of the
716SV you passed to C<newRV_inc> has been incremented to two. Now you
07fa94a1
JO
717return the reference from the XSUB routine and forget about the SV.
718But Perl hasn't! Whenever the returned reference is destroyed, the
719reference count of the original SV is decreased to one and nothing happens.
720The SV will hang around without any way to access it until Perl itself
721terminates. This is a memory leak.
5f05dabc
PP
722
723The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
724C<newRV_inc>. Then, if and when the last reference is destroyed,
725the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 726stopping any memory leak.
55497cff 727
5f05dabc 728There are some convenience functions available that can help with the
54310121 729destruction of xVs. These functions introduce the concept of "mortality".
07fa94a1
JO
730An xV that is mortal has had its reference count marked to be decremented,
731but not actually decremented, until "a short time later". Generally the
732term "short time later" means a single Perl statement, such as a call to
54310121 733an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
JO
734reference count decremented depends on two macros, SAVETMPS and FREETMPS.
735See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
PP
736
737"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
738However, if you mortalize a variable twice, the reference count will
739later be decremented twice.
740
00aadd71
NIS
741"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
742For example an SV which is created just to pass a number to a called sub
06f6df17
RGS
743is made mortal to have it cleaned up automatically when it's popped off
744the stack. Similarly, results returned by XSUBs (which are pushed on the
745stack) are often made mortal.
a0d0e21e
LW
746
747To create a mortal variable, use the functions:
748
749 SV* sv_newmortal()
750 SV* sv_2mortal(SV*)
751 SV* sv_mortalcopy(SV*)
752
00aadd71 753The first call creates a mortal SV (with no value), the second converts an existing
5f05dabc
PP
754SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
755third creates a mortal copy of an existing SV.
00aadd71 756Because C<sv_newmortal> gives the new SV no value,it must normally be given one
9a68f1db 757via C<sv_setpv>, C<sv_setiv>, etc. :
00aadd71
NIS
758
759 SV *tmp = sv_newmortal();
760 sv_setiv(tmp, an_integer);
761
762As that is multiple C statements it is quite common so see this idiom instead:
763
764 SV *tmp = sv_2mortal(newSViv(an_integer));
765
766
767You should be careful about creating mortal variables. Strange things
768can happen if you make the same value mortal within multiple contexts,
769or if you make a variable mortal multiple times. Thinking of "Mortalization"
770as deferred C<SvREFCNT_dec> should help to minimize such problems.
771For example if you are passing an SV which you I<know> has high enough REFCNT
772to survive its use on the stack you need not do any mortalization.
773If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
774making a C<sv_mortalcopy> is safer.
a0d0e21e 775
54310121 776The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 777made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 778C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 779
5f05dabc 780=head2 Stashes and Globs
a0d0e21e 781
06f6df17
RGS
782A B<stash> is a hash that contains all variables that are defined
783within a package. Each key of the stash is a symbol
aa689395
PP
784name (shared by all the different types of objects that have the same
785name), and each value in the hash table is a GV (Glob Value). This GV
786in turn contains references to the various objects of that name,
787including (but not limited to) the following:
cb1a09d0 788
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789 Scalar Value
790 Array Value
791 Hash Value
a3cb178b 792 I/O Handle
a0d0e21e
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793 Format
794 Subroutine
795
06f6df17
RGS
796There is a single stash called C<PL_defstash> that holds the items that exist
797in the C<main> package. To get at the items in other packages, append the
798string "::" to the package name. The items in the C<Foo> package are in
799the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
800in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 801
d1b91892 802To get the stash pointer for a particular package, use the function:
a0d0e21e 803
08105a92 804 HV* gv_stashpv(const char* name, I32 create)
a0d0e21e
LW
805 HV* gv_stashsv(SV*, I32 create)
806
807The first function takes a literal string, the second uses the string stored
d1b91892 808in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 809C<HV*>. The C<create> flag will create a new package if it is set.
a0d0e21e
LW
810
811The name that C<gv_stash*v> wants is the name of the package whose symbol table
812you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
813packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
814language itself.
a0d0e21e
LW
815
816Alternately, if you have an SV that is a blessed reference, you can find
817out the stash pointer by using:
818
819 HV* SvSTASH(SvRV(SV*));
820
821then use the following to get the package name itself:
822
823 char* HvNAME(HV* stash);
824
5f05dabc
PP
825If you need to bless or re-bless an object you can use the following
826function:
a0d0e21e
LW
827
828 SV* sv_bless(SV*, HV* stash)
829
830where the first argument, an C<SV*>, must be a reference, and the second
831argument is a stash. The returned C<SV*> can now be used in the same way
832as any other SV.
833
d1b91892
AD
834For more information on references and blessings, consult L<perlref>.
835
54310121 836=head2 Double-Typed SVs
0a753a76
PP
837
838Scalar variables normally contain only one type of value, an integer,
839double, pointer, or reference. Perl will automatically convert the
840actual scalar data from the stored type into the requested type.
841
842Some scalar variables contain more than one type of scalar data. For
843example, the variable C<$!> contains either the numeric value of C<errno>
844or its string equivalent from either C<strerror> or C<sys_errlist[]>.
845
846To force multiple data values into an SV, you must do two things: use the
847C<sv_set*v> routines to add the additional scalar type, then set a flag
848so that Perl will believe it contains more than one type of data. The
849four macros to set the flags are:
850
851 SvIOK_on
852 SvNOK_on
853 SvPOK_on
854 SvROK_on
855
856The particular macro you must use depends on which C<sv_set*v> routine
857you called first. This is because every C<sv_set*v> routine turns on
858only the bit for the particular type of data being set, and turns off
859all the rest.
860
861For example, to create a new Perl variable called "dberror" that contains
862both the numeric and descriptive string error values, you could use the
863following code:
864
865 extern int dberror;
866 extern char *dberror_list;
867
4929bf7b 868 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
869 sv_setiv(sv, (IV) dberror);
870 sv_setpv(sv, dberror_list[dberror]);
871 SvIOK_on(sv);
872
873If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
874macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
875
876=head2 Magic Variables
a0d0e21e 877
d1b91892
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878[This section still under construction. Ignore everything here. Post no
879bills. Everything not permitted is forbidden.]
880
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881Any SV may be magical, that is, it has special features that a normal
882SV does not have. These features are stored in the SV structure in a
5f05dabc 883linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
884
885 struct magic {
886 MAGIC* mg_moremagic;
887 MGVTBL* mg_virtual;
888 U16 mg_private;
889 char mg_type;
890 U8 mg_flags;
891 SV* mg_obj;
892 char* mg_ptr;
893 I32 mg_len;
894 };
895
896Note this is current as of patchlevel 0, and could change at any time.
897
898=head2 Assigning Magic
899
900Perl adds magic to an SV using the sv_magic function:
901
08105a92 902 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
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903
904The C<sv> argument is a pointer to the SV that is to acquire a new magical
905feature.
906
907If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
645c22ef
DM
908convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
909to the beginning of the linked list of magical features. Any prior entry
910of the same type of magic is deleted. Note that this can be overridden,
911and multiple instances of the same type of magic can be associated with an
912SV.
d1b91892 913
54310121
PP
914The C<name> and C<namlen> arguments are used to associate a string with
915the magic, typically the name of a variable. C<namlen> is stored in the
9b5bb84f 916C<mg_len> field and if C<name> is non-null and C<namlen> E<gt>= 0 a malloc'd
d1b91892
AD
917copy of the name is stored in C<mg_ptr> field.
918
919The sv_magic function uses C<how> to determine which, if any, predefined
920"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
06f6df17 921See the L<Magic Virtual Tables> section below. The C<how> argument is also
14befaf4 922stored in the C<mg_type> field. The value of C<how> should be chosen
06f6df17 923from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 924these macros were added, Perl internals used to directly use character
14befaf4 925literals, so you may occasionally come across old code or documentation
75d0f26d 926referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
927
928The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
929structure. If it is not the same as the C<sv> argument, the reference
930count of the C<obj> object is incremented. If it is the same, or if
645c22ef 931the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 932then C<obj> is merely stored, without the reference count being incremented.
d1b91892 933
cb1a09d0
AD
934There is also a function to add magic to an C<HV>:
935
936 void hv_magic(HV *hv, GV *gv, int how);
937
938This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
939
940To remove the magic from an SV, call the function sv_unmagic:
941
942 void sv_unmagic(SV *sv, int type);
943
944The C<type> argument should be equal to the C<how> value when the C<SV>
945was initially made magical.
