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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*);
5f05dabc
PP
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
a0d0e21e
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
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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)
1169 % PERL_MAGIC_rhash (none) extra data for restricted
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
FC
1174 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1175 : PERL_MAGIC_symtab (none) extra data for symbol
1176 tables
1177 < PERL_MAGIC_backref vtbl_backref for weak ref data
1178 @ PERL_MAGIC_arylen_p (none) to move arylen out of XPVAV
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
1206 r PERL_MAGIC_qr vtbl_regexp precompiled qr// regex
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
1221 ] PERL_MAGIC_checkcall vtbl_checkcall inlining/mutation of call
1222 to this CV
1223 ~ PERL_MAGIC_ext (none) Available for use by
1224 extensions
0cbee0a4 1225
f1f5ddd7 1226=for mg_vtable.pl end
d1b91892 1227
68dc0745 1228When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1229uppercase letter is typically used to represent some kind of composite type
1230(a list or a hash), and the lowercase letter is used to represent an element
10e2eb10 1231of that composite type. Some internals code makes use of this case
92f0c265 1232relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1233
1234The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1235specifically for use by extensions and will not be used by perl itself.
1236Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1237to variables (typically objects). This is especially useful because
1238there is no way for normal perl code to corrupt this private information
1239(unlike using extra elements of a hash object).
1240
1241Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1242C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1243C<mg_ptr> field points to a C<ufuncs> structure:
1244
1245 struct ufuncs {
a9402793
AB
1246 I32 (*uf_val)(pTHX_ IV, SV*);
1247 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1248 IV uf_index;
1249 };
1250
1251When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1252function will be called with C<uf_index> as the first arg and a pointer to
1253the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1254magic is shown below. Note that the ufuncs structure is copied by
1255sv_magic, so you can safely allocate it on the stack.
1256
1257 void
1258 Umagic(sv)
1259 SV *sv;
1260 PREINIT:
1261 struct ufuncs uf;
1262 CODE:
1263 uf.uf_val = &my_get_fn;
1264 uf.uf_set = &my_set_fn;
1265 uf.uf_index = 0;
14befaf4 1266 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1267
1e73acc8
AS
1268Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1269
1270For hashes there is a specialized hook that gives control over hash
1271keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1272if the "set" function in the C<ufuncs> structure is NULL. The hook
1273is activated whenever the hash is accessed with a key specified as
1274an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1275C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1276through the functions without the C<..._ent> suffix circumvents the
4509d391 1277hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1e73acc8 1278
14befaf4
DM
1279Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1280or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1281extra care to avoid conflict. Typically only using the magic on
1282objects blessed into the same class as the extension is sufficient.
2f07f21a
FR
1283For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1284C<MGVTBL>, even if all its fields will be C<0>, so that individual
1285C<MAGIC> pointers can be identified as a particular kind of magic
10e2eb10 1286using their magic virtual table. C<mg_findext> provides an easy way
f6ee7b17 1287to do that:
2f07f21a
FR
1288
1289 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1290
1291 MAGIC *mg;
f6ee7b17
FR
1292 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1293 /* this is really ours, not another module's PERL_MAGIC_ext */
1294 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1295 ...
2f07f21a 1296 }
5f05dabc 1297
ef50df4b
GS
1298Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1299earlier do B<not> invoke 'set' magic on their targets. This must
1300be done by the user either by calling the C<SvSETMAGIC()> macro after
1301calling these functions, or by using one of the C<sv_set*_mg()> or
1302C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1303C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1304obtained from external sources in functions that don't handle magic.
4a4eefd0 1305See L<perlapi> for a description of these functions.
189b2af5
GS
1306For example, calls to the C<sv_cat*()> functions typically need to be
1307followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1308since their implementation handles 'get' magic.
1309
d1b91892
AD
1310=head2 Finding Magic
1311
a9b0660e
KW
1312 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1313 * type */
f6ee7b17
FR
1314
1315This routine returns a pointer to a C<MAGIC> structure stored in the SV.
10e2eb10
FC
1316If the SV does not have that magical
1317feature, C<NULL> is returned. If the
f6ee7b17 1318SV has multiple instances of that magical feature, the first one will be
10e2eb10
FC
1319returned. C<mg_findext> can be used
1320to find a C<MAGIC> structure of an SV
da8c5729 1321based on both its magic type and its magic virtual table:
f6ee7b17
FR
1322
1323 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
d1b91892 1324
f6ee7b17
FR
1325Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1326SVt_PVMG, Perl may core dump.
d1b91892 1327
08105a92 1328 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1329
1330This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1331field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1332the mg_type field is changed to be the lowercase letter.
a0d0e21e 1333
04343c6d
GS
1334=head2 Understanding the Magic of Tied Hashes and Arrays
1335
14befaf4
DM
1336Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1337magic type.
9edb2b46
GS
1338
1339WARNING: As of the 5.004 release, proper usage of the array and hash
1340access functions requires understanding a few caveats. Some
1341of these caveats are actually considered bugs in the API, to be fixed
10e2eb10 1342in later releases, and are bracketed with [MAYCHANGE] below. If
9edb2b46
GS
1343you find yourself actually applying such information in this section, be
1344aware that the behavior may change in the future, umm, without warning.
04343c6d 1345
1526ead6 1346The perl tie function associates a variable with an object that implements
9a68f1db 1347the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1348tie function from an XSUB, you must mimic this behaviour. The code below
1349carries out the necessary steps - firstly it creates a new hash, and then
1350creates a second hash which it blesses into the class which will implement
10e2eb10 1351the tie methods. Lastly it ties the two hashes together, and returns a
1526ead6
AB
1352reference to the new tied hash. Note that the code below does NOT call the
1353TIEHASH method in the MyTie class -
1354see L<Calling Perl Routines from within C Programs> for details on how
1355to do this.
1356
1357 SV*
1358 mytie()
1359 PREINIT:
1360 HV *hash;
1361 HV *stash;
1362 SV *tie;
1363 CODE:
1364 hash = newHV();
1365 tie = newRV_noinc((SV*)newHV());
da51bb9b 1366 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1367 sv_bless(tie, stash);
899e16d0 1368 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1369 RETVAL = newRV_noinc(hash);
1370 OUTPUT:
1371 RETVAL
1372
04343c6d
GS
1373The C<av_store> function, when given a tied array argument, merely
1374copies the magic of the array onto the value to be "stored", using
1375C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1376actually need to be stored in the array. [MAYCHANGE] After a call to
1377C<av_store> on a tied array, the caller will usually need to call
1378C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1379TIEARRAY object. If C<av_store> did return NULL, a call to
1380C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1381leak. [/MAYCHANGE]
04343c6d
GS
1382
1383The previous paragraph is applicable verbatim to tied hash access using the
1384C<hv_store> and C<hv_store_ent> functions as well.
1385
1386C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1387C<hv_fetch_ent> actually return an undefined mortal value whose magic
1388has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1389need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1390need to call C<mg_get()> on the returned value in order to actually invoke
1391the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1392you may also call C<mg_set()> on the return value after possibly assigning
1393a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1394method on the TIE object. [/MAYCHANGE]
04343c6d 1395
9edb2b46 1396[MAYCHANGE]
04343c6d
GS
1397In other words, the array or hash fetch/store functions don't really
1398fetch and store actual values in the case of tied arrays and hashes. They
1399merely call C<mg_copy> to attach magic to the values that were meant to be
1400"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1401do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1402the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1403and hashes.
1404
1405Currently (as of perl version 5.004), use of the hash and array access
1406functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1407"normal" hashes and arrays, or on their tied variants. The API may be
1408changed to provide more transparent access to both tied and normal data
1409types in future versions.
1410[/MAYCHANGE]
04343c6d
GS
1411
1412You would do well to understand that the TIEARRAY and TIEHASH interfaces
1413are mere sugar to invoke some perl method calls while using the uniform hash
1414and array syntax. The use of this sugar imposes some overhead (typically
1415about two to four extra opcodes per FETCH/STORE operation, in addition to
1416the creation of all the mortal variables required to invoke the methods).
1417This overhead will be comparatively small if the TIE methods are themselves
1418substantial, but if they are only a few statements long, the overhead
1419will not be insignificant.
1420
d1c897a1
IZ
1421=head2 Localizing changes
1422
1423Perl has a very handy construction
1424
1425 {
1426 local $var = 2;
1427 ...
1428 }
1429
1430This construction is I<approximately> equivalent to
1431
1432 {
1433 my $oldvar = $var;
1434 $var = 2;
1435 ...
1436 $var = $oldvar;
1437 }
1438
1439The biggest difference is that the first construction would
1440reinstate the initial value of $var, irrespective of how control exits
10e2eb10 1441the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1442more efficient as well.
