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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
1e422769
PP
551=head2 Hash API Extensions
552
553Beginning with version 5.004, the following functions are also supported:
554
555 HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
556 HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
c47ff5f1 557
1e422769
PP
558 bool hv_exists_ent (HV* tb, SV* key, U32 hash);
559 SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
c47ff5f1 560
1e422769
PP
561 SV* hv_iterkeysv (HE* entry);
562
563Note that these functions take C<SV*> keys, which simplifies writing
564of extension code that deals with hash structures. These functions
565also allow passing of C<SV*> keys to C<tie> functions without forcing
566you to stringify the keys (unlike the previous set of functions).
567
568They also return and accept whole hash entries (C<HE*>), making their
569use more efficient (since the hash number for a particular string
4a4eefd0
GS
570doesn't have to be recomputed every time). See L<perlapi> for detailed
571descriptions.
1e422769
PP
572
573The following macros must always be used to access the contents of hash
574entries. Note that the arguments to these macros must be simple
575variables, since they may get evaluated more than once. See
4a4eefd0 576L<perlapi> for detailed descriptions of these macros.
1e422769
PP
577
578 HePV(HE* he, STRLEN len)
579 HeVAL(HE* he)
580 HeHASH(HE* he)
581 HeSVKEY(HE* he)
582 HeSVKEY_force(HE* he)
583 HeSVKEY_set(HE* he, SV* sv)
584
585These two lower level macros are defined, but must only be used when
586dealing with keys that are not C<SV*>s:
587
588 HeKEY(HE* he)
589 HeKLEN(HE* he)
590
04343c6d
GS
591Note that both C<hv_store> and C<hv_store_ent> do not increment the
592reference count of the stored C<val>, which is the caller's responsibility.
593If these functions return a NULL value, the caller will usually have to
594decrement the reference count of C<val> to avoid a memory leak.
1e422769 595
a9381218
MHM
596=head2 AVs, HVs and undefined values
597
10e2eb10
FC
598Sometimes you have to store undefined values in AVs or HVs. Although
599this may be a rare case, it can be tricky. That's because you're
a9381218
MHM
600used to using C<&PL_sv_undef> if you need an undefined SV.
601
602For example, intuition tells you that this XS code:
603
604 AV *av = newAV();
605 av_store( av, 0, &PL_sv_undef );
606
607is equivalent to this Perl code:
608
609 my @av;
610 $av[0] = undef;
611
f3c4ec28 612Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
a9381218
MHM
613for indicating that an array element has not yet been initialized.
614Thus, C<exists $av[0]> would be true for the above Perl code, but
f3c4ec28
FC
615false for the array generated by the XS code. In perl 5.20, storing
616&PL_sv_undef will create a read-only element, because the scalar
617&PL_sv_undef itself is stored, not a copy.
a9381218 618
f3c4ec28 619Similar problems can occur when storing C<&PL_sv_undef> in HVs:
a9381218
MHM
620
621 hv_store( hv, "key", 3, &PL_sv_undef, 0 );
622
623This will indeed make the value C<undef>, but if you try to modify
624the value of C<key>, you'll get the following error:
625
626 Modification of non-creatable hash value attempted
627
628In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
10e2eb10 629in restricted hashes. This caused such hash entries not to appear
a9381218
MHM
630when iterating over the hash or when checking for the keys
631with the C<hv_exists> function.
632
8abccac8 633You can run into similar problems when you store C<&PL_sv_yes> or
10e2eb10 634C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
a9381218
MHM
635will give you the following error:
636
637 Modification of a read-only value attempted
638
639To make a long story short, you can use the special variables
8abccac8 640C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
a9381218
MHM
641HVs, but you have to make sure you know what you're doing.
642
643Generally, if you want to store an undefined value in an AV
644or HV, you should not use C<&PL_sv_undef>, but rather create a
645new undefined value using the C<newSV> function, for example:
646
647 av_store( av, 42, newSV(0) );
648 hv_store( hv, "foo", 3, newSV(0), 0 );
649
a0d0e21e
LW
650=head2 References
651
d1b91892 652References are a special type of scalar that point to other data types
a9b0660e 653(including other references).
a0d0e21e 654
07fa94a1 655To create a reference, use either of the following functions:
a0d0e21e 656
5f05dabc
PP
657 SV* newRV_inc((SV*) thing);
658 SV* newRV_noinc((SV*) thing);
a0d0e21e 659
5f05dabc 660The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The
07fa94a1
JO
661functions are identical except that C<newRV_inc> increments the reference
662count of the C<thing>, while C<newRV_noinc> does not. For historical
663reasons, C<newRV> is a synonym for C<newRV_inc>.
664
665Once you have a reference, you can use the following macro to dereference
666the reference:
a0d0e21e
LW
667
668 SvRV(SV*)
669
670then call the appropriate routines, casting the returned C<SV*> to either an
d1b91892 671C<AV*> or C<HV*>, if required.
a0d0e21e 672
d1b91892 673To determine if an SV is a reference, you can use the following macro:
a0d0e21e
LW
674
675 SvROK(SV*)
676
07fa94a1
JO
677To discover what type of value the reference refers to, use the following
678macro and then check the return value.
d1b91892
AD
679
680 SvTYPE(SvRV(SV*))
681
682The most useful types that will be returned are:
683
a5e62da0
FC
684 < SVt_PVAV Scalar
685 SVt_PVAV Array
686 SVt_PVHV Hash
687 SVt_PVCV Code
688 SVt_PVGV Glob (possibly a file handle)
689
690See L<perlapi/svtype> for more details.
d1b91892 691
cb1a09d0
AD
692=head2 Blessed References and Class Objects
693
06f6df17 694References are also used to support object-oriented programming. In perl's
cb1a09d0
AD
695OO lexicon, an object is simply a reference that has been blessed into a
696package (or class). Once blessed, the programmer may now use the reference
697to access the various methods in the class.
698
699A reference can be blessed into a package with the following function:
700
701 SV* sv_bless(SV* sv, HV* stash);
702
06f6df17
RGS
703The C<sv> argument must be a reference value. The C<stash> argument
704specifies which class the reference will belong to. See
5a0de581 705L</Stashes and Globs> for information on converting class names into stashes.
cb1a09d0
AD
706
707/* Still under construction */
708
ddd2cc91
DM
709The following function upgrades rv to reference if not already one.
710Creates a new SV for rv to point to. If C<classname> is non-null, the SV
711is blessed into the specified class. SV is returned.
cb1a09d0 712
08105a92 713 SV* newSVrv(SV* rv, const char* classname);
cb1a09d0 714
ddd2cc91
DM
715The following three functions copy integer, unsigned integer or double
716into an SV whose reference is C<rv>. SV is blessed if C<classname> is
717non-null.
cb1a09d0 718
08105a92 719 SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
e1c57cef 720 SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
08105a92 721 SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
cb1a09d0 722
ddd2cc91
DM
723The following function copies the pointer value (I<the address, not the
724string!>) into an SV whose reference is rv. SV is blessed if C<classname>
725is non-null.
cb1a09d0 726
ddd2cc91 727 SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
cb1a09d0 728
a9b0660e 729The following function copies a string into an SV whose reference is C<rv>.
ddd2cc91
DM
730Set length to 0 to let Perl calculate the string length. SV is blessed if
731C<classname> is non-null.
cb1a09d0 732
a9b0660e
KW
733 SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
734 STRLEN length);
cb1a09d0 735
ddd2cc91
DM
736The following function tests whether the SV is blessed into the specified
737class. It does not check inheritance relationships.
9abd00ed 738
08105a92 739 int sv_isa(SV* sv, const char* name);
9abd00ed 740
ddd2cc91 741The following function tests whether the SV is a reference to a blessed object.
9abd00ed
GS
742
743 int sv_isobject(SV* sv);
744
ddd2cc91 745The following function tests whether the SV is derived from the specified
10e2eb10
FC
746class. SV can be either a reference to a blessed object or a string
747containing a class name. This is the function implementing the
ddd2cc91 748C<UNIVERSAL::isa> functionality.
9abd00ed 749
08105a92 750 bool sv_derived_from(SV* sv, const char* name);
9abd00ed 751
00aadd71 752To check if you've got an object derived from a specific class you have
9abd00ed
GS
753to write:
754
755 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
cb1a09d0 756
5f05dabc 757=head2 Creating New Variables
cb1a09d0 758
5f05dabc
PP
759To create a new Perl variable with an undef value which can be accessed from
760your Perl script, use the following routines, depending on the variable type.
cb1a09d0 761
64ace3f8 762 SV* get_sv("package::varname", GV_ADD);
cbfd0a87 763 AV* get_av("package::varname", GV_ADD);
6673a63c 764 HV* get_hv("package::varname", GV_ADD);
cb1a09d0 765
058a5f6c 766Notice the use of GV_ADD as the second parameter. The new variable can now
cb1a09d0
AD
767be set, using the routines appropriate to the data type.
768
5f05dabc 769There are additional macros whose values may be bitwise OR'ed with the
058a5f6c 770C<GV_ADD> argument to enable certain extra features. Those bits are:
cb1a09d0 771
9a68f1db
SB
772=over
773
774=item GV_ADDMULTI
775
776Marks the variable as multiply defined, thus preventing the:
777
778 Name <varname> used only once: possible typo
779
780warning.
781
9a68f1db
SB
782=item GV_ADDWARN
783
784Issues the warning:
785
786 Had to create <varname> unexpectedly
787
788if the variable did not exist before the function was called.
789
790=back
cb1a09d0 791
07fa94a1
JO
792If you do not specify a package name, the variable is created in the current
793package.
cb1a09d0 794
5f05dabc 795=head2 Reference Counts and Mortality
a0d0e21e 796
10e2eb10 797Perl uses a reference count-driven garbage collection mechanism. SVs,
54310121 798AVs, or HVs (xV for short in the following) start their life with a
55497cff 799reference count of 1. If the reference count of an xV ever drops to 0,
07fa94a1 800then it will be destroyed and its memory made available for reuse.
3d2ba989
Z
801At the most basic internal level, reference counts can be manipulated
802with the following macros:
55497cff
PP
803
804 int SvREFCNT(SV* sv);
5f05dabc 805 SV* SvREFCNT_inc(SV* sv);
55497cff
PP
806 void SvREFCNT_dec(SV* sv);
807
3d2ba989
Z
808(There are also suffixed versions of the increment and decrement macros,
809for situations where the full generality of these basic macros can be
810exchanged for some performance.)
811
812However, the way a programmer should think about references is not so
813much in terms of the bare reference count, but in terms of I<ownership>
814of references. A reference to an xV can be owned by any of a variety
815of entities: another xV, the Perl interpreter, an XS data structure,
816a piece of running code, or a dynamic scope. An xV generally does not
817know what entities own the references to it; it only knows how many
818references there are, which is the reference count.
819
820To correctly maintain reference counts, it is essential to keep track
821of what references the XS code is manipulating. The programmer should
822always know where a reference has come from and who owns it, and be
823aware of any creation or destruction of references, and any transfers
824of ownership. Because ownership isn't represented explicitly in the xV
825data structures, only the reference count need be actually maintained
826by the code, and that means that this understanding of ownership is not
827actually evident in the code. For example, transferring ownership of a
828reference from one owner to another doesn't change the reference count
829at all, so may be achieved with no actual code. (The transferring code
830doesn't touch the referenced object, but does need to ensure that the
831former owner knows that it no longer owns the reference, and that the
832new owner knows that it now does.)
833
834An xV that is visible at the Perl level should not become unreferenced
835and thus be destroyed. Normally, an object will only become unreferenced
836when it is no longer visible, often by the same means that makes it
837invisible. For example, a Perl reference value (RV) owns a reference to
838its referent, so if the RV is overwritten that reference gets destroyed,
839and the no-longer-reachable referent may be destroyed as a result.
840
841Many functions have some kind of reference manipulation as
842part of their purpose. Sometimes this is documented in terms
843of ownership of references, and sometimes it is (less helpfully)
844documented in terms of changes to reference counts. For example, the
845L<newRV_inc()|perlapi/newRV_inc> function is documented to create a new RV
846(with reference count 1) and increment the reference count of the referent
847that was supplied by the caller. This is best understood as creating
848a new reference to the referent, which is owned by the created RV,
849and returning to the caller ownership of the sole reference to the RV.
850The L<newRV_noinc()|perlapi/newRV_noinc> function instead does not
851increment the reference count of the referent, but the RV nevertheless
852ends up owning a reference to the referent. It is therefore implied
853that the caller of C<newRV_noinc()> is relinquishing a reference to the
854referent, making this conceptually a more complicated operation even
855though it does less to the data structures.
856
857For example, imagine you want to return a reference from an XSUB
858function. Inside the XSUB routine, you create an SV which initially
859has just a single reference, owned by the XSUB routine. This reference
860needs to be disposed of before the routine is complete, otherwise it
861will leak, preventing the SV from ever being destroyed. So to create
862an RV referencing the SV, it is most convenient to pass the SV to
863C<newRV_noinc()>, which consumes that reference. Now the XSUB routine
864no longer owns a reference to the SV, but does own a reference to the RV,
865which in turn owns a reference to the SV. The ownership of the reference
866to the RV is then transferred by the process of returning the RV from
867the XSUB.
55497cff 868
5f05dabc 869There are some convenience functions available that can help with the
54310121 870destruction of xVs. These functions introduce the concept of "mortality".
3d2ba989
Z
871Much documentation speaks of an xV itself being mortal, but this is
872misleading. It is really I<a reference to> an xV that is mortal, and it
873is possible for there to be more than one mortal reference to a single xV.
874For a reference to be mortal means that it is owned by the temps stack,
875one of perl's many internal stacks, which will destroy that reference
876"a short time later". Usually the "short time later" is the end of
877the current Perl statement. However, it gets more complicated around
878dynamic scopes: there can be multiple sets of mortal references hanging
879around at the same time, with different death dates. Internally, the
880actual determinant for when mortal xV references are destroyed depends
881on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs>
882for more details on these macros.
883
884Mortal references are mainly used for xVs that are placed on perl's
885main stack. The stack is problematic for reference tracking, because it
886contains a lot of xV references, but doesn't own those references: they
887are not counted. Currently, there are many bugs resulting from xVs being
888destroyed while referenced by the stack, because the stack's uncounted
889references aren't enough to keep the xVs alive. So when putting an
890(uncounted) reference on the stack, it is vitally important to ensure that
891there will be a counted reference to the same xV that will last at least
892as long as the uncounted reference. But it's also important that that
893counted reference be cleaned up at an appropriate time, and not unduly
894prolong the xV's life. For there to be a mortal reference is often the
895best way to satisfy this requirement, especially if the xV was created
896especially to be put on the stack and would otherwise be unreferenced.
