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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)>).
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161
162If you have an SV and want to know what kind of data Perl thinks is stored
163in it, you can use the following macros to check the type of SV you have.
164
165 SvIOK(SV*)
166 SvNOK(SV*)
167 SvPOK(SV*)
168
169You can get and set the current length of the string stored in an SV with
170the following macros:
171
172 SvCUR(SV*)
173 SvCUR_set(SV*, I32 val)
174
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175You can also get a pointer to the end of the string stored in the SV
176with the macro:
177
178 SvEND(SV*)
179
180But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 181
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182If you want to append something to the end of string stored in an C<SV*>,
183you can use the following functions:
184
08105a92 185 void sv_catpv(SV*, const char*);
e65f3abd 186 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 187 void sv_catpvf(SV*, const char*, ...);
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188 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
189 I32, bool);
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190 void sv_catsv(SV*, SV*);
191
192The first function calculates the length of the string to be appended by
193using C<strlen>. In the second, you specify the length of the string
46fc3d4c 194yourself. The third function processes its arguments like C<sprintf> and
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195appends the formatted output. The fourth function works like C<vsprintf>.
196You can specify the address and length of an array of SVs instead of the
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197va_list argument. The fifth function
198extends the string stored in the first
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199SV with the string stored in the second SV. It also forces the second SV
200to be interpreted as a string.
201
202The C<sv_cat*()> functions are not generic enough to operate on values that
203have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 204
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205If you know the name of a scalar variable, you can get a pointer to its SV
206by using the following:
207
64ace3f8 208 SV* get_sv("package::varname", 0);
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209
210This returns NULL if the variable does not exist.
211
d1b91892 212If you want to know if this variable (or any other SV) is actually C<defined>,
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213you can call:
214
215 SvOK(SV*)
216
06f6df17 217The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
9adebda4 218
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219Its address can be used whenever an C<SV*> is needed. Make sure that
220you don't try to compare a random sv with C<&PL_sv_undef>. For example
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221when interfacing Perl code, it'll work correctly for:
222
223 foo(undef);
224
225But won't work when called as:
226
227 $x = undef;
228 foo($x);
229
230So to repeat always use SvOK() to check whether an sv is defined.
231
232Also you have to be careful when using C<&PL_sv_undef> as a value in
233AVs or HVs (see L<AVs, HVs and undefined values>).
a0d0e21e 234
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235There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
236boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
237addresses can be used whenever an C<SV*> is needed.
a0d0e21e 238
9cde0e7f 239Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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240Take this code:
241
242 SV* sv = (SV*) 0;
243 if (I-am-to-return-a-real-value) {
244 sv = sv_2mortal(newSViv(42));
245 }
246 sv_setsv(ST(0), sv);
247
248This code tries to return a new SV (which contains the value 42) if it should
04343c6d 249return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 250pointer which, somewhere down the line, will cause a segmentation violation,
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251bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
252first line and all will be well.
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253
254To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 255call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 256
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257=head2 Offsets
258
259Perl provides the function C<sv_chop> to efficiently remove characters
260from the beginning of a string; you give it an SV and a pointer to
da75cd15 261somewhere inside the PV, and it discards everything before the
10e2eb10 262pointer. The efficiency comes by means of a little hack: instead of
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263actually removing the characters, C<sv_chop> sets the flag C<OOK>
264(offset OK) to signal to other functions that the offset hack is in
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265effect, and it moves the PV pointer (called C<SvPVX>) forward
266by the number of bytes chopped off, and adjusts C<SvCUR> and C<SvLEN>
267accordingly. (A portion of the space between the old and new PV
268pointers is used to store the count of chopped bytes.)
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269
270Hence, at this point, the start of the buffer that we allocated lives
271at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
272into the middle of this allocated storage.
273
274This is best demonstrated by example:
275
276 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
277 SV = PVIV(0x8128450) at 0x81340f0
278 REFCNT = 1
279 FLAGS = (POK,OOK,pPOK)
280 IV = 1 (OFFSET)
281 PV = 0x8135781 ( "1" . ) "2345"\0
282 CUR = 4
283 LEN = 5
284
285Here the number of bytes chopped off (1) is put into IV, and
10e2eb10 286C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
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287portion of the string between the "real" and the "fake" beginnings is
288shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
289the fake beginning, not the real one.
290
fe854a6f 291Something similar to the offset hack is performed on AVs to enable
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292efficient shifting and splicing off the beginning of the array; while
293C<AvARRAY> points to the first element in the array that is visible from
10e2eb10 294Perl, C<AvALLOC> points to the real start of the C array. These are
319cef53 295usually the same, but a C<shift> operation can be carried out by
6de131f0 296increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
319cef53 297Again, the location of the real start of the C array only comes into
10e2eb10 298play when freeing the array. See C<av_shift> in F<av.c>.
319cef53 299
d1b91892 300=head2 What's Really Stored in an SV?
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301
302Recall that the usual method of determining the type of scalar you have is
5f05dabc 303to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 304usually these macros will always return TRUE and calling the C<Sv*V>
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305macros will do the appropriate conversion of string to integer/double or
306integer/double to string.
307
308If you I<really> need to know if you have an integer, double, or string
309pointer in an SV, you can use the following three macros instead:
310
311 SvIOKp(SV*)
312 SvNOKp(SV*)
313 SvPOKp(SV*)
314
315These will tell you if you truly have an integer, double, or string pointer
d1b91892 316stored in your SV. The "p" stands for private.
a0d0e21e 317
da8c5729 318There are various ways in which the private and public flags may differ.
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319For example, in perl 5.16 and earlier a tied SV may have a valid
320underlying value in the IV slot (so SvIOKp is true), but the data
321should be accessed via the FETCH routine rather than directly,
322so SvIOK is false. (In perl 5.18 onwards, tied scalars use
323the flags the same way as untied scalars.) Another is when
d7f8936a 324numeric conversion has occurred and precision has been lost: only the
10e2eb10 325private flag is set on 'lossy' values. So when an NV is converted to an
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326IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
327
07fa94a1 328In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 329
54310121 330=head2 Working with AVs
a0d0e21e 331
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332There are two ways to create and load an AV. The first method creates an
333empty AV:
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334
335 AV* newAV();
336
54310121 337The second method both creates the AV and initially populates it with SVs:
a0d0e21e 338
c70927a6 339 AV* av_make(SSize_t num, SV **ptr);
a0d0e21e 340
5f05dabc 341The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 342AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 343
da8c5729 344Once the AV has been created, the following operations are possible on it:
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345
346 void av_push(AV*, SV*);
347 SV* av_pop(AV*);
348 SV* av_shift(AV*);
c70927a6 349 void av_unshift(AV*, SSize_t num);
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350
351These should be familiar operations, with the exception of C<av_unshift>.
352This routine adds C<num> elements at the front of the array with the C<undef>
353value. You must then use C<av_store> (described below) to assign values
354to these new elements.
355
356Here are some other functions:
357
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358 SSize_t av_top_index(AV*);
359 SV** av_fetch(AV*, SSize_t key, I32 lval);
360 SV** av_store(AV*, SSize_t key, SV* val);
a0d0e21e 361
dab460cd 362The C<av_top_index> function returns the highest index value in an array (just
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363like $#array in Perl). If the array is empty, -1 is returned. The
364C<av_fetch> function returns the value at index C<key>, but if C<lval>
365is non-zero, then C<av_fetch> will store an undef value at that index.
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366The C<av_store> function stores the value C<val> at index C<key>, and does
367not increment the reference count of C<val>. Thus the caller is responsible
368for taking care of that, and if C<av_store> returns NULL, the caller will
369have to decrement the reference count to avoid a memory leak. Note that
370C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
371return value.
d1b91892 372
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373A few more:
374
a0d0e21e 375 void av_clear(AV*);
a0d0e21e 376 void av_undef(AV*);
c70927a6 377 void av_extend(AV*, SSize_t key);
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378
379The C<av_clear> function deletes all the elements in the AV* array, but
380does not actually delete the array itself. The C<av_undef> function will
381delete all the elements in the array plus the array itself. The
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382C<av_extend> function extends the array so that it contains at least C<key+1>
383elements. If C<key+1> is less than the currently allocated length of the array,
384then nothing is done.
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385
386If you know the name of an array variable, you can get a pointer to its AV
387by using the following:
388
cbfd0a87 389 AV* get_av("package::varname", 0);
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390
391This returns NULL if the variable does not exist.
392
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393See L<Understanding the Magic of Tied Hashes and Arrays> for more
394information on how to use the array access functions on tied arrays.
395
54310121 396=head2 Working with HVs
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397
398To create an HV, you use the following routine:
399
400 HV* newHV();
401
da8c5729 402Once the HV has been created, the following operations are possible on it:
a0d0e21e 403
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404 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
405 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 406
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407The C<klen> parameter is the length of the key being passed in (Note that
408you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
409length of the key). The C<val> argument contains the SV pointer to the
54310121 410scalar being stored, and C<hash> is the precomputed hash value (zero if
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411you want C<hv_store> to calculate it for you). The C<lval> parameter
412indicates whether this fetch is actually a part of a store operation, in
413which case a new undefined value will be added to the HV with the supplied
414key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 415
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416Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
417C<SV*>. To access the scalar value, you must first dereference the return
418value. However, you should check to make sure that the return value is
419not NULL before dereferencing it.
a0d0e21e 420
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421The first of these two functions checks if a hash table entry exists, and the
422second deletes it.
a0d0e21e 423
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424 bool hv_exists(HV*, const char* key, U32 klen);
425 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 426
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427If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
428create and return a mortal copy of the deleted value.
429
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430And more miscellaneous functions:
431
432 void hv_clear(HV*);
a0d0e21e 433 void hv_undef(HV*);
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434
435Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
436table but does not actually delete the hash table. The C<hv_undef> deletes
437both the entries and the hash table itself.
a0d0e21e 438
a9b0660e 439Perl keeps the actual data in a linked list of structures with a typedef of HE.
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440These contain the actual key and value pointers (plus extra administrative
441overhead). The key is a string pointer; the value is an C<SV*>. However,
442once you have an C<HE*>, to get the actual key and value, use the routines
443specified below.
444
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445 I32 hv_iterinit(HV*);
446 /* Prepares starting point to traverse hash table */
447 HE* hv_iternext(HV*);
448 /* Get the next entry, and return a pointer to a
449 structure that has both the key and value */
450 char* hv_iterkey(HE* entry, I32* retlen);
451 /* Get the key from an HE structure and also return
452 the length of the key string */
cb1a09d0 453 SV* hv_iterval(HV*, HE* entry);
d1be9408 454 /* Return an SV pointer to the value of the HE
a0d0e21e 455 structure */
cb1a09d0 456 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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457 /* This convenience routine combines hv_iternext,
458 hv_iterkey, and hv_iterval. The key and retlen
459 arguments are return values for the key and its
460 length. The value is returned in the SV* argument */
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461
462If you know the name of a hash variable, you can get a pointer to its HV
463by using the following:
464
6673a63c 465 HV* get_hv("package::varname", 0);
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466
467This returns NULL if the variable does not exist.
468
a43e7901 469The hash algorithm is defined in the C<PERL_HASH> macro:
a0d0e21e 470
a43e7901 471 PERL_HASH(hash, key, klen)
ab192400 472
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473The exact implementation of this macro varies by architecture and version
474of perl, and the return value may change per invocation, so the value
475is only valid for the duration of a single perl process.
a0d0e21e 476
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477See L<Understanding the Magic of Tied Hashes and Arrays> for more
478information on how to use the hash access functions on tied hashes.
479
1e422769
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480=head2 Hash API Extensions
481
482Beginning with version 5.004, the following functions are also supported:
483
484 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
485 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 486
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PP
487 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
488 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 489
1e422769
PP
490 SV* hv_iterkeysv (HE* entry);
491
492Note that these functions take C<SV*> keys, which simplifies writing
493of extension code that deals with hash structures. These functions
494also allow passing of C<SV*> keys to C<tie> functions without forcing
495you to stringify the keys (unlike the previous set of functions).
496
497They also return and accept whole hash entries (C<HE*>), making their
498use more efficient (since the hash number for a particular string
4a4eefd0
GS
499doesn't have to be recomputed every time). See L<perlapi> for detailed
500descriptions.
1e422769
PP
501
502The following macros must always be used to access the contents of hash
503entries. Note that the arguments to these macros must be simple
504variables, since they may get evaluated more than once. See
4a4eefd0 505L<perlapi> for detailed descriptions of these macros.
1e422769
PP
506
507 HePV(HE* he, STRLEN len)
508 HeVAL(HE* he)
509 HeHASH(HE* he)
510 HeSVKEY(HE* he)
511 HeSVKEY_force(HE* he)
512 HeSVKEY_set(HE* he, SV* sv)
513
514These two lower level macros are defined, but must only be used when
515dealing with keys that are not C<SV*>s:
516
517 HeKEY(HE* he)
518 HeKLEN(HE* he)
519
04343c6d
GS
520Note that both C<hv_store> and C<hv_store_ent> do not increment the
521reference count of the stored C<val>, which is the caller's responsibility.
522If these functions return a NULL value, the caller will usually have to
523decrement the reference count of C<val> to avoid a memory leak.
