perlembed
PERLEMBED(1) Perl Programmers Reference Guide PERLEMBED(1)
NAME
perlembed - how to embed perl in your C program
DESCRIPTION
PREAMBLE
Do you want to:
Use C from Perl?
Read perlxstut, perlxs, h2xs, perlguts, and perlapi.
Use a Unix program from Perl?
Read about back-quotes and about "system" and "exec" in perlfunc.
Use Perl from Perl?
Read about "do" in perlfunc and "eval" in perlfunc and "require"
in perlfunc and "use" in perlfunc.
Use C from C?
Rethink your design.
Use Perl from C?
Read on...
ROADMAP
· Compiling your C program
· Adding a Perl interpreter to your C program
· Calling a Perl subroutine from your C program
· Evaluating a Perl statement from your C program
· Performing Perl pattern matches and substitutions from your C
program
· Fiddling with the Perl stack from your C program
· Maintaining a persistent interpreter
· Maintaining multiple interpreter instances
· Using Perl modules, which themselves use C libraries, from your C
program
· Embedding Perl under Win32
Compiling your C program
If you have trouble compiling the scripts in this documentation,
you’re not alone. The cardinal rule: COMPILE THE PROGRAMS IN EXACTLY
THE SAME WAY THAT YOUR PERL WAS COMPILED. (Sorry for yelling.)
Also, every C program that uses Perl must link in the perl library.
What’s that, you ask? Perl is itself written in C; the perl library
is the collection of compiled C programs that were used to create your
perl executable (/usr/bin/perl or equivalent). (Corollary: you can’t
use Perl from your C program unless Perl has been compiled on your
machine, or installed properly--that’s why you shouldn’t blithely copy
Perl executables from machine to machine without also copying the lib
directory.)
When you use Perl from C, your C program will--usually--allocate,
"run", and deallocate a PerlInterpreter object, which is defined by
the perl library.
If your copy of Perl is recent enough to contain this documentation
(version 5.002 or later), then the perl library (and EXTERN.h and
perl.h, which you’ll also need) will reside in a directory that looks
like this:
/usr/local/lib/perl5/your_architecture_here/CORE
or perhaps just
/usr/local/lib/perl5/CORE
or maybe something like
/usr/opt/perl5/CORE
Execute this statement for a hint about where to find CORE:
perl -MConfig -e ’print $Config{archlib}’
Here’s how you’d compile the example in the next section, "Adding a
Perl interpreter to your C program", on my Linux box:
% gcc -O2 -Dbool=char -DHAS_BOOL -I/usr/local/include
-I/usr/local/lib/perl5/i586-linux/5.003/CORE
-L/usr/local/lib/perl5/i586-linux/5.003/CORE
-o interp interp.c -lperl -lm
(That’s all one line.) On my DEC Alpha running old 5.003_05, the
incantation is a bit different:
% cc -O2 -Olimit 2900 -DSTANDARD_C -I/usr/local/include
-I/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE
-L/usr/local/lib/perl5/alpha-dec_osf/5.00305/CORE -L/usr/local/lib
-D__LANGUAGE_C__ -D_NO_PROTO -o interp interp.c -lperl -lm
How can you figure out what to add? Assuming your Perl is post-5.001,
execute a "perl -V" command and pay special attention to the "cc" and
"ccflags" information.
You’ll have to choose the appropriate compiler (cc, gcc, et al.) for
your machine: "perl -MConfig -e ’print $Config{cc}’" will tell you
what to use.
You’ll also have to choose the appropriate library directory
(/usr/local/lib/...) for your machine. If your compiler complains
that certain functions are undefined, or that it can’t locate -lperl,
then you need to change the path following the "-L". If it complains
that it can’t find EXTERN.h and perl.h, you need to change the path
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-mod-
ule/ExtUtils/ (If this documentation came from your Perl distribution,
then you’re running 5.004 or better and you already have it.)
The ExtUtils::Embed kit on CPAN also contains all source code for the
examples in this document, tests, additional examples and other infor-
mation you may find useful.