897
898To create a mortal reference, use the functions:
a0d0e21e
LW
899
900 SV* sv_newmortal()
a0d0e21e 901 SV* sv_mortalcopy(SV*)
3d2ba989 902 SV* sv_2mortal(SV*)
a0d0e21e 903
3d2ba989
Z
904C<sv_newmortal()> creates an SV (with the undefined value) whose sole
905reference is mortal. C<sv_mortalcopy()> creates an xV whose value is a
906copy of a supplied xV and whose sole reference is mortal. C<sv_2mortal()>
907mortalises an existing xV reference: it transfers ownership of a reference
908from the caller to the temps stack. Because C<sv_newmortal> gives the new
909SV no value, it must normally be given one via C<sv_setpv>, C<sv_setiv>,
910etc. :
00aadd71
NIS
911
912 SV *tmp = sv_newmortal();
913 sv_setiv(tmp, an_integer);
914
915As that is multiple C statements it is quite common so see this idiom instead:
916
917 SV *tmp = sv_2mortal(newSViv(an_integer));
918
ac036724 919The mortal routines are not just for SVs; AVs and HVs can be
faed5253 920made mortal by passing their address (type-casted to C<SV*>) to the
07fa94a1 921C<sv_2mortal> or C<sv_mortalcopy> routines.
a0d0e21e 922
5f05dabc 923=head2 Stashes and Globs
a0d0e21e 924
06f6df17
RGS
925A B<stash> is a hash that contains all variables that are defined
926within a package. Each key of the stash is a symbol
aa689395
PP
927name (shared by all the different types of objects that have the same
928name), and each value in the hash table is a GV (Glob Value). This GV
929in turn contains references to the various objects of that name,
930including (but not limited to) the following:
cb1a09d0 931
a0d0e21e
LW
932 Scalar Value
933 Array Value
934 Hash Value
a3cb178b 935 I/O Handle
a0d0e21e
LW
936 Format
937 Subroutine
938
06f6df17
RGS
939There is a single stash called C<PL_defstash> that holds the items that exist
940in the C<main> package. To get at the items in other packages, append the
941string "::" to the package name. The items in the C<Foo> package are in
942the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
943in the stash C<Baz::> in C<Bar::>'s stash.
a0d0e21e 944
d1b91892 945To get the stash pointer for a particular package, use the function:
a0d0e21e 946
da51bb9b
NC
947 HV* gv_stashpv(const char* name, I32 flags)
948 HV* gv_stashsv(SV*, I32 flags)
a0d0e21e
LW
949
950The first function takes a literal string, the second uses the string stored
d1b91892 951in the SV. Remember that a stash is just a hash table, so you get back an
da51bb9b 952C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
a0d0e21e
LW
953
954The name that C<gv_stash*v> wants is the name of the package whose symbol table
955you want. The default package is called C<main>. If you have multiply nested
d1b91892
AD
956packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl
957language itself.
a0d0e21e
LW
958
959Alternately, if you have an SV that is a blessed reference, you can find
960out the stash pointer by using:
961
962 HV* SvSTASH(SvRV(SV*));
963
964then use the following to get the package name itself:
965
966 char* HvNAME(HV* stash);
967
5f05dabc
PP
968If you need to bless or re-bless an object you can use the following
969function:
a0d0e21e
LW
970
971 SV* sv_bless(SV*, HV* stash)
972
973where the first argument, an C<SV*>, must be a reference, and the second
974argument is a stash. The returned C<SV*> can now be used in the same way
975as any other SV.
976
d1b91892
AD
977For more information on references and blessings, consult L<perlref>.
978
54310121 979=head2 Double-Typed SVs
0a753a76
PP
980
981Scalar variables normally contain only one type of value, an integer,
982double, pointer, or reference. Perl will automatically convert the
983actual scalar data from the stored type into the requested type.
984
985Some scalar variables contain more than one type of scalar data. For
986example, the variable C<$!> contains either the numeric value of C<errno>
987or its string equivalent from either C<strerror> or C<sys_errlist[]>.
988
989To force multiple data values into an SV, you must do two things: use the
990C<sv_set*v> routines to add the additional scalar type, then set a flag
991so that Perl will believe it contains more than one type of data. The
992four macros to set the flags are:
993
994 SvIOK_on
995 SvNOK_on
996 SvPOK_on
997 SvROK_on
998
999The particular macro you must use depends on which C<sv_set*v> routine
1000you called first. This is because every C<sv_set*v> routine turns on
1001only the bit for the particular type of data being set, and turns off
1002all the rest.
1003
1004For example, to create a new Perl variable called "dberror" that contains
1005both the numeric and descriptive string error values, you could use the
1006following code:
1007
1008 extern int dberror;
1009 extern char *dberror_list;
1010
64ace3f8 1011 SV* sv = get_sv("dberror", GV_ADD);
0a753a76
PP
1012 sv_setiv(sv, (IV) dberror);
1013 sv_setpv(sv, dberror_list[dberror]);
1014 SvIOK_on(sv);
1015
1016If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
1017macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
1018
4f4531b8
FC
1019=head2 Read-Only Values
1020
1021In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
1022flag bit with read-only scalars. So the only way to test whether
1023C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
1024in those versions is:
1025
1026 SvREADONLY(sv) && !SvIsCOW(sv)
1027
1028Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
1029and, under 5.20, copy-on-write scalars can also be read-only, so the above
1030check is incorrect. You just want:
1031
1032 SvREADONLY(sv)
1033
1034If you need to do this check often, define your own macro like this:
1035
1036 #if PERL_VERSION >= 18
1037 # define SvTRULYREADONLY(sv) SvREADONLY(sv)
1038 #else
1039 # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
1040 #endif
1041
1042=head2 Copy on Write
1043
1044Perl implements a copy-on-write (COW) mechanism for scalars, in which
1045string copies are not immediately made when requested, but are deferred
1046until made necessary by one or the other scalar changing. This is mostly
1047transparent, but one must take care not to modify string buffers that are
1048shared by multiple SVs.
1049
1050You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
1051
1052You 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).
1053
1054If you want to make the SV drop its string buffer, use
1055C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
1056C<sv_setsv(sv, NULL)>.
1057
1058All of these functions will croak on read-only scalars (see the previous
1059section for more on those).
1060
1061To test that your code is behaving correctly and not modifying COW buffers,
1062on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
1063C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
1064into crashes. You will find it to be marvellously slow, so you may want to
1065skip perl's own tests.
1066
0a753a76 1067=head2 Magic Variables
a0d0e21e 1068
d1b91892
AD
1069[This section still under construction. Ignore everything here. Post no
1070bills. Everything not permitted is forbidden.]
1071
d1b91892
AD
1072Any SV may be magical, that is, it has special features that a normal
1073SV does not have. These features are stored in the SV structure in a
5f05dabc 1074linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.
d1b91892
AD
1075
1076 struct magic {
1077 MAGIC* mg_moremagic;
1078 MGVTBL* mg_virtual;
1079 U16 mg_private;
1080 char mg_type;
1081 U8 mg_flags;
b205eb13 1082 I32 mg_len;
d1b91892
AD
1083 SV* mg_obj;
1084 char* mg_ptr;
d1b91892
AD
1085 };
1086
1087Note this is current as of patchlevel 0, and could change at any time.
1088
1089=head2 Assigning Magic
1090
1091Perl adds magic to an SV using the sv_magic function:
1092
a9b0660e 1093 void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
d1b91892
AD
1094
1095The C<sv> argument is a pointer to the SV that is to acquire a new magical
1096feature.
1097
1098If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
10e2eb10
FC
1099convert C<sv> to type C<SVt_PVMG>.
1100Perl then continues by adding new magic
645c22ef
DM
1101to the beginning of the linked list of magical features. Any prior entry
1102of the same type of magic is deleted. Note that this can be overridden,
1103and multiple instances of the same type of magic can be associated with an
1104SV.
d1b91892 1105
54310121 1106The C<name> and C<namlen> arguments are used to associate a string with
10e2eb10 1107the magic, typically the name of a variable. C<namlen> is stored in the
2d8d5d5a
SH
1108C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
1109C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
1110whether C<namlen> is greater than zero or equal to zero respectively. As a
1111special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
1112to contain an C<SV*> and is stored as-is with its REFCNT incremented.
d1b91892
AD
1113
1114The sv_magic function uses C<how> to determine which, if any, predefined
1115"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
5a0de581 1116See the L</Magic Virtual Tables> section below. The C<how> argument is also
10e2eb10
FC
1117stored in the C<mg_type> field. The value of
1118C<how> should be chosen from the set of macros
1119C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
645c22ef 1120these macros were added, Perl internals used to directly use character
14befaf4 1121literals, so you may occasionally come across old code or documentation
75d0f26d 1122referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
d1b91892
AD
1123
1124The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
1125structure. If it is not the same as the C<sv> argument, the reference
1126count of the C<obj> object is incremented. If it is the same, or if
27deb0cf
YO
1127the C<how> argument is C<PERL_MAGIC_arylen>, C<PERL_MAGIC_regdatum>,
1128C<PERL_MAGIC_regdata>, or if it is a NULL pointer, then C<obj> is merely
1129stored, without the reference count being incremented.
d1b91892 1130
2d8d5d5a
SH
1131See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
1132to an SV.
1133
cb1a09d0
AD
1134There is also a function to add magic to an C<HV>:
1135
1136 void hv_magic(HV *hv, GV *gv, int how);
1137
1138This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.
1139
1140To remove the magic from an SV, call the function sv_unmagic:
1141
70a53b35 1142 int sv_unmagic(SV *sv, int type);
cb1a09d0
AD
1143
1144The C<type> argument should be equal to the C<how> value when the C<SV>
1145was initially made magical.
1146
f6ee7b17 1147However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
10e2eb10
FC
1148C<SV>. If you want to remove only certain
1149magic of a C<type> based on the magic
f6ee7b17
FR
1150virtual table, use C<sv_unmagicext> instead:
1151
1152 int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
1153
d1b91892
AD
1154=head2 Magic Virtual Tables
1155
d1be9408 1156The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
d1b91892
AD
1157C<MGVTBL>, which is a structure of function pointers and stands for
1158"Magic Virtual Table" to handle the various operations that might be
1159applied to that variable.
1160
301cb7e8
DM
1161The C<MGVTBL> has five (or sometimes eight) pointers to the following
1162routine types:
d1b91892 1163
e97ca230
DM
1164 int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
1165 int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
1166 U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
1167 int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
1168 int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
d1b91892 1169
e97ca230 1170 int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
a9b0660e 1171 const char *name, I32 namlen);
e97ca230
DM
1172 int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
1173 int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
301cb7e8
DM
1174
1175
06f6df17 1176This MGVTBL structure is set at compile-time in F<perl.h> and there are
b7a0f54c
SM
1177currently 32 types. These different structures contain pointers to various
1178routines that perform additional actions depending on which function is
1179being called.
d1b91892 1180
a9b0660e
KW
1181 Function pointer Action taken
1182 ---------------- ------------
1183 svt_get Do something before the value of the SV is
1184 retrieved.
1185 svt_set Do something after the SV is assigned a value.
1186 svt_len Report on the SV's length.
1187 svt_clear Clear something the SV represents.
1188 svt_free Free any extra storage associated with the SV.
d1b91892 1189
a9b0660e
KW
1190 svt_copy copy tied variable magic to a tied element
1191 svt_dup duplicate a magic structure during thread cloning
1192 svt_local copy magic to local value during 'local'
301cb7e8 1193
d1b91892 1194For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
14befaf4 1195to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
d1b91892
AD
1196
1197 { magic_get, magic_set, magic_len, 0, 0 }
1198
14befaf4
DM
1199Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
1200if a get operation is being performed, the routine C<magic_get> is
1201called. All the various routines for the various magical types begin
1202with C<magic_>. NOTE: the magic routines are not considered part of
1203the Perl API, and may not be exported by the Perl library.
d1b91892 1204
301cb7e8
DM
1205The last three slots are a recent addition, and for source code
1206compatibility they are only checked for if one of the three flags
10e2eb10
FC
1207MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
1208This means that most code can continue declaring
1209a vtable as a 5-element value. These three are
301cb7e8
DM
1210currently used exclusively by the threading code, and are highly subject
1211to change.
1212
d1b91892
AD
1213The current kinds of Magic Virtual Tables are:
1214
f1f5ddd7
FC
1215=for comment
1216This table is generated by regen/mg_vtable.pl. Any changes made here
1217will be lost.
1218
1219=for mg_vtable.pl begin
1220
a9b0660e 1221 mg_type
bd6e6c12
FC
1222 (old-style char and macro) MGVTBL Type of magic
1223 -------------------------- ------ -------------
1224 \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
1225 # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
e5e1ee61 1226 % PERL_MAGIC_rhash (none) Extra data for restricted
bd6e6c12 1227 hashes
a6d69523
TC
1228 * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
1229 vars
bd6e6c12 1230 . PERL_MAGIC_pos vtbl_pos pos() lvalue
e5e1ee61 1231 : PERL_MAGIC_symtab (none) Extra data for symbol
bd6e6c12 1232 tables
e5e1ee61
FC
1233 < PERL_MAGIC_backref vtbl_backref For weak ref data
1234 @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
bd6e6c12
FC
1235 B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
1236 (fast string search)
1237 c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
1238 (AMT) on stash
1239 D PERL_MAGIC_regdata vtbl_regdata Regex match position data
1240 (@+ and @- vars)
1241 d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
1242 element
1243 E PERL_MAGIC_env vtbl_env %ENV hash
1244 e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
eccba044 1245 f PERL_MAGIC_fm vtbl_regexp Formline
bd6e6c12 1246 ('compiled' format)
bd6e6c12
FC
1247 g PERL_MAGIC_regex_global vtbl_mglob m//g target
1248 H PERL_MAGIC_hints vtbl_hints %^H hash
1249 h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
1250 I PERL_MAGIC_isa vtbl_isa @ISA array
1251 i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
1252 k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
1253 L PERL_MAGIC_dbfile (none) Debugger %_<filename
1254 l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
1255 element
1256 N PERL_MAGIC_shared (none) Shared between threads
1257 n PERL_MAGIC_shared_scalar (none) Shared between threads
1258 o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
1259 P PERL_MAGIC_tied vtbl_pack Tied array or hash
1260 p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
1261 q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
e5e1ee61 1262 r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
bd6e6c12
FC
1263 S PERL_MAGIC_sig (none) %SIG hash
1264 s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
1265 t PERL_MAGIC_taint vtbl_taint Taintedness
1266 U PERL_MAGIC_uvar vtbl_uvar Available for use by
1267 extensions
1268 u PERL_MAGIC_uvar_elem (none) Reserved for use by
1269 extensions
4499db73 1270 V PERL_MAGIC_vstring (none) SV was vstring literal
bd6e6c12
FC
1271 v PERL_MAGIC_vec vtbl_vec vec() lvalue
1272 w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
1273 x PERL_MAGIC_substr vtbl_substr substr() lvalue
1f1dcfb5
FC
1274 Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
1275 exist
bd6e6c12
FC
1276 y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
1277 variable / smart parameter
1278 vivification
baabe3fb
FC
1279 \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
1280 constructor
e5e1ee61 1281 ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
bd6e6c12
FC
1282 to this CV
1283 ~ PERL_MAGIC_ext (none) Available for use by
1284 extensions
0cbee0a4 1285
f1f5ddd7 1286=for mg_vtable.pl end
d1b91892 1287
68dc0745 1288When an uppercase and lowercase letter both exist in the table, then the
92f0c265
JP
1289uppercase letter is typically used to represent some kind of composite type
1290(a list or a hash), and the lowercase letter is used to represent an element
10e2eb10 1291of that composite type. Some internals code makes use of this case
92f0c265 1292relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
14befaf4
DM
1293
1294The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
1295specifically for use by extensions and will not be used by perl itself.
1296Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
1297to variables (typically objects). This is especially useful because
1298there is no way for normal perl code to corrupt this private information
1299(unlike using extra elements of a hash object).
1300
1301Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
1302C function any time a scalar's value is used or changed. The C<MAGIC>'s
bdbeb323
SM
1303C<mg_ptr> field points to a C<ufuncs> structure:
1304
1305 struct ufuncs {
a9402793
AB
1306 I32 (*uf_val)(pTHX_ IV, SV*);
1307 I32 (*uf_set)(pTHX_ IV, SV*);
bdbeb323
SM
1308 IV uf_index;
1309 };
1310
1311When the SV is read from or written to, the C<uf_val> or C<uf_set>
14befaf4
DM
1312function will be called with C<uf_index> as the first arg and a pointer to
1313the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
1526ead6
AB
1314magic is shown below. Note that the ufuncs structure is copied by
1315sv_magic, so you can safely allocate it on the stack.
