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Re: 5.005 - a sneak preview
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1=head1 NAME
2
cb1a09d0 3perlembed - how to embed perl in your C program
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4
5=head1 DESCRIPTION
6
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7=head2 PREAMBLE
8
9Do you want to:
10
11=over 5
12
96dbc785 13=item B<Use C from Perl?>
cb1a09d0 14
ee580363 15Read L<perlxs>, L<perlxstut> and L<h2xs>.
cb1a09d0 16
54310121 17=item B<Use a Unix program from Perl?>
cb1a09d0 18
5f05dabc 19Read about back-quotes and about C<system> and C<exec> in L<perlfunc>.
cb1a09d0 20
96dbc785 21=item B<Use Perl from Perl?>
cb1a09d0 22
7b8d334a 23Read about do(), eval(), require(), and use() in L<perlfunc>.
cb1a09d0 24
96dbc785 25=item B<Use C from C?>
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26
27Rethink your design.
28
96dbc785 29=item B<Use Perl from C?>
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30
31Read on...
32
33=back
34
35=head2 ROADMAP
36
7b8d334a 37Compiling your C program
cb1a09d0 38
53f52f58 39There's one example in each of the nine sections:
cb1a09d0 40
7b8d334a 41=over 4
cb1a09d0 42
7b8d334a 43=item *
cb1a09d0 44
7b8d334a 45Adding a Perl interpreter to your C program
cb1a09d0 46
7b8d334a 47=item *
cb1a09d0 48
7b8d334a 49Calling a Perl subroutine from your C program
cb1a09d0 50
7b8d334a 51=item *
a6006777 52
7b8d334a 53Evaluating a Perl statement from your C program
8ebc5c01 54
7b8d334a 55=item *
96dbc785 56
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57Performing Perl pattern matches and substitutions from your C program
58
59=item *
60
61Fiddling with the Perl stack from your C program
62
63=item *
64
65Maintaining a persistent interpreter
66
67=item *
68
69Maintaining multiple interpreter instances
70
71=item *
72
73Using Perl modules, which themselves use C libraries, from your C program
74
75=item *
76
77Embedding Perl under Win32
78
79=back
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80
81=head2 Compiling your C program
82
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83If you have trouble compiling the scripts in this documentation,
84you're not alone. The cardinal rule: COMPILE THE PROGRAMS IN EXACTLY
85THE SAME WAY THAT YOUR PERL WAS COMPILED. (Sorry for yelling.)
cb1a09d0 86
8a7dc658 87Also, every C program that uses Perl must link in the I<perl library>.
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88What's that, you ask? Perl is itself written in C; the perl library
89is the collection of compiled C programs that were used to create your
90perl executable (I</usr/bin/perl> or equivalent). (Corollary: you
91can't use Perl from your C program unless Perl has been compiled on
92your machine, or installed properly--that's why you shouldn't blithely
93copy Perl executables from machine to machine without also copying the
94I<lib> directory.)
95
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96When you use Perl from C, your C program will--usually--allocate,
97"run", and deallocate a I<PerlInterpreter> object, which is defined by
98the perl library.
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99
100If your copy of Perl is recent enough to contain this documentation
a6006777 101(version 5.002 or later), then the perl library (and I<EXTERN.h> and
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102I<perl.h>, which you'll also need) will reside in a directory
103that looks like this:
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104
105 /usr/local/lib/perl5/your_architecture_here/CORE
106
107or perhaps just
108
109 /usr/local/lib/perl5/CORE
110
111or maybe something like
112
113 /usr/opt/perl5/CORE
114
115Execute this statement for a hint about where to find CORE:
116
96dbc785 117 perl -MConfig -e 'print $Config{archlib}'
cb1a09d0 118
54310121 119Here's how you'd compile the example in the next section,
7b8d334a 120Adding a Perl interpreter to your C program, on my Linux box:
cb1a09d0 121
54310121 122 % gcc -O2 -Dbool=char -DHAS_BOOL -I/usr/local/include
8a7dc658 123 -I/usr/local/lib/perl5/i586-linux/5.003/CORE
54310121 124 -L/usr/local/lib/perl5/i586-linux/5.003/CORE
8a7dc658 125 -o interp interp.c -lperl -lm
cb1a09d0 126
54310121 127(That's all one line.) On my DEC Alpha running 5.003_05, the incantation
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128is a bit different:
129
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130 % cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
131 -I/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE
132 -L/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE -L/usr/local/lib
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133 -D__LANGUAGE_C__ -D_NO_PROTO -o interp interp.c -lperl -lm
134
135How can you figure out what to add? Assuming your Perl is post-5.001,
136execute a C<perl -V> command and pay special attention to the "cc" and
54310121 137"ccflags" information.
8a7dc658 138
54310121 139You'll have to choose the appropriate compiler (I<cc>, I<gcc>, et al.) for
8a7dc658 140your machine: C<perl -MConfig -e 'print $Config{cc}'> will tell you what
54310121 141to use.
