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X11R7.4 - man page for perlembed (x11r4 section 1)

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

       perlembed - how to embed perl in your C program


       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...


       o    Compiling your C program

       o    Adding a Perl interpreter to your C program

       o    Calling a Perl subroutine from your C program

       o    Evaluating a Perl statement from your C program

       o    Performing Perl pattern matches and substitutions from your C program

       o    Fiddling with the Perl stack from your C program

       o    Maintaining a persistent interpreter

       o    Maintaining multiple interpreter instances

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

       o    Embedding Perl under Win32

       Compiling your C program

       If you have trouble compiling the scripts in this documentation, you're not alone.  The
       (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:


       or perhaps just


       or maybe something like


       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
	   -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

	   % cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
	   -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, non-portable version of miniperlmain.c containing the essentials of

	   #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();
	       PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
	       perl_parse(my_perl, NULL, argc, argv, (char **)NULL);

       Notice that we don't use the "env" pointer.  Normally handed to "perl_parse" as its final
       argument, "env" here is replaced by "NULL", which means that the current environment will
       be used.  The macros PERL_SYS_INIT3() and PERL_SYS_TERM() provide system-specific tune up
       of the C runtime environment necessary to run Perl interpreters; since PERL_SYS_INIT3()
       may change "env", it may be more appropriate to provide "env" as an argument to

       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;
	   Pretty Good Perl
	   10890 - 9801 is 1089


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

       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 };
	       my_perl = perl_alloc();

	       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);


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

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

	   print "I shan't be printed.";

	   sub showtime {
	       print time;

       Simple enough.  Now compile and run:

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

	   % showtime showtime.pl

       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 manipulate the Perl stack.  That's demonstrated in "Fiddling
       with the Perl stack from your C program".

       Evaluating a Perl statement from your C program

       Perl provides two API functions to evaluate pieces of Perl code.  These are "eval_sv" in
       perlapi and "eval_pv" in perlapi.

       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 "use" in perlfunc, "require" in perlfunc, and "do" in perlfunc
       to include external Perl files.

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

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

	  static PerlInterpreter *my_perl;

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

	      my_perl = perl_alloc();
	      perl_construct( my_perl );

	      perl_parse(my_perl, NULL, 3, embedding, NULL);
	      PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

	      /** 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_nolen(get_sv("a", FALSE)));


       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'ed expression.  It is also possible and in most cases a better strategy
       to fetch the return value from eval_pv() instead.  Example:

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

       This way, we avoid namespace pollution by not creating global variables and we've simpli-
       fied 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 func-
       tions that use it to "specialize" in matches and substitutions: match(), substitute(), and

	  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

	  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)
	    SV* retval;

	    eval_sv(sv, G_SCALAR);

	    retval = POPs;

	    if (croak_on_error && SvTRUE(ERRSV))

	    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(0), *retval;

	    sv_setpvf(command, "my $string = '%s'; $string =~ %s",
		     SvPV_nolen(string), pattern);

	    retval = my_eval_sv(command, TRUE);

	    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(0), *retval;

	    sv_setpvf(command, "$string = '%s'; ($string =~ %s)",
		     SvPV_nolen(*string), pattern);

	    retval = my_eval_sv(command, TRUE);

	    *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(0);
	    I32 num_matches;

	    sv_setpvf(command, "my $string = '%s'; @array = ($string =~ %s)",
		     SvPV_nolen(string), pattern);

	    my_eval_sv(command, TRUE);

	    *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;

	    my_perl = perl_alloc();
	    perl_parse(my_perl, NULL, 3, embedding, NULL);
	    PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

	    text = newSV(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. "

	    if (match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
	       printf("match: Text contains the word 'quarter'.\n\n");
	       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");
	       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_nolen(*av_fetch(match_list, i, FALSE)));

	    /** Remove all vowels from text **/
	    num_matches = substitute(&text, "s/[aeiou]//gi");
	    if (num_matches) {
	       printf("substitute: s/[aeiou]//gi...%d substitutions made.\n",
	       printf("Now text is: %s\n\n", SvPV_nolen(text));

	    /** Attempt a substitution **/
	    if (!substitute(&text, "s/Perl/C/")) {
	       printf("substitute: s/Perl/C...No substitution made.\n\n");

	    PL_perl_destruct_level = 1;

       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 argu-
       ments onto "the Perl stack", shut its eyes while some magic happens, and then pop the
       results--the return value of your Perl subroutine--off the stack.

       First you'll need to know how to convert between C types and Perl types, with newSViv()
       and sv_setnv() and newAV() and all their friends.  They're described in perlguts and per-

       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 ** opera-
       tor 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);
	     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);
	     PL_exit_flags |= PERL_EXIT_DESTRUCT_END;

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


       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 inter-
       preter 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 dan-

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

       One way to avoid namespace collisions in this scenario is to translate the filename into a
       guaranteed-unique package name, and then compile the code into that package using "eval"
       in perlfunc.  In the example below, each file will only be compiled once.  Or, the appli-
       cation 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::Persis-
       tent::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 ta-
       ble 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 possi-

	package Embed::Persistent;

	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
		   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, $/;



	/* 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

	#define BUFFER_SIZE 1024

	static PerlInterpreter *my_perl = NULL;

	main(int argc, char **argv, char **env)
	    char *embedding[] = { "", "persistent.pl" };
	    char *args[] = { "", DO_CLEAN, NULL };
	    char filename[BUFFER_SIZE];
	    int exitstatus = 0;

	    if((my_perl = perl_alloc()) == NULL) {
	       fprintf(stderr, "no memory!");

	    PL_origalen = 1; /* don't let $0 assignment update the proctitle or embedding[0] */
	    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;
				  G_DISCARD | G_EVAL, args);

		   /* check $@ */
		       fprintf(stderr, "eval error: %s\n", SvPV_nolen(ERRSV));

	    PL_perl_destruct_level = 0;

       Now compile:

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

       Here's an example script file:

	my $string = "hello";

	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 prob-
       lems 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

       $0 assignments

       When a perl script assigns a value to $0 then the perl runtime will try to make this value
       show up as the program name reported by "ps" by updating the memory pointed to by the argv
       passed to perl_parse() and also calling API functions like setproctitle() where available.
       This behaviour might not be appropriate when embedding perl and can be disabled by assign-
       ing the value 1 to the variable "PL_origalen" before perl_parse() is called.