1443
1444There is a way to achieve a similar task from C via Perl API: create a
1445I<pseudo-block>, and arrange for some changes to be automatically
1446undone at the end of it, either explicit, or via a non-local exit (via
10e2eb10 1447die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1448C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1449Such a construct may be created specially for some important localized
1450task, or an existing one (like boundaries of enclosing Perl
1451subroutine/block, or an existing pair for freeing TMPs) may be
10e2eb10
FC
1452used. (In the second case the overhead of additional localization must
1453be almost negligible.) Note that any XSUB is automatically enclosed in
b687b08b 1454an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1455
1456Inside such a I<pseudo-block> the following service is available:
1457
13a2d996 1458=over 4
d1c897a1
IZ
1459
1460=item C<SAVEINT(int i)>
1461
1462=item C<SAVEIV(IV i)>
1463
1464=item C<SAVEI32(I32 i)>
1465
1466=item C<SAVELONG(long i)>
1467
1468These macros arrange things to restore the value of integer variable
1469C<i> at the end of enclosing I<pseudo-block>.
1470
1471=item C<SAVESPTR(s)>
1472
1473=item C<SAVEPPTR(p)>
1474
1475These macros arrange things to restore the value of pointers C<s> and
10e2eb10 1476C<p>. C<s> must be a pointer of a type which survives conversion to
d1c897a1
IZ
1477C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1478and back.
1479
1480=item C<SAVEFREESV(SV *sv)>
1481
1482The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1483I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1484mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1485extends the lifetime of C<sv> until the beginning of the next statement,
1486C<SAVEFREESV> extends it until the end of the enclosing scope. These
1487lifetimes can be wildly different.
1488
1489Also compare C<SAVEMORTALIZESV>.
1490
1491=item C<SAVEMORTALIZESV(SV *sv)>
1492
1493Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1494scope instead of decrementing its reference count. This usually has the
1495effect of keeping C<sv> alive until the statement that called the currently
1496live scope has finished executing.
d1c897a1
IZ
1497
1498=item C<SAVEFREEOP(OP *op)>
1499
1500The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1501
1502=item C<SAVEFREEPV(p)>
1503
1504The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1505end of I<pseudo-block>.
1506
1507=item C<SAVECLEARSV(SV *sv)>
1508
1509Clears a slot in the current scratchpad which corresponds to C<sv> at
1510the end of I<pseudo-block>.
1511
1512=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1513
10e2eb10 1514The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
d1c897a1
IZ
1515string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1516short-lived storage, the corresponding string may be reallocated like
1517this:
1518
9cde0e7f 1519 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1520
c76ac1ee 1521=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1522
1523At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1524only argument C<p>.
1525
1526=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1527
1528At the end of I<pseudo-block> the function C<f> is called with the
1529implicit context argument (if any), and C<p>.
d1c897a1
IZ
1530
1531=item C<SAVESTACK_POS()>
1532
1533The current offset on the Perl internal stack (cf. C<SP>) is restored
1534at the end of I<pseudo-block>.
1535
1536=back
1537
1538The following API list contains functions, thus one needs to
1539provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1540or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1541function takes C<int *>.
1542
13a2d996 1543=over 4
d1c897a1
IZ
1544
1545=item C<SV* save_scalar(GV *gv)>
1546
1547Equivalent to Perl code C<local $gv>.
1548
1549=item C<AV* save_ary(GV *gv)>
1550
1551=item C<HV* save_hash(GV *gv)>
1552
1553Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1554
1555=item C<void save_item(SV *item)>
1556
1557Duplicates the current value of C<SV>, on the exit from the current
1558C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
10e2eb10 1559using the stored value. It doesn't handle magic. Use C<save_scalar> if
038fcae3 1560magic is affected.
d1c897a1
IZ
1561
1562=item C<void save_list(SV **sarg, I32 maxsarg)>
1563
1564A variant of C<save_item> which takes multiple arguments via an array
1565C<sarg> of C<SV*> of length C<maxsarg>.
1566
1567=item C<SV* save_svref(SV **sptr)>
1568
d1be9408 1569Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1570
1571=item C<void save_aptr(AV **aptr)>
1572
1573=item C<void save_hptr(HV **hptr)>
1574
1575Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1576
1577=back
1578
1579The C<Alias> module implements localization of the basic types within the
1580I<caller's scope>. People who are interested in how to localize things in
1581the containing scope should take a look there too.
1582
0a753a76 1583=head1 Subroutines
a0d0e21e 1584
68dc0745 1585=head2 XSUBs and the Argument Stack
5f05dabc
PP
1586
1587The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1588An XSUB routine will have a stack that contains the arguments from the Perl
1589program, and a way to map from the Perl data structures to a C equivalent.
1590
1591The stack arguments are accessible through the C<ST(n)> macro, which returns
1592the C<n>'th stack argument. Argument 0 is the first argument passed in the
1593Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1594an C<SV*> is used.
1595
1596Most of the time, output from the C routine can be handled through use of
1597the RETVAL and OUTPUT directives. However, there are some cases where the
1598argument stack is not already long enough to handle all the return values.
1599An example is the POSIX tzname() call, which takes no arguments, but returns
1600two, the local time zone's standard and summer time abbreviations.
1601
1602To handle this situation, the PPCODE directive is used and the stack is
1603extended using the macro:
1604
924508f0 1605 EXTEND(SP, num);
5f05dabc 1606
924508f0
GS
1607where C<SP> is the macro that represents the local copy of the stack pointer,
1608and C<num> is the number of elements the stack should be extended by.
5f05dabc 1609
00aadd71 1610Now that there is room on the stack, values can be pushed on it using C<PUSHs>
10e2eb10 1611macro. The pushed values will often need to be "mortal" (See
d82b684c 1612L</Reference Counts and Mortality>):
5f05dabc 1613
00aadd71 1614 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1615 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1616 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1617 PUSHs(sv_2mortal(newSVpv("Some String",0)))
a9b0660e
KW
1618 /* Although the last example is better written as the more
1619 * efficient: */
a3179684 1620 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
5f05dabc
PP
1621
1622And now the Perl program calling C<tzname>, the two values will be assigned
1623as in:
1624
1625 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1626
1627An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1628to use the macro:
5f05dabc 1629
5f05dabc
PP
1630 XPUSHs(SV*)
1631
da8c5729 1632This macro automatically adjusts the stack for you, if needed. Thus, you
5f05dabc 1633do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1634
1635Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1636C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1637For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1638C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1639
1640For more information, consult L<perlxs> and L<perlxstut>.
1641
5b36e945
FC
1642=head2 Autoloading with XSUBs
1643
1644If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1645fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1646of the XSUB's package.
1647
1648But it also puts the same information in certain fields of the XSUB itself:
1649
1650 HV *stash = CvSTASH(cv);
1651 const char *subname = SvPVX(cv);
1652 STRLEN name_length = SvCUR(cv); /* in bytes */
1653 U32 is_utf8 = SvUTF8(cv);
f703fc96 1654
5b36e945 1655C<SvPVX(cv)> contains just the sub name itself, not including the package.
d8893903
FC
1656For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1657C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
5b36e945
FC
1658
1659B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1660XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1661XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1662to support 5.8-5.14, use the XSUB's fields.
1663
5f05dabc 1664=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1665
1666There are four routines that can be used to call a Perl subroutine from
1667within a C program. These four are:
1668
954c1994
GS
1669 I32 call_sv(SV*, I32);
1670 I32 call_pv(const char*, I32);
1671 I32 call_method(const char*, I32);
5aaab254 1672 I32 call_argv(const char*, I32, char**);
a0d0e21e 1673
954c1994 1674The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1675contains either the name of the Perl subroutine to be called, or a
1676reference to the subroutine. The second argument consists of flags
1677that control the context in which the subroutine is called, whether
1678or not the subroutine is being passed arguments, how errors should be
1679trapped, and how to treat return values.
a0d0e21e
LW
1680
1681All four routines return the number of arguments that the subroutine returned
1682on the Perl stack.
1683
9a68f1db 1684These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1685but those names are now deprecated; macros of the same name are provided for
1686compatibility.
1687
1688When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1689must manipulate the Perl stack. These include the following macros and
1690functions:
a0d0e21e
LW
1691
1692 dSP
924508f0 1693 SP
a0d0e21e
LW
1694 PUSHMARK()
1695 PUTBACK
1696 SPAGAIN
1697 ENTER
1698 SAVETMPS
1699 FREETMPS
1700 LEAVE
1701 XPUSH*()
cb1a09d0 1702 POP*()
a0d0e21e 1703
5f05dabc
PP
1704For a detailed description of calling conventions from C to Perl,
1705consult L<perlcall>.
a0d0e21e 1706
8ebc5c01 1707=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1708
1709A lot of opcodes (this is an elementary operation in the internal perl
10e2eb10
FC
1710stack machine) put an SV* on the stack. However, as an optimization
1711the corresponding SV is (usually) not recreated each time. The opcodes
ce3d39e2
IZ
1712reuse specially assigned SVs (I<target>s) which are (as a corollary)
1713not constantly freed/created.
