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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
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129 SV *s;
130 STRLEN len;
131 char * ptr;
132 ptr = SvPV(s, len);
133 foo(ptr, len);
ce2f5d8f 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
64ace3f8 194 SV* get_sv("package::varname", 0);
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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
6de131f0 281increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
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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
d7f8936a 307numeric conversion has occurred and precision has been lost: only the
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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
cbfd0a87 370 AV* get_av("package::varname", 0);
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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
6673a63c 445 HV* get_hv("package::varname", 0);
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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
MHM
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
LW
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:
a0d0e21e
LW
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
AD
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
3f2c382a 603See the F<sv.h> header file for more details.
d1b91892 604
cb1a09d0
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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
AD
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
64ace3f8 670 SV* get_sv("package::varname", GV_ADD);
cbfd0a87 671 AV* get_av("package::varname", GV_ADD);
6673a63c 672 HV* get_hv("package::varname", GV_ADD);
cb1a09d0
AD
673
674Notice the use of TRUE as the second parameter. The new variable can now
675be set, using the routines appropriate to the data type.
676
5f05dabc
PP
677There are additional macros whose values may be bitwise OR'ed with the
678C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 679
9a68f1db
SB
680=over
681
682=item GV_ADDMULTI
683
684Marks the variable as multiply defined, thus preventing the:
685
686 Name <varname> used only once: possible typo
687
688warning.
689
9a68f1db
SB
690=item GV_ADDWARN
691
692Issues the warning:
693
694 Had to create <varname> unexpectedly
695
696if the variable did not exist before the function was called.
697
698=back
cb1a09d0 699
07fa94a1
JO
700If you do not specify a package name, the variable is created in the current
701package.
cb1a09d0 702
5f05dabc 703=head2 Reference Counts and Mortality
a0d0e21e 704
d1be9408 705Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 706AVs, or HVs (xV for short in the following) start their life with a
55497cff 707reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 708then it will be destroyed and its memory made available for reuse.
55497cff
PP
709
710This normally doesn't happen at the Perl level unless a variable is
5f05dabc
PP
711undef'ed or the last variable holding a reference to it is changed or
712overwritten. At the internal level, however, reference counts can be
55497cff
PP
713manipulated with the following macros:
714
715 int SvREFCNT(SV* sv);
5f05dabc 716 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
717 void SvREFCNT_dec(SV* sv);
718
719However, there is one other function which manipulates the reference
07fa94a1
JO
720count of its argument. The C<newRV_inc> function, you will recall,
721creates a reference to the specified argument. As a side effect,
722it increments the argument's reference count. If this is not what
723you want, use C<newRV_noinc> instead.
724
725For example, imagine you want to return a reference from an XSUB function.
726Inside the XSUB routine, you create an SV which initially has a reference
727count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
PP
728This returns the reference as a new SV, but the reference count of the
729SV you passed to C<newRV_inc> has been incremented to two. Now you
07fa94a1
JO
730return the reference from the XSUB routine and forget about the SV.
731But Perl hasn't! Whenever the returned reference is destroyed, the
732reference count of the original SV is decreased to one and nothing happens.
733The SV will hang around without any way to access it until Perl itself
734terminates. This is a memory leak.
5f05dabc
PP
735
736The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
737C<newRV_inc>. Then, if and when the last reference is destroyed,
738the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 739stopping any memory leak.
55497cff 740
5f05dabc 741There are some convenience functions available that can help with the
54310121 742destruction of xVs. These functions introduce the concept of "mortality".
07fa94a1
JO
743An xV that is mortal has had its reference count marked to be decremented,
744but not actually decremented, until "a short time later". Generally the
745term "short time later" means a single Perl statement, such as a call to
54310121 746an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
JO
747reference count decremented depends on two macros, SAVETMPS and FREETMPS.
748See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
PP
749
750"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
751However, if you mortalize a variable twice, the reference count will
752later be decremented twice.
753
00aadd71
NIS
754"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
755For example an SV which is created just to pass a number to a called sub
06f6df17
RGS
756is made mortal to have it cleaned up automatically when it's popped off
757the stack. Similarly, results returned by XSUBs (which are pushed on the
758stack) are often made mortal.
a0d0e21e
LW
759
760To create a mortal variable, use the functions:
761
762 SV* sv_newmortal()
763 SV* sv_2mortal(SV*)
764 SV* sv_mortalcopy(SV*)
765
00aadd71 766The first call creates a mortal SV (with no value), the second converts an existing
5f05dabc
PP
767SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
768third creates a mortal copy of an existing SV.
00aadd71 769Because C<sv_newmortal> gives the new SV no value,it must normally be given one
9a68f1db 770via C<sv_setpv>, C<sv_setiv>, etc. :
00aadd71
NIS
771
772 SV *tmp = sv_newmortal();
773 sv_setiv(tmp, an_integer);
774
775As that is multiple C statements it is quite common so see this idiom instead:
776
777 SV *tmp = sv_2mortal(newSViv(an_integer));
778
779
780You should be careful about creating mortal variables. Strange things
781can happen if you make the same value mortal within multiple contexts,
782or if you make a variable mortal multiple times. Thinking of "Mortalization"
783as deferred C<SvREFCNT_dec> should help to minimize such problems.
784For example if you are passing an SV which you I<know> has high enough REFCNT
785to survive its use on the stack you need not do any mortalization.
786If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
787making a C<sv_mortalcopy> is safer.
a0d0e21e 788
ac036724 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
a0d0e21e
LW
802 Scalar Value
803 Array Value
804 Hash Value
a3cb178b 805 I/O Handle
a0d0e21e
LW
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
da51bb9b
NC
817 HV* gv_stashpv(const char* name, I32 flags)
818 HV* gv_stashsv(SV*, I32 flags)
a0d0e21e
LW
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
da51bb9b 822C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
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
64ace3f8 881 SV* sv = get_sv("dberror", GV_ADD);
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
AD
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;
b205eb13 904 I32 mg_len;
d1b91892
AD
905 SV* mg_obj;
906 char* mg_ptr;
d1b91892
AD
907 };
908
909Note this is current as of patchlevel 0, and could change at any time.
910
911=head2 Assigning Magic
912
913Perl adds magic to an SV using the sv_magic function:
914
08105a92 915 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
916
917The C<sv> argument is a pointer to the SV that is to acquire a new magical
918feature.
919
920If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
645c22ef
DM
921convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
922to the beginning of the linked list of magical features. Any prior entry
923of the same type of magic is deleted. Note that this can be overridden,
924and multiple instances of the same type of magic can be associated with an
925SV.
d1b91892 926
54310121
PP
927The C<name> and C<namlen> arguments are used to associate a string with
928the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
929C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
930C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
931whether C<namlen> is greater than zero or equal to zero respectively. As a
932special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
933to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
934
935The sv_magic function uses C<how> to determine which, if any, predefined
936"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
06f6df17 937See the L<Magic Virtual Tables> section below. The C<how> argument is also
14befaf4 938stored in the C<mg_type> field. The value of C<how> should be chosen
06f6df17 939from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 940these macros were added, Perl internals used to directly use character
14befaf4 941literals, so you may occasionally come across old code or documentation
75d0f26d 942referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
943
944The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
945structure. If it is not the same as the C<sv> argument, the reference
946count of the C<obj> object is incremented. If it is the same, or if
645c22ef 947the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 948then C<obj> is merely stored, without the reference count being incremented.
d1b91892 949
2d8d5d5a
SH
950See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
951to an SV.
952
cb1a09d0
AD
953There is also a function to add magic to an C<HV>:
954
955 void hv_magic(HV *hv, GV *gv, int how);
956
957This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
958
959To remove the magic from an SV, call the function sv_unmagic:
960
961 void sv_unmagic(SV *sv, int type);
962
963The C<type> argument should be equal to the C<how> value when the C<SV>
964was initially made magical.
965
d1b91892
AD
966=head2 Magic Virtual Tables
967
d1be9408 968The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
969C<MGVTBL>, which is a structure of function pointers and stands for
970"Magic Virtual Table" to handle the various operations that might be
971applied to that variable.
