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NAME
perlembed - how to embed perl in your C program
---------------------------------------------------------------------------
DESCRIPTION
---------------------------------------------------------------------------
PREAMBLE
Do you want to:
Use C from Perl?
Read the perlcall manpage and the perlxs manpage .
Use a UNIX program from Perl?
Read about backquotes and system and exec .
Use Perl from Perl?
Read about do and eval and use and require .
Use C from C?
Rethink your design.
Use Perl from C?
Read on...
---------------------------------------------------------------------------
ROADMAP
Compiling your C program
There's one example in each of the five sections:
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
This documentation is UNIX specific.
---------------------------------------------------------------------------
Compiling your C program
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.)
Your C program will--usually--allocate, ``run'', and deallocate a
PerlInterpreter object, which is defined in the perl library.
If your copy of Perl is recent enough to contain this documentation (5.002
or later), then the perl library (and EXTERN.h and perl.h, which you'll
also need) will reside in a directory resembling this:
/usr/local/lib/perl5/your_architecture_here/CORE
or perhaps just
/usr/local/lib/perl5/CORE
or maybe something like
/usr/opt/perl5/CORE
Execute this statement for a hint about where to find CORE:
perl -e 'use Config; print $Config{archlib}'
Here's how you might compile the example in the next section, Adding a Perl
interpreter to your C program , on a DEC Alpha running the OSF operating
system:
% cc -o interp interp.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
You'll have to choose the appropriate compiler (cc, gcc, et al.) and
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 or 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 -e 'use Config; print $Config{libs}'
---------------------------------------------------------------------------
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, from the
source distribution. Here's a bastardized, non-portable version of
miniperlmain.c containing the essentials of embedding:
#include <stdio.h>
#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, env);
perl_run(my_perl);
perl_destruct(my_perl);
perl_free(my_perl);
}
Now compile this program (I'll call it interp.c) into an executable:
% cc -o interp interp.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
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'll need to remove the call to
perl_run() and replace it with a call to perl_call_argv().
That's shown below, in a program I'll call showtime.c.
#include <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int main(int argc, char **argv, char **env)
{
my_perl = perl_alloc();
perl_construct(my_perl);
perl_parse(my_perl, NULL, argc, argv, env);
/*** This replaces perl_run() ***/
perl_call_argv("showtime", G_DISCARD | G_NOARGS, argv);
perl_destruct(my_perl);
perl_free(my_perl);
}
where showtime is a Perl subroutine that takes no arguments (that's the
G_NOARGS) and for which I'll ignore the return value (that's the
G_DISCARD). Those flags, and others, are discussed in the perlcall manpage
..
I'll define the showtime subroutine in a file called showtime.pl:
print "I shan't be printed.";
sub showtime {
print time;
}
Simple enough. Now compile and run:
% cc -o showtime showtime.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
% 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.
If you want to pass some arguments to the Perl subroutine, or you want to
access the return value, you'll need to manipulate the Perl stack,
demonstrated in the last section of this document: Fiddling with the Perl
stack from your C program
---------------------------------------------------------------------------
Evaluating a Perl statement from your C program
NOTE: This section, and the next, employ some very brittle techniques for
evaluting strings of Perl code. Perl 5.002 contains some nifty features
that enable A Better Way (such as with perl_eval_sv). Look for updates to
this document soon.
One way to evaluate a Perl string is to define a function (we'll call ours
perl_eval()) that wraps around Perl's eval .
Arguably, this is the only routine you'll ever need to execute snippets of
Perl code from within your C program. Your string can be as long as you
wish; it can contain multiple statements; it can use require or do to
include external Perl files.