1e422769 524
a9381218
MHM
525=head2 AVs, HVs and undefined values
526
10e2eb10
FC
527Sometimes you have to store undefined values in AVs or HVs. Although
528this may be a rare case, it can be tricky. That's because you're
a9381218
MHM
529used to using C<&PL_sv_undef> if you need an undefined SV.
530
531For example, intuition tells you that this XS code:
532
533 AV *av = newAV();
534 av_store( av, 0, &PL_sv_undef );
535
536is equivalent to this Perl code:
537
538 my @av;
539 $av[0] = undef;
540
f3c4ec28 541Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
a9381218
MHM
542for indicating that an array element has not yet been initialized.
543Thus, C<exists $av[0]> would be true for the above Perl code, but
f3c4ec28
FC
544false for the array generated by the XS code. In perl 5.20, storing
545&PL_sv_undef will create a read-only element, because the scalar
546&PL_sv_undef itself is stored, not a copy.
a9381218 547
f3c4ec28 548Similar problems can occur when storing C<&PL_sv_undef> in HVs:
a9381218
MHM
549
550 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
551
552This will indeed make the value C<undef>, but if you try to modify
553the value of C<key>, you'll get the following error:
554
555 Modification of non-creatable hash value attempted
556
557In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
10e2eb10 558in restricted hashes. This caused such hash entries not to appear
a9381218
MHM
559when iterating over the hash or when checking for the keys
560with the C<hv_exists> function.
561
8abccac8 562You can run into similar problems when you store C<&PL_sv_yes> or
10e2eb10 563C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
a9381218
MHM
564will give you the following error:
565
566 Modification of a read-only value attempted
567
568To make a long story short, you can use the special variables
8abccac8 569C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
a9381218
MHM
570HVs, but you have to make sure you know what you're doing.
571
572Generally, if you want to store an undefined value in an AV
573or HV, you should not use C<&PL_sv_undef>, but rather create a
574new undefined value using the C<newSV> function, for example:
575
576 av_store( av, 42, newSV(0) );
577 hv_store( hv, "foo", 3, newSV(0), 0 );
578
a0d0e21e
LW
579=head2 References
580
d1b91892 581References are a special type of scalar that point to other data types
a9b0660e 582(including other references).
a0d0e21e 583
07fa94a1 584To create a reference, use either of the following functions:
a0d0e21e 585
5f05dabc
PP
586 SV* newRV_inc((SV*) thing);
587 SV* newRV_noinc((SV*) thing);
a0d0e21e 588
5f05dabc 589The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
590functions are identical except that C<newRV_inc> increments the reference
591count of the C<thing>, while C<newRV_noinc> does not. For historical
592reasons, C<newRV> is a synonym for C<newRV_inc>.
593
594Once you have a reference, you can use the following macro to dereference
595the reference:
a0d0e21e
LW
596
597 SvRV(SV*)
598
599then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 600C<AV*> or C<HV*>, if required.
a0d0e21e 601
d1b91892 602To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
603
604 SvROK(SV*)
605
07fa94a1
JO
606To discover what type of value the reference refers to, use the following
607macro and then check the return value.
d1b91892
AD
608
609 SvTYPE(SvRV(SV*))
610
611The most useful types that will be returned are:
612
a5e62da0
FC
613 < SVt_PVAV Scalar
614 SVt_PVAV Array
615 SVt_PVHV Hash
616 SVt_PVCV Code
617 SVt_PVGV Glob (possibly a file handle)
618
619See L<perlapi/svtype> for more details.
d1b91892 620
cb1a09d0
AD
621=head2 Blessed References and Class Objects
622
06f6df17 623References are also used to support object-oriented programming. In perl's
cb1a09d0
AD
624OO lexicon, an object is simply a reference that has been blessed into a
625package (or class). Once blessed, the programmer may now use the reference
626to access the various methods in the class.
627
628A reference can be blessed into a package with the following function:
629
630 SV* sv_bless(SV* sv, HV* stash);
631
06f6df17
RGS
632The C<sv> argument must be a reference value. The C<stash> argument
633specifies which class the reference will belong to. See
2ae324a7 634L<Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
AD
635
636/* Still under construction */
637
ddd2cc91
DM
638The following function upgrades rv to reference if not already one.
639Creates a new SV for rv to point to. If C<classname> is non-null, the SV
640is blessed into the specified class. SV is returned.
cb1a09d0 641
08105a92 642 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 643
ddd2cc91
DM
644The following three functions copy integer, unsigned integer or double
645into an SV whose reference is C<rv>. SV is blessed if C<classname> is
646non-null.
cb1a09d0 647
08105a92 648 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 649 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 650 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 651
ddd2cc91
DM
652The following function copies the pointer value (I<the address, not the
653string!>) into an SV whose reference is rv. SV is blessed if C<classname>
654is non-null.
cb1a09d0 655
ddd2cc91 656 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
cb1a09d0 657
a9b0660e 658The following function copies a string into an SV whose reference is C<rv>.
ddd2cc91
DM
659Set length to 0 to let Perl calculate the string length. SV is blessed if
660C<classname> is non-null.
cb1a09d0 661
a9b0660e
KW
662 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
663 STRLEN length);
cb1a09d0 664
ddd2cc91
DM
665The following function tests whether the SV is blessed into the specified
666class. It does not check inheritance relationships.
9abd00ed 667
08105a92 668 int sv_isa(SV* sv, const char* name);
9abd00ed 669
ddd2cc91 670The following function tests whether the SV is a reference to a blessed object.
9abd00ed
GS
671
672 int sv_isobject(SV* sv);
673
ddd2cc91 674The following function tests whether the SV is derived from the specified
10e2eb10
FC
675class. SV can be either a reference to a blessed object or a string
676containing a class name. This is the function implementing the
ddd2cc91 677C<UNIVERSAL::isa> functionality.
9abd00ed 678
08105a92 679 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 680
00aadd71 681To check if you've got an object derived from a specific class you have
9abd00ed
GS
682to write:
683
684 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 685
5f05dabc 686=head2 Creating New Variables
cb1a09d0 687
5f05dabc
PP
688To create a new Perl variable with an undef value which can be accessed from
689your Perl script, use the following routines, depending on the variable type.
cb1a09d0 690
64ace3f8 691 SV* get_sv("package::varname", GV_ADD);
cbfd0a87 692 AV* get_av("package::varname", GV_ADD);
6673a63c 693 HV* get_hv("package::varname", GV_ADD);
cb1a09d0 694
058a5f6c 695Notice the use of GV_ADD as the second parameter. The new variable can now
cb1a09d0
AD
696be set, using the routines appropriate to the data type.
697
5f05dabc 698There are additional macros whose values may be bitwise OR'ed with the
058a5f6c 699C<GV_ADD> argument to enable certain extra features. Those bits are:
cb1a09d0 700
9a68f1db
SB
701=over
702
703=item GV_ADDMULTI
704
705Marks the variable as multiply defined, thus preventing the:
706
707 Name <varname> used only once: possible typo
708
709warning.
710
9a68f1db
SB
711=item GV_ADDWARN
712
713Issues the warning:
714
715 Had to create <varname> unexpectedly
716
717if the variable did not exist before the function was called.
718
719=back
cb1a09d0 720
07fa94a1
JO
721If you do not specify a package name, the variable is created in the current
722package.
cb1a09d0 723
5f05dabc 724=head2 Reference Counts and Mortality
a0d0e21e 725
10e2eb10 726Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 727AVs, or HVs (xV for short in the following) start their life with a
55497cff 728reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 729then it will be destroyed and its memory made available for reuse.
55497cff
PP
730
731This normally doesn't happen at the Perl level unless a variable is
5f05dabc
PP
732undef'ed or the last variable holding a reference to it is changed or
733overwritten. At the internal level, however, reference counts can be
55497cff
PP
734manipulated with the following macros:
735
736 int SvREFCNT(SV* sv);
5f05dabc 737 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
738 void SvREFCNT_dec(SV* sv);
739
740However, there is one other function which manipulates the reference
07fa94a1
JO
741count of its argument. The C<newRV_inc> function, you will recall,
742creates a reference to the specified argument. As a side effect,
743it increments the argument's reference count. If this is not what
744you want, use C<newRV_noinc> instead.
745
746For example, imagine you want to return a reference from an XSUB function.
747Inside the XSUB routine, you create an SV which initially has a reference
748count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
PP
749This returns the reference as a new SV, but the reference count of the
750SV you passed to C<newRV_inc> has been incremented to two. Now you
07fa94a1
JO
751return the reference from the XSUB routine and forget about the SV.
752But Perl hasn't! Whenever the returned reference is destroyed, the
753reference count of the original SV is decreased to one and nothing happens.
754The SV will hang around without any way to access it until Perl itself
755terminates. This is a memory leak.
5f05dabc
PP
756
757The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
758C<newRV_inc>. Then, if and when the last reference is destroyed,
759the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 760stopping any memory leak.
55497cff 761
5f05dabc 762There are some convenience functions available that can help with the
54310121 763destruction of xVs. These functions introduce the concept of "mortality".
07fa94a1
JO
764An xV that is mortal has had its reference count marked to be decremented,
765but not actually decremented, until "a short time later". Generally the
766term "short time later" means a single Perl statement, such as a call to
54310121 767an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
JO
768reference count decremented depends on two macros, SAVETMPS and FREETMPS.
769See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
PP
770
771"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
772However, if you mortalize a variable twice, the reference count will
773later be decremented twice.
774
00aadd71
NIS
775"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
776For example an SV which is created just to pass a number to a called sub
06f6df17 777is made mortal to have it cleaned up automatically when it's popped off
10e2eb10 778the stack. Similarly, results returned by XSUBs (which are pushed on the
06f6df17 779stack) are often made mortal.
a0d0e21e
LW
780
781To create a mortal variable, use the functions:
782
783 SV* sv_newmortal()
784 SV* sv_2mortal(SV*)
785 SV* sv_mortalcopy(SV*)
786
00aadd71 787The first call creates a mortal SV (with no value), the second converts an existing
5f05dabc
PP
788SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
789third creates a mortal copy of an existing SV.
da8c5729 790Because C<sv_newmortal> gives the new SV no value, it must normally be given one
9a68f1db 791via C<sv_setpv>, C<sv_setiv>, etc. :
00aadd71
NIS
792
793 SV *tmp = sv_newmortal();
794 sv_setiv(tmp, an_integer);
795
796As that is multiple C statements it is quite common so see this idiom instead:
797
798 SV *tmp = sv_2mortal(newSViv(an_integer));
799
800
801You should be careful about creating mortal variables. Strange things
802can happen if you make the same value mortal within multiple contexts,
10e2eb10
FC
803or if you make a variable mortal multiple
804times. Thinking of "Mortalization"
00aadd71 805as deferred C<SvREFCNT_dec> should help to minimize such problems.
da8c5729 806For example if you are passing an SV which you I<know> has a high enough REFCNT
00aadd71
NIS
807to survive its use on the stack you need not do any mortalization.
808If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
809making a C<sv_mortalcopy> is safer.
a0d0e21e 810
ac036724 811The mortal routines are not just for SVs; AVs and HVs can be
faed5253 812made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 813C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 814
5f05dabc 815=head2 Stashes and Globs
a0d0e21e 816
06f6df17
RGS
817A B<stash> is a hash that contains all variables that are defined
818within a package. Each key of the stash is a symbol
aa689395
PP
819name (shared by all the different types of objects that have the same
820name), and each value in the hash table is a GV (Glob Value). This GV
821in turn contains references to the various objects of that name,
822including (but not limited to) the following:
cb1a09d0 823
a0d0e21e
LW
824 Scalar Value
825 Array Value
826 Hash Value
a3cb178b 827 I/O Handle
a0d0e21e
LW
828 Format
829 Subroutine
830
06f6df17
RGS
831There is a single stash called C<PL_defstash> that holds the items that exist
832in the C<main> package. To get at the items in other packages, append the
833string "::" to the package name. The items in the C<Foo> package are in
834the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
835in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 836
d1b91892 837To get the stash pointer for a particular package, use the function:
a0d0e21e 838
da51bb9b
NC
839 HV* gv_stashpv(const char* name, I32 flags)
840 HV* gv_stashsv(SV*, I32 flags)
a0d0e21e
LW
841
842The first function takes a literal string, the second uses the string stored
d1b91892 843in the SV. Remember that a stash is just a hash table, so you get back an
da51bb9b 844C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
a0d0e21e
LW
845
846The name that C<gv_stash*v> wants is the name of the package whose symbol table
847you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
848packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
849language itself.
a0d0e21e
LW
850
851Alternately, if you have an SV that is a blessed reference, you can find
852out the stash pointer by using:
853
854 HV* SvSTASH(SvRV(SV*));
855
856then use the following to get the package name itself:
857
858 char* HvNAME(HV* stash);
859
5f05dabc
PP
860If you need to bless or re-bless an object you can use the following
861function:
a0d0e21e
LW
862
863 SV* sv_bless(SV*, HV* stash)
864
865where the first argument, an C<SV*>, must be a reference, and the second
866argument is a stash. The returned C<SV*> can now be used in the same way
867as any other SV.
868
d1b91892
AD
869For more information on references and blessings, consult L<perlref>.
