path: root/pod/perlembed.pod

=head1 NAME

perlembed - how to embed perl in your C program

=head1 DESCRIPTION

=head2 PREAMBLE

Do you want to:

=over 5

=item B<Use C from Perl?>

Read L<perlcall> and L<perlxs>.

=item B<Use a UNIX program from Perl?>

Read about back-quotes and about C<system> and C<exec> in L<perlfunc>.

=item B<Use Perl from Perl?>

Read about C<do> and C<eval> in L<perlfunc/do> and L<perlfunc/eval> and C<use>
and C<require> in L<perlmod> and L<perlfunc/require>, L<perlfunc/use>.

=item B<Use C from C?>

Rethink your design.

=item B<Use Perl from C?>

Read on...

=back

=head2 ROADMAP

L<Compiling your C program>

There's one example in each of the eight sections:

L<Adding a Perl interpreter to your C program>

L<Calling a Perl subroutine from your C program>

L<Evaluating a Perl statement from your C program>

L<Performing Perl pattern matches and substitutions from your C program>

L<Fiddling with the Perl stack from your C program>

L<Maintaining a persistent interpreter>

L<Maintaining multiple interpreter instances>

L<Using Perl modules, which themselves use C libraries, from your C program>

This documentation is UNIX specific.

=head2 Compiling your C program

Every C program that uses Perl must link in the I<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 (I</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
I<lib> directory.)

Your C program will--usually--allocate, "run", and deallocate a
I<PerlInterpreter> object, which is defined in 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 I<EXTERN.h> and
I<perl.h>, which you'll also need) will
reside in a directory resembling 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 might compile the example in the next section,
L<Adding a Perl interpreter to your C program>,
on a DEC Alpha running the OSF operating system:

    % cc -o interp interp.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
    -I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm

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

We strongly recommend you use the B<ExtUtils::Embed> module to determine 
all of this information for you:

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


If the B<ExtUtils::Embed> module is not part of your perl kit's
distribution you can retrieve it from:
http://www.perl.com/cgi-bin/cpan_mod?module=ExtUtils::Embed.


=head2 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 I<miniperlmain.c>,
from the source distribution.  Here's a bastardized, non-portable version of
I<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);
    }

Note that we do not use the C<env> pointer here or in any of the
following examples.
Normally handed to C<perl_parse> as its final argument,
we hand it a B<NULL> instead, in which case the current environment
is used.

Now compile this program (I'll call it I<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 I<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 I<argv[1]> before
calling I<perl_run()>.

=head2 Calling a Perl subroutine from your C program

To call individual Perl subroutines, you can use any of the B<perl_call_*>
functions documented in the L<perlcall> man page.
In this example we'll use I<perl_call_argv>.

That's shown below, in a program I'll call I<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() ***/

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

        perl_destruct(my_perl);
        perl_free(my_perl);
    }

where I<showtime> is a Perl subroutine that takes no arguments (that's the
I<G_NOARGS>) and for which I'll ignore the return value (that's the
I<G_DISCARD>).  Those flags, and others, are discussed in L<perlcall>.

I'll define the I<showtime> subroutine in a file called I<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.

Note that in this particular case we are not required to call I<perl_run>,
however, in general it's considered good practice to ensure proper 
initialization of library code including execution of all object C<DESTROY>
methods and package C<END {}> blocks.

If you want to pass some arguments to the Perl subroutine, you may add
strings to the C<NULL> terminated C<args> list passed to I<perl_call_argv>.
In order to pass arguments of another data type and/or examine return values
of the subroutine you'll need to manipulate the
Perl stack, demonstrated in the last section of this document:
L<Fiddling with the Perl stack from your C program>

=head2 Evaluating a Perl statement from your C program

One way to evaluate pieces of Perl code is to use L<perlguts/perl_eval_sv>.
We have wrapped this function with our own I<perl_eval()> function, which
converts a command string to an SV, passing this and the L<perlcall/G_DISCARD>
flag to L<perlguts/perl_eval_sv>.

Arguably, this is the only routine you'll ever need to execute
snippets of Perl code from within your C program.  Your string can be
as long as you wish; it can contain multiple statements; it can
include L<perlfunc/use>, L<perlfunc/require> and L<perlfunc/do> to
include external Perl files.

