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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
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8to provide some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
b3b6085d 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
10e2eb10 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).
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40("PV" stands for "Pointer Value". You might think that it is misnamed
41because it is described as pointing only to strings. However, it is
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42possible to have it point to other things For example, it could point
43to an array of UVs. But,
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44using it for non-strings requires care, as the underlying assumption of
45much of the internals is that PVs are just for strings. Often, for
6602b933 46example, a trailing C<NUL> is tacked on automatically. The non-string use
61984ee1 47is documented only in this paragraph.)
a0d0e21e 48
20dbd849 49The seven routines are:
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50
51 SV* newSViv(IV);
20dbd849 52 SV* newSVuv(UV);
a0d0e21e 53 SV* newSVnv(double);
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54 SV* newSVpv(const char*, STRLEN);
55 SV* newSVpvn(const char*, STRLEN);
46fc3d4c 56 SV* newSVpvf(const char*, ...);
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57 SV* newSVsv(SV*);
58
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59C<STRLEN> is an integer type (Size_t, usually defined as size_t in
60F<config.h>) guaranteed to be large enough to represent the size of
61any string that perl can handle.
62
3bf17896 63In the unlikely case of a SV requiring more complex initialization, you
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64can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
65type NULL is returned, else an SV of type PV is returned with len + 1 (for
6602b933 66the C<NUL>) bytes of storage allocated, accessible via SvPVX. In both cases
da8c5729 67the SV has the undef value.
20dbd849 68
06f6df17 69 SV *sv = newSV(0); /* no storage allocated */
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70 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
71 * allocated */
20dbd849 72
06f6df17 73To change the value of an I<already-existing> SV, there are eight routines:
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74
75 void sv_setiv(SV*, IV);
deb3007b 76 void sv_setuv(SV*, UV);
a0d0e21e 77 void sv_setnv(SV*, double);
08105a92 78 void sv_setpv(SV*, const char*);
06f6df17 79 void sv_setpvn(SV*, const char*, STRLEN)
46fc3d4c 80 void sv_setpvf(SV*, const char*, ...);
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81 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
82 SV **, I32, bool *);
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83 void sv_setsv(SV*, SV*);
84
85Notice that you can choose to specify the length of the string to be
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86assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
87allow Perl to calculate the length by using C<sv_setpv> or by specifying
880 as the second argument to C<newSVpv>. Be warned, though, that Perl will
89determine the string's length by using C<strlen>, which depends on the
6602b933 90string terminating with a C<NUL> character, and not otherwise containing
a9b0660e 91NULs.
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92
93The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
94formatted output becomes the value.
95
328bf373 96C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
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97either a pointer to a variable argument list or the address and length of
98an array of SVs. The last argument points to a boolean; on return, if that
99boolean is true, then locale-specific information has been used to format
c2611fb3 100the string, and the string's contents are therefore untrustworthy (see
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101L<perlsec>). This pointer may be NULL if that information is not
102important. Note that this function requires you to specify the length of
103the format.
104
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105The C<sv_set*()> functions are not generic enough to operate on values
106that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 107
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108All SVs that contain strings should be terminated with a C<NUL> character.
109If it is not C<NUL>-terminated there is a risk of
5f05dabc 110core dumps and corruptions from code which passes the string to C
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111functions or system calls which expect a C<NUL>-terminated string.
112Perl's own functions typically add a trailing C<NUL> for this reason.
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113Nevertheless, you should be very careful when you pass a string stored
114in an SV to a C function or system call.
115
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116To access the actual value that an SV points to, you can use the macros:
117
118 SvIV(SV*)
954c1994 119 SvUV(SV*)
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120 SvNV(SV*)
121 SvPV(SV*, STRLEN len)
1fa8b10d 122 SvPV_nolen(SV*)
a0d0e21e 123
954c1994 124which will automatically coerce the actual scalar type into an IV, UV, double,
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125or string.
126
127In the C<SvPV> macro, the length of the string returned is placed into the
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128variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
129not care what the length of the data is, use the C<SvPV_nolen> macro.
130Historically the C<SvPV> macro with the global variable C<PL_na> has been
131used in this case. But that can be quite inefficient because C<PL_na> must
132be accessed in thread-local storage in threaded Perl. In any case, remember
133that Perl allows arbitrary strings of data that may both contain NULs and
6602b933 134might not be terminated by a C<NUL>.
a0d0e21e 135
ce2f5d8f 136Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
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137len);>. It might work with your
138compiler, but it won't work for everyone.
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139Break this sort of statement up into separate assignments:
140
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141 SV *s;
142 STRLEN len;
61955433 143 char *ptr;
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144 ptr = SvPV(s, len);
145 foo(ptr, len);
ce2f5d8f 146
07fa94a1 147If you want to know if the scalar value is TRUE, you can use:
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148
149 SvTRUE(SV*)
150
151Although Perl will automatically grow strings for you, if you need to force
152Perl to allocate more memory for your SV, you can use the macro
153
154 SvGROW(SV*, STRLEN newlen)
155
156which will determine if more memory needs to be allocated. If so, it will
157call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
5f05dabc 158decrease, the allocated memory of an SV and that it does not automatically
6602b933 159add space for the trailing C<NUL> byte (perl's own string functions typically do
8ebc5c01 160C<SvGROW(sv, len + 1)>).
a0d0e21e 161
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162If you want to write to an existing SV's buffer and set its value to a
163string, use SvPV_force() or one of its variants to force the SV to be
164a PV. This will remove any of various types of non-stringness from
165the SV while preserving the content of the SV in the PV. This can be
166used, for example, to append data from an API function to a buffer
167without extra copying:
168
169 (void)SvPVbyte_force(sv, len);
170 s = SvGROW(sv, len + needlen + 1);
171 /* something that modifies up to needlen bytes at s+len, but
172 modifies newlen bytes
173 eg. newlen = read(fd, s + len, needlen);
174 ignoring errors for these examples
175 */
176 s[len + newlen] = '\0';
177 SvCUR_set(sv, len + newlen);
178 SvUTF8_off(sv);
179 SvSETMAGIC(sv);
180
181If you already have the data in memory or if you want to keep your
182code simple, you can use one of the sv_cat*() variants, such as
183sv_catpvn(). If you want to insert anywhere in the string you can use
184sv_insert() or sv_insert_flags().
185
186If you don't need the existing content of the SV, you can avoid some
187copying with:
188
189 sv_setpvn(sv, "", 0);
190 s = SvGROW(sv, needlen + 1);
191 /* something that modifies up to needlen bytes at s, but modifies
192 newlen bytes
193 eg. newlen = read(fd, s. needlen);
194 */
195 s[newlen] = '\0';
196 SvCUR_set(sv, newlen);
197 SvPOK_only(sv); /* also clears SVf_UTF8 */
198 SvSETMAGIC(sv);
199
200Again, if you already have the data in memory or want to avoid the
201complexity of the above, you can use sv_setpvn().
202
203If you have a buffer allocated with Newx() and want to set that as the
204SV's value, you can use sv_usepvn_flags(). That has some requirements
205if you want to avoid perl re-allocating the buffer to fit the trailing
206NUL:
207
208 Newx(buf, somesize+1, char);
209 /* ... fill in buf ... */
210 buf[somesize] = '\0';
211 sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
212 /* buf now belongs to perl, don't release it */
213
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214If you have an SV and want to know what kind of data Perl thinks is stored
215in it, you can use the following macros to check the type of SV you have.
216
217 SvIOK(SV*)
218 SvNOK(SV*)
219 SvPOK(SV*)
220
221You can get and set the current length of the string stored in an SV with
222the following macros:
223
224 SvCUR(SV*)
225 SvCUR_set(SV*, I32 val)
226
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227You can also get a pointer to the end of the string stored in the SV
228with the macro:
229
230 SvEND(SV*)
231
232But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 233
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234If you want to append something to the end of string stored in an C<SV*>,
235you can use the following functions:
236
08105a92 237 void sv_catpv(SV*, const char*);
e65f3abd 238 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 239 void sv_catpvf(SV*, const char*, ...);
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240 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
241 I32, bool);
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242 void sv_catsv(SV*, SV*);
243
244The first function calculates the length of the string to be appended by
245using C<strlen>. In the second, you specify the length of the string
46fc3d4c 246yourself. The third function processes its arguments like C<sprintf> and
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247appends the formatted output. The fourth function works like C<vsprintf>.
248You can specify the address and length of an array of SVs instead of the
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249va_list argument. The fifth function
250extends the string stored in the first
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251SV with the string stored in the second SV. It also forces the second SV
252to be interpreted as a string.
253
254The C<sv_cat*()> functions are not generic enough to operate on values that
255have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 256
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257If you know the name of a scalar variable, you can get a pointer to its SV
258by using the following:
259
64ace3f8 260 SV* get_sv("package::varname", 0);
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261
262This returns NULL if the variable does not exist.
263
d1b91892 264If you want to know if this variable (or any other SV) is actually C<defined>,
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265you can call:
266
267 SvOK(SV*)
268
06f6df17 269The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
9adebda4 270
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271Its address can be used whenever an C<SV*> is needed. Make sure that
272you don't try to compare a random sv with C<&PL_sv_undef>. For example
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273when interfacing Perl code, it'll work correctly for:
274
275 foo(undef);
276
277But won't work when called as:
278
279 $x = undef;
280 foo($x);
281
282So to repeat always use SvOK() to check whether an sv is defined.
283
284Also you have to be careful when using C<&PL_sv_undef> as a value in
285AVs or HVs (see L<AVs, HVs and undefined values>).
a0d0e21e 286
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287There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
288boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
289addresses can be used whenever an C<SV*> is needed.
a0d0e21e 290
9cde0e7f 291Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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292Take this code:
293
294 SV* sv = (SV*) 0;
295 if (I-am-to-return-a-real-value) {
296 sv = sv_2mortal(newSViv(42));
297 }
298 sv_setsv(ST(0), sv);
299
300This code tries to return a new SV (which contains the value 42) if it should
04343c6d 301return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 302pointer which, somewhere down the line, will cause a segmentation violation,
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303bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
304first line and all will be well.
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305
306To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 307call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 308
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309=head2 Offsets
310
311Perl provides the function C<sv_chop> to efficiently remove characters
312from the beginning of a string; you give it an SV and a pointer to
da75cd15 313somewhere inside the PV, and it discards everything before the
10e2eb10 314pointer. The efficiency comes by means of a little hack: instead of
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315actually removing the characters, C<sv_chop> sets the flag C<OOK>
316(offset OK) to signal to other functions that the offset hack is in
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317effect, and it moves the PV pointer (called C<SvPVX>) forward
318by the number of bytes chopped off, and adjusts C<SvCUR> and C<SvLEN>
319accordingly. (A portion of the space between the old and new PV
320pointers is used to store the count of chopped bytes.)
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321
322Hence, at this point, the start of the buffer that we allocated lives
323at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
324into the middle of this allocated storage.
325
326This is best demonstrated by example:
327
328 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
329 SV = PVIV(0x8128450) at 0x81340f0
330 REFCNT = 1
331 FLAGS = (POK,OOK,pPOK)
332 IV = 1 (OFFSET)
333 PV = 0x8135781 ( "1" . ) "2345"\0
334 CUR = 4
335 LEN = 5
336
337Here the number of bytes chopped off (1) is put into IV, and
10e2eb10 338C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
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339portion of the string between the "real" and the "fake" beginnings is
340shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
341the fake beginning, not the real one.
342
fe854a6f 343Something similar to the offset hack is performed on AVs to enable
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344efficient shifting and splicing off the beginning of the array; while
345C<AvARRAY> points to the first element in the array that is visible from
10e2eb10 346Perl, C<AvALLOC> points to the real start of the C array. These are
319cef53 347usually the same, but a C<shift> operation can be carried out by
6de131f0 348increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
319cef53 349Again, the location of the real start of the C array only comes into
10e2eb10 350play when freeing the array. See C<av_shift> in F<av.c>.
319cef53 351
d1b91892 352=head2 What's Really Stored in an SV?
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353
354Recall that the usual method of determining the type of scalar you have is
5f05dabc 355to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 356usually these macros will always return TRUE and calling the C<Sv*V>
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357macros will do the appropriate conversion of string to integer/double or
358integer/double to string.
359
360If you I<really> need to know if you have an integer, double, or string
361pointer in an SV, you can use the following three macros instead:
362
363 SvIOKp(SV*)
364 SvNOKp(SV*)
365 SvPOKp(SV*)
366
367These will tell you if you truly have an integer, double, or string pointer
d1b91892 368stored in your SV. The "p" stands for private.
a0d0e21e 369
da8c5729 370There are various ways in which the private and public flags may differ.
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371For example, in perl 5.16 and earlier a tied SV may have a valid
372underlying value in the IV slot (so SvIOKp is true), but the data
373should be accessed via the FETCH routine rather than directly,
374so SvIOK is false. (In perl 5.18 onwards, tied scalars use
375the flags the same way as untied scalars.) Another is when
d7f8936a 376numeric conversion has occurred and precision has been lost: only the
10e2eb10 377private flag is set on 'lossy' values. So when an NV is converted to an
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378IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
379
07fa94a1 380In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 381
54310121 382=head2 Working with AVs
a0d0e21e 383
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384There are two ways to create and load an AV. The first method creates an
385empty AV:
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386
387 AV* newAV();
388
54310121 389The second method both creates the AV and initially populates it with SVs:
a0d0e21e 390
c70927a6 391 AV* av_make(SSize_t num, SV **ptr);
a0d0e21e 392
5f05dabc 393The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 394AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 395
da8c5729 396Once the AV has been created, the following operations are possible on it:
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397
398 void av_push(AV*, SV*);
399 SV* av_pop(AV*);
400 SV* av_shift(AV*);
c70927a6 401 void av_unshift(AV*, SSize_t num);
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402
403These should be familiar operations, with the exception of C<av_unshift>.
404This routine adds C<num> elements at the front of the array with the C<undef>
405value. You must then use C<av_store> (described below) to assign values
406to these new elements.
407
408Here are some other functions:
409
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410 SSize_t av_top_index(AV*);
411 SV** av_fetch(AV*, SSize_t key, I32 lval);
412 SV** av_store(AV*, SSize_t key, SV* val);
a0d0e21e 413
dab460cd 414The C<av_top_index> function returns the highest index value in an array (just
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415like $#array in Perl). If the array is empty, -1 is returned. The
416C<av_fetch> function returns the value at index C<key>, but if C<lval>
417is non-zero, then C<av_fetch> will store an undef value at that index.