Adding a Perl interpreter to your C program
In a sense, perl (the C program) is a good example of embedding Perl
(the language), so I’ll demonstrate embedding with miniperlmain.c,
included in the source distribution. Here’s a bastardized, non-
portable version of miniperlmain.c containing the essentials of embed-
ding:
#include <EXTERN.h> /* from the Perl distribution */
#include <perl.h> /* from the Perl distribution */
static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
int main(int argc, char **argv, char **env)
{
PERL_SYS_INIT3(&argc,&argv,&env);
my_perl = perl_alloc();
perl_construct(my_perl);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
perl_run(my_perl);
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
}
Notice that we don’t use the "env" pointer. Normally handed to
"perl_parse" as its final argument, "env" here is replaced by "NULL",
which means that the current environment will be used. The macros
PERL_SYS_INIT3() and PERL_SYS_TERM() provide system-specific tune up
of the C runtime environment necessary to run Perl interpreters; since
PERL_SYS_INIT3() may change "env", it may be more appropriate to pro-
vide "env" as an argument to perl_parse().
Now compile this program (I’ll call it interp.c) into an executable:
% cc -o interp interp.c ‘perl -MExtUtils::Embed -e ccopts -e ldopts‘
After a successful compilation, you’ll be able to use interp just like
perl itself:
% interp
print "Pretty Good Perl \n";
print "10890 - 9801 is ", 10890 - 9801;
<CTRL-D>
Pretty Good Perl
10890 - 9801 is 1089
or
% interp -e ’printf("%x", 3735928559)’
deadbeef
You can also read and execute Perl statements from a file while in the
midst of your C program, by placing the filename in argv[1] before
calling perl_run.
Calling a Perl subroutine from your C program
To call individual Perl subroutines, you can use any of the call_*
functions documented in perlcall. In this example we’ll use
"call_argv".
That’s shown below, in a program I’ll call showtime.c.
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int main(int argc, char **argv, char **env)
{
char *args[] = { NULL };
PERL_SYS_INIT3(&argc,&argv,&env);
my_perl = perl_alloc();
perl_construct(my_perl);
perl_parse(my_perl, NULL, argc, argv, NULL);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
/*** skipping perl_run() ***/
call_argv("showtime", G_DISCARD │ G_NOARGS, args);
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
}
where showtime is a Perl subroutine that takes no arguments (that’s
the G_NOARGS) and for which I’ll ignore the return value (that’s the
G_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
818284590
yielding the number of seconds that elapsed between January 1, 1970
(the beginning of the Unix epoch), and the moment I began writing this
sentence.
In this particular case we don’t have to call perl_run, as we set the
PL_exit_flag PERL_EXIT_DESTRUCT_END which executes END blocks in
perl_destruct.
If you want to pass arguments to the Perl subroutine, you can add
strings to the "NULL"-terminated "args" list passed to call_argv. For
other data types, or to examine return values, you’ll need to manipu-
late the Perl stack. That’s demonstrated in "Fiddling with the Perl
stack from your C program".
Evaluating a Perl statement from your C program
Perl provides two API functions to evaluate pieces of Perl code.
These are "eval_sv" in perlapi and "eval_pv" in perlapi.
Arguably, these are the only routines you’ll ever need to execute
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)
{
STRLEN n_a;
char *embedding[] = { "", "-e", "0" };
PERL_SYS_INIT3(&argc,&argv,&env);
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 3, embedding, NULL);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
perl_run(my_perl);
/** Treat $a as an integer **/
eval_pv("$a = 3; $a **= 2", TRUE);
printf("a = %d\n", SvIV(get_sv("a", FALSE)));
/** Treat $a as a float **/
eval_pv("$a = 3.14; $a **= 2", TRUE);
printf("a = %f\n", SvNV(get_sv("a", FALSE)));
/** Treat $a as a string **/
eval_pv("$a = ’rekcaH lreP rehtonA tsuJ’; $a = reverse($a);", TRUE);
printf("a = %s\n", SvPV(get_sv("a", FALSE), n_a));
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
}
All of those strange functions with sv in their names help convert
Perl scalars to C types. They’re described in perlguts and perlapi.
If you compile and run string.c, you’ll see the results of using
SvIV() to create an "int", SvNV() to create a "float", and SvPV() to
create a string:
a = 9
a = 9.859600
a = Just Another Perl Hacker
In the example above, we’ve created a global variable to temporarily
store the computed value of our eval’d expression. It is also possi-
ble and in most cases a better strategy to fetch the return value from
eval_pv() instead. Example:
...
