path: root/pod/perlembed.pod

=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)
    {
        my_perl = perl_alloc();
        perl_construct(my_perl);
        perl_parse(my_perl, NULL, argc, argv, (char **) NULL);
        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)
    {
        my_perl = perl_alloc();
        perl_construct(my_perl);
    
        perl_parse(my_perl, NULL, argc, argv, (char **) NULL);
    
                                     /*** 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 *embedding[] = { "", "-e", "sub _eval_ { eval $_[0] }" };
     STRLEN length;
   
     my_perl = perl_alloc();
     perl_construct( my_perl );
   
     perl_parse(my_perl, NULL, 3, embedding, (char **) NULL);
   
                                       /** 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_sv("a", FALSE), length));
   
     perl_destruct(my_perl);
     perl_free(my_perl);
   }

All of those strange functions with I<sv> in their names help convert Perl scalars to C types.  They're described in L<perlguts>.

If you compile and run I<string.c>, you'll see the results of using
I<SvIV()> to create an C<int>, I<SvNV()> to create a C<float>, and
I<SvPV()> to create a string:

   a = 9
   a = 9.859600
   a = Just Another Perl Hacker


=head2 Performing Perl pattern matches and substitutions from your C program

Our I<perl_eval()> lets us evaluate strings of Perl code, so we can
define some functions that use it to "specialize" in matches and
substitutions: I<match()>, I<substitute()>, and I<matches()>.

   char match(char *string, char *pattern); 

Given a string and a pattern (e.g. "m/clasp/" or "/\b\w*\b/", which in
your program might be represented as C<"/\\b\\w*\\b/">),
returns 1 if the string matches the pattern and 0 otherwise.


   int substitute(char *string[], char *pattern);

Given a pointer to a string and an "=~" operation (e.g. "s/bob/robert/g" or 
"tr[A-Z][a-z]"), modifies the string according to the operation,
returning the number of substitutions made.

   int matches(char *string, char *pattern, char **matches[]);

Given a string, a pattern, and a pointer to an empty array of strings,
evaluates C<$string =~ $pattern> in an array context, and fills in
I<matches> with the array elements (allocating memory as it does so), 
returning the number of matches found.

Here's a sample program, I<match.c>, that uses all three:

   #include <stdio.h>
   #include <EXTERN.h>
   #include <perl.h>
   
   static PerlInterpreter *my_perl;
   
   int eval(char *string)
   {
     char *argv[2];
     argv[0] = string;
     argv[1] = NULL;
     perl_call_argv("_eval_", 0, argv);
   }
   
   /** match(string, pattern)
    ** 
    ** Used for matches in a scalar context.
    **
    ** Returns 1 if the match was successful; 0 otherwise. 
    **/
   char match(char *string, char *pattern) 
   {
     char *command;
     command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 37);
     sprintf(command, "$string = '%s'; $return = $string =~ %s", 
	string, pattern); 
     perl_eval(command);
     free(command);
     return SvIV(perl_get_sv("return", FALSE));
   }
   
   /** substitute(string, pattern)
    **
    ** Used for =~ operations that modify their left-hand side (s/// and tr///)
    **
    ** Returns the number of successful matches, and
    ** modifies the input string if there were any.
    **/
   int substitute(char *string[], char *pattern) 
   {
     char *command;
     STRLEN length;
     command = malloc(sizeof(char) * strlen(*string) + strlen(pattern) + 35);
     sprintf(command, "$string = '%s'; $ret = ($string =~ %s)", 
	*string, pattern); 
     perl_eval(command);
     free(command);
     *string = SvPV(perl_get_sv("string", FALSE), length);
     return SvIV(perl_get_sv("ret", FALSE));
   }
   
   /** matches(string, pattern, matches)
    ** 
    ** Used for matches in an array context.
    **
    ** Returns the number of matches, 
    ** and fills in **matches with the matching substrings (allocates memory!)
    **/
   int matches(char *string, char *pattern, char **matches[]) 
   {
     char *command;
     SV *current_match;
     AV *array;
     I32 num_matches;
     STRLEN length;
     int i;
   
     command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 38);
     sprintf(command, "$string = '%s'; @array = ($string =~ %s)", 
	string, pattern); 
     perl_eval(command);
     free(command);
     array = perl_get_av("array", FALSE);
     num_matches = av_len(array) + 1; /** assume $[ is 0 **/
     *matches = (char **) malloc(sizeof(char *) * num_matches);
     for (i = 0; i <= num_matches; i++) {  
       current_match = av_shift(array);
       (*matches)[i] = SvPV(current_match, length);
     }
     return num_matches;
   }
   
   main (int argc, char **argv)
   {
     char *embedding[] = { "", "-e", "sub _eval_ { eval $_[0] }" };
     char *text, **matches;
     int num_matches, i;
     int j;
   
     my_perl = perl_alloc();
     perl_construct( my_perl );
   
     perl_parse(my_perl, NULL, 3, embedding, (char **) NULL);
   
     text = (char *) malloc(sizeof(char) * 486); /** A long string follows! **/
     sprintf(text, "%s", "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");  
   
     if (perl_match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
       printf("perl_match: Text contains the word 'quarter'.\n\n");
     else 
       printf("perl_match: Text doesn't contain the word 'quarter'.\n\n");
   
     if (perl_match(text, "m/eighth/")) /** Does text contain 'eighth'? **/
       printf("perl_match: Text contains the word 'eighth'.\n\n");
     else 
       printf("perl_match: Text doesn't contain the word 'eighth'.\n\n");
   