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418The C<av_store> function stores the value C<val> at index C<key>, and does
419not increment the reference count of C<val>. Thus the caller is responsible
420for taking care of that, and if C<av_store> returns NULL, the caller will
421have to decrement the reference count to avoid a memory leak. Note that
422C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
423return value.
d1b91892 424
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425A few more:
426
a0d0e21e 427 void av_clear(AV*);
a0d0e21e 428 void av_undef(AV*);
c70927a6 429 void av_extend(AV*, SSize_t key);
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430
431The C<av_clear> function deletes all the elements in the AV* array, but
432does not actually delete the array itself. The C<av_undef> function will
433delete all the elements in the array plus the array itself. The
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434C<av_extend> function extends the array so that it contains at least C<key+1>
435elements. If C<key+1> is less than the currently allocated length of the array,
436then nothing is done.
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437
438If you know the name of an array variable, you can get a pointer to its AV
439by using the following:
440
cbfd0a87 441 AV* get_av("package::varname", 0);
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442
443This returns NULL if the variable does not exist.
444
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445See L<Understanding the Magic of Tied Hashes and Arrays> for more
446information on how to use the array access functions on tied arrays.
447
54310121 448=head2 Working with HVs
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449
450To create an HV, you use the following routine:
451
452 HV* newHV();
453
da8c5729 454Once the HV has been created, the following operations are possible on it:
a0d0e21e 455
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456 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
457 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 458
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459The C<klen> parameter is the length of the key being passed in (Note that
460you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
461length of the key). The C<val> argument contains the SV pointer to the
54310121 462scalar being stored, and C<hash> is the precomputed hash value (zero if
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463you want C<hv_store> to calculate it for you). The C<lval> parameter
464indicates whether this fetch is actually a part of a store operation, in
465which case a new undefined value will be added to the HV with the supplied
466key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 467
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468Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
469C<SV*>. To access the scalar value, you must first dereference the return
470value. However, you should check to make sure that the return value is
471not NULL before dereferencing it.
a0d0e21e 472
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473The first of these two functions checks if a hash table entry exists, and the
474second deletes it.
a0d0e21e 475
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476 bool hv_exists(HV*, const char* key, U32 klen);
477 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 478
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479If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
480create and return a mortal copy of the deleted value.
481
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482And more miscellaneous functions:
483
484 void hv_clear(HV*);
a0d0e21e 485 void hv_undef(HV*);
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486
487Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
488table but does not actually delete the hash table. The C<hv_undef> deletes
489both the entries and the hash table itself.
a0d0e21e 490
a9b0660e 491Perl keeps the actual data in a linked list of structures with a typedef of HE.
d1b91892
AD
492These contain the actual key and value pointers (plus extra administrative
493overhead). The key is a string pointer; the value is an C<SV*>. However,
494once you have an C<HE*>, to get the actual key and value, use the routines
495specified below.
496
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LW
497 I32 hv_iterinit(HV*);
498 /* Prepares starting point to traverse hash table */
499 HE* hv_iternext(HV*);
500 /* Get the next entry, and return a pointer to a
501 structure that has both the key and value */
502 char* hv_iterkey(HE* entry, I32* retlen);
503 /* Get the key from an HE structure and also return
504 the length of the key string */
cb1a09d0 505 SV* hv_iterval(HV*, HE* entry);
d1be9408 506 /* Return an SV pointer to the value of the HE
a0d0e21e 507 structure */
cb1a09d0 508 SV* hv_iternextsv(HV*, char** key, I32* retlen);
d1b91892
AD
509 /* This convenience routine combines hv_iternext,
510 hv_iterkey, and hv_iterval. The key and retlen
511 arguments are return values for the key and its
512 length. The value is returned in the SV* argument */
a0d0e21e
LW
513
514If you know the name of a hash variable, you can get a pointer to its HV
515by using the following:
516
6673a63c 517 HV* get_hv("package::varname", 0);
a0d0e21e
LW
518
519This returns NULL if the variable does not exist.
520
a43e7901 521The hash algorithm is defined in the C<PERL_HASH> macro:
a0d0e21e 522
a43e7901 523 PERL_HASH(hash, key, klen)
ab192400 524
a43e7901
YO
525The exact implementation of this macro varies by architecture and version
526of perl, and the return value may change per invocation, so the value
527is only valid for the duration of a single perl process.
a0d0e21e 528
04343c6d
GS
529See L<Understanding the Magic of Tied Hashes and Arrays> for more
530information on how to use the hash access functions on tied hashes.
531
1e422769
PP
532=head2 Hash API Extensions
533
534Beginning with version 5.004, the following functions are also supported:
535
536 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
537 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 538
1e422769
PP
539 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
540 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 541
1e422769
PP
542 SV* hv_iterkeysv (HE* entry);
543
544Note that these functions take C<SV*> keys, which simplifies writing
545of extension code that deals with hash structures. These functions
546also allow passing of C<SV*> keys to C<tie> functions without forcing
547you to stringify the keys (unlike the previous set of functions).
548
549They also return and accept whole hash entries (C<HE*>), making their
550use more efficient (since the hash number for a particular string
4a4eefd0
GS
551doesn't have to be recomputed every time). See L<perlapi> for detailed
552descriptions.
1e422769
PP
553
554The following macros must always be used to access the contents of hash
555entries. Note that the arguments to these macros must be simple
556variables, since they may get evaluated more than once. See
4a4eefd0 557L<perlapi> for detailed descriptions of these macros.
1e422769
PP
558
559 HePV(HE* he, STRLEN len)
560 HeVAL(HE* he)
561 HeHASH(HE* he)
562 HeSVKEY(HE* he)
563 HeSVKEY_force(HE* he)
564 HeSVKEY_set(HE* he, SV* sv)
565
566These two lower level macros are defined, but must only be used when
567dealing with keys that are not C<SV*>s:
568
569 HeKEY(HE* he)
570 HeKLEN(HE* he)
571
04343c6d
GS
572Note that both C<hv_store> and C<hv_store_ent> do not increment the
573reference count of the stored C<val>, which is the caller's responsibility.
574If these functions return a NULL value, the caller will usually have to
575decrement the reference count of C<val> to avoid a memory leak.
1e422769 576
a9381218
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577=head2 AVs, HVs and undefined values
578
10e2eb10
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579Sometimes you have to store undefined values in AVs or HVs. Although
580this may be a rare case, it can be tricky. That's because you're
a9381218
MHM
581used to using C<&PL_sv_undef> if you need an undefined SV.
582
583For example, intuition tells you that this XS code:
584
585 AV *av = newAV();
586 av_store( av, 0, &PL_sv_undef );
587
588is equivalent to this Perl code:
589
590 my @av;
591 $av[0] = undef;
592
f3c4ec28 593Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
a9381218
MHM
594for indicating that an array element has not yet been initialized.
595Thus, C<exists $av[0]> would be true for the above Perl code, but
f3c4ec28
FC
596false for the array generated by the XS code. In perl 5.20, storing
597&PL_sv_undef will create a read-only element, because the scalar
598&PL_sv_undef itself is stored, not a copy.
a9381218 599
f3c4ec28 600Similar problems can occur when storing C<&PL_sv_undef> in HVs:
a9381218
MHM
601
602 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
603
604This will indeed make the value C<undef>, but if you try to modify
605the value of C<key>, you'll get the following error:
606
607 Modification of non-creatable hash value attempted
608
609In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
10e2eb10 610in restricted hashes. This caused such hash entries not to appear
a9381218
MHM
611when iterating over the hash or when checking for the keys
612with the C<hv_exists> function.
613
8abccac8 614You can run into similar problems when you store C<&PL_sv_yes> or
10e2eb10 615C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
a9381218
MHM
616will give you the following error:
617
618 Modification of a read-only value attempted
619
620To make a long story short, you can use the special variables
8abccac8 621C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
a9381218
MHM
622HVs, but you have to make sure you know what you're doing.
623
624Generally, if you want to store an undefined value in an AV
625or HV, you should not use C<&PL_sv_undef>, but rather create a
626new undefined value using the C<newSV> function, for example:
627
628 av_store( av, 42, newSV(0) );
629 hv_store( hv, "foo", 3, newSV(0), 0 );
630
a0d0e21e
LW
631=head2 References
632
d1b91892 633References are a special type of scalar that point to other data types
a9b0660e 634(including other references).
a0d0e21e 635
07fa94a1 636To create a reference, use either of the following functions:
a0d0e21e 637
5f05dabc
PP
638 SV* newRV_inc((SV*) thing);
639 SV* newRV_noinc((SV*) thing);
a0d0e21e 640
5f05dabc 641The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
642functions are identical except that C<newRV_inc> increments the reference
643count of the C<thing>, while C<newRV_noinc> does not. For historical
644reasons, C<newRV> is a synonym for C<newRV_inc>.
645
646Once you have a reference, you can use the following macro to dereference
647the reference:
a0d0e21e
LW
648
649 SvRV(SV*)
650
651then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 652C<AV*> or C<HV*>, if required.
a0d0e21e 653
d1b91892 654To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
655
656 SvROK(SV*)
657
07fa94a1
JO
658To discover what type of value the reference refers to, use the following
659macro and then check the return value.
d1b91892
AD
660
661 SvTYPE(SvRV(SV*))
662
663The most useful types that will be returned are:
664
a5e62da0
FC
665 < SVt_PVAV Scalar
666 SVt_PVAV Array
667 SVt_PVHV Hash
668 SVt_PVCV Code
669 SVt_PVGV Glob (possibly a file handle)
670
671See L<perlapi/svtype> for more details.
d1b91892 672
cb1a09d0
AD
673=head2 Blessed References and Class Objects
674
06f6df17 675References are also used to support object-oriented programming. In perl's
cb1a09d0
AD
676OO lexicon, an object is simply a reference that has been blessed into a
677package (or class). Once blessed, the programmer may now use the reference
678to access the various methods in the class.
679
680A reference can be blessed into a package with the following function:
681
682 SV* sv_bless(SV* sv, HV* stash);
683
06f6df17
RGS
684The C<sv> argument must be a reference value. The C<stash> argument
685specifies which class the reference will belong to. See
2ae324a7 686L<Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
AD
687
688/* Still under construction */
689
ddd2cc91
DM
690The following function upgrades rv to reference if not already one.
691Creates a new SV for rv to point to. If C<classname> is non-null, the SV
692is blessed into the specified class. SV is returned.
cb1a09d0 693
08105a92 694 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 695
ddd2cc91
DM
696The following three functions copy integer, unsigned integer or double
697into an SV whose reference is C<rv>. SV is blessed if C<classname> is
698non-null.
cb1a09d0 699
08105a92 700 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 701 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 702 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 703
ddd2cc91
DM
704The following function copies the pointer value (I<the address, not the
705string!>) into an SV whose reference is rv. SV is blessed if C<classname>
706is non-null.
cb1a09d0 707
ddd2cc91 708 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
cb1a09d0 709
a9b0660e 710The following function copies a string into an SV whose reference is C<rv>.
ddd2cc91
DM
711Set length to 0 to let Perl calculate the string length. SV is blessed if
712C<classname> is non-null.
cb1a09d0 713
a9b0660e
KW
714 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
715 STRLEN length);
cb1a09d0 716
ddd2cc91
DM
717The following function tests whether the SV is blessed into the specified
718class. It does not check inheritance relationships.
9abd00ed 719
08105a92 720 int sv_isa(SV* sv, const char* name);
9abd00ed 721
ddd2cc91 722The following function tests whether the SV is a reference to a blessed object.
9abd00ed
GS
723
724 int sv_isobject(SV* sv);
725
ddd2cc91 726The following function tests whether the SV is derived from the specified
10e2eb10
FC
727class. SV can be either a reference to a blessed object or a string
728containing a class name. This is the function implementing the
ddd2cc91 729C<UNIVERSAL::isa> functionality.
9abd00ed 730
08105a92 731 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 732
00aadd71 733To check if you've got an object derived from a specific class you have
9abd00ed
GS
734to write:
735
736 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 737
5f05dabc 738=head2 Creating New Variables
cb1a09d0 739
5f05dabc
PP
740To create a new Perl variable with an undef value which can be accessed from
741your Perl script, use the following routines, depending on the variable type.
cb1a09d0 742
64ace3f8 743 SV* get_sv("package::varname", GV_ADD);
cbfd0a87 744 AV* get_av("package::varname", GV_ADD);
6673a63c 745 HV* get_hv("package::varname", GV_ADD);
cb1a09d0 746
058a5f6c 747Notice the use of GV_ADD as the second parameter. The new variable can now
cb1a09d0
AD
748be set, using the routines appropriate to the data type.
749
5f05dabc 750There are additional macros whose values may be bitwise OR'ed with the
058a5f6c 751C<GV_ADD> argument to enable certain extra features. Those bits are:
cb1a09d0 752
9a68f1db
SB
753=over
754
755=item GV_ADDMULTI
756
757Marks the variable as multiply defined, thus preventing the:
758
759 Name <varname> used only once: possible typo
760
761warning.
762
9a68f1db
SB
763=item GV_ADDWARN
764
765Issues the warning:
766
767 Had to create <varname> unexpectedly
768
769if the variable did not exist before the function was called.
770
771=back
cb1a09d0 772
07fa94a1
JO
773If you do not specify a package name, the variable is created in the current
774package.
cb1a09d0 775
5f05dabc 776=head2 Reference Counts and Mortality
a0d0e21e 777
10e2eb10 778Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 779AVs, or HVs (xV for short in the following) start their life with a
55497cff 780reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 781then it will be destroyed and its memory made available for reuse.
55497cff
PP
782
783This normally doesn't happen at the Perl level unless a variable is
5f05dabc
PP
784undef'ed or the last variable holding a reference to it is changed or
785overwritten. At the internal level, however, reference counts can be
55497cff
PP
786manipulated with the following macros:
787
788 int SvREFCNT(SV* sv);
5f05dabc 789 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
790 void SvREFCNT_dec(SV* sv);
791
792However, there is one other function which manipulates the reference
07fa94a1
JO
793count of its argument. The C<newRV_inc> function, you will recall,
794creates a reference to the specified argument. As a side effect,
795it increments the argument's reference count. If this is not what
796you want, use C<newRV_noinc> instead.