972
301cb7e8
DM
973The C<MGVTBL> has five (or sometimes eight) pointers to the following
974routine types:
d1b91892
AD
975
976 int (*svt_get)(SV* sv, MAGIC* mg);
977 int (*svt_set)(SV* sv, MAGIC* mg);
978 U32 (*svt_len)(SV* sv, MAGIC* mg);
979 int (*svt_clear)(SV* sv, MAGIC* mg);
980 int (*svt_free)(SV* sv, MAGIC* mg);
981
3468c7ea 982 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, I32 namlen);
301cb7e8
DM
983 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
984 int (*svt_local)(SV *nsv, MAGIC *mg);
985
986
06f6df17 987This MGVTBL structure is set at compile-time in F<perl.h> and there are
b7a0f54c
SM
988currently 32 types. These different structures contain pointers to various
989routines that perform additional actions depending on which function is
990being called.
d1b91892
AD
991
992 Function pointer Action taken
993 ---------------- ------------
8b0711c3 994 svt_get Do something before the value of the SV is retrieved.
d1b91892
AD
995 svt_set Do something after the SV is assigned a value.
996 svt_len Report on the SV's length.
805b34a4 997 svt_clear Clear something the SV represents.
d1b91892
AD
998 svt_free Free any extra storage associated with the SV.
999
805b34a4
AB
1000 svt_copy copy tied variable magic to a tied element
1001 svt_dup duplicate a magic structure during thread cloning
1002 svt_local copy magic to local value during 'local'
301cb7e8 1003
d1b91892 1004For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 1005to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
1006
1007 { magic_get, magic_set, magic_len, 0, 0 }
1008
14befaf4
DM
1009Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1010if a get operation is being performed, the routine C<magic_get> is
1011called. All the various routines for the various magical types begin
1012with C<magic_>. NOTE: the magic routines are not considered part of
1013the Perl API, and may not be exported by the Perl library.
d1b91892 1014
301cb7e8
DM
1015The last three slots are a recent addition, and for source code
1016compatibility they are only checked for if one of the three flags
1017MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
1018code can continue declaring a vtable as a 5-element value. These three are
1019currently used exclusively by the threading code, and are highly subject
1020to change.
1021
d1b91892
AD
1022The current kinds of Magic Virtual Tables are:
1023
14befaf4 1024 mg_type
5633a428
NC
1025 (old-style char and macro) MGVTBL Type of magic
1026 -------------------------- ------ -------------
805b34a4
AB
1027 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1028 A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
14befaf4 1029 a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
805b34a4
AB
1030 c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
1031 on stash
1032 B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
1033 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1034 (@+ and @- vars)
1035 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1036 element
1037 E PERL_MAGIC_env vtbl_env %ENV hash
1038 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
1039 f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
1040 g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
f747ebd6 1041 H PERL_MAGIC_hints vtbl_hints %^H hash
805b34a4
AB
1042 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1043 I PERL_MAGIC_isa vtbl_isa @ISA array
1044 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1045 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1046 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1047 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
805b34a4
AB
1048 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
1049 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1050 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1051 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1052 r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
1053 S PERL_MAGIC_sig vtbl_sig %SIG hash
1054 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1055 t PERL_MAGIC_taint vtbl_taint Taintedness
1056 U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
1057 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1058 V PERL_MAGIC_vstring (none) v-string scalars
1059 w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
1060 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1061 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1062 variable / smart parameter
1063 vivification
1064 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1065 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1066 < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref
1067 ~ PERL_MAGIC_ext (none) Available for use by extensions
1068 : PERL_MAGIC_symtab (none) hash used as symbol table
1069 % PERL_MAGIC_rhash (none) hash used as restricted hash
1070 @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a
0cbee0a4 1071
d1b91892 1072
68dc0745 1073When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1074uppercase letter is typically used to represent some kind of composite type
1075(a list or a hash), and the lowercase letter is used to represent an element
1076of that composite type. Some internals code makes use of this case
1077relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1078
1079The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1080specifically for use by extensions and will not be used by perl itself.
1081Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1082to variables (typically objects). This is especially useful because
1083there is no way for normal perl code to corrupt this private information
1084(unlike using extra elements of a hash object).
1085
1086Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1087C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1088C<mg_ptr> field points to a C<ufuncs> structure:
1089
1090 struct ufuncs {
a9402793
AB
1091 I32 (*uf_val)(pTHX_ IV, SV*);
1092 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1093 IV uf_index;
1094 };
1095
1096When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1097function will be called with C<uf_index> as the first arg and a pointer to
1098the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1099magic is shown below. Note that the ufuncs structure is copied by
1100sv_magic, so you can safely allocate it on the stack.
1101
1102 void
1103 Umagic(sv)
1104 SV *sv;
1105 PREINIT:
1106 struct ufuncs uf;
1107 CODE:
1108 uf.uf_val = &my_get_fn;
1109 uf.uf_set = &my_set_fn;
1110 uf.uf_index = 0;
14befaf4 1111 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1112
1e73acc8
AS
1113Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1114
1115For hashes there is a specialized hook that gives control over hash
1116keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1117if the "set" function in the C<ufuncs> structure is NULL. The hook
1118is activated whenever the hash is accessed with a key specified as
1119an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1120C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1121through the functions without the C<..._ent> suffix circumvents the
1122hook. See L<Hash::Util::Fieldhash/Guts> for a detailed description.
1123
14befaf4
DM
1124Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1125or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1126extra care to avoid conflict. Typically only using the magic on
1127objects blessed into the same class as the extension is sufficient.
1128For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
1129'signature' at the top of the private data area and check that.
5f05dabc 1130
ef50df4b
GS
1131Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1132earlier do B<not> invoke 'set' magic on their targets. This must
1133be done by the user either by calling the C<SvSETMAGIC()> macro after
1134calling these functions, or by using one of the C<sv_set*_mg()> or
1135C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1136C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1137obtained from external sources in functions that don't handle magic.
4a4eefd0 1138See L<perlapi> for a description of these functions.
189b2af5
GS
1139For example, calls to the C<sv_cat*()> functions typically need to be
1140followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1141since their implementation handles 'get' magic.
1142
d1b91892
AD
1143=head2 Finding Magic
1144
1145 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
1146
1147This routine returns a pointer to the C<MAGIC> structure stored in the SV.
1148If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 1149if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 1150
08105a92 1151 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1152
1153This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1154field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1155the mg_type field is changed to be the lowercase letter.
a0d0e21e 1156
04343c6d
GS
1157=head2 Understanding the Magic of Tied Hashes and Arrays
1158
14befaf4
DM
1159Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1160magic type.
9edb2b46
GS
1161
1162WARNING: As of the 5.004 release, proper usage of the array and hash
1163access functions requires understanding a few caveats. Some
1164of these caveats are actually considered bugs in the API, to be fixed
1165in later releases, and are bracketed with [MAYCHANGE] below. If
1166you find yourself actually applying such information in this section, be
1167aware that the behavior may change in the future, umm, without warning.
04343c6d 1168
1526ead6 1169The perl tie function associates a variable with an object that implements
9a68f1db 1170the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1171tie function from an XSUB, you must mimic this behaviour. The code below
1172carries out the necessary steps - firstly it creates a new hash, and then
1173creates a second hash which it blesses into the class which will implement
1174the tie methods. Lastly it ties the two hashes together, and returns a
1175reference to the new tied hash. Note that the code below does NOT call the
1176TIEHASH method in the MyTie class -
1177see L<Calling Perl Routines from within C Programs> for details on how
1178to do this.
1179
1180 SV*
1181 mytie()
1182 PREINIT:
1183 HV *hash;
1184 HV *stash;
1185 SV *tie;
1186 CODE:
1187 hash = newHV();
1188 tie = newRV_noinc((SV*)newHV());
da51bb9b 1189 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1190 sv_bless(tie, stash);
899e16d0 1191 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1192 RETVAL = newRV_noinc(hash);
1193 OUTPUT:
1194 RETVAL
1195
04343c6d
GS
1196The C<av_store> function, when given a tied array argument, merely
1197copies the magic of the array onto the value to be "stored", using
1198C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1199actually need to be stored in the array. [MAYCHANGE] After a call to
1200C<av_store> on a tied array, the caller will usually need to call
1201C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1202TIEARRAY object. If C<av_store> did return NULL, a call to
1203C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1204leak. [/MAYCHANGE]
04343c6d
GS
1205
1206The previous paragraph is applicable verbatim to tied hash access using the
1207C<hv_store> and C<hv_store_ent> functions as well.
1208
1209C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1210C<hv_fetch_ent> actually return an undefined mortal value whose magic
1211has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1212need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1213need to call C<mg_get()> on the returned value in order to actually invoke
1214the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1215you may also call C<mg_set()> on the return value after possibly assigning
1216a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1217method on the TIE object. [/MAYCHANGE]
04343c6d 1218
9edb2b46 1219[MAYCHANGE]
04343c6d
GS
1220In other words, the array or hash fetch/store functions don't really
1221fetch and store actual values in the case of tied arrays and hashes. They
1222merely call C<mg_copy> to attach magic to the values that were meant to be
1223"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1224do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1225the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1226and hashes.