1714
0a753a76 1715Each of the targets is created only once (but see
ce3d39e2
IZ
1716L<Scratchpads and recursion> below), and when an opcode needs to put
1717an integer, a double, or a string on stack, it just sets the
1718corresponding parts of its I<target> and puts the I<target> on stack.
1719
1720The macro to put this target on stack is C<PUSHTARG>, and it is
1721directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1722others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1723
1bd1c0d5 1724Because the target is reused, you must be careful when pushing multiple
10e2eb10 1725values on the stack. The following code will not do what you think:
1bd1c0d5
SC
1726
1727 XPUSHi(10);
1728 XPUSHi(20);
1729
1730This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1731the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1732At the end of the operation, the stack does not contain the values 10
1733and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1734to 20.
1bd1c0d5 1735
d82b684c
SH
1736If you need to push multiple different values then you should either use
1737the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1738none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1739SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1740will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1741this a little easier to achieve by creating a new mortal for you (via
1742C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1743in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1744Thus, instead of writing this to "fix" the example above:
1745
1746 XPUSHs(sv_2mortal(newSViv(10)))
1747 XPUSHs(sv_2mortal(newSViv(20)))
1748
1749you can simply write:
1750
1751 mXPUSHi(10)
1752 mXPUSHi(20)
1753
1754On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1755need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1756macros can make use of the local variable C<TARG>. See also C<dTARGET>
1757and C<dXSTARG>.
1bd1c0d5 1758
8ebc5c01 1759=head2 Scratchpads
ce3d39e2 1760
54310121 1761The question remains on when the SVs which are I<target>s for opcodes
10e2eb10 1762are created. The answer is that they are created when the current
ac036724 1763unit--a subroutine or a file (for opcodes for statements outside of
10e2eb10 1764subroutines)--is compiled. During this time a special anonymous Perl
ac036724 1765array is created, which is called a scratchpad for the current unit.
ce3d39e2 1766
54310121 1767A scratchpad keeps SVs which are lexicals for the current unit and are
d777b41a
FC
1768targets for opcodes. A previous version of this document
1769stated that one can deduce that an SV lives on a scratchpad
ce3d39e2 1770by looking on its flags: lexicals have C<SVs_PADMY> set, and
d777b41a
FC
1771I<target>s have C<SVs_PADTMP> set. But this have never been fully true.
1772C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1773While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
1774that have never resided in a pad, but nonetheless act like I<target>s.
ce3d39e2 1775
10e2eb10 1776The correspondence between OPs and I<target>s is not 1-to-1. Different
54310121 1777OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1778would not conflict with the expected life of the temporary.
1779
2ae324a7 1780=head2 Scratchpads and recursion
ce3d39e2
IZ
1781
1782In fact it is not 100% true that a compiled unit contains a pointer to
10e2eb10
FC
1783the scratchpad AV. In fact it contains a pointer to an AV of
1784(initially) one element, and this element is the scratchpad AV. Why do
ce3d39e2
IZ
1785we need an extra level of indirection?
1786
10e2eb10 1787The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2 1788these can create several execution pointers going into the same
10e2eb10 1789subroutine. For the subroutine-child not write over the temporaries
ce3d39e2
IZ
1790for the subroutine-parent (lifespan of which covers the call to the
1791child), the parent and the child should have different
10e2eb10 1792scratchpads. (I<And> the lexicals should be separate anyway!)
ce3d39e2 1793
5f05dabc
PP
1794So each subroutine is born with an array of scratchpads (of length 1).
1795On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1796depth of the recursion is not more than the length of this array, and
1797if it is, new scratchpad is created and pushed into the array.
1798
1799The I<target>s on this scratchpad are C<undef>s, but they are already
1800marked with correct flags.
1801
22d36020
FC
1802=head1 Memory Allocation
1803
1804=head2 Allocation
1805
1806All memory meant to be used with the Perl API functions should be manipulated
1807using the macros described in this section. The macros provide the necessary
1808transparency between differences in the actual malloc implementation that is
1809used within perl.
1810
1811It is suggested that you enable the version of malloc that is distributed
1812with Perl. It keeps pools of various sizes of unallocated memory in
1813order to satisfy allocation requests more quickly. However, on some
1814platforms, it may cause spurious malloc or free errors.
1815
1816The following three macros are used to initially allocate memory :
1817
1818 Newx(pointer, number, type);
1819 Newxc(pointer, number, type, cast);
1820 Newxz(pointer, number, type);
1821
1822The first argument C<pointer> should be the name of a variable that will
1823point to the newly allocated memory.
1824
1825The second and third arguments C<number> and C<type> specify how many of
1826the specified type of data structure should be allocated. The argument
1827C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1828should be used if the C<pointer> argument is different from the C<type>
1829argument.
1830
1831Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1832to zero out all the newly allocated memory.
1833
1834=head2 Reallocation
1835
1836 Renew(pointer, number, type);
1837 Renewc(pointer, number, type, cast);
1838 Safefree(pointer)
1839
1840These three macros are used to change a memory buffer size or to free a
1841piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1842match those of C<New> and C<Newc> with the exception of not needing the
1843"magic cookie" argument.
1844
1845=head2 Moving
1846
1847 Move(source, dest, number, type);
1848 Copy(source, dest, number, type);
1849 Zero(dest, number, type);
1850
1851These three macros are used to move, copy, or zero out previously allocated
1852memory. The C<source> and C<dest> arguments point to the source and
1853destination starting points. Perl will move, copy, or zero out C<number>
1854instances of the size of the C<type> data structure (using the C<sizeof>
1855function).
1856
1857=head1 PerlIO
1858
1859The most recent development releases of Perl have been experimenting with
1860removing Perl's dependency on the "normal" standard I/O suite and allowing
1861other stdio implementations to be used. This involves creating a new
1862abstraction layer that then calls whichever implementation of stdio Perl
1863was compiled with. All XSUBs should now use the functions in the PerlIO
1864abstraction layer and not make any assumptions about what kind of stdio
1865is being used.
1866
1867For a complete description of the PerlIO abstraction, consult L<perlapio>.
1868
0a753a76
PP
1869=head1 Compiled code
1870
1871=head2 Code tree
1872
1873Here we describe the internal form your code is converted to by
10e2eb10 1874Perl. Start with a simple example:
0a753a76
PP
1875
1876 $a = $b + $c;
1877
1878This is converted to a tree similar to this one:
1879
1880 assign-to
1881 / \
1882 + $a
1883 / \
1884 $b $c
1885
7b8d334a 1886(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1887parsed your code, but has nothing to do with the execution order.
1888There is an additional "thread" going through the nodes of the tree
1889which shows the order of execution of the nodes. In our simplified
1890example above it looks like:
1891
1892 $b ---> $c ---> + ---> $a ---> assign-to
1893
1894But with the actual compile tree for C<$a = $b + $c> it is different:
1895some nodes I<optimized away>. As a corollary, though the actual tree
1896contains more nodes than our simplified example, the execution order
1897is the same as in our example.
1898
1899=head2 Examining the tree
1900
06f6df17
RGS
1901If you have your perl compiled for debugging (usually done with
1902C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1903compiled tree by specifying C<-Dx> on the Perl command line. The
1904output takes several lines per node, and for C<$b+$c> it looks like
1905this:
1906
1907 5 TYPE = add ===> 6
1908 TARG = 1
1909 FLAGS = (SCALAR,KIDS)
1910 {
1911 TYPE = null ===> (4)
1912 (was rv2sv)
1913 FLAGS = (SCALAR,KIDS)
1914 {
1915 3 TYPE = gvsv ===> 4
1916 FLAGS = (SCALAR)
1917 GV = main::b
1918 }
1919 }
1920 {
1921 TYPE = null ===> (5)
1922 (was rv2sv)
1923 FLAGS = (SCALAR,KIDS)
1924 {
1925 4 TYPE = gvsv ===> 5
1926 FLAGS = (SCALAR)
1927 GV = main::c
1928 }
1929 }
1930
1931This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1932not optimized away (one per number in the left column). The immediate
1933children of the given node correspond to C<{}> pairs on the same level
1934of indentation, thus this listing corresponds to the tree:
1935
1936 add
1937 / \
1938 null null
1939 | |
1940 gvsv gvsv
1941
1942The execution order is indicated by C<===E<gt>> marks, thus it is C<3
19434 5 6> (node C<6> is not included into above listing), i.e.,
1944C<gvsv gvsv add whatever>.
1945
9afa14e3 1946Each of these nodes represents an op, a fundamental operation inside the
10e2eb10 1947Perl core. The code which implements each operation can be found in the
9afa14e3 1948F<pp*.c> files; the function which implements the op with type C<gvsv>
10e2eb10 1949is C<pp_gvsv>, and so on. As the tree above shows, different ops have
9afa14e3 1950different numbers of children: C<add> is a binary operator, as one would
10e2eb10 1951expect, and so has two children. To accommodate the various different
9afa14e3
SC
1952numbers of children, there are various types of op data structure, and
1953they link together in different ways.