Our perl_eval() 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 <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int perl_eval(char *string)
{
char *argv[2];
argv[0] = string;
argv[1] = NULL;
perl_call_argv("_eval_", 0, argv);
}
main (int argc, char **argv, char **env)
{
char *embedding[] = { "", "-e", "sub _eval_ { eval $_[0] }" };
STRLEN length;
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 3, embedding, env);
/** Treat $a as an integer **/
perl_eval("$a = 3; $a **= 2");
printf("a = %d\n", SvIV(perl_get_sv("a", FALSE)));
/** Treat $a as a float **/
perl_eval("$a = 3.14; $a **= 2");
printf("a = %f\n", SvNV(perl_get_sv("a", FALSE)));
/** Treat $a as a string **/
perl_eval("$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a); ");
printf("a = %s\n", SvPV(perl_get_sv("a", FALSE), length));
perl_destruct(my_perl);
perl_free(my_perl);
}
All of those strange functions with sv in their names help convert Perl
scalars to C types. They're described in the perlguts manpage .
If you compile and run string.c, you'll see the results of using SvIV() to
create an int , SvNV() to create a float, and SvPV() to create a string:
a = 9
a = 9.859600
a = Just Another Perl Hacker
---------------------------------------------------------------------------
Performing Perl pattern matches and substitutions from your C program
Our perl_eval() 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().
char match(char *string, char *pattern);
Given a string and a pattern (e.g. ``m/clasp/'' or ``/\b\w*\b/'', which in
your program might be represented as ``/\\b\\w*\\b/''), returns 1 if the
string matches the pattern and 0 otherwise.
int substitute(char *string[], char *pattern);
Given a pointer to a string and an ``=~'' operation (e.g.
``s/bob/robert/g'' or ``tr[A-Z][a-z]''), modifies the string according to
the operation, returning the number of substitutions made.
int matches(char *string, char *pattern, char **matches[]);
Given a string, a pattern, and a pointer to an empty array of strings,
evaluates $string =~ $pattern in an array context, and fills in matches
with the array elements (allocating memory as it does so), returning the
number of matches found.
Here's a sample program, match.c, that uses all three:
#include <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int eval(char *string)
{
char *argv[2];
argv[0] = string;
argv[1] = NULL;
perl_call_argv("_eval_", 0, argv);
}
/** match(string, pattern)
**
** Used for matches in a scalar context.
**
** Returns 1 if the match was successful; 0 otherwise.
**/
char match(char *string, char *pattern)
{
char *command;
command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 37);
sprintf(command, "$string = '%s'; $return = $string =~ %s",
string, pattern);
perl_eval(command);
free(command);
return SvIV(perl_get_sv("return", FALSE));
}
/** substitute(string, pattern)
**
** Used for =~ operations that modify their left-hand side (s/// and tr///)
**
** Returns the number of successful matches, and
** modifies the input string if there were any.
**/
int substitute(char *string[], char *pattern)
{
char *command;
STRLEN length;
command = malloc(sizeof(char) * strlen(*string) + strlen(pattern) + 35);
sprintf(command, "$string = '%s'; $ret = ($string =~ %s)",
*string, pattern);
perl_eval(command);
free(command);
*string = SvPV(perl_get_sv("string", FALSE), length);
return SvIV(perl_get_sv("ret", FALSE));
}
/** matches(string, pattern, matches)