797
798For example, imagine you want to return a reference from an XSUB function.
799Inside the XSUB routine, you create an SV which initially has a reference
800count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
PP
801This returns the reference as a new SV, but the reference count of the
802SV you passed to C<newRV_inc> has been incremented to two. Now you
07fa94a1
JO
803return the reference from the XSUB routine and forget about the SV.
804But Perl hasn't! Whenever the returned reference is destroyed, the
805reference count of the original SV is decreased to one and nothing happens.
806The SV will hang around without any way to access it until Perl itself
807terminates. This is a memory leak.
5f05dabc
PP
808
809The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
810C<newRV_inc>. Then, if and when the last reference is destroyed,
811the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 812stopping any memory leak.
55497cff 813
5f05dabc 814There are some convenience functions available that can help with the
54310121 815destruction of xVs. These functions introduce the concept of "mortality".
07fa94a1
JO
816An xV that is mortal has had its reference count marked to be decremented,
817but not actually decremented, until "a short time later". Generally the
818term "short time later" means a single Perl statement, such as a call to
54310121 819an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
JO
820reference count decremented depends on two macros, SAVETMPS and FREETMPS.
821See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
PP
822
823"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
824However, if you mortalize a variable twice, the reference count will
825later be decremented twice.
826
00aadd71
NIS
827"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
828For example an SV which is created just to pass a number to a called sub
06f6df17 829is made mortal to have it cleaned up automatically when it's popped off
10e2eb10 830the stack. Similarly, results returned by XSUBs (which are pushed on the
06f6df17 831stack) are often made mortal.
a0d0e21e
LW
832
833To create a mortal variable, use the functions:
834
835 SV* sv_newmortal()
836 SV* sv_2mortal(SV*)
837 SV* sv_mortalcopy(SV*)
838
00aadd71 839The first call creates a mortal SV (with no value), the second converts an existing
5f05dabc
PP
840SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
841third creates a mortal copy of an existing SV.
da8c5729 842Because C<sv_newmortal> gives the new SV no value, it must normally be given one
9a68f1db 843via C<sv_setpv>, C<sv_setiv>, etc. :
00aadd71
NIS
844
845 SV *tmp = sv_newmortal();
846 sv_setiv(tmp, an_integer);
847
848As that is multiple C statements it is quite common so see this idiom instead:
849
850 SV *tmp = sv_2mortal(newSViv(an_integer));
851
852
853You should be careful about creating mortal variables. Strange things
854can happen if you make the same value mortal within multiple contexts,
10e2eb10
FC
855or if you make a variable mortal multiple
856times. Thinking of "Mortalization"
00aadd71 857as deferred C<SvREFCNT_dec> should help to minimize such problems.
da8c5729 858For example if you are passing an SV which you I<know> has a high enough REFCNT
00aadd71
NIS
859to survive its use on the stack you need not do any mortalization.
860If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
861making a C<sv_mortalcopy> is safer.
a0d0e21e 862
ac036724 863The mortal routines are not just for SVs; AVs and HVs can be
faed5253 864made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 865C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 866
5f05dabc 867=head2 Stashes and Globs
a0d0e21e 868
06f6df17
RGS
869A B<stash> is a hash that contains all variables that are defined
870within a package. Each key of the stash is a symbol
aa689395
PP
871name (shared by all the different types of objects that have the same
872name), and each value in the hash table is a GV (Glob Value). This GV
873in turn contains references to the various objects of that name,
874including (but not limited to) the following:
cb1a09d0 875
a0d0e21e
LW
876 Scalar Value
877 Array Value
878 Hash Value
a3cb178b 879 I/O Handle
a0d0e21e
LW
880 Format
881 Subroutine
882
06f6df17
RGS
883There is a single stash called C<PL_defstash> that holds the items that exist
884in the C<main> package. To get at the items in other packages, append the
885string "::" to the package name. The items in the C<Foo> package are in
886the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
887in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 888
d1b91892 889To get the stash pointer for a particular package, use the function:
a0d0e21e 890
da51bb9b
NC
891 HV* gv_stashpv(const char* name, I32 flags)
892 HV* gv_stashsv(SV*, I32 flags)
a0d0e21e
LW
893
894The first function takes a literal string, the second uses the string stored
d1b91892 895in the SV. Remember that a stash is just a hash table, so you get back an
da51bb9b 896C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
a0d0e21e
LW
897
898The name that C<gv_stash*v> wants is the name of the package whose symbol table
899you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
900packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
901language itself.
a0d0e21e
LW
902
903Alternately, if you have an SV that is a blessed reference, you can find
904out the stash pointer by using:
905
906 HV* SvSTASH(SvRV(SV*));
907
908then use the following to get the package name itself:
909
910 char* HvNAME(HV* stash);
911
5f05dabc
PP
912If you need to bless or re-bless an object you can use the following
913function:
a0d0e21e
LW
914
915 SV* sv_bless(SV*, HV* stash)
916
917where the first argument, an C<SV*>, must be a reference, and the second
918argument is a stash. The returned C<SV*> can now be used in the same way
919as any other SV.
920
d1b91892
AD
921For more information on references and blessings, consult L<perlref>.
922
54310121 923=head2 Double-Typed SVs
0a753a76
PP
924
925Scalar variables normally contain only one type of value, an integer,
926double, pointer, or reference. Perl will automatically convert the
927actual scalar data from the stored type into the requested type.
928
929Some scalar variables contain more than one type of scalar data. For
930example, the variable C<$!> contains either the numeric value of C<errno>
931or its string equivalent from either C<strerror> or C<sys_errlist[]>.
932
933To force multiple data values into an SV, you must do two things: use the
934C<sv_set*v> routines to add the additional scalar type, then set a flag
935so that Perl will believe it contains more than one type of data. The
936four macros to set the flags are:
937
938 SvIOK_on
939 SvNOK_on
940 SvPOK_on
941 SvROK_on
942
943The particular macro you must use depends on which C<sv_set*v> routine
944you called first. This is because every C<sv_set*v> routine turns on
945only the bit for the particular type of data being set, and turns off
946all the rest.
947
948For example, to create a new Perl variable called "dberror" that contains
949both the numeric and descriptive string error values, you could use the
950following code:
951
952 extern int dberror;
953 extern char *dberror_list;
954
64ace3f8 955 SV* sv = get_sv("dberror", GV_ADD);
0a753a76
PP
956 sv_setiv(sv, (IV) dberror);
957 sv_setpv(sv, dberror_list[dberror]);
958 SvIOK_on(sv);
959
960If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
961macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
962
4f4531b8
FC
963=head2 Read-Only Values
964
965In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
966flag bit with read-only scalars. So the only way to test whether
967C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
968in those versions is:
969
970 SvREADONLY(sv) && !SvIsCOW(sv)
971
972Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
973and, under 5.20, copy-on-write scalars can also be read-only, so the above
974check is incorrect. You just want:
975
976 SvREADONLY(sv)
977
978If you need to do this check often, define your own macro like this:
979
980 #if PERL_VERSION >= 18
981 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
982 #else
983 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
984 #endif
985
986=head2 Copy on Write
987
988Perl implements a copy-on-write (COW) mechanism for scalars, in which
989string copies are not immediately made when requested, but are deferred
990until made necessary by one or the other scalar changing. This is mostly
991transparent, but one must take care not to modify string buffers that are
992shared by multiple SVs.
993
994You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
995
996You can force an SV to make its own copy of its string buffer by calling C<sv_force_normal(sv)> or SvPV_force_nolen(sv).
997
998If you want to make the SV drop its string buffer, use
999C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
1000C<sv_setsv(sv, NULL)>.
1001
1002All of these functions will croak on read-only scalars (see the previous
1003section for more on those).
1004
1005To test that your code is behaving correctly and not modifying COW buffers,
1006on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
1007C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
1008into crashes. You will find it to be marvellously slow, so you may want to
1009skip perl's own tests.
1010
0a753a76 1011=head2 Magic Variables
a0d0e21e 1012
d1b91892
AD
1013[This section still under construction. Ignore everything here. Post no
1014bills. Everything not permitted is forbidden.]
1015
d1b91892
AD
1016Any SV may be magical, that is, it has special features that a normal
1017SV does not have. These features are stored in the SV structure in a
5f05dabc 1018linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
1019
1020 struct magic {
1021 MAGIC* mg_moremagic;
1022 MGVTBL* mg_virtual;
1023 U16 mg_private;
1024 char mg_type;
1025 U8 mg_flags;
b205eb13 1026 I32 mg_len;
d1b91892
AD
1027 SV* mg_obj;
1028 char* mg_ptr;
d1b91892
AD
1029 };
1030
1031Note this is current as of patchlevel 0, and could change at any time.
1032
1033=head2 Assigning Magic
1034
1035Perl adds magic to an SV using the sv_magic function:
1036
a9b0660e 1037 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
1038
1039The C<sv> argument is a pointer to the SV that is to acquire a new magical
1040feature.
1041
1042If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
10e2eb10
FC
1043convert C<sv> to type C<SVt_PVMG>.
1044Perl then continues by adding new magic
645c22ef
DM
1045to the beginning of the linked list of magical features. Any prior entry
1046of the same type of magic is deleted. Note that this can be overridden,
1047and multiple instances of the same type of magic can be associated with an
1048SV.
d1b91892 1049
54310121 1050The C<name> and C<namlen> arguments are used to associate a string with
10e2eb10 1051the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
1052C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
1053C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
1054whether C<namlen> is greater than zero or equal to zero respectively. As a
1055special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
1056to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
1057
1058The sv_magic function uses C<how> to determine which, if any, predefined
1059"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
06f6df17 1060See the L<Magic Virtual Tables> section below. The C<how> argument is also
10e2eb10
FC
1061stored in the C<mg_type> field. The value of
1062C<how> should be chosen from the set of macros
1063C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 1064these macros were added, Perl internals used to directly use character
14befaf4 1065literals, so you may occasionally come across old code or documentation
75d0f26d 1066referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
1067
1068The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
1069structure. If it is not the same as the C<sv> argument, the reference
1070count of the C<obj> object is incremented. If it is the same, or if
645c22ef 1071the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 1072then C<obj> is merely stored, without the reference count being incremented.
d1b91892 1073
2d8d5d5a
SH
1074See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
1075to an SV.
1076
cb1a09d0
AD
1077There is also a function to add magic to an C<HV>:
1078
1079 void hv_magic(HV *hv, GV *gv, int how);
1080
1081This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
1082
1083To remove the magic from an SV, call the function sv_unmagic:
1084
70a53b35 1085 int sv_unmagic(SV *sv, int type);
cb1a09d0
AD
1086
1087The C<type> argument should be equal to the C<how> value when the C<SV>
1088was initially made magical.
1089
f6ee7b17 1090However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
10e2eb10
FC
1091C<SV>. If you want to remove only certain
1092magic of a C<type> based on the magic
f6ee7b17
FR
1093virtual table, use C<sv_unmagicext> instead:
1094
1095 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1096
d1b91892
AD
1097=head2 Magic Virtual Tables
1098
d1be9408 1099The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
1100C<MGVTBL>, which is a structure of function pointers and stands for
1101"Magic Virtual Table" to handle the various operations that might be
1102applied to that variable.
1103
301cb7e8
DM
1104The C<MGVTBL> has five (or sometimes eight) pointers to the following
1105routine types:
d1b91892
AD
1106
1107 int (*svt_get)(SV* sv, MAGIC* mg);
1108 int (*svt_set)(SV* sv, MAGIC* mg);
1109 U32 (*svt_len)(SV* sv, MAGIC* mg);
1110 int (*svt_clear)(SV* sv, MAGIC* mg);
1111 int (*svt_free)(SV* sv, MAGIC* mg);
1112
a9b0660e
KW
1113 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
1114 const char *name, I32 namlen);
301cb7e8
DM
1115 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
1116 int (*svt_local)(SV *nsv, MAGIC *mg);
1117
1118
06f6df17 1119This MGVTBL structure is set at compile-time in F<perl.h> and there are
b7a0f54c
SM
1120currently 32 types. These different structures contain pointers to various
1121routines that perform additional actions depending on which function is
1122being called.
d1b91892 1123
a9b0660e
KW
1124 Function pointer Action taken
1125 ---------------- ------------
1126 svt_get Do something before the value of the SV is
1127 retrieved.
1128 svt_set Do something after the SV is assigned a value.
1129 svt_len Report on the SV's length.
1130 svt_clear Clear something the SV represents.
1131 svt_free Free any extra storage associated with the SV.
d1b91892 1132
a9b0660e
KW
1133 svt_copy copy tied variable magic to a tied element
1134 svt_dup duplicate a magic structure during thread cloning
1135 svt_local copy magic to local value during 'local'
301cb7e8 1136
d1b91892 1137For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 1138to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
1139
1140 { magic_get, magic_set, magic_len, 0, 0 }
1141
14befaf4
DM
1142Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1143if a get operation is being performed, the routine C<magic_get> is
1144called. All the various routines for the various magical types begin
1145with C<magic_>. NOTE: the magic routines are not considered part of
1146the Perl API, and may not be exported by the Perl library.
d1b91892 1147
301cb7e8
DM
1148The last three slots are a recent addition, and for source code
1149compatibility they are only checked for if one of the three flags
10e2eb10
FC
1150MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
1151This means that most code can continue declaring
1152a vtable as a 5-element value. These three are
301cb7e8
DM
1153currently used exclusively by the threading code, and are highly subject
1154to change.
1155
d1b91892
AD
1156The current kinds of Magic Virtual Tables are:
1157
f1f5ddd7
FC
1158=for comment
1159This table is generated by regen/mg_vtable.pl. Any changes made here
1160will be lost.