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

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

   static PerlInterpreter *my_perl;

   I32 perl_eval(char *string)
   {
     return perl_eval_sv(newSVpv(string,0), G_DISCARD);
   }

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

     my_perl = perl_alloc();
     perl_construct( my_perl );

     perl_parse(my_perl, NULL, 3, embedding, NULL);
     perl_run(my_perl);
                                       /** Treat $a as an integer **/
     perl_eval("$a = 3; $a **= 2");
     printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));

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

                                       /** Treat $a as a string **/
     perl_eval("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a); ");
     printf("a = %s\n", SvPV(perl_get_sv("a", FALSE), length));

     perl_destruct(my_perl);
     perl_free(my_perl);
   }

All of those strange functions with I<sv> in their names help convert Perl scalars to C types.  They're described in L<perlguts>.

If you compile and run I<string.c>, you'll see the results of using
I<SvIV()> to create an C<int>, I<SvNV()> to create a C<float>, and
I<SvPV()> to create a string:

   a = 9
   a = 9.859600
   a = Just Another Perl Hacker


=head2 Performing Perl pattern matches and substitutions from your C program

Our I<perl_eval()> lets us evaluate strings of Perl code, so we can
define some functions that use it to "specialize" in matches and
substitutions: I<match()>, I<substitute()>, and I<matches()>.

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

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


   int substitute(char *string[], char *pattern);

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

   int matches(char *string, char *pattern, char **matches[]);

Given a string, a pattern, and a pointer to an empty array of strings,
evaluates C<$string =~ $pattern> in an array context, and fills in
I<matches> with the array elements (allocating memory as it does so),
returning the number of matches found.

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

   #include <EXTERN.h>
   #include <perl.h>
   static PerlInterpreter *my_perl;
   I32 perl_eval(char *string)
   {
      return perl_eval_sv(newSVpv(string,0), G_DISCARD);
   }
   /** match(string, pattern)
   **
   ** Used for matches in a scalar context.
   **
   ** Returns 1 if the match was successful; 0 otherwise.
   **/
   char match(char *string, char *pattern)
   {
     char *command;
     command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 37);
     sprintf(command, "$string = '%s'; $return = $string =~ %s",
                      string, pattern);
     perl_eval(command);
     free(command);
     return SvIV(perl_get_sv("return", FALSE));
   }
   /** substitute(string, pattern)
   **
   ** Used for =~ operations that modify their left-hand side (s/// and tr///)
   **
   ** Returns the number of successful matches, and
   ** modifies the input string if there were any.
   **/
   int substitute(char *string[], char *pattern)
   {
     char *command;
     STRLEN length;
     command = malloc(sizeof(char) * strlen(*string) + strlen(pattern) + 35);
     sprintf(command, "$string = '%s'; $ret = ($string =~ %s)",
                      *string, pattern);
     perl_eval(command);
     free(command);
     *string = SvPV(perl_get_sv("string", FALSE), length);
     return SvIV(perl_get_sv("ret", FALSE));
   }
   /** matches(string, pattern, matches)
   **
   ** Used for matches in an array context.
   **
   ** Returns the number of matches,
   ** and fills in **matches with the matching substrings (allocates memory!)
   **/
   int matches(char *string, char *pattern, char **match_list[])
   {
     char *command;
     SV *current_match;
     AV *array;
     I32 num_matches;
     STRLEN length;
     int i;
     command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 38);
     sprintf(command, "$string = '%s'; @array = ($string =~ %s)",
                      string, pattern);
     perl_eval(command);
     free(command);
     array = perl_get_av("array", FALSE);
     num_matches = av_len(array) + 1; /** assume $[ is 0 **/
     *match_list = (char **) malloc(sizeof(char *) * num_matches);
     for (i = 0; i <= num_matches; i++) {
       current_match = av_shift(array);
       (*match_list)[i] = SvPV(current_match, length);
     }
     return num_matches;
   }
   main (int argc, char **argv, char **env)
   {
     char *embedding[] = { "", "-e", "0" };
     char *text, **match_list;
     int num_matches, i;
     int j;
     my_perl = perl_alloc();
     perl_construct( my_perl );
     perl_parse(my_perl, NULL, 3, embedding, NULL);
     perl_run(my_perl);

