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 perlxstut manpage, the perlxs manpage, the
            h2xs manpage, and the perlguts manpage.

       Use a Unix program from Perl?
            Read about back-quotes and about system and exec 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
            require entry in the perlfunc manpage and the use
            entry in the perlfunc manpage.

       Use C from C?
            Rethink your design.

       Use Perl from C?
            Read on...

       ROADMAP

       the section on Compiling your C program

       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

       the section on Maintaining a persistent interpreter

       the section on Maintaining multiple interpreter instances

       the section on Using Perl modules, which themselves use C
       libraries, from your C program

       the section on Embedding Perl under Win32



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       Compiling your C program

       If you have trouble compiling the scripts in this
       documentation, you're not alone.  The cardinal rule:
       COMPILE THE PROGRAMS IN EXACTLY THE SAME WAY THAT YOUR
       PERL WAS COMPILED.  (Sorry for yelling.)

       Also, 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.)

       When you use Perl from C, your C program
       will--usually--allocate, "run", and deallocate a
       PerlInterpreter object, which is defined by the perl
       library.

       If your copy of Perl is recent enough to contain this
       documentation (version 5.002 or later), then the perl
       library (and EXTERN.h and perl.h, which you'll also need)
       will reside in a directory that looks like 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 -MConfig -e 'print $Config{archlib}'

       Here's how you'd compile the example in the next section,
       the section on Adding a Perl interpreter to your C
       program, on my Linux box:

           % gcc -O2 -Dbool=char -DHAS_BOOL -I/usr/local/include
           -I/usr/local/lib/perl5/i586-linux/5.003/CORE
           -L/usr/local/lib/perl5/i586-linux/5.003/CORE
           -o interp interp.c -lperl -lm

       (That's all one line.)  On my DEC Alpha running old
       5.003_05, the incantation is a bit different:




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           % cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
           -I/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE
           -L/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE -L/usr/local/lib
           -D__LANGUAGE_C__ -D_NO_PROTO -o interp interp.c -lperl -lm

       How can you figure out what to add?  Assuming your Perl is
       post-5.001, execute a perl -V command and pay special
       attention to the "cc" and "ccflags" information.

       You'll have to choose the appropriate compiler (cc, gcc,
       et al.) for your machine: perl -MConfig -e 'print
       $Config{cc}' will tell you what to use.

       You'll also have to choose the appropriate 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 and 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 -MConfig -e 'print $Config{libs}'

       Provided your perl binary was properly configured and
       installed the ExtUtils::Embed module will determine all of
       this information for you:

          % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

       If the ExtUtils::Embed module isn't part of your Perl
       distribution, you can retrieve it from
       http://www.perl.com/perl/CPAN/modules/by-module/ExtUtils/.
       (If this documentation came from your Perl distribution,
       then you're running 5.004 or better and you already have
       it.)

       The ExtUtils::Embed kit on CPAN also contains all source
       code for the examples in this document, tests, additional
       examples and other information you may find useful.

       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, included in the source
       distribution.  Here's a bastardized, nonportable version
       of miniperlmain.c containing the essentials of embedding:

           #include <EXTERN.h>               /* from the Perl distribution     */
           #include <perl.h>                 /* from the Perl distribution     */




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           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, (char **)NULL);
               perl_run(my_perl);
               perl_destruct(my_perl);
               perl_free(my_perl);
           }

       Notice that we don't use the env pointer.  Normally handed
       to perl_parse as its final argument, env here is replaced
       by NULL, which means that the current environment will be
       used.

       Now compile this program (I'll call it interp.c) into an
       executable:

           % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

       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 can use any of
       the perl_call_* functions documented in the perlcall
       manpage.  In this example we'll use perl_call_argv.

       That's shown below, in a program I'll call showtime.c.

           #include <EXTERN.h>
           #include <perl.h>

           static PerlInterpreter *my_perl;




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           int main(int argc, char **argv, char **env)
           {
               char *args[] = { NULL };
               my_perl = perl_alloc();
               perl_construct(my_perl);

               perl_parse(my_perl, NULL, argc, argv, NULL);

               /*** skipping perl_run() ***/

               perl_call_argv("showtime", G_DISCARD | G_NOARGS, args);

               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 `perl -MExtUtils::Embed -e ccopts -e ldopts`

           % 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.

       In this particular case we don't have to call perl_run,
       but in general it's considered good practice to ensure
       proper initialization of library code, including execution
       of all object DESTROY methods and package END {} blocks.