1316
1317 void
1318 Umagic(sv)
1319 SV *sv;
1320 PREINIT:
1321 struct ufuncs uf;
1322 CODE:
1323 uf.uf_val = &my_get_fn;
1324 uf.uf_set = &my_set_fn;
1325 uf.uf_index = 0;
14befaf4 1326 sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
5f05dabc 1327
1e73acc8
AS
1328Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
1329
1330For hashes there is a specialized hook that gives control over hash
1331keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
1332if the "set" function in the C<ufuncs> structure is NULL. The hook
1333is activated whenever the hash is accessed with a key specified as
1334an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
1335C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
1336through the functions without the C<..._ent> suffix circumvents the
4509d391 1337hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
1e73acc8 1338
14befaf4
DM
1339Note that because multiple extensions may be using C<PERL_MAGIC_ext>
1340or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
1341extra care to avoid conflict. Typically only using the magic on
1342objects blessed into the same class as the extension is sufficient.
2f07f21a
FR
1343For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
1344C<MGVTBL>, even if all its fields will be C<0>, so that individual
1345C<MAGIC> pointers can be identified as a particular kind of magic
10e2eb10 1346using their magic virtual table. C<mg_findext> provides an easy way
f6ee7b17 1347to do that:
2f07f21a
FR
1348
1349 STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
1350
1351 MAGIC *mg;
f6ee7b17
FR
1352 if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
1353 /* this is really ours, not another module's PERL_MAGIC_ext */
1354 my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
1355 ...
2f07f21a 1356 }
5f05dabc 1357
ef50df4b
GS
1358Also note that the C<sv_set*()> and C<sv_cat*()> functions described
1359earlier do B<not> invoke 'set' magic on their targets. This must
1360be done by the user either by calling the C<SvSETMAGIC()> macro after
1361calling these functions, or by using one of the C<sv_set*_mg()> or
1362C<sv_cat*_mg()> functions. Similarly, generic C code must call the
1363C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV
1364obtained from external sources in functions that don't handle magic.
4a4eefd0 1365See L<perlapi> for a description of these functions.
189b2af5
GS
1366For example, calls to the C<sv_cat*()> functions typically need to be
1367followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>
1368since their implementation handles 'get' magic.
1369
d1b91892
AD
1370=head2 Finding Magic
1371
a9b0660e
KW
1372 MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
1373 * type */
f6ee7b17
FR
1374
1375This routine returns a pointer to a C<MAGIC> structure stored in the SV.
10e2eb10
FC
1376If the SV does not have that magical
1377feature, C<NULL> is returned. If the
f6ee7b17 1378SV has multiple instances of that magical feature, the first one will be
10e2eb10
FC
1379returned. C<mg_findext> can be used
1380to find a C<MAGIC> structure of an SV
da8c5729 1381based on both its magic type and its magic virtual table:
f6ee7b17
FR
1382
1383 MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
d1b91892 1384
f6ee7b17
FR
1385Also, if the SV passed to C<mg_find> or C<mg_findext> is not of type
1386SVt_PVMG, Perl may core dump.
d1b91892 1387
08105a92 1388 int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
d1b91892
AD
1389
1390This routine checks to see what types of magic C<sv> has. If the mg_type
68dc0745
PP
1391field is an uppercase letter, then the mg_obj is copied to C<nsv>, but
1392the mg_type field is changed to be the lowercase letter.
a0d0e21e 1393
04343c6d
GS
1394=head2 Understanding the Magic of Tied Hashes and Arrays
1395
14befaf4
DM
1396Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
1397magic type.
9edb2b46
GS
1398
1399WARNING: As of the 5.004 release, proper usage of the array and hash
1400access functions requires understanding a few caveats. Some
1401of these caveats are actually considered bugs in the API, to be fixed
10e2eb10 1402in later releases, and are bracketed with [MAYCHANGE] below. If
9edb2b46
GS
1403you find yourself actually applying such information in this section, be
1404aware that the behavior may change in the future, umm, without warning.
04343c6d 1405
1526ead6 1406The perl tie function associates a variable with an object that implements
9a68f1db 1407the various GET, SET, etc methods. To perform the equivalent of the perl
1526ead6 1408tie function from an XSUB, you must mimic this behaviour. The code below
61ad4b94 1409carries out the necessary steps -- firstly it creates a new hash, and then
1526ead6 1410creates a second hash which it blesses into the class which will implement
10e2eb10 1411the tie methods. Lastly it ties the two hashes together, and returns a
1526ead6
AB
1412reference to the new tied hash. Note that the code below does NOT call the
1413TIEHASH method in the MyTie class -
5a0de581 1414see L</Calling Perl Routines from within C Programs> for details on how
1526ead6
AB
1415to do this.
1416
1417 SV*
1418 mytie()
1419 PREINIT:
1420 HV *hash;
1421 HV *stash;
1422 SV *tie;
1423 CODE:
1424 hash = newHV();
1425 tie = newRV_noinc((SV*)newHV());
da51bb9b 1426 stash = gv_stashpv("MyTie", GV_ADD);
1526ead6 1427 sv_bless(tie, stash);
899e16d0 1428 hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
1526ead6
AB
1429 RETVAL = newRV_noinc(hash);
1430 OUTPUT:
1431 RETVAL
1432
04343c6d
GS
1433The C<av_store> function, when given a tied array argument, merely
1434copies the magic of the array onto the value to be "stored", using
1435C<mg_copy>. It may also return NULL, indicating that the value did not
9edb2b46
GS
1436actually need to be stored in the array. [MAYCHANGE] After a call to
1437C<av_store> on a tied array, the caller will usually need to call
1438C<mg_set(val)> to actually invoke the perl level "STORE" method on the
1439TIEARRAY object. If C<av_store> did return NULL, a call to
1440C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory
1441leak. [/MAYCHANGE]
04343c6d
GS
1442
1443The previous paragraph is applicable verbatim to tied hash access using the
1444C<hv_store> and C<hv_store_ent> functions as well.
1445
1446C<av_fetch> and the corresponding hash functions C<hv_fetch> and
1447C<hv_fetch_ent> actually return an undefined mortal value whose magic
1448has been initialized using C<mg_copy>. Note the value so returned does not
9edb2b46
GS
1449need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
1450need to call C<mg_get()> on the returned value in order to actually invoke
1451the perl level "FETCH" method on the underlying TIE object. Similarly,
04343c6d
GS
1452you may also call C<mg_set()> on the return value after possibly assigning
1453a suitable value to it using C<sv_setsv>, which will invoke the "STORE"
9edb2b46 1454method on the TIE object. [/MAYCHANGE]
04343c6d 1455
9edb2b46 1456[MAYCHANGE]
04343c6d
GS
1457In other words, the array or hash fetch/store functions don't really
1458fetch and store actual values in the case of tied arrays and hashes. They
1459merely call C<mg_copy> to attach magic to the values that were meant to be
1460"stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually
1461do the job of invoking the TIE methods on the underlying objects. Thus
9edb2b46 1462the magic mechanism currently implements a kind of lazy access to arrays
04343c6d
GS
1463and hashes.
1464
1465Currently (as of perl version 5.004), use of the hash and array access
1466functions requires the user to be aware of whether they are operating on
9edb2b46
GS
1467"normal" hashes and arrays, or on their tied variants. The API may be
1468changed to provide more transparent access to both tied and normal data
1469types in future versions.
1470[/MAYCHANGE]
04343c6d
GS
1471
1472You would do well to understand that the TIEARRAY and TIEHASH interfaces
1473are mere sugar to invoke some perl method calls while using the uniform hash
1474and array syntax. The use of this sugar imposes some overhead (typically
1475about two to four extra opcodes per FETCH/STORE operation, in addition to
1476the creation of all the mortal variables required to invoke the methods).
1477This overhead will be comparatively small if the TIE methods are themselves
1478substantial, but if they are only a few statements long, the overhead
1479will not be insignificant.
1480
d1c897a1
IZ
1481=head2 Localizing changes
1482
1483Perl has a very handy construction
1484
1485 {
1486 local $var = 2;
1487 ...
1488 }
1489
1490This construction is I<approximately> equivalent to
1491
1492 {
1493 my $oldvar = $var;
1494 $var = 2;
1495 ...
1496 $var = $oldvar;
1497 }
1498
1499The biggest difference is that the first construction would
1500reinstate the initial value of $var, irrespective of how control exits
10e2eb10 1501the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
d1c897a1
IZ
1502more efficient as well.
1503
1504There is a way to achieve a similar task from C via Perl API: create a
1505I<pseudo-block>, and arrange for some changes to be automatically
1506undone at the end of it, either explicit, or via a non-local exit (via
10e2eb10 1507die()). A I<block>-like construct is created by a pair of
b687b08b
TC
1508C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
1509Such a construct may be created specially for some important localized
1510task, or an existing one (like boundaries of enclosing Perl
1511subroutine/block, or an existing pair for freeing TMPs) may be
10e2eb10
FC
1512used. (In the second case the overhead of additional localization must
1513be almost negligible.) Note that any XSUB is automatically enclosed in
b687b08b 1514an C<ENTER>/C<LEAVE> pair.
d1c897a1
IZ
1515
1516Inside such a I<pseudo-block> the following service is available:
1517
13a2d996 1518=over 4
d1c897a1
IZ
1519
1520=item C<SAVEINT(int i)>
1521
1522=item C<SAVEIV(IV i)>
1523
1524=item C<SAVEI32(I32 i)>
1525
1526=item C<SAVELONG(long i)>
1527
1528These macros arrange things to restore the value of integer variable
1529C<i> at the end of enclosing I<pseudo-block>.
1530
1531=item C<SAVESPTR(s)>
1532
1533=item C<SAVEPPTR(p)>
1534
1535These macros arrange things to restore the value of pointers C<s> and
10e2eb10 1536C<p>. C<s> must be a pointer of a type which survives conversion to
d1c897a1
IZ
1537C<SV*> and back, C<p> should be able to survive conversion to C<char*>
1538and back.
1539
1540=item C<SAVEFREESV(SV *sv)>
1541
06f1e0b6 1542The refcount of C<sv> will be decremented at the end of
26d9b02f
JH
1543I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
1544mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
1545extends the lifetime of C<sv> until the beginning of the next statement,
1546C<SAVEFREESV> extends it until the end of the enclosing scope. These
1547lifetimes can be wildly different.
1548
1549Also compare C<SAVEMORTALIZESV>.
1550
1551=item C<SAVEMORTALIZESV(SV *sv)>
1552
1553Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
1554scope instead of decrementing its reference count. This usually has the
1555effect of keeping C<sv> alive until the statement that called the currently
1556live scope has finished executing.
d1c897a1
IZ
1557
1558=item C<SAVEFREEOP(OP *op)>
1559
1560The C<OP *> is op_free()ed at the end of I<pseudo-block>.
1561
1562=item C<SAVEFREEPV(p)>
1563
1564The chunk of memory which is pointed to by C<p> is Safefree()ed at the
1565end of I<pseudo-block>.
1566
1567=item C<SAVECLEARSV(SV *sv)>
1568
1569Clears a slot in the current scratchpad which corresponds to C<sv> at
1570the end of I<pseudo-block>.
1571
1572=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
1573
10e2eb10 1574The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
d1c897a1
IZ
1575string pointed to by C<key> is Safefree()ed. If one has a I<key> in
1576short-lived storage, the corresponding string may be reallocated like
1577this:
1578
9cde0e7f 1579 SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
d1c897a1 1580
c76ac1ee 1581=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
d1c897a1
IZ
1582
1583At the end of I<pseudo-block> the function C<f> is called with the
c76ac1ee
GS
1584only argument C<p>.
1585
1586=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
1587
1588At the end of I<pseudo-block> the function C<f> is called with the
1589implicit context argument (if any), and C<p>.
d1c897a1
IZ
1590
1591=item C<SAVESTACK_POS()>
1592
1593The current offset on the Perl internal stack (cf. C<SP>) is restored
1594at the end of I<pseudo-block>.
1595
1596=back
1597
1598The following API list contains functions, thus one needs to
1599provide pointers to the modifiable data explicitly (either C pointers,
00aadd71 1600or Perlish C<GV *>s). Where the above macros take C<int>, a similar
d1c897a1
IZ
1601function takes C<int *>.
1602
13a2d996 1603=over 4
d1c897a1
IZ
1604
1605=item C<SV* save_scalar(GV *gv)>
1606
1607Equivalent to Perl code C<local $gv>.
1608
1609=item C<AV* save_ary(GV *gv)>
1610
1611=item C<HV* save_hash(GV *gv)>
1612
1613Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
1614
1615=item C<void save_item(SV *item)>
1616
1617Duplicates the current value of C<SV>, on the exit from the current
1618C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
10e2eb10 1619using the stored value. It doesn't handle magic. Use C<save_scalar> if
038fcae3 1620magic is affected.
d1c897a1
IZ
1621
1622=item C<void save_list(SV **sarg, I32 maxsarg)>
1623
1624A variant of C<save_item> which takes multiple arguments via an array
1625C<sarg> of C<SV*> of length C<maxsarg>.
1626
1627=item C<SV* save_svref(SV **sptr)>
1628
d1be9408 1629Similar to C<save_scalar>, but will reinstate an C<SV *>.
d1c897a1
IZ
1630
1631=item C<void save_aptr(AV **aptr)>
1632
1633=item C<void save_hptr(HV **hptr)>
1634
1635Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
1636
1637=back
1638
1639The C<Alias> module implements localization of the basic types within the
1640I<caller's scope>. People who are interested in how to localize things in
1641the containing scope should take a look there too.
1642
0a753a76 1643=head1 Subroutines
a0d0e21e 1644
68dc0745 1645=head2 XSUBs and the Argument Stack
5f05dabc
PP
1646
1647The XSUB mechanism is a simple way for Perl programs to access C subroutines.
1648An XSUB routine will have a stack that contains the arguments from the Perl
1649program, and a way to map from the Perl data structures to a C equivalent.
1650
1651The stack arguments are accessible through the C<ST(n)> macro, which returns
1652the C<n>'th stack argument. Argument 0 is the first argument passed in the
1653Perl subroutine call. These arguments are C<SV*>, and can be used anywhere
1654an C<SV*> is used.
1655
1656Most of the time, output from the C routine can be handled through use of
1657the RETVAL and OUTPUT directives. However, there are some cases where the
1658argument stack is not already long enough to handle all the return values.
1659An example is the POSIX tzname() call, which takes no arguments, but returns
1660two, the local time zone's standard and summer time abbreviations.
1661
1662To handle this situation, the PPCODE directive is used and the stack is
1663extended using the macro:
1664
924508f0 1665 EXTEND(SP, num);
5f05dabc 1666
924508f0
GS
1667where C<SP> is the macro that represents the local copy of the stack pointer,
1668and C<num> is the number of elements the stack should be extended by.
5f05dabc 1669
00aadd71 1670Now that there is room on the stack, values can be pushed on it using C<PUSHs>
10e2eb10 1671macro. The pushed values will often need to be "mortal" (See
d82b684c 1672L</Reference Counts and Mortality>):
5f05dabc 1673
00aadd71 1674 PUSHs(sv_2mortal(newSViv(an_integer)))
d82b684c
SH
1675 PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
1676 PUSHs(sv_2mortal(newSVnv(a_double)))
00aadd71 1677 PUSHs(sv_2mortal(newSVpv("Some String",0)))
a9b0660e
KW
1678 /* Although the last example is better written as the more
1679 * efficient: */
a3179684 1680 PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
5f05dabc
PP
1681
1682And now the Perl program calling C<tzname>, the two values will be assigned
1683as in:
1684
1685 ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
1686
1687An alternate (and possibly simpler) method to pushing values on the stack is
00aadd71 1688to use the macro:
5f05dabc 1689
5f05dabc
PP
1690 XPUSHs(SV*)
1691
da8c5729 1692This macro automatically adjusts the stack for you, if needed. Thus, you
5f05dabc 1693do not need to call C<EXTEND> to extend the stack.
00aadd71
NIS
1694
1695Despite their suggestions in earlier versions of this document the macros
d82b684c
SH
1696C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
1697For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
1698C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
5f05dabc
PP
1699
1700For more information, consult L<perlxs> and L<perlxstut>.