       The persistent.c example above is for instance likely to segfault when $0 is assigned to
       if the "PL_origalen = 1;" assignment is removed.  This because perl will try to write to
       the read only memory of the "embedding[]" strings.

       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

       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;
	    /* clean and reset _everything_ during perl_destruct */
	    PL_perl_destruct_level = 1;
	    /* let's go do it again! */

       When perl_destruct() is called, the interpreter's syntax parse tree and symbol tables are
       cleaned up, and global variables are reset.  The second assignment to
       "PL_perl_destruct_level" is needed because perl_construct resets it to 0.

       Now suppose we have more than one interpreter instance 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 Con-
       figure 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 "-Duse-
       multiplicity" option otherwise some interpreter variables may not be initialized correctly
       between consecutive runs and your application may crash.

       See also "Thread-aware system interfaces" in perlxs.

       Using "-Dusethreads -Duseithreads" rather than "-Dusemultiplicity" is more appropriate if
       you intend to run multiple interpreters concurrently in different threads, because it
       enables support for linking in the thread libraries of your system with the interpreter.

       Let's give it a try:

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

	/* 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 };

	    one_perl = perl_alloc();
	    two_perl = perl_alloc();


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




       Note the calls to PERL_SET_CONTEXT().  These are necessary to initialize the global state
       that tracks which interpreter is the "current" one on the particular process or thread
       that may be running it.	It should always be used if you have more than one interpreter
       and are making perl API calls on both interpreters in an interleaved fashion.

       PERL_SET_CONTEXT(interp) should also be called whenever "interp" is used by a thread that
       did not create it (using either perl_alloc(), or the more esoteric perl_clone()).

       Compile as usual:

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

       Run it, Run it:

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

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

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

	Can't load module Socket, dynamic loading not available in this perl.
	 (You may need to build a new perl executable which either supports
	 dynamic loading or has the Socket module statically linked into it.)

       What's wrong?

       Your interpreter doesn't know how to communicate with these extensions on its own.  A lit-
       tle glue will help.  Up until now you've been calling perl_parse(), handing it NULL for
       the second argument:

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

       That's where the glue code can be inserted to create the initial contact between Perl and
       linked C/C++ routines.  Let's take a look some pieces of perlmain.c to see how Perl does

	static void xs_init (pTHX);

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

	EXTERN_C void
	       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
       Module::bootstrap() and is invoked when you say use Module.  In turn, this hooks into an
       XSUB, boot_Module, which creates a Perl counterpart for each of the extension's XSUBs.
       Don't worry about this part; leave that to the xsubpp and extension authors.  If your
       extension is dynamically loaded, DynaLoader creates Module::bootstrap() for you on the
       fly.  In fact, if you have a working DynaLoader then there is rarely any need to link in
       any other extensions statically.

       Once you have this code, slap it into the second argument of perl_parse():

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

       Then compile:

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

	% interp
	  use Socket;
	  use SomeDynamicallyLoadedModule;

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

       ExtUtils::Embed can also automate writing the xs_init glue code.

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

       Consult perlxs, perlguts, and perlapi for more details.

Embedding Perl under Win32
       In general, all of the source code shown here should work unmodified under Windows.

       However, there are some caveats about the command-line examples shown.  For starters,
       backticks won't work under the Win32 native command shell.  The ExtUtils::Embed kit on
       CPAN ships with a script called genmake, which generates a simple makefile to build a pro-
       gram from a single C source file.  It can be used like this:

	C:\ExtUtils-Embed\eg> perl genmake interp.c
	C:\ExtUtils-Embed\eg> nmake
	C:\ExtUtils-Embed\eg> interp -e "print qq{I'm embedded in Win32!\n}"

       You may wish to use a more robust environment such as the Microsoft Developer Studio.  In
       this case, run this to generate perlxsi.c:

	perl -MExtUtils::Embed -e xsinit

       Create a new project and Insert -> Files into Project: perlxsi.c, perl.lib, and your own
       source files, e.g. interp.c.  Typically you'll find perl.lib in C:\perl\lib\CORE, if not,
       you should see the CORE 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

	   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 pre-
       processor 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

       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.

       Jon Orwant <orwant@media.mit.edu> and Doug MacEachern <dougm@covalent.net>, with small
       contributions from Tim Bunce, Tom Christiansen, Guy Decoux, Hallvard Furuseth, Dov
       Grobgeld, and Ilya Zakharevich.

       Doug MacEachern has an article on embedding in Volume 1, Issue 4 of The Perl Journal (
       http://www.tpj.com/ ).  Doug is also the developer of the most widely-used Perl embedding:
       the mod_perl system (perl.apache.org), which embeds Perl in the Apache web server.  Ora-
       cle, Binary Evolution, ActiveState, and Ben Sugars's nsapi_perl have used this model for
       Oracle, Netscape and Internet Information Server Perl plugins.

       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 pro-
       vided 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 modi-
       fied versions, that the authors' names and title are unchanged (though subtitles and addi-
       tional 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 documentation into
       another language, under the above conditions for modified versions.

perl v5.8.9				    2007-11-17				     PERLEMBED(1)

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