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1=head1 NAME
2
954c1994 3perlguts - Introduction to the Perl API
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4
5=head1 DESCRIPTION
6
b3b6085d 7This document attempts to describe how to use the Perl API, as well as
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8to provide some info on the basic workings of the Perl core. It is far
9from complete and probably contains many errors. Please refer any
b3b6085d 10questions or comments to the author below.
a0d0e21e 11
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12=head1 Variables
13
5f05dabc 14=head2 Datatypes
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15
16Perl has three typedefs that handle Perl's three main data types:
17
18 SV Scalar Value
19 AV Array Value
20 HV Hash Value
21
d1b91892 22Each typedef has specific routines that manipulate the various data types.
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23
24=head2 What is an "IV"?
25
954c1994 26Perl uses a special typedef IV which is a simple signed integer type that is
5f05dabc 27guaranteed to be large enough to hold a pointer (as well as an integer).
954c1994 28Additionally, there is the UV, which is simply an unsigned IV.
a0d0e21e 29
d1b91892 30Perl also uses two special typedefs, I32 and I16, which will always be at
10e2eb10 31least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
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32as well.) They will usually be exactly 32 and 16 bits long, but on Crays
33they will both be 64 bits.
a0d0e21e 34
54310121 35=head2 Working with SVs
a0d0e21e 36
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37An SV can be created and loaded with one command. There are five types of
38values that can be loaded: an integer value (IV), an unsigned integer
39value (UV), a double (NV), a string (PV), and another scalar (SV).
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40("PV" stands for "Pointer Value". You might think that it is misnamed
41because it is described as pointing only to strings. However, it is
3ee1a09c 42possible to have it point to other things. For example, it could point
d6605d24 43to an array of UVs. But,
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44using it for non-strings requires care, as the underlying assumption of
45much of the internals is that PVs are just for strings. Often, for
6602b933 46example, a trailing C<NUL> is tacked on automatically. The non-string use
61984ee1 47is documented only in this paragraph.)
a0d0e21e 48
20dbd849 49The seven routines are:
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50
51 SV* newSViv(IV);
20dbd849 52 SV* newSVuv(UV);
a0d0e21e 53 SV* newSVnv(double);
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54 SV* newSVpv(const char*, STRLEN);
55 SV* newSVpvn(const char*, STRLEN);
46fc3d4c 56 SV* newSVpvf(const char*, ...);
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57 SV* newSVsv(SV*);
58
e613617c 59C<STRLEN> is an integer type (C<Size_t>, usually defined as C<size_t> in
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60F<config.h>) guaranteed to be large enough to represent the size of
61any string that perl can handle.
62
3bf17896 63In the unlikely case of a SV requiring more complex initialization, you
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64can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
65type NULL is returned, else an SV of type PV is returned with len + 1 (for
6602b933 66the C<NUL>) bytes of storage allocated, accessible via SvPVX. In both cases
da8c5729 67the SV has the undef value.
20dbd849 68
06f6df17 69 SV *sv = newSV(0); /* no storage allocated */
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70 SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
71 * allocated */
20dbd849 72
06f6df17 73To change the value of an I<already-existing> SV, there are eight routines:
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74
75 void sv_setiv(SV*, IV);
deb3007b 76 void sv_setuv(SV*, UV);
a0d0e21e 77 void sv_setnv(SV*, double);
08105a92 78 void sv_setpv(SV*, const char*);
06f6df17 79 void sv_setpvn(SV*, const char*, STRLEN)
46fc3d4c 80 void sv_setpvf(SV*, const char*, ...);
a9b0660e 81 void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
03a22d83 82 SV **, Size_t, bool *);
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83 void sv_setsv(SV*, SV*);
84
85Notice that you can choose to specify the length of the string to be
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86assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may
87allow Perl to calculate the length by using C<sv_setpv> or by specifying
880 as the second argument to C<newSVpv>. Be warned, though, that Perl will
89determine the string's length by using C<strlen>, which depends on the
6602b933 90string terminating with a C<NUL> character, and not otherwise containing
a9b0660e 91NULs.
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92
93The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
94formatted output becomes the value.
95
328bf373 96C<sv_vsetpvfn> is an analogue of C<vsprintf>, but it allows you to specify
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97either a pointer to a variable argument list or the address and length of
98an array of SVs. The last argument points to a boolean; on return, if that
99boolean is true, then locale-specific information has been used to format
c2611fb3 100the string, and the string's contents are therefore untrustworthy (see
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101L<perlsec>). This pointer may be NULL if that information is not
102important. Note that this function requires you to specify the length of
103the format.
104
9da1e3b5 105The C<sv_set*()> functions are not generic enough to operate on values
5a0de581 106that have "magic". See L</Magic Virtual Tables> later in this document.
a0d0e21e 107
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108All SVs that contain strings should be terminated with a C<NUL> character.
109If it is not C<NUL>-terminated there is a risk of
5f05dabc 110core dumps and corruptions from code which passes the string to C
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111functions or system calls which expect a C<NUL>-terminated string.
112Perl's own functions typically add a trailing C<NUL> for this reason.
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113Nevertheless, you should be very careful when you pass a string stored
114in an SV to a C function or system call.
115
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116To access the actual value that an SV points to, you can use the macros:
117
118 SvIV(SV*)
954c1994 119 SvUV(SV*)
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120 SvNV(SV*)
121 SvPV(SV*, STRLEN len)
1fa8b10d 122 SvPV_nolen(SV*)
a0d0e21e 123
954c1994 124which will automatically coerce the actual scalar type into an IV, UV, double,
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125or string.
126
127In the C<SvPV> macro, the length of the string returned is placed into the
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128variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
129not care what the length of the data is, use the C<SvPV_nolen> macro.
130Historically the C<SvPV> macro with the global variable C<PL_na> has been
131used in this case. But that can be quite inefficient because C<PL_na> must
132be accessed in thread-local storage in threaded Perl. In any case, remember
133that Perl allows arbitrary strings of data that may both contain NULs and
6602b933 134might not be terminated by a C<NUL>.
a0d0e21e 135
ce2f5d8f 136Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
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137len);>. It might work with your
138compiler, but it won't work for everyone.
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139Break this sort of statement up into separate assignments:
140
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141 SV *s;
142 STRLEN len;
61955433 143 char *ptr;
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144 ptr = SvPV(s, len);
145 foo(ptr, len);
ce2f5d8f 146
07fa94a1 147If you want to know if the scalar value is TRUE, you can use:
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148
149 SvTRUE(SV*)
150
151Although Perl will automatically grow strings for you, if you need to force
152Perl to allocate more memory for your SV, you can use the macro
153
154 SvGROW(SV*, STRLEN newlen)
155
156which will determine if more memory needs to be allocated. If so, it will
157call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
5f05dabc 158decrease, the allocated memory of an SV and that it does not automatically
6602b933 159add space for the trailing C<NUL> byte (perl's own string functions typically do
8ebc5c01 160C<SvGROW(sv, len + 1)>).
a0d0e21e 161
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162If you want to write to an existing SV's buffer and set its value to a
163string, use SvPV_force() or one of its variants to force the SV to be
164a PV. This will remove any of various types of non-stringness from
165the SV while preserving the content of the SV in the PV. This can be
166used, for example, to append data from an API function to a buffer
167without extra copying:
168
169 (void)SvPVbyte_force(sv, len);
170 s = SvGROW(sv, len + needlen + 1);
171 /* something that modifies up to needlen bytes at s+len, but
172 modifies newlen bytes
173 eg. newlen = read(fd, s + len, needlen);
174 ignoring errors for these examples
175 */
176 s[len + newlen] = '\0';
177 SvCUR_set(sv, len + newlen);
178 SvUTF8_off(sv);
179 SvSETMAGIC(sv);
180
181If you already have the data in memory or if you want to keep your
182code simple, you can use one of the sv_cat*() variants, such as
183sv_catpvn(). If you want to insert anywhere in the string you can use
184sv_insert() or sv_insert_flags().
185
186If you don't need the existing content of the SV, you can avoid some
187copying with:
188
5b1fede8 189 SvPVCLEAR(sv);
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190 s = SvGROW(sv, needlen + 1);
191 /* something that modifies up to needlen bytes at s, but modifies
192 newlen bytes
193 eg. newlen = read(fd, s. needlen);
194 */
195 s[newlen] = '\0';
196 SvCUR_set(sv, newlen);
197 SvPOK_only(sv); /* also clears SVf_UTF8 */
198 SvSETMAGIC(sv);
199
200Again, if you already have the data in memory or want to avoid the
201complexity of the above, you can use sv_setpvn().
202
203If you have a buffer allocated with Newx() and want to set that as the
204SV's value, you can use sv_usepvn_flags(). That has some requirements
205if you want to avoid perl re-allocating the buffer to fit the trailing
206NUL:
207
208 Newx(buf, somesize+1, char);
209 /* ... fill in buf ... */
210 buf[somesize] = '\0';
211 sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
212 /* buf now belongs to perl, don't release it */
213
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214If you have an SV and want to know what kind of data Perl thinks is stored
215in it, you can use the following macros to check the type of SV you have.
216
217 SvIOK(SV*)
218 SvNOK(SV*)
219 SvPOK(SV*)
220
221You can get and set the current length of the string stored in an SV with
222the following macros:
223
224 SvCUR(SV*)
225 SvCUR_set(SV*, I32 val)
226
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227You can also get a pointer to the end of the string stored in the SV
228with the macro:
229
230 SvEND(SV*)
231
232But note that these last three macros are valid only if C<SvPOK()> is true.
a0d0e21e 233
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234If you want to append something to the end of string stored in an C<SV*>,
235you can use the following functions:
236
08105a92 237 void sv_catpv(SV*, const char*);
e65f3abd 238 void sv_catpvn(SV*, const char*, STRLEN);
46fc3d4c 239 void sv_catpvf(SV*, const char*, ...);
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240 void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
241 I32, bool);
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242 void sv_catsv(SV*, SV*);
243
244The first function calculates the length of the string to be appended by
245using C<strlen>. In the second, you specify the length of the string
46fc3d4c 246yourself. The third function processes its arguments like C<sprintf> and
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247appends the formatted output. The fourth function works like C<vsprintf>.
248You can specify the address and length of an array of SVs instead of the
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249va_list argument. The fifth function
250extends the string stored in the first
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251SV with the string stored in the second SV. It also forces the second SV
252to be interpreted as a string.
253
254The C<sv_cat*()> functions are not generic enough to operate on values that
5a0de581 255have "magic". See L</Magic Virtual Tables> later in this document.
d1b91892 256
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257If you know the name of a scalar variable, you can get a pointer to its SV
258by using the following:
259
64ace3f8 260 SV* get_sv("package::varname", 0);
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261
262This returns NULL if the variable does not exist.
263
d1b91892 264If you want to know if this variable (or any other SV) is actually C<defined>,
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265you can call:
266
267 SvOK(SV*)
268
06f6df17 269The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
9adebda4 270
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271Its address can be used whenever an C<SV*> is needed. Make sure that
272you don't try to compare a random sv with C<&PL_sv_undef>. For example
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273when interfacing Perl code, it'll work correctly for:
274
275 foo(undef);
276
277But won't work when called as:
278
279 $x = undef;
280 foo($x);
281
282So to repeat always use SvOK() to check whether an sv is defined.
283
284Also you have to be careful when using C<&PL_sv_undef> as a value in
5a0de581 285AVs or HVs (see L</AVs, HVs and undefined values>).
a0d0e21e 286
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287There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
288boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
289addresses can be used whenever an C<SV*> is needed.
a0d0e21e 290
9cde0e7f 291Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
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292Take this code:
293
294 SV* sv = (SV*) 0;
295 if (I-am-to-return-a-real-value) {
296 sv = sv_2mortal(newSViv(42));
297 }
298 sv_setsv(ST(0), sv);
299
300This code tries to return a new SV (which contains the value 42) if it should
04343c6d 301return a real value, or undef otherwise. Instead it has returned a NULL
a0d0e21e 302pointer which, somewhere down the line, will cause a segmentation violation,
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303bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
304first line and all will be well.
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305
306To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
5a0de581 307call is not necessary (see L</Reference Counts and Mortality>).
a0d0e21e 308
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309=head2 Offsets
310
311Perl provides the function C<sv_chop> to efficiently remove characters
312from the beginning of a string; you give it an SV and a pointer to
da75cd15 313somewhere inside the PV, and it discards everything before the
10e2eb10 314pointer. The efficiency comes by means of a little hack: instead of
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315actually removing the characters, C<sv_chop> sets the flag C<OOK>
316(offset OK) to signal to other functions that the offset hack is in
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317effect, and it moves the PV pointer (called C<SvPVX>) forward
318by the number of bytes chopped off, and adjusts C<SvCUR> and C<SvLEN>
319accordingly. (A portion of the space between the old and new PV
320pointers is used to store the count of chopped bytes.)
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321
322Hence, at this point, the start of the buffer that we allocated lives
323at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
324into the middle of this allocated storage.
325
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326This is best demonstrated by example. Normally copy-on-write will prevent
327the substitution from operator from using this hack, but if you can craft a
328string for which copy-on-write is not possible, you can see it in play. In
329the current implementation, the final byte of a string buffer is used as a
330copy-on-write reference count. If the buffer is not big enough, then
331copy-on-write is skipped. First have a look at an empty string:
332
333 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
334 SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
335 REFCNT = 1
336 FLAGS = (POK,pPOK)
337 PV = 0x7ffb7bc05b50 ""\0
338 CUR = 0
339 LEN = 10
340
341Notice here the LEN is 10. (It may differ on your platform.) Extend the
342length of the string to one less than 10, and do a substitution:
94dde4fb 343
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344 % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
345 Dump($a)'
346 SV = PV(0x7ffa04008a70) at 0x7ffa04030390
347 REFCNT = 1
348 FLAGS = (POK,OOK,pPOK)
349 OFFSET = 1
350 PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
351 CUR = 8
352 LEN = 9
94dde4fb 353
f942a0df 354Here the number of bytes chopped off (1) is shown next as the OFFSET. The
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355portion of the string between the "real" and the "fake" beginnings is
356shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
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357the fake beginning, not the real one. (The first character of the string
358buffer happens to have changed to "\1" here, not "1", because the current
359implementation stores the offset count in the string buffer. This is
360subject to change.)
94dde4fb 361
fe854a6f 362Something similar to the offset hack is performed on AVs to enable
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363efficient shifting and splicing off the beginning of the array; while
364C<AvARRAY> points to the first element in the array that is visible from
10e2eb10 365Perl, C<AvALLOC> points to the real start of the C array. These are
319cef53 366usually the same, but a C<shift> operation can be carried out by
6de131f0 367increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
319cef53 368Again, the location of the real start of the C array only comes into
10e2eb10 369play when freeing the array. See C<av_shift> in F<av.c>.
319cef53 370
d1b91892 371=head2 What's Really Stored in an SV?
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372
373Recall that the usual method of determining the type of scalar you have is
5f05dabc 374to use C<Sv*OK> macros. Because a scalar can be both a number and a string,
d1b91892 375usually these macros will always return TRUE and calling the C<Sv*V>
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376macros will do the appropriate conversion of string to integer/double or
377integer/double to string.
378
379If you I<really> need to know if you have an integer, double, or string
380pointer in an SV, you can use the following three macros instead:
381
382 SvIOKp(SV*)
383 SvNOKp(SV*)
384 SvPOKp(SV*)
385
386These will tell you if you truly have an integer, double, or string pointer
d1b91892 387stored in your SV. The "p" stands for private.
a0d0e21e 388
da8c5729 389There are various ways in which the private and public flags may differ.
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390For example, in perl 5.16 and earlier a tied SV may have a valid
391underlying value in the IV slot (so SvIOKp is true), but the data
392should be accessed via the FETCH routine rather than directly,
393so SvIOK is false. (In perl 5.18 onwards, tied scalars use
394the flags the same way as untied scalars.) Another is when
d7f8936a 395numeric conversion has occurred and precision has been lost: only the
10e2eb10 396private flag is set on 'lossy' values. So when an NV is converted to an
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397IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
398
07fa94a1 399In general, though, it's best to use the C<Sv*V> macros.
a0d0e21e 400
54310121 401=head2 Working with AVs
a0d0e21e 402
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403There are two ways to create and load an AV. The first method creates an
404empty AV:
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405
406 AV* newAV();
407
54310121 408The second method both creates the AV and initially populates it with SVs:
a0d0e21e 409
c70927a6 410 AV* av_make(SSize_t num, SV **ptr);
a0d0e21e 411
5f05dabc 412The second argument points to an array containing C<num> C<SV*>'s. Once the
54310121 413AV has been created, the SVs can be destroyed, if so desired.
a0d0e21e 414
da8c5729 415Once the AV has been created, the following operations are possible on it:
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416
417 void av_push(AV*, SV*);
418 SV* av_pop(AV*);
419 SV* av_shift(AV*);
c70927a6 420 void av_unshift(AV*, SSize_t num);
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421
422These should be familiar operations, with the exception of C<av_unshift>.
423This routine adds C<num> elements at the front of the array with the C<undef>
424value. You must then use C<av_store> (described below) to assign values
425to these new elements.
426
427Here are some other functions:
428
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429 SSize_t av_top_index(AV*);
430 SV** av_fetch(AV*, SSize_t key, I32 lval);
431 SV** av_store(AV*, SSize_t key, SV* val);
a0d0e21e 432
dab460cd 433The C<av_top_index> function returns the highest index value in an array (just
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434like $#array in Perl). If the array is empty, -1 is returned. The
435C<av_fetch> function returns the value at index C<key>, but if C<lval>
436is non-zero, then C<av_fetch> will store an undef value at that index.
