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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
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7This document attempts to describe how to use the Perl API, as well as
8containing some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl has three typedefs that handle Perl's three main data types:
17
18 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21
d1b91892 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
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
91c54917 41The seven routines are:
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42
43 SV* newSViv(IV);
91c54917 44 SV* newSVuv(UV);
a0d0e21e 45 SV* newSVnv(double);
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46 SV* newSVpv(const char*, int);
47 SV* newSVpvn(const char*, int);
46fc3d4c 48 SV* newSVpvf(const char*, ...);
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49 SV* newSVsv(SV*);
50
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51If you require more complex initialisation you can create an empty SV with
52newSV(len). If C<len> is 0 an empty SV of type NULL is returned, else an
53SV of type PV is returned with len + 1 (for the NUL) bytes of storage
54allocated, accessible via SvPVX. In both cases the SV has value undef.
55
56 SV* newSV(0); /* no storage allocated */
57 SV* newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
58
59To change the value of an *already-existing* SV, there are eight routines:
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60
61 void sv_setiv(SV*, IV);
deb3007b 62 void sv_setuv(SV*, UV);
a0d0e21e 63 void sv_setnv(SV*, double);
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64 void sv_setpv(SV*, const char*);
65 void sv_setpvn(SV*, const char*, int)
46fc3d4c 66 void sv_setpvf(SV*, const char*, ...);
5b3218b9 67 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
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68 void sv_setsv(SV*, SV*);
69
70Notice that you can choose to specify the length of the string to be
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71assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
72allow Perl to calculate the length by using C<sv_setpv> or by specifying
730 as the second argument to C<newSVpv>. Be warned, though, that Perl will
74determine the string's length by using C<strlen>, which depends on the
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75string terminating with a NUL character.
76
77The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
78formatted output becomes the value.
79
328bf373 80C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
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81either a pointer to a variable argument list or the address and length of
82an array of SVs. The last argument points to a boolean; on return, if that
83boolean is true, then locale-specific information has been used to format
c2611fb3 84the string, and the string's contents are therefore untrustworthy (see
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85L<perlsec>). This pointer may be NULL if that information is not
86important. Note that this function requires you to specify the length of
87the format.
88
7c906a97 89STRLEN is an integer type (Size_t, usually defined as size_t in
00aadd71 90config.h) guaranteed to be large enough to represent the size of
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91any string that perl can handle.
92
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93The C<sv_set*()> functions are not generic enough to operate on values
94that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 95
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96All SVs that contain strings should be terminated with a NUL character.
97If it is not NUL-terminated there is a risk of
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98core dumps and corruptions from code which passes the string to C
99functions or system calls which expect a NUL-terminated string.
100Perl's own functions typically add a trailing NUL for this reason.
101Nevertheless, you should be very careful when you pass a string stored
102in an SV to a C function or system call.
103
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104To access the actual value that an SV points to, you can use the macros:
105
106 SvIV(SV*)
954c1994 107 SvUV(SV*)
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108 SvNV(SV*)
109 SvPV(SV*, STRLEN len)
1fa8b10d 110 SvPV_nolen(SV*)
a0d0e21e 111
954c1994 112which will automatically coerce the actual scalar type into an IV, UV, double,
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113or string.
114
115In the C<SvPV> macro, the length of the string returned is placed into the
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116variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
117not care what the length of the data is, use the C<SvPV_nolen> macro.
118Historically the C<SvPV> macro with the global variable C<PL_na> has been
119used in this case. But that can be quite inefficient because C<PL_na> must
120be accessed in thread-local storage in threaded Perl. In any case, remember
121that Perl allows arbitrary strings of data that may both contain NULs and
122might not be terminated by a NUL.
a0d0e21e 123
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124Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
125len);>. It might work with your compiler, but it won't work for everyone.
126Break this sort of statement up into separate assignments:
127
b2f5ed49 128 SV *s;
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129 STRLEN len;
130 char * ptr;
b2f5ed49 131 ptr = SvPV(s, len);
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132 foo(ptr, len);
133
07fa94a1 134If you want to know if the scalar value is TRUE, you can use:
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135
136 SvTRUE(SV*)
137
138Although Perl will automatically grow strings for you, if you need to force
139Perl to allocate more memory for your SV, you can use the macro
140
141 SvGROW(SV*, STRLEN newlen)
142
143which will determine if more memory needs to be allocated. If so, it will
144call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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145decrease, the allocated memory of an SV and that it does not automatically
146add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 147C<SvGROW(sv, len + 1)>).
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148
149If you have an SV and want to know what kind of data Perl thinks is stored
150in it, you can use the following macros to check the type of SV you have.
151
152 SvIOK(SV*)
153 SvNOK(SV*)
154 SvPOK(SV*)
155
156You can get and set the current length of the string stored in an SV with
157the following macros:
158
159 SvCUR(SV*)
160 SvCUR_set(SV*, I32 val)
161
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162You can also get a pointer to the end of the string stored in the SV
163with the macro:
164
165 SvEND(SV*)
166
167But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 168
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169If you want to append something to the end of string stored in an C<SV*>,
170you can use the following functions:
171
08105a92 172 void sv_catpv(SV*, const char*);
e65f3abd 173 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 174 void sv_catpvf(SV*, const char*, ...);
328bf373 175 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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176 void sv_catsv(SV*, SV*);
177
178The first function calculates the length of the string to be appended by
179using C<strlen>. In the second, you specify the length of the string
46fc3d4c 180yourself. The third function processes its arguments like C<sprintf> and
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181appends the formatted output. The fourth function works like C<vsprintf>.
182You can specify the address and length of an array of SVs instead of the
183va_list argument. The fifth function extends the string stored in the first
184SV with the string stored in the second SV. It also forces the second SV
185to be interpreted as a string.
186
187The C<sv_cat*()> functions are not generic enough to operate on values that
188have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 189
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190If you know the name of a scalar variable, you can get a pointer to its SV
191by using the following:
192
4929bf7b 193 SV* get_sv("package::varname", FALSE);
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194
195This returns NULL if the variable does not exist.
196
d1b91892 197If you want to know if this variable (or any other SV) is actually C<defined>,
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198you can call:
199
200 SvOK(SV*)
201
9cde0e7f 202The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its
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203address can be used whenever an C<SV*> is needed.
204
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205There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean
206TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can
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207be used whenever an C<SV*> is needed.
208
9cde0e7f 209Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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210Take this code:
211
212 SV* sv = (SV*) 0;
213 if (I-am-to-return-a-real-value) {
214 sv = sv_2mortal(newSViv(42));
215 }
216 sv_setsv(ST(0), sv);
217
218This code tries to return a new SV (which contains the value 42) if it should
04343c6d 219return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 220pointer which, somewhere down the line, will cause a segmentation violation,
9cde0e7f 221bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first
5f05dabc 222line and all will be well.
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223
224To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 225call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 226
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227=head2 Offsets
228
229Perl provides the function C<sv_chop> to efficiently remove characters
230from the beginning of a string; you give it an SV and a pointer to
da75cd15 231somewhere inside the PV, and it discards everything before the
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232pointer. The efficiency comes by means of a little hack: instead of
233actually removing the characters, C<sv_chop> sets the flag C<OOK>
234(offset OK) to signal to other functions that the offset hack is in
235effect, and it puts the number of bytes chopped off into the IV field
236of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
00aadd71 237many bytes, and adjusts C<SvCUR> and C<SvLEN>.
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238
239Hence, at this point, the start of the buffer that we allocated lives
240at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
241into the middle of this allocated storage.
242
243This is best demonstrated by example:
244
245 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
246 SV = PVIV(0x8128450) at 0x81340f0
247 REFCNT = 1
248 FLAGS = (POK,OOK,pPOK)
249 IV = 1 (OFFSET)
250 PV = 0x8135781 ( "1" . ) "2345"\0
251 CUR = 4
252 LEN = 5
253
254Here the number of bytes chopped off (1) is put into IV, and
255C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
256portion of the string between the "real" and the "fake" beginnings is
257shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
258the fake beginning, not the real one.
259
fe854a6f 260Something similar to the offset hack is performed on AVs to enable
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261efficient shifting and splicing off the beginning of the array; while
262C<AvARRAY> points to the first element in the array that is visible from
263Perl, C<AvALLOC> points to the real start of the C array. These are
264usually the same, but a C<shift> operation can be carried out by
265increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
266Again, the location of the real start of the C array only comes into
267play when freeing the array. See C<av_shift> in F<av.c>.
268
d1b91892 269=head2 What's Really Stored in an SV?
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270
271Recall that the usual method of determining the type of scalar you have is
5f05dabc 272to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 273usually these macros will always return TRUE and calling the C<Sv*V>
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274macros will do the appropriate conversion of string to integer/double or
275integer/double to string.
276
277If you I<really> need to know if you have an integer, double, or string
278pointer in an SV, you can use the following three macros instead:
279
280 SvIOKp(SV*)
281 SvNOKp(SV*)
282 SvPOKp(SV*)
283
284These will tell you if you truly have an integer, double, or string pointer
d1b91892 285stored in your SV. The "p" stands for private.
a0d0e21e 286
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287The are various ways in which the private and public flags may differ.
288For example, a tied SV may have a valid underlying value in the IV slot
289(so SvIOKp is true), but the data should be accessed via the FETCH
290routine rather than directly, so SvIOK is false. Another is when
291numeric conversion has occured and precision has been lost: only the
292private flag is set on 'lossy' values. So when an NV is converted to an
293IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
294
07fa94a1 295In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 296
54310121 297=head2 Working with AVs
a0d0e21e 298
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299There are two ways to create and load an AV. The first method creates an
300empty AV:
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301
302 AV* newAV();
303
54310121 304The second method both creates the AV and initially populates it with SVs:
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305
306 AV* av_make(I32 num, SV **ptr);
307
5f05dabc 308The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 309AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 310
54310121 311Once the AV has been created, the following operations are possible on AVs:
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312
313 void av_push(AV*, SV*);
314 SV* av_pop(AV*);
315 SV* av_shift(AV*);
316 void av_unshift(AV*, I32 num);
317
318These should be familiar operations, with the exception of C<av_unshift>.
319This routine adds C<num> elements at the front of the array with the C<undef>
320value. You must then use C<av_store> (described below) to assign values
321to these new elements.
322
323Here are some other functions:
324
5f05dabc 325 I32 av_len(AV*);
a0d0e21e 326 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 327 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 328
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329The C<av_len> function returns the highest index value in array (just
330like $#array in Perl). If the array is empty, -1 is returned. The
331C<av_fetch> function returns the value at index C<key>, but if C<lval>
332is non-zero, then C<av_fetch> will store an undef value at that index.
