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NAME
       perlembed - how to embed perl in your C program

DESCRIPTION
   PREAMBLE
       Do you want to:

       Use C from Perl?
            Read perlxstut, perlxs, h2xs, perlguts, and perlapi.

       Use a Unix program from Perl?
            Read about back-quotes and about "system" and "exec"
            in perlfunc.

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

       Use C from C?
            Rethink your design.

       Use Perl from C?
            Read on...

   ROADMAP
       o    Compiling your C program

       o    Adding a Perl interpreter to your C program

       o    Calling a Perl subroutine from your C program

       o    Evaluating a Perl statement from your C program

       o    Performing Perl pattern matches and substitutions
            from your C program

       o    Fiddling with the Perl stack from your C program

       o    Maintaining a persistent interpreter

       o    Maintaining multiple interpreter instances

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

       o    Embedding Perl under Win32

   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



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       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,
       "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:

           % 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.



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       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, non-portable version
       of miniperlmain.c containing the essentials of embedding:

           #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)
           {
               PERL_SYS_INIT3(&argc,&argv,&env);
               my_perl = perl_alloc();
               perl_construct(my_perl);
               PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
               perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
               perl_run(my_perl);



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               perl_destruct(my_perl);
               perl_free(my_perl);
               PERL_SYS_TERM();
           }

       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.

       The macros PERL_SYS_INIT3() and PERL_SYS_TERM() provide
       system-specific tune up of the C runtime environment
       necessary to run Perl interpreters; they should only be
       called once regardless of how many interpreters you create
       or destroy. Call PERL_SYS_INIT3() before you create your
       first interpreter, and PERL_SYS_TERM() after you free your
       last interpreter.

       Since PERL_SYS_INIT3() may change "env", it may be more
       appropriate to provide "env" as an argument to
       perl_parse().

       Also notice that no matter what arguments you pass to
       perl_parse(), PERL_SYS_INIT3() must be invoked on the C
       main() argc, argv and env and only once.

       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 call_* functions documented in perlcall.  In this
       example we'll use "call_argv".



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

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

           static PerlInterpreter *my_perl;

           int main(int argc, char **argv, char **env)
           {
               char *args[] = { NULL };
               PERL_SYS_INIT3(&argc,&argv,&env);
               my_perl = perl_alloc();
               perl_construct(my_perl);

               perl_parse(my_perl, NULL, argc, argv, NULL);
               PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

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

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

               perl_destruct(my_perl);
               perl_free(my_perl);
               PERL_SYS_TERM();
           }

       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 perlcall.

       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, as
       we set the PL_exit_flag PERL_EXIT_DESTRUCT_END which
       executes END blocks in perl_destruct.



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

   Evaluating a Perl statement from your C program
       Perl provides two API functions to evaluate pieces of Perl
       code.  These are "eval_sv" in perlapi and "eval_pv" in
       perlapi.

       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 "use" in perlfunc,
       "require" in perlfunc, and "do" in perlfunc to include
       external Perl files.

       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)
          {
              char *embedding[] = { "", "-e", "0" };

              PERL_SYS_INIT3(&argc,&argv,&env);
              my_perl = perl_alloc();
              perl_construct( my_perl );

              perl_parse(my_perl, NULL, 3, embedding, NULL);
              PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
              perl_run(my_perl);

              /** Treat $a as an integer **/
              eval_pv("$a = 3; $a **= 2", TRUE);
              printf("a = %d\n", SvIV(get_sv("a", 0)));

              /** Treat $a as a float **/
              eval_pv("$a = 3.14; $a **= 2", TRUE);
              printf("a = %f\n", SvNV(get_sv("a", 0)));

              /** Treat $a as a string **/
              eval_pv("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
              printf("a = %s\n", SvPV_nolen(get_sv("a", 0)));




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              perl_destruct(my_perl);
              perl_free(my_perl);
              PERL_SYS_TERM();
          }

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

       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'ed
       expression.  It is also possible and in most cases a
       better strategy to fetch the return value from eval_pv()
       instead.  Example:

          ...
          SV *val = eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
          printf("%s\n", SvPV_nolen(val));
          ...

       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 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 "SV" as according to the operation,
       returning the number of substitutions made.