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437The C<av_store> function stores the value C<val> at index C<key>, and does
438not increment the reference count of C<val>. Thus the caller is responsible
439for taking care of that, and if C<av_store> returns NULL, the caller will
440have to decrement the reference count to avoid a memory leak. Note that
441C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their
442return value.
d1b91892 443
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444A few more:
445
a0d0e21e 446 void av_clear(AV*);
a0d0e21e 447 void av_undef(AV*);
c70927a6 448 void av_extend(AV*, SSize_t key);
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449
450The C<av_clear> function deletes all the elements in the AV* array, but
451does not actually delete the array itself. The C<av_undef> function will
452delete all the elements in the array plus the array itself. The
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453C<av_extend> function extends the array so that it contains at least C<key+1>
454elements. If C<key+1> is less than the currently allocated length of the array,
455then nothing is done.
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456
457If you know the name of an array variable, you can get a pointer to its AV
458by using the following:
459
cbfd0a87 460 AV* get_av("package::varname", 0);
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461
462This returns NULL if the variable does not exist.
463
5a0de581 464See L</Understanding the Magic of Tied Hashes and Arrays> for more
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465information on how to use the array access functions on tied arrays.
466
54310121 467=head2 Working with HVs
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468
469To create an HV, you use the following routine:
470
471 HV* newHV();
472
da8c5729 473Once the HV has been created, the following operations are possible on it:
a0d0e21e 474
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475 SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
476 SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
a0d0e21e 477
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478The C<klen> parameter is the length of the key being passed in (Note that
479you cannot pass 0 in as a value of C<klen> to tell Perl to measure the
480length of the key). The C<val> argument contains the SV pointer to the
54310121 481scalar being stored, and C<hash> is the precomputed hash value (zero if
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482you want C<hv_store> to calculate it for you). The C<lval> parameter
483indicates whether this fetch is actually a part of a store operation, in
484which case a new undefined value will be added to the HV with the supplied
485key and C<hv_fetch> will return as if the value had already existed.
a0d0e21e 486
5f05dabc
PP
487Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
488C<SV*>. To access the scalar value, you must first dereference the return
489value. However, you should check to make sure that the return value is
490not NULL before dereferencing it.
a0d0e21e 491
da8c5729
MH
492The first of these two functions checks if a hash table entry exists, and the
493second deletes it.
a0d0e21e 494
08105a92
GS
495 bool hv_exists(HV*, const char* key, U32 klen);
496 SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
a0d0e21e 497
5f05dabc
PP
498If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will
499create and return a mortal copy of the deleted value.
500
a0d0e21e
LW
501And more miscellaneous functions:
502
503 void hv_clear(HV*);
a0d0e21e 504 void hv_undef(HV*);
5f05dabc
PP
505
506Like their AV counterparts, C<hv_clear> deletes all the entries in the hash
507table but does not actually delete the hash table. The C<hv_undef> deletes
508both the entries and the hash table itself.
a0d0e21e 509
a9b0660e 510Perl keeps the actual data in a linked list of structures with a typedef of HE.
d1b91892
AD
511These contain the actual key and value pointers (plus extra administrative
512overhead). The key is a string pointer; the value is an C<SV*>. However,
513once you have an C<HE*>, to get the actual key and value, use the routines
514specified below.
515
a0d0e21e
LW
516 I32 hv_iterinit(HV*);
517 /* Prepares starting point to traverse hash table */
518 HE* hv_iternext(HV*);
519 /* Get the next entry, and return a pointer to a
520 structure that has both the key and value */
521 char* hv_iterkey(HE* entry, I32* retlen);
522 /* Get the key from an HE structure and also return
523 the length of the key string */
cb1a09d0 524 SV* hv_iterval(HV*, HE* entry);
d1be9408 525 /* Return an SV pointer to the value of the HE
a0d0e21e 526 structure */
cb1a09d0 527 SV* hv_iternextsv(HV*, char** key, I32* retlen);
d1b91892
AD
528 /* This convenience routine combines hv_iternext,
529 hv_iterkey, and hv_iterval. The key and retlen
530 arguments are return values for the key and its
531 length. The value is returned in the SV* argument */
a0d0e21e
LW
532
533If you know the name of a hash variable, you can get a pointer to its HV
534by using the following:
535
6673a63c 536 HV* get_hv("package::varname", 0);
a0d0e21e
LW
537
538This returns NULL if the variable does not exist.
539
a43e7901 540The hash algorithm is defined in the C<PERL_HASH> macro:
a0d0e21e 541
a43e7901 542 PERL_HASH(hash, key, klen)
ab192400 543
a43e7901
YO
544The exact implementation of this macro varies by architecture and version
545of perl, and the return value may change per invocation, so the value
546is only valid for the duration of a single perl process.
a0d0e21e 547
5a0de581 548See L</Understanding the Magic of Tied Hashes and Arrays> for more
04343c6d
GS
549information on how to use the hash access functions on tied hashes.
550
4f313521
KW
551=for apidoc Amh|void|PERL_HASH|U32 hash|char *key|STRLEN klen
552
1e422769
PP
553=head2 Hash API Extensions
554
555Beginning with version 5.004, the following functions are also supported:
556
557 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
558 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 559
1e422769
PP
560 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
561 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 562
1e422769
PP
563 SV* hv_iterkeysv (HE* entry);
564
565Note that these functions take C<SV*> keys, which simplifies writing
566of extension code that deals with hash structures. These functions
567also allow passing of C<SV*> keys to C<tie> functions without forcing
568you to stringify the keys (unlike the previous set of functions).
569
570They also return and accept whole hash entries (C<HE*>), making their
571use more efficient (since the hash number for a particular string
4a4eefd0
GS
572doesn't have to be recomputed every time). See L<perlapi> for detailed
573descriptions.
1e422769
PP
574
575The following macros must always be used to access the contents of hash
576entries. Note that the arguments to these macros must be simple
577variables, since they may get evaluated more than once. See
4a4eefd0 578L<perlapi> for detailed descriptions of these macros.
1e422769
PP
579
580 HePV(HE* he, STRLEN len)
581 HeVAL(HE* he)
582 HeHASH(HE* he)
583 HeSVKEY(HE* he)
584 HeSVKEY_force(HE* he)
585 HeSVKEY_set(HE* he, SV* sv)
586
587These two lower level macros are defined, but must only be used when
588dealing with keys that are not C<SV*>s:
589
590 HeKEY(HE* he)
591 HeKLEN(HE* he)
592
04343c6d
GS
593Note that both C<hv_store> and C<hv_store_ent> do not increment the
594reference count of the stored C<val>, which is the caller's responsibility.
595If these functions return a NULL value, the caller will usually have to
596decrement the reference count of C<val> to avoid a memory leak.
1e422769 597
a9381218
MHM
598=head2 AVs, HVs and undefined values
599
10e2eb10
FC
600Sometimes you have to store undefined values in AVs or HVs. Although
601this may be a rare case, it can be tricky. That's because you're
a9381218
MHM
602used to using C<&PL_sv_undef> if you need an undefined SV.
603
604For example, intuition tells you that this XS code:
605
606 AV *av = newAV();
607 av_store( av, 0, &PL_sv_undef );
608
609is equivalent to this Perl code:
610
611 my @av;
612 $av[0] = undef;
613
f3c4ec28 614Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
a9381218
MHM
615for indicating that an array element has not yet been initialized.
616Thus, C<exists $av[0]> would be true for the above Perl code, but
f3c4ec28
FC
617false for the array generated by the XS code. In perl 5.20, storing
618&PL_sv_undef will create a read-only element, because the scalar
619&PL_sv_undef itself is stored, not a copy.
a9381218 620
f3c4ec28 621Similar problems can occur when storing C<&PL_sv_undef> in HVs:
a9381218
MHM
622
623 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
624
625This will indeed make the value C<undef>, but if you try to modify
626the value of C<key>, you'll get the following error:
627
628 Modification of non-creatable hash value attempted
629
630In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
10e2eb10 631in restricted hashes. This caused such hash entries not to appear
a9381218
MHM
632when iterating over the hash or when checking for the keys
633with the C<hv_exists> function.
634
8abccac8 635You can run into similar problems when you store C<&PL_sv_yes> or
10e2eb10 636C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
a9381218
MHM
637will give you the following error:
638
639 Modification of a read-only value attempted
640
641To make a long story short, you can use the special variables
8abccac8 642C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
a9381218
MHM
643HVs, but you have to make sure you know what you're doing.
644
645Generally, if you want to store an undefined value in an AV
646or HV, you should not use C<&PL_sv_undef>, but rather create a
647new undefined value using the C<newSV> function, for example:
648
649 av_store( av, 42, newSV(0) );
650 hv_store( hv, "foo", 3, newSV(0), 0 );
651
a0d0e21e
LW
652=head2 References
653
d1b91892 654References are a special type of scalar that point to other data types
a9b0660e 655(including other references).
a0d0e21e 656
07fa94a1 657To create a reference, use either of the following functions:
a0d0e21e 658
5f05dabc
PP
659 SV* newRV_inc((SV*) thing);
660 SV* newRV_noinc((SV*) thing);
a0d0e21e 661
5f05dabc 662The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
663functions are identical except that C<newRV_inc> increments the reference
664count of the C<thing>, while C<newRV_noinc> does not. For historical
665reasons, C<newRV> is a synonym for C<newRV_inc>.
666
667Once you have a reference, you can use the following macro to dereference
668the reference:
a0d0e21e
LW
669
670 SvRV(SV*)
671
672then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 673C<AV*> or C<HV*>, if required.
a0d0e21e 674
d1b91892 675To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
676
677 SvROK(SV*)
678
07fa94a1
JO
679To discover what type of value the reference refers to, use the following
680macro and then check the return value.
d1b91892
AD
681
682 SvTYPE(SvRV(SV*))
683
684The most useful types that will be returned are:
685
a5e62da0
FC
686 SVt_PVAV Array
687 SVt_PVHV Hash
688 SVt_PVCV Code
689 SVt_PVGV Glob (possibly a file handle)
690
2d0e7d1f
DM
691Any numerical value returned which is less than SVt_PVAV will be a scalar
692of some form.
693
a5e62da0 694See L<perlapi/svtype> for more details.
d1b91892 695
cb1a09d0
AD
696=head2 Blessed References and Class Objects
697
06f6df17 698References are also used to support object-oriented programming. In perl's
cb1a09d0
AD
699OO lexicon, an object is simply a reference that has been blessed into a
700package (or class). Once blessed, the programmer may now use the reference
701to access the various methods in the class.
702
703A reference can be blessed into a package with the following function:
704
705 SV* sv_bless(SV* sv, HV* stash);
706
06f6df17
RGS
707The C<sv> argument must be a reference value. The C<stash> argument
708specifies which class the reference will belong to. See
5a0de581 709L</Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
AD
710
711/* Still under construction */
712
ddd2cc91
DM
713The following function upgrades rv to reference if not already one.
714Creates a new SV for rv to point to. If C<classname> is non-null, the SV
715is blessed into the specified class. SV is returned.
cb1a09d0 716
08105a92 717 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 718
ddd2cc91
DM
719The following three functions copy integer, unsigned integer or double
720into an SV whose reference is C<rv>. SV is blessed if C<classname> is
721non-null.
cb1a09d0 722
08105a92 723 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 724 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 725 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 726
ddd2cc91
DM
727The following function copies the pointer value (I<the address, not the
728string!>) into an SV whose reference is rv. SV is blessed if C<classname>
729is non-null.
cb1a09d0 730
ddd2cc91 731 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
cb1a09d0 732
a9b0660e 733The following function copies a string into an SV whose reference is C<rv>.
ddd2cc91
DM
734Set length to 0 to let Perl calculate the string length. SV is blessed if
735C<classname> is non-null.
cb1a09d0 736
a9b0660e
KW
737 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
738 STRLEN length);
cb1a09d0 739
ddd2cc91
DM
740The following function tests whether the SV is blessed into the specified
741class. It does not check inheritance relationships.
9abd00ed 742
08105a92 743 int sv_isa(SV* sv, const char* name);
9abd00ed 744
ddd2cc91 745The following function tests whether the SV is a reference to a blessed object.
9abd00ed
GS
746
747 int sv_isobject(SV* sv);
748
ddd2cc91 749The following function tests whether the SV is derived from the specified
10e2eb10
FC
750class. SV can be either a reference to a blessed object or a string
751containing a class name. This is the function implementing the
ddd2cc91 752C<UNIVERSAL::isa> functionality.
9abd00ed 753
08105a92 754 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 755
00aadd71 756To check if you've got an object derived from a specific class you have
9abd00ed
GS
757to write:
758
759 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 760
5f05dabc 761=head2 Creating New Variables
cb1a09d0 762
5f05dabc
PP
763To create a new Perl variable with an undef value which can be accessed from
764your Perl script, use the following routines, depending on the variable type.
cb1a09d0 765
64ace3f8 766 SV* get_sv("package::varname", GV_ADD);
cbfd0a87 767 AV* get_av("package::varname", GV_ADD);
6673a63c 768 HV* get_hv("package::varname", GV_ADD);
cb1a09d0 769
058a5f6c 770Notice the use of GV_ADD as the second parameter. The new variable can now
cb1a09d0
AD
771be set, using the routines appropriate to the data type.
772
5f05dabc 773There are additional macros whose values may be bitwise OR'ed with the
058a5f6c 774C<GV_ADD> argument to enable certain extra features. Those bits are:
cb1a09d0 775
9a68f1db
SB
776=over
777
778=item GV_ADDMULTI
779
780Marks the variable as multiply defined, thus preventing the:
781
782 Name <varname> used only once: possible typo
783
784warning.
785
9a68f1db
SB
786=item GV_ADDWARN
787
788Issues the warning:
789
790 Had to create <varname> unexpectedly
791
792if the variable did not exist before the function was called.
793
794=back
cb1a09d0 795
07fa94a1
JO
796If you do not specify a package name, the variable is created in the current
797package.
cb1a09d0 798
5f05dabc 799=head2 Reference Counts and Mortality
a0d0e21e 800
10e2eb10 801Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 802AVs, or HVs (xV for short in the following) start their life with a
55497cff 803reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 804then it will be destroyed and its memory made available for reuse.
3d2ba989
Z
805At the most basic internal level, reference counts can be manipulated
806with the following macros:
55497cff
PP
807
808 int SvREFCNT(SV* sv);
5f05dabc 809 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
810 void SvREFCNT_dec(SV* sv);
811
3d2ba989
Z
812(There are also suffixed versions of the increment and decrement macros,
813for situations where the full generality of these basic macros can be
814exchanged for some performance.)
815
816However, the way a programmer should think about references is not so
817much in terms of the bare reference count, but in terms of I<ownership>
818of references. A reference to an xV can be owned by any of a variety
819of entities: another xV, the Perl interpreter, an XS data structure,
820a piece of running code, or a dynamic scope. An xV generally does not
821know what entities own the references to it; it only knows how many
822references there are, which is the reference count.
823
824To correctly maintain reference counts, it is essential to keep track
825of what references the XS code is manipulating. The programmer should
826always know where a reference has come from and who owns it, and be
827aware of any creation or destruction of references, and any transfers
828of ownership. Because ownership isn't represented explicitly in the xV
829data structures, only the reference count need be actually maintained
830by the code, and that means that this understanding of ownership is not
831actually evident in the code. For example, transferring ownership of a
832reference from one owner to another doesn't change the reference count
833at all, so may be achieved with no actual code. (The transferring code
834doesn't touch the referenced object, but does need to ensure that the
835former owner knows that it no longer owns the reference, and that the
836new owner knows that it now does.)
837
838An xV that is visible at the Perl level should not become unreferenced
839and thus be destroyed. Normally, an object will only become unreferenced
840when it is no longer visible, often by the same means that makes it
841invisible. For example, a Perl reference value (RV) owns a reference to
842its referent, so if the RV is overwritten that reference gets destroyed,
843and the no-longer-reachable referent may be destroyed as a result.
844
845Many functions have some kind of reference manipulation as
846part of their purpose. Sometimes this is documented in terms
847of ownership of references, and sometimes it is (less helpfully)
848documented in terms of changes to reference counts. For example, the
849L<newRV_inc()|perlapi/newRV_inc> function is documented to create a new RV
850(with reference count 1) and increment the reference count of the referent
851that was supplied by the caller. This is best understood as creating
852a new reference to the referent, which is owned by the created RV,
853and returning to the caller ownership of the sole reference to the RV.
854The L<newRV_noinc()|perlapi/newRV_noinc> function instead does not
855increment the reference count of the referent, but the RV nevertheless
856ends up owning a reference to the referent. It is therefore implied
857that the caller of C<newRV_noinc()> is relinquishing a reference to the
858referent, making this conceptually a more complicated operation even
859though it does less to the data structures.
860
861For example, imagine you want to return a reference from an XSUB
862function. Inside the XSUB routine, you create an SV which initially
863has just a single reference, owned by the XSUB routine. This reference
864needs to be disposed of before the routine is complete, otherwise it
865will leak, preventing the SV from ever being destroyed. So to create
866an RV referencing the SV, it is most convenient to pass the SV to
867C<newRV_noinc()>, which consumes that reference. Now the XSUB routine
868no longer owns a reference to the SV, but does own a reference to the RV,
869which in turn owns a reference to the SV. The ownership of the reference
870to the RV is then transferred by the process of returning the RV from
871the XSUB.
55497cff 872
5f05dabc 873There are some convenience functions available that can help with the
54310121 874destruction of xVs. These functions introduce the concept of "mortality".