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333The C<av_store> function stores the value C<val> at index C<key>, and does
334not increment the reference count of C<val>. Thus the caller is responsible
335for taking care of that, and if C<av_store> returns NULL, the caller will
336have to decrement the reference count to avoid a memory leak. Note that
337C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
338return value.
d1b91892 339
a0d0e21e 340 void av_clear(AV*);
a0d0e21e 341 void av_undef(AV*);
cb1a09d0 342 void av_extend(AV*, I32 key);
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343
344The C<av_clear> function deletes all the elements in the AV* array, but
345does not actually delete the array itself. The C<av_undef> function will
346delete all the elements in the array plus the array itself. The
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347C<av_extend> function extends the array so that it contains at least C<key+1>
348elements. If C<key+1> is less than the currently allocated length of the array,
349then nothing is done.
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350
351If you know the name of an array variable, you can get a pointer to its AV
352by using the following:
353
4929bf7b 354 AV* get_av("package::varname", FALSE);
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355
356This returns NULL if the variable does not exist.
357
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358See L<Understanding the Magic of Tied Hashes and Arrays> for more
359information on how to use the array access functions on tied arrays.
360
54310121 361=head2 Working with HVs
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362
363To create an HV, you use the following routine:
364
365 HV* newHV();
366
54310121 367Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 368
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369 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
370 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 371
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372The C<klen> parameter is the length of the key being passed in (Note that
373you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
374length of the key). The C<val> argument contains the SV pointer to the
54310121 375scalar being stored, and C<hash> is the precomputed hash value (zero if
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376you want C<hv_store> to calculate it for you). The C<lval> parameter
377indicates whether this fetch is actually a part of a store operation, in
378which case a new undefined value will be added to the HV with the supplied
379key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 380
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381Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
382C<SV*>. To access the scalar value, you must first dereference the return
383value. However, you should check to make sure that the return value is
384not NULL before dereferencing it.
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385
386These two functions check if a hash table entry exists, and deletes it.
387
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388 bool hv_exists(HV*, const char* key, U32 klen);
389 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 390
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391If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
392create and return a mortal copy of the deleted value.
393
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394And more miscellaneous functions:
395
396 void hv_clear(HV*);
a0d0e21e 397 void hv_undef(HV*);
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398
399Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
400table but does not actually delete the hash table. The C<hv_undef> deletes
401both the entries and the hash table itself.
a0d0e21e 402
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403Perl keeps the actual data in linked list of structures with a typedef of HE.
404These contain the actual key and value pointers (plus extra administrative
405overhead). The key is a string pointer; the value is an C<SV*>. However,
406once you have an C<HE*>, to get the actual key and value, use the routines
407specified below.
408
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409 I32 hv_iterinit(HV*);
410 /* Prepares starting point to traverse hash table */
411 HE* hv_iternext(HV*);
412 /* Get the next entry, and return a pointer to a
413 structure that has both the key and value */
414 char* hv_iterkey(HE* entry, I32* retlen);
415 /* Get the key from an HE structure and also return
416 the length of the key string */
cb1a09d0 417 SV* hv_iterval(HV*, HE* entry);
d1be9408 418 /* Return an SV pointer to the value of the HE
a0d0e21e 419 structure */
cb1a09d0 420 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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421 /* This convenience routine combines hv_iternext,
422 hv_iterkey, and hv_iterval. The key and retlen
423 arguments are return values for the key and its
424 length. The value is returned in the SV* argument */
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425
426If you know the name of a hash variable, you can get a pointer to its HV
427by using the following:
428
4929bf7b 429 HV* get_hv("package::varname", FALSE);
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430
431This returns NULL if the variable does not exist.
432
8ebc5c01 433The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 434
a0d0e21e 435 hash = 0;
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436 while (klen--)
437 hash = (hash * 33) + *key++;
87275199 438 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 439
87275199 440The last step was added in version 5.6 to improve distribution of
ab192400 441lower bits in the resulting hash value.
a0d0e21e 442
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443See L<Understanding the Magic of Tied Hashes and Arrays> for more
444information on how to use the hash access functions on tied hashes.
445
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446=head2 Hash API Extensions
447
448Beginning with version 5.004, the following functions are also supported:
449
450 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
451 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 452
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453 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
454 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 455
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456 SV* hv_iterkeysv (HE* entry);
457
458Note that these functions take C<SV*> keys, which simplifies writing
459of extension code that deals with hash structures. These functions
460also allow passing of C<SV*> keys to C<tie> functions without forcing
461you to stringify the keys (unlike the previous set of functions).
462
463They also return and accept whole hash entries (C<HE*>), making their
464use more efficient (since the hash number for a particular string
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465doesn't have to be recomputed every time). See L<perlapi> for detailed
466descriptions.
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467
468The following macros must always be used to access the contents of hash
469entries. Note that the arguments to these macros must be simple
470variables, since they may get evaluated more than once. See
4a4eefd0 471L<perlapi> for detailed descriptions of these macros.
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472
473 HePV(HE* he, STRLEN len)
474 HeVAL(HE* he)
475 HeHASH(HE* he)
476 HeSVKEY(HE* he)
477 HeSVKEY_force(HE* he)
478 HeSVKEY_set(HE* he, SV* sv)
479
480These two lower level macros are defined, but must only be used when
481dealing with keys that are not C<SV*>s:
482
483 HeKEY(HE* he)
484 HeKLEN(HE* he)
485
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486Note that both C<hv_store> and C<hv_store_ent> do not increment the
487reference count of the stored C<val>, which is the caller's responsibility.
488If these functions return a NULL value, the caller will usually have to
489decrement the reference count of C<val> to avoid a memory leak.
1e422769 490
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491=head2 References
492
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493References are a special type of scalar that point to other data types
494(including references).
a0d0e21e 495
07fa94a1 496To create a reference, use either of the following functions:
a0d0e21e 497
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498 SV* newRV_inc((SV*) thing);
499 SV* newRV_noinc((SV*) thing);
a0d0e21e 500
5f05dabc 501The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
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502functions are identical except that C<newRV_inc> increments the reference
503count of the C<thing>, while C<newRV_noinc> does not. For historical
504reasons, C<newRV> is a synonym for C<newRV_inc>.
505
506Once you have a reference, you can use the following macro to dereference
507the reference:
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508
509 SvRV(SV*)
510
511then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 512C<AV*> or C<HV*>, if required.
a0d0e21e 513
d1b91892 514To determine if an SV is a reference, you can use the following macro:
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515
516 SvROK(SV*)
517
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518To discover what type of value the reference refers to, use the following
519macro and then check the return value.
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520
521 SvTYPE(SvRV(SV*))
522
523The most useful types that will be returned are:
524
525 SVt_IV Scalar
526 SVt_NV Scalar
527 SVt_PV Scalar
5f05dabc 528 SVt_RV Scalar
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529 SVt_PVAV Array
530 SVt_PVHV Hash
531 SVt_PVCV Code
5f05dabc
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532 SVt_PVGV Glob (possible a file handle)
533 SVt_PVMG Blessed or Magical Scalar
534
535 See the sv.h header file for more details.
d1b91892 536
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537=head2 Blessed References and Class Objects
538
539References are also used to support object-oriented programming. In the
540OO lexicon, an object is simply a reference that has been blessed into a
541package (or class). Once blessed, the programmer may now use the reference
542to access the various methods in the class.
543
544A reference can be blessed into a package with the following function:
545
546 SV* sv_bless(SV* sv, HV* stash);
547
548The C<sv> argument must be a reference. The C<stash> argument specifies
3fe9a6f1 549which class the reference will belong to. See
2ae324a7 550L<Stashes and Globs> for information on converting class names into stashes.
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551
552/* Still under construction */
553
554Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
PP
555point to. If C<classname> is non-null, the SV is blessed into the specified
556class. SV is returned.
cb1a09d0 557
08105a92 558 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 559
e1c57cef 560Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 561if C<classname> is non-null.
cb1a09d0 562
08105a92 563 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 564 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 565 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 566
5f05dabc 567Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 568reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 569
08105a92 570 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 571
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572Copies string into an SV whose reference is C<rv>. Set length to 0 to let
573Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 574
e65f3abd 575 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 576
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577Tests whether the SV is blessed into the specified class. It does not
578check inheritance relationships.
579
08105a92 580 int sv_isa(SV* sv, const char* name);
9abd00ed
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581
582Tests whether the SV is a reference to a blessed object.
583
584 int sv_isobject(SV* sv);
585
586Tests whether the SV is derived from the specified class. SV can be either
587a reference to a blessed object or a string containing a class name. This
588is the function implementing the C<UNIVERSAL::isa> functionality.
589
08105a92 590 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 591
00aadd71 592To check if you've got an object derived from a specific class you have
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593to write:
594
595 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 596
5f05dabc 597=head2 Creating New Variables
cb1a09d0 598
5f05dabc
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599To create a new Perl variable with an undef value which can be accessed from
600your Perl script, use the following routines, depending on the variable type.
cb1a09d0 601
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602 SV* get_sv("package::varname", TRUE);
603 AV* get_av("package::varname", TRUE);
604 HV* get_hv("package::varname", TRUE);
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605
606Notice the use of TRUE as the second parameter. The new variable can now
607be set, using the routines appropriate to the data type.
608
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609There are additional macros whose values may be bitwise OR'ed with the
610C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 611
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612=over
613
614=item GV_ADDMULTI
615
616Marks the variable as multiply defined, thus preventing the:
617
618 Name <varname> used only once: possible typo
619
620warning.
621
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622=item GV_ADDWARN
623
624Issues the warning:
625
626 Had to create <varname> unexpectedly
627
628if the variable did not exist before the function was called.
629
630=back
cb1a09d0 631
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632If you do not specify a package name, the variable is created in the current
633package.
cb1a09d0 634
5f05dabc 635=head2 Reference Counts and Mortality
a0d0e21e 636
d1be9408 637Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 638AVs, or HVs (xV for short in the following) start their life with a
55497cff 639reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 640then it will be destroyed and its memory made available for reuse.
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641
642This normally doesn't happen at the Perl level unless a variable is
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643undef'ed or the last variable holding a reference to it is changed or
644overwritten. At the internal level, however, reference counts can be
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645manipulated with the following macros:
646
647 int SvREFCNT(SV* sv);
5f05dabc 648 SV* SvREFCNT_inc(SV* sv);
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649 void SvREFCNT_dec(SV* sv);
650
651However, there is one other function which manipulates the reference
07fa94a1
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652count of its argument. The C<newRV_inc> function, you will recall,
653creates a reference to the specified argument. As a side effect,
654it increments the argument's reference count. If this is not what
655you want, use C<newRV_noinc> instead.
