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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
JO
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
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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
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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
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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
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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
0cbee0a4 1049 < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref
14befaf4 1050 ~ PERL_MAGIC_ext (none) Available for use by extensions
0cbee0a4
DM
1051 : PERL_MAGIC_symtab (none) hash used as symbol table
1052 % PERL_MAGIC_rhash (none) hash used as restricted hash
1053 @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a
1054
d1b91892 1055
68dc0745 1056When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1057uppercase letter is typically used to represent some kind of composite type
1058(a list or a hash), and the lowercase letter is used to represent an element
1059of that composite type. Some internals code makes use of this case
1060relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1061
1062The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1063specifically for use by extensions and will not be used by perl itself.
1064Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1065to variables (typically objects). This is especially useful because
1066there is no way for normal perl code to corrupt this private information
1067(unlike using extra elements of a hash object).
1068
1069Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1070C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1071C<mg_ptr> field points to a C<ufuncs> structure:
1072
1073 struct ufuncs {
a9402793
AB
1074 I32 (*uf_val)(pTHX_ IV, SV*);
1075 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1076 IV uf_index;
1077 };
1078
1079When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1080function will be called with C<uf_index> as the first arg and a pointer to
1081the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1082magic is shown below. Note that the ufuncs structure is copied by
1083sv_magic, so you can safely allocate it on the stack.
1084
1085 void
1086 Umagic(sv)
1087 SV *sv;
1088 PREINIT:
1089 struct ufuncs uf;
1090 CODE:
1091 uf.uf_val = &my_get_fn;
1092 uf.uf_set = &my_set_fn;
1093 uf.uf_index = 0;
14befaf4 1094 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1095
14befaf4
DM
1096Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1097or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1098extra care to avoid conflict. Typically only using the magic on
1099objects blessed into the same class as the extension is sufficient.
1100For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1101'signature' at the top of the private data area and check that.
5f05dabc 1102
ef50df4b
GS
1103Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1104earlier do B<not> invoke 'set' magic on their targets. This must
1105be done by the user either by calling the C<SvSETMAGIC()> macro after
1106calling these functions, or by using one of the C<sv_set*_mg()> or
1107C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1108C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1109obtained from external sources in functions that don't handle magic.
4a4eefd0 1110See L<perlapi> for a description of these functions.
189b2af5
GS
1111For example, calls to the C<sv_cat*()> functions typically need to be
1112followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1113since their implementation handles 'get' magic.
1114
d1b91892
AD
1115=head2 Finding Magic
1116
1117 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1118
1119This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1120If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1121if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1122
08105a92 1123 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1124
1125This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1126field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1127the mg_type field is changed to be the lowercase letter.
a0d0e21e 1128
04343c6d
GS
1129=head2 Understanding the Magic of Tied Hashes and Arrays
1130
14befaf4
DM
1131Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1132magic type.
9edb2b46
GS
1133
1134WARNING: As of the 5.004 release, proper usage of the array and hash
1135access functions requires understanding a few caveats. Some
1136of these caveats are actually considered bugs in the API, to be fixed
1137in later releases, and are bracketed with [MAYCHANGE] below. If
1138you find yourself actually applying such information in this section, be
1139aware that the behavior may change in the future, umm, without warning.
04343c6d 1140
1526ead6 1141The perl tie function associates a variable with an object that implements
9a68f1db 1142the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1143tie function from an XSUB, you must mimic this behaviour. The code below
1144carries out the necessary steps - firstly it creates a new hash, and then
1145creates a second hash which it blesses into the class which will implement
1146the tie methods. Lastly it ties the two hashes together, and returns a
1147reference to the new tied hash. Note that the code below does NOT call the
1148TIEHASH method in the MyTie class -
1149see L<Calling Perl Routines from within C Programs> for details on how
1150to do this.
1151
1152 SV*
1153 mytie()
1154 PREINIT:
1155 HV *hash;
1156 HV *stash;
1157 SV *tie;
1158 CODE:
1159 hash = newHV();
1160 tie = newRV_noinc((SV*)newHV());
1161 stash = gv_stashpv("MyTie", TRUE);
1162 sv_bless(tie, stash);
899e16d0 1163 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1164 RETVAL = newRV_noinc(hash);
1165 OUTPUT:
1166 RETVAL
1167
04343c6d
GS
1168The C<av_store> function, when given a tied array argument, merely
1169copies the magic of the array onto the value to be "stored", using
1170C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1171actually need to be stored in the array. [MAYCHANGE] After a call to
1172C<av_store> on a tied array, the caller will usually need to call
1173C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1174TIEARRAY object. If C<av_store> did return NULL, a call to
1175C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1176leak. [/MAYCHANGE]
04343c6d
GS
1177
1178The previous paragraph is applicable verbatim to tied hash access using the
1179C<hv_store> and C<hv_store_ent> functions as well.
1180
1181C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1182C<hv_fetch_ent> actually return an undefined mortal value whose magic
1183has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1184need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1185need to call C<mg_get()> on the returned value in order to actually invoke
1186the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1187you may also call C<mg_set()> on the return value after possibly assigning
1188a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1189method on the TIE object. [/MAYCHANGE]
04343c6d 1190
9edb2b46 1191[MAYCHANGE]
04343c6d
GS
1192In other words, the array or hash fetch/store functions don't really
1193fetch and store actual values in the case of tied arrays and hashes. They
1194merely call C<mg_copy> to attach magic to the values that were meant to be
1195"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1196do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1197the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1198and hashes.
1199
1200Currently (as of perl version 5.004), use of the hash and array access
1201functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1202"normal" hashes and arrays, or on their tied variants. The API may be
1203changed to provide more transparent access to both tied and normal data
1204types in future versions.
1205[/MAYCHANGE]
04343c6d
GS
1206
1207You would do well to understand that the TIEARRAY and TIEHASH interfaces
1208are mere sugar to invoke some perl method calls while using the uniform hash
1209and array syntax. The use of this sugar imposes some overhead (typically
1210about two to four extra opcodes per FETCH/STORE operation, in addition to
1211the creation of all the mortal variables required to invoke the methods).
1212This overhead will be comparatively small if the TIE methods are themselves
1213substantial, but if they are only a few statements long, the overhead
1214will not be insignificant.
1215
d1c897a1
IZ
1216=head2 Localizing changes
1217
1218Perl has a very handy construction
1219
1220 {
1221 local $var = 2;
1222 ...
1223 }
1224
1225This construction is I<approximately> equivalent to
1226
1227 {
1228 my $oldvar = $var;
1229 $var = 2;
1230 ...
1231 $var = $oldvar;
1232 }
1233
1234The biggest difference is that the first construction would
1235reinstate the initial value of $var, irrespective of how control exits
9a68f1db 1236the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1237more efficient as well.