946
d1b91892
AD
947=head2 Magic Virtual Tables
948
d1be9408 949The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
950C<MGVTBL>, which is a structure of function pointers and stands for
951"Magic Virtual Table" to handle the various operations that might be
952applied to that variable.
953
954The C<MGVTBL> has five pointers to the following routine types:
955
956 int (*svt_get)(SV* sv, MAGIC* mg);
957 int (*svt_set)(SV* sv, MAGIC* mg);
958 U32 (*svt_len)(SV* sv, MAGIC* mg);
959 int (*svt_clear)(SV* sv, MAGIC* mg);
960 int (*svt_free)(SV* sv, MAGIC* mg);
961
06f6df17 962This MGVTBL structure is set at compile-time in F<perl.h> and there are
d1b91892
AD
963currently 19 types (or 21 with overloading turned on). These different
964structures contain pointers to various routines that perform additional
965actions depending on which function is being called.
966
967 Function pointer Action taken
968 ---------------- ------------
8b0711c3 969 svt_get Do something before the value of the SV is retrieved.
d1b91892
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970 svt_set Do something after the SV is assigned a value.
971 svt_len Report on the SV's length.
972 svt_clear Clear something the SV represents.
973 svt_free Free any extra storage associated with the SV.
974
975For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 976to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
977
978 { magic_get, magic_set, magic_len, 0, 0 }
979
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DM
980Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
981if a get operation is being performed, the routine C<magic_get> is
982called. All the various routines for the various magical types begin
983with C<magic_>. NOTE: the magic routines are not considered part of
984the Perl API, and may not be exported by the Perl library.
d1b91892
AD
985
986The current kinds of Magic Virtual Tables are:
987
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DM
988 mg_type
989 (old-style char and macro) MGVTBL Type of magic
990 -------------------------- ------ ----------------------------
991 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
992 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
993 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
994 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
995 on stash
996 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
997 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
998 (@+ and @- vars)
999 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1000 element
1001 E PERL_MAGIC_env vtbl_env %ENV hash
1002 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1003 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
1004 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
1005 I PERL_MAGIC_isa vtbl_isa @ISA array
1006 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1007 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1008 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1009 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
1010 m PERL_MAGIC_mutex vtbl_mutex ???
645c22ef 1011 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
14befaf4
DM
1012 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1013 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1014 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1015 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
1016 S PERL_MAGIC_sig vtbl_sig %SIG hash
1017 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1018 t PERL_MAGIC_taint vtbl_taint Taintedness
1019 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
1020 v PERL_MAGIC_vec vtbl_vec vec() lvalue
92f0c265 1021 V PERL_MAGIC_vstring (none) v-string scalars
836995da 1022 w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
14befaf4
DM
1023 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1024 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1025 variable / smart parameter
1026 vivification
1027 * PERL_MAGIC_glob vtbl_glob GV (typeglob)
1028 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1029 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1030 < PERL_MAGIC_backref vtbl_backref ???
1031 ~ PERL_MAGIC_ext (none) Available for use by extensions
d1b91892 1032
68dc0745 1033When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1034uppercase letter is typically used to represent some kind of composite type
1035(a list or a hash), and the lowercase letter is used to represent an element
1036of that composite type. Some internals code makes use of this case
1037relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1038
1039The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1040specifically for use by extensions and will not be used by perl itself.
1041Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1042to variables (typically objects). This is especially useful because
1043there is no way for normal perl code to corrupt this private information
1044(unlike using extra elements of a hash object).
1045
1046Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1047C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1048C<mg_ptr> field points to a C<ufuncs> structure:
1049
1050 struct ufuncs {
a9402793
AB
1051 I32 (*uf_val)(pTHX_ IV, SV*);
1052 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1053 IV uf_index;
1054 };
1055
1056When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1057function will be called with C<uf_index> as the first arg and a pointer to
1058the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1059magic is shown below. Note that the ufuncs structure is copied by
1060sv_magic, so you can safely allocate it on the stack.
1061
1062 void
1063 Umagic(sv)
1064 SV *sv;
1065 PREINIT:
1066 struct ufuncs uf;
1067 CODE:
1068 uf.uf_val = &my_get_fn;
1069 uf.uf_set = &my_set_fn;
1070 uf.uf_index = 0;
14befaf4 1071 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1072
14befaf4
DM
1073Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1074or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1075extra care to avoid conflict. Typically only using the magic on
1076objects blessed into the same class as the extension is sufficient.
1077For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1078'signature' at the top of the private data area and check that.
5f05dabc 1079
ef50df4b
GS
1080Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1081earlier do B<not> invoke 'set' magic on their targets. This must
1082be done by the user either by calling the C<SvSETMAGIC()> macro after
1083calling these functions, or by using one of the C<sv_set*_mg()> or
1084C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1085C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1086obtained from external sources in functions that don't handle magic.
4a4eefd0 1087See L<perlapi> for a description of these functions.
189b2af5
GS
1088For example, calls to the C<sv_cat*()> functions typically need to be
1089followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1090since their implementation handles 'get' magic.
1091
d1b91892
AD
1092=head2 Finding Magic
1093
1094 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1095
1096This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1097If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1098if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1099
08105a92 1100 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1101
1102This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1103field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1104the mg_type field is changed to be the lowercase letter.
a0d0e21e 1105
04343c6d
GS
1106=head2 Understanding the Magic of Tied Hashes and Arrays
1107
14befaf4
DM
1108Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1109magic type.
9edb2b46
GS
1110
1111WARNING: As of the 5.004 release, proper usage of the array and hash
1112access functions requires understanding a few caveats. Some
1113of these caveats are actually considered bugs in the API, to be fixed
1114in later releases, and are bracketed with [MAYCHANGE] below. If
1115you find yourself actually applying such information in this section, be
1116aware that the behavior may change in the future, umm, without warning.
04343c6d 1117
1526ead6 1118The perl tie function associates a variable with an object that implements
9a68f1db 1119the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1120tie function from an XSUB, you must mimic this behaviour. The code below
1121carries out the necessary steps - firstly it creates a new hash, and then
1122creates a second hash which it blesses into the class which will implement
1123the tie methods. Lastly it ties the two hashes together, and returns a
1124reference to the new tied hash. Note that the code below does NOT call the
1125TIEHASH method in the MyTie class -
1126see L<Calling Perl Routines from within C Programs> for details on how
1127to do this.
1128
1129 SV*
1130 mytie()
1131 PREINIT:
1132 HV *hash;
1133 HV *stash;
1134 SV *tie;
1135 CODE:
1136 hash = newHV();
1137 tie = newRV_noinc((SV*)newHV());
1138 stash = gv_stashpv("MyTie", TRUE);
1139 sv_bless(tie, stash);
899e16d0 1140 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1141 RETVAL = newRV_noinc(hash);
1142 OUTPUT:
1143 RETVAL
1144
04343c6d
GS
1145The C<av_store> function, when given a tied array argument, merely
1146copies the magic of the array onto the value to be "stored", using
1147C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1148actually need to be stored in the array. [MAYCHANGE] After a call to
1149C<av_store> on a tied array, the caller will usually need to call
1150C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1151TIEARRAY object. If C<av_store> did return NULL, a call to
1152C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1153leak. [/MAYCHANGE]
04343c6d
GS
1154
1155The previous paragraph is applicable verbatim to tied hash access using the
1156C<hv_store> and C<hv_store_ent> functions as well.