656
657For example, imagine you want to return a reference from an XSUB function.
658Inside the XSUB routine, you create an SV which initially has a reference
659count of one. Then you call C<newRV_inc>, passing it the just-created SV.
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660This returns the reference as a new SV, but the reference count of the
661SV you passed to C<newRV_inc> has been incremented to two. Now you
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662return the reference from the XSUB routine and forget about the SV.
663But Perl hasn't! Whenever the returned reference is destroyed, the
664reference count of the original SV is decreased to one and nothing happens.
665The SV will hang around without any way to access it until Perl itself
666terminates. This is a memory leak.
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667
668The correct procedure, then, is to use C<newRV_noinc> instead of
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669C<newRV_inc>. Then, if and when the last reference is destroyed,
670the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 671stopping any memory leak.
55497cff 672
5f05dabc 673There are some convenience functions available that can help with the
54310121 674destruction of xVs. These functions introduce the concept of "mortality".
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675An xV that is mortal has had its reference count marked to be decremented,
676but not actually decremented, until "a short time later". Generally the
677term "short time later" means a single Perl statement, such as a call to
54310121 678an XSUB function. The actual determinant for when mortal xVs have their
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679reference count decremented depends on two macros, SAVETMPS and FREETMPS.
680See L<perlcall> and L<perlxs> for more details on these macros.
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681
682"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
683However, if you mortalize a variable twice, the reference count will
684later be decremented twice.
685
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686"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
687For example an SV which is created just to pass a number to a called sub
688is made mortal to have it cleaned up automatically when stack is popped.
689Similarly results returned by XSUBs (which go in the stack) are often
690made mortal.
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LW
691
692To create a mortal variable, use the functions:
693
694 SV* sv_newmortal()
695 SV* sv_2mortal(SV*)
696 SV* sv_mortalcopy(SV*)
697
00aadd71 698The first call creates a mortal SV (with no value), the second converts an existing
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699SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
700third creates a mortal copy of an existing SV.
00aadd71 701Because C<sv_newmortal> gives the new SV no value,it must normally be given one
9a68f1db 702via C<sv_setpv>, C<sv_setiv>, etc. :
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703
704 SV *tmp = sv_newmortal();
705 sv_setiv(tmp, an_integer);
706
707As that is multiple C statements it is quite common so see this idiom instead:
708
709 SV *tmp = sv_2mortal(newSViv(an_integer));
710
711
712You should be careful about creating mortal variables. Strange things
713can happen if you make the same value mortal within multiple contexts,
714or if you make a variable mortal multiple times. Thinking of "Mortalization"
715as deferred C<SvREFCNT_dec> should help to minimize such problems.
716For example if you are passing an SV which you I<know> has high enough REFCNT
717to survive its use on the stack you need not do any mortalization.
718If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
719making a C<sv_mortalcopy> is safer.
a0d0e21e 720
54310121 721The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 722made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 723C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 724
5f05dabc 725=head2 Stashes and Globs
a0d0e21e 726
aa689395
PP
727A "stash" is a hash that contains all of the different objects that
728are contained within a package. Each key of the stash is a symbol
729name (shared by all the different types of objects that have the same
730name), and each value in the hash table is a GV (Glob Value). This GV
731in turn contains references to the various objects of that name,
732including (but not limited to) the following:
cb1a09d0 733
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734 Scalar Value
735 Array Value
736 Hash Value
a3cb178b 737 I/O Handle
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738 Format
739 Subroutine
740
9cde0e7f 741There is a single stash called "PL_defstash" that holds the items that exist
5f05dabc
PP
742in the "main" package. To get at the items in other packages, append the
743string "::" to the package name. The items in the "Foo" package are in
9cde0e7f 744the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are
5f05dabc 745in the stash "Baz::" in "Bar::"'s stash.
a0d0e21e 746
d1b91892 747To get the stash pointer for a particular package, use the function:
a0d0e21e 748
08105a92 749 HV* gv_stashpv(const char* name, I32 create)
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750 HV* gv_stashsv(SV*, I32 create)
751
752The first function takes a literal string, the second uses the string stored
d1b91892 753in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 754C<HV*>. The C<create> flag will create a new package if it is set.
a0d0e21e
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755
756The name that C<gv_stash*v> wants is the name of the package whose symbol table
757you want. The default package is called C<main>. If you have multiply nested
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AD
758packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
759language itself.
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760
761Alternately, if you have an SV that is a blessed reference, you can find
762out the stash pointer by using:
763
764 HV* SvSTASH(SvRV(SV*));
765
766then use the following to get the package name itself:
767
768 char* HvNAME(HV* stash);
769
5f05dabc
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770If you need to bless or re-bless an object you can use the following
771function:
a0d0e21e
LW
772
773 SV* sv_bless(SV*, HV* stash)
774
775where the first argument, an C<SV*>, must be a reference, and the second
776argument is a stash. The returned C<SV*> can now be used in the same way
777as any other SV.
778
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AD
779For more information on references and blessings, consult L<perlref>.
780
54310121 781=head2 Double-Typed SVs
0a753a76
PP
782
783Scalar variables normally contain only one type of value, an integer,
784double, pointer, or reference. Perl will automatically convert the
785actual scalar data from the stored type into the requested type.
786
787Some scalar variables contain more than one type of scalar data. For
788example, the variable C<$!> contains either the numeric value of C<errno>
789or its string equivalent from either C<strerror> or C<sys_errlist[]>.
790
791To force multiple data values into an SV, you must do two things: use the
792C<sv_set*v> routines to add the additional scalar type, then set a flag
793so that Perl will believe it contains more than one type of data. The
794four macros to set the flags are:
795
796 SvIOK_on
797 SvNOK_on
798 SvPOK_on
799 SvROK_on
800
801The particular macro you must use depends on which C<sv_set*v> routine
802you called first. This is because every C<sv_set*v> routine turns on
803only the bit for the particular type of data being set, and turns off
804all the rest.
805
806For example, to create a new Perl variable called "dberror" that contains
807both the numeric and descriptive string error values, you could use the
808following code:
809
810 extern int dberror;
811 extern char *dberror_list;
812
4929bf7b 813 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
814 sv_setiv(sv, (IV) dberror);
815 sv_setpv(sv, dberror_list[dberror]);
816 SvIOK_on(sv);
817
818If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
819macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
820
821=head2 Magic Variables
a0d0e21e 822
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823[This section still under construction. Ignore everything here. Post no
824bills. Everything not permitted is forbidden.]
825
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826Any SV may be magical, that is, it has special features that a normal
827SV does not have. These features are stored in the SV structure in a
5f05dabc 828linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
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829
830 struct magic {
831 MAGIC* mg_moremagic;
832 MGVTBL* mg_virtual;
833 U16 mg_private;
834 char mg_type;
835 U8 mg_flags;
836 SV* mg_obj;
837 char* mg_ptr;
838 I32 mg_len;
839 };
840
841Note this is current as of patchlevel 0, and could change at any time.
842
843=head2 Assigning Magic
844
845Perl adds magic to an SV using the sv_magic function:
846
08105a92 847 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
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848
849The C<sv> argument is a pointer to the SV that is to acquire a new magical
850feature.
851
852If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
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853convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
854to the beginning of the linked list of magical features. Any prior entry
855of the same type of magic is deleted. Note that this can be overridden,
856and multiple instances of the same type of magic can be associated with an
857SV.
d1b91892 858
54310121
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859The C<name> and C<namlen> arguments are used to associate a string with
860the magic, typically the name of a variable. C<namlen> is stored in the
9b5bb84f 861C<mg_len> field and if C<name> is non-null and C<namlen> E<gt>= 0 a malloc'd
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862copy of the name is stored in C<mg_ptr> field.
863
864The sv_magic function uses C<how> to determine which, if any, predefined
865"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
cb1a09d0 866See the "Magic Virtual Table" section below. The C<how> argument is also
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DM
867stored in the C<mg_type> field. The value of C<how> should be chosen
868from the set of macros C<PERL_MAGIC_foo> found perl.h. Note that before
645c22ef 869these macros were added, Perl internals used to directly use character
14befaf4 870literals, so you may occasionally come across old code or documentation
75d0f26d 871referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
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872
873The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
874structure. If it is not the same as the C<sv> argument, the reference
875count of the C<obj> object is incremented. If it is the same, or if
645c22ef 876the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 877then C<obj> is merely stored, without the reference count being incremented.
d1b91892 878
cb1a09d0
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879There is also a function to add magic to an C<HV>:
880
881 void hv_magic(HV *hv, GV *gv, int how);
882
883This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
884
885To remove the magic from an SV, call the function sv_unmagic:
886
887 void sv_unmagic(SV *sv, int type);
888
889The C<type> argument should be equal to the C<how> value when the C<SV>
890was initially made magical.
891
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892=head2 Magic Virtual Tables
893
d1be9408 894The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
895C<MGVTBL>, which is a structure of function pointers and stands for
896"Magic Virtual Table" to handle the various operations that might be
897applied to that variable.
898
899The C<MGVTBL> has five pointers to the following routine types:
900
901 int (*svt_get)(SV* sv, MAGIC* mg);
902 int (*svt_set)(SV* sv, MAGIC* mg);
903 U32 (*svt_len)(SV* sv, MAGIC* mg);
904 int (*svt_clear)(SV* sv, MAGIC* mg);
905 int (*svt_free)(SV* sv, MAGIC* mg);
906
907This MGVTBL structure is set at compile-time in C<perl.h> and there are
908currently 19 types (or 21 with overloading turned on). These different
909structures contain pointers to various routines that perform additional
910actions depending on which function is being called.
911
912 Function pointer Action taken
913 ---------------- ------------
8b0711c3 914 svt_get Do something before the value of the SV is retrieved.
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915 svt_set Do something after the SV is assigned a value.
916 svt_len Report on the SV's length.
917 svt_clear Clear something the SV represents.
918 svt_free Free any extra storage associated with the SV.
919
920For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 921to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
922
923 { magic_get, magic_set, magic_len, 0, 0 }
924
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925Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
926if a get operation is being performed, the routine C<magic_get> is
927called. All the various routines for the various magical types begin
928with C<magic_>. NOTE: the magic routines are not considered part of
929the Perl API, and may not be exported by the Perl library.
d1b91892
AD
930
931The current kinds of Magic Virtual Tables are:
932
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933 mg_type
934 (old-style char and macro) MGVTBL Type of magic
935 -------------------------- ------ ----------------------------
936 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
937 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
938 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
939 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
940 on stash
941 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
942 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
943 (@+ and @- vars)
944 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
945 element
946 E PERL_MAGIC_env vtbl_env %ENV hash
947 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
948 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
949 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
950 I PERL_MAGIC_isa vtbl_isa @ISA array
951 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
952 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
953 L PERL_MAGIC_dbfile (none) Debugger %_<filename
954 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
955 m PERL_MAGIC_mutex vtbl_mutex ???