1161
1162=for mg_vtable.pl begin
1163
a9b0660e 1164 mg_type
bd6e6c12
FC
1165 (old-style char and macro) MGVTBL Type of magic
1166 -------------------------- ------ -------------
1167 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1168 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
e5e1ee61 1169 % PERL_MAGIC_rhash (none) Extra data for restricted
bd6e6c12 1170 hashes
2a388207 1171 & PERL_MAGIC_proto (none) my sub prototype CV
a6d69523
TC
1172 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1173 vars
bd6e6c12 1174 . PERL_MAGIC_pos vtbl_pos pos() lvalue
e5e1ee61 1175 : PERL_MAGIC_symtab (none) Extra data for symbol
bd6e6c12 1176 tables
e5e1ee61
FC
1177 < PERL_MAGIC_backref vtbl_backref For weak ref data
1178 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
bd6e6c12
FC
1179 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1180 (fast string search)
1181 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1182 (AMT) on stash
1183 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1184 (@+ and @- vars)
1185 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1186 element
1187 E PERL_MAGIC_env vtbl_env %ENV hash
1188 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
eccba044 1189 f PERL_MAGIC_fm vtbl_regexp Formline
bd6e6c12 1190 ('compiled' format)
bd6e6c12
FC
1191 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1192 H PERL_MAGIC_hints vtbl_hints %^H hash
1193 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1194 I PERL_MAGIC_isa vtbl_isa @ISA array
1195 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1196 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1197 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1198 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1199 element
1200 N PERL_MAGIC_shared (none) Shared between threads
1201 n PERL_MAGIC_shared_scalar (none) Shared between threads
1202 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1203 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1204 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1205 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
e5e1ee61 1206 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
bd6e6c12
FC
1207 S PERL_MAGIC_sig (none) %SIG hash
1208 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1209 t PERL_MAGIC_taint vtbl_taint Taintedness
1210 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1211 extensions
1212 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1213 extensions
4499db73 1214 V PERL_MAGIC_vstring (none) SV was vstring literal
bd6e6c12
FC
1215 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1216 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1217 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1218 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1219 variable / smart parameter
1220 vivification
9cce4f9a
FC
1221 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference in list
1222 assignment
e5e1ee61 1223 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
bd6e6c12
FC
1224 to this CV
1225 ~ PERL_MAGIC_ext (none) Available for use by
1226 extensions
0cbee0a4 1227
f1f5ddd7 1228=for mg_vtable.pl end
d1b91892 1229
68dc0745 1230When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1231uppercase letter is typically used to represent some kind of composite type
1232(a list or a hash), and the lowercase letter is used to represent an element
10e2eb10 1233of that composite type. Some internals code makes use of this case
92f0c265 1234relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1235
1236The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1237specifically for use by extensions and will not be used by perl itself.
1238Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1239to variables (typically objects). This is especially useful because
1240there is no way for normal perl code to corrupt this private information
1241(unlike using extra elements of a hash object).
1242
1243Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1244C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1245C<mg_ptr> field points to a C<ufuncs> structure:
1246
1247 struct ufuncs {
a9402793
AB
1248 I32 (*uf_val)(pTHX_ IV, SV*);
1249 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1250 IV uf_index;
1251 };
1252
1253When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1254function will be called with C<uf_index> as the first arg and a pointer to
1255the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1256magic is shown below. Note that the ufuncs structure is copied by
1257sv_magic, so you can safely allocate it on the stack.
1258
1259 void
1260 Umagic(sv)
1261 SV *sv;
1262 PREINIT:
1263 struct ufuncs uf;
1264 CODE:
1265 uf.uf_val = &my_get_fn;
1266 uf.uf_set = &my_set_fn;
1267 uf.uf_index = 0;
14befaf4 1268 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1269
1e73acc8
AS
1270Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1271
1272For hashes there is a specialized hook that gives control over hash
1273keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1274if the "set" function in the C<ufuncs> structure is NULL. The hook
1275is activated whenever the hash is accessed with a key specified as
1276an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1277C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1278through the functions without the C<..._ent> suffix circumvents the
4509d391 1279hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1e73acc8 1280
14befaf4
DM
1281Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1282or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1283extra care to avoid conflict. Typically only using the magic on
1284objects blessed into the same class as the extension is sufficient.
2f07f21a
FR
1285For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1286C<MGVTBL>, even if all its fields will be C<0>, so that individual
1287C<MAGIC> pointers can be identified as a particular kind of magic
10e2eb10 1288using their magic virtual table. C<mg_findext> provides an easy way
f6ee7b17 1289to do that:
2f07f21a
FR
1290
1291 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1292
1293 MAGIC *mg;
f6ee7b17
FR
1294 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1295 /* this is really ours, not another module's PERL_MAGIC_ext */
1296 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1297 ...
2f07f21a 1298 }
5f05dabc 1299
ef50df4b
GS
1300Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1301earlier do B<not> invoke 'set' magic on their targets. This must
1302be done by the user either by calling the C<SvSETMAGIC()> macro after
1303calling these functions, or by using one of the C<sv_set*_mg()> or
1304C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1305C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1306obtained from external sources in functions that don't handle magic.
4a4eefd0 1307See L<perlapi> for a description of these functions.
189b2af5
GS
1308For example, calls to the C<sv_cat*()> functions typically need to be
1309followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1310since their implementation handles 'get' magic.
1311
d1b91892
AD
1312=head2 Finding Magic
1313
a9b0660e
KW
1314 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1315 * type */
f6ee7b17
FR
1316
1317This routine returns a pointer to a C<MAGIC> structure stored in the SV.
10e2eb10
FC
1318If the SV does not have that magical
1319feature, C<NULL> is returned. If the
f6ee7b17 1320SV has multiple instances of that magical feature, the first one will be
10e2eb10
FC
1321returned. C<mg_findext> can be used
1322to find a C<MAGIC> structure of an SV
da8c5729 1323based on both its magic type and its magic virtual table:
f6ee7b17
FR
1324
1325 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
d1b91892 1326
f6ee7b17
FR
1327Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1328SVt_PVMG, Perl may core dump.
d1b91892 1329
08105a92 1330 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1331
1332This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1333field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1334the mg_type field is changed to be the lowercase letter.
a0d0e21e 1335
04343c6d
GS
1336=head2 Understanding the Magic of Tied Hashes and Arrays
1337
14befaf4
DM
1338Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1339magic type.
9edb2b46
GS
1340
1341WARNING: As of the 5.004 release, proper usage of the array and hash
1342access functions requires understanding a few caveats. Some
1343of these caveats are actually considered bugs in the API, to be fixed
10e2eb10 1344in later releases, and are bracketed with [MAYCHANGE] below. If
9edb2b46
GS
1345you find yourself actually applying such information in this section, be
1346aware that the behavior may change in the future, umm, without warning.
04343c6d 1347
1526ead6 1348The perl tie function associates a variable with an object that implements
9a68f1db 1349the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1350tie function from an XSUB, you must mimic this behaviour. The code below
1351carries out the necessary steps - firstly it creates a new hash, and then
1352creates a second hash which it blesses into the class which will implement
10e2eb10 1353the tie methods. Lastly it ties the two hashes together, and returns a
1526ead6
AB
1354reference to the new tied hash. Note that the code below does NOT call the
1355TIEHASH method in the MyTie class -
1356see L<Calling Perl Routines from within C Programs> for details on how
1357to do this.
1358
1359 SV*
1360 mytie()
1361 PREINIT:
1362 HV *hash;
1363 HV *stash;
1364 SV *tie;
1365 CODE:
1366 hash = newHV();
1367 tie = newRV_noinc((SV*)newHV());
da51bb9b 1368 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1369 sv_bless(tie, stash);
899e16d0 1370 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1371 RETVAL = newRV_noinc(hash);
1372 OUTPUT:
1373 RETVAL
1374
04343c6d
GS
1375The C<av_store> function, when given a tied array argument, merely
1376copies the magic of the array onto the value to be "stored", using
1377C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1378actually need to be stored in the array. [MAYCHANGE] After a call to
1379C<av_store> on a tied array, the caller will usually need to call
1380C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1381TIEARRAY object. If C<av_store> did return NULL, a call to
1382C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1383leak. [/MAYCHANGE]
04343c6d
GS
1384
1385The previous paragraph is applicable verbatim to tied hash access using the
1386C<hv_store> and C<hv_store_ent> functions as well.
1387
1388C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1389C<hv_fetch_ent> actually return an undefined mortal value whose magic
1390has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1391need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1392need to call C<mg_get()> on the returned value in order to actually invoke
1393the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1394you may also call C<mg_set()> on the return value after possibly assigning
1395a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1396method on the TIE object. [/MAYCHANGE]
04343c6d 1397
9edb2b46 1398[MAYCHANGE]
04343c6d
GS
1399In other words, the array or hash fetch/store functions don't really
1400fetch and store actual values in the case of tied arrays and hashes. They
1401merely call C<mg_copy> to attach magic to the values that were meant to be
1402"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1403do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1404the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1405and hashes.
1406
1407Currently (as of perl version 5.004), use of the hash and array access
1408functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1409"normal" hashes and arrays, or on their tied variants. The API may be
1410changed to provide more transparent access to both tied and normal data
1411types in future versions.
1412[/MAYCHANGE]
04343c6d
GS
1413
1414You would do well to understand that the TIEARRAY and TIEHASH interfaces
1415are mere sugar to invoke some perl method calls while using the uniform hash
1416and array syntax. The use of this sugar imposes some overhead (typically
1417about two to four extra opcodes per FETCH/STORE operation, in addition to
1418the creation of all the mortal variables required to invoke the methods).
1419This overhead will be comparatively small if the TIE methods are themselves
1420substantial, but if they are only a few statements long, the overhead
1421will not be insignificant.
1422
d1c897a1
IZ
1423=head2 Localizing changes
1424
1425Perl has a very handy construction
1426
1427 {
1428 local $var = 2;
1429 ...
1430 }
1431
1432This construction is I<approximately> equivalent to
1433
1434 {
1435 my $oldvar = $var;
1436 $var = 2;
1437 ...
1438 $var = $oldvar;
1439 }
1440
1441The biggest difference is that the first construction would
1442reinstate the initial value of $var, irrespective of how control exits
10e2eb10 1443the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1444more efficient as well.
1445
1446There is a way to achieve a similar task from C via Perl API: create a
1447I<pseudo-block>, and arrange for some changes to be automatically
1448undone at the end of it, either explicit, or via a non-local exit (via
10e2eb10 1449die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1450C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1451Such a construct may be created specially for some important localized
1452task, or an existing one (like boundaries of enclosing Perl
1453subroutine/block, or an existing pair for freeing TMPs) may be
10e2eb10
FC
1454used. (In the second case the overhead of additional localization must
1455be almost negligible.) Note that any XSUB is automatically enclosed in
b687b08b 1456an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1457
1458Inside such a I<pseudo-block> the following service is available:
1459
13a2d996 1460=over 4
d1c897a1
IZ
1461
1462=item C<SAVEINT(int i)>
1463
1464=item C<SAVEIV(IV i)>
1465
1466=item C<SAVEI32(I32 i)>
1467
1468=item C<SAVELONG(long i)>
1469
1470These macros arrange things to restore the value of integer variable
1471C<i> at the end of enclosing I<pseudo-block>.
1472
1473=item C<SAVESPTR(s)>
1474
1475=item C<SAVEPPTR(p)>
1476
1477These macros arrange things to restore the value of pointers C<s> and
10e2eb10 1478C<p>. C<s> must be a pointer of a type which survives conversion to
d1c897a1
IZ
1479C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1480and back.
1481
1482=item C<SAVEFREESV(SV *sv)>
1483
1484The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1485I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1486mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1487extends the lifetime of C<sv> until the beginning of the next statement,
1488C<SAVEFREESV> extends it until the end of the enclosing scope. These
1489lifetimes can be wildly different.
1490
1491Also compare C<SAVEMORTALIZESV>.
1492
1493=item C<SAVEMORTALIZESV(SV *sv)>
1494
1495Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1496scope instead of decrementing its reference count. This usually has the
1497effect of keeping C<sv> alive until the statement that called the currently
1498live scope has finished executing.
d1c897a1
IZ
1499
1500=item C<SAVEFREEOP(OP *op)>
1501
1502The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1503
1504=item C<SAVEFREEPV(p)>
1505
1506The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1507end of I<pseudo-block>.
1508
1509=item C<SAVECLEARSV(SV *sv)>
1510
1511Clears a slot in the current scratchpad which corresponds to C<sv> at
1512the end of I<pseudo-block>.
1513
1514=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1515
10e2eb10 1516The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
d1c897a1
IZ
1517string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1518short-lived storage, the corresponding string may be reallocated like
1519this:
1520
9cde0e7f 1521 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1522
c76ac1ee 1523=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1524
1525At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1526only argument C<p>.
1527
1528=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1529
1530At the end of I<pseudo-block> the function C<f> is called with the
1531implicit context argument (if any), and C<p>.
d1c897a1
IZ
1532
1533=item C<SAVESTACK_POS()>
1534
1535The current offset on the Perl internal stack (cf. C<SP>) is restored
1536at the end of I<pseudo-block>.
1537
1538=back
1539
1540The following API list contains functions, thus one needs to
1541provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1542or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1543function takes C<int *>.
1544
13a2d996 1545=over 4
d1c897a1
IZ
1546
1547=item C<SV* save_scalar(GV *gv)>
1548
1549Equivalent to Perl code C<local $gv>.
1550
1551=item C<AV* save_ary(GV *gv)>
1552
1553=item C<HV* save_hash(GV *gv)>
1554
1555Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1556
1557=item C<void save_item(SV *item)>
1558
1559Duplicates the current value of C<SV>, on the exit from the current
1560C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
10e2eb10 1561using the stored value. It doesn't handle magic. Use C<save_scalar> if
038fcae3 1562magic is affected.
d1c897a1
IZ
1563
1564=item C<void save_list(SV **sarg, I32 maxsarg)>
1565
1566A variant of C<save_item> which takes multiple arguments via an array
1567C<sarg> of C<SV*> of length C<maxsarg>.
1568
1569=item C<SV* save_svref(SV **sptr)>
1570
d1be9408 1571Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1572
1573=item C<void save_aptr(AV **aptr)>
1574
1575=item C<void save_hptr(HV **hptr)>
1576
1577Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1578
1579=back
1580
1581The C<Alias> module implements localization of the basic types within the
1582I<caller's scope>. People who are interested in how to localize things in
1583the containing scope should take a look there too.
1584
0a753a76 1585=head1 Subroutines
a0d0e21e 1586
68dc0745 1587=head2 XSUBs and the Argument Stack
5f05dabc
PP
1588
1589The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1590An XSUB routine will have a stack that contains the arguments from the Perl
1591program, and a way to map from the Perl data structures to a C equivalent.
