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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
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7This document attempts to describe how to use the Perl API, as well as
8containing some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl has three typedefs that handle Perl's three main data types:
17
18 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21
d1b91892 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
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31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
32as well.)
a0d0e21e 33
54310121 34=head2 Working with SVs
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35
36An SV can be created and loaded with one command. There are four types of
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37values that can be loaded: an integer value (IV), a double (NV),
38a string (PV), and another scalar (SV).
a0d0e21e 39
9da1e3b5 40The six routines are:
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41
42 SV* newSViv(IV);
43 SV* newSVnv(double);
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44 SV* newSVpv(const char*, int);
45 SV* newSVpvn(const char*, int);
46fc3d4c 46 SV* newSVpvf(const char*, ...);
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47 SV* newSVsv(SV*);
48
deb3007b 49To change the value of an *already-existing* SV, there are seven routines:
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50
51 void sv_setiv(SV*, IV);
deb3007b 52 void sv_setuv(SV*, UV);
a0d0e21e 53 void sv_setnv(SV*, double);
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54 void sv_setpv(SV*, const char*);
55 void sv_setpvn(SV*, const char*, int)
46fc3d4c 56 void sv_setpvf(SV*, const char*, ...);
9abd00ed 57 void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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58 void sv_setsv(SV*, SV*);
59
60Notice that you can choose to specify the length of the string to be
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61assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
62allow Perl to calculate the length by using C<sv_setpv> or by specifying
630 as the second argument to C<newSVpv>. Be warned, though, that Perl will
64determine the string's length by using C<strlen>, which depends on the
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65string terminating with a NUL character.
66
67The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
68formatted output becomes the value.
69
70C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify
71either a pointer to a variable argument list or the address and length of
72an array of SVs. The last argument points to a boolean; on return, if that
73boolean is true, then locale-specific information has been used to format
c2611fb3 74the string, and the string's contents are therefore untrustworthy (see
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75L<perlsec>). This pointer may be NULL if that information is not
76important. Note that this function requires you to specify the length of
77the format.
78
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79STRLEN is an integer type (Size_t, usually defined as size_t in
80config.h) guaranteed to be large enough to represent the size of
81any string that perl can handle.
82
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83The C<sv_set*()> functions are not generic enough to operate on values
84that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 85
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86All SVs that contain strings should be terminated with a NUL character.
87If it is not NUL-terminated there is a risk of
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88core dumps and corruptions from code which passes the string to C
89functions or system calls which expect a NUL-terminated string.
90Perl's own functions typically add a trailing NUL for this reason.
91Nevertheless, you should be very careful when you pass a string stored
92in an SV to a C function or system call.
93
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94To access the actual value that an SV points to, you can use the macros:
95
96 SvIV(SV*)
954c1994 97 SvUV(SV*)
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98 SvNV(SV*)
99 SvPV(SV*, STRLEN len)
1fa8b10d 100 SvPV_nolen(SV*)
a0d0e21e 101
954c1994 102which will automatically coerce the actual scalar type into an IV, UV, double,
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103or string.
104
105In the C<SvPV> macro, the length of the string returned is placed into the
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106variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
107not care what the length of the data is, use the C<SvPV_nolen> macro.
108Historically the C<SvPV> macro with the global variable C<PL_na> has been
109used in this case. But that can be quite inefficient because C<PL_na> must
110be accessed in thread-local storage in threaded Perl. In any case, remember
111that Perl allows arbitrary strings of data that may both contain NULs and
112might not be terminated by a NUL.
a0d0e21e 113
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114Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
115len);>. It might work with your compiler, but it won't work for everyone.
116Break this sort of statement up into separate assignments:
117
b2f5ed49 118 SV *s;
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119 STRLEN len;
120 char * ptr;
b2f5ed49 121 ptr = SvPV(s, len);
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122 foo(ptr, len);
123
07fa94a1 124If you want to know if the scalar value is TRUE, you can use:
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125
126 SvTRUE(SV*)
127
128Although Perl will automatically grow strings for you, if you need to force
129Perl to allocate more memory for your SV, you can use the macro
130
131 SvGROW(SV*, STRLEN newlen)
132
133which will determine if more memory needs to be allocated. If so, it will
134call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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135decrease, the allocated memory of an SV and that it does not automatically
136add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 137C<SvGROW(sv, len + 1)>).
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138
139If you have an SV and want to know what kind of data Perl thinks is stored
140in it, you can use the following macros to check the type of SV you have.
141
142 SvIOK(SV*)
143 SvNOK(SV*)
144 SvPOK(SV*)
145
146You can get and set the current length of the string stored in an SV with
147the following macros:
148
149 SvCUR(SV*)
150 SvCUR_set(SV*, I32 val)
151
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152You can also get a pointer to the end of the string stored in the SV
153with the macro:
154
155 SvEND(SV*)
156
157But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 158
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159If you want to append something to the end of string stored in an C<SV*>,
160you can use the following functions:
161
08105a92 162 void sv_catpv(SV*, const char*);
e65f3abd 163 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 164 void sv_catpvf(SV*, const char*, ...);
9abd00ed 165 void sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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166 void sv_catsv(SV*, SV*);
167
168The first function calculates the length of the string to be appended by
169using C<strlen>. In the second, you specify the length of the string
46fc3d4c 170yourself. The third function processes its arguments like C<sprintf> and
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171appends the formatted output. The fourth function works like C<vsprintf>.
172You can specify the address and length of an array of SVs instead of the
173va_list argument. The fifth function extends the string stored in the first
174SV with the string stored in the second SV. It also forces the second SV
175to be interpreted as a string.
176
177The C<sv_cat*()> functions are not generic enough to operate on values that
178have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 179
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180If you know the name of a scalar variable, you can get a pointer to its SV
181by using the following:
182
4929bf7b 183 SV* get_sv("package::varname", FALSE);
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184
185This returns NULL if the variable does not exist.
186
d1b91892 187If you want to know if this variable (or any other SV) is actually C<defined>,
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188you can call:
189
190 SvOK(SV*)
191
9cde0e7f 192The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its
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193address can be used whenever an C<SV*> is needed.
194
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195There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean
196TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can
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197be used whenever an C<SV*> is needed.
198
9cde0e7f 199Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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200Take this code:
201
202 SV* sv = (SV*) 0;
203 if (I-am-to-return-a-real-value) {
204 sv = sv_2mortal(newSViv(42));
205 }
206 sv_setsv(ST(0), sv);
207
208This code tries to return a new SV (which contains the value 42) if it should
04343c6d 209return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 210pointer which, somewhere down the line, will cause a segmentation violation,
9cde0e7f 211bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first
5f05dabc 212line and all will be well.
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213
214To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 215call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 216
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217=head2 Offsets
218
219Perl provides the function C<sv_chop> to efficiently remove characters
220from the beginning of a string; you give it an SV and a pointer to
221somewhere inside the the PV, and it discards everything before the
222pointer. The efficiency comes by means of a little hack: instead of
223actually removing the characters, C<sv_chop> sets the flag C<OOK>
224(offset OK) to signal to other functions that the offset hack is in
225effect, and it puts the number of bytes chopped off into the IV field
226of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
227many bytes, and adjusts C<SvCUR> and C<SvLEN>.
228
229Hence, at this point, the start of the buffer that we allocated lives
230at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
231into the middle of this allocated storage.
232
233This is best demonstrated by example:
234
235 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
236 SV = PVIV(0x8128450) at 0x81340f0
237 REFCNT = 1
238 FLAGS = (POK,OOK,pPOK)
239 IV = 1 (OFFSET)
240 PV = 0x8135781 ( "1" . ) "2345"\0
241 CUR = 4
242 LEN = 5
243
244Here the number of bytes chopped off (1) is put into IV, and
245C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
246portion of the string between the "real" and the "fake" beginnings is
247shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
248the fake beginning, not the real one.
249
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250Something similar to the offset hack is perfomed on AVs to enable
251efficient shifting and splicing off the beginning of the array; while
252C<AvARRAY> points to the first element in the array that is visible from
253Perl, C<AvALLOC> points to the real start of the C array. These are
254usually the same, but a C<shift> operation can be carried out by
255increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
256Again, the location of the real start of the C array only comes into
257play when freeing the array. See C<av_shift> in F<av.c>.
258
d1b91892 259=head2 What's Really Stored in an SV?
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260
261Recall that the usual method of determining the type of scalar you have is
5f05dabc 262to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 263usually these macros will always return TRUE and calling the C<Sv*V>
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264macros will do the appropriate conversion of string to integer/double or
265integer/double to string.
266
267If you I<really> need to know if you have an integer, double, or string
268pointer in an SV, you can use the following three macros instead:
269
270 SvIOKp(SV*)
271 SvNOKp(SV*)
272 SvPOKp(SV*)
273
274These will tell you if you truly have an integer, double, or string pointer
d1b91892 275stored in your SV. The "p" stands for private.
a0d0e21e 276
07fa94a1 277In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 278
54310121 279=head2 Working with AVs
a0d0e21e 280
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281There are two ways to create and load an AV. The first method creates an
282empty AV:
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283
284 AV* newAV();
285
54310121 286The second method both creates the AV and initially populates it with SVs:
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287
288 AV* av_make(I32 num, SV **ptr);
289
5f05dabc 290The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 291AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 292
54310121 293Once the AV has been created, the following operations are possible on AVs:
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294
295 void av_push(AV*, SV*);
296 SV* av_pop(AV*);
297 SV* av_shift(AV*);
298 void av_unshift(AV*, I32 num);
299
300These should be familiar operations, with the exception of C<av_unshift>.
301This routine adds C<num> elements at the front of the array with the C<undef>
302value. You must then use C<av_store> (described below) to assign values
303to these new elements.
304
305Here are some other functions:
306
5f05dabc 307 I32 av_len(AV*);
a0d0e21e 308 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 309 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 310
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311The C<av_len> function returns the highest index value in array (just
312like $#array in Perl). If the array is empty, -1 is returned. The
313C<av_fetch> function returns the value at index C<key>, but if C<lval>
314is non-zero, then C<av_fetch> will store an undef value at that index.
