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    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

    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 the system entry in the
                perlfunc manpage and the exec entry in the perlfunc
                manpage.

           Use Perl from Perl?
                Read about the do entry in the perlfunc manpage and
                the eval entry in the perlfunc manpage and the use
                entry in the perlmod manpage and the require entry in
                the perlmod manpage.

           Use C from C?
                Rethink your design.

           Use Perl from C?
                Read on...

           ROADMAP

           the section on Compiling your C program

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

           the section on Adding a Perl interpreter to your C program

           the section on Calling a Perl subroutine from your C
           program

           the section on Evaluating a Perl statement from your C
           program

           the section on Performing Perl pattern matches and
           substitutions from your C program

           the section on 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.

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    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           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, the section on 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}'

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    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           Adding a Perl interpreter to youur 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 yyour 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().

    30/Jan/96                perl 5.003 with                        3

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           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: the section on Fiddling with the
           Perl stack from your C program

    30/Jan/96                perl 5.003 with                        4

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           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 the perl_eval_sv entry in the perlguts
           manpage).  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 the
           eval entry in the perlfunc manpage.

           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 the require entry in the
           perlmod manpage or the do entry in the perlfunc manpage 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.

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    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

              #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

    30/Jan/96                perl 5.003 with                        6

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           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:

    30/Jan/96                perl 5.003 with                        7

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

              #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.

    30/Jan/96                perl 5.003 with                        8

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

               **
               ** 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../ **/

    30/Jan/96                perl 5.003 with                        9

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

                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

    30/Jan/96                perl 5.003 with                       10

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

           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...

    30/Jan/96                perl 5.003 with                       11

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

               #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.

    30/Jan/96                perl 5.003 with                       12

    PERLEMBED(1)     Perl Programmers Reference Guide    PERLEMBED(1)

    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.

    30/Jan/96                perl 5.003 with                       13

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