1954
10e2eb10 1955The simplest type of op structure is C<OP>: this has no children. Unary
9afa14e3 1956operators, C<UNOP>s, have one child, and this is pointed to by the
10e2eb10
FC
1957C<op_first> field. Binary operators (C<BINOP>s) have not only an
1958C<op_first> field but also an C<op_last> field. The most complex type of
1959op is a C<LISTOP>, which has any number of children. In this case, the
9afa14e3 1960first child is pointed to by C<op_first> and the last child by
10e2eb10 1961C<op_last>. The children in between can be found by iteratively
29e61fd9
DM
1962following the C<op_sibling> pointer from the first child to the last 9but
1963see below).
9afa14e3 1964
29e61fd9 1965There are also some other op types: a C<PMOP> holds a regular expression,
10e2eb10
FC
1966and has no children, and a C<LOOP> may or may not have children. If the
1967C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
9afa14e3
SC
1968complicate matters, if a C<UNOP> is actually a C<null> op after
1969optimization (see L</Compile pass 2: context propagation>) it will still
1970have children in accordance with its former type.
1971
29e61fd9
DM
1972Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
1973or more children, but it doesn't have an C<op_last> field: so you have to
1974follow C<op_first> and then the C<op_sibling> chain itself to find the
1975last child. Instead it has an C<op_other> field, which is comparable to
1976the C<op_next> field described below, and represents an alternate
1977execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
1978that in general, C<op_other> may not point to any of the direct children
1979of the C<LOGOP>.
1980
1981Starting in version 5.21.2, perls built with the experimental
1982define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
1983C<op_lastsib>. When set, this indicates that this is the last op in an
1984C<op_sibling> chain. This frees up the C<op_sibling> field on the last
1985sibling to point back to the parent op. The macro C<OP_SIBLING(o)> wraps
1986this special behaviour, and always returns NULL on the last sibling.
1987With this build the C<op_parent(o)> function can be used to find the
1988parent of any op.
1989
06f6df17
RGS
1990Another way to examine the tree is to use a compiler back-end module, such
1991as L<B::Concise>.
1992
0a753a76
PP
1993=head2 Compile pass 1: check routines
1994
8870b5c7 1995The tree is created by the compiler while I<yacc> code feeds it
10e2eb10 1996the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1997the first pass of perl compilation.
1998
1999What makes this pass interesting for perl developers is that some
2000optimization may be performed on this pass. This is optimization by
8870b5c7 2001so-called "check routines". The correspondence between node names
0a753a76
PP
2002and corresponding check routines is described in F<opcode.pl> (do not
2003forget to run C<make regen_headers> if you modify this file).
2004
2005A check routine is called when the node is fully constructed except
7b8d334a 2006for the execution-order thread. Since at this time there are no
0a753a76
PP
2007back-links to the currently constructed node, one can do most any
2008operation to the top-level node, including freeing it and/or creating
2009new nodes above/below it.
2010
2011The check routine returns the node which should be inserted into the
2012tree (if the top-level node was not modified, check routine returns
2013its argument).
2014
10e2eb10 2015By convention, check routines have names C<ck_*>. They are usually
0a753a76
PP
2016called from C<new*OP> subroutines (or C<convert>) (which in turn are
2017called from F<perly.y>).
2018
2019=head2 Compile pass 1a: constant folding
2020
2021Immediately after the check routine is called the returned node is
2022checked for being compile-time executable. If it is (the value is
2023judged to be constant) it is immediately executed, and a I<constant>
2024node with the "return value" of the corresponding subtree is
2025substituted instead. The subtree is deleted.
2026
2027If constant folding was not performed, the execution-order thread is
2028created.
2029
2030=head2 Compile pass 2: context propagation
2031
2032When a context for a part of compile tree is known, it is propagated
a3cb178b 2033down through the tree. At this time the context can have 5 values
0a753a76
PP
2034(instead of 2 for runtime context): void, boolean, scalar, list, and
2035lvalue. In contrast with the pass 1 this pass is processed from top
2036to bottom: a node's context determines the context for its children.
2037
2038Additional context-dependent optimizations are performed at this time.
2039Since at this moment the compile tree contains back-references (via
2040"thread" pointers), nodes cannot be free()d now. To allow
2041optimized-away nodes at this stage, such nodes are null()ified instead
2042of free()ing (i.e. their type is changed to OP_NULL).
2043
2044=head2 Compile pass 3: peephole optimization
2045
2046After the compile tree for a subroutine (or for an C<eval> or a file)
10e2eb10 2047is created, an additional pass over the code is performed. This pass
0a753a76 2048is neither top-down or bottom-up, but in the execution order (with
9ea12537
Z
2049additional complications for conditionals). Optimizations performed
2050at this stage are subject to the same restrictions as in the pass 2.
2051
2052Peephole optimizations are done by calling the function pointed to
2053by the global variable C<PL_peepp>. By default, C<PL_peepp> just
2054calls the function pointed to by the global variable C<PL_rpeepp>.
2055By default, that performs some basic op fixups and optimisations along
2056the execution-order op chain, and recursively calls C<PL_rpeepp> for
2057each side chain of ops (resulting from conditionals). Extensions may
2058provide additional optimisations or fixups, hooking into either the
2059per-subroutine or recursive stage, like this:
2060
2061 static peep_t prev_peepp;
2062 static void my_peep(pTHX_ OP *o)
2063 {
2064 /* custom per-subroutine optimisation goes here */
f0358462 2065 prev_peepp(aTHX_ o);
9ea12537
Z
2066 /* custom per-subroutine optimisation may also go here */
2067 }
2068 BOOT:
2069 prev_peepp = PL_peepp;
2070 PL_peepp = my_peep;
2071
2072 static peep_t prev_rpeepp;
2073 static void my_rpeep(pTHX_ OP *o)
2074 {
2075 OP *orig_o = o;
2076 for(; o; o = o->op_next) {
2077 /* custom per-op optimisation goes here */
2078 }
f0358462 2079 prev_rpeepp(aTHX_ orig_o);
9ea12537
Z
2080 }
2081 BOOT:
2082 prev_rpeepp = PL_rpeepp;
2083 PL_rpeepp = my_rpeep;
0a753a76 2084
1ba7f851
PJ
2085=head2 Pluggable runops
2086
2087The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
2088functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2089with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2090control over the execution of the compile tree it is possible to provide
2091your own runops function.
1ba7f851
PJ
2092
2093It's probably best to copy one of the existing runops functions and
2094change it to suit your needs. Then, in the BOOT section of your XS
2095file, add the line:
2096
2097 PL_runops = my_runops;
2098
2099This function should be as efficient as possible to keep your programs
2100running as fast as possible.
2101
fd85fad2
BM
2102=head2 Compile-time scope hooks
2103
2104As of perl 5.14 it is possible to hook into the compile-time lexical
10e2eb10 2105scope mechanism using C<Perl_blockhook_register>. This is used like
fd85fad2
BM
2106this:
2107
2108 STATIC void my_start_hook(pTHX_ int full);
2109 STATIC BHK my_hooks;
2110
2111 BOOT:
a88d97bf 2112 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
fd85fad2
BM
2113 Perl_blockhook_register(aTHX_ &my_hooks);
2114
2115This will arrange to have C<my_start_hook> called at the start of
10e2eb10 2116compiling every lexical scope. The available hooks are:
fd85fad2
BM
2117
2118=over 4
2119
a88d97bf 2120=item C<void bhk_start(pTHX_ int full)>
fd85fad2 2121
10e2eb10 2122This is called just after starting a new lexical scope. Note that Perl
fd85fad2
BM
2123code like
2124
2125 if ($x) { ... }
2126
2127creates two scopes: the first starts at the C<(> and has C<full == 1>,
10e2eb10
FC
2128the second starts at the C<{> and has C<full == 0>. Both end at the
2129C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
fd85fad2
BM
2130pushed onto the save stack by this hook will be popped just before the
2131scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2132
a88d97bf 2133=item C<void bhk_pre_end(pTHX_ OP **o)>
fd85fad2
BM
2134
2135This is called at the end of a lexical scope, just before unwinding the
10e2eb10 2136stack. I<o> is the root of the optree representing the scope; it is a
fd85fad2
BM
2137double pointer so you can replace the OP if you need to.
2138
a88d97bf 2139=item C<void bhk_post_end(pTHX_ OP **o)>
fd85fad2
BM
2140
2141This is called at the end of a lexical scope, just after unwinding the
10e2eb10 2142stack. I<o> is as above. Note that it is possible for calls to C<pre_>
fd85fad2
BM
2143and C<post_end> to nest, if there is something on the save stack that
2144calls string eval.