1238
1239There is a way to achieve a similar task from C via Perl API: create a
1240I<pseudo-block>, and arrange for some changes to be automatically
1241undone at the end of it, either explicit, or via a non-local exit (via
1242die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1243C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1244Such a construct may be created specially for some important localized
1245task, or an existing one (like boundaries of enclosing Perl
1246subroutine/block, or an existing pair for freeing TMPs) may be
1247used. (In the second case the overhead of additional localization must
1248be almost negligible.) Note that any XSUB is automatically enclosed in
1249an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1250
1251Inside such a I<pseudo-block> the following service is available:
1252
13a2d996 1253=over 4
d1c897a1
IZ
1254
1255=item C<SAVEINT(int i)>
1256
1257=item C<SAVEIV(IV i)>
1258
1259=item C<SAVEI32(I32 i)>
1260
1261=item C<SAVELONG(long i)>
1262
1263These macros arrange things to restore the value of integer variable
1264C<i> at the end of enclosing I<pseudo-block>.
1265
1266=item C<SAVESPTR(s)>
1267
1268=item C<SAVEPPTR(p)>
1269
1270These macros arrange things to restore the value of pointers C<s> and
1271C<p>. C<s> must be a pointer of a type which survives conversion to
1272C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1273and back.
1274
1275=item C<SAVEFREESV(SV *sv)>
1276
1277The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1278I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1279mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1280extends the lifetime of C<sv> until the beginning of the next statement,
1281C<SAVEFREESV> extends it until the end of the enclosing scope. These
1282lifetimes can be wildly different.
1283
1284Also compare C<SAVEMORTALIZESV>.
1285
1286=item C<SAVEMORTALIZESV(SV *sv)>
1287
1288Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1289scope instead of decrementing its reference count. This usually has the
1290effect of keeping C<sv> alive until the statement that called the currently
1291live scope has finished executing.
d1c897a1
IZ
1292
1293=item C<SAVEFREEOP(OP *op)>
1294
1295The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1296
1297=item C<SAVEFREEPV(p)>
1298
1299The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1300end of I<pseudo-block>.
1301
1302=item C<SAVECLEARSV(SV *sv)>
1303
1304Clears a slot in the current scratchpad which corresponds to C<sv> at
1305the end of I<pseudo-block>.
1306
1307=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1308
1309The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1310string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1311short-lived storage, the corresponding string may be reallocated like
1312this:
1313
9cde0e7f 1314 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1315
c76ac1ee 1316=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1317
1318At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1319only argument C<p>.
1320
1321=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1322
1323At the end of I<pseudo-block> the function C<f> is called with the
1324implicit context argument (if any), and C<p>.
d1c897a1
IZ
1325
1326=item C<SAVESTACK_POS()>
1327
1328The current offset on the Perl internal stack (cf. C<SP>) is restored
1329at the end of I<pseudo-block>.
1330
1331=back
1332
1333The following API list contains functions, thus one needs to
1334provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1335or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1336function takes C<int *>.
1337
13a2d996 1338=over 4
d1c897a1
IZ
1339
1340=item C<SV* save_scalar(GV *gv)>
1341
1342Equivalent to Perl code C<local $gv>.
1343
1344=item C<AV* save_ary(GV *gv)>
1345
1346=item C<HV* save_hash(GV *gv)>
1347
1348Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1349
1350=item C<void save_item(SV *item)>
1351
1352Duplicates the current value of C<SV>, on the exit from the current
1353C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
038fcae3
SB
1354using the stored value. It doesn't handle magic. Use C<save_scalar> if
1355magic is affected.
d1c897a1
IZ
1356
1357=item C<void save_list(SV **sarg, I32 maxsarg)>
1358
1359A variant of C<save_item> which takes multiple arguments via an array
1360C<sarg> of C<SV*> of length C<maxsarg>.
1361
1362=item C<SV* save_svref(SV **sptr)>
1363
d1be9408 1364Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1365
1366=item C<void save_aptr(AV **aptr)>
1367
1368=item C<void save_hptr(HV **hptr)>
1369
1370Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1371
1372=back
1373
1374The C<Alias> module implements localization of the basic types within the
1375I<caller's scope>. People who are interested in how to localize things in
1376the containing scope should take a look there too.
1377
0a753a76 1378=head1 Subroutines
a0d0e21e 1379
68dc0745 1380=head2 XSUBs and the Argument Stack
5f05dabc
PP
1381
1382The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1383An XSUB routine will have a stack that contains the arguments from the Perl
1384program, and a way to map from the Perl data structures to a C equivalent.
1385
1386The stack arguments are accessible through the C<ST(n)> macro, which returns
1387the C<n>'th stack argument. Argument 0 is the first argument passed in the
1388Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1389an C<SV*> is used.
1390
1391Most of the time, output from the C routine can be handled through use of
1392the RETVAL and OUTPUT directives. However, there are some cases where the
1393argument stack is not already long enough to handle all the return values.
1394An example is the POSIX tzname() call, which takes no arguments, but returns
1395two, the local time zone's standard and summer time abbreviations.
1396
1397To handle this situation, the PPCODE directive is used and the stack is
1398extended using the macro:
1399
924508f0 1400 EXTEND(SP, num);
5f05dabc 1401
924508f0
GS
1402where C<SP> is the macro that represents the local copy of the stack pointer,
1403and C<num> is the number of elements the stack should be extended by.
5f05dabc 1404
00aadd71 1405Now that there is room on the stack, values can be pushed on it using C<PUSHs>
06f6df17 1406macro. The pushed values will often need to be "mortal" (See
d82b684c 1407L</Reference Counts and Mortality>):
5f05dabc 1408
00aadd71 1409 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1410 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1411 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1412 PUSHs(sv_2mortal(newSVpv("Some String",0)))
5f05dabc
PP
1413
1414And now the Perl program calling C<tzname>, the two values will be assigned
1415as in:
1416
1417 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1418
1419An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1420to use the macro:
5f05dabc 1421
5f05dabc
PP
1422 XPUSHs(SV*)
1423
00aadd71 1424This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1425do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1426
1427Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1428C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1429For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1430C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1431
1432For more information, consult L<perlxs> and L<perlxstut>.
1433
1434=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1435
1436There are four routines that can be used to call a Perl subroutine from
1437within a C program. These four are:
1438
954c1994
GS
1439 I32 call_sv(SV*, I32);
1440 I32 call_pv(const char*, I32);
1441 I32 call_method(const char*, I32);
1442 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1443
954c1994 1444The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1445contains either the name of the Perl subroutine to be called, or a
1446reference to the subroutine. The second argument consists of flags
1447that control the context in which the subroutine is called, whether
1448or not the subroutine is being passed arguments, how errors should be
1449trapped, and how to treat return values.
a0d0e21e
LW
1450
1451All four routines return the number of arguments that the subroutine returned
1452on the Perl stack.
1453
9a68f1db 1454These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1455but those names are now deprecated; macros of the same name are provided for
1456compatibility.