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142
143You'll also have to choose the appropriate library directory
144(I</usr/local/lib/...>) for your machine. If your compiler complains
145that certain functions are undefined, or that it can't locate
146I<-lperl>, then you need to change the path following the C<-L>. If it
147complains that it can't find I<EXTERN.h> and I<perl.h>, you need to
148change the path following the C<-I>.
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149
150You may have to add extra libraries as well. Which ones?
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151Perhaps those printed by
152
153 perl -MConfig -e 'print $Config{libs}'
154
54310121 155Provided your perl binary was properly configured and installed the
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156B<ExtUtils::Embed> module will determine all of this information for
157you:
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158
159 % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
160
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161If the B<ExtUtils::Embed> module isn't part of your Perl distribution,
162you can retrieve it from
163http://www.perl.com/perl/CPAN/modules/by-module/ExtUtils::Embed. (If
164this documentation came from your Perl distribution, then you're
165running 5.004 or better and you already have it.)
96dbc785 166
8a7dc658 167The B<ExtUtils::Embed> kit on CPAN also contains all source code for
54310121 168the examples in this document, tests, additional examples and other
8a7dc658 169information you may find useful.
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170
171=head2 Adding a Perl interpreter to your C program
172
173In a sense, perl (the C program) is a good example of embedding Perl
174(the language), so I'll demonstrate embedding with I<miniperlmain.c>,
54310121 175from the source distribution. Here's a bastardized, nonportable
8a7dc658 176version of I<miniperlmain.c> containing the essentials of embedding:
cb1a09d0 177
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178 #include <EXTERN.h> /* from the Perl distribution */
179 #include <perl.h> /* from the Perl distribution */
96dbc785 180
cb1a09d0 181 static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
96dbc785 182
c07a80fd 183 int main(int argc, char **argv, char **env)
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184 {
185 my_perl = perl_alloc();
186 perl_construct(my_perl);
96dbc785 187 perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
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188 perl_run(my_perl);
189 perl_destruct(my_perl);
190 perl_free(my_perl);
191 }
192
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193Notice that we don't use the C<env> pointer. Normally handed to
194C<perl_parse> as its final argument, C<env> here is replaced by
195C<NULL>, which means that the current environment will be used.
96dbc785 196
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197Now compile this program (I'll call it I<interp.c>) into an executable:
198
96dbc785 199 % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
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200
201After a successful compilation, you'll be able to use I<interp> just
202like perl itself:
203
204 % interp
205 print "Pretty Good Perl \n";
206 print "10890 - 9801 is ", 10890 - 9801;
207 <CTRL-D>
208 Pretty Good Perl
209 10890 - 9801 is 1089
210
211or
212
213 % interp -e 'printf("%x", 3735928559)'
214 deadbeef
215
216You can also read and execute Perl statements from a file while in the
217midst of your C program, by placing the filename in I<argv[1]> before
96dbc785 218calling I<perl_run()>.
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219
220=head2 Calling a Perl subroutine from your C program
221
8ebc5c01 222To call individual Perl subroutines, you can use any of the B<perl_call_*>
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223functions documented in L<perlcall>.
224In this example we'll use perl_call_argv().
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225
226That's shown below, in a program I'll call I<showtime.c>.
227
cb1a09d0 228 #include <EXTERN.h>
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229 #include <perl.h>
230
231 static PerlInterpreter *my_perl;
232
c07a80fd 233 int main(int argc, char **argv, char **env)
cb1a09d0 234 {
8ebc5c01 235 char *args[] = { NULL };
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236 my_perl = perl_alloc();
237 perl_construct(my_perl);
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238
239 perl_parse(my_perl, NULL, argc, argv, NULL);
240
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241 /*** skipping perl_run() ***/
242
243 perl_call_argv("showtime", G_DISCARD | G_NOARGS, args);
244
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245 perl_destruct(my_perl);
246 perl_free(my_perl);
247 }
248
249where I<showtime> is a Perl subroutine that takes no arguments (that's the
96dbc785 250I<G_NOARGS>) and for which I'll ignore the return value (that's the
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251I<G_DISCARD>). Those flags, and others, are discussed in L<perlcall>.
252
253I'll define the I<showtime> subroutine in a file called I<showtime.pl>:
254
255 print "I shan't be printed.";
96dbc785 256
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257 sub showtime {
258 print time;
259 }
260
261Simple enough. Now compile and run:
262
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263 % cc -o showtime showtime.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
264
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265 % showtime showtime.pl
266 818284590
267
268yielding the number of seconds that elapsed between January 1, 1970
8a7dc658 269(the beginning of the Unix epoch), and the moment I began writing this
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270sentence.
271
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272In this particular case we don't have to call I<perl_run>, but in
273general it's considered good practice to ensure proper initialization
274of library code, including execution of all object C<DESTROY> methods
275and package C<END {}> blocks.
8ebc5c01 276
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277If you want to pass arguments to the Perl subroutine, you can add
278strings to the C<NULL>-terminated C<args> list passed to
279I<perl_call_argv>. For other data types, or to examine return values,
280you'll need to manipulate the Perl stack. That's demonstrated in the
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281last section of this document: Fiddling with the Perl stack from
282your C program.