645c22ef 956 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
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957 P PERL_MAGIC_tied vtbl_pack Tied array or hash
958 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
959 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
960 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
961 S PERL_MAGIC_sig vtbl_sig %SIG hash
962 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
963 t PERL_MAGIC_taint vtbl_taint Taintedness
964 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
965 v PERL_MAGIC_vec vtbl_vec vec() lvalue
966 x PERL_MAGIC_substr vtbl_substr substr() lvalue
967 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
968 variable / smart parameter
969 vivification
970 * PERL_MAGIC_glob vtbl_glob GV (typeglob)
971 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
972 . PERL_MAGIC_pos vtbl_pos pos() lvalue
973 < PERL_MAGIC_backref vtbl_backref ???
974 ~ PERL_MAGIC_ext (none) Available for use by extensions
d1b91892 975
68dc0745
PP
976When an uppercase and lowercase letter both exist in the table, then the
977uppercase letter is used to represent some kind of composite type (a list
978or a hash), and the lowercase letter is used to represent an element of
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DM
979that composite type. Some internals code makes use of this case
980relationship.
981
982The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
983specifically for use by extensions and will not be used by perl itself.
984Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
985to variables (typically objects). This is especially useful because
986there is no way for normal perl code to corrupt this private information
987(unlike using extra elements of a hash object).
988
989Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
990C function any time a scalar's value is used or changed. The C<MAGIC>'s
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991C<mg_ptr> field points to a C<ufuncs> structure:
992
993 struct ufuncs {
a9402793
AB
994 I32 (*uf_val)(pTHX_ IV, SV*);
995 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
996 IV uf_index;
997 };
998
999When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1000function will be called with C<uf_index> as the first arg and a pointer to
1001the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1002magic is shown below. Note that the ufuncs structure is copied by
1003sv_magic, so you can safely allocate it on the stack.
1004
1005 void
1006 Umagic(sv)
1007 SV *sv;
1008 PREINIT:
1009 struct ufuncs uf;
1010 CODE:
1011 uf.uf_val = &my_get_fn;
1012 uf.uf_set = &my_set_fn;
1013 uf.uf_index = 0;
14befaf4 1014 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1015
14befaf4
DM
1016Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1017or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1018extra care to avoid conflict. Typically only using the magic on
1019objects blessed into the same class as the extension is sufficient.
1020For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1021'signature' at the top of the private data area and check that.
5f05dabc 1022
ef50df4b
GS
1023Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1024earlier do B<not> invoke 'set' magic on their targets. This must
1025be done by the user either by calling the C<SvSETMAGIC()> macro after
1026calling these functions, or by using one of the C<sv_set*_mg()> or
1027C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1028C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1029obtained from external sources in functions that don't handle magic.
4a4eefd0 1030See L<perlapi> for a description of these functions.
189b2af5
GS
1031For example, calls to the C<sv_cat*()> functions typically need to be
1032followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1033since their implementation handles 'get' magic.
1034
d1b91892
AD
1035=head2 Finding Magic
1036
1037 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1038
1039This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1040If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1041if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1042
08105a92 1043 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1044
1045This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1046field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1047the mg_type field is changed to be the lowercase letter.
a0d0e21e 1048
04343c6d
GS
1049=head2 Understanding the Magic of Tied Hashes and Arrays
1050
14befaf4
DM
1051Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1052magic type.
9edb2b46
GS
1053
1054WARNING: As of the 5.004 release, proper usage of the array and hash
1055access functions requires understanding a few caveats. Some
1056of these caveats are actually considered bugs in the API, to be fixed
1057in later releases, and are bracketed with [MAYCHANGE] below. If
1058you find yourself actually applying such information in this section, be
1059aware that the behavior may change in the future, umm, without warning.
04343c6d 1060
1526ead6 1061The perl tie function associates a variable with an object that implements
9a68f1db 1062the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1063tie function from an XSUB, you must mimic this behaviour. The code below
1064carries out the necessary steps - firstly it creates a new hash, and then
1065creates a second hash which it blesses into the class which will implement
1066the tie methods. Lastly it ties the two hashes together, and returns a
1067reference to the new tied hash. Note that the code below does NOT call the
1068TIEHASH method in the MyTie class -
1069see L<Calling Perl Routines from within C Programs> for details on how
1070to do this.
1071
1072 SV*
1073 mytie()
1074 PREINIT:
1075 HV *hash;
1076 HV *stash;
1077 SV *tie;
1078 CODE:
1079 hash = newHV();
1080 tie = newRV_noinc((SV*)newHV());
1081 stash = gv_stashpv("MyTie", TRUE);
1082 sv_bless(tie, stash);
899e16d0 1083 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1084 RETVAL = newRV_noinc(hash);
1085 OUTPUT:
1086 RETVAL
1087
04343c6d
GS
1088The C<av_store> function, when given a tied array argument, merely
1089copies the magic of the array onto the value to be "stored", using
1090C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1091actually need to be stored in the array. [MAYCHANGE] After a call to
1092C<av_store> on a tied array, the caller will usually need to call
1093C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1094TIEARRAY object. If C<av_store> did return NULL, a call to
1095C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1096leak. [/MAYCHANGE]
04343c6d
GS
1097
1098The previous paragraph is applicable verbatim to tied hash access using the
1099C<hv_store> and C<hv_store_ent> functions as well.
1100
1101C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1102C<hv_fetch_ent> actually return an undefined mortal value whose magic
1103has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1104need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1105need to call C<mg_get()> on the returned value in order to actually invoke
1106the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1107you may also call C<mg_set()> on the return value after possibly assigning
1108a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1109method on the TIE object. [/MAYCHANGE]
04343c6d 1110
9edb2b46 1111[MAYCHANGE]
04343c6d
GS
1112In other words, the array or hash fetch/store functions don't really
1113fetch and store actual values in the case of tied arrays and hashes. They
1114merely call C<mg_copy> to attach magic to the values that were meant to be
1115"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1116do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1117the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1118and hashes.
1119
1120Currently (as of perl version 5.004), use of the hash and array access
1121functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1122"normal" hashes and arrays, or on their tied variants. The API may be
1123changed to provide more transparent access to both tied and normal data
1124types in future versions.
1125[/MAYCHANGE]
04343c6d
GS
1126
1127You would do well to understand that the TIEARRAY and TIEHASH interfaces
1128are mere sugar to invoke some perl method calls while using the uniform hash
1129and array syntax. The use of this sugar imposes some overhead (typically
1130about two to four extra opcodes per FETCH/STORE operation, in addition to
1131the creation of all the mortal variables required to invoke the methods).
1132This overhead will be comparatively small if the TIE methods are themselves
1133substantial, but if they are only a few statements long, the overhead
1134will not be insignificant.
1135
d1c897a1
IZ
1136=head2 Localizing changes
1137
1138Perl has a very handy construction
1139
1140 {
1141 local $var = 2;
1142 ...
1143 }
1144
1145This construction is I<approximately> equivalent to
1146
1147 {
1148 my $oldvar = $var;
1149 $var = 2;
1150 ...
1151 $var = $oldvar;
1152 }
1153
1154The biggest difference is that the first construction would
1155reinstate the initial value of $var, irrespective of how control exits
9a68f1db 1156the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1157more efficient as well.
1158
1159There is a way to achieve a similar task from C via Perl API: create a
1160I<pseudo-block>, and arrange for some changes to be automatically
1161undone at the end of it, either explicit, or via a non-local exit (via
1162die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1163C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1164Such a construct may be created specially for some important localized
1165task, or an existing one (like boundaries of enclosing Perl
1166subroutine/block, or an existing pair for freeing TMPs) may be
1167used. (In the second case the overhead of additional localization must
1168be almost negligible.) Note that any XSUB is automatically enclosed in
1169an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1170
1171Inside such a I<pseudo-block> the following service is available:
1172
13a2d996 1173=over 4
d1c897a1
IZ
1174
1175=item C<SAVEINT(int i)>
1176
1177=item C<SAVEIV(IV i)>
1178
1179=item C<SAVEI32(I32 i)>
1180
1181=item C<SAVELONG(long i)>
1182
1183These macros arrange things to restore the value of integer variable
1184C<i> at the end of enclosing I<pseudo-block>.
1185
1186=item C<SAVESPTR(s)>
1187
1188=item C<SAVEPPTR(p)>
1189
1190These macros arrange things to restore the value of pointers C<s> and
1191C<p>. C<s> must be a pointer of a type which survives conversion to
1192C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1193and back.
1194
1195=item C<SAVEFREESV(SV *sv)>
1196
1197The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1198I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1199mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1200extends the lifetime of C<sv> until the beginning of the next statement,
1201C<SAVEFREESV> extends it until the end of the enclosing scope. These
1202lifetimes can be wildly different.
1203
1204Also compare C<SAVEMORTALIZESV>.
1205
1206=item C<SAVEMORTALIZESV(SV *sv)>
1207
1208Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1209scope instead of decrementing its reference count. This usually has the
1210effect of keeping C<sv> alive until the statement that called the currently
1211live scope has finished executing.
d1c897a1
IZ
1212
1213=item C<SAVEFREEOP(OP *op)>
1214
1215The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1216
1217=item C<SAVEFREEPV(p)>
1218
1219The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1220end of I<pseudo-block>.
1221
1222=item C<SAVECLEARSV(SV *sv)>
1223
1224Clears a slot in the current scratchpad which corresponds to C<sv> at
1225the end of I<pseudo-block>.
1226
1227=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1228
1229The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1230string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1231short-lived storage, the corresponding string may be reallocated like
1232this:
1233
9cde0e7f 1234 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1235
c76ac1ee 1236=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1237
1238At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1239only argument C<p>.
1240
1241=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1242
1243At the end of I<pseudo-block> the function C<f> is called with the
1244implicit context argument (if any), and C<p>.
d1c897a1
IZ
1245
1246=item C<SAVESTACK_POS()>
1247
1248The current offset on the Perl internal stack (cf. C<SP>) is restored
1249at the end of I<pseudo-block>.
1250
1251=back
1252
1253The following API list contains functions, thus one needs to
1254provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1255or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1256function takes C<int *>.
