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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
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7This document attempts to describe how to use the Perl API, as well as
8containing some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl has three typedefs that handle Perl's three main data types:
17
18 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21
d1b91892 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
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31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
32as well.)
a0d0e21e 33
54310121 34=head2 Working with SVs
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35
36An SV can be created and loaded with one command. There are four types of
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37values that can be loaded: an integer value (IV), a double (NV),
38a string (PV), and another scalar (SV).
a0d0e21e 39
9da1e3b5 40The six routines are:
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41
42 SV* newSViv(IV);
43 SV* newSVnv(double);
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44 SV* newSVpv(const char*, int);
45 SV* newSVpvn(const char*, int);
46fc3d4c 46 SV* newSVpvf(const char*, ...);
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47 SV* newSVsv(SV*);
48
deb3007b 49To change the value of an *already-existing* SV, there are seven routines:
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50
51 void sv_setiv(SV*, IV);
deb3007b 52 void sv_setuv(SV*, UV);
a0d0e21e 53 void sv_setnv(SV*, double);
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54 void sv_setpv(SV*, const char*);
55 void sv_setpvn(SV*, const char*, int)
46fc3d4c 56 void sv_setpvf(SV*, const char*, ...);
9abd00ed 57 void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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58 void sv_setsv(SV*, SV*);
59
60Notice that you can choose to specify the length of the string to be
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61assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
62allow Perl to calculate the length by using C<sv_setpv> or by specifying
630 as the second argument to C<newSVpv>. Be warned, though, that Perl will
64determine the string's length by using C<strlen>, which depends on the
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65string terminating with a NUL character.
66
67The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
68formatted output becomes the value.
69
70C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify
71either a pointer to a variable argument list or the address and length of
72an array of SVs. The last argument points to a boolean; on return, if that
73boolean is true, then locale-specific information has been used to format
c2611fb3 74the string, and the string's contents are therefore untrustworthy (see
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75L<perlsec>). This pointer may be NULL if that information is not
76important. Note that this function requires you to specify the length of
77the format.
78
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79STRLEN is an integer type (Size_t, usually defined as size_t in
80config.h) guaranteed to be large enough to represent the size of
81any string that perl can handle.
82
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83The C<sv_set*()> functions are not generic enough to operate on values
84that have "magic". See L<Magic Virtual Tables> later in this document.
a0d0e21e 85
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86All SVs that contain strings should be terminated with a NUL character.
87If it is not NUL-terminated there is a risk of
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88core dumps and corruptions from code which passes the string to C
89functions or system calls which expect a NUL-terminated string.
90Perl's own functions typically add a trailing NUL for this reason.
91Nevertheless, you should be very careful when you pass a string stored
92in an SV to a C function or system call.
93
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94To access the actual value that an SV points to, you can use the macros:
95
96 SvIV(SV*)
954c1994 97 SvUV(SV*)
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98 SvNV(SV*)
99 SvPV(SV*, STRLEN len)
1fa8b10d 100 SvPV_nolen(SV*)
a0d0e21e 101
954c1994 102which will automatically coerce the actual scalar type into an IV, UV, double,
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103or string.
104
105In the C<SvPV> macro, the length of the string returned is placed into the
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106variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
107not care what the length of the data is, use the C<SvPV_nolen> macro.
108Historically the C<SvPV> macro with the global variable C<PL_na> has been
109used in this case. But that can be quite inefficient because C<PL_na> must
110be accessed in thread-local storage in threaded Perl. In any case, remember
111that Perl allows arbitrary strings of data that may both contain NULs and
112might not be terminated by a NUL.
a0d0e21e 113
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114Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
115len);>. It might work with your compiler, but it won't work for everyone.
116Break this sort of statement up into separate assignments:
117
b2f5ed49 118 SV *s;
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119 STRLEN len;
120 char * ptr;
b2f5ed49 121 ptr = SvPV(s, len);
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122 foo(ptr, len);
123
07fa94a1 124If you want to know if the scalar value is TRUE, you can use:
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125
126 SvTRUE(SV*)
127
128Although Perl will automatically grow strings for you, if you need to force
129Perl to allocate more memory for your SV, you can use the macro
130
131 SvGROW(SV*, STRLEN newlen)
132
133which will determine if more memory needs to be allocated. If so, it will
134call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
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135decrease, the allocated memory of an SV and that it does not automatically
136add a byte for the a trailing NUL (perl's own string functions typically do
8ebc5c01 137C<SvGROW(sv, len + 1)>).
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138
139If you have an SV and want to know what kind of data Perl thinks is stored
140in it, you can use the following macros to check the type of SV you have.
141
142 SvIOK(SV*)
143 SvNOK(SV*)
144 SvPOK(SV*)
145
146You can get and set the current length of the string stored in an SV with
147the following macros:
148
149 SvCUR(SV*)
150 SvCUR_set(SV*, I32 val)
151
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152You can also get a pointer to the end of the string stored in the SV
153with the macro:
154
155 SvEND(SV*)
156
157But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 158
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159If you want to append something to the end of string stored in an C<SV*>,
160you can use the following functions:
161
08105a92 162 void sv_catpv(SV*, const char*);
e65f3abd 163 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 164 void sv_catpvf(SV*, const char*, ...);
9abd00ed 165 void sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
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166 void sv_catsv(SV*, SV*);
167
168The first function calculates the length of the string to be appended by
169using C<strlen>. In the second, you specify the length of the string
46fc3d4c 170yourself. The third function processes its arguments like C<sprintf> and
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171appends the formatted output. The fourth function works like C<vsprintf>.
172You can specify the address and length of an array of SVs instead of the
173va_list argument. The fifth function extends the string stored in the first
174SV with the string stored in the second SV. It also forces the second SV
175to be interpreted as a string.
176
177The C<sv_cat*()> functions are not generic enough to operate on values that
178have "magic". See L<Magic Virtual Tables> later in this document.
d1b91892 179
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180If you know the name of a scalar variable, you can get a pointer to its SV
181by using the following:
182
4929bf7b 183 SV* get_sv("package::varname", FALSE);
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184
185This returns NULL if the variable does not exist.
186
d1b91892 187If you want to know if this variable (or any other SV) is actually C<defined>,
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188you can call:
189
190 SvOK(SV*)
191
9cde0e7f 192The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its
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193address can be used whenever an C<SV*> is needed.
194
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195There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean
196TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can
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197be used whenever an C<SV*> is needed.
198
9cde0e7f 199Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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200Take this code:
201
202 SV* sv = (SV*) 0;
203 if (I-am-to-return-a-real-value) {
204 sv = sv_2mortal(newSViv(42));
205 }
206 sv_setsv(ST(0), sv);
207
208This code tries to return a new SV (which contains the value 42) if it should
04343c6d 209return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 210pointer which, somewhere down the line, will cause a segmentation violation,
9cde0e7f 211bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first
5f05dabc 212line and all will be well.
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213
214To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
3fe9a6f1 215call is not necessary (see L<Reference Counts and Mortality>).
a0d0e21e 216
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217=head2 Offsets
218
219Perl provides the function C<sv_chop> to efficiently remove characters
220from the beginning of a string; you give it an SV and a pointer to
221somewhere inside the the PV, and it discards everything before the
222pointer. The efficiency comes by means of a little hack: instead of
223actually removing the characters, C<sv_chop> sets the flag C<OOK>
224(offset OK) to signal to other functions that the offset hack is in
225effect, and it puts the number of bytes chopped off into the IV field
226of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
227many bytes, and adjusts C<SvCUR> and C<SvLEN>.
228
229Hence, at this point, the start of the buffer that we allocated lives
230at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
231into the middle of this allocated storage.
232
233This is best demonstrated by example:
234
235 % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
236 SV = PVIV(0x8128450) at 0x81340f0
237 REFCNT = 1
238 FLAGS = (POK,OOK,pPOK)
239 IV = 1 (OFFSET)
240 PV = 0x8135781 ( "1" . ) "2345"\0
241 CUR = 4
242 LEN = 5
243
244Here the number of bytes chopped off (1) is put into IV, and
245C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
246portion of the string between the "real" and the "fake" beginnings is
247shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
248the fake beginning, not the real one.
249
d1b91892 250=head2 What's Really Stored in an SV?
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251
252Recall that the usual method of determining the type of scalar you have is
5f05dabc 253to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 254usually these macros will always return TRUE and calling the C<Sv*V>
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255macros will do the appropriate conversion of string to integer/double or
256integer/double to string.
257
258If you I<really> need to know if you have an integer, double, or string
259pointer in an SV, you can use the following three macros instead:
260
261 SvIOKp(SV*)
262 SvNOKp(SV*)
263 SvPOKp(SV*)
264
265These will tell you if you truly have an integer, double, or string pointer
d1b91892 266stored in your SV. The "p" stands for private.
a0d0e21e 267
07fa94a1 268In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 269
54310121 270=head2 Working with AVs
a0d0e21e 271
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272There are two ways to create and load an AV. The first method creates an
273empty AV:
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274
275 AV* newAV();
276
54310121 277The second method both creates the AV and initially populates it with SVs:
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278
279 AV* av_make(I32 num, SV **ptr);
280
5f05dabc 281The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 282AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 283
54310121 284Once the AV has been created, the following operations are possible on AVs:
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285
286 void av_push(AV*, SV*);
287 SV* av_pop(AV*);
288 SV* av_shift(AV*);
289 void av_unshift(AV*, I32 num);
290
291These should be familiar operations, with the exception of C<av_unshift>.
292This routine adds C<num> elements at the front of the array with the C<undef>
293value. You must then use C<av_store> (described below) to assign values
294to these new elements.
295
296Here are some other functions:
297
5f05dabc 298 I32 av_len(AV*);
a0d0e21e 299 SV** av_fetch(AV*, I32 key, I32 lval);
a0d0e21e 300 SV** av_store(AV*, I32 key, SV* val);
a0d0e21e 301
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302The C<av_len> function returns the highest index value in array (just
303like $#array in Perl). If the array is empty, -1 is returned. The
304C<av_fetch> function returns the value at index C<key>, but if C<lval>
305is non-zero, then C<av_fetch> will store an undef value at that index.