	                               /** Match all occurrences of /wi../ **/
     num_matches = perl_matches(text, "m/(wi..)/g", &matches);
     
     printf("perl_matches: m/(wi..)/g found %d matches...\n", num_matches);
     for (i = 0; i < num_matches; i++) 
       printf("match: %s\n", matches[i]);
     printf("\n");
     for (i = 0; i < num_matches; i++) {
       free(matches[i]);
     }
     free(matches);
   
                                       /** Remove all vowels from text **/
     num_matches = perl_substitute(&text, "s/[aeiou]//gi");
     if (num_matches) {
       printf("perl_substitute: s/[aeiou]//gi...%d substitutions made.\n", 
	num_matches);
       printf("Now text is: %s\n\n", text);
     }
   
                                       /** Attempt a substitution
     if (!perl_substitute(&text, "s/Perl/C/")) {
       printf("perl_substitute: s/Perl/C...No substitution made.\n\n");
     }
   
     free(text);
   
     perl_destruct(my_perl);
     perl_free(my_perl);
   }

which produces the output

   perl_match: Text contains the word 'quarter'.
   
   perl_match: Text doesn't contain the word 'eighth'.
   
   perl_matches: m/(wi..)/g found 2 matches...
   match: will
   match: with
   
   perl_substitute: s/[aeiou]//gi...139 substitutions made.
   Now text is: Whn h s t  cnvnnc str nd th bll cms t sm mnt lk 76 cnts, Mynrd s wr tht thr s smthng h *shld* d, smthng tht wll nbl hm t gt bck  qrtr, bt h hs n d *wht*.  H fmbls thrgh hs rd sqzy chngprs nd gvs th by thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt.  Th by gvs hm bck tw f hs wn pnns nd thn th bg shny qrtr tht s hs prz. -RCHH
   
   perl_substitute: s/Perl/C...No substitution made.
      
=head2 Fiddling with the Perl stack from your C program

When trying to explain stacks, most computer science textbooks mumble
something about spring-loaded columns of cafeteria plates: the last
thing you pushed on the stack is the first thing you pop off.  That'll
do for our purposes: your C program will push some arguments onto "the Perl
stack", shut its eyes while some magic happens, and then pop the
results--the return value of your Perl subroutine--off the stack.
   
First you'll need to know how to convert between C types and Perl
types, with newSViv() and sv_setnv() and newAV() and all their
friends.  They're described in L<perlguts>.

Then you'll need to know how to manipulate the Perl stack.  That's
described in L<perlcall>.

Once you've understood those, embedding Perl in C is easy.  

Since C has no built-in function for integer exponentiation, let's
make Perl's ** operator available to it (this is less useful than it
sounds, since Perl implements ** with C's I<pow()> function).  First
I'll create a stub exponentiation function in I<power.pl>:

    sub expo {
        my ($a, $b) = @_;
        return $a ** $b;
    }

Now I'll create a C program, I<power.c>, with a function
I<PerlPower()> that contains all the perlguts necessary to push the
two arguments into I<expo()> and to pop the return value out.  Take a
deep breath...

    #include <stdio.h>
    #include <EXTERN.h>
    #include <perl.h>
    
    static PerlInterpreter *my_perl;
    
    static void
    PerlPower(int a, int b)
    {
      dSP;                            /* initialize stack pointer      */
      ENTER;                          /* everything created after here */
      SAVETMPS;                       /* ...is a temporary variable.   */
      PUSHMARK(sp);                   /* remember the stack pointer    */
      XPUSHs(sv_2mortal(newSViv(a))); /* push the base onto the stack  */
      XPUSHs(sv_2mortal(newSViv(b))); /* push the exponent onto stack  */
      PUTBACK;                      /* make local stack pointer global */
      perl_call_pv("expo", G_SCALAR); /* call the function             */
      SPAGAIN;                        /* refresh stack pointer         */
                                    /* pop the return value from stack */
      printf ("%d to the %dth power is %d.\n", a, b, POPi);
      PUTBACK;                               
      FREETMPS;                       /* free that return value        */
      LEAVE;                       /* ...and the XPUSHed "mortal" args.*/
    }
    
    int main (int argc, char **argv) 
    {
      char *my_argv[2];
    
      my_perl = perl_alloc();
      perl_construct( my_perl );
    
      my_argv[1] = (char *) malloc(10);
      sprintf(my_argv[1], "power.pl");
    
      perl_parse(my_perl, NULL, argc, my_argv, (char **) NULL);
      
      PerlPower(3, 4);                      /*** Compute 3 ** 4 ***/
    
      perl_destruct(my_perl);
      perl_free(my_perl);
    }
    


Compile and run:

    % cc -o power power.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE 
    -I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
    
    % power 
    3 to the 4th power is 81.

=head1 MORAL

You can sometimes I<write faster code> in C, but
you can always I<write code faster> in Perl.  Since you can use
each from the other, combine them as you wish.


=head1 AUTHOR

Jon Orwant F<E<lt>orwant@media.mit.eduE<gt>>, with contributions from
Tim Bunce, Tom Christiansen, Dov Grobgeld, and Ilya Zakharevich.

December 18, 1995

Some of this material is excerpted from my book: I<Perl 5 Interactive>, 
Waite Group Press, 1996 (ISBN 1-57169-064-6) and appears
courtesy of Waite Group Press.