870
54310121 871=head2 Double-Typed SVs
0a753a76
PP
872
873Scalar variables normally contain only one type of value, an integer,
874double, pointer, or reference. Perl will automatically convert the
875actual scalar data from the stored type into the requested type.
876
877Some scalar variables contain more than one type of scalar data. For
878example, the variable C<$!> contains either the numeric value of C<errno>
879or its string equivalent from either C<strerror> or C<sys_errlist[]>.
880
881To force multiple data values into an SV, you must do two things: use the
882C<sv_set*v> routines to add the additional scalar type, then set a flag
883so that Perl will believe it contains more than one type of data. The
884four macros to set the flags are:
885
886 SvIOK_on
887 SvNOK_on
888 SvPOK_on
889 SvROK_on
890
891The particular macro you must use depends on which C<sv_set*v> routine
892you called first. This is because every C<sv_set*v> routine turns on
893only the bit for the particular type of data being set, and turns off
894all the rest.
895
896For example, to create a new Perl variable called "dberror" that contains
897both the numeric and descriptive string error values, you could use the
898following code:
899
900 extern int dberror;
901 extern char *dberror_list;
902
64ace3f8 903 SV* sv = get_sv("dberror", GV_ADD);
0a753a76
PP
904 sv_setiv(sv, (IV) dberror);
905 sv_setpv(sv, dberror_list[dberror]);
906 SvIOK_on(sv);
907
908If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
909macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
910
4f4531b8
FC
911=head2 Read-Only Values
912
913In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
914flag bit with read-only scalars. So the only way to test whether
915C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
916in those versions is:
917
918 SvREADONLY(sv) && !SvIsCOW(sv)
919
920Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
921and, under 5.20, copy-on-write scalars can also be read-only, so the above
922check is incorrect. You just want:
923
924 SvREADONLY(sv)
925
926If you need to do this check often, define your own macro like this:
927
928 #if PERL_VERSION >= 18
929 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
930 #else
931 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
932 #endif
933
934=head2 Copy on Write
935
936Perl implements a copy-on-write (COW) mechanism for scalars, in which
937string copies are not immediately made when requested, but are deferred
938until made necessary by one or the other scalar changing. This is mostly
939transparent, but one must take care not to modify string buffers that are
940shared by multiple SVs.
941
942You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
943
944You 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).
945
946If you want to make the SV drop its string buffer, use
947C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
948C<sv_setsv(sv, NULL)>.
949
950All of these functions will croak on read-only scalars (see the previous
951section for more on those).
952
953To test that your code is behaving correctly and not modifying COW buffers,
954on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
955C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
956into crashes. You will find it to be marvellously slow, so you may want to
957skip perl's own tests.
958
0a753a76 959=head2 Magic Variables
a0d0e21e 960
d1b91892
AD
961[This section still under construction. Ignore everything here. Post no
962bills. Everything not permitted is forbidden.]
963
d1b91892
AD
964Any SV may be magical, that is, it has special features that a normal
965SV does not have. These features are stored in the SV structure in a
5f05dabc 966linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
967
968 struct magic {
969 MAGIC* mg_moremagic;
970 MGVTBL* mg_virtual;
971 U16 mg_private;
972 char mg_type;
973 U8 mg_flags;
b205eb13 974 I32 mg_len;
d1b91892
AD
975 SV* mg_obj;
976 char* mg_ptr;
d1b91892
AD
977 };
978
979Note this is current as of patchlevel 0, and could change at any time.
980
981=head2 Assigning Magic
982
983Perl adds magic to an SV using the sv_magic function:
984
a9b0660e 985 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
986
987The C<sv> argument is a pointer to the SV that is to acquire a new magical
988feature.
989
990If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
10e2eb10
FC
991convert C<sv> to type C<SVt_PVMG>.
992Perl then continues by adding new magic
645c22ef
DM
993to the beginning of the linked list of magical features. Any prior entry
994of the same type of magic is deleted. Note that this can be overridden,
995and multiple instances of the same type of magic can be associated with an
996SV.
d1b91892 997
54310121 998The C<name> and C<namlen> arguments are used to associate a string with
10e2eb10 999the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
1000C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
1001C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
1002whether C<namlen> is greater than zero or equal to zero respectively. As a
1003special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
1004to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
1005
1006The sv_magic function uses C<how> to determine which, if any, predefined
1007"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
06f6df17 1008See the L<Magic Virtual Tables> section below. The C<how> argument is also
10e2eb10
FC
1009stored in the C<mg_type> field. The value of
1010C<how> should be chosen from the set of macros
1011C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 1012these macros were added, Perl internals used to directly use character
14befaf4 1013literals, so you may occasionally come across old code or documentation
75d0f26d 1014referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
1015
1016The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
1017structure. If it is not the same as the C<sv> argument, the reference
1018count of the C<obj> object is incremented. If it is the same, or if
645c22ef 1019the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
14befaf4 1020then C<obj> is merely stored, without the reference count being incremented.
d1b91892 1021
2d8d5d5a
SH
1022See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
1023to an SV.
1024
cb1a09d0
AD
1025There is also a function to add magic to an C<HV>:
1026
1027 void hv_magic(HV *hv, GV *gv, int how);
1028
1029This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
1030
1031To remove the magic from an SV, call the function sv_unmagic:
1032
70a53b35 1033 int sv_unmagic(SV *sv, int type);
cb1a09d0
AD
1034
1035The C<type> argument should be equal to the C<how> value when the C<SV>
1036was initially made magical.
1037
f6ee7b17 1038However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
10e2eb10
FC
1039C<SV>. If you want to remove only certain
1040magic of a C<type> based on the magic
f6ee7b17
FR
1041virtual table, use C<sv_unmagicext> instead:
1042
1043 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1044
d1b91892
AD
1045=head2 Magic Virtual Tables
1046
d1be9408 1047The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
1048C<MGVTBL>, which is a structure of function pointers and stands for
1049"Magic Virtual Table" to handle the various operations that might be
1050applied to that variable.
1051
301cb7e8
DM
1052The C<MGVTBL> has five (or sometimes eight) pointers to the following
1053routine types:
d1b91892
AD
1054
1055 int (*svt_get)(SV* sv, MAGIC* mg);
1056 int (*svt_set)(SV* sv, MAGIC* mg);
1057 U32 (*svt_len)(SV* sv, MAGIC* mg);
1058 int (*svt_clear)(SV* sv, MAGIC* mg);
1059 int (*svt_free)(SV* sv, MAGIC* mg);
1060
a9b0660e
KW
1061 int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
1062 const char *name, I32 namlen);
301cb7e8
DM
1063 int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
1064 int (*svt_local)(SV *nsv, MAGIC *mg);
1065
1066
06f6df17 1067This MGVTBL structure is set at compile-time in F<perl.h> and there are
b7a0f54c
SM
1068currently 32 types. These different structures contain pointers to various
1069routines that perform additional actions depending on which function is
1070being called.
d1b91892 1071
a9b0660e
KW
1072 Function pointer Action taken
1073 ---------------- ------------
1074 svt_get Do something before the value of the SV is
1075 retrieved.
1076 svt_set Do something after the SV is assigned a value.
1077 svt_len Report on the SV's length.
1078 svt_clear Clear something the SV represents.
1079 svt_free Free any extra storage associated with the SV.
d1b91892 1080
a9b0660e
KW
1081 svt_copy copy tied variable magic to a tied element
1082 svt_dup duplicate a magic structure during thread cloning
1083 svt_local copy magic to local value during 'local'
301cb7e8 1084
d1b91892 1085For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 1086to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
1087
1088 { magic_get, magic_set, magic_len, 0, 0 }
1089
14befaf4
DM
1090Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1091if a get operation is being performed, the routine C<magic_get> is
1092called. All the various routines for the various magical types begin
1093with C<magic_>. NOTE: the magic routines are not considered part of
1094the Perl API, and may not be exported by the Perl library.
d1b91892 1095
301cb7e8
DM
1096The last three slots are a recent addition, and for source code
1097compatibility they are only checked for if one of the three flags
10e2eb10
FC
1098MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
1099This means that most code can continue declaring
1100a vtable as a 5-element value. These three are
301cb7e8
DM
1101currently used exclusively by the threading code, and are highly subject
1102to change.
1103
d1b91892
AD
1104The current kinds of Magic Virtual Tables are:
1105
f1f5ddd7
FC
1106=for comment
1107This table is generated by regen/mg_vtable.pl. Any changes made here
1108will be lost.
1109
1110=for mg_vtable.pl begin
1111
a9b0660e 1112 mg_type
bd6e6c12
FC
1113 (old-style char and macro) MGVTBL Type of magic
1114 -------------------------- ------ -------------
1115 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1116 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
1117 % PERL_MAGIC_rhash (none) extra data for restricted
1118 hashes
2a388207 1119 & PERL_MAGIC_proto (none) my sub prototype CV
bd6e6c12
FC
1120 . PERL_MAGIC_pos vtbl_pos pos() lvalue
1121 : PERL_MAGIC_symtab (none) extra data for symbol
1122 tables
1123 < PERL_MAGIC_backref vtbl_backref for weak ref data
1124 @ PERL_MAGIC_arylen_p (none) to move arylen out of XPVAV
1125 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1126 (fast string search)
1127 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1128 (AMT) on stash
1129 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1130 (@+ and @- vars)
1131 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1132 element
1133 E PERL_MAGIC_env vtbl_env %ENV hash
1134 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
eccba044 1135 f PERL_MAGIC_fm vtbl_regexp Formline
bd6e6c12 1136 ('compiled' format)
bd6e6c12
FC
1137 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1138 H PERL_MAGIC_hints vtbl_hints %^H hash
1139 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1140 I PERL_MAGIC_isa vtbl_isa @ISA array
1141 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1142 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1143 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1144 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1145 element
1146 N PERL_MAGIC_shared (none) Shared between threads
1147 n PERL_MAGIC_shared_scalar (none) Shared between threads
1148 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1149 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1150 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1151 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
1152 r PERL_MAGIC_qr vtbl_regexp precompiled qr// regex
1153 S PERL_MAGIC_sig (none) %SIG hash
1154 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1155 t PERL_MAGIC_taint vtbl_taint Taintedness
1156 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1157 extensions
1158 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1159 extensions
4499db73 1160 V PERL_MAGIC_vstring (none) SV was vstring literal
bd6e6c12
FC
1161 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1162 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1163 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1164 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1165 variable / smart parameter
1166 vivification
1167 ] PERL_MAGIC_checkcall vtbl_checkcall inlining/mutation of call
1168 to this CV
1169 ~ PERL_MAGIC_ext (none) Available for use by
1170 extensions
0cbee0a4 1171
f1f5ddd7 1172=for mg_vtable.pl end
d1b91892 1173
68dc0745 1174When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1175uppercase letter is typically used to represent some kind of composite type
1176(a list or a hash), and the lowercase letter is used to represent an element
10e2eb10 1177of that composite type. Some internals code makes use of this case
92f0c265 1178relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1179
1180The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1181specifically for use by extensions and will not be used by perl itself.
1182Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1183to variables (typically objects). This is especially useful because
1184there is no way for normal perl code to corrupt this private information
1185(unlike using extra elements of a hash object).
1186
1187Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1188C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1189C<mg_ptr> field points to a C<ufuncs> structure:
1190
1191 struct ufuncs {
a9402793
AB
1192 I32 (*uf_val)(pTHX_ IV, SV*);
1193 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1194 IV uf_index;
1195 };
1196
1197When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1198function will be called with C<uf_index> as the first arg and a pointer to
1199the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1200magic is shown below. Note that the ufuncs structure is copied by
1201sv_magic, so you can safely allocate it on the stack.
1202
1203 void
1204 Umagic(sv)
1205 SV *sv;
1206 PREINIT:
1207 struct ufuncs uf;
1208 CODE:
1209 uf.uf_val = &my_get_fn;
1210 uf.uf_set = &my_set_fn;
1211 uf.uf_index = 0;
14befaf4 1212 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1213
1e73acc8
AS
1214Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1215
1216For hashes there is a specialized hook that gives control over hash
1217keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1218if the "set" function in the C<ufuncs> structure is NULL. The hook
1219is activated whenever the hash is accessed with a key specified as
1220an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1221C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1222through the functions without the C<..._ent> suffix circumvents the
4509d391 1223hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1e73acc8 1224
14befaf4
DM
1225Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1226or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1227extra care to avoid conflict. Typically only using the magic on
1228objects blessed into the same class as the extension is sufficient.
2f07f21a
FR
1229For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1230C<MGVTBL>, even if all its fields will be C<0>, so that individual
1231C<MAGIC> pointers can be identified as a particular kind of magic
10e2eb10 1232using their magic virtual table. C<mg_findext> provides an easy way
f6ee7b17 1233to do that:
2f07f21a
FR
1234
1235 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1236
1237 MAGIC *mg;
f6ee7b17
FR
1238 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1239 /* this is really ours, not another module's PERL_MAGIC_ext */
1240 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1241 ...
2f07f21a 1242 }
5f05dabc 1243
ef50df4b
GS
1244Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1245earlier do B<not> invoke 'set' magic on their targets. This must
1246be done by the user either by calling the C<SvSETMAGIC()> macro after
1247calling these functions, or by using one of the C<sv_set*_mg()> or
1248C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1249C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1250obtained from external sources in functions that don't handle magic.