       If you want to pass arguments to the Perl subroutine, you
       can add strings to the NULL-terminated args list passed to
       perl_call_argv.  For other data types, or to examine
       return values, you'll need to manipulate the Perl stack.
       That's demonstrated in the last section of this document:
       the section on Fiddling with the Perl stack from your C
       program.




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       Evaluating a Perl statement from your C program

       Perl provides two API functions to evaluate pieces of Perl
       code.  These are the perl_eval_sv entry in the perlguts
       manpage and the perl_eval_pv entry in the perlguts
       manpage.

       Arguably, these are the only routines you'll ever need to
       execute snippets of Perl code from within your C program.
       Your code can be as long as you wish; it can contain
       multiple statements; it can employ the use entry in the
       perlfunc manpage, the require entry in the perlfunc
       manpage, and the do entry in the perlfunc manpage to
       include external Perl files.

       perl_eval_pv 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 <EXTERN.h>
          #include <perl.h>

          static PerlInterpreter *my_perl;

          main (int argc, char **argv, char **env)
          {
              STRLEN n_a;
              char *embedding[] = { "", "-e", "0" };

              my_perl = perl_alloc();
              perl_construct( my_perl );

              perl_parse(my_perl, NULL, 3, embedding, NULL);
              perl_run(my_perl);

              /** Treat $a as an integer **/
              perl_eval_pv("$a = 3; $a **= 2", TRUE);
              printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));

              /** Treat $a as a float **/
              perl_eval_pv("$a = 3.14; $a **= 2", TRUE);
              printf("a = %f\n", SvNV(perl_get_sv("a", FALSE)));

              /** Treat $a as a string **/
              perl_eval_pv("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
              printf("a = %s\n", SvPV(perl_get_sv("a", FALSE), n_a));

              perl_destruct(my_perl);
              perl_free(my_perl);
          }

       All of those strange functions with sv in their names help



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       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

       In the example above, we've created a global variable to
       temporarily store the computed value of our eval'd
       expression.  It is also possible and in most cases a
       better strategy to fetch the return value from
       perl_eval_pv() instead.  Example:

          ...
          STRLEN n_a;
          SV *val = perl_eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
          printf("%s\n", SvPV(val,n_a));
          ...

       This way, we avoid namespace pollution by not creating
       global variables and we've simplified our code as well.

       Performing Perl pattern matches and substitutions from
       your C program

       The perl_eval_sv() function 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().

          I32 match(SV *string, char *pattern);

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

          int substitute(SV **string, char *pattern);

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

          int matches(SV *string, char *pattern, AV **matches);

       Given an SV, a pattern, and a pointer to an empty AV,
       matches() evaluates $string =~ $pattern in an array
       context, and fills in matches with the array elements,
       returning the number of matches found.



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       Here's a sample program, match.c, that uses all three
       (long lines have been wrapped here):























































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        #include <EXTERN.h>
        #include <perl.h>

        /** my_perl_eval_sv(code, error_check)
        ** kinda like perl_eval_sv(),
        ** but we pop the return value off the stack
        **/
        SV* my_perl_eval_sv(SV *sv, I32 croak_on_error)
        {
            dSP;
            SV* retval;
            STRLEN n_a;

            PUSHMARK(SP);
            perl_eval_sv(sv, G_SCALAR);

            SPAGAIN;
            retval = POPs;
            PUTBACK;

            if (croak_on_error && SvTRUE(ERRSV))
               croak(SvPVx(ERRSV, n_a));

            return retval;
        }

        /** match(string, pattern)
        **
        ** Used for matches in a scalar context.
        **
        ** Returns 1 if the match was successful; 0 otherwise.
        **/

        I32 match(SV *string, char *pattern)
        {
            SV *command = NEWSV(1099, 0), *retval;
            STRLEN n_a;

            sv_setpvf(command, "my $string = '%s'; $string =~ %s",
                     SvPV(string,n_a), pattern);

            retval = my_perl_eval_sv(command, TRUE);
            SvREFCNT_dec(command);

            return SvIV(retval);
        }

        /** 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.
        **/



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        I32 substitute(SV **string, char *pattern)
        {
            SV *command = NEWSV(1099, 0), *retval;
            STRLEN n_a;

            sv_setpvf(command, "$string = '%s'; ($string =~ %s)",
                     SvPV(*string,n_a), pattern);

            retval = my_perl_eval_sv(command, TRUE);
            SvREFCNT_dec(command);