3d2ba989
Z
875Much documentation speaks of an xV itself being mortal, but this is
876misleading. It is really I<a reference to> an xV that is mortal, and it
877is possible for there to be more than one mortal reference to a single xV.
878For a reference to be mortal means that it is owned by the temps stack,
879one of perl's many internal stacks, which will destroy that reference
880"a short time later". Usually the "short time later" is the end of
881the current Perl statement. However, it gets more complicated around
882dynamic scopes: there can be multiple sets of mortal references hanging
883around at the same time, with different death dates. Internally, the
884actual determinant for when mortal xV references are destroyed depends
885on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs>
886for more details on these macros.
887
888Mortal references are mainly used for xVs that are placed on perl's
889main stack. The stack is problematic for reference tracking, because it
890contains a lot of xV references, but doesn't own those references: they
891are not counted. Currently, there are many bugs resulting from xVs being
892destroyed while referenced by the stack, because the stack's uncounted
893references aren't enough to keep the xVs alive. So when putting an
894(uncounted) reference on the stack, it is vitally important to ensure that
895there will be a counted reference to the same xV that will last at least
896as long as the uncounted reference. But it's also important that that
897counted reference be cleaned up at an appropriate time, and not unduly
898prolong the xV's life. For there to be a mortal reference is often the
899best way to satisfy this requirement, especially if the xV was created
900especially to be put on the stack and would otherwise be unreferenced.
901
902To create a mortal reference, use the functions:
a0d0e21e
LW
903
904 SV* sv_newmortal()
a0d0e21e 905 SV* sv_mortalcopy(SV*)
3d2ba989 906 SV* sv_2mortal(SV*)
a0d0e21e 907
3d2ba989
Z
908C<sv_newmortal()> creates an SV (with the undefined value) whose sole
909reference is mortal. C<sv_mortalcopy()> creates an xV whose value is a
910copy of a supplied xV and whose sole reference is mortal. C<sv_2mortal()>
911mortalises an existing xV reference: it transfers ownership of a reference
912from the caller to the temps stack. Because C<sv_newmortal> gives the new
913SV no value, it must normally be given one via C<sv_setpv>, C<sv_setiv>,
914etc. :
00aadd71
NIS
915
916 SV *tmp = sv_newmortal();
917 sv_setiv(tmp, an_integer);
918
919As that is multiple C statements it is quite common so see this idiom instead:
920
921 SV *tmp = sv_2mortal(newSViv(an_integer));
922
ac036724 923The mortal routines are not just for SVs; AVs and HVs can be
faed5253 924made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 925C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 926
5f05dabc 927=head2 Stashes and Globs
a0d0e21e 928
06f6df17
RGS
929A B<stash> is a hash that contains all variables that are defined
930within a package. Each key of the stash is a symbol
aa689395
PP
931name (shared by all the different types of objects that have the same
932name), and each value in the hash table is a GV (Glob Value). This GV
933in turn contains references to the various objects of that name,
934including (but not limited to) the following:
cb1a09d0 935
a0d0e21e
LW
936 Scalar Value
937 Array Value
938 Hash Value
a3cb178b 939 I/O Handle
a0d0e21e
LW
940 Format
941 Subroutine
942
06f6df17
RGS
943There is a single stash called C<PL_defstash> that holds the items that exist
944in the C<main> package. To get at the items in other packages, append the
945string "::" to the package name. The items in the C<Foo> package are in
946the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
947in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 948
d1b91892 949To get the stash pointer for a particular package, use the function:
a0d0e21e 950
da51bb9b
NC
951 HV* gv_stashpv(const char* name, I32 flags)
952 HV* gv_stashsv(SV*, I32 flags)
a0d0e21e
LW
953
954The first function takes a literal string, the second uses the string stored
d1b91892 955in the SV. Remember that a stash is just a hash table, so you get back an
da51bb9b 956C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
a0d0e21e
LW
957
958The name that C<gv_stash*v> wants is the name of the package whose symbol table
959you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
960packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
961language itself.
a0d0e21e
LW
962
963Alternately, if you have an SV that is a blessed reference, you can find
964out the stash pointer by using:
965
966 HV* SvSTASH(SvRV(SV*));
967
968then use the following to get the package name itself:
969
970 char* HvNAME(HV* stash);
971
5f05dabc
PP
972If you need to bless or re-bless an object you can use the following
973function:
a0d0e21e
LW
974
975 SV* sv_bless(SV*, HV* stash)
976
977where the first argument, an C<SV*>, must be a reference, and the second
978argument is a stash. The returned C<SV*> can now be used in the same way
979as any other SV.
980
d1b91892
AD
981For more information on references and blessings, consult L<perlref>.
982
54310121 983=head2 Double-Typed SVs
0a753a76
PP
984
985Scalar variables normally contain only one type of value, an integer,
986double, pointer, or reference. Perl will automatically convert the
987actual scalar data from the stored type into the requested type.
988
989Some scalar variables contain more than one type of scalar data. For
990example, the variable C<$!> contains either the numeric value of C<errno>
991or its string equivalent from either C<strerror> or C<sys_errlist[]>.
992
993To force multiple data values into an SV, you must do two things: use the
994C<sv_set*v> routines to add the additional scalar type, then set a flag
995so that Perl will believe it contains more than one type of data. The
996four macros to set the flags are:
997
998 SvIOK_on
999 SvNOK_on
1000 SvPOK_on
1001 SvROK_on
1002
1003The particular macro you must use depends on which C<sv_set*v> routine
1004you called first. This is because every C<sv_set*v> routine turns on
1005only the bit for the particular type of data being set, and turns off
1006all the rest.
1007
1008For example, to create a new Perl variable called "dberror" that contains
1009both the numeric and descriptive string error values, you could use the
1010following code:
1011
1012 extern int dberror;
1013 extern char *dberror_list;
1014
64ace3f8 1015 SV* sv = get_sv("dberror", GV_ADD);
0a753a76
PP
1016 sv_setiv(sv, (IV) dberror);
1017 sv_setpv(sv, dberror_list[dberror]);
1018 SvIOK_on(sv);
1019
1020If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
1021macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
1022
4f4531b8
FC
1023=head2 Read-Only Values
1024
1025In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
1026flag bit with read-only scalars. So the only way to test whether
1027C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
1028in those versions is:
1029
1030 SvREADONLY(sv) && !SvIsCOW(sv)
1031
1032Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
1033and, under 5.20, copy-on-write scalars can also be read-only, so the above
1034check is incorrect. You just want:
1035
1036 SvREADONLY(sv)
1037
1038If you need to do this check often, define your own macro like this:
1039
1040 #if PERL_VERSION >= 18
1041 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
1042 #else
1043 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
1044 #endif
1045
1046=head2 Copy on Write
1047
1048Perl implements a copy-on-write (COW) mechanism for scalars, in which
1049string copies are not immediately made when requested, but are deferred
1050until made necessary by one or the other scalar changing. This is mostly
1051transparent, but one must take care not to modify string buffers that are
1052shared by multiple SVs.
1053
1054You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
1055
1056You can force an SV to make its own copy of its string buffer by calling C<sv_force_normal(sv)> or SvPV_force_nolen(sv).
1057
1058If you want to make the SV drop its string buffer, use
1059C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
1060C<sv_setsv(sv, NULL)>.
1061
1062All of these functions will croak on read-only scalars (see the previous
1063section for more on those).
1064
1065To test that your code is behaving correctly and not modifying COW buffers,
1066on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
1067C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
1068into crashes. You will find it to be marvellously slow, so you may want to
1069skip perl's own tests.
1070
0a753a76 1071=head2 Magic Variables
a0d0e21e 1072
d1b91892
AD
1073[This section still under construction. Ignore everything here. Post no
1074bills. Everything not permitted is forbidden.]
1075
d1b91892
AD
1076Any SV may be magical, that is, it has special features that a normal
1077SV does not have. These features are stored in the SV structure in a
5f05dabc 1078linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
1079
1080 struct magic {
1081 MAGIC* mg_moremagic;
1082 MGVTBL* mg_virtual;
1083 U16 mg_private;
1084 char mg_type;
1085 U8 mg_flags;
b205eb13 1086 I32 mg_len;
d1b91892
AD
1087 SV* mg_obj;
1088 char* mg_ptr;
d1b91892
AD
1089 };
1090
1091Note this is current as of patchlevel 0, and could change at any time.
1092
1093=head2 Assigning Magic
1094
1095Perl adds magic to an SV using the sv_magic function:
1096
a9b0660e 1097 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
1098
1099The C<sv> argument is a pointer to the SV that is to acquire a new magical
1100feature.
1101
1102If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
10e2eb10
FC
1103convert C<sv> to type C<SVt_PVMG>.
1104Perl then continues by adding new magic
645c22ef
DM
1105to the beginning of the linked list of magical features. Any prior entry
1106of the same type of magic is deleted. Note that this can be overridden,
1107and multiple instances of the same type of magic can be associated with an
1108SV.
d1b91892 1109
54310121 1110The C<name> and C<namlen> arguments are used to associate a string with
10e2eb10 1111the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
1112C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
1113C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
1114whether C<namlen> is greater than zero or equal to zero respectively. As a
1115special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
1116to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
1117
1118The sv_magic function uses C<how> to determine which, if any, predefined
1119"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
5a0de581 1120See the L</Magic Virtual Tables> section below. The C<how> argument is also
10e2eb10
FC
1121stored in the C<mg_type> field. The value of
1122C<how> should be chosen from the set of macros
1123C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 1124these macros were added, Perl internals used to directly use character
14befaf4 1125literals, so you may occasionally come across old code or documentation
75d0f26d 1126referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
1127
1128The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
1129structure. If it is not the same as the C<sv> argument, the reference
1130count of the C<obj> object is incremented. If it is the same, or if
27deb0cf
YO
1131the C<how> argument is C<PERL_MAGIC_arylen>, C<PERL_MAGIC_regdatum>,
1132C<PERL_MAGIC_regdata>, or if it is a NULL pointer, then C<obj> is merely
1133stored, without the reference count being incremented.
d1b91892 1134
2d8d5d5a
SH
1135See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
1136to an SV.
1137
cb1a09d0
AD
1138There is also a function to add magic to an C<HV>:
1139
1140 void hv_magic(HV *hv, GV *gv, int how);
1141
1142This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
1143
1144To remove the magic from an SV, call the function sv_unmagic:
1145
70a53b35 1146 int sv_unmagic(SV *sv, int type);
cb1a09d0
AD
1147
1148The C<type> argument should be equal to the C<how> value when the C<SV>
1149was initially made magical.
1150
f6ee7b17 1151However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
10e2eb10
FC
1152C<SV>. If you want to remove only certain
1153magic of a C<type> based on the magic
f6ee7b17
FR
1154virtual table, use C<sv_unmagicext> instead:
1155
1156 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1157
d1b91892
AD
1158=head2 Magic Virtual Tables
1159
d1be9408 1160The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
1161C<MGVTBL>, which is a structure of function pointers and stands for
1162"Magic Virtual Table" to handle the various operations that might be
1163applied to that variable.
1164
301cb7e8
DM
1165The C<MGVTBL> has five (or sometimes eight) pointers to the following
1166routine types:
d1b91892 1167
e97ca230
DM
1168 int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
1169 int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
1170 U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
1171 int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
1172 int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
d1b91892 1173
e97ca230 1174 int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
a9b0660e 1175 const char *name, I32 namlen);
e97ca230
DM
1176 int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
1177 int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
301cb7e8
DM
1178
1179
06f6df17 1180This MGVTBL structure is set at compile-time in F<perl.h> and there are
b7a0f54c
SM
1181currently 32 types. These different structures contain pointers to various
1182routines that perform additional actions depending on which function is
1183being called.
d1b91892 1184
a9b0660e
KW
1185 Function pointer Action taken
1186 ---------------- ------------
1187 svt_get Do something before the value of the SV is
1188 retrieved.
1189 svt_set Do something after the SV is assigned a value.
1190 svt_len Report on the SV's length.
1191 svt_clear Clear something the SV represents.
1192 svt_free Free any extra storage associated with the SV.
d1b91892 1193
a9b0660e
KW
1194 svt_copy copy tied variable magic to a tied element
1195 svt_dup duplicate a magic structure during thread cloning
1196 svt_local copy magic to local value during 'local'
301cb7e8 1197
d1b91892 1198For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 1199to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
1200
1201 { magic_get, magic_set, magic_len, 0, 0 }
1202
14befaf4
DM
1203Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1204if a get operation is being performed, the routine C<magic_get> is
1205called. All the various routines for the various magical types begin
1206with C<magic_>. NOTE: the magic routines are not considered part of
1207the Perl API, and may not be exported by the Perl library.
d1b91892 1208
301cb7e8
DM
1209The last three slots are a recent addition, and for source code
1210compatibility they are only checked for if one of the three flags
10e2eb10
FC
1211MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
1212This means that most code can continue declaring
1213a vtable as a 5-element value. These three are
301cb7e8
DM
1214currently used exclusively by the threading code, and are highly subject
1215to change.
1216
d1b91892
AD
1217The current kinds of Magic Virtual Tables are:
1218
f1f5ddd7
FC
1219=for comment
1220This table is generated by regen/mg_vtable.pl. Any changes made here
1221will be lost.
1222
1223=for mg_vtable.pl begin
1224
a9b0660e 1225 mg_type
bd6e6c12
FC
1226 (old-style char and macro) MGVTBL Type of magic
1227 -------------------------- ------ -------------
1228 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1229 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
e5e1ee61 1230 % PERL_MAGIC_rhash (none) Extra data for restricted
bd6e6c12 1231 hashes
a6d69523
TC
1232 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1233 vars
bd6e6c12 1234 . PERL_MAGIC_pos vtbl_pos pos() lvalue
e5e1ee61 1235 : PERL_MAGIC_symtab (none) Extra data for symbol
bd6e6c12 1236 tables
e5e1ee61
FC
1237 < PERL_MAGIC_backref vtbl_backref For weak ref data
1238 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
bd6e6c12
FC
1239 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1240 (fast string search)
1241 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1242 (AMT) on stash
1243 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1244 (@+ and @- vars)
1245 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1246 element
1247 E PERL_MAGIC_env vtbl_env %ENV hash
1248 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
eccba044 1249 f PERL_MAGIC_fm vtbl_regexp Formline
bd6e6c12 1250 ('compiled' format)
bd6e6c12
FC
1251 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1252 H PERL_MAGIC_hints vtbl_hints %^H hash
1253 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1254 I PERL_MAGIC_isa vtbl_isa @ISA array
1255 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1256 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1257 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1258 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1259 element
1260 N PERL_MAGIC_shared (none) Shared between threads
1261 n PERL_MAGIC_shared_scalar (none) Shared between threads
1262 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1263 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1264 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1265 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
e5e1ee61 1266 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
bd6e6c12
FC
1267 S PERL_MAGIC_sig (none) %SIG hash
1268 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1269 t PERL_MAGIC_taint vtbl_taint Taintedness
1270 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1271 extensions
1272 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1273 extensions
4499db73 1274 V PERL_MAGIC_vstring (none) SV was vstring literal
bd6e6c12
FC
1275 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1276 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1277 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1f1dcfb5
FC
1278 Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
1279 exist
bd6e6c12
FC
1280 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1281 variable / smart parameter
1282 vivification
baabe3fb
FC
1283 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
1284 constructor
e5e1ee61 1285 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
bd6e6c12
FC
1286 to this CV
1287 ~ PERL_MAGIC_ext (none) Available for use by
1288 extensions
0cbee0a4 1289
f2f5335a
KW
1290
1291=for apidoc Amnh||PERL_MAGIC_sv
1292=for apidoc Amnh||PERL_MAGIC_arylen
1293=for apidoc Amnh||PERL_MAGIC_rhash
1294=for apidoc Amnh||PERL_MAGIC_debugvar
1295=for apidoc Amnh||PERL_MAGIC_pos
1296=for apidoc Amnh||PERL_MAGIC_symtab
1297=for apidoc Amnh||PERL_MAGIC_backref
1298=for apidoc Amnh||PERL_MAGIC_arylen_p
1299=for apidoc Amnh||PERL_MAGIC_bm
1300=for apidoc Amnh||PERL_MAGIC_overload_table
1301=for apidoc Amnh||PERL_MAGIC_regdata
1302=for apidoc Amnh||PERL_MAGIC_regdatum
1303=for apidoc Amnh||PERL_MAGIC_env
1304=for apidoc Amnh||PERL_MAGIC_envelem
1305=for apidoc Amnh||PERL_MAGIC_fm
1306=for apidoc Amnh||PERL_MAGIC_regex_global
1307=for apidoc Amnh||PERL_MAGIC_hints
1308=for apidoc Amnh||PERL_MAGIC_hintselem
1309=for apidoc Amnh||PERL_MAGIC_isa
1310=for apidoc Amnh||PERL_MAGIC_isaelem
1311=for apidoc Amnh||PERL_MAGIC_nkeys
1312=for apidoc Amnh||PERL_MAGIC_dbfile
1313=for apidoc Amnh||PERL_MAGIC_dbline
1314=for apidoc Amnh||PERL_MAGIC_shared
1315=for apidoc Amnh||PERL_MAGIC_shared_scalar
1316=for apidoc Amnh||PERL_MAGIC_collxfrm
1317=for apidoc Amnh||PERL_MAGIC_tied
1318=for apidoc Amnh||PERL_MAGIC_tiedelem
1319=for apidoc Amnh||PERL_MAGIC_tiedscalar
1320=for apidoc Amnh||PERL_MAGIC_qr
1321=for apidoc Amnh||PERL_MAGIC_sig
1322=for apidoc Amnh||PERL_MAGIC_sigelem
1323=for apidoc Amnh||PERL_MAGIC_taint
1324=for apidoc Amnh||PERL_MAGIC_uvar
1325=for apidoc Amnh||PERL_MAGIC_uvar_elem
1326=for apidoc Amnh||PERL_MAGIC_vstring
1327=for apidoc Amnh||PERL_MAGIC_vec
1328=for apidoc Amnh||PERL_MAGIC_utf8
1329=for apidoc Amnh||PERL_MAGIC_substr
1330=for apidoc Amnh||PERL_MAGIC_nonelem
1331=for apidoc Amnh||PERL_MAGIC_defelem
1332=for apidoc Amnh||PERL_MAGIC_lvref
1333=for apidoc Amnh||PERL_MAGIC_checkcall
1334=for apidoc Amnh||PERL_MAGIC_ext
1335
f1f5ddd7 1336=for mg_vtable.pl end
d1b91892 1337
68dc0745 1338When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1339uppercase letter is typically used to represent some kind of composite type
1340(a list or a hash), and the lowercase letter is used to represent an element
10e2eb10 1341of that composite type. Some internals code makes use of this case
92f0c265 1342relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1343
1344The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1345specifically for use by extensions and will not be used by perl itself.
