MirOS Manual: perlembed(1)


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 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

     +    Compiling your C program

     +    Adding a Perl interpreter to your C program

     +    Calling a Perl subroutine from your C program

     +    Evaluating a Perl statement from your C program

     +    Performing Perl pattern matches and substitutions from
          your C program

     +    Fiddling with the Perl stack from your C program

     +    Maintaining a persistent interpreter

     +    Maintaining multiple interpreter instances

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

     +    Embedding Perl under Win32

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     Compiling your C program

     If you have trouble compiling the scripts in this documenta-
     tion, you're not alone.  The cardinal rule: COMPILE THE PRO-
     GRAMS 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 copy-
     ing 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 docu-
     mentation (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:

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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 fol-
     lowing 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 dis-
     tribution, 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 embed-
     ding 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 con-
     taining the essentials of embedding:

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

     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.

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     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".

     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

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     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 exe-
     cutes END blocks in perl_destruct.

     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 per-
     lapi.

     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 multi-
     ple 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)
        {
            STRLEN n_a;
            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);

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

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

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

            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 perl-
     guts 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'd expres-
     sion.  It is also possible and in most cases a better stra-
     tegy to fetch the return value from eval_pv() instead.
     Example:

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

     This way, we avoid namespace pollution by not creating glo-
     bal 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 "spe-
     cialize" in matches and substitutions: match(), substi-
     tute(), and matches().

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        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);

     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;
          STRLEN n_a;

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

          SPAGAIN;
          retval = POPs;
          PUTBACK;

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

          return retval;
      }

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      /** 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_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.
      **/

      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_eval_sv(command, TRUE);
          SvREFCNT_dec(command);

          *string = get_sv("string", FALSE);
          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
      **/

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      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_eval_sv(command, TRUE);
          SvREFCNT_dec(command);

          *match_list = get_av("array", FALSE);
          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;
          SV *text;
          STRLEN n_a;

          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(1099,0);
          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");

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          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);
          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

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        substitute: s/Perl/C...No substitution made.

     Fiddling with the Perl stack from your C program

     When trying to explain stacks, most computer science text-
     books 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 per-
     lapi.

     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 exponentia-
     tion, 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 exponentia-
     tion function in power.pl:

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

     Now I'll create a C program, power.c, with a function Perl-
     Power() 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 */
           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.

     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.

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     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 occasion-
     ally 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 sub-
     routines 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 {
          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;

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          # 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;
          STRLEN n_a;

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

          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(ERRSV,n_a));
             }
          }

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

     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,
     when perl is not built with any special options, the global
     variable "PL_perl_destruct_level" is set to 0, since extra

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     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 run-
     ning 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 ini-
     tialized correctly between consecutive runs and your appli-
     cation may crash.

     Using "-Dusethreads -Duseithreads" rather than "-Dusemulti-
     plicity" 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>

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      /* 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 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()).

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     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)
      {
             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 execut-
     able (determined during its initial configuration on your
     computer or when adding a new extension), a Perl subroutine

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     is created to incorporate the extension's routines.  Nor-
     mally, 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 exam-
     ples 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 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}"

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     You may wish to use a more robust environment such as the
     Microsoft Developer Studio.  In this case, run this to gen-
     erate 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 explana-
     tion of the "aTHX_"." in perlguts )  Hiding the short forms
     is very useful for avoiding all sorts of nasty (C preproces-
     sor 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 con-
     flict.)

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,
     Issue 4 of The Perl Journal ( http://www.tpj.com/ ).  Doug
     is also the developer of the most widely-used Perl embed-
     ding: 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

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     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 ver-
     sions of this documentation under the conditions for verba-
     tim 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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