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283
284=head2 Evaluating a Perl statement from your C program
285
137443ea 286Perl provides two API functions to evaluate pieces of Perl code.
7b8d334a 287These are perl_eval_sv() and perl_eval_pv().
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288
289Arguably, these are the only routines you'll ever need to execute
290snippets of Perl code from within your C program. Your code can be
8a7dc658 291as long as you wish; it can contain multiple statements; it can employ
7b8d334a 292use(), require(), and do() to include external Perl files.
cb1a09d0 293
7b8d334a 294perl_eval_pv() lets us evaluate individual Perl strings, and then
96dbc785 295extract variables for coercion into C types. The following program,
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296I<string.c>, executes three Perl strings, extracting an C<int> from
297the first, a C<float> from the second, and a C<char *> from the third.
298
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299 #include <EXTERN.h>
300 #include <perl.h>
137443ea 301
cb1a09d0 302 static PerlInterpreter *my_perl;
137443ea 303
c07a80fd 304 main (int argc, char **argv, char **env)
cb1a09d0 305 {
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306 char *embedding[] = { "", "-e", "0" };
307
308 my_perl = perl_alloc();
309 perl_construct( my_perl );
310
311 perl_parse(my_perl, NULL, 3, embedding, NULL);
312 perl_run(my_perl);
313
314 /** Treat $a as an integer **/
315 perl_eval_pv("$a = 3; $a **= 2", TRUE);
316 printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));
317
318 /** Treat $a as a float **/
319 perl_eval_pv("$a = 3.14; $a **= 2", TRUE);
320 printf("a = %f\n", SvNV(perl_get_sv("a", FALSE)));
321
322 /** Treat $a as a string **/
323 perl_eval_pv("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
324 printf("a = %s\n", SvPV(perl_get_sv("a", FALSE), na));
325
326 perl_destruct(my_perl);
327 perl_free(my_perl);
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328 }
329
330All of those strange functions with I<sv> in their names help convert Perl scalars to C types. They're described in L<perlguts>.
331
332If you compile and run I<string.c>, you'll see the results of using
333I<SvIV()> to create an C<int>, I<SvNV()> to create a C<float>, and
334I<SvPV()> to create a string:
335
336 a = 9
337 a = 9.859600
338 a = Just Another Perl Hacker
339
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340In the example above, we've created a global variable to temporarily
341store the computed value of our eval'd expression. It is also
342possible and in most cases a better strategy to fetch the return value
7b8d334a 343from perl_eval_pv() instead. Example:
8f183262 344
8f183262 345 ...
137443ea 346 SV *val = perl_eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
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347 printf("%s\n", SvPV(val,na));
348 ...
349
350This way, we avoid namespace pollution by not creating global
351variables and we've simplified our code as well.
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352
353=head2 Performing Perl pattern matches and substitutions from your C program
354
1f05cdcd 355The I<perl_eval_sv()> function lets us evaluate chunks of Perl code, so we can
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356define some functions that use it to "specialize" in matches and
357substitutions: I<match()>, I<substitute()>, and I<matches()>.
358
1f05cdcd 359 char match(SV *string, char *pattern);
cb1a09d0 360
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361Given a string and a pattern (e.g., C<m/clasp/> or C</\b\w*\b/>, which
362in your C program might appear as "/\\b\\w*\\b/"), match()
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363returns 1 if the string matches the pattern and 0 otherwise.
364
1f05cdcd 365 int substitute(SV **string, char *pattern);
cb1a09d0 366
1f05cdcd 367Given a pointer to an C<SV> and an C<=~> operation (e.g.,
8a7dc658 368C<s/bob/robert/g> or C<tr[A-Z][a-z]>), substitute() modifies the string
1f05cdcd 369within the C<AV> at according to the operation, returning the number of substitutions
8a7dc658 370made.
cb1a09d0 371
1f05cdcd 372 int matches(SV *string, char *pattern, AV **matches);
cb1a09d0 373
1f05cdcd 374Given an C<SV>, a pattern, and a pointer to an empty C<AV>,
8a7dc658 375matches() evaluates C<$string =~ $pattern> in an array context, and
1f05cdcd 376fills in I<matches> with the array elements, returning the number of matches found.
cb1a09d0 377
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378Here's a sample program, I<match.c>, that uses all three (long lines have
379been wrapped here):
cb1a09d0 380
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381 #include <EXTERN.h>
382 #include <perl.h>
383
384 /** my_perl_eval_sv(code, error_check)
385 ** kinda like perl_eval_sv(),
386 ** but we pop the return value off the stack
387 **/
388 SV* my_perl_eval_sv(SV *sv, I32 croak_on_error)
389 {
390 dSP;
391 SV* retval;
392
924508f0 393 PUSHMARK(SP);
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394 perl_eval_sv(sv, G_SCALAR);
395
396 SPAGAIN;
397 retval = POPs;
398 PUTBACK;
399
400 if (croak_on_error && SvTRUE(GvSV(errgv)))
401 croak(SvPVx(GvSV(errgv), na));
402
403 return retval;
404 }
405
406 /** match(string, pattern)
407 **
408 ** Used for matches in a scalar context.