1346Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1347to variables (typically objects). This is especially useful because
1348there is no way for normal perl code to corrupt this private information
1349(unlike using extra elements of a hash object).
1350
1351Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1352C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1353C<mg_ptr> field points to a C<ufuncs> structure:
1354
1355 struct ufuncs {
a9402793
AB
1356 I32 (*uf_val)(pTHX_ IV, SV*);
1357 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1358 IV uf_index;
1359 };
1360
1361When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1362function will be called with C<uf_index> as the first arg and a pointer to
1363the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1364magic is shown below. Note that the ufuncs structure is copied by
1365sv_magic, so you can safely allocate it on the stack.
1366
1367 void
1368 Umagic(sv)
1369 SV *sv;
1370 PREINIT:
1371 struct ufuncs uf;
1372 CODE:
1373 uf.uf_val = &my_get_fn;
1374 uf.uf_set = &my_set_fn;
1375 uf.uf_index = 0;
14befaf4 1376 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1377
1e73acc8
AS
1378Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1379
1380For hashes there is a specialized hook that gives control over hash
1381keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1382if the "set" function in the C<ufuncs> structure is NULL. The hook
1383is activated whenever the hash is accessed with a key specified as
1384an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1385C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1386through the functions without the C<..._ent> suffix circumvents the
4509d391 1387hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1e73acc8 1388
14befaf4
DM
1389Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1390or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1391extra care to avoid conflict. Typically only using the magic on
1392objects blessed into the same class as the extension is sufficient.
2f07f21a
FR
1393For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1394C<MGVTBL>, even if all its fields will be C<0>, so that individual
1395C<MAGIC> pointers can be identified as a particular kind of magic
10e2eb10 1396using their magic virtual table. C<mg_findext> provides an easy way
f6ee7b17 1397to do that:
2f07f21a
FR
1398
1399 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1400
1401 MAGIC *mg;
f6ee7b17
FR
1402 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1403 /* this is really ours, not another module's PERL_MAGIC_ext */
1404 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1405 ...
2f07f21a 1406 }
5f05dabc 1407
ef50df4b
GS
1408Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1409earlier do B<not> invoke 'set' magic on their targets. This must
1410be done by the user either by calling the C<SvSETMAGIC()> macro after
1411calling these functions, or by using one of the C<sv_set*_mg()> or
1412C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1413C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1414obtained from external sources in functions that don't handle magic.
4a4eefd0 1415See L<perlapi> for a description of these functions.
189b2af5
GS
1416For example, calls to the C<sv_cat*()> functions typically need to be
1417followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1418since their implementation handles 'get' magic.
1419
d1b91892
AD
1420=head2 Finding Magic
1421
a9b0660e
KW
1422 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1423 * type */
f6ee7b17
FR
1424
1425This routine returns a pointer to a C<MAGIC> structure stored in the SV.
10e2eb10
FC
1426If the SV does not have that magical
1427feature, C<NULL> is returned. If the
f6ee7b17 1428SV has multiple instances of that magical feature, the first one will be
10e2eb10
FC
1429returned. C<mg_findext> can be used
1430to find a C<MAGIC> structure of an SV
da8c5729 1431based on both its magic type and its magic virtual table:
f6ee7b17
FR
1432
1433 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
d1b91892 1434
f6ee7b17
FR
1435Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1436SVt_PVMG, Perl may core dump.
d1b91892 1437
08105a92 1438 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1439
1440This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1441field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1442the mg_type field is changed to be the lowercase letter.
a0d0e21e 1443
04343c6d
GS
1444=head2 Understanding the Magic of Tied Hashes and Arrays
1445
14befaf4
DM
1446Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1447magic type.
9edb2b46
GS
1448
1449WARNING: As of the 5.004 release, proper usage of the array and hash
1450access functions requires understanding a few caveats. Some
1451of these caveats are actually considered bugs in the API, to be fixed
10e2eb10 1452in later releases, and are bracketed with [MAYCHANGE] below. If
9edb2b46
GS
1453you find yourself actually applying such information in this section, be
1454aware that the behavior may change in the future, umm, without warning.
04343c6d 1455
1526ead6 1456The perl tie function associates a variable with an object that implements
9a68f1db 1457the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6 1458tie function from an XSUB, you must mimic this behaviour. The code below
61ad4b94 1459carries out the necessary steps -- firstly it creates a new hash, and then
1526ead6 1460creates a second hash which it blesses into the class which will implement
10e2eb10 1461the tie methods. Lastly it ties the two hashes together, and returns a
1526ead6
AB
1462reference to the new tied hash. Note that the code below does NOT call the
1463TIEHASH method in the MyTie class -
5a0de581 1464see L</Calling Perl Routines from within C Programs> for details on how
1526ead6
AB
1465to do this.
1466
1467 SV*
1468 mytie()
1469 PREINIT:
1470 HV *hash;
1471 HV *stash;
1472 SV *tie;
1473 CODE:
1474 hash = newHV();
1475 tie = newRV_noinc((SV*)newHV());
da51bb9b 1476 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1477 sv_bless(tie, stash);
899e16d0 1478 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1479 RETVAL = newRV_noinc(hash);
1480 OUTPUT:
1481 RETVAL
1482
04343c6d
GS
1483The C<av_store> function, when given a tied array argument, merely
1484copies the magic of the array onto the value to be "stored", using
1485C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1486actually need to be stored in the array. [MAYCHANGE] After a call to
1487C<av_store> on a tied array, the caller will usually need to call
1488C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1489TIEARRAY object. If C<av_store> did return NULL, a call to
1490C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1491leak. [/MAYCHANGE]
04343c6d
GS
1492
1493The previous paragraph is applicable verbatim to tied hash access using the
1494C<hv_store> and C<hv_store_ent> functions as well.
1495
1496C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1497C<hv_fetch_ent> actually return an undefined mortal value whose magic
1498has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1499need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1500need to call C<mg_get()> on the returned value in order to actually invoke
1501the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1502you may also call C<mg_set()> on the return value after possibly assigning
1503a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1504method on the TIE object. [/MAYCHANGE]
04343c6d 1505
9edb2b46 1506[MAYCHANGE]
04343c6d
GS
1507In other words, the array or hash fetch/store functions don't really
1508fetch and store actual values in the case of tied arrays and hashes. They
1509merely call C<mg_copy> to attach magic to the values that were meant to be
1510"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1511do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1512the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1513and hashes.
1514
1515Currently (as of perl version 5.004), use of the hash and array access
1516functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1517"normal" hashes and arrays, or on their tied variants. The API may be
1518changed to provide more transparent access to both tied and normal data
1519types in future versions.
1520[/MAYCHANGE]
04343c6d
GS
1521
1522You would do well to understand that the TIEARRAY and TIEHASH interfaces
1523are mere sugar to invoke some perl method calls while using the uniform hash
1524and array syntax. The use of this sugar imposes some overhead (typically
1525about two to four extra opcodes per FETCH/STORE operation, in addition to
1526the creation of all the mortal variables required to invoke the methods).
1527This overhead will be comparatively small if the TIE methods are themselves
1528substantial, but if they are only a few statements long, the overhead
1529will not be insignificant.
1530
d1c897a1
IZ
1531=head2 Localizing changes
1532
1533Perl has a very handy construction
1534
1535 {
1536 local $var = 2;
1537 ...
1538 }
1539
1540This construction is I<approximately> equivalent to
1541
1542 {
1543 my $oldvar = $var;
1544 $var = 2;
1545 ...
1546 $var = $oldvar;
1547 }
1548
1549The biggest difference is that the first construction would
1550reinstate the initial value of $var, irrespective of how control exits
10e2eb10 1551the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1552more efficient as well.
1553
1554There is a way to achieve a similar task from C via Perl API: create a
1555I<pseudo-block>, and arrange for some changes to be automatically
1556undone at the end of it, either explicit, or via a non-local exit (via
10e2eb10 1557die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1558C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1559Such a construct may be created specially for some important localized
1560task, or an existing one (like boundaries of enclosing Perl
1561subroutine/block, or an existing pair for freeing TMPs) may be
10e2eb10
FC
1562used. (In the second case the overhead of additional localization must
1563be almost negligible.) Note that any XSUB is automatically enclosed in
b687b08b 1564an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1565
1566Inside such a I<pseudo-block> the following service is available:
1567
13a2d996 1568=over 4
d1c897a1
IZ
1569
1570=item C<SAVEINT(int i)>
1571
1572=item C<SAVEIV(IV i)>
1573
1574=item C<SAVEI32(I32 i)>
1575
1576=item C<SAVELONG(long i)>
1577
1578These macros arrange things to restore the value of integer variable
1579C<i> at the end of enclosing I<pseudo-block>.
1580
1581=item C<SAVESPTR(s)>
1582
1583=item C<SAVEPPTR(p)>
1584
1585These macros arrange things to restore the value of pointers C<s> and
10e2eb10 1586C<p>. C<s> must be a pointer of a type which survives conversion to
d1c897a1
IZ
1587C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1588and back.
1589
1590=item C<SAVEFREESV(SV *sv)>
1591
06f1e0b6 1592The refcount of C<sv> will be decremented at the end of
26d9b02f
JH
1593I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1594mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1595extends the lifetime of C<sv> until the beginning of the next statement,
1596C<SAVEFREESV> extends it until the end of the enclosing scope. These
1597lifetimes can be wildly different.
1598
1599Also compare C<SAVEMORTALIZESV>.
1600
1601=item C<SAVEMORTALIZESV(SV *sv)>
1602
1603Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1604scope instead of decrementing its reference count. This usually has the
1605effect of keeping C<sv> alive until the statement that called the currently
1606live scope has finished executing.
d1c897a1
IZ
1607
1608=item C<SAVEFREEOP(OP *op)>
1609
1610The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1611
1612=item C<SAVEFREEPV(p)>
1613
1614The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1615end of I<pseudo-block>.
1616
1617=item C<SAVECLEARSV(SV *sv)>
1618
1619Clears a slot in the current scratchpad which corresponds to C<sv> at
1620the end of I<pseudo-block>.
1621
1622=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1623
10e2eb10 1624The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
d1c897a1
IZ
1625string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1626short-lived storage, the corresponding string may be reallocated like
1627this:
1628
9cde0e7f 1629 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1630
c76ac1ee 1631=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1632
1633At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1634only argument C<p>.
1635
1636=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1637
1638At the end of I<pseudo-block> the function C<f> is called with the
1639implicit context argument (if any), and C<p>.
d1c897a1
IZ
1640
1641=item C<SAVESTACK_POS()>
1642
1643The current offset on the Perl internal stack (cf. C<SP>) is restored
1644at the end of I<pseudo-block>.
1645
1646=back
1647
1648The following API list contains functions, thus one needs to
1649provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1650or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1651function takes C<int *>.
1652
13a2d996 1653=over 4
d1c897a1
IZ
1654
1655=item C<SV* save_scalar(GV *gv)>
1656
4f313521
KW
1657=for apidoc save_scalar
1658
d1c897a1
IZ
1659Equivalent to Perl code C<local $gv>.
1660
1661=item C<AV* save_ary(GV *gv)>
1662
4f313521
KW
1663=for apidoc save_ary
1664
d1c897a1
IZ
1665=item C<HV* save_hash(GV *gv)>
1666
4f313521
KW
1667=for apidoc save_hash
1668
d1c897a1
IZ
1669Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1670
1671=item C<void save_item(SV *item)>
1672
53dedf6f
KW
1673=for apidoc save_item
1674
1675Duplicates the current value of C<SV>. On the exit from the current
1676C<ENTER>/C<LEAVE> I<pseudo-block> the value of C<SV> will be restored
10e2eb10 1677using the stored value. It doesn't handle magic. Use C<save_scalar> if
038fcae3 1678magic is affected.
d1c897a1
IZ
1679
1680=item C<void save_list(SV **sarg, I32 maxsarg)>
1681
4f313521
KW
1682=for apidoc save_list
1683
d1c897a1
IZ
1684A variant of C<save_item> which takes multiple arguments via an array
1685C<sarg> of C<SV*> of length C<maxsarg>.
1686
1687=item C<SV* save_svref(SV **sptr)>
1688
4f313521
KW
1689=for apidoc save_svref
1690
d1be9408 1691Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1692
1693=item C<void save_aptr(AV **aptr)>
1694
1695=item C<void save_hptr(HV **hptr)>
1696
4f313521
KW
1697=for apidoc save_aptr
1698=for apidoc save_hptr
1699
d1c897a1
IZ
1700Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1701
1702=back
1703
1704The C<Alias> module implements localization of the basic types within the
1705I<caller's scope>. People who are interested in how to localize things in
1706the containing scope should take a look there too.
1707
0a753a76 1708=head1 Subroutines
a0d0e21e 1709
68dc0745 1710=head2 XSUBs and the Argument Stack
5f05dabc
PP
1711
1712The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1713An XSUB routine will have a stack that contains the arguments from the Perl
1714program, and a way to map from the Perl data structures to a C equivalent.
1715
1716The stack arguments are accessible through the C<ST(n)> macro, which returns
1717the C<n>'th stack argument. Argument 0 is the first argument passed in the
1718Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1719an C<SV*> is used.
1720
1721Most of the time, output from the C routine can be handled through use of
1722the RETVAL and OUTPUT directives. However, there are some cases where the
1723argument stack is not already long enough to handle all the return values.
1724An example is the POSIX tzname() call, which takes no arguments, but returns
1725two, the local time zone's standard and summer time abbreviations.
1726
1727To handle this situation, the PPCODE directive is used and the stack is
1728extended using the macro:
1729
924508f0 1730 EXTEND(SP, num);
5f05dabc 1731
924508f0
GS
1732where C<SP> is the macro that represents the local copy of the stack pointer,
1733and C<num> is the number of elements the stack should be extended by.
5f05dabc 1734
00aadd71 1735Now that there is room on the stack, values can be pushed on it using C<PUSHs>
10e2eb10 1736macro. The pushed values will often need to be "mortal" (See
d82b684c 1737L</Reference Counts and Mortality>):
5f05dabc 1738
00aadd71 1739 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1740 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1741 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1742 PUSHs(sv_2mortal(newSVpv("Some String",0)))
a9b0660e
KW
1743 /* Although the last example is better written as the more
1744 * efficient: */
a3179684 1745 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
5f05dabc
PP
1746
1747And now the Perl program calling C<tzname>, the two values will be assigned
1748as in:
1749
1750 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1751
1752An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1753to use the macro:
5f05dabc 1754
5f05dabc
PP
1755 XPUSHs(SV*)
1756
da8c5729 1757This macro automatically adjusts the stack for you, if needed. Thus, you
5f05dabc 1758do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1759
1760Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1761C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1762For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1763C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1764
1765For more information, consult L<perlxs> and L<perlxstut>.
1766
5b36e945
FC
1767=head2 Autoloading with XSUBs
1768
1769If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1770fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1771of the XSUB's package.
1772
1773But it also puts the same information in certain fields of the XSUB itself:
1774
1775 HV *stash = CvSTASH(cv);
1776 const char *subname = SvPVX(cv);
1777 STRLEN name_length = SvCUR(cv); /* in bytes */
1778 U32 is_utf8 = SvUTF8(cv);
f703fc96 1779
5b36e945 1780C<SvPVX(cv)> contains just the sub name itself, not including the package.
d8893903
FC
1781For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1782C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
5b36e945
FC
1783
1784B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1785XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1786XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1787to support 5.8-5.14, use the XSUB's fields.
1788
5f05dabc 1789=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1790
1791There are four routines that can be used to call a Perl subroutine from
1792within a C program. These four are:
1793
954c1994
GS
1794 I32 call_sv(SV*, I32);
1795 I32 call_pv(const char*, I32);
1796 I32 call_method(const char*, I32);
5aaab254 1797 I32 call_argv(const char*, I32, char**);
a0d0e21e 1798
954c1994 1799The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1800contains either the name of the Perl subroutine to be called, or a
1801reference to the subroutine. The second argument consists of flags
1802that control the context in which the subroutine is called, whether
1803or not the subroutine is being passed arguments, how errors should be
1804trapped, and how to treat return values.
a0d0e21e
LW
1805
1806All four routines return the number of arguments that the subroutine returned
1807on the Perl stack.
1808
9a68f1db 1809These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1810but those names are now deprecated; macros of the same name are provided for
1811compatibility.