2145
a88d97bf 2146=item C<void bhk_eval(pTHX_ OP *const o)>
fd85fad2
BM
2147
2148This is called just before starting to compile an C<eval STRING>, C<do
10e2eb10 2149FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
fd85fad2
BM
2150OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2151C<OP_DOFILE> or C<OP_REQUIRE>.
2152
2153=back
2154
2155Once you have your hook functions, you need a C<BHK> structure to put
10e2eb10
FC
2156them in. It's best to allocate it statically, since there is no way to
2157free it once it's registered. The function pointers should be inserted
fd85fad2 2158into this structure using the C<BhkENTRY_set> macro, which will also set
10e2eb10 2159flags indicating which entries are valid. If you do need to allocate
fd85fad2
BM
2160your C<BHK> dynamically for some reason, be sure to zero it before you
2161start.
2162
2163Once registered, there is no mechanism to switch these hooks off, so if
10e2eb10 2164that is necessary you will need to do this yourself. An entry in C<%^H>
a3e07c87
BM
2165is probably the best way, so the effect is lexically scoped; however it
2166is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
10e2eb10 2167temporarily switch entries on and off. You should also be aware that
a3e07c87
BM
2168generally speaking at least one scope will have opened before your
2169extension is loaded, so you will see some C<pre/post_end> pairs that
2170didn't have a matching C<start>.
fd85fad2 2171
9afa14e3
SC
2172=head1 Examining internal data structures with the C<dump> functions
2173
2174To aid debugging, the source file F<dump.c> contains a number of
2175functions which produce formatted output of internal data structures.
2176
2177The most commonly used of these functions is C<Perl_sv_dump>; it's used
10e2eb10 2178for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
9afa14e3 2179C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 2180module should already be familiar with its format.
9afa14e3
SC
2181
2182C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 2183derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
2184C<Perl_dump_eval> will dump the main root of the code being evaluated,
2185exactly like C<-Dx>.
2186
2187Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2188op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2189subroutines in a package like so: (Thankfully, these are all xsubs, so
2190there is no op tree)
2191
2192 (gdb) print Perl_dump_packsubs(PL_defstash)
2193
2194 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2195
2196 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2197
2198 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2199
2200 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2201
2202 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2203
2204and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2205the op tree of the main root.
2206
954c1994 2207=head1 How multiple interpreters and concurrency are supported
ee072b34 2208
ee072b34
GS
2209=head2 Background and PERL_IMPLICIT_CONTEXT
2210
2211The Perl interpreter can be regarded as a closed box: it has an API
2212for feeding it code or otherwise making it do things, but it also has
2213functions for its own use. This smells a lot like an object, and
2214there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
2215interpreters, with one interpreter represented either as a C structure,
2216or inside a thread-specific structure. These structures contain all
2217the context, the state of that interpreter.
2218
10e2eb10 2219One macro controls the major Perl build flavor: MULTIPLICITY. The
7b52221d 2220MULTIPLICITY build has a C structure that packages all the interpreter
10e2eb10 2221state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
7b52221d 2222normally defined, and enables the support for passing in a "hidden" first
10e2eb10 2223argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 2224multi-threaded perls possible (with the ithreads threading model, related
7b52221d 2225to the macro USE_ITHREADS.)
54aff467 2226
27da23d5
JH
2227Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2228PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2229former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2230internal variables of Perl to be wrapped inside a single global struct,
2231struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2232the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2233one step further, there is still a single struct (allocated in main()
2234either from heap or from stack) but there are no global data symbols
3bf17896 2235pointing to it. In either case the global struct should be initialized
27da23d5
JH
2236as the very first thing in main() using Perl_init_global_struct() and
2237correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2238please see F<miniperlmain.c> for usage details. You may also need
2239to use C<dVAR> in your coding to "declare the global variables"
2240when you are using them. dTHX does this for you automatically.
2241
9aa97215
JH
2242To see whether you have non-const data you can use a BSD (or GNU)
2243compatible C<nm>:
bc028b6b
JH
2244
2245 nm libperl.a | grep -v ' [TURtr] '
2246
9aa97215
JH
2247If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
2248you have non-const data. The symbols the C<grep> removed are as follows:
2249C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
2250and the C<U> is <undefined>, external symbols referred to.
2251
2252The test F<t/porting/libperl.t> does this kind of symbol sanity
2253checking on C<libperl.a>.
bc028b6b 2254
27da23d5
JH
2255For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2256doesn't actually hide all symbols inside a big global struct: some
2257PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2258then hides everything (see how the PERLIO_FUNCS_DECL is used).
2259
54aff467 2260All this obviously requires a way for the Perl internal functions to be
acfe0abc 2261either subroutines taking some kind of structure as the first
ee072b34 2262argument, or subroutines taking nothing as the first argument. To
acfe0abc 2263enable these two very different ways of building the interpreter,
ee072b34
GS
2264the Perl source (as it does in so many other situations) makes heavy
2265use of macros and subroutine naming conventions.
2266
54aff467 2267First problem: deciding which functions will be public API functions and
00aadd71 2268which will be private. All functions whose names begin C<S_> are private
954c1994
GS
2269(think "S" for "secret" or "static"). All other functions begin with
2270"Perl_", but just because a function begins with "Perl_" does not mean it is
10e2eb10
FC
2271part of the API. (See L</Internal
2272Functions>.) The easiest way to be B<sure> a
00aadd71
NIS
2273function is part of the API is to find its entry in L<perlapi>.
2274If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2275think it should be (i.e., you need it for your extension), send mail via
a422fd2d 2276L<perlbug> explaining why you think it should be.
ee072b34
GS
2277
2278Second problem: there must be a syntax so that the same subroutine
2279declarations and calls can pass a structure as their first argument,
2280or pass nothing. To solve this, the subroutines are named and
2281declared in a particular way. Here's a typical start of a static
2282function used within the Perl guts:
2283
2284 STATIC void
2285 S_incline(pTHX_ char *s)
2286
acfe0abc 2287STATIC becomes "static" in C, and may be #define'd to nothing in some
da8c5729 2288configurations in the future.
ee072b34 2289
651a3225
GS
2290A public function (i.e. part of the internal API, but not necessarily
2291sanctioned for use in extensions) begins like this:
ee072b34
GS
2292
2293 void
2307c6d0 2294 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 2295
0147cd53 2296C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
2297details of the interpreter's context. THX stands for "thread", "this",
2298or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2299The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
2300or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2301their variants.
ee072b34 2302
a7486cbb
JH
2303When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2304first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
2305in the pTHX_ macro indicates that the macro expansion needs a comma
2306after the context argument because other arguments follow it. If
2307PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
2308subroutine is not prototyped to take the extra argument. The form of the
2309macro without the trailing underscore is used when there are no additional
ee072b34
GS
2310explicit arguments.
2311
54aff467 2312When a core function calls another, it must pass the context. This
2307c6d0 2313is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
2314something like this:
2315
2307c6d0
SB
2316 #ifdef PERL_IMPLICIT_CONTEXT
2317 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 2318 /* can't do this for vararg functions, see below */
2307c6d0
SB
2319 #else
2320 #define sv_setiv Perl_sv_setiv
2321 #endif
ee072b34
GS
2322
2323This works well, and means that XS authors can gleefully write:
2324
2307c6d0 2325 sv_setiv(foo, bar);
ee072b34
GS
2326
2327and still have it work under all the modes Perl could have been
2328compiled with.
2329
ee072b34
GS
2330This doesn't work so cleanly for varargs functions, though, as macros
2331imply that the number of arguments is known in advance. Instead we
2332either need to spell them out fully, passing C<aTHX_> as the first
2333argument (the Perl core tends to do this with functions like
2334Perl_warner), or use a context-free version.
2335
2336The context-free version of Perl_warner is called
2337Perl_warner_nocontext, and does not take the extra argument. Instead
2338it does dTHX; to get the context from thread-local storage. We
2339C<#define warner Perl_warner_nocontext> so that extensions get source
2340compatibility at the expense of performance. (Passing an arg is
2341cheaper than grabbing it from thread-local storage.)
2342
acfe0abc 2343You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2344Those are strictly for use within the core. Extensions and embedders
2345need only be aware of [pad]THX.
2346
a7486cbb
JH
2347=head2 So what happened to dTHR?
2348
2349C<dTHR> was introduced in perl 5.005 to support the older thread model.
2350The older thread model now uses the C<THX> mechanism to pass context
2351pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2352later still have it for backward source compatibility, but it is defined
2353to be a no-op.
2354
ee072b34
GS
2355=head2 How do I use all this in extensions?
2356
2357When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2358any functions in the Perl API will need to pass the initial context
2359argument somehow. The kicker is that you will need to write it in
2360such a way that the extension still compiles when Perl hasn't been
2361built with PERL_IMPLICIT_CONTEXT enabled.
2362
2363There are three ways to do this. First, the easy but inefficient way,
2364which is also the default, in order to maintain source compatibility
0147cd53 2365with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2366and aTHX_ macros to call a function that will return the context.