            *string = perl_get_sv("string", FALSE);
            return SvIV(retval);
        }

        /** matches(string, pattern, matches)
        **
        ** Used for matches in an array context.
        **
        ** Returns the number of matches,
        ** and fills in **matches with the matching substrings
        **/

        I32 matches(SV *string, char *pattern, AV **match_list)
        {
            SV *command = NEWSV(1099, 0);
            I32 num_matches;
            STRLEN n_a;

            sv_setpvf(command, "my $string = '%s'; @array = ($string =~ %s)",
                     SvPV(string,n_a), pattern);

            my_perl_eval_sv(command, TRUE);
            SvREFCNT_dec(command);

            *match_list = perl_get_av("array", FALSE);
            num_matches = av_len(*match_list) + 1; /** assume $[ is 0 **/

            return num_matches;
        }

        main (int argc, char **argv, char **env)
        {
            PerlInterpreter *my_perl = perl_alloc();
            char *embedding[] = { "", "-e", "0" };
            AV *match_list;
            I32 num_matches, i;
            SV *text = NEWSV(1099,0);
            STRLEN n_a;

            perl_construct(my_perl);
            perl_parse(my_perl, NULL, 3, embedding, NULL);




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            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");

            if (match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
               printf("match: Text contains the word 'quarter'.\n\n");
            else
               printf("match: Text doesn't contain the word 'quarter'.\n\n");

            if (match(text, "m/eighth/")) /** Does text contain 'eighth'? **/
               printf("match: Text contains the word 'eighth'.\n\n");
            else
               printf("match: Text doesn't contain the word 'eighth'.\n\n");

            /** Match all occurrences of /wi../ **/
            num_matches = matches(text, "m/(wi..)/g", &match_list);
            printf("matches: m/(wi..)/g found %d matches...\n", num_matches);

            for (i = 0; i < num_matches; i++)
               printf("match: %s\n", SvPV(*av_fetch(match_list, i, FALSE),n_a));
            printf("\n");

            /** Remove all vowels from text **/
            num_matches = substitute(&text, "s/[aeiou]//gi");
            if (num_matches) {
               printf("substitute: s/[aeiou]//gi...%d substitutions made.\n",
                      num_matches);
               printf("Now text is: %s\n\n", SvPV(text,n_a));
            }

            /** Attempt a substitution **/
            if (!substitute(&text, "s/Perl/C/")) {
               printf("substitute: s/Perl/C...No substitution made.\n\n");
            }

            SvREFCNT_dec(text);
            PL_perl_destruct_level = 1;
            perl_destruct(my_perl);
            perl_free(my_perl);
        }

       which produces the output (again, long lines have been
       wrapped here)

          match: Text contains the word 'quarter'.

          match: Text doesn't contain the word 'eighth'.

          matches: m/(wi..)/g found 2 matches...
          match: will
          match: with








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          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

          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.

       Because C has no builtin function for integer
       exponentiation, let's make Perl's ** operator available to
       it (this is less useful than it sounds, because 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 <EXTERN.h>
           #include <perl.h>

           static PerlInterpreter *my_perl;






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           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[] = { "", "power.pl" };

             my_perl = perl_alloc();
             perl_construct( my_perl );

             perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
             perl_run(my_perl);

             PerlPower(3, 4);                      /*** Compute 3 ** 4 ***/

             perl_destruct(my_perl);
             perl_free(my_perl);
           }

       Compile and run:

           % cc -o power power.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

           % power
           3 to the 4th power is 81.


       Maintaining a persistent interpreter

       When developing interactive and/or potentially long-
       running applications, it's a good idea to maintain a
       persistent interpreter rather than allocating and
       constructing a new interpreter multiple times.  The major
       reason is speed: since Perl will only be loaded into
       memory once.

       However, you have to be more cautious with namespace and
       variable scoping when using a persistent interpreter.  In



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       previous examples we've been using global variables in the
       default package main.  We knew exactly what code would be
       run, and assumed we could avoid variable collisions and
       outrageous symbol table growth.

       Let's say your application is a server that will
       occasionally run Perl code from some arbitrary file.  Your
       server has no way of knowing what code it's going to run.
       Very dangerous.