1812
1813When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1814must manipulate the Perl stack. These include the following macros and
1815functions:
a0d0e21e
LW
1816
1817 dSP
924508f0 1818 SP
a0d0e21e
LW
1819 PUSHMARK()
1820 PUTBACK
1821 SPAGAIN
1822 ENTER
1823 SAVETMPS
1824 FREETMPS
1825 LEAVE
1826 XPUSH*()
cb1a09d0 1827 POP*()
a0d0e21e 1828
5f05dabc
PP
1829For a detailed description of calling conventions from C to Perl,
1830consult L<perlcall>.
a0d0e21e 1831
8ebc5c01 1832=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1833
1834A lot of opcodes (this is an elementary operation in the internal perl
10e2eb10
FC
1835stack machine) put an SV* on the stack. However, as an optimization
1836the corresponding SV is (usually) not recreated each time. The opcodes
ce3d39e2
IZ
1837reuse specially assigned SVs (I<target>s) which are (as a corollary)
1838not constantly freed/created.
1839
0a753a76 1840Each of the targets is created only once (but see
5a0de581 1841L</Scratchpads and recursion> below), and when an opcode needs to put
ce3d39e2
IZ
1842an integer, a double, or a string on stack, it just sets the
1843corresponding parts of its I<target> and puts the I<target> on stack.
1844
1845The macro to put this target on stack is C<PUSHTARG>, and it is
1846directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1847others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1848
1bd1c0d5 1849Because the target is reused, you must be careful when pushing multiple
10e2eb10 1850values on the stack. The following code will not do what you think:
1bd1c0d5
SC
1851
1852 XPUSHi(10);
1853 XPUSHi(20);
1854
1855This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1856the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1857At the end of the operation, the stack does not contain the values 10
1858and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1859to 20.
1bd1c0d5 1860
d82b684c
SH
1861If you need to push multiple different values then you should either use
1862the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1863none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1864SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1865will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1866this a little easier to achieve by creating a new mortal for you (via
1867C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1868in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1869Thus, instead of writing this to "fix" the example above:
1870
1871 XPUSHs(sv_2mortal(newSViv(10)))
1872 XPUSHs(sv_2mortal(newSViv(20)))
1873
1874you can simply write:
1875
1876 mXPUSHi(10)
1877 mXPUSHi(20)
1878
1879On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1880need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1881macros can make use of the local variable C<TARG>. See also C<dTARGET>
1882and C<dXSTARG>.
1bd1c0d5 1883
8ebc5c01 1884=head2 Scratchpads
ce3d39e2 1885
54310121 1886The question remains on when the SVs which are I<target>s for opcodes
10e2eb10 1887are created. The answer is that they are created when the current
ac036724 1888unit--a subroutine or a file (for opcodes for statements outside of
10e2eb10 1889subroutines)--is compiled. During this time a special anonymous Perl
ac036724 1890array is created, which is called a scratchpad for the current unit.
ce3d39e2 1891
54310121 1892A scratchpad keeps SVs which are lexicals for the current unit and are
d777b41a
FC
1893targets for opcodes. A previous version of this document
1894stated that one can deduce that an SV lives on a scratchpad
ce3d39e2 1895by looking on its flags: lexicals have C<SVs_PADMY> set, and
eee3e302 1896I<target>s have C<SVs_PADTMP> set. But this has never been fully true.
d777b41a
FC
1897C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1898While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
eed77337
FC
1899that have never resided in a pad, but nonetheless act like I<target>s. As
1900of perl 5.21.5, the C<SVs_PADMY> flag is no longer used and is defined as
19010. C<SvPADMY()> now returns true for anything without C<SVs_PADTMP>.
ce3d39e2 1902
10e2eb10 1903The correspondence between OPs and I<target>s is not 1-to-1. Different
54310121 1904OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1905would not conflict with the expected life of the temporary.
1906
2ae324a7 1907=head2 Scratchpads and recursion
ce3d39e2
IZ
1908
1909In fact it is not 100% true that a compiled unit contains a pointer to
10e2eb10
FC
1910the scratchpad AV. In fact it contains a pointer to an AV of
1911(initially) one element, and this element is the scratchpad AV. Why do
ce3d39e2
IZ
1912we need an extra level of indirection?
1913
10e2eb10 1914The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2 1915these can create several execution pointers going into the same
10e2eb10 1916subroutine. For the subroutine-child not write over the temporaries
ce3d39e2
IZ
1917for the subroutine-parent (lifespan of which covers the call to the
1918child), the parent and the child should have different
10e2eb10 1919scratchpads. (I<And> the lexicals should be separate anyway!)
ce3d39e2 1920
5f05dabc
PP
1921So each subroutine is born with an array of scratchpads (of length 1).
1922On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1923depth of the recursion is not more than the length of this array, and
1924if it is, new scratchpad is created and pushed into the array.
1925
1926The I<target>s on this scratchpad are C<undef>s, but they are already
1927marked with correct flags.
1928
22d36020
FC
1929=head1 Memory Allocation
1930
1931=head2 Allocation
1932
1933All memory meant to be used with the Perl API functions should be manipulated
1934using the macros described in this section. The macros provide the necessary
1935transparency between differences in the actual malloc implementation that is
1936used within perl.
1937
1938It is suggested that you enable the version of malloc that is distributed
1939with Perl. It keeps pools of various sizes of unallocated memory in
1940order to satisfy allocation requests more quickly. However, on some
1941platforms, it may cause spurious malloc or free errors.
1942
1943The following three macros are used to initially allocate memory :
1944
1945 Newx(pointer, number, type);
1946 Newxc(pointer, number, type, cast);
1947 Newxz(pointer, number, type);
1948
1949The first argument C<pointer> should be the name of a variable that will
1950point to the newly allocated memory.
1951
1952The second and third arguments C<number> and C<type> specify how many of
1953the specified type of data structure should be allocated. The argument
1954C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1955should be used if the C<pointer> argument is different from the C<type>
1956argument.
1957
1958Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1959to zero out all the newly allocated memory.
1960
1961=head2 Reallocation
1962
1963 Renew(pointer, number, type);
1964 Renewc(pointer, number, type, cast);
1965 Safefree(pointer)
1966
1967These three macros are used to change a memory buffer size or to free a
1968piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1969match those of C<New> and C<Newc> with the exception of not needing the
1970"magic cookie" argument.
1971
1972=head2 Moving
1973
1974 Move(source, dest, number, type);
1975 Copy(source, dest, number, type);
1976 Zero(dest, number, type);
1977
1978These three macros are used to move, copy, or zero out previously allocated
1979memory. The C<source> and C<dest> arguments point to the source and
1980destination starting points. Perl will move, copy, or zero out C<number>
1981instances of the size of the C<type> data structure (using the C<sizeof>
1982function).
1983
1984=head1 PerlIO
1985
1986The most recent development releases of Perl have been experimenting with
1987removing Perl's dependency on the "normal" standard I/O suite and allowing
1988other stdio implementations to be used. This involves creating a new
1989abstraction layer that then calls whichever implementation of stdio Perl
1990was compiled with. All XSUBs should now use the functions in the PerlIO
1991abstraction layer and not make any assumptions about what kind of stdio
1992is being used.
1993
1994For a complete description of the PerlIO abstraction, consult L<perlapio>.
1995
0a753a76
PP
1996=head1 Compiled code
1997
1998=head2 Code tree
1999
2000Here we describe the internal form your code is converted to by
10e2eb10 2001Perl. Start with a simple example:
0a753a76
PP
2002
2003 $a = $b + $c;
2004
2005This is converted to a tree similar to this one:
2006
2007 assign-to
2008 / \
2009 + $a
2010 / \
2011 $b $c
2012
7b8d334a 2013(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
2014parsed your code, but has nothing to do with the execution order.
2015There is an additional "thread" going through the nodes of the tree
2016which shows the order of execution of the nodes. In our simplified
2017example above it looks like:
2018
2019 $b ---> $c ---> + ---> $a ---> assign-to
2020
2021But with the actual compile tree for C<$a = $b + $c> it is different:
2022some nodes I<optimized away>. As a corollary, though the actual tree
2023contains more nodes than our simplified example, the execution order
2024is the same as in our example.
2025
2026=head2 Examining the tree
2027
06f6df17
RGS
2028If you have your perl compiled for debugging (usually done with
2029C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
2030compiled tree by specifying C<-Dx> on the Perl command line. The
2031output takes several lines per node, and for C<$b+$c> it looks like
2032this:
2033
2034 5 TYPE = add ===> 6
2035 TARG = 1
2036 FLAGS = (SCALAR,KIDS)
2037 {
2038 TYPE = null ===> (4)
2039 (was rv2sv)
2040 FLAGS = (SCALAR,KIDS)
2041 {
2042 3 TYPE = gvsv ===> 4
2043 FLAGS = (SCALAR)
2044 GV = main::b
2045 }
2046 }
2047 {
2048 TYPE = null ===> (5)
2049 (was rv2sv)
2050 FLAGS = (SCALAR,KIDS)
2051 {
2052 4 TYPE = gvsv ===> 5
2053 FLAGS = (SCALAR)
2054 GV = main::c
2055 }
2056 }
2057
2058This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
2059not optimized away (one per number in the left column). The immediate
2060children of the given node correspond to C<{}> pairs on the same level
2061of indentation, thus this listing corresponds to the tree:
2062
2063 add
2064 / \
2065 null null
2066 | |
2067 gvsv gvsv
2068
2069The execution order is indicated by C<===E<gt>> marks, thus it is C<3
20704 5 6> (node C<6> is not included into above listing), i.e.,
2071C<gvsv gvsv add whatever>.
2072
9afa14e3 2073Each of these nodes represents an op, a fundamental operation inside the
10e2eb10 2074Perl core. The code which implements each operation can be found in the
9afa14e3 2075F<pp*.c> files; the function which implements the op with type C<gvsv>
10e2eb10 2076is C<pp_gvsv>, and so on. As the tree above shows, different ops have
9afa14e3 2077different numbers of children: C<add> is a binary operator, as one would
10e2eb10 2078expect, and so has two children. To accommodate the various different
9afa14e3
SC
2079numbers of children, there are various types of op data structure, and
2080they link together in different ways.
2081
10e2eb10 2082The simplest type of op structure is C<OP>: this has no children. Unary
9afa14e3 2083operators, C<UNOP>s, have one child, and this is pointed to by the
10e2eb10
FC
2084C<op_first> field. Binary operators (C<BINOP>s) have not only an
2085C<op_first> field but also an C<op_last> field. The most complex type of
2086op is a C<LISTOP>, which has any number of children. In this case, the
9afa14e3 2087first child is pointed to by C<op_first> and the last child by
10e2eb10 2088C<op_last>. The children in between can be found by iteratively
86cd3a13 2089following the C<OpSIBLING> pointer from the first child to the last (but
29e61fd9 2090see below).
9afa14e3 2091
29e61fd9 2092There are also some other op types: a C<PMOP> holds a regular expression,
10e2eb10
FC
2093and has no children, and a C<LOOP> may or may not have children. If the
2094C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
9afa14e3
SC
2095complicate matters, if a C<UNOP> is actually a C<null> op after
2096optimization (see L</Compile pass 2: context propagation>) it will still
2097have children in accordance with its former type.
2098
29e61fd9
DM
2099Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
2100or more children, but it doesn't have an C<op_last> field: so you have to
86cd3a13 2101follow C<op_first> and then the C<OpSIBLING> chain itself to find the
29e61fd9
DM
2102last child. Instead it has an C<op_other> field, which is comparable to
2103the C<op_next> field described below, and represents an alternate
2104execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
2105that in general, C<op_other> may not point to any of the direct children
2106of the C<LOGOP>.
2107
2108Starting in version 5.21.2, perls built with the experimental
2109define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
87b5a8b9 2110C<op_moresib>. When not set, this indicates that this is the last op in an
86cd3a13
DM
2111C<OpSIBLING> chain. This frees up the C<op_sibling> field on the last
2112sibling to point back to the parent op. Under this build, that field is
2113also renamed C<op_sibparent> to reflect its joint role. The macro
2114C<OpSIBLING(o)> wraps this special behaviour, and always returns NULL on
2115the last sibling. With this build the C<op_parent(o)> function can be
2116used to find the parent of any op. Thus for forward compatibility, you
2117should always use the C<OpSIBLING(o)> macro rather than accessing
2118C<op_sibling> directly.
29e61fd9 2119
06f6df17
RGS
2120Another way to examine the tree is to use a compiler back-end module, such
2121as L<B::Concise>.
2122
0a753a76
PP
2123=head2 Compile pass 1: check routines
2124
8870b5c7 2125The tree is created by the compiler while I<yacc> code feeds it
10e2eb10 2126the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
2127the first pass of perl compilation.
2128
2129What makes this pass interesting for perl developers is that some
2130optimization may be performed on this pass. This is optimization by
8870b5c7 2131so-called "check routines". The correspondence between node names
0a753a76
PP
2132and corresponding check routines is described in F<opcode.pl> (do not
2133forget to run C<make regen_headers> if you modify this file).
2134
2135A check routine is called when the node is fully constructed except
7b8d334a 2136for the execution-order thread. Since at this time there are no
0a753a76
PP
2137back-links to the currently constructed node, one can do most any
2138operation to the top-level node, including freeing it and/or creating
2139new nodes above/below it.
2140
2141The check routine returns the node which should be inserted into the
2142tree (if the top-level node was not modified, check routine returns
2143its argument).
2144
10e2eb10 2145By convention, check routines have names C<ck_*>. They are usually
0a753a76
PP
2146called from C<new*OP> subroutines (or C<convert>) (which in turn are
2147called from F<perly.y>).
2148
2149=head2 Compile pass 1a: constant folding
2150
2151Immediately after the check routine is called the returned node is
2152checked for being compile-time executable. If it is (the value is
2153judged to be constant) it is immediately executed, and a I<constant>
2154node with the "return value" of the corresponding subtree is
2155substituted instead. The subtree is deleted.
2156
2157If constant folding was not performed, the execution-order thread is
2158created.
2159
2160=head2 Compile pass 2: context propagation
2161
2162When a context for a part of compile tree is known, it is propagated
a3cb178b 2163down through the tree. At this time the context can have 5 values
0a753a76
PP
2164(instead of 2 for runtime context): void, boolean, scalar, list, and
2165lvalue. In contrast with the pass 1 this pass is processed from top
2166to bottom: a node's context determines the context for its children.
2167
2168Additional context-dependent optimizations are performed at this time.
2169Since at this moment the compile tree contains back-references (via
2170"thread" pointers), nodes cannot be free()d now. To allow
2171optimized-away nodes at this stage, such nodes are null()ified instead
2172of free()ing (i.e. their type is changed to OP_NULL).
2173
2174=head2 Compile pass 3: peephole optimization
2175
2176After the compile tree for a subroutine (or for an C<eval> or a file)
10e2eb10 2177is created, an additional pass over the code is performed. This pass
0a753a76 2178is neither top-down or bottom-up, but in the execution order (with
9ea12537
Z
2179additional complications for conditionals). Optimizations performed
2180at this stage are subject to the same restrictions as in the pass 2.
2181
2182Peephole optimizations are done by calling the function pointed to
2183by the global variable C<PL_peepp>. By default, C<PL_peepp> just
2184calls the function pointed to by the global variable C<PL_rpeepp>.
2185By default, that performs some basic op fixups and optimisations along
2186the execution-order op chain, and recursively calls C<PL_rpeepp> for
2187each side chain of ops (resulting from conditionals). Extensions may
2188provide additional optimisations or fixups, hooking into either the
2189per-subroutine or recursive stage, like this:
2190
2191 static peep_t prev_peepp;
2192 static void my_peep(pTHX_ OP *o)
2193 {
2194 /* custom per-subroutine optimisation goes here */
f0358462 2195 prev_peepp(aTHX_ o);
9ea12537
Z
2196 /* custom per-subroutine optimisation may also go here */
2197 }
2198 BOOT:
2199 prev_peepp = PL_peepp;
2200 PL_peepp = my_peep;
2201
2202 static peep_t prev_rpeepp;
2203 static void my_rpeep(pTHX_ OP *o)
2204 {
2205 OP *orig_o = o;
2206 for(; o; o = o->op_next) {
2207 /* custom per-op optimisation goes here */
2208 }
f0358462 2209 prev_rpeepp(aTHX_ orig_o);
9ea12537
Z
2210 }
2211 BOOT:
2212 prev_rpeepp = PL_rpeepp;
2213 PL_rpeepp = my_rpeep;
0a753a76 2214
1ba7f851
PJ
2215=head2 Pluggable runops
2216
2217The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
2218functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2219with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2220control over the execution of the compile tree it is possible to provide
2221your own runops function.
1ba7f851
PJ
2222
2223It's probably best to copy one of the existing runops functions and
2224change it to suit your needs. Then, in the BOOT section of your XS
2225file, add the line:
2226
2227 PL_runops = my_runops;
2228
2229This function should be as efficient as possible to keep your programs
2230running as fast as possible.
