@node perlembed, perlapio, perlbook, Top
@unnumberedsec NAME
perlembed - how to embed perl in your C program

@unnumberedsec DESCRIPTION
@unnumberedsubsec PREAMBLE
Do you want to:

@table @asis
@item @strong{Use C from Perl?}
Read @xref{perlcall,Perlcall}, and @xref{perlxs,Perlxs},.

@item @strong{Use a Unix program from Perl?}
Read about back-quotes and about system and exec in @xref{perlfunc,Perlfunc}.

@item @strong{Use Perl from Perl?}
Read about @samp{do}, @xref{perlfunc,Perlfunc}, and @samp{eval}, @xref{perlfunc,Perlfunc}, and @samp{require}, @xref{perlfunc,Perlfunc},
and @samp{use}, @xref{perlfunc,Perlfunc}.

@item @strong{Use C from C?}
Rethink your design.

@item @strong{Use Perl from C?}
Read on...

@end table
@unnumberedsubsec ROADMAP
@samp{Compiling your C program} in this node

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

@samp{Adding a Perl interpreter to your C program} in this node

@samp{Calling a Perl subroutine from your C program} in this node

@samp{Evaluating a Perl statement from your C program} in this node

@samp{Performing Perl pattern matches and substitutions from your C program} in this node

@samp{Fiddling with the Perl stack from your C program} in this node

@samp{Maintaining a persistent interpreter} in this node

@samp{Maintaining multiple interpreter instances} in this node

@samp{Using Perl modules, which themselves use C libraries, from your C program} in this node

This documentation is Unix specific; if you have information about how
to embed Perl on other platforms, please send e-mail to <@file{orwant@@tpj.com}>.

@unnumberedsubsec 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 @emph{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 (@emph{/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 @emph{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 @emph{EXTERN.h} and
@emph{perl.h}, which you@'ll also need) will reside in a directory
that looks like this:


@example
/usr/local/lib/perl5/your_architecture_here/CORE
@end example

or perhaps just


@example
/usr/local/lib/perl5/CORE
@end example

or maybe something like


@example
/usr/opt/perl5/CORE
@end example

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


@example
perl -MConfig -e @'print $Config@{archlib@}@'
@end example

Here@'s how you@'d compile the example in the next section,
@samp{Adding a Perl interpreter to your C program} in this node, on my Linux box:


@example
% 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
@end example

(That@'s all one line.)  On my DEC Alpha running 5.003_05, the incantation
is a bit different:


@example
% 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
@end example

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

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

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


@example
perl -MConfig -e @'print $Config@{libs@}@'
@end example

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


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

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::Embed.  (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.

@unnumberedsubsec 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 @emph{miniperlmain.c},
from the source distribution.  Here@'s a bastardized, nonportable
version of @emph{miniperlmain.c} containing the essentials of embedding:


@example
#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);
@}
@end example

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

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


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

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


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

or


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

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

@unnumberedsubsec Calling a Perl subroutine from your C program
To call individual Perl subroutines, you can use any of the @strong{perl_call_*}
functions documented in the @xref{perlcall,Perlcall}, manpage.
In this example we@'ll use perl_call_argv.

That@'s shown below, in a program I@'ll call @emph{showtime.c}.


@example
#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);
    @}
@end example

where @emph{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 @xref{perlcall,Perlcall}.

I@'ll define the @emph{showtime} subroutine in a file called @emph{showtime.pl}:


@example
print "I shan@'t be printed.";

sub showtime @{
    print time;
@}
@end example

Simple enough.  Now compile and run:


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

% showtime showtime.pl
818284590
@end example

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 @code{END @{@}} blocks.

If you want to pass arguments to the Perl subroutine, you can add
strings to the NULL-terminated @code{args} list passed to
perl_call_argv.  For other data types, or to examine return values,
you@'ll need to manipulate the Perl stack.  That@'s demonstrated in the
last section of this document: @samp{Fiddling with the Perl stack from
your C program} in this node.