1457
1458When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1459must manipulate the Perl stack. These include the following macros and
1460functions:
a0d0e21e
LW
1461
1462 dSP
924508f0 1463 SP
a0d0e21e
LW
1464 PUSHMARK()
1465 PUTBACK
1466 SPAGAIN
1467 ENTER
1468 SAVETMPS
1469 FREETMPS
1470 LEAVE
1471 XPUSH*()
cb1a09d0 1472 POP*()
a0d0e21e 1473
5f05dabc
PP
1474For a detailed description of calling conventions from C to Perl,
1475consult L<perlcall>.
a0d0e21e 1476
5f05dabc 1477=head2 Memory Allocation
a0d0e21e 1478
06f6df17
RGS
1479=head3 Allocation
1480
86058a2d
GS
1481All memory meant to be used with the Perl API functions should be manipulated
1482using the macros described in this section. The macros provide the necessary
1483transparency between differences in the actual malloc implementation that is
1484used within perl.
1485
1486It is suggested that you enable the version of malloc that is distributed
5f05dabc 1487with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1488order to satisfy allocation requests more quickly. However, on some
1489platforms, it may cause spurious malloc or free errors.
d1b91892 1490
06f6df17
RGS
1491The following three macros are used to initially allocate memory :
1492
d1b91892
AD
1493 New(x, pointer, number, type);
1494 Newc(x, pointer, number, type, cast);
1495 Newz(x, pointer, number, type);
1496
5f05dabc
PP
1497The first argument C<x> was a "magic cookie" that was used to keep track
1498of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1499the current code makes no use of this feature (most Perl developers now
1500use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1501
1502The second argument C<pointer> should be the name of a variable that will
1503point to the newly allocated memory.
d1b91892 1504
d1b91892
AD
1505The third and fourth arguments C<number> and C<type> specify how many of
1506the specified type of data structure should be allocated. The argument
1507C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1508should be used if the C<pointer> argument is different from the C<type>
1509argument.
1510
1511Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1512to zero out all the newly allocated memory.
1513
06f6df17
RGS
1514=head3 Reallocation
1515
d1b91892
AD
1516 Renew(pointer, number, type);
1517 Renewc(pointer, number, type, cast);
1518 Safefree(pointer)
1519
1520These three macros are used to change a memory buffer size or to free a
1521piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1522match those of C<New> and C<Newc> with the exception of not needing the
1523"magic cookie" argument.
1524
06f6df17
RGS
1525=head3 Moving
1526
d1b91892
AD
1527 Move(source, dest, number, type);
1528 Copy(source, dest, number, type);
1529 Zero(dest, number, type);
1530
1531These three macros are used to move, copy, or zero out previously allocated
1532memory. The C<source> and C<dest> arguments point to the source and
1533destination starting points. Perl will move, copy, or zero out C<number>
1534instances of the size of the C<type> data structure (using the C<sizeof>
1535function).
a0d0e21e 1536
5f05dabc 1537=head2 PerlIO
ce3d39e2 1538
5f05dabc
PP
1539The most recent development releases of Perl has been experimenting with
1540removing Perl's dependency on the "normal" standard I/O suite and allowing
1541other stdio implementations to be used. This involves creating a new
1542abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1543was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1544abstraction layer and not make any assumptions about what kind of stdio
1545is being used.
1546
1547For a complete description of the PerlIO abstraction, consult L<perlapio>.
1548
8ebc5c01 1549=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1550
1551A lot of opcodes (this is an elementary operation in the internal perl
1552stack machine) put an SV* on the stack. However, as an optimization
1553the corresponding SV is (usually) not recreated each time. The opcodes
1554reuse specially assigned SVs (I<target>s) which are (as a corollary)
1555not constantly freed/created.
1556
0a753a76 1557Each of the targets is created only once (but see
ce3d39e2
IZ
1558L<Scratchpads and recursion> below), and when an opcode needs to put
1559an integer, a double, or a string on stack, it just sets the
1560corresponding parts of its I<target> and puts the I<target> on stack.
1561
1562The macro to put this target on stack is C<PUSHTARG>, and it is
1563directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1564others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1565
1bd1c0d5
SC
1566Because the target is reused, you must be careful when pushing multiple
1567values on the stack. The following code will not do what you think:
1568
1569 XPUSHi(10);
1570 XPUSHi(20);
1571
1572This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1573the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1574At the end of the operation, the stack does not contain the values 10
1575and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1576to 20.
1bd1c0d5 1577
d82b684c
SH
1578If you need to push multiple different values then you should either use
1579the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1580none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1581SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1582will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1583this a little easier to achieve by creating a new mortal for you (via
1584C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1585in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1586Thus, instead of writing this to "fix" the example above:
1587
1588 XPUSHs(sv_2mortal(newSViv(10)))
1589 XPUSHs(sv_2mortal(newSViv(20)))
1590
1591you can simply write:
1592
1593 mXPUSHi(10)
1594 mXPUSHi(20)
1595
1596On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1597need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1598macros can make use of the local variable C<TARG>. See also C<dTARGET>
1599and C<dXSTARG>.
1bd1c0d5 1600
8ebc5c01 1601=head2 Scratchpads
ce3d39e2 1602
54310121 1603The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1604are created. The answer is that they are created when the current unit --
1605a subroutine or a file (for opcodes for statements outside of
1606subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1607array is created, which is called a scratchpad for the current
1608unit.
1609
54310121 1610A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1611targets for opcodes. One can deduce that an SV lives on a scratchpad
1612by looking on its flags: lexicals have C<SVs_PADMY> set, and
1613I<target>s have C<SVs_PADTMP> set.
1614
54310121
PP
1615The correspondence between OPs and I<target>s is not 1-to-1. Different
1616OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1617would not conflict with the expected life of the temporary.
1618
2ae324a7 1619=head2 Scratchpads and recursion
ce3d39e2
IZ
1620
1621In fact it is not 100% true that a compiled unit contains a pointer to
1622the scratchpad AV. In fact it contains a pointer to an AV of
1623(initially) one element, and this element is the scratchpad AV. Why do
1624we need an extra level of indirection?
1625
9a68f1db 1626The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2
IZ
1627these can create several execution pointers going into the same
1628subroutine. For the subroutine-child not write over the temporaries
1629for the subroutine-parent (lifespan of which covers the call to the
1630child), the parent and the child should have different
1631scratchpads. (I<And> the lexicals should be separate anyway!)
1632
5f05dabc
PP
1633So each subroutine is born with an array of scratchpads (of length 1).
1634On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1635depth of the recursion is not more than the length of this array, and
1636if it is, new scratchpad is created and pushed into the array.
1637
1638The I<target>s on this scratchpad are C<undef>s, but they are already
1639marked with correct flags.
1640
0a753a76
PP
1641=head1 Compiled code
1642
1643=head2 Code tree
1644
1645Here we describe the internal form your code is converted to by
1646Perl. Start with a simple example:
1647
1648 $a = $b + $c;
1649
1650This is converted to a tree similar to this one:
1651
1652 assign-to
1653 / \
1654 + $a
1655 / \
1656 $b $c
1657
7b8d334a 1658(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1659parsed your code, but has nothing to do with the execution order.