1257
13a2d996 1258=over 4
d1c897a1
IZ
1259
1260=item C<SV* save_scalar(GV *gv)>
1261
1262Equivalent to Perl code C<local $gv>.
1263
1264=item C<AV* save_ary(GV *gv)>
1265
1266=item C<HV* save_hash(GV *gv)>
1267
1268Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1269
1270=item C<void save_item(SV *item)>
1271
1272Duplicates the current value of C<SV>, on the exit from the current
1273C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1274using the stored value.
1275
1276=item C<void save_list(SV **sarg, I32 maxsarg)>
1277
1278A variant of C<save_item> which takes multiple arguments via an array
1279C<sarg> of C<SV*> of length C<maxsarg>.
1280
1281=item C<SV* save_svref(SV **sptr)>
1282
d1be9408 1283Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1284
1285=item C<void save_aptr(AV **aptr)>
1286
1287=item C<void save_hptr(HV **hptr)>
1288
1289Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1290
1291=back
1292
1293The C<Alias> module implements localization of the basic types within the
1294I<caller's scope>. People who are interested in how to localize things in
1295the containing scope should take a look there too.
1296
0a753a76 1297=head1 Subroutines
a0d0e21e 1298
68dc0745 1299=head2 XSUBs and the Argument Stack
5f05dabc
PP
1300
1301The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1302An XSUB routine will have a stack that contains the arguments from the Perl
1303program, and a way to map from the Perl data structures to a C equivalent.
1304
1305The stack arguments are accessible through the C<ST(n)> macro, which returns
1306the C<n>'th stack argument. Argument 0 is the first argument passed in the
1307Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1308an C<SV*> is used.
1309
1310Most of the time, output from the C routine can be handled through use of
1311the RETVAL and OUTPUT directives. However, there are some cases where the
1312argument stack is not already long enough to handle all the return values.
1313An example is the POSIX tzname() call, which takes no arguments, but returns
1314two, the local time zone's standard and summer time abbreviations.
1315
1316To handle this situation, the PPCODE directive is used and the stack is
1317extended using the macro:
1318
924508f0 1319 EXTEND(SP, num);
5f05dabc 1320
924508f0
GS
1321where C<SP> is the macro that represents the local copy of the stack pointer,
1322and C<num> is the number of elements the stack should be extended by.
5f05dabc 1323
00aadd71 1324Now that there is room on the stack, values can be pushed on it using C<PUSHs>
484ce0c5 1325macro. The values pushed will often need to be "mortal" (See L</Reference Counts and Mortality>).
5f05dabc 1326
00aadd71
NIS
1327 PUSHs(sv_2mortal(newSViv(an_integer)))
1328 PUSHs(sv_2mortal(newSVpv("Some String",0)))
1329 PUSHs(sv_2mortal(newSVnv(3.141592)))
5f05dabc
PP
1330
1331And now the Perl program calling C<tzname>, the two values will be assigned
1332as in:
1333
1334 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1335
1336An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1337to use the macro:
5f05dabc 1338
5f05dabc
PP
1339 XPUSHs(SV*)
1340
00aadd71 1341This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1342do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1343
1344Despite their suggestions in earlier versions of this document the macros
1345C<PUSHi>, C<PUSHn> and C<PUSHp> are I<not> suited to XSUBs which return
1346multiple results, see L</Putting a C value on Perl stack>.
5f05dabc
PP
1347
1348For more information, consult L<perlxs> and L<perlxstut>.
1349
1350=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1351
1352There are four routines that can be used to call a Perl subroutine from
1353within a C program. These four are:
1354
954c1994
GS
1355 I32 call_sv(SV*, I32);
1356 I32 call_pv(const char*, I32);
1357 I32 call_method(const char*, I32);
1358 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1359
954c1994 1360The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1361contains either the name of the Perl subroutine to be called, or a
1362reference to the subroutine. The second argument consists of flags
1363that control the context in which the subroutine is called, whether
1364or not the subroutine is being passed arguments, how errors should be
1365trapped, and how to treat return values.
a0d0e21e
LW
1366
1367All four routines return the number of arguments that the subroutine returned
1368on the Perl stack.
1369
9a68f1db 1370These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1371but those names are now deprecated; macros of the same name are provided for
1372compatibility.
1373
1374When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1375must manipulate the Perl stack. These include the following macros and
1376functions:
a0d0e21e
LW
1377
1378 dSP
924508f0 1379 SP
a0d0e21e
LW
1380 PUSHMARK()
1381 PUTBACK
1382 SPAGAIN
1383 ENTER
1384 SAVETMPS
1385 FREETMPS
1386 LEAVE
1387 XPUSH*()
cb1a09d0 1388 POP*()
a0d0e21e 1389
5f05dabc
PP
1390For a detailed description of calling conventions from C to Perl,
1391consult L<perlcall>.
a0d0e21e 1392
5f05dabc 1393=head2 Memory Allocation
a0d0e21e 1394
86058a2d
GS
1395All memory meant to be used with the Perl API functions should be manipulated
1396using the macros described in this section. The macros provide the necessary
1397transparency between differences in the actual malloc implementation that is
1398used within perl.
1399
1400It is suggested that you enable the version of malloc that is distributed
5f05dabc 1401with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1402order to satisfy allocation requests more quickly. However, on some
1403platforms, it may cause spurious malloc or free errors.
d1b91892
AD
1404
1405 New(x, pointer, number, type);
1406 Newc(x, pointer, number, type, cast);
1407 Newz(x, pointer, number, type);
1408
07fa94a1 1409These three macros are used to initially allocate memory.
5f05dabc
PP
1410
1411The first argument C<x> was a "magic cookie" that was used to keep track
1412of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1413the current code makes no use of this feature (most Perl developers now
1414use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1415
1416The second argument C<pointer> should be the name of a variable that will
1417point to the newly allocated memory.
d1b91892 1418
d1b91892
AD
1419The third and fourth arguments C<number> and C<type> specify how many of
1420the specified type of data structure should be allocated. The argument
1421C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1422should be used if the C<pointer> argument is different from the C<type>
1423argument.
1424
1425Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1426to zero out all the newly allocated memory.
1427
1428 Renew(pointer, number, type);
1429 Renewc(pointer, number, type, cast);
1430 Safefree(pointer)
1431
1432These three macros are used to change a memory buffer size or to free a
1433piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1434match those of C<New> and C<Newc> with the exception of not needing the
1435"magic cookie" argument.
1436
1437 Move(source, dest, number, type);
1438 Copy(source, dest, number, type);
1439 Zero(dest, number, type);
1440
1441These three macros are used to move, copy, or zero out previously allocated
1442memory. The C<source> and C<dest> arguments point to the source and
1443destination starting points. Perl will move, copy, or zero out C<number>
1444instances of the size of the C<type> data structure (using the C<sizeof>
1445function).
a0d0e21e 1446
5f05dabc 1447=head2 PerlIO
ce3d39e2 1448
5f05dabc
PP
1449The most recent development releases of Perl has been experimenting with
1450removing Perl's dependency on the "normal" standard I/O suite and allowing
1451other stdio implementations to be used. This involves creating a new
1452abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1453was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1454abstraction layer and not make any assumptions about what kind of stdio
1455is being used.
1456
1457For a complete description of the PerlIO abstraction, consult L<perlapio>.
1458
8ebc5c01 1459=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1460
1461A lot of opcodes (this is an elementary operation in the internal perl
1462stack machine) put an SV* on the stack. However, as an optimization
1463the corresponding SV is (usually) not recreated each time. The opcodes
1464reuse specially assigned SVs (I<target>s) which are (as a corollary)
1465not constantly freed/created.
1466
0a753a76 1467Each of the targets is created only once (but see
ce3d39e2
IZ
1468L<Scratchpads and recursion> below), and when an opcode needs to put
1469an integer, a double, or a string on stack, it just sets the
1470corresponding parts of its I<target> and puts the I<target> on stack.
1471
1472The macro to put this target on stack is C<PUSHTARG>, and it is
1473directly used in some opcodes, as well as indirectly in zillions of
1474others, which use it via C<(X)PUSH[pni]>.
1475
1bd1c0d5
SC
1476Because the target is reused, you must be careful when pushing multiple
1477values on the stack. The following code will not do what you think:
1478
1479 XPUSHi(10);
1480 XPUSHi(20);
1481
1482This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1483the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1484At the end of the operation, the stack does not contain the values 10
1485and 20, but actually contains two pointers to C<TARG>, which we have set
1486to 20. If you need to push multiple different values, use C<XPUSHs>,
1487which bypasses C<TARG>.
1488
1489On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
1490need a C<dTARG> in your variable declarations so that the C<*PUSH*>
00aadd71 1491macros can make use of the local variable C<TARG>.
1bd1c0d5 1492
8ebc5c01 1493=head2 Scratchpads
ce3d39e2 1494
54310121 1495The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1496are created. The answer is that they are created when the current unit --
1497a subroutine or a file (for opcodes for statements outside of
1498subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1499array is created, which is called a scratchpad for the current
1500unit.
1501
54310121 1502A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1503targets for opcodes. One can deduce that an SV lives on a scratchpad
1504by looking on its flags: lexicals have C<SVs_PADMY> set, and
1505I<target>s have C<SVs_PADTMP> set.
1506
54310121
PP
1507The correspondence between OPs and I<target>s is not 1-to-1. Different
1508OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1509would not conflict with the expected life of the temporary.
1510
2ae324a7 1511=head2 Scratchpads and recursion
ce3d39e2
IZ
1512
1513In fact it is not 100% true that a compiled unit contains a pointer to
1514the scratchpad AV. In fact it contains a pointer to an AV of
1515(initially) one element, and this element is the scratchpad AV. Why do
1516we need an extra level of indirection?
1517
9a68f1db 1518The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2
IZ
1519these can create several execution pointers going into the same
1520subroutine. For the subroutine-child not write over the temporaries
1521for the subroutine-parent (lifespan of which covers the call to the
1522child), the parent and the child should have different
1523scratchpads. (I<And> the lexicals should be separate anyway!)
1524
5f05dabc
PP
1525So each subroutine is born with an array of scratchpads (of length 1).
1526On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1527depth of the recursion is not more than the length of this array, and
1528if it is, new scratchpad is created and pushed into the array.
1529
1530The I<target>s on this scratchpad are C<undef>s, but they are already
1531marked with correct flags.
