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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
b3b6085d 7This document attempts to describe how to use the Perl API, as well as
06f6df17 8to provide some info on the basic workings of the Perl core. It is far
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9from complete and probably contains many errors. Please refer any
10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl has three typedefs that handle Perl's three main data types:
17
18 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21
d1b91892 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
954c1994 31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
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32as well.) They will usually be exactly 32 and 16 bits long, but on Crays
33they will both be 64 bits.
a0d0e21e 34
54310121 35=head2 Working with SVs
a0d0e21e 36
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37An SV can be created and loaded with one command. There are five types of
38values that can be loaded: an integer value (IV), an unsigned integer
39value (UV), a double (NV), a string (PV), and another scalar (SV).
a0d0e21e 40
20dbd849 41The seven routines are:
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42
43 SV* newSViv(IV);
20dbd849 44 SV* newSVuv(UV);
a0d0e21e 45 SV* newSVnv(double);
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46 SV* newSVpv(const char*, STRLEN);
47 SV* newSVpvn(const char*, STRLEN);
46fc3d4c 48 SV* newSVpvf(const char*, ...);
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49 SV* newSVsv(SV*);
50
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51C<STRLEN> is an integer type (Size_t, usually defined as size_t in
52F<config.h>) guaranteed to be large enough to represent the size of
53any string that perl can handle.
54
55In the unlikely case of a SV requiring more complex initialisation, you
56can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
57type NULL is returned, else an SV of type PV is returned with len + 1 (for
58the NUL) bytes of storage allocated, accessible via SvPVX. In both cases
59the SV has value undef.
20dbd849 60
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61 SV *sv = newSV(0); /* no storage allocated */
62 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
20dbd849 63
06f6df17 64To change the value of an I<already-existing> SV, there are eight routines:
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65
66 void sv_setiv(SV*, IV);
deb3007b 67 void sv_setuv(SV*, UV);
a0d0e21e 68 void sv_setnv(SV*, double);
08105a92 69 void sv_setpv(SV*, const char*);
06f6df17 70 void sv_setpvn(SV*, const char*, STRLEN)
46fc3d4c 71 void sv_setpvf(SV*, const char*, ...);
5b3218b9 72 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
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73 void sv_setsv(SV*, SV*);
74
75Notice that you can choose to specify the length of the string to be
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76assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
77allow Perl to calculate the length by using C<sv_setpv> or by specifying
780 as the second argument to C<newSVpv>. Be warned, though, that Perl will
79determine the string's length by using C<strlen>, which depends on the
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80string terminating with a NUL character.
81
82The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
83formatted output becomes the value.
84
328bf373 85C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
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86either a pointer to a variable argument list or the address and length of
87an array of SVs. The last argument points to a boolean; on return, if that
88boolean is true, then locale-specific information has been used to format
c2611fb3 89the string, and the string's contents are therefore untrustworthy (see
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90L<perlsec>). This pointer may be NULL if that information is not
91important. Note that this function requires you to specify the length of
92the format.
93
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94The C<sv_set*()> functions are not generic enough to operate on values
95that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 96
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97All SVs that contain strings should be terminated with a NUL character.
98If it is not NUL-terminated there is a risk of
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99core dumps and corruptions from code which passes the string to C
100functions or system calls which expect a NUL-terminated string.
101Perl's own functions typically add a trailing NUL for this reason.
102Nevertheless, you should be very careful when you pass a string stored
103in an SV to a C function or system call.
104
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105To access the actual value that an SV points to, you can use the macros:
106
107 SvIV(SV*)
954c1994 108 SvUV(SV*)
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109 SvNV(SV*)
110 SvPV(SV*, STRLEN len)
1fa8b10d 111 SvPV_nolen(SV*)
a0d0e21e 112
954c1994 113which will automatically coerce the actual scalar type into an IV, UV, double,
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114or string.
115
116In the C<SvPV> macro, the length of the string returned is placed into the
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117variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
118not care what the length of the data is, use the C<SvPV_nolen> macro.
119Historically the C<SvPV> macro with the global variable C<PL_na> has been
120used in this case. But that can be quite inefficient because C<PL_na> must
121be accessed in thread-local storage in threaded Perl. In any case, remember
122that Perl allows arbitrary strings of data that may both contain NULs and
123might not be terminated by a NUL.
a0d0e21e 124
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125Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
126len);>. It might work with your compiler, but it won't work for everyone.
127Break this sort of statement up into separate assignments:
128
b2f5ed49 129 SV *s;
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130 STRLEN len;
131 char * ptr;
b2f5ed49 132 ptr = SvPV(s, len);
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133 foo(ptr, len);
134
07fa94a1 135If you want to know if the scalar value is TRUE, you can use:
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136
137 SvTRUE(SV*)
138
139Although Perl will automatically grow strings for you, if you need to force
140Perl to allocate more memory for your SV, you can use the macro
141
142 SvGROW(SV*, STRLEN newlen)
143
144which will determine if more memory needs to be allocated. If so, it will
145call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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146decrease, the allocated memory of an SV and that it does not automatically
147add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 148C<SvGROW(sv, len + 1)>).
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149
150If you have an SV and want to know what kind of data Perl thinks is stored
151in it, you can use the following macros to check the type of SV you have.
152
153 SvIOK(SV*)
154 SvNOK(SV*)
155 SvPOK(SV*)
156
157You can get and set the current length of the string stored in an SV with
158the following macros:
159
160 SvCUR(SV*)
161 SvCUR_set(SV*, I32 val)
162
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163You can also get a pointer to the end of the string stored in the SV
164with the macro:
165
166 SvEND(SV*)
167
168But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 169
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170If you want to append something to the end of string stored in an C<SV*>,
171you can use the following functions:
172
08105a92 173 void sv_catpv(SV*, const char*);
e65f3abd 174 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 175 void sv_catpvf(SV*, const char*, ...);
328bf373 176 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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177 void sv_catsv(SV*, SV*);
178
179The first function calculates the length of the string to be appended by
180using C<strlen>. In the second, you specify the length of the string
46fc3d4c 181yourself. The third function processes its arguments like C<sprintf> and
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182appends the formatted output. The fourth function works like C<vsprintf>.
183You can specify the address and length of an array of SVs instead of the
184va_list argument. The fifth function extends the string stored in the first
185SV with the string stored in the second SV. It also forces the second SV
186to be interpreted as a string.
187
188The C<sv_cat*()> functions are not generic enough to operate on values that
189have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 190
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191If you know the name of a scalar variable, you can get a pointer to its SV
192by using the following:
193
4929bf7b 194 SV* get_sv("package::varname", FALSE);
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195
196This returns NULL if the variable does not exist.
197
d1b91892 198If you want to know if this variable (or any other SV) is actually C<defined>,
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199you can call:
200
201 SvOK(SV*)
202
06f6df17 203The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
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204
205Its address can be used whenever an C<SV*> is needed. Make sure that
206you don't try to compare a random sv with C<&PL_sv_undef>. For example
207when interfacing Perl code, it'll work correctly for:
208
209 foo(undef);
210
211But won't work when called as:
212
213 $x = undef;
214 foo($x);
215
216So to repeat always use SvOK() to check whether an sv is defined.
217
218Also you have to be careful when using C<&PL_sv_undef> as a value in
219AVs or HVs (see L<AVs, HVs and undefined values>).
a0d0e21e 220
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221There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
222boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
223addresses can be used whenever an C<SV*> is needed.
a0d0e21e 224
9cde0e7f 225Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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226Take this code:
227
228 SV* sv = (SV*) 0;
229 if (I-am-to-return-a-real-value) {
230 sv = sv_2mortal(newSViv(42));
231 }
232 sv_setsv(ST(0), sv);
233
234This code tries to return a new SV (which contains the value 42) if it should
04343c6d 235return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 236pointer which, somewhere down the line, will cause a segmentation violation,
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237bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
238first line and all will be well.
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239
240To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 241call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 242
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243=head2 Offsets
244
245Perl provides the function C<sv_chop> to efficiently remove characters
246from the beginning of a string; you give it an SV and a pointer to
da75cd15 247somewhere inside the PV, and it discards everything before the
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248pointer. The efficiency comes by means of a little hack: instead of
249actually removing the characters, C<sv_chop> sets the flag C<OOK>
250(offset OK) to signal to other functions that the offset hack is in
251effect, and it puts the number of bytes chopped off into the IV field
252of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
00aadd71 253many bytes, and adjusts C<SvCUR> and C<SvLEN>.
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254
255Hence, at this point, the start of the buffer that we allocated lives
256at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
257into the middle of this allocated storage.
258
259This is best demonstrated by example:
260
261 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
262 SV = PVIV(0x8128450) at 0x81340f0
263 REFCNT = 1
264 FLAGS = (POK,OOK,pPOK)
265 IV = 1 (OFFSET)
266 PV = 0x8135781 ( "1" . ) "2345"\0
267 CUR = 4
268 LEN = 5
269
270Here the number of bytes chopped off (1) is put into IV, and
271C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
272portion of the string between the "real" and the "fake" beginnings is
273shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
274the fake beginning, not the real one.
275
fe854a6f 276Something similar to the offset hack is performed on AVs to enable
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277efficient shifting and splicing off the beginning of the array; while
278C<AvARRAY> points to the first element in the array that is visible from
279Perl, C<AvALLOC> points to the real start of the C array. These are
280usually the same, but a C<shift> operation can be carried out by
281increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.
282Again, the location of the real start of the C array only comes into
283play when freeing the array. See C<av_shift> in F<av.c>.
284
d1b91892 285=head2 What's Really Stored in an SV?
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286
287Recall that the usual method of determining the type of scalar you have is
5f05dabc 288to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 289usually these macros will always return TRUE and calling the C<Sv*V>
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290macros will do the appropriate conversion of string to integer/double or
291integer/double to string.
292
293If you I<really> need to know if you have an integer, double, or string
294pointer in an SV, you can use the following three macros instead:
295
296 SvIOKp(SV*)
297 SvNOKp(SV*)
298 SvPOKp(SV*)
299
300These will tell you if you truly have an integer, double, or string pointer
d1b91892 301stored in your SV. The "p" stands for private.
a0d0e21e 302
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303The are various ways in which the private and public flags may differ.
304For example, a tied SV may have a valid underlying value in the IV slot
305(so SvIOKp is true), but the data should be accessed via the FETCH
306routine rather than directly, so SvIOK is false. Another is when
307numeric conversion has occured and precision has been lost: only the
308private flag is set on 'lossy' values. So when an NV is converted to an
309IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
310
07fa94a1 311In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 312
54310121 313=head2 Working with AVs
a0d0e21e 314
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315There are two ways to create and load an AV. The first method creates an
316empty AV:
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317
318 AV* newAV();
319
54310121 320The second method both creates the AV and initially populates it with SVs:
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321
322 AV* av_make(I32 num, SV **ptr);
323
5f05dabc 324The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 325AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 326
54310121 327Once the AV has been created, the following operations are possible on AVs:
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328
329 void av_push(AV*, SV*);
330 SV* av_pop(AV*);
331 SV* av_shift(AV*);
332 void av_unshift(AV*, I32 num);
333
334These should be familiar operations, with the exception of C<av_unshift>.