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306The C<av_store> function stores the value C<val> at index C<key>, and does
307not increment the reference count of C<val>. Thus the caller is responsible
308for taking care of that, and if C<av_store> returns NULL, the caller will
309have to decrement the reference count to avoid a memory leak. Note that
310C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
311return value.
d1b91892 312
a0d0e21e 313 void av_clear(AV*);
a0d0e21e 314 void av_undef(AV*);
cb1a09d0 315 void av_extend(AV*, I32 key);
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316
317The C<av_clear> function deletes all the elements in the AV* array, but
318does not actually delete the array itself. The C<av_undef> function will
319delete all the elements in the array plus the array itself. The
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320C<av_extend> function extends the array so that it contains at least C<key+1>
321elements. If C<key+1> is less than the currently allocated length of the array,
322then nothing is done.
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323
324If you know the name of an array variable, you can get a pointer to its AV
325by using the following:
326
4929bf7b 327 AV* get_av("package::varname", FALSE);
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328
329This returns NULL if the variable does not exist.
330
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331See L<Understanding the Magic of Tied Hashes and Arrays> for more
332information on how to use the array access functions on tied arrays.
333
54310121 334=head2 Working with HVs
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335
336To create an HV, you use the following routine:
337
338 HV* newHV();
339
54310121 340Once the HV has been created, the following operations are possible on HVs:
a0d0e21e 341
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342 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
343 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 344
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345The C<klen> parameter is the length of the key being passed in (Note that
346you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
347length of the key). The C<val> argument contains the SV pointer to the
54310121 348scalar being stored, and C<hash> is the precomputed hash value (zero if
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349you want C<hv_store> to calculate it for you). The C<lval> parameter
350indicates whether this fetch is actually a part of a store operation, in
351which case a new undefined value will be added to the HV with the supplied
352key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 353
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354Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
355C<SV*>. To access the scalar value, you must first dereference the return
356value. However, you should check to make sure that the return value is
357not NULL before dereferencing it.
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358
359These two functions check if a hash table entry exists, and deletes it.
360
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361 bool hv_exists(HV*, const char* key, U32 klen);
362 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 363
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364If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
365create and return a mortal copy of the deleted value.
366
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367And more miscellaneous functions:
368
369 void hv_clear(HV*);
a0d0e21e 370 void hv_undef(HV*);
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371
372Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
373table but does not actually delete the hash table. The C<hv_undef> deletes
374both the entries and the hash table itself.
a0d0e21e 375
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376Perl keeps the actual data in linked list of structures with a typedef of HE.
377These contain the actual key and value pointers (plus extra administrative
378overhead). The key is a string pointer; the value is an C<SV*>. However,
379once you have an C<HE*>, to get the actual key and value, use the routines
380specified below.
381
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382 I32 hv_iterinit(HV*);
383 /* Prepares starting point to traverse hash table */
384 HE* hv_iternext(HV*);
385 /* Get the next entry, and return a pointer to a
386 structure that has both the key and value */
387 char* hv_iterkey(HE* entry, I32* retlen);
388 /* Get the key from an HE structure and also return
389 the length of the key string */
cb1a09d0 390 SV* hv_iterval(HV*, HE* entry);
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391 /* Return a SV pointer to the value of the HE
392 structure */
cb1a09d0 393 SV* hv_iternextsv(HV*, char** key, I32* retlen);
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394 /* This convenience routine combines hv_iternext,
395 hv_iterkey, and hv_iterval. The key and retlen
396 arguments are return values for the key and its
397 length. The value is returned in the SV* argument */
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398
399If you know the name of a hash variable, you can get a pointer to its HV
400by using the following:
401
4929bf7b 402 HV* get_hv("package::varname", FALSE);
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403
404This returns NULL if the variable does not exist.
405
8ebc5c01 406The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:
a0d0e21e 407
a0d0e21e 408 hash = 0;
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409 while (klen--)
410 hash = (hash * 33) + *key++;
87275199 411 hash = hash + (hash >> 5); /* after 5.6 */
ab192400 412
87275199 413The last step was added in version 5.6 to improve distribution of
ab192400 414lower bits in the resulting hash value.
a0d0e21e 415
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416See L<Understanding the Magic of Tied Hashes and Arrays> for more
417information on how to use the hash access functions on tied hashes.
418
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419=head2 Hash API Extensions
420
421Beginning with version 5.004, the following functions are also supported:
422
423 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
424 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 425
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426 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
427 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 428
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429 SV* hv_iterkeysv (HE* entry);
430
431Note that these functions take C<SV*> keys, which simplifies writing
432of extension code that deals with hash structures. These functions
433also allow passing of C<SV*> keys to C<tie> functions without forcing
434you to stringify the keys (unlike the previous set of functions).
435
436They also return and accept whole hash entries (C<HE*>), making their
437use more efficient (since the hash number for a particular string
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438doesn't have to be recomputed every time). See L<perlapi> for detailed
439descriptions.
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440
441The following macros must always be used to access the contents of hash
442entries. Note that the arguments to these macros must be simple
443variables, since they may get evaluated more than once. See
4a4eefd0 444L<perlapi> for detailed descriptions of these macros.
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445
446 HePV(HE* he, STRLEN len)
447 HeVAL(HE* he)
448 HeHASH(HE* he)
449 HeSVKEY(HE* he)
450 HeSVKEY_force(HE* he)
451 HeSVKEY_set(HE* he, SV* sv)
452
453These two lower level macros are defined, but must only be used when
454dealing with keys that are not C<SV*>s:
455
456 HeKEY(HE* he)
457 HeKLEN(HE* he)
458
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459Note that both C<hv_store> and C<hv_store_ent> do not increment the
460reference count of the stored C<val>, which is the caller's responsibility.
461If these functions return a NULL value, the caller will usually have to
462decrement the reference count of C<val> to avoid a memory leak.
1e422769 463
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464=head2 References
465
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466References are a special type of scalar that point to other data types
467(including references).
a0d0e21e 468
07fa94a1 469To create a reference, use either of the following functions:
a0d0e21e 470
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471 SV* newRV_inc((SV*) thing);
472 SV* newRV_noinc((SV*) thing);
a0d0e21e 473
5f05dabc 474The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
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475functions are identical except that C<newRV_inc> increments the reference
476count of the C<thing>, while C<newRV_noinc> does not. For historical
477reasons, C<newRV> is a synonym for C<newRV_inc>.
478
479Once you have a reference, you can use the following macro to dereference
480the reference:
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481
482 SvRV(SV*)
483
484then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 485C<AV*> or C<HV*>, if required.
a0d0e21e 486
d1b91892 487To determine if an SV is a reference, you can use the following macro:
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488
489 SvROK(SV*)
490
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491To discover what type of value the reference refers to, use the following
492macro and then check the return value.
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493
494 SvTYPE(SvRV(SV*))
495
496The most useful types that will be returned are:
497
498 SVt_IV Scalar
499 SVt_NV Scalar
500 SVt_PV Scalar
5f05dabc 501 SVt_RV Scalar
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502 SVt_PVAV Array
503 SVt_PVHV Hash
504 SVt_PVCV Code
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505 SVt_PVGV Glob (possible a file handle)
506 SVt_PVMG Blessed or Magical Scalar
507
508 See the sv.h header file for more details.
d1b91892 509
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510=head2 Blessed References and Class Objects
511
512References are also used to support object-oriented programming. In the
513OO lexicon, an object is simply a reference that has been blessed into a
514package (or class). Once blessed, the programmer may now use the reference
515to access the various methods in the class.
516
517A reference can be blessed into a package with the following function:
518
519 SV* sv_bless(SV* sv, HV* stash);
520
521The C<sv> argument must be a reference. The C<stash> argument specifies
3fe9a6f1 522which class the reference will belong to. See
2ae324a7 523L<Stashes and Globs> for information on converting class names into stashes.
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524
525/* Still under construction */
526
527Upgrades rv to reference if not already one. Creates new SV for rv to
8ebc5c01
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528point to. If C<classname> is non-null, the SV is blessed into the specified
529class. SV is returned.
cb1a09d0 530
08105a92 531 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 532
e1c57cef 533Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
8ebc5c01 534if C<classname> is non-null.
cb1a09d0 535
08105a92 536 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 537 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 538 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 539
5f05dabc 540Copies the pointer value (I<the address, not the string!>) into an SV whose
8ebc5c01 541reference is rv. SV is blessed if C<classname> is non-null.
cb1a09d0 542
08105a92 543 SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
cb1a09d0 544
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545Copies string into an SV whose reference is C<rv>. Set length to 0 to let
546Perl calculate the string length. SV is blessed if C<classname> is non-null.
cb1a09d0 547
e65f3abd 548 SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
cb1a09d0 549
9abd00ed
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550Tests whether the SV is blessed into the specified class. It does not
551check inheritance relationships.
552
08105a92 553 int sv_isa(SV* sv, const char* name);
9abd00ed
GS
554
555Tests whether the SV is a reference to a blessed object.
556
557 int sv_isobject(SV* sv);
558
559Tests whether the SV is derived from the specified class. SV can be either
560a reference to a blessed object or a string containing a class name. This
561is the function implementing the C<UNIVERSAL::isa> functionality.
562
08105a92 563 bool sv_derived_from(SV* sv, const char* name);
9abd00ed
GS
564
565To check if you've got an object derived from a specific class you have
566to write:
567
568 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 569
5f05dabc 570=head2 Creating New Variables
cb1a09d0 571
5f05dabc
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572To create a new Perl variable with an undef value which can be accessed from
573your Perl script, use the following routines, depending on the variable type.
cb1a09d0 574
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575 SV* get_sv("package::varname", TRUE);
576 AV* get_av("package::varname", TRUE);
577 HV* get_hv("package::varname", TRUE);
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578
579Notice the use of TRUE as the second parameter. The new variable can now
580be set, using the routines appropriate to the data type.
581
5f05dabc
PP
582There are additional macros whose values may be bitwise OR'ed with the
583C<TRUE> argument to enable certain extra features. Those bits are:
cb1a09d0 584
5f05dabc 585 GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
54310121 586 "Name <varname> used only once: possible typo" warning.
07fa94a1
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587 GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
588 the variable did not exist before the function was called.
cb1a09d0 589
07fa94a1
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590If you do not specify a package name, the variable is created in the current
591package.
cb1a09d0 592
5f05dabc 593=head2 Reference Counts and Mortality
a0d0e21e 594
54310121
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595Perl uses an reference count-driven garbage collection mechanism. SVs,
596AVs, or HVs (xV for short in the following) start their life with a
55497cff 597reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 598then it will be destroyed and its memory made available for reuse.