2231
fd85fad2
BM
2232=head2 Compile-time scope hooks
2233
2234As of perl 5.14 it is possible to hook into the compile-time lexical
10e2eb10 2235scope mechanism using C<Perl_blockhook_register>. This is used like
fd85fad2
BM
2236this:
2237
2238 STATIC void my_start_hook(pTHX_ int full);
2239 STATIC BHK my_hooks;
2240
2241 BOOT:
a88d97bf 2242 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
fd85fad2
BM
2243 Perl_blockhook_register(aTHX_ &my_hooks);
2244
2245This will arrange to have C<my_start_hook> called at the start of
10e2eb10 2246compiling every lexical scope. The available hooks are:
fd85fad2
BM
2247
2248=over 4
2249
a88d97bf 2250=item C<void bhk_start(pTHX_ int full)>
fd85fad2 2251
10e2eb10 2252This is called just after starting a new lexical scope. Note that Perl
fd85fad2
BM
2253code like
2254
2255 if ($x) { ... }
2256
2257creates two scopes: the first starts at the C<(> and has C<full == 1>,
10e2eb10 2258the second starts at the C<{> and has C<full == 0>. Both end at the
f185f654 2259C<}>, so calls to C<start> and C<pre>/C<post_end> will match. Anything
fd85fad2
BM
2260pushed onto the save stack by this hook will be popped just before the
2261scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2262
a88d97bf 2263=item C<void bhk_pre_end(pTHX_ OP **o)>
fd85fad2
BM
2264
2265This is called at the end of a lexical scope, just before unwinding the
10e2eb10 2266stack. I<o> is the root of the optree representing the scope; it is a
fd85fad2
BM
2267double pointer so you can replace the OP if you need to.
2268
a88d97bf 2269=item C<void bhk_post_end(pTHX_ OP **o)>
fd85fad2
BM
2270
2271This is called at the end of a lexical scope, just after unwinding the
10e2eb10 2272stack. I<o> is as above. Note that it is possible for calls to C<pre_>
fd85fad2
BM
2273and C<post_end> to nest, if there is something on the save stack that
2274calls string eval.
2275
a88d97bf 2276=item C<void bhk_eval(pTHX_ OP *const o)>
fd85fad2
BM
2277
2278This is called just before starting to compile an C<eval STRING>, C<do
10e2eb10 2279FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
fd85fad2
BM
2280OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2281C<OP_DOFILE> or C<OP_REQUIRE>.
2282
2283=back
2284
2285Once you have your hook functions, you need a C<BHK> structure to put
10e2eb10
FC
2286them in. It's best to allocate it statically, since there is no way to
2287free it once it's registered. The function pointers should be inserted
fd85fad2 2288into this structure using the C<BhkENTRY_set> macro, which will also set
10e2eb10 2289flags indicating which entries are valid. If you do need to allocate
fd85fad2
BM
2290your C<BHK> dynamically for some reason, be sure to zero it before you
2291start.
2292
2293Once registered, there is no mechanism to switch these hooks off, so if
10e2eb10 2294that is necessary you will need to do this yourself. An entry in C<%^H>
a3e07c87
BM
2295is probably the best way, so the effect is lexically scoped; however it
2296is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
10e2eb10 2297temporarily switch entries on and off. You should also be aware that
a3e07c87 2298generally speaking at least one scope will have opened before your
f185f654 2299extension is loaded, so you will see some C<pre>/C<post_end> pairs that
a3e07c87 2300didn't have a matching C<start>.
fd85fad2 2301
9afa14e3
SC
2302=head1 Examining internal data structures with the C<dump> functions
2303
2304To aid debugging, the source file F<dump.c> contains a number of
2305functions which produce formatted output of internal data structures.
2306
2307The most commonly used of these functions is C<Perl_sv_dump>; it's used
10e2eb10 2308for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
9afa14e3 2309C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 2310module should already be familiar with its format.
9afa14e3
SC
2311
2312C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 2313derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
2314C<Perl_dump_eval> will dump the main root of the code being evaluated,
2315exactly like C<-Dx>.
2316
2317Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2318op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2319subroutines in a package like so: (Thankfully, these are all xsubs, so
2320there is no op tree)
2321
2322 (gdb) print Perl_dump_packsubs(PL_defstash)
2323
2324 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2325
2326 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2327
2328 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2329
2330 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2331
2332 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2333
2334and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2335the op tree of the main root.
2336
954c1994 2337=head1 How multiple interpreters and concurrency are supported
ee072b34 2338
ee072b34
GS
2339=head2 Background and PERL_IMPLICIT_CONTEXT
2340
2341The Perl interpreter can be regarded as a closed box: it has an API
2342for feeding it code or otherwise making it do things, but it also has
2343functions for its own use. This smells a lot like an object, and
2344there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
2345interpreters, with one interpreter represented either as a C structure,
2346or inside a thread-specific structure. These structures contain all
2347the context, the state of that interpreter.
2348
10e2eb10 2349One macro controls the major Perl build flavor: MULTIPLICITY. The
7b52221d 2350MULTIPLICITY build has a C structure that packages all the interpreter
10e2eb10 2351state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
7b52221d 2352normally defined, and enables the support for passing in a "hidden" first
10e2eb10 2353argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 2354multi-threaded perls possible (with the ithreads threading model, related
7b52221d 2355to the macro USE_ITHREADS.)
54aff467 2356
27da23d5
JH
2357Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2358PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2359former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2360internal variables of Perl to be wrapped inside a single global struct,
2361struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2362the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2363one step further, there is still a single struct (allocated in main()
2364either from heap or from stack) but there are no global data symbols
3bf17896 2365pointing to it. In either case the global struct should be initialized
27da23d5
JH
2366as the very first thing in main() using Perl_init_global_struct() and
2367correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2368please see F<miniperlmain.c> for usage details. You may also need
2369to use C<dVAR> in your coding to "declare the global variables"
2370when you are using them. dTHX does this for you automatically.
2371
4f313521
KW
2372=for apidoc Amnh||dVAR
2373
9aa97215
JH
2374To see whether you have non-const data you can use a BSD (or GNU)
2375compatible C<nm>:
bc028b6b
JH
2376
2377 nm libperl.a | grep -v ' [TURtr] '
2378
9aa97215
JH
2379If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
2380you have non-const data. The symbols the C<grep> removed are as follows:
2381C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
2382and the C<U> is <undefined>, external symbols referred to.
2383
2384The test F<t/porting/libperl.t> does this kind of symbol sanity
2385checking on C<libperl.a>.
bc028b6b 2386
27da23d5
JH
2387For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2388doesn't actually hide all symbols inside a big global struct: some
2389PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2390then hides everything (see how the PERLIO_FUNCS_DECL is used).
2391
54aff467 2392All this obviously requires a way for the Perl internal functions to be
acfe0abc 2393either subroutines taking some kind of structure as the first
ee072b34 2394argument, or subroutines taking nothing as the first argument. To
acfe0abc 2395enable these two very different ways of building the interpreter,
ee072b34
GS
2396the Perl source (as it does in so many other situations) makes heavy
2397use of macros and subroutine naming conventions.
2398
54aff467 2399First problem: deciding which functions will be public API functions and
00aadd71 2400which will be private. All functions whose names begin C<S_> are private
954c1994
GS
2401(think "S" for "secret" or "static"). All other functions begin with
2402"Perl_", but just because a function begins with "Perl_" does not mean it is
10e2eb10
FC
2403part of the API. (See L</Internal
2404Functions>.) The easiest way to be B<sure> a
00aadd71
NIS
2405function is part of the API is to find its entry in L<perlapi>.
2406If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2407think it should be (i.e., you need it for your extension), send mail via
a422fd2d 2408L<perlbug> explaining why you think it should be.
ee072b34
GS
2409
2410Second problem: there must be a syntax so that the same subroutine
2411declarations and calls can pass a structure as their first argument,
2412or pass nothing. To solve this, the subroutines are named and
2413declared in a particular way. Here's a typical start of a static
2414function used within the Perl guts:
2415
2416 STATIC void
2417 S_incline(pTHX_ char *s)
2418
acfe0abc 2419STATIC becomes "static" in C, and may be #define'd to nothing in some
da8c5729 2420configurations in the future.
ee072b34 2421
651a3225
GS
2422A public function (i.e. part of the internal API, but not necessarily
2423sanctioned for use in extensions) begins like this:
ee072b34
GS
2424
2425 void
2307c6d0 2426 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 2427
0147cd53 2428C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
2429details of the interpreter's context. THX stands for "thread", "this",
2430or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2431The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
2432or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2433their variants.
ee072b34 2434
4f313521
KW
2435=for apidoc Amnh||aTHX
2436=for apidoc Amnh||aTHX_
2437=for apidoc Amnh||dTHX
2438=for apidoc Amnh||pTHX
2439=for apidoc Amnh||pTHX_
2440
a7486cbb
JH
2441When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2442first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
2443in the pTHX_ macro indicates that the macro expansion needs a comma
2444after the context argument because other arguments follow it. If
2445PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
2446subroutine is not prototyped to take the extra argument. The form of the
2447macro without the trailing underscore is used when there are no additional
ee072b34
GS
2448explicit arguments.
2449
54aff467 2450When a core function calls another, it must pass the context. This
2307c6d0 2451is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
2452something like this:
2453
2307c6d0
SB
2454 #ifdef PERL_IMPLICIT_CONTEXT
2455 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 2456 /* can't do this for vararg functions, see below */
2307c6d0
SB
2457 #else
2458 #define sv_setiv Perl_sv_setiv
2459 #endif
ee072b34
GS
2460
2461This works well, and means that XS authors can gleefully write:
2462
2307c6d0 2463 sv_setiv(foo, bar);
ee072b34
GS
2464
2465and still have it work under all the modes Perl could have been
2466compiled with.
2467
ee072b34
GS
2468This doesn't work so cleanly for varargs functions, though, as macros
2469imply that the number of arguments is known in advance. Instead we
2470either need to spell them out fully, passing C<aTHX_> as the first
2471argument (the Perl core tends to do this with functions like
2472Perl_warner), or use a context-free version.
2473
2474The context-free version of Perl_warner is called
2475Perl_warner_nocontext, and does not take the extra argument. Instead
2476it does dTHX; to get the context from thread-local storage. We
2477C<#define warner Perl_warner_nocontext> so that extensions get source
2478compatibility at the expense of performance. (Passing an arg is
2479cheaper than grabbing it from thread-local storage.)
2480
acfe0abc 2481You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2482Those are strictly for use within the core. Extensions and embedders
2483need only be aware of [pad]THX.
2484
a7486cbb
JH
2485=head2 So what happened to dTHR?
2486
4f313521
KW
2487=for apidoc Amnh||dTHR
2488
a7486cbb
JH
2489C<dTHR> was introduced in perl 5.005 to support the older thread model.
2490The older thread model now uses the C<THX> mechanism to pass context
2491pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2492later still have it for backward source compatibility, but it is defined
2493to be a no-op.
2494
ee072b34
GS
2495=head2 How do I use all this in extensions?
2496
2497When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2498any functions in the Perl API will need to pass the initial context
2499argument somehow. The kicker is that you will need to write it in
2500such a way that the extension still compiles when Perl hasn't been
2501built with PERL_IMPLICIT_CONTEXT enabled.
2502
2503There are three ways to do this. First, the easy but inefficient way,
2504which is also the default, in order to maintain source compatibility
0147cd53 2505with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2506and aTHX_ macros to call a function that will return the context.
2507Thus, something like:
2508
2307c6d0 2509 sv_setiv(sv, num);
ee072b34 2510
4375e838 2511in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2512in effect:
ee072b34 2513
2307c6d0 2514 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2515
54aff467 2516or to this otherwise:
ee072b34 2517
2307c6d0 2518 Perl_sv_setiv(sv, num);
ee072b34 2519
da8c5729 2520You don't have to do anything new in your extension to get this; since
2fa86c13 2521the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2522work.
2523
2524The second, more efficient way is to use the following template for
2525your Foo.xs:
2526
c52f9dcd
JH
2527 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2528 #include "EXTERN.h"
2529 #include "perl.h"
2530 #include "XSUB.h"
ee072b34 2531
fd061412 2532 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2533
fd061412 2534 STATIC void
c52f9dcd
JH
2535 my_private_function(int arg1, int arg2)
2536 {
2537 dTHX; /* fetch context */
2538 ... call many Perl API functions ...
2539 }
ee072b34
GS
2540
2541 [... etc ...]
2542
c52f9dcd 2543 MODULE = Foo PACKAGE = Foo
ee072b34 2544
c52f9dcd 2545 /* typical XSUB */
ee072b34 2546
c52f9dcd
JH
2547 void
2548 my_xsub(arg)
2549 int arg
2550 CODE:
2551 my_private_function(arg, 10);
ee072b34
GS
2552
2553Note that the only two changes from the normal way of writing an
2554extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2555including the Perl headers, followed by a C<dTHX;> declaration at
2556the start of every function that will call the Perl API. (You'll
2557know which functions need this, because the C compiler will complain
2558that there's an undeclared identifier in those functions.) No changes
2559are needed for the XSUBs themselves, because the XS() macro is
2560correctly defined to pass in the implicit context if needed.
2561
2562The third, even more efficient way is to ape how it is done within
2563the Perl guts:
2564
2565
c52f9dcd
JH
2566 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2567 #include "EXTERN.h"
2568 #include "perl.h"
2569 #include "XSUB.h"
ee072b34
GS
2570
2571 /* pTHX_ only needed for functions that call Perl API */
fd061412 2572 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2573
fd061412 2574 STATIC void
c52f9dcd
JH
2575 my_private_function(pTHX_ int arg1, int arg2)
2576 {
2577 /* dTHX; not needed here, because THX is an argument */
2578 ... call Perl API functions ...
2579 }
ee072b34
GS
2580
2581 [... etc ...]
2582
c52f9dcd 2583 MODULE = Foo PACKAGE = Foo
ee072b34 2584
c52f9dcd 2585 /* typical XSUB */
ee072b34 2586
c52f9dcd
JH
2587 void
2588 my_xsub(arg)
2589 int arg
2590 CODE:
2591 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2592
2593This implementation never has to fetch the context using a function
2594call, since it is always passed as an extra argument. Depending on
2595your needs for simplicity or efficiency, you may mix the previous
2596two approaches freely.
2597
651a3225
GS
2598Never add a comma after C<pTHX> yourself--always use the form of the
2599macro with the underscore for functions that take explicit arguments,
2600or the form without the argument for functions with no explicit arguments.
ee072b34 2601
27da23d5
JH
2602If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2603definition is needed if the Perl global variables (see F<perlvars.h>
2604or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2605used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2606the need for C<dVAR> only with the said compile-time define, because
2607otherwise the Perl global variables are visible as-is.
2608
a7486cbb
JH
2609=head2 Should I do anything special if I call perl from multiple threads?
2610
2611If you create interpreters in one thread and then proceed to call them in
2612another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2613initialized correctly in each of those threads.
2614
2615The C<perl_alloc> and C<perl_clone> API functions will automatically set
2616the TLS slot to the interpreter they created, so that there is no need to do
2617anything special if the interpreter is always accessed in the same thread that
2618created it, and that thread did not create or call any other interpreters
2619afterwards. If that is not the case, you have to set the TLS slot of the
2620thread before calling any functions in the Perl API on that particular
2621interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2622thread as the first thing you do:
2623
2624 /* do this before doing anything else with some_perl */
2625 PERL_SET_CONTEXT(some_perl);
2626
2627 ... other Perl API calls on some_perl go here ...
2628
ee072b34
GS
2629=head2 Future Plans and PERL_IMPLICIT_SYS
2630
2631Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2632that the interpreter knows about itself and pass it around, so too are
2633there plans to allow the interpreter to bundle up everything it knows
2634about the environment it's running on. This is enabled with the
7b52221d
RGS
2635PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2636Windows.
ee072b34
GS
2637
2638This allows the ability to provide an extra pointer (called the "host"
2639environment) for all the system calls. This makes it possible for
2640all the system stuff to maintain their own state, broken down into
2641seven C structures. These are thin wrappers around the usual system
0147cd53 2642calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2643more ambitious host (like the one that would do fork() emulation) all
2644the extra work needed to pretend that different interpreters are
2645actually different "processes", would be done here.
2646
2647The Perl engine/interpreter and the host are orthogonal entities.
2648There could be one or more interpreters in a process, and one or
2649more "hosts", with free association between them.
2650
a422fd2d
SC
2651=head1 Internal Functions
2652
2653All of Perl's internal functions which will be exposed to the outside
06f6df17 2654world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d 2655functions or functions used in a program in which Perl is embedded.
10e2eb10 2656Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2657static functions start with C<S_>.)
a422fd2d 2658
0972ecdf
DM
2659Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2660either with or without the C<Perl_> prefix, thanks to a bunch of defines
10e2eb10 2661that live in F<embed.h>. Note that extension code should I<not> set
0972ecdf
DM
2662C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2663breakage of the XS in each new perl release.
2664
2665The file F<embed.h> is generated automatically from
10e2eb10 2666F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
dc9b1d22 2667header files for the internal functions, generates the documentation
10e2eb10 2668and a lot of other bits and pieces. It's important that when you add
dc9b1d22 2669a new function to the core or change an existing one, you change the
10e2eb10 2670data in the table in F<embed.fnc> as well. Here's a sample entry from
dc9b1d22 2671that table:
a422fd2d
SC
2672
2673 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2674
790ba721
KW
2675The first column is a set of flags, the second column the return type,
2676the third column the name. Columns after that are the arguments.
2677The flags are documented at the top of F<embed.fnc>.
a422fd2d 2678
dc9b1d22
MHM
2679If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2680C<make regen_headers> to force a rebuild of F<embed.h> and other
2681auto-generated files.
a422fd2d 2682
6b4667fc 2683=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2684
6b4667fc
A
2685If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2686formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2687following macros for portability
9dd9db0b 2688
c52f9dcd
JH
2689 IVdf IV in decimal
2690 UVuf UV in decimal
2691 UVof UV in octal
2692 UVxf UV in hexadecimal
2693 NVef NV %e-like
2694 NVff NV %f-like
2695 NVgf NV %g-like
9dd9db0b 2696
4f313521
KW
2697=for apidoc Amnh||IVdf
2698=for apidoc Amnh||UVuf
2699=for apidoc Amnh||UVof
2700=for apidoc Amnh||UVxf
2701=for apidoc Amnh||NVef
2702=for apidoc Amnh||NVff
2703=for apidoc Amnh||NVgf
2704
6b4667fc
A
2705These will take care of 64-bit integers and long doubles.