409 **
410 ** Returns 1 if the match was successful; 0 otherwise.
411 **/
412
413 I32 match(SV *string, char *pattern)
414 {
8c52afec 415 SV *command = NEWSV(1099, 0), *retval;
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416
417 sv_setpvf(command, "my $string = '%s'; $string =~ %s",
418 SvPV(string,na), pattern);
419
420 retval = my_perl_eval_sv(command, TRUE);
421 SvREFCNT_dec(command);
422
423 return SvIV(retval);
424 }
425
426 /** substitute(string, pattern)
427 **
428 ** Used for =~ operations that modify their left-hand side (s/// and tr///)
429 **
430 ** Returns the number of successful matches, and
431 ** modifies the input string if there were any.
432 **/
433
434 I32 substitute(SV **string, char *pattern)
435 {
8c52afec 436 SV *command = NEWSV(1099, 0), *retval;
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437
438 sv_setpvf(command, "$string = '%s'; ($string =~ %s)",
439 SvPV(*string,na), pattern);
440
441 retval = my_perl_eval_sv(command, TRUE);
442 SvREFCNT_dec(command);
443
444 *string = perl_get_sv("string", FALSE);
445 return SvIV(retval);
446 }
447
448 /** matches(string, pattern, matches)
449 **
450 ** Used for matches in an array context.
451 **
452 ** Returns the number of matches,
453 ** and fills in **matches with the matching substrings
454 **/
455
456 I32 matches(SV *string, char *pattern, AV **match_list)
457 {
8c52afec 458 SV *command = NEWSV(1099, 0);
cb1a09d0 459 I32 num_matches;
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460
461 sv_setpvf(command, "my $string = '%s'; @array = ($string =~ %s)",
462 SvPV(string,na), pattern);
463
464 my_perl_eval_sv(command, TRUE);
465 SvREFCNT_dec(command);
466
467 *match_list = perl_get_av("array", FALSE);
468 num_matches = av_len(*match_list) + 1; /** assume $[ is 0 **/
469
cb1a09d0 470 return num_matches;
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471 }
472
473 main (int argc, char **argv, char **env)
474 {
475 PerlInterpreter *my_perl = perl_alloc();
a6006777 476 char *embedding[] = { "", "-e", "0" };
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477 AV *match_list;
478 I32 num_matches, i;
8c52afec 479 SV *text = NEWSV(1099,0);
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480
481 perl_construct(my_perl);
96dbc785 482 perl_parse(my_perl, NULL, 3, embedding, NULL);
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483
484 sv_setpv(text, "When he is at a convenience store and the bill comes to some amount like 76 cents, Maynard is aware that there is something he *should* do, something that will enable him to get back a quarter, but he has no idea *what*. He fumbles through his red squeezey changepurse and gives the boy three extra pennies with his dollar, hoping that he might luck into the correct amount. The boy gives him back two of his own pennies and then the big shiny quarter that is his prize. -RICHH");
485
96dbc785 486 if (match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
1f05cdcd 487 printf("match: Text contains the word 'quarter'.\n\n");
96dbc785 488 else
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489 printf("match: Text doesn't contain the word 'quarter'.\n\n");
490
96dbc785 491 if (match(text, "m/eighth/")) /** Does text contain 'eighth'? **/
1f05cdcd 492 printf("match: Text contains the word 'eighth'.\n\n");
96dbc785 493 else
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494 printf("match: Text doesn't contain the word 'eighth'.\n\n");
495
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496 /** Match all occurrences of /wi../ **/
497 num_matches = matches(text, "m/(wi..)/g", &match_list);
498 printf("matches: m/(wi..)/g found %d matches...\n", num_matches);
1f05cdcd 499
96dbc785 500 for (i = 0; i < num_matches; i++)
1f05cdcd 501 printf("match: %s\n", SvPV(*av_fetch(match_list, i, FALSE),na));
cb1a09d0 502 printf("\n");
1f05cdcd 503
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504 /** Remove all vowels from text **/
505 num_matches = substitute(&text, "s/[aeiou]//gi");
cb1a09d0 506 if (num_matches) {
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507 printf("substitute: s/[aeiou]//gi...%d substitutions made.\n",
508 num_matches);
509 printf("Now text is: %s\n\n", SvPV(text,na));
cb1a09d0 510 }
1f05cdcd 511
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512 /** Attempt a substitution **/
513 if (!substitute(&text, "s/Perl/C/")) {
1f05cdcd 514 printf("substitute: s/Perl/C...No substitution made.\n\n");
cb1a09d0 515 }
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516
517 SvREFCNT_dec(text);
518 perl_destruct_level = 1;
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519 perl_destruct(my_perl);
520 perl_free(my_perl);
1f05cdcd 521 }
cb1a09d0 522
96dbc785 523which produces the output (again, long lines have been wrapped here)
cb1a09d0 524
8a7dc658 525 match: Text contains the word 'quarter'.
96dbc785 526
8a7dc658 527 match: Text doesn't contain the word 'eighth'.
96dbc785 528
8a7dc658 529 matches: m/(wi..)/g found 2 matches...
