perlembed

# NAME

perlembed - how to embed perl in your C program

# DESCRIPTION

## PREAMBLE

Do you want to:

Use C from Perl?
  Read the perlcall manpage and the perlxs manpage .

Use a UNIX program from Perl?
  Read about backquotes and system and exec .

Use Perl from Perl?
  Read about do and eval and use and require .

Use C from C?
  Rethink your design.

Use Perl from C?
  Read on...

## ROADMAP

Compiling your C program

There's one example in each of the five sections:

Adding a Perl interpreter to your C program

Calling a Perl subroutine from your C program

Evaluating a Perl statement from your C program

Performing Perl pattern matches and substitutions from your C program

Fiddling with the Perl stack from your C program

This documentation is UNIX specific.

## Compiling your C program

Every C program that uses Perl must link in the 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 (/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 lib directory.)

Your C program will--usually--allocate, ``run'', and deallocate a 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 EXTERN.h and 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, 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 (cc, gcc, et al.) and library directory (/usr/local/lib/...) for your machine. If your compiler complains that certain functions are undefined, or that it can't locate -lperl, then you need to change the path following the -L. If it complains that it can't find EXTERN.h or 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}'

## 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 miniperlmain.c, from the source distribution. Here's a bastardized, non-portable version of 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 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 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 argv[1] before calling perl_run().

## Calling a Perl subroutine from your C program

To call individual Perl subroutines, you'll need to remove the call to perl_run() and replace it with a call to perl_call_argv().

That's shown below, in a program I'll call 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 showtime is a Perl subroutine that takes no arguments (that's the G_NOARGS) and for which I'll ignore the return value (that's the G_DISCARD). Those flags, and others, are discussed in the perlcall manpage .

I'll define the showtime subroutine in a file called 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: Fiddling with the Perl stack from your C program

## 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 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 perl_eval()) that wraps around Perl's 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 require or do to include external Perl files.

Our perl_eval() lets us evaluate individual Perl strings, and then extract variables for coercion into C types. The following program, string.c, executes three Perl strings, extracting an int from the first, a `float` from the second, and a `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_sv("a", FALSE), length));

         perl_destruct(my_perl);
         perl_free(my_perl);
       }

All of those strange functions with sv in their names help convert Perl scalars to C types. They're described in the perlguts manpage .

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

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

## Performing Perl pattern matches and substitutions from your C program

Our perl_eval() lets us evaluate strings of Perl code, so we can define some functions that use it to ``specialize'' in matches and substitutions: match(), substitute(), and 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 ```/\\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 `$string =~ $pattern` in an array context, and fills in matches with the array elements (allocating memory as it does so), returning the number of matches found.

Here's a sample program, 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 **env)
       {
         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, env);

         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.

## 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 the perlguts manpage .

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

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 pow() function). First I'll create a stub exponentiation function in power.pl:

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

Now I'll create a C program, power.c, with a function PerlPower() that contains all the perlguts necessary to push the two arguments into 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 **env)
        {
          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, env);

          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.

# MORAL

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

# AUTHOR

Jon Orwant <orwant@media.mit.edu>, with contributions from Tim Bunce, Tom Christiansen, Dov Grobgeld, and Ilya Zakharevich.

December 18, 1995

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