1701
5b36e945
FC
1702=head2 Autoloading with XSUBs
1703
1704If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
1705fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
1706of the XSUB's package.
1707
1708But it also puts the same information in certain fields of the XSUB itself:
1709
1710 HV *stash = CvSTASH(cv);
1711 const char *subname = SvPVX(cv);
1712 STRLEN name_length = SvCUR(cv); /* in bytes */
1713 U32 is_utf8 = SvUTF8(cv);
f703fc96 1714
5b36e945 1715C<SvPVX(cv)> contains just the sub name itself, not including the package.
d8893903
FC
1716For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
1717C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
5b36e945
FC
1718
1719B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
1720XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
1721XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
1722to support 5.8-5.14, use the XSUB's fields.
1723
5f05dabc 1724=head2 Calling Perl Routines from within C Programs
a0d0e21e
LW
1725
1726There are four routines that can be used to call a Perl subroutine from
1727within a C program. These four are:
1728
954c1994
GS
1729 I32 call_sv(SV*, I32);
1730 I32 call_pv(const char*, I32);
1731 I32 call_method(const char*, I32);
5aaab254 1732 I32 call_argv(const char*, I32, char**);
a0d0e21e 1733
954c1994 1734The routine most often used is C<call_sv>. The C<SV*> argument
d1b91892
AD
1735contains either the name of the Perl subroutine to be called, or a
1736reference to the subroutine. The second argument consists of flags
1737that control the context in which the subroutine is called, whether
1738or not the subroutine is being passed arguments, how errors should be
1739trapped, and how to treat return values.
a0d0e21e
LW
1740
1741All four routines return the number of arguments that the subroutine returned
1742on the Perl stack.
1743
9a68f1db 1744These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
954c1994
GS
1745but those names are now deprecated; macros of the same name are provided for
1746compatibility.
1747
1748When using any of these routines (except C<call_argv>), the programmer
d1b91892
AD
1749must manipulate the Perl stack. These include the following macros and
1750functions:
a0d0e21e
LW
1751
1752 dSP
924508f0 1753 SP
a0d0e21e
LW
1754 PUSHMARK()
1755 PUTBACK
1756 SPAGAIN
1757 ENTER
1758 SAVETMPS
1759 FREETMPS
1760 LEAVE
1761 XPUSH*()
cb1a09d0 1762 POP*()
a0d0e21e 1763
5f05dabc
PP
1764For a detailed description of calling conventions from C to Perl,
1765consult L<perlcall>.
a0d0e21e 1766
8ebc5c01 1767=head2 Putting a C value on Perl stack
ce3d39e2
IZ
1768
1769A lot of opcodes (this is an elementary operation in the internal perl
10e2eb10
FC
1770stack machine) put an SV* on the stack. However, as an optimization
1771the corresponding SV is (usually) not recreated each time. The opcodes
ce3d39e2
IZ
1772reuse specially assigned SVs (I<target>s) which are (as a corollary)
1773not constantly freed/created.
1774
0a753a76 1775Each of the targets is created only once (but see
5a0de581 1776L</Scratchpads and recursion> below), and when an opcode needs to put
ce3d39e2
IZ
1777an integer, a double, or a string on stack, it just sets the
1778corresponding parts of its I<target> and puts the I<target> on stack.
1779
1780The macro to put this target on stack is C<PUSHTARG>, and it is
1781directly used in some opcodes, as well as indirectly in zillions of
d82b684c 1782others, which use it via C<(X)PUSH[iunp]>.
ce3d39e2 1783
1bd1c0d5 1784Because the target is reused, you must be careful when pushing multiple
10e2eb10 1785values on the stack. The following code will not do what you think:
1bd1c0d5
SC
1786
1787 XPUSHi(10);
1788 XPUSHi(20);
1789
1790This translates as "set C<TARG> to 10, push a pointer to C<TARG> onto
1791the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".
1792At the end of the operation, the stack does not contain the values 10
1793and 20, but actually contains two pointers to C<TARG>, which we have set
d82b684c 1794to 20.
1bd1c0d5 1795
d82b684c
SH
1796If you need to push multiple different values then you should either use
1797the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
1798none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
1799SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
1800will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
1801this a little easier to achieve by creating a new mortal for you (via
1802C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
1803in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
1804Thus, instead of writing this to "fix" the example above:
1805
1806 XPUSHs(sv_2mortal(newSViv(10)))
1807 XPUSHs(sv_2mortal(newSViv(20)))
1808
1809you can simply write:
1810
1811 mXPUSHi(10)
1812 mXPUSHi(20)
1813
1814On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to
1bd1c0d5 1815need a C<dTARG> in your variable declarations so that the C<*PUSH*>
d82b684c
SH
1816macros can make use of the local variable C<TARG>. See also C<dTARGET>
1817and C<dXSTARG>.
1bd1c0d5 1818
8ebc5c01 1819=head2 Scratchpads
ce3d39e2 1820
54310121 1821The question remains on when the SVs which are I<target>s for opcodes
10e2eb10 1822are created. The answer is that they are created when the current
ac036724 1823unit--a subroutine or a file (for opcodes for statements outside of
10e2eb10 1824subroutines)--is compiled. During this time a special anonymous Perl
ac036724 1825array is created, which is called a scratchpad for the current unit.
ce3d39e2 1826
54310121 1827A scratchpad keeps SVs which are lexicals for the current unit and are
d777b41a
FC
1828targets for opcodes. A previous version of this document
1829stated that one can deduce that an SV lives on a scratchpad
ce3d39e2 1830by looking on its flags: lexicals have C<SVs_PADMY> set, and
eee3e302 1831I<target>s have C<SVs_PADTMP> set. But this has never been fully true.
d777b41a
FC
1832C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
1833While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
eed77337
FC
1834that have never resided in a pad, but nonetheless act like I<target>s. As
1835of perl 5.21.5, the C<SVs_PADMY> flag is no longer used and is defined as
18360. C<SvPADMY()> now returns true for anything without C<SVs_PADTMP>.
ce3d39e2 1837
10e2eb10 1838The correspondence between OPs and I<target>s is not 1-to-1. Different
54310121 1839OPs in the compile tree of the unit can use the same target, if this
ce3d39e2
IZ
1840would not conflict with the expected life of the temporary.
1841
2ae324a7 1842=head2 Scratchpads and recursion
ce3d39e2
IZ
1843
1844In fact it is not 100% true that a compiled unit contains a pointer to
10e2eb10
FC
1845the scratchpad AV. In fact it contains a pointer to an AV of
1846(initially) one element, and this element is the scratchpad AV. Why do
ce3d39e2
IZ
1847we need an extra level of indirection?
1848
10e2eb10 1849The answer is B<recursion>, and maybe B<threads>. Both
ce3d39e2 1850these can create several execution pointers going into the same
10e2eb10 1851subroutine. For the subroutine-child not write over the temporaries
ce3d39e2
IZ
1852for the subroutine-parent (lifespan of which covers the call to the
1853child), the parent and the child should have different
10e2eb10 1854scratchpads. (I<And> the lexicals should be separate anyway!)
ce3d39e2 1855
5f05dabc
PP
1856So each subroutine is born with an array of scratchpads (of length 1).
1857On each entry to the subroutine it is checked that the current
ce3d39e2
IZ
1858depth of the recursion is not more than the length of this array, and
1859if it is, new scratchpad is created and pushed into the array.
1860
1861The I<target>s on this scratchpad are C<undef>s, but they are already
1862marked with correct flags.
1863
22d36020
FC
1864=head1 Memory Allocation
1865
1866=head2 Allocation
1867
1868All memory meant to be used with the Perl API functions should be manipulated
1869using the macros described in this section. The macros provide the necessary
1870transparency between differences in the actual malloc implementation that is
1871used within perl.
1872
1873It is suggested that you enable the version of malloc that is distributed
1874with Perl. It keeps pools of various sizes of unallocated memory in
1875order to satisfy allocation requests more quickly. However, on some
1876platforms, it may cause spurious malloc or free errors.
1877
1878The following three macros are used to initially allocate memory :
1879
1880 Newx(pointer, number, type);
1881 Newxc(pointer, number, type, cast);
1882 Newxz(pointer, number, type);
1883
1884The first argument C<pointer> should be the name of a variable that will
1885point to the newly allocated memory.
1886
1887The second and third arguments C<number> and C<type> specify how many of
1888the specified type of data structure should be allocated. The argument
1889C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
1890should be used if the C<pointer> argument is different from the C<type>
1891argument.
1892
1893Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
1894to zero out all the newly allocated memory.
1895
1896=head2 Reallocation
1897
1898 Renew(pointer, number, type);
1899 Renewc(pointer, number, type, cast);
1900 Safefree(pointer)
1901
1902These three macros are used to change a memory buffer size or to free a
1903piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
1904match those of C<New> and C<Newc> with the exception of not needing the
1905"magic cookie" argument.
1906
1907=head2 Moving
1908
1909 Move(source, dest, number, type);
1910 Copy(source, dest, number, type);
1911 Zero(dest, number, type);
1912
1913These three macros are used to move, copy, or zero out previously allocated
1914memory. The C<source> and C<dest> arguments point to the source and
1915destination starting points. Perl will move, copy, or zero out C<number>
1916instances of the size of the C<type> data structure (using the C<sizeof>
1917function).
1918
1919=head1 PerlIO
1920
1921The most recent development releases of Perl have been experimenting with
1922removing Perl's dependency on the "normal" standard I/O suite and allowing
1923other stdio implementations to be used. This involves creating a new
1924abstraction layer that then calls whichever implementation of stdio Perl
1925was compiled with. All XSUBs should now use the functions in the PerlIO
1926abstraction layer and not make any assumptions about what kind of stdio
1927is being used.
1928
1929For a complete description of the PerlIO abstraction, consult L<perlapio>.
1930
0a753a76
PP
1931=head1 Compiled code
1932
1933=head2 Code tree
1934
1935Here we describe the internal form your code is converted to by
10e2eb10 1936Perl. Start with a simple example:
0a753a76
PP
1937
1938 $a = $b + $c;
1939
1940This is converted to a tree similar to this one:
1941
1942 assign-to
1943 / \
1944 + $a
1945 / \
1946 $b $c
1947
7b8d334a 1948(but slightly more complicated). This tree reflects the way Perl
0a753a76
PP
1949parsed your code, but has nothing to do with the execution order.
1950There is an additional "thread" going through the nodes of the tree
1951which shows the order of execution of the nodes. In our simplified
1952example above it looks like:
1953
1954 $b ---> $c ---> + ---> $a ---> assign-to
1955
1956But with the actual compile tree for C<$a = $b + $c> it is different:
1957some nodes I<optimized away>. As a corollary, though the actual tree
1958contains more nodes than our simplified example, the execution order
1959is the same as in our example.
1960
1961=head2 Examining the tree
1962
06f6df17
RGS
1963If you have your perl compiled for debugging (usually done with
1964C<-DDEBUGGING> on the C<Configure> command line), you may examine the
0a753a76
PP
1965compiled tree by specifying C<-Dx> on the Perl command line. The
1966output takes several lines per node, and for C<$b+$c> it looks like
1967this:
1968
1969 5 TYPE = add ===> 6
1970 TARG = 1
1971 FLAGS = (SCALAR,KIDS)
1972 {
1973 TYPE = null ===> (4)
1974 (was rv2sv)
1975 FLAGS = (SCALAR,KIDS)
1976 {
1977 3 TYPE = gvsv ===> 4
1978 FLAGS = (SCALAR)
1979 GV = main::b
1980 }
1981 }
1982 {
1983 TYPE = null ===> (5)
1984 (was rv2sv)
1985 FLAGS = (SCALAR,KIDS)
1986 {
1987 4 TYPE = gvsv ===> 5
1988 FLAGS = (SCALAR)
1989 GV = main::c
1990 }
1991 }
1992
1993This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are
1994not optimized away (one per number in the left column). The immediate
1995children of the given node correspond to C<{}> pairs on the same level
1996of indentation, thus this listing corresponds to the tree:
1997
1998 add
1999 / \
2000 null null
2001 | |
2002 gvsv gvsv
2003
2004The execution order is indicated by C<===E<gt>> marks, thus it is C<3
20054 5 6> (node C<6> is not included into above listing), i.e.,
2006C<gvsv gvsv add whatever>.
2007
9afa14e3 2008Each of these nodes represents an op, a fundamental operation inside the
10e2eb10 2009Perl core. The code which implements each operation can be found in the
9afa14e3 2010F<pp*.c> files; the function which implements the op with type C<gvsv>
10e2eb10 2011is C<pp_gvsv>, and so on. As the tree above shows, different ops have
9afa14e3 2012different numbers of children: C<add> is a binary operator, as one would
10e2eb10 2013expect, and so has two children. To accommodate the various different
9afa14e3
SC
2014numbers of children, there are various types of op data structure, and
2015they link together in different ways.
2016
10e2eb10 2017The simplest type of op structure is C<OP>: this has no children. Unary
9afa14e3 2018operators, C<UNOP>s, have one child, and this is pointed to by the
10e2eb10
FC
2019C<op_first> field. Binary operators (C<BINOP>s) have not only an
2020C<op_first> field but also an C<op_last> field. The most complex type of
2021op is a C<LISTOP>, which has any number of children. In this case, the
9afa14e3 2022first child is pointed to by C<op_first> and the last child by
10e2eb10 2023C<op_last>. The children in between can be found by iteratively
86cd3a13 2024following the C<OpSIBLING> pointer from the first child to the last (but
29e61fd9 2025see below).
9afa14e3 2026
29e61fd9 2027There are also some other op types: a C<PMOP> holds a regular expression,
10e2eb10
FC
2028and has no children, and a C<LOOP> may or may not have children. If the
2029C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
9afa14e3
SC
2030complicate matters, if a C<UNOP> is actually a C<null> op after
2031optimization (see L</Compile pass 2: context propagation>) it will still
2032have children in accordance with its former type.
2033
29e61fd9
DM
2034Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
2035or more children, but it doesn't have an C<op_last> field: so you have to
86cd3a13 2036follow C<op_first> and then the C<OpSIBLING> chain itself to find the
29e61fd9
DM
2037last child. Instead it has an C<op_other> field, which is comparable to
2038the C<op_next> field described below, and represents an alternate
2039execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
2040that in general, C<op_other> may not point to any of the direct children
2041of the C<LOGOP>.
2042
2043Starting in version 5.21.2, perls built with the experimental
2044define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
87b5a8b9 2045C<op_moresib>. When not set, this indicates that this is the last op in an
86cd3a13
DM
2046C<OpSIBLING> chain. This frees up the C<op_sibling> field on the last
2047sibling to point back to the parent op. Under this build, that field is
2048also renamed C<op_sibparent> to reflect its joint role. The macro
2049C<OpSIBLING(o)> wraps this special behaviour, and always returns NULL on
2050the last sibling. With this build the C<op_parent(o)> function can be
2051used to find the parent of any op. Thus for forward compatibility, you
2052should always use the C<OpSIBLING(o)> macro rather than accessing
2053C<op_sibling> directly.
29e61fd9 2054
06f6df17
RGS
2055Another way to examine the tree is to use a compiler back-end module, such
2056as L<B::Concise>.
2057
0a753a76
PP
2058=head2 Compile pass 1: check routines
2059
8870b5c7 2060The tree is created by the compiler while I<yacc> code feeds it
10e2eb10 2061the constructions it recognizes. Since I<yacc> works bottom-up, so does
0a753a76
PP
2062the first pass of perl compilation.
2063
2064What makes this pass interesting for perl developers is that some
2065optimization may be performed on this pass. This is optimization by
8870b5c7 2066so-called "check routines". The correspondence between node names
0a753a76
PP
2067and corresponding check routines is described in F<opcode.pl> (do not
2068forget to run C<make regen_headers> if you modify this file).