55497cff
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599
600This normally doesn't happen at the Perl level unless a variable is
5f05dabc
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601undef'ed or the last variable holding a reference to it is changed or
602overwritten. At the internal level, however, reference counts can be
55497cff
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603manipulated with the following macros:
604
605 int SvREFCNT(SV* sv);
5f05dabc 606 SV* SvREFCNT_inc(SV* sv);
55497cff
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607 void SvREFCNT_dec(SV* sv);
608
609However, there is one other function which manipulates the reference
07fa94a1
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610count of its argument. The C<newRV_inc> function, you will recall,
611creates a reference to the specified argument. As a side effect,
612it increments the argument's reference count. If this is not what
613you want, use C<newRV_noinc> instead.
614
615For example, imagine you want to return a reference from an XSUB function.
616Inside the XSUB routine, you create an SV which initially has a reference
617count of one. Then you call C<newRV_inc>, passing it the just-created SV.
5f05dabc
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618This returns the reference as a new SV, but the reference count of the
619SV you passed to C<newRV_inc> has been incremented to two. Now you
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620return the reference from the XSUB routine and forget about the SV.
621But Perl hasn't! Whenever the returned reference is destroyed, the
622reference count of the original SV is decreased to one and nothing happens.
623The SV will hang around without any way to access it until Perl itself
624terminates. This is a memory leak.
5f05dabc
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625
626The correct procedure, then, is to use C<newRV_noinc> instead of
faed5253
JO
627C<newRV_inc>. Then, if and when the last reference is destroyed,
628the reference count of the SV will go to zero and it will be destroyed,
07fa94a1 629stopping any memory leak.
55497cff 630
5f05dabc 631There are some convenience functions available that can help with the
54310121 632destruction of xVs. These functions introduce the concept of "mortality".
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633An xV that is mortal has had its reference count marked to be decremented,
634but not actually decremented, until "a short time later". Generally the
635term "short time later" means a single Perl statement, such as a call to
54310121 636an XSUB function. The actual determinant for when mortal xVs have their
07fa94a1
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637reference count decremented depends on two macros, SAVETMPS and FREETMPS.
638See L<perlcall> and L<perlxs> for more details on these macros.
55497cff
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639
640"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
641However, if you mortalize a variable twice, the reference count will
642later be decremented twice.
643
644You should be careful about creating mortal variables. Strange things
645can happen if you make the same value mortal within multiple contexts,
5f05dabc 646or if you make a variable mortal multiple times.
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647
648To create a mortal variable, use the functions:
649
650 SV* sv_newmortal()
651 SV* sv_2mortal(SV*)
652 SV* sv_mortalcopy(SV*)
653
5f05dabc
PP
654The first call creates a mortal SV, the second converts an existing
655SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
656third creates a mortal copy of an existing SV.
a0d0e21e 657
54310121 658The mortal routines are not just for SVs -- AVs and HVs can be
faed5253 659made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 660C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 661
5f05dabc 662=head2 Stashes and Globs
a0d0e21e 663
aa689395
PP
664A "stash" is a hash that contains all of the different objects that
665are contained within a package. Each key of the stash is a symbol
666name (shared by all the different types of objects that have the same
667name), and each value in the hash table is a GV (Glob Value). This GV
668in turn contains references to the various objects of that name,
669including (but not limited to) the following:
cb1a09d0 670
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671 Scalar Value
672 Array Value
673 Hash Value
a3cb178b 674 I/O Handle
a0d0e21e
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675 Format
676 Subroutine
677
9cde0e7f 678There is a single stash called "PL_defstash" that holds the items that exist
5f05dabc
PP
679in the "main" package. To get at the items in other packages, append the
680string "::" to the package name. The items in the "Foo" package are in
9cde0e7f 681the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are
5f05dabc 682in the stash "Baz::" in "Bar::"'s stash.
a0d0e21e 683
d1b91892 684To get the stash pointer for a particular package, use the function:
a0d0e21e 685
08105a92 686 HV* gv_stashpv(const char* name, I32 create)
a0d0e21e
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687 HV* gv_stashsv(SV*, I32 create)
688
689The first function takes a literal string, the second uses the string stored
d1b91892 690in the SV. Remember that a stash is just a hash table, so you get back an
cb1a09d0 691C<HV*>. The C<create> flag will create a new package if it is set.
a0d0e21e
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692
693The name that C<gv_stash*v> wants is the name of the package whose symbol table
694you want. The default package is called C<main>. If you have multiply nested
d1b91892
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695packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
696language itself.
a0d0e21e
LW
697
698Alternately, if you have an SV that is a blessed reference, you can find
699out the stash pointer by using:
700
701 HV* SvSTASH(SvRV(SV*));
702
703then use the following to get the package name itself:
704
705 char* HvNAME(HV* stash);
706
5f05dabc
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707If you need to bless or re-bless an object you can use the following
708function:
a0d0e21e
LW
709
710 SV* sv_bless(SV*, HV* stash)
711
712where the first argument, an C<SV*>, must be a reference, and the second
713argument is a stash. The returned C<SV*> can now be used in the same way
714as any other SV.
715
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716For more information on references and blessings, consult L<perlref>.
717
54310121 718=head2 Double-Typed SVs
0a753a76
PP
719
720Scalar variables normally contain only one type of value, an integer,
721double, pointer, or reference. Perl will automatically convert the
722actual scalar data from the stored type into the requested type.
723
724Some scalar variables contain more than one type of scalar data. For
725example, the variable C<$!> contains either the numeric value of C<errno>
726or its string equivalent from either C<strerror> or C<sys_errlist[]>.
727
728To force multiple data values into an SV, you must do two things: use the
729C<sv_set*v> routines to add the additional scalar type, then set a flag
730so that Perl will believe it contains more than one type of data. The
731four macros to set the flags are:
732
733 SvIOK_on
734 SvNOK_on
735 SvPOK_on
736 SvROK_on
737
738The particular macro you must use depends on which C<sv_set*v> routine
739you called first. This is because every C<sv_set*v> routine turns on
740only the bit for the particular type of data being set, and turns off
741all the rest.
742
743For example, to create a new Perl variable called "dberror" that contains
744both the numeric and descriptive string error values, you could use the
745following code:
746
747 extern int dberror;
748 extern char *dberror_list;
749
4929bf7b 750 SV* sv = get_sv("dberror", TRUE);
0a753a76
PP
751 sv_setiv(sv, (IV) dberror);
752 sv_setpv(sv, dberror_list[dberror]);
753 SvIOK_on(sv);
754
755If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
756macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
757
758=head2 Magic Variables
a0d0e21e 759
d1b91892
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760[This section still under construction. Ignore everything here. Post no
761bills. Everything not permitted is forbidden.]
762
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763Any SV may be magical, that is, it has special features that a normal
764SV does not have. These features are stored in the SV structure in a
5f05dabc 765linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
766
767 struct magic {
768 MAGIC* mg_moremagic;
769 MGVTBL* mg_virtual;
770 U16 mg_private;
771 char mg_type;
772 U8 mg_flags;
773 SV* mg_obj;
774 char* mg_ptr;
775 I32 mg_len;
776 };
777
778Note this is current as of patchlevel 0, and could change at any time.
779
780=head2 Assigning Magic
781
782Perl adds magic to an SV using the sv_magic function:
783
08105a92 784 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
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785
786The C<sv> argument is a pointer to the SV that is to acquire a new magical
787feature.
788
789If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
790set the C<SVt_PVMG> flag for the C<sv>. Perl then continues by adding
791it to the beginning of the linked list of magical features. Any prior
792entry of the same type of magic is deleted. Note that this can be
5fb8527f 793overridden, and multiple instances of the same type of magic can be
d1b91892
AD
794associated with an SV.
795
54310121
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796The C<name> and C<namlen> arguments are used to associate a string with
797the magic, typically the name of a variable. C<namlen> is stored in the
798C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd
d1b91892
AD
799copy of the name is stored in C<mg_ptr> field.
800
801The sv_magic function uses C<how> to determine which, if any, predefined
802"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
cb1a09d0
AD
803See the "Magic Virtual Table" section below. The C<how> argument is also
804stored in the C<mg_type> field.
d1b91892
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805
806The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
807structure. If it is not the same as the C<sv> argument, the reference
808count of the C<obj> object is incremented. If it is the same, or if
04343c6d 809the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is
d1b91892
AD
810merely stored, without the reference count being incremented.
811
cb1a09d0
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812There is also a function to add magic to an C<HV>:
813
814 void hv_magic(HV *hv, GV *gv, int how);
815
816This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
817
818To remove the magic from an SV, call the function sv_unmagic:
819
820 void sv_unmagic(SV *sv, int type);
821
822The C<type> argument should be equal to the C<how> value when the C<SV>
823was initially made magical.
824
d1b91892
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825=head2 Magic Virtual Tables
826
827The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a
828C<MGVTBL>, which is a structure of function pointers and stands for
829"Magic Virtual Table" to handle the various operations that might be
830applied to that variable.
831
832The C<MGVTBL> has five pointers to the following routine types:
833
834 int (*svt_get)(SV* sv, MAGIC* mg);
835 int (*svt_set)(SV* sv, MAGIC* mg);
836 U32 (*svt_len)(SV* sv, MAGIC* mg);
837 int (*svt_clear)(SV* sv, MAGIC* mg);
838 int (*svt_free)(SV* sv, MAGIC* mg);
839
840This MGVTBL structure is set at compile-time in C<perl.h> and there are
841currently 19 types (or 21 with overloading turned on). These different
842structures contain pointers to various routines that perform additional
843actions depending on which function is being called.
844
845 Function pointer Action taken
846 ---------------- ------------
847 svt_get Do something after the value of the SV is retrieved.
848 svt_set Do something after the SV is assigned a value.
849 svt_len Report on the SV's length.
850 svt_clear Clear something the SV represents.
851 svt_free Free any extra storage associated with the SV.
852
853For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
854to an C<mg_type> of '\0') contains:
855
856 { magic_get, magic_set, magic_len, 0, 0 }
857
858Thus, when an SV is determined to be magical and of type '\0', if a get
859operation is being performed, the routine C<magic_get> is called. All
860the various routines for the various magical types begin with C<magic_>.