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315The C<av_store> function stores the value C<val> at index C<key>, and does
316not increment the reference count of C<val>. Thus the caller is responsible
317for taking care of that, and if C<av_store> returns NULL, the caller will
318have to decrement the reference count to avoid a memory leak. Note that
319C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
320return value.
d1b91892 321
a0d0e21e 322 void av_clear(AV*);
a0d0e21e 323 void av_undef(AV*);
cb1a09d0 324 void av_extend(AV*, I32 key);
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325
326The C<av_clear> function deletes all the elements in the AV* array, but
327does not actually delete the array itself. The C<av_undef> function will
328delete all the elements in the array plus the array itself. The
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329C<av_extend> function extends the array so that it contains at least C<key+1>
330elements. If C<key+1> is less than the currently allocated length of the array,
331then nothing is done.
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332
333If you know the name of an array variable, you can get a pointer to its AV
334by using the following:
335
4929bf7b 336 AV* get_av("package::varname", FALSE);
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337
338This returns NULL if the variable does not exist.
339
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340See L<Understanding the Magic of Tied Hashes and Arrays> for more
341information on how to use the array access functions on tied arrays.
342
54310121 343=head2 Working with HVs
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344
345To create an HV, you use the following routine:
346
347 HV* newHV();
348
54310121 349Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 350
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351 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
352 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 353
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354The C<klen> parameter is the length of the key being passed in (Note that
355you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
356length of the key). The C<val> argument contains the SV pointer to the
54310121 357scalar being stored, and C<hash> is the precomputed hash value (zero if
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358you want C<hv_store> to calculate it for you). The C<lval> parameter
359indicates whether this fetch is actually a part of a store operation, in
360which case a new undefined value will be added to the HV with the supplied
361key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 362
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363Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
364C<SV*>. To access the scalar value, you must first dereference the return
365value. However, you should check to make sure that the return value is
366not NULL before dereferencing it.
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367
368These two functions check if a hash table entry exists, and deletes it.
369
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370 bool hv_exists(HV*, const char* key, U32 klen);
371 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 372
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373If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
374create and return a mortal copy of the deleted value.
375
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376And more miscellaneous functions:
377
378 void hv_clear(HV*);
a0d0e21e 379 void hv_undef(HV*);
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380
381Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
382table but does not actually delete the hash table. The C<hv_undef> deletes
383both the entries and the hash table itself.
a0d0e21e 384
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385Perl keeps the actual data in linked list of structures with a typedef of HE.
386These contain the actual key and value pointers (plus extra administrative
387overhead). The key is a string pointer; the value is an C<SV*>. However,
388once you have an C<HE*>, to get the actual key and value, use the routines
389specified below.
390
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391 I32 hv_iterinit(HV*);
392 /* Prepares starting point to traverse hash table */
393 HE* hv_iternext(HV*);
394 /* Get the next entry, and return a pointer to a
395 structure that has both the key and value */
396 char* hv_iterkey(HE* entry, I32* retlen);
397 /* Get the key from an HE structure and also return
398 the length of the key string */
cb1a09d0 399 SV* hv_iterval(HV*, HE* entry);
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400 /* Return a SV pointer to the value of the HE
401 structure */
cb1a09d0 402 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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403 /* This convenience routine combines hv_iternext,
404 hv_iterkey, and hv_iterval. The key and retlen
405 arguments are return values for the key and its
406 length. The value is returned in the SV* argument */
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407
408If you know the name of a hash variable, you can get a pointer to its HV
409by using the following:
410
4929bf7b 411 HV* get_hv("package::varname", FALSE);
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412
413This returns NULL if the variable does not exist.
414
8ebc5c01 415The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 416
a0d0e21e 417 hash = 0;
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418 while (klen--)
419 hash = (hash * 33) + *key++;
87275199 420 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 421
87275199 422The last step was added in version 5.6 to improve distribution of
ab192400 423lower bits in the resulting hash value.
a0d0e21e 424
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425See L<Understanding the Magic of Tied Hashes and Arrays> for more
426information on how to use the hash access functions on tied hashes.
427
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428=head2 Hash API Extensions
429
430Beginning with version 5.004, the following functions are also supported:
431
432 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
433 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 434
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435 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
436 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 437
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438 SV* hv_iterkeysv (HE* entry);
439
440Note that these functions take C<SV*> keys, which simplifies writing
441of extension code that deals with hash structures. These functions
442also allow passing of C<SV*> keys to C<tie> functions without forcing
443you to stringify the keys (unlike the previous set of functions).
444
445They also return and accept whole hash entries (C<HE*>), making their
446use more efficient (since the hash number for a particular string
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447doesn't have to be recomputed every time). See L<perlapi> for detailed
448descriptions.
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449
450The following macros must always be used to access the contents of hash
451entries. Note that the arguments to these macros must be simple
452variables, since they may get evaluated more than once. See
4a4eefd0 453L<perlapi> for detailed descriptions of these macros.
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454
455 HePV(HE* he, STRLEN len)
456 HeVAL(HE* he)
457 HeHASH(HE* he)
458 HeSVKEY(HE* he)
459 HeSVKEY_force(HE* he)
460 HeSVKEY_set(HE* he, SV* sv)
461
462These two lower level macros are defined, but must only be used when
463dealing with keys that are not C<SV*>s:
464
465 HeKEY(HE* he)
466 HeKLEN(HE* he)
467
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468Note that both C<hv_store> and C<hv_store_ent> do not increment the
469reference count of the stored C<val>, which is the caller's responsibility.
470If these functions return a NULL value, the caller will usually have to
471decrement the reference count of C<val> to avoid a memory leak.
1e422769 472
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473=head2 References
474
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475References are a special type of scalar that point to other data types
476(including references).
a0d0e21e 477
07fa94a1 478To create a reference, use either of the following functions:
a0d0e21e 479
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480 SV* newRV_inc((SV*) thing);
481 SV* newRV_noinc((SV*) thing);
a0d0e21e 482
5f05dabc 483The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
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484functions are identical except that C<newRV_inc> increments the reference
485count of the C<thing>, while C<newRV_noinc> does not. For historical
486reasons, C<newRV> is a synonym for C<newRV_inc>.
487
488Once you have a reference, you can use the following macro to dereference
489the reference:
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490
491 SvRV(SV*)
492
493then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 494C<AV*> or C<HV*>, if required.
a0d0e21e 495
d1b91892 496To determine if an SV is a reference, you can use the following macro:
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497
498 SvROK(SV*)
499
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500To discover what type of value the reference refers to, use the following
501macro and then check the return value.
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502
503 SvTYPE(SvRV(SV*))
504
505The most useful types that will be returned are:
506
507 SVt_IV Scalar
508 SVt_NV Scalar
509 SVt_PV Scalar
5f05dabc 510 SVt_RV Scalar
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511 SVt_PVAV Array
512 SVt_PVHV Hash
513 SVt_PVCV Code
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514 SVt_PVGV Glob (possible a file handle)
515 SVt_PVMG Blessed or Magical Scalar
516
517 See the sv.h header file for more details.
d1b91892 518
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519=head2 Blessed References and Class Objects
520
521References are also used to support object-oriented programming. In the
522OO lexicon, an object is simply a reference that has been blessed into a
523package (or class). Once blessed, the programmer may now use the reference
524to access the various methods in the class.
525
526A reference can be blessed into a package with the following function:
527
528 SV* sv_bless(SV* sv, HV* stash);
529
530The C<sv> argument must be a reference. The C<stash> argument specifies
3fe9a6f1 531which class the reference will belong to. See
2ae324a7 532L<Stashes and Globs> for information on converting class names into stashes.
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533
534/* Still under construction */
535
536Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
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537point to. If C<classname> is non-null, the SV is blessed into the specified
538class. SV is returned.
cb1a09d0 539
08105a92 540 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 541
e1c57cef 542Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 543if C<classname> is non-null.
cb1a09d0 544
08105a92 545 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 546 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 547 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 548
5f05dabc 549Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 550reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 551
08105a92 552 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 553
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554Copies string into an SV whose reference is C<rv>. Set length to 0 to let
555Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 556
e65f3abd 557 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 558
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559Tests whether the SV is blessed into the specified class. It does not
560check inheritance relationships.
561
08105a92 562 int sv_isa(SV* sv, const char* name);
9abd00ed
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563
564Tests whether the SV is a reference to a blessed object.
565
566 int sv_isobject(SV* sv);
567
568Tests whether the SV is derived from the specified class. SV can be either
569a reference to a blessed object or a string containing a class name. This
570is the function implementing the C<UNIVERSAL::isa> functionality.
571
08105a92 572 bool sv_derived_from(SV* sv, const char* name);
9abd00ed
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573
574To check if you've got an object derived from a specific class you have
575to write:
576
577 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 578
5f05dabc 579=head2 Creating New Variables
cb1a09d0 580
5f05dabc
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581To create a new Perl variable with an undef value which can be accessed from
582your Perl script, use the following routines, depending on the variable type.
cb1a09d0 583
4929bf7b
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584 SV* get_sv("package::varname", TRUE);
585 AV* get_av("package::varname", TRUE);
586 HV* get_hv("package::varname", TRUE);
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587
588Notice the use of TRUE as the second parameter. The new variable can now
589be set, using the routines appropriate to the data type.
590
5f05dabc
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591There are additional macros whose values may be bitwise OR'ed with the
592C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 593
5f05dabc 594 GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
54310121 595 "Name <varname> used only once: possible typo" warning.
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596 GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
597 the variable did not exist before the function was called.
cb1a09d0 598
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599If you do not specify a package name, the variable is created in the current
600package.
cb1a09d0 601
5f05dabc 602=head2 Reference Counts and Mortality
a0d0e21e 603
54310121
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604Perl uses an reference count-driven garbage collection mechanism. SVs,
605AVs, or HVs (xV for short in the following) start their life with a
55497cff 606reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 607then it will be destroyed and its memory made available for reuse.