1157
1158C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1159C<hv_fetch_ent> actually return an undefined mortal value whose magic
1160has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1161need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1162need to call C<mg_get()> on the returned value in order to actually invoke
1163the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1164you may also call C<mg_set()> on the return value after possibly assigning
1165a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1166method on the TIE object. [/MAYCHANGE]
04343c6d 1167
9edb2b46 1168[MAYCHANGE]
04343c6d
GS
1169In other words, the array or hash fetch/store functions don't really
1170fetch and store actual values in the case of tied arrays and hashes. They
1171merely call C<mg_copy> to attach magic to the values that were meant to be
1172"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1173do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1174the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1175and hashes.
1176
1177Currently (as of perl version 5.004), use of the hash and array access
1178functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1179"normal" hashes and arrays, or on their tied variants. The API may be
1180changed to provide more transparent access to both tied and normal data
1181types in future versions.
1182[/MAYCHANGE]
04343c6d
GS
1183
1184You would do well to understand that the TIEARRAY and TIEHASH interfaces
1185are mere sugar to invoke some perl method calls while using the uniform hash
1186and array syntax. The use of this sugar imposes some overhead (typically
1187about two to four extra opcodes per FETCH/STORE operation, in addition to
1188the creation of all the mortal variables required to invoke the methods).
1189This overhead will be comparatively small if the TIE methods are themselves
1190substantial, but if they are only a few statements long, the overhead
1191will not be insignificant.
1192
d1c897a1
IZ
1193=head2 Localizing changes
1194
1195Perl has a very handy construction
1196
1197 {
1198 local $var = 2;
1199 ...
1200 }
1201
1202This construction is I<approximately> equivalent to
1203
1204 {
1205 my $oldvar = $var;
1206 $var = 2;
1207 ...
1208 $var = $oldvar;
1209 }
1210
1211The biggest difference is that the first construction would
1212reinstate the initial value of $var, irrespective of how control exits
9a68f1db 1213the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1214more efficient as well.
1215
1216There is a way to achieve a similar task from C via Perl API: create a
1217I<pseudo-block>, and arrange for some changes to be automatically
1218undone at the end of it, either explicit, or via a non-local exit (via
1219die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1220C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1221Such a construct may be created specially for some important localized
1222task, or an existing one (like boundaries of enclosing Perl
1223subroutine/block, or an existing pair for freeing TMPs) may be
1224used. (In the second case the overhead of additional localization must
1225be almost negligible.) Note that any XSUB is automatically enclosed in
1226an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1227
1228Inside such a I<pseudo-block> the following service is available:
1229
13a2d996 1230=over 4
d1c897a1
IZ
1231
1232=item C<SAVEINT(int i)>
1233
1234=item C<SAVEIV(IV i)>
1235
1236=item C<SAVEI32(I32 i)>
1237
1238=item C<SAVELONG(long i)>
1239
1240These macros arrange things to restore the value of integer variable
1241C<i> at the end of enclosing I<pseudo-block>.
1242
1243=item C<SAVESPTR(s)>
1244
1245=item C<SAVEPPTR(p)>
1246
1247These macros arrange things to restore the value of pointers C<s> and
1248C<p>. C<s> must be a pointer of a type which survives conversion to
1249C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1250and back.
1251
1252=item C<SAVEFREESV(SV *sv)>
1253
1254The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1255I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1256mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1257extends the lifetime of C<sv> until the beginning of the next statement,
1258C<SAVEFREESV> extends it until the end of the enclosing scope. These
1259lifetimes can be wildly different.
1260
1261Also compare C<SAVEMORTALIZESV>.
1262
1263=item C<SAVEMORTALIZESV(SV *sv)>
1264
1265Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1266scope instead of decrementing its reference count. This usually has the
1267effect of keeping C<sv> alive until the statement that called the currently
1268live scope has finished executing.
d1c897a1
IZ
1269
1270=item C<SAVEFREEOP(OP *op)>
1271
1272The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1273
1274=item C<SAVEFREEPV(p)>
1275
1276The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1277end of I<pseudo-block>.
1278
1279=item C<SAVECLEARSV(SV *sv)>
1280
1281Clears a slot in the current scratchpad which corresponds to C<sv> at
1282the end of I<pseudo-block>.
1283
1284=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1285
1286The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1287string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1288short-lived storage, the corresponding string may be reallocated like
1289this:
1290
9cde0e7f 1291 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1292
c76ac1ee 1293=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1294
1295At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1296only argument C<p>.
1297
1298=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1299
1300At the end of I<pseudo-block> the function C<f> is called with the
1301implicit context argument (if any), and C<p>.
d1c897a1
IZ
1302
1303=item C<SAVESTACK_POS()>
1304
1305The current offset on the Perl internal stack (cf. C<SP>) is restored
1306at the end of I<pseudo-block>.
1307
1308=back
1309
1310The following API list contains functions, thus one needs to
1311provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1312or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1313function takes C<int *>.
1314
13a2d996 1315=over 4
d1c897a1
IZ
1316
1317=item C<SV* save_scalar(GV *gv)>
1318
1319Equivalent to Perl code C<local $gv>.
1320
1321=item C<AV* save_ary(GV *gv)>
1322
1323=item C<HV* save_hash(GV *gv)>
1324
1325Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1326
1327=item C<void save_item(SV *item)>
1328
1329Duplicates the current value of C<SV>, on the exit from the current
1330C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
038fcae3
SB
1331using the stored value. It doesn't handle magic. Use C<save_scalar> if
1332magic is affected.
d1c897a1
IZ
1333
1334=item C<void save_list(SV **sarg, I32 maxsarg)>
1335
1336A variant of C<save_item> which takes multiple arguments via an array
1337C<sarg> of C<SV*> of length C<maxsarg>.
1338
1339=item C<SV* save_svref(SV **sptr)>
1340
d1be9408 1341Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1342
1343=item C<void save_aptr(AV **aptr)>
1344
1345=item C<void save_hptr(HV **hptr)>
1346
1347Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1348
1349=back
1350
1351The C<Alias> module implements localization of the basic types within the
1352I<caller's scope>. People who are interested in how to localize things in
1353the containing scope should take a look there too.
1354
0a753a76 1355=head1 Subroutines
a0d0e21e 1356
68dc0745 1357=head2 XSUBs and the Argument Stack
5f05dabc
PP
1358
1359The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1360An XSUB routine will have a stack that contains the arguments from the Perl
1361program, and a way to map from the Perl data structures to a C equivalent.
1362
1363The stack arguments are accessible through the C<ST(n)> macro, which returns
1364the C<n>'th stack argument. Argument 0 is the first argument passed in the
1365Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1366an C<SV*> is used.
1367
1368Most of the time, output from the C routine can be handled through use of
1369the RETVAL and OUTPUT directives. However, there are some cases where the
1370argument stack is not already long enough to handle all the return values.
1371An example is the POSIX tzname() call, which takes no arguments, but returns
1372two, the local time zone's standard and summer time abbreviations.
1373
1374To handle this situation, the PPCODE directive is used and the stack is
1375extended using the macro:
1376
924508f0 1377 EXTEND(SP, num);
5f05dabc 1378
924508f0
GS
1379where C<SP> is the macro that represents the local copy of the stack pointer,
1380and C<num> is the number of elements the stack should be extended by.
5f05dabc 1381
00aadd71 1382Now that there is room on the stack, values can be pushed on it using C<PUSHs>
06f6df17
RGS
1383macro. The pushed values will often need to be "mortal" (See
1384L</Reference Counts and Mortality>).
5f05dabc 1385
00aadd71
NIS
1386 PUSHs(sv_2mortal(newSViv(an_integer)))
1387 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1388 PUSHs(sv_2mortal(newSVnv(3.141592)))
5f05dabc
PP
1389
1390And now the Perl program calling C<tzname>, the two values will be assigned
1391as in:
1392
1393 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1394
1395An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1396to use the macro:
5f05dabc 1397
5f05dabc
PP
1398 XPUSHs(SV*)
1399
00aadd71 1400This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1401do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1402
1403Despite their suggestions in earlier versions of this document the macros
1404C<PUSHi>, C<PUSHn> and C<PUSHp> are I<not> suited to XSUBs which return
1405multiple results, see L</Putting a C value on Perl stack>.