STRLEN n_a;
SV *val = eval_pv("reverse ’rekcaH lreP rehtonA tsuJ’", TRUE);
printf("%s\n", SvPV(val,n_a));
...
This way, we avoid namespace pollution by not creating global vari-
ables and we’ve simplified our code as well.
Performing Perl pattern matches and substitutions from your C program
The eval_sv() function lets us evaluate strings of Perl code, so we
can define some functions that use it to "specialize" in matches and
substitutions: match(), substitute(), and matches().
I32 match(SV *string, char *pattern);
Given a string and a pattern (e.g., "m/clasp/" or "/\b\w*\b/", which
in your C program might appear as "/\\b\\w*\\b/"), match() returns 1
if the string matches the pattern and 0 otherwise.
int substitute(SV **string, char *pattern);
Given a pointer to an "SV" and an "=~" operation (e.g.,
"s/bob/robert/g" or "tr[A-Z][a-z]"), substitute() modifies the string
within the "SV" as according to the operation, returning the number of
substitutions made.
int matches(SV *string, char *pattern, AV **matches);
Given an "SV", a pattern, and a pointer to an empty "AV", matches()
evaluates "$string =~ $pattern" in a list context, and fills in
matches with the array elements, returning the number of matches
found.
Here’s a sample program, match.c, that uses all three (long lines have
been wrapped here):
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
/** my_eval_sv(code, error_check)
** kinda like eval_sv(),
** but we pop the return value off the stack
**/
SV* my_eval_sv(SV *sv, I32 croak_on_error)
{
dSP;
SV* retval;
STRLEN n_a;
PUSHMARK(SP);
eval_sv(sv, G_SCALAR);
SPAGAIN;
retval = POPs;
PUTBACK;
if (croak_on_error && SvTRUE(ERRSV))
croak(SvPVx(ERRSV, n_a));
return retval;
}
/** match(string, pattern)
**
** Used for matches in a scalar context.
**
** Returns 1 if the match was successful; 0 otherwise.
**/
I32 match(SV *string, char *pattern)
{
SV *command = NEWSV(1099, 0), *retval;
STRLEN n_a;
sv_setpvf(command, "my $string = ’%s’; $string =~ %s",
SvPV(string,n_a), pattern);
retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
return SvIV(retval);
}
/** substitute(string, pattern)
**
** Used for =~ operations that modify their left-hand side (s/// and tr///)
**
** Returns the number of successful matches, and
** modifies the input string if there were any.
**/
I32 substitute(SV **string, char *pattern)
{
SV *command = NEWSV(1099, 0), *retval;
STRLEN n_a;
sv_setpvf(command, "$string = ’%s’; ($string =~ %s)",
SvPV(*string,n_a), pattern);
retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
*string = get_sv("string", FALSE);
return SvIV(retval);
}
/** matches(string, pattern, matches)
**
** Used for matches in a list context.
**
** Returns the number of matches,
** and fills in **matches with the matching substrings
**/
I32 matches(SV *string, char *pattern, AV **match_list)
{
SV *command = NEWSV(1099, 0);
I32 num_matches;
STRLEN n_a;
sv_setpvf(command, "my $string = ’%s’; @array = ($string =~ %s)",
SvPV(string,n_a), pattern);
my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
*match_list = get_av("array", FALSE);
num_matches = av_len(*match_list) + 1; /** assume $[ is 0 **/
return num_matches;
}
main (int argc, char **argv, char **env)
{
char *embedding[] = { "", "-e", "0" };
AV *match_list;
I32 num_matches, i;
SV *text;
STRLEN n_a;
PERL_SYS_INIT3(&argc,&argv,&env);
my_perl = perl_alloc();
perl_construct(my_perl);
perl_parse(my_perl, NULL, 3, embedding, NULL);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
text = NEWSV(1099,0);
sv_setpv(text, "When he is at a convenience store and the "
"bill comes to some amount like 76 cents, Maynard is "
"aware that there is something he *should* do, something "
"that will enable him to get back a quarter, but he has "
"no idea *what*. He fumbles through his red squeezey "
"changepurse and gives the boy three extra pennies with "
"his dollar, hoping that he might luck into the correct "
"amount. The boy gives him back two of his own pennies "
"and then the big shiny quarter that is his prize. "
"-RICHH");
if (match(text, "m/quarter/")) /** Does text contain ’quarter’? **/