1532
0a753a76
PP
1533=head1 Compiled code
1534
1535=head2 Code tree
1536
1537Here we describe the internal form your code is converted to by
1538Perl. Start with a simple example:
1539
1540 $a = $b + $c;
1541
1542This is converted to a tree similar to this one:
1543
1544 assign-to
1545 / \
1546 + $a
1547 / \
1548 $b $c
1549
7b8d334a 1550(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1551parsed your code, but has nothing to do with the execution order.
1552There is an additional "thread" going through the nodes of the tree
1553which shows the order of execution of the nodes. In our simplified
1554example above it looks like:
1555
1556 $b ---> $c ---> + ---> $a ---> assign-to
1557
1558But with the actual compile tree for C<$a = $b + $c> it is different:
1559some nodes I<optimized away>. As a corollary, though the actual tree
1560contains more nodes than our simplified example, the execution order
1561is the same as in our example.
1562
1563=head2 Examining the tree
1564
1565If you have your perl compiled for debugging (usually done with C<-D
1566optimize=-g> on C<Configure> command line), you may examine the
1567compiled tree by specifying C<-Dx> on the Perl command line. The
1568output takes several lines per node, and for C<$b+$c> it looks like
1569this:
1570
1571 5 TYPE = add ===> 6
1572 TARG = 1
1573 FLAGS = (SCALAR,KIDS)
1574 {
1575 TYPE = null ===> (4)
1576 (was rv2sv)
1577 FLAGS = (SCALAR,KIDS)
1578 {
1579 3 TYPE = gvsv ===> 4
1580 FLAGS = (SCALAR)
1581 GV = main::b
1582 }
1583 }
1584 {
1585 TYPE = null ===> (5)
1586 (was rv2sv)
1587 FLAGS = (SCALAR,KIDS)
1588 {
1589 4 TYPE = gvsv ===> 5
1590 FLAGS = (SCALAR)
1591 GV = main::c
1592 }
1593 }
1594
1595This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1596not optimized away (one per number in the left column). The immediate
1597children of the given node correspond to C<{}> pairs on the same level
1598of indentation, thus this listing corresponds to the tree:
1599
1600 add
1601 / \
1602 null null
1603 | |
1604 gvsv gvsv
1605
1606The execution order is indicated by C<===E<gt>> marks, thus it is C<3
16074 5 6> (node C<6> is not included into above listing), i.e.,
1608C<gvsv gvsv add whatever>.
1609
9afa14e3
SC
1610Each of these nodes represents an op, a fundamental operation inside the
1611Perl core. The code which implements each operation can be found in the
1612F<pp*.c> files; the function which implements the op with type C<gvsv>
1613is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1614different numbers of children: C<add> is a binary operator, as one would
1615expect, and so has two children. To accommodate the various different
1616numbers of children, there are various types of op data structure, and
1617they link together in different ways.
1618
1619The simplest type of op structure is C<OP>: this has no children. Unary
1620operators, C<UNOP>s, have one child, and this is pointed to by the
1621C<op_first> field. Binary operators (C<BINOP>s) have not only an
1622C<op_first> field but also an C<op_last> field. The most complex type of
1623op is a C<LISTOP>, which has any number of children. In this case, the
1624first child is pointed to by C<op_first> and the last child by
1625C<op_last>. The children in between can be found by iteratively
1626following the C<op_sibling> pointer from the first child to the last.
1627
1628There are also two other op types: a C<PMOP> holds a regular expression,
1629and has no children, and a C<LOOP> may or may not have children. If the
1630C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1631complicate matters, if a C<UNOP> is actually a C<null> op after
1632optimization (see L</Compile pass 2: context propagation>) it will still
1633have children in accordance with its former type.
1634
0a753a76
PP
1635=head2 Compile pass 1: check routines
1636
8870b5c7
GS
1637The tree is created by the compiler while I<yacc> code feeds it
1638the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1639the first pass of perl compilation.
1640
1641What makes this pass interesting for perl developers is that some
1642optimization may be performed on this pass. This is optimization by
8870b5c7 1643so-called "check routines". The correspondence between node names
0a753a76
PP
1644and corresponding check routines is described in F<opcode.pl> (do not
1645forget to run C<make regen_headers> if you modify this file).
1646
1647A check routine is called when the node is fully constructed except
7b8d334a 1648for the execution-order thread. Since at this time there are no
0a753a76
PP
1649back-links to the currently constructed node, one can do most any
1650operation to the top-level node, including freeing it and/or creating
1651new nodes above/below it.
1652
1653The check routine returns the node which should be inserted into the
1654tree (if the top-level node was not modified, check routine returns
1655its argument).
1656
1657By convention, check routines have names C<ck_*>. They are usually
1658called from C<new*OP> subroutines (or C<convert>) (which in turn are
1659called from F<perly.y>).
1660
1661=head2 Compile pass 1a: constant folding
1662
1663Immediately after the check routine is called the returned node is
1664checked for being compile-time executable. If it is (the value is
1665judged to be constant) it is immediately executed, and a I<constant>
1666node with the "return value" of the corresponding subtree is
1667substituted instead. The subtree is deleted.
1668
1669If constant folding was not performed, the execution-order thread is
1670created.
1671
1672=head2 Compile pass 2: context propagation
1673
1674When a context for a part of compile tree is known, it is propagated
a3cb178b 1675down through the tree. At this time the context can have 5 values
0a753a76
PP
1676(instead of 2 for runtime context): void, boolean, scalar, list, and
1677lvalue. In contrast with the pass 1 this pass is processed from top
1678to bottom: a node's context determines the context for its children.
1679
1680Additional context-dependent optimizations are performed at this time.
1681Since at this moment the compile tree contains back-references (via
1682"thread" pointers), nodes cannot be free()d now. To allow
1683optimized-away nodes at this stage, such nodes are null()ified instead
1684of free()ing (i.e. their type is changed to OP_NULL).
1685
1686=head2 Compile pass 3: peephole optimization
1687
1688After the compile tree for a subroutine (or for an C<eval> or a file)
1689is created, an additional pass over the code is performed. This pass
1690is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1691additional complications for conditionals). These optimizations are
0a753a76
PP
1692done in the subroutine peep(). Optimizations performed at this stage
1693are subject to the same restrictions as in the pass 2.
1694
1ba7f851
PJ
1695=head2 Pluggable runops
1696
1697The compile tree is executed in a runops function. There are two runops
1698functions in F<run.c>. C<Perl_runops_debug> is used with DEBUGGING and
1699C<Perl_runops_standard> is used otherwise. For fine control over the
1700execution of the compile tree it is possible to provide your own runops
1701function.
1702
1703It's probably best to copy one of the existing runops functions and
1704change it to suit your needs. Then, in the BOOT section of your XS
1705file, add the line:
1706
1707 PL_runops = my_runops;
1708
1709This function should be as efficient as possible to keep your programs
1710running as fast as possible.
1711
9afa14e3
SC
1712=head1 Examining internal data structures with the C<dump> functions
1713
1714To aid debugging, the source file F<dump.c> contains a number of
1715functions which produce formatted output of internal data structures.
1716
1717The most commonly used of these functions is C<Perl_sv_dump>; it's used
1718for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1719C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1720module should already be familiar with its format.
9afa14e3
SC
1721
1722C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 1723derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
1724C<Perl_dump_eval> will dump the main root of the code being evaluated,
1725exactly like C<-Dx>.
1726
1727Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1728op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1729subroutines in a package like so: (Thankfully, these are all xsubs, so
1730there is no op tree)
1731
1732 (gdb) print Perl_dump_packsubs(PL_defstash)
1733
1734 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1735
1736 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1737
1738 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1739
1740 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1741
1742 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1743
1744and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1745the op tree of the main root.
1746
954c1994 1747=head1 How multiple interpreters and concurrency are supported
ee072b34 1748
ee072b34
GS
1749=head2 Background and PERL_IMPLICIT_CONTEXT
1750
1751The Perl interpreter can be regarded as a closed box: it has an API
1752for feeding it code or otherwise making it do things, but it also has
1753functions for its own use. This smells a lot like an object, and
1754there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
1755interpreters, with one interpreter represented either as a C structure,
1756or inside a thread-specific structure. These structures contain all
1757the context, the state of that interpreter.
1758
9a68f1db 1759Two macros control the major Perl build flavors: MULTIPLICITY and
acfe0abc
GS
1760USE_5005THREADS. The MULTIPLICITY build has a C structure
1761that packages all the interpreter state, and there is a similar thread-specific
1762data structure under USE_5005THREADS. In both cases,
54aff467
GS
1763PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1764support for passing in a "hidden" first argument that represents all three
651a3225 1765data structures.
54aff467
GS
1766
1767All this obviously requires a way for the Perl internal functions to be
acfe0abc 1768either subroutines taking some kind of structure as the first
ee072b34 1769argument, or subroutines taking nothing as the first argument. To
acfe0abc 1770enable these two very different ways of building the interpreter,
ee072b34
GS
1771the Perl source (as it does in so many other situations) makes heavy
1772use of macros and subroutine naming conventions.
1773
54aff467 1774First problem: deciding which functions will be public API functions and
00aadd71 1775which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1776(think "S" for "secret" or "static"). All other functions begin with
1777"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1778part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1779function is part of the API is to find its entry in L<perlapi>.
1780If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1781think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1782L<perlbug> explaining why you think it should be.
ee072b34
GS
1783
1784Second problem: there must be a syntax so that the same subroutine
1785declarations and calls can pass a structure as their first argument,
1786or pass nothing. To solve this, the subroutines are named and
1787declared in a particular way. Here's a typical start of a static
1788function used within the Perl guts:
1789
1790 STATIC void
1791 S_incline(pTHX_ char *s)
1792
acfe0abc
GS
1793STATIC becomes "static" in C, and may be #define'd to nothing in some
1794configurations in future.
ee072b34 1795
651a3225
GS
1796A public function (i.e. part of the internal API, but not necessarily
1797sanctioned for use in extensions) begins like this:
ee072b34
GS
1798
1799 void
1800 Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)
1801
1802C<pTHX_> is one of a number of macros (in perl.h) that hide the
1803details of the interpreter's context. THX stands for "thread", "this",
1804or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1805The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1806or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1807their variants.
ee072b34 1808
a7486cbb
JH
1809When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1810first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1811in the pTHX_ macro indicates that the macro expansion needs a comma
1812after the context argument because other arguments follow it. If
1813PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1814subroutine is not prototyped to take the extra argument. The form of the
1815macro without the trailing underscore is used when there are no additional
ee072b34
GS
1816explicit arguments.