4a4eefd0 1251See L<perlapi> for a description of these functions.
189b2af5
GS
1252For example, calls to the C<sv_cat*()> functions typically need to be
1253followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1254since their implementation handles 'get' magic.
1255
d1b91892
AD
1256=head2 Finding Magic
1257
a9b0660e
KW
1258 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1259 * type */
f6ee7b17
FR
1260
1261This routine returns a pointer to a C<MAGIC> structure stored in the SV.
10e2eb10
FC
1262If the SV does not have that magical
1263feature, C<NULL> is returned. If the
f6ee7b17 1264SV has multiple instances of that magical feature, the first one will be
10e2eb10
FC
1265returned. C<mg_findext> can be used
1266to find a C<MAGIC> structure of an SV
da8c5729 1267based on both its magic type and its magic virtual table:
f6ee7b17
FR
1268
1269 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
d1b91892 1270
f6ee7b17
FR
1271Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1272SVt_PVMG, Perl may core dump.
d1b91892 1273
08105a92 1274 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1275
1276This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1277field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1278the mg_type field is changed to be the lowercase letter.
a0d0e21e 1279
04343c6d
GS
1280=head2 Understanding the Magic of Tied Hashes and Arrays
1281
14befaf4
DM
1282Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1283magic type.
9edb2b46
GS
1284
1285WARNING: As of the 5.004 release, proper usage of the array and hash
1286access functions requires understanding a few caveats. Some
1287of these caveats are actually considered bugs in the API, to be fixed
10e2eb10 1288in later releases, and are bracketed with [MAYCHANGE] below. If
9edb2b46
GS
1289you find yourself actually applying such information in this section, be
1290aware that the behavior may change in the future, umm, without warning.
04343c6d 1291
1526ead6 1292The perl tie function associates a variable with an object that implements
9a68f1db 1293the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6
AB
1294tie function from an XSUB, you must mimic this behaviour. The code below
1295carries out the necessary steps - firstly it creates a new hash, and then
1296creates a second hash which it blesses into the class which will implement
10e2eb10 1297the tie methods. Lastly it ties the two hashes together, and returns a
1526ead6
AB
1298reference to the new tied hash. Note that the code below does NOT call the
1299TIEHASH method in the MyTie class -
1300see L<Calling Perl Routines from within C Programs> for details on how
1301to do this.
1302
1303 SV*
1304 mytie()
1305 PREINIT:
1306 HV *hash;
1307 HV *stash;
1308 SV *tie;
1309 CODE:
1310 hash = newHV();
1311 tie = newRV_noinc((SV*)newHV());
da51bb9b 1312 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1313 sv_bless(tie, stash);
899e16d0 1314 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1315 RETVAL = newRV_noinc(hash);
1316 OUTPUT:
1317 RETVAL
1318
04343c6d
GS
1319The C<av_store> function, when given a tied array argument, merely
1320copies the magic of the array onto the value to be "stored", using
1321C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1322actually need to be stored in the array. [MAYCHANGE] After a call to
1323C<av_store> on a tied array, the caller will usually need to call
1324C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1325TIEARRAY object. If C<av_store> did return NULL, a call to
1326C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1327leak. [/MAYCHANGE]
04343c6d
GS
1328
1329The previous paragraph is applicable verbatim to tied hash access using the
1330C<hv_store> and C<hv_store_ent> functions as well.
1331
1332C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1333C<hv_fetch_ent> actually return an undefined mortal value whose magic
1334has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1335need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1336need to call C<mg_get()> on the returned value in order to actually invoke
1337the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1338you may also call C<mg_set()> on the return value after possibly assigning
1339a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1340method on the TIE object. [/MAYCHANGE]
04343c6d 1341
9edb2b46 1342[MAYCHANGE]
04343c6d
GS
1343In other words, the array or hash fetch/store functions don't really
1344fetch and store actual values in the case of tied arrays and hashes. They
1345merely call C<mg_copy> to attach magic to the values that were meant to be
1346"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1347do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1348the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1349and hashes.
1350
1351Currently (as of perl version 5.004), use of the hash and array access
1352functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1353"normal" hashes and arrays, or on their tied variants. The API may be
1354changed to provide more transparent access to both tied and normal data
1355types in future versions.
1356[/MAYCHANGE]
04343c6d
GS
1357
1358You would do well to understand that the TIEARRAY and TIEHASH interfaces
1359are mere sugar to invoke some perl method calls while using the uniform hash
1360and array syntax. The use of this sugar imposes some overhead (typically
1361about two to four extra opcodes per FETCH/STORE operation, in addition to
1362the creation of all the mortal variables required to invoke the methods).
1363This overhead will be comparatively small if the TIE methods are themselves
1364substantial, but if they are only a few statements long, the overhead
1365will not be insignificant.
1366
d1c897a1
IZ
1367=head2 Localizing changes
1368
1369Perl has a very handy construction
1370
1371 {
1372 local $var = 2;
1373 ...
1374 }
1375
1376This construction is I<approximately> equivalent to
1377
1378 {
1379 my $oldvar = $var;
1380 $var = 2;
1381 ...
1382 $var = $oldvar;
1383 }
1384
1385The biggest difference is that the first construction would
1386reinstate the initial value of $var, irrespective of how control exits
10e2eb10 1387the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1388more efficient as well.
1389
1390There is a way to achieve a similar task from C via Perl API: create a
1391I<pseudo-block>, and arrange for some changes to be automatically
1392undone at the end of it, either explicit, or via a non-local exit (via
10e2eb10 1393die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1394C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1395Such a construct may be created specially for some important localized
1396task, or an existing one (like boundaries of enclosing Perl
1397subroutine/block, or an existing pair for freeing TMPs) may be
10e2eb10
FC
1398used. (In the second case the overhead of additional localization must
1399be almost negligible.) Note that any XSUB is automatically enclosed in
b687b08b 1400an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1401
1402Inside such a I<pseudo-block> the following service is available:
1403
13a2d996 1404=over 4
d1c897a1
IZ
1405
1406=item C<SAVEINT(int i)>
1407
1408=item C<SAVEIV(IV i)>
1409
1410=item C<SAVEI32(I32 i)>
1411
1412=item C<SAVELONG(long i)>
1413
1414These macros arrange things to restore the value of integer variable
1415C<i> at the end of enclosing I<pseudo-block>.
1416
1417=item C<SAVESPTR(s)>
1418
1419=item C<SAVEPPTR(p)>
1420
1421These macros arrange things to restore the value of pointers C<s> and
10e2eb10 1422C<p>. C<s> must be a pointer of a type which survives conversion to
d1c897a1
IZ
1423C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1424and back.
1425
1426=item C<SAVEFREESV(SV *sv)>
1427
1428The refcount of C<sv> would be decremented at the end of
26d9b02f
JH
1429I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1430mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1431extends the lifetime of C<sv> until the beginning of the next statement,
1432C<SAVEFREESV> extends it until the end of the enclosing scope. These
1433lifetimes can be wildly different.
1434
1435Also compare C<SAVEMORTALIZESV>.
1436
1437=item C<SAVEMORTALIZESV(SV *sv)>
1438
1439Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1440scope instead of decrementing its reference count. This usually has the
1441effect of keeping C<sv> alive until the statement that called the currently
1442live scope has finished executing.
d1c897a1
IZ
1443
1444=item C<SAVEFREEOP(OP *op)>
1445
1446The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1447
1448=item C<SAVEFREEPV(p)>
1449
1450The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1451end of I<pseudo-block>.
1452
1453=item C<SAVECLEARSV(SV *sv)>
1454
1455Clears a slot in the current scratchpad which corresponds to C<sv> at
1456the end of I<pseudo-block>.
1457
1458=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1459
10e2eb10 1460The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
d1c897a1
IZ
1461string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1462short-lived storage, the corresponding string may be reallocated like
1463this:
1464
9cde0e7f 1465 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1466
c76ac1ee 1467=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1468
1469At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1470only argument C<p>.
1471
1472=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1473
1474At the end of I<pseudo-block> the function C<f> is called with the
1475implicit context argument (if any), and C<p>.
d1c897a1
IZ
1476
1477=item C<SAVESTACK_POS()>
1478
1479The current offset on the Perl internal stack (cf. C<SP>) is restored
1480at the end of I<pseudo-block>.
1481
1482=back
1483
1484The following API list contains functions, thus one needs to
1485provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1486or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1487function takes C<int *>.
1488
13a2d996 1489=over 4
d1c897a1
IZ
1490
1491=item C<SV* save_scalar(GV *gv)>
1492
1493Equivalent to Perl code C<local $gv>.
1494
1495=item C<AV* save_ary(GV *gv)>
1496
1497=item C<HV* save_hash(GV *gv)>
1498
1499Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1500
1501=item C<void save_item(SV *item)>
1502
1503Duplicates the current value of C<SV>, on the exit from the current
1504C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
10e2eb10 1505using the stored value. It doesn't handle magic. Use C<save_scalar> if
038fcae3 1506magic is affected.
d1c897a1
IZ
1507
1508=item C<void save_list(SV **sarg, I32 maxsarg)>
1509
1510A variant of C<save_item> which takes multiple arguments via an array
1511C<sarg> of C<SV*> of length C<maxsarg>.
1512
1513=item C<SV* save_svref(SV **sptr)>
1514
d1be9408 1515Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1516
1517=item C<void save_aptr(AV **aptr)>
1518
1519=item C<void save_hptr(HV **hptr)>
1520
1521Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1522
1523=back
1524
1525The C<Alias> module implements localization of the basic types within the
1526I<caller's scope>. People who are interested in how to localize things in
1527the containing scope should take a look there too.
1528
0a753a76 1529=head1 Subroutines
a0d0e21e 1530
68dc0745 1531=head2 XSUBs and the Argument Stack
5f05dabc
PP
1532
1533The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1534An XSUB routine will have a stack that contains the arguments from the Perl
1535program, and a way to map from the Perl data structures to a C equivalent.
1536
1537The stack arguments are accessible through the C<ST(n)> macro, which returns
1538the C<n>'th stack argument. Argument 0 is the first argument passed in the
1539Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1540an C<SV*> is used.
1541
1542Most of the time, output from the C routine can be handled through use of
1543the RETVAL and OUTPUT directives. However, there are some cases where the
1544argument stack is not already long enough to handle all the return values.
1545An example is the POSIX tzname() call, which takes no arguments, but returns
1546two, the local time zone's standard and summer time abbreviations.
1547
1548To handle this situation, the PPCODE directive is used and the stack is
1549extended using the macro:
1550
924508f0 1551 EXTEND(SP, num);
5f05dabc 1552
924508f0
GS
1553where C<SP> is the macro that represents the local copy of the stack pointer,
1554and C<num> is the number of elements the stack should be extended by.
5f05dabc 1555
00aadd71 1556Now that there is room on the stack, values can be pushed on it using C<PUSHs>
10e2eb10 1557macro. The pushed values will often need to be "mortal" (See
d82b684c 1558L</Reference Counts and Mortality>):
5f05dabc 1559
00aadd71 1560 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1561 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1562 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1563 PUSHs(sv_2mortal(newSVpv("Some String",0)))
a9b0660e
KW
1564 /* Although the last example is better written as the more
1565 * efficient: */
a3179684 1566 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
5f05dabc
PP
1567
1568And now the Perl program calling C<tzname>, the two values will be assigned
1569as in:
1570
1571 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1572
1573An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1574to use the macro:
5f05dabc 1575
5f05dabc
PP
1576 XPUSHs(SV*)
1577
da8c5729 1578This macro automatically adjusts the stack for you, if needed. Thus, you
5f05dabc 1579do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1580
1581Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1582C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1583For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1584C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1585
1586For more information, consult L<perlxs> and L<perlxstut>.
1587
5b36e945
FC
1588=head2 Autoloading with XSUBs
1589
1590If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1591fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1592of the XSUB's package.
1593
1594But it also puts the same information in certain fields of the XSUB itself:
1595
1596 HV *stash = CvSTASH(cv);
1597 const char *subname = SvPVX(cv);
1598 STRLEN name_length = SvCUR(cv); /* in bytes */
1599 U32 is_utf8 = SvUTF8(cv);
f703fc96 1600
5b36e945 1601C<SvPVX(cv)> contains just the sub name itself, not including the package.
d8893903
FC
1602For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1603C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
5b36e945
FC
1604
1605B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1606XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1607XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1608to support 5.8-5.14, use the XSUB's fields.
1609
5f05dabc 1610=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1611
1612There are four routines that can be used to call a Perl subroutine from
1613within a C program. These four are:
1614
954c1994
GS
1615 I32 call_sv(SV*, I32);
1616 I32 call_pv(const char*, I32);
1617 I32 call_method(const char*, I32);
5aaab254 1618 I32 call_argv(const char*, I32, char**);
a0d0e21e 1619
954c1994 1620The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1621contains either the name of the Perl subroutine to be called, or a
1622reference to the subroutine. The second argument consists of flags
1623that control the context in which the subroutine is called, whether
1624or not the subroutine is being passed arguments, how errors should be
1625trapped, and how to treat return values.
a0d0e21e
LW
1626
1627All four routines return the number of arguments that the subroutine returned
1628on the Perl stack.
1629
9a68f1db 1630These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1631but those names are now deprecated; macros of the same name are provided for
1632compatibility.