printf("match: Text contains the word ’quarter’.\n\n");
else
printf("match: Text doesn’t contain the word ’quarter’.\n\n");
if (match(text, "m/eighth/")) /** Does text contain ’eighth’? **/
printf("match: Text contains the word ’eighth’.\n\n");
else
printf("match: Text doesn’t contain the word ’eighth’.\n\n");
/** Match all occurrences of /wi../ **/
num_matches = matches(text, "m/(wi..)/g", &match_list);
printf("matches: m/(wi..)/g found %d matches...\n", num_matches);
for (i = 0; i < num_matches; i++)
printf("match: %s\n", SvPV(*av_fetch(match_list, i, FALSE),n_a));
printf("\n");
/** Remove all vowels from text **/
num_matches = substitute(&text, "s/[aeiou]//gi");
if (num_matches) {
printf("substitute: s/[aeiou]//gi...%d substitutions made.\n",
num_matches);
printf("Now text is: %s\n\n", SvPV(text,n_a));
}
/** Attempt a substitution **/
if (!substitute(&text, "s/Perl/C/")) {
printf("substitute: s/Perl/C...No substitution made.\n\n");
}
SvREFCNT_dec(text);
PL_perl_destruct_level = 1;
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
}
which produces the output (again, long lines have been wrapped here)
match: Text contains the word ’quarter’.
match: Text doesn’t contain the word ’eighth’.
matches: m/(wi..)/g found 2 matches...
match: will
match: with
substitute: s/[aeiou]//gi...139 substitutions made.
Now text is: Whn h s t cnvnnc str nd th bll cms t sm mnt lk 76 cnts,
Mynrd s wr tht thr s smthng h *shld* d, smthng tht wll nbl hm t gt bck
qrtr, bt h hs n d *wht*. H fmbls thrgh hs rd sqzy chngprs nd gvs th by
thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt. Th by gvs
hm bck tw f hs wn pnns nd thn th bg shny qrtr tht s hs prz. -RCHH
substitute: s/Perl/C...No substitution made.
Fiddling with the Perl stack from your C program
When trying to explain stacks, most computer science textbooks mumble
something about spring-loaded columns of cafeteria plates: the last
thing you pushed on the stack is the first thing you pop off. That’ll
do for our purposes: your C program will push some arguments onto "the
Perl stack", shut its eyes while some magic happens, and then pop the
results--the return value of your Perl subroutine--off the stack.
First you’ll need to know how to convert between C types and Perl
types, with newSViv() and sv_setnv() and newAV() and all their
friends. They’re described in perlguts and perlapi.
Then you’ll need to know how to manipulate the Perl stack. That’s
described in perlcall.
Once you’ve understood those, embedding Perl in C is easy.
Because C has no builtin function for integer exponentiation, let’s
make Perl’s ** operator available to it (this is less useful than it
sounds, because Perl implements ** with C’s pow() function). First
I’ll create a stub exponentiation function in power.pl:
sub expo {
my ($a, $b) = @_;
return $a ** $b;
}
Now I’ll create a C program, power.c, with a function PerlPower() that
contains all the perlguts necessary to push the two arguments into
expo() and to pop the return value out. Take a deep breath...
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
static void
PerlPower(int a, int b)
{
dSP; /* initialize stack pointer */
ENTER; /* everything created after here */
SAVETMPS; /* ...is a temporary variable. */
PUSHMARK(SP); /* remember the stack pointer */
XPUSHs(sv_2mortal(newSViv(a))); /* push the base onto the stack */
XPUSHs(sv_2mortal(newSViv(b))); /* push the exponent onto stack */
PUTBACK; /* make local stack pointer global */
call_pv("expo", G_SCALAR); /* call the function */
SPAGAIN; /* refresh stack pointer */
/* pop the return value from stack */
printf ("%d to the %dth power is %d.\n", a, b, POPi);
PUTBACK;
FREETMPS; /* free that return value */
LEAVE; /* ...and the XPUSHed "mortal" args.*/
}
int main (int argc, char **argv, char **env)
{
char *my_argv[] = { "", "power.pl" };
PERL_SYS_INIT3(&argc,&argv,&env);
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
perl_run(my_perl);
PerlPower(3, 4); /*** Compute 3 ** 4 ***/
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
}
Compile and run:
% cc -o power power.c ‘perl -MExtUtils::Embed -e ccopts -e ldopts‘
% power
3 to the 4th power is 81.