1227
1228Currently (as of perl version 5.004), use of the hash and array access
1229functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1230"normal" hashes and arrays, or on their tied variants. The API may be
1231changed to provide more transparent access to both tied and normal data
1232types in future versions.
1233[/MAYCHANGE]
04343c6d
GS
1234
1235You would do well to understand that the TIEARRAY and TIEHASH interfaces
1236are mere sugar to invoke some perl method calls while using the uniform hash
1237and array syntax. The use of this sugar imposes some overhead (typically
1238about two to four extra opcodes per FETCH/STORE operation, in addition to
1239the creation of all the mortal variables required to invoke the methods).
1240This overhead will be comparatively small if the TIE methods are themselves
1241substantial, but if they are only a few statements long, the overhead
1242will not be insignificant.
1243
d1c897a1
IZ
1244=head2 Localizing changes
1245
1246Perl has a very handy construction
1247
1248 {
1249 local $var = 2;
1250 ...
1251 }
1252
1253This construction is I<approximately> equivalent to
1254
1255 {
1256 my $oldvar = $var;
1257 $var = 2;
1258 ...
1259 $var = $oldvar;
1260 }
1261
1262The biggest difference is that the first construction would
1263reinstate the initial value of $var, irrespective of how control exits
9a68f1db 1264the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1265more efficient as well.
1266
1267There is a way to achieve a similar task from C via Perl API: create a
1268I<pseudo-block>, and arrange for some changes to be automatically
1269undone at the end of it, either explicit, or via a non-local exit (via
1270die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1271C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1272Such a construct may be created specially for some important localized
1273task, or an existing one (like boundaries of enclosing Perl
1274subroutine/block, or an existing pair for freeing TMPs) may be
1275used. (In the second case the overhead of additional localization must
1276be almost negligible.) Note that any XSUB is automatically enclosed in
1277an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1278
1279Inside such a I<pseudo-block> the following service is available:
1280
13a2d996 1281=over 4
d1c897a1
IZ
1282
1283=item C<SAVEINT(int i)>
1284
1285=item C<SAVEIV(IV i)>
1286
1287=item C<SAVEI32(I32 i)>
1288
1289=item C<SAVELONG(long i)>
1290
1291These macros arrange things to restore the value of integer variable
1292C<i> at the end of enclosing I<pseudo-block>.
1293
1294=item C<SAVESPTR(s)>
1295
1296=item C<SAVEPPTR(p)>
1297
1298These macros arrange things to restore the value of pointers C<s> and
1299C<p>. C<s> must be a pointer of a type which survives conversion to
1300C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1301and back.
1302
1303=item C<SAVEFREESV(SV *sv)>
1304
1305The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1306I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1307mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1308extends the lifetime of C<sv> until the beginning of the next statement,
1309C<SAVEFREESV> extends it until the end of the enclosing scope. These
1310lifetimes can be wildly different.
1311
1312Also compare C<SAVEMORTALIZESV>.
1313
1314=item C<SAVEMORTALIZESV(SV *sv)>
1315
1316Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1317scope instead of decrementing its reference count. This usually has the
1318effect of keeping C<sv> alive until the statement that called the currently
1319live scope has finished executing.
d1c897a1
IZ
1320
1321=item C<SAVEFREEOP(OP *op)>
1322
1323The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1324
1325=item C<SAVEFREEPV(p)>
1326
1327The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1328end of I<pseudo-block>.
1329
1330=item C<SAVECLEARSV(SV *sv)>
1331
1332Clears a slot in the current scratchpad which corresponds to C<sv> at
1333the end of I<pseudo-block>.
1334
1335=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1336
1337The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1338string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1339short-lived storage, the corresponding string may be reallocated like
1340this:
1341
9cde0e7f 1342 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1343
c76ac1ee 1344=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1345
1346At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1347only argument C<p>.
1348
1349=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1350
1351At the end of I<pseudo-block> the function C<f> is called with the
1352implicit context argument (if any), and C<p>.
d1c897a1
IZ
1353
1354=item C<SAVESTACK_POS()>
1355
1356The current offset on the Perl internal stack (cf. C<SP>) is restored
1357at the end of I<pseudo-block>.
1358
1359=back
1360
1361The following API list contains functions, thus one needs to
1362provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1363or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1364function takes C<int *>.
1365
13a2d996 1366=over 4
d1c897a1
IZ
1367
1368=item C<SV* save_scalar(GV *gv)>
1369
1370Equivalent to Perl code C<local $gv>.
1371
1372=item C<AV* save_ary(GV *gv)>
1373
1374=item C<HV* save_hash(GV *gv)>
1375
1376Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1377
1378=item C<void save_item(SV *item)>
1379
1380Duplicates the current value of C<SV>, on the exit from the current
1381C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
038fcae3
SB
1382using the stored value. It doesn't handle magic. Use C<save_scalar> if
1383magic is affected.
d1c897a1
IZ
1384
1385=item C<void save_list(SV **sarg, I32 maxsarg)>
1386
1387A variant of C<save_item> which takes multiple arguments via an array
1388C<sarg> of C<SV*> of length C<maxsarg>.
1389
1390=item C<SV* save_svref(SV **sptr)>
1391
d1be9408 1392Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1393
1394=item C<void save_aptr(AV **aptr)>
1395
1396=item C<void save_hptr(HV **hptr)>
1397
1398Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1399
1400=back
1401
1402The C<Alias> module implements localization of the basic types within the
1403I<caller's scope>. People who are interested in how to localize things in
1404the containing scope should take a look there too.
1405
0a753a76 1406=head1 Subroutines
a0d0e21e 1407
68dc0745 1408=head2 XSUBs and the Argument Stack
5f05dabc
PP
1409
1410The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1411An XSUB routine will have a stack that contains the arguments from the Perl
1412program, and a way to map from the Perl data structures to a C equivalent.
1413
1414The stack arguments are accessible through the C<ST(n)> macro, which returns
1415the C<n>'th stack argument. Argument 0 is the first argument passed in the
1416Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1417an C<SV*> is used.
1418
1419Most of the time, output from the C routine can be handled through use of
1420the RETVAL and OUTPUT directives. However, there are some cases where the
1421argument stack is not already long enough to handle all the return values.
1422An example is the POSIX tzname() call, which takes no arguments, but returns
1423two, the local time zone's standard and summer time abbreviations.
1424
1425To handle this situation, the PPCODE directive is used and the stack is
1426extended using the macro:
1427
924508f0 1428 EXTEND(SP, num);
5f05dabc 1429
924508f0
GS
1430where C<SP> is the macro that represents the local copy of the stack pointer,
1431and C<num> is the number of elements the stack should be extended by.
5f05dabc 1432
00aadd71 1433Now that there is room on the stack, values can be pushed on it using C<PUSHs>
06f6df17 1434macro. The pushed values will often need to be "mortal" (See
d82b684c 1435L</Reference Counts and Mortality>):
5f05dabc 1436
00aadd71 1437 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1438 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1439 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1440 PUSHs(sv_2mortal(newSVpv("Some String",0)))
5f05dabc
PP
1441
1442And now the Perl program calling C<tzname>, the two values will be assigned
1443as in:
1444
1445 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1446
1447An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1448to use the macro:
5f05dabc 1449
5f05dabc
PP
1450 XPUSHs(SV*)
1451
00aadd71 1452This macro automatically adjust the stack for you, if needed. Thus, you
5f05dabc 1453do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1454
1455Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1456C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1457For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1458C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1459
1460For more information, consult L<perlxs> and L<perlxstut>.
1461
1462=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1463
1464There are four routines that can be used to call a Perl subroutine from
1465within a C program. These four are:
1466
954c1994
GS
1467 I32 call_sv(SV*, I32);
1468 I32 call_pv(const char*, I32);
1469 I32 call_method(const char*, I32);
1470 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1471
954c1994 1472The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1473contains either the name of the Perl subroutine to be called, or a
1474reference to the subroutine. The second argument consists of flags
1475that control the context in which the subroutine is called, whether
1476or not the subroutine is being passed arguments, how errors should be
1477trapped, and how to treat return values.
a0d0e21e
LW
1478
1479All four routines return the number of arguments that the subroutine returned
1480on the Perl stack.
1481
9a68f1db 1482These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1483but those names are now deprecated; macros of the same name are provided for
1484compatibility.