@unnumberedsubsec Evaluating a Perl statement from your C program
Perl provides two API functions to evaluate pieces of Perl code.
These are @samp{perl_eval_sv()}, @xref{perlguts,Perlguts}, and @samp{perl_eval_pv()}, @xref{perlguts,Perlguts},.

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
@samp{use}, @xref{perlfunc,Perlfunc}, @samp{require}, @xref{perlfunc,Perlfunc}, and @samp{do}, @xref{perlfunc,Perlfunc}, to include
external Perl files.

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


@example
#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);
    perl_run(my_perl);

    /** Treat $a as an integer **/
    perl_eval_pv("$a = 3; $a **= 2", TRUE);
    printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));

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

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

    perl_destruct(my_perl);
    perl_free(my_perl);
@}
@end example

All of those strange functions with @emph{sv} in their names help convert Perl scalars to C types.  They@'re described in @xref{perlguts,Perlguts}.

If you compile and run @emph{string.c}, you@'ll see the results of using
@emph{SvIV()} to create an int, @emph{SvNV()} to create a @code{float}, and
@emph{SvPV()} to create a string:


@example
a = 9
a = 9.859600
a = Just Another Perl Hacker
@end example

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 @samp{perl_eval_pv} in this node instead.  Example:


@example
...
SV *val = perl_eval_pv("reverse @'rekcaH lreP rehtonA tsuJ@'", TRUE);
printf("%s\n", SvPV(val,na));
...
@end example

This way, we avoid namespace pollution by not creating global
variables and we@'ve simplified our code as well.

@unnumberedsubsec Performing Perl pattern matches and substitutions from your C program
The @emph{perl_eval_pv()} function lets us evaluate strings of Perl code, so we can
define some functions that use it to "specialize" in matches and
substitutions: @emph{match()}, @emph{substitute()}, and @emph{matches()}.


@example
char match(char *string, char *pattern);
@end example

Given a string and a pattern (e.g., @code{m/clasp/} or @code{/\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.


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

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


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

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

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


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

static PerlInterpreter *my_perl;

/** 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 = $string =~ %s",
                   string, pattern);
  perl_eval_pv(command, TRUE);
  free(command);
  return SvIV(perl_get_sv("return", FALSE));
@}
/** substitute(string, pattern)
**
** Used for =~ operations that modify their left-hand side (s/// and tr///)
**
** Returns the number of successful matches, and
** modifies the input string if there were any.
**/
int substitute(char *string[], char *pattern)
@{
  char *command;
  STRLEN length;
  command = malloc(sizeof(char) * strlen(*string) + strlen(pattern) + 35);
  sprintf(command, "$string = @'%s@'; $ret = ($string =~ %s)",
                   *string, pattern);
  perl_eval_pv(command, TRUE);
  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_pv(command, TRUE);
  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);
   @}
@end example

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


@example
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.
@end example

@unnumberedsubsec 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 @xref{perlguts,Perlguts}.

Then you@'ll need to know how to manipulate the Perl stack.  That@'s
described in @xref{perlcall,Perlcall}.

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

Because C has no builtin function for integer exponentiation, let@'s
make Perl@'s ** operator available to it (this is less useful than it
sounds, because Perl implements ** with C@'s @emph{pow()} function).  First
I@'ll create a stub exponentiation function in @emph{power.pl}:


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

Now I@'ll create a C program, @emph{power.c}, with a function
@emph{PerlPower()} that contains all the perlguts necessary to push the
two arguments into @emph{expo()} and to pop the return value out.  Take a
deep breath...