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




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

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

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

        static PerlInterpreter *my_perl;

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


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

            SPAGAIN;
            retval = POPs;
            PUTBACK;

            if (croak_on_error && SvTRUE(ERRSV))
               croak(SvPVx_nolen(ERRSV));

            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(0), *retval;

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

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

            return SvIV(retval);



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        }

        /** 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.
        **/

        I32 substitute(SV **string, char *pattern)
        {
            SV *command = newSV(0), *retval;

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

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

            *string = get_sv("string", 0);
            return SvIV(retval);
        }

        /** matches(string, pattern, matches)
        **
        ** Used for matches in a list 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(0);
            I32 num_matches;

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

            my_eval_sv(command, TRUE);
            SvREFCNT_dec(command);

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

            return num_matches;
        }

        main (int argc, char **argv, char **env)
        {
            char *embedding[] = { "", "-e", "0" };
            AV *match_list;
            I32 num_matches, i;



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            SV *text;

            PERL_SYS_INIT3(&argc,&argv,&env);
            my_perl = perl_alloc();
            perl_construct(my_perl);
            perl_parse(my_perl, NULL, 3, embedding, NULL);
            PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

            text = newSV(0);
            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_nolen(*av_fetch(match_list, i, FALSE)));
            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_nolen(text));
            }

            /** 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);



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            perl_free(my_perl);
            PERL_SYS_TERM();
        }

       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

          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 perlguts and
       perlapi.

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

       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;
           }



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       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;

           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 */
             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" };

             PERL_SYS_INIT3(&argc,&argv,&env);
             my_perl = perl_alloc();
             perl_construct( my_perl );

             perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
             PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
             perl_run(my_perl);

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

             perl_destruct(my_perl);
             perl_free(my_perl);
             PERL_SYS_TERM();
           }

       Compile and run:

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

           % power
           3 to the 4th power is 81.



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   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
       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
       "eval" in perlfunc.  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 "call_argv" in
       perlapi, 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 "AUTOLOAD"ed
       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 "my" in perlfunc whenever possible.

        package Embed::Persistent;
        #persistent.pl

        use strict;
        our %Cache;
        use Symbol qw(delete_package);

        sub valid_package_name {



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            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;
        }

        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__



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        /* 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

        #define BUFFER_SIZE 1024

        static PerlInterpreter *my_perl = NULL;

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

            PERL_SYS_INIT3(&argc,&argv,&env);
            if((my_perl = perl_alloc()) == NULL) {
               fprintf(stderr, "no memory!");
               exit(1);
            }
            perl_construct(my_perl);

            PL_origalen = 1; /* don't let $0 assignment update the proctitle or embedding[0] */
            exitstatus = perl_parse(my_perl, NULL, 2, embedding, NULL);
            PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
            if(!exitstatus) {
               exitstatus = perl_run(my_perl);

               while(printf("Enter file name: ") &&
                     fgets(filename, BUFFER_SIZE, stdin)) {

                   filename[strlen(filename)-1] = '\0'; /* strip \n */
                   /* call the subroutine, passing it the filename as an argument */
                   args[0] = filename;
                   call_argv("Embed::Persistent::eval_file",
                                  G_DISCARD | G_EVAL, args);

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

            PL_perl_destruct_level = 0;
            perl_destruct(my_perl);
            perl_free(my_perl);
            PERL_SYS_TERM();



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            exit(exitstatus);
        }

       Now compile:

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

       Here's an example script file:

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

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

       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

   Execution of END blocks
       Traditionally END blocks have been executed at the end of
       the perl_run.  This causes problems for applications that
       never call perl_run. Since perl 5.7.2 you can specify
       "PL_exit_flags |= PERL_EXIT_DESTRUCT_END" to get the new
       behaviour. This also enables the running of END blocks if
       the perl_parse fails and "perl_destruct" will return the
       exit value.

   $0 assignments
       When a perl script assigns a value to $0 then the perl
       runtime will try to make this value show up as the program
       name reported by "ps" by updating the memory pointed to by
       the argv passed to perl_parse() and also calling API
       functions like setproctitle() where available.  This
       behaviour might not be appropriate when embedding perl and
       can be disabled by assigning the value 1 to the variable
       "PL_origalen" before perl_parse() is called.

       The persistent.c example above is for instance likely to
       segfault when $0 is assigned to if the "PL_origalen = 1;"
       assignment is removed.  This because perl will try to
       write to the read only memory of the "embedding[]"
       strings.

   Maintaining multiple interpreter instances
       Some rare applications will need to create more than one



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       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,
       when perl is not built with any special options, the
       global variable "PL_perl_destruct_level" is set to 0,
       since extra cleaning isn't usually needed when a program
       only ever creates a single interpreter in its entire
       lifetime.