Maintaining a persistent interpreter
When developing interactive and/or potentially long-running applica-
tions, it’s a good idea to maintain a persistent interpreter rather
than allocating and constructing a new interpreter multiple times.
The major reason is speed: since Perl will only be loaded into memory
once.
However, you have to be more cautious with namespace and variable
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 vari-
able collisions and outrageous symbol table growth.
Let’s say your application is a server that will occasionally run Perl
code from some arbitrary file. Your server has no way of knowing what
code it’s going to run. Very dangerous.
If the file is pulled in by "perl_parse()", compiled into a newly con-
structed interpreter, and subsequently cleaned out with
"perl_destruct()" afterwards, you’re shielded from most namespace
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 application
might choose to clean out the symbol table associated with the file
after it’s no longer needed. Using "call_argv" in perlapi, We’ll call
the subroutine "Embed::Persistent::eval_file" which lives in the file
"persistent.pl" and pass the filename and boolean cleanup/cache flag
as arguments.
Note that the process will continue to grow for each file that it
uses. In addition, there might be "AUTOLOAD"ed subroutines and other
conditions that cause Perl’s symbol table to grow. You might want to
add some logic that keeps track of the process size, or restarts
itself after a certain number of requests, to ensure that memory con-
sumption is minimized. You’ll also want to scope your variables with
"my" in perlfunc whenever possible.
package Embed::Persistent;
#persistent.pl
use strict;
our %Cache;
use Symbol qw(delete_package);
sub valid_package_name {
my($string) = @_;
$string =~ s/([^A-Za-z0-9\/])/sprintf("_%2x",unpack("C",$1))/eg;
# second pass only for words starting with a digit
$string =~ s│/(\d)│sprintf("/_%2x",unpack("C",$1))│eg;
# Dress it up as a real package name
$string =~ s│/│::│g;
return "Embed" . $string;
}
sub eval_file {
my($filename, $delete) = @_;
my $package = valid_package_name($filename);
my $mtime = -M $filename;
if(defined $Cache{$package}{mtime}
&&
$Cache{$package}{mtime} <= $mtime)
{
# we have compiled this subroutine already,
# it has not been updated on disk, nothing left to do
print STDERR "already compiled $package->handler\n";
}
else {
local *FH;
open FH, $filename or die "open ’$filename’ $!";
local($/) = undef;
my $sub = <FH>;
close FH;
#wrap the code into a subroutine inside our unique package
my $eval = qq{package $package; sub handler { $sub; }};
{
# hide our variables within this block
my($filename,$mtime,$package,$sub);
eval $eval;
}
die $@ if $@;
#cache it unless we’re cleaning out each time
$Cache{$package}{mtime} = $mtime unless $delete;
}
eval {$package->handler;};
die $@ if $@;
delete_package($package) if $delete;
#take a look if you want
#print Devel::Symdump->rnew($package)->as_string, $/;
}
1;
__END__
/* persistent.c */
#include <EXTERN.h>
#include <perl.h>
/* 1 = clean out filename’s symbol table after each request, 0 = don’t */
#ifndef DO_CLEAN
#define DO_CLEAN 0
#endif
#define BUFFER_SIZE 1024
static PerlInterpreter *my_perl = NULL;
int
main(int argc, char **argv, char **env)
{
char *embedding[] = { "", "persistent.pl" };
char *args[] = { "", DO_CLEAN, NULL };
char filename[BUFFER_SIZE];
int exitstatus = 0;
STRLEN n_a;
PERL_SYS_INIT3(&argc,&argv,&env);
if((my_perl = perl_alloc()) == NULL) {
fprintf(stderr, "no memory!");
exit(1);
}
perl_construct(my_perl);
exitstatus = perl_parse(my_perl, NULL, 2, embedding, NULL);
PL_exit_flags │= PERL_EXIT_DESTRUCT_END;
if(!exitstatus) {
exitstatus = perl_run(my_perl);
while(printf("Enter file name: ") &&
fgets(filename, BUFFER_SIZE, stdin)) {
filename[strlen(filename)-1] = ’\0’; /* strip \n */
/* call the subroutine, passing it the filename as an argument */
args[0] = filename;
call_argv("Embed::Persistent::eval_file",
G_DISCARD │ G_EVAL, args);
/* check $@ */
if(SvTRUE(ERRSV))
fprintf(stderr, "eval error: %s\n", SvPV(ERRSV,n_a));
}
}
PL_perl_destruct_level = 0;