1485
1486When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1487must manipulate the Perl stack. These include the following macros and
1488functions:
a0d0e21e
LW
1489
1490 dSP
924508f0 1491 SP
a0d0e21e
LW
1492 PUSHMARK()
1493 PUTBACK
1494 SPAGAIN
1495 ENTER
1496 SAVETMPS
1497 FREETMPS
1498 LEAVE
1499 XPUSH*()
cb1a09d0 1500 POP*()
a0d0e21e 1501
5f05dabc
PP
1502For a detailed description of calling conventions from C to Perl,
1503consult L<perlcall>.
a0d0e21e 1504
5f05dabc 1505=head2 Memory Allocation
a0d0e21e 1506
06f6df17
RGS
1507=head3 Allocation
1508
86058a2d
GS
1509All memory meant to be used with the Perl API functions should be manipulated
1510using the macros described in this section. The macros provide the necessary
1511transparency between differences in the actual malloc implementation that is
1512used within perl.
1513
1514It is suggested that you enable the version of malloc that is distributed
5f05dabc 1515with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1516order to satisfy allocation requests more quickly. However, on some
1517platforms, it may cause spurious malloc or free errors.
d1b91892 1518
06f6df17
RGS
1519The following three macros are used to initially allocate memory :
1520
9f653bb5
SH
1521 Newx(pointer, number, type);
1522 Newxc(pointer, number, type, cast);
1523 Newxz(pointer, number, type);
d1b91892 1524
9f653bb5 1525The first argument C<pointer> should be the name of a variable that will
5f05dabc 1526point to the newly allocated memory.
d1b91892 1527
9f653bb5 1528The second and third arguments C<number> and C<type> specify how many of
d1b91892 1529the specified type of data structure should be allocated. The argument
9f653bb5 1530C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
d1b91892
AD
1531should be used if the C<pointer> argument is different from the C<type>
1532argument.
1533
9f653bb5 1534Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
d1b91892
AD
1535to zero out all the newly allocated memory.
1536
06f6df17
RGS
1537=head3 Reallocation
1538
d1b91892
AD
1539 Renew(pointer, number, type);
1540 Renewc(pointer, number, type, cast);
1541 Safefree(pointer)
1542
1543These three macros are used to change a memory buffer size or to free a
1544piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1545match those of C<New> and C<Newc> with the exception of not needing the
1546"magic cookie" argument.
1547
06f6df17
RGS
1548=head3 Moving
1549
d1b91892
AD
1550 Move(source, dest, number, type);
1551 Copy(source, dest, number, type);
1552 Zero(dest, number, type);
1553
1554These three macros are used to move, copy, or zero out previously allocated
1555memory. The C<source> and C<dest> arguments point to the source and
1556destination starting points. Perl will move, copy, or zero out C<number>
1557instances of the size of the C<type> data structure (using the C<sizeof>
1558function).
a0d0e21e 1559
5f05dabc 1560=head2 PerlIO
ce3d39e2 1561
5f05dabc
PP
1562The most recent development releases of Perl has been experimenting with
1563removing Perl's dependency on the "normal" standard I/O suite and allowing
1564other stdio implementations to be used. This involves creating a new
1565abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1566was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1567abstraction layer and not make any assumptions about what kind of stdio
1568is being used.
1569
1570For a complete description of the PerlIO abstraction, consult L<perlapio>.
1571
8ebc5c01 1572=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1573
1574A lot of opcodes (this is an elementary operation in the internal perl
1575stack machine) put an SV* on the stack. However, as an optimization
1576the corresponding SV is (usually) not recreated each time. The opcodes
1577reuse specially assigned SVs (I<target>s) which are (as a corollary)
1578not constantly freed/created.
1579
0a753a76 1580Each of the targets is created only once (but see
ce3d39e2
IZ
1581L<Scratchpads and recursion> below), and when an opcode needs to put
1582an integer, a double, or a string on stack, it just sets the
1583corresponding parts of its I<target> and puts the I<target> on stack.
1584
1585The macro to put this target on stack is C<PUSHTARG>, and it is
1586directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1587others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1588
1bd1c0d5
SC
1589Because the target is reused, you must be careful when pushing multiple
1590values on the stack. The following code will not do what you think:
1591
1592 XPUSHi(10);
1593 XPUSHi(20);
1594
1595This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1596the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1597At the end of the operation, the stack does not contain the values 10
1598and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1599to 20.
1bd1c0d5 1600
d82b684c
SH
1601If you need to push multiple different values then you should either use
1602the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1603none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1604SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1605will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1606this a little easier to achieve by creating a new mortal for you (via
1607C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1608in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1609Thus, instead of writing this to "fix" the example above:
1610
1611 XPUSHs(sv_2mortal(newSViv(10)))
1612 XPUSHs(sv_2mortal(newSViv(20)))
1613
1614you can simply write:
1615
1616 mXPUSHi(10)
1617 mXPUSHi(20)
1618
1619On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1620need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1621macros can make use of the local variable C<TARG>. See also C<dTARGET>
1622and C<dXSTARG>.
1bd1c0d5 1623
8ebc5c01 1624=head2 Scratchpads
ce3d39e2 1625
54310121 1626The question remains on when the SVs which are I<target>s for opcodes
ac036724 1627are created. The answer is that they are created when the current
1628unit--a subroutine or a file (for opcodes for statements outside of
1629subroutines)--is compiled. During this time a special anonymous Perl
1630array is created, which is called a scratchpad for the current unit.
ce3d39e2 1631
54310121 1632A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1633targets for opcodes. One can deduce that an SV lives on a scratchpad
1634by looking on its flags: lexicals have C<SVs_PADMY> set, and
1635I<target>s have C<SVs_PADTMP> set.
1636
54310121
PP
1637The correspondence between OPs and I<target>s is not 1-to-1. Different
1638OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1639would not conflict with the expected life of the temporary.
1640
2ae324a7 1641=head2 Scratchpads and recursion
ce3d39e2
IZ
1642
1643In fact it is not 100% true that a compiled unit contains a pointer to
1644the scratchpad AV. In fact it contains a pointer to an AV of
1645(initially) one element, and this element is the scratchpad AV. Why do
1646we need an extra level of indirection?
1647
9a68f1db 1648The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2
IZ
1649these can create several execution pointers going into the same
1650subroutine. For the subroutine-child not write over the temporaries
1651for the subroutine-parent (lifespan of which covers the call to the
1652child), the parent and the child should have different
1653scratchpads. (I<And> the lexicals should be separate anyway!)
1654
5f05dabc
PP
1655So each subroutine is born with an array of scratchpads (of length 1).
1656On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1657depth of the recursion is not more than the length of this array, and
1658if it is, new scratchpad is created and pushed into the array.
1659
1660The I<target>s on this scratchpad are C<undef>s, but they are already
1661marked with correct flags.
1662
0a753a76
PP
1663=head1 Compiled code
1664
1665=head2 Code tree
1666
1667Here we describe the internal form your code is converted to by
1668Perl. Start with a simple example:
1669
1670 $a = $b + $c;
1671
1672This is converted to a tree similar to this one:
1673
1674 assign-to
1675 / \
1676 + $a
1677 / \
1678 $b $c
1679
7b8d334a 1680(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1681parsed your code, but has nothing to do with the execution order.
1682There is an additional "thread" going through the nodes of the tree
1683which shows the order of execution of the nodes. In our simplified
1684example above it looks like:
1685
1686 $b ---> $c ---> + ---> $a ---> assign-to
1687
1688But with the actual compile tree for C<$a = $b + $c> it is different:
1689some nodes I<optimized away>. As a corollary, though the actual tree
1690contains more nodes than our simplified example, the execution order
1691is the same as in our example.
1692
1693=head2 Examining the tree
1694
06f6df17
RGS
1695If you have your perl compiled for debugging (usually done with
1696C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1697compiled tree by specifying C<-Dx> on the Perl command line. The
1698output takes several lines per node, and for C<$b+$c> it looks like
1699this:
1700
1701 5 TYPE = add ===> 6
1702 TARG = 1
1703 FLAGS = (SCALAR,KIDS)
1704 {
1705 TYPE = null ===> (4)
1706 (was rv2sv)
1707 FLAGS = (SCALAR,KIDS)
1708 {
1709 3 TYPE = gvsv ===> 4
1710 FLAGS = (SCALAR)
1711 GV = main::b
1712 }
1713 }
1714 {
1715 TYPE = null ===> (5)
1716 (was rv2sv)
1717 FLAGS = (SCALAR,KIDS)
1718 {
1719 4 TYPE = gvsv ===> 5
1720 FLAGS = (SCALAR)
1721 GV = main::c
1722 }
1723 }
1724
1725This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1726not optimized away (one per number in the left column). The immediate
1727children of the given node correspond to C<{}> pairs on the same level
1728of indentation, thus this listing corresponds to the tree:
1729
1730 add
1731 / \
1732 null null
1733 | |
1734 gvsv gvsv
1735
1736The execution order is indicated by C<===E<gt>> marks, thus it is C<3
17374 5 6> (node C<6> is not included into above listing), i.e.,
1738C<gvsv gvsv add whatever>.