2367Thus, something like:
2368
2307c6d0 2369 sv_setiv(sv, num);
ee072b34 2370
4375e838 2371in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2372in effect:
ee072b34 2373
2307c6d0 2374 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2375
54aff467 2376or to this otherwise:
ee072b34 2377
2307c6d0 2378 Perl_sv_setiv(sv, num);
ee072b34 2379
da8c5729 2380You don't have to do anything new in your extension to get this; since
2fa86c13 2381the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2382work.
2383
2384The second, more efficient way is to use the following template for
2385your Foo.xs:
2386
c52f9dcd
JH
2387 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2388 #include "EXTERN.h"
2389 #include "perl.h"
2390 #include "XSUB.h"
ee072b34 2391
fd061412 2392 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2393
fd061412 2394 STATIC void
c52f9dcd
JH
2395 my_private_function(int arg1, int arg2)
2396 {
2397 dTHX; /* fetch context */
2398 ... call many Perl API functions ...
2399 }
ee072b34
GS
2400
2401 [... etc ...]
2402
c52f9dcd 2403 MODULE = Foo PACKAGE = Foo
ee072b34 2404
c52f9dcd 2405 /* typical XSUB */
ee072b34 2406
c52f9dcd
JH
2407 void
2408 my_xsub(arg)
2409 int arg
2410 CODE:
2411 my_private_function(arg, 10);
ee072b34
GS
2412
2413Note that the only two changes from the normal way of writing an
2414extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2415including the Perl headers, followed by a C<dTHX;> declaration at
2416the start of every function that will call the Perl API. (You'll
2417know which functions need this, because the C compiler will complain
2418that there's an undeclared identifier in those functions.) No changes
2419are needed for the XSUBs themselves, because the XS() macro is
2420correctly defined to pass in the implicit context if needed.
2421
2422The third, even more efficient way is to ape how it is done within
2423the Perl guts:
2424
2425
c52f9dcd
JH
2426 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2427 #include "EXTERN.h"
2428 #include "perl.h"
2429 #include "XSUB.h"
ee072b34
GS
2430
2431 /* pTHX_ only needed for functions that call Perl API */
fd061412 2432 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2433
fd061412 2434 STATIC void
c52f9dcd
JH
2435 my_private_function(pTHX_ int arg1, int arg2)
2436 {
2437 /* dTHX; not needed here, because THX is an argument */
2438 ... call Perl API functions ...
2439 }
ee072b34
GS
2440
2441 [... etc ...]
2442
c52f9dcd 2443 MODULE = Foo PACKAGE = Foo
ee072b34 2444
c52f9dcd 2445 /* typical XSUB */
ee072b34 2446
c52f9dcd
JH
2447 void
2448 my_xsub(arg)
2449 int arg
2450 CODE:
2451 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2452
2453This implementation never has to fetch the context using a function
2454call, since it is always passed as an extra argument. Depending on
2455your needs for simplicity or efficiency, you may mix the previous
2456two approaches freely.
2457
651a3225
GS
2458Never add a comma after C<pTHX> yourself--always use the form of the
2459macro with the underscore for functions that take explicit arguments,
2460or the form without the argument for functions with no explicit arguments.
ee072b34 2461
27da23d5
JH
2462If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2463definition is needed if the Perl global variables (see F<perlvars.h>
2464or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2465used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2466the need for C<dVAR> only with the said compile-time define, because
2467otherwise the Perl global variables are visible as-is.
2468
a7486cbb
JH
2469=head2 Should I do anything special if I call perl from multiple threads?
2470
2471If you create interpreters in one thread and then proceed to call them in
2472another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2473initialized correctly in each of those threads.
2474
2475The C<perl_alloc> and C<perl_clone> API functions will automatically set
2476the TLS slot to the interpreter they created, so that there is no need to do
2477anything special if the interpreter is always accessed in the same thread that
2478created it, and that thread did not create or call any other interpreters
2479afterwards. If that is not the case, you have to set the TLS slot of the
2480thread before calling any functions in the Perl API on that particular
2481interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2482thread as the first thing you do:
2483
2484 /* do this before doing anything else with some_perl */
2485 PERL_SET_CONTEXT(some_perl);
2486
2487 ... other Perl API calls on some_perl go here ...
2488
ee072b34
GS
2489=head2 Future Plans and PERL_IMPLICIT_SYS
2490
2491Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2492that the interpreter knows about itself and pass it around, so too are
2493there plans to allow the interpreter to bundle up everything it knows
2494about the environment it's running on. This is enabled with the
7b52221d
RGS
2495PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2496Windows.
ee072b34
GS
2497
2498This allows the ability to provide an extra pointer (called the "host"
2499environment) for all the system calls. This makes it possible for
2500all the system stuff to maintain their own state, broken down into
2501seven C structures. These are thin wrappers around the usual system
0147cd53 2502calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2503more ambitious host (like the one that would do fork() emulation) all
2504the extra work needed to pretend that different interpreters are
2505actually different "processes", would be done here.
2506
2507The Perl engine/interpreter and the host are orthogonal entities.
2508There could be one or more interpreters in a process, and one or
2509more "hosts", with free association between them.
2510
a422fd2d
SC
2511=head1 Internal Functions
2512
2513All of Perl's internal functions which will be exposed to the outside
06f6df17 2514world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d 2515functions or functions used in a program in which Perl is embedded.
10e2eb10 2516Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2517static functions start with C<S_>.)
a422fd2d 2518
0972ecdf
DM
2519Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2520either with or without the C<Perl_> prefix, thanks to a bunch of defines
10e2eb10 2521that live in F<embed.h>. Note that extension code should I<not> set
0972ecdf
DM
2522C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2523breakage of the XS in each new perl release.
2524
2525The file F<embed.h> is generated automatically from
10e2eb10 2526F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
dc9b1d22 2527header files for the internal functions, generates the documentation
10e2eb10 2528and a lot of other bits and pieces. It's important that when you add
dc9b1d22 2529a new function to the core or change an existing one, you change the
10e2eb10 2530data in the table in F<embed.fnc> as well. Here's a sample entry from
dc9b1d22 2531that table:
a422fd2d
SC
2532
2533 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2534
10e2eb10
FC
2535The second column is the return type, the third column the name. Columns
2536after that are the arguments. The first column is a set of flags:
a422fd2d
SC
2537
2538=over 3
2539
2540=item A
2541
10e2eb10
FC
2542This function is a part of the public
2543API. All such functions should also
1aa6ea50 2544have 'd', very few do not.
a422fd2d
SC
2545
2546=item p
2547
1aa6ea50
JC
2548This function has a C<Perl_> prefix; i.e. it is defined as
2549C<Perl_av_fetch>.
a422fd2d
SC
2550
2551=item d
2552
2553This function has documentation using the C<apidoc> feature which we'll
1aa6ea50 2554look at in a second. Some functions have 'd' but not 'A'; docs are good.
a422fd2d
SC
2555
2556=back
2557
2558Other available flags are:
2559
2560=over 3
2561
2562=item s
2563
1aa6ea50
JC
2564This is a static function and is defined as C<STATIC S_whatever>, and
2565usually called within the sources as C<whatever(...)>.
a422fd2d
SC
2566
2567=item n
2568
da8c5729 2569This does not need an interpreter context, so the definition has no
1aa6ea50 2570C<pTHX>, and it follows that callers don't use C<aTHX>. (See
d3a43cd8 2571L</Background and PERL_IMPLICIT_CONTEXT>.)
a422fd2d
SC
2572
2573=item r
2574
2575This function never returns; C<croak>, C<exit> and friends.
2576
2577=item f
2578
2579This function takes a variable number of arguments, C<printf> style.
2580The argument list should end with C<...>, like this:
2581
2582 Afprd |void |croak |const char* pat|...
2583
a7486cbb 2584=item M
a422fd2d 2585
00aadd71 2586This function is part of the experimental development API, and may change
a422fd2d
SC
2587or disappear without notice.
2588
2589=item o
2590
2591This function should not have a compatibility macro to define, say,
10e2eb10 2592C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
a422fd2d 2593
a422fd2d
SC
2594=item x
2595
2596This function isn't exported out of the Perl core.
2597
dc9b1d22
MHM
2598=item m
2599
2600This is implemented as a macro.
2601
2602=item X
2603
2604This function is explicitly exported.
2605
2606=item E
2607
2608This function is visible to extensions included in the Perl core.
2609
2610=item b
2611
2612Binary backward compatibility; this function is a macro but also has
2613a C<Perl_> implementation (which is exported).
2614
1aa6ea50
JC
2615=item others
2616
2617See the comments at the top of C<embed.fnc> for others.