1592
1593The stack arguments are accessible through the C<ST(n)> macro, which returns
1594the C<n>'th stack argument. Argument 0 is the first argument passed in the
1595Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1596an C<SV*> is used.
1597
1598Most of the time, output from the C routine can be handled through use of
1599the RETVAL and OUTPUT directives. However, there are some cases where the
1600argument stack is not already long enough to handle all the return values.
1601An example is the POSIX tzname() call, which takes no arguments, but returns
1602two, the local time zone's standard and summer time abbreviations.
1603
1604To handle this situation, the PPCODE directive is used and the stack is
1605extended using the macro:
1606
924508f0 1607 EXTEND(SP, num);
5f05dabc 1608
924508f0
GS
1609where C<SP> is the macro that represents the local copy of the stack pointer,
1610and C<num> is the number of elements the stack should be extended by.
5f05dabc 1611
00aadd71 1612Now that there is room on the stack, values can be pushed on it using C<PUSHs>
10e2eb10 1613macro. The pushed values will often need to be "mortal" (See
d82b684c 1614L</Reference Counts and Mortality>):
5f05dabc 1615
00aadd71 1616 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1617 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1618 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1619 PUSHs(sv_2mortal(newSVpv("Some String",0)))
a9b0660e
KW
1620 /* Although the last example is better written as the more
1621 * efficient: */
a3179684 1622 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
5f05dabc
PP
1623
1624And now the Perl program calling C<tzname>, the two values will be assigned
1625as in:
1626
1627 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1628
1629An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1630to use the macro:
5f05dabc 1631
5f05dabc
PP
1632 XPUSHs(SV*)
1633
da8c5729 1634This macro automatically adjusts the stack for you, if needed. Thus, you
5f05dabc 1635do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1636
1637Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1638C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1639For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1640C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1641
1642For more information, consult L<perlxs> and L<perlxstut>.
1643
5b36e945
FC
1644=head2 Autoloading with XSUBs
1645
1646If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1647fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1648of the XSUB's package.
1649
1650But it also puts the same information in certain fields of the XSUB itself:
1651
1652 HV *stash = CvSTASH(cv);
1653 const char *subname = SvPVX(cv);
1654 STRLEN name_length = SvCUR(cv); /* in bytes */
1655 U32 is_utf8 = SvUTF8(cv);
f703fc96 1656
5b36e945 1657C<SvPVX(cv)> contains just the sub name itself, not including the package.
d8893903
FC
1658For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1659C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
5b36e945
FC
1660
1661B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1662XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1663XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1664to support 5.8-5.14, use the XSUB's fields.
1665
5f05dabc 1666=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1667
1668There are four routines that can be used to call a Perl subroutine from
1669within a C program. These four are:
1670
954c1994
GS
1671 I32 call_sv(SV*, I32);
1672 I32 call_pv(const char*, I32);
1673 I32 call_method(const char*, I32);
5aaab254 1674 I32 call_argv(const char*, I32, char**);
a0d0e21e 1675
954c1994 1676The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1677contains either the name of the Perl subroutine to be called, or a
1678reference to the subroutine. The second argument consists of flags
1679that control the context in which the subroutine is called, whether
1680or not the subroutine is being passed arguments, how errors should be
1681trapped, and how to treat return values.
a0d0e21e
LW
1682
1683All four routines return the number of arguments that the subroutine returned
1684on the Perl stack.
1685
9a68f1db 1686These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1687but those names are now deprecated; macros of the same name are provided for
1688compatibility.
1689
1690When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1691must manipulate the Perl stack. These include the following macros and
1692functions:
a0d0e21e
LW
1693
1694 dSP
924508f0 1695 SP
a0d0e21e
LW
1696 PUSHMARK()
1697 PUTBACK
1698 SPAGAIN
1699 ENTER
1700 SAVETMPS
1701 FREETMPS
1702 LEAVE
1703 XPUSH*()
cb1a09d0 1704 POP*()
a0d0e21e 1705
5f05dabc
PP
1706For a detailed description of calling conventions from C to Perl,
1707consult L<perlcall>.
a0d0e21e 1708
8ebc5c01 1709=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1710
1711A lot of opcodes (this is an elementary operation in the internal perl
10e2eb10
FC
1712stack machine) put an SV* on the stack. However, as an optimization
1713the corresponding SV is (usually) not recreated each time. The opcodes
ce3d39e2
IZ
1714reuse specially assigned SVs (I<target>s) which are (as a corollary)
1715not constantly freed/created.
1716
0a753a76 1717Each of the targets is created only once (but see
ce3d39e2
IZ
1718L<Scratchpads and recursion> below), and when an opcode needs to put
1719an integer, a double, or a string on stack, it just sets the
1720corresponding parts of its I<target> and puts the I<target> on stack.
1721
1722The macro to put this target on stack is C<PUSHTARG>, and it is
1723directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1724others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1725
1bd1c0d5 1726Because the target is reused, you must be careful when pushing multiple
10e2eb10 1727values on the stack. The following code will not do what you think:
1bd1c0d5
SC
1728
1729 XPUSHi(10);
1730 XPUSHi(20);
1731
1732This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1733the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1734At the end of the operation, the stack does not contain the values 10
1735and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1736to 20.
1bd1c0d5 1737
d82b684c
SH
1738If you need to push multiple different values then you should either use
1739the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1740none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1741SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1742will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1743this a little easier to achieve by creating a new mortal for you (via
1744C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1745in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1746Thus, instead of writing this to "fix" the example above:
1747
1748 XPUSHs(sv_2mortal(newSViv(10)))
1749 XPUSHs(sv_2mortal(newSViv(20)))
1750
1751you can simply write:
1752
1753 mXPUSHi(10)
1754 mXPUSHi(20)
1755
1756On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1757need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1758macros can make use of the local variable C<TARG>. See also C<dTARGET>
1759and C<dXSTARG>.
1bd1c0d5 1760
8ebc5c01 1761=head2 Scratchpads
ce3d39e2 1762
54310121 1763The question remains on when the SVs which are I<target>s for opcodes
10e2eb10 1764are created. The answer is that they are created when the current
ac036724 1765unit--a subroutine or a file (for opcodes for statements outside of
10e2eb10 1766subroutines)--is compiled. During this time a special anonymous Perl
ac036724 1767array is created, which is called a scratchpad for the current unit.
ce3d39e2 1768
54310121 1769A scratchpad keeps SVs which are lexicals for the current unit and are
d777b41a
FC
1770targets for opcodes. A previous version of this document
1771stated that one can deduce that an SV lives on a scratchpad
ce3d39e2 1772by looking on its flags: lexicals have C<SVs_PADMY> set, and
d777b41a
FC
1773I<target>s have C<SVs_PADTMP> set. But this have never been fully true.
1774C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1775While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
1776that have never resided in a pad, but nonetheless act like I<target>s.
ce3d39e2 1777
10e2eb10 1778The correspondence between OPs and I<target>s is not 1-to-1. Different
54310121 1779OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1780would not conflict with the expected life of the temporary.
1781
2ae324a7 1782=head2 Scratchpads and recursion
ce3d39e2
IZ
1783
1784In fact it is not 100% true that a compiled unit contains a pointer to
10e2eb10
FC
1785the scratchpad AV. In fact it contains a pointer to an AV of
1786(initially) one element, and this element is the scratchpad AV. Why do
ce3d39e2
IZ
1787we need an extra level of indirection?
1788
10e2eb10 1789The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2 1790these can create several execution pointers going into the same
10e2eb10 1791subroutine. For the subroutine-child not write over the temporaries
ce3d39e2
IZ
1792for the subroutine-parent (lifespan of which covers the call to the
1793child), the parent and the child should have different
10e2eb10 1794scratchpads. (I<And> the lexicals should be separate anyway!)
ce3d39e2 1795
5f05dabc
PP
1796So each subroutine is born with an array of scratchpads (of length 1).
1797On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1798depth of the recursion is not more than the length of this array, and
1799if it is, new scratchpad is created and pushed into the array.
1800
1801The I<target>s on this scratchpad are C<undef>s, but they are already
1802marked with correct flags.
1803
22d36020
FC
1804=head1 Memory Allocation
1805
1806=head2 Allocation
1807
1808All memory meant to be used with the Perl API functions should be manipulated
1809using the macros described in this section. The macros provide the necessary
1810transparency between differences in the actual malloc implementation that is
1811used within perl.
1812
1813It is suggested that you enable the version of malloc that is distributed
1814with Perl. It keeps pools of various sizes of unallocated memory in
1815order to satisfy allocation requests more quickly. However, on some
1816platforms, it may cause spurious malloc or free errors.
1817
1818The following three macros are used to initially allocate memory :
1819
1820 Newx(pointer, number, type);
1821 Newxc(pointer, number, type, cast);
1822 Newxz(pointer, number, type);
1823
1824The first argument C<pointer> should be the name of a variable that will
1825point to the newly allocated memory.
1826
1827The second and third arguments C<number> and C<type> specify how many of
1828the specified type of data structure should be allocated. The argument
1829C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1830should be used if the C<pointer> argument is different from the C<type>
1831argument.
1832
1833Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1834to zero out all the newly allocated memory.
1835
1836=head2 Reallocation
1837
1838 Renew(pointer, number, type);
1839 Renewc(pointer, number, type, cast);
1840 Safefree(pointer)
1841
1842These three macros are used to change a memory buffer size or to free a
1843piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1844match those of C<New> and C<Newc> with the exception of not needing the
1845"magic cookie" argument.
1846
1847=head2 Moving
1848
1849 Move(source, dest, number, type);
1850 Copy(source, dest, number, type);
1851 Zero(dest, number, type);
1852
1853These three macros are used to move, copy, or zero out previously allocated
1854memory. The C<source> and C<dest> arguments point to the source and
1855destination starting points. Perl will move, copy, or zero out C<number>
1856instances of the size of the C<type> data structure (using the C<sizeof>
1857function).
1858
1859=head1 PerlIO
1860
1861The most recent development releases of Perl have been experimenting with
1862removing Perl's dependency on the "normal" standard I/O suite and allowing
1863other stdio implementations to be used. This involves creating a new
1864abstraction layer that then calls whichever implementation of stdio Perl
1865was compiled with. All XSUBs should now use the functions in the PerlIO
1866abstraction layer and not make any assumptions about what kind of stdio
1867is being used.
1868
1869For a complete description of the PerlIO abstraction, consult L<perlapio>.
1870
0a753a76
PP
1871=head1 Compiled code
1872
1873=head2 Code tree
1874
1875Here we describe the internal form your code is converted to by
10e2eb10 1876Perl. Start with a simple example:
0a753a76
PP
1877
1878 $a = $b + $c;
1879
1880This is converted to a tree similar to this one:
1881
1882 assign-to
1883 / \
1884 + $a
1885 / \
1886 $b $c
1887
7b8d334a 1888(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1889parsed your code, but has nothing to do with the execution order.
1890There is an additional "thread" going through the nodes of the tree
1891which shows the order of execution of the nodes. In our simplified
1892example above it looks like:
1893
1894 $b ---> $c ---> + ---> $a ---> assign-to
1895
1896But with the actual compile tree for C<$a = $b + $c> it is different:
1897some nodes I<optimized away>. As a corollary, though the actual tree
1898contains more nodes than our simplified example, the execution order
1899is the same as in our example.
1900
1901=head2 Examining the tree
1902
06f6df17
RGS
1903If you have your perl compiled for debugging (usually done with
1904C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1905compiled tree by specifying C<-Dx> on the Perl command line. The
1906output takes several lines per node, and for C<$b+$c> it looks like
1907this:
1908
1909 5 TYPE = add ===> 6
1910 TARG = 1
1911 FLAGS = (SCALAR,KIDS)
1912 {
1913 TYPE = null ===> (4)
1914 (was rv2sv)
1915 FLAGS = (SCALAR,KIDS)
1916 {
1917 3 TYPE = gvsv ===> 4
1918 FLAGS = (SCALAR)
1919 GV = main::b
1920 }
1921 }
1922 {
1923 TYPE = null ===> (5)
1924 (was rv2sv)
1925 FLAGS = (SCALAR,KIDS)
1926 {
1927 4 TYPE = gvsv ===> 5
1928 FLAGS = (SCALAR)
1929 GV = main::c
1930 }
1931 }
1932
1933This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1934not optimized away (one per number in the left column). The immediate
1935children of the given node correspond to C<{}> pairs on the same level
1936of indentation, thus this listing corresponds to the tree:
1937
1938 add
1939 / \
1940 null null
1941 | |
1942 gvsv gvsv
1943
1944The execution order is indicated by C<===E<gt>> marks, thus it is C<3
19454 5 6> (node C<6> is not included into above listing), i.e.,
1946C<gvsv gvsv add whatever>.
1947
9afa14e3 1948Each of these nodes represents an op, a fundamental operation inside the
10e2eb10 1949Perl core. The code which implements each operation can be found in the
9afa14e3 1950F<pp*.c> files; the function which implements the op with type C<gvsv>
10e2eb10 1951is C<pp_gvsv>, and so on. As the tree above shows, different ops have
9afa14e3 1952different numbers of children: C<add> is a binary operator, as one would
10e2eb10 1953expect, and so has two children. To accommodate the various different
9afa14e3
SC
1954numbers of children, there are various types of op data structure, and
1955they link together in different ways.
1956
10e2eb10 1957The simplest type of op structure is C<OP>: this has no children. Unary
9afa14e3 1958operators, C<UNOP>s, have one child, and this is pointed to by the
10e2eb10
FC
1959C<op_first> field. Binary operators (C<BINOP>s) have not only an
1960C<op_first> field but also an C<op_last> field. The most complex type of
1961op is a C<LISTOP>, which has any number of children. In this case, the
9afa14e3 1962first child is pointed to by C<op_first> and the last child by
10e2eb10 1963C<op_last>. The children in between can be found by iteratively
29e61fd9
DM
1964following the C<op_sibling> pointer from the first child to the last 9but
1965see below).
9afa14e3 1966
29e61fd9 1967There are also some other op types: a C<PMOP> holds a regular expression,
10e2eb10
FC
1968and has no children, and a C<LOOP> may or may not have children. If the
1969C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
9afa14e3
SC
1970complicate matters, if a C<UNOP> is actually a C<null> op after
1971optimization (see L</Compile pass 2: context propagation>) it will still
1972have children in accordance with its former type.