1739
9afa14e3
SC
1740Each of these nodes represents an op, a fundamental operation inside the
1741Perl core. The code which implements each operation can be found in the
1742F<pp*.c> files; the function which implements the op with type C<gvsv>
1743is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1744different numbers of children: C<add> is a binary operator, as one would
1745expect, and so has two children. To accommodate the various different
1746numbers of children, there are various types of op data structure, and
1747they link together in different ways.
1748
1749The simplest type of op structure is C<OP>: this has no children. Unary
1750operators, C<UNOP>s, have one child, and this is pointed to by the
1751C<op_first> field. Binary operators (C<BINOP>s) have not only an
1752C<op_first> field but also an C<op_last> field. The most complex type of
1753op is a C<LISTOP>, which has any number of children. In this case, the
1754first child is pointed to by C<op_first> and the last child by
1755C<op_last>. The children in between can be found by iteratively
1756following the C<op_sibling> pointer from the first child to the last.
1757
1758There are also two other op types: a C<PMOP> holds a regular expression,
1759and has no children, and a C<LOOP> may or may not have children. If the
1760C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1761complicate matters, if a C<UNOP> is actually a C<null> op after
1762optimization (see L</Compile pass 2: context propagation>) it will still
1763have children in accordance with its former type.
1764
06f6df17
RGS
1765Another way to examine the tree is to use a compiler back-end module, such
1766as L<B::Concise>.
1767
0a753a76
PP
1768=head2 Compile pass 1: check routines
1769
8870b5c7
GS
1770The tree is created by the compiler while I<yacc> code feeds it
1771the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1772the first pass of perl compilation.
1773
1774What makes this pass interesting for perl developers is that some
1775optimization may be performed on this pass. This is optimization by
8870b5c7 1776so-called "check routines". The correspondence between node names
0a753a76
PP
1777and corresponding check routines is described in F<opcode.pl> (do not
1778forget to run C<make regen_headers> if you modify this file).
1779
1780A check routine is called when the node is fully constructed except
7b8d334a 1781for the execution-order thread. Since at this time there are no
0a753a76
PP
1782back-links to the currently constructed node, one can do most any
1783operation to the top-level node, including freeing it and/or creating
1784new nodes above/below it.
1785
1786The check routine returns the node which should be inserted into the
1787tree (if the top-level node was not modified, check routine returns
1788its argument).
1789
1790By convention, check routines have names C<ck_*>. They are usually
1791called from C<new*OP> subroutines (or C<convert>) (which in turn are
1792called from F<perly.y>).
1793
1794=head2 Compile pass 1a: constant folding
1795
1796Immediately after the check routine is called the returned node is
1797checked for being compile-time executable. If it is (the value is
1798judged to be constant) it is immediately executed, and a I<constant>
1799node with the "return value" of the corresponding subtree is
1800substituted instead. The subtree is deleted.
1801
1802If constant folding was not performed, the execution-order thread is
1803created.
1804
1805=head2 Compile pass 2: context propagation
1806
1807When a context for a part of compile tree is known, it is propagated
a3cb178b 1808down through the tree. At this time the context can have 5 values
0a753a76
PP
1809(instead of 2 for runtime context): void, boolean, scalar, list, and
1810lvalue. In contrast with the pass 1 this pass is processed from top
1811to bottom: a node's context determines the context for its children.
1812
1813Additional context-dependent optimizations are performed at this time.
1814Since at this moment the compile tree contains back-references (via
1815"thread" pointers), nodes cannot be free()d now. To allow
1816optimized-away nodes at this stage, such nodes are null()ified instead
1817of free()ing (i.e. their type is changed to OP_NULL).
1818
1819=head2 Compile pass 3: peephole optimization
1820
1821After the compile tree for a subroutine (or for an C<eval> or a file)
1822is created, an additional pass over the code is performed. This pass
1823is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1824additional complications for conditionals). These optimizations are
0a753a76
PP
1825done in the subroutine peep(). Optimizations performed at this stage
1826are subject to the same restrictions as in the pass 2.
1827
1ba7f851
PJ
1828=head2 Pluggable runops
1829
1830The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
1831functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
1832with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
1833control over the execution of the compile tree it is possible to provide
1834your own runops function.
1ba7f851
PJ
1835
1836It's probably best to copy one of the existing runops functions and
1837change it to suit your needs. Then, in the BOOT section of your XS
1838file, add the line:
1839
1840 PL_runops = my_runops;
1841
1842This function should be as efficient as possible to keep your programs
1843running as fast as possible.
1844
9afa14e3
SC
1845=head1 Examining internal data structures with the C<dump> functions
1846
1847To aid debugging, the source file F<dump.c> contains a number of
1848functions which produce formatted output of internal data structures.
1849
1850The most commonly used of these functions is C<Perl_sv_dump>; it's used
1851for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1852C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 1853module should already be familiar with its format.
9afa14e3
SC
1854
1855C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 1856derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
1857C<Perl_dump_eval> will dump the main root of the code being evaluated,
1858exactly like C<-Dx>.
1859
1860Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1861op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1862subroutines in a package like so: (Thankfully, these are all xsubs, so
1863there is no op tree)
1864
1865 (gdb) print Perl_dump_packsubs(PL_defstash)
1866
1867 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1868
1869 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1870
1871 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1872
1873 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1874
1875 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1876
1877and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1878the op tree of the main root.
1879
954c1994 1880=head1 How multiple interpreters and concurrency are supported
ee072b34 1881
ee072b34
GS
1882=head2 Background and PERL_IMPLICIT_CONTEXT
1883
1884The Perl interpreter can be regarded as a closed box: it has an API
1885for feeding it code or otherwise making it do things, but it also has
1886functions for its own use. This smells a lot like an object, and
1887there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
1888interpreters, with one interpreter represented either as a C structure,
1889or inside a thread-specific structure. These structures contain all
1890the context, the state of that interpreter.
1891
7b52221d
RGS
1892One macro controls the major Perl build flavor: MULTIPLICITY. The
1893MULTIPLICITY build has a C structure that packages all the interpreter
1894state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
1895normally defined, and enables the support for passing in a "hidden" first
1896argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 1897multi-threaded perls possible (with the ithreads threading model, related
7b52221d 1898to the macro USE_ITHREADS.)
54aff467 1899
27da23d5
JH
1900Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
1901PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
1902former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
1903internal variables of Perl to be wrapped inside a single global struct,
1904struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
1905the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
1906one step further, there is still a single struct (allocated in main()
1907either from heap or from stack) but there are no global data symbols
1908pointing to it. In either case the global struct should be initialised
1909as the very first thing in main() using Perl_init_global_struct() and
1910correspondingly tear it down after perl_free() using Perl_free_global_struct(),
1911please see F<miniperlmain.c> for usage details. You may also need
1912to use C<dVAR> in your coding to "declare the global variables"
1913when you are using them. dTHX does this for you automatically.
1914
bc028b6b
JH
1915To see whether you have non-const data you can use a BSD-compatible C<nm>:
1916
1917 nm libperl.a | grep -v ' [TURtr] '
1918
1919If this displays any C<D> or C<d> symbols, you have non-const data.
1920
27da23d5
JH
1921For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
1922doesn't actually hide all symbols inside a big global struct: some
1923PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
1924then hides everything (see how the PERLIO_FUNCS_DECL is used).
1925
54aff467 1926All this obviously requires a way for the Perl internal functions to be
acfe0abc 1927either subroutines taking some kind of structure as the first
ee072b34 1928argument, or subroutines taking nothing as the first argument. To
acfe0abc 1929enable these two very different ways of building the interpreter,
ee072b34
GS
1930the Perl source (as it does in so many other situations) makes heavy
1931use of macros and subroutine naming conventions.
1932
54aff467 1933First problem: deciding which functions will be public API functions and
00aadd71 1934which will be private. All functions whose names begin C<S_> are private
954c1994
GS
1935(think "S" for "secret" or "static"). All other functions begin with
1936"Perl_", but just because a function begins with "Perl_" does not mean it is
00aadd71
NIS
1937part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1938function is part of the API is to find its entry in L<perlapi>.