1633
1634When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1635must manipulate the Perl stack. These include the following macros and
1636functions:
a0d0e21e
LW
1637
1638 dSP
924508f0 1639 SP
a0d0e21e
LW
1640 PUSHMARK()
1641 PUTBACK
1642 SPAGAIN
1643 ENTER
1644 SAVETMPS
1645 FREETMPS
1646 LEAVE
1647 XPUSH*()
cb1a09d0 1648 POP*()
a0d0e21e 1649
5f05dabc
PP
1650For a detailed description of calling conventions from C to Perl,
1651consult L<perlcall>.
a0d0e21e 1652
8ebc5c01 1653=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1654
1655A lot of opcodes (this is an elementary operation in the internal perl
10e2eb10
FC
1656stack machine) put an SV* on the stack. However, as an optimization
1657the corresponding SV is (usually) not recreated each time. The opcodes
ce3d39e2
IZ
1658reuse specially assigned SVs (I<target>s) which are (as a corollary)
1659not constantly freed/created.
1660
0a753a76 1661Each of the targets is created only once (but see
ce3d39e2
IZ
1662L<Scratchpads and recursion> below), and when an opcode needs to put
1663an integer, a double, or a string on stack, it just sets the
1664corresponding parts of its I<target> and puts the I<target> on stack.
1665
1666The macro to put this target on stack is C<PUSHTARG>, and it is
1667directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1668others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1669
1bd1c0d5 1670Because the target is reused, you must be careful when pushing multiple
10e2eb10 1671values on the stack. The following code will not do what you think:
1bd1c0d5
SC
1672
1673 XPUSHi(10);
1674 XPUSHi(20);
1675
1676This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1677the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1678At the end of the operation, the stack does not contain the values 10
1679and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1680to 20.
1bd1c0d5 1681
d82b684c
SH
1682If you need to push multiple different values then you should either use
1683the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1684none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1685SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1686will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1687this a little easier to achieve by creating a new mortal for you (via
1688C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1689in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1690Thus, instead of writing this to "fix" the example above:
1691
1692 XPUSHs(sv_2mortal(newSViv(10)))
1693 XPUSHs(sv_2mortal(newSViv(20)))
1694
1695you can simply write:
1696
1697 mXPUSHi(10)
1698 mXPUSHi(20)
1699
1700On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1701need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1702macros can make use of the local variable C<TARG>. See also C<dTARGET>
1703and C<dXSTARG>.
1bd1c0d5 1704
8ebc5c01 1705=head2 Scratchpads
ce3d39e2 1706
54310121 1707The question remains on when the SVs which are I<target>s for opcodes
10e2eb10 1708are created. The answer is that they are created when the current
ac036724 1709unit--a subroutine or a file (for opcodes for statements outside of
10e2eb10 1710subroutines)--is compiled. During this time a special anonymous Perl
ac036724 1711array is created, which is called a scratchpad for the current unit.
ce3d39e2 1712
54310121 1713A scratchpad keeps SVs which are lexicals for the current unit and are
d777b41a
FC
1714targets for opcodes. A previous version of this document
1715stated that one can deduce that an SV lives on a scratchpad
ce3d39e2 1716by looking on its flags: lexicals have C<SVs_PADMY> set, and
d777b41a
FC
1717I<target>s have C<SVs_PADTMP> set. But this have never been fully true.
1718C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1719While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
1720that have never resided in a pad, but nonetheless act like I<target>s.
ce3d39e2 1721
10e2eb10 1722The correspondence between OPs and I<target>s is not 1-to-1. Different
54310121 1723OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1724would not conflict with the expected life of the temporary.
1725
2ae324a7 1726=head2 Scratchpads and recursion
ce3d39e2
IZ
1727
1728In fact it is not 100% true that a compiled unit contains a pointer to
10e2eb10
FC
1729the scratchpad AV. In fact it contains a pointer to an AV of
1730(initially) one element, and this element is the scratchpad AV. Why do
ce3d39e2
IZ
1731we need an extra level of indirection?
1732
10e2eb10 1733The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2 1734these can create several execution pointers going into the same
10e2eb10 1735subroutine. For the subroutine-child not write over the temporaries
ce3d39e2
IZ
1736for the subroutine-parent (lifespan of which covers the call to the
1737child), the parent and the child should have different
10e2eb10 1738scratchpads. (I<And> the lexicals should be separate anyway!)
ce3d39e2 1739
5f05dabc
PP
1740So each subroutine is born with an array of scratchpads (of length 1).
1741On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1742depth of the recursion is not more than the length of this array, and
1743if it is, new scratchpad is created and pushed into the array.
1744
1745The I<target>s on this scratchpad are C<undef>s, but they are already
1746marked with correct flags.
1747
22d36020
FC
1748=head1 Memory Allocation
1749
1750=head2 Allocation
1751
1752All memory meant to be used with the Perl API functions should be manipulated
1753using the macros described in this section. The macros provide the necessary
1754transparency between differences in the actual malloc implementation that is
1755used within perl.
1756
1757It is suggested that you enable the version of malloc that is distributed
1758with Perl. It keeps pools of various sizes of unallocated memory in
1759order to satisfy allocation requests more quickly. However, on some
1760platforms, it may cause spurious malloc or free errors.
1761
1762The following three macros are used to initially allocate memory :
1763
1764 Newx(pointer, number, type);
1765 Newxc(pointer, number, type, cast);
1766 Newxz(pointer, number, type);
1767
1768The first argument C<pointer> should be the name of a variable that will
1769point to the newly allocated memory.
1770
1771The second and third arguments C<number> and C<type> specify how many of
1772the specified type of data structure should be allocated. The argument
1773C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1774should be used if the C<pointer> argument is different from the C<type>
1775argument.
1776
1777Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1778to zero out all the newly allocated memory.
1779
1780=head2 Reallocation
1781
1782 Renew(pointer, number, type);
1783 Renewc(pointer, number, type, cast);
1784 Safefree(pointer)
1785
1786These three macros are used to change a memory buffer size or to free a
1787piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1788match those of C<New> and C<Newc> with the exception of not needing the
1789"magic cookie" argument.
1790
1791=head2 Moving
1792
1793 Move(source, dest, number, type);
1794 Copy(source, dest, number, type);
1795 Zero(dest, number, type);
1796
1797These three macros are used to move, copy, or zero out previously allocated
1798memory. The C<source> and C<dest> arguments point to the source and
1799destination starting points. Perl will move, copy, or zero out C<number>
1800instances of the size of the C<type> data structure (using the C<sizeof>
1801function).
1802
1803=head1 PerlIO
1804
1805The most recent development releases of Perl have been experimenting with
1806removing Perl's dependency on the "normal" standard I/O suite and allowing
1807other stdio implementations to be used. This involves creating a new
1808abstraction layer that then calls whichever implementation of stdio Perl
1809was compiled with. All XSUBs should now use the functions in the PerlIO
1810abstraction layer and not make any assumptions about what kind of stdio
1811is being used.
1812
1813For a complete description of the PerlIO abstraction, consult L<perlapio>.
1814
0a753a76
PP
1815=head1 Compiled code
1816
1817=head2 Code tree
1818
1819Here we describe the internal form your code is converted to by
10e2eb10 1820Perl. Start with a simple example:
0a753a76
PP
1821
1822 $a = $b + $c;
1823
1824This is converted to a tree similar to this one:
1825
1826 assign-to
1827 / \
1828 + $a
1829 / \
1830 $b $c
1831
7b8d334a 1832(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1833parsed your code, but has nothing to do with the execution order.
1834There is an additional "thread" going through the nodes of the tree
1835which shows the order of execution of the nodes. In our simplified
1836example above it looks like:
1837
1838 $b ---> $c ---> + ---> $a ---> assign-to
1839
1840But with the actual compile tree for C<$a = $b + $c> it is different:
1841some nodes I<optimized away>. As a corollary, though the actual tree
1842contains more nodes than our simplified example, the execution order
1843is the same as in our example.
1844
1845=head2 Examining the tree
1846
06f6df17
RGS
1847If you have your perl compiled for debugging (usually done with
1848C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1849compiled tree by specifying C<-Dx> on the Perl command line. The
1850output takes several lines per node, and for C<$b+$c> it looks like
1851this:
1852
1853 5 TYPE = add ===> 6
1854 TARG = 1
1855 FLAGS = (SCALAR,KIDS)
1856 {
1857 TYPE = null ===> (4)
1858 (was rv2sv)
1859 FLAGS = (SCALAR,KIDS)
1860 {
1861 3 TYPE = gvsv ===> 4
1862 FLAGS = (SCALAR)
1863 GV = main::b
1864 }
1865 }
1866 {
1867 TYPE = null ===> (5)
1868 (was rv2sv)
1869 FLAGS = (SCALAR,KIDS)
1870 {
1871 4 TYPE = gvsv ===> 5
1872 FLAGS = (SCALAR)
1873 GV = main::c
1874 }
1875 }
1876
1877This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1878not optimized away (one per number in the left column). The immediate
1879children of the given node correspond to C<{}> pairs on the same level
1880of indentation, thus this listing corresponds to the tree:
1881
1882 add
1883 / \
1884 null null
1885 | |
1886 gvsv gvsv
1887
1888The execution order is indicated by C<===E<gt>> marks, thus it is C<3
18894 5 6> (node C<6> is not included into above listing), i.e.,
1890C<gvsv gvsv add whatever>.
1891
9afa14e3 1892Each of these nodes represents an op, a fundamental operation inside the
10e2eb10 1893Perl core. The code which implements each operation can be found in the
9afa14e3 1894F<pp*.c> files; the function which implements the op with type C<gvsv>
10e2eb10 1895is C<pp_gvsv>, and so on. As the tree above shows, different ops have
9afa14e3 1896different numbers of children: C<add> is a binary operator, as one would
10e2eb10 1897expect, and so has two children. To accommodate the various different
9afa14e3
SC
1898numbers of children, there are various types of op data structure, and
1899they link together in different ways.
1900
10e2eb10 1901The simplest type of op structure is C<OP>: this has no children. Unary
9afa14e3 1902operators, C<UNOP>s, have one child, and this is pointed to by the
10e2eb10
FC
1903C<op_first> field. Binary operators (C<BINOP>s) have not only an
1904C<op_first> field but also an C<op_last> field. The most complex type of
1905op is a C<LISTOP>, which has any number of children. In this case, the
9afa14e3 1906first child is pointed to by C<op_first> and the last child by
10e2eb10 1907C<op_last>. The children in between can be found by iteratively
9afa14e3
SC
1908following the C<op_sibling> pointer from the first child to the last.
1909
1910There are also two other op types: a C<PMOP> holds a regular expression,
10e2eb10
FC
1911and has no children, and a C<LOOP> may or may not have children. If the
1912C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
9afa14e3
SC
1913complicate matters, if a C<UNOP> is actually a C<null> op after
1914optimization (see L</Compile pass 2: context propagation>) it will still
1915have children in accordance with its former type.
1916
06f6df17
RGS
1917Another way to examine the tree is to use a compiler back-end module, such
1918as L<B::Concise>.
1919
0a753a76
PP
1920=head2 Compile pass 1: check routines
1921
8870b5c7 1922The tree is created by the compiler while I<yacc> code feeds it
10e2eb10 1923the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1924the first pass of perl compilation.
1925
1926What makes this pass interesting for perl developers is that some
1927optimization may be performed on this pass. This is optimization by
8870b5c7 1928so-called "check routines". The correspondence between node names
0a753a76
PP
1929and corresponding check routines is described in F<opcode.pl> (do not
1930forget to run C<make regen_headers> if you modify this file).
1931
1932A check routine is called when the node is fully constructed except
7b8d334a 1933for the execution-order thread. Since at this time there are no
0a753a76
PP
1934back-links to the currently constructed node, one can do most any
1935operation to the top-level node, including freeing it and/or creating
1936new nodes above/below it.
1937
1938The check routine returns the node which should be inserted into the
1939tree (if the top-level node was not modified, check routine returns
1940its argument).
1941
10e2eb10 1942By convention, check routines have names C<ck_*>. They are usually
0a753a76
PP
1943called from C<new*OP> subroutines (or C<convert>) (which in turn are
1944called from F<perly.y>).
1945
1946=head2 Compile pass 1a: constant folding
1947
1948Immediately after the check routine is called the returned node is
1949checked for being compile-time executable. If it is (the value is
1950judged to be constant) it is immediately executed, and a I<constant>
1951node with the "return value" of the corresponding subtree is
1952substituted instead. The subtree is deleted.
1953
1954If constant folding was not performed, the execution-order thread is
1955created.
1956
1957=head2 Compile pass 2: context propagation
1958
1959When a context for a part of compile tree is known, it is propagated
a3cb178b 1960down through the tree. At this time the context can have 5 values
0a753a76
PP
1961(instead of 2 for runtime context): void, boolean, scalar, list, and
1962lvalue. In contrast with the pass 1 this pass is processed from top
1963to bottom: a node's context determines the context for its children.
1964
1965Additional context-dependent optimizations are performed at this time.
1966Since at this moment the compile tree contains back-references (via
1967"thread" pointers), nodes cannot be free()d now. To allow
1968optimized-away nodes at this stage, such nodes are null()ified instead
1969of free()ing (i.e. their type is changed to OP_NULL).