1973
29e61fd9
DM
1974Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
1975or more children, but it doesn't have an C<op_last> field: so you have to
1976follow C<op_first> and then the C<op_sibling> chain itself to find the
1977last child. Instead it has an C<op_other> field, which is comparable to
1978the C<op_next> field described below, and represents an alternate
1979execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
1980that in general, C<op_other> may not point to any of the direct children
1981of the C<LOGOP>.
1982
1983Starting in version 5.21.2, perls built with the experimental
1984define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
1985C<op_lastsib>. When set, this indicates that this is the last op in an
1986C<op_sibling> chain. This frees up the C<op_sibling> field on the last
1987sibling to point back to the parent op. The macro C<OP_SIBLING(o)> wraps
1988this special behaviour, and always returns NULL on the last sibling.
1989With this build the C<op_parent(o)> function can be used to find the
1990parent of any op.
1991
06f6df17
RGS
1992Another way to examine the tree is to use a compiler back-end module, such
1993as L<B::Concise>.
1994
0a753a76
PP
1995=head2 Compile pass 1: check routines
1996
8870b5c7 1997The tree is created by the compiler while I<yacc> code feeds it
10e2eb10 1998the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1999the first pass of perl compilation.
2000
2001What makes this pass interesting for perl developers is that some
2002optimization may be performed on this pass. This is optimization by
8870b5c7 2003so-called "check routines". The correspondence between node names
0a753a76
PP
2004and corresponding check routines is described in F<opcode.pl> (do not
2005forget to run C<make regen_headers> if you modify this file).
2006
2007A check routine is called when the node is fully constructed except
7b8d334a 2008for the execution-order thread. Since at this time there are no
0a753a76
PP
2009back-links to the currently constructed node, one can do most any
2010operation to the top-level node, including freeing it and/or creating
2011new nodes above/below it.
2012
2013The check routine returns the node which should be inserted into the
2014tree (if the top-level node was not modified, check routine returns
2015its argument).
2016
10e2eb10 2017By convention, check routines have names C<ck_*>. They are usually
0a753a76
PP
2018called from C<new*OP> subroutines (or C<convert>) (which in turn are
2019called from F<perly.y>).
2020
2021=head2 Compile pass 1a: constant folding
2022
2023Immediately after the check routine is called the returned node is
2024checked for being compile-time executable. If it is (the value is
2025judged to be constant) it is immediately executed, and a I<constant>
2026node with the "return value" of the corresponding subtree is
2027substituted instead. The subtree is deleted.
2028
2029If constant folding was not performed, the execution-order thread is
2030created.
2031
2032=head2 Compile pass 2: context propagation
2033
2034When a context for a part of compile tree is known, it is propagated
a3cb178b 2035down through the tree. At this time the context can have 5 values
0a753a76
PP
2036(instead of 2 for runtime context): void, boolean, scalar, list, and
2037lvalue. In contrast with the pass 1 this pass is processed from top
2038to bottom: a node's context determines the context for its children.
2039
2040Additional context-dependent optimizations are performed at this time.
2041Since at this moment the compile tree contains back-references (via
2042"thread" pointers), nodes cannot be free()d now. To allow
2043optimized-away nodes at this stage, such nodes are null()ified instead
2044of free()ing (i.e. their type is changed to OP_NULL).
2045
2046=head2 Compile pass 3: peephole optimization
2047
2048After the compile tree for a subroutine (or for an C<eval> or a file)
10e2eb10 2049is created, an additional pass over the code is performed. This pass
0a753a76 2050is neither top-down or bottom-up, but in the execution order (with
9ea12537
Z
2051additional complications for conditionals). Optimizations performed
2052at this stage are subject to the same restrictions as in the pass 2.
2053
2054Peephole optimizations are done by calling the function pointed to
2055by the global variable C<PL_peepp>. By default, C<PL_peepp> just
2056calls the function pointed to by the global variable C<PL_rpeepp>.
2057By default, that performs some basic op fixups and optimisations along
2058the execution-order op chain, and recursively calls C<PL_rpeepp> for
2059each side chain of ops (resulting from conditionals). Extensions may
2060provide additional optimisations or fixups, hooking into either the
2061per-subroutine or recursive stage, like this:
2062
2063 static peep_t prev_peepp;
2064 static void my_peep(pTHX_ OP *o)
2065 {
2066 /* custom per-subroutine optimisation goes here */
f0358462 2067 prev_peepp(aTHX_ o);
9ea12537
Z
2068 /* custom per-subroutine optimisation may also go here */
2069 }
2070 BOOT:
2071 prev_peepp = PL_peepp;
2072 PL_peepp = my_peep;
2073
2074 static peep_t prev_rpeepp;
2075 static void my_rpeep(pTHX_ OP *o)
2076 {
2077 OP *orig_o = o;
2078 for(; o; o = o->op_next) {
2079 /* custom per-op optimisation goes here */
2080 }
f0358462 2081 prev_rpeepp(aTHX_ orig_o);
9ea12537
Z
2082 }
2083 BOOT:
2084 prev_rpeepp = PL_rpeepp;
2085 PL_rpeepp = my_rpeep;
0a753a76 2086
1ba7f851
PJ
2087=head2 Pluggable runops
2088
2089The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
2090functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2091with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2092control over the execution of the compile tree it is possible to provide
2093your own runops function.
1ba7f851
PJ
2094
2095It's probably best to copy one of the existing runops functions and
2096change it to suit your needs. Then, in the BOOT section of your XS
2097file, add the line:
2098
2099 PL_runops = my_runops;
2100
2101This function should be as efficient as possible to keep your programs
2102running as fast as possible.
2103
fd85fad2
BM
2104=head2 Compile-time scope hooks
2105
2106As of perl 5.14 it is possible to hook into the compile-time lexical
10e2eb10 2107scope mechanism using C<Perl_blockhook_register>. This is used like
fd85fad2
BM
2108this:
2109
2110 STATIC void my_start_hook(pTHX_ int full);
2111 STATIC BHK my_hooks;
2112
2113 BOOT:
a88d97bf 2114 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
fd85fad2
BM
2115 Perl_blockhook_register(aTHX_ &my_hooks);
2116
2117This will arrange to have C<my_start_hook> called at the start of
10e2eb10 2118compiling every lexical scope. The available hooks are:
fd85fad2
BM
2119
2120=over 4
2121
a88d97bf 2122=item C<void bhk_start(pTHX_ int full)>
fd85fad2 2123
10e2eb10 2124This is called just after starting a new lexical scope. Note that Perl
fd85fad2
BM
2125code like
2126
2127 if ($x) { ... }
2128
2129creates two scopes: the first starts at the C<(> and has C<full == 1>,
10e2eb10
FC
2130the second starts at the C<{> and has C<full == 0>. Both end at the
2131C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
fd85fad2
BM
2132pushed onto the save stack by this hook will be popped just before the
2133scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2134
a88d97bf 2135=item C<void bhk_pre_end(pTHX_ OP **o)>
fd85fad2
BM
2136
2137This is called at the end of a lexical scope, just before unwinding the
10e2eb10 2138stack. I<o> is the root of the optree representing the scope; it is a
fd85fad2
BM
2139double pointer so you can replace the OP if you need to.
2140
a88d97bf 2141=item C<void bhk_post_end(pTHX_ OP **o)>
fd85fad2
BM
2142
2143This is called at the end of a lexical scope, just after unwinding the
10e2eb10 2144stack. I<o> is as above. Note that it is possible for calls to C<pre_>
fd85fad2
BM
2145and C<post_end> to nest, if there is something on the save stack that
2146calls string eval.
2147
a88d97bf 2148=item C<void bhk_eval(pTHX_ OP *const o)>
fd85fad2
BM
2149
2150This is called just before starting to compile an C<eval STRING>, C<do
10e2eb10 2151FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
fd85fad2
BM
2152OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2153C<OP_DOFILE> or C<OP_REQUIRE>.
2154
2155=back
2156
2157Once you have your hook functions, you need a C<BHK> structure to put
10e2eb10
FC
2158them in. It's best to allocate it statically, since there is no way to
2159free it once it's registered. The function pointers should be inserted
fd85fad2 2160into this structure using the C<BhkENTRY_set> macro, which will also set
10e2eb10 2161flags indicating which entries are valid. If you do need to allocate
fd85fad2
BM
2162your C<BHK> dynamically for some reason, be sure to zero it before you
2163start.
2164
2165Once registered, there is no mechanism to switch these hooks off, so if
10e2eb10 2166that is necessary you will need to do this yourself. An entry in C<%^H>
a3e07c87
BM
2167is probably the best way, so the effect is lexically scoped; however it
2168is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
10e2eb10 2169temporarily switch entries on and off. You should also be aware that
a3e07c87
BM
2170generally speaking at least one scope will have opened before your
2171extension is loaded, so you will see some C<pre/post_end> pairs that
2172didn't have a matching C<start>.
fd85fad2 2173
9afa14e3
SC
2174=head1 Examining internal data structures with the C<dump> functions
2175
2176To aid debugging, the source file F<dump.c> contains a number of
2177functions which produce formatted output of internal data structures.
2178
2179The most commonly used of these functions is C<Perl_sv_dump>; it's used
10e2eb10 2180for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
9afa14e3 2181C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 2182module should already be familiar with its format.
9afa14e3
SC
2183
2184C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 2185derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
2186C<Perl_dump_eval> will dump the main root of the code being evaluated,
2187exactly like C<-Dx>.
2188
2189Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2190op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2191subroutines in a package like so: (Thankfully, these are all xsubs, so
2192there is no op tree)
2193
2194 (gdb) print Perl_dump_packsubs(PL_defstash)
2195
2196 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2197
2198 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2199
2200 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2201
2202 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2203
2204 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2205
2206and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2207the op tree of the main root.
2208
954c1994 2209=head1 How multiple interpreters and concurrency are supported
ee072b34 2210
ee072b34
GS
2211=head2 Background and PERL_IMPLICIT_CONTEXT
2212
2213The Perl interpreter can be regarded as a closed box: it has an API
2214for feeding it code or otherwise making it do things, but it also has
2215functions for its own use. This smells a lot like an object, and
2216there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
2217interpreters, with one interpreter represented either as a C structure,
2218or inside a thread-specific structure. These structures contain all
2219the context, the state of that interpreter.
2220
10e2eb10 2221One macro controls the major Perl build flavor: MULTIPLICITY. The
7b52221d 2222MULTIPLICITY build has a C structure that packages all the interpreter
10e2eb10 2223state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
7b52221d 2224normally defined, and enables the support for passing in a "hidden" first
10e2eb10 2225argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 2226multi-threaded perls possible (with the ithreads threading model, related
7b52221d 2227to the macro USE_ITHREADS.)
54aff467 2228
27da23d5
JH
2229Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2230PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2231former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2232internal variables of Perl to be wrapped inside a single global struct,
2233struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2234the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2235one step further, there is still a single struct (allocated in main()
2236either from heap or from stack) but there are no global data symbols
3bf17896 2237pointing to it. In either case the global struct should be initialized
27da23d5
JH
2238as the very first thing in main() using Perl_init_global_struct() and
2239correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2240please see F<miniperlmain.c> for usage details. You may also need
2241to use C<dVAR> in your coding to "declare the global variables"
2242when you are using them. dTHX does this for you automatically.
2243
9aa97215
JH
2244To see whether you have non-const data you can use a BSD (or GNU)
2245compatible C<nm>:
bc028b6b
JH
2246
2247 nm libperl.a | grep -v ' [TURtr] '
2248
9aa97215
JH
2249If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
2250you have non-const data. The symbols the C<grep> removed are as follows:
2251C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
2252and the C<U> is <undefined>, external symbols referred to.
2253
2254The test F<t/porting/libperl.t> does this kind of symbol sanity
2255checking on C<libperl.a>.
bc028b6b 2256
27da23d5
JH
2257For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2258doesn't actually hide all symbols inside a big global struct: some
2259PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2260then hides everything (see how the PERLIO_FUNCS_DECL is used).
2261
54aff467 2262All this obviously requires a way for the Perl internal functions to be
acfe0abc 2263either subroutines taking some kind of structure as the first
ee072b34 2264argument, or subroutines taking nothing as the first argument. To
acfe0abc 2265enable these two very different ways of building the interpreter,
ee072b34
GS
2266the Perl source (as it does in so many other situations) makes heavy
2267use of macros and subroutine naming conventions.
2268
54aff467 2269First problem: deciding which functions will be public API functions and
00aadd71 2270which will be private. All functions whose names begin C<S_> are private
954c1994
GS
2271(think "S" for "secret" or "static"). All other functions begin with
2272"Perl_", but just because a function begins with "Perl_" does not mean it is
10e2eb10
FC
2273part of the API. (See L</Internal
2274Functions>.) The easiest way to be B<sure> a
00aadd71
NIS
2275function is part of the API is to find its entry in L<perlapi>.
2276If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2277think it should be (i.e., you need it for your extension), send mail via
a422fd2d 2278L<perlbug> explaining why you think it should be.
ee072b34
GS
2279
2280Second problem: there must be a syntax so that the same subroutine
2281declarations and calls can pass a structure as their first argument,
2282or pass nothing. To solve this, the subroutines are named and
2283declared in a particular way. Here's a typical start of a static
2284function used within the Perl guts:
2285
2286 STATIC void
2287 S_incline(pTHX_ char *s)
2288
acfe0abc 2289STATIC becomes "static" in C, and may be #define'd to nothing in some
da8c5729 2290configurations in the future.
ee072b34 2291
651a3225
GS
2292A public function (i.e. part of the internal API, but not necessarily
2293sanctioned for use in extensions) begins like this:
ee072b34
GS
2294
2295 void
2307c6d0 2296 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 2297
0147cd53 2298C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
2299details of the interpreter's context. THX stands for "thread", "this",
2300or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2301The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
2302or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2303their variants.
ee072b34 2304
a7486cbb
JH
2305When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2306first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
2307in the pTHX_ macro indicates that the macro expansion needs a comma
2308after the context argument because other arguments follow it. If
2309PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
2310subroutine is not prototyped to take the extra argument. The form of the
2311macro without the trailing underscore is used when there are no additional
ee072b34
GS
2312explicit arguments.