@example
#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);
    @}
@end example

Compile and run:


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

% power
3 to the 4th power is 81.
@end example

@unnumberedsubsec 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 @code{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 @code{perl_parse()}, compiled into a newly
constructed interpreter, and subsequently cleaned out with
@code{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 @samp{eval}, @xref{perlfunc,Perlfunc}.  In the example
below, each file will only be compiled once.  Or, the application
might choose to clean out the symbol table associated with the file
after it@'s no longer needed.  Using @samp{perl_call_argv}, @xref{perlcall,Perlcall}, We@'ll
call the subroutine @code{Embed::Persistent::eval_file} which lives in the
file @code{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 @code{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 @samp{my}, @xref{perlfunc,Perlfunc}, whenever possible.


@example
package Embed::Persistent;
#persistent.pl

use strict;
use vars @'%Cache@';

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);
 @}
@end example

Now compile:


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

Here@'s a example script file:


@example
#test.pl
my $string = "hello";
foo($string);

sub foo @{
    print "foo says: @@_\n";
@}
@end example

Now run:


@example
% 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
@end example

@unnumberedsubsec 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 @emph{before}
the next interpreter is constructed.  By default, the global variable
@code{perl_destruct_level} is set to @code{0}, since extra cleaning isn@'t
needed when a program has only one interpreter.

Setting @code{perl_destruct_level} to 1 makes everything squeaky clean:


@example
perl_destruct_level = 1;

while(1) @{
    ...
    /* reset global variables here with 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! */
@}
@end example

When @emph{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
@code{-DMULTIPLICITY} flag when building Perl.  By default, that sets
@code{perl_destruct_level} to 1.

Let@'s give it a try:


@example
#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);
 @}
@end example

Compile as usual:


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

Run it, Run it:


@example
% multiplicity
Hi, I@'m one_perl
Hi, I@'m two_perl
@end example

@unnumberedsubsec 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 @emph{use()}s a Perl module (such as Socket) which itself uses a C or C++ library,
this probably happened:


@example
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.)
@end example

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 @emph{perl_parse()}, handing it NULL for the second argument:


@example
perl_parse(my_perl, NULL, argc, my_argv, NULL);
@end example

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 @emph{perlmain.c}
to see how Perl does this:


@example
#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);
@}
@end example

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
@emph{Module::bootstrap()} and is invoked when you say @emph{use Module}.  In
turn, this hooks into an XSUB, @emph{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 @emph{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 @emph{perl_parse()}:


@example
perl_parse(my_perl, xs_init, argc, my_argv, NULL);
@end example

Then compile:


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

% interp
  use Socket;
  use SomeDynamicallyLoadedModule;

print "Now I can use extensions!\n"@'
@end example

ExtUtils::Embed can also automate writing the @emph{xs_init} glue code.


@example
% 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@`
@end example

Consult @xref{perlxs,Perlxs}, and @xref{perlguts,Perlguts}, for more details.

@unnumberedsec MORAL
You can sometimes @emph{write faster code} in C, but
you can always @emph{write code faster} in Perl.  Because you can use
each from the other, combine them as you wish.

@unnumberedsec AUTHOR
Jon Orwant and <@file{orwant@@tpj.com}> and Doug MacEachern <@file{dougm@@osf.org}>,
with small contributions from Tim Bunce, Tom Christiansen, Hallvard Furuseth,
Dov Grobgeld, and Ilya Zakharevich.

Check out Doug@'s article on embedding in Volume 1, Issue 4 of The Perl
Journal.  Info about TPJ is available from http://tpj.com.

April 14, 1997

Some of this material is excerpted from Jon Orwant@'s book: @emph{Perl 5
Interactive}, Waite Group Press, 1996 (ISBN 1-57169-064-6) and appears
courtesy of Waite Group Press.

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

Although destined for release with the standard Perl distribution,
this document is not public domain, nor is any of Perl and its
documentation.  Permission is granted to freely distribute verbatim
copies of this document provided that no modifications outside of
formatting be made, and that this notice remain intact.  You are
permitted and encouraged to use its code and derivatives thereof in
your own source code for fun or for profit as you see fit.