       If the file is pulled in by perl_parse(), compiled into a
       newly constructed interpreter, and subsequently cleaned
       out with perl_destruct() afterwards, you're shielded from
       most namespace troubles.

       One way to avoid namespace collisions in this scenario is
       to translate the filename into a guaranteed-unique package
       name, and then compile the code into that package using
       the eval entry in the perlfunc manpage.  In the example
       below, each file will only be compiled once.  Or, the
       application might choose to clean out the symbol table
       associated with the file after it's no longer needed.
       Using the perl_call_argv entry in the perlcall manpage,
       We'll call the subroutine Embed::Persistent::eval_file
       which lives in the file persistent.pl and pass the
       filename and boolean cleanup/cache flag as arguments.

       Note that the process will continue to grow for each file
       that it uses.  In addition, there might be AUTOLOADed
       subroutines and other conditions that cause Perl's symbol
       table to grow.  You might want to add some logic that
       keeps track of the process size, or restarts itself after
       a certain number of requests, to ensure that memory
       consumption is minimized.  You'll also want to scope your
       variables with the my entry in the perlfunc manpage
       whenever possible.

        package Embed::Persistent;
        #persistent.pl

        use strict;
        use vars '%Cache';
        use Symbol qw(delete_package);

        sub valid_package_name {
            my($string) = @_;
            $string =~ s/([^A-Za-z0-9\/])/sprintf("_%2x",unpack("C",$1))/eg;
            # second pass only for words starting with a digit
            $string =~ s|/(\d)|sprintf("/_%2x",unpack("C",$1))|eg;

            # Dress it up as a real package name
            $string =~ s|/|::|g;
            return "Embed" . $string;
        }



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        sub eval_file {
            my($filename, $delete) = @_;
            my $package = valid_package_name($filename);
            my $mtime = -M $filename;
            if(defined $Cache{$package}{mtime}
               &&
               $Cache{$package}{mtime} <= $mtime)
            {
               # we have compiled this subroutine already,
               # it has not been updated on disk, nothing left to do
               print STDERR "already compiled $package->handler\n";
            }
            else {
               local *FH;
               open FH, $filename or die "open '$filename' $!";
               local($/) = undef;
               my $sub = <FH>;
               close FH;

               #wrap the code into a subroutine inside our unique package
               my $eval = qq{package $package; sub handler { $sub; }};
               {
                   # hide our variables within this block
                   my($filename,$mtime,$package,$sub);
                   eval $eval;
               }
               die $@ if $@;

               #cache it unless we're cleaning out each time
               $Cache{$package}{mtime} = $mtime unless $delete;
            }

            eval {$package->handler;};
            die $@ if $@;

            delete_package($package) if $delete;

            #take a look if you want
            #print Devel::Symdump->rnew($package)->as_string, $/;
        }

        1;

        __END__

        /* persistent.c */
        #include <EXTERN.h>
        #include <perl.h>

        /* 1 = clean out filename's symbol table after each request, 0 = don't */
        #ifndef DO_CLEAN
        #define DO_CLEAN 0
        #endif




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


        static PerlInterpreter *perl = NULL;

        int
        main(int argc, char **argv, char **env)
        {
            char *embedding[] = { "", "persistent.pl" };
            char *args[] = { "", DO_CLEAN, NULL };
            char filename [1024];
            int exitstatus = 0;
            STRLEN n_a;

            if((perl = perl_alloc()) == NULL) {
               fprintf(stderr, "no memory!");
               exit(1);
            }
            perl_construct(perl);

            exitstatus = perl_parse(perl, NULL, 2, embedding, NULL);

            if(!exitstatus) {
               exitstatus = perl_run(perl);

               while(printf("Enter file name: ") && gets(filename)) {

                   /* call the subroutine, passing it the filename as an argument */
                   args[0] = filename;
                   perl_call_argv("Embed::Persistent::eval_file",
                                  G_DISCARD | G_EVAL, args);

                   /* check $@ */
                   if(SvTRUE(ERRSV))
                       fprintf(stderr, "eval error: %s\n", SvPV(ERRSV,n_a));
               }
            }

            PL_perl_destruct_level = 0;
            perl_destruct(perl);
            perl_free(perl);
            exit(exitstatus);
        }

       Now compile:

        % cc -o persistent persistent.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

       Here's a example script file:

        #test.pl
        my $string = "hello";
        foo($string);

        sub foo {
            print "foo says: @_\n";
        }



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


       Now run:

        % persistent
        Enter file name: test.pl
        foo says: hello
        Enter file name: test.pl
        already compiled Embed::test_2epl->handler
        foo says: hello
        Enter file name: ^C


       Maintaining multiple interpreter instances

       Some rare applications will need to create more than one
       interpreter during a session.  Such an application might
       sporadically decide to release any resources associated
       with the interpreter.