2618
a422fd2d
SC
2619=back
2620
dc9b1d22
MHM
2621If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2622C<make regen_headers> to force a rebuild of F<embed.h> and other
2623auto-generated files.
a422fd2d 2624
6b4667fc 2625=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2626
6b4667fc
A
2627If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2628formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2629following macros for portability
9dd9db0b 2630
c52f9dcd
JH
2631 IVdf IV in decimal
2632 UVuf UV in decimal
2633 UVof UV in octal
2634 UVxf UV in hexadecimal
2635 NVef NV %e-like
2636 NVff NV %f-like
2637 NVgf NV %g-like
9dd9db0b 2638
6b4667fc
A
2639These will take care of 64-bit integers and long doubles.
2640For example:
2641
c52f9dcd 2642 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2643
2644The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2645
aacf4ea2
JH
2646Note that there are different "long doubles": Perl will use
2647whatever the compiler has.
2648
8908e76d
JH
2649If you are printing addresses of pointers, use UVxf combined
2650with PTR2UV(), do not use %lx or %p.
2651
2652=head2 Pointer-To-Integer and Integer-To-Pointer
2653
2654Because pointer size does not necessarily equal integer size,
2655use the follow macros to do it right.
2656
c52f9dcd
JH
2657 PTR2UV(pointer)
2658 PTR2IV(pointer)
2659 PTR2NV(pointer)
2660 INT2PTR(pointertotype, integer)
8908e76d
JH
2661
2662For example:
2663
c52f9dcd
JH
2664 IV iv = ...;
2665 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2666
2667and
2668
c52f9dcd
JH
2669 AV *av = ...;
2670 UV uv = PTR2UV(av);
8908e76d 2671
0ca3a874
MHM
2672=head2 Exception Handling
2673
9b5c3821 2674There are a couple of macros to do very basic exception handling in XS
10e2eb10 2675modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
9b5c3821
MHM
2676be able to use these macros:
2677
2678 #define NO_XSLOCKS
2679 #include "XSUB.h"
2680
2681You can use these macros if you call code that may croak, but you need
10e2eb10 2682to do some cleanup before giving control back to Perl. For example:
0ca3a874 2683
d7f8936a 2684 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2685
2686 XCPT_TRY_START {
2687 code_that_may_croak();
2688 } XCPT_TRY_END
2689
2690 XCPT_CATCH
2691 {
2692 /* do cleanup here */
2693 XCPT_RETHROW;
2694 }
2695
2696Note that you always have to rethrow an exception that has been
10e2eb10
FC
2697caught. Using these macros, it is not possible to just catch the
2698exception and ignore it. If you have to ignore the exception, you
0ca3a874
MHM
2699have to use the C<call_*> function.
2700
2701The advantage of using the above macros is that you don't have
2702to setup an extra function for C<call_*>, and that using these
2703macros is faster than using C<call_*>.
2704
a422fd2d
SC
2705=head2 Source Documentation
2706
2707There's an effort going on to document the internal functions and
2708automatically produce reference manuals from them - L<perlapi> is one
2709such manual which details all the functions which are available to XS
10e2eb10 2710writers. L<perlintern> is the autogenerated manual for the functions
a422fd2d
SC
2711which are not part of the API and are supposedly for internal use only.
2712
2713Source documentation is created by putting POD comments into the C
2714source, like this:
2715
2716 /*
2717 =for apidoc sv_setiv
2718
2719 Copies an integer into the given SV. Does not handle 'set' magic. See
2720 C<sv_setiv_mg>.
2721
2722 =cut
2723 */
2724
2725Please try and supply some documentation if you add functions to the
2726Perl core.
2727
0d098d33
MHM
2728=head2 Backwards compatibility
2729
10e2eb10
FC
2730The Perl API changes over time. New functions are
2731added or the interfaces of existing functions are
2732changed. The C<Devel::PPPort> module tries to
0d098d33
MHM
2733provide compatibility code for some of these changes, so XS writers don't
2734have to code it themselves when supporting multiple versions of Perl.
2735
2736C<Devel::PPPort> generates a C header file F<ppport.h> that can also
10e2eb10 2737be run as a Perl script. To generate F<ppport.h>, run:
0d098d33
MHM
2738
2739 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2740
2741Besides checking existing XS code, the script can also be used to retrieve
2742compatibility information for various API calls using the C<--api-info>
10e2eb10 2743command line switch. For example:
0d098d33
MHM
2744
2745 % perl ppport.h --api-info=sv_magicext
2746
2747For details, see C<perldoc ppport.h>.
2748
a422fd2d
SC
2749=head1 Unicode Support
2750
10e2eb10 2751Perl 5.6.0 introduced Unicode support. It's important for porters and XS
a422fd2d
SC
2752writers to understand this support and make sure that the code they
2753write does not corrupt Unicode data.
2754
2755=head2 What B<is> Unicode, anyway?
2756
10e2eb10
FC
2757In the olden, less enlightened times, we all used to use ASCII. Most of
2758us did, anyway. The big problem with ASCII is that it's American. Well,
a422fd2d 2759no, that's not actually the problem; the problem is that it's not
10e2eb10 2760particularly useful for people who don't use the Roman alphabet. What
a422fd2d 2761used to happen was that particular languages would stick their own
10e2eb10 2762alphabet in the upper range of the sequence, between 128 and 255. Of
a422fd2d
SC
2763course, we then ended up with plenty of variants that weren't quite
2764ASCII, and the whole point of it being a standard was lost.
2765
2766Worse still, if you've got a language like Chinese or
2767Japanese that has hundreds or thousands of characters, then you really
2768can't fit them into a mere 256, so they had to forget about ASCII
2769altogether, and build their own systems using pairs of numbers to refer
2770to one character.
2771
2772To fix this, some people formed Unicode, Inc. and
2773produced a new character set containing all the characters you can
10e2eb10
FC
2774possibly think of and more. There are several ways of representing these
2775characters, and the one Perl uses is called UTF-8. UTF-8 uses
2776a variable number of bytes to represent a character. You can learn more
2575c402 2777about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2778
1e54db1a 2779=head2 How can I recognise a UTF-8 string?
a422fd2d 2780
10e2eb10
FC
2781You can't. This is because UTF-8 data is stored in bytes just like
2782non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d 2783capital E with a grave accent, is represented by the two bytes
10e2eb10
FC
2784C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2785has that byte sequence as well. So you can't tell just by looking - this
a422fd2d
SC
2786is what makes Unicode input an interesting problem.
2787
2575c402
JW
2788In general, you either have to know what you're dealing with, or you
2789have to guess. The API function C<is_utf8_string> can help; it'll tell
10e2eb10
FC
2790you if a string contains only valid UTF-8 characters. However, it can't
2791do the work for you. On a character-by-character basis,
6302f837 2792C<isUTF8_CHAR>
2575c402 2793will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2794
1e54db1a 2795=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2796
1e54db1a 2797As mentioned above, UTF-8 uses a variable number of bytes to store a
10e2eb10
FC
2798character. Characters with values 0...127 are stored in one
2799byte, just like good ol' ASCII. Character 128 is stored as
2800C<v194.128>; this continues up to character 191, which is
2801C<v194.191>. Now we've run out of bits (191 is binary
2802C<10111111>) so we move on; 192 is C<v195.128>. And
a422fd2d
SC
2803so it goes on, moving to three bytes at character 2048.
2804
1e54db1a 2805Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2806how long the first character in it is with the C<UTF8SKIP> macro:
2807
2808 char *utf = "\305\233\340\240\201";
2809 I32 len;
2810
2811 len = UTF8SKIP(utf); /* len is 2 here */
2812 utf += len;
2813 len = UTF8SKIP(utf); /* len is 3 here */
2814
1e54db1a 2815Another way to skip over characters in a UTF-8 string is to use
a422fd2d 2816C<utf8_hop>, which takes a string and a number of characters to skip
10e2eb10 2817over. You're on your own about bounds checking, though, so don't use it
a422fd2d
SC
2818lightly.
2819
1e54db1a 2820All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2821so you can test if you need to do something special with this
2822character like this (the UTF8_IS_INVARIANT() is a macro that tests
9f98c7fe 2823whether the byte is encoded as a single byte even in UTF-8):
a422fd2d 2824
3a2263fe 2825 U8 *utf;
4b88fb76 2826 U8 *utf_end; /* 1 beyond buffer pointed to by utf */
3a2263fe 2827 UV uv; /* Note: a UV, not a U8, not a char */
95701e00 2828 STRLEN len; /* length of character in bytes */
a422fd2d 2829
3a2263fe 2830 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2831 /* Must treat this as UTF-8 */
4b88fb76 2832 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
a422fd2d
SC
2833 else
2834 /* OK to treat this character as a byte */
2835 uv = *utf;
2836
4b88fb76 2837You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
95701e00 2838value of the character; the inverse function C<uvchr_to_utf8> is available
1e54db1a 2839for putting a UV into UTF-8:
a422fd2d 2840
3a2263fe 2841 if (!UTF8_IS_INVARIANT(uv))
a422fd2d 2842 /* Must treat this as UTF8 */
95701e00 2843 utf8 = uvchr_to_utf8(utf8, uv);
a422fd2d
SC
2844 else
2845 /* OK to treat this character as a byte */
2846 *utf8++ = uv;
2847
2848You B<must> convert characters to UVs using the above functions if
1e54db1a 2849you're ever in a situation where you have to match UTF-8 and non-UTF-8
10e2eb10 2850characters. You may not skip over UTF-8 characters in this case. If you
1e54db1a
JH
2851do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2852for instance, if your UTF-8 string contains C<v196.172>, and you skip
2853that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2854So don't do that!