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530 match: will
531 match: with
96dbc785 532
8a7dc658 533 substitute: s/[aeiou]//gi...139 substitutions made.
54310121 534 Now text is: Whn h s t cnvnnc str nd th bll cms t sm mnt lk 76 cnts,
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535 Mynrd s wr tht thr s smthng h *shld* d, smthng tht wll nbl hm t gt bck
536 qrtr, bt h hs n d *wht*. H fmbls thrgh hs rd sqzy chngprs nd gvs th by
537 thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt. Th by gvs
538 hm bck tw f hs wn pnns nd thn th bg shny qrtr tht s hs prz. -RCHH
539
8a7dc658 540 substitute: s/Perl/C...No substitution made.
96dbc785 541
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542=head2 Fiddling with the Perl stack from your C program
543
544When trying to explain stacks, most computer science textbooks mumble
545something about spring-loaded columns of cafeteria plates: the last
546thing you pushed on the stack is the first thing you pop off. That'll
547do for our purposes: your C program will push some arguments onto "the Perl
548stack", shut its eyes while some magic happens, and then pop the
549results--the return value of your Perl subroutine--off the stack.
96dbc785 550
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551First you'll need to know how to convert between C types and Perl
552types, with newSViv() and sv_setnv() and newAV() and all their
553friends. They're described in L<perlguts>.
554
555Then you'll need to know how to manipulate the Perl stack. That's
556described in L<perlcall>.
557
96dbc785 558Once you've understood those, embedding Perl in C is easy.
cb1a09d0 559
54310121 560Because C has no builtin function for integer exponentiation, let's
cb1a09d0 561make Perl's ** operator available to it (this is less useful than it
5f05dabc 562sounds, because Perl implements ** with C's I<pow()> function). First
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563I'll create a stub exponentiation function in I<power.pl>:
564
565 sub expo {
566 my ($a, $b) = @_;
567 return $a ** $b;
568 }
569
570Now I'll create a C program, I<power.c>, with a function
571I<PerlPower()> that contains all the perlguts necessary to push the
572two arguments into I<expo()> and to pop the return value out. Take a
573deep breath...
574
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575 #include <EXTERN.h>
576 #include <perl.h>
96dbc785 577
cb1a09d0 578 static PerlInterpreter *my_perl;
96dbc785 579
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580 static void
581 PerlPower(int a, int b)
582 {
583 dSP; /* initialize stack pointer */
584 ENTER; /* everything created after here */
585 SAVETMPS; /* ...is a temporary variable. */
924508f0 586 PUSHMARK(SP); /* remember the stack pointer */
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587 XPUSHs(sv_2mortal(newSViv(a))); /* push the base onto the stack */
588 XPUSHs(sv_2mortal(newSViv(b))); /* push the exponent onto stack */
589 PUTBACK; /* make local stack pointer global */
590 perl_call_pv("expo", G_SCALAR); /* call the function */
591 SPAGAIN; /* refresh stack pointer */
592 /* pop the return value from stack */
593 printf ("%d to the %dth power is %d.\n", a, b, POPi);
96dbc785 594 PUTBACK;
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595 FREETMPS; /* free that return value */
596 LEAVE; /* ...and the XPUSHed "mortal" args.*/
597 }
96dbc785
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598
599 int main (int argc, char **argv, char **env)
cb1a09d0 600 {
95b76e31 601 char *my_argv[] = { "", "power.pl" };
96dbc785 602
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603 my_perl = perl_alloc();
604 perl_construct( my_perl );
96dbc785 605
95b76e31 606 perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
8ebc5c01 607 perl_run(my_perl);
96dbc785 608
cb1a09d0 609 PerlPower(3, 4); /*** Compute 3 ** 4 ***/
96dbc785 610
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611 perl_destruct(my_perl);
612 perl_free(my_perl);
613 }
96dbc785 614
cb1a09d0
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615
616
617Compile and run:
618
96dbc785
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619 % cc -o power power.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
620
621 % power
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622 3 to the 4th power is 81.
623
a6006777
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624=head2 Maintaining a persistent interpreter
625
8a7dc658
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626When developing interactive and/or potentially long-running
627applications, it's a good idea to maintain a persistent interpreter
628rather than allocating and constructing a new interpreter multiple
629times. The major reason is speed: since Perl will only be loaded into
54310121 630memory once.
8a7dc658
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631
632However, you have to be more cautious with namespace and variable
633scoping when using a persistent interpreter. In previous examples
634we've been using global variables in the default package C<main>. We
635knew exactly what code would be run, and assumed we could avoid
636variable collisions and outrageous symbol table growth.
637
638Let's say your application is a server that will occasionally run Perl
639code from some arbitrary file. Your server has no way of knowing what
640code it's going to run. Very dangerous.
641
642If the file is pulled in by C<perl_parse()>, compiled into a newly
643constructed interpreter, and subsequently cleaned out with
644C<perl_destruct()> afterwards, you're shielded from most namespace
645troubles.