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608
609This normally doesn't happen at the Perl level unless a variable is
5f05dabc
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610undef'ed or the last variable holding a reference to it is changed or
611overwritten. At the internal level, however, reference counts can be
55497cff
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612manipulated with the following macros:
613
614 int SvREFCNT(SV* sv);
5f05dabc 615 SV* SvREFCNT_inc(SV* sv);
55497cff
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616 void SvREFCNT_dec(SV* sv);
617
618However, there is one other function which manipulates the reference
07fa94a1
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619count of its argument. The C<newRV_inc> function, you will recall,
620creates a reference to the specified argument. As a side effect,
621it increments the argument's reference count. If this is not what
622you want, use C<newRV_noinc> instead.
623
624For example, imagine you want to return a reference from an XSUB function.
625Inside the XSUB routine, you create an SV which initially has a reference
626count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
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627This returns the reference as a new SV, but the reference count of the
628SV you passed to C<newRV_inc> has been incremented to two. Now you
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629return the reference from the XSUB routine and forget about the SV.
630But Perl hasn't! Whenever the returned reference is destroyed, the
631reference count of the original SV is decreased to one and nothing happens.
632The SV will hang around without any way to access it until Perl itself
633terminates. This is a memory leak.
5f05dabc
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634
635The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
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636C<newRV_inc>. Then, if and when the last reference is destroyed,
637the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 638stopping any memory leak.
55497cff 639
5f05dabc 640There are some convenience functions available that can help with the
54310121 641destruction of xVs. These functions introduce the concept of "mortality".
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642An xV that is mortal has had its reference count marked to be decremented,
643but not actually decremented, until "a short time later". Generally the
644term "short time later" means a single Perl statement, such as a call to
54310121 645an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
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646reference count decremented depends on two macros, SAVETMPS and FREETMPS.
647See L<perlcall> and L<perlxs> for more details on these macros.
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648
649"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
650However, if you mortalize a variable twice, the reference count will
651later be decremented twice.
652
653You should be careful about creating mortal variables. Strange things
654can happen if you make the same value mortal within multiple contexts,
5f05dabc 655or if you make a variable mortal multiple times.
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656
657To create a mortal variable, use the functions:
658
659 SV* sv_newmortal()
660 SV* sv_2mortal(SV*)
661 SV* sv_mortalcopy(SV*)
662
5f05dabc
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663The first call creates a mortal SV, the second converts an existing
664SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
665third creates a mortal copy of an existing SV.
a0d0e21e 666
54310121 667The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 668made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 669C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 670
5f05dabc 671=head2 Stashes and Globs
a0d0e21e 672
aa689395
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673A "stash" is a hash that contains all of the different objects that
674are contained within a package. Each key of the stash is a symbol
675name (shared by all the different types of objects that have the same
676name), and each value in the hash table is a GV (Glob Value). This GV
677in turn contains references to the various objects of that name,
678including (but not limited to) the following:
cb1a09d0 679
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680 Scalar Value
681 Array Value
682 Hash Value
a3cb178b 683 I/O Handle
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684 Format
685 Subroutine
686
9cde0e7f 687There is a single stash called "PL_defstash" that holds the items that exist
5f05dabc
PP
688in the "main" package. To get at the items in other packages, append the
689string "::" to the package name. The items in the "Foo" package are in
9cde0e7f 690the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are
5f05dabc 691in the stash "Baz::" in "Bar::"'s stash.
a0d0e21e 692
d1b91892 693To get the stash pointer for a particular package, use the function:
a0d0e21e 694
08105a92 695 HV* gv_stashpv(const char* name, I32 create)
a0d0e21e
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696 HV* gv_stashsv(SV*, I32 create)
697
698The first function takes a literal string, the second uses the string stored
d1b91892 699in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 700C<HV*>. The C<create> flag will create a new package if it is set.
a0d0e21e
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701
702The name that C<gv_stash*v> wants is the name of the package whose symbol table
703you want. The default package is called C<main>. If you have multiply nested
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704packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
705language itself.
a0d0e21e
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706
707Alternately, if you have an SV that is a blessed reference, you can find
708out the stash pointer by using:
709
710 HV* SvSTASH(SvRV(SV*));
711
712then use the following to get the package name itself:
713
714 char* HvNAME(HV* stash);
715
5f05dabc
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716If you need to bless or re-bless an object you can use the following
717function:
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LW
718
719 SV* sv_bless(SV*, HV* stash)
720
721where the first argument, an C<SV*>, must be a reference, and the second
722argument is a stash. The returned C<SV*> can now be used in the same way
723as any other SV.
724
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725For more information on references and blessings, consult L<perlref>.
726
54310121 727=head2 Double-Typed SVs
0a753a76
PP
728
729Scalar variables normally contain only one type of value, an integer,
730double, pointer, or reference. Perl will automatically convert the
731actual scalar data from the stored type into the requested type.
732
733Some scalar variables contain more than one type of scalar data. For
734example, the variable C<$!> contains either the numeric value of C<errno>
735or its string equivalent from either C<strerror> or C<sys_errlist[]>.
736
737To force multiple data values into an SV, you must do two things: use the
738C<sv_set*v> routines to add the additional scalar type, then set a flag
739so that Perl will believe it contains more than one type of data. The
740four macros to set the flags are:
741
742 SvIOK_on
743 SvNOK_on
744 SvPOK_on
745 SvROK_on
746
747The particular macro you must use depends on which C<sv_set*v> routine
748you called first. This is because every C<sv_set*v> routine turns on
749only the bit for the particular type of data being set, and turns off
750all the rest.
751
752For example, to create a new Perl variable called "dberror" that contains
753both the numeric and descriptive string error values, you could use the
754following code:
755
756 extern int dberror;
757 extern char *dberror_list;
758
4929bf7b 759 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
760 sv_setiv(sv, (IV) dberror);
761 sv_setpv(sv, dberror_list[dberror]);
762 SvIOK_on(sv);
763
764If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
765macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
766
767=head2 Magic Variables
a0d0e21e 768
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769[This section still under construction. Ignore everything here. Post no
770bills. Everything not permitted is forbidden.]
771
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772Any SV may be magical, that is, it has special features that a normal
773SV does not have. These features are stored in the SV structure in a
5f05dabc 774linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
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775
776 struct magic {
777 MAGIC* mg_moremagic;
778 MGVTBL* mg_virtual;
779 U16 mg_private;
780 char mg_type;
781 U8 mg_flags;
782 SV* mg_obj;
783 char* mg_ptr;
784 I32 mg_len;
785 };
786
787Note this is current as of patchlevel 0, and could change at any time.
788
789=head2 Assigning Magic
790
791Perl adds magic to an SV using the sv_magic function:
792
08105a92 793 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
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794
795The C<sv> argument is a pointer to the SV that is to acquire a new magical
796feature.
797
798If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
799set the C<SVt_PVMG> flag for the C<sv>. Perl then continues by adding
800it to the beginning of the linked list of magical features. Any prior
801entry of the same type of magic is deleted. Note that this can be
5fb8527f 802overridden, and multiple instances of the same type of magic can be
d1b91892
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803associated with an SV.
804
54310121
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805The C<name> and C<namlen> arguments are used to associate a string with
806the magic, typically the name of a variable. C<namlen> is stored in the
807C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd
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808copy of the name is stored in C<mg_ptr> field.
809
810The sv_magic function uses C<how> to determine which, if any, predefined
811"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
cb1a09d0
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812See the "Magic Virtual Table" section below. The C<how> argument is also
813stored in the C<mg_type> field.
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814
815The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
816structure. If it is not the same as the C<sv> argument, the reference
817count of the C<obj> object is incremented. If it is the same, or if
04343c6d 818the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is
d1b91892
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819merely stored, without the reference count being incremented.
820
cb1a09d0
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821There is also a function to add magic to an C<HV>:
822
823 void hv_magic(HV *hv, GV *gv, int how);
824
825This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
826
827To remove the magic from an SV, call the function sv_unmagic:
828
829 void sv_unmagic(SV *sv, int type);
830
831The C<type> argument should be equal to the C<how> value when the C<SV>
832was initially made magical.
833
d1b91892
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834=head2 Magic Virtual Tables
835
836The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a
837C<MGVTBL>, which is a structure of function pointers and stands for
838"Magic Virtual Table" to handle the various operations that might be
839applied to that variable.
840
841The C<MGVTBL> has five pointers to the following routine types:
842
843 int (*svt_get)(SV* sv, MAGIC* mg);
844 int (*svt_set)(SV* sv, MAGIC* mg);
845 U32 (*svt_len)(SV* sv, MAGIC* mg);
846 int (*svt_clear)(SV* sv, MAGIC* mg);
847 int (*svt_free)(SV* sv, MAGIC* mg);
848
849This MGVTBL structure is set at compile-time in C<perl.h> and there are
850currently 19 types (or 21 with overloading turned on). These different
851structures contain pointers to various routines that perform additional
852actions depending on which function is being called.
853
854 Function pointer Action taken
855 ---------------- ------------
856 svt_get Do something after the value of the SV is retrieved.
857 svt_set Do something after the SV is assigned a value.
858 svt_len Report on the SV's length.
859 svt_clear Clear something the SV represents.
860 svt_free Free any extra storage associated with the SV.
861
862For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
863to an C<mg_type> of '\0') contains:
864
865 { magic_get, magic_set, magic_len, 0, 0 }
866
867Thus, when an SV is determined to be magical and of type '\0', if a get
868operation is being performed, the routine C<magic_get> is called. All
869the various routines for the various magical types begin with C<magic_>.
954c1994
GS
870NOTE: the magic routines are not considered part of the Perl API, and may
871not be exported by the Perl library.