2855
1e54db1a 2856=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2857
2858Currently, Perl deals with Unicode strings and non-Unicode strings
10e2eb10
FC
2859slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2860string is internally encoded as UTF-8. Without it, the byte value is the
2575c402 2861codepoint number and vice versa (in other words, the string is encoded
e1b711da 2862as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
c31cc9fc
FC
2863semantics). This flag is only meaningful if the SV is C<SvPOK>
2864or immediately after stringification via C<SvPV> or a similar
2865macro. You can check and manipulate this flag with the
2575c402 2866following macros:
a422fd2d
SC
2867
2868 SvUTF8(sv)
2869 SvUTF8_on(sv)
2870 SvUTF8_off(sv)
2871
2872This flag has an important effect on Perl's treatment of the string: if
2873Unicode data is not properly distinguished, regular expressions,
2874C<length>, C<substr> and other string handling operations will have
2875undesirable results.
2876
2877The problem comes when you have, for instance, a string that isn't
2575c402 2878flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
1e54db1a 2879especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2880
2881Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2882need be sure you don't accidentally knock it off while you're
10e2eb10 2883manipulating SVs. More specifically, you cannot expect to do this:
a422fd2d
SC
2884
2885 SV *sv;
2886 SV *nsv;
2887 STRLEN len;
2888 char *p;
2889
2890 p = SvPV(sv, len);
2891 frobnicate(p);
2892 nsv = newSVpvn(p, len);
2893
2894The C<char*> string does not tell you the whole story, and you can't
10e2eb10 2895copy or reconstruct an SV just by copying the string value. Check if the
c31cc9fc
FC
2896old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2897accordingly:
a422fd2d
SC
2898
2899 p = SvPV(sv, len);
2900 frobnicate(p);
2901 nsv = newSVpvn(p, len);
2902 if (SvUTF8(sv))
2903 SvUTF8_on(nsv);
2904
2905In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2906not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2907appropriately.
2908
3a2263fe 2909Since just passing an SV to an XS function and copying the data of
2575c402 2910the SV is not enough to copy the UTF8 flags, even less right is just
3a2263fe
RGS
2911passing a C<char *> to an XS function.
2912
1e54db1a 2913=head2 How do I convert a string to UTF-8?
a422fd2d 2914
2575c402 2915If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
10e2eb10 2916one of the strings to UTF-8. If you've got an SV, the easiest way to do
2575c402 2917this is:
a422fd2d
SC
2918
2919 sv_utf8_upgrade(sv);
2920
2921However, you must not do this, for example:
2922
2923 if (!SvUTF8(left))
2924 sv_utf8_upgrade(left);
2925
2926If you do this in a binary operator, you will actually change one of the
b1866b2d 2927strings that came into the operator, and, while it shouldn't be noticeable
2575c402 2928by the end user, it can cause problems in deficient code.
a422fd2d 2929
1e54db1a 2930Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
10e2eb10
FC
2931string argument. This is useful for having the data available for
2932comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2933C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2934the string contains any characters above 255 that can't be represented
2935in a single byte.
2936
2937=head2 Is there anything else I need to know?
2938
10e2eb10 2939Not really. Just remember these things:
a422fd2d
SC
2940
2941=over 3
2942
2943=item *
2944
10e2eb10 2945There's no way to tell if a string is UTF-8 or not. You can tell if an SV
c31cc9fc
FC
2946is UTF-8 by looking at its C<SvUTF8> flag after stringifying it
2947with C<SvPV> or a similar macro. Don't forget to set the flag if
10e2eb10 2948something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2949it's not - if you pass on the PV to somewhere, pass on the flag too.
2950
2951=item *
2952
4b88fb76 2953If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
3a2263fe 2954unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2955
2956=item *
2957
1e54db1a 2958When writing a character C<uv> to a UTF-8 string, B<always> use
95701e00 2959C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
3a2263fe 2960you can use C<*s = uv>.
a422fd2d
SC
2961
2962=item *
2963
10e2eb10
FC
2964Mixing UTF-8 and non-UTF-8 strings is
2965tricky. Use C<bytes_to_utf8> to get
2bbc8d55 2966a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
2967
2968=back
2969
53e06cf0
SC
2970=head1 Custom Operators
2971
2a0fd0f1 2972Custom operator support is an experimental feature that allows you to
10e2eb10 2973define your own ops. This is primarily to allow the building of
53e06cf0
SC
2974interpreters for other languages in the Perl core, but it also allows
2975optimizations through the creation of "macro-ops" (ops which perform the
2976functions of multiple ops which are usually executed together, such as
1aa6ea50 2977C<gvsv, gvsv, add>.)
53e06cf0 2978
10e2eb10 2979This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0 2980core does not "know" anything special about this op type, and so it will
10e2eb10 2981not be involved in any optimizations. This also means that you can
53e06cf0
SC
2982define your custom ops to be any op structure - unary, binary, list and
2983so on - you like.
2984
10e2eb10
FC
2985It's important to know what custom operators won't do for you. They
2986won't let you add new syntax to Perl, directly. They won't even let you
2987add new keywords, directly. In fact, they won't change the way Perl
2988compiles a program at all. You have to do those changes yourself, after
2989Perl has compiled the program. You do this either by manipulating the op
53e06cf0
SC
2990tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2991a custom peephole optimizer with the C<optimize> module.
2992
2993When you do this, you replace ordinary Perl ops with custom ops by
407f86e1 2994creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
10e2eb10
FC
2995PP function. This should be defined in XS code, and should look like
2996the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
53e06cf0
SC
2997takes the appropriate number of values from the stack, and you are
2998responsible for adding stack marks if necessary.
2999
3000You should also "register" your op with the Perl interpreter so that it
10e2eb10 3001can produce sensible error and warning messages. Since it is possible to
53e06cf0 3002have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
9733086d 3003Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
10e2eb10 3004it is dealing with. You should create an C<XOP> structure for each
9733086d
BM
3005ppaddr you use, set the properties of the custom op with
3006C<XopENTRY_set>, and register the structure against the ppaddr using
10e2eb10 3007C<Perl_custom_op_register>. A trivial example might look like:
9733086d
BM
3008
3009 static XOP my_xop;
3010 static OP *my_pp(pTHX);
3011
3012 BOOT:
3013 XopENTRY_set(&my_xop, xop_name, "myxop");
3014 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3015 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3016
3017The available fields in the structure are:
3018
3019=over 4
3020
3021=item xop_name
3022
10e2eb10 3023A short name for your op. This will be included in some error messages,
9733086d
BM
3024and will also be returned as C<< $op->name >> by the L<B|B> module, so
3025it will appear in the output of module like L<B::Concise|B::Concise>.
3026
3027=item xop_desc
3028
3029A short description of the function of the op.
3030
3031=item xop_class
3032
10e2eb10 3033Which of the various C<*OP> structures this op uses. This should be one of
9733086d
BM
3034the C<OA_*> constants from F<op.h>, namely
3035
3036=over 4
3037
3038=item OA_BASEOP
3039
3040=item OA_UNOP
3041
3042=item OA_BINOP
3043
3044=item OA_LOGOP
3045
3046=item OA_LISTOP
3047
3048=item OA_PMOP
3049
3050=item OA_SVOP
3051
3052=item OA_PADOP
3053
3054=item OA_PVOP_OR_SVOP
3055
10e2eb10 3056This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
9733086d
BM
3057the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
3058
3059=item OA_LOOP
3060
3061=item OA_COP
3062
3063=back
3064
3065The other C<OA_*> constants should not be used.
3066
3067=item xop_peep
3068
3069This member is of type C<Perl_cpeep_t>, which expands to C<void
10e2eb10 3070(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
9733086d 3071will be called from C<Perl_rpeep> when ops of this type are encountered
10e2eb10 3072by the peephole optimizer. I<o> is the OP that needs optimizing;
9733086d
BM
3073I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
3074
3075=back
53e06cf0 3076
e7d4c058 3077C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 3078
954c1994 3079=head1 AUTHORS
e89caa19 3080
954c1994 3081Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
3082E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
3083itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 3084
954c1994
GS
3085With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3086Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3087Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3088Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 3089
954c1994 3090=head1 SEE ALSO
cb1a09d0 3091
ba555bf5 3092L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>