1970
1971=head2 Compile pass 3: peephole optimization
1972
1973After the compile tree for a subroutine (or for an C<eval> or a file)
10e2eb10 1974is created, an additional pass over the code is performed. This pass
0a753a76 1975is neither top-down or bottom-up, but in the execution order (with
9ea12537
Z
1976additional complications for conditionals). Optimizations performed
1977at this stage are subject to the same restrictions as in the pass 2.
1978
1979Peephole optimizations are done by calling the function pointed to
1980by the global variable C<PL_peepp>. By default, C<PL_peepp> just
1981calls the function pointed to by the global variable C<PL_rpeepp>.
1982By default, that performs some basic op fixups and optimisations along
1983the execution-order op chain, and recursively calls C<PL_rpeepp> for
1984each side chain of ops (resulting from conditionals). Extensions may
1985provide additional optimisations or fixups, hooking into either the
1986per-subroutine or recursive stage, like this:
1987
1988 static peep_t prev_peepp;
1989 static void my_peep(pTHX_ OP *o)
1990 {
1991 /* custom per-subroutine optimisation goes here */
f0358462 1992 prev_peepp(aTHX_ o);
9ea12537
Z
1993 /* custom per-subroutine optimisation may also go here */
1994 }
1995 BOOT:
1996 prev_peepp = PL_peepp;
1997 PL_peepp = my_peep;
1998
1999 static peep_t prev_rpeepp;
2000 static void my_rpeep(pTHX_ OP *o)
2001 {
2002 OP *orig_o = o;
2003 for(; o; o = o->op_next) {
2004 /* custom per-op optimisation goes here */
2005 }
f0358462 2006 prev_rpeepp(aTHX_ orig_o);
9ea12537
Z
2007 }
2008 BOOT:
2009 prev_rpeepp = PL_rpeepp;
2010 PL_rpeepp = my_rpeep;
0a753a76 2011
1ba7f851
PJ
2012=head2 Pluggable runops
2013
2014The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
2015functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2016with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2017control over the execution of the compile tree it is possible to provide
2018your own runops function.
1ba7f851
PJ
2019
2020It's probably best to copy one of the existing runops functions and
2021change it to suit your needs. Then, in the BOOT section of your XS
2022file, add the line:
2023
2024 PL_runops = my_runops;
2025
2026This function should be as efficient as possible to keep your programs
2027running as fast as possible.
2028
fd85fad2
BM
2029=head2 Compile-time scope hooks
2030
2031As of perl 5.14 it is possible to hook into the compile-time lexical
10e2eb10 2032scope mechanism using C<Perl_blockhook_register>. This is used like
fd85fad2
BM
2033this:
2034
2035 STATIC void my_start_hook(pTHX_ int full);
2036 STATIC BHK my_hooks;
2037
2038 BOOT:
a88d97bf 2039 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
fd85fad2
BM
2040 Perl_blockhook_register(aTHX_ &my_hooks);
2041
2042This will arrange to have C<my_start_hook> called at the start of
10e2eb10 2043compiling every lexical scope. The available hooks are:
fd85fad2
BM
2044
2045=over 4
2046
a88d97bf 2047=item C<void bhk_start(pTHX_ int full)>
fd85fad2 2048
10e2eb10 2049This is called just after starting a new lexical scope. Note that Perl
fd85fad2
BM
2050code like
2051
2052 if ($x) { ... }
2053
2054creates two scopes: the first starts at the C<(> and has C<full == 1>,
10e2eb10
FC
2055the second starts at the C<{> and has C<full == 0>. Both end at the
2056C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
fd85fad2
BM
2057pushed onto the save stack by this hook will be popped just before the
2058scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2059
a88d97bf 2060=item C<void bhk_pre_end(pTHX_ OP **o)>
fd85fad2
BM
2061
2062This is called at the end of a lexical scope, just before unwinding the
10e2eb10 2063stack. I<o> is the root of the optree representing the scope; it is a
fd85fad2
BM
2064double pointer so you can replace the OP if you need to.
2065
a88d97bf 2066=item C<void bhk_post_end(pTHX_ OP **o)>
fd85fad2
BM
2067
2068This is called at the end of a lexical scope, just after unwinding the
10e2eb10 2069stack. I<o> is as above. Note that it is possible for calls to C<pre_>
fd85fad2
BM
2070and C<post_end> to nest, if there is something on the save stack that
2071calls string eval.
2072
a88d97bf 2073=item C<void bhk_eval(pTHX_ OP *const o)>
fd85fad2
BM
2074
2075This is called just before starting to compile an C<eval STRING>, C<do
10e2eb10 2076FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
fd85fad2
BM
2077OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2078C<OP_DOFILE> or C<OP_REQUIRE>.
2079
2080=back
2081
2082Once you have your hook functions, you need a C<BHK> structure to put
10e2eb10
FC
2083them in. It's best to allocate it statically, since there is no way to
2084free it once it's registered. The function pointers should be inserted
fd85fad2 2085into this structure using the C<BhkENTRY_set> macro, which will also set
10e2eb10 2086flags indicating which entries are valid. If you do need to allocate
fd85fad2
BM
2087your C<BHK> dynamically for some reason, be sure to zero it before you
2088start.
2089
2090Once registered, there is no mechanism to switch these hooks off, so if
10e2eb10 2091that is necessary you will need to do this yourself. An entry in C<%^H>
a3e07c87
BM
2092is probably the best way, so the effect is lexically scoped; however it
2093is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
10e2eb10 2094temporarily switch entries on and off. You should also be aware that
a3e07c87
BM
2095generally speaking at least one scope will have opened before your
2096extension is loaded, so you will see some C<pre/post_end> pairs that
2097didn't have a matching C<start>.
fd85fad2 2098
9afa14e3
SC
2099=head1 Examining internal data structures with the C<dump> functions
2100
2101To aid debugging, the source file F<dump.c> contains a number of
2102functions which produce formatted output of internal data structures.
2103
2104The most commonly used of these functions is C<Perl_sv_dump>; it's used
10e2eb10 2105for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
9afa14e3 2106C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 2107module should already be familiar with its format.
9afa14e3
SC
2108
2109C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 2110derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
2111C<Perl_dump_eval> will dump the main root of the code being evaluated,
2112exactly like C<-Dx>.
2113
2114Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2115op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2116subroutines in a package like so: (Thankfully, these are all xsubs, so
2117there is no op tree)
2118
2119 (gdb) print Perl_dump_packsubs(PL_defstash)
2120
2121 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2122
2123 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2124
2125 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2126
2127 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2128
2129 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2130
2131and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2132the op tree of the main root.
2133
954c1994 2134=head1 How multiple interpreters and concurrency are supported
ee072b34 2135
ee072b34
GS
2136=head2 Background and PERL_IMPLICIT_CONTEXT
2137
2138The Perl interpreter can be regarded as a closed box: it has an API
2139for feeding it code or otherwise making it do things, but it also has
2140functions for its own use. This smells a lot like an object, and
2141there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
2142interpreters, with one interpreter represented either as a C structure,
2143or inside a thread-specific structure. These structures contain all
2144the context, the state of that interpreter.
2145
10e2eb10 2146One macro controls the major Perl build flavor: MULTIPLICITY. The
7b52221d 2147MULTIPLICITY build has a C structure that packages all the interpreter
10e2eb10 2148state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
7b52221d 2149normally defined, and enables the support for passing in a "hidden" first
10e2eb10 2150argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 2151multi-threaded perls possible (with the ithreads threading model, related
7b52221d 2152to the macro USE_ITHREADS.)
54aff467 2153
27da23d5
JH
2154Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2155PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2156former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2157internal variables of Perl to be wrapped inside a single global struct,
2158struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2159the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2160one step further, there is still a single struct (allocated in main()
2161either from heap or from stack) but there are no global data symbols
3bf17896 2162pointing to it. In either case the global struct should be initialized
27da23d5
JH
2163as the very first thing in main() using Perl_init_global_struct() and
2164correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2165please see F<miniperlmain.c> for usage details. You may also need
2166to use C<dVAR> in your coding to "declare the global variables"
2167when you are using them. dTHX does this for you automatically.
2168
bc028b6b
JH
2169To see whether you have non-const data you can use a BSD-compatible C<nm>:
2170
2171 nm libperl.a | grep -v ' [TURtr] '
2172
2173If this displays any C<D> or C<d> symbols, you have non-const data.
2174
27da23d5
JH
2175For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2176doesn't actually hide all symbols inside a big global struct: some
2177PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2178then hides everything (see how the PERLIO_FUNCS_DECL is used).
2179
54aff467 2180All this obviously requires a way for the Perl internal functions to be
acfe0abc 2181either subroutines taking some kind of structure as the first
ee072b34 2182argument, or subroutines taking nothing as the first argument. To
acfe0abc 2183enable these two very different ways of building the interpreter,
ee072b34
GS
2184the Perl source (as it does in so many other situations) makes heavy
2185use of macros and subroutine naming conventions.
2186
54aff467 2187First problem: deciding which functions will be public API functions and
00aadd71 2188which will be private. All functions whose names begin C<S_> are private
954c1994
GS
2189(think "S" for "secret" or "static"). All other functions begin with
2190"Perl_", but just because a function begins with "Perl_" does not mean it is
10e2eb10
FC
2191part of the API. (See L</Internal
2192Functions>.) The easiest way to be B<sure> a
00aadd71
NIS
2193function is part of the API is to find its entry in L<perlapi>.
2194If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2195think it should be (i.e., you need it for your extension), send mail via
a422fd2d 2196L<perlbug> explaining why you think it should be.
ee072b34
GS
2197
2198Second problem: there must be a syntax so that the same subroutine
2199declarations and calls can pass a structure as their first argument,
2200or pass nothing. To solve this, the subroutines are named and
2201declared in a particular way. Here's a typical start of a static
2202function used within the Perl guts:
2203
2204 STATIC void
2205 S_incline(pTHX_ char *s)
2206
acfe0abc 2207STATIC becomes "static" in C, and may be #define'd to nothing in some
da8c5729 2208configurations in the future.
ee072b34 2209
651a3225
GS
2210A public function (i.e. part of the internal API, but not necessarily
2211sanctioned for use in extensions) begins like this:
ee072b34
GS
2212
2213 void
2307c6d0 2214 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 2215
0147cd53 2216C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
2217details of the interpreter's context. THX stands for "thread", "this",
2218or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2219The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
2220or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2221their variants.
ee072b34 2222
a7486cbb
JH
2223When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2224first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
2225in the pTHX_ macro indicates that the macro expansion needs a comma
2226after the context argument because other arguments follow it. If
2227PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
2228subroutine is not prototyped to take the extra argument. The form of the
2229macro without the trailing underscore is used when there are no additional
ee072b34
GS
2230explicit arguments.
2231
54aff467 2232When a core function calls another, it must pass the context. This
2307c6d0 2233is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
2234something like this:
2235
2307c6d0
SB
2236 #ifdef PERL_IMPLICIT_CONTEXT
2237 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 2238 /* can't do this for vararg functions, see below */
2307c6d0
SB
2239 #else
2240 #define sv_setiv Perl_sv_setiv
2241 #endif
ee072b34
GS
2242
2243This works well, and means that XS authors can gleefully write:
2244
2307c6d0 2245 sv_setiv(foo, bar);
ee072b34
GS
2246
2247and still have it work under all the modes Perl could have been
2248compiled with.
2249
ee072b34
GS
2250This doesn't work so cleanly for varargs functions, though, as macros
2251imply that the number of arguments is known in advance. Instead we
2252either need to spell them out fully, passing C<aTHX_> as the first
2253argument (the Perl core tends to do this with functions like
2254Perl_warner), or use a context-free version.
2255
2256The context-free version of Perl_warner is called
2257Perl_warner_nocontext, and does not take the extra argument. Instead
2258it does dTHX; to get the context from thread-local storage. We
2259C<#define warner Perl_warner_nocontext> so that extensions get source
2260compatibility at the expense of performance. (Passing an arg is
2261cheaper than grabbing it from thread-local storage.)
2262
acfe0abc 2263You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2264Those are strictly for use within the core. Extensions and embedders
2265need only be aware of [pad]THX.
2266
a7486cbb
JH
2267=head2 So what happened to dTHR?
2268
2269C<dTHR> was introduced in perl 5.005 to support the older thread model.
2270The older thread model now uses the C<THX> mechanism to pass context
2271pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2272later still have it for backward source compatibility, but it is defined
2273to be a no-op.
2274
ee072b34
GS
2275=head2 How do I use all this in extensions?
2276
2277When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2278any functions in the Perl API will need to pass the initial context
2279argument somehow. The kicker is that you will need to write it in
2280such a way that the extension still compiles when Perl hasn't been
2281built with PERL_IMPLICIT_CONTEXT enabled.
2282
2283There are three ways to do this. First, the easy but inefficient way,
2284which is also the default, in order to maintain source compatibility
0147cd53 2285with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2286and aTHX_ macros to call a function that will return the context.