       Setting "PL_perl_destruct_level" to 1 makes everything
       squeaky clean:

        while(1) {
            ...
            /* reset global variables here with PL_perl_destruct_level = 1 */
            PL_perl_destruct_level = 1;
            perl_construct(my_perl);
            ...
            /* clean and reset _everything_ during perl_destruct */
            PL_perl_destruct_level = 1;
            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.  The second assignment to
       "PL_perl_destruct_level" is needed because perl_construct
       resets it to 0.

       Now suppose we have more than one interpreter instance
       running at the same time.  This is feasible, but only if
       you used the Configure option "-Dusemultiplicity" or the
       options "-Dusethreads -Duseithreads" when building perl.
       By default, enabling one of these Configure options sets
       the per-interpreter global variable
       "PL_perl_destruct_level" to 1, so that thorough cleaning
       is automatic and interpreter variables are initialized
       correctly.  Even if you don't intend to run two or more
       interpreters at the same time, but to run them
       sequentially, like in the above example, it is recommended
       to build perl with the "-Dusemultiplicity" option
       otherwise some interpreter variables may not be
       initialized correctly between consecutive runs and your
       application may crash.

       See also "Thread-aware system interfaces" in perlxs.

       Using "-Dusethreads -Duseithreads" rather than



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       "-Dusemultiplicity" is more appropriate if you intend to
       run multiple interpreters concurrently in different
       threads, because it enables support for linking in the
       thread libraries of your system with the interpreter.

       Let's give it a try:

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

        /* 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, *two_perl;
            char *one_args[] = { "one_perl", SAY_HELLO };
            char *two_args[] = { "two_perl", SAY_HELLO };

            PERL_SYS_INIT3(&argc,&argv,&env);
            one_perl = perl_alloc();
            two_perl = perl_alloc();

            PERL_SET_CONTEXT(one_perl);
            perl_construct(one_perl);
            PERL_SET_CONTEXT(two_perl);
            perl_construct(two_perl);

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

            PERL_SET_CONTEXT(one_perl);
            perl_run(one_perl);
            PERL_SET_CONTEXT(two_perl);
            perl_run(two_perl);

            PERL_SET_CONTEXT(one_perl);
            perl_destruct(one_perl);
            PERL_SET_CONTEXT(two_perl);
            perl_destruct(two_perl);

            PERL_SET_CONTEXT(one_perl);
            perl_free(one_perl);
            PERL_SET_CONTEXT(two_perl);
            perl_free(two_perl);
            PERL_SYS_TERM();
        }

       Note the calls to PERL_SET_CONTEXT().  These are necessary
       to initialize the global state that tracks which



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       interpreter is the "current" one on the particular process
       or thread that may be running it.  It should always be
       used if you have more than one interpreter and are making
       perl API calls on both interpreters in an interleaved
       fashion.

       PERL_SET_CONTEXT(interp) should also be called whenever
       "interp" is used by a thread that did not create it (using
       either perl_alloc(), or the more esoteric perl_clone()).

       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?

       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:

        static void xs_init (pTHX);

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


        EXTERN_C void
        xs_init(pTHX)
        {



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               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:

        % 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 perlxs, perlguts, and perlapi for more details.

Embedding Perl under Win32
       In general, all of the source code shown here should work
       unmodified under Windows.

       However, there are some caveats about the command-line
       examples shown.  For starters, backticks won't work under
       the Win32 native command shell.  The ExtUtils::Embed kit
       on CPAN ships with a script called genmake, which



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       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
       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.

Hiding Perl_
       If you completely hide the short forms forms of the Perl
       public API, add -DPERL_NO_SHORT_NAMES to the compilation
       flags.  This means that for example instead of writing

           warn("%d bottles of beer on the wall", bottlecount);

       you will have to write the explicit full form

           Perl_warn(aTHX_ "%d bottles of beer on the wall", bottlecount);

       (See "Background and PERL_IMPLICIT_CONTEXT for the
       explanation of the "aTHX_"." in perlguts )  Hiding the
       short forms is very useful for avoiding all sorts of nasty
       (C preprocessor or otherwise) conflicts with other
       software packages (Perl defines about 2400 APIs with these
       short names, take or leave few hundred, so there certainly
       is room for conflict.)

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@media.mit.edu> and Doug MacEachern
       <dougm@covalent.net>, 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,



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       Issue 4 of The Perl Journal ( http://www.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.

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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