perlembed



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 pro-
            gram

       ·    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

       Compiling your C program

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

       Also, every C program that uses Perl must link in the perl library.
       What’s that, you ask?  Perl is itself written in C; the perl library is
       the collection of compiled C programs that were used to create your
       perl executable (/usr/bin/perl or equivalent).  (Corollary: you can’t
       use Perl from your C program unless Perl has been compiled on your
       machine, or installed properly--that’s why you shouldn’t blithely copy
       Perl executables from machine to machine without also copying the lib
       directory.)

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

       If your copy of Perl is recent enough to contain this documentation
       (version 5.002 or later), then the perl library (and EXTERN.h and
       perl.h, which you’ll also need) will reside in a directory that looks
       like this:

           /usr/local/lib/perl5/your_architecture_here/CORE

       or perhaps just

           /usr/local/lib/perl5/CORE

       or maybe something like

           /usr/opt/perl5/CORE

       Execute this statement for a hint about where to find CORE:

           perl -MConfig -e ’print $Config{archlib}’

       Here’s how you’d compile the example in the next section, "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.

       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 follow-
       ing 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-mod-
       ule/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 infor-
       mation you may find useful.

       Adding a Perl interpreter to your C program

       In a sense, perl (the C program) is a good example of embedding Perl
       (the language), so I’ll demonstrate embedding with miniperlmain.c,
       included in the source distribution.  Here’s a bastardized, nonportable
       version of miniperlmain.c containing the essentials of embedding:

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

           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 pro-
       vide "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.

       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_DIS-
       CARD).  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.

       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 manipu-
       late 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 snip-
       pets 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 exter-
       nal 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);

              /** 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 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’d expression.  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 "specialize" in matches and sub-
       stitutions: 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);

       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;
        }

        /** 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
        **/

        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");

            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

          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;
           }

       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.

       Maintaining a persistent interpreter

       When developing interactive and/or potentially long-running applica-
       tions, 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 scop-
       ing 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 con-
       structed interpreter, and subsequently cleaned out with
       "perl_destruct()" afterwards, you’re shielded from most namespace trou-
       bles.

       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 "per-
       sistent.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 condi-
       tions 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;

            # 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>

        /* 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));
               }
            }

            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 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 sec-
       ond assignment to "PL_perl_destruct_level" is needed because perl_con-
       struct 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 "-Dusemul-
       tiplicity" option otherwise some interpreter variables may not be ini-
       tialized correctly between consecutive runs and your application may
       crash.

       Using "-Dusethreads -Duseithreads" rather than "-Dusemultiplicity" is
       more appropriate if you intend to run multiple interpreters concur-
       rently 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 initial-
       ize 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()).

       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 pro-
       gram

       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 con-
       tact 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 executable (deter-
       mined 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::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 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 gen-
       make, which generates a simple makefile to build a program from a sin-
       gle 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@cova-
       lent.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 embedding: the mod_perl system
       (perl.apache.org), which embeds Perl in the Apache web server.  Oracle,
       Binary Evolution, ActiveState, and Ben Sugars’s nsapi_perl have used
       this model for Oracle, Netscape and Internet Information Server Perl
       plugins.

       July 22, 1998


COPYRIGHT

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

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

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

       Permission is granted to copy and distribute translations of this docu-
       mentation into another language, under the above conditions for modi-
       fied versions.



perl v5.8.6                       2004-11-05                      PERLEMBED(1)

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