954c1994
GS
861NOTE: the magic routines are not considered part of the Perl API, and may
862not be exported by the Perl library.
d1b91892
AD
863
864The current kinds of Magic Virtual Tables are:
865
bdbeb323 866 mg_type MGVTBL Type of magic
5f05dabc 867 ------- ------ ----------------------------
bdbeb323
SM
868 \0 vtbl_sv Special scalar variable
869 A vtbl_amagic %OVERLOAD hash
870 a vtbl_amagicelem %OVERLOAD hash element
871 c (none) Holds overload table (AMT) on stash
872 B vtbl_bm Boyer-Moore (fast string search)
c2e66d9e
GS
873 D vtbl_regdata Regex match position data (@+ and @- vars)
874 d vtbl_regdatum Regex match position data element
d1b91892
AD
875 E vtbl_env %ENV hash
876 e vtbl_envelem %ENV hash element
bdbeb323
SM
877 f vtbl_fm Formline ('compiled' format)
878 g vtbl_mglob m//g target / study()ed string
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AD
879 I vtbl_isa @ISA array
880 i vtbl_isaelem @ISA array element
bdbeb323
SM
881 k vtbl_nkeys scalar(keys()) lvalue
882 L (none) Debugger %_<filename
883 l vtbl_dbline Debugger %_<filename element
44a8e56a 884 o vtbl_collxfrm Locale transformation
bdbeb323
SM
885 P vtbl_pack Tied array or hash
886 p vtbl_packelem Tied array or hash element
887 q vtbl_packelem Tied scalar or handle
888 S vtbl_sig %SIG hash
889 s vtbl_sigelem %SIG hash element
d1b91892 890 t vtbl_taint Taintedness
bdbeb323
SM
891 U vtbl_uvar Available for use by extensions
892 v vtbl_vec vec() lvalue
893 x vtbl_substr substr() lvalue
894 y vtbl_defelem Shadow "foreach" iterator variable /
895 smart parameter vivification
896 * vtbl_glob GV (typeglob)
897 # vtbl_arylen Array length ($#ary)
898 . vtbl_pos pos() lvalue
899 ~ (none) Available for use by extensions
d1b91892 900
68dc0745
PP
901When an uppercase and lowercase letter both exist in the table, then the
902uppercase letter is used to represent some kind of composite type (a list
903or a hash), and the lowercase letter is used to represent an element of
d1b91892
AD
904that composite type.
905
bdbeb323
SM
906The '~' and 'U' magic types are defined specifically for use by
907extensions and will not be used by perl itself. Extensions can use
908'~' magic to 'attach' private information to variables (typically
909objects). This is especially useful because there is no way for
910normal perl code to corrupt this private information (unlike using
911extra elements of a hash object).
912
913Similarly, 'U' magic can be used much like tie() to call a C function
914any time a scalar's value is used or changed. The C<MAGIC>'s
915C<mg_ptr> field points to a C<ufuncs> structure:
916
917 struct ufuncs {
918 I32 (*uf_val)(IV, SV*);
919 I32 (*uf_set)(IV, SV*);
920 IV uf_index;
921 };
922
923When the SV is read from or written to, the C<uf_val> or C<uf_set>
924function will be called with C<uf_index> as the first arg and a
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AB
925pointer to the SV as the second. A simple example of how to add 'U'
926magic is shown below. Note that the ufuncs structure is copied by
927sv_magic, so you can safely allocate it on the stack.
928
929 void
930 Umagic(sv)
931 SV *sv;
932 PREINIT:
933 struct ufuncs uf;
934 CODE:
935 uf.uf_val = &my_get_fn;
936 uf.uf_set = &my_set_fn;
937 uf.uf_index = 0;
938 sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));
5f05dabc 939
bdbeb323
SM
940Note that because multiple extensions may be using '~' or 'U' magic,
941it is important for extensions to take extra care to avoid conflict.
942Typically only using the magic on objects blessed into the same class
943as the extension is sufficient. For '~' magic, it may also be
944appropriate to add an I32 'signature' at the top of the private data
945area and check that.
5f05dabc 946
ef50df4b
GS
947Also note that the C<sv_set*()> and C<sv_cat*()> functions described
948earlier do B<not> invoke 'set' magic on their targets. This must
949be done by the user either by calling the C<SvSETMAGIC()> macro after
950calling these functions, or by using one of the C<sv_set*_mg()> or
951C<sv_cat*_mg()> functions. Similarly, generic C code must call the
952C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
953obtained from external sources in functions that don't handle magic.
4a4eefd0 954See L<perlapi> for a description of these functions.
189b2af5
GS
955For example, calls to the C<sv_cat*()> functions typically need to be
956followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
957since their implementation handles 'get' magic.
958
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959=head2 Finding Magic
960
961 MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
962
963This routine returns a pointer to the C<MAGIC> structure stored in the SV.
964If the SV does not have that magical feature, C<NULL> is returned. Also,
54310121 965if the SV is not of type SVt_PVMG, Perl may core dump.
d1b91892 966
08105a92 967 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
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968
969This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
970field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
971the mg_type field is changed to be the lowercase letter.
a0d0e21e 972
04343c6d
GS
973=head2 Understanding the Magic of Tied Hashes and Arrays
974
975Tied hashes and arrays are magical beasts of the 'P' magic type.
9edb2b46
GS
976
977WARNING: As of the 5.004 release, proper usage of the array and hash
978access functions requires understanding a few caveats. Some
979of these caveats are actually considered bugs in the API, to be fixed
980in later releases, and are bracketed with [MAYCHANGE] below. If
981you find yourself actually applying such information in this section, be
982aware that the behavior may change in the future, umm, without warning.
04343c6d 983
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984The perl tie function associates a variable with an object that implements
985the various GET, SET etc methods. To perform the equivalent of the perl
986tie function from an XSUB, you must mimic this behaviour. The code below
987carries out the necessary steps - firstly it creates a new hash, and then
988creates a second hash which it blesses into the class which will implement
989the tie methods. Lastly it ties the two hashes together, and returns a
990reference to the new tied hash. Note that the code below does NOT call the
991TIEHASH method in the MyTie class -
992see L<Calling Perl Routines from within C Programs> for details on how
993to do this.
994
995 SV*
996 mytie()
997 PREINIT:
998 HV *hash;
999 HV *stash;
1000 SV *tie;
1001 CODE:
1002 hash = newHV();
1003 tie = newRV_noinc((SV*)newHV());
1004 stash = gv_stashpv("MyTie", TRUE);
1005 sv_bless(tie, stash);
1006 hv_magic(hash, tie, 'P');
1007 RETVAL = newRV_noinc(hash);
1008 OUTPUT:
1009 RETVAL
1010
04343c6d
GS
1011The C<av_store> function, when given a tied array argument, merely
1012copies the magic of the array onto the value to be "stored", using
1013C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1014actually need to be stored in the array. [MAYCHANGE] After a call to
1015C<av_store> on a tied array, the caller will usually need to call
1016C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1017TIEARRAY object. If C<av_store> did return NULL, a call to
1018C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1019leak. [/MAYCHANGE]
04343c6d
GS
1020
1021The previous paragraph is applicable verbatim to tied hash access using the
1022C<hv_store> and C<hv_store_ent> functions as well.
1023
1024C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1025C<hv_fetch_ent> actually return an undefined mortal value whose magic
1026has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1027need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1028need to call C<mg_get()> on the returned value in order to actually invoke
1029the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1030you may also call C<mg_set()> on the return value after possibly assigning
1031a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1032method on the TIE object. [/MAYCHANGE]
04343c6d 1033
9edb2b46 1034[MAYCHANGE]
04343c6d
GS
1035In other words, the array or hash fetch/store functions don't really
1036fetch and store actual values in the case of tied arrays and hashes. They
1037merely call C<mg_copy> to attach magic to the values that were meant to be
1038"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1039do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1040the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1041and hashes.
1042
1043Currently (as of perl version 5.004), use of the hash and array access
1044functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1045"normal" hashes and arrays, or on their tied variants. The API may be
1046changed to provide more transparent access to both tied and normal data
1047types in future versions.
1048[/MAYCHANGE]
04343c6d
GS
1049
1050You would do well to understand that the TIEARRAY and TIEHASH interfaces
1051are mere sugar to invoke some perl method calls while using the uniform hash
1052and array syntax. The use of this sugar imposes some overhead (typically
1053about two to four extra opcodes per FETCH/STORE operation, in addition to
1054the creation of all the mortal variables required to invoke the methods).
1055This overhead will be comparatively small if the TIE methods are themselves
1056substantial, but if they are only a few statements long, the overhead
1057will not be insignificant.
1058
d1c897a1
IZ
1059=head2 Localizing changes
1060
1061Perl has a very handy construction
1062
1063 {
1064 local $var = 2;
1065 ...
1066 }
1067
1068This construction is I<approximately> equivalent to
1069
1070 {
1071 my $oldvar = $var;
1072 $var = 2;
1073 ...
1074 $var = $oldvar;
1075 }
1076
1077The biggest difference is that the first construction would
1078reinstate the initial value of $var, irrespective of how control exits
1079the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit
1080more efficient as well.
1081
1082There is a way to achieve a similar task from C via Perl API: create a
1083I<pseudo-block>, and arrange for some changes to be automatically
1084undone at the end of it, either explicit, or via a non-local exit (via
1085die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1086C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1087Such a construct may be created specially for some important localized
1088task, or an existing one (like boundaries of enclosing Perl
1089subroutine/block, or an existing pair for freeing TMPs) may be
1090used. (In the second case the overhead of additional localization must
1091be almost negligible.) Note that any XSUB is automatically enclosed in
1092an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1093
1094Inside such a I<pseudo-block> the following service is available:
1095
13a2d996 1096=over 4
d1c897a1
IZ
1097
1098=item C<SAVEINT(int i)>
1099
1100=item C<SAVEIV(IV i)>
1101
1102=item C<SAVEI32(I32 i)>
1103
1104=item C<SAVELONG(long i)>
1105
1106These macros arrange things to restore the value of integer variable
1107C<i> at the end of enclosing I<pseudo-block>.
1108
1109=item C<SAVESPTR(s)>
1110
1111=item C<SAVEPPTR(p)>
1112
1113These macros arrange things to restore the value of pointers C<s> and
1114C<p>. C<s> must be a pointer of a type which survives conversion to
1115C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1116and back.
1117
1118=item C<SAVEFREESV(SV *sv)>
1119
1120The refcount of C<sv> would be decremented at the end of
1121I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be
1122used instead.
1123
1124=item C<SAVEFREEOP(OP *op)>
1125
1126The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1127
1128=item C<SAVEFREEPV(p)>
1129
1130The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1131end of I<pseudo-block>.
1132
1133=item C<SAVECLEARSV(SV *sv)>
1134
1135Clears a slot in the current scratchpad which corresponds to C<sv> at
1136the end of I<pseudo-block>.
1137
1138=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1139
1140The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
1141string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1142short-lived storage, the corresponding string may be reallocated like
1143this:
1144
9cde0e7f 1145 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1146
c76ac1ee 1147=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1148
1149At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1150only argument C<p>.