1660There is an additional "thread" going through the nodes of the tree
1661which shows the order of execution of the nodes. In our simplified
1662example above it looks like:
1663
1664 $b ---> $c ---> + ---> $a ---> assign-to
1665
1666But with the actual compile tree for C<$a = $b + $c> it is different:
1667some nodes I<optimized away>. As a corollary, though the actual tree
1668contains more nodes than our simplified example, the execution order
1669is the same as in our example.
1670
1671=head2 Examining the tree
1672
06f6df17
RGS
1673If you have your perl compiled for debugging (usually done with
1674C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1675compiled tree by specifying C<-Dx> on the Perl command line. The
1676output takes several lines per node, and for C<$b+$c> it looks like
1677this:
1678
1679 5 TYPE = add ===> 6
1680 TARG = 1
1681 FLAGS = (SCALAR,KIDS)
1682 {
1683 TYPE = null ===> (4)
1684 (was rv2sv)
1685 FLAGS = (SCALAR,KIDS)
1686 {
1687 3 TYPE = gvsv ===> 4
1688 FLAGS = (SCALAR)
1689 GV = main::b
1690 }
1691 }
1692 {
1693 TYPE = null ===> (5)
1694 (was rv2sv)
1695 FLAGS = (SCALAR,KIDS)
1696 {
1697 4 TYPE = gvsv ===> 5
1698 FLAGS = (SCALAR)
1699 GV = main::c
1700 }
1701 }
1702
1703This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1704not optimized away (one per number in the left column). The immediate
1705children of the given node correspond to C<{}> pairs on the same level
1706of indentation, thus this listing corresponds to the tree:
1707
1708 add
1709 / \
1710 null null
1711 | |
1712 gvsv gvsv
1713
1714The execution order is indicated by C<===E<gt>> marks, thus it is C<3
17154 5 6> (node C<6> is not included into above listing), i.e.,
1716C<gvsv gvsv add whatever>.
1717
9afa14e3
SC
1718Each of these nodes represents an op, a fundamental operation inside the
1719Perl core. The code which implements each operation can be found in the
1720F<pp*.c> files; the function which implements the op with type C<gvsv>
1721is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1722different numbers of children: C<add> is a binary operator, as one would
1723expect, and so has two children. To accommodate the various different
1724numbers of children, there are various types of op data structure, and
1725they link together in different ways.
1726
1727The simplest type of op structure is C<OP>: this has no children. Unary
1728operators, C<UNOP>s, have one child, and this is pointed to by the
1729C<op_first> field. Binary operators (C<BINOP>s) have not only an
1730C<op_first> field but also an C<op_last> field. The most complex type of
1731op is a C<LISTOP>, which has any number of children. In this case, the
1732first child is pointed to by C<op_first> and the last child by
1733C<op_last>. The children in between can be found by iteratively
1734following the C<op_sibling> pointer from the first child to the last.
1735
1736There are also two other op types: a C<PMOP> holds a regular expression,
1737and has no children, and a C<LOOP> may or may not have children. If the
1738C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1739complicate matters, if a C<UNOP> is actually a C<null> op after
1740optimization (see L</Compile pass 2: context propagation>) it will still
1741have children in accordance with its former type.
1742
06f6df17
RGS
1743Another way to examine the tree is to use a compiler back-end module, such
1744as L<B::Concise>.
1745
0a753a76
PP
1746=head2 Compile pass 1: check routines
1747
8870b5c7
GS
1748The tree is created by the compiler while I<yacc> code feeds it
1749the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1750the first pass of perl compilation.
1751
1752What makes this pass interesting for perl developers is that some
1753optimization may be performed on this pass. This is optimization by
8870b5c7 1754so-called "check routines". The correspondence between node names
0a753a76
PP
1755and corresponding check routines is described in F<opcode.pl> (do not
1756forget to run C<make regen_headers> if you modify this file).
1757
1758A check routine is called when the node is fully constructed except
7b8d334a 1759for the execution-order thread. Since at this time there are no
0a753a76
PP
1760back-links to the currently constructed node, one can do most any
1761operation to the top-level node, including freeing it and/or creating
1762new nodes above/below it.
1763
1764The check routine returns the node which should be inserted into the
1765tree (if the top-level node was not modified, check routine returns
1766its argument).
1767
1768By convention, check routines have names C<ck_*>. They are usually
1769called from C<new*OP> subroutines (or C<convert>) (which in turn are
1770called from F<perly.y>).
1771
1772=head2 Compile pass 1a: constant folding
1773
1774Immediately after the check routine is called the returned node is
1775checked for being compile-time executable. If it is (the value is
1776judged to be constant) it is immediately executed, and a I<constant>
1777node with the "return value" of the corresponding subtree is
1778substituted instead. The subtree is deleted.
1779
1780If constant folding was not performed, the execution-order thread is
1781created.
1782
1783=head2 Compile pass 2: context propagation
1784
1785When a context for a part of compile tree is known, it is propagated
a3cb178b 1786down through the tree. At this time the context can have 5 values
0a753a76
PP
1787(instead of 2 for runtime context): void, boolean, scalar, list, and
1788lvalue. In contrast with the pass 1 this pass is processed from top
1789to bottom: a node's context determines the context for its children.
1790
1791Additional context-dependent optimizations are performed at this time.
1792Since at this moment the compile tree contains back-references (via
1793"thread" pointers), nodes cannot be free()d now. To allow
1794optimized-away nodes at this stage, such nodes are null()ified instead
1795of free()ing (i.e. their type is changed to OP_NULL).
1796
1797=head2 Compile pass 3: peephole optimization
1798
1799After the compile tree for a subroutine (or for an C<eval> or a file)
1800is created, an additional pass over the code is performed. This pass
1801is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1802additional complications for conditionals). These optimizations are
0a753a76
PP
1803done in the subroutine peep(). Optimizations performed at this stage
1804are subject to the same restrictions as in the pass 2.
1805
1ba7f851
PJ
1806=head2 Pluggable runops
1807
1808The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
1809functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
1810with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
1811control over the execution of the compile tree it is possible to provide
1812your own runops function.
1ba7f851
PJ
1813
1814It's probably best to copy one of the existing runops functions and
1815change it to suit your needs. Then, in the BOOT section of your XS
1816file, add the line:
1817
1818 PL_runops = my_runops;
1819
1820This function should be as efficient as possible to keep your programs
1821running as fast as possible.
1822
9afa14e3
SC
1823=head1 Examining internal data structures with the C<dump> functions
1824
1825To aid debugging, the source file F<dump.c> contains a number of
1826functions which produce formatted output of internal data structures.
1827
1828The most commonly used of these functions is C<Perl_sv_dump>; it's used
1829for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1830C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1831module should already be familiar with its format.
9afa14e3
SC
1832
1833C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 1834derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
1835C<Perl_dump_eval> will dump the main root of the code being evaluated,
1836exactly like C<-Dx>.
1837
1838Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1839op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1840subroutines in a package like so: (Thankfully, these are all xsubs, so
1841there is no op tree)
1842
1843 (gdb) print Perl_dump_packsubs(PL_defstash)
1844
1845 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1846
1847 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1848
1849 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1850
1851 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1852
1853 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1854
1855and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1856the op tree of the main root.