     text = (char *) malloc(sizeof(char) * 486); /** A long string follows! **/
     sprintf(text, "%s", "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", match_list[i]);
     printf("\n");
     for (i = 0; i < num_matches; i++) {
       free(match_list[i]);
     }
     free(match_list);
     /** 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", text);
     }
     /** Attempt a substitution **/
     if (!substitute(&text, "s/Perl/C/")) {
       printf("substitute: s/Perl/C...No substitution made.\n\n");
     }
     free(text);
     perl_destruct(my_perl);
     perl_free(my_perl);
   }

which produces the output (again, long lines have been wrapped here)

   perl_match: Text contains the word 'quarter'.

   perl_match: Text doesn't contain the word 'eighth'.

   perl_matches: m/(wi..)/g found 2 matches...
   match: will
   match: with

   perl_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

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

=head2 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 L<perlguts>.

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

Once you've understood those, embedding Perl in C is easy.

Because C has no built-in 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 I<pow()> function).  First
I'll create a stub exponentiation function in I<power.pl>:

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

Now I'll create a C program, I<power.c>, with a function
I<PerlPower()> that contains all the perlguts necessary to push the
two arguments into I<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 */
      perl_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[2];

      my_perl = perl_alloc();
      perl_construct( my_perl );

      my_argv[1] = (char *) malloc(10);
      sprintf(my_argv[1], "power.pl");

      perl_parse(my_perl, NULL, argc, my_argv, 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.

=head2 Maintaining a persistent interpreter

When developing interactive, 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 gain here is
speed, avoiding the penalty of Perl start-up time.  However, a persistent
interpreter will require you to be more cautious in your use of namespace
and variable scoping.  In previous examples we've been using global variables
in the default package B<main>.  We knew exactly what code would be run, 
making it safe to assume we'd avoid any variable collision or outrageous 
symbol table growth.  

Let's say your application is a server, which must run perl code from an 
arbitrary file during each transaction.  Your server has no way of knowing
what code is inside anyone of these files.  
If the file was pulled in by B<perl_parse()>, compiled into a newly 
constructed interpreter, then cleaned out with B<perl_destruct()> after the
the transaction, you'd be shielded from most namespace troubles.

One way to avoid namespace collisions in this scenerio, is to translate the
file name into a valid Perl package name, which is most likely to be unique,
then compile the code into that package using L<perlfunc/eval>.
In the example below, each file will only be compiled once, unless it is
updated on disk.  
Optionally, the application may choose to clean out the symbol table
associated with the file after we are done with it.  We'll call the subroutine
B<Embed::Persistent::eval_file> which lives in the file B<persistent.pl>, with
L<perlcall/perl_call_argv>, passing the filename and boolean cleanup/cache
flag as arguments.

Note that the process will continue to grow for each file that is compiled,
and each file it pulls in via L<perlfunc/require>, L<perlfunc/use> or
L<perlfunc/do>.  In addition, there maybe B<AUTOLOAD>ed subroutines and 
other conditions that cause Perl's symbol table to grow.  You may wish to
add logic which keeps track of process size or restarts itself after n number
of requests to ensure memory consumption is kept to a minimum.  You also need
to consider the importance of variable scoping with L<perlfunc/my> to futher
reduce symbol table growth.

 
 package Embed::Persistent;
 #persistent.pl
 
 use strict;
 use vars '%Cache';
 
 #use Devel::Symdump ();
 
 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;
 }
 
 #borrowed from Safe.pm
 sub delete_package {
     my $pkg = shift;
     my ($stem, $leaf);
 
     no strict 'refs';
     $pkg = "main::$pkg\::";    # expand to full symbol table name
     ($stem, $leaf) = $pkg =~ m/(.*::)(\w+::)$/;
 
     my $stem_symtab = *{$stem}{HASH};
 
     delete $stem_symtab->{$leaf};
 }
 
 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;
 
     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;
            perl_call_argv("Embed::Persistent::eval_file", 
                           G_DISCARD | G_EVAL, args);
 
            /* check $@ */
            if(SvTRUE(GvSV(errgv))) 
                fprintf(stderr, "eval error: %s\n", SvPV(GvSV(errgv),na));
        }
     }
     
     perl_destruct_level = 0;
     perl_destruct(perl); 
     perl_free(perl); 
     exit(exitstatus);
 }

 
Now compile:

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

=head2 Maintaining multiple interpreter instances

The previous examples have gone through several steps to startup, use and
shutdown an embedded Perl interpreter.  Certain applications may require
more than one instance of an interpreter to be created during the lifespan
of a single process.  Such an application may take different approaches in
it's use of interpreter objects.  For example, a particular transaction may
want to create an interpreter instance, then release any resources associated
with the object once the transaction is completed.  When a single process 
does this once, resources are released upon exit of the program and the next
time it starts, the interpreter's global state is fresh.