1151
1152=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1153
1154At the end of I<pseudo-block> the function C<f> is called with the
1155implicit context argument (if any), and C<p>.
d1c897a1
IZ
1156
1157=item C<SAVESTACK_POS()>
1158
1159The current offset on the Perl internal stack (cf. C<SP>) is restored
1160at the end of I<pseudo-block>.
1161
1162=back
1163
1164The following API list contains functions, thus one needs to
1165provide pointers to the modifiable data explicitly (either C pointers,
1166or Perlish C<GV *>s). Where the above macros take C<int>, a similar
1167function takes C<int *>.
1168
13a2d996 1169=over 4
d1c897a1
IZ
1170
1171=item C<SV* save_scalar(GV *gv)>
1172
1173Equivalent to Perl code C<local $gv>.
1174
1175=item C<AV* save_ary(GV *gv)>
1176
1177=item C<HV* save_hash(GV *gv)>
1178
1179Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1180
1181=item C<void save_item(SV *item)>
1182
1183Duplicates the current value of C<SV>, on the exit from the current
1184C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
1185using the stored value.
1186
1187=item C<void save_list(SV **sarg, I32 maxsarg)>
1188
1189A variant of C<save_item> which takes multiple arguments via an array
1190C<sarg> of C<SV*> of length C<maxsarg>.
1191
1192=item C<SV* save_svref(SV **sptr)>
1193
1194Similar to C<save_scalar>, but will reinstate a C<SV *>.
1195
1196=item C<void save_aptr(AV **aptr)>
1197
1198=item C<void save_hptr(HV **hptr)>
1199
1200Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1201
1202=back
1203
1204The C<Alias> module implements localization of the basic types within the
1205I<caller's scope>. People who are interested in how to localize things in
1206the containing scope should take a look there too.
1207
0a753a76 1208=head1 Subroutines
a0d0e21e 1209
68dc0745 1210=head2 XSUBs and the Argument Stack
5f05dabc
PP
1211
1212The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1213An XSUB routine will have a stack that contains the arguments from the Perl
1214program, and a way to map from the Perl data structures to a C equivalent.
1215
1216The stack arguments are accessible through the C<ST(n)> macro, which returns
1217the C<n>'th stack argument. Argument 0 is the first argument passed in the
1218Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1219an C<SV*> is used.
1220
1221Most of the time, output from the C routine can be handled through use of
1222the RETVAL and OUTPUT directives. However, there are some cases where the
1223argument stack is not already long enough to handle all the return values.
1224An example is the POSIX tzname() call, which takes no arguments, but returns
1225two, the local time zone's standard and summer time abbreviations.
1226
1227To handle this situation, the PPCODE directive is used and the stack is
1228extended using the macro:
1229
924508f0 1230 EXTEND(SP, num);
5f05dabc 1231
924508f0
GS
1232where C<SP> is the macro that represents the local copy of the stack pointer,
1233and C<num> is the number of elements the stack should be extended by.
5f05dabc
PP
1234
1235Now that there is room on the stack, values can be pushed on it using the
54310121 1236macros to push IVs, doubles, strings, and SV pointers respectively:
5f05dabc
PP
1237
1238 PUSHi(IV)
1239 PUSHn(double)
1240 PUSHp(char*, I32)
1241 PUSHs(SV*)
1242
1243And now the Perl program calling C<tzname>, the two values will be assigned
1244as in:
1245
1246 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1247
1248An alternate (and possibly simpler) method to pushing values on the stack is
1249to use the macros:
1250
1251 XPUSHi(IV)
1252 XPUSHn(double)
1253 XPUSHp(char*, I32)
1254 XPUSHs(SV*)
1255
1256These macros automatically adjust the stack for you, if needed. Thus, you
1257do not need to call C<EXTEND> to extend the stack.
1bd1c0d5 1258However, see L</Putting a C value on Perl stack>
5f05dabc
PP
1259
1260For more information, consult L<perlxs> and L<perlxstut>.
1261
1262=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1263
1264There are four routines that can be used to call a Perl subroutine from
1265within a C program. These four are:
1266
954c1994
GS
1267 I32 call_sv(SV*, I32);
1268 I32 call_pv(const char*, I32);
1269 I32 call_method(const char*, I32);
1270 I32 call_argv(const char*, I32, register char**);
a0d0e21e 1271
954c1994 1272The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1273contains either the name of the Perl subroutine to be called, or a
1274reference to the subroutine. The second argument consists of flags
1275that control the context in which the subroutine is called, whether
1276or not the subroutine is being passed arguments, how errors should be
1277trapped, and how to treat return values.
a0d0e21e
LW
1278
1279All four routines return the number of arguments that the subroutine returned
1280on the Perl stack.
1281
954c1994
GS
1282These routines used to be called C<perl_call_sv> etc., before Perl v5.6.0,
1283but those names are now deprecated; macros of the same name are provided for
1284compatibility.
1285
1286When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1287must manipulate the Perl stack. These include the following macros and
1288functions:
a0d0e21e
LW
1289
1290 dSP
924508f0 1291 SP
a0d0e21e
LW
1292 PUSHMARK()
1293 PUTBACK
1294 SPAGAIN
1295 ENTER
1296 SAVETMPS
1297 FREETMPS
1298 LEAVE
1299 XPUSH*()
cb1a09d0 1300 POP*()
a0d0e21e 1301
5f05dabc
PP
1302For a detailed description of calling conventions from C to Perl,
1303consult L<perlcall>.
a0d0e21e 1304
5f05dabc 1305=head2 Memory Allocation
a0d0e21e 1306
86058a2d
GS
1307All memory meant to be used with the Perl API functions should be manipulated
1308using the macros described in this section. The macros provide the necessary
1309transparency between differences in the actual malloc implementation that is
1310used within perl.
1311
1312It is suggested that you enable the version of malloc that is distributed
5f05dabc 1313with Perl. It keeps pools of various sizes of unallocated memory in
07fa94a1
JO
1314order to satisfy allocation requests more quickly. However, on some
1315platforms, it may cause spurious malloc or free errors.
d1b91892
AD
1316
1317 New(x, pointer, number, type);
1318 Newc(x, pointer, number, type, cast);
1319 Newz(x, pointer, number, type);
1320
07fa94a1 1321These three macros are used to initially allocate memory.
5f05dabc
PP
1322
1323The first argument C<x> was a "magic cookie" that was used to keep track
1324of who called the macro, to help when debugging memory problems. However,
07fa94a1
JO
1325the current code makes no use of this feature (most Perl developers now
1326use run-time memory checkers), so this argument can be any number.
5f05dabc
PP
1327
1328The second argument C<pointer> should be the name of a variable that will
1329point to the newly allocated memory.
d1b91892 1330
d1b91892
AD
1331The third and fourth arguments C<number> and C<type> specify how many of
1332the specified type of data structure should be allocated. The argument
1333C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
1334should be used if the C<pointer> argument is different from the C<type>
1335argument.
1336
1337Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
1338to zero out all the newly allocated memory.
1339
1340 Renew(pointer, number, type);
1341 Renewc(pointer, number, type, cast);
1342 Safefree(pointer)
1343
1344These three macros are used to change a memory buffer size or to free a
1345piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1346match those of C<New> and C<Newc> with the exception of not needing the
1347"magic cookie" argument.
1348
1349 Move(source, dest, number, type);
1350 Copy(source, dest, number, type);
1351 Zero(dest, number, type);
1352
1353These three macros are used to move, copy, or zero out previously allocated
1354memory. The C<source> and C<dest> arguments point to the source and
1355destination starting points. Perl will move, copy, or zero out C<number>
1356instances of the size of the C<type> data structure (using the C<sizeof>
1357function).
a0d0e21e 1358
5f05dabc 1359=head2 PerlIO
ce3d39e2 1360
5f05dabc
PP
1361The most recent development releases of Perl has been experimenting with
1362removing Perl's dependency on the "normal" standard I/O suite and allowing
1363other stdio implementations to be used. This involves creating a new
1364abstraction layer that then calls whichever implementation of stdio Perl
68dc0745 1365was compiled with. All XSUBs should now use the functions in the PerlIO
5f05dabc
PP
1366abstraction layer and not make any assumptions about what kind of stdio
1367is being used.
1368
1369For a complete description of the PerlIO abstraction, consult L<perlapio>.
1370
8ebc5c01 1371=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1372
1373A lot of opcodes (this is an elementary operation in the internal perl
1374stack machine) put an SV* on the stack. However, as an optimization
1375the corresponding SV is (usually) not recreated each time. The opcodes
1376reuse specially assigned SVs (I<target>s) which are (as a corollary)
1377not constantly freed/created.
1378
0a753a76 1379Each of the targets is created only once (but see
ce3d39e2
IZ
1380L<Scratchpads and recursion> below), and when an opcode needs to put
1381an integer, a double, or a string on stack, it just sets the
1382corresponding parts of its I<target> and puts the I<target> on stack.
1383
1384The macro to put this target on stack is C<PUSHTARG>, and it is
1385directly used in some opcodes, as well as indirectly in zillions of
1386others, which use it via C<(X)PUSH[pni]>.
1387
1bd1c0d5
SC
1388Because the target is reused, you must be careful when pushing multiple
1389values on the stack. The following code will not do what you think:
1390
1391 XPUSHi(10);
1392 XPUSHi(20);
1393
1394This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1395the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1396At the end of the operation, the stack does not contain the values 10
1397and 20, but actually contains two pointers to C<TARG>, which we have set
1398to 20. If you need to push multiple different values, use C<XPUSHs>,
1399which bypasses C<TARG>.
1400
1401On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
1402need a C<dTARG> in your variable declarations so that the C<*PUSH*>
1403macros can make use of the local variable C<TARG>.
1404
8ebc5c01 1405=head2 Scratchpads
ce3d39e2 1406
54310121 1407The question remains on when the SVs which are I<target>s for opcodes
5f05dabc
PP
1408are created. The answer is that they are created when the current unit --
1409a subroutine or a file (for opcodes for statements outside of
1410subroutines) -- is compiled. During this time a special anonymous Perl
ce3d39e2
IZ
1411array is created, which is called a scratchpad for the current
1412unit.
1413
54310121 1414A scratchpad keeps SVs which are lexicals for the current unit and are
ce3d39e2
IZ
1415targets for opcodes. One can deduce that an SV lives on a scratchpad
1416by looking on its flags: lexicals have C<SVs_PADMY> set, and
1417I<target>s have C<SVs_PADTMP> set.