646
647One way to avoid namespace collisions in this scenario is to translate
648the filename into a guaranteed-unique package name, and then compile
7b8d334a 649the code into that package using eval(). In the example
8a7dc658
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650below, each file will only be compiled once. Or, the application
651might choose to clean out the symbol table associated with the file
7b8d334a 652after it's no longer needed. Using perl_call_argv(), We'll
8a7dc658
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653call the subroutine C<Embed::Persistent::eval_file> which lives in the
654file C<persistent.pl> and pass the filename and boolean cleanup/cache
a6006777
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655flag as arguments.
656
8a7dc658
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657Note that the process will continue to grow for each file that it
658uses. In addition, there might be C<AUTOLOAD>ed subroutines and other
659conditions that cause Perl's symbol table to grow. You might want to
660add some logic that keeps track of the process size, or restarts
661itself after a certain number of requests, to ensure that memory
662consumption is minimized. You'll also want to scope your variables
7b8d334a 663with my() whenever possible.
a6006777 664
54310121 665
a6006777
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666 package Embed::Persistent;
667 #persistent.pl
54310121 668
a6006777
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669 use strict;
670 use vars '%Cache';
1ee082b7 671 use Symbol qw(delete_package);
54310121 672
a6006777
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673 sub valid_package_name {
674 my($string) = @_;
675 $string =~ s/([^A-Za-z0-9\/])/sprintf("_%2x",unpack("C",$1))/eg;
676 # second pass only for words starting with a digit
677 $string =~ s|/(\d)|sprintf("/_%2x",unpack("C",$1))|eg;
54310121 678
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679 # Dress it up as a real package name
680 $string =~ s|/|::|g;
681 return "Embed" . $string;
682 }
54310121 683
a6006777
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684 sub eval_file {
685 my($filename, $delete) = @_;
686 my $package = valid_package_name($filename);
687 my $mtime = -M $filename;
688 if(defined $Cache{$package}{mtime}
689 &&
54310121 690 $Cache{$package}{mtime} <= $mtime)
a6006777 691 {
54310121 692 # we have compiled this subroutine already,
8ebc5c01
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693 # it has not been updated on disk, nothing left to do
694 print STDERR "already compiled $package->handler\n";
a6006777
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695 }
696 else {
8ebc5c01
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697 local *FH;
698 open FH, $filename or die "open '$filename' $!";
699 local($/) = undef;
700 my $sub = <FH>;
701 close FH;
54310121 702
8ebc5c01
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703 #wrap the code into a subroutine inside our unique package
704 my $eval = qq{package $package; sub handler { $sub; }};
705 {
706 # hide our variables within this block
707 my($filename,$mtime,$package,$sub);
708 eval $eval;
709 }
710 die $@ if $@;
54310121 711
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712 #cache it unless we're cleaning out each time
713 $Cache{$package}{mtime} = $mtime unless $delete;
a6006777 714 }
54310121 715
a6006777
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716 eval {$package->handler;};
717 die $@ if $@;
54310121 718
a6006777 719 delete_package($package) if $delete;
54310121 720
a6006777
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721 #take a look if you want
722 #print Devel::Symdump->rnew($package)->as_string, $/;
723 }
54310121 724
a6006777 725 1;
54310121 726
a6006777
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727 __END__
728
729 /* persistent.c */
54310121
PP
730 #include <EXTERN.h>
731 #include <perl.h>
732
a6006777
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733 /* 1 = clean out filename's symbol table after each request, 0 = don't */
734 #ifndef DO_CLEAN
735 #define DO_CLEAN 0
736 #endif
54310121 737
a6006777 738 static PerlInterpreter *perl = NULL;
54310121 739
a6006777
PP
740 int
741 main(int argc, char **argv, char **env)
742 {
743 char *embedding[] = { "", "persistent.pl" };
744 char *args[] = { "", DO_CLEAN, NULL };
745 char filename [1024];
746 int exitstatus = 0;
54310121 747
a6006777 748 if((perl = perl_alloc()) == NULL) {
8ebc5c01
PP
749 fprintf(stderr, "no memory!");
750 exit(1);
a6006777 751 }
54310121
PP
752 perl_construct(perl);
753
a6006777 754 exitstatus = perl_parse(perl, NULL, 2, embedding, NULL);
54310121
PP
755
756 if(!exitstatus) {
8ebc5c01 757 exitstatus = perl_run(perl);
54310121 758
8ebc5c01 759 while(printf("Enter file name: ") && gets(filename)) {
54310121 760
8ebc5c01
PP
761 /* call the subroutine, passing it the filename as an argument */
762 args[0] = filename;
54310121 763 perl_call_argv("Embed::Persistent::eval_file",
8ebc5c01 764 G_DISCARD | G_EVAL, args);
54310121 765
8ebc5c01 766 /* check $@ */
54310121 767 if(SvTRUE(GvSV(errgv)))
8ebc5c01
PP
768 fprintf(stderr, "eval error: %s\n", SvPV(GvSV(errgv),na));
769 }
a6006777 770 }
54310121 771
a6006777 772 perl_destruct_level = 0;
54310121
PP
773 perl_destruct(perl);
774 perl_free(perl);
a6006777
PP
775 exit(exitstatus);
776 }
777
a6006777
PP
778Now compile:
779
54310121 780 % cc -o persistent persistent.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
a6006777
PP
781
782Here's a example script file:
783
784 #test.pl
785 my $string = "hello";
786 foo($string);
787
788 sub foo {
789 print "foo says: @_\n";
790 }
791
792Now run:
793
794 % persistent
795 Enter file name: test.pl
796 foo says: hello
797 Enter file name: test.pl
798 already compiled Embed::test_2epl->handler
799 foo says: hello
800 Enter file name: ^C
801
8ebc5c01
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802=head2 Maintaining multiple interpreter instances
803
8a7dc658
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804Some rare applications will need to create more than one interpreter
805during a session. Such an application might sporadically decide to
54310121 806release any resources associated with the interpreter.