1857
954c1994 1858=head1 How multiple interpreters and concurrency are supported
ee072b34 1859
ee072b34
GS
1860=head2 Background and PERL_IMPLICIT_CONTEXT
1861
1862The Perl interpreter can be regarded as a closed box: it has an API
1863for feeding it code or otherwise making it do things, but it also has
1864functions for its own use. This smells a lot like an object, and
1865there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
1866interpreters, with one interpreter represented either as a C structure,
1867or inside a thread-specific structure. These structures contain all
1868the context, the state of that interpreter.
1869
9a68f1db 1870Two macros control the major Perl build flavors: MULTIPLICITY and
acfe0abc
GS
1871USE_5005THREADS. The MULTIPLICITY build has a C structure
1872that packages all the interpreter state, and there is a similar thread-specific
1873data structure under USE_5005THREADS. In both cases,
54aff467
GS
1874PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1875support for passing in a "hidden" first argument that represents all three
651a3225 1876data structures.
54aff467 1877
27da23d5
JH
1878Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
1879PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
1880former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
1881internal variables of Perl to be wrapped inside a single global struct,
1882struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
1883the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
1884one step further, there is still a single struct (allocated in main()
1885either from heap or from stack) but there are no global data symbols
1886pointing to it. In either case the global struct should be initialised
1887as the very first thing in main() using Perl_init_global_struct() and
1888correspondingly tear it down after perl_free() using Perl_free_global_struct(),
1889please see F<miniperlmain.c> for usage details. You may also need
1890to use C<dVAR> in your coding to "declare the global variables"
1891when you are using them. dTHX does this for you automatically.
1892
1893For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
1894doesn't actually hide all symbols inside a big global struct: some
1895PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
1896then hides everything (see how the PERLIO_FUNCS_DECL is used).
1897
54aff467 1898All this obviously requires a way for the Perl internal functions to be
acfe0abc 1899either subroutines taking some kind of structure as the first
ee072b34 1900argument, or subroutines taking nothing as the first argument. To
acfe0abc 1901enable these two very different ways of building the interpreter,
ee072b34
GS
1902the Perl source (as it does in so many other situations) makes heavy
1903use of macros and subroutine naming conventions.
1904
54aff467 1905First problem: deciding which functions will be public API functions and
00aadd71 1906which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1907(think "S" for "secret" or "static"). All other functions begin with
1908"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1909part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1910function is part of the API is to find its entry in L<perlapi>.
1911If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1912think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1913L<perlbug> explaining why you think it should be.
ee072b34
GS
1914
1915Second problem: there must be a syntax so that the same subroutine
1916declarations and calls can pass a structure as their first argument,
1917or pass nothing. To solve this, the subroutines are named and
1918declared in a particular way. Here's a typical start of a static
1919function used within the Perl guts:
1920
1921 STATIC void
1922 S_incline(pTHX_ char *s)
1923
acfe0abc
GS
1924STATIC becomes "static" in C, and may be #define'd to nothing in some
1925configurations in future.
ee072b34 1926
651a3225
GS
1927A public function (i.e. part of the internal API, but not necessarily
1928sanctioned for use in extensions) begins like this:
ee072b34
GS
1929
1930 void
2307c6d0 1931 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34
GS
1932
1933C<pTHX_> is one of a number of macros (in perl.h) that hide the
1934details of the interpreter's context. THX stands for "thread", "this",
1935or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1936The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1937or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1938their variants.
ee072b34 1939
a7486cbb
JH
1940When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1941first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1942in the pTHX_ macro indicates that the macro expansion needs a comma
1943after the context argument because other arguments follow it. If
1944PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1945subroutine is not prototyped to take the extra argument. The form of the
1946macro without the trailing underscore is used when there are no additional
ee072b34
GS
1947explicit arguments.
1948
54aff467 1949When a core function calls another, it must pass the context. This
2307c6d0 1950is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
1951something like this:
1952
2307c6d0
SB
1953 #ifdef PERL_IMPLICIT_CONTEXT
1954 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 1955 /* can't do this for vararg functions, see below */
2307c6d0
SB
1956 #else
1957 #define sv_setiv Perl_sv_setiv
1958 #endif
ee072b34
GS
1959
1960This works well, and means that XS authors can gleefully write:
1961
2307c6d0 1962 sv_setiv(foo, bar);
ee072b34
GS
1963
1964and still have it work under all the modes Perl could have been
1965compiled with.
1966
ee072b34
GS
1967This doesn't work so cleanly for varargs functions, though, as macros
1968imply that the number of arguments is known in advance. Instead we
1969either need to spell them out fully, passing C<aTHX_> as the first
1970argument (the Perl core tends to do this with functions like
1971Perl_warner), or use a context-free version.
1972
1973The context-free version of Perl_warner is called
1974Perl_warner_nocontext, and does not take the extra argument. Instead
1975it does dTHX; to get the context from thread-local storage. We
1976C<#define warner Perl_warner_nocontext> so that extensions get source
1977compatibility at the expense of performance. (Passing an arg is
1978cheaper than grabbing it from thread-local storage.)
1979
acfe0abc 1980You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
1981Those are strictly for use within the core. Extensions and embedders
1982need only be aware of [pad]THX.
1983
a7486cbb
JH
1984=head2 So what happened to dTHR?
1985
1986C<dTHR> was introduced in perl 5.005 to support the older thread model.
1987The older thread model now uses the C<THX> mechanism to pass context
1988pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
1989later still have it for backward source compatibility, but it is defined
1990to be a no-op.
1991
ee072b34
GS
1992=head2 How do I use all this in extensions?
1993
1994When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1995any functions in the Perl API will need to pass the initial context
1996argument somehow. The kicker is that you will need to write it in
1997such a way that the extension still compiles when Perl hasn't been
1998built with PERL_IMPLICIT_CONTEXT enabled.
1999
2000There are three ways to do this. First, the easy but inefficient way,
2001which is also the default, in order to maintain source compatibility
2002with extensions: whenever XSUB.h is #included, it redefines the aTHX
2003and aTHX_ macros to call a function that will return the context.
2004Thus, something like:
2005
2307c6d0 2006 sv_setiv(sv, num);
ee072b34 2007
4375e838 2008in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2009in effect:
ee072b34 2010
2307c6d0 2011 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2012
54aff467 2013or to this otherwise:
ee072b34 2014
2307c6d0 2015 Perl_sv_setiv(sv, num);
ee072b34
GS
2016
2017You have to do nothing new in your extension to get this; since
2fa86c13 2018the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2019work.
2020
2021The second, more efficient way is to use the following template for
2022your Foo.xs:
2023
c52f9dcd
JH
2024 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2025 #include "EXTERN.h"
2026 #include "perl.h"
2027 #include "XSUB.h"
ee072b34
GS
2028
2029 static my_private_function(int arg1, int arg2);
2030
c52f9dcd
JH
2031 static SV *
2032 my_private_function(int arg1, int arg2)
2033 {
2034 dTHX; /* fetch context */
2035 ... call many Perl API functions ...