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872
873The current kinds of Magic Virtual Tables are:
874
bdbeb323 875 mg_type MGVTBL Type of magic
5f05dabc 876 ------- ------ ----------------------------
bdbeb323
SM
877 \0 vtbl_sv Special scalar variable
878 A vtbl_amagic %OVERLOAD hash
879 a vtbl_amagicelem %OVERLOAD hash element
880 c (none) Holds overload table (AMT) on stash
881 B vtbl_bm Boyer-Moore (fast string search)
c2e66d9e
GS
882 D vtbl_regdata Regex match position data (@+ and @- vars)
883 d vtbl_regdatum Regex match position data element
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884 E vtbl_env %ENV hash
885 e vtbl_envelem %ENV hash element
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SM
886 f vtbl_fm Formline ('compiled' format)
887 g vtbl_mglob m//g target / study()ed string
d1b91892
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888 I vtbl_isa @ISA array
889 i vtbl_isaelem @ISA array element
bdbeb323
SM
890 k vtbl_nkeys scalar(keys()) lvalue
891 L (none) Debugger %_<filename
892 l vtbl_dbline Debugger %_<filename element
44a8e56a 893 o vtbl_collxfrm Locale transformation
bdbeb323
SM
894 P vtbl_pack Tied array or hash
895 p vtbl_packelem Tied array or hash element
896 q vtbl_packelem Tied scalar or handle
897 S vtbl_sig %SIG hash
898 s vtbl_sigelem %SIG hash element
d1b91892 899 t vtbl_taint Taintedness
bdbeb323
SM
900 U vtbl_uvar Available for use by extensions
901 v vtbl_vec vec() lvalue
902 x vtbl_substr substr() lvalue
903 y vtbl_defelem Shadow "foreach" iterator variable /
904 smart parameter vivification
905 * vtbl_glob GV (typeglob)
906 # vtbl_arylen Array length ($#ary)
907 . vtbl_pos pos() lvalue
908 ~ (none) Available for use by extensions
d1b91892 909
68dc0745
PP
910When an uppercase and lowercase letter both exist in the table, then the
911uppercase letter is used to represent some kind of composite type (a list
912or a hash), and the lowercase letter is used to represent an element of
d1b91892
AD
913that composite type.
914
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915The '~' and 'U' magic types are defined specifically for use by
916extensions and will not be used by perl itself. Extensions can use
917'~' magic to 'attach' private information to variables (typically
918objects). This is especially useful because there is no way for
919normal perl code to corrupt this private information (unlike using
920extra elements of a hash object).
921
922Similarly, 'U' magic can be used much like tie() to call a C function
923any time a scalar's value is used or changed. The C<MAGIC>'s
924C<mg_ptr> field points to a C<ufuncs> structure:
925
926 struct ufuncs {
927 I32 (*uf_val)(IV, SV*);
928 I32 (*uf_set)(IV, SV*);
929 IV uf_index;
930 };
931
932When the SV is read from or written to, the C<uf_val> or C<uf_set>
933function will be called with C<uf_index> as the first arg and a
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934pointer to the SV as the second. A simple example of how to add 'U'
935magic is shown below. Note that the ufuncs structure is copied by
936sv_magic, so you can safely allocate it on the stack.
937
938 void
939 Umagic(sv)
940 SV *sv;
941 PREINIT:
942 struct ufuncs uf;
943 CODE:
944 uf.uf_val = &my_get_fn;
945 uf.uf_set = &my_set_fn;
946 uf.uf_index = 0;
947 sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));
5f05dabc 948
bdbeb323
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949Note that because multiple extensions may be using '~' or 'U' magic,
950it is important for extensions to take extra care to avoid conflict.
951Typically only using the magic on objects blessed into the same class
952as the extension is sufficient. For '~' magic, it may also be
953appropriate to add an I32 'signature' at the top of the private data
954area and check that.
5f05dabc 955
ef50df4b
GS
956Also note that the C<sv_set*()> and C<sv_cat*()> functions described
957earlier do B<not> invoke 'set' magic on their targets. This must
958be done by the user either by calling the C<SvSETMAGIC()> macro after
959calling these functions, or by using one of the C<sv_set*_mg()> or
960C<sv_cat*_mg()> functions. Similarly, generic C code must call the
961C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
962obtained from external sources in functions that don't handle magic.
4a4eefd0 963See L<perlapi> for a description of these functions.
189b2af5
GS
964For example, calls to the C<sv_cat*()> functions typically need to be
965followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
966since their implementation handles 'get' magic.
967
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968=head2 Finding Magic
969
970 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
971
972This routine returns a pointer to the C<MAGIC> structure stored in the SV.
973If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 974if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 975
08105a92 976 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
977
978This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
979field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
980the mg_type field is changed to be the lowercase letter.
a0d0e21e 981
04343c6d
GS
982=head2 Understanding the Magic of Tied Hashes and Arrays
983
984Tied hashes and arrays are magical beasts of the 'P' magic type.
9edb2b46
GS
985
986WARNING: As of the 5.004 release, proper usage of the array and hash
987access functions requires understanding a few caveats. Some
988of these caveats are actually considered bugs in the API, to be fixed
989in later releases, and are bracketed with [MAYCHANGE] below. If
990you find yourself actually applying such information in this section, be
991aware that the behavior may change in the future, umm, without warning.
04343c6d 992
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993The perl tie function associates a variable with an object that implements
994the various GET, SET etc methods. To perform the equivalent of the perl
995tie function from an XSUB, you must mimic this behaviour. The code below
996carries out the necessary steps - firstly it creates a new hash, and then
997creates a second hash which it blesses into the class which will implement
998the tie methods. Lastly it ties the two hashes together, and returns a
999reference to the new tied hash. Note that the code below does NOT call the
1000TIEHASH method in the MyTie class -
1001see L<Calling Perl Routines from within C Programs> for details on how
1002to do this.
1003
1004 SV*
1005 mytie()
1006 PREINIT:
1007 HV *hash;
1008 HV *stash;
1009 SV *tie;
1010 CODE:
1011 hash = newHV();
1012 tie = newRV_noinc((SV*)newHV());
1013 stash = gv_stashpv("MyTie", TRUE);
1014 sv_bless(tie, stash);
1015 hv_magic(hash, tie, 'P');
1016 RETVAL = newRV_noinc(hash);
1017 OUTPUT:
1018 RETVAL
1019
04343c6d
GS
1020The C<av_store> function, when given a tied array argument, merely
1021copies the magic of the array onto the value to be "stored", using
1022C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1023actually need to be stored in the array. [MAYCHANGE] After a call to
1024C<av_store> on a tied array, the caller will usually need to call
1025C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1026TIEARRAY object. If C<av_store> did return NULL, a call to
1027C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1028leak. [/MAYCHANGE]
04343c6d
GS
1029
1030The previous paragraph is applicable verbatim to tied hash access using the
1031C<hv_store> and C<hv_store_ent> functions as well.
1032
1033C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1034C<hv_fetch_ent> actually return an undefined mortal value whose magic
1035has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1036need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1037need to call C<mg_get()> on the returned value in order to actually invoke
1038the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1039you may also call C<mg_set()> on the return value after possibly assigning
1040a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1041method on the TIE object. [/MAYCHANGE]
04343c6d 1042
9edb2b46 1043[MAYCHANGE]
04343c6d
GS
1044In other words, the array or hash fetch/store functions don't really
1045fetch and store actual values in the case of tied arrays and hashes. They
1046merely call C<mg_copy> to attach magic to the values that were meant to be
1047"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1048do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1049the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1050and hashes.
1051
1052Currently (as of perl version 5.004), use of the hash and array access
1053functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1054"normal" hashes and arrays, or on their tied variants. The API may be
1055changed to provide more transparent access to both tied and normal data
1056types in future versions.
1057[/MAYCHANGE]
04343c6d
GS
1058
1059You would do well to understand that the TIEARRAY and TIEHASH interfaces
1060are mere sugar to invoke some perl method calls while using the uniform hash
1061and array syntax. The use of this sugar imposes some overhead (typically
1062about two to four extra opcodes per FETCH/STORE operation, in addition to
1063the creation of all the mortal variables required to invoke the methods).
1064This overhead will be comparatively small if the TIE methods are themselves
1065substantial, but if they are only a few statements long, the overhead
1066will not be insignificant.
1067
d1c897a1
IZ
1068=head2 Localizing changes
1069
1070Perl has a very handy construction
1071
1072 {
1073 local $var = 2;
1074 ...
1075 }
1076
1077This construction is I<approximately> equivalent to
1078
1079 {
1080 my $oldvar = $var;
1081 $var = 2;
1082 ...
1083 $var = $oldvar;
1084 }
1085
1086The biggest difference is that the first construction would
1087reinstate the initial value of $var, irrespective of how control exits
1088the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit
1089more efficient as well.
1090
1091There is a way to achieve a similar task from C via Perl API: create a
1092I<pseudo-block>, and arrange for some changes to be automatically
1093undone at the end of it, either explicit, or via a non-local exit (via
1094die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1095C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1096Such a construct may be created specially for some important localized
1097task, or an existing one (like boundaries of enclosing Perl
1098subroutine/block, or an existing pair for freeing TMPs) may be
1099used. (In the second case the overhead of additional localization must
1100be almost negligible.) Note that any XSUB is automatically enclosed in
1101an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1102
1103Inside such a I<pseudo-block> the following service is available:
1104
13a2d996 1105=over 4
d1c897a1
IZ
1106
1107=item C<SAVEINT(int i)>
1108
1109=item C<SAVEIV(IV i)>
1110
1111=item C<SAVEI32(I32 i)>
1112
1113=item C<SAVELONG(long i)>
1114
1115These macros arrange things to restore the value of integer variable
1116C<i> at the end of enclosing I<pseudo-block>.
1117
1118=item C<SAVESPTR(s)>
1119
1120=item C<SAVEPPTR(p)>
1121
1122These macros arrange things to restore the value of pointers C<s> and
1123C<p>. C<s> must be a pointer of a type which survives conversion to
1124C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1125and back.
1126
1127=item C<SAVEFREESV(SV *sv)>
1128
1129The refcount of C<sv> would be decremented at the end of
1130I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be
1131used instead.
1132
1133=item C<SAVEFREEOP(OP *op)>
1134
1135The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1136
1137=item C<SAVEFREEPV(p)>
1138
1139The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1140end of I<pseudo-block>.
1141
1142=item C<SAVECLEARSV(SV *sv)>
1143
1144Clears a slot in the current scratchpad which corresponds to C<sv> at
1145the end of I<pseudo-block>.
1146
1147=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1148
1149The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1150string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1151short-lived storage, the corresponding string may be reallocated like
1152this:
1153
9cde0e7f 1154 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1155
c76ac1ee 1156=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1157
1158At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1159only argument C<p>.