2706For example:
2707
c52f9dcd 2708 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2709
2710The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2711
aacf4ea2
JH
2712Note that there are different "long doubles": Perl will use
2713whatever the compiler has.
2714
8908e76d
JH
2715If you are printing addresses of pointers, use UVxf combined
2716with PTR2UV(), do not use %lx or %p.
2717
e613617c 2718=head2 Formatted Printing of C<Size_t> and C<SSize_t>
5862f74e
KW
2719
2720The most general way to do this is to cast them to a UV or IV, and
2721print as in the
2722L<previous section|/Formatted Printing of IVs, UVs, and NVs>.
2723
2724But if you're using C<PerlIO_printf()>, it's less typing and visual
2725clutter to use the C<"%z"> length modifier (for I<siZe>):
2726
2727 PerlIO_printf("STRLEN is %zu\n", len);
2728
2729This modifier is not portable, so its use should be restricted to
2730C<PerlIO_printf()>.
2731
8908e76d
JH
2732=head2 Pointer-To-Integer and Integer-To-Pointer
2733
2734Because pointer size does not necessarily equal integer size,
2735use the follow macros to do it right.
2736
c52f9dcd
JH
2737 PTR2UV(pointer)
2738 PTR2IV(pointer)
2739 PTR2NV(pointer)
2740 INT2PTR(pointertotype, integer)
8908e76d 2741
4f313521
KW
2742=for apidoc Amh|void *|INT2PTR|type|int value
2743=for apidoc Amh|UV|PTR2UV|void *
2744=for apidoc Amh|IV|PTR2IV|void *
2745=for apidoc Amh|NV|PTR2NV|void *
2746
8908e76d
JH
2747For example:
2748
c52f9dcd
JH
2749 IV iv = ...;
2750 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2751
2752and
2753
c52f9dcd
JH
2754 AV *av = ...;
2755 UV uv = PTR2UV(av);
8908e76d 2756
0ca3a874
MHM
2757=head2 Exception Handling
2758
9b5c3821 2759There are a couple of macros to do very basic exception handling in XS
10e2eb10 2760modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
9b5c3821
MHM
2761be able to use these macros:
2762
2763 #define NO_XSLOCKS
2764 #include "XSUB.h"
2765
2766You can use these macros if you call code that may croak, but you need
10e2eb10 2767to do some cleanup before giving control back to Perl. For example:
0ca3a874 2768
d7f8936a 2769 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2770
2771 XCPT_TRY_START {
2772 code_that_may_croak();
2773 } XCPT_TRY_END
2774
2775 XCPT_CATCH
2776 {
2777 /* do cleanup here */
2778 XCPT_RETHROW;
2779 }
2780
2781Note that you always have to rethrow an exception that has been
10e2eb10
FC
2782caught. Using these macros, it is not possible to just catch the
2783exception and ignore it. If you have to ignore the exception, you
0ca3a874
MHM
2784have to use the C<call_*> function.
2785
2786The advantage of using the above macros is that you don't have
2787to setup an extra function for C<call_*>, and that using these
2788macros is faster than using C<call_*>.
2789
a422fd2d
SC
2790=head2 Source Documentation
2791
2792There's an effort going on to document the internal functions and
61ad4b94 2793automatically produce reference manuals from them -- L<perlapi> is one
a422fd2d 2794such manual which details all the functions which are available to XS
10e2eb10 2795writers. L<perlintern> is the autogenerated manual for the functions
a422fd2d
SC
2796which are not part of the API and are supposedly for internal use only.
2797
2798Source documentation is created by putting POD comments into the C
2799source, like this:
2800
2801 /*
2802 =for apidoc sv_setiv
2803
2804 Copies an integer into the given SV. Does not handle 'set' magic. See
a95b3d6a 2805 L<perlapi/sv_setiv_mg>.
a422fd2d
SC
2806
2807 =cut
2808 */
2809
2810Please try and supply some documentation if you add functions to the
2811Perl core.
2812
0d098d33
MHM
2813=head2 Backwards compatibility
2814
10e2eb10
FC
2815The Perl API changes over time. New functions are
2816added or the interfaces of existing functions are
2817changed. The C<Devel::PPPort> module tries to
0d098d33
MHM
2818provide compatibility code for some of these changes, so XS writers don't
2819have to code it themselves when supporting multiple versions of Perl.
2820
2821C<Devel::PPPort> generates a C header file F<ppport.h> that can also
10e2eb10 2822be run as a Perl script. To generate F<ppport.h>, run:
0d098d33
MHM
2823
2824 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2825
2826Besides checking existing XS code, the script can also be used to retrieve
2827compatibility information for various API calls using the C<--api-info>
10e2eb10 2828command line switch. For example:
0d098d33
MHM
2829
2830 % perl ppport.h --api-info=sv_magicext
2831
2832For details, see C<perldoc ppport.h>.
2833
a422fd2d
SC
2834=head1 Unicode Support
2835
10e2eb10 2836Perl 5.6.0 introduced Unicode support. It's important for porters and XS
a422fd2d
SC
2837writers to understand this support and make sure that the code they
2838write does not corrupt Unicode data.
2839
2840=head2 What B<is> Unicode, anyway?
2841
10e2eb10
FC
2842In the olden, less enlightened times, we all used to use ASCII. Most of
2843us did, anyway. The big problem with ASCII is that it's American. Well,
a422fd2d 2844no, that's not actually the problem; the problem is that it's not
10e2eb10 2845particularly useful for people who don't use the Roman alphabet. What
a422fd2d 2846used to happen was that particular languages would stick their own
10e2eb10 2847alphabet in the upper range of the sequence, between 128 and 255. Of
a422fd2d
SC
2848course, we then ended up with plenty of variants that weren't quite
2849ASCII, and the whole point of it being a standard was lost.
2850
2851Worse still, if you've got a language like Chinese or
2852Japanese that has hundreds or thousands of characters, then you really
2853can't fit them into a mere 256, so they had to forget about ASCII
2854altogether, and build their own systems using pairs of numbers to refer
2855to one character.
2856
2857To fix this, some people formed Unicode, Inc. and
2858produced a new character set containing all the characters you can
10e2eb10
FC
2859possibly think of and more. There are several ways of representing these
2860characters, and the one Perl uses is called UTF-8. UTF-8 uses
2861a variable number of bytes to represent a character. You can learn more
2575c402 2862about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2863
3ad86f0e
KW
2864(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
2865UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
2866UTF-EBCDIC is like UTF-8, but the details are different. The macros
2867hide the differences from you, just remember that the particular numbers
2868and bit patterns presented below will differ in UTF-EBCDIC.)
2869
1e54db1a 2870=head2 How can I recognise a UTF-8 string?
a422fd2d 2871
10e2eb10
FC
2872You can't. This is because UTF-8 data is stored in bytes just like
2873non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d 2874capital E with a grave accent, is represented by the two bytes
10e2eb10 2875C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
61ad4b94 2876has that byte sequence as well. So you can't tell just by looking -- this
a422fd2d
SC
2877is what makes Unicode input an interesting problem.
2878
2575c402
JW
2879In general, you either have to know what you're dealing with, or you
2880have to guess. The API function C<is_utf8_string> can help; it'll tell
61ad4b94
KW
2881you if a string contains only valid UTF-8 characters, and the chances
2882of a non-UTF-8 string looking like valid UTF-8 become very small very
2883quickly with increasing string length. On a character-by-character
2884basis, C<isUTF8_CHAR>
2575c402 2885will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2886
1e54db1a 2887=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2888
1e54db1a 2889As mentioned above, UTF-8 uses a variable number of bytes to store a
10e2eb10
FC
2890character. Characters with values 0...127 are stored in one
2891byte, just like good ol' ASCII. Character 128 is stored as
2892C<v194.128>; this continues up to character 191, which is
2893C<v194.191>. Now we've run out of bits (191 is binary
61ad4b94 2894C<10111111>) so we move on; character 192 is C<v195.128>. And
a422fd2d 2895so it goes on, moving to three bytes at character 2048.
6e31cdd1 2896L<perlunicode/Unicode Encodings> has pictures of how this works.
a422fd2d 2897
1e54db1a 2898Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2899how long the first character in it is with the C<UTF8SKIP> macro:
2900
2901 char *utf = "\305\233\340\240\201";
2902 I32 len;
2903
2904 len = UTF8SKIP(utf); /* len is 2 here */
2905 utf += len;
2906 len = UTF8SKIP(utf); /* len is 3 here */
2907
1e54db1a 2908Another way to skip over characters in a UTF-8 string is to use
a422fd2d 2909C<utf8_hop>, which takes a string and a number of characters to skip
10e2eb10 2910over. You're on your own about bounds checking, though, so don't use it
a422fd2d
SC
2911lightly.
2912
1e54db1a 2913All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe 2914so you can test if you need to do something special with this
61ad4b94 2915character like this (the C<UTF8_IS_INVARIANT()> is a macro that tests
9f98c7fe 2916whether the byte is encoded as a single byte even in UTF-8):
a422fd2d 2917
32128a7f
KW
2918 U8 *utf; /* Initialize this to point to the beginning of the
2919 sequence to convert */
2920 U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
2921 pointed to by 'utf' */
2922 UV uv; /* Returned code point; note: a UV, not a U8, not a
2923 char */
2924 STRLEN len; /* Returned length of character in bytes */
a422fd2d 2925
3a2263fe 2926 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2927 /* Must treat this as UTF-8 */
4b88fb76 2928 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
a422fd2d
SC
2929 else
2930 /* OK to treat this character as a byte */
2931 uv = *utf;
2932
4b88fb76 2933You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
95701e00 2934value of the character; the inverse function C<uvchr_to_utf8> is available
1e54db1a 2935for putting a UV into UTF-8:
a422fd2d 2936
61ad4b94 2937 if (!UVCHR_IS_INVARIANT(uv))
a422fd2d 2938 /* Must treat this as UTF8 */
95701e00 2939 utf8 = uvchr_to_utf8(utf8, uv);
a422fd2d
SC
2940 else
2941 /* OK to treat this character as a byte */
2942 *utf8++ = uv;
2943
2944You B<must> convert characters to UVs using the above functions if
1e54db1a 2945you're ever in a situation where you have to match UTF-8 and non-UTF-8
10e2eb10 2946characters. You may not skip over UTF-8 characters in this case. If you
1e54db1a
JH
2947do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2948for instance, if your UTF-8 string contains C<v196.172>, and you skip
2949that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2950So don't do that!
2951
61ad4b94
KW
2952(Note that we don't have to test for invariant characters in the
2953examples above. The functions work on any well-formed UTF-8 input.
2954It's just that its faster to avoid the function overhead when it's not
2955needed.)
2956
1e54db1a 2957=head2 How does Perl store UTF-8 strings?
a422fd2d 2958
61ad4b94 2959Currently, Perl deals with UTF-8 strings and non-UTF-8 strings
10e2eb10
FC
2960slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2961string is internally encoded as UTF-8. Without it, the byte value is the
61ad4b94
KW
2962codepoint number and vice versa. This flag is only meaningful if the SV
2963is C<SvPOK> or immediately after stringification via C<SvPV> or a
2964similar macro. You can check and manipulate this flag with the
2575c402 2965following macros:
a422fd2d
SC
2966
2967 SvUTF8(sv)
2968 SvUTF8_on(sv)
2969 SvUTF8_off(sv)
2970
2971This flag has an important effect on Perl's treatment of the string: if
61ad4b94 2972UTF-8 data is not properly distinguished, regular expressions,
a422fd2d 2973C<length>, C<substr> and other string handling operations will have
61ad4b94 2974undesirable (wrong) results.
a422fd2d
SC
2975
2976The problem comes when you have, for instance, a string that isn't
61ad4b94 2977flagged as UTF-8, and contains a byte sequence that could be UTF-8 --
1e54db1a 2978especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d 2979
61ad4b94
KW
2980Never forget that the C<SVf_UTF8> flag is separate from the PV value; you
2981need to be sure you don't accidentally knock it off while you're
10e2eb10 2982manipulating SVs. More specifically, you cannot expect to do this:
a422fd2d
SC
2983
2984 SV *sv;
2985 SV *nsv;
2986 STRLEN len;
2987 char *p;
2988
2989 p = SvPV(sv, len);
2990 frobnicate(p);
2991 nsv = newSVpvn(p, len);
2992
2993The C<char*> string does not tell you the whole story, and you can't
10e2eb10 2994copy or reconstruct an SV just by copying the string value. Check if the
c31cc9fc
FC
2995old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2996accordingly:
a422fd2d
SC
2997
2998 p = SvPV(sv, len);
6db25795
KW
2999 is_utf8 = SvUTF8(sv);
3000 frobnicate(p, is_utf8);
a422fd2d 3001 nsv = newSVpvn(p, len);
6db25795 3002 if (is_utf8)
a422fd2d
SC
3003 SvUTF8_on(nsv);
3004
6db25795
KW
3005In the above, your C<frobnicate> function has been changed to be made
3006aware of whether or not it's dealing with UTF-8 data, so that it can
3007handle the string appropriately.
a422fd2d 3008
3a2263fe 3009Since just passing an SV to an XS function and copying the data of
2575c402 3010the SV is not enough to copy the UTF8 flags, even less right is just
61ad4b94 3011passing a S<C<char *>> to an XS function.
3a2263fe 3012
dc83bf8e 3013For full generality, use the L<C<DO_UTF8>|perlapi/DO_UTF8> macro to see if the
6db25795
KW
3014string in an SV is to be I<treated> as UTF-8. This takes into account
3015if the call to the XS function is being made from within the scope of
3016L<S<C<use bytes>>|bytes>. If so, the underlying bytes that comprise the
3017UTF-8 string are to be exposed, rather than the character they
3018represent. But this pragma should only really be used for debugging and
3019perhaps low-level testing at the byte level. Hence most XS code need
3020not concern itself with this, but various areas of the perl core do need
3021to support it.
3022
3023And this isn't the whole story. Starting in Perl v5.12, strings that
3024aren't encoded in UTF-8 may also be treated as Unicode under various
6e31cdd1 3025conditions (see L<perlunicode/ASCII Rules versus Unicode Rules>).
6db25795
KW
3026This is only really a problem for characters whose ordinals are between
3027128 and 255, and their behavior varies under ASCII versus Unicode rules
3028in ways that your code cares about (see L<perlunicode/The "Unicode Bug">).
3029There is no published API for dealing with this, as it is subject to
3030change, but you can look at the code for C<pp_lc> in F<pp.c> for an
3031example as to how it's currently done.
3032
1e54db1a 3033=head2 How do I convert a string to UTF-8?
a422fd2d 3034
2575c402 3035If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
61ad4b94 3036the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do
2575c402 3037this is:
a422fd2d
SC
3038
3039 sv_utf8_upgrade(sv);
3040
3041However, you must not do this, for example:
3042
3043 if (!SvUTF8(left))
3044 sv_utf8_upgrade(left);
3045
3046If you do this in a binary operator, you will actually change one of the
b1866b2d 3047strings that came into the operator, and, while it shouldn't be noticeable
2575c402 3048by the end user, it can cause problems in deficient code.
a422fd2d 3049
1e54db1a 3050Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
10e2eb10
FC
3051string argument. This is useful for having the data available for
3052comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
3053C<utf8_to_bytes> to go the other way, but naturally, this will fail if
3054the string contains any characters above 255 that can't be represented
3055in a single byte.
3056
6db25795
KW
3057=head2 How do I compare strings?
3058
3059L<perlapi/sv_cmp> and L<perlapi/sv_cmp_flags> do a lexigraphic
3060comparison of two SV's, and handle UTF-8ness properly. Note, however,
3061that Unicode specifies a much fancier mechanism for collation, available
3062via the L<Unicode::Collate> module.
3063
3064To just compare two strings for equality/non-equality, you can just use
3065L<C<memEQ()>|perlapi/memEQ> and L<C<memNE()>|perlapi/memEQ> as usual,
3066except the strings must be both UTF-8 or not UTF-8 encoded.
3067
3068To compare two strings case-insensitively, use
3069L<C<foldEQ_utf8()>|perlapi/foldEQ_utf8> (the strings don't have to have
3070the same UTF-8ness).
3071
a422fd2d
SC
3072=head2 Is there anything else I need to know?
3073
10e2eb10 3074Not really. Just remember these things:
a422fd2d
SC
3075
3076=over 3
3077
3078=item *
3079
6db25795
KW
3080There's no way to tell if a S<C<char *>> or S<C<U8 *>> string is UTF-8
3081or not. But you can tell if an SV is to be treated as UTF-8 by calling
3082C<DO_UTF8> on it, after stringifying it with C<SvPV> or a similar
3083macro. And, you can tell if SV is actually UTF-8 (even if it is not to
3084be treated as such) by looking at its C<SvUTF8> flag (again after
3085stringifying it). Don't forget to set the flag if something should be
3086UTF-8.