2036 }
ee072b34
GS
2037
2038 [... etc ...]
2039
c52f9dcd 2040 MODULE = Foo PACKAGE = Foo
ee072b34 2041
c52f9dcd 2042 /* typical XSUB */
ee072b34 2043
c52f9dcd
JH
2044 void
2045 my_xsub(arg)
2046 int arg
2047 CODE:
2048 my_private_function(arg, 10);
ee072b34
GS
2049
2050Note that the only two changes from the normal way of writing an
2051extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2052including the Perl headers, followed by a C<dTHX;> declaration at
2053the start of every function that will call the Perl API. (You'll
2054know which functions need this, because the C compiler will complain
2055that there's an undeclared identifier in those functions.) No changes
2056are needed for the XSUBs themselves, because the XS() macro is
2057correctly defined to pass in the implicit context if needed.
2058
2059The third, even more efficient way is to ape how it is done within
2060the Perl guts:
2061
2062
c52f9dcd
JH
2063 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2064 #include "EXTERN.h"
2065 #include "perl.h"
2066 #include "XSUB.h"
ee072b34
GS
2067
2068 /* pTHX_ only needed for functions that call Perl API */
2069 static my_private_function(pTHX_ int arg1, int arg2);
2070
c52f9dcd
JH
2071 static SV *
2072 my_private_function(pTHX_ int arg1, int arg2)
2073 {
2074 /* dTHX; not needed here, because THX is an argument */
2075 ... call Perl API functions ...
2076 }
ee072b34
GS
2077
2078 [... etc ...]
2079
c52f9dcd 2080 MODULE = Foo PACKAGE = Foo
ee072b34 2081
c52f9dcd 2082 /* typical XSUB */
ee072b34 2083
c52f9dcd
JH
2084 void
2085 my_xsub(arg)
2086 int arg
2087 CODE:
2088 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2089
2090This implementation never has to fetch the context using a function
2091call, since it is always passed as an extra argument. Depending on
2092your needs for simplicity or efficiency, you may mix the previous
2093two approaches freely.
2094
651a3225
GS
2095Never add a comma after C<pTHX> yourself--always use the form of the
2096macro with the underscore for functions that take explicit arguments,
2097or the form without the argument for functions with no explicit arguments.
ee072b34 2098
27da23d5
JH
2099If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2100definition is needed if the Perl global variables (see F<perlvars.h>
2101or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2102used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2103the need for C<dVAR> only with the said compile-time define, because
2104otherwise the Perl global variables are visible as-is.
2105
a7486cbb
JH
2106=head2 Should I do anything special if I call perl from multiple threads?
2107
2108If you create interpreters in one thread and then proceed to call them in
2109another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2110initialized correctly in each of those threads.
2111
2112The C<perl_alloc> and C<perl_clone> API functions will automatically set
2113the TLS slot to the interpreter they created, so that there is no need to do
2114anything special if the interpreter is always accessed in the same thread that
2115created it, and that thread did not create or call any other interpreters
2116afterwards. If that is not the case, you have to set the TLS slot of the
2117thread before calling any functions in the Perl API on that particular
2118interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2119thread as the first thing you do:
2120
2121 /* do this before doing anything else with some_perl */
2122 PERL_SET_CONTEXT(some_perl);
2123
2124 ... other Perl API calls on some_perl go here ...
2125
ee072b34
GS
2126=head2 Future Plans and PERL_IMPLICIT_SYS
2127
2128Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2129that the interpreter knows about itself and pass it around, so too are
2130there plans to allow the interpreter to bundle up everything it knows
2131about the environment it's running on. This is enabled with the
acfe0abc 2132PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS
4d1ff10f 2133and USE_5005THREADS on Windows (see inside iperlsys.h).
ee072b34
GS
2134
2135This allows the ability to provide an extra pointer (called the "host"
2136environment) for all the system calls. This makes it possible for
2137all the system stuff to maintain their own state, broken down into
2138seven C structures. These are thin wrappers around the usual system
2139calls (see win32/perllib.c) for the default perl executable, but for a
2140more ambitious host (like the one that would do fork() emulation) all
2141the extra work needed to pretend that different interpreters are
2142actually different "processes", would be done here.
2143
2144The Perl engine/interpreter and the host are orthogonal entities.
2145There could be one or more interpreters in a process, and one or
2146more "hosts", with free association between them.
2147
a422fd2d
SC
2148=head1 Internal Functions
2149
2150All of Perl's internal functions which will be exposed to the outside
06f6df17 2151world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d
SC
2152functions or functions used in a program in which Perl is embedded.
2153Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2154static functions start with C<S_>.)
a422fd2d
SC
2155
2156Inside the Perl core, you can get at the functions either with or
2157without the C<Perl_> prefix, thanks to a bunch of defines that live in
2158F<embed.h>. This header file is generated automatically from
dc9b1d22
MHM
2159F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2160header files for the internal functions, generates the documentation
2161and a lot of other bits and pieces. It's important that when you add
2162a new function to the core or change an existing one, you change the
2163data in the table in F<embed.fnc> as well. Here's a sample entry from
2164that table:
a422fd2d
SC
2165
2166 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2167
2168The second column is the return type, the third column the name. Columns
2169after that are the arguments. The first column is a set of flags:
2170
2171=over 3
2172
2173=item A
2174
2175This function is a part of the public API.
2176
2177=item p
2178
2179This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
2180
2181=item d
2182
2183This function has documentation using the C<apidoc> feature which we'll
2184look at in a second.
2185
2186=back
2187
2188Other available flags are:
2189
2190=over 3
2191
2192=item s
2193
a7486cbb
JH
2194This is a static function and is defined as C<S_whatever>, and usually
2195called within the sources as C<whatever(...)>.
a422fd2d
SC
2196
2197=item n
2198
2199This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
2200L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2201
2202=item r
2203
2204This function never returns; C<croak>, C<exit> and friends.
2205
2206=item f
2207
2208This function takes a variable number of arguments, C<printf> style.
2209The argument list should end with C<...>, like this:
2210
2211 Afprd |void |croak |const char* pat|...
2212
a7486cbb 2213=item M
a422fd2d 2214
00aadd71 2215This function is part of the experimental development API, and may change
a422fd2d
SC
2216or disappear without notice.
2217
2218=item o
2219
2220This function should not have a compatibility macro to define, say,
2221C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2222
a422fd2d
SC
2223=item x
2224
2225This function isn't exported out of the Perl core.
2226
dc9b1d22
MHM
2227=item m
2228
2229This is implemented as a macro.
2230
2231=item X
2232
2233This function is explicitly exported.
2234
2235=item E
2236
2237This function is visible to extensions included in the Perl core.
2238
2239=item b
2240
2241Binary backward compatibility; this function is a macro but also has
2242a C<Perl_> implementation (which is exported).