2287Thus, something like:
2288
2307c6d0 2289 sv_setiv(sv, num);
ee072b34 2290
4375e838 2291in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2292in effect:
ee072b34 2293
2307c6d0 2294 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2295
54aff467 2296or to this otherwise:
ee072b34 2297
2307c6d0 2298 Perl_sv_setiv(sv, num);
ee072b34 2299
da8c5729 2300You don't have to do anything new in your extension to get this; since
2fa86c13 2301the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2302work.
2303
2304The second, more efficient way is to use the following template for
2305your Foo.xs:
2306
c52f9dcd
JH
2307 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2308 #include "EXTERN.h"
2309 #include "perl.h"
2310 #include "XSUB.h"
ee072b34 2311
fd061412 2312 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2313
fd061412 2314 STATIC void
c52f9dcd
JH
2315 my_private_function(int arg1, int arg2)
2316 {
2317 dTHX; /* fetch context */
2318 ... call many Perl API functions ...
2319 }
ee072b34
GS
2320
2321 [... etc ...]
2322
c52f9dcd 2323 MODULE = Foo PACKAGE = Foo
ee072b34 2324
c52f9dcd 2325 /* typical XSUB */
ee072b34 2326
c52f9dcd
JH
2327 void
2328 my_xsub(arg)
2329 int arg
2330 CODE:
2331 my_private_function(arg, 10);
ee072b34
GS
2332
2333Note that the only two changes from the normal way of writing an
2334extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2335including the Perl headers, followed by a C<dTHX;> declaration at
2336the start of every function that will call the Perl API. (You'll
2337know which functions need this, because the C compiler will complain
2338that there's an undeclared identifier in those functions.) No changes
2339are needed for the XSUBs themselves, because the XS() macro is
2340correctly defined to pass in the implicit context if needed.
2341
2342The third, even more efficient way is to ape how it is done within
2343the Perl guts:
2344
2345
c52f9dcd
JH
2346 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2347 #include "EXTERN.h"
2348 #include "perl.h"
2349 #include "XSUB.h"
ee072b34
GS
2350
2351 /* pTHX_ only needed for functions that call Perl API */
fd061412 2352 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2353
fd061412 2354 STATIC void
c52f9dcd
JH
2355 my_private_function(pTHX_ int arg1, int arg2)
2356 {
2357 /* dTHX; not needed here, because THX is an argument */
2358 ... call Perl API functions ...
2359 }
ee072b34
GS
2360
2361 [... etc ...]
2362
c52f9dcd 2363 MODULE = Foo PACKAGE = Foo
ee072b34 2364
c52f9dcd 2365 /* typical XSUB */
ee072b34 2366
c52f9dcd
JH
2367 void
2368 my_xsub(arg)
2369 int arg
2370 CODE:
2371 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2372
2373This implementation never has to fetch the context using a function
2374call, since it is always passed as an extra argument. Depending on
2375your needs for simplicity or efficiency, you may mix the previous
2376two approaches freely.
2377
651a3225
GS
2378Never add a comma after C<pTHX> yourself--always use the form of the
2379macro with the underscore for functions that take explicit arguments,
2380or the form without the argument for functions with no explicit arguments.
ee072b34 2381
27da23d5
JH
2382If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2383definition is needed if the Perl global variables (see F<perlvars.h>
2384or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2385used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2386the need for C<dVAR> only with the said compile-time define, because
2387otherwise the Perl global variables are visible as-is.
2388
a7486cbb
JH
2389=head2 Should I do anything special if I call perl from multiple threads?
2390
2391If you create interpreters in one thread and then proceed to call them in
2392another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2393initialized correctly in each of those threads.
2394
2395The C<perl_alloc> and C<perl_clone> API functions will automatically set
2396the TLS slot to the interpreter they created, so that there is no need to do
2397anything special if the interpreter is always accessed in the same thread that
2398created it, and that thread did not create or call any other interpreters
2399afterwards. If that is not the case, you have to set the TLS slot of the
2400thread before calling any functions in the Perl API on that particular
2401interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2402thread as the first thing you do:
2403
2404 /* do this before doing anything else with some_perl */
2405 PERL_SET_CONTEXT(some_perl);
2406
2407 ... other Perl API calls on some_perl go here ...
2408
ee072b34
GS
2409=head2 Future Plans and PERL_IMPLICIT_SYS
2410
2411Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2412that the interpreter knows about itself and pass it around, so too are
2413there plans to allow the interpreter to bundle up everything it knows
2414about the environment it's running on. This is enabled with the
7b52221d
RGS
2415PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2416Windows.
ee072b34
GS
2417
2418This allows the ability to provide an extra pointer (called the "host"
2419environment) for all the system calls. This makes it possible for
2420all the system stuff to maintain their own state, broken down into
2421seven C structures. These are thin wrappers around the usual system
0147cd53 2422calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2423more ambitious host (like the one that would do fork() emulation) all
2424the extra work needed to pretend that different interpreters are
2425actually different "processes", would be done here.
2426
2427The Perl engine/interpreter and the host are orthogonal entities.
2428There could be one or more interpreters in a process, and one or
2429more "hosts", with free association between them.
2430
a422fd2d
SC
2431=head1 Internal Functions
2432
2433All of Perl's internal functions which will be exposed to the outside
06f6df17 2434world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d 2435functions or functions used in a program in which Perl is embedded.
10e2eb10 2436Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2437static functions start with C<S_>.)
a422fd2d 2438
0972ecdf
DM
2439Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2440either with or without the C<Perl_> prefix, thanks to a bunch of defines
10e2eb10 2441that live in F<embed.h>. Note that extension code should I<not> set
0972ecdf
DM
2442C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2443breakage of the XS in each new perl release.
2444
2445The file F<embed.h> is generated automatically from
10e2eb10 2446F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
dc9b1d22 2447header files for the internal functions, generates the documentation
10e2eb10 2448and a lot of other bits and pieces. It's important that when you add
dc9b1d22 2449a new function to the core or change an existing one, you change the
10e2eb10 2450data in the table in F<embed.fnc> as well. Here's a sample entry from
dc9b1d22 2451that table:
a422fd2d
SC
2452
2453 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2454
10e2eb10
FC
2455The second column is the return type, the third column the name. Columns
2456after that are the arguments. The first column is a set of flags:
a422fd2d
SC
2457
2458=over 3
2459
2460=item A
2461
10e2eb10
FC
2462This function is a part of the public
2463API. All such functions should also
1aa6ea50 2464have 'd', very few do not.
a422fd2d
SC
2465
2466=item p
2467
1aa6ea50
JC
2468This function has a C<Perl_> prefix; i.e. it is defined as
2469C<Perl_av_fetch>.
a422fd2d
SC
2470
2471=item d
2472
2473This function has documentation using the C<apidoc> feature which we'll
1aa6ea50 2474look at in a second. Some functions have 'd' but not 'A'; docs are good.
a422fd2d
SC
2475
2476=back
2477
2478Other available flags are:
2479
2480=over 3
2481
2482=item s
2483
1aa6ea50
JC
2484This is a static function and is defined as C<STATIC S_whatever>, and
2485usually called within the sources as C<whatever(...)>.
a422fd2d
SC
2486
2487=item n
2488
da8c5729 2489This does not need an interpreter context, so the definition has no
1aa6ea50 2490C<pTHX>, and it follows that callers don't use C<aTHX>. (See
d3a43cd8 2491L</Background and PERL_IMPLICIT_CONTEXT>.)
a422fd2d
SC
2492
2493=item r
2494
2495This function never returns; C<croak>, C<exit> and friends.
2496
2497=item f
2498
2499This function takes a variable number of arguments, C<printf> style.
2500The argument list should end with C<...>, like this:
2501
2502 Afprd |void |croak |const char* pat|...
2503
a7486cbb 2504=item M
a422fd2d 2505
00aadd71 2506This function is part of the experimental development API, and may change
a422fd2d
SC
2507or disappear without notice.
2508
2509=item o
2510
2511This function should not have a compatibility macro to define, say,
10e2eb10 2512C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
a422fd2d 2513
a422fd2d
SC
2514=item x
2515
2516This function isn't exported out of the Perl core.
2517
dc9b1d22
MHM
2518=item m
2519
2520This is implemented as a macro.
2521
2522=item X
2523
2524This function is explicitly exported.
2525
2526=item E
2527
2528This function is visible to extensions included in the Perl core.
2529
2530=item b
2531
2532Binary backward compatibility; this function is a macro but also has
2533a C<Perl_> implementation (which is exported).
2534
1aa6ea50
JC
2535=item others
2536
2537See the comments at the top of C<embed.fnc> for others.
2538
a422fd2d
SC
2539=back
2540
dc9b1d22
MHM
2541If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2542C<make regen_headers> to force a rebuild of F<embed.h> and other
2543auto-generated files.
a422fd2d 2544
6b4667fc 2545=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2546
6b4667fc
A
2547If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2548formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2549following macros for portability
9dd9db0b 2550
c52f9dcd
JH
2551 IVdf IV in decimal
2552 UVuf UV in decimal
2553 UVof UV in octal
2554 UVxf UV in hexadecimal
2555 NVef NV %e-like
2556 NVff NV %f-like
2557 NVgf NV %g-like
9dd9db0b 2558
6b4667fc
A
2559These will take care of 64-bit integers and long doubles.
2560For example:
2561
c52f9dcd 2562 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2563
2564The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2565
8908e76d
JH
2566If you are printing addresses of pointers, use UVxf combined
2567with PTR2UV(), do not use %lx or %p.
2568
2569=head2 Pointer-To-Integer and Integer-To-Pointer
2570
2571Because pointer size does not necessarily equal integer size,
2572use the follow macros to do it right.
2573
c52f9dcd
JH
2574 PTR2UV(pointer)
2575 PTR2IV(pointer)
2576 PTR2NV(pointer)
2577 INT2PTR(pointertotype, integer)
8908e76d
JH
2578
2579For example:
2580
c52f9dcd
JH
2581 IV iv = ...;
2582 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2583
2584and
2585
c52f9dcd
JH
2586 AV *av = ...;
2587 UV uv = PTR2UV(av);
8908e76d 2588
0ca3a874
MHM
2589=head2 Exception Handling
2590
9b5c3821 2591There are a couple of macros to do very basic exception handling in XS
10e2eb10 2592modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
9b5c3821
MHM
2593be able to use these macros:
2594
2595 #define NO_XSLOCKS
2596 #include "XSUB.h"
2597
2598You can use these macros if you call code that may croak, but you need
10e2eb10 2599to do some cleanup before giving control back to Perl. For example:
0ca3a874 2600
d7f8936a 2601 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2602
2603 XCPT_TRY_START {
2604 code_that_may_croak();
2605 } XCPT_TRY_END
2606
2607 XCPT_CATCH
2608 {
2609 /* do cleanup here */
2610 XCPT_RETHROW;
2611 }
2612
2613Note that you always have to rethrow an exception that has been
10e2eb10
FC
2614caught. Using these macros, it is not possible to just catch the
2615exception and ignore it. If you have to ignore the exception, you
0ca3a874
MHM
2616have to use the C<call_*> function.
2617
2618The advantage of using the above macros is that you don't have
2619to setup an extra function for C<call_*>, and that using these
2620macros is faster than using C<call_*>.
2621
a422fd2d
SC
2622=head2 Source Documentation
2623
2624There's an effort going on to document the internal functions and
2625automatically produce reference manuals from them - L<perlapi> is one
2626such manual which details all the functions which are available to XS
10e2eb10 2627writers. L<perlintern> is the autogenerated manual for the functions
a422fd2d
SC
2628which are not part of the API and are supposedly for internal use only.
2629
2630Source documentation is created by putting POD comments into the C
2631source, like this:
2632
2633 /*
2634 =for apidoc sv_setiv
2635
2636 Copies an integer into the given SV. Does not handle 'set' magic. See
2637 C<sv_setiv_mg>.
2638
2639 =cut
2640 */
2641
2642Please try and supply some documentation if you add functions to the
2643Perl core.
2644
0d098d33
MHM
2645=head2 Backwards compatibility
2646
10e2eb10
FC
2647The Perl API changes over time. New functions are
2648added or the interfaces of existing functions are
2649changed. The C<Devel::PPPort> module tries to
0d098d33
MHM
2650provide compatibility code for some of these changes, so XS writers don't
2651have to code it themselves when supporting multiple versions of Perl.
2652
2653C<Devel::PPPort> generates a C header file F<ppport.h> that can also
10e2eb10 2654be run as a Perl script. To generate F<ppport.h>, run:
0d098d33
MHM
2655
2656 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2657
2658Besides checking existing XS code, the script can also be used to retrieve
2659compatibility information for various API calls using the C<--api-info>
10e2eb10 2660command line switch. For example:
0d098d33
MHM
2661
2662 % perl ppport.h --api-info=sv_magicext
2663
2664For details, see C<perldoc ppport.h>.
2665
a422fd2d
SC
2666=head1 Unicode Support
2667
10e2eb10 2668Perl 5.6.0 introduced Unicode support. It's important for porters and XS
a422fd2d
SC
2669writers to understand this support and make sure that the code they
2670write does not corrupt Unicode data.
2671
2672=head2 What B<is> Unicode, anyway?