1418
54310121
PP
1419The correspondence between OPs and I<target>s is not 1-to-1. Different
1420OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1421would not conflict with the expected life of the temporary.
1422
2ae324a7 1423=head2 Scratchpads and recursion
ce3d39e2
IZ
1424
1425In fact it is not 100% true that a compiled unit contains a pointer to
1426the scratchpad AV. In fact it contains a pointer to an AV of
1427(initially) one element, and this element is the scratchpad AV. Why do
1428we need an extra level of indirection?
1429
1430The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both
1431these can create several execution pointers going into the same
1432subroutine. For the subroutine-child not write over the temporaries
1433for the subroutine-parent (lifespan of which covers the call to the
1434child), the parent and the child should have different
1435scratchpads. (I<And> the lexicals should be separate anyway!)
1436
5f05dabc
PP
1437So each subroutine is born with an array of scratchpads (of length 1).
1438On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1439depth of the recursion is not more than the length of this array, and
1440if it is, new scratchpad is created and pushed into the array.
1441
1442The I<target>s on this scratchpad are C<undef>s, but they are already
1443marked with correct flags.
1444
0a753a76
PP
1445=head1 Compiled code
1446
1447=head2 Code tree
1448
1449Here we describe the internal form your code is converted to by
1450Perl. Start with a simple example:
1451
1452 $a = $b + $c;
1453
1454This is converted to a tree similar to this one:
1455
1456 assign-to
1457 / \
1458 + $a
1459 / \
1460 $b $c
1461
7b8d334a 1462(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1463parsed your code, but has nothing to do with the execution order.
1464There is an additional "thread" going through the nodes of the tree
1465which shows the order of execution of the nodes. In our simplified
1466example above it looks like:
1467
1468 $b ---> $c ---> + ---> $a ---> assign-to
1469
1470But with the actual compile tree for C<$a = $b + $c> it is different:
1471some nodes I<optimized away>. As a corollary, though the actual tree
1472contains more nodes than our simplified example, the execution order
1473is the same as in our example.
1474
1475=head2 Examining the tree
1476
1477If you have your perl compiled for debugging (usually done with C<-D
1478optimize=-g> on C<Configure> command line), you may examine the
1479compiled tree by specifying C<-Dx> on the Perl command line. The
1480output takes several lines per node, and for C<$b+$c> it looks like
1481this:
1482
1483 5 TYPE = add ===> 6
1484 TARG = 1
1485 FLAGS = (SCALAR,KIDS)
1486 {
1487 TYPE = null ===> (4)
1488 (was rv2sv)
1489 FLAGS = (SCALAR,KIDS)
1490 {
1491 3 TYPE = gvsv ===> 4
1492 FLAGS = (SCALAR)
1493 GV = main::b
1494 }
1495 }
1496 {
1497 TYPE = null ===> (5)
1498 (was rv2sv)
1499 FLAGS = (SCALAR,KIDS)
1500 {
1501 4 TYPE = gvsv ===> 5
1502 FLAGS = (SCALAR)
1503 GV = main::c
1504 }
1505 }
1506
1507This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1508not optimized away (one per number in the left column). The immediate
1509children of the given node correspond to C<{}> pairs on the same level
1510of indentation, thus this listing corresponds to the tree:
1511
1512 add
1513 / \
1514 null null
1515 | |
1516 gvsv gvsv
1517
1518The execution order is indicated by C<===E<gt>> marks, thus it is C<3
15194 5 6> (node C<6> is not included into above listing), i.e.,
1520C<gvsv gvsv add whatever>.
1521
9afa14e3
SC
1522Each of these nodes represents an op, a fundamental operation inside the
1523Perl core. The code which implements each operation can be found in the
1524F<pp*.c> files; the function which implements the op with type C<gvsv>
1525is C<pp_gvsv>, and so on. As the tree above shows, different ops have
1526different numbers of children: C<add> is a binary operator, as one would
1527expect, and so has two children. To accommodate the various different
1528numbers of children, there are various types of op data structure, and
1529they link together in different ways.
1530
1531The simplest type of op structure is C<OP>: this has no children. Unary
1532operators, C<UNOP>s, have one child, and this is pointed to by the
1533C<op_first> field. Binary operators (C<BINOP>s) have not only an
1534C<op_first> field but also an C<op_last> field. The most complex type of
1535op is a C<LISTOP>, which has any number of children. In this case, the
1536first child is pointed to by C<op_first> and the last child by
1537C<op_last>. The children in between can be found by iteratively
1538following the C<op_sibling> pointer from the first child to the last.
1539
1540There are also two other op types: a C<PMOP> holds a regular expression,
1541and has no children, and a C<LOOP> may or may not have children. If the
1542C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
1543complicate matters, if a C<UNOP> is actually a C<null> op after
1544optimization (see L</Compile pass 2: context propagation>) it will still
1545have children in accordance with its former type.
1546
0a753a76
PP
1547=head2 Compile pass 1: check routines
1548
8870b5c7
GS
1549The tree is created by the compiler while I<yacc> code feeds it
1550the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
1551the first pass of perl compilation.
1552
1553What makes this pass interesting for perl developers is that some
1554optimization may be performed on this pass. This is optimization by
8870b5c7 1555so-called "check routines". The correspondence between node names
0a753a76
PP
1556and corresponding check routines is described in F<opcode.pl> (do not
1557forget to run C<make regen_headers> if you modify this file).
1558
1559A check routine is called when the node is fully constructed except
7b8d334a 1560for the execution-order thread. Since at this time there are no
0a753a76
PP
1561back-links to the currently constructed node, one can do most any
1562operation to the top-level node, including freeing it and/or creating
1563new nodes above/below it.
1564
1565The check routine returns the node which should be inserted into the
1566tree (if the top-level node was not modified, check routine returns
1567its argument).
1568
1569By convention, check routines have names C<ck_*>. They are usually
1570called from C<new*OP> subroutines (or C<convert>) (which in turn are
1571called from F<perly.y>).
1572
1573=head2 Compile pass 1a: constant folding
1574
1575Immediately after the check routine is called the returned node is
1576checked for being compile-time executable. If it is (the value is
1577judged to be constant) it is immediately executed, and a I<constant>
1578node with the "return value" of the corresponding subtree is
1579substituted instead. The subtree is deleted.
1580
1581If constant folding was not performed, the execution-order thread is
1582created.
1583
1584=head2 Compile pass 2: context propagation
1585
1586When a context for a part of compile tree is known, it is propagated
a3cb178b 1587down through the tree. At this time the context can have 5 values
0a753a76
PP
1588(instead of 2 for runtime context): void, boolean, scalar, list, and
1589lvalue. In contrast with the pass 1 this pass is processed from top
1590to bottom: a node's context determines the context for its children.
1591
1592Additional context-dependent optimizations are performed at this time.
1593Since at this moment the compile tree contains back-references (via
1594"thread" pointers), nodes cannot be free()d now. To allow
1595optimized-away nodes at this stage, such nodes are null()ified instead
1596of free()ing (i.e. their type is changed to OP_NULL).
1597
1598=head2 Compile pass 3: peephole optimization
1599
1600After the compile tree for a subroutine (or for an C<eval> or a file)
1601is created, an additional pass over the code is performed. This pass
1602is neither top-down or bottom-up, but in the execution order (with
7b8d334a 1603additional complications for conditionals). These optimizations are
0a753a76
PP
1604done in the subroutine peep(). Optimizations performed at this stage
1605are subject to the same restrictions as in the pass 2.
1606
9afa14e3
SC
1607=head1 Examining internal data structures with the C<dump> functions
1608
1609To aid debugging, the source file F<dump.c> contains a number of
1610functions which produce formatted output of internal data structures.
1611
1612The most commonly used of these functions is C<Perl_sv_dump>; it's used
1613for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
1614C<sv_dump> to produce debugging output from Perl-space, so users of that
1615module should already be familiar with its format.
1616
1617C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
1618derivatives, and produces output similiar to C<perl -Dx>; in fact,
1619C<Perl_dump_eval> will dump the main root of the code being evaluated,
1620exactly like C<-Dx>.
1621
1622Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
1623op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
1624subroutines in a package like so: (Thankfully, these are all xsubs, so
1625there is no op tree)
1626
1627 (gdb) print Perl_dump_packsubs(PL_defstash)
1628
1629 SUB attributes::bootstrap = (xsub 0x811fedc 0)
1630
1631 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
1632
1633 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
1634
1635 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
1636
1637 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
1638
1639and C<Perl_dump_all>, which dumps all the subroutines in the stash and
1640the op tree of the main root.
1641
954c1994 1642=head1 How multiple interpreters and concurrency are supported
ee072b34 1643
ee072b34
GS
1644=head2 Background and PERL_IMPLICIT_CONTEXT
1645
1646The Perl interpreter can be regarded as a closed box: it has an API
1647for feeding it code or otherwise making it do things, but it also has
1648functions for its own use. This smells a lot like an object, and
1649there are ways for you to build Perl so that you can have multiple
1650interpreters, with one interpreter represented either as a C++ object,
1651a C structure, or inside a thread. The thread, the C structure, or
1652the C++ object will contain all the context, the state of that
1653interpreter.
1654
54aff467
GS
1655Three macros control the major Perl build flavors: MULTIPLICITY,
1656USE_THREADS and PERL_OBJECT. The MULTIPLICITY build has a C structure
1657that packages all the interpreter state, there is a similar thread-specific
1658data structure under USE_THREADS, and the PERL_OBJECT build has a C++
1659class to maintain interpreter state. In all three cases,
1660PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
1661support for passing in a "hidden" first argument that represents all three
651a3225 1662data structures.
54aff467
GS
1663
1664All this obviously requires a way for the Perl internal functions to be
ee072b34
GS
1665C++ methods, subroutines taking some kind of structure as the first
1666argument, or subroutines taking nothing as the first argument. To
1667enable these three very different ways of building the interpreter,
1668the Perl source (as it does in so many other situations) makes heavy
1669use of macros and subroutine naming conventions.
1670
54aff467 1671First problem: deciding which functions will be public API functions and
954c1994
GS
1672which will be private. All functions whose names begin C<S_> are private
1673(think "S" for "secret" or "static"). All other functions begin with
1674"Perl_", but just because a function begins with "Perl_" does not mean it is
a422fd2d
SC
1675part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
1676function is part of the API is to find its entry in L<perlapi>.