2313
54aff467 2314When a core function calls another, it must pass the context. This
2307c6d0 2315is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
2316something like this:
2317
2307c6d0
SB
2318 #ifdef PERL_IMPLICIT_CONTEXT
2319 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 2320 /* can't do this for vararg functions, see below */
2307c6d0
SB
2321 #else
2322 #define sv_setiv Perl_sv_setiv
2323 #endif
ee072b34
GS
2324
2325This works well, and means that XS authors can gleefully write:
2326
2307c6d0 2327 sv_setiv(foo, bar);
ee072b34
GS
2328
2329and still have it work under all the modes Perl could have been
2330compiled with.
2331
ee072b34
GS
2332This doesn't work so cleanly for varargs functions, though, as macros
2333imply that the number of arguments is known in advance. Instead we
2334either need to spell them out fully, passing C<aTHX_> as the first
2335argument (the Perl core tends to do this with functions like
2336Perl_warner), or use a context-free version.
2337
2338The context-free version of Perl_warner is called
2339Perl_warner_nocontext, and does not take the extra argument. Instead
2340it does dTHX; to get the context from thread-local storage. We
2341C<#define warner Perl_warner_nocontext> so that extensions get source
2342compatibility at the expense of performance. (Passing an arg is
2343cheaper than grabbing it from thread-local storage.)
2344
acfe0abc 2345You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2346Those are strictly for use within the core. Extensions and embedders
2347need only be aware of [pad]THX.
2348
a7486cbb
JH
2349=head2 So what happened to dTHR?
2350
2351C<dTHR> was introduced in perl 5.005 to support the older thread model.
2352The older thread model now uses the C<THX> mechanism to pass context
2353pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2354later still have it for backward source compatibility, but it is defined
2355to be a no-op.
2356
ee072b34
GS
2357=head2 How do I use all this in extensions?
2358
2359When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2360any functions in the Perl API will need to pass the initial context
2361argument somehow. The kicker is that you will need to write it in
2362such a way that the extension still compiles when Perl hasn't been
2363built with PERL_IMPLICIT_CONTEXT enabled.
2364
2365There are three ways to do this. First, the easy but inefficient way,
2366which is also the default, in order to maintain source compatibility
0147cd53 2367with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2368and aTHX_ macros to call a function that will return the context.
2369Thus, something like:
2370
2307c6d0 2371 sv_setiv(sv, num);
ee072b34 2372
4375e838 2373in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2374in effect:
ee072b34 2375
2307c6d0 2376 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2377
54aff467 2378or to this otherwise:
ee072b34 2379
2307c6d0 2380 Perl_sv_setiv(sv, num);
ee072b34 2381
da8c5729 2382You don't have to do anything new in your extension to get this; since
2fa86c13 2383the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2384work.
2385
2386The second, more efficient way is to use the following template for
2387your Foo.xs:
2388
c52f9dcd
JH
2389 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2390 #include "EXTERN.h"
2391 #include "perl.h"
2392 #include "XSUB.h"
ee072b34 2393
fd061412 2394 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2395
fd061412 2396 STATIC void
c52f9dcd
JH
2397 my_private_function(int arg1, int arg2)
2398 {
2399 dTHX; /* fetch context */
2400 ... call many Perl API functions ...
2401 }
ee072b34
GS
2402
2403 [... etc ...]
2404
c52f9dcd 2405 MODULE = Foo PACKAGE = Foo
ee072b34 2406
c52f9dcd 2407 /* typical XSUB */
ee072b34 2408
c52f9dcd
JH
2409 void
2410 my_xsub(arg)
2411 int arg
2412 CODE:
2413 my_private_function(arg, 10);
ee072b34
GS
2414
2415Note that the only two changes from the normal way of writing an
2416extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2417including the Perl headers, followed by a C<dTHX;> declaration at
2418the start of every function that will call the Perl API. (You'll
2419know which functions need this, because the C compiler will complain
2420that there's an undeclared identifier in those functions.) No changes
2421are needed for the XSUBs themselves, because the XS() macro is
2422correctly defined to pass in the implicit context if needed.
2423
2424The third, even more efficient way is to ape how it is done within
2425the Perl guts:
2426
2427
c52f9dcd
JH
2428 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2429 #include "EXTERN.h"
2430 #include "perl.h"
2431 #include "XSUB.h"
ee072b34
GS
2432
2433 /* pTHX_ only needed for functions that call Perl API */
fd061412 2434 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2435
fd061412 2436 STATIC void
c52f9dcd
JH
2437 my_private_function(pTHX_ int arg1, int arg2)
2438 {
2439 /* dTHX; not needed here, because THX is an argument */
2440 ... call Perl API functions ...
2441 }
ee072b34
GS
2442
2443 [... etc ...]
2444
c52f9dcd 2445 MODULE = Foo PACKAGE = Foo
ee072b34 2446
c52f9dcd 2447 /* typical XSUB */
ee072b34 2448
c52f9dcd
JH
2449 void
2450 my_xsub(arg)
2451 int arg
2452 CODE:
2453 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2454
2455This implementation never has to fetch the context using a function
2456call, since it is always passed as an extra argument. Depending on
2457your needs for simplicity or efficiency, you may mix the previous
2458two approaches freely.
2459
651a3225
GS
2460Never add a comma after C<pTHX> yourself--always use the form of the
2461macro with the underscore for functions that take explicit arguments,
2462or the form without the argument for functions with no explicit arguments.
ee072b34 2463
27da23d5
JH
2464If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2465definition is needed if the Perl global variables (see F<perlvars.h>
2466or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2467used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2468the need for C<dVAR> only with the said compile-time define, because
2469otherwise the Perl global variables are visible as-is.
2470
a7486cbb
JH
2471=head2 Should I do anything special if I call perl from multiple threads?
2472
2473If you create interpreters in one thread and then proceed to call them in
2474another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2475initialized correctly in each of those threads.
2476
2477The C<perl_alloc> and C<perl_clone> API functions will automatically set
2478the TLS slot to the interpreter they created, so that there is no need to do
2479anything special if the interpreter is always accessed in the same thread that
2480created it, and that thread did not create or call any other interpreters
2481afterwards. If that is not the case, you have to set the TLS slot of the
2482thread before calling any functions in the Perl API on that particular
2483interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2484thread as the first thing you do:
2485
2486 /* do this before doing anything else with some_perl */
2487 PERL_SET_CONTEXT(some_perl);
2488
2489 ... other Perl API calls on some_perl go here ...
2490
ee072b34
GS
2491=head2 Future Plans and PERL_IMPLICIT_SYS
2492
2493Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2494that the interpreter knows about itself and pass it around, so too are
2495there plans to allow the interpreter to bundle up everything it knows
2496about the environment it's running on. This is enabled with the
7b52221d
RGS
2497PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2498Windows.
ee072b34
GS
2499
2500This allows the ability to provide an extra pointer (called the "host"
2501environment) for all the system calls. This makes it possible for
2502all the system stuff to maintain their own state, broken down into
2503seven C structures. These are thin wrappers around the usual system
0147cd53 2504calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2505more ambitious host (like the one that would do fork() emulation) all
2506the extra work needed to pretend that different interpreters are
2507actually different "processes", would be done here.
2508
2509The Perl engine/interpreter and the host are orthogonal entities.
2510There could be one or more interpreters in a process, and one or
2511more "hosts", with free association between them.
2512
a422fd2d
SC
2513=head1 Internal Functions
2514
2515All of Perl's internal functions which will be exposed to the outside
06f6df17 2516world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d 2517functions or functions used in a program in which Perl is embedded.
10e2eb10 2518Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2519static functions start with C<S_>.)
a422fd2d 2520
0972ecdf
DM
2521Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2522either with or without the C<Perl_> prefix, thanks to a bunch of defines
10e2eb10 2523that live in F<embed.h>. Note that extension code should I<not> set
0972ecdf
DM
2524C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2525breakage of the XS in each new perl release.
2526
2527The file F<embed.h> is generated automatically from
10e2eb10 2528F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
dc9b1d22 2529header files for the internal functions, generates the documentation
10e2eb10 2530and a lot of other bits and pieces. It's important that when you add
dc9b1d22 2531a new function to the core or change an existing one, you change the
10e2eb10 2532data in the table in F<embed.fnc> as well. Here's a sample entry from
dc9b1d22 2533that table:
a422fd2d
SC
2534
2535 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2536
10e2eb10
FC
2537The second column is the return type, the third column the name. Columns
2538after that are the arguments. The first column is a set of flags:
a422fd2d
SC
2539
2540=over 3
2541
2542=item A
2543
10e2eb10
FC
2544This function is a part of the public
2545API. All such functions should also
1aa6ea50 2546have 'd', very few do not.
a422fd2d
SC
2547
2548=item p
2549
1aa6ea50
JC
2550This function has a C<Perl_> prefix; i.e. it is defined as
2551C<Perl_av_fetch>.
a422fd2d
SC
2552
2553=item d
2554
2555This function has documentation using the C<apidoc> feature which we'll
1aa6ea50 2556look at in a second. Some functions have 'd' but not 'A'; docs are good.
a422fd2d
SC
2557
2558=back
2559
2560Other available flags are:
2561
2562=over 3
2563
2564=item s
2565
1aa6ea50
JC
2566This is a static function and is defined as C<STATIC S_whatever>, and
2567usually called within the sources as C<whatever(...)>.
a422fd2d
SC
2568
2569=item n
2570
da8c5729 2571This does not need an interpreter context, so the definition has no
1aa6ea50 2572C<pTHX>, and it follows that callers don't use C<aTHX>. (See
d3a43cd8 2573L</Background and PERL_IMPLICIT_CONTEXT>.)
a422fd2d
SC
2574
2575=item r
2576
2577This function never returns; C<croak>, C<exit> and friends.
2578
2579=item f
2580
2581This function takes a variable number of arguments, C<printf> style.
2582The argument list should end with C<...>, like this:
2583
2584 Afprd |void |croak |const char* pat|...
2585
a7486cbb 2586=item M
a422fd2d 2587
00aadd71 2588This function is part of the experimental development API, and may change
a422fd2d
SC
2589or disappear without notice.
2590
2591=item o
2592
2593This function should not have a compatibility macro to define, say,
10e2eb10 2594C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
a422fd2d 2595
a422fd2d
SC
2596=item x
2597
2598This function isn't exported out of the Perl core.
2599
dc9b1d22
MHM
2600=item m
2601
2602This is implemented as a macro.
2603
2604=item X
2605
2606This function is explicitly exported.
2607
2608=item E
2609
2610This function is visible to extensions included in the Perl core.
2611
2612=item b
2613
2614Binary backward compatibility; this function is a macro but also has
2615a C<Perl_> implementation (which is exported).
2616
1aa6ea50
JC
2617=item others
2618
2619See the comments at the top of C<embed.fnc> for others.
2620
a422fd2d
SC
2621=back
2622
dc9b1d22
MHM
2623If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2624C<make regen_headers> to force a rebuild of F<embed.h> and other
2625auto-generated files.
a422fd2d 2626
6b4667fc 2627=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2628
6b4667fc
A
2629If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2630formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2631following macros for portability
9dd9db0b 2632
c52f9dcd
JH
2633 IVdf IV in decimal
2634 UVuf UV in decimal
2635 UVof UV in octal
2636 UVxf UV in hexadecimal
2637 NVef NV %e-like
2638 NVff NV %f-like
2639 NVgf NV %g-like
9dd9db0b 2640
6b4667fc
A
2641These will take care of 64-bit integers and long doubles.
2642For example:
2643
c52f9dcd 2644 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2645
2646The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2647
aacf4ea2
JH
2648Note that there are different "long doubles": Perl will use
2649whatever the compiler has.
2650
8908e76d
JH
2651If you are printing addresses of pointers, use UVxf combined
2652with PTR2UV(), do not use %lx or %p.
2653
2654=head2 Pointer-To-Integer and Integer-To-Pointer
2655
2656Because pointer size does not necessarily equal integer size,
2657use the follow macros to do it right.
2658
c52f9dcd
JH
2659 PTR2UV(pointer)
2660 PTR2IV(pointer)
2661 PTR2NV(pointer)
2662 INT2PTR(pointertotype, integer)
8908e76d
JH
2663
2664For example:
2665
c52f9dcd
JH
2666 IV iv = ...;
2667 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2668
2669and
2670
c52f9dcd
JH
2671 AV *av = ...;
2672 UV uv = PTR2UV(av);
8908e76d 2673
0ca3a874
MHM
2674=head2 Exception Handling
2675
9b5c3821 2676There are a couple of macros to do very basic exception handling in XS
10e2eb10 2677modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
9b5c3821
MHM
2678be able to use these macros:
2679
2680 #define NO_XSLOCKS
2681 #include "XSUB.h"
2682
2683You can use these macros if you call code that may croak, but you need
10e2eb10 2684to do some cleanup before giving control back to Perl. For example:
0ca3a874 2685
d7f8936a 2686 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2687
2688 XCPT_TRY_START {
2689 code_that_may_croak();
2690 } XCPT_TRY_END
2691
2692 XCPT_CATCH
2693 {
2694 /* do cleanup here */
2695 XCPT_RETHROW;
2696 }
2697
2698Note that you always have to rethrow an exception that has been
10e2eb10
FC
2699caught. Using these macros, it is not possible to just catch the
2700exception and ignore it. If you have to ignore the exception, you
0ca3a874
MHM
2701have to use the C<call_*> function.
2702
2703The advantage of using the above macros is that you don't have
2704to setup an extra function for C<call_*>, and that using these
2705macros is faster than using C<call_*>.
2706
a422fd2d
SC
2707=head2 Source Documentation
2708
2709There's an effort going on to document the internal functions and
2710automatically produce reference manuals from them - L<perlapi> is one
2711such manual which details all the functions which are available to XS
10e2eb10 2712writers. L<perlintern> is the autogenerated manual for the functions
a422fd2d
SC
2713which are not part of the API and are supposedly for internal use only.
2714
2715Source documentation is created by putting POD comments into the C
2716source, like this:
2717
2718 /*
2719 =for apidoc sv_setiv
2720
2721 Copies an integer into the given SV. Does not handle 'set' magic. See
2722 C<sv_setiv_mg>.
2723
2724 =cut
2725 */
2726
2727Please try and supply some documentation if you add functions to the
2728Perl core.
