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1996-06-28
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=head1 NAME
perlembed - how to embed perl in your C program
=head1 DESCRIPTION
=head2 PREAMBLE
Do you want to:
=over 5
=item B<Use C from Perl?>
Read L<perlcall> and L<perlxs>.
=item B<Use a UNIX program from Perl?>
Read about backquotes and L<perlfunc/system> and L<perlfunc/exec>.
=item B<Use Perl from Perl?>
Read about L<perlfunc/do> and L<perlfunc/eval> and L<perlmod/use>
and L<perlmod/require>.
=item B<Use C from C?>
Rethink your design.
=item B<Use Perl from C?>
Read on...
=back
=head2 ROADMAP
L<Compiling your C program>
There's one example in each of the five sections:
L<Adding a Perl interpreter to your C program>
L<Calling a Perl subroutine from your C program>
L<Evaluating a Perl statement from your C program>
L<Performing Perl pattern matches and substitutions from your C program>
L<Fiddling with the Perl stack from your C program>
This documentation is UNIX specific.
=head2 Compiling your C program
Every C program that uses Perl must link in the I<perl library>.
What's that, you ask? Perl is itself written in C; the perl library
is the collection of compiled C programs that were used to create your
perl executable (I</usr/bin/perl> or equivalent). (Corollary: you
can't use Perl from your C program unless Perl has been compiled on
your machine, or installed properly--that's why you shouldn't blithely
copy Perl executables from machine to machine without also copying the
I<lib> directory.)
Your C program will--usually--allocate, "run", and deallocate a
I<PerlInterpreter> object, which is defined in the perl library.
If your copy of Perl is recent enough to contain this documentation
(5.002 or later), then the perl library (and I<EXTERN.h> and
I<perl.h>, which you'll also need) will
reside in a directory resembling this:
/usr/local/lib/perl5/your_architecture_here/CORE
or perhaps just
/usr/local/lib/perl5/CORE
or maybe something like
/usr/opt/perl5/CORE
Execute this statement for a hint about where to find CORE:
perl -e 'use Config; print $Config{archlib}'
Here's how you might compile the example in the next section,
L<Adding a Perl interpreter to your C program>,
on a DEC Alpha running the OSF operating system:
% cc -o interp interp.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
You'll have to choose the appropriate compiler (I<cc>, I<gcc>, et al.) and
library directory (I</usr/local/lib/...>) for your machine. If your
compiler complains that certain functions are undefined, or that it
can't locate I<-lperl>, then you need to change the path following the
-L. If it complains that it can't find I<EXTERN.h> or I<perl.h>, you need
to change the path following the -I.
You may have to add extra libraries as well. Which ones?
Perhaps those printed by
perl -e 'use Config; print $Config{libs}'
=head2 Adding a Perl interpreter to your C program
In a sense, perl (the C program) is a good example of embedding Perl
(the language), so I'll demonstrate embedding with I<miniperlmain.c>,
from the source distribution. Here's a bastardized, non-portable version of
I<miniperlmain.c> containing the essentials of embedding:
#include <stdio.h>
#include <EXTERN.h> /* from the Perl distribution */
#include <perl.h> /* from the Perl distribution */
static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
int main(int argc, char **argv, char **env)
{
my_perl = perl_alloc();
perl_construct(my_perl);
perl_parse(my_perl, NULL, argc, argv, env);
perl_run(my_perl);
perl_destruct(my_perl);
perl_free(my_perl);
}
Now compile this program (I'll call it I<interp.c>) into an executable:
% cc -o interp interp.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
After a successful compilation, you'll be able to use I<interp> just
like perl itself:
% interp
print "Pretty Good Perl \n";
print "10890 - 9801 is ", 10890 - 9801;
<CTRL-D>
Pretty Good Perl
10890 - 9801 is 1089
or
% interp -e 'printf("%x", 3735928559)'
deadbeef
You can also read and execute Perl statements from a file while in the
midst of your C program, by placing the filename in I<argv[1]> before
calling I<perl_run()>.
=head2 Calling a Perl subroutine from your C program
To call individual Perl subroutines, you'll need to remove the call to
I<perl_run()> and replace it with a call to I<perl_call_argv()>.
That's shown below, in a program I'll call I<showtime.c>.
#include <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int main(int argc, char **argv, char **env)
{
my_perl = perl_alloc();
perl_construct(my_perl);
perl_parse(my_perl, NULL, argc, argv, env);
/*** This replaces perl_run() ***/
perl_call_argv("showtime", G_DISCARD | G_NOARGS, argv);
perl_destruct(my_perl);
perl_free(my_perl);
}
where I<showtime> is a Perl subroutine that takes no arguments (that's the
I<G_NOARGS>) and for which I'll ignore the return value (that's the
I<G_DISCARD>). Those flags, and others, are discussed in L<perlcall>.
I'll define the I<showtime> subroutine in a file called I<showtime.pl>:
print "I shan't be printed.";
sub showtime {
print time;
}
Simple enough. Now compile and run:
% cc -o showtime showtime.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
% showtime showtime.pl
818284590
yielding the number of seconds that elapsed between January 1, 1970
(the beginning of the UNIX epoch), and the moment I began writing this
sentence.
If you want to pass some arguments to the Perl subroutine, or
you want to access the return value, you'll need to manipulate the
Perl stack, demonstrated in the last section of this document:
L<Fiddling with the Perl stack from your C program>
=head2 Evaluating a Perl statement from your C program
NOTE: This section, and the next, employ some very brittle techniques
for evaluting strings of Perl code. Perl 5.002 contains some nifty
features that enable A Better Way (such as with L<perlguts/perl_eval_sv>).
Look for updates to this document soon.
One way to evaluate a Perl string is to define a function (we'll call
ours I<perl_eval()>) that wraps around Perl's L<perlfunc/eval>.
Arguably, this is the only routine you'll ever need to execute
snippets of Perl code from within your C program. Your string can be
as long as you wish; it can contain multiple statements; it can
use L<perlmod/require> or L<perlfunc/do> to include external Perl
files.
Our I<perl_eval()> lets us evaluate individual Perl strings, and then
extract variables for coercion into C types. The following program,
I<string.c>, executes three Perl strings, extracting an C<int> from
the first, a C<float> from the second, and a C<char *> from the third.
#include <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int perl_eval(char *string)
{
char *argv[2];
argv[0] = string;
argv[1] = NULL;
perl_call_argv("_eval_", 0, argv);
}
main (int argc, char **argv, char **env)
{
char *embedding[] = { "", "-e", "sub _eval_ { eval $_[0] }" };
STRLEN length;
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 3, embedding, env);
/** Treat $a as an integer **/
perl_eval("$a = 3; $a **= 2");
printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));
/** Treat $a as a float **/
perl_eval("$a = 3.14; $a **= 2");
printf("a = %f\n", SvNV(perl_get_sv("a", FALSE)));
/** Treat $a as a string **/
perl_eval("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a); ");
printf("a = %s\n", SvPV(perl_get_