1677If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
1678think it should be (i.e., you need it for your extension), send mail via
1679L<perlbug> explaining why you think it should be.
ee072b34
GS
1680
1681Second problem: there must be a syntax so that the same subroutine
1682declarations and calls can pass a structure as their first argument,
1683or pass nothing. To solve this, the subroutines are named and
1684declared in a particular way. Here's a typical start of a static
1685function used within the Perl guts:
1686
1687 STATIC void
1688 S_incline(pTHX_ char *s)
1689
1690STATIC becomes "static" in C, and is #define'd to nothing in C++.
1691
651a3225
GS
1692A public function (i.e. part of the internal API, but not necessarily
1693sanctioned for use in extensions) begins like this:
ee072b34
GS
1694
1695 void
1696 Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)
1697
1698C<pTHX_> is one of a number of macros (in perl.h) that hide the
1699details of the interpreter's context. THX stands for "thread", "this",
1700or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
1701The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
1702or 'd' for B<d>eclaration.
1703
1704When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first
1705argument containing the interpreter's context. The trailing underscore
1706in the pTHX_ macro indicates that the macro expansion needs a comma
1707after the context argument because other arguments follow it. If
1708PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
1709subroutine is not prototyped to take the extra argument. The form of the
1710macro without the trailing underscore is used when there are no additional
ee072b34
GS
1711explicit arguments.
1712
54aff467 1713When a core function calls another, it must pass the context. This
ee072b34
GS
1714is normally hidden via macros. Consider C<sv_setsv>. It expands
1715something like this:
1716
1717 ifdef PERL_IMPLICIT_CONTEXT
c52f9dcd 1718 define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
ee072b34
GS
1719 /* can't do this for vararg functions, see below */
1720 else
c52f9dcd 1721 define sv_setsv Perl_sv_setsv
ee072b34
GS
1722 endif
1723
1724This works well, and means that XS authors can gleefully write:
1725
1726 sv_setsv(foo, bar);
1727
1728and still have it work under all the modes Perl could have been
1729compiled with.
1730
1731Under PERL_OBJECT in the core, that will translate to either:
1732
1733 CPerlObj::Perl_sv_setsv(foo,bar); # in CPerlObj functions,
1734 # C++ takes care of 'this'
1735 or
1736
1737 pPerl->Perl_sv_setsv(foo,bar); # in truly static functions,
1738 # see objXSUB.h
1739
1740Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
1741MULTIPLICITY/USE_THREADS w/ PERL_IMPLICIT_CONTEXT in both core
1742and extensions, it will be:
1743
1744 Perl_sv_setsv(aTHX_ foo, bar); # the canonical Perl "API"
1745 # for all build flavors
1746
1747This doesn't work so cleanly for varargs functions, though, as macros
1748imply that the number of arguments is known in advance. Instead we
1749either need to spell them out fully, passing C<aTHX_> as the first
1750argument (the Perl core tends to do this with functions like
1751Perl_warner), or use a context-free version.
1752
1753The context-free version of Perl_warner is called
1754Perl_warner_nocontext, and does not take the extra argument. Instead
1755it does dTHX; to get the context from thread-local storage. We
1756C<#define warner Perl_warner_nocontext> so that extensions get source
1757compatibility at the expense of performance. (Passing an arg is
1758cheaper than grabbing it from thread-local storage.)
1759
1760You can ignore [pad]THX[xo] when browsing the Perl headers/sources.
1761Those are strictly for use within the core. Extensions and embedders
1762need only be aware of [pad]THX.
1763
1764=head2 How do I use all this in extensions?
1765
1766When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
1767any functions in the Perl API will need to pass the initial context
1768argument somehow. The kicker is that you will need to write it in
1769such a way that the extension still compiles when Perl hasn't been
1770built with PERL_IMPLICIT_CONTEXT enabled.
1771
1772There are three ways to do this. First, the easy but inefficient way,
1773which is also the default, in order to maintain source compatibility
1774with extensions: whenever XSUB.h is #included, it redefines the aTHX
1775and aTHX_ macros to call a function that will return the context.
1776Thus, something like:
1777
1778 sv_setsv(asv, bsv);
1779
4375e838 1780in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 1781in effect:
ee072b34 1782
2fa86c13 1783 Perl_sv_setsv(Perl_get_context(), asv, bsv);
ee072b34 1784
54aff467 1785or to this otherwise:
ee072b34
GS
1786
1787 Perl_sv_setsv(asv, bsv);
1788
1789You have to do nothing new in your extension to get this; since
2fa86c13 1790the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
1791work.
1792
1793The second, more efficient way is to use the following template for
1794your Foo.xs:
1795
c52f9dcd
JH
1796 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1797 #include "EXTERN.h"
1798 #include "perl.h"
1799 #include "XSUB.h"
ee072b34
GS
1800
1801 static my_private_function(int arg1, int arg2);
1802
c52f9dcd
JH
1803 static SV *
1804 my_private_function(int arg1, int arg2)
1805 {
1806 dTHX; /* fetch context */
1807 ... call many Perl API functions ...
1808 }
ee072b34
GS
1809
1810 [... etc ...]
1811
c52f9dcd 1812 MODULE = Foo PACKAGE = Foo
ee072b34 1813
c52f9dcd 1814 /* typical XSUB */
ee072b34 1815
c52f9dcd
JH
1816 void
1817 my_xsub(arg)
1818 int arg
1819 CODE:
1820 my_private_function(arg, 10);
ee072b34
GS
1821
1822Note that the only two changes from the normal way of writing an
1823extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
1824including the Perl headers, followed by a C<dTHX;> declaration at
1825the start of every function that will call the Perl API. (You'll
1826know which functions need this, because the C compiler will complain
1827that there's an undeclared identifier in those functions.) No changes
1828are needed for the XSUBs themselves, because the XS() macro is
1829correctly defined to pass in the implicit context if needed.
1830
1831The third, even more efficient way is to ape how it is done within
1832the Perl guts:
1833
1834
c52f9dcd
JH
1835 #define PERL_NO_GET_CONTEXT /* we want efficiency */
1836 #include "EXTERN.h"
1837 #include "perl.h"
1838 #include "XSUB.h"
ee072b34
GS
1839
1840 /* pTHX_ only needed for functions that call Perl API */
1841 static my_private_function(pTHX_ int arg1, int arg2);
1842
c52f9dcd
JH
1843 static SV *
1844 my_private_function(pTHX_ int arg1, int arg2)
1845 {
1846 /* dTHX; not needed here, because THX is an argument */
1847 ... call Perl API functions ...
1848 }
ee072b34
GS
1849
1850 [... etc ...]
1851
c52f9dcd 1852 MODULE = Foo PACKAGE = Foo
ee072b34 1853
c52f9dcd 1854 /* typical XSUB */
ee072b34 1855
c52f9dcd
JH
1856 void
1857 my_xsub(arg)
1858 int arg
1859 CODE:
1860 my_private_function(aTHX_ arg, 10);
ee072b34
GS
1861
1862This implementation never has to fetch the context using a function
1863call, since it is always passed as an extra argument. Depending on
1864your needs for simplicity or efficiency, you may mix the previous
1865two approaches freely.
1866
651a3225
GS
1867Never add a comma after C<pTHX> yourself--always use the form of the
1868macro with the underscore for functions that take explicit arguments,
1869or the form without the argument for functions with no explicit arguments.
ee072b34
GS
1870
1871=head2 Future Plans and PERL_IMPLICIT_SYS
1872
1873Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
1874that the interpreter knows about itself and pass it around, so too are
1875there plans to allow the interpreter to bundle up everything it knows
1876about the environment it's running on. This is enabled with the
1877PERL_IMPLICIT_SYS macro. Currently it only works with PERL_OBJECT,
1878but is mostly there for MULTIPLICITY and USE_THREADS (see inside
1879iperlsys.h).
1880
1881This allows the ability to provide an extra pointer (called the "host"
1882environment) for all the system calls. This makes it possible for
1883all the system stuff to maintain their own state, broken down into
1884seven C structures. These are thin wrappers around the usual system
1885calls (see win32/perllib.c) for the default perl executable, but for a
1886more ambitious host (like the one that would do fork() emulation) all
1887the extra work needed to pretend that different interpreters are
1888actually different "processes", would be done here.
1889
1890The Perl engine/interpreter and the host are orthogonal entities.
1891There could be one or more interpreters in a process, and one or
1892more "hosts", with free association between them.
1893
a422fd2d
SC
1894=head1 Internal Functions
1895
1896All of Perl's internal functions which will be exposed to the outside
1897world are be prefixed by C<Perl_> so that they will not conflict with XS
1898functions or functions used in a program in which Perl is embedded.
1899Similarly, all global variables begin with C<PL_>. (By convention,
1900static functions start with C<S_>)
1901
1902Inside the Perl core, you can get at the functions either with or
1903without the C<Perl_> prefix, thanks to a bunch of defines that live in
1904F<embed.h>. This header file is generated automatically from
1905F<embed.pl>. F<embed.pl> also creates the prototyping header files for
1906the internal functions, generates the documentation and a lot of other
1907bits and pieces. It's important that when you add a new function to the
1908core or change an existing one, you change the data in the table at the
1909end of F<embed.pl> as well. Here's a sample entry from that table:
1910
1911 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
1912
1913The second column is the return type, the third column the name. Columns
1914after that are the arguments. The first column is a set of flags:
1915
1916=over 3
1917
1918=item A
1919
1920This function is a part of the public API.
1921
1922=item p
1923
1924This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
1925
1926=item d
1927
1928This function has documentation using the C<apidoc> feature which we'll
1929look at in a second.
1930
1931=back
1932
1933Other available flags are:
1934
1935=over 3
1936
1937=item s
1938
1939This is a static function and is defined as C<S_whatever>.
1940
1941=item n
1942
1943This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
1944L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
1945
1946=item r
1947
1948This function never returns; C<croak>, C<exit> and friends.
1949
1950=item f
1951
1952This function takes a variable number of arguments, C<printf> style.
1953The argument list should end with C<...>, like this:
1954
1955 Afprd |void |croak |const char* pat|...
1956
1957=item m
1958
1959This function is part of the experimental development API, and may change
1960or disappear without notice.
1961
1962=item o
1963
1964This function should not have a compatibility macro to define, say,
1965C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
1966
1967=item j
1968
1969This function is not a member of C<CPerlObj>. If you don't know
1970what this means, don't use it.
1971
1972=item x
1973
1974This function isn't exported out of the Perl core.
1975
1976=back
1977
1978If you edit F<embed.pl>, you will need to run C<make regen_headers> to
1979force a rebuild of F<embed.h> and other auto-generated files.