1939If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1940think it should be (i.e., you need it for your extension), send mail via
a422fd2d 1941L<perlbug> explaining why you think it should be.
ee072b34
GS
1942
1943Second problem: there must be a syntax so that the same subroutine
1944declarations and calls can pass a structure as their first argument,
1945or pass nothing. To solve this, the subroutines are named and
1946declared in a particular way. Here's a typical start of a static
1947function used within the Perl guts:
1948
1949 STATIC void
1950 S_incline(pTHX_ char *s)
1951
acfe0abc
GS
1952STATIC becomes "static" in C, and may be #define'd to nothing in some
1953configurations in future.
ee072b34 1954
651a3225
GS
1955A public function (i.e. part of the internal API, but not necessarily
1956sanctioned for use in extensions) begins like this:
ee072b34
GS
1957
1958 void
2307c6d0 1959 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 1960
0147cd53 1961C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
1962details of the interpreter's context. THX stands for "thread", "this",
1963or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1964The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
1965or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
1966their variants.
ee072b34 1967
a7486cbb
JH
1968When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
1969first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
1970in the pTHX_ macro indicates that the macro expansion needs a comma
1971after the context argument because other arguments follow it. If
1972PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1973subroutine is not prototyped to take the extra argument. The form of the
1974macro without the trailing underscore is used when there are no additional
ee072b34
GS
1975explicit arguments.
1976
54aff467 1977When a core function calls another, it must pass the context. This
2307c6d0 1978is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
1979something like this:
1980
2307c6d0
SB
1981 #ifdef PERL_IMPLICIT_CONTEXT
1982 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 1983 /* can't do this for vararg functions, see below */
2307c6d0
SB
1984 #else
1985 #define sv_setiv Perl_sv_setiv
1986 #endif
ee072b34
GS
1987
1988This works well, and means that XS authors can gleefully write:
1989
2307c6d0 1990 sv_setiv(foo, bar);
ee072b34
GS
1991
1992and still have it work under all the modes Perl could have been
1993compiled with.
1994
ee072b34
GS
1995This doesn't work so cleanly for varargs functions, though, as macros
1996imply that the number of arguments is known in advance. Instead we
1997either need to spell them out fully, passing C<aTHX_> as the first
1998argument (the Perl core tends to do this with functions like
1999Perl_warner), or use a context-free version.
2000
2001The context-free version of Perl_warner is called
2002Perl_warner_nocontext, and does not take the extra argument. Instead
2003it does dTHX; to get the context from thread-local storage. We
2004C<#define warner Perl_warner_nocontext> so that extensions get source
2005compatibility at the expense of performance. (Passing an arg is
2006cheaper than grabbing it from thread-local storage.)
2007
acfe0abc 2008You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2009Those are strictly for use within the core. Extensions and embedders
2010need only be aware of [pad]THX.
2011
a7486cbb
JH
2012=head2 So what happened to dTHR?
2013
2014C<dTHR> was introduced in perl 5.005 to support the older thread model.
2015The older thread model now uses the C<THX> mechanism to pass context
2016pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2017later still have it for backward source compatibility, but it is defined
2018to be a no-op.
2019
ee072b34
GS
2020=head2 How do I use all this in extensions?
2021
2022When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2023any functions in the Perl API will need to pass the initial context
2024argument somehow. The kicker is that you will need to write it in
2025such a way that the extension still compiles when Perl hasn't been
2026built with PERL_IMPLICIT_CONTEXT enabled.
2027
2028There are three ways to do this. First, the easy but inefficient way,
2029which is also the default, in order to maintain source compatibility
0147cd53 2030with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2031and aTHX_ macros to call a function that will return the context.
2032Thus, something like:
2033
2307c6d0 2034 sv_setiv(sv, num);
ee072b34 2035
4375e838 2036in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2037in effect:
ee072b34 2038
2307c6d0 2039 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2040
54aff467 2041or to this otherwise:
ee072b34 2042
2307c6d0 2043 Perl_sv_setiv(sv, num);
ee072b34
GS
2044
2045You have to do nothing new in your extension to get this; since
2fa86c13 2046the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2047work.
2048
2049The second, more efficient way is to use the following template for
2050your Foo.xs:
2051
c52f9dcd
JH
2052 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2053 #include "EXTERN.h"
2054 #include "perl.h"
2055 #include "XSUB.h"
ee072b34 2056
fd061412 2057 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2058
fd061412 2059 STATIC void
c52f9dcd
JH
2060 my_private_function(int arg1, int arg2)
2061 {
2062 dTHX; /* fetch context */
2063 ... call many Perl API functions ...
2064 }
ee072b34
GS
2065
2066 [... etc ...]
2067
c52f9dcd 2068 MODULE = Foo PACKAGE = Foo
ee072b34 2069
c52f9dcd 2070 /* typical XSUB */
ee072b34 2071
c52f9dcd
JH
2072 void
2073 my_xsub(arg)
2074 int arg
2075 CODE:
2076 my_private_function(arg, 10);
ee072b34
GS
2077
2078Note that the only two changes from the normal way of writing an
2079extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2080including the Perl headers, followed by a C<dTHX;> declaration at
2081the start of every function that will call the Perl API. (You'll
2082know which functions need this, because the C compiler will complain
2083that there's an undeclared identifier in those functions.) No changes
2084are needed for the XSUBs themselves, because the XS() macro is
2085correctly defined to pass in the implicit context if needed.
2086
2087The third, even more efficient way is to ape how it is done within
2088the Perl guts:
2089
2090
c52f9dcd
JH
2091 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2092 #include "EXTERN.h"
2093 #include "perl.h"
2094 #include "XSUB.h"
ee072b34
GS
2095
2096 /* pTHX_ only needed for functions that call Perl API */
fd061412 2097 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2098
fd061412 2099 STATIC void
c52f9dcd
JH
2100 my_private_function(pTHX_ int arg1, int arg2)
2101 {
2102 /* dTHX; not needed here, because THX is an argument */
2103 ... call Perl API functions ...
2104 }
ee072b34
GS
2105
2106 [... etc ...]
2107
c52f9dcd 2108 MODULE = Foo PACKAGE = Foo
ee072b34 2109
c52f9dcd 2110 /* typical XSUB */
ee072b34 2111
c52f9dcd
JH
2112 void
2113 my_xsub(arg)
2114 int arg
2115 CODE:
2116 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2117
2118This implementation never has to fetch the context using a function
2119call, since it is always passed as an extra argument. Depending on
2120your needs for simplicity or efficiency, you may mix the previous
2121two approaches freely.
2122
651a3225
GS
2123Never add a comma after C<pTHX> yourself--always use the form of the
2124macro with the underscore for functions that take explicit arguments,
2125or the form without the argument for functions with no explicit arguments.
ee072b34 2126
27da23d5
JH
2127If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2128definition is needed if the Perl global variables (see F<perlvars.h>
2129or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2130used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2131the need for C<dVAR> only with the said compile-time define, because
2132otherwise the Perl global variables are visible as-is.
2133
a7486cbb
JH
2134=head2 Should I do anything special if I call perl from multiple threads?
2135
2136If you create interpreters in one thread and then proceed to call them in
2137another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2138initialized correctly in each of those threads.
2139
2140The C<perl_alloc> and C<perl_clone> API functions will automatically set
2141the TLS slot to the interpreter they created, so that there is no need to do
2142anything special if the interpreter is always accessed in the same thread that
2143created it, and that thread did not create or call any other interpreters
2144afterwards. If that is not the case, you have to set the TLS slot of the
2145thread before calling any functions in the Perl API on that particular
2146interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2147thread as the first thing you do:
2148
2149 /* do this before doing anything else with some_perl */
2150 PERL_SET_CONTEXT(some_perl);
2151
2152 ... other Perl API calls on some_perl go here ...
2153
ee072b34
GS
2154=head2 Future Plans and PERL_IMPLICIT_SYS
2155
2156Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2157that the interpreter knows about itself and pass it around, so too are
2158there plans to allow the interpreter to bundle up everything it knows
2159about the environment it's running on. This is enabled with the
7b52221d
RGS
2160PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2161Windows.
ee072b34
GS
2162
2163This allows the ability to provide an extra pointer (called the "host"
2164environment) for all the system calls. This makes it possible for
2165all the system stuff to maintain their own state, broken down into
2166seven C structures. These are thin wrappers around the usual system
0147cd53 2167calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2168more ambitious host (like the one that would do fork() emulation) all
2169the extra work needed to pretend that different interpreters are
2170actually different "processes", would be done here.
2171
2172The Perl engine/interpreter and the host are orthogonal entities.
2173There could be one or more interpreters in a process, and one or
2174more "hosts", with free association between them.