3087Treat the flag as part of the PV, even though it's not -- if you pass on
3088the PV to somewhere, pass on the flag too.
a422fd2d
SC
3089
3090=item *
3091
4b88fb76 3092If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
3a2263fe 3093unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
3094
3095=item *
3096
61ad4b94
KW
3097When writing a character UV to a UTF-8 string, B<always> use
3098C<uvchr_to_utf8>, unless C<UVCHR_IS_INVARIANT(uv))> in which case
3a2263fe 3099you can use C<*s = uv>.
a422fd2d
SC
3100
3101=item *
3102
10e2eb10
FC
3103Mixing UTF-8 and non-UTF-8 strings is
3104tricky. Use C<bytes_to_utf8> to get
2bbc8d55 3105a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
3106
3107=back
3108
53e06cf0
SC
3109=head1 Custom Operators
3110
2a0fd0f1 3111Custom operator support is an experimental feature that allows you to
10e2eb10 3112define your own ops. This is primarily to allow the building of
53e06cf0
SC
3113interpreters for other languages in the Perl core, but it also allows
3114optimizations through the creation of "macro-ops" (ops which perform the
3115functions of multiple ops which are usually executed together, such as
1aa6ea50 3116C<gvsv, gvsv, add>.)
53e06cf0 3117
10e2eb10 3118This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0 3119core does not "know" anything special about this op type, and so it will
10e2eb10 3120not be involved in any optimizations. This also means that you can
61ad4b94
KW
3121define your custom ops to be any op structure -- unary, binary, list and
3122so on -- you like.
53e06cf0 3123
10e2eb10
FC
3124It's important to know what custom operators won't do for you. They
3125won't let you add new syntax to Perl, directly. They won't even let you
3126add new keywords, directly. In fact, they won't change the way Perl
3127compiles a program at all. You have to do those changes yourself, after
3128Perl has compiled the program. You do this either by manipulating the op
53e06cf0
SC
3129tree using a C<CHECK> block and the C<B::Generate> module, or by adding
3130a custom peephole optimizer with the C<optimize> module.
3131
3132When you do this, you replace ordinary Perl ops with custom ops by
407f86e1 3133creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
10e2eb10
FC
3134PP function. This should be defined in XS code, and should look like
3135the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
53e06cf0
SC
3136takes the appropriate number of values from the stack, and you are
3137responsible for adding stack marks if necessary.
3138
3139You should also "register" your op with the Perl interpreter so that it
10e2eb10 3140can produce sensible error and warning messages. Since it is possible to
53e06cf0 3141have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
9733086d 3142Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
10e2eb10 3143it is dealing with. You should create an C<XOP> structure for each
9733086d
BM
3144ppaddr you use, set the properties of the custom op with
3145C<XopENTRY_set>, and register the structure against the ppaddr using
10e2eb10 3146C<Perl_custom_op_register>. A trivial example might look like:
9733086d
BM
3147
3148 static XOP my_xop;
3149 static OP *my_pp(pTHX);
3150
3151 BOOT:
3152 XopENTRY_set(&my_xop, xop_name, "myxop");
3153 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3154 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3155
3156The available fields in the structure are:
3157
3158=over 4
3159
3160=item xop_name
3161
10e2eb10 3162A short name for your op. This will be included in some error messages,
9733086d
BM
3163and will also be returned as C<< $op->name >> by the L<B|B> module, so
3164it will appear in the output of module like L<B::Concise|B::Concise>.
3165
3166=item xop_desc
3167
3168A short description of the function of the op.
3169
3170=item xop_class
3171
10e2eb10 3172Which of the various C<*OP> structures this op uses. This should be one of
9733086d
BM
3173the C<OA_*> constants from F<op.h>, namely
3174
3175=over 4
3176
3177=item OA_BASEOP
3178
3179=item OA_UNOP
3180
3181=item OA_BINOP
3182
3183=item OA_LOGOP
3184
3185=item OA_LISTOP
3186
3187=item OA_PMOP
3188
3189=item OA_SVOP
3190
3191=item OA_PADOP
3192
3193=item OA_PVOP_OR_SVOP
3194
10e2eb10 3195This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
9733086d
BM
3196the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
3197
3198=item OA_LOOP
3199
3200=item OA_COP
3201
3202=back
3203
3204The other C<OA_*> constants should not be used.
3205
3206=item xop_peep
3207
3208This member is of type C<Perl_cpeep_t>, which expands to C<void
10e2eb10 3209(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
9733086d 3210will be called from C<Perl_rpeep> when ops of this type are encountered
10e2eb10 3211by the peephole optimizer. I<o> is the OP that needs optimizing;
9733086d
BM
3212I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
3213
3214=back
53e06cf0 3215
e7d4c058 3216C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 3217
bf5e9371
DM
3218
3219=head1 Dynamic Scope and the Context Stack
3220
3221B<Note:> this section describes a non-public internal API that is subject
3222to change without notice.
3223
3224=head2 Introduction to the context stack
3225
3226In Perl, dynamic scoping refers to the runtime nesting of things like
3227subroutine calls, evals etc, as well as the entering and exiting of block
3228scopes. For example, the restoring of a C<local>ised variable is
3229determined by the dynamic scope.
3230
3231Perl tracks the dynamic scope by a data structure called the context
3232stack, which is an array of C<PERL_CONTEXT> structures, and which is
3233itself a big union for all the types of context. Whenever a new scope is
3234entered (such as a block, a C<for> loop, or a subroutine call), a new
3235context entry is pushed onto the stack. Similarly when leaving a block or
3236returning from a subroutine call etc. a context is popped. Since the
3237context stack represents the current dynamic scope, it can be searched.
3238For example, C<next LABEL> searches back through the stack looking for a
3239loop context that matches the label; C<return> pops contexts until it
3240finds a sub or eval context or similar; C<caller> examines sub contexts on
3241the stack.
3242
3243Each context entry is labelled with a context type, C<cx_type>. Typical
3244context types are C<CXt_SUB>, C<CXt_EVAL> etc., as well as C<CXt_BLOCK>
3245and C<CXt_NULL> which represent a basic scope (as pushed by C<pp_enter>)
3246and a sort block. The type determines which part of the context union are
3247valid.
3248
3249The main division in the context struct is between a substitution scope
3250(C<CXt_SUBST>) and block scopes, which are everything else. The former is
d7c7f8cb 3251just used while executing C<s///e>, and won't be discussed further
bf5e9371
DM
3252here.
3253
3254All the block scope types share a common base, which corresponds to
3255C<CXt_BLOCK>. This stores the old values of various scope-related
3256variables like C<PL_curpm>, as well as information about the current
3257scope, such as C<gimme>. On scope exit, the old variables are restored.
3258
3259Particular block scope types store extra per-type information. For
3260example, C<CXt_SUB> stores the currently executing CV, while the various
3261for loop types might hold the original loop variable SV. On scope exit,
3262the per-type data is processed; for example the CV has its reference count
3263decremented, and the original loop variable is restored.
3264
3265The macro C<cxstack> returns the base of the current context stack, while
3266C<cxstack_ix> is the index of the current frame within that stack.
3267
3268In fact, the context stack is actually part of a stack-of-stacks system;
3269whenever something unusual is done such as calling a C<DESTROY> or tie
3270handler, a new stack is pushed, then popped at the end.
3271
3272Note that the API described here changed considerably in perl 5.24; prior
3273to that, big macros like C<PUSHBLOCK> and C<POPSUB> were used; in 5.24
3274they were replaced by the inline static functions described below. In
3275addition, the ordering and detail of how these macros/function work
3276changed in many ways, often subtly. In particular they didn't handle
3277saving the savestack and temps stack positions, and required additional
3278C<ENTER>, C<SAVETMPS> and C<LEAVE> compared to the new functions. The
3279old-style macros will not be described further.
3280
3281
3282=head2 Pushing contexts
3283
3284For pushing a new context, the two basic functions are
3285C<cx = cx_pushblock()>, which pushes a new basic context block and returns
3286its address, and a family of similar functions with names like
3287C<cx_pushsub(cx)> which populate the additional type-dependent fields in
3288the C<cx> struct. Note that C<CXt_NULL> and C<CXt_BLOCK> don't have their
3289own push functions, as they don't store any data beyond that pushed by
3290C<cx_pushblock>.
3291
3292The fields of the context struct and the arguments to the C<cx_*>
3293functions are subject to change between perl releases, representing
3294whatever is convenient or efficient for that release.
3295
3296A typical context stack pushing can be found in C<pp_entersub>; the
3297following shows a simplified and stripped-down example of a non-XS call,
3298along with comments showing roughly what each function does.
3299
61f554bd
KW
3300 dMARK;
3301 U8 gimme = GIMME_V;
3302 bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
3303 OP *retop = PL_op->op_next;
3304 I32 old_ss_ix = PL_savestack_ix;
3305 CV *cv = ....;
3306
3307 /* ... make mortal copies of stack args which are PADTMPs here ... */
3308
3309 /* ... do any additional savestack pushes here ... */
3310
3311 /* Now push a new context entry of type 'CXt_SUB'; initially just
3312 * doing the actions common to all block types: */
3313
3314 cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
3315
3316 /* this does (approximately):
3317 CXINC; /* cxstack_ix++ (grow if necessary) */
3318 cx = CX_CUR(); /* and get the address of new frame */
3319 cx->cx_type = CXt_SUB;
3320 cx->blk_gimme = gimme;
3321 cx->blk_oldsp = MARK - PL_stack_base;
3322 cx->blk_oldsaveix = old_ss_ix;
3323 cx->blk_oldcop = PL_curcop;
3324 cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
3325 cx->blk_oldscopesp = PL_scopestack_ix;
3326 cx->blk_oldpm = PL_curpm;
3327 cx->blk_old_tmpsfloor = PL_tmps_floor;
3328
3329 PL_tmps_floor = PL_tmps_ix;
3330 */
bf5e9371
DM
3331
3332
61f554bd
KW
3333 /* then update the new context frame with subroutine-specific info,
3334 * such as the CV about to be executed: */
bf5e9371 3335
61f554bd 3336 cx_pushsub(cx, cv, retop, hasargs);
bf5e9371 3337
61f554bd
KW
3338 /* this does (approximately):
3339 cx->blk_sub.cv = cv;
3340 cx->blk_sub.olddepth = CvDEPTH(cv);
3341 cx->blk_sub.prevcomppad = PL_comppad;
3342 cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
3343 cx->blk_sub.retop = retop;
3344 SvREFCNT_inc_simple_void_NN(cv);
3345 */
bf5e9371
DM
3346
3347Note that C<cx_pushblock()> sets two new floors: for the args stack (to
3348C<MARK>) and the temps stack (to C<PL_tmps_ix>). While executing at this
3349scope level, every C<nextstate> (amongst others) will reset the args and
3350tmps stack levels to these floors. Note that since C<cx_pushblock> uses
3351the current value of C<PL_tmps_ix> rather than it being passed as an arg,
3352this dictates at what point C<cx_pushblock> should be called. In
3353particular, any new mortals which should be freed only on scope exit
3354(rather than at the next C<nextstate>) should be created first.
3355
3356Most callers of C<cx_pushblock> simply set the new args stack floor to the
3357top of the previous stack frame, but for C<CXt_LOOP_LIST> it stores the
3358items being iterated over on the stack, and so sets C<blk_oldsp> to the
3359top of these items instead. Note that, contrary to its name, C<blk_oldsp>
3360doesn't always represent the value to restore C<PL_stack_sp> to on scope
3361exit.
3362
3363Note the early capture of C<PL_savestack_ix> to C<old_ss_ix>, which is
3364later passed as an arg to C<cx_pushblock>. In the case of C<pp_entersub>,
3365this is because, although most values needing saving are stored in fields
3366of the context struct, an extra value needs saving only when the debugger
3367is running, and it doesn't make sense to bloat the struct for this rare
3368case. So instead it is saved on the savestack. Since this value gets
3369calculated and saved before the context is pushed, it is necessary to pass
3370the old value of C<PL_savestack_ix> to C<cx_pushblock>, to ensure that the
3371saved value gets freed during scope exit. For most users of
3372C<cx_pushblock>, where nothing needs pushing on the save stack,
3373C<PL_savestack_ix> is just passed directly as an arg to C<cx_pushblock>.
3374
3375Note that where possible, values should be saved in the context struct
3376rather than on the save stack; it's much faster that way.
3377
3378Normally C<cx_pushblock> should be immediately followed by the appropriate
3379C<cx_pushfoo>, with nothing between them; this is because if code
3380in-between could die (e.g. a warning upgraded to fatal), then the context
3381stack unwinding code in C<dounwind> would see (in the example above) a
3382C<CXt_SUB> context frame, but without all the subroutine-specific fields
3383set, and crashes would soon ensue.
3384
3385Where the two must be separate, initially set the type to C<CXt_NULL> or
3386C<CXt_BLOCK>, and later change it to C<CXt_foo> when doing the
3387C<cx_pushfoo>. This is exactly what C<pp_enteriter> does, once it's
3388determined which type of loop it's pushing.
3389
3390=head2 Popping contexts
3391
3392Contexts are popped using C<cx_popsub()> etc. and C<cx_popblock()>. Note
3393however, that unlike C<cx_pushblock>, neither of these functions actually
3394decrement the current context stack index; this is done separately using
3395C<CX_POP()>.
3396
3397There are two main ways that contexts are popped. During normal execution
3398as scopes are exited, functions like C<pp_leave>, C<pp_leaveloop> and
3399C<pp_leavesub> process and pop just one context using C<cx_popfoo> and
3400C<cx_popblock>. On the other hand, things like C<pp_return> and C<next>
3401may have to pop back several scopes until a sub or loop context is found,
3402and exceptions (such as C<die>) need to pop back contexts until an eval
3403context is found. Both of these are accomplished by C<dounwind()>, which
3404is capable of processing and popping all contexts above the target one.
3405
3406Here is a typical example of context popping, as found in C<pp_leavesub>
3407(simplified slightly):
3408
61f554bd
KW
3409 U8 gimme;
3410 PERL_CONTEXT *cx;
3411 SV **oldsp;
3412 OP *retop;
bf5e9371 3413
61f554bd 3414 cx = CX_CUR();
bf5e9371 3415
61f554bd
KW
3416 gimme = cx->blk_gimme;
3417 oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
bf5e9371 3418
61f554bd
KW
3419 if (gimme == G_VOID)
3420 PL_stack_sp = oldsp;
3421 else
3422 leave_adjust_stacks(oldsp, oldsp, gimme, 0);
bf5e9371 3423
61f554bd
KW
3424 CX_LEAVE_SCOPE(cx);
3425 cx_popsub(cx);
3426 cx_popblock(cx);
3427 retop = cx->blk_sub.retop;
3428 CX_POP(cx);
bf5e9371 3429
61f554bd 3430 return retop;
bf5e9371
DM
3431
3432The steps above are in a very specific order, designed to be the reverse
3433order of when the context was pushed. The first thing to do is to copy
3434and/or protect any any return arguments and free any temps in the current
3435scope. Scope exits like an rvalue sub normally return a mortal copy of
3436their return args (as opposed to lvalue subs). It is important to make
3437this copy before the save stack is popped or variables are restored, or
3438bad things like the following can happen:
3439
3440 sub f { my $x =...; $x } # $x freed before we get to copy it
3441 sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
3442
3443Although we wish to free any temps at the same time, we have to be careful
3444not to free any temps which are keeping return args alive; nor to free the
3445temps we have just created while mortal copying return args. Fortunately,
3446C<leave_adjust_stacks()> is capable of making mortal copies of return args,
3447shifting args down the stack, and only processing those entries on the
3448temps stack that are safe to do so.
3449
3450In void context no args are returned, so it's more efficient to skip
3451calling C<leave_adjust_stacks()>. Also in void context, a C<nextstate> op
3452is likely to be imminently called which will do a C<FREETMPS>, so there's
3453no need to do that either.
3454
3455The next step is to pop savestack entries: C<CX_LEAVE_SCOPE(cx)> is just
4b93f2ab 3456defined as C<< LEAVE_SCOPE(cx->blk_oldsaveix) >>. Note that during the
bf5e9371
DM
3457popping, it's possible for perl to call destructors, call C<STORE> to undo
3458localisations of tied vars, and so on. Any of these can die or call
3459C<exit()>. In this case, C<dounwind()> will be called, and the current
3460context stack frame will be re-processed. Thus it is vital that all steps
3461in popping a context are done in such a way to support reentrancy. The
3462other alternative, of decrementing C<cxstack_ix> I<before> processing the
3463frame, would lead to leaks and the like if something died halfway through,
3464or overwriting of the current frame.
3465
3466C<CX_LEAVE_SCOPE> itself is safely re-entrant: if only half the savestack
3467items have been popped before dying and getting trapped by eval, then the
3468C<CX_LEAVE_SCOPE>s in C<dounwind> or C<pp_leaveeval> will continue where
3469the first one left off.
3470
3471The next step is the type-specific context processing; in this case
3472C<cx_popsub>. In part, this looks like:
3473
3474 cv = cx->blk_sub.cv;
3475 CvDEPTH(cv) = cx->blk_sub.olddepth;
3476 cx->blk_sub.cv = NULL;
3477 SvREFCNT_dec(cv);
3478
3479where its processing the just-executed CV. Note that before it decrements
3480the CV's reference count, it nulls the C<blk_sub.cv>. This means that if
3481it re-enters, the CV won't be freed twice. It also means that you can't
3482rely on such type-specific fields having useful values after the return
3483from C<cx_popfoo>.
3484
3485Next, C<cx_popblock> restores all the various interpreter vars to their
3486previous values or previous high water marks; it expands to:
3487
3488 PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
3489 PL_scopestack_ix = cx->blk_oldscopesp;
3490 PL_curpm = cx->blk_oldpm;
3491 PL_curcop = cx->blk_oldcop;
3492 PL_tmps_floor = cx->blk_old_tmpsfloor;
3493
3494Note that it I<doesn't> restore C<PL_stack_sp>; as mentioned earlier,