Next: perlfaq | Previous: perlebcdic |  | [Table of Contents] | [Index]

# perlembed

- NAME
- DESCRIPTION
  - PREAMBLE
  - 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
- MORAL
- AUTHOR
- COPYRIGHT

## 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

- 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

### Compiling your C program

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

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

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

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

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

    or perhaps just

        /usr/local/lib/perl5/CORE

    or maybe something like

        /usr/opt/perl5/CORE

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

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

    Here's how you'd compile the example in the next section,
    the 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:

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

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

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

You'll also have to choose the appropriate library directory (/usr/local/lib/...) for your machine. If your compiler complains that certain functions are undefined, or that it can't locate -lperl, then you need to change the path following the `-L`. If it complains that it can't find EXTERN.h and perl.h, you need to change the path following the `-I`.

    You may have to add extra libraries as well.  Which ones?
    Perhaps those printed by

       perl -MConfig -e 'print $Config{libs}'

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

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

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

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

### Adding a Perl interpreter to your C program

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

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

        static PerlInterpreter *my_perl;  /***    The Perl interpreter    ***/

        int main(int argc, char **argv, char **env)
        {
            my_perl = perl_alloc();
            perl_construct(my_perl);
            perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
            perl_run(my_perl);
            perl_destruct(my_perl);
            perl_free(my_perl);
        }

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

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

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

    After a successful compilation, you'll be able to use interp just
    like perl itself:

        % interp
        print "Pretty Good Perl \n";
        print "10890 - 9801 is ", 10890 - 9801;
        <CTRL-D>
        Pretty Good Perl
        10890 - 9801 is 1089

    or

        % interp -e 'printf("%x", 3735928559)'
        deadbeef

You can also read and execute Perl statements from a file while in the midst of your C program, by placing the filename in argv[1] before calling perl_run.

### Calling a Perl subroutine from your C program

To call individual Perl subroutines, you can use any of the 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;

        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 arguments (that's the G_NOARGS) and for which I'll ignore the return value (that's the G_DISCARD). Those flags, and others, are discussed in the perlcall manpage.

    I'll define the showtime subroutine in a file called showtime.pl:

        print "I shan't be printed.";

        sub showtime {
            print time;
        }

    Simple enough.  Now compile and run:

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

        % showtime showtime.pl
        818284590

yielding the number of seconds that elapsed between January 1, 1970 (the beginning of the Unix epoch), and the moment I began writing this sentence.

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

If you want to pass arguments to the Perl subroutine, you can add strings to the `NULL`-terminated `args` list passed to 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.

### 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 multiple statements; it can employ the use entry in the perlfunc manpage, the require entry in the perlfunc manpage, and the do entry in the perlfunc manpage to include external Perl files.

    eval_pv lets us evaluate individual Perl strings, and then
    extract variables for coercion into C types.  The following program,
    string.c, executes three Perl strings, extracting an int from
    the first, a float from the second, and a char * from the third.

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

       static PerlInterpreter *my_perl;

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

           my_perl = perl_alloc();
           perl_construct( my_perl );

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

           /** Treat $a as an integer **/
           eval_pv("$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 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 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
    substitutions: match(), substitute(), and matches().

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

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

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

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

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

Given an `SV`, a pattern, and a pointer to an empty `AV`, matches() evaluates `$string =~ $pattern` in 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>

     /** 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)
     {
         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)

       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 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 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" };

          my_perl = perl_alloc();
          perl_construct( my_perl );

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

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

          perl_destruct(my_perl);
          perl_free(my_perl);
        }

    Compile and run:

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

        % power
        3 to the 4th power is 81.

### Maintaining a persistent interpreter

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

However, you have to be more cautious with namespace and variable scoping when using a persistent interpreter. In previous examples we've been using global variables in the default package `main`. We knew exactly what code would be run, and assumed we could avoid variable collisions and outrageous symbol table growth.

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

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

One way to avoid namespace collisions in this scenario is to translate the filename into a guaranteed-unique package name, and then compile the code into that package using the eval entry in the perlfunc manpage. In the example below, each file will only be compiled once. Or, the application might choose to clean out the symbol table associated with the file after it's no longer needed. Using the 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 consumption is minimized. You'll also want to scope your variables with the my entry in the perlfunc manpage whenever possible.

     package Embed::Persistent;
     #persistent.pl

     use strict;
     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

     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:

     #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, the global variable `PL_perl_destruct_level` is set to ``, since extra cleaning isn't needed when a program has only one interpreter.

    Setting PL_perl_destruct_level to 1 makes everything squeaky clean:

     PL_perl_destruct_level = 1;

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

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

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

Let's give it a try:

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

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

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

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

         perl_construct(one_perl);
         perl_construct(two_perl);

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

         perl_run(one_perl);
         perl_run(two_perl);

         perl_destruct(one_perl);
         perl_destruct(two_perl);

         perl_free(one_perl);
         perl_free(two_perl);
     }

    Compile as usual:

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

    Run it, Run it:

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

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

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

     Can't load module Socket, dynamic loading not available in this perl.
      (You may need to build a new p