2069
2070A check routine is called when the node is fully constructed except
7b8d334a 2071for the execution-order thread. Since at this time there are no
0a753a76
PP
2072back-links to the currently constructed node, one can do most any
2073operation to the top-level node, including freeing it and/or creating
2074new nodes above/below it.
2075
2076The check routine returns the node which should be inserted into the
2077tree (if the top-level node was not modified, check routine returns
2078its argument).
2079
10e2eb10 2080By convention, check routines have names C<ck_*>. They are usually
0a753a76
PP
2081called from C<new*OP> subroutines (or C<convert>) (which in turn are
2082called from F<perly.y>).
2083
2084=head2 Compile pass 1a: constant folding
2085
2086Immediately after the check routine is called the returned node is
2087checked for being compile-time executable. If it is (the value is
2088judged to be constant) it is immediately executed, and a I<constant>
2089node with the "return value" of the corresponding subtree is
2090substituted instead. The subtree is deleted.
2091
2092If constant folding was not performed, the execution-order thread is
2093created.
2094
2095=head2 Compile pass 2: context propagation
2096
2097When a context for a part of compile tree is known, it is propagated
a3cb178b 2098down through the tree. At this time the context can have 5 values
0a753a76
PP
2099(instead of 2 for runtime context): void, boolean, scalar, list, and
2100lvalue. In contrast with the pass 1 this pass is processed from top
2101to bottom: a node's context determines the context for its children.
2102
2103Additional context-dependent optimizations are performed at this time.
2104Since at this moment the compile tree contains back-references (via
2105"thread" pointers), nodes cannot be free()d now. To allow
2106optimized-away nodes at this stage, such nodes are null()ified instead
2107of free()ing (i.e. their type is changed to OP_NULL).
2108
2109=head2 Compile pass 3: peephole optimization
2110
2111After the compile tree for a subroutine (or for an C<eval> or a file)
10e2eb10 2112is created, an additional pass over the code is performed. This pass
0a753a76 2113is neither top-down or bottom-up, but in the execution order (with
9ea12537
Z
2114additional complications for conditionals). Optimizations performed
2115at this stage are subject to the same restrictions as in the pass 2.
2116
2117Peephole optimizations are done by calling the function pointed to
2118by the global variable C<PL_peepp>. By default, C<PL_peepp> just
2119calls the function pointed to by the global variable C<PL_rpeepp>.
2120By default, that performs some basic op fixups and optimisations along
2121the execution-order op chain, and recursively calls C<PL_rpeepp> for
2122each side chain of ops (resulting from conditionals). Extensions may
2123provide additional optimisations or fixups, hooking into either the
2124per-subroutine or recursive stage, like this:
2125
2126 static peep_t prev_peepp;
2127 static void my_peep(pTHX_ OP *o)
2128 {
2129 /* custom per-subroutine optimisation goes here */
f0358462 2130 prev_peepp(aTHX_ o);
9ea12537
Z
2131 /* custom per-subroutine optimisation may also go here */
2132 }
2133 BOOT:
2134 prev_peepp = PL_peepp;
2135 PL_peepp = my_peep;
2136
2137 static peep_t prev_rpeepp;
2138 static void my_rpeep(pTHX_ OP *o)
2139 {
2140 OP *orig_o = o;
2141 for(; o; o = o->op_next) {
2142 /* custom per-op optimisation goes here */
2143 }
f0358462 2144 prev_rpeepp(aTHX_ orig_o);
9ea12537
Z
2145 }
2146 BOOT:
2147 prev_rpeepp = PL_rpeepp;
2148 PL_rpeepp = my_rpeep;
0a753a76 2149
1ba7f851
PJ
2150=head2 Pluggable runops
2151
2152The compile tree is executed in a runops function. There are two runops
1388f78e
RGS
2153functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
2154with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
2155control over the execution of the compile tree it is possible to provide
2156your own runops function.
1ba7f851
PJ
2157
2158It's probably best to copy one of the existing runops functions and
2159change it to suit your needs. Then, in the BOOT section of your XS
2160file, add the line:
2161
2162 PL_runops = my_runops;
2163
2164This function should be as efficient as possible to keep your programs
2165running as fast as possible.
2166
fd85fad2
BM
2167=head2 Compile-time scope hooks
2168
2169As of perl 5.14 it is possible to hook into the compile-time lexical
10e2eb10 2170scope mechanism using C<Perl_blockhook_register>. This is used like
fd85fad2
BM
2171this:
2172
2173 STATIC void my_start_hook(pTHX_ int full);
2174 STATIC BHK my_hooks;
2175
2176 BOOT:
a88d97bf 2177 BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
fd85fad2
BM
2178 Perl_blockhook_register(aTHX_ &my_hooks);
2179
2180This will arrange to have C<my_start_hook> called at the start of
10e2eb10 2181compiling every lexical scope. The available hooks are:
fd85fad2
BM
2182
2183=over 4
2184
a88d97bf 2185=item C<void bhk_start(pTHX_ int full)>
fd85fad2 2186
10e2eb10 2187This is called just after starting a new lexical scope. Note that Perl
fd85fad2
BM
2188code like
2189
2190 if ($x) { ... }
2191
2192creates two scopes: the first starts at the C<(> and has C<full == 1>,
10e2eb10 2193the second starts at the C<{> and has C<full == 0>. Both end at the
f185f654 2194C<}>, so calls to C<start> and C<pre>/C<post_end> will match. Anything
fd85fad2
BM
2195pushed onto the save stack by this hook will be popped just before the
2196scope ends (between the C<pre_> and C<post_end> hooks, in fact).
2197
a88d97bf 2198=item C<void bhk_pre_end(pTHX_ OP **o)>
fd85fad2
BM
2199
2200This is called at the end of a lexical scope, just before unwinding the
10e2eb10 2201stack. I<o> is the root of the optree representing the scope; it is a
fd85fad2
BM
2202double pointer so you can replace the OP if you need to.
2203
a88d97bf 2204=item C<void bhk_post_end(pTHX_ OP **o)>
fd85fad2
BM
2205
2206This is called at the end of a lexical scope, just after unwinding the
10e2eb10 2207stack. I<o> is as above. Note that it is possible for calls to C<pre_>
fd85fad2
BM
2208and C<post_end> to nest, if there is something on the save stack that
2209calls string eval.
2210
a88d97bf 2211=item C<void bhk_eval(pTHX_ OP *const o)>
fd85fad2
BM
2212
2213This is called just before starting to compile an C<eval STRING>, C<do
10e2eb10 2214FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
fd85fad2
BM
2215OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
2216C<OP_DOFILE> or C<OP_REQUIRE>.
2217
2218=back
2219
2220Once you have your hook functions, you need a C<BHK> structure to put
10e2eb10
FC
2221them in. It's best to allocate it statically, since there is no way to
2222free it once it's registered. The function pointers should be inserted
fd85fad2 2223into this structure using the C<BhkENTRY_set> macro, which will also set
10e2eb10 2224flags indicating which entries are valid. If you do need to allocate
fd85fad2
BM
2225your C<BHK> dynamically for some reason, be sure to zero it before you
2226start.
2227
2228Once registered, there is no mechanism to switch these hooks off, so if
10e2eb10 2229that is necessary you will need to do this yourself. An entry in C<%^H>
a3e07c87
BM
2230is probably the best way, so the effect is lexically scoped; however it
2231is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
10e2eb10 2232temporarily switch entries on and off. You should also be aware that
a3e07c87 2233generally speaking at least one scope will have opened before your
f185f654 2234extension is loaded, so you will see some C<pre>/C<post_end> pairs that
a3e07c87 2235didn't have a matching C<start>.
fd85fad2 2236
9afa14e3
SC
2237=head1 Examining internal data structures with the C<dump> functions
2238
2239To aid debugging, the source file F<dump.c> contains a number of
2240functions which produce formatted output of internal data structures.
2241
2242The most commonly used of these functions is C<Perl_sv_dump>; it's used
10e2eb10 2243for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
9afa14e3 2244C<sv_dump> to produce debugging output from Perl-space, so users of that
00aadd71 2245module should already be familiar with its format.
9afa14e3
SC
2246
2247C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
210b36aa 2248derivatives, and produces output similar to C<perl -Dx>; in fact,
9afa14e3
SC
2249C<Perl_dump_eval> will dump the main root of the code being evaluated,
2250exactly like C<-Dx>.
2251
2252Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
2253op tree, C<Perl_dump_packsubs> which calls C<Perl_dump_sub> on all the
2254subroutines in a package like so: (Thankfully, these are all xsubs, so
2255there is no op tree)
2256
2257 (gdb) print Perl_dump_packsubs(PL_defstash)
2258
2259 SUB attributes::bootstrap = (xsub 0x811fedc 0)
2260
2261 SUB UNIVERSAL::can = (xsub 0x811f50c 0)
2262
2263 SUB UNIVERSAL::isa = (xsub 0x811f304 0)
2264
2265 SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
2266
2267 SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
2268
2269and C<Perl_dump_all>, which dumps all the subroutines in the stash and
2270the op tree of the main root.
2271
954c1994 2272=head1 How multiple interpreters and concurrency are supported
ee072b34 2273
ee072b34
GS
2274=head2 Background and PERL_IMPLICIT_CONTEXT
2275
2276The Perl interpreter can be regarded as a closed box: it has an API
2277for feeding it code or otherwise making it do things, but it also has
2278functions for its own use. This smells a lot like an object, and
2279there are ways for you to build Perl so that you can have multiple
acfe0abc
GS
2280interpreters, with one interpreter represented either as a C structure,
2281or inside a thread-specific structure. These structures contain all
2282the context, the state of that interpreter.
2283
10e2eb10 2284One macro controls the major Perl build flavor: MULTIPLICITY. The
7b52221d 2285MULTIPLICITY build has a C structure that packages all the interpreter
10e2eb10 2286state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
7b52221d 2287normally defined, and enables the support for passing in a "hidden" first
10e2eb10 2288argument that represents all three data structures. MULTIPLICITY makes
1a64a5e6 2289multi-threaded perls possible (with the ithreads threading model, related
7b52221d 2290to the macro USE_ITHREADS.)
54aff467 2291
27da23d5
JH
2292Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
2293PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
2294former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
2295internal variables of Perl to be wrapped inside a single global struct,
2296struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
2297the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
2298one step further, there is still a single struct (allocated in main()
2299either from heap or from stack) but there are no global data symbols
3bf17896 2300pointing to it. In either case the global struct should be initialized
27da23d5
JH
2301as the very first thing in main() using Perl_init_global_struct() and
2302correspondingly tear it down after perl_free() using Perl_free_global_struct(),
2303please see F<miniperlmain.c> for usage details. You may also need
2304to use C<dVAR> in your coding to "declare the global variables"
2305when you are using them. dTHX does this for you automatically.
2306
9aa97215
JH
2307To see whether you have non-const data you can use a BSD (or GNU)
2308compatible C<nm>:
bc028b6b
JH
2309
2310 nm libperl.a | grep -v ' [TURtr] '
2311
9aa97215
JH
2312If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
2313you have non-const data. The symbols the C<grep> removed are as follows:
2314C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
2315and the C<U> is <undefined>, external symbols referred to.
2316
2317The test F<t/porting/libperl.t> does this kind of symbol sanity
2318checking on C<libperl.a>.
bc028b6b 2319
27da23d5
JH
2320For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
2321doesn't actually hide all symbols inside a big global struct: some
2322PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
2323then hides everything (see how the PERLIO_FUNCS_DECL is used).
2324
54aff467 2325All this obviously requires a way for the Perl internal functions to be
acfe0abc 2326either subroutines taking some kind of structure as the first
ee072b34 2327argument, or subroutines taking nothing as the first argument. To
acfe0abc 2328enable these two very different ways of building the interpreter,
ee072b34
GS
2329the Perl source (as it does in so many other situations) makes heavy
2330use of macros and subroutine naming conventions.
2331
54aff467 2332First problem: deciding which functions will be public API functions and
00aadd71 2333which will be private. All functions whose names begin C<S_> are private
954c1994
GS
2334(think "S" for "secret" or "static"). All other functions begin with
2335"Perl_", but just because a function begins with "Perl_" does not mean it is
10e2eb10
FC
2336part of the API. (See L</Internal
2337Functions>.) The easiest way to be B<sure> a
00aadd71
NIS
2338function is part of the API is to find its entry in L<perlapi>.
2339If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
2340think it should be (i.e., you need it for your extension), send mail via
a422fd2d 2341L<perlbug> explaining why you think it should be.
ee072b34
GS
2342
2343Second problem: there must be a syntax so that the same subroutine
2344declarations and calls can pass a structure as their first argument,
2345or pass nothing. To solve this, the subroutines are named and
2346declared in a particular way. Here's a typical start of a static
2347function used within the Perl guts:
2348
2349 STATIC void
2350 S_incline(pTHX_ char *s)
2351
acfe0abc 2352STATIC becomes "static" in C, and may be #define'd to nothing in some
da8c5729 2353configurations in the future.
ee072b34 2354
651a3225
GS
2355A public function (i.e. part of the internal API, but not necessarily
2356sanctioned for use in extensions) begins like this:
ee072b34
GS
2357
2358 void
2307c6d0 2359 Perl_sv_setiv(pTHX_ SV* dsv, IV num)
ee072b34 2360
0147cd53 2361C<pTHX_> is one of a number of macros (in F<perl.h>) that hide the
ee072b34
GS
2362details of the interpreter's context. THX stands for "thread", "this",
2363or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
2364The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
a7486cbb
JH
2365or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
2366their variants.
ee072b34 2367
a7486cbb
JH
2368When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
2369first argument containing the interpreter's context. The trailing underscore
ee072b34
GS
2370in the pTHX_ macro indicates that the macro expansion needs a comma
2371after the context argument because other arguments follow it. If
2372PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
54aff467
GS
2373subroutine is not prototyped to take the extra argument. The form of the
2374macro without the trailing underscore is used when there are no additional
ee072b34
GS
2375explicit arguments.
2376
54aff467 2377When a core function calls another, it must pass the context. This
2307c6d0 2378is normally hidden via macros. Consider C<sv_setiv>. It expands into
ee072b34
GS
2379something like this:
2380
2307c6d0
SB
2381 #ifdef PERL_IMPLICIT_CONTEXT
2382 #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
ee072b34 2383 /* can't do this for vararg functions, see below */
2307c6d0
SB
2384 #else
2385 #define sv_setiv Perl_sv_setiv
2386 #endif
ee072b34
GS
2387
2388This works well, and means that XS authors can gleefully write:
2389
2307c6d0 2390 sv_setiv(foo, bar);
ee072b34
GS
2391
2392and still have it work under all the modes Perl could have been
2393compiled with.
2394
ee072b34
GS
2395This doesn't work so cleanly for varargs functions, though, as macros
2396imply that the number of arguments is known in advance. Instead we
2397either need to spell them out fully, passing C<aTHX_> as the first
2398argument (the Perl core tends to do this with functions like
2399Perl_warner), or use a context-free version.
2400
2401The context-free version of Perl_warner is called
2402Perl_warner_nocontext, and does not take the extra argument. Instead
2403it does dTHX; to get the context from thread-local storage. We
2404C<#define warner Perl_warner_nocontext> so that extensions get source
2405compatibility at the expense of performance. (Passing an arg is
2406cheaper than grabbing it from thread-local storage.)
2407
acfe0abc 2408You can ignore [pad]THXx when browsing the Perl headers/sources.
ee072b34
GS
2409Those are strictly for use within the core. Extensions and embedders
2410need only be aware of [pad]THX.
2411
a7486cbb
JH
2412=head2 So what happened to dTHR?
2413
2414C<dTHR> was introduced in perl 5.005 to support the older thread model.