**
** Used for matches in an array context.
**
** Returns the number of matches,
** and fills in **matches with the matching substrings (allocates memory!)
**/
int matches(char *string, char *pattern, char **matches[])
{
char *command;
SV *current_match;
AV *array;
I32 num_matches;
STRLEN length;
int i;
command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 38);
sprintf(command, "$string = '%s'; @array = ($string =~ %s)",
string, pattern);
perl_eval(command);
free(command);
array = perl_get_av("array", FALSE);
num_matches = av_len(array) + 1; /** assume $[ is 0 **/
*matches = (char **) malloc(sizeof(char *) * num_matches);
for (i = 0; i <= num_matches; i++) {
current_match = av_shift(array);
(*matches)[i] = SvPV(current_match, length);
}
return num_matches;
}
main (int argc, char **argv, char **env)
{
char *embedding[] = { "", "-e", "sub _eval_ { eval $_[0] }" };
char *text, **matches;
int num_matches, i;
int j;
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 3, embedding, env);
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. -
if (perl_match(text, "m/quarter/")) /** Does text contain 'quarter'? **/
printf("perl_match: Text contains the word 'quarter'.\n\n");
else
printf("perl_match: Text doesn't contain the word 'quarter'.\n\n");
if (perl_match(text, "m/eighth/")) /** Does text contain 'eighth'? **/
printf("perl_match: Text contains the word 'eighth'.\n\n");
else
printf("perl_match: Text doesn't contain the word 'eighth'.\n\n");
/** Match all occurrences of /wi../ **/
num_matches = perl_matches(text, "m/(wi..)/g", &matches);
printf("perl_matches: m/(wi..)/g found %d matches...\n", num_matches);
for (i = 0; i < num_matches; i++)
printf("match: %s\n", matches[i]);
printf("\n");
for (i = 0; i < num_matches; i++) {
free(matches[i]);
}
free(matches);
/** Remove all vowels from text **/
num_matches = perl_substitute(&text, "s/[aeiou]//gi");
if (num_matches) {
printf("perl_substitute: s/[aeiou]//gi...%d substitutions made.\n",
num_matches);
printf("Now text is: %s\n\n", text);
}
/** Attempt a substitution
if (!perl_substitute(&text, "s/Perl/C/")) {
printf("perl_substitute: s/Perl/C...No substitution made.\n\n");
}
free(text);
perl_destruct(my_perl);
perl_free(my_perl);
}
which produces the output
perl_match: Text contains the word 'quarter'.
perl_match: Text doesn't contain the word 'eighth'.
perl_matches: m/(wi..)/g found 2 matches...
match: will
match: with
perl_substitute: s/[aeiou]//gi...139 substitutions made.
Now text is: Whn h s t cnvnnc str nd th bll cms t sm mnt lk 76 cnts, Mynrd s wr tht thr s smthng h *shld* d, smthng tht wll nbl hm t gt bck qrtr, bt h hs n d *wht*. H fmbls thrgh hs rd sqzy chngprs nd gvs th by thr xtr pnns wth hs dllr, hpng tht h mght lck nt th crrct mnt. Th by gvs hm bck tw f hs wn pnns nd thn th bg shny qrtr tht s hs prz. -RCHH
perl_substitute: s/Perl/C...No substitution made.
---------------------------------------------------------------------------
Fiddling with the Perl stack from your C program
When trying to explain stacks, most computer science textbooks mumble
something about spring-loaded columns of cafeteria plates: the last thing
you pushed on the stack is the first thing you pop off. That'll do for our
purposes: your C program will push some arguments onto "the Perl stack",
shut its eyes while some magic happens, and then pop the results--the
return value of your Perl subroutine--off the stack. First you'll need to
know how to convert between C types and Perl types, with newSViv() and
sv_setnv() and newAV() and all their friends. They're described in the
perlguts manpage .
Then you'll need to know how to manipulate the Perl stack. That's described
in the perlcall manpage .
Once you've understood those, embedding Perl in C is easy.
Since C has no built-in function for integer exponentiation, let's make
Perl's ** operator available to it (this is less useful than it sounds,
since 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 <stdio.h>
#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, env);
PerlPower(3, 4); /*** Compute 3 ** 4 ***/
perl_destruct(my_perl);
perl_free(my_perl);
}
Compile and run:
% cc -o power power.c -L/usr/local/lib/perl5/alpha-dec_osf/CORE
-I/usr/local/lib/perl5/alpha-dec_osf/CORE -lperl -lm
% power
3 to the 4th power is 81.
---------------------------------------------------------------------------
MORAL
You can sometimes write faster code in C, but you can always write code
faster in Perl. Since you can use each from the other, combine them as you
wish.
---------------------------------------------------------------------------
AUTHOR
Jon Orwant <orwant@media.mit.edu>, with contributions from Tim Bunce, Tom
Christiansen, Dov Grobgeld, and Ilya Zakharevich.
December 18, 1995
Some of this material is excerpted from my book: Perl 5 Interactive, Waite
Group Press, 1996 (ISBN 1-57169-064-6) and appears courtesy of Waite Group
Press.
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