335This routine adds C<num> elements at the front of the array with the C<undef>
336value. You must then use C<av_store> (described below) to assign values
337to these new elements.
338
339Here are some other functions:
340
5f05dabc 341 I32 av_len(AV*);
a0d0e21e 342 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 343 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 344
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345The C<av_len> function returns the highest index value in array (just
346like $#array in Perl). If the array is empty, -1 is returned. The
347C<av_fetch> function returns the value at index C<key>, but if C<lval>
348is non-zero, then C<av_fetch> will store an undef value at that index.
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349The C<av_store> function stores the value C<val> at index C<key>, and does
350not increment the reference count of C<val>. Thus the caller is responsible
351for taking care of that, and if C<av_store> returns NULL, the caller will
352have to decrement the reference count to avoid a memory leak. Note that
353C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
354return value.
d1b91892 355
a0d0e21e 356 void av_clear(AV*);
a0d0e21e 357 void av_undef(AV*);
cb1a09d0 358 void av_extend(AV*, I32 key);
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359
360The C<av_clear> function deletes all the elements in the AV* array, but
361does not actually delete the array itself. The C<av_undef> function will
362delete all the elements in the array plus the array itself. The
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363C<av_extend> function extends the array so that it contains at least C<key+1>
364elements. If C<key+1> is less than the currently allocated length of the array,
365then nothing is done.
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366
367If you know the name of an array variable, you can get a pointer to its AV
368by using the following:
369
4929bf7b 370 AV* get_av("package::varname", FALSE);
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371
372This returns NULL if the variable does not exist.
373
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374See L<Understanding the Magic of Tied Hashes and Arrays> for more
375information on how to use the array access functions on tied arrays.
376
54310121 377=head2 Working with HVs
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378
379To create an HV, you use the following routine:
380
381 HV* newHV();
382
54310121 383Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 384
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385 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
386 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 387
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388The C<klen> parameter is the length of the key being passed in (Note that
389you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
390length of the key). The C<val> argument contains the SV pointer to the
54310121 391scalar being stored, and C<hash> is the precomputed hash value (zero if
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392you want C<hv_store> to calculate it for you). The C<lval> parameter
393indicates whether this fetch is actually a part of a store operation, in
394which case a new undefined value will be added to the HV with the supplied
395key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 396
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397Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
398C<SV*>. To access the scalar value, you must first dereference the return
399value. However, you should check to make sure that the return value is
400not NULL before dereferencing it.
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401
402These two functions check if a hash table entry exists, and deletes it.
403
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404 bool hv_exists(HV*, const char* key, U32 klen);
405 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 406
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407If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
408create and return a mortal copy of the deleted value.
409
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410And more miscellaneous functions:
411
412 void hv_clear(HV*);
a0d0e21e 413 void hv_undef(HV*);
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414
415Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
416table but does not actually delete the hash table. The C<hv_undef> deletes
417both the entries and the hash table itself.
a0d0e21e 418
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419Perl keeps the actual data in linked list of structures with a typedef of HE.
420These contain the actual key and value pointers (plus extra administrative
421overhead). The key is a string pointer; the value is an C<SV*>. However,
422once you have an C<HE*>, to get the actual key and value, use the routines
423specified below.
424
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425 I32 hv_iterinit(HV*);
426 /* Prepares starting point to traverse hash table */
427 HE* hv_iternext(HV*);
428 /* Get the next entry, and return a pointer to a
429 structure that has both the key and value */
430 char* hv_iterkey(HE* entry, I32* retlen);
431 /* Get the key from an HE structure and also return
432 the length of the key string */
cb1a09d0 433 SV* hv_iterval(HV*, HE* entry);
d1be9408 434 /* Return an SV pointer to the value of the HE
a0d0e21e 435 structure */
cb1a09d0 436 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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437 /* This convenience routine combines hv_iternext,
438 hv_iterkey, and hv_iterval. The key and retlen
439 arguments are return values for the key and its
440 length. The value is returned in the SV* argument */
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441
442If you know the name of a hash variable, you can get a pointer to its HV
443by using the following:
444
4929bf7b 445 HV* get_hv("package::varname", FALSE);
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446
447This returns NULL if the variable does not exist.
448
8ebc5c01 449The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 450
a0d0e21e 451 hash = 0;
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452 while (klen--)
453 hash = (hash * 33) + *key++;
87275199 454 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 455
87275199 456The last step was added in version 5.6 to improve distribution of
ab192400 457lower bits in the resulting hash value.
a0d0e21e 458
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459See L<Understanding the Magic of Tied Hashes and Arrays> for more
460information on how to use the hash access functions on tied hashes.
461
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462=head2 Hash API Extensions
463
464Beginning with version 5.004, the following functions are also supported:
465
466 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
467 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 468
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469 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
470 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 471
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472 SV* hv_iterkeysv (HE* entry);
473
474Note that these functions take C<SV*> keys, which simplifies writing
475of extension code that deals with hash structures. These functions
476also allow passing of C<SV*> keys to C<tie> functions without forcing
477you to stringify the keys (unlike the previous set of functions).
478
479They also return and accept whole hash entries (C<HE*>), making their
480use more efficient (since the hash number for a particular string
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481doesn't have to be recomputed every time). See L<perlapi> for detailed
482descriptions.
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483
484The following macros must always be used to access the contents of hash
485entries. Note that the arguments to these macros must be simple
486variables, since they may get evaluated more than once. See
4a4eefd0 487L<perlapi> for detailed descriptions of these macros.
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488
489 HePV(HE* he, STRLEN len)
490 HeVAL(HE* he)
491 HeHASH(HE* he)
492 HeSVKEY(HE* he)
493 HeSVKEY_force(HE* he)
494 HeSVKEY_set(HE* he, SV* sv)
495
496These two lower level macros are defined, but must only be used when
497dealing with keys that are not C<SV*>s:
498
499 HeKEY(HE* he)
500 HeKLEN(HE* he)
501
04343c6d
GS
502Note that both C<hv_store> and C<hv_store_ent> do not increment the
503reference count of the stored C<val>, which is the caller's responsibility.
504If these functions return a NULL value, the caller will usually have to
505decrement the reference count of C<val> to avoid a memory leak.
1e422769 506
a9381218
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507=head2 AVs, HVs and undefined values
508
509Sometimes you have to store undefined values in AVs or HVs. Although
510this may be a rare case, it can be tricky. That's because you're
511used to using C<&PL_sv_undef> if you need an undefined SV.
512
513For example, intuition tells you that this XS code:
514
515 AV *av = newAV();
516 av_store( av, 0, &PL_sv_undef );
517
518is equivalent to this Perl code:
519
520 my @av;
521 $av[0] = undef;
522
523Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
524for indicating that an array element has not yet been initialized.
525Thus, C<exists $av[0]> would be true for the above Perl code, but
526false for the array generated by the XS code.
527
528Other problems can occur when storing C<&PL_sv_undef> in HVs:
529
530 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
531
532This will indeed make the value C<undef>, but if you try to modify
533the value of C<key>, you'll get the following error:
534
535 Modification of non-creatable hash value attempted
536
537In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
538in restricted hashes. This caused such hash entries not to appear
539when iterating over the hash or when checking for the keys
540with the C<hv_exists> function.
541
542You can run into similar problems when you store C<&PL_sv_true> or
543C<&PL_sv_false> into AVs or HVs. Trying to modify such elements
544will give you the following error:
545
546 Modification of a read-only value attempted
547
548To make a long story short, you can use the special variables
549C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> with AVs and
550HVs, but you have to make sure you know what you're doing.
551
552Generally, if you want to store an undefined value in an AV
553or HV, you should not use C<&PL_sv_undef>, but rather create a
554new undefined value using the C<newSV> function, for example:
555
556 av_store( av, 42, newSV(0) );
557 hv_store( hv, "foo", 3, newSV(0), 0 );
558
a0d0e21e
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559=head2 References
560
d1b91892
AD
561References are a special type of scalar that point to other data types
562(including references).
a0d0e21e 563
07fa94a1 564To create a reference, use either of the following functions:
a0d0e21e 565
5f05dabc
PP
566 SV* newRV_inc((SV*) thing);
567 SV* newRV_noinc((SV*) thing);
a0d0e21e 568
5f05dabc 569The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
570functions are identical except that C<newRV_inc> increments the reference
571count of the C<thing>, while C<newRV_noinc> does not. For historical
572reasons, C<newRV> is a synonym for C<newRV_inc>.
573
574Once you have a reference, you can use the following macro to dereference
575the reference:
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576
577 SvRV(SV*)
578
579then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 580C<AV*> or C<HV*>, if required.
a0d0e21e 581
d1b91892 582To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
583
584 SvROK(SV*)
585
07fa94a1
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586To discover what type of value the reference refers to, use the following
587macro and then check the return value.
d1b91892
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588
589 SvTYPE(SvRV(SV*))
590
591The most useful types that will be returned are:
592
593 SVt_IV Scalar
594 SVt_NV Scalar
595 SVt_PV Scalar
5f05dabc 596 SVt_RV Scalar
d1b91892
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597 SVt_PVAV Array
598 SVt_PVHV Hash
599 SVt_PVCV Code
5f05dabc
PP
600 SVt_PVGV Glob (possible a file handle)
601 SVt_PVMG Blessed or Magical Scalar
602
603 See the sv.h header file for more details.
d1b91892 604
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605=head2 Blessed References and Class Objects
606
06f6df17 607References are also used to support object-oriented programming. In perl's
cb1a09d0
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608OO lexicon, an object is simply a reference that has been blessed into a
609package (or class). Once blessed, the programmer may now use the reference
610to access the various methods in the class.
611
612A reference can be blessed into a package with the following function:
613
614 SV* sv_bless(SV* sv, HV* stash);
615
06f6df17
RGS
616The C<sv> argument must be a reference value. The C<stash> argument
617specifies which class the reference will belong to. See
2ae324a7 618L<Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
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619
620/* Still under construction */
621
622Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
PP
623point to. If C<classname> is non-null, the SV is blessed into the specified
624class. SV is returned.
cb1a09d0 625
08105a92 626 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 627
e1c57cef 628Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 629if C<classname> is non-null.
cb1a09d0 630
08105a92 631 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 632 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 633 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 634
5f05dabc 635Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 636reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 637
08105a92 638 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 639
8ebc5c01
PP
640Copies string into an SV whose reference is C<rv>. Set length to 0 to let
641Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 642
e65f3abd 643 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 644
9abd00ed
GS
645Tests whether the SV is blessed into the specified class. It does not
646check inheritance relationships.
647
08105a92 648 int sv_isa(SV* sv, const char* name);
9abd00ed
GS
649
650Tests whether the SV is a reference to a blessed object.
651
652 int sv_isobject(SV* sv);
653
654Tests whether the SV is derived from the specified class. SV can be either
655a reference to a blessed object or a string containing a class name. This
656is the function implementing the C<UNIVERSAL::isa> functionality.
657
08105a92 658 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 659
00aadd71 660To check if you've got an object derived from a specific class you have
9abd00ed
GS
661to write:
662
663 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 664
5f05dabc 665=head2 Creating New Variables
cb1a09d0 666
5f05dabc
PP
667To create a new Perl variable with an undef value which can be accessed from
668your Perl script, use the following routines, depending on the variable type.
cb1a09d0 669
4929bf7b
GS
670 SV* get_sv("package::varname", TRUE);
671 AV* get_av("package::varname", TRUE);
672 HV* get_hv("package::varname", TRUE);
cb1a09d0
AD
673
674Notice the use of TRUE as the second parameter. The new variable can now
675be set, using the routines appropriate to the data type.