2673
10e2eb10
FC
2674In the olden, less enlightened times, we all used to use ASCII. Most of
2675us did, anyway. The big problem with ASCII is that it's American. Well,
a422fd2d 2676no, that's not actually the problem; the problem is that it's not
10e2eb10 2677particularly useful for people who don't use the Roman alphabet. What
a422fd2d 2678used to happen was that particular languages would stick their own
10e2eb10 2679alphabet in the upper range of the sequence, between 128 and 255. Of
a422fd2d
SC
2680course, we then ended up with plenty of variants that weren't quite
2681ASCII, and the whole point of it being a standard was lost.
2682
2683Worse still, if you've got a language like Chinese or
2684Japanese that has hundreds or thousands of characters, then you really
2685can't fit them into a mere 256, so they had to forget about ASCII
2686altogether, and build their own systems using pairs of numbers to refer
2687to one character.
2688
2689To fix this, some people formed Unicode, Inc. and
2690produced a new character set containing all the characters you can
10e2eb10
FC
2691possibly think of and more. There are several ways of representing these
2692characters, and the one Perl uses is called UTF-8. UTF-8 uses
2693a variable number of bytes to represent a character. You can learn more
2575c402 2694about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2695
1e54db1a 2696=head2 How can I recognise a UTF-8 string?
a422fd2d 2697
10e2eb10
FC
2698You can't. This is because UTF-8 data is stored in bytes just like
2699non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d 2700capital E with a grave accent, is represented by the two bytes
10e2eb10
FC
2701C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2702has that byte sequence as well. So you can't tell just by looking - this
a422fd2d
SC
2703is what makes Unicode input an interesting problem.
2704
2575c402
JW
2705In general, you either have to know what you're dealing with, or you
2706have to guess. The API function C<is_utf8_string> can help; it'll tell
10e2eb10
FC
2707you if a string contains only valid UTF-8 characters. However, it can't
2708do the work for you. On a character-by-character basis,
6302f837 2709C<isUTF8_CHAR>
2575c402 2710will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2711
1e54db1a 2712=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2713
1e54db1a 2714As mentioned above, UTF-8 uses a variable number of bytes to store a
10e2eb10
FC
2715character. Characters with values 0...127 are stored in one
2716byte, just like good ol' ASCII. Character 128 is stored as
2717C<v194.128>; this continues up to character 191, which is
2718C<v194.191>. Now we've run out of bits (191 is binary
2719C<10111111>) so we move on; 192 is C<v195.128>. And
a422fd2d
SC
2720so it goes on, moving to three bytes at character 2048.
2721
1e54db1a 2722Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2723how long the first character in it is with the C<UTF8SKIP> macro:
2724
2725 char *utf = "\305\233\340\240\201";
2726 I32 len;
2727
2728 len = UTF8SKIP(utf); /* len is 2 here */
2729 utf += len;
2730 len = UTF8SKIP(utf); /* len is 3 here */
2731
1e54db1a 2732Another way to skip over characters in a UTF-8 string is to use
a422fd2d 2733C<utf8_hop>, which takes a string and a number of characters to skip
10e2eb10 2734over. You're on your own about bounds checking, though, so don't use it
a422fd2d
SC
2735lightly.
2736
1e54db1a 2737All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe
RGS
2738so you can test if you need to do something special with this
2739character like this (the UTF8_IS_INVARIANT() is a macro that tests
9f98c7fe 2740whether the byte is encoded as a single byte even in UTF-8):
a422fd2d 2741
3a2263fe 2742 U8 *utf;
4b88fb76 2743 U8 *utf_end; /* 1 beyond buffer pointed to by utf */
3a2263fe 2744 UV uv; /* Note: a UV, not a U8, not a char */
95701e00 2745 STRLEN len; /* length of character in bytes */
a422fd2d 2746
3a2263fe 2747 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2748 /* Must treat this as UTF-8 */
4b88fb76 2749 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
a422fd2d
SC
2750 else
2751 /* OK to treat this character as a byte */
2752 uv = *utf;
2753
4b88fb76 2754You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
95701e00 2755value of the character; the inverse function C<uvchr_to_utf8> is available
1e54db1a 2756for putting a UV into UTF-8:
a422fd2d 2757
3a2263fe 2758 if (!UTF8_IS_INVARIANT(uv))
a422fd2d 2759 /* Must treat this as UTF8 */
95701e00 2760 utf8 = uvchr_to_utf8(utf8, uv);
a422fd2d
SC
2761 else
2762 /* OK to treat this character as a byte */
2763 *utf8++ = uv;
2764
2765You B<must> convert characters to UVs using the above functions if
1e54db1a 2766you're ever in a situation where you have to match UTF-8 and non-UTF-8
10e2eb10 2767characters. You may not skip over UTF-8 characters in this case. If you
1e54db1a
JH
2768do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2769for instance, if your UTF-8 string contains C<v196.172>, and you skip
2770that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2771So don't do that!
2772
1e54db1a 2773=head2 How does Perl store UTF-8 strings?
a422fd2d
SC
2774
2775Currently, Perl deals with Unicode strings and non-Unicode strings
10e2eb10
FC
2776slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2777string is internally encoded as UTF-8. Without it, the byte value is the
2575c402 2778codepoint number and vice versa (in other words, the string is encoded
e1b711da 2779as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
c31cc9fc
FC
2780semantics). This flag is only meaningful if the SV is C<SvPOK>
2781or immediately after stringification via C<SvPV> or a similar
2782macro. You can check and manipulate this flag with the
2575c402 2783following macros:
a422fd2d
SC
2784
2785 SvUTF8(sv)
2786 SvUTF8_on(sv)
2787 SvUTF8_off(sv)
2788
2789This flag has an important effect on Perl's treatment of the string: if
2790Unicode data is not properly distinguished, regular expressions,
2791C<length>, C<substr> and other string handling operations will have
2792undesirable results.
2793
2794The problem comes when you have, for instance, a string that isn't
2575c402 2795flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
1e54db1a 2796especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d
SC
2797
2798Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2799need be sure you don't accidentally knock it off while you're
10e2eb10 2800manipulating SVs. More specifically, you cannot expect to do this:
a422fd2d
SC
2801
2802 SV *sv;
2803 SV *nsv;
2804 STRLEN len;
2805 char *p;
2806
2807 p = SvPV(sv, len);
2808 frobnicate(p);
2809 nsv = newSVpvn(p, len);
2810
2811The C<char*> string does not tell you the whole story, and you can't
10e2eb10 2812copy or reconstruct an SV just by copying the string value. Check if the
c31cc9fc
FC
2813old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2814accordingly:
a422fd2d
SC
2815
2816 p = SvPV(sv, len);
2817 frobnicate(p);
2818 nsv = newSVpvn(p, len);
2819 if (SvUTF8(sv))
2820 SvUTF8_on(nsv);
2821
2822In fact, your C<frobnicate> function should be made aware of whether or
1e54db1a 2823not it's dealing with UTF-8 data, so that it can handle the string
a422fd2d
SC
2824appropriately.
2825
3a2263fe 2826Since just passing an SV to an XS function and copying the data of
2575c402 2827the SV is not enough to copy the UTF8 flags, even less right is just
3a2263fe
RGS
2828passing a C<char *> to an XS function.
2829
1e54db1a 2830=head2 How do I convert a string to UTF-8?
a422fd2d 2831
2575c402 2832If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
10e2eb10 2833one of the strings to UTF-8. If you've got an SV, the easiest way to do
2575c402 2834this is:
a422fd2d
SC
2835
2836 sv_utf8_upgrade(sv);
2837
2838However, you must not do this, for example:
2839
2840 if (!SvUTF8(left))
2841 sv_utf8_upgrade(left);
2842
2843If you do this in a binary operator, you will actually change one of the
b1866b2d 2844strings that came into the operator, and, while it shouldn't be noticeable
2575c402 2845by the end user, it can cause problems in deficient code.
a422fd2d 2846
1e54db1a 2847Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
10e2eb10
FC
2848string argument. This is useful for having the data available for
2849comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2850C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2851the string contains any characters above 255 that can't be represented
2852in a single byte.
2853
2854=head2 Is there anything else I need to know?
2855
10e2eb10 2856Not really. Just remember these things:
a422fd2d
SC
2857
2858=over 3
2859
2860=item *
2861
10e2eb10 2862There's no way to tell if a string is UTF-8 or not. You can tell if an SV
c31cc9fc
FC
2863is UTF-8 by looking at its C<SvUTF8> flag after stringifying it
2864with C<SvPV> or a similar macro. Don't forget to set the flag if
10e2eb10 2865something should be UTF-8. Treat the flag as part of the PV, even though
a422fd2d
SC
2866it's not - if you pass on the PV to somewhere, pass on the flag too.
2867
2868=item *
2869
4b88fb76 2870If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
3a2263fe 2871unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
2872
2873=item *
2874
1e54db1a 2875When writing a character C<uv> to a UTF-8 string, B<always> use
95701e00 2876C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
3a2263fe 2877you can use C<*s = uv>.
a422fd2d
SC
2878
2879=item *
2880
10e2eb10
FC
2881Mixing UTF-8 and non-UTF-8 strings is
2882tricky. Use C<bytes_to_utf8> to get
2bbc8d55 2883a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
2884
2885=back
2886
53e06cf0
SC
2887=head1 Custom Operators
2888
2a0fd0f1 2889Custom operator support is an experimental feature that allows you to
10e2eb10 2890define your own ops. This is primarily to allow the building of
53e06cf0
SC
2891interpreters for other languages in the Perl core, but it also allows
2892optimizations through the creation of "macro-ops" (ops which perform the
2893functions of multiple ops which are usually executed together, such as
1aa6ea50 2894C<gvsv, gvsv, add>.)
53e06cf0 2895
10e2eb10 2896This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0 2897core does not "know" anything special about this op type, and so it will
10e2eb10 2898not be involved in any optimizations. This also means that you can
53e06cf0
SC
2899define your custom ops to be any op structure - unary, binary, list and
2900so on - you like.
2901
10e2eb10
FC
2902It's important to know what custom operators won't do for you. They
2903won't let you add new syntax to Perl, directly. They won't even let you
2904add new keywords, directly. In fact, they won't change the way Perl
2905compiles a program at all. You have to do those changes yourself, after
2906Perl has compiled the program. You do this either by manipulating the op
53e06cf0
SC
2907tree using a C<CHECK> block and the C<B::Generate> module, or by adding
2908a custom peephole optimizer with the C<optimize> module.
2909
2910When you do this, you replace ordinary Perl ops with custom ops by
407f86e1 2911creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
10e2eb10
FC
2912PP function. This should be defined in XS code, and should look like
2913the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
53e06cf0
SC
2914takes the appropriate number of values from the stack, and you are
2915responsible for adding stack marks if necessary.
2916
2917You should also "register" your op with the Perl interpreter so that it
10e2eb10 2918can produce sensible error and warning messages. Since it is possible to
53e06cf0 2919have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
9733086d 2920Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
10e2eb10 2921it is dealing with. You should create an C<XOP> structure for each
9733086d
BM
2922ppaddr you use, set the properties of the custom op with
2923C<XopENTRY_set>, and register the structure against the ppaddr using
10e2eb10 2924C<Perl_custom_op_register>. A trivial example might look like:
9733086d
BM
2925
2926 static XOP my_xop;
2927 static OP *my_pp(pTHX);
2928
2929 BOOT:
2930 XopENTRY_set(&my_xop, xop_name, "myxop");
2931 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
2932 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
2933
2934The available fields in the structure are:
2935
2936=over 4
2937
2938=item xop_name
2939
10e2eb10 2940A short name for your op. This will be included in some error messages,
9733086d
BM
2941and will also be returned as C<< $op->name >> by the L<B|B> module, so
2942it will appear in the output of module like L<B::Concise|B::Concise>.
2943
2944=item xop_desc
2945
2946A short description of the function of the op.
2947
2948=item xop_class
2949
10e2eb10 2950Which of the various C<*OP> structures this op uses. This should be one of
9733086d
BM
2951the C<OA_*> constants from F<op.h>, namely
2952
2953=over 4
2954
2955=item OA_BASEOP
2956
2957=item OA_UNOP
2958
2959=item OA_BINOP
2960
2961=item OA_LOGOP
2962
2963=item OA_LISTOP
2964
2965=item OA_PMOP
2966
2967=item OA_SVOP
2968
2969=item OA_PADOP
2970
2971=item OA_PVOP_OR_SVOP
2972
10e2eb10 2973This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
9733086d
BM
2974the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
2975
2976=item OA_LOOP
2977
2978=item OA_COP
2979
2980=back
2981
2982The other C<OA_*> constants should not be used.
2983
2984=item xop_peep
2985
2986This member is of type C<Perl_cpeep_t>, which expands to C<void
10e2eb10 2987(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
9733086d 2988will be called from C<Perl_rpeep> when ops of this type are encountered
10e2eb10 2989by the peephole optimizer. I<o> is the OP that needs optimizing;
9733086d
BM
2990I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
2991
2992=back
53e06cf0 2993
e7d4c058 2994C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 2995
954c1994 2996=head1 AUTHORS
e89caa19 2997
954c1994 2998Until May 1997, this document was maintained by Jeff Okamoto
9b5bb84f
SB
2999E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
3000itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
cb1a09d0 3001
954c1994
GS
3002With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
3003Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
3004Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
3005Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 3006
954c1994 3007=head1 SEE ALSO
cb1a09d0 3008
ba555bf5 3009L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>