5f05dabc
PP
1406
1407For more information, consult L<perlxs> and L<perlxstut>.
1408
1409=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1410
1411There are four routines that can be used to call a Perl subroutine from
1412within a C program. These four are:
1413
954c1994
GS
1414 I32 call_sv(SV*, I32);
1415 I32 call_pv(const char*, I32);
1416 I32 call_method(const char*, I32);
1417 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1418
954c1994 1419The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1420contains either the name of the Perl subroutine to be called, or a
1421reference to the subroutine. The second argument consists of flags
1422that control the context in which the subroutine is called, whether
1423or not the subroutine is being passed arguments, how errors should be
1424trapped, and how to treat return values.
a0d0e21e
LW
1425
1426All four routines return the number of arguments that the subroutine returned
1427on the Perl stack.
1428
9a68f1db 1429These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1430but those names are now deprecated; macros of the same name are provided for
1431compatibility.
1432
1433When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1434must manipulate the Perl stack. These include the following macros and
1435functions:
a0d0e21e
LW
1436
1437 dSP
924508f0 1438 SP
a0d0e21e
LW
1439 PUSHMARK()
1440 PUTBACK
1441 SPAGAIN
1442 ENTER
1443 SAVETMPS
1444 FREETMPS
1445 LEAVE
1446 XPUSH*()
cb1a09d0 1447 POP*()
a0d0e21e 1448
5f05dabc
PP
1449For a detailed description of calling conventions from C to Perl,
1450consult L<perlcall>.
a0d0e21e 1451
5f05dabc 1452=head2 Memory Allocation
a0d0e21e 1453
06f6df17
RGS
1454=head3 Allocation
1455
86058a2d
GS
1456All memory meant to be used with the Perl API functions should be manipulated
1457using the macros described in this section. The macros provide the necessary
1458transparency between differences in the actual malloc implementation that is
1459used within perl.
1460
1461It is suggested that you enable the version of malloc that is distributed
5f05dabc 1462with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1463order to satisfy allocation requests more quickly. However, on some
1464platforms, it may cause spurious malloc or free errors.
d1b91892 1465
06f6df17
RGS
1466The following three macros are used to initially allocate memory :
1467
d1b91892
AD
1468 New(x, pointer, number, type);
1469 Newc(x, pointer, number, type, cast);
1470 Newz(x, pointer, number, type);
1471
5f05dabc
PP
1472The first argument C<x> was a "magic cookie" that was used to keep track
1473of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1474the current code makes no use of this feature (most Perl developers now
1475use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1476
1477The second argument C<pointer> should be the name of a variable that will
1478point to the newly allocated memory.
d1b91892 1479
d1b91892
AD
1480The third and fourth arguments C<number> and C<type> specify how many of
1481the specified type of data structure should be allocated. The argument
1482C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1483should be used if the C<pointer> argument is different from the C<type>
1484argument.
1485
1486Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1487to zero out all the newly allocated memory.
1488
06f6df17
RGS
1489=head3 Reallocation
1490
d1b91892
AD
1491 Renew(pointer, number, type);
1492 Renewc(pointer, number, type, cast);
1493 Safefree(pointer)
1494
1495These three macros are used to change a memory buffer size or to free a
1496piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1497match those of C<New> and C<Newc> with the exception of not needing the
1498"magic cookie" argument.
1499
06f6df17
RGS
1500=head3 Moving
1501
d1b91892
AD
1502 Move(source, dest, number, type);
1503 Copy(source, dest, number, type);
1504 Zero(dest, number, type);
1505
1506These three macros are used to move, copy, or zero out previously allocated
1507memory. The C<source> and C<dest> arguments point to the source and
1508destination starting points. Perl will move, copy, or zero out C<number>
1509instances of the size of the C<type> data structure (using the C<sizeof>
1510function).
a0d0e21e 1511
5f05dabc 1512=head2 PerlIO
ce3d39e2 1513
5f05dabc
PP
1514The most recent development releases of Perl has been experimenting with
1515removing Perl's dependency on the "normal" standard I/O suite and allowing
1516other stdio implementations to be used. This involves creating a new
1517abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1518was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1519abstraction layer and not make any assumptions about what kind of stdio
1520is being used.
1521
1522For a complete description of the PerlIO abstraction, consult L<perlapio>.
1523
8ebc5c01 1524=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1525
1526A lot of opcodes (this is an elementary operation in the internal perl
1527stack machine) put an SV* on the stack. However, as an optimization
1528the corresponding SV is (usually) not recreated each time. The opcodes
1529reuse specially assigned SVs (I<target>s) which are (as a corollary)
1530not constantly freed/created.
1531
0a753a76 1532Each of the targets is created only once (but see
ce3d39e2
IZ
1533L<Scratchpads and recursion> below), and when an opcode needs to put
1534an integer, a double, or a string on stack, it just sets the
1535corresponding parts of its I<target> and puts the I<target> on stack.
1536
1537The macro to put this target on stack is C<PUSHTARG>, and it is
1538directly used in some opcodes, as well as indirectly in zillions of
1539others, which use it via C<(X)PUSH[pni]>.
1540
1bd1c0d5
SC
1541Because the target is reused, you must be careful when pushing multiple
1542values on the stack. The following code will not do what you think:
1543
1544 XPUSHi(10);
1545 XPUSHi(20);
1546
1547This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1548the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1549At the end of the operation, the stack does not contain the values 10
1550and 20, but actually contains two pointers to C<TARG>, which we have set
1551to 20. If you need to push multiple different values, use C<XPUSHs>,
1552which bypasses C<TARG>.
1553
1554On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
1555need a C<dTARG> in your variable declarations so that the C<*PUSH*>
00aadd71 1556macros can make use of the local variable C<TARG>.
1bd1c0d5 1557
8ebc5c01 1558=head2 Scratchpads
ce3d39e2 1559
54310121 1560The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1561are created. The answer is that they are created when the current unit --
1562a subroutine or a file (for opcodes for statements outside of
1563subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1564array is created, which is called a scratchpad for the current
1565unit.
1566
54310121 1567A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1568targets for opcodes. One can deduce that an SV lives on a scratchpad
1569by looking on its flags: lexicals have C<SVs_PADMY> set, and
1570I<target>s have C<SVs_PADTMP> set.
1571
54310121
PP
1572The correspondence between OPs and I<target>s is not 1-to-1. Different
1573OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1574would not conflict with the expected life of the temporary.
1575
2ae324a7 1576=head2 Scratchpads and recursion
ce3d39e2
IZ
1577
1578In fact it is not 100% true that a compiled unit contains a pointer to
1579the scratchpad AV. In fact it contains a pointer to an AV of
1580(initially) one element, and this element is the scratchpad AV. Why do
1581we need an extra level of indirection?
1582
9a68f1db 1583The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2
IZ
1584these can create several execution pointers going into the same
1585subroutine. For the subroutine-child not write over the temporaries
1586for the subroutine-parent (lifespan of which covers the call to the
1587child), the parent and the child should have different
1588scratchpads. (I<And> the lexicals should be separate anyway!)
1589
5f05dabc
PP
1590So each subroutine is born with an array of scratchpads (of length 1).
1591On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1592depth of the recursion is not more than the length of this array, and
1593if it is, new scratchpad is created and pushed into the array.
1594
1595The I<target>s on this scratchpad are C<undef>s, but they are already
1596marked with correct flags.
1597
0a753a76
PP
1598=head1 Compiled code
1599
1600=head2 Code tree
1601
1602Here we describe the internal form your code is converted to by
1603Perl. Start with a simple example:
1604
1605 $a = $b + $c;
1606
1607This is converted to a tree similar to this one:
1608
1609 assign-to
1610 / \
1611 + $a
1612 / \
1613 $b $c
1614
7b8d334a 1615(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1616parsed your code, but has nothing to do with the execution order.