8a7dc658
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807
808The program must take care to ensure that this takes place I<before>
809the next interpreter is constructed. By default, the global variable
810C<perl_destruct_level> is set to C<0>, since extra cleaning isn't
811needed when a program has only one interpreter.
812
813Setting C<perl_destruct_level> to C<1> makes everything squeaky clean:
814
54310121 815 perl_destruct_level = 1;
8ebc5c01 816
8ebc5c01
PP
817 while(1) {
818 ...
819 /* reset global variables here with perl_destruct_level = 1 */
54310121 820 perl_construct(my_perl);
8ebc5c01
PP
821 ...
822 /* clean and reset _everything_ during perl_destruct */
54310121
PP
823 perl_destruct(my_perl);
824 perl_free(my_perl);
8ebc5c01
PP
825 ...
826 /* let's go do it again! */
827 }
828
54310121
PP
829When I<perl_destruct()> is called, the interpreter's syntax parse tree
830and symbol tables are cleaned up, and global variables are reset.
8ebc5c01 831
8a7dc658
JO
832Now suppose we have more than one interpreter instance running at the
833same time. This is feasible, but only if you used the
834C<-DMULTIPLICITY> flag when building Perl. By default, that sets
835C<perl_destruct_level> to C<1>.
8ebc5c01
PP
836
837Let's give it a try:
838
839
840 #include <EXTERN.h>
8a7dc658 841 #include <perl.h>
8ebc5c01
PP
842
843 /* we're going to embed two interpreters */
844 /* we're going to embed two interpreters */
845
8ebc5c01
PP
846 #define SAY_HELLO "-e", "print qq(Hi, I'm $^X\n)"
847
8ebc5c01
PP
848 int main(int argc, char **argv, char **env)
849 {
54310121 850 PerlInterpreter
8ebc5c01 851 *one_perl = perl_alloc(),
54310121 852 *two_perl = perl_alloc();
8ebc5c01
PP
853 char *one_args[] = { "one_perl", SAY_HELLO };
854 char *two_args[] = { "two_perl", SAY_HELLO };
855
856 perl_construct(one_perl);
857 perl_construct(two_perl);
858
859 perl_parse(one_perl, NULL, 3, one_args, (char **)NULL);
860 perl_parse(two_perl, NULL, 3, two_args, (char **)NULL);
861
862 perl_run(one_perl);
863 perl_run(two_perl);
864
865 perl_destruct(one_perl);
866 perl_destruct(two_perl);
867
868 perl_free(one_perl);
869 perl_free(two_perl);
870 }
871
872
873Compile as usual:
874
875 % cc -o multiplicity multiplicity.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
876
877Run it, Run it:
878
879 % multiplicity
880 Hi, I'm one_perl
881 Hi, I'm two_perl
882
96dbc785
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883=head2 Using Perl modules, which themselves use C libraries, from your C program
884
885If you've played with the examples above and tried to embed a script
886that I<use()>s a Perl module (such as I<Socket>) which itself uses a C or C++ library,
887this probably happened:
888
889
890 Can't load module Socket, dynamic loading not available in this perl.
891 (You may need to build a new perl executable which either supports
892 dynamic loading or has the Socket module statically linked into it.)
893
894
895What's wrong?
896
897Your interpreter doesn't know how to communicate with these extensions
898on its own. A little glue will help. Up until now you've been
899calling I<perl_parse()>, handing it NULL for the second argument:
900
901 perl_parse(my_perl, NULL, argc, my_argv, NULL);
902
903That's where the glue code can be inserted to create the initial contact between
904Perl and linked C/C++ routines. Let's take a look some pieces of I<perlmain.c>
905to see how Perl does this:
906
907
908 #ifdef __cplusplus
909 # define EXTERN_C extern "C"
910 #else
911 # define EXTERN_C extern
912 #endif
913
914 static void xs_init _((void));
915
916 EXTERN_C void boot_DynaLoader _((CV* cv));
917 EXTERN_C void boot_Socket _((CV* cv));
918
919
920 EXTERN_C void
921 xs_init()
922 {
923 char *file = __FILE__;
924 /* DynaLoader is a special case */
925 newXS("DynaLoader::boot_DynaLoader", boot_DynaLoader, file);
926 newXS("Socket::bootstrap", boot_Socket, file);
927 }
928
929Simply put: for each extension linked with your Perl executable
930(determined during its initial configuration on your
931computer or when adding a new extension),
932a Perl subroutine is created to incorporate the extension's
933routines. Normally, that subroutine is named
934I<Module::bootstrap()> and is invoked when you say I<use Module>. In
935turn, this hooks into an XSUB, I<boot_Module>, which creates a Perl
936counterpart for each of the extension's XSUBs. Don't worry about this
937part; leave that to the I<xsubpp> and extension authors. If your
938extension is dynamically loaded, DynaLoader creates I<Module::bootstrap()>
939for you on the fly. In fact, if you have a working DynaLoader then there
5f05dabc 940is rarely any need to link in any other extensions statically.