1817
54aff467 1818When a core function calls another, it must pass the context. This
a7486cbb 1819is normally hidden via macros. Consider C<sv_setsv>. It expands into
ee072b34
GS
1820something like this:
1821
1822 ifdef PERL_IMPLICIT_CONTEXT
c52f9dcd 1823 define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
ee072b34
GS
1824 /* can't do this for vararg functions, see below */
1825 else
c52f9dcd 1826 define sv_setsv Perl_sv_setsv
ee072b34
GS
1827 endif
1828
1829This works well, and means that XS authors can gleefully write:
1830
1831 sv_setsv(foo, bar);
1832
1833and still have it work under all the modes Perl could have been
1834compiled with.
1835
ee072b34
GS
1836This doesn't work so cleanly for varargs functions, though, as macros
1837imply that the number of arguments is known in advance. Instead we
1838either need to spell them out fully, passing C<aTHX_> as the first
1839argument (the Perl core tends to do this with functions like
1840Perl_warner), or use a context-free version.
1841
1842The context-free version of Perl_warner is called
1843Perl_warner_nocontext, and does not take the extra argument. Instead
1844it does dTHX; to get the context from thread-local storage. We
1845C<#define warner Perl_warner_nocontext> so that extensions get source
1846compatibility at the expense of performance. (Passing an arg is
1847cheaper than grabbing it from thread-local storage.)
1848
acfe0abc 1849You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
1850Those are strictly for use within the core. Extensions and embedders
1851need only be aware of [pad]THX.
1852
a7486cbb
JH
1853=head2 So what happened to dTHR?
1854
1855C<dTHR> was introduced in perl 5.005 to support the older thread model.
1856The older thread model now uses the C<THX> mechanism to pass context
1857pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
1858later still have it for backward source compatibility, but it is defined
1859to be a no-op.
1860
ee072b34
GS
1861=head2 How do I use all this in extensions?
1862
1863When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1864any functions in the Perl API will need to pass the initial context
1865argument somehow. The kicker is that you will need to write it in
1866such a way that the extension still compiles when Perl hasn't been
1867built with PERL_IMPLICIT_CONTEXT enabled.
1868
1869There are three ways to do this. First, the easy but inefficient way,
1870which is also the default, in order to maintain source compatibility
1871with extensions: whenever XSUB.h is #included, it redefines the aTHX
1872and aTHX_ macros to call a function that will return the context.
1873Thus, something like:
1874
1875 sv_setsv(asv, bsv);
1876
4375e838 1877in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1878in effect:
ee072b34 1879
2fa86c13 1880 Perl_sv_setsv(Perl_get_context(), asv, bsv);
ee072b34 1881
54aff467 1882or to this otherwise:
ee072b34
GS
1883
1884 Perl_sv_setsv(asv, bsv);
1885
1886You have to do nothing new in your extension to get this; since
2fa86c13 1887the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1888work.
1889
1890The second, more efficient way is to use the following template for
1891your Foo.xs:
1892
c52f9dcd
JH
1893 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1894 #include "EXTERN.h"
1895 #include "perl.h"
1896 #include "XSUB.h"
ee072b34
GS
1897
1898 static my_private_function(int arg1, int arg2);
1899
c52f9dcd
JH
1900 static SV *
1901 my_private_function(int arg1, int arg2)
1902 {
1903 dTHX; /* fetch context */
1904 ... call many Perl API functions ...
1905 }
ee072b34
GS
1906
1907 [... etc ...]
1908
c52f9dcd 1909 MODULE = Foo PACKAGE = Foo
ee072b34 1910
c52f9dcd 1911 /* typical XSUB */
ee072b34 1912
c52f9dcd
JH
1913 void
1914 my_xsub(arg)
1915 int arg
1916 CODE:
1917 my_private_function(arg, 10);
ee072b34
GS
1918
1919Note that the only two changes from the normal way of writing an
1920extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
1921including the Perl headers, followed by a C<dTHX;> declaration at
1922the start of every function that will call the Perl API. (You'll
1923know which functions need this, because the C compiler will complain
1924that there's an undeclared identifier in those functions.) No changes
1925are needed for the XSUBs themselves, because the XS() macro is
1926correctly defined to pass in the implicit context if needed.
1927
1928The third, even more efficient way is to ape how it is done within
1929the Perl guts:
1930
1931
c52f9dcd
JH
1932 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1933 #include "EXTERN.h"
1934 #include "perl.h"
1935 #include "XSUB.h"
ee072b34
GS
1936
1937 /* pTHX_ only needed for functions that call Perl API */
1938 static my_private_function(pTHX_ int arg1, int arg2);
1939
c52f9dcd
JH
1940 static SV *
1941 my_private_function(pTHX_ int arg1, int arg2)
1942 {
1943 /* dTHX; not needed here, because THX is an argument */
1944 ... call Perl API functions ...
1945 }
ee072b34
GS
1946
1947 [... etc ...]
1948
c52f9dcd 1949 MODULE = Foo PACKAGE = Foo
ee072b34 1950
c52f9dcd 1951 /* typical XSUB */
ee072b34 1952
c52f9dcd
JH
1953 void
1954 my_xsub(arg)
1955 int arg
1956 CODE:
1957 my_private_function(aTHX_ arg, 10);
ee072b34
GS
1958
1959This implementation never has to fetch the context using a function
1960call, since it is always passed as an extra argument. Depending on
1961your needs for simplicity or efficiency, you may mix the previous
1962two approaches freely.
1963
651a3225
GS
1964Never add a comma after C<pTHX> yourself--always use the form of the
1965macro with the underscore for functions that take explicit arguments,
1966or the form without the argument for functions with no explicit arguments.
ee072b34 1967
a7486cbb
JH
1968=head2 Should I do anything special if I call perl from multiple threads?
1969
1970If you create interpreters in one thread and then proceed to call them in
1971another, you need to make sure perl's own Thread Local Storage (TLS) slot is
1972initialized correctly in each of those threads.
1973
1974The C<perl_alloc> and C<perl_clone> API functions will automatically set
1975the TLS slot to the interpreter they created, so that there is no need to do
1976anything special if the interpreter is always accessed in the same thread that
1977created it, and that thread did not create or call any other interpreters
1978afterwards. If that is not the case, you have to set the TLS slot of the
1979thread before calling any functions in the Perl API on that particular
1980interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
1981thread as the first thing you do:
1982
1983 /* do this before doing anything else with some_perl */
1984 PERL_SET_CONTEXT(some_perl);
1985
1986 ... other Perl API calls on some_perl go here ...
1987
ee072b34
GS
1988=head2 Future Plans and PERL_IMPLICIT_SYS
1989
1990Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
1991that the interpreter knows about itself and pass it around, so too are
1992there plans to allow the interpreter to bundle up everything it knows
1993about the environment it's running on. This is enabled with the
acfe0abc 1994PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS
4d1ff10f 1995and USE_5005THREADS on Windows (see inside iperlsys.h).
ee072b34
GS
1996
1997This allows the ability to provide an extra pointer (called the "host"
1998environment) for all the system calls. This makes it possible for
1999all the system stuff to maintain their own state, broken down into
2000seven C structures. These are thin wrappers around the usual system
2001calls (see win32/perllib.c) for the default perl executable, but for a
2002more ambitious host (like the one that would do fork() emulation) all
2003the extra work needed to pretend that different interpreters are
2004actually different "processes", would be done here.
2005
2006The Perl engine/interpreter and the host are orthogonal entities.
2007There could be one or more interpreters in a process, and one or
2008more "hosts", with free association between them.
2009
a422fd2d
SC
2010=head1 Internal Functions
2011
2012All of Perl's internal functions which will be exposed to the outside
2013world are be prefixed by C<Perl_> so that they will not conflict with XS
2014functions or functions used in a program in which Perl is embedded.
2015Similarly, all global variables begin with C<PL_>. (By convention,
2016static functions start with C<S_>)
2017
2018Inside the Perl core, you can get at the functions either with or
2019without the C<Perl_> prefix, thanks to a bunch of defines that live in
2020F<embed.h>. This header file is generated automatically from
2021F<embed.pl>. F<embed.pl> also creates the prototyping header files for
2022the internal functions, generates the documentation and a lot of other
2023bits and pieces. It's important that when you add a new function to the
2024core or change an existing one, you change the data in the table at the
2025end of F<embed.pl> as well. Here's a sample entry from that table:
2026
2027 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2028
2029The second column is the return type, the third column the name. Columns
2030after that are the arguments. The first column is a set of flags:
2031
2032=over 3
2033
2034=item A
2035
2036This function is a part of the public API.
2037
2038=item p
2039
2040This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
2041
2042=item d
2043
2044This function has documentation using the C<apidoc> feature which we'll
2045look at in a second.
2046
2047=back
2048
2049Other available flags are:
2050
2051=over 3
2052
2053=item s
2054
a7486cbb
JH
2055This is a static function and is defined as C<S_whatever>, and usually
2056called within the sources as C<whatever(...)>.
a422fd2d
SC
2057
2058=item n
2059
2060This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
2061L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2062
2063=item r
2064
2065This function never returns; C<croak>, C<exit> and friends.
2066
2067=item f
2068
2069This function takes a variable number of arguments, C<printf> style.
2070The argument list should end with C<...>, like this:
2071
2072 Afprd |void |croak |const char* pat|...
2073
a7486cbb 2074=item M
a422fd2d 2075
00aadd71 2076This function is part of the experimental development API, and may change
a422fd2d
SC
2077or disappear without notice.
2078
2079=item o
2080
2081This function should not have a compatibility macro to define, say,
2082C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2083
2084=item j
2085
2086This function is not a member of C<CPerlObj>. If you don't know
2087what this means, don't use it.
2088
2089=item x
2090
2091This function isn't exported out of the Perl core.
2092
2093=back
2094
2095If you edit F<embed.pl>, you will need to run C<make regen_headers> to
2096force a rebuild of F<embed.h> and other auto-generated files.
2097
6b4667fc 2098=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2099
6b4667fc
A
2100If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2101formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2102following macros for portability
9dd9db0b 2103
c52f9dcd
JH
2104 IVdf IV in decimal
2105 UVuf UV in decimal
2106 UVof UV in octal
2107 UVxf UV in hexadecimal
2108 NVef NV %e-like
2109 NVff NV %f-like
2110 NVgf NV %g-like
9dd9db0b 2111
6b4667fc
A
2112These will take care of 64-bit integers and long doubles.
2113For example:
2114
c52f9dcd 2115 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2116
2117The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2118
8908e76d
JH
2119If you are printing addresses of pointers, use UVxf combined
2120with PTR2UV(), do not use %lx or %p.
2121
2122=head2 Pointer-To-Integer and Integer-To-Pointer
2123
2124Because pointer size does not necessarily equal integer size,
2125use the follow macros to do it right.