2243
a422fd2d
SC
2244=back
2245
dc9b1d22
MHM
2246If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2247C<make regen_headers> to force a rebuild of F<embed.h> and other
2248auto-generated files.
a422fd2d 2249
6b4667fc 2250=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2251
6b4667fc
A
2252If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2253formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2254following macros for portability
9dd9db0b 2255
c52f9dcd
JH
2256 IVdf IV in decimal
2257 UVuf UV in decimal
2258 UVof UV in octal
2259 UVxf UV in hexadecimal
2260 NVef NV %e-like
2261 NVff NV %f-like
2262 NVgf NV %g-like
9dd9db0b 2263
6b4667fc
A
2264These will take care of 64-bit integers and long doubles.
2265For example:
2266
c52f9dcd 2267 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2268
2269The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2270
8908e76d
JH
2271If you are printing addresses of pointers, use UVxf combined
2272with PTR2UV(), do not use %lx or %p.
2273
2274=head2 Pointer-To-Integer and Integer-To-Pointer
2275
2276Because pointer size does not necessarily equal integer size,
2277use the follow macros to do it right.
2278
c52f9dcd
JH
2279 PTR2UV(pointer)
2280 PTR2IV(pointer)
2281 PTR2NV(pointer)
2282 INT2PTR(pointertotype, integer)
8908e76d
JH
2283
2284For example:
2285
c52f9dcd
JH
2286 IV iv = ...;
2287 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2288
2289and
2290
c52f9dcd
JH
2291 AV *av = ...;
2292 UV uv = PTR2UV(av);
8908e76d 2293
0ca3a874
MHM
2294=head2 Exception Handling
2295
9b5c3821
MHM
2296There are a couple of macros to do very basic exception handling in XS
2297modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2298be able to use these macros:
2299
2300 #define NO_XSLOCKS
2301 #include "XSUB.h"
2302
2303You can use these macros if you call code that may croak, but you need
2304to do some cleanup before giving control back to Perl. For example:
0ca3a874
MHM
2305
2306 dXCPT; /* set up neccessary variables */
2307
2308 XCPT_TRY_START {
2309 code_that_may_croak();
2310 } XCPT_TRY_END
2311
2312 XCPT_CATCH
2313 {
2314 /* do cleanup here */
2315 XCPT_RETHROW;
2316 }
2317
2318Note that you always have to rethrow an exception that has been
2319caught. Using these macros, it is not possible to just catch the
2320exception and ignore it. If you have to ignore the exception, you
2321have to use the C<call_*> function.
2322
2323The advantage of using the above macros is that you don't have
2324to setup an extra function for C<call_*>, and that using these
2325macros is faster than using C<call_*>.
2326
a422fd2d
SC
2327=head2 Source Documentation
2328
2329There's an effort going on to document the internal functions and
2330automatically produce reference manuals from them - L<perlapi> is one
2331such manual which details all the functions which are available to XS
2332writers. L<perlintern> is the autogenerated manual for the functions
2333which are not part of the API and are supposedly for internal use only.
2334
2335Source documentation is created by putting POD comments into the C
2336source, like this:
2337
2338 /*
2339 =for apidoc sv_setiv
2340
2341 Copies an integer into the given SV. Does not handle 'set' magic. See
2342 C<sv_setiv_mg>.
2343
2344 =cut
2345 */
2346
2347Please try and supply some documentation if you add functions to the
2348Perl core.
2349
0d098d33
MHM
2350=head2 Backwards compatibility
2351
2352The Perl API changes over time. New functions are added or the interfaces
2353of existing functions are changed. The C<Devel::PPPort> module tries to
2354provide compatibility code for some of these changes, so XS writers don't
2355have to code it themselves when supporting multiple versions of Perl.
2356
2357C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2358be run as a Perl script. To generate F<ppport.h>, run:
2359
2360 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2361
2362Besides checking existing XS code, the script can also be used to retrieve
2363compatibility information for various API calls using the C<--api-info>
2364command line switch. For example:
2365
2366 % perl ppport.h --api-info=sv_magicext
2367
2368For details, see C<perldoc ppport.h>.
2369
a422fd2d
SC
2370=head1 Unicode Support
2371
2372Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2373writers to understand this support and make sure that the code they
2374write does not corrupt Unicode data.
2375
2376=head2 What B<is> Unicode, anyway?
2377
2378In the olden, less enlightened times, we all used to use ASCII. Most of
2379us did, anyway. The big problem with ASCII is that it's American. Well,
2380no, that's not actually the problem; the problem is that it's not
2381particularly useful for people who don't use the Roman alphabet. What
2382used to happen was that particular languages would stick their own
2383alphabet in the upper range of the sequence, between 128 and 255. Of
2384course, we then ended up with plenty of variants that weren't quite
2385ASCII, and the whole point of it being a standard was lost.
2386
2387Worse still, if you've got a language like Chinese or
2388Japanese that has hundreds or thousands of characters, then you really
2389can't fit them into a mere 256, so they had to forget about ASCII
2390altogether, and build their own systems using pairs of numbers to refer
2391to one character.
2392
2393To fix this, some people formed Unicode, Inc. and
2394produced a new character set containing all the characters you can
2395possibly think of and more. There are several ways of representing these
1e54db1a 2396characters, and the one Perl uses is called UTF-8. UTF-8 uses
a422fd2d 2397a variable number of bytes to represent a character, instead of just
b3b6085d 2398one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d 2399
1e54db1a 2400=head2 How can I recognise a UTF-8 string?
a422fd2d 2401
1e54db1a
JH
2402You can't. This is because UTF-8 data is stored in bytes just like
2403non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d
SC
2404capital E with a grave accent, is represented by the two bytes
2405C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2406has that byte sequence as well. So you can't tell just by looking - this
2407is what makes Unicode input an interesting problem.
2408
2409The API function C<is_utf8_string> can help; it'll tell you if a string
1e54db1a 2410contains only valid UTF-8 characters. However, it can't do the work for
a422fd2d 2411you. On a character-by-character basis, C<is_utf8_char> will tell you
1e54db1a 2412whether the current character in a string is valid UTF-8.
a422fd2d 2413
1e54db1a 2414=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2415
1e54db1a 2416As mentioned above, UTF-8 uses a variable number of bytes to store a
a422fd2d
SC
2417character. Characters with values 1...128 are stored in one byte, just
2418like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2419continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2420bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2421so it goes on, moving to three bytes at character 2048.
2422
1e54db1a 2423Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2424how long the first character in it is with the C<UTF8SKIP> macro:
2425
2426 char *utf = "\305\233\340\240\201";
2427 I32 len;
2428
2429 len = UTF8SKIP(utf); /* len is 2 here */
2430 utf += len;
2431 len = UTF8SKIP(utf); /* len is 3 here */
2432
1e54db1a 2433Another way to skip over characters in a UTF-8 string is to use
a422fd2d
SC
2434C<utf8_hop>, which takes a string and a number of characters to skip
2435over. You're on your own about bounds checking, though, so don't use it
2436lightly.