676
5f05dabc
PP
677There are additional macros whose values may be bitwise OR'ed with the
678C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 679
9a68f1db
SB
680=over
681
682=item GV_ADDMULTI
683
684Marks the variable as multiply defined, thus preventing the:
685
686 Name <varname> used only once: possible typo
687
688warning.
689
9a68f1db
SB
690=item GV_ADDWARN
691
692Issues the warning:
693
694 Had to create <varname> unexpectedly
695
696if the variable did not exist before the function was called.
697
698=back
cb1a09d0 699
07fa94a1
JO
700If you do not specify a package name, the variable is created in the current
701package.
cb1a09d0 702
5f05dabc 703=head2 Reference Counts and Mortality
a0d0e21e 704
d1be9408 705Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 706AVs, or HVs (xV for short in the following) start their life with a
55497cff 707reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 708then it will be destroyed and its memory made available for reuse.
55497cff
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709
710This normally doesn't happen at the Perl level unless a variable is
5f05dabc
PP
711undef'ed or the last variable holding a reference to it is changed or
712overwritten. At the internal level, however, reference counts can be
55497cff
PP
713manipulated with the following macros:
714
715 int SvREFCNT(SV* sv);
5f05dabc 716 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
717 void SvREFCNT_dec(SV* sv);
718
719However, there is one other function which manipulates the reference
07fa94a1
JO
720count of its argument. The C<newRV_inc> function, you will recall,
721creates a reference to the specified argument. As a side effect,
722it increments the argument's reference count. If this is not what
723you want, use C<newRV_noinc> instead.
724
725For example, imagine you want to return a reference from an XSUB function.
726Inside the XSUB routine, you create an SV which initially has a reference
727count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
PP
728This returns the reference as a new SV, but the reference count of the
729SV you passed to C<newRV_inc> has been incremented to two. Now you
07fa94a1
JO
730return the reference from the XSUB routine and forget about the SV.
731But Perl hasn't! Whenever the returned reference is destroyed, the
732reference count of the original SV is decreased to one and nothing happens.
733The SV will hang around without any way to access it until Perl itself
734terminates. This is a memory leak.
5f05dabc
PP
735
736The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
737C<newRV_inc>. Then, if and when the last reference is destroyed,
738the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 739stopping any memory leak.
55497cff 740
5f05dabc 741There are some convenience functions available that can help with the
54310121 742destruction of xVs. These functions introduce the concept of "mortality".
07fa94a1
JO
743An xV that is mortal has had its reference count marked to be decremented,
744but not actually decremented, until "a short time later". Generally the
745term "short time later" means a single Perl statement, such as a call to
54310121 746an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
JO
747reference count decremented depends on two macros, SAVETMPS and FREETMPS.
748See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
PP
749
750"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
751However, if you mortalize a variable twice, the reference count will
752later be decremented twice.
753
00aadd71
NIS
754"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
755For example an SV which is created just to pass a number to a called sub
06f6df17
RGS
756is made mortal to have it cleaned up automatically when it's popped off
757the stack. Similarly, results returned by XSUBs (which are pushed on the
758stack) are often made mortal.
a0d0e21e
LW
759
760To create a mortal variable, use the functions:
761
762 SV* sv_newmortal()
763 SV* sv_2mortal(SV*)
764 SV* sv_mortalcopy(SV*)
765
00aadd71 766The first call creates a mortal SV (with no value), the second converts an existing
5f05dabc
PP
767SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
768third creates a mortal copy of an existing SV.
00aadd71 769Because C<sv_newmortal> gives the new SV no value,it must normally be given one
9a68f1db 770via C<sv_setpv>, C<sv_setiv>, etc. :
00aadd71
NIS
771
772 SV *tmp = sv_newmortal();
773 sv_setiv(tmp, an_integer);
774
775As that is multiple C statements it is quite common so see this idiom instead:
776
777 SV *tmp = sv_2mortal(newSViv(an_integer));
778
779
780You should be careful about creating mortal variables. Strange things
781can happen if you make the same value mortal within multiple contexts,
782or if you make a variable mortal multiple times. Thinking of "Mortalization"
783as deferred C<SvREFCNT_dec> should help to minimize such problems.
784For example if you are passing an SV which you I<know> has high enough REFCNT
785to survive its use on the stack you need not do any mortalization.
786If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
787making a C<sv_mortalcopy> is safer.
a0d0e21e 788
54310121 789The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 790made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 791C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 792
5f05dabc 793=head2 Stashes and Globs
a0d0e21e 794
06f6df17
RGS
795A B<stash> is a hash that contains all variables that are defined
796within a package. Each key of the stash is a symbol
aa689395
PP
797name (shared by all the different types of objects that have the same
798name), and each value in the hash table is a GV (Glob Value). This GV
799in turn contains references to the various objects of that name,
800including (but not limited to) the following:
cb1a09d0 801
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LW
802 Scalar Value
803 Array Value
804 Hash Value
a3cb178b 805 I/O Handle
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806 Format
807 Subroutine
808
06f6df17
RGS
809There is a single stash called C<PL_defstash> that holds the items that exist
810in the C<main> package. To get at the items in other packages, append the
811string "::" to the package name. The items in the C<Foo> package are in
812the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
813in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 814
d1b91892 815To get the stash pointer for a particular package, use the function:
a0d0e21e 816
08105a92 817 HV* gv_stashpv(const char* name, I32 create)
a0d0e21e
LW
818 HV* gv_stashsv(SV*, I32 create)
819
820The first function takes a literal string, the second uses the string stored
d1b91892 821in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 822C<HV*>. The C<create> flag will create a new package if it is set.
a0d0e21e
LW
823
824The name that C<gv_stash*v> wants is the name of the package whose symbol table
825you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
826packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
827language itself.
a0d0e21e
LW
828
829Alternately, if you have an SV that is a blessed reference, you can find
830out the stash pointer by using:
831
832 HV* SvSTASH(SvRV(SV*));
833
834then use the following to get the package name itself:
835
836 char* HvNAME(HV* stash);
837
5f05dabc
PP
838If you need to bless or re-bless an object you can use the following
839function:
a0d0e21e
LW
840
841 SV* sv_bless(SV*, HV* stash)
842
843where the first argument, an C<SV*>, must be a reference, and the second
844argument is a stash. The returned C<SV*> can now be used in the same way
845as any other SV.
846
d1b91892
AD
847For more information on references and blessings, consult L<perlref>.
848
54310121 849=head2 Double-Typed SVs
0a753a76
PP
850
851Scalar variables normally contain only one type of value, an integer,
852double, pointer, or reference. Perl will automatically convert the
853actual scalar data from the stored type into the requested type.
854
855Some scalar variables contain more than one type of scalar data. For
856example, the variable C<$!> contains either the numeric value of C<errno>
857or its string equivalent from either C<strerror> or C<sys_errlist[]>.
858
859To force multiple data values into an SV, you must do two things: use the
860C<sv_set*v> routines to add the additional scalar type, then set a flag
861so that Perl will believe it contains more than one type of data. The
862four macros to set the flags are:
863
864 SvIOK_on
865 SvNOK_on
866 SvPOK_on
867 SvROK_on
868
869The particular macro you must use depends on which C<sv_set*v> routine
870you called first. This is because every C<sv_set*v> routine turns on
871only the bit for the particular type of data being set, and turns off
872all the rest.
873
874For example, to create a new Perl variable called "dberror" that contains
875both the numeric and descriptive string error values, you could use the
876following code:
877
878 extern int dberror;
879 extern char *dberror_list;
880
4929bf7b 881 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
882 sv_setiv(sv, (IV) dberror);
883 sv_setpv(sv, dberror_list[dberror]);
884 SvIOK_on(sv);
885
886If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
887macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
888
889=head2 Magic Variables
a0d0e21e 890
d1b91892
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891[This section still under construction. Ignore everything here. Post no
892bills. Everything not permitted is forbidden.]
893
d1b91892
AD
894Any SV may be magical, that is, it has special features that a normal
895SV does not have. These features are stored in the SV structure in a
5f05dabc 896linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
897
898 struct magic {
899 MAGIC* mg_moremagic;
900 MGVTBL* mg_virtual;
901 U16 mg_private;
902 char mg_type;
903 U8 mg_flags;
904 SV* mg_obj;
905 char* mg_ptr;
906 I32 mg_len;
907 };
908
909Note this is current as of patchlevel 0, and could change at any time.
910
911=head2 Assigning Magic
912
913Perl adds magic to an SV using the sv_magic function:
914
08105a92 915 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
916
917The C<sv> argument is a pointer to the SV that is to acquire a new magical
918feature.
919
920If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
645c22ef
DM
921convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
922to the beginning of the linked list of magical features. Any prior entry
923of the same type of magic is deleted. Note that this can be overridden,
924and multiple instances of the same type of magic can be associated with an
925SV.
d1b91892 926
54310121
PP
927The C<name> and C<namlen> arguments are used to associate a string with
928the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
929C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
930C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
931whether C<namlen> is greater than zero or equal to zero respectively. As a
932special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
933to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
934
935The sv_magic function uses C<how> to determine which, if any, predefined
936"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
06f6df17 937See the L<Magic Virtual Tables> section below. The C<how> argument is also
14befaf4 938stored in the C<mg_type> field. The value of C<how> should be chosen
06f6df17 939from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 940these macros were added, Perl internals used to directly use character
14befaf4 941literals, so you may occasionally come across old code or documentation
75d0f26d 942referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
943
944The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
945structure. If it is not the same as the C<sv> argument, the reference
946count of the C<obj> object is incremented. If it is the same, or if
645c22ef 947the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 948then C<obj> is merely stored, without the reference count being incremented.
d1b91892 949
2d8d5d5a
SH
950See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
951to an SV.
952
cb1a09d0
AD
953There is also a function to add magic to an C<HV>:
954
955 void hv_magic(HV *hv, GV *gv, int how);
956
957This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
958
959To remove the magic from an SV, call the function sv_unmagic:
960
961 void sv_unmagic(SV *sv, int type);
962
963The C<type> argument should be equal to the C<how> value when the C<SV>
964was initially made magical.
965
d1b91892
AD
966=head2 Magic Virtual Tables
967
d1be9408 968The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
969C<MGVTBL>, which is a structure of function pointers and stands for
970"Magic Virtual Table" to handle the various operations that might be
971applied to that variable.
972
973The C<MGVTBL> has five pointers to the following routine types:
974
975 int (*svt_get)(SV* sv, MAGIC* mg);
976 int (*svt_set)(SV* sv, MAGIC* mg);
977 U32 (*svt_len)(SV* sv, MAGIC* mg);
978 int (*svt_clear)(SV* sv, MAGIC* mg);
979 int (*svt_free)(SV* sv, MAGIC* mg);
980
06f6df17 981This MGVTBL structure is set at compile-time in F<perl.h> and there are
d1b91892
AD
982currently 19 types (or 21 with overloading turned on). These different
983structures contain pointers to various routines that perform additional
984actions depending on which function is being called.