1617There is an additional "thread" going through the nodes of the tree
1618which shows the order of execution of the nodes. In our simplified
1619example above it looks like:
1620
1621 $b ---> $c ---> + ---> $a ---> assign-to
1622
1623But with the actual compile tree for C<$a = $b + $c> it is different:
1624some nodes I<optimized away>. As a corollary, though the actual tree
1625contains more nodes than our simplified example, the execution order
1626is the same as in our example.
1627
1628=head2 Examining the tree
1629
06f6df17
RGS
1630If you have your perl compiled for debugging (usually done with
1631C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1632compiled tree by specifying C<-Dx> on the Perl command line. The
1633output takes several lines per node, and for C<$b+$c> it looks like
1634this:
1635
1636 5 TYPE = add ===> 6
1637 TARG = 1
1638 FLAGS = (SCALAR,KIDS)
1639 {
1640 TYPE = null ===> (4)
1641 (was rv2sv)
1642 FLAGS = (SCALAR,KIDS)
1643 {
1644 3 TYPE = gvsv ===> 4
1645 FLAGS = (SCALAR)
1646 GV = main::b
1647 }
1648 }
1649 {
1650 TYPE = null ===> (5)
1651 (was rv2sv)
1652 FLAGS = (SCALAR,KIDS)
1653 {
1654 4 TYPE = gvsv ===> 5
1655 FLAGS = (SCALAR)
1656 GV = main::c
1657 }
1658 }
1659
1660This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1661not optimized away (one per number in the left column). The immediate
1662children of the given node correspond to C<{}> pairs on the same level
1663of indentation, thus this listing corresponds to the tree:
1664
1665 add
1666 / \
1667 null null
1668 | |
1669 gvsv gvsv
1670
1671The execution order is indicated by C<===E<gt>> marks, thus it is C<3
16724 5 6> (node C<6> is not included into above listing), i.e.,
1673C<gvsv gvsv add whatever>.
1674
9afa14e3
SC
1675Each of these nodes represents an op, a fundamental operation inside the
1676Perl core. The code which implements each operation can be found in the
1677F<pp*.c> files; the function which implements the op with type C<gvsv>
1678is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1679different numbers of children: C<add> is a binary operator, as one would
1680expect, and so has two children. To accommodate the various different
1681numbers of children, there are various types of op data structure, and
1682they link together in different ways.
1683
1684The simplest type of op structure is C<OP>: this has no children. Unary
1685operators, C<UNOP>s, have one child, and this is pointed to by the
1686C<op_first> field. Binary operators (C<BINOP>s) have not only an
1687C<op_first> field but also an C<op_last> field. The most complex type of
1688op is a C<LISTOP>, which has any number of children. In this case, the
1689first child is pointed to by C<op_first> and the last child by
1690C<op_last>. The children in between can be found by iteratively
1691following the C<op_sibling> pointer from the first child to the last.
1692
1693There are also two other op types: a C<PMOP> holds a regular expression,
1694and has no children, and a C<LOOP> may or may not have children. If the
1695C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1696complicate matters, if a C<UNOP> is actually a C<null> op after
1697optimization (see L</Compile pass 2: context propagation>) it will still
1698have children in accordance with its former type.
1699
06f6df17
RGS
1700Another way to examine the tree is to use a compiler back-end module, such
1701as L<B::Concise>.
1702
0a753a76
PP
1703=head2 Compile pass 1: check routines
1704
8870b5c7
GS
1705The tree is created by the compiler while I<yacc> code feeds it
1706the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1707the first pass of perl compilation.
1708
1709What makes this pass interesting for perl developers is that some
1710optimization may be performed on this pass. This is optimization by
8870b5c7 1711so-called "check routines". The correspondence between node names
0a753a76
PP
1712and corresponding check routines is described in F<opcode.pl> (do not
1713forget to run C<make regen_headers> if you modify this file).
1714
1715A check routine is called when the node is fully constructed except
7b8d334a 1716for the execution-order thread. Since at this time there are no
0a753a76
PP
1717back-links to the currently constructed node, one can do most any
1718operation to the top-level node, including freeing it and/or creating
1719new nodes above/below it.
1720
1721The check routine returns the node which should be inserted into the
1722tree (if the top-level node was not modified, check routine returns
1723its argument).
1724
1725By convention, check routines have names C<ck_*>. They are usually
1726called from C<new*OP> subroutines (or C<convert>) (which in turn are
1727called from F<perly.y>).
1728
1729=head2 Compile pass 1a: constant folding
1730
1731Immediately after the check routine is called the returned node is
1732checked for being compile-time executable. If it is (the value is
1733judged to be constant) it is immediately executed, and a I<constant>
1734node with the "return value" of the corresponding subtree is
1735substituted instead. The subtree is deleted.
1736
1737If constant folding was not performed, the execution-order thread is
1738created.
1739
1740=head2 Compile pass 2: context propagation
1741
1742When a context for a part of compile tree is known, it is propagated
a3cb178b 1743down through the tree. At this time the context can have 5 values
0a753a76
PP
1744(instead of 2 for runtime context): void, boolean, scalar, list, and
1745lvalue. In contrast with the pass 1 this pass is processed from top
1746to bottom: a node's context determines the context for its children.
1747
1748Additional context-dependent optimizations are performed at this time.
1749Since at this moment the compile tree contains back-references (via
1750"thread" pointers), nodes cannot be free()d now. To allow
1751optimized-away nodes at this stage, such nodes are null()ified instead
1752of free()ing (i.e. their type is changed to OP_NULL).
1753
1754=head2 Compile pass 3: peephole optimization
1755
1756After the compile tree for a subroutine (or for an C<eval> or a file)
1757is created, an additional pass over the code is performed. This pass
1758is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1759additional complications for conditionals). These optimizations are
0a753a76
PP
1760done in the subroutine peep(). Optimizations performed at this stage
1761are subject to the same restrictions as in the pass 2.
1762
1ba7f851
PJ
1763=head2 Pluggable runops
1764
1765The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
1766functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
1767with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
1768control over the execution of the compile tree it is possible to provide
1769your own runops function.
1ba7f851
PJ
1770
1771It's probably best to copy one of the existing runops functions and
1772change it to suit your needs. Then, in the BOOT section of your XS
1773file, add the line:
1774
1775 PL_runops = my_runops;
1776
1777This function should be as efficient as possible to keep your programs
1778running as fast as possible.
1779
9afa14e3
SC
1780=head1 Examining internal data structures with the C<dump> functions
1781
1782To aid debugging, the source file F<dump.c> contains a number of
1783functions which produce formatted output of internal data structures.
1784
1785The most commonly used of these functions is C<Perl_sv_dump>; it's used
1786for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1787C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1788module should already be familiar with its format.
9afa14e3
SC
1789
1790C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 1791derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
1792C<Perl_dump_eval> will dump the main root of the code being evaluated,
1793exactly like C<-Dx>.
1794
1795Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1796op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1797subroutines in a package like so: (Thankfully, these are all xsubs, so
1798there is no op tree)
1799
1800 (gdb) print Perl_dump_packsubs(PL_defstash)
1801
1802 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1803
1804 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1805
1806 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1807
1808 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1809
1810 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1811
1812and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1813the op tree of the main root.
1814
954c1994 1815=head1 How multiple interpreters and concurrency are supported
ee072b34 1816
ee072b34
GS
1817=head2 Background and PERL_IMPLICIT_CONTEXT
1818
1819The Perl interpreter can be regarded as a closed box: it has an API
1820for feeding it code or otherwise making it do things, but it also has
1821functions for its own use. This smells a lot like an object, and
1822there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
1823interpreters, with one interpreter represented either as a C structure,
1824or inside a thread-specific structure. These structures contain all
1825the context, the state of that interpreter.
1826
9a68f1db 1827Two macros control the major Perl build flavors: MULTIPLICITY and
acfe0abc
GS
1828USE_5005THREADS. The MULTIPLICITY build has a C structure
1829that packages all the interpreter state, and there is a similar thread-specific
1830data structure under USE_5005THREADS. In both cases,
54aff467
GS
1831PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1832support for passing in a "hidden" first argument that represents all three
651a3225 1833data structures.