In the same process, the program must take care to ensure that these
actions take place before constructing a new interpreter.  By default, the 
global variable C<perl_destruct_level> is set to C<0> since extra cleaning
is not needed when a program constructs a single interpreter, such as the 
perl executable itself in C</usr/bin/perl> or some such.

You can tell Perl to make everything squeeky clean by setting 
C<perl_destruct_level> to C<1>.

 perl_destruct_level = 1; /* perl global variable */
 while(1) {
     ...
     /* reset global variables here with perl_destruct_level = 1 */
     perl_contruct(my_perl); 
     ...
     /* clean and reset _everything_ during perl_destruct */
     perl_destruct(my_perl); /* ah, nice and fresh */
     perl_free(my_perl);      
     ...
     /* let's go do it again! */
 }

Now, when I<perl_destruct()> is called, the interpreter's syntax parsetree 
and symbol tables are cleaned out, along with reseting global variables.  

So, we've seen how to startup and shutdown an interpreter more than once
in the same process, but there was only one instance in existance at any
one time.  Hmm, wonder if we can have more than one interpreter instance 
running at the _same_ time?  
Indeed this is possible, however when you build Perl, you must compile with
C<-DMULTIPLICITY>.  

It's a little tricky for the Perl runtime to handle multiple interpreters, 
introducing some overhead that most programs with a single interpreter don't
get burdened with.  When you compile with C<-DMULTIPLICITY>, by default, 
C<perl_destruct_level> is set to C<1> for each 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_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

=head2 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 I<use()>s a Perl module (such as I<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 I<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 I<perlmain.c>
to see how Perl does this:


 #ifdef __cplusplus
 #  define EXTERN_C extern "C"
 #else
 #  define EXTERN_C extern
 #endif

 static void xs_init _((void));

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


 EXTERN_C void
 xs_init()
 {
        char *file = __FILE__;
        /* DynaLoader is a special case */
        newXS("DynaLoader::boot_DynaLoader", boot_DynaLoader, file);
        newXS("Socket::bootstrap", boot_Socket, file);
 }

Simply put: for each extension linked with your Perl executable
(determined during its initial configuration on your
computer or when adding a new extension),
a Perl subroutine is created to incorporate the extension's
routines.  Normally, that subroutine is named
I<Module::bootstrap()> and is invoked when you say I<use Module>.  In
turn, this hooks into an XSUB, I<boot_Module>, which creates a Perl
counterpart for each of the extension's XSUBs.  Don't worry about this
part; leave that to the I<xsubpp> and extension authors.  If your
extension is dynamically loaded, DynaLoader creates I<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 I<perl_parse()>:


 perl_parse(my_perl, xs_init, argc, my_argv, NULL);


Then compile:

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

 % interp
   use Socket;
   use SomeDynamicallyLoadedModule;

   print "Now I can use extensions!\n"'

B<ExtUtils::Embed> can also automate writing the I<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 L<perlxs> and L<perlguts> for more details.


=head1 MORAL

You can sometimes I<write faster code> in C, but
you can always I<write code faster> in Perl.  Because you can use
each from the other, combine them as you wish.


=head1 AUTHOR

Jon Orwant F<E<lt>orwant@media.mit.eduE<gt>>, 
co-authored by Doug MacEachern F<E<lt>dougm@osf.orgE<gt>>, 
with contributions from
Tim Bunce, Tom Christiansen, Dov Grobgeld, and Ilya
Zakharevich.

June 17, 1996

Some of this material is excerpted from my book: I<Perl 5 Interactive>,
Waite Group Press, 1996 (ISBN 1-57169-064-6) and appears
courtesy of Waite Group Press.