2175
a422fd2d
SC
2176=head1 Internal Functions
2177
2178All of Perl's internal functions which will be exposed to the outside
06f6df17 2179world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d
SC
2180functions or functions used in a program in which Perl is embedded.
2181Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2182static functions start with C<S_>.)
a422fd2d 2183
0972ecdf
DM
2184Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2185either with or without the C<Perl_> prefix, thanks to a bunch of defines
2186that live in F<embed.h>. Note that extension code should I<not> set
2187C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2188breakage of the XS in each new perl release.
2189
2190The file F<embed.h> is generated automatically from
dc9b1d22
MHM
2191F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
2192header files for the internal functions, generates the documentation
2193and a lot of other bits and pieces. It's important that when you add
2194a new function to the core or change an existing one, you change the
2195data in the table in F<embed.fnc> as well. Here's a sample entry from
2196that table:
a422fd2d
SC
2197
2198 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2199
2200The second column is the return type, the third column the name. Columns
2201after that are the arguments. The first column is a set of flags:
2202
2203=over 3
2204
2205=item A
2206
1aa6ea50
JC
2207This function is a part of the public API. All such functions should also
2208have 'd', very few do not.
a422fd2d
SC
2209
2210=item p
2211
1aa6ea50
JC
2212This function has a C<Perl_> prefix; i.e. it is defined as
2213C<Perl_av_fetch>.
a422fd2d
SC
2214
2215=item d
2216
2217This function has documentation using the C<apidoc> feature which we'll
1aa6ea50 2218look at in a second. Some functions have 'd' but not 'A'; docs are good.
a422fd2d
SC
2219
2220=back
2221
2222Other available flags are:
2223
2224=over 3
2225
2226=item s
2227
1aa6ea50
JC
2228This is a static function and is defined as C<STATIC S_whatever>, and
2229usually called within the sources as C<whatever(...)>.
a422fd2d
SC
2230
2231=item n
2232
1aa6ea50
JC
2233This does not need a interpreter context, so the definition has no
2234C<pTHX>, and it follows that callers don't use C<aTHX>. (See
a422fd2d
SC
2235L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
2236
2237=item r
2238
2239This function never returns; C<croak>, C<exit> and friends.
2240
2241=item f
2242
2243This function takes a variable number of arguments, C<printf> style.
2244The argument list should end with C<...>, like this:
2245
2246 Afprd |void |croak |const char* pat|...
2247
a7486cbb 2248=item M
a422fd2d 2249
00aadd71 2250This function is part of the experimental development API, and may change
a422fd2d
SC
2251or disappear without notice.
2252
2253=item o
2254
2255This function should not have a compatibility macro to define, say,
2256C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
2257
a422fd2d
SC
2258=item x
2259
2260This function isn't exported out of the Perl core.
2261
dc9b1d22
MHM
2262=item m
2263
2264This is implemented as a macro.
2265
2266=item X
2267
2268This function is explicitly exported.
2269
2270=item E
2271
2272This function is visible to extensions included in the Perl core.
2273
2274=item b
2275
2276Binary backward compatibility; this function is a macro but also has
2277a C<Perl_> implementation (which is exported).
2278
1aa6ea50
JC
2279=item others
2280
2281See the comments at the top of C<embed.fnc> for others.
2282
a422fd2d
SC
2283=back
2284
dc9b1d22
MHM
2285If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2286C<make regen_headers> to force a rebuild of F<embed.h> and other
2287auto-generated files.
a422fd2d 2288
6b4667fc 2289=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2290
6b4667fc
A
2291If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2292formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2293following macros for portability
9dd9db0b 2294
c52f9dcd
JH
2295 IVdf IV in decimal
2296 UVuf UV in decimal
2297 UVof UV in octal
2298 UVxf UV in hexadecimal
2299 NVef NV %e-like
2300 NVff NV %f-like
2301 NVgf NV %g-like
9dd9db0b 2302
6b4667fc
A
2303These will take care of 64-bit integers and long doubles.
2304For example:
2305
c52f9dcd 2306 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2307
2308The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2309
8908e76d
JH
2310If you are printing addresses of pointers, use UVxf combined
2311with PTR2UV(), do not use %lx or %p.
2312
2313=head2 Pointer-To-Integer and Integer-To-Pointer
2314
2315Because pointer size does not necessarily equal integer size,
2316use the follow macros to do it right.
2317
c52f9dcd
JH
2318 PTR2UV(pointer)
2319 PTR2IV(pointer)
2320 PTR2NV(pointer)
2321 INT2PTR(pointertotype, integer)
8908e76d
JH
2322
2323For example:
2324
c52f9dcd
JH
2325 IV iv = ...;
2326 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2327
2328and
2329
c52f9dcd
JH
2330 AV *av = ...;
2331 UV uv = PTR2UV(av);
8908e76d 2332
0ca3a874
MHM
2333=head2 Exception Handling
2334
9b5c3821
MHM
2335There are a couple of macros to do very basic exception handling in XS
2336modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
2337be able to use these macros:
2338
2339 #define NO_XSLOCKS
2340 #include "XSUB.h"
2341
2342You can use these macros if you call code that may croak, but you need
2343to do some cleanup before giving control back to Perl. For example:
0ca3a874 2344
d7f8936a 2345 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2346
2347 XCPT_TRY_START {
2348 code_that_may_croak();
2349 } XCPT_TRY_END
2350
2351 XCPT_CATCH
2352 {
2353 /* do cleanup here */
2354 XCPT_RETHROW;
2355 }
2356
2357Note that you always have to rethrow an exception that has been
2358caught. Using these macros, it is not possible to just catch the
2359exception and ignore it. If you have to ignore the exception, you
2360have to use the C<call_*> function.
2361
2362The advantage of using the above macros is that you don't have
2363to setup an extra function for C<call_*>, and that using these
2364macros is faster than using C<call_*>.
2365
a422fd2d
SC
2366=head2 Source Documentation
2367
2368There's an effort going on to document the internal functions and
2369automatically produce reference manuals from them - L<perlapi> is one
2370such manual which details all the functions which are available to XS
2371writers. L<perlintern> is the autogenerated manual for the functions
2372which are not part of the API and are supposedly for internal use only.
2373
2374Source documentation is created by putting POD comments into the C
2375source, like this:
2376
2377 /*
2378 =for apidoc sv_setiv
2379
2380 Copies an integer into the given SV. Does not handle 'set' magic. See
2381 C<sv_setiv_mg>.
2382
2383 =cut
2384 */
2385
2386Please try and supply some documentation if you add functions to the
2387Perl core.
2388
0d098d33
MHM
2389=head2 Backwards compatibility
2390
2391The Perl API changes over time. New functions are added or the interfaces
2392of existing functions are changed. The C<Devel::PPPort> module tries to
2393provide compatibility code for some of these changes, so XS writers don't
2394have to code it themselves when supporting multiple versions of Perl.
2395
2396C<Devel::PPPort> generates a C header file F<ppport.h> that can also
2397be run as a Perl script. To generate F<ppport.h>, run:
2398
2399 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2400
2401Besides checking existing XS code, the script can also be used to retrieve
2402compatibility information for various API calls using the C<--api-info>
2403command line switch. For example:
2404
2405 % perl ppport.h --api-info=sv_magicext
2406
2407For details, see C<perldoc ppport.h>.
2408
a422fd2d
SC
2409=head1 Unicode Support
2410
2411Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2412writers to understand this support and make sure that the code they
2413write does not corrupt Unicode data.
2414
2415=head2 What B<is> Unicode, anyway?
2416
2417In the olden, less enlightened times, we all used to use ASCII. Most of
2418us did, anyway. The big problem with ASCII is that it's American. Well,
2419no, that's not actually the problem; the problem is that it's not
2420particularly useful for people who don't use the Roman alphabet. What
2421used to happen was that particular languages would stick their own
2422alphabet in the upper range of the sequence, between 128 and 255. Of
2423course, we then ended up with plenty of variants that weren't quite
2424ASCII, and the whole point of it being a standard was lost.
2425
2426Worse still, if you've got a language like Chinese or
2427Japanese that has hundreds or thousands of characters, then you really
2428can't fit them into a mere 256, so they had to forget about ASCII
2429altogether, and build their own systems using pairs of numbers to refer
2430to one character.
2431
2432To fix this, some people formed Unicode, Inc. and
2433produced a new character set containing all the characters you can
2434possibly think of and more. There are several ways of representing these
1e54db1a 2435characters, and the one Perl uses is called UTF-8. UTF-8 uses
2575c402
JW
2436a variable number of bytes to represent a character. You can learn more
2437about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2438
1e54db1a 2439=head2 How can I recognise a UTF-8 string?
a422fd2d 2440
1e54db1a
JH
2441You can't. This is because UTF-8 data is stored in bytes just like
2442non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d
SC
2443capital E with a grave accent, is represented by the two bytes
2444C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2445has that byte sequence as well. So you can't tell just by looking - this
2446is what makes Unicode input an interesting problem.