1980
6b4667fc 1981=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 1982
6b4667fc
A
1983If you are printing IVs, UVs, or NVS instead of the stdio(3) style
1984formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
1985following macros for portability
9dd9db0b 1986
c52f9dcd
JH
1987 IVdf IV in decimal
1988 UVuf UV in decimal
1989 UVof UV in octal
1990 UVxf UV in hexadecimal
1991 NVef NV %e-like
1992 NVff NV %f-like
1993 NVgf NV %g-like
9dd9db0b 1994
6b4667fc
A
1995These will take care of 64-bit integers and long doubles.
1996For example:
1997
c52f9dcd 1998 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
1999
2000The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2001
8908e76d
JH
2002If you are printing addresses of pointers, use UVxf combined
2003with PTR2UV(), do not use %lx or %p.
2004
2005=head2 Pointer-To-Integer and Integer-To-Pointer
2006
2007Because pointer size does not necessarily equal integer size,
2008use the follow macros to do it right.
2009
c52f9dcd
JH
2010 PTR2UV(pointer)
2011 PTR2IV(pointer)
2012 PTR2NV(pointer)
2013 INT2PTR(pointertotype, integer)
8908e76d
JH
2014
2015For example:
2016
c52f9dcd
JH
2017 IV iv = ...;
2018 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2019
2020and
2021
c52f9dcd
JH
2022 AV *av = ...;
2023 UV uv = PTR2UV(av);
8908e76d 2024
a422fd2d
SC
2025=head2 Source Documentation
2026
2027There's an effort going on to document the internal functions and
2028automatically produce reference manuals from them - L<perlapi> is one
2029such manual which details all the functions which are available to XS
2030writers. L<perlintern> is the autogenerated manual for the functions
2031which are not part of the API and are supposedly for internal use only.
2032
2033Source documentation is created by putting POD comments into the C
2034source, like this:
2035
2036 /*
2037 =for apidoc sv_setiv
2038
2039 Copies an integer into the given SV. Does not handle 'set' magic. See
2040 C<sv_setiv_mg>.
2041
2042 =cut
2043 */
2044
2045Please try and supply some documentation if you add functions to the
2046Perl core.
2047
2048=head1 Unicode Support
2049
2050Perl 5.6.0 introduced Unicode support. It's important for porters and XS
2051writers to understand this support and make sure that the code they
2052write does not corrupt Unicode data.
2053
2054=head2 What B<is> Unicode, anyway?
2055
2056In the olden, less enlightened times, we all used to use ASCII. Most of
2057us did, anyway. The big problem with ASCII is that it's American. Well,
2058no, that's not actually the problem; the problem is that it's not
2059particularly useful for people who don't use the Roman alphabet. What
2060used to happen was that particular languages would stick their own
2061alphabet in the upper range of the sequence, between 128 and 255. Of
2062course, we then ended up with plenty of variants that weren't quite
2063ASCII, and the whole point of it being a standard was lost.
2064
2065Worse still, if you've got a language like Chinese or
2066Japanese that has hundreds or thousands of characters, then you really
2067can't fit them into a mere 256, so they had to forget about ASCII
2068altogether, and build their own systems using pairs of numbers to refer
2069to one character.
2070
2071To fix this, some people formed Unicode, Inc. and
2072produced a new character set containing all the characters you can
2073possibly think of and more. There are several ways of representing these
2074characters, and the one Perl uses is called UTF8. UTF8 uses
2075a variable number of bytes to represent a character, instead of just
b3b6085d 2076one. You can learn more about Unicode at http://www.unicode.org/
a422fd2d
SC
2077
2078=head2 How can I recognise a UTF8 string?
2079
2080You can't. This is because UTF8 data is stored in bytes just like
2081non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
2082capital E with a grave accent, is represented by the two bytes
2083C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
2084has that byte sequence as well. So you can't tell just by looking - this
2085is what makes Unicode input an interesting problem.
2086
2087The API function C<is_utf8_string> can help; it'll tell you if a string
2088contains only valid UTF8 characters. However, it can't do the work for
2089you. On a character-by-character basis, C<is_utf8_char> will tell you
2090whether the current character in a string is valid UTF8.
2091
2092=head2 How does UTF8 represent Unicode characters?
2093
2094As mentioned above, UTF8 uses a variable number of bytes to store a
2095character. Characters with values 1...128 are stored in one byte, just
2096like good ol' ASCII. Character 129 is stored as C<v194.129>; this
a31a806a 2097continues up to character 191, which is C<v194.191>. Now we've run out of
a422fd2d
SC
2098bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
2099so it goes on, moving to three bytes at character 2048.
2100
2101Assuming you know you're dealing with a UTF8 string, you can find out
2102how long the first character in it is with the C<UTF8SKIP> macro:
2103
2104 char *utf = "\305\233\340\240\201";
2105 I32 len;
2106
2107 len = UTF8SKIP(utf); /* len is 2 here */
2108 utf += len;
2109 len = UTF8SKIP(utf); /* len is 3 here */
2110
2111Another way to skip over characters in a UTF8 string is to use
2112C<utf8_hop>, which takes a string and a number of characters to skip
2113over. You're on your own about bounds checking, though, so don't use it
2114lightly.
2115
2116All bytes in a multi-byte UTF8 character will have the high bit set, so
2117you can test if you need to do something special with this character
2118like this:
2119
2120 UV uv;
2121
2122 if (utf & 0x80)
2123 /* Must treat this as UTF8 */
2124 uv = utf8_to_uv(utf);
2125 else
2126 /* OK to treat this character as a byte */
2127 uv = *utf;
2128
2129You can also see in that example that we use C<utf8_to_uv> to get the
2130value of the character; the inverse function C<uv_to_utf8> is available
2131for putting a UV into UTF8:
2132
2133 if (uv > 0x80)
2134 /* Must treat this as UTF8 */
2135 utf8 = uv_to_utf8(utf8, uv);
2136 else
2137 /* OK to treat this character as a byte */
2138 *utf8++ = uv;
2139
2140You B<must> convert characters to UVs using the above functions if
2141you're ever in a situation where you have to match UTF8 and non-UTF8
2142characters. You may not skip over UTF8 characters in this case. If you
2143do this, you'll lose the ability to match hi-bit non-UTF8 characters;
2144for instance, if your UTF8 string contains C<v196.172>, and you skip
2145that character, you can never match a C<chr(200)> in a non-UTF8 string.
2146So don't do that!
2147
2148=head2 How does Perl store UTF8 strings?
2149
2150Currently, Perl deals with Unicode strings and non-Unicode strings
2151slightly differently. If a string has been identified as being UTF-8
2152encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
2153manipulate this flag with the following macros:
2154
2155 SvUTF8(sv)
2156 SvUTF8_on(sv)
2157 SvUTF8_off(sv)
2158
2159This flag has an important effect on Perl's treatment of the string: if
2160Unicode data is not properly distinguished, regular expressions,
2161C<length>, C<substr> and other string handling operations will have
2162undesirable results.
2163
2164The problem comes when you have, for instance, a string that isn't
2165flagged is UTF8, and contains a byte sequence that could be UTF8 -
2166especially when combining non-UTF8 and UTF8 strings.
2167
2168Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
2169need be sure you don't accidentally knock it off while you're
2170manipulating SVs. More specifically, you cannot expect to do this:
2171
2172 SV *sv;
2173 SV *nsv;
2174 STRLEN len;
2175 char *p;
2176
2177 p = SvPV(sv, len);
2178 frobnicate(p);
2179 nsv = newSVpvn(p, len);
2180
2181The C<char*> string does not tell you the whole story, and you can't
2182copy or reconstruct an SV just by copying the string value. Check if the
2183old SV has the UTF8 flag set, and act accordingly:
2184
2185 p = SvPV(sv, len);
2186 frobnicate(p);
2187 nsv = newSVpvn(p, len);
2188 if (SvUTF8(sv))
2189 SvUTF8_on(nsv);
2190
2191In fact, your C<frobnicate> function should be made aware of whether or
2192not it's dealing with UTF8 data, so that it can handle the string
2193appropriately.
2194
2195=head2 How do I convert a string to UTF8?
2196
2197If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
2198to upgrade one of the strings to UTF8. If you've got an SV, the easiest
2199way to do this is:
2200
2201 sv_utf8_upgrade(sv);
2202
2203However, you must not do this, for example:
2204
2205 if (!SvUTF8(left))
2206 sv_utf8_upgrade(left);
2207
2208If you do this in a binary operator, you will actually change one of the
b1866b2d 2209strings that came into the operator, and, while it shouldn't be noticeable
a422fd2d
SC
2210by the end user, it can cause problems.
2211
2212Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
2213string argument. This is useful for having the data available for
b1866b2d 2214comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
2215C<utf8_to_bytes> to go the other way, but naturally, this will fail if
2216the string contains any characters above 255 that can't be represented
2217in a single byte.
2218
2219=head2 Is there anything else I need to know?
2220
2221Not really. Just remember these things:
2222
2223=over 3
2224
2225=item *
2226
2227There's no way to tell if a string is UTF8 or not. You can tell if an SV
2228is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
2229something should be UTF8. Treat the flag as part of the PV, even though
2230it's not - if you pass on the PV to somewhere, pass on the flag too.
2231
2232=item *
2233
2234If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
2235unless C<!(*s & 0x80)> in which case you can use C<*s>.
2236
2237=item *
2238
2239When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
2240C<uv < 0x80> in which case you can use C<*s = uv>.
2241
2242=item *
2243
2244Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
2245a new string which is UTF8 encoded. There are tricks you can use to
2246delay deciding whether you need to use a UTF8 string until you get to a
2247high character - C<HALF_UPGRADE> is one of those.
2248
2249=back
2250
954c1994 2251=head1 AUTHORS
e89caa19 2252
954c1994
GS
2253Until May 1997, this document was maintained by Jeff Okamoto
2254<okamoto@corp.hp.com>. It is now maintained as part of Perl itself
2255by the Perl 5 Porters <perl5-porters@perl.org>.
cb1a09d0 2256
954c1994
GS
2257With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
2258Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
2259Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
2260Stephen McCamant, and Gurusamy Sarathy.
cb1a09d0 2261
954c1994 2262API Listing originally by Dean Roehrich <roehrich@cray.com>.
cb1a09d0 2263
954c1994
GS
2264Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
2265Stuhl.
cb1a09d0 2266
954c1994 2267=head1 SEE ALSO
cb1a09d0 2268
954c1994 2269perlapi(1), perlintern(1), perlxs(1), perlembed(1)