perl_destruct(my_perl);
perl_free(my_perl);
PERL_SYS_TERM();
exit(exitstatus);
}
Now compile:
% cc -o persistent persistent.c ‘perl -MExtUtils::Embed -e ccopts -e ldopts‘
Here’s an example script file:
#test.pl
my $string = "hello";
foo($string);
sub foo {
print "foo says: @_\n";
}
Now run:
% persistent
Enter file name: test.pl
foo says: hello
Enter file name: test.pl
already compiled Embed::test_2epl->handler
foo says: hello
Enter file name: ^C
Execution of END blocks
Traditionally END blocks have been executed at the end of the
perl_run. This causes problems for applications that never call
perl_run. Since perl 5.7.2 you can specify "PL_exit_flags │=
PERL_EXIT_DESTRUCT_END" to get the new behaviour. This also enables
the running of END blocks if the perl_parse fails and "perl_destruct"
will return the exit value.
Maintaining multiple interpreter instances
Some rare applications will need to create more than one interpreter
during a session. Such an application might sporadically decide to
release any resources associated with the interpreter.
The program must take care to ensure that this takes place before the
next interpreter is constructed. By default, when perl is not built
with any special options, the global variable "PL_perl_destruct_level"
is set to 0, since extra cleaning isn’t usually needed when a program
only ever creates a single interpreter in its entire lifetime.
Setting "PL_perl_destruct_level" to 1 makes everything squeaky clean:
while(1) {
...
/* reset global variables here with PL_perl_destruct_level = 1 */
PL_perl_destruct_level = 1;
perl_construct(my_perl);
...
/* clean and reset _everything_ during perl_destruct */
PL_perl_destruct_level = 1;
perl_destruct(my_perl);
perl_free(my_perl);
...
/* let’s go do it again! */
}
When perl_destruct() is called, the interpreter’s syntax parse tree
and symbol tables are cleaned up, and global variables are reset. The
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 Configure
options sets the per-interpreter global variable
"PL_perl_destruct_level" to 1, so that thorough cleaning is automatic
and interpreter variables are initialized correctly. Even if you
don’t intend to run two or more interpreters at the same time, but to
run them sequentially, like in the above example, it is recommended to
build perl with the "-Dusemultiplicity" option otherwise some inter-
preter variables may not be initialized correctly between consecutive
runs and your application may crash.
Using "-Dusethreads -Duseithreads" rather than "-Dusemultiplicity" is
more appropriate if you intend to run multiple interpreters concur-
rently in different threads, because it enables support for linking in
the thread libraries of your system with the interpreter.
Let’s give it a try:
#include <EXTERN.h>
#include <perl.h>
/* we’re going to embed two interpreters */
/* we’re going to embed two interpreters */
#define SAY_HELLO "-e", "print qq(Hi, I’m $^X\n)"
int main(int argc, char **argv, char **env)
{
PerlInterpreter *one_perl, *two_perl;
char *one_args[] = { "one_perl", SAY_HELLO };
char *two_args[] = { "two_perl", SAY_HELLO };
PERL_SYS_INIT3(&argc,&argv,&env);
one_perl = perl_alloc();
two_perl = perl_alloc();
PERL_SET_CONTEXT(one_perl);
perl_construct(one_perl);
PERL_SET_CONTEXT(two_perl);
perl_construct(two_perl);
PERL_SET_CONTEXT(one_perl);
perl_parse(one_perl, NULL, 3, one_args, (char **)NULL);
PERL_SET_CONTEXT(two_perl);
perl_parse(two_perl, NULL, 3, two_args, (char **)NULL);
PERL_SET_CONTEXT(one_perl);
perl_run(one_perl);
PERL_SET_CONTEXT(two_perl);
perl_run(two_perl);
PERL_SET_CONTEXT(one_perl);
perl_destruct(one_perl);
PERL_SET_CONTEXT(two_perl);
perl_destruct(two_perl);
PERL_SET_CONTEXT(one_perl);
perl_free(one_perl);
PERL_SET_CONTEXT(two_perl);
perl_free(two_perl);
PERL_SYS_TERM();
}
Note the calls to PERL_SET_CONTEXT(). These are necessary to initial-
ize the global state that tracks which interpreter is the "current"
one on the particular process or thread that may be running it. It
should always be used if you have more than one interpreter and are
making perl API calls on both interpreters in an interleaved fashion.