2415The older thread model now uses the C<THX> mechanism to pass context
2416pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
2417later still have it for backward source compatibility, but it is defined
2418to be a no-op.
2419
ee072b34
GS
2420=head2 How do I use all this in extensions?
2421
2422When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
2423any functions in the Perl API will need to pass the initial context
2424argument somehow. The kicker is that you will need to write it in
2425such a way that the extension still compiles when Perl hasn't been
2426built with PERL_IMPLICIT_CONTEXT enabled.
2427
2428There are three ways to do this. First, the easy but inefficient way,
2429which is also the default, in order to maintain source compatibility
0147cd53 2430with extensions: whenever F<XSUB.h> is #included, it redefines the aTHX
ee072b34
GS
2431and aTHX_ macros to call a function that will return the context.
2432Thus, something like:
2433
2307c6d0 2434 sv_setiv(sv, num);
ee072b34 2435
4375e838 2436in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
54aff467 2437in effect:
ee072b34 2438
2307c6d0 2439 Perl_sv_setiv(Perl_get_context(), sv, num);
ee072b34 2440
54aff467 2441or to this otherwise:
ee072b34 2442
2307c6d0 2443 Perl_sv_setiv(sv, num);
ee072b34 2444
da8c5729 2445You don't have to do anything new in your extension to get this; since
2fa86c13 2446the Perl library provides Perl_get_context(), it will all just
ee072b34
GS
2447work.
2448
2449The second, more efficient way is to use the following template for
2450your Foo.xs:
2451
c52f9dcd
JH
2452 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2453 #include "EXTERN.h"
2454 #include "perl.h"
2455 #include "XSUB.h"
ee072b34 2456
fd061412 2457 STATIC void my_private_function(int arg1, int arg2);
ee072b34 2458
fd061412 2459 STATIC void
c52f9dcd
JH
2460 my_private_function(int arg1, int arg2)
2461 {
2462 dTHX; /* fetch context */
2463 ... call many Perl API functions ...
2464 }
ee072b34
GS
2465
2466 [... etc ...]
2467
c52f9dcd 2468 MODULE = Foo PACKAGE = Foo
ee072b34 2469
c52f9dcd 2470 /* typical XSUB */
ee072b34 2471
c52f9dcd
JH
2472 void
2473 my_xsub(arg)
2474 int arg
2475 CODE:
2476 my_private_function(arg, 10);
ee072b34
GS
2477
2478Note that the only two changes from the normal way of writing an
2479extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before
2480including the Perl headers, followed by a C<dTHX;> declaration at
2481the start of every function that will call the Perl API. (You'll
2482know which functions need this, because the C compiler will complain
2483that there's an undeclared identifier in those functions.) No changes
2484are needed for the XSUBs themselves, because the XS() macro is
2485correctly defined to pass in the implicit context if needed.
2486
2487The third, even more efficient way is to ape how it is done within
2488the Perl guts:
2489
2490
c52f9dcd
JH
2491 #define PERL_NO_GET_CONTEXT /* we want efficiency */
2492 #include "EXTERN.h"
2493 #include "perl.h"
2494 #include "XSUB.h"
ee072b34
GS
2495
2496 /* pTHX_ only needed for functions that call Perl API */
fd061412 2497 STATIC void my_private_function(pTHX_ int arg1, int arg2);
ee072b34 2498
fd061412 2499 STATIC void
c52f9dcd
JH
2500 my_private_function(pTHX_ int arg1, int arg2)
2501 {
2502 /* dTHX; not needed here, because THX is an argument */
2503 ... call Perl API functions ...
2504 }
ee072b34
GS
2505
2506 [... etc ...]
2507
c52f9dcd 2508 MODULE = Foo PACKAGE = Foo
ee072b34 2509
c52f9dcd 2510 /* typical XSUB */
ee072b34 2511
c52f9dcd
JH
2512 void
2513 my_xsub(arg)
2514 int arg
2515 CODE:
2516 my_private_function(aTHX_ arg, 10);
ee072b34
GS
2517
2518This implementation never has to fetch the context using a function
2519call, since it is always passed as an extra argument. Depending on
2520your needs for simplicity or efficiency, you may mix the previous
2521two approaches freely.
2522
651a3225
GS
2523Never add a comma after C<pTHX> yourself--always use the form of the
2524macro with the underscore for functions that take explicit arguments,
2525or the form without the argument for functions with no explicit arguments.
ee072b34 2526
27da23d5
JH
2527If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
2528definition is needed if the Perl global variables (see F<perlvars.h>
2529or F<globvar.sym>) are accessed in the function and C<dTHX> is not
2530used (the C<dTHX> includes the C<dVAR> if necessary). One notices
2531the need for C<dVAR> only with the said compile-time define, because
2532otherwise the Perl global variables are visible as-is.
2533
a7486cbb
JH
2534=head2 Should I do anything special if I call perl from multiple threads?
2535
2536If you create interpreters in one thread and then proceed to call them in
2537another, you need to make sure perl's own Thread Local Storage (TLS) slot is
2538initialized correctly in each of those threads.
2539
2540The C<perl_alloc> and C<perl_clone> API functions will automatically set
2541the TLS slot to the interpreter they created, so that there is no need to do
2542anything special if the interpreter is always accessed in the same thread that
2543created it, and that thread did not create or call any other interpreters
2544afterwards. If that is not the case, you have to set the TLS slot of the
2545thread before calling any functions in the Perl API on that particular
2546interpreter. This is done by calling the C<PERL_SET_CONTEXT> macro in that
2547thread as the first thing you do:
2548
2549 /* do this before doing anything else with some_perl */
2550 PERL_SET_CONTEXT(some_perl);
2551
2552 ... other Perl API calls on some_perl go here ...
2553
ee072b34
GS
2554=head2 Future Plans and PERL_IMPLICIT_SYS
2555
2556Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
2557that the interpreter knows about itself and pass it around, so too are
2558there plans to allow the interpreter to bundle up everything it knows
2559about the environment it's running on. This is enabled with the
7b52221d
RGS
2560PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
2561Windows.
ee072b34
GS
2562
2563This allows the ability to provide an extra pointer (called the "host"
2564environment) for all the system calls. This makes it possible for
2565all the system stuff to maintain their own state, broken down into
2566seven C structures. These are thin wrappers around the usual system
0147cd53 2567calls (see F<win32/perllib.c>) for the default perl executable, but for a
ee072b34
GS
2568more ambitious host (like the one that would do fork() emulation) all
2569the extra work needed to pretend that different interpreters are
2570actually different "processes", would be done here.
2571
2572The Perl engine/interpreter and the host are orthogonal entities.
2573There could be one or more interpreters in a process, and one or
2574more "hosts", with free association between them.
2575
a422fd2d
SC
2576=head1 Internal Functions
2577
2578All of Perl's internal functions which will be exposed to the outside
06f6df17 2579world are prefixed by C<Perl_> so that they will not conflict with XS
a422fd2d 2580functions or functions used in a program in which Perl is embedded.
10e2eb10 2581Similarly, all global variables begin with C<PL_>. (By convention,
06f6df17 2582static functions start with C<S_>.)
a422fd2d 2583
0972ecdf
DM
2584Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
2585either with or without the C<Perl_> prefix, thanks to a bunch of defines
10e2eb10 2586that live in F<embed.h>. Note that extension code should I<not> set
0972ecdf
DM
2587C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
2588breakage of the XS in each new perl release.
2589
2590The file F<embed.h> is generated automatically from
10e2eb10 2591F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
dc9b1d22 2592header files for the internal functions, generates the documentation
10e2eb10 2593and a lot of other bits and pieces. It's important that when you add
dc9b1d22 2594a new function to the core or change an existing one, you change the
10e2eb10 2595data in the table in F<embed.fnc> as well. Here's a sample entry from
dc9b1d22 2596that table:
a422fd2d
SC
2597
2598 Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
2599
10e2eb10
FC
2600The second column is the return type, the third column the name. Columns
2601after that are the arguments. The first column is a set of flags:
a422fd2d
SC
2602
2603=over 3
2604
2605=item A
2606
10e2eb10
FC
2607This function is a part of the public
2608API. All such functions should also
1aa6ea50 2609have 'd', very few do not.
a422fd2d
SC
2610
2611=item p
2612
1aa6ea50
JC
2613This function has a C<Perl_> prefix; i.e. it is defined as
2614C<Perl_av_fetch>.
a422fd2d
SC
2615
2616=item d
2617
2618This function has documentation using the C<apidoc> feature which we'll
1aa6ea50 2619look at in a second. Some functions have 'd' but not 'A'; docs are good.
a422fd2d
SC
2620
2621=back
2622
2623Other available flags are:
2624
2625=over 3
2626
2627=item s
2628
1aa6ea50
JC
2629This is a static function and is defined as C<STATIC S_whatever>, and
2630usually called within the sources as C<whatever(...)>.
a422fd2d
SC
2631
2632=item n
2633
da8c5729 2634This does not need an interpreter context, so the definition has no
1aa6ea50 2635C<pTHX>, and it follows that callers don't use C<aTHX>. (See
d3a43cd8 2636L</Background and PERL_IMPLICIT_CONTEXT>.)
a422fd2d
SC
2637
2638=item r
2639
2640This function never returns; C<croak>, C<exit> and friends.
2641
2642=item f
2643
2644This function takes a variable number of arguments, C<printf> style.
2645The argument list should end with C<...>, like this:
2646
2647 Afprd |void |croak |const char* pat|...
2648
a7486cbb 2649=item M
a422fd2d 2650
00aadd71 2651This function is part of the experimental development API, and may change
a422fd2d
SC
2652or disappear without notice.
2653
2654=item o
2655
2656This function should not have a compatibility macro to define, say,
10e2eb10 2657C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
a422fd2d 2658
a422fd2d
SC
2659=item x
2660
2661This function isn't exported out of the Perl core.
2662
dc9b1d22
MHM
2663=item m
2664
2665This is implemented as a macro.
2666
2667=item X
2668
2669This function is explicitly exported.
2670
2671=item E
2672
2673This function is visible to extensions included in the Perl core.
2674
2675=item b
2676
2677Binary backward compatibility; this function is a macro but also has
2678a C<Perl_> implementation (which is exported).
2679
1aa6ea50
JC
2680=item others
2681
2682See the comments at the top of C<embed.fnc> for others.
2683
a422fd2d
SC
2684=back
2685
dc9b1d22
MHM
2686If you edit F<embed.pl> or F<embed.fnc>, you will need to run
2687C<make regen_headers> to force a rebuild of F<embed.h> and other
2688auto-generated files.
a422fd2d 2689
6b4667fc 2690=head2 Formatted Printing of IVs, UVs, and NVs
9dd9db0b 2691
6b4667fc
A
2692If you are printing IVs, UVs, or NVS instead of the stdio(3) style
2693formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
2694following macros for portability
9dd9db0b 2695
c52f9dcd
JH
2696 IVdf IV in decimal
2697 UVuf UV in decimal
2698 UVof UV in octal
2699 UVxf UV in hexadecimal
2700 NVef NV %e-like
2701 NVff NV %f-like
2702 NVgf NV %g-like
9dd9db0b 2703
6b4667fc
A
2704These will take care of 64-bit integers and long doubles.
2705For example:
2706
c52f9dcd 2707 printf("IV is %"IVdf"\n", iv);
6b4667fc
A
2708
2709The IVdf will expand to whatever is the correct format for the IVs.
9dd9db0b 2710
aacf4ea2
JH
2711Note that there are different "long doubles": Perl will use
2712whatever the compiler has.
2713
8908e76d
JH
2714If you are printing addresses of pointers, use UVxf combined
2715with PTR2UV(), do not use %lx or %p.
2716
e613617c 2717=head2 Formatted Printing of C<Size_t> and C<SSize_t>
5862f74e
KW
2718
2719The most general way to do this is to cast them to a UV or IV, and
2720print as in the
2721L<previous section|/Formatted Printing of IVs, UVs, and NVs>.
2722
2723But if you're using C<PerlIO_printf()>, it's less typing and visual
2724clutter to use the C<"%z"> length modifier (for I<siZe>):
2725
2726 PerlIO_printf("STRLEN is %zu\n", len);
2727
2728This modifier is not portable, so its use should be restricted to
2729C<PerlIO_printf()>.
2730
8908e76d
JH
2731=head2 Pointer-To-Integer and Integer-To-Pointer
2732
2733Because pointer size does not necessarily equal integer size,
2734use the follow macros to do it right.
2735
c52f9dcd
JH
2736 PTR2UV(pointer)
2737 PTR2IV(pointer)
2738 PTR2NV(pointer)
2739 INT2PTR(pointertotype, integer)
8908e76d
JH
2740
2741For example:
2742
c52f9dcd
JH
2743 IV iv = ...;
2744 SV *sv = INT2PTR(SV*, iv);
8908e76d
JH
2745
2746and
2747
c52f9dcd
JH
2748 AV *av = ...;
2749 UV uv = PTR2UV(av);
8908e76d 2750
0ca3a874
MHM
2751=head2 Exception Handling
2752
9b5c3821 2753There are a couple of macros to do very basic exception handling in XS
10e2eb10 2754modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
9b5c3821
MHM
2755be able to use these macros:
2756
2757 #define NO_XSLOCKS
2758 #include "XSUB.h"
2759
2760You can use these macros if you call code that may croak, but you need
10e2eb10 2761to do some cleanup before giving control back to Perl. For example:
0ca3a874 2762
d7f8936a 2763 dXCPT; /* set up necessary variables */
0ca3a874
MHM
2764
2765 XCPT_TRY_START {
2766 code_that_may_croak();
2767 } XCPT_TRY_END
2768
2769 XCPT_CATCH
2770 {
2771 /* do cleanup here */
2772 XCPT_RETHROW;
2773 }
2774
2775Note that you always have to rethrow an exception that has been
10e2eb10
FC
2776caught. Using these macros, it is not possible to just catch the
2777exception and ignore it. If you have to ignore the exception, you
0ca3a874
MHM
2778have to use the C<call_*> function.
2779
2780The advantage of using the above macros is that you don't have
2781to setup an extra function for C<call_*>, and that using these
2782macros is faster than using C<call_*>.
2783
a422fd2d
SC
2784=head2 Source Documentation
2785
2786There's an effort going on to document the internal functions and
61ad4b94 2787automatically produce reference manuals from them -- L<perlapi> is one
a422fd2d 2788such manual which details all the functions which are available to XS
10e2eb10 2789writers. L<perlintern> is the autogenerated manual for the functions
a422fd2d
SC
2790which are not part of the API and are supposedly for internal use only.
2791
2792Source documentation is created by putting POD comments into the C
2793source, like this:
2794
2795 /*
2796 =for apidoc sv_setiv
2797
2798 Copies an integer into the given SV. Does not handle 'set' magic. See
a95b3d6a 2799 L<perlapi/sv_setiv_mg>.
a422fd2d
SC
2800
2801 =cut
2802 */
2803
2804Please try and supply some documentation if you add functions to the
2805Perl core.
2806
0d098d33
MHM
2807=head2 Backwards compatibility
2808
10e2eb10
FC
2809The Perl API changes over time. New functions are
2810added or the interfaces of existing functions are
2811changed. The C<Devel::PPPort> module tries to
0d098d33
MHM
2812provide compatibility code for some of these changes, so XS writers don't
2813have to code it themselves when supporting multiple versions of Perl.
2814
2815C<Devel::PPPort> generates a C header file F<ppport.h> that can also
10e2eb10 2816be run as a Perl script. To generate F<ppport.h>, run:
0d098d33
MHM
2817
2818 perl -MDevel::PPPort -eDevel::PPPort::WriteFile
2819
2820Besides checking existing XS code, the script can also be used to retrieve
2821compatibility information for various API calls using the C<--api-info>
10e2eb10 2822command line switch. For example:
0d098d33
MHM
2823
2824 % perl ppport.h --api-info=sv_magicext
2825
2826For details, see C<perldoc ppport.h>.