       The program must take care to ensure that this takes place
       before the next interpreter is constructed.  By default,
       the global variable PL_perl_destruct_level is set to 0,
       since extra cleaning isn't needed when a program has only
       one interpreter.

       Setting PL_perl_destruct_level to 1 makes everything
       squeaky clean:

        PL_perl_destruct_level = 1;

        while(1) {
            ...
            /* reset global variables here with PL_perl_destruct_level = 1 */
            perl_construct(my_perl);
            ...
            /* clean and reset _everything_ during perl_destruct */
            perl_destruct(my_perl);
            perl_free(my_perl);
            ...
            /* let's go do it again! */
        }

       When perl_destruct() is called, the interpreter's syntax
       parse tree and symbol tables are cleaned up, and global
       variables are reset.

       Now suppose we have more than one interpreter instance
       running at the same time.  This is feasible, but only if
       you used the -DMULTIPLICITY flag when building Perl.  By
       default, that sets PL_perl_destruct_level to 1.

       Let's give it a try:

        #include <EXTERN.h>
        #include <perl.h>



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


        /* we're going to embed two interpreters */
        /* we're going to embed two interpreters */

        #define SAY_HELLO "-e", "print qq(Hi, I'm $^X\n)"

        int main(int argc, char **argv, char **env)
        {
            PerlInterpreter
                *one_perl = perl_alloc(),
                *two_perl = perl_alloc();
            char *one_args[] = { "one_perl", SAY_HELLO };
            char *two_args[] = { "two_perl", SAY_HELLO };

            perl_construct(one_perl);
            perl_construct(two_perl);

            perl_parse(one_perl, NULL, 3, one_args, (char **)NULL);
            perl_parse(two_perl, NULL, 3, two_args, (char **)NULL);

            perl_run(one_perl);
            perl_run(two_perl);

            perl_destruct(one_perl);
            perl_destruct(two_perl);

            perl_free(one_perl);
            perl_free(two_perl);
        }

       Compile as usual:

        % cc -o multiplicity multiplicity.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

       Run it, Run it:

        % multiplicity
        Hi, I'm one_perl
        Hi, I'm two_perl


       Using Perl modules, which themselves use C libraries, from
       your C program

       If you've played with the examples above and tried to
       embed a script that use()s a Perl module (such as Socket)
       which itself uses a C or C++ library, this probably
       happened:

        Can't load module Socket, dynamic loading not available in this perl.
         (You may need to build a new perl executable which either supports
         dynamic loading or has the Socket module statically linked into it.)

       What's wrong?




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


       Your interpreter doesn't know how to communicate with
       these extensions on its own.  A little glue will help.  Up
       until now you've been calling perl_parse(), handing it
       NULL for the second argument:

        perl_parse(my_perl, NULL, argc, my_argv, NULL);

       That's where the glue code can be inserted to create the
       initial contact between Perl and linked C/C++ routines.
       Let's take a look some pieces of perlmain.c to see how
       Perl does this:

        #ifdef __cplusplus
        #  define EXTERN_C extern "C"
        #else
        #  define EXTERN_C extern
        #endif

        static void xs_init _((void));

        EXTERN_C void boot_DynaLoader _((CV* cv));
        EXTERN_C void boot_Socket _((CV* cv));

        EXTERN_C void
        xs_init()
        {
               char *file = __FILE__;
               /* DynaLoader is a special case */
               newXS("DynaLoader::boot_DynaLoader", boot_DynaLoader, file);
               newXS("Socket::bootstrap", boot_Socket, file);
        }

       Simply put: for each extension linked with your Perl
       executable (determined during its initial configuration on
       your computer or when adding a new extension), a Perl
       subroutine is created to incorporate the extension's
       routines.  Normally, that subroutine is named
       Module::bootstrap() and is invoked when you say use
       Module.  In turn, this hooks into an XSUB, boot_Module,
       which creates a Perl counterpart for each of the
       extension's XSUBs.  Don't worry about this part; leave
       that to the xsubpp and extension authors.  If your
       extension is dynamically loaded, DynaLoader creates
       Module::bootstrap() for you on the fly.  In fact, if you
       have a working DynaLoader then there is rarely any need to
       link in any other extensions statically.