2729
0d098d33
MHM
2730=head2 Backwards compatibility
2731
10e2eb10
FC
2732The Perl API changes over time. New functions are
2733added or the interfaces of existing functions are
2734changed. The C<Devel::PPPort> module tries to
0d098d33
MHM
2735provide compatibility code for some of these changes, so XS writers don't
2736have to code it themselves when supporting multiple versions of Perl.
2737
2738C<Devel::PPPort> generates a C header file F<ppport.h> that can also
10e2eb10 2739be run as a Perl script. To generate F<ppport.h>, run:
0d098d33
MHM
2740
2741 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2742
2743Besides checking existing XS code, the script can also be used to retrieve
2744compatibility information for various API calls using the C<--api-info>
10e2eb10 2745command line switch. For example:
0d098d33
MHM
2746
2747 % perl ppport.h --api-info=sv_magicext
2748
2749For details, see C<perldoc ppport.h>.
2750
a422fd2d
SC
2751=head1 Unicode Support
2752
10e2eb10 2753Perl 5.6.0 introduced Unicode support. It's important for porters and XS
a422fd2d
SC
2754writers to understand this support and make sure that the code they
2755write does not corrupt Unicode data.
2756
2757=head2 What B<is> Unicode, anyway?
2758
10e2eb10
FC
2759In the olden, less enlightened times, we all used to use ASCII. Most of
2760us did, anyway. The big problem with ASCII is that it's American. Well,
a422fd2d 2761no, that's not actually the problem; the problem is that it's not
10e2eb10 2762particularly useful for people who don't use the Roman alphabet. What
a422fd2d 2763used to happen was that particular languages would stick their own
10e2eb10 2764alphabet in the upper range of the sequence, between 128 and 255. Of
a422fd2d
SC
2765course, we then ended up with plenty of variants that weren't quite
2766ASCII, and the whole point of it being a standard was lost.
2767
2768Worse still, if you've got a language like Chinese or
2769Japanese that has hundreds or thousands of characters, then you really
2770can't fit them into a mere 256, so they had to forget about ASCII
2771altogether, and build their own systems using pairs of numbers to refer
2772to one character.
2773
2774To fix this, some people formed Unicode, Inc. and
2775produced a new character set containing all the characters you can
10e2eb10
FC
2776possibly think of and more. There are several ways of representing these
2777characters, and the one Perl uses is called UTF-8. UTF-8 uses
2778a variable number of bytes to represent a character. You can learn more
2575c402 2779about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2780
1e54db1a 2781=head2 How can I recognise a UTF-8 string?
a422fd2d 2782
10e2eb10
FC
2783You can't. This is because UTF-8 data is stored in bytes just like
2784non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d 2785capital E with a grave accent, is represented by the two bytes
10e2eb10
FC
2786C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2787has that byte sequence as well. So you can't tell just by looking - this
a422fd2d
SC
2788is what makes Unicode input an interesting problem.
2789
2575c402
JW
2790In general, you either have to know what you're dealing with, or you
2791have to guess. The API function C<is_utf8_string> can help; it'll tell
10e2eb10
FC
2792you if a string contains only valid UTF-8 characters. However, it can't
2793do the work for you. On a character-by-character basis,
6302f837 2794C<isUTF8_CHAR>
2575c402 2795will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2796
1e54db1a 2797=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2798
1e54db1a 2799As mentioned above, UTF-8 uses a variable number of bytes to store a
10e2eb10
FC
2800character. Characters with values 0...127 are stored in one
2801byte, just like good ol' ASCII. Character 128 is stored as
2802C<v194.128>; this continues up to character 191, which is
2803C<v194.191>. Now we've run out of bits (191 is binary
2804C<10111111>) so we move on; 192 is C<v195.128>. And
a422fd2d
SC
2805so it goes on, moving to three bytes at character 2048.
2806
1e54db1a 2807Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2808how long the first character in it is with the C<UTF8SKIP> macro:
2809
2810 char *utf = "\305\233\340\240\201";
2811 I32 len;
2812
2813 len = UTF8SKIP(utf); /* len is 2 here */
2814 utf += len;
2815 len = UTF8SKIP(utf); /* len is 3 here */
2816
1e54db1a 2817Another way to skip over characters in a UTF-8 string is to use
a422fd2d 2818C<utf8_hop>, which takes a string and a number of characters to skip
10e2eb10 2819over. You're on your own about bounds checking, though, so don't use it
a422fd2d
SC
2820lightly.
2821
1e54db1a 2822All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2823so you can test if you need to do something special with this
2824character like this (the UTF8_IS_INVARIANT() is a macro that tests
9f98c7fe 2825whether the byte is encoded as a single byte even in UTF-8):
a422fd2d 2826
3a2263fe 2827 U8 *utf;
4b88fb76 2828 U8 *utf_end; /* 1 beyond buffer pointed to by utf */
3a2263fe 2829 UV uv; /* Note: a UV, not a U8, not a char */
95701e00 2830 STRLEN len; /* length of character in bytes */
a422fd2d 2831
3a2263fe 2832 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2833 /* Must treat this as UTF-8 */
4b88fb76 2834 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
a422fd2d
SC
2835 else
2836 /* OK to treat this character as a byte */
2837 uv = *utf;
2838
4b88fb76 2839You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
95701e00 2840value of the character; the inverse function C<uvchr_to_utf8> is available
1e54db1a 2841for putting a UV into UTF-8:
a422fd2d 2842
3a2263fe 2843 if (!UTF8_IS_INVARIANT(uv))
a422fd2d 2844 /* Must treat this as UTF8 */
95701e00 2845 utf8 = uvchr_to_utf8(utf8, uv);
a422fd2d
SC
2846 else
2847 /* OK to treat this character as a byte */
2848 *utf8++ = uv;
2849
2850You B<must> convert characters to UVs using the above functions if
1e54db1a 2851you're ever in a situation where you have to match UTF-8 and non-UTF-8
10e2eb10 2852characters. You may not skip over UTF-8 characters in this case. If you
1e54db1a
JH
2853do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2854for instance, if your UTF-8 string contains C<v196.172>, and you skip
2855that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2856So don't do that!
2857
1e54db1a 2858=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2859
2860Currently, Perl deals with Unicode strings and non-Unicode strings
10e2eb10
FC
2861slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2862string is internally encoded as UTF-8. Without it, the byte value is the
2575c402 2863codepoint number and vice versa (in other words, the string is encoded
e1b711da 2864as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
c31cc9fc
FC
2865semantics). This flag is only meaningful if the SV is C<SvPOK>
2866or immediately after stringification via C<SvPV> or a similar
2867macro. You can check and manipulate this flag with the
2575c402 2868following macros:
a422fd2d
SC
2869
2870 SvUTF8(sv)
2871 SvUTF8_on(sv)
2872 SvUTF8_off(sv)
2873
2874This flag has an important effect on Perl's treatment of the string: if
2875Unicode data is not properly distinguished, regular expressions,
2876C<length>, C<substr> and other string handling operations will have
2877undesirable results.
2878
2879The problem comes when you have, for instance, a string that isn't
2575c402 2880flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
1e54db1a 2881especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2882
2883Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2884need be sure you don't accidentally knock it off while you're
10e2eb10 2885manipulating SVs. More specifically, you cannot expect to do this:
a422fd2d
SC
2886
2887 SV *sv;
2888 SV *nsv;
2889 STRLEN len;
2890 char *p;
2891
2892 p = SvPV(sv, len);
2893 frobnicate(p);
2894 nsv = newSVpvn(p, len);
2895
2896The C<char*> string does not tell you the whole story, and you can't
10e2eb10 2897copy or reconstruct an SV just by copying the string value. Check if the
c31cc9fc
FC
2898old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2899accordingly:
a422fd2d
SC
2900
2901 p = SvPV(sv, len);
2902 frobnicate(p);
2903 nsv = newSVpvn(p, len);
2904 if (SvUTF8(sv))
2905 SvUTF8_on(nsv);
2906
2907In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2908not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2909appropriately.
2910
3a2263fe 2911Since just passing an SV to an XS function and copying the data of
2575c402 2912the SV is not enough to copy the UTF8 flags, even less right is just
3a2263fe
RGS
2913passing a C<char *> to an XS function.
2914
1e54db1a 2915=head2 How do I convert a string to UTF-8?
a422fd2d 2916
2575c402 2917If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
10e2eb10 2918one of the strings to UTF-8. If you've got an SV, the easiest way to do
2575c402 2919this is:
a422fd2d
SC
2920
2921 sv_utf8_upgrade(sv);
2922
2923However, you must not do this, for example:
2924
2925 if (!SvUTF8(left))
2926 sv_utf8_upgrade(left);
2927
2928If you do this in a binary operator, you will actually change one of the
b1866b2d 2929strings that came into the operator, and, while it shouldn't be noticeable
2575c402 2930by the end user, it can cause problems in deficient code.
a422fd2d 2931
1e54db1a 2932Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
10e2eb10
FC
2933string argument. This is useful for having the data available for
2934comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2935C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2936the string contains any characters above 255 that can't be represented
2937in a single byte.
2938
2939=head2 Is there anything else I need to know?
2940
10e2eb10 2941Not really. Just remember these things:
a422fd2d
SC
2942
2943=over 3
2944
2945=item *
2946
10e2eb10 2947There's no way to tell if a string is UTF-8 or not. You can tell if an SV
c31cc9fc
FC
2948is UTF-8 by looking at its C<SvUTF8> flag after stringifying it
2949with C<SvPV> or a similar macro. Don't forget to set the flag if
10e2eb10 2950something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2951it's not - if you pass on the PV to somewhere, pass on the flag too.
2952
2953=item *
2954
4b88fb76 2955If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
3a2263fe 2956unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2957
2958=item *
2959
1e54db1a 2960When writing a character C<uv> to a UTF-8 string, B<always> use
95701e00 2961C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
3a2263fe 2962you can use C<*s = uv>.
a422fd2d
SC
2963
2964=item *
2965
10e2eb10
FC
2966Mixing UTF-8 and non-UTF-8 strings is
2967tricky. Use C<bytes_to_utf8> to get
2bbc8d55 2968a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
2969
2970=back
2971
53e06cf0
SC
2972=head1 Custom Operators
2973
2a0fd0f1 2974Custom operator support is an experimental feature that allows you to
10e2eb10 2975define your own ops. This is primarily to allow the building of
53e06cf0
SC
2976interpreters for other languages in the Perl core, but it also allows
2977optimizations through the creation of "macro-ops" (ops which perform the
2978functions of multiple ops which are usually executed together, such as
1aa6ea50 2979C<gvsv, gvsv, add>.)
53e06cf0 2980
10e2eb10 2981This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0 2982core does not "know" anything special about this op type, and so it will
10e2eb10 2983not be involved in any optimizations. This also means that you can
53e06cf0
SC
2984define your custom ops to be any op structure - unary, binary, list and
2985so on - you like.
2986
10e2eb10
FC
2987It's important to know what custom operators won't do for you. They
2988won't let you add new syntax to Perl, directly. They won't even let you
2989add new keywords, directly. In fact, they won't change the way Perl
2990compiles a program at all. You have to do those changes yourself, after
2991Perl has compiled the program. You do this either by manipulating the op
53e06cf0
SC
2992tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2993a custom peephole optimizer with the C<optimize> module.
2994
2995When you do this, you replace ordinary Perl ops with custom ops by
407f86e1 2996creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
10e2eb10
FC
2997PP function. This should be defined in XS code, and should look like
2998the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
53e06cf0
SC
2999takes the appropriate number of values from the stack, and you are
3000responsible for adding stack marks if necessary.
3001
3002You should also "register" your op with the Perl interpreter so that it
10e2eb10 3003can produce sensible error and warning messages. Since it is possible to
53e06cf0 3004have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
9733086d 3005Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
10e2eb10 3006it is dealing with. You should create an C<XOP> structure for each
9733086d
BM
3007ppaddr you use, set the properties of the custom op with
3008C<XopENTRY_set>, and register the structure against the ppaddr using
10e2eb10 3009C<Perl_custom_op_register>. A trivial example might look like:
9733086d
BM
3010
3011 static XOP my_xop;
3012 static OP *my_pp(pTHX);
3013
3014 BOOT:
3015 XopENTRY_set(&my_xop, xop_name, "myxop");
3016 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3017 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3018
3019The available fields in the structure are:
3020
3021=over 4
3022
3023=item xop_name
3024
10e2eb10 3025A short name for your op. This will be included in some error messages,
9733086d
BM
3026and will also be returned as C<< $op->name >> by the L<B|B> module, so
3027it will appear in the output of module like L<B::Concise|B::Concise>.
3028
3029=item xop_desc
3030
3031A short description of the function of the op.
3032
3033=item xop_class
3034
10e2eb10 3035Which of the various C<*OP> structures this op uses. This should be one of
9733086d
BM
3036the C<OA_*> constants from F<op.h>, namely
3037
3038=over 4
3039
3040=item OA_BASEOP
3041
3042=item OA_UNOP
3043
3044=item OA_BINOP
3045
3046=item OA_LOGOP
3047
3048=item OA_LISTOP
3049
3050=item OA_PMOP
3051
3052=item OA_SVOP
3053
3054=item OA_PADOP
3055
3056=item OA_PVOP_OR_SVOP
3057
10e2eb10 3058This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
9733086d
BM
3059the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
3060
3061=item OA_LOOP
3062
3063=item OA_COP
3064
3065=back
3066
3067The other C<OA_*> constants should not be used.
3068
3069=item xop_peep
3070
3071This member is of type C<Perl_cpeep_t>, which expands to C<void
10e2eb10 3072(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
9733086d 3073will be called from C<Perl_rpeep> when ops of this type are encountered
10e2eb10 3074by the peephole optimizer. I<o> is the OP that needs optimizing;
9733086d
BM
3075I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
3076
3077=back
53e06cf0 3078
e7d4c058 3079C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 3080
954c1994 3081=head1 AUTHORS
e89caa19 3082
954c1994 3083Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
3084E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
3085itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 3086
954c1994
GS
3087With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3088Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3089Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3090Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 3091
954c1994 3092=head1 SEE ALSO
cb1a09d0 3093
ba555bf5 3094L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>