2827
a422fd2d
SC
2828=head1 Unicode Support
2829
10e2eb10 2830Perl 5.6.0 introduced Unicode support. It's important for porters and XS
a422fd2d
SC
2831writers to understand this support and make sure that the code they
2832write does not corrupt Unicode data.
2833
2834=head2 What B<is> Unicode, anyway?
2835
10e2eb10
FC
2836In the olden, less enlightened times, we all used to use ASCII. Most of
2837us did, anyway. The big problem with ASCII is that it's American. Well,
a422fd2d 2838no, that's not actually the problem; the problem is that it's not
10e2eb10 2839particularly useful for people who don't use the Roman alphabet. What
a422fd2d 2840used to happen was that particular languages would stick their own
10e2eb10 2841alphabet in the upper range of the sequence, between 128 and 255. Of
a422fd2d
SC
2842course, we then ended up with plenty of variants that weren't quite
2843ASCII, and the whole point of it being a standard was lost.
2844
2845Worse still, if you've got a language like Chinese or
2846Japanese that has hundreds or thousands of characters, then you really
2847can't fit them into a mere 256, so they had to forget about ASCII
2848altogether, and build their own systems using pairs of numbers to refer
2849to one character.
2850
2851To fix this, some people formed Unicode, Inc. and
2852produced a new character set containing all the characters you can
10e2eb10
FC
2853possibly think of and more. There are several ways of representing these
2854characters, and the one Perl uses is called UTF-8. UTF-8 uses
2855a variable number of bytes to represent a character. You can learn more
2575c402 2856about Unicode and Perl's Unicode model in L<perlunicode>.
a422fd2d 2857
3ad86f0e
KW
2858(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
2859UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
2860UTF-EBCDIC is like UTF-8, but the details are different. The macros
2861hide the differences from you, just remember that the particular numbers
2862and bit patterns presented below will differ in UTF-EBCDIC.)
2863
1e54db1a 2864=head2 How can I recognise a UTF-8 string?
a422fd2d 2865
10e2eb10
FC
2866You can't. This is because UTF-8 data is stored in bytes just like
2867non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
a422fd2d 2868capital E with a grave accent, is represented by the two bytes
10e2eb10 2869C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
61ad4b94 2870has that byte sequence as well. So you can't tell just by looking -- this
a422fd2d
SC
2871is what makes Unicode input an interesting problem.
2872
2575c402
JW
2873In general, you either have to know what you're dealing with, or you
2874have to guess. The API function C<is_utf8_string> can help; it'll tell
61ad4b94
KW
2875you if a string contains only valid UTF-8 characters, and the chances
2876of a non-UTF-8 string looking like valid UTF-8 become very small very
2877quickly with increasing string length. On a character-by-character
2878basis, C<isUTF8_CHAR>
2575c402 2879will tell you whether the current character in a string is valid UTF-8.
a422fd2d 2880
1e54db1a 2881=head2 How does UTF-8 represent Unicode characters?
a422fd2d 2882
1e54db1a 2883As mentioned above, UTF-8 uses a variable number of bytes to store a
10e2eb10
FC
2884character. Characters with values 0...127 are stored in one
2885byte, just like good ol' ASCII. Character 128 is stored as
2886C<v194.128>; this continues up to character 191, which is
2887C<v194.191>. Now we've run out of bits (191 is binary
61ad4b94 2888C<10111111>) so we move on; character 192 is C<v195.128>. And
a422fd2d 2889so it goes on, moving to three bytes at character 2048.
6e31cdd1 2890L<perlunicode/Unicode Encodings> has pictures of how this works.
a422fd2d 2891
1e54db1a 2892Assuming you know you're dealing with a UTF-8 string, you can find out
a422fd2d
SC
2893how long the first character in it is with the C<UTF8SKIP> macro:
2894
2895 char *utf = "\305\233\340\240\201";
2896 I32 len;
2897
2898 len = UTF8SKIP(utf); /* len is 2 here */
2899 utf += len;
2900 len = UTF8SKIP(utf); /* len is 3 here */
2901
1e54db1a 2902Another way to skip over characters in a UTF-8 string is to use
a422fd2d 2903C<utf8_hop>, which takes a string and a number of characters to skip
10e2eb10 2904over. You're on your own about bounds checking, though, so don't use it
a422fd2d
SC
2905lightly.
2906
1e54db1a 2907All bytes in a multi-byte UTF-8 character will have the high bit set,
3a2263fe 2908so you can test if you need to do something special with this
61ad4b94 2909character like this (the C<UTF8_IS_INVARIANT()> is a macro that tests
9f98c7fe 2910whether the byte is encoded as a single byte even in UTF-8):
a422fd2d 2911
32128a7f
KW
2912 U8 *utf; /* Initialize this to point to the beginning of the
2913 sequence to convert */
2914 U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
2915 pointed to by 'utf' */
2916 UV uv; /* Returned code point; note: a UV, not a U8, not a
2917 char */
2918 STRLEN len; /* Returned length of character in bytes */
a422fd2d 2919
3a2263fe 2920 if (!UTF8_IS_INVARIANT(*utf))
1e54db1a 2921 /* Must treat this as UTF-8 */
4b88fb76 2922 uv = utf8_to_uvchr_buf(utf, utf_end, &len);
a422fd2d
SC
2923 else
2924 /* OK to treat this character as a byte */
2925 uv = *utf;
2926
4b88fb76 2927You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
95701e00 2928value of the character; the inverse function C<uvchr_to_utf8> is available
1e54db1a 2929for putting a UV into UTF-8:
a422fd2d 2930
61ad4b94 2931 if (!UVCHR_IS_INVARIANT(uv))
a422fd2d 2932 /* Must treat this as UTF8 */
95701e00 2933 utf8 = uvchr_to_utf8(utf8, uv);
a422fd2d
SC
2934 else
2935 /* OK to treat this character as a byte */
2936 *utf8++ = uv;
2937
2938You B<must> convert characters to UVs using the above functions if
1e54db1a 2939you're ever in a situation where you have to match UTF-8 and non-UTF-8
10e2eb10 2940characters. You may not skip over UTF-8 characters in this case. If you
1e54db1a
JH
2941do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
2942for instance, if your UTF-8 string contains C<v196.172>, and you skip
2943that character, you can never match a C<chr(200)> in a non-UTF-8 string.
a422fd2d
SC
2944So don't do that!
2945
61ad4b94
KW
2946(Note that we don't have to test for invariant characters in the
2947examples above. The functions work on any well-formed UTF-8 input.
2948It's just that its faster to avoid the function overhead when it's not
2949needed.)
2950
1e54db1a 2951=head2 How does Perl store UTF-8 strings?
a422fd2d 2952
61ad4b94 2953Currently, Perl deals with UTF-8 strings and non-UTF-8 strings
10e2eb10
FC
2954slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
2955string is internally encoded as UTF-8. Without it, the byte value is the
61ad4b94
KW
2956codepoint number and vice versa. This flag is only meaningful if the SV
2957is C<SvPOK> or immediately after stringification via C<SvPV> or a
2958similar macro. You can check and manipulate this flag with the
2575c402 2959following macros:
a422fd2d
SC
2960
2961 SvUTF8(sv)
2962 SvUTF8_on(sv)
2963 SvUTF8_off(sv)
2964
2965This flag has an important effect on Perl's treatment of the string: if
61ad4b94 2966UTF-8 data is not properly distinguished, regular expressions,
a422fd2d 2967C<length>, C<substr> and other string handling operations will have
61ad4b94 2968undesirable (wrong) results.
a422fd2d
SC
2969
2970The problem comes when you have, for instance, a string that isn't
61ad4b94 2971flagged as UTF-8, and contains a byte sequence that could be UTF-8 --
1e54db1a 2972especially when combining non-UTF-8 and UTF-8 strings.
a422fd2d 2973
61ad4b94
KW
2974Never forget that the C<SVf_UTF8> flag is separate from the PV value; you
2975need to be sure you don't accidentally knock it off while you're
10e2eb10 2976manipulating SVs. More specifically, you cannot expect to do this:
a422fd2d
SC
2977
2978 SV *sv;
2979 SV *nsv;
2980 STRLEN len;
2981 char *p;
2982
2983 p = SvPV(sv, len);
2984 frobnicate(p);
2985 nsv = newSVpvn(p, len);
2986
2987The C<char*> string does not tell you the whole story, and you can't
10e2eb10 2988copy or reconstruct an SV just by copying the string value. Check if the
c31cc9fc
FC
2989old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
2990accordingly:
a422fd2d
SC
2991
2992 p = SvPV(sv, len);
6db25795
KW
2993 is_utf8 = SvUTF8(sv);
2994 frobnicate(p, is_utf8);
a422fd2d 2995 nsv = newSVpvn(p, len);
6db25795 2996 if (is_utf8)
a422fd2d
SC
2997 SvUTF8_on(nsv);
2998
6db25795
KW
2999In the above, your C<frobnicate> function has been changed to be made
3000aware of whether or not it's dealing with UTF-8 data, so that it can
3001handle the string appropriately.
a422fd2d 3002
3a2263fe 3003Since just passing an SV to an XS function and copying the data of
2575c402 3004the SV is not enough to copy the UTF8 flags, even less right is just
61ad4b94 3005passing a S<C<char *>> to an XS function.
3a2263fe 3006
dc83bf8e 3007For full generality, use the L<C<DO_UTF8>|perlapi/DO_UTF8> macro to see if the
6db25795
KW
3008string in an SV is to be I<treated> as UTF-8. This takes into account
3009if the call to the XS function is being made from within the scope of
3010L<S<C<use bytes>>|bytes>. If so, the underlying bytes that comprise the
3011UTF-8 string are to be exposed, rather than the character they
3012represent. But this pragma should only really be used for debugging and
3013perhaps low-level testing at the byte level. Hence most XS code need
3014not concern itself with this, but various areas of the perl core do need
3015to support it.
3016
3017And this isn't the whole story. Starting in Perl v5.12, strings that
3018aren't encoded in UTF-8 may also be treated as Unicode under various
6e31cdd1 3019conditions (see L<perlunicode/ASCII Rules versus Unicode Rules>).
6db25795
KW
3020This is only really a problem for characters whose ordinals are between
3021128 and 255, and their behavior varies under ASCII versus Unicode rules
3022in ways that your code cares about (see L<perlunicode/The "Unicode Bug">).
3023There is no published API for dealing with this, as it is subject to
3024change, but you can look at the code for C<pp_lc> in F<pp.c> for an
3025example as to how it's currently done.
3026
1e54db1a 3027=head2 How do I convert a string to UTF-8?
a422fd2d 3028
2575c402 3029If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
61ad4b94 3030the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do
2575c402 3031this is:
a422fd2d
SC
3032
3033 sv_utf8_upgrade(sv);
3034
3035However, you must not do this, for example:
3036
3037 if (!SvUTF8(left))
3038 sv_utf8_upgrade(left);
3039
3040If you do this in a binary operator, you will actually change one of the
b1866b2d 3041strings that came into the operator, and, while it shouldn't be noticeable
2575c402 3042by the end user, it can cause problems in deficient code.
a422fd2d 3043
1e54db1a 3044Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
10e2eb10
FC
3045string argument. This is useful for having the data available for
3046comparisons and so on, without harming the original SV. There's also
a422fd2d
SC
3047C<utf8_to_bytes> to go the other way, but naturally, this will fail if
3048the string contains any characters above 255 that can't be represented
3049in a single byte.
3050
6db25795
KW
3051=head2 How do I compare strings?
3052
3053L<perlapi/sv_cmp> and L<perlapi/sv_cmp_flags> do a lexigraphic
3054comparison of two SV's, and handle UTF-8ness properly. Note, however,
3055that Unicode specifies a much fancier mechanism for collation, available
3056via the L<Unicode::Collate> module.
3057
3058To just compare two strings for equality/non-equality, you can just use
3059L<C<memEQ()>|perlapi/memEQ> and L<C<memNE()>|perlapi/memEQ> as usual,
3060except the strings must be both UTF-8 or not UTF-8 encoded.
3061
3062To compare two strings case-insensitively, use
3063L<C<foldEQ_utf8()>|perlapi/foldEQ_utf8> (the strings don't have to have
3064the same UTF-8ness).
3065
a422fd2d
SC
3066=head2 Is there anything else I need to know?
3067
10e2eb10 3068Not really. Just remember these things:
a422fd2d
SC
3069
3070=over 3
3071
3072=item *
3073
6db25795
KW
3074There's no way to tell if a S<C<char *>> or S<C<U8 *>> string is UTF-8
3075or not. But you can tell if an SV is to be treated as UTF-8 by calling
3076C<DO_UTF8> on it, after stringifying it with C<SvPV> or a similar
3077macro. And, you can tell if SV is actually UTF-8 (even if it is not to
3078be treated as such) by looking at its C<SvUTF8> flag (again after
3079stringifying it). Don't forget to set the flag if something should be
3080UTF-8.
3081Treat the flag as part of the PV, even though it's not -- if you pass on
3082the PV to somewhere, pass on the flag too.
a422fd2d
SC
3083
3084=item *
3085
4b88fb76 3086If a string is UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
3a2263fe 3087unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
a422fd2d
SC
3088
3089=item *
3090
61ad4b94
KW
3091When writing a character UV to a UTF-8 string, B<always> use
3092C<uvchr_to_utf8>, unless C<UVCHR_IS_INVARIANT(uv))> in which case
3a2263fe 3093you can use C<*s = uv>.
a422fd2d
SC
3094
3095=item *
3096
10e2eb10
FC
3097Mixing UTF-8 and non-UTF-8 strings is
3098tricky. Use C<bytes_to_utf8> to get
2bbc8d55 3099a new string which is UTF-8 encoded, and then combine them.
a422fd2d
SC
3100
3101=back
3102
53e06cf0
SC
3103=head1 Custom Operators
3104
2a0fd0f1 3105Custom operator support is an experimental feature that allows you to
10e2eb10 3106define your own ops. This is primarily to allow the building of
53e06cf0
SC
3107interpreters for other languages in the Perl core, but it also allows
3108optimizations through the creation of "macro-ops" (ops which perform the
3109functions of multiple ops which are usually executed together, such as
1aa6ea50 3110C<gvsv, gvsv, add>.)
53e06cf0 3111
10e2eb10 3112This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
53e06cf0 3113core does not "know" anything special about this op type, and so it will
10e2eb10 3114not be involved in any optimizations. This also means that you can
61ad4b94
KW
3115define your custom ops to be any op structure -- unary, binary, list and
3116so on -- you like.
53e06cf0 3117
10e2eb10
FC
3118It's important to know what custom operators won't do for you. They
3119won't let you add new syntax to Perl, directly. They won't even let you
3120add new keywords, directly. In fact, they won't change the way Perl
3121compiles a program at all. You have to do those changes yourself, after
3122Perl has compiled the program. You do this either by manipulating the op
53e06cf0
SC
3123tree using a C<CHECK> block and the C<B::Generate> module, or by adding
3124a custom peephole optimizer with the C<optimize> module.
3125
3126When you do this, you replace ordinary Perl ops with custom ops by
407f86e1 3127creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
10e2eb10
FC
3128PP function. This should be defined in XS code, and should look like
3129the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
53e06cf0
SC
3130takes the appropriate number of values from the stack, and you are
3131responsible for adding stack marks if necessary.
