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 the perlcall manpage and the perlxs manpage.
Use a UNIX program from Perl?
Read about backquotes and the system entry in the
perlfunc manpage and the exec entry in the perlfunc
manpage.
Use Perl from Perl?
Read about the do entry in the perlfunc manpage and
the eval entry in the perlfunc manpage and the use
entry in the perlmod manpage and the require entry in
the perlmod manpage.
Use C from C?
Rethink your design.
Use Perl from C?
Read on...
ROADMAP
the section on Compiling your C program
There's one example in each of the five sections:
the section on Adding a Perl interpreter to your C program
the section on Calling a Perl subroutine from your C
program
the section on Evaluating a Perl statement from your C
program
the section on Performing Perl pattern matches and
substitutions from your C program
the section on 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.
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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, the section on 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}'
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Adding a Perl interpreter to youur 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 yyour 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().
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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: the section on Fiddling with the
Perl stack from your C program
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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 the perl_eval_sv entry in the perlguts
manpage). 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 the
eval entry in the perlfunc manpage.
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 the require entry in the
perlmod manpage or the do entry in the perlfunc manpage 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.
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#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
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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 =~ $pattern in an array
context, and fills in matches with the array elements
(allocating memory as it does so), returning the number of
matches found.
Here's a sample program, match.c, that uses all three:
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#include <stdio.h>
#include <EXTERN.h>
#include <perl.h>
static PerlInterpreter *my_perl;
int eval(char *string)
{
char *argv[2];
argv[0] = string;
argv[1] = NULL;
perl_call_argv("_eval_", 0, argv);
}
/** match(string, pattern)
**
** Used for matches in a scalar context.
**
** Returns 1 if the match was successful; 0 otherwise.
**/
char match(char *string, char *pattern)
{
char *command;
command = malloc(sizeof(char) * strlen(string) + strlen(pattern) + 37);
sprintf(command, "$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.
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**
** 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. -RICHH");
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../ **/
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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.
=head2 Fiddling with the Perl stack from your C program
When trying to explain stacks, most computer science
textbooks mumble something about spring-loaded columns of
cafeteria plates: the last thing you pushed on the stack
is the first thing you pop off. That'll do for our
purposes: your C program will push some arguments onto
"the Perl stack", shut its eyes while some magic happens,
and then pop the results--the return value of your Perl
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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...
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#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.
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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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