54aff467
GS
1834
1835All this obviously requires a way for the Perl internal functions to be
acfe0abc 1836either subroutines taking some kind of structure as the first
ee072b34 1837argument, or subroutines taking nothing as the first argument. To
acfe0abc 1838enable these two very different ways of building the interpreter,
ee072b34
GS
1839the Perl source (as it does in so many other situations) makes heavy
1840use of macros and subroutine naming conventions.
1841
54aff467 1842First problem: deciding which functions will be public API functions and
00aadd71 1843which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1844(think "S" for "secret" or "static"). All other functions begin with
1845"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1846part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1847function is part of the API is to find its entry in L<perlapi>.
1848If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1849think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1850L<perlbug> explaining why you think it should be.
ee072b34
GS
1851
1852Second problem: there must be a syntax so that the same subroutine
1853declarations and calls can pass a structure as their first argument,
1854or pass nothing. To solve this, the subroutines are named and
1855declared in a particular way. Here's a typical start of a static
1856function used within the Perl guts:
1857
1858 STATIC void
1859 S_incline(pTHX_ char *s)
1860
acfe0abc
GS
1861STATIC becomes "static" in C, and may be #define'd to nothing in some
1862configurations in future.
ee072b34 1863
651a3225
GS
1864A public function (i.e. part of the internal API, but not necessarily
1865sanctioned for use in extensions) begins like this:
ee072b34
GS
1866
1867 void
2307c6d0 1868 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34
GS
1869
1870C<pTHX_> is one of a number of macros (in perl.h) that hide the
1871details of the interpreter's context. THX stands for "thread", "this",
1872or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1873The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1874or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1875their variants.
ee072b34 1876
a7486cbb
JH
1877When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1878first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1879in the pTHX_ macro indicates that the macro expansion needs a comma
1880after the context argument because other arguments follow it. If
1881PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1882subroutine is not prototyped to take the extra argument. The form of the
1883macro without the trailing underscore is used when there are no additional
ee072b34
GS
1884explicit arguments.
1885
54aff467 1886When a core function calls another, it must pass the context. This
2307c6d0 1887is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
1888something like this:
1889
2307c6d0
SB
1890 #ifdef PERL_IMPLICIT_CONTEXT
1891 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 1892 /* can't do this for vararg functions, see below */
2307c6d0
SB
1893 #else
1894 #define sv_setiv Perl_sv_setiv
1895 #endif
ee072b34
GS
1896
1897This works well, and means that XS authors can gleefully write:
1898
2307c6d0 1899 sv_setiv(foo, bar);
ee072b34
GS
1900
1901and still have it work under all the modes Perl could have been
1902compiled with.
1903
ee072b34
GS
1904This doesn't work so cleanly for varargs functions, though, as macros
1905imply that the number of arguments is known in advance. Instead we
1906either need to spell them out fully, passing C<aTHX_> as the first
1907argument (the Perl core tends to do this with functions like
1908Perl_warner), or use a context-free version.
1909
1910The context-free version of Perl_warner is called
1911Perl_warner_nocontext, and does not take the extra argument. Instead
1912it does dTHX; to get the context from thread-local storage. We
1913C<#define warner Perl_warner_nocontext> so that extensions get source
1914compatibility at the expense of performance. (Passing an arg is
1915cheaper than grabbing it from thread-local storage.)
1916
acfe0abc 1917You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
1918Those are strictly for use within the core. Extensions and embedders
1919need only be aware of [pad]THX.
1920
a7486cbb
JH
1921=head2 So what happened to dTHR?
1922
1923C<dTHR> was introduced in perl 5.005 to support the older thread model.
1924The older thread model now uses the C<THX> mechanism to pass context
1925pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
1926later still have it for backward source compatibility, but it is defined
1927to be a no-op.
1928
ee072b34
GS
1929=head2 How do I use all this in extensions?
1930
1931When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1932any functions in the Perl API will need to pass the initial context
1933argument somehow. The kicker is that you will need to write it in
1934such a way that the extension still compiles when Perl hasn't been
1935built with PERL_IMPLICIT_CONTEXT enabled.
1936
1937There are three ways to do this. First, the easy but inefficient way,
1938which is also the default, in order to maintain source compatibility
1939with extensions: whenever XSUB.h is #included, it redefines the aTHX
1940and aTHX_ macros to call a function that will return the context.
1941Thus, something like:
1942
2307c6d0 1943 sv_setiv(sv, num);
ee072b34 1944
4375e838 1945in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1946in effect:
ee072b34 1947
2307c6d0 1948 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 1949
54aff467 1950or to this otherwise:
ee072b34 1951
2307c6d0 1952 Perl_sv_setiv(sv, num);
ee072b34
GS
1953
1954You have to do nothing new in your extension to get this; since
2fa86c13 1955the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1956work.
1957
1958The second, more efficient way is to use the following template for
1959your Foo.xs:
1960
c52f9dcd
JH
1961 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1962 #include "EXTERN.h"
1963 #include "perl.h"
1964 #include "XSUB.h"
ee072b34
GS
1965
1966 static my_private_function(int arg1, int arg2);
1967
c52f9dcd
JH
1968 static SV *
1969 my_private_function(int arg1, int arg2)
1970 {
1971 dTHX; /* fetch context */
1972 ... call many Perl API functions ...
1973 }
ee072b34
GS
1974
1975 [... etc ...]
1976
c52f9dcd 1977 MODULE = Foo PACKAGE = Foo
ee072b34 1978
c52f9dcd 1979 /* typical XSUB */
ee072b34 1980
c52f9dcd
JH
1981 void
1982 my_xsub(arg)
1983 int arg
1984 CODE:
1985 my_private_function(arg, 10);
ee072b34
GS
1986
1987Note that the only two changes from the normal way of writing an
1988extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
1989including the Perl headers, followed by a C<dTHX;> declaration at
1990the start of every function that will call the Perl API. (You'll
1991know which functions need this, because the C compiler will complain
1992that there's an undeclared identifier in those functions.) No changes
1993are needed for the XSUBs themselves, because the XS() macro is
1994correctly defined to pass in the implicit context if needed.
1995
1996The third, even more efficient way is to ape how it is done within
1997the Perl guts:
1998
1999
c52f9dcd
JH
2000 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2001 #include "EXTERN.h"
2002 #include "perl.h"
2003 #include "XSUB.h"
ee072b34
GS
2004
2005 /* pTHX_ only needed for functions that call Perl API */
2006 static my_private_function(pTHX_ int arg1, int arg2);
2007
c52f9dcd
JH
2008 static SV *
2009 my_private_function(pTHX_ int arg1, int arg2)
2010 {
2011 /* dTHX; not needed here, because THX is an argument */
2012 ... call Perl API functions ...
2013 }
ee072b34
GS
2014
2015 [... etc ...]
2016
c52f9dcd 2017 MODULE = Foo PACKAGE = Foo
ee072b34 2018
c52f9dcd 2019 /* typical XSUB */
ee072b34 2020
c52f9dcd
JH
2021 void
2022 my_xsub(arg)
2023 int arg
2024 CODE:
2025 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2026
2027This implementation never has to fetch the context using a function
2028call, since it is always passed as an extra argument. Depending on
2029your needs for simplicity or efficiency, you may mix the previous
2030two approaches freely.
2031
651a3225
GS
2032Never add a comma after C<pTHX> yourself--always use the form of the
2033macro with the underscore for functions that take explicit arguments,
2034or the form without the argument for functions with no explicit arguments.
ee072b34 2035
a7486cbb
JH
2036=head2 Should I do anything special if I call perl from multiple threads?
2037
2038If you create interpreters in one thread and then proceed to call them in
2039another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2040initialized correctly in each of those threads.