985
986 Function pointer Action taken
987 ---------------- ------------
8b0711c3 988 svt_get Do something before the value of the SV is retrieved.
d1b91892
AD
989 svt_set Do something after the SV is assigned a value.
990 svt_len Report on the SV's length.
991 svt_clear Clear something the SV represents.
992 svt_free Free any extra storage associated with the SV.
993
994For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 995to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
996
997 { magic_get, magic_set, magic_len, 0, 0 }
998
14befaf4
DM
999Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1000if a get operation is being performed, the routine C<magic_get> is
1001called. All the various routines for the various magical types begin
1002with C<magic_>. NOTE: the magic routines are not considered part of
1003the Perl API, and may not be exported by the Perl library.
d1b91892
AD
1004
1005The current kinds of Magic Virtual Tables are:
1006
14befaf4
DM
1007 mg_type
1008 (old-style char and macro) MGVTBL Type of magic
1009 -------------------------- ------ ----------------------------
1010 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1011 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
1012 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
1013 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
1014 on stash
1015 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
1016 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1017 (@+ and @- vars)
1018 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1019 element
1020 E PERL_MAGIC_env vtbl_env %ENV hash
1021 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1022 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
1023 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
1024 I PERL_MAGIC_isa vtbl_isa @ISA array
1025 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1026 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1027 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1028 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
1029 m PERL_MAGIC_mutex vtbl_mutex ???
645c22ef 1030 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
14befaf4
DM
1031 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1032 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1033 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1034 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
1035 S PERL_MAGIC_sig vtbl_sig %SIG hash
1036 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1037 t PERL_MAGIC_taint vtbl_taint Taintedness
1038 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
1039 v PERL_MAGIC_vec vtbl_vec vec() lvalue
92f0c265 1040 V PERL_MAGIC_vstring (none) v-string scalars
836995da 1041 w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
14befaf4
DM
1042 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1043 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1044 variable / smart parameter
1045 vivification
1046 * PERL_MAGIC_glob vtbl_glob GV (typeglob)
1047 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1048 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1049 < PERL_MAGIC_backref vtbl_backref ???
1050 ~ PERL_MAGIC_ext (none) Available for use by extensions
d1b91892 1051
68dc0745 1052When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1053uppercase letter is typically used to represent some kind of composite type
1054(a list or a hash), and the lowercase letter is used to represent an element
1055of that composite type. Some internals code makes use of this case
1056relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1057
1058The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1059specifically for use by extensions and will not be used by perl itself.
1060Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1061to variables (typically objects). This is especially useful because
1062there is no way for normal perl code to corrupt this private information
1063(unlike using extra elements of a hash object).
1064
1065Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1066C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1067C<mg_ptr> field points to a C<ufuncs> structure:
1068
1069 struct ufuncs {
a9402793
AB
1070 I32 (*uf_val)(pTHX_ IV, SV*);
1071 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1072 IV uf_index;
1073 };
1074
1075When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1076function will be called with C<uf_index> as the first arg and a pointer to
1077the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1078magic is shown below. Note that the ufuncs structure is copied by
1079sv_magic, so you can safely allocate it on the stack.
1080
1081 void
1082 Umagic(sv)
1083 SV *sv;
1084 PREINIT:
1085 struct ufuncs uf;
1086 CODE:
1087 uf.uf_val = &my_get_fn;
1088 uf.uf_set = &my_set_fn;
1089 uf.uf_index = 0;
14befaf4 1090 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1091
14befaf4
DM
1092Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1093or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1094extra care to avoid conflict. Typically only using the magic on
1095objects blessed into the same class as the extension is sufficient.
1096For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1097'signature' at the top of the private data area and check that.
5f05dabc 1098
ef50df4b
GS
1099Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1100earlier do B<not> invoke 'set' magic on their targets. This must
1101be done by the user either by calling the C<SvSETMAGIC()> macro after
1102calling these functions, or by using one of the C<sv_set*_mg()> or
1103C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1104C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1105obtained from external sources in functions that don't handle magic.
4a4eefd0 1106See L<perlapi> for a description of these functions.
189b2af5
GS
1107For example, calls to the C<sv_cat*()> functions typically need to be
1108followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1109since their implementation handles 'get' magic.
1110
d1b91892
AD
1111=head2 Finding Magic
1112
1113 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1114
1115This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1116If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1117if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1118
08105a92 1119 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1120
1121This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1122field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1123the mg_type field is changed to be the lowercase letter.
a0d0e21e 1124
04343c6d
GS
1125=head2 Understanding the Magic of Tied Hashes and Arrays
1126
14befaf4
DM
1127Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1128magic type.
9edb2b46
GS
1129
1130WARNING: As of the 5.004 release, proper usage of the array and hash
1131access functions requires understanding a few caveats. Some
1132of these caveats are actually considered bugs in the API, to be fixed
1133in later releases, and are bracketed with [MAYCHANGE] below. If
1134you find yourself actually applying such information in this section, be
1135aware that the behavior may change in the future, umm, without warning.
04343c6d 1136
1526ead6 1137The perl tie function associates a variable with an object that implements
9a68f1db 1138the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1139tie function from an XSUB, you must mimic this behaviour. The code below
1140carries out the necessary steps - firstly it creates a new hash, and then
1141creates a second hash which it blesses into the class which will implement
1142the tie methods. Lastly it ties the two hashes together, and returns a
1143reference to the new tied hash. Note that the code below does NOT call the
1144TIEHASH method in the MyTie class -
1145see L<Calling Perl Routines from within C Programs> for details on how
1146to do this.
1147
1148 SV*
1149 mytie()
1150 PREINIT:
1151 HV *hash;
1152 HV *stash;
1153 SV *tie;
1154 CODE:
1155 hash = newHV();
1156 tie = newRV_noinc((SV*)newHV());
1157 stash = gv_stashpv("MyTie", TRUE);
1158 sv_bless(tie, stash);
899e16d0 1159 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1160 RETVAL = newRV_noinc(hash);
1161 OUTPUT:
1162 RETVAL
1163
04343c6d
GS
1164The C<av_store> function, when given a tied array argument, merely
1165copies the magic of the array onto the value to be "stored", using
1166C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1167actually need to be stored in the array. [MAYCHANGE] After a call to
1168C<av_store> on a tied array, the caller will usually need to call
1169C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1170TIEARRAY object. If C<av_store> did return NULL, a call to
1171C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1172leak. [/MAYCHANGE]
04343c6d
GS
1173
1174The previous paragraph is applicable verbatim to tied hash access using the
1175C<hv_store> and C<hv_store_ent> functions as well.
1176
1177C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1178C<hv_fetch_ent> actually return an undefined mortal value whose magic
1179has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1180need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1181need to call C<mg_get()> on the returned value in order to actually invoke
1182the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1183you may also call C<mg_set()> on the return value after possibly assigning
1184a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1185method on the TIE object. [/MAYCHANGE]
04343c6d 1186
9edb2b46 1187[MAYCHANGE]
04343c6d
GS
1188In other words, the array or hash fetch/store functions don't really
1189fetch and store actual values in the case of tied arrays and hashes. They
1190merely call C<mg_copy> to attach magic to the values that were meant to be
1191"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1192do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1193the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1194and hashes.
1195
1196Currently (as of perl version 5.004), use of the hash and array access
1197functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1198"normal" hashes and arrays, or on their tied variants. The API may be
1199changed to provide more transparent access to both tied and normal data
1200types in future versions.
1201[/MAYCHANGE]
04343c6d
GS
1202
1203You would do well to understand that the TIEARRAY and TIEHASH interfaces
1204are mere sugar to invoke some perl method calls while using the uniform hash
1205and array syntax. The use of this sugar imposes some overhead (typically
1206about two to four extra opcodes per FETCH/STORE operation, in addition to
1207the creation of all the mortal variables required to invoke the methods).
1208This overhead will be comparatively small if the TIE methods are themselves
1209substantial, but if they are only a few statements long, the overhead
1210will not be insignificant.
1211
d1c897a1
IZ
1212=head2 Localizing changes
1213
1214Perl has a very handy construction
1215
1216 {
1217 local $var = 2;
1218 ...
1219 }
1220
1221This construction is I<approximately> equivalent to
1222
1223 {
1224 my $oldvar = $var;
1225 $var = 2;
1226 ...
1227 $var = $oldvar;
1228 }
1229
1230The biggest difference is that the first construction would
1231reinstate the initial value of $var, irrespective of how control exits
9a68f1db 1232the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1233more efficient as well.
1234
1235There is a way to achieve a similar task from C via Perl API: create a
1236I<pseudo-block>, and arrange for some changes to be automatically
1237undone at the end of it, either explicit, or via a non-local exit (via
1238die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1239C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1240Such a construct may be created specially for some important localized
1241task, or an existing one (like boundaries of enclosing Perl
1242subroutine/block, or an existing pair for freeing TMPs) may be
1243used. (In the second case the overhead of additional localization must
1244be almost negligible.) Note that any XSUB is automatically enclosed in
1245an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1246
1247Inside such a I<pseudo-block> the following service is available:
1248
13a2d996 1249=over 4
d1c897a1
IZ
1250
1251=item C<SAVEINT(int i)>
1252
1253=item C<SAVEIV(IV i)>
1254
1255=item C<SAVEI32(I32 i)>
1256
1257=item C<SAVELONG(long i)>
1258
1259These macros arrange things to restore the value of integer variable
1260C<i> at the end of enclosing I<pseudo-block>.
1261
1262=item C<SAVESPTR(s)>
1263
1264=item C<SAVEPPTR(p)>
1265
1266These macros arrange things to restore the value of pointers C<s> and
1267C<p>. C<s> must be a pointer of a type which survives conversion to
1268C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1269and back.
1270
1271=item C<SAVEFREESV(SV *sv)>
1272
1273The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1274I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1275mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1276extends the lifetime of C<sv> until the beginning of the next statement,
1277C<SAVEFREESV> extends it until the end of the enclosing scope. These
1278lifetimes can be wildly different.
1279
1280Also compare C<SAVEMORTALIZESV>.
1281
1282=item C<SAVEMORTALIZESV(SV *sv)>
1283
1284Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1285scope instead of decrementing its reference count. This usually has the
1286effect of keeping C<sv> alive until the statement that called the currently
1287live scope has finished executing.
d1c897a1
IZ
1288
1289=item C<SAVEFREEOP(OP *op)>
1290
1291The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1292
1293=item C<SAVEFREEPV(p)>
1294
1295The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1296end of I<pseudo-block>.
1297
1298=item C<SAVECLEARSV(SV *sv)>
1299
1300Clears a slot in the current scratchpad which corresponds to C<sv> at
1301the end of I<pseudo-block>.
1302
1303=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1304
1305The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1306string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1307short-lived storage, the corresponding string may be reallocated like
1308this:
1309
9cde0e7f 1310 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1311
c76ac1ee 1312=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1313
1314At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1315only argument C<p>.
1316
1317=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1318
1319At the end of I<pseudo-block> the function C<f> is called with the
1320implicit context argument (if any), and C<p>.
d1c897a1
IZ
1321
1322=item C<SAVESTACK_POS()>
1323
1324The current offset on the Perl internal stack (cf. C<SP>) is restored
1325at the end of I<pseudo-block>.