       Once you have this code, slap it into the second argument
       of perl_parse():

        perl_parse(my_perl, xs_init, argc, my_argv, NULL);

       Then compile:




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


        % cc -o interp interp.c `perl -MExtUtils::Embed -e ccopts -e ldopts`

        % interp
          use Socket;
          use SomeDynamicallyLoadedModule;

          print "Now I can use extensions!\n"'

       ExtUtils::Embed can also automate writing the xs_init glue
       code.

        % perl -MExtUtils::Embed -e xsinit -- -o perlxsi.c
        % cc -c perlxsi.c `perl -MExtUtils::Embed -e ccopts`
        % cc -c interp.c  `perl -MExtUtils::Embed -e ccopts`
        % cc -o interp perlxsi.o interp.o `perl -MExtUtils::Embed -e ldopts`

       Consult the perlxs manpage and the perlguts manpage for
       more details.

Embedding Perl under Win32
       At the time of this writing (5.004), there are two
       versions of Perl which run under Win32.  (The two versions
       are merging in 5.005.)  Interfacing to ActiveState's Perl
       library is quite different from the examples in this
       documentation, as significant changes were made to the
       internal Perl API.  However, it is possible to embed
       ActiveState's Perl runtime.  For details, see the Perl for
       Win32 FAQ at
       http://www.perl.com/CPAN/doc/FAQs/win32/perlwin32faq.html.

       With the "official" Perl version 5.004 or higher, all the
       examples within this documentation will compile and run
       untouched, although the build process is slightly
       different between Unix and Win32.

       For starters, backticks don't work under the Win32 native
       command shell.  The ExtUtils::Embed kit on CPAN ships with
       a script called genmake, which generates a simple makefile
       to build a program from a single C source file.  It can be
       used like this:

        C:\ExtUtils-Embed\eg> perl genmake interp.c
        C:\ExtUtils-Embed\eg> nmake
        C:\ExtUtils-Embed\eg> interp -e "print qq{I'm embedded in Win32!\n}"

       You may wish to use a more robust environment such as the
       Microsoft Developer Studio.  In this case, run this to
       generate perlxsi.c:

        perl -MExtUtils::Embed -e xsinit

       Create a new project and Insert -> Files into Project:
       perlxsi.c, perl.lib, and your own source files, e.g.
       interp.c.  Typically you'll find perl.lib in



29/Apr/1999            perl 5.005, patch 03                    20





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


       C:\perl\lib\CORE, if not, you should see the CORE
       directory relative to perl -V:archlib.  The studio will
       also need this path so it knows where to find Perl include
       files.  This path can be added via the Tools -> Options ->
       Directories menu.  Finally, select Build -> Build
       interp.exe and you're ready to go.

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

AUTHOR
       Jon Orwant <orwant@tpj.com> and Doug MacEachern
       <dougm@osf.org>, with small contributions from Tim Bunce,
       Tom Christiansen, Guy Decoux, Hallvard Furuseth, Dov
       Grobgeld, and Ilya Zakharevich.

       Doug MacEachern has an article on embedding in Volume 1,
       Issue 4 of The Perl Journal (http://tpj.com).  Doug is
       also the developer of the most widely-used Perl embedding:
       the mod_perl system (perl.apache.org), which embeds Perl
       in the Apache web server.  Oracle, Binary Evolution,
       ActiveState, and Ben Sugars's nsapi_perl have used this
       model for Oracle, Netscape and Internet Information Server
       Perl plugins.

       July 22, 1998

COPYRIGHT
       Copyright (C) 1995, 1996, 1997, 1998 Doug MacEachern and
       Jon Orwant.  All Rights Reserved.

       Permission is granted to make and distribute verbatim
       copies of this documentation provided the copyright notice
       and this permission notice are preserved on all copies.

       Permission is granted to copy and distribute modified
       versions of this documentation under the conditions for
       verbatim copying, provided also that they are marked
       clearly as modified versions, that the authors' names and
       title are unchanged (though subtitles and additional
       authors' names may be added), and that the entire
       resulting derived work is distributed under the terms of a
       permission notice identical to this one.

       Permission is granted to copy and distribute translations
       of this documentation into another language, under the
       above conditions for modified versions.








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