1160
1161=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1162
1163At the end of I<pseudo-block> the function C<f> is called with the
1164implicit context argument (if any), and C<p>.
d1c897a1
IZ
1165
1166=item C<SAVESTACK_POS()>
1167
1168The current offset on the Perl internal stack (cf. C<SP>) is restored
1169at the end of I<pseudo-block>.
1170
1171=back
1172
1173The following API list contains functions, thus one needs to
1174provide pointers to the modifiable data explicitly (either C pointers,
1175or Perlish C<GV *>s). Where the above macros take C<int>, a similar
1176function takes C<int *>.
1177
13a2d996 1178=over 4
d1c897a1
IZ
1179
1180=item C<SV* save_scalar(GV *gv)>
1181
1182Equivalent to Perl code C<local $gv>.
1183
1184=item C<AV* save_ary(GV *gv)>
1185
1186=item C<HV* save_hash(GV *gv)>
1187
1188Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1189
1190=item C<void save_item(SV *item)>
1191
1192Duplicates the current value of C<SV>, on the exit from the current
1193C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1194using the stored value.
1195
1196=item C<void save_list(SV **sarg, I32 maxsarg)>
1197
1198A variant of C<save_item> which takes multiple arguments via an array
1199C<sarg> of C<SV*> of length C<maxsarg>.
1200
1201=item C<SV* save_svref(SV **sptr)>
1202
1203Similar to C<save_scalar>, but will reinstate a C<SV *>.
1204
1205=item C<void save_aptr(AV **aptr)>
1206
1207=item C<void save_hptr(HV **hptr)>
1208
1209Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1210
1211=back
1212
1213The C<Alias> module implements localization of the basic types within the
1214I<caller's scope>. People who are interested in how to localize things in
1215the containing scope should take a look there too.
1216
0a753a76 1217=head1 Subroutines
a0d0e21e 1218
68dc0745 1219=head2 XSUBs and the Argument Stack
5f05dabc
PP
1220
1221The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1222An XSUB routine will have a stack that contains the arguments from the Perl
1223program, and a way to map from the Perl data structures to a C equivalent.
1224
1225The stack arguments are accessible through the C<ST(n)> macro, which returns
1226the C<n>'th stack argument. Argument 0 is the first argument passed in the
1227Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1228an C<SV*> is used.
1229
1230Most of the time, output from the C routine can be handled through use of
1231the RETVAL and OUTPUT directives. However, there are some cases where the
1232argument stack is not already long enough to handle all the return values.
1233An example is the POSIX tzname() call, which takes no arguments, but returns
1234two, the local time zone's standard and summer time abbreviations.
1235
1236To handle this situation, the PPCODE directive is used and the stack is
1237extended using the macro:
1238
924508f0 1239 EXTEND(SP, num);
5f05dabc 1240
924508f0
GS
1241where C<SP> is the macro that represents the local copy of the stack pointer,
1242and C<num> is the number of elements the stack should be extended by.
5f05dabc
PP
1243
1244Now that there is room on the stack, values can be pushed on it using the
54310121 1245macros to push IVs, doubles, strings, and SV pointers respectively:
5f05dabc
PP
1246
1247 PUSHi(IV)
1248 PUSHn(double)
1249 PUSHp(char*, I32)
1250 PUSHs(SV*)
1251
1252And now the Perl program calling C<tzname>, the two values will be assigned
1253as in:
1254
1255 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1256
1257An alternate (and possibly simpler) method to pushing values on the stack is
1258to use the macros:
1259
1260 XPUSHi(IV)
1261 XPUSHn(double)
1262 XPUSHp(char*, I32)
1263 XPUSHs(SV*)
1264
1265These macros automatically adjust the stack for you, if needed. Thus, you
1266do not need to call C<EXTEND> to extend the stack.
1bd1c0d5 1267However, see L</Putting a C value on Perl stack>
5f05dabc
PP
1268
1269For more information, consult L<perlxs> and L<perlxstut>.
1270
1271=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1272
1273There are four routines that can be used to call a Perl subroutine from
1274within a C program. These four are:
1275
954c1994
GS
1276 I32 call_sv(SV*, I32);
1277 I32 call_pv(const char*, I32);
1278 I32 call_method(const char*, I32);
1279 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1280
954c1994 1281The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1282contains either the name of the Perl subroutine to be called, or a
1283reference to the subroutine. The second argument consists of flags
1284that control the context in which the subroutine is called, whether
1285or not the subroutine is being passed arguments, how errors should be
1286trapped, and how to treat return values.
a0d0e21e
LW
1287
1288All four routines return the number of arguments that the subroutine returned
1289on the Perl stack.
1290
954c1994
GS
1291These routines used to be called C<perl_call_sv> etc., before Perl v5.6.0,
1292but those names are now deprecated; macros of the same name are provided for
1293compatibility.
1294
1295When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1296must manipulate the Perl stack. These include the following macros and
1297functions:
a0d0e21e
LW
1298
1299 dSP
924508f0 1300 SP
a0d0e21e
LW
1301 PUSHMARK()
1302 PUTBACK
1303 SPAGAIN
1304 ENTER
1305 SAVETMPS
1306 FREETMPS
1307 LEAVE
1308 XPUSH*()
cb1a09d0 1309 POP*()
a0d0e21e 1310
5f05dabc
PP
1311For a detailed description of calling conventions from C to Perl,
1312consult L<perlcall>.
a0d0e21e 1313
5f05dabc 1314=head2 Memory Allocation
a0d0e21e 1315
86058a2d
GS
1316All memory meant to be used with the Perl API functions should be manipulated
1317using the macros described in this section. The macros provide the necessary
1318transparency between differences in the actual malloc implementation that is
1319used within perl.
1320
1321It is suggested that you enable the version of malloc that is distributed
5f05dabc 1322with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1323order to satisfy allocation requests more quickly. However, on some
1324platforms, it may cause spurious malloc or free errors.
d1b91892
AD
1325
1326 New(x, pointer, number, type);
1327 Newc(x, pointer, number, type, cast);
1328 Newz(x, pointer, number, type);
1329
07fa94a1 1330These three macros are used to initially allocate memory.
5f05dabc
PP
1331
1332The first argument C<x> was a "magic cookie" that was used to keep track
1333of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1334the current code makes no use of this feature (most Perl developers now
1335use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1336
1337The second argument C<pointer> should be the name of a variable that will
1338point to the newly allocated memory.
d1b91892 1339
d1b91892
AD
1340The third and fourth arguments C<number> and C<type> specify how many of
1341the specified type of data structure should be allocated. The argument
1342C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1343should be used if the C<pointer> argument is different from the C<type>
1344argument.
1345
1346Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1347to zero out all the newly allocated memory.
1348
1349 Renew(pointer, number, type);
1350 Renewc(pointer, number, type, cast);
1351 Safefree(pointer)
1352
1353These three macros are used to change a memory buffer size or to free a
1354piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1355match those of C<New> and C<Newc> with the exception of not needing the
1356"magic cookie" argument.
1357
1358 Move(source, dest, number, type);
1359 Copy(source, dest, number, type);
1360 Zero(dest, number, type);
1361
1362These three macros are used to move, copy, or zero out previously allocated
1363memory. The C<source> and C<dest> arguments point to the source and
1364destination starting points. Perl will move, copy, or zero out C<number>
1365instances of the size of the C<type> data structure (using the C<sizeof>
1366function).
a0d0e21e 1367
5f05dabc 1368=head2 PerlIO
ce3d39e2 1369
5f05dabc
PP
1370The most recent development releases of Perl has been experimenting with
1371removing Perl's dependency on the "normal" standard I/O suite and allowing
1372other stdio implementations to be used. This involves creating a new
1373abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1374was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1375abstraction layer and not make any assumptions about what kind of stdio
1376is being used.
1377
1378For a complete description of the PerlIO abstraction, consult L<perlapio>.
1379
8ebc5c01 1380=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1381
1382A lot of opcodes (this is an elementary operation in the internal perl
1383stack machine) put an SV* on the stack. However, as an optimization
1384the corresponding SV is (usually) not recreated each time. The opcodes
1385reuse specially assigned SVs (I<target>s) which are (as a corollary)
1386not constantly freed/created.
1387
0a753a76 1388Each of the targets is created only once (but see
ce3d39e2
IZ
1389L<Scratchpads and recursion> below), and when an opcode needs to put
1390an integer, a double, or a string on stack, it just sets the
1391corresponding parts of its I<target> and puts the I<target> on stack.
1392
1393The macro to put this target on stack is C<PUSHTARG>, and it is
1394directly used in some opcodes, as well as indirectly in zillions of
1395others, which use it via C<(X)PUSH[pni]>.
1396
1bd1c0d5
SC
1397Because the target is reused, you must be careful when pushing multiple
1398values on the stack. The following code will not do what you think:
1399
1400 XPUSHi(10);
1401 XPUSHi(20);
1402
1403This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1404the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1405At the end of the operation, the stack does not contain the values 10
1406and 20, but actually contains two pointers to C<TARG>, which we have set
1407to 20. If you need to push multiple different values, use C<XPUSHs>,
1408which bypasses C<TARG>.
1409
1410On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
1411need a C<dTARG> in your variable declarations so that the C<*PUSH*>
1412macros can make use of the local variable C<TARG>.
1413
8ebc5c01 1414=head2 Scratchpads
ce3d39e2 1415
54310121 1416The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1417are created. The answer is that they are created when the current unit --
1418a subroutine or a file (for opcodes for statements outside of
1419subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1420array is created, which is called a scratchpad for the current
1421unit.
1422
54310121 1423A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1424targets for opcodes. One can deduce that an SV lives on a scratchpad
1425by looking on its flags: lexicals have C<SVs_PADMY> set, and
1426I<target>s have C<SVs_PADTMP> set.
1427
54310121
PP
1428The correspondence between OPs and I<target>s is not 1-to-1. Different
1429OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1430would not conflict with the expected life of the temporary.