1326
1327=back
1328
1329The following API list contains functions, thus one needs to
1330provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1331or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1332function takes C<int *>.
1333
13a2d996 1334=over 4
d1c897a1
IZ
1335
1336=item C<SV* save_scalar(GV *gv)>
1337
1338Equivalent to Perl code C<local $gv>.
1339
1340=item C<AV* save_ary(GV *gv)>
1341
1342=item C<HV* save_hash(GV *gv)>
1343
1344Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1345
1346=item C<void save_item(SV *item)>
1347
1348Duplicates the current value of C<SV>, on the exit from the current
1349C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
038fcae3
SB
1350using the stored value. It doesn't handle magic. Use C<save_scalar> if
1351magic is affected.
d1c897a1
IZ
1352
1353=item C<void save_list(SV **sarg, I32 maxsarg)>
1354
1355A variant of C<save_item> which takes multiple arguments via an array
1356C<sarg> of C<SV*> of length C<maxsarg>.
1357
1358=item C<SV* save_svref(SV **sptr)>
1359
d1be9408 1360Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1361
1362=item C<void save_aptr(AV **aptr)>
1363
1364=item C<void save_hptr(HV **hptr)>
1365
1366Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1367
1368=back
1369
1370The C<Alias> module implements localization of the basic types within the
1371I<caller's scope>. People who are interested in how to localize things in
1372the containing scope should take a look there too.
1373
0a753a76 1374=head1 Subroutines
a0d0e21e 1375
68dc0745 1376=head2 XSUBs and the Argument Stack
5f05dabc
PP
1377
1378The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1379An XSUB routine will have a stack that contains the arguments from the Perl
1380program, and a way to map from the Perl data structures to a C equivalent.
1381
1382The stack arguments are accessible through the C<ST(n)> macro, which returns
1383the C<n>'th stack argument. Argument 0 is the first argument passed in the
1384Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1385an C<SV*> is used.
1386
1387Most of the time, output from the C routine can be handled through use of
1388the RETVAL and OUTPUT directives. However, there are some cases where the
1389argument stack is not already long enough to handle all the return values.
1390An example is the POSIX tzname() call, which takes no arguments, but returns
1391two, the local time zone's standard and summer time abbreviations.
1392
1393To handle this situation, the PPCODE directive is used and the stack is
1394extended using the macro:
1395
924508f0 1396 EXTEND(SP, num);
5f05dabc 1397
924508f0
GS
1398where C<SP> is the macro that represents the local copy of the stack pointer,
1399and C<num> is the number of elements the stack should be extended by.
5f05dabc 1400
00aadd71 1401Now that there is room on the stack, values can be pushed on it using C<PUSHs>
06f6df17 1402macro. The pushed values will often need to be "mortal" (See
d82b684c 1403L</Reference Counts and Mortality>):
5f05dabc 1404
00aadd71 1405 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1406 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1407 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1408 PUSHs(sv_2mortal(newSVpv("Some String",0)))
5f05dabc
PP
1409
1410And now the Perl program calling C<tzname>, the two values will be assigned
1411as in:
1412
1413 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1414
1415An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1416to use the macro:
5f05dabc 1417
5f05dabc
PP
1418 XPUSHs(SV*)
1419
00aadd71 1420This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1421do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1422
1423Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1424C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1425For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1426C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1427
1428For more information, consult L<perlxs> and L<perlxstut>.
1429
1430=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1431
1432There are four routines that can be used to call a Perl subroutine from
1433within a C program. These four are:
1434
954c1994
GS
1435 I32 call_sv(SV*, I32);
1436 I32 call_pv(const char*, I32);
1437 I32 call_method(const char*, I32);
1438 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1439
954c1994 1440The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1441contains either the name of the Perl subroutine to be called, or a
1442reference to the subroutine. The second argument consists of flags
1443that control the context in which the subroutine is called, whether
1444or not the subroutine is being passed arguments, how errors should be
1445trapped, and how to treat return values.
a0d0e21e
LW
1446
1447All four routines return the number of arguments that the subroutine returned
1448on the Perl stack.
1449
9a68f1db 1450These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1451but those names are now deprecated; macros of the same name are provided for
1452compatibility.
1453
1454When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1455must manipulate the Perl stack. These include the following macros and
1456functions:
a0d0e21e
LW
1457
1458 dSP
924508f0 1459 SP
a0d0e21e
LW
1460 PUSHMARK()
1461 PUTBACK
1462 SPAGAIN
1463 ENTER
1464 SAVETMPS
1465 FREETMPS
1466 LEAVE
1467 XPUSH*()
cb1a09d0 1468 POP*()
a0d0e21e 1469
5f05dabc
PP
1470For a detailed description of calling conventions from C to Perl,
1471consult L<perlcall>.
a0d0e21e 1472
5f05dabc 1473=head2 Memory Allocation
a0d0e21e 1474
06f6df17
RGS
1475=head3 Allocation
1476
86058a2d
GS
1477All memory meant to be used with the Perl API functions should be manipulated
1478using the macros described in this section. The macros provide the necessary
1479transparency between differences in the actual malloc implementation that is
1480used within perl.
1481
1482It is suggested that you enable the version of malloc that is distributed
5f05dabc 1483with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1484order to satisfy allocation requests more quickly. However, on some
1485platforms, it may cause spurious malloc or free errors.
d1b91892 1486
06f6df17
RGS
1487The following three macros are used to initially allocate memory :
1488
d1b91892
AD
1489 New(x, pointer, number, type);
1490 Newc(x, pointer, number, type, cast);
1491 Newz(x, pointer, number, type);
1492
5f05dabc
PP
1493The first argument C<x> was a "magic cookie" that was used to keep track
1494of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1495the current code makes no use of this feature (most Perl developers now
1496use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1497
1498The second argument C<pointer> should be the name of a variable that will
1499point to the newly allocated memory.
d1b91892 1500
d1b91892
AD
1501The third and fourth arguments C<number> and C<type> specify how many of
1502the specified type of data structure should be allocated. The argument
1503C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1504should be used if the C<pointer> argument is different from the C<type>
1505argument.
1506
1507Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1508to zero out all the newly allocated memory.
1509
06f6df17
RGS
1510=head3 Reallocation
1511
d1b91892
AD
1512 Renew(pointer, number, type);
1513 Renewc(pointer, number, type, cast);
1514 Safefree(pointer)
1515
1516These three macros are used to change a memory buffer size or to free a
1517piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1518match those of C<New> and C<Newc> with the exception of not needing the
1519"magic cookie" argument.
1520
06f6df17
RGS
1521=head3 Moving
1522
d1b91892
AD
1523 Move(source, dest, number, type);
1524 Copy(source, dest, number, type);
1525 Zero(dest, number, type);
1526
1527These three macros are used to move, copy, or zero out previously allocated
1528memory. The C<source> and C<dest> arguments point to the source and
1529destination starting points. Perl will move, copy, or zero out C<number>
1530instances of the size of the C<type> data structure (using the C<sizeof>
1531function).
a0d0e21e 1532
5f05dabc 1533=head2 PerlIO
ce3d39e2 1534
5f05dabc
PP
1535The most recent development releases of Perl has been experimenting with
1536removing Perl's dependency on the "normal" standard I/O suite and allowing
1537other stdio implementations to be used. This involves creating a new
1538abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1539was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1540abstraction layer and not make any assumptions about what kind of stdio
1541is being used.
1542
1543For a complete description of the PerlIO abstraction, consult L<perlapio>.
1544
8ebc5c01 1545=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1546
1547A lot of opcodes (this is an elementary operation in the internal perl
1548stack machine) put an SV* on the stack. However, as an optimization
1549the corresponding SV is (usually) not recreated each time. The opcodes
1550reuse specially assigned SVs (I<target>s) which are (as a corollary)
1551not constantly freed/created.
1552
0a753a76 1553Each of the targets is created only once (but see
ce3d39e2
IZ
1554L<Scratchpads and recursion> below), and when an opcode needs to put
1555an integer, a double, or a string on stack, it just sets the
1556corresponding parts of its I<target> and puts the I<target> on stack.
1557
1558The macro to put this target on stack is C<PUSHTARG>, and it is
1559directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1560others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1561
1bd1c0d5
SC
1562Because the target is reused, you must be careful when pushing multiple
1563values on the stack. The following code will not do what you think:
1564
1565 XPUSHi(10);
1566 XPUSHi(20);
1567
1568This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1569the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1570At the end of the operation, the stack does not contain the values 10
1571and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1572to 20.
1bd1c0d5 1573
d82b684c
SH
1574If you need to push multiple different values then you should either use
1575the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1576none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1577SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1578will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1579this a little easier to achieve by creating a new mortal for you (via
1580C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1581in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1582Thus, instead of writing this to "fix" the example above:
1583
1584 XPUSHs(sv_2mortal(newSViv(10)))
1585 XPUSHs(sv_2mortal(newSViv(20)))
1586
1587you can simply write:
1588
1589 mXPUSHi(10)
1590 mXPUSHi(20)
1591
1592On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1593need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1594macros can make use of the local variable C<TARG>. See also C<dTARGET>
1595and C<dXSTARG>.
1bd1c0d5 1596
8ebc5c01 1597=head2 Scratchpads
ce3d39e2 1598
54310121 1599The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1600are created. The answer is that they are created when the current unit --
1601a subroutine or a file (for opcodes for statements outside of
1602subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1603array is created, which is called a scratchpad for the current
1604unit.
1605
54310121 1606A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1607targets for opcodes. One can deduce that an SV lives on a scratchpad
1608by looking on its flags: lexicals have C<SVs_PADMY> set, and
1609I<target>s have C<SVs_PADTMP> set.
1610
54310121
PP
1611The correspondence between OPs and I<target>s is not 1-to-1. Different
1612OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1613would not conflict with the expected life of the temporary.
1614
2ae324a7 1615=head2 Scratchpads and recursion
ce3d39e2
IZ
1616
1617In fact it is not 100% true that a compiled unit contains a pointer to
1618the scratchpad AV. In fact it contains a pointer to an AV of
1619(initially) one element, and this element is the scratchpad AV. Why do
1620we need an extra level of indirection?
1621
9a68f1db 1622The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2
IZ
1623these can create several execution pointers going into the same
1624subroutine. For the subroutine-child not write over the temporaries
1625for the subroutine-parent (lifespan of which covers the call to the
1626child), the parent and the child should have different
1627scratchpads. (I<And> the lexicals should be separate anyway!)
1628
5f05dabc
PP
1629So each subroutine is born with an array of scratchpads (of length 1).
1630On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1631depth of the recursion is not more than the length of this array, and
1632if it is, new scratchpad is created and pushed into the array.
1633
1634The I<target>s on this scratchpad are C<undef>s, but they are already
1635marked with correct flags.
1636
0a753a76
PP
1637=head1 Compiled code
1638
1639=head2 Code tree
1640
1641Here we describe the internal form your code is converted to by
1642Perl. Start with a simple example:
1643
1644 $a = $b + $c;
1645
1646This is converted to a tree similar to this one:
1647
1648 assign-to
1649 / \
1650 + $a
1651 / \
1652 $b $c
1653
7b8d334a 1654(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1655parsed your code, but has nothing to do with the execution order.