2447
2575c402
JW
2448In general, you either have to know what you're dealing with, or you
2449have to guess. The API function C<is_utf8_string> can help; it'll tell
2450you if a string contains only valid UTF-8 characters. However, it can't
2451do the work for you. On a character-by-character basis, C<is_utf8_char>
2452will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2453
1e54db1a 2454=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2455
1e54db1a 2456As mentioned above, UTF-8 uses a variable number of bytes to store a
2575c402
JW
2457character. Characters with values 0...127 are stored in one byte, just
2458like good ol' ASCII. Character 128 is stored as C<v194.128>; this
a31a806a 2459continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2460bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2461so it goes on, moving to three bytes at character 2048.
2462
1e54db1a 2463Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2464how long the first character in it is with the C<UTF8SKIP> macro:
2465
2466 char *utf = "\305\233\340\240\201";
2467 I32 len;
2468
2469 len = UTF8SKIP(utf); /* len is 2 here */
2470 utf += len;
2471 len = UTF8SKIP(utf); /* len is 3 here */
2472
1e54db1a 2473Another way to skip over characters in a UTF-8 string is to use
a422fd2d
SC
2474C<utf8_hop>, which takes a string and a number of characters to skip
2475over. You're on your own about bounds checking, though, so don't use it
2476lightly.
2477
1e54db1a 2478All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2479so you can test if you need to do something special with this
2480character like this (the UTF8_IS_INVARIANT() is a macro that tests
2481whether the byte can be encoded as a single byte even in UTF-8):
a422fd2d 2482
3a2263fe
RGS
2483 U8 *utf;
2484 UV uv; /* Note: a UV, not a U8, not a char */
a422fd2d 2485
3a2263fe 2486 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2487 /* Must treat this as UTF-8 */
a422fd2d
SC
2488 uv = utf8_to_uv(utf);
2489 else
2490 /* OK to treat this character as a byte */
2491 uv = *utf;
2492
2493You can also see in that example that we use C<utf8_to_uv> to get the
2494value of the character; the inverse function C<uv_to_utf8> is available
1e54db1a 2495for putting a UV into UTF-8:
a422fd2d 2496
3a2263fe 2497 if (!UTF8_IS_INVARIANT(uv))
a422fd2d
SC
2498 /* Must treat this as UTF8 */
2499 utf8 = uv_to_utf8(utf8, uv);
2500 else
2501 /* OK to treat this character as a byte */
2502 *utf8++ = uv;
2503
2504You B<must> convert characters to UVs using the above functions if
1e54db1a
JH
2505you're ever in a situation where you have to match UTF-8 and non-UTF-8
2506characters. You may not skip over UTF-8 characters in this case. If you
2507do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2508for instance, if your UTF-8 string contains C<v196.172>, and you skip
2509that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2510So don't do that!
2511
1e54db1a 2512=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2513
2514Currently, Perl deals with Unicode strings and non-Unicode strings
2575c402
JW
2515slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2516string is internally encoded as UTF-8. Without it, the byte value is the
2517codepoint number and vice versa (in other words, the string is encoded
e1b711da
KW
2518as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
2519semantics). You can check and manipulate this flag with the
2575c402 2520following macros:
a422fd2d
SC
2521
2522 SvUTF8(sv)
2523 SvUTF8_on(sv)
2524 SvUTF8_off(sv)
2525
2526This flag has an important effect on Perl's treatment of the string: if
2527Unicode data is not properly distinguished, regular expressions,
2528C<length>, C<substr> and other string handling operations will have
2529undesirable results.
2530
2531The problem comes when you have, for instance, a string that isn't
2575c402 2532flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
1e54db1a 2533especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2534
2535Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2536need be sure you don't accidentally knock it off while you're
2537manipulating SVs. More specifically, you cannot expect to do this:
2538
2539 SV *sv;
2540 SV *nsv;
2541 STRLEN len;
2542 char *p;
2543
2544 p = SvPV(sv, len);
2545 frobnicate(p);
2546 nsv = newSVpvn(p, len);
2547
2548The C<char*> string does not tell you the whole story, and you can't
2549copy or reconstruct an SV just by copying the string value. Check if the
2575c402 2550old SV has the UTF8 flag set, and act accordingly:
a422fd2d
SC
2551
2552 p = SvPV(sv, len);
2553 frobnicate(p);
2554 nsv = newSVpvn(p, len);
2555 if (SvUTF8(sv))
2556 SvUTF8_on(nsv);
2557
2558In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2559not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2560appropriately.
2561
3a2263fe 2562Since just passing an SV to an XS function and copying the data of
2575c402 2563the SV is not enough to copy the UTF8 flags, even less right is just
3a2263fe
RGS
2564passing a C<char *> to an XS function.
2565
1e54db1a 2566=head2 How do I convert a string to UTF-8?
a422fd2d 2567
2575c402
JW
2568If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
2569one of the strings to UTF-8. If you've got an SV, the easiest way to do
2570this is:
a422fd2d
SC
2571
2572 sv_utf8_upgrade(sv);
2573
2574However, you must not do this, for example:
2575
2576 if (!SvUTF8(left))
2577 sv_utf8_upgrade(left);
2578
2579If you do this in a binary operator, you will actually change one of the
b1866b2d 2580strings that came into the operator, and, while it shouldn't be noticeable
2575c402 2581by the end user, it can cause problems in deficient code.
a422fd2d 2582
1e54db1a 2583Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
a422fd2d 2584string argument. This is useful for having the data available for
b1866b2d 2585comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2586C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2587the string contains any characters above 255 that can't be represented
2588in a single byte.
2589
2590=head2 Is there anything else I need to know?
2591
2592Not really. Just remember these things:
2593
2594=over 3
2595
2596=item *
2597
1e54db1a
JH
2598There's no way to tell if a string is UTF-8 or not. You can tell if an SV
2599is UTF-8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2600something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2601it's not - if you pass on the PV to somewhere, pass on the flag too.
2602
2603=item *
2604
1e54db1a 2605If a string is UTF-8, B<always> use C<utf8_to_uv> to get at the value,
3a2263fe 2606unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2607
2608=item *
2609
1e54db1a 2610When writing a character C<uv> to a UTF-8 string, B<always> use
3a2263fe
RGS
2611C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
2612you can use C<*s = uv>.
a422fd2d
SC
2613
2614=item *
2615
1e54db1a 2616Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
2bbc8d55 2617a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
2618
2619=back
2620
53e06cf0
SC
2621=head1 Custom Operators
2622
9a68f1db 2623Custom operator support is a new experimental feature that allows you to
53e06cf0
SC
2624define your own ops. This is primarily to allow the building of
2625interpreters for other languages in the Perl core, but it also allows
2626optimizations through the creation of "macro-ops" (ops which perform the
2627functions of multiple ops which are usually executed together, such as
1aa6ea50 2628C<gvsv, gvsv, add>.)
53e06cf0 2629
b455bf3f 2630This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0
SC
2631core does not "know" anything special about this op type, and so it will
2632not be involved in any optimizations. This also means that you can
2633define your custom ops to be any op structure - unary, binary, list and
2634so on - you like.
2635
2636It's important to know what custom operators won't do for you. They
2637won't let you add new syntax to Perl, directly. They won't even let you
2638add new keywords, directly. In fact, they won't change the way Perl
2639compiles a program at all. You have to do those changes yourself, after
2640Perl has compiled the program. You do this either by manipulating the op
2641tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2642a custom peephole optimizer with the C<optimize> module.
2643
2644When you do this, you replace ordinary Perl ops with custom ops by
2645creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
2646PP function. This should be defined in XS code, and should look like
2647the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
2648takes the appropriate number of values from the stack, and you are
2649responsible for adding stack marks if necessary.
2650
2651You should also "register" your op with the Perl interpreter so that it
2652can produce sensible error and warning messages. Since it is possible to
2653have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
2654Perl uses the value of C<< o->op_ppaddr >> as a key into the
2655C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
2656need to enter a name and description for your op at the appropriate
2657place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
2658
e7d4c058 2659C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 2660
954c1994 2661=head1 AUTHORS
e89caa19 2662
954c1994 2663Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2664E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
2665itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 2666
954c1994
GS
2667With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2668Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2669Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2670Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2671
954c1994 2672=head1 SEE ALSO
cb1a09d0 2673
ba555bf5 2674L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>