1431
2ae324a7 1432=head2 Scratchpads and recursion
ce3d39e2
IZ
1433
1434In fact it is not 100% true that a compiled unit contains a pointer to
1435the scratchpad AV. In fact it contains a pointer to an AV of
1436(initially) one element, and this element is the scratchpad AV. Why do
1437we need an extra level of indirection?
1438
1439The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both
1440these can create several execution pointers going into the same
1441subroutine. For the subroutine-child not write over the temporaries
1442for the subroutine-parent (lifespan of which covers the call to the
1443child), the parent and the child should have different
1444scratchpads. (I<And> the lexicals should be separate anyway!)
1445
5f05dabc
PP
1446So each subroutine is born with an array of scratchpads (of length 1).
1447On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1448depth of the recursion is not more than the length of this array, and
1449if it is, new scratchpad is created and pushed into the array.
1450
1451The I<target>s on this scratchpad are C<undef>s, but they are already
1452marked with correct flags.
1453
0a753a76
PP
1454=head1 Compiled code
1455
1456=head2 Code tree
1457
1458Here we describe the internal form your code is converted to by
1459Perl. Start with a simple example:
1460
1461 $a = $b + $c;
1462
1463This is converted to a tree similar to this one:
1464
1465 assign-to
1466 / \
1467 + $a
1468 / \
1469 $b $c
1470
7b8d334a 1471(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1472parsed your code, but has nothing to do with the execution order.
1473There is an additional "thread" going through the nodes of the tree
1474which shows the order of execution of the nodes. In our simplified
1475example above it looks like:
1476
1477 $b ---> $c ---> + ---> $a ---> assign-to
1478
1479But with the actual compile tree for C<$a = $b + $c> it is different:
1480some nodes I<optimized away>. As a corollary, though the actual tree
1481contains more nodes than our simplified example, the execution order
1482is the same as in our example.
1483
1484=head2 Examining the tree
1485
1486If you have your perl compiled for debugging (usually done with C<-D
1487optimize=-g> on C<Configure> command line), you may examine the
1488compiled tree by specifying C<-Dx> on the Perl command line. The
1489output takes several lines per node, and for C<$b+$c> it looks like
1490this:
1491
1492 5 TYPE = add ===> 6
1493 TARG = 1
1494 FLAGS = (SCALAR,KIDS)
1495 {
1496 TYPE = null ===> (4)
1497 (was rv2sv)
1498 FLAGS = (SCALAR,KIDS)
1499 {
1500 3 TYPE = gvsv ===> 4
1501 FLAGS = (SCALAR)
1502 GV = main::b
1503 }
1504 }
1505 {
1506 TYPE = null ===> (5)
1507 (was rv2sv)
1508 FLAGS = (SCALAR,KIDS)
1509 {
1510 4 TYPE = gvsv ===> 5
1511 FLAGS = (SCALAR)
1512 GV = main::c
1513 }
1514 }
1515
1516This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1517not optimized away (one per number in the left column). The immediate
1518children of the given node correspond to C<{}> pairs on the same level
1519of indentation, thus this listing corresponds to the tree:
1520
1521 add
1522 / \
1523 null null
1524 | |
1525 gvsv gvsv
1526
1527The execution order is indicated by C<===E<gt>> marks, thus it is C<3
15284 5 6> (node C<6> is not included into above listing), i.e.,
1529C<gvsv gvsv add whatever>.
1530
9afa14e3
SC
1531Each of these nodes represents an op, a fundamental operation inside the
1532Perl core. The code which implements each operation can be found in the
1533F<pp*.c> files; the function which implements the op with type C<gvsv>
1534is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1535different numbers of children: C<add> is a binary operator, as one would
1536expect, and so has two children. To accommodate the various different
1537numbers of children, there are various types of op data structure, and
1538they link together in different ways.
1539
1540The simplest type of op structure is C<OP>: this has no children. Unary
1541operators, C<UNOP>s, have one child, and this is pointed to by the
1542C<op_first> field. Binary operators (C<BINOP>s) have not only an
1543C<op_first> field but also an C<op_last> field. The most complex type of
1544op is a C<LISTOP>, which has any number of children. In this case, the
1545first child is pointed to by C<op_first> and the last child by
1546C<op_last>. The children in between can be found by iteratively
1547following the C<op_sibling> pointer from the first child to the last.
1548
1549There are also two other op types: a C<PMOP> holds a regular expression,
1550and has no children, and a C<LOOP> may or may not have children. If the
1551C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1552complicate matters, if a C<UNOP> is actually a C<null> op after
1553optimization (see L</Compile pass 2: context propagation>) it will still
1554have children in accordance with its former type.
1555
0a753a76
PP
1556=head2 Compile pass 1: check routines
1557
8870b5c7
GS
1558The tree is created by the compiler while I<yacc> code feeds it
1559the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1560the first pass of perl compilation.
1561
1562What makes this pass interesting for perl developers is that some
1563optimization may be performed on this pass. This is optimization by
8870b5c7 1564so-called "check routines". The correspondence between node names
0a753a76
PP
1565and corresponding check routines is described in F<opcode.pl> (do not
1566forget to run C<make regen_headers> if you modify this file).
1567
1568A check routine is called when the node is fully constructed except
7b8d334a 1569for the execution-order thread. Since at this time there are no
0a753a76
PP
1570back-links to the currently constructed node, one can do most any
1571operation to the top-level node, including freeing it and/or creating
1572new nodes above/below it.
1573
1574The check routine returns the node which should be inserted into the
1575tree (if the top-level node was not modified, check routine returns
1576its argument).
1577
1578By convention, check routines have names C<ck_*>. They are usually
1579called from C<new*OP> subroutines (or C<convert>) (which in turn are
1580called from F<perly.y>).
1581
1582=head2 Compile pass 1a: constant folding
1583
1584Immediately after the check routine is called the returned node is
1585checked for being compile-time executable. If it is (the value is
1586judged to be constant) it is immediately executed, and a I<constant>
1587node with the "return value" of the corresponding subtree is
1588substituted instead. The subtree is deleted.
1589
1590If constant folding was not performed, the execution-order thread is
1591created.
1592
1593=head2 Compile pass 2: context propagation
1594
1595When a context for a part of compile tree is known, it is propagated
a3cb178b 1596down through the tree. At this time the context can have 5 values
0a753a76
PP
1597(instead of 2 for runtime context): void, boolean, scalar, list, and
1598lvalue. In contrast with the pass 1 this pass is processed from top
1599to bottom: a node's context determines the context for its children.
1600
1601Additional context-dependent optimizations are performed at this time.
1602Since at this moment the compile tree contains back-references (via
1603"thread" pointers), nodes cannot be free()d now. To allow
1604optimized-away nodes at this stage, such nodes are null()ified instead
1605of free()ing (i.e. their type is changed to OP_NULL).
1606
1607=head2 Compile pass 3: peephole optimization
1608
1609After the compile tree for a subroutine (or for an C<eval> or a file)
1610is created, an additional pass over the code is performed. This pass
1611is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1612additional complications for conditionals). These optimizations are
0a753a76
PP
1613done in the subroutine peep(). Optimizations performed at this stage
1614are subject to the same restrictions as in the pass 2.
1615
9afa14e3
SC
1616=head1 Examining internal data structures with the C<dump> functions
1617
1618To aid debugging, the source file F<dump.c> contains a number of
1619functions which produce formatted output of internal data structures.
1620
1621The most commonly used of these functions is C<Perl_sv_dump>; it's used
1622for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1623C<sv_dump> to produce debugging output from Perl-space, so users of that
1624module should already be familiar with its format.
1625
1626C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
1627derivatives, and produces output similiar to C<perl -Dx>; in fact,
1628C<Perl_dump_eval> will dump the main root of the code being evaluated,
1629exactly like C<-Dx>.
1630
1631Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1632op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1633subroutines in a package like so: (Thankfully, these are all xsubs, so
1634there is no op tree)
1635
1636 (gdb) print Perl_dump_packsubs(PL_defstash)
1637
1638 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1639
1640 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1641
1642 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1643
1644 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1645
1646 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1647
1648and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1649the op tree of the main root.
1650
954c1994 1651=head1 How multiple interpreters and concurrency are supported
ee072b34 1652
ee072b34
GS
1653=head2 Background and PERL_IMPLICIT_CONTEXT
1654
1655The Perl interpreter can be regarded as a closed box: it has an API
1656for feeding it code or otherwise making it do things, but it also has
1657functions for its own use. This smells a lot like an object, and
1658there are ways for you to build Perl so that you can have multiple
1659interpreters, with one interpreter represented either as a C++ object,
1660a C structure, or inside a thread. The thread, the C structure, or
1661the C++ object will contain all the context, the state of that
1662interpreter.
1663
54aff467
GS
1664Three macros control the major Perl build flavors: MULTIPLICITY,
1665USE_THREADS and PERL_OBJECT. The MULTIPLICITY build has a C structure
1666that packages all the interpreter state, there is a similar thread-specific
1667data structure under USE_THREADS, and the PERL_OBJECT build has a C++
1668class to maintain interpreter state. In all three cases,
1669PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1670support for passing in a "hidden" first argument that represents all three
651a3225 1671data structures.
54aff467
GS
1672
1673All this obviously requires a way for the Perl internal functions to be
ee072b34
GS
1674C++ methods, subroutines taking some kind of structure as the first
1675argument, or subroutines taking nothing as the first argument. To
1676enable these three very different ways of building the interpreter,
1677the Perl source (as it does in so many other situations) makes heavy
1678use of macros and subroutine naming conventions.
1679
54aff467 1680First problem: deciding which functions will be public API functions and
954c1994
GS
1681which will be private. All functions whose names begin C<S_> are private
1682(think "S" for "secret" or "static"). All other functions begin with
1683"Perl_", but just because a function begins with "Perl_" does not mean it is
a422fd2d
SC
1684part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1685function is part of the API is to find its entry in L<perlapi>.
1686If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1687think it should be (i.e., you need it for your extension), send mail via
1688L<perlbug> explaining why you think it should be.
ee072b34
GS
1689
1690Second problem: there must be a syntax so that the same subroutine
1691declarations and calls can pass a structure as their first argument,
1692or pass nothing. To solve this, the subroutines are named and
1693declared in a particular way. Here's a typical start of a static
1694function used within the Perl guts:
1695
1696 STATIC void
1697 S_incline(pTHX_ char *s)
1698
1699STATIC becomes "static" in C, and is #define'd to nothing in C++.