2041
2042The C<perl_alloc> and C<perl_clone> API functions will automatically set
2043the TLS slot to the interpreter they created, so that there is no need to do
2044anything special if the interpreter is always accessed in the same thread that
2045created it, and that thread did not create or call any other interpreters
2046afterwards. If that is not the case, you have to set the TLS slot of the
2047thread before calling any functions in the Perl API on that particular
2048interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2049thread as the first thing you do:
2050
2051 /* do this before doing anything else with some_perl */
2052 PERL_SET_CONTEXT(some_perl);
2053
2054 ... other Perl API calls on some_perl go here ...
2055
ee072b34
GS
2056=head2 Future Plans and PERL_IMPLICIT_SYS
2057
2058Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2059that the interpreter knows about itself and pass it around, so too are
2060there plans to allow the interpreter to bundle up everything it knows
2061about the environment it's running on. This is enabled with the
acfe0abc 2062PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS
4d1ff10f 2063and USE_5005THREADS on Windows (see inside iperlsys.h).
ee072b34
GS
2064
2065This allows the ability to provide an extra pointer (called the "host"
2066environment) for all the system calls. This makes it possible for
2067all the system stuff to maintain their own state, broken down into
2068seven C structures. These are thin wrappers around the usual system
2069calls (see win32/perllib.c) for the default perl executable, but for a
2070more ambitious host (like the one that would do fork() emulation) all
2071the extra work needed to pretend that different interpreters are
2072actually different "processes", would be done here.
2073
2074The Perl engine/interpreter and the host are orthogonal entities.
2075There could be one or more interpreters in a process, and one or
2076more "hosts", with free association between them.
2077
a422fd2d
SC
2078=head1 Internal Functions
2079
2080All of Perl's internal functions which will be exposed to the outside
06f6df17 2081world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d
SC
2082functions or functions used in a program in which Perl is embedded.
2083Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2084static functions start with C<S_>.)
a422fd2d
SC
2085
2086Inside the Perl core, you can get at the functions either with or
2087without the C<Perl_> prefix, thanks to a bunch of defines that live in
2088F<embed.h>. This header file is generated automatically from
dc9b1d22
MHM
2089F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2090header files for the internal functions, generates the documentation
2091and a lot of other bits and pieces. It's important that when you add
2092a new function to the core or change an existing one, you change the
2093data in the table in F<embed.fnc> as well. Here's a sample entry from
2094that table:
a422fd2d
SC
2095
2096 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2097
2098The second column is the return type, the third column the name. Columns
2099after that are the arguments. The first column is a set of flags:
2100
2101=over 3
2102
2103=item A
2104
2105This function is a part of the public API.
2106
2107=item p
2108
2109This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
2110
2111=item d
2112
2113This function has documentation using the C<apidoc> feature which we'll
2114look at in a second.
2115
2116=back
2117
2118Other available flags are:
2119
2120=over 3
2121
2122=item s
2123
a7486cbb
JH
2124This is a static function and is defined as C<S_whatever>, and usually
2125called within the sources as C<whatever(...)>.
a422fd2d
SC
2126
2127=item n
2128
2129This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
2130L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2131
2132=item r
2133
2134This function never returns; C<croak>, C<exit> and friends.
2135
2136=item f
2137
2138This function takes a variable number of arguments, C<printf> style.
2139The argument list should end with C<...>, like this:
2140
2141 Afprd |void |croak |const char* pat|...
2142
a7486cbb 2143=item M
a422fd2d 2144
00aadd71 2145This function is part of the experimental development API, and may change
a422fd2d
SC
2146or disappear without notice.
2147
2148=item o
2149
2150This function should not have a compatibility macro to define, say,
2151C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2152
a422fd2d
SC
2153=item x
2154
2155This function isn't exported out of the Perl core.
2156
dc9b1d22
MHM
2157=item m
2158
2159This is implemented as a macro.
2160
2161=item X
2162
2163This function is explicitly exported.
2164
2165=item E
2166
2167This function is visible to extensions included in the Perl core.
2168
2169=item b
2170
2171Binary backward compatibility; this function is a macro but also has
2172a C<Perl_> implementation (which is exported).
2173
a422fd2d
SC
2174=back
2175
dc9b1d22
MHM
2176If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2177C<make regen_headers> to force a rebuild of F<embed.h> and other
2178auto-generated files.
a422fd2d 2179
6b4667fc 2180=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2181
6b4667fc
A
2182If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2183formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2184following macros for portability
9dd9db0b 2185
c52f9dcd
JH
2186 IVdf IV in decimal
2187 UVuf UV in decimal
2188 UVof UV in octal
2189 UVxf UV in hexadecimal
2190 NVef NV %e-like
2191 NVff NV %f-like
2192 NVgf NV %g-like
9dd9db0b 2193
6b4667fc
A
2194These will take care of 64-bit integers and long doubles.
2195For example:
2196
c52f9dcd 2197 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2198
2199The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2200
8908e76d
JH
2201If you are printing addresses of pointers, use UVxf combined
2202with PTR2UV(), do not use %lx or %p.
2203
2204=head2 Pointer-To-Integer and Integer-To-Pointer
2205
2206Because pointer size does not necessarily equal integer size,
2207use the follow macros to do it right.
2208
c52f9dcd
JH
2209 PTR2UV(pointer)
2210 PTR2IV(pointer)
2211 PTR2NV(pointer)
2212 INT2PTR(pointertotype, integer)
8908e76d
JH
2213
2214For example:
2215
c52f9dcd
JH
2216 IV iv = ...;
2217 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2218
2219and
2220
c52f9dcd
JH
2221 AV *av = ...;
2222 UV uv = PTR2UV(av);
8908e76d 2223
a422fd2d
SC
2224=head2 Source Documentation
2225
2226There's an effort going on to document the internal functions and
2227automatically produce reference manuals from them - L<perlapi> is one
2228such manual which details all the functions which are available to XS
2229writers. L<perlintern> is the autogenerated manual for the functions
2230which are not part of the API and are supposedly for internal use only.
2231
2232Source documentation is created by putting POD comments into the C
2233source, like this:
2234
2235 /*
2236 =for apidoc sv_setiv
2237
2238 Copies an integer into the given SV. Does not handle 'set' magic. See
2239 C<sv_setiv_mg>.
2240
2241 =cut
2242 */
2243
2244Please try and supply some documentation if you add functions to the
2245Perl core.
2246
2247=head1 Unicode Support
2248
2249Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2250writers to understand this support and make sure that the code they
2251write does not corrupt Unicode data.
2252
2253=head2 What B<is> Unicode, anyway?
2254
2255In the olden, less enlightened times, we all used to use ASCII. Most of
2256us did, anyway. The big problem with ASCII is that it's American. Well,
2257no, that's not actually the problem; the problem is that it's not
2258particularly useful for people who don't use the Roman alphabet. What
2259used to happen was that particular languages would stick their own
2260alphabet in the upper range of the sequence, between 128 and 255. Of
2261course, we then ended up with plenty of variants that weren't quite
2262ASCII, and the whole point of it being a standard was lost.
2263
2264Worse still, if you've got a language like Chinese or
2265Japanese that has hundreds or thousands of characters, then you really
2266can't fit them into a mere 256, so they had to forget about ASCII
2267altogether, and build their own systems using pairs of numbers to refer
2268to one character.
2269
2270To fix this, some people formed Unicode, Inc. and
2271produced a new character set containing all the characters you can
2272possibly think of and more. There are several ways of representing these
1e54db1a 2273characters, and the one Perl uses is called UTF-8. UTF-8 uses
a422fd2d 2274a variable number of bytes to represent a character, instead of just
b3b6085d 2275one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d 2276
1e54db1a 2277=head2 How can I recognise a UTF-8 string?
a422fd2d 2278
1e54db1a
JH
2279You can't. This is because UTF-8 data is stored in bytes just like
2280non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d
SC
2281capital E with a grave accent, is represented by the two bytes
2282C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2283has that byte sequence as well. So you can't tell just by looking - this
2284is what makes Unicode input an interesting problem.