3132
3133You should also "register" your op with the Perl interpreter so that it
10e2eb10 3134can produce sensible error and warning messages. Since it is possible to
53e06cf0 3135have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
9733086d 3136Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
10e2eb10 3137it is dealing with. You should create an C<XOP> structure for each
9733086d
BM
3138ppaddr you use, set the properties of the custom op with
3139C<XopENTRY_set>, and register the structure against the ppaddr using
10e2eb10 3140C<Perl_custom_op_register>. A trivial example might look like:
9733086d
BM
3141
3142 static XOP my_xop;
3143 static OP *my_pp(pTHX);
3144
3145 BOOT:
3146 XopENTRY_set(&my_xop, xop_name, "myxop");
3147 XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
3148 Perl_custom_op_register(aTHX_ my_pp, &my_xop);
3149
3150The available fields in the structure are:
3151
3152=over 4
3153
3154=item xop_name
3155
10e2eb10 3156A short name for your op. This will be included in some error messages,
9733086d
BM
3157and will also be returned as C<< $op->name >> by the L<B|B> module, so
3158it will appear in the output of module like L<B::Concise|B::Concise>.
3159
3160=item xop_desc
3161
3162A short description of the function of the op.
3163
3164=item xop_class
3165
10e2eb10 3166Which of the various C<*OP> structures this op uses. This should be one of
9733086d
BM
3167the C<OA_*> constants from F<op.h>, namely
3168
3169=over 4
3170
3171=item OA_BASEOP
3172
3173=item OA_UNOP
3174
3175=item OA_BINOP
3176
3177=item OA_LOGOP
3178
3179=item OA_LISTOP
3180
3181=item OA_PMOP
3182
3183=item OA_SVOP
3184
3185=item OA_PADOP
3186
3187=item OA_PVOP_OR_SVOP
3188
10e2eb10 3189This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
9733086d
BM
3190the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
3191
3192=item OA_LOOP
3193
3194=item OA_COP
3195
3196=back
3197
3198The other C<OA_*> constants should not be used.
3199
3200=item xop_peep
3201
3202This member is of type C<Perl_cpeep_t>, which expands to C<void
10e2eb10 3203(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
9733086d 3204will be called from C<Perl_rpeep> when ops of this type are encountered
10e2eb10 3205by the peephole optimizer. I<o> is the OP that needs optimizing;
9733086d
BM
3206I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
3207
3208=back
53e06cf0 3209
e7d4c058 3210C<B::Generate> directly supports the creation of custom ops by name.
53e06cf0 3211
bf5e9371
DM
3212
3213=head1 Dynamic Scope and the Context Stack
3214
3215B<Note:> this section describes a non-public internal API that is subject
3216to change without notice.
3217
3218=head2 Introduction to the context stack
3219
3220In Perl, dynamic scoping refers to the runtime nesting of things like
3221subroutine calls, evals etc, as well as the entering and exiting of block
3222scopes. For example, the restoring of a C<local>ised variable is
3223determined by the dynamic scope.
3224
3225Perl tracks the dynamic scope by a data structure called the context
3226stack, which is an array of C<PERL_CONTEXT> structures, and which is
3227itself a big union for all the types of context. Whenever a new scope is
3228entered (such as a block, a C<for> loop, or a subroutine call), a new
3229context entry is pushed onto the stack. Similarly when leaving a block or
3230returning from a subroutine call etc. a context is popped. Since the
3231context stack represents the current dynamic scope, it can be searched.
3232For example, C<next LABEL> searches back through the stack looking for a
3233loop context that matches the label; C<return> pops contexts until it
3234finds a sub or eval context or similar; C<caller> examines sub contexts on
3235the stack.
3236
3237Each context entry is labelled with a context type, C<cx_type>. Typical
3238context types are C<CXt_SUB>, C<CXt_EVAL> etc., as well as C<CXt_BLOCK>
3239and C<CXt_NULL> which represent a basic scope (as pushed by C<pp_enter>)
3240and a sort block. The type determines which part of the context union are
3241valid.
3242
3243The main division in the context struct is between a substitution scope
3244(C<CXt_SUBST>) and block scopes, which are everything else. The former is
d7c7f8cb 3245just used while executing C<s///e>, and won't be discussed further
bf5e9371
DM
3246here.
3247
3248All the block scope types share a common base, which corresponds to
3249C<CXt_BLOCK>. This stores the old values of various scope-related
3250variables like C<PL_curpm>, as well as information about the current
3251scope, such as C<gimme>. On scope exit, the old variables are restored.
3252
3253Particular block scope types store extra per-type information. For
3254example, C<CXt_SUB> stores the currently executing CV, while the various
3255for loop types might hold the original loop variable SV. On scope exit,
3256the per-type data is processed; for example the CV has its reference count
3257decremented, and the original loop variable is restored.
3258
3259The macro C<cxstack> returns the base of the current context stack, while
3260C<cxstack_ix> is the index of the current frame within that stack.
3261
3262In fact, the context stack is actually part of a stack-of-stacks system;
3263whenever something unusual is done such as calling a C<DESTROY> or tie
3264handler, a new stack is pushed, then popped at the end.
3265
3266Note that the API described here changed considerably in perl 5.24; prior
3267to that, big macros like C<PUSHBLOCK> and C<POPSUB> were used; in 5.24
3268they were replaced by the inline static functions described below. In
3269addition, the ordering and detail of how these macros/function work
3270changed in many ways, often subtly. In particular they didn't handle
3271saving the savestack and temps stack positions, and required additional
3272C<ENTER>, C<SAVETMPS> and C<LEAVE> compared to the new functions. The
3273old-style macros will not be described further.
3274
3275
3276=head2 Pushing contexts
3277
3278For pushing a new context, the two basic functions are
3279C<cx = cx_pushblock()>, which pushes a new basic context block and returns
3280its address, and a family of similar functions with names like
3281C<cx_pushsub(cx)> which populate the additional type-dependent fields in
3282the C<cx> struct. Note that C<CXt_NULL> and C<CXt_BLOCK> don't have their
3283own push functions, as they don't store any data beyond that pushed by
3284C<cx_pushblock>.
3285
3286The fields of the context struct and the arguments to the C<cx_*>
3287functions are subject to change between perl releases, representing
3288whatever is convenient or efficient for that release.
3289
3290A typical context stack pushing can be found in C<pp_entersub>; the
3291following shows a simplified and stripped-down example of a non-XS call,
3292along with comments showing roughly what each function does.
3293
61f554bd
KW
3294 dMARK;
3295 U8 gimme = GIMME_V;
3296 bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
3297 OP *retop = PL_op->op_next;
3298 I32 old_ss_ix = PL_savestack_ix;
3299 CV *cv = ....;
3300
3301 /* ... make mortal copies of stack args which are PADTMPs here ... */
3302
3303 /* ... do any additional savestack pushes here ... */
3304
3305 /* Now push a new context entry of type 'CXt_SUB'; initially just
3306 * doing the actions common to all block types: */
3307
3308 cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
3309
3310 /* this does (approximately):
3311 CXINC; /* cxstack_ix++ (grow if necessary) */
3312 cx = CX_CUR(); /* and get the address of new frame */
3313 cx->cx_type = CXt_SUB;
3314 cx->blk_gimme = gimme;
3315 cx->blk_oldsp = MARK - PL_stack_base;
3316 cx->blk_oldsaveix = old_ss_ix;
3317 cx->blk_oldcop = PL_curcop;
3318 cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
3319 cx->blk_oldscopesp = PL_scopestack_ix;
3320 cx->blk_oldpm = PL_curpm;
3321 cx->blk_old_tmpsfloor = PL_tmps_floor;
3322
3323 PL_tmps_floor = PL_tmps_ix;
3324 */
bf5e9371
DM
3325
3326
61f554bd
KW
3327 /* then update the new context frame with subroutine-specific info,
3328 * such as the CV about to be executed: */
bf5e9371 3329
61f554bd 3330 cx_pushsub(cx, cv, retop, hasargs);
bf5e9371 3331
61f554bd
KW
3332 /* this does (approximately):
3333 cx->blk_sub.cv = cv;
3334 cx->blk_sub.olddepth = CvDEPTH(cv);
3335 cx->blk_sub.prevcomppad = PL_comppad;
3336 cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
3337 cx->blk_sub.retop = retop;
3338 SvREFCNT_inc_simple_void_NN(cv);
3339 */
bf5e9371
DM
3340
3341Note that C<cx_pushblock()> sets two new floors: for the args stack (to
3342C<MARK>) and the temps stack (to C<PL_tmps_ix>). While executing at this
3343scope level, every C<nextstate> (amongst others) will reset the args and
3344tmps stack levels to these floors. Note that since C<cx_pushblock> uses
3345the current value of C<PL_tmps_ix> rather than it being passed as an arg,
3346this dictates at what point C<cx_pushblock> should be called. In
3347particular, any new mortals which should be freed only on scope exit
3348(rather than at the next C<nextstate>) should be created first.
3349
3350Most callers of C<cx_pushblock> simply set the new args stack floor to the
3351top of the previous stack frame, but for C<CXt_LOOP_LIST> it stores the
3352items being iterated over on the stack, and so sets C<blk_oldsp> to the
3353top of these items instead. Note that, contrary to its name, C<blk_oldsp>
3354doesn't always represent the value to restore C<PL_stack_sp> to on scope
3355exit.
3356
3357Note the early capture of C<PL_savestack_ix> to C<old_ss_ix>, which is
3358later passed as an arg to C<cx_pushblock>. In the case of C<pp_entersub>,
3359this is because, although most values needing saving are stored in fields
3360of the context struct, an extra value needs saving only when the debugger
3361is running, and it doesn't make sense to bloat the struct for this rare
3362case. So instead it is saved on the savestack. Since this value gets
3363calculated and saved before the context is pushed, it is necessary to pass
3364the old value of C<PL_savestack_ix> to C<cx_pushblock>, to ensure that the
3365saved value gets freed during scope exit. For most users of
3366C<cx_pushblock>, where nothing needs pushing on the save stack,
3367C<PL_savestack_ix> is just passed directly as an arg to C<cx_pushblock>.
3368
3369Note that where possible, values should be saved in the context struct
3370rather than on the save stack; it's much faster that way.
3371
3372Normally C<cx_pushblock> should be immediately followed by the appropriate
3373C<cx_pushfoo>, with nothing between them; this is because if code
3374in-between could die (e.g. a warning upgraded to fatal), then the context
3375stack unwinding code in C<dounwind> would see (in the example above) a
3376C<CXt_SUB> context frame, but without all the subroutine-specific fields
3377set, and crashes would soon ensue.
3378
3379Where the two must be separate, initially set the type to C<CXt_NULL> or
3380C<CXt_BLOCK>, and later change it to C<CXt_foo> when doing the
3381C<cx_pushfoo>. This is exactly what C<pp_enteriter> does, once it's
3382determined which type of loop it's pushing.
3383
3384=head2 Popping contexts
3385
3386Contexts are popped using C<cx_popsub()> etc. and C<cx_popblock()>. Note
3387however, that unlike C<cx_pushblock>, neither of these functions actually
3388decrement the current context stack index; this is done separately using
3389C<CX_POP()>.
3390
3391There are two main ways that contexts are popped. During normal execution
3392as scopes are exited, functions like C<pp_leave>, C<pp_leaveloop> and
3393C<pp_leavesub> process and pop just one context using C<cx_popfoo> and
3394C<cx_popblock>. On the other hand, things like C<pp_return> and C<next>
3395may have to pop back several scopes until a sub or loop context is found,
3396and exceptions (such as C<die>) need to pop back contexts until an eval
3397context is found. Both of these are accomplished by C<dounwind()>, which
3398is capable of processing and popping all contexts above the target one.
3399
3400Here is a typical example of context popping, as found in C<pp_leavesub>
3401(simplified slightly):
3402
61f554bd
KW
3403 U8 gimme;
3404 PERL_CONTEXT *cx;
3405 SV **oldsp;
3406 OP *retop;
bf5e9371 3407
61f554bd 3408 cx = CX_CUR();
bf5e9371 3409
61f554bd
KW
3410 gimme = cx->blk_gimme;
3411 oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
bf5e9371 3412
61f554bd
KW
3413 if (gimme == G_VOID)
3414 PL_stack_sp = oldsp;
3415 else
3416 leave_adjust_stacks(oldsp, oldsp, gimme, 0);
bf5e9371 3417
61f554bd
KW
3418 CX_LEAVE_SCOPE(cx);
3419 cx_popsub(cx);
3420 cx_popblock(cx);
3421 retop = cx->blk_sub.retop;
3422 CX_POP(cx);
bf5e9371 3423
61f554bd 3424 return retop;
bf5e9371
DM
3425
3426The steps above are in a very specific order, designed to be the reverse
3427order of when the context was pushed. The first thing to do is to copy
3428and/or protect any any return arguments and free any temps in the current
3429scope. Scope exits like an rvalue sub normally return a mortal copy of
3430their return args (as opposed to lvalue subs). It is important to make
3431this copy before the save stack is popped or variables are restored, or
3432bad things like the following can happen:
3433
3434 sub f { my $x =...; $x } # $x freed before we get to copy it
3435 sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
3436
3437Although we wish to free any temps at the same time, we have to be careful
3438not to free any temps which are keeping return args alive; nor to free the
3439temps we have just created while mortal copying return args. Fortunately,
3440C<leave_adjust_stacks()> is capable of making mortal copies of return args,
3441shifting args down the stack, and only processing those entries on the
3442temps stack that are safe to do so.
3443
3444In void context no args are returned, so it's more efficient to skip
3445calling C<leave_adjust_stacks()>. Also in void context, a C<nextstate> op
3446is likely to be imminently called which will do a C<FREETMPS>, so there's
3447no need to do that either.
3448
3449The next step is to pop savestack entries: C<CX_LEAVE_SCOPE(cx)> is just
3450defined as C<<LEAVE_SCOPE(cx->blk_oldsaveix)>>. Note that during the
3451popping, it's possible for perl to call destructors, call C<STORE> to undo
3452localisations of tied vars, and so on. Any of these can die or call
3453C<exit()>. In this case, C<dounwind()> will be called, and the current
3454context stack frame will be re-processed. Thus it is vital that all steps
3455in popping a context are done in such a way to support reentrancy. The
3456other alternative, of decrementing C<cxstack_ix> I<before> processing the
3457frame, would lead to leaks and the like if something died halfway through,
3458or overwriting of the current frame.
3459
3460C<CX_LEAVE_SCOPE> itself is safely re-entrant: if only half the savestack
3461items have been popped before dying and getting trapped by eval, then the
3462C<CX_LEAVE_SCOPE>s in C<dounwind> or C<pp_leaveeval> will continue where
3463the first one left off.
3464
3465The next step is the type-specific context processing; in this case
3466C<cx_popsub>. In part, this looks like:
3467
3468 cv = cx->blk_sub.cv;
3469 CvDEPTH(cv) = cx->blk_sub.olddepth;
3470 cx->blk_sub.cv = NULL;
3471 SvREFCNT_dec(cv);
3472
3473where its processing the just-executed CV. Note that before it decrements
3474the CV's reference count, it nulls the C<blk_sub.cv>. This means that if
3475it re-enters, the CV won't be freed twice. It also means that you can't
3476rely on such type-specific fields having useful values after the return
3477from C<cx_popfoo>.
3478
3479Next, C<cx_popblock> restores all the various interpreter vars to their
3480previous values or previous high water marks; it expands to:
3481
3482 PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
3483 PL_scopestack_ix = cx->blk_oldscopesp;
3484 PL_curpm = cx->blk_oldpm;
3485 PL_curcop = cx->blk_oldcop;
3486 PL_tmps_floor = cx->blk_old_tmpsfloor;
3487
3488Note that it I<doesn't> restore C<PL_stack_sp>; as mentioned earlier,
3489which value to restore it to depends on the context type (specifically
3490C<for (list) {}>), and what args (if any) it returns; and that will
3491already have been sorted out earlier by C<leave_adjust_stacks()>.
3492
3493Finally, the context stack pointer is actually decremented by C<CX_POP(cx)>.
3494After this point, it's possible that that the current context frame could
3495be overwritten by other contexts being pushed. Although things like ties
3496and C<DESTROY> are supposed to work within a new context stack, it's best
3497not to assume this. Indeed on debugging builds, C<CX_POP(cx)> deliberately