1656There is an additional "thread" going through the nodes of the tree
1657which shows the order of execution of the nodes. In our simplified
1658example above it looks like:
1659
1660 $b ---> $c ---> + ---> $a ---> assign-to
1661
1662But with the actual compile tree for C<$a = $b + $c> it is different:
1663some nodes I<optimized away>. As a corollary, though the actual tree
1664contains more nodes than our simplified example, the execution order
1665is the same as in our example.
1666
1667=head2 Examining the tree
1668
06f6df17
RGS
1669If you have your perl compiled for debugging (usually done with
1670C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1671compiled tree by specifying C<-Dx> on the Perl command line. The
1672output takes several lines per node, and for C<$b+$c> it looks like
1673this:
1674
1675 5 TYPE = add ===> 6
1676 TARG = 1
1677 FLAGS = (SCALAR,KIDS)
1678 {
1679 TYPE = null ===> (4)
1680 (was rv2sv)
1681 FLAGS = (SCALAR,KIDS)
1682 {
1683 3 TYPE = gvsv ===> 4
1684 FLAGS = (SCALAR)
1685 GV = main::b
1686 }
1687 }
1688 {
1689 TYPE = null ===> (5)
1690 (was rv2sv)
1691 FLAGS = (SCALAR,KIDS)
1692 {
1693 4 TYPE = gvsv ===> 5
1694 FLAGS = (SCALAR)
1695 GV = main::c
1696 }
1697 }
1698
1699This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1700not optimized away (one per number in the left column). The immediate
1701children of the given node correspond to C<{}> pairs on the same level
1702of indentation, thus this listing corresponds to the tree:
1703
1704 add
1705 / \
1706 null null
1707 | |
1708 gvsv gvsv
1709
1710The execution order is indicated by C<===E<gt>> marks, thus it is C<3
17114 5 6> (node C<6> is not included into above listing), i.e.,
1712C<gvsv gvsv add whatever>.
1713
9afa14e3
SC
1714Each of these nodes represents an op, a fundamental operation inside the
1715Perl core. The code which implements each operation can be found in the
1716F<pp*.c> files; the function which implements the op with type C<gvsv>
1717is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1718different numbers of children: C<add> is a binary operator, as one would
1719expect, and so has two children. To accommodate the various different
1720numbers of children, there are various types of op data structure, and
1721they link together in different ways.
1722
1723The simplest type of op structure is C<OP>: this has no children. Unary
1724operators, C<UNOP>s, have one child, and this is pointed to by the
1725C<op_first> field. Binary operators (C<BINOP>s) have not only an
1726C<op_first> field but also an C<op_last> field. The most complex type of
1727op is a C<LISTOP>, which has any number of children. In this case, the
1728first child is pointed to by C<op_first> and the last child by
1729C<op_last>. The children in between can be found by iteratively
1730following the C<op_sibling> pointer from the first child to the last.
1731
1732There are also two other op types: a C<PMOP> holds a regular expression,
1733and has no children, and a C<LOOP> may or may not have children. If the
1734C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1735complicate matters, if a C<UNOP> is actually a C<null> op after
1736optimization (see L</Compile pass 2: context propagation>) it will still
1737have children in accordance with its former type.
1738
06f6df17
RGS
1739Another way to examine the tree is to use a compiler back-end module, such
1740as L<B::Concise>.
1741
0a753a76
PP
1742=head2 Compile pass 1: check routines
1743
8870b5c7
GS
1744The tree is created by the compiler while I<yacc> code feeds it
1745the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1746the first pass of perl compilation.
1747
1748What makes this pass interesting for perl developers is that some
1749optimization may be performed on this pass. This is optimization by
8870b5c7 1750so-called "check routines". The correspondence between node names
0a753a76
PP
1751and corresponding check routines is described in F<opcode.pl> (do not
1752forget to run C<make regen_headers> if you modify this file).
1753
1754A check routine is called when the node is fully constructed except
7b8d334a 1755for the execution-order thread. Since at this time there are no
0a753a76
PP
1756back-links to the currently constructed node, one can do most any
1757operation to the top-level node, including freeing it and/or creating
1758new nodes above/below it.
1759
1760The check routine returns the node which should be inserted into the
1761tree (if the top-level node was not modified, check routine returns
1762its argument).
1763
1764By convention, check routines have names C<ck_*>. They are usually
1765called from C<new*OP> subroutines (or C<convert>) (which in turn are
1766called from F<perly.y>).
1767
1768=head2 Compile pass 1a: constant folding
1769
1770Immediately after the check routine is called the returned node is
1771checked for being compile-time executable. If it is (the value is
1772judged to be constant) it is immediately executed, and a I<constant>
1773node with the "return value" of the corresponding subtree is
1774substituted instead. The subtree is deleted.
1775
1776If constant folding was not performed, the execution-order thread is
1777created.
1778
1779=head2 Compile pass 2: context propagation
1780
1781When a context for a part of compile tree is known, it is propagated
a3cb178b 1782down through the tree. At this time the context can have 5 values
0a753a76
PP
1783(instead of 2 for runtime context): void, boolean, scalar, list, and
1784lvalue. In contrast with the pass 1 this pass is processed from top
1785to bottom: a node's context determines the context for its children.
1786
1787Additional context-dependent optimizations are performed at this time.
1788Since at this moment the compile tree contains back-references (via
1789"thread" pointers), nodes cannot be free()d now. To allow
1790optimized-away nodes at this stage, such nodes are null()ified instead
1791of free()ing (i.e. their type is changed to OP_NULL).
1792
1793=head2 Compile pass 3: peephole optimization
1794
1795After the compile tree for a subroutine (or for an C<eval> or a file)
1796is created, an additional pass over the code is performed. This pass
1797is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1798additional complications for conditionals). These optimizations are
0a753a76
PP
1799done in the subroutine peep(). Optimizations performed at this stage
1800are subject to the same restrictions as in the pass 2.
1801
1ba7f851
PJ
1802=head2 Pluggable runops
1803
1804The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
1805functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
1806with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
1807control over the execution of the compile tree it is possible to provide
1808your own runops function.
1ba7f851
PJ
1809
1810It's probably best to copy one of the existing runops functions and
1811change it to suit your needs. Then, in the BOOT section of your XS
1812file, add the line:
1813
1814 PL_runops = my_runops;
1815
1816This function should be as efficient as possible to keep your programs
1817running as fast as possible.
1818
9afa14e3
SC
1819=head1 Examining internal data structures with the C<dump> functions
1820
1821To aid debugging, the source file F<dump.c> contains a number of
1822functions which produce formatted output of internal data structures.
1823
1824The most commonly used of these functions is C<Perl_sv_dump>; it's used
1825for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1826C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1827module should already be familiar with its format.
9afa14e3
SC
1828
1829C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 1830derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
1831C<Perl_dump_eval> will dump the main root of the code being evaluated,
1832exactly like C<-Dx>.
1833
1834Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1835op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1836subroutines in a package like so: (Thankfully, these are all xsubs, so
1837there is no op tree)
1838
1839 (gdb) print Perl_dump_packsubs(PL_defstash)
1840
1841 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1842
1843 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1844
1845 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1846
1847 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1848
1849 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1850
1851and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1852the op tree of the main root.
1853
954c1994 1854=head1 How multiple interpreters and concurrency are supported
ee072b34 1855
ee072b34
GS
1856=head2 Background and PERL_IMPLICIT_CONTEXT
1857
1858The Perl interpreter can be regarded as a closed box: it has an API
1859for feeding it code or otherwise making it do things, but it also has
1860functions for its own use. This smells a lot like an object, and
1861there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
1862interpreters, with one interpreter represented either as a C structure,
1863or inside a thread-specific structure. These structures contain all
1864the context, the state of that interpreter.
1865
9a68f1db 1866Two macros control the major Perl build flavors: MULTIPLICITY and
acfe0abc
GS
1867USE_5005THREADS. The MULTIPLICITY build has a C structure
1868that packages all the interpreter state, and there is a similar thread-specific
1869data structure under USE_5005THREADS. In both cases,
54aff467
GS
1870PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1871support for passing in a "hidden" first argument that represents all three
651a3225 1872data structures.
54aff467
GS
1873
1874All this obviously requires a way for the Perl internal functions to be
acfe0abc 1875either subroutines taking some kind of structure as the first
ee072b34 1876argument, or subroutines taking nothing as the first argument. To
acfe0abc 1877enable these two very different ways of building the interpreter,
ee072b34
GS
1878the Perl source (as it does in so many other situations) makes heavy
1879use of macros and subroutine naming conventions.
1880
54aff467 1881First problem: deciding which functions will be public API functions and
00aadd71 1882which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1883(think "S" for "secret" or "static"). All other functions begin with
1884"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1885part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1886function is part of the API is to find its entry in L<perlapi>.
1887If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1888think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1889L<perlbug> explaining why you think it should be.
ee072b34
GS
1890
1891Second problem: there must be a syntax so that the same subroutine
1892declarations and calls can pass a structure as their first argument,
1893or pass nothing. To solve this, the subroutines are named and
1894declared in a particular way. Here's a typical start of a static
1895function used within the Perl guts:
1896
1897 STATIC void
1898 S_incline(pTHX_ char *s)
1899
acfe0abc
GS
1900STATIC becomes "static" in C, and may be #define'd to nothing in some
1901configurations in future.
ee072b34 1902
651a3225
GS
1903A public function (i.e. part of the internal API, but not necessarily
1904sanctioned for use in extensions) begins like this:
ee072b34
GS
1905
1906 void
2307c6d0 1907 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34
GS
1908
1909C<pTHX_> is one of a number of macros (in perl.h) that hide the
1910details of the interpreter's context. THX stands for "thread", "this",
1911or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1912The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1913or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1914their variants.
ee072b34 1915
a7486cbb
JH
1916When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1917first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1918in the pTHX_ macro indicates that the macro expansion needs a comma
1919after the context argument because other arguments follow it. If
1920PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1921subroutine is not prototyped to take the extra argument. The form of the
1922macro without the trailing underscore is used when there are no additional
ee072b34
GS
1923explicit arguments.
1924
54aff467 1925When a core function calls another, it must pass the context. This
2307c6d0 1926is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
1927something like this:
1928
2307c6d0
SB
1929 #ifdef PERL_IMPLICIT_CONTEXT
1930 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 1931 /* can't do this for vararg functions, see below */
2307c6d0
SB
1932 #else
1933 #define sv_setiv Perl_sv_setiv
1934 #endif
ee072b34
GS
1935
1936This works well, and means that XS authors can gleefully write:
1937
2307c6d0 1938 sv_setiv(foo, bar);
ee072b34
GS
1939
1940and still have it work under all the modes Perl could have been
1941compiled with.
1942
ee072b34
GS
1943This doesn't work so cleanly for varargs functions, though, as macros
1944imply that the number of arguments is known in advance. Instead we
1945either need to spell them out fully, passing C<aTHX_> as the first
1946argument (the Perl core tends to do this with functions like
1947Perl_warner), or use a context-free version.