2285
2286The API function C<is_utf8_string> can help; it'll tell you if a string
1e54db1a 2287contains only valid UTF-8 characters. However, it can't do the work for
a422fd2d 2288you. On a character-by-character basis, C<is_utf8_char> will tell you
1e54db1a 2289whether the current character in a string is valid UTF-8.
a422fd2d 2290
1e54db1a 2291=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2292
1e54db1a 2293As mentioned above, UTF-8 uses a variable number of bytes to store a
a422fd2d
SC
2294character. Characters with values 1...128 are stored in one byte, just
2295like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2296continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2297bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2298so it goes on, moving to three bytes at character 2048.
2299
1e54db1a 2300Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2301how long the first character in it is with the C<UTF8SKIP> macro:
2302
2303 char *utf = "\305\233\340\240\201";
2304 I32 len;
2305
2306 len = UTF8SKIP(utf); /* len is 2 here */
2307 utf += len;
2308 len = UTF8SKIP(utf); /* len is 3 here */
2309
1e54db1a 2310Another way to skip over characters in a UTF-8 string is to use
a422fd2d
SC
2311C<utf8_hop>, which takes a string and a number of characters to skip
2312over. You're on your own about bounds checking, though, so don't use it
2313lightly.
2314
1e54db1a 2315All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2316so you can test if you need to do something special with this
2317character like this (the UTF8_IS_INVARIANT() is a macro that tests
2318whether the byte can be encoded as a single byte even in UTF-8):
a422fd2d 2319
3a2263fe
RGS
2320 U8 *utf;
2321 UV uv; /* Note: a UV, not a U8, not a char */
a422fd2d 2322
3a2263fe 2323 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2324 /* Must treat this as UTF-8 */
a422fd2d
SC
2325 uv = utf8_to_uv(utf);
2326 else
2327 /* OK to treat this character as a byte */
2328 uv = *utf;
2329
2330You can also see in that example that we use C<utf8_to_uv> to get the
2331value of the character; the inverse function C<uv_to_utf8> is available
1e54db1a 2332for putting a UV into UTF-8:
a422fd2d 2333
3a2263fe 2334 if (!UTF8_IS_INVARIANT(uv))
a422fd2d
SC
2335 /* Must treat this as UTF8 */
2336 utf8 = uv_to_utf8(utf8, uv);
2337 else
2338 /* OK to treat this character as a byte */
2339 *utf8++ = uv;
2340
2341You B<must> convert characters to UVs using the above functions if
1e54db1a
JH
2342you're ever in a situation where you have to match UTF-8 and non-UTF-8
2343characters. You may not skip over UTF-8 characters in this case. If you
2344do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2345for instance, if your UTF-8 string contains C<v196.172>, and you skip
2346that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2347So don't do that!
2348
1e54db1a 2349=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2350
2351Currently, Perl deals with Unicode strings and non-Unicode strings
2352slightly differently. If a string has been identified as being UTF-8
2353encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2354manipulate this flag with the following macros:
2355
2356 SvUTF8(sv)
2357 SvUTF8_on(sv)
2358 SvUTF8_off(sv)
2359
2360This flag has an important effect on Perl's treatment of the string: if
2361Unicode data is not properly distinguished, regular expressions,
2362C<length>, C<substr> and other string handling operations will have
2363undesirable results.
2364
2365The problem comes when you have, for instance, a string that isn't
1e54db1a
JH
2366flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
2367especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2368
2369Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2370need be sure you don't accidentally knock it off while you're
2371manipulating SVs. More specifically, you cannot expect to do this:
2372
2373 SV *sv;
2374 SV *nsv;
2375 STRLEN len;
2376 char *p;
2377
2378 p = SvPV(sv, len);
2379 frobnicate(p);
2380 nsv = newSVpvn(p, len);
2381
2382The C<char*> string does not tell you the whole story, and you can't
2383copy or reconstruct an SV just by copying the string value. Check if the
1e54db1a 2384old SV has the UTF-8 flag set, and act accordingly:
a422fd2d
SC
2385
2386 p = SvPV(sv, len);
2387 frobnicate(p);
2388 nsv = newSVpvn(p, len);
2389 if (SvUTF8(sv))
2390 SvUTF8_on(nsv);
2391
2392In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2393not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2394appropriately.
2395
3a2263fe 2396Since just passing an SV to an XS function and copying the data of
1e54db1a 2397the SV is not enough to copy the UTF-8 flags, even less right is just
3a2263fe
RGS
2398passing a C<char *> to an XS function.
2399
1e54db1a 2400=head2 How do I convert a string to UTF-8?
a422fd2d 2401
1e54db1a
JH
2402If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary
2403to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
a422fd2d
SC
2404way to do this is:
2405
2406 sv_utf8_upgrade(sv);
2407
2408However, you must not do this, for example:
2409
2410 if (!SvUTF8(left))
2411 sv_utf8_upgrade(left);
2412
2413If you do this in a binary operator, you will actually change one of the
b1866b2d 2414strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2415by the end user, it can cause problems.
2416
1e54db1a 2417Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
a422fd2d 2418string argument. This is useful for having the data available for
b1866b2d 2419comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2420C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2421the string contains any characters above 255 that can't be represented
2422in a single byte.
2423
2424=head2 Is there anything else I need to know?
2425
2426Not really. Just remember these things:
2427
2428=over 3
2429
2430=item *
2431
1e54db1a
JH
2432There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2433is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2434something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2435it's not - if you pass on the PV to somewhere, pass on the flag too.
2436
2437=item *
2438
1e54db1a 2439If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
3a2263fe 2440unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2441
2442=item *
2443
1e54db1a 2444When writing a character C<uv> to a UTF-8 string, B<always> use
3a2263fe
RGS
2445C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2446you can use C<*s = uv>.
a422fd2d
SC
2447
2448=item *
2449
1e54db1a
JH
2450Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2451a new string which is UTF-8 encoded. There are tricks you can use to
2452delay deciding whether you need to use a UTF-8 string until you get to a
a422fd2d
SC
2453high character - C<HALF_UPGRADE> is one of those.
2454
2455=back
2456
53e06cf0
SC
2457=head1 Custom Operators
2458
9a68f1db 2459Custom operator support is a new experimental feature that allows you to
53e06cf0
SC
2460define your own ops. This is primarily to allow the building of
2461interpreters for other languages in the Perl core, but it also allows
2462optimizations through the creation of "macro-ops" (ops which perform the
2463functions of multiple ops which are usually executed together, such as
b7cb320d 2464C<gvsv, gvsv, add>.)
53e06cf0 2465
b455bf3f 2466This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0
SC
2467core does not "know" anything special about this op type, and so it will
2468not be involved in any optimizations. This also means that you can
2469define your custom ops to be any op structure - unary, binary, list and
2470so on - you like.
2471
2472It's important to know what custom operators won't do for you. They
2473won't let you add new syntax to Perl, directly. They won't even let you
2474add new keywords, directly. In fact, they won't change the way Perl
2475compiles a program at all. You have to do those changes yourself, after
2476Perl has compiled the program. You do this either by manipulating the op
2477tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2478a custom peephole optimizer with the C<optimize> module.
2479
2480When you do this, you replace ordinary Perl ops with custom ops by
2481creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2482PP function. This should be defined in XS code, and should look like
2483the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2484takes the appropriate number of values from the stack, and you are
2485responsible for adding stack marks if necessary.
2486
2487You should also "register" your op with the Perl interpreter so that it
2488can produce sensible error and warning messages. Since it is possible to
2489have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2490Perl uses the value of C<< o->op_ppaddr >> as a key into the
2491C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2492need to enter a name and description for your op at the appropriate
2493place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2494
2495Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2496directly support the creation of custom ops by name; C<Opcodes::Custom>
2497will provide functions which make it trivial to "register" custom ops to
2498the Perl interpreter.
2499
954c1994 2500=head1 AUTHORS
e89caa19 2501
954c1994 2502Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2503E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2504itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 2505
954c1994
GS
2506With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2507Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2508Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2509Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2510
954c1994 2511=head1 SEE ALSO
cb1a09d0 2512
954c1994 2513perlapi(1), perlintern(1), perlxs(1), perlembed(1)