PERL_SET_CONTEXT(interp) should also be called whenever "interp" is
used by a thread that did not create it (using either perl_alloc(), or
the more esoteric perl_clone()).
Compile as usual:
% cc -o multiplicity multiplicity.c ‘perl -MExtUtils::Embed -e ccopts -e ldopts‘
Run it, Run it:
% multiplicity
Hi, I’m one_perl
Hi, I’m two_perl
Using Perl modules, which themselves use C libraries, from your C pro-
gram
If you’ve played with the examples above and tried to embed a script
that use()s a Perl module (such as Socket) which itself uses a C or
C++ library, this probably happened:
Can’t load module Socket, dynamic loading not available in this perl.
(You may need to build a new perl executable which either supports
dynamic loading or has the Socket module statically linked into it.)
What’s wrong?
Your interpreter doesn’t know how to communicate with these extensions
on its own. A little glue will help. Up until now you’ve been call-
ing perl_parse(), handing it NULL for the second argument:
perl_parse(my_perl, NULL, argc, my_argv, NULL);
That’s where the glue code can be inserted to create the initial con-
tact between Perl and linked C/C++ routines. Let’s take a look some
pieces of perlmain.c to see how Perl does this:
static void xs_init (pTHX);
EXTERN_C void boot_DynaLoader (pTHX_ CV* cv);
EXTERN_C void boot_Socket (pTHX_ CV* cv);
EXTERN_C void
xs_init(pTHX)
{
char *file = __FILE__;
/* DynaLoader is a special case */
newXS("DynaLoader::boot_DynaLoader", boot_DynaLoader, file);
newXS("Socket::bootstrap", boot_Socket, file);
}
Simply put: for each extension linked with your Perl executable
(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 Mod-
ule::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 dynam-
ically 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 program 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 writing
warn("%d bottles of beer on the wall", bottlecount);
you will have to write the explicit full form
Perl_warn(aTHX_ "%d bottles of beer on the wall", bottlecount);
(See "Background and PERL_IMPLICIT_CONTEXT for the explanation of the
"aTHX_"." in perlguts ) Hiding the short forms is very useful for
avoiding all sorts of nasty (C preprocessor or otherwise) conflicts
with other software packages (Perl defines about 2400 APIs with these
short names, take or leave few hundred, so there certainly is room for
conflict.)
MORAL
You can sometimes write faster code in C, but you can always write
code faster in Perl. Because you can use each from the other, combine
them as you wish.
AUTHOR
Jon Orwant <orwant@media.mit.edu> and Doug MacEachern <dougm@cova-
lent.net>, with small contributions from Tim Bunce, Tom Christiansen,
Guy Decoux, Hallvard Furuseth, Dov Grobgeld, and Ilya Zakharevich.
Doug MacEachern has an article on embedding in Volume 1, Issue 4 of
The Perl Journal ( http://www.tpj.com/ ). Doug is also the developer
of the most widely-used Perl embedding: the mod_perl system
(perl.apache.org), which embeds Perl in the Apache web server. 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
Copyright (C) 1995, 1996, 1997, 1998 Doug MacEachern and Jon Orwant.
All Rights Reserved.
Permission is granted to make and distribute verbatim copies of this
documentation provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of this
documentation under the conditions for verbatim copying, provided also
that they are marked clearly as modified versions, that the authors’
names and title are unchanged (though subtitles and additional
authors’ names may be added), and that the entire resulting derived
work is distributed under the terms of a permission notice identical
to this one.
Permission is granted to copy and distribute translations of this doc-
umentation into another language, under the above conditions for modi-
fied versions.
perl v5.8.8 2006-01-07 PERLEMBED(1)