2437
1e54db1a 2438All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2439so you can test if you need to do something special with this
2440character like this (the UTF8_IS_INVARIANT() is a macro that tests
2441whether the byte can be encoded as a single byte even in UTF-8):
a422fd2d 2442
3a2263fe
RGS
2443 U8 *utf;
2444 UV uv; /* Note: a UV, not a U8, not a char */
a422fd2d 2445
3a2263fe 2446 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2447 /* Must treat this as UTF-8 */
a422fd2d
SC
2448 uv = utf8_to_uv(utf);
2449 else
2450 /* OK to treat this character as a byte */
2451 uv = *utf;
2452
2453You can also see in that example that we use C<utf8_to_uv> to get the
2454value of the character; the inverse function C<uv_to_utf8> is available
1e54db1a 2455for putting a UV into UTF-8:
a422fd2d 2456
3a2263fe 2457 if (!UTF8_IS_INVARIANT(uv))
a422fd2d
SC
2458 /* Must treat this as UTF8 */
2459 utf8 = uv_to_utf8(utf8, uv);
2460 else
2461 /* OK to treat this character as a byte */
2462 *utf8++ = uv;
2463
2464You B<must> convert characters to UVs using the above functions if
1e54db1a
JH
2465you're ever in a situation where you have to match UTF-8 and non-UTF-8
2466characters. You may not skip over UTF-8 characters in this case. If you
2467do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2468for instance, if your UTF-8 string contains C<v196.172>, and you skip
2469that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2470So don't do that!
2471
1e54db1a 2472=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2473
2474Currently, Perl deals with Unicode strings and non-Unicode strings
2475slightly differently. If a string has been identified as being UTF-8
2476encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2477manipulate this flag with the following macros:
2478
2479 SvUTF8(sv)
2480 SvUTF8_on(sv)
2481 SvUTF8_off(sv)
2482
2483This flag has an important effect on Perl's treatment of the string: if
2484Unicode data is not properly distinguished, regular expressions,
2485C<length>, C<substr> and other string handling operations will have
2486undesirable results.
2487
2488The problem comes when you have, for instance, a string that isn't
1e54db1a
JH
2489flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
2490especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2491
2492Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2493need be sure you don't accidentally knock it off while you're
2494manipulating SVs. More specifically, you cannot expect to do this:
2495
2496 SV *sv;
2497 SV *nsv;
2498 STRLEN len;
2499 char *p;
2500
2501 p = SvPV(sv, len);
2502 frobnicate(p);
2503 nsv = newSVpvn(p, len);
2504
2505The C<char*> string does not tell you the whole story, and you can't
2506copy or reconstruct an SV just by copying the string value. Check if the
1e54db1a 2507old SV has the UTF-8 flag set, and act accordingly:
a422fd2d
SC
2508
2509 p = SvPV(sv, len);
2510 frobnicate(p);
2511 nsv = newSVpvn(p, len);
2512 if (SvUTF8(sv))
2513 SvUTF8_on(nsv);
2514
2515In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2516not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2517appropriately.
2518
3a2263fe 2519Since just passing an SV to an XS function and copying the data of
1e54db1a 2520the SV is not enough to copy the UTF-8 flags, even less right is just
3a2263fe
RGS
2521passing a C<char *> to an XS function.
2522
1e54db1a 2523=head2 How do I convert a string to UTF-8?
a422fd2d 2524
1e54db1a
JH
2525If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary
2526to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
a422fd2d
SC
2527way to do this is:
2528
2529 sv_utf8_upgrade(sv);
2530
2531However, you must not do this, for example:
2532
2533 if (!SvUTF8(left))
2534 sv_utf8_upgrade(left);
2535
2536If you do this in a binary operator, you will actually change one of the
b1866b2d 2537strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2538by the end user, it can cause problems.
2539
1e54db1a 2540Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
a422fd2d 2541string argument. This is useful for having the data available for
b1866b2d 2542comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2543C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2544the string contains any characters above 255 that can't be represented
2545in a single byte.
2546
2547=head2 Is there anything else I need to know?
2548
2549Not really. Just remember these things:
2550
2551=over 3
2552
2553=item *
2554
1e54db1a
JH
2555There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2556is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2557something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2558it's not - if you pass on the PV to somewhere, pass on the flag too.
2559
2560=item *
2561
1e54db1a 2562If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
3a2263fe 2563unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2564
2565=item *
2566
1e54db1a 2567When writing a character C<uv> to a UTF-8 string, B<always> use
3a2263fe
RGS
2568C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2569you can use C<*s = uv>.
a422fd2d
SC
2570
2571=item *
2572
1e54db1a
JH
2573Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2574a new string which is UTF-8 encoded. There are tricks you can use to
2575delay deciding whether you need to use a UTF-8 string until you get to a
a422fd2d
SC
2576high character - C<HALF_UPGRADE> is one of those.
2577
2578=back
2579
53e06cf0
SC
2580=head1 Custom Operators
2581
9a68f1db 2582Custom operator support is a new experimental feature that allows you to
53e06cf0
SC
2583define your own ops. This is primarily to allow the building of
2584interpreters for other languages in the Perl core, but it also allows
2585optimizations through the creation of "macro-ops" (ops which perform the
2586functions of multiple ops which are usually executed together, such as
b7cb320d 2587C<gvsv, gvsv, add>.)
53e06cf0 2588
b455bf3f 2589This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0
SC
2590core does not "know" anything special about this op type, and so it will
2591not be involved in any optimizations. This also means that you can
2592define your custom ops to be any op structure - unary, binary, list and
2593so on - you like.
2594
2595It's important to know what custom operators won't do for you. They
2596won't let you add new syntax to Perl, directly. They won't even let you
2597add new keywords, directly. In fact, they won't change the way Perl
2598compiles a program at all. You have to do those changes yourself, after
2599Perl has compiled the program. You do this either by manipulating the op
2600tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2601a custom peephole optimizer with the C<optimize> module.
2602
2603When you do this, you replace ordinary Perl ops with custom ops by
2604creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2605PP function. This should be defined in XS code, and should look like
2606the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2607takes the appropriate number of values from the stack, and you are
2608responsible for adding stack marks if necessary.
2609
2610You should also "register" your op with the Perl interpreter so that it
2611can produce sensible error and warning messages. Since it is possible to
2612have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2613Perl uses the value of C<< o->op_ppaddr >> as a key into the
2614C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2615need to enter a name and description for your op at the appropriate
2616place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2617
2618Forthcoming versions of C<B::Generate> (version 1.0 and above) should
2619directly support the creation of custom ops by name; C<Opcodes::Custom>
2620will provide functions which make it trivial to "register" custom ops to
2621the Perl interpreter.
2622
954c1994 2623=head1 AUTHORS
e89caa19 2624
954c1994 2625Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2626E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2627itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 2628
954c1994
GS
2629With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2630Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2631Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2632Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2633
954c1994 2634=head1 SEE ALSO
cb1a09d0 2635
954c1994 2636perlapi(1), perlintern(1), perlxs(1), perlembed(1)