PERLEMBED
NAME DESCRIPTION Embedding Perl under Win32 MORAL AUTHOR COPYRIGHT
perlembed - how to embed perl in your C program
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
or perhaps just
/usr/local/lib/perl5/CORE
or maybe something like
/usr/opt/perl5/CORE
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
You may have to add extra libraries as well. Which ones? Perhaps those printed by
perl -MConfig -e 'print $Config{libs}'
If the !ExtUtils?::Embed module isn't part of your Perl distribution, you can retrieve it from 
http://www.perl.com/perl/CPAN/modules/by-module/!ExtUtils/. (If this documentation came from your Perl distribution, then you're running 5.004 or better and you already have it.)
The !ExtUtils?::Embed kit on CPAN also contains all source code for the examples in this document, tests, additional examples and other information you may find useful.
Adding a Perl interpreter to your C program
In a sense, perl (the C program) is a good example of embedding Perl (the language), so I'll demonstrate embedding with miniperlmain.c, included in the source distribution. Here's a bastardized, nonportable version of miniperlmain.c containing the essentials of
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); } 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.
Now compile this program (I'll call it interp.c) into
print or
% interp -e 'printf(
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.
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() ***/ call_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 perlcall.
I'll define the showtime subroutine in a file called
print time; }
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, but in general it's considered good practice to ensure proper initialization of library code, including execution of all object DESTROY methods and package END {} blocks.
If you want to pass arguments to the Perl subroutine, you can add strings to the NULL-terminated args list passed to call_argv. For other data types, or to examine return values, you'll need to manipulate the Perl stack. That's demonstrated in ``Fiddling with the Perl stack from your C program''.
Evaluating a Perl statement from your C program
Perl provides two API functions to evaluate pieces of Perl code. These are ``eval_sv in perlapi and ``eval_pv in perlapi.
Arguably, these are the only routines you'll ever need to execute snippets of Perl code from within your C program. Your code can be as long as you wish; it can contain multiple statements; it can employ ``use in perlfunc, ``require in perlfunc, and ``do'' in perlfunc to include external Perl files.
eval_pv lets us evaluate individual Perl strings, and then extract variables for coercion into C types. The following program, string.c, executes three Perl strings, extracting an int from the first, a float from the second, and a char * from the third.
static !PerlInterpreter? *my_perl; main (int argc, char **argv, char **env)
{ STRLEN n_a; char *embedding[? = {
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 **/
eval_pv(
/** Treat $a as a float **/
eval_pv(
/** Treat $a as a string **/
eval_pv(
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 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()
a = 9.859600 a = Just Another Perl Hacker
STRLEN n_a; SV *val = eval_pv( This way, we avoid namespace pollution by not creating global variables 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 /bw*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 AV at 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
/** 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 return retval;
}
/** match(string, pattern)
*
*
I32 match(SV *string, char *pattern)
{ SV *command = NEWSV(1099, 0), *retval; STRLEN n_a;
sv_setpvf(command, retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
return SvIV(retval);
}
/** substitute(string, pattern)
*
*
I32 substitute(SV **string, char *pattern)
{ SV *command = NEWSV(1099, 0), *retval; STRLEN n_a;
sv_setpvf(command, retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
- string = get_sv(
/** matches(string, pattern, matches)
*
*
I32 matches(SV *string, char *pattern, AV **match_list)
{ SV *command = NEWSV(1099, 0); I32 num_matches; STRLEN n_a;
sv_setpvf(command, my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
- match_list = get_av(
return num_matches;
}
main (int argc, char **argv, char **env)
{
char *embedding[? = {
perl_construct(my_perl);
perl_parse(my_perl, NULL, 3, embedding, NULL);
sv_setpv(text, if