96dbc785
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941
942
943Once you have this code, slap it into the second argument of I<perl_parse()>:
944
945
946 perl_parse(my_perl, xs_init, argc, my_argv, NULL);
947
948
949Then compile:
950
8a7dc658 951 % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
96dbc785
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952
953 % interp
954 use Socket;
955 use SomeDynamicallyLoadedModule;
956
957 print "Now I can use extensions!\n"'
958
959B<ExtUtils::Embed> can also automate writing the I<xs_init> glue code.
960
8a7dc658 961 % perl -MExtUtils::Embed -e xsinit -- -o perlxsi.c
96dbc785
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962 % cc -c perlxsi.c `perl -MExtUtils::Embed -e ccopts`
963 % cc -c interp.c `perl -MExtUtils::Embed -e ccopts`
8a7dc658 964 % cc -o interp perlxsi.o interp.o `perl -MExtUtils::Embed -e ldopts`
96dbc785
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965
966Consult L<perlxs> and L<perlguts> for more details.
967
53f52f58
DM
968=head1 Embedding Perl under Win32
969
970At the time of this writing, there are two versions of Perl which run
971under Win32. Interfacing to Activeware's Perl library is quite
972different from the examples in this documentation, as significant
973changes were made to the internal Perl API. However, it is possible
974to embed Activeware's Perl runtime, see the Perl for Win32 FAQ:
975http://www.perl.com/perl/faq/win32/Perl_for_Win32_FAQ.html
976
977With the "official" Perl version 5.004 or higher, all the examples
978within this documentation will compile and run untouched, although,
979the build process is slightly different between Unix and Win32.
980
981For starters, backticks don't work under the Win32 native command shell!
982The ExtUtils::Embed kit on CPAN ships with a script called
983B<genmake>, which generates a simple makefile to build a program from
984a single C source file. It can be used like so:
985
986 C:\ExtUtils-Embed\eg> perl genmake interp.c
987 C:\ExtUtils-Embed\eg> nmake
988 C:\ExtUtils-Embed\eg> interp -e "print qq{I'm embedded in Win32!\n}"
989
990You may wish to use a more robust environment such as the MS Developer
991stdio. In this case, to generate perlxsi.c run:
992
993 perl -MExtUtils::Embed -e xsinit
994
995Create a new project, Insert -> Files into Project: perlxsi.c, perl.lib,
996and your own source files, e.g. interp.c. Typically you'll find
997perl.lib in B<C:\perl\lib\CORE>, if not, you should see the B<CORE>
998directory relative to C<perl -V:archlib>.
999The studio will also need this path so it knows where to find Perl
1000include files. This path can be added via the Tools -> Options ->
1001Directories menu. Finnally, select Build -> Build interp.exe and
1002you're ready to go!
96dbc785 1003
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1004=head1 MORAL
1005
1006You can sometimes I<write faster code> in C, but
5f05dabc 1007you can always I<write code faster> in Perl. Because you can use
cb1a09d0
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1008each from the other, combine them as you wish.
1009
1010
1011=head1 AUTHOR
1012
9607fc9c
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1013Jon Orwant and <F<orwant@tpj.com>> and Doug MacEachern <F<dougm@osf.org>>,
1014with small contributions from Tim Bunce, Tom Christiansen, Hallvard Furuseth,
1015Dov Grobgeld, and Ilya Zakharevich.
8a7dc658
JO
1016
1017Check out Doug's article on embedding in Volume 1, Issue 4 of The Perl
1018Journal. Info about TPJ is available from http://tpj.com.
cb1a09d0 1019
84902520 1020July 17, 1997
cb1a09d0 1021
8a7dc658
JO
1022Some of this material is excerpted from Jon Orwant's book: I<Perl 5
1023Interactive>, Waite Group Press, 1996 (ISBN 1-57169-064-6) and appears
cb1a09d0 1024courtesy of Waite Group Press.
8a7dc658
JO
1025
1026=head1 COPYRIGHT
1027
1028Copyright (C) 1995, 1996, 1997 Doug MacEachern and Jon Orwant. All
1029Rights Reserved.
1030
1031Although destined for release with the standard Perl distribution,
1032this document is not public domain, nor is any of Perl and its
1033documentation. Permission is granted to freely distribute verbatim
1034copies of this document provided that no modifications outside of
1035formatting be made, and that this notice remain intact. You are
1036permitted and encouraged to use its code and derivatives thereof in
1037your own source code for fun or for profit as you see fit.