2126
c52f9dcd
JH
2127 PTR2UV(pointer)
2128 PTR2IV(pointer)
2129 PTR2NV(pointer)
2130 INT2PTR(pointertotype, integer)
8908e76d
JH
2131
2132For example:
2133
c52f9dcd
JH
2134 IV iv = ...;
2135 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2136
2137and
2138
c52f9dcd
JH
2139 AV *av = ...;
2140 UV uv = PTR2UV(av);
8908e76d 2141
a422fd2d
SC
2142=head2 Source Documentation
2143
2144There's an effort going on to document the internal functions and
2145automatically produce reference manuals from them - L<perlapi> is one
2146such manual which details all the functions which are available to XS
2147writers. L<perlintern> is the autogenerated manual for the functions
2148which are not part of the API and are supposedly for internal use only.
2149
2150Source documentation is created by putting POD comments into the C
2151source, like this:
2152
2153 /*
2154 =for apidoc sv_setiv
2155
2156 Copies an integer into the given SV. Does not handle 'set' magic. See
2157 C<sv_setiv_mg>.
2158
2159 =cut
2160 */
2161
2162Please try and supply some documentation if you add functions to the
2163Perl core.
2164
2165=head1 Unicode Support
2166
2167Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2168writers to understand this support and make sure that the code they
2169write does not corrupt Unicode data.
2170
2171=head2 What B<is> Unicode, anyway?
2172
2173In the olden, less enlightened times, we all used to use ASCII. Most of
2174us did, anyway. The big problem with ASCII is that it's American. Well,
2175no, that's not actually the problem; the problem is that it's not
2176particularly useful for people who don't use the Roman alphabet. What
2177used to happen was that particular languages would stick their own
2178alphabet in the upper range of the sequence, between 128 and 255. Of
2179course, we then ended up with plenty of variants that weren't quite
2180ASCII, and the whole point of it being a standard was lost.
2181
2182Worse still, if you've got a language like Chinese or
2183Japanese that has hundreds or thousands of characters, then you really
2184can't fit them into a mere 256, so they had to forget about ASCII
2185altogether, and build their own systems using pairs of numbers to refer
2186to one character.
2187
2188To fix this, some people formed Unicode, Inc. and
2189produced a new character set containing all the characters you can
2190possibly think of and more. There are several ways of representing these
2191characters, and the one Perl uses is called UTF8. UTF8 uses
2192a variable number of bytes to represent a character, instead of just
b3b6085d 2193one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d
SC
2194
2195=head2 How can I recognise a UTF8 string?
2196
2197You can't. This is because UTF8 data is stored in bytes just like
2198non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
2199capital E with a grave accent, is represented by the two bytes
2200C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2201has that byte sequence as well. So you can't tell just by looking - this
2202is what makes Unicode input an interesting problem.
2203
2204The API function C<is_utf8_string> can help; it'll tell you if a string
2205contains only valid UTF8 characters. However, it can't do the work for
2206you. On a character-by-character basis, C<is_utf8_char> will tell you
2207whether the current character in a string is valid UTF8.
2208
2209=head2 How does UTF8 represent Unicode characters?
2210
2211As mentioned above, UTF8 uses a variable number of bytes to store a
2212character. Characters with values 1...128 are stored in one byte, just
2213like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2214continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2215bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2216so it goes on, moving to three bytes at character 2048.
2217
2218Assuming you know you're dealing with a UTF8 string, you can find out
2219how long the first character in it is with the C<UTF8SKIP> macro:
2220
2221 char *utf = "\305\233\340\240\201";
2222 I32 len;
2223
2224 len = UTF8SKIP(utf); /* len is 2 here */
2225 utf += len;
2226 len = UTF8SKIP(utf); /* len is 3 here */
2227
2228Another way to skip over characters in a UTF8 string is to use
2229C<utf8_hop>, which takes a string and a number of characters to skip
2230over. You're on your own about bounds checking, though, so don't use it
2231lightly.
2232
2233All bytes in a multi-byte UTF8 character will have the high bit set, so
2234you can test if you need to do something special with this character
2235like this:
2236
2237 UV uv;
2238
2239 if (utf & 0x80)
2240 /* Must treat this as UTF8 */
2241 uv = utf8_to_uv(utf);
2242 else
2243 /* OK to treat this character as a byte */
2244 uv = *utf;
2245
2246You can also see in that example that we use C<utf8_to_uv> to get the
2247value of the character; the inverse function C<uv_to_utf8> is available
2248for putting a UV into UTF8:
2249
2250 if (uv > 0x80)
2251 /* Must treat this as UTF8 */
2252 utf8 = uv_to_utf8(utf8, uv);
2253 else
2254 /* OK to treat this character as a byte */
2255 *utf8++ = uv;
2256
2257You B<must> convert characters to UVs using the above functions if
2258you're ever in a situation where you have to match UTF8 and non-UTF8
2259characters. You may not skip over UTF8 characters in this case. If you
2260do this, you'll lose the ability to match hi-bit non-UTF8 characters;
2261for instance, if your UTF8 string contains C<v196.172>, and you skip
2262that character, you can never match a C<chr(200)> in a non-UTF8 string.
2263So don't do that!
2264
2265=head2 How does Perl store UTF8 strings?
2266
2267Currently, Perl deals with Unicode strings and non-Unicode strings
2268slightly differently. If a string has been identified as being UTF-8
2269encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2270manipulate this flag with the following macros:
2271
2272 SvUTF8(sv)
2273 SvUTF8_on(sv)
2274 SvUTF8_off(sv)
2275
2276This flag has an important effect on Perl's treatment of the string: if
2277Unicode data is not properly distinguished, regular expressions,
2278C<length>, C<substr> and other string handling operations will have
2279undesirable results.
2280
2281The problem comes when you have, for instance, a string that isn't
2282flagged is UTF8, and contains a byte sequence that could be UTF8 -
2283especially when combining non-UTF8 and UTF8 strings.
2284
2285Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2286need be sure you don't accidentally knock it off while you're
2287manipulating SVs. More specifically, you cannot expect to do this:
2288
2289 SV *sv;
2290 SV *nsv;
2291 STRLEN len;
2292 char *p;
2293
2294 p = SvPV(sv, len);
2295 frobnicate(p);
2296 nsv = newSVpvn(p, len);
2297
2298The C<char*> string does not tell you the whole story, and you can't
2299copy or reconstruct an SV just by copying the string value. Check if the
2300old SV has the UTF8 flag set, and act accordingly:
2301
2302 p = SvPV(sv, len);
2303 frobnicate(p);
2304 nsv = newSVpvn(p, len);
2305 if (SvUTF8(sv))
2306 SvUTF8_on(nsv);
2307
2308In fact, your C<frobnicate> function should be made aware of whether or
2309not it's dealing with UTF8 data, so that it can handle the string
2310appropriately.
2311
2312=head2 How do I convert a string to UTF8?
2313
2314If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
2315to upgrade one of the strings to UTF8. If you've got an SV, the easiest
2316way to do this is:
2317
2318 sv_utf8_upgrade(sv);
2319
2320However, you must not do this, for example:
2321
2322 if (!SvUTF8(left))
2323 sv_utf8_upgrade(left);
2324
2325If you do this in a binary operator, you will actually change one of the
b1866b2d 2326strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2327by the end user, it can cause problems.
2328
2329Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
2330string argument. This is useful for having the data available for
b1866b2d 2331comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2332C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2333the string contains any characters above 255 that can't be represented
2334in a single byte.
2335
2336=head2 Is there anything else I need to know?
2337
2338Not really. Just remember these things:
2339
2340=over 3
2341
2342=item *
2343
2344There's no way to tell if a string is UTF8 or not. You can tell if an SV
2345is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2346something should be UTF8. Treat the flag as part of the PV, even though
2347it's not - if you pass on the PV to somewhere, pass on the flag too.
2348
2349=item *
2350
2351If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
2352unless C<!(*s & 0x80)> in which case you can use C<*s>.
2353
2354=item *
2355
2356When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
2357C<uv < 0x80> in which case you can use C<*s = uv>.
2358
2359=item *
2360
2361Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
2362a new string which is UTF8 encoded. There are tricks you can use to
2363delay deciding whether you need to use a UTF8 string until you get to a
2364high character - C<HALF_UPGRADE> is one of those.
2365
2366=back
2367
53e06cf0
SC
2368=head1 Custom Operators
2369
9a68f1db 2370Custom operator support is a new experimental feature that allows you to
53e06cf0
SC
2371define your own ops. This is primarily to allow the building of
2372interpreters for other languages in the Perl core, but it also allows
2373optimizations through the creation of "macro-ops" (ops which perform the
2374functions of multiple ops which are usually executed together, such as
b7cb320d 2375C<gvsv, gvsv, add>.)
53e06cf0 2376
b455bf3f 2377This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0
SC
2378core does not "know" anything special about this op type, and so it will
2379not be involved in any optimizations. This also means that you can
2380define your custom ops to be any op structure - unary, binary, list and
2381so on - you like.
2382
2383It's important to know what custom operators won't do for you. They
2384won't let you add new syntax to Perl, directly. They won't even let you
2385add new keywords, directly. In fact, they won't change the way Perl
2386compiles a program at all. You have to do those changes yourself, after
2387Perl has compiled the program. You do this either by manipulating the op
2388tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2389a custom peephole optimizer with the C<optimize> module.
2390
2391When you do this, you replace ordinary Perl ops with custom ops by
2392creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2393PP function. This should be defined in XS code, and should look like
2394the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2395takes the appropriate number of values from the stack, and you are
2396responsible for adding stack marks if necessary.
2397
2398You should also "register" your op with the Perl interpreter so that it
2399can produce sensible error and warning messages. Since it is possible to
2400have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2401Perl uses the value of C<< o->op_ppaddr >> as a key into the
2402C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2403need to enter a name and description for your op at the appropriate
2404place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2405
2406Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2407directly support the creation of custom ops by name; C<Opcodes::Custom>
2408will provide functions which make it trivial to "register" custom ops to
2409the Perl interpreter.
2410
954c1994 2411=head1 AUTHORS
e89caa19 2412
954c1994 2413Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2414E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2415itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 2416
954c1994
GS
2417With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2418Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2419Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2420Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2421
9b5bb84f 2422API Listing originally by Dean Roehrich E<lt>roehrich@cray.comE<gt>.
cb1a09d0 2423
954c1994
GS
2424Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
2425Stuhl.
cb1a09d0 2426
954c1994 2427=head1 SEE ALSO
cb1a09d0 2428
954c1994 2429perlapi(1), perlintern(1), perlxs(1), perlembed(1)