1948
1949The context-free version of Perl_warner is called
1950Perl_warner_nocontext, and does not take the extra argument. Instead
1951it does dTHX; to get the context from thread-local storage. We
1952C<#define warner Perl_warner_nocontext> so that extensions get source
1953compatibility at the expense of performance. (Passing an arg is
1954cheaper than grabbing it from thread-local storage.)
1955
acfe0abc 1956You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
1957Those are strictly for use within the core. Extensions and embedders
1958need only be aware of [pad]THX.
1959
a7486cbb
JH
1960=head2 So what happened to dTHR?
1961
1962C<dTHR> was introduced in perl 5.005 to support the older thread model.
1963The older thread model now uses the C<THX> mechanism to pass context
1964pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
1965later still have it for backward source compatibility, but it is defined
1966to be a no-op.
1967
ee072b34
GS
1968=head2 How do I use all this in extensions?
1969
1970When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1971any functions in the Perl API will need to pass the initial context
1972argument somehow. The kicker is that you will need to write it in
1973such a way that the extension still compiles when Perl hasn't been
1974built with PERL_IMPLICIT_CONTEXT enabled.
1975
1976There are three ways to do this. First, the easy but inefficient way,
1977which is also the default, in order to maintain source compatibility
1978with extensions: whenever XSUB.h is #included, it redefines the aTHX
1979and aTHX_ macros to call a function that will return the context.
1980Thus, something like:
1981
2307c6d0 1982 sv_setiv(sv, num);
ee072b34 1983
4375e838 1984in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1985in effect:
ee072b34 1986
2307c6d0 1987 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 1988
54aff467 1989or to this otherwise:
ee072b34 1990
2307c6d0 1991 Perl_sv_setiv(sv, num);
ee072b34
GS
1992
1993You have to do nothing new in your extension to get this; since
2fa86c13 1994the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1995work.
1996
1997The second, more efficient way is to use the following template for
1998your Foo.xs:
1999
c52f9dcd
JH
2000 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2001 #include "EXTERN.h"
2002 #include "perl.h"
2003 #include "XSUB.h"
ee072b34
GS
2004
2005 static my_private_function(int arg1, int arg2);
2006
c52f9dcd
JH
2007 static SV *
2008 my_private_function(int arg1, int arg2)
2009 {
2010 dTHX; /* fetch context */
2011 ... call many Perl API functions ...
2012 }
ee072b34
GS
2013
2014 [... etc ...]
2015
c52f9dcd 2016 MODULE = Foo PACKAGE = Foo
ee072b34 2017
c52f9dcd 2018 /* typical XSUB */
ee072b34 2019
c52f9dcd
JH
2020 void
2021 my_xsub(arg)
2022 int arg
2023 CODE:
2024 my_private_function(arg, 10);
ee072b34
GS
2025
2026Note that the only two changes from the normal way of writing an
2027extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2028including the Perl headers, followed by a C<dTHX;> declaration at
2029the start of every function that will call the Perl API. (You'll
2030know which functions need this, because the C compiler will complain
2031that there's an undeclared identifier in those functions.) No changes
2032are needed for the XSUBs themselves, because the XS() macro is
2033correctly defined to pass in the implicit context if needed.
2034
2035The third, even more efficient way is to ape how it is done within
2036the Perl guts:
2037
2038
c52f9dcd
JH
2039 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2040 #include "EXTERN.h"
2041 #include "perl.h"
2042 #include "XSUB.h"
ee072b34
GS
2043
2044 /* pTHX_ only needed for functions that call Perl API */
2045 static my_private_function(pTHX_ int arg1, int arg2);
2046
c52f9dcd
JH
2047 static SV *
2048 my_private_function(pTHX_ int arg1, int arg2)
2049 {
2050 /* dTHX; not needed here, because THX is an argument */
2051 ... call Perl API functions ...
2052 }
ee072b34
GS
2053
2054 [... etc ...]
2055
c52f9dcd 2056 MODULE = Foo PACKAGE = Foo
ee072b34 2057
c52f9dcd 2058 /* typical XSUB */
ee072b34 2059
c52f9dcd
JH
2060 void
2061 my_xsub(arg)
2062 int arg
2063 CODE:
2064 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2065
2066This implementation never has to fetch the context using a function
2067call, since it is always passed as an extra argument. Depending on
2068your needs for simplicity or efficiency, you may mix the previous
2069two approaches freely.
2070
651a3225
GS
2071Never add a comma after C<pTHX> yourself--always use the form of the
2072macro with the underscore for functions that take explicit arguments,
2073or the form without the argument for functions with no explicit arguments.
ee072b34 2074
a7486cbb
JH
2075=head2 Should I do anything special if I call perl from multiple threads?
2076
2077If you create interpreters in one thread and then proceed to call them in
2078another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2079initialized correctly in each of those threads.
2080
2081The C<perl_alloc> and C<perl_clone> API functions will automatically set
2082the TLS slot to the interpreter they created, so that there is no need to do
2083anything special if the interpreter is always accessed in the same thread that
2084created it, and that thread did not create or call any other interpreters
2085afterwards. If that is not the case, you have to set the TLS slot of the
2086thread before calling any functions in the Perl API on that particular
2087interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2088thread as the first thing you do:
2089
2090 /* do this before doing anything else with some_perl */
2091 PERL_SET_CONTEXT(some_perl);
2092
2093 ... other Perl API calls on some_perl go here ...
2094
ee072b34
GS
2095=head2 Future Plans and PERL_IMPLICIT_SYS
2096
2097Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2098that the interpreter knows about itself and pass it around, so too are
2099there plans to allow the interpreter to bundle up everything it knows
2100about the environment it's running on. This is enabled with the
acfe0abc 2101PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS
4d1ff10f 2102and USE_5005THREADS on Windows (see inside iperlsys.h).
ee072b34
GS
2103
2104This allows the ability to provide an extra pointer (called the "host"
2105environment) for all the system calls. This makes it possible for
2106all the system stuff to maintain their own state, broken down into
2107seven C structures. These are thin wrappers around the usual system
2108calls (see win32/perllib.c) for the default perl executable, but for a
2109more ambitious host (like the one that would do fork() emulation) all
2110the extra work needed to pretend that different interpreters are
2111actually different "processes", would be done here.
2112
2113The Perl engine/interpreter and the host are orthogonal entities.
2114There could be one or more interpreters in a process, and one or
2115more "hosts", with free association between them.
2116
a422fd2d
SC
2117=head1 Internal Functions
2118
2119All of Perl's internal functions which will be exposed to the outside
06f6df17 2120world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d
SC
2121functions or functions used in a program in which Perl is embedded.
2122Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2123static functions start with C<S_>.)
a422fd2d
SC
2124
2125Inside the Perl core, you can get at the functions either with or
2126without the C<Perl_> prefix, thanks to a bunch of defines that live in
2127F<embed.h>. This header file is generated automatically from
dc9b1d22
MHM
2128F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2129header files for the internal functions, generates the documentation
2130and a lot of other bits and pieces. It's important that when you add
2131a new function to the core or change an existing one, you change the
2132data in the table in F<embed.fnc> as well. Here's a sample entry from
2133that table:
a422fd2d
SC
2134
2135 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2136
2137The second column is the return type, the third column the name. Columns
2138after that are the arguments. The first column is a set of flags:
2139
2140=over 3
2141
2142=item A
2143
2144This function is a part of the public API.
2145
2146=item p
2147
2148This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
2149
2150=item d
2151
2152This function has documentation using the C<apidoc> feature which we'll
2153look at in a second.
2154
2155=back
2156
2157Other available flags are:
2158
2159=over 3
2160
2161=item s
2162
a7486cbb
JH
2163This is a static function and is defined as C<S_whatever>, and usually
2164called within the sources as C<whatever(...)>.
a422fd2d
SC
2165
2166=item n
2167
2168This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
2169L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2170
2171=item r
2172
2173This function never returns; C<croak>, C<exit> and friends.
2174
2175=item f
2176
2177This function takes a variable number of arguments, C<printf> style.
2178The argument list should end with C<...>, like this:
2179
2180 Afprd |void |croak |const char* pat|...
2181
a7486cbb 2182=item M
a422fd2d 2183
00aadd71 2184This function is part of the experimental development API, and may change
a422fd2d
SC
2185or disappear without notice.
2186
2187=item o
2188
2189This function should not have a compatibility macro to define, say,
2190C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2191
a422fd2d
SC
2192=item x
2193
2194This function isn't exported out of the Perl core.
2195
dc9b1d22
MHM
2196=item m
2197
2198This is implemented as a macro.
2199
2200=item X
2201
2202This function is explicitly exported.
2203
2204=item E
2205
2206This function is visible to extensions included in the Perl core.
2207
2208=item b
2209
2210Binary backward compatibility; this function is a macro but also has
2211a C<Perl_> implementation (which is exported).
2212
a422fd2d
SC
2213=back
2214
dc9b1d22
MHM
2215If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2216C<make regen_headers> to force a rebuild of F<embed.h> and other
2217auto-generated files.
a422fd2d 2218
6b4667fc 2219=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2220
6b4667fc
A
2221If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2222formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2223following macros for portability
9dd9db0b 2224
c52f9dcd
JH
2225 IVdf IV in decimal
2226 UVuf UV in decimal
2227 UVof UV in octal
2228 UVxf UV in hexadecimal
2229 NVef NV %e-like
2230 NVff NV %f-like
2231 NVgf NV %g-like
9dd9db0b 2232
6b4667fc
A
2233These will take care of 64-bit integers and long doubles.
2234For example:
2235
c52f9dcd 2236 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2237
2238The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2239
8908e76d
JH
2240If you are printing addresses of pointers, use UVxf combined
2241with PTR2UV(), do not use %lx or %p.
2242
2243=head2 Pointer-To-Integer and Integer-To-Pointer
2244
2245Because pointer size does not necessarily equal integer size,
2246use the follow macros to do it right.
2247
c52f9dcd
JH
2248 PTR2UV(pointer)
2249 PTR2IV(pointer)
2250 PTR2NV(pointer)
2251 INT2PTR(pointertotype, integer)
8908e76d
JH
2252
2253For example:
2254
c52f9dcd
JH
2255 IV iv = ...;
2256 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2257
2258and
2259
c52f9dcd
JH
2260 AV *av = ...;
2261 UV uv = PTR2UV(av);
8908e76d 2262
0ca3a874
MHM
2263=head2 Exception Handling
2264
9b5c3821
MHM
2265There are a couple of macros to do very basic exception handling in XS
2266modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2267be able to use these macros:
2268
2269 #define NO_XSLOCKS
2270 #include "XSUB.h"
2271
2272You can use these macros if you call code that may croak, but you need
2273to do some cleanup before giving control back to Perl. For example:
0ca3a874
MHM
2274
2275 dXCPT; /* set up neccessary variables */
2276
2277 XCPT_TRY_START {
2278 code_that_may_croak();
2279 } XCPT_TRY_END
2280
2281 XCPT_CATCH
2282 {
2283 /* do cleanup here */
2284 XCPT_RETHROW;
2285 }
2286
2287Note that you always have to rethrow an exception that has been
2288caught. Using these macros, it is not possible to just catch the
2289exception and ignore it. If you have to ignore the exception, you
2290have to use the C<call_*> function.
2291
2292The advantage of using the above macros is that you don't have
2293to setup an extra function for C<call_*>, and that using these
2294macros is faster than using C<call_*>.