1700
651a3225
GS
1701A public function (i.e. part of the internal API, but not necessarily
1702sanctioned for use in extensions) begins like this:
ee072b34
GS
1703
1704 void
1705 Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)
1706
1707C<pTHX_> is one of a number of macros (in perl.h) that hide the
1708details of the interpreter's context. THX stands for "thread", "this",
1709or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1710The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
1711or 'd' for B<d>eclaration.
1712
1713When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first
1714argument containing the interpreter's context. The trailing underscore
1715in the pTHX_ macro indicates that the macro expansion needs a comma
1716after the context argument because other arguments follow it. If
1717PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1718subroutine is not prototyped to take the extra argument. The form of the
1719macro without the trailing underscore is used when there are no additional
ee072b34
GS
1720explicit arguments.
1721
54aff467 1722When a core function calls another, it must pass the context. This
ee072b34
GS
1723is normally hidden via macros. Consider C<sv_setsv>. It expands
1724something like this:
1725
1726 ifdef PERL_IMPLICIT_CONTEXT
c52f9dcd 1727 define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
ee072b34
GS
1728 /* can't do this for vararg functions, see below */
1729 else
c52f9dcd 1730 define sv_setsv Perl_sv_setsv
ee072b34
GS
1731 endif
1732
1733This works well, and means that XS authors can gleefully write:
1734
1735 sv_setsv(foo, bar);
1736
1737and still have it work under all the modes Perl could have been
1738compiled with.
1739
1740Under PERL_OBJECT in the core, that will translate to either:
1741
1742 CPerlObj::Perl_sv_setsv(foo,bar); # in CPerlObj functions,
1743 # C++ takes care of 'this'
1744 or
1745
1746 pPerl->Perl_sv_setsv(foo,bar); # in truly static functions,
1747 # see objXSUB.h
1748
1749Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
1750MULTIPLICITY/USE_THREADS w/ PERL_IMPLICIT_CONTEXT in both core
1751and extensions, it will be:
1752
1753 Perl_sv_setsv(aTHX_ foo, bar); # the canonical Perl "API"
1754 # for all build flavors
1755
1756This doesn't work so cleanly for varargs functions, though, as macros
1757imply that the number of arguments is known in advance. Instead we
1758either need to spell them out fully, passing C<aTHX_> as the first
1759argument (the Perl core tends to do this with functions like
1760Perl_warner), or use a context-free version.
1761
1762The context-free version of Perl_warner is called
1763Perl_warner_nocontext, and does not take the extra argument. Instead
1764it does dTHX; to get the context from thread-local storage. We
1765C<#define warner Perl_warner_nocontext> so that extensions get source
1766compatibility at the expense of performance. (Passing an arg is
1767cheaper than grabbing it from thread-local storage.)
1768
1769You can ignore [pad]THX[xo] when browsing the Perl headers/sources.
1770Those are strictly for use within the core. Extensions and embedders
1771need only be aware of [pad]THX.
1772
1773=head2 How do I use all this in extensions?
1774
1775When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1776any functions in the Perl API will need to pass the initial context
1777argument somehow. The kicker is that you will need to write it in
1778such a way that the extension still compiles when Perl hasn't been
1779built with PERL_IMPLICIT_CONTEXT enabled.
1780
1781There are three ways to do this. First, the easy but inefficient way,
1782which is also the default, in order to maintain source compatibility
1783with extensions: whenever XSUB.h is #included, it redefines the aTHX
1784and aTHX_ macros to call a function that will return the context.
1785Thus, something like:
1786
1787 sv_setsv(asv, bsv);
1788
4375e838 1789in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1790in effect:
ee072b34 1791
2fa86c13 1792 Perl_sv_setsv(Perl_get_context(), asv, bsv);
ee072b34 1793
54aff467 1794or to this otherwise:
ee072b34
GS
1795
1796 Perl_sv_setsv(asv, bsv);
1797
1798You have to do nothing new in your extension to get this; since
2fa86c13 1799the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1800work.
1801
1802The second, more efficient way is to use the following template for
1803your Foo.xs:
1804
c52f9dcd
JH
1805 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1806 #include "EXTERN.h"
1807 #include "perl.h"
1808 #include "XSUB.h"
ee072b34
GS
1809
1810 static my_private_function(int arg1, int arg2);
1811
c52f9dcd
JH
1812 static SV *
1813 my_private_function(int arg1, int arg2)
1814 {
1815 dTHX; /* fetch context */
1816 ... call many Perl API functions ...
1817 }
ee072b34
GS
1818
1819 [... etc ...]
1820
c52f9dcd 1821 MODULE = Foo PACKAGE = Foo
ee072b34 1822
c52f9dcd 1823 /* typical XSUB */
ee072b34 1824
c52f9dcd
JH
1825 void
1826 my_xsub(arg)
1827 int arg
1828 CODE:
1829 my_private_function(arg, 10);
ee072b34
GS
1830
1831Note that the only two changes from the normal way of writing an
1832extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
1833including the Perl headers, followed by a C<dTHX;> declaration at
1834the start of every function that will call the Perl API. (You'll
1835know which functions need this, because the C compiler will complain
1836that there's an undeclared identifier in those functions.) No changes
1837are needed for the XSUBs themselves, because the XS() macro is
1838correctly defined to pass in the implicit context if needed.
1839
1840The third, even more efficient way is to ape how it is done within
1841the Perl guts:
1842
1843
c52f9dcd
JH
1844 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1845 #include "EXTERN.h"
1846 #include "perl.h"
1847 #include "XSUB.h"
ee072b34
GS
1848
1849 /* pTHX_ only needed for functions that call Perl API */
1850 static my_private_function(pTHX_ int arg1, int arg2);
1851
c52f9dcd
JH
1852 static SV *
1853 my_private_function(pTHX_ int arg1, int arg2)
1854 {
1855 /* dTHX; not needed here, because THX is an argument */
1856 ... call Perl API functions ...
1857 }
ee072b34
GS
1858
1859 [... etc ...]
1860
c52f9dcd 1861 MODULE = Foo PACKAGE = Foo
ee072b34 1862
c52f9dcd 1863 /* typical XSUB */
ee072b34 1864
c52f9dcd
JH
1865 void
1866 my_xsub(arg)
1867 int arg
1868 CODE:
1869 my_private_function(aTHX_ arg, 10);
ee072b34
GS
1870
1871This implementation never has to fetch the context using a function
1872call, since it is always passed as an extra argument. Depending on
1873your needs for simplicity or efficiency, you may mix the previous
1874two approaches freely.
1875
651a3225
GS
1876Never add a comma after C<pTHX> yourself--always use the form of the
1877macro with the underscore for functions that take explicit arguments,
1878or the form without the argument for functions with no explicit arguments.
ee072b34
GS
1879
1880=head2 Future Plans and PERL_IMPLICIT_SYS
1881
1882Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
1883that the interpreter knows about itself and pass it around, so too are
1884there plans to allow the interpreter to bundle up everything it knows
1885about the environment it's running on. This is enabled with the
1886PERL_IMPLICIT_SYS macro. Currently it only works with PERL_OBJECT,
1887but is mostly there for MULTIPLICITY and USE_THREADS (see inside
1888iperlsys.h).
1889
1890This allows the ability to provide an extra pointer (called the "host"
1891environment) for all the system calls. This makes it possible for
1892all the system stuff to maintain their own state, broken down into
1893seven C structures. These are thin wrappers around the usual system
1894calls (see win32/perllib.c) for the default perl executable, but for a
1895more ambitious host (like the one that would do fork() emulation) all
1896the extra work needed to pretend that different interpreters are
1897actually different "processes", would be done here.
1898
1899The Perl engine/interpreter and the host are orthogonal entities.
1900There could be one or more interpreters in a process, and one or
1901more "hosts", with free association between them.
1902
a422fd2d
SC
1903=head1 Internal Functions
1904
1905All of Perl's internal functions which will be exposed to the outside
1906world are be prefixed by C<Perl_> so that they will not conflict with XS
1907functions or functions used in a program in which Perl is embedded.
1908Similarly, all global variables begin with C<PL_>. (By convention,
1909static functions start with C<S_>)
1910
1911Inside the Perl core, you can get at the functions either with or
1912without the C<Perl_> prefix, thanks to a bunch of defines that live in
1913F<embed.h>. This header file is generated automatically from
1914F<embed.pl>. F<embed.pl> also creates the prototyping header files for
1915the internal functions, generates the documentation and a lot of other
1916bits and pieces. It's important that when you add a new function to the
1917core or change an existing one, you change the data in the table at the
1918end of F<embed.pl> as well. Here's a sample entry from that table:
1919
1920 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
1921
1922The second column is the return type, the third column the name. Columns
1923after that are the arguments. The first column is a set of flags:
1924
1925=over 3
1926
1927=item A
1928
1929This function is a part of the public API.
1930
1931=item p
1932
1933This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
1934
1935=item d
1936
1937This function has documentation using the C<apidoc> feature which we'll
1938look at in a second.
1939
1940=back
1941
1942Other available flags are:
1943
1944=over 3
1945
1946=item s
1947
1948This is a static function and is defined as C<S_whatever>.
1949
1950=item n
1951
1952This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
1953L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
1954
1955=item r
1956
1957This function never returns; C<croak>, C<exit> and friends.
1958
1959=item f
1960
1961This function takes a variable number of arguments, C<printf> style.
1962The argument list should end with C<...>, like this:
1963
1964 Afprd |void |croak |const char* pat|...
1965
1966=item m
1967
1968This function is part of the experimental development API, and may change
1969or disappear without notice.
1970
1971=item o
1972
1973This function should not have a compatibility macro to define, say,
1974C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
1975
1976=item j
1977
1978This function is not a member of C<CPerlObj>. If you don't know
1979what this means, don't use it.
1980
1981=item x
1982
1983This function isn't exported out of the Perl core.
