


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, the perlguts manpage, and the perlapi
            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


       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




2001-05-24                 perl v5.6.1                          1





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


       Compiling your C program

       If you have trouble compiling the scripts in this documen-
       tation, you're not alone.  The cardinal rule: COMPILE THE
       PROGRAMS IN EXACTLY THE SAME WAY THAT YOUR PERL WAS COM-
       PILED.  (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 pro-
       grams 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--usu-
       ally--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 doc-
       umentation (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 Adding a Perl interpreter to your C program entry
       elsewhere in this document, 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:





2001-05-24                 perl v5.6.1                          2





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


           % 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 $Con-
       fig{cc}'" will tell you what to use.

       You'll also have to choose the appropriate library direc-
       tory (/usr/local/lib/...) for your machine.  If your com-
       piler 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 fol-
       lowing 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
       embedding Perl (the language), so I'll demonstrate embed-
       ding with miniperlmain.c, included in the source distribu-
       tion.  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     */




2001-05-24                 perl v5.6.1                          3





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


           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 envi-
       ronment 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 file-
       name 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 the perlcall manpage.
       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;




2001-05-24                 perl v5.6.1                          4





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


           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() ***/

               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 argu-
       ments (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 show-
       time.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 Jan-
       uary 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 call_argv.  For other data types, or to examine
       return values, you'll need to manipulate the Perl stack.
       That's demonstrated in the Fiddling with the Perl stack
       from your C program entry elsewhere in this document.




2001-05-24                 perl v5.6.1                          5





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


       Evaluating a Perl statement from your C program

       Perl provides two API functions to evaluate pieces of Perl
       code.  These are the eval_sv entry in the perlapi manpage
       and the eval_pv entry in the perlapi 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 mul-
       tiple statements; it can employ the use entry in the perl-
       func manpage, the require entry in the perlfunc manpage,
       and the do entry in the perlfunc manpage to include exter-
       nal Perl files.

       eval_pv lets us evaluate individual Perl strings, and then
       extract variables for coercion into C types.  The follow-
       ing 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 $a as an integer **/
              eval_pv("$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);
          }

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



2001-05-24                 perl v5.6.1                          6





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


       perlguts manpage and the perlapi 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 expres-
       sion.  It is also possible and in most cases a better
       strategy 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
       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 "spe-
       cialize" in matches and substitutions: match(), substi-
       tute(), 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 a list con-
       text, and fills in matches with the array elements,
       returning the number of matches found.




2001-05-24                 perl v5.6.1                          7





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


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

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

        /** 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;
        }

        /** 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);
        }








2001-05-24                 perl v5.6.1                          8





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


        /** 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
        **/

        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;
        }









2001-05-24                 perl v5.6.1                          9





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


        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);

            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)




2001-05-24                 perl v5.6.1                         10





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


          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 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 pur-
       poses: 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 sub-
       routine--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 and the perlapi 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 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 expo-
       nentiation 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...



2001-05-24                 perl v5.6.1                         11





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


           #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" };

             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-run-
       ning applications, it's a good idea to maintain a persis-
       tent interpreter rather than allocating and constructing a
       new interpreter multiple times.  The major reason is



2001-05-24                 perl v5.6.1                         12





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


       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 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
       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 call_argv entry in the perlapi 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 "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 con-
       sumption is minimized.  You'll also want to scope your
       variables with the my entry in the perlfunc manpage when-
       ever 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;




2001-05-24                 perl v5.6.1                         13





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


            # 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__

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




2001-05-24                 perl v5.6.1                         14





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


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

        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;
                   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:






2001-05-24                 perl v5.6.1                         15





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


        #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


       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 cleaning isn't usually needed when a program
       only ever creates a single interpreter in its entire life-
       time.

       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.



2001-05-24                 perl v5.6.1                         16





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


       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.

       Using "-Dusethreads -Duseithreads" rather than "-Dusemul-
       tiplicity" is more appropriate if you intend to run multi-
       ple 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 = perl_alloc(),
                *two_perl = perl_alloc();
            char *one_args[] = { "one_perl", SAY_HELLO };
            char *two_args[] = { "two_perl", SAY_HELLO };

            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);





2001-05-24                 perl v5.6.1                         17





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


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

       Note the calls to PERL_SET_CONTEXT().  These are necessary
       to initialize the global state that tracks which inter-
       preter 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 hap-
       pened:

        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:




2001-05-24                 perl v5.6.1                         18





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


        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 exe-
       cutable (determined during its initial configuration on
       your computer or when adding a new extension), a Perl sub-
       routine is created to incorporate the extension's rou-
       tines.  Normally, that subroutine is named Module::boot-
       strap() 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 exten-
       sion 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 stati-
       cally.

       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 the perlxs manpage, the perlguts manpage, and the
       perlapi manpage for more details.



2001-05-24                 perl v5.6.1                         19





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


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 gener-
       ates 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 direc-
       tory 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





2001-05-24                 perl v5.6.1                         20





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


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 ver-
       batim 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.





































2001-05-24                 perl v5.6.1                         21