2295
a422fd2d
SC
2296=head2 Source Documentation
2297
2298There's an effort going on to document the internal functions and
2299automatically produce reference manuals from them - L<perlapi> is one
2300such manual which details all the functions which are available to XS
2301writers. L<perlintern> is the autogenerated manual for the functions
2302which are not part of the API and are supposedly for internal use only.
2303
2304Source documentation is created by putting POD comments into the C
2305source, like this:
2306
2307 /*
2308 =for apidoc sv_setiv
2309
2310 Copies an integer into the given SV. Does not handle 'set' magic. See
2311 C<sv_setiv_mg>.
2312
2313 =cut
2314 */
2315
2316Please try and supply some documentation if you add functions to the
2317Perl core.
2318
0d098d33
MHM
2319=head2 Backwards compatibility
2320
2321The Perl API changes over time. New functions are added or the interfaces
2322of existing functions are changed. The C<Devel::PPPort> module tries to
2323provide compatibility code for some of these changes, so XS writers don't
2324have to code it themselves when supporting multiple versions of Perl.
2325
2326C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2327be run as a Perl script. To generate F<ppport.h>, run:
2328
2329 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2330
2331Besides checking existing XS code, the script can also be used to retrieve
2332compatibility information for various API calls using the C<--api-info>
2333command line switch. For example:
2334
2335 % perl ppport.h --api-info=sv_magicext
2336
2337For details, see C<perldoc ppport.h>.
2338
a422fd2d
SC
2339=head1 Unicode Support
2340
2341Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2342writers to understand this support and make sure that the code they
2343write does not corrupt Unicode data.
2344
2345=head2 What B<is> Unicode, anyway?
2346
2347In the olden, less enlightened times, we all used to use ASCII. Most of
2348us did, anyway. The big problem with ASCII is that it's American. Well,
2349no, that's not actually the problem; the problem is that it's not
2350particularly useful for people who don't use the Roman alphabet. What
2351used to happen was that particular languages would stick their own
2352alphabet in the upper range of the sequence, between 128 and 255. Of
2353course, we then ended up with plenty of variants that weren't quite
2354ASCII, and the whole point of it being a standard was lost.
2355
2356Worse still, if you've got a language like Chinese or
2357Japanese that has hundreds or thousands of characters, then you really
2358can't fit them into a mere 256, so they had to forget about ASCII
2359altogether, and build their own systems using pairs of numbers to refer
2360to one character.
2361
2362To fix this, some people formed Unicode, Inc. and
2363produced a new character set containing all the characters you can
2364possibly think of and more. There are several ways of representing these
1e54db1a 2365characters, and the one Perl uses is called UTF-8. UTF-8 uses
a422fd2d 2366a variable number of bytes to represent a character, instead of just
b3b6085d 2367one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d 2368
1e54db1a 2369=head2 How can I recognise a UTF-8 string?
a422fd2d 2370
1e54db1a
JH
2371You can't. This is because UTF-8 data is stored in bytes just like
2372non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d
SC
2373capital E with a grave accent, is represented by the two bytes
2374C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2375has that byte sequence as well. So you can't tell just by looking - this
2376is what makes Unicode input an interesting problem.
2377
2378The API function C<is_utf8_string> can help; it'll tell you if a string
1e54db1a 2379contains only valid UTF-8 characters. However, it can't do the work for
a422fd2d 2380you. On a character-by-character basis, C<is_utf8_char> will tell you
1e54db1a 2381whether the current character in a string is valid UTF-8.
a422fd2d 2382
1e54db1a 2383=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2384
1e54db1a 2385As mentioned above, UTF-8 uses a variable number of bytes to store a
a422fd2d
SC
2386character. Characters with values 1...128 are stored in one byte, just
2387like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2388continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2389bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2390so it goes on, moving to three bytes at character 2048.
2391
1e54db1a 2392Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2393how long the first character in it is with the C<UTF8SKIP> macro:
2394
2395 char *utf = "\305\233\340\240\201";
2396 I32 len;
2397
2398 len = UTF8SKIP(utf); /* len is 2 here */
2399 utf += len;
2400 len = UTF8SKIP(utf); /* len is 3 here */
2401
1e54db1a 2402Another way to skip over characters in a UTF-8 string is to use
a422fd2d
SC
2403C<utf8_hop>, which takes a string and a number of characters to skip
2404over. You're on your own about bounds checking, though, so don't use it
2405lightly.
2406
1e54db1a 2407All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2408so you can test if you need to do something special with this
2409character like this (the UTF8_IS_INVARIANT() is a macro that tests
2410whether the byte can be encoded as a single byte even in UTF-8):
a422fd2d 2411
3a2263fe
RGS
2412 U8 *utf;
2413 UV uv; /* Note: a UV, not a U8, not a char */
a422fd2d 2414
3a2263fe 2415 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2416 /* Must treat this as UTF-8 */
a422fd2d
SC
2417 uv = utf8_to_uv(utf);
2418 else
2419 /* OK to treat this character as a byte */
2420 uv = *utf;
2421
2422You can also see in that example that we use C<utf8_to_uv> to get the
2423value of the character; the inverse function C<uv_to_utf8> is available
1e54db1a 2424for putting a UV into UTF-8:
a422fd2d 2425
3a2263fe 2426 if (!UTF8_IS_INVARIANT(uv))
a422fd2d
SC
2427 /* Must treat this as UTF8 */
2428 utf8 = uv_to_utf8(utf8, uv);
2429 else
2430 /* OK to treat this character as a byte */
2431 *utf8++ = uv;
2432
2433You B<must> convert characters to UVs using the above functions if
1e54db1a
JH
2434you're ever in a situation where you have to match UTF-8 and non-UTF-8
2435characters. You may not skip over UTF-8 characters in this case. If you
2436do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2437for instance, if your UTF-8 string contains C<v196.172>, and you skip
2438that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2439So don't do that!
2440
1e54db1a 2441=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2442
2443Currently, Perl deals with Unicode strings and non-Unicode strings
2444slightly differently. If a string has been identified as being UTF-8
2445encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2446manipulate this flag with the following macros:
2447
2448 SvUTF8(sv)
2449 SvUTF8_on(sv)
2450 SvUTF8_off(sv)
2451
2452This flag has an important effect on Perl's treatment of the string: if
2453Unicode data is not properly distinguished, regular expressions,
2454C<length>, C<substr> and other string handling operations will have
2455undesirable results.
2456
2457The problem comes when you have, for instance, a string that isn't
1e54db1a
JH
2458flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
2459especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2460
2461Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2462need be sure you don't accidentally knock it off while you're
2463manipulating SVs. More specifically, you cannot expect to do this:
2464
2465 SV *sv;
2466 SV *nsv;
2467 STRLEN len;
2468 char *p;
2469
2470 p = SvPV(sv, len);
2471 frobnicate(p);
2472 nsv = newSVpvn(p, len);
2473
2474The C<char*> string does not tell you the whole story, and you can't
2475copy or reconstruct an SV just by copying the string value. Check if the
1e54db1a 2476old SV has the UTF-8 flag set, and act accordingly:
a422fd2d
SC
2477
2478 p = SvPV(sv, len);
2479 frobnicate(p);
2480 nsv = newSVpvn(p, len);
2481 if (SvUTF8(sv))
2482 SvUTF8_on(nsv);
2483
2484In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2485not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2486appropriately.
2487
3a2263fe 2488Since just passing an SV to an XS function and copying the data of
1e54db1a 2489the SV is not enough to copy the UTF-8 flags, even less right is just
3a2263fe
RGS
2490passing a C<char *> to an XS function.
2491
1e54db1a 2492=head2 How do I convert a string to UTF-8?
a422fd2d 2493
1e54db1a
JH
2494If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary
2495to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
a422fd2d
SC
2496way to do this is:
2497
2498 sv_utf8_upgrade(sv);
2499
2500However, you must not do this, for example:
2501
2502 if (!SvUTF8(left))
2503 sv_utf8_upgrade(left);
2504
2505If you do this in a binary operator, you will actually change one of the
b1866b2d 2506strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2507by the end user, it can cause problems.
2508
1e54db1a 2509Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
a422fd2d 2510string argument. This is useful for having the data available for
b1866b2d 2511comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2512C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2513the string contains any characters above 255 that can't be represented
2514in a single byte.
2515
2516=head2 Is there anything else I need to know?
2517
2518Not really. Just remember these things:
2519
2520=over 3
2521
2522=item *
2523
1e54db1a
JH
2524There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2525is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2526something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2527it's not - if you pass on the PV to somewhere, pass on the flag too.
2528
2529=item *
2530
1e54db1a 2531If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
3a2263fe 2532unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2533
2534=item *
2535
1e54db1a 2536When writing a character C<uv> to a UTF-8 string, B<always> use
3a2263fe
RGS
2537C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2538you can use C<*s = uv>.
a422fd2d
SC
2539
2540=item *
2541
1e54db1a
JH
2542Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2543a new string which is UTF-8 encoded. There are tricks you can use to
2544delay deciding whether you need to use a UTF-8 string until you get to a
a422fd2d
SC
2545high character - C<HALF_UPGRADE> is one of those.
2546
2547=back
2548
53e06cf0
SC
2549=head1 Custom Operators
2550
9a68f1db 2551Custom operator support is a new experimental feature that allows you to
53e06cf0
SC
2552define your own ops. This is primarily to allow the building of
2553interpreters for other languages in the Perl core, but it also allows
2554optimizations through the creation of "macro-ops" (ops which perform the
2555functions of multiple ops which are usually executed together, such as
b7cb320d 2556C<gvsv, gvsv, add>.)
53e06cf0 2557
b455bf3f 2558This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0
SC
2559core does not "know" anything special about this op type, and so it will
2560not be involved in any optimizations. This also means that you can
2561define your custom ops to be any op structure - unary, binary, list and
2562so on - you like.
2563
2564It's important to know what custom operators won't do for you. They
2565won't let you add new syntax to Perl, directly. They won't even let you
2566add new keywords, directly. In fact, they won't change the way Perl
2567compiles a program at all. You have to do those changes yourself, after
2568Perl has compiled the program. You do this either by manipulating the op
2569tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2570a custom peephole optimizer with the C<optimize> module.
2571
2572When you do this, you replace ordinary Perl ops with custom ops by
2573creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2574PP function. This should be defined in XS code, and should look like
2575the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2576takes the appropriate number of values from the stack, and you are
2577responsible for adding stack marks if necessary.
2578
2579You should also "register" your op with the Perl interpreter so that it
2580can produce sensible error and warning messages. Since it is possible to
2581have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2582Perl uses the value of C<< o->op_ppaddr >> as a key into the
2583C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2584need to enter a name and description for your op at the appropriate
2585place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2586
2587Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2588directly support the creation of custom ops by name; C<Opcodes::Custom>
2589will provide functions which make it trivial to "register" custom ops to
2590the Perl interpreter.
2591
954c1994 2592=head1 AUTHORS
e89caa19 2593
954c1994 2594Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2595E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2596itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 2597
954c1994
GS
2598With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2599Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2600Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2601Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2602
954c1994 2603=head1 SEE ALSO
cb1a09d0 2604
954c1994 2605perlapi(1), perlintern(1), perlxs(1), perlembed(1)