1984
1985=back
1986
1987If you edit F<embed.pl>, you will need to run C<make regen_headers> to
1988force a rebuild of F<embed.h> and other auto-generated files.
1989
6b4667fc 1990=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 1991
6b4667fc
A
1992If you are printing IVs, UVs, or NVS instead of the stdio(3) style
1993formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
1994following macros for portability
9dd9db0b 1995
c52f9dcd
JH
1996 IVdf IV in decimal
1997 UVuf UV in decimal
1998 UVof UV in octal
1999 UVxf UV in hexadecimal
2000 NVef NV %e-like
2001 NVff NV %f-like
2002 NVgf NV %g-like
9dd9db0b 2003
6b4667fc
A
2004These will take care of 64-bit integers and long doubles.
2005For example:
2006
c52f9dcd 2007 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2008
2009The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2010
8908e76d
JH
2011If you are printing addresses of pointers, use UVxf combined
2012with PTR2UV(), do not use %lx or %p.
2013
2014=head2 Pointer-To-Integer and Integer-To-Pointer
2015
2016Because pointer size does not necessarily equal integer size,
2017use the follow macros to do it right.
2018
c52f9dcd
JH
2019 PTR2UV(pointer)
2020 PTR2IV(pointer)
2021 PTR2NV(pointer)
2022 INT2PTR(pointertotype, integer)
8908e76d
JH
2023
2024For example:
2025
c52f9dcd
JH
2026 IV iv = ...;
2027 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2028
2029and
2030
c52f9dcd
JH
2031 AV *av = ...;
2032 UV uv = PTR2UV(av);
8908e76d 2033
a422fd2d
SC
2034=head2 Source Documentation
2035
2036There's an effort going on to document the internal functions and
2037automatically produce reference manuals from them - L<perlapi> is one
2038such manual which details all the functions which are available to XS
2039writers. L<perlintern> is the autogenerated manual for the functions
2040which are not part of the API and are supposedly for internal use only.
2041
2042Source documentation is created by putting POD comments into the C
2043source, like this:
2044
2045 /*
2046 =for apidoc sv_setiv
2047
2048 Copies an integer into the given SV. Does not handle 'set' magic. See
2049 C<sv_setiv_mg>.
2050
2051 =cut
2052 */
2053
2054Please try and supply some documentation if you add functions to the
2055Perl core.
2056
2057=head1 Unicode Support
2058
2059Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2060writers to understand this support and make sure that the code they
2061write does not corrupt Unicode data.
2062
2063=head2 What B<is> Unicode, anyway?
2064
2065In the olden, less enlightened times, we all used to use ASCII. Most of
2066us did, anyway. The big problem with ASCII is that it's American. Well,
2067no, that's not actually the problem; the problem is that it's not
2068particularly useful for people who don't use the Roman alphabet. What
2069used to happen was that particular languages would stick their own
2070alphabet in the upper range of the sequence, between 128 and 255. Of
2071course, we then ended up with plenty of variants that weren't quite
2072ASCII, and the whole point of it being a standard was lost.
2073
2074Worse still, if you've got a language like Chinese or
2075Japanese that has hundreds or thousands of characters, then you really
2076can't fit them into a mere 256, so they had to forget about ASCII
2077altogether, and build their own systems using pairs of numbers to refer
2078to one character.
2079
2080To fix this, some people formed Unicode, Inc. and
2081produced a new character set containing all the characters you can
2082possibly think of and more. There are several ways of representing these
2083characters, and the one Perl uses is called UTF8. UTF8 uses
2084a variable number of bytes to represent a character, instead of just
b3b6085d 2085one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d
SC
2086
2087=head2 How can I recognise a UTF8 string?
2088
2089You can't. This is because UTF8 data is stored in bytes just like
2090non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
2091capital E with a grave accent, is represented by the two bytes
2092C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2093has that byte sequence as well. So you can't tell just by looking - this
2094is what makes Unicode input an interesting problem.
2095
2096The API function C<is_utf8_string> can help; it'll tell you if a string
2097contains only valid UTF8 characters. However, it can't do the work for
2098you. On a character-by-character basis, C<is_utf8_char> will tell you
2099whether the current character in a string is valid UTF8.
2100
2101=head2 How does UTF8 represent Unicode characters?
2102
2103As mentioned above, UTF8 uses a variable number of bytes to store a
2104character. Characters with values 1...128 are stored in one byte, just
2105like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2106continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2107bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2108so it goes on, moving to three bytes at character 2048.
2109
2110Assuming you know you're dealing with a UTF8 string, you can find out
2111how long the first character in it is with the C<UTF8SKIP> macro:
2112
2113 char *utf = "\305\233\340\240\201";
2114 I32 len;
2115
2116 len = UTF8SKIP(utf); /* len is 2 here */
2117 utf += len;
2118 len = UTF8SKIP(utf); /* len is 3 here */
2119
2120Another way to skip over characters in a UTF8 string is to use
2121C<utf8_hop>, which takes a string and a number of characters to skip
2122over. You're on your own about bounds checking, though, so don't use it
2123lightly.
2124
2125All bytes in a multi-byte UTF8 character will have the high bit set, so
2126you can test if you need to do something special with this character
2127like this:
2128
2129 UV uv;
2130
2131 if (utf & 0x80)
2132 /* Must treat this as UTF8 */
2133 uv = utf8_to_uv(utf);
2134 else
2135 /* OK to treat this character as a byte */
2136 uv = *utf;
2137
2138You can also see in that example that we use C<utf8_to_uv> to get the
2139value of the character; the inverse function C<uv_to_utf8> is available
2140for putting a UV into UTF8:
2141
2142 if (uv > 0x80)
2143 /* Must treat this as UTF8 */
2144 utf8 = uv_to_utf8(utf8, uv);
2145 else
2146 /* OK to treat this character as a byte */
2147 *utf8++ = uv;
2148
2149You B<must> convert characters to UVs using the above functions if
2150you're ever in a situation where you have to match UTF8 and non-UTF8
2151characters. You may not skip over UTF8 characters in this case. If you
2152do this, you'll lose the ability to match hi-bit non-UTF8 characters;
2153for instance, if your UTF8 string contains C<v196.172>, and you skip
2154that character, you can never match a C<chr(200)> in a non-UTF8 string.
2155So don't do that!
2156
2157=head2 How does Perl store UTF8 strings?
2158
2159Currently, Perl deals with Unicode strings and non-Unicode strings
2160slightly differently. If a string has been identified as being UTF-8
2161encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2162manipulate this flag with the following macros:
2163
2164 SvUTF8(sv)
2165 SvUTF8_on(sv)
2166 SvUTF8_off(sv)
2167
2168This flag has an important effect on Perl's treatment of the string: if
2169Unicode data is not properly distinguished, regular expressions,
2170C<length>, C<substr> and other string handling operations will have
2171undesirable results.
2172
2173The problem comes when you have, for instance, a string that isn't
2174flagged is UTF8, and contains a byte sequence that could be UTF8 -
2175especially when combining non-UTF8 and UTF8 strings.
2176
2177Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2178need be sure you don't accidentally knock it off while you're
2179manipulating SVs. More specifically, you cannot expect to do this:
2180
2181 SV *sv;
2182 SV *nsv;
2183 STRLEN len;
2184 char *p;
2185
2186 p = SvPV(sv, len);
2187 frobnicate(p);
2188 nsv = newSVpvn(p, len);
2189
2190The C<char*> string does not tell you the whole story, and you can't
2191copy or reconstruct an SV just by copying the string value. Check if the
2192old SV has the UTF8 flag set, and act accordingly:
2193
2194 p = SvPV(sv, len);
2195 frobnicate(p);
2196 nsv = newSVpvn(p, len);
2197 if (SvUTF8(sv))
2198 SvUTF8_on(nsv);
2199
2200In fact, your C<frobnicate> function should be made aware of whether or
2201not it's dealing with UTF8 data, so that it can handle the string
2202appropriately.
2203
2204=head2 How do I convert a string to UTF8?
2205
2206If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
2207to upgrade one of the strings to UTF8. If you've got an SV, the easiest
2208way to do this is:
2209
2210 sv_utf8_upgrade(sv);
2211
2212However, you must not do this, for example:
2213
2214 if (!SvUTF8(left))
2215 sv_utf8_upgrade(left);
2216
2217If you do this in a binary operator, you will actually change one of the
b1866b2d 2218strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2219by the end user, it can cause problems.
2220
2221Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
2222string argument. This is useful for having the data available for
b1866b2d 2223comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2224C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2225the string contains any characters above 255 that can't be represented
2226in a single byte.
2227
2228=head2 Is there anything else I need to know?
2229
2230Not really. Just remember these things:
2231
2232=over 3
2233
2234=item *
2235
2236There's no way to tell if a string is UTF8 or not. You can tell if an SV
2237is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2238something should be UTF8. Treat the flag as part of the PV, even though
2239it's not - if you pass on the PV to somewhere, pass on the flag too.
2240
2241=item *
2242
2243If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
2244unless C<!(*s & 0x80)> in which case you can use C<*s>.
2245
2246=item *
2247
2248When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
2249C<uv < 0x80> in which case you can use C<*s = uv>.
2250
2251=item *
2252
2253Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
2254a new string which is UTF8 encoded. There are tricks you can use to
2255delay deciding whether you need to use a UTF8 string until you get to a
2256high character - C<HALF_UPGRADE> is one of those.
2257
2258=back
2259
954c1994 2260=head1 AUTHORS
e89caa19 2261
954c1994
GS
2262Until May 1997, this document was maintained by Jeff Okamoto
2263<okamoto@corp.hp.com>. It is now maintained as part of Perl itself
2264by the Perl 5 Porters <perl5-porters@perl.org>.
cb1a09d0 2265
954c1994
GS
2266With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2267Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2268Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2269Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2270
954c1994 2271API Listing originally by Dean Roehrich <roehrich@cray.com>.
cb1a09d0 2272
954c1994
GS
2273Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
2274Stuhl.
cb1a09d0 2275
954c1994 2276=head1 SEE ALSO
cb1a09d0 2277
954c1994 2278perlapi(1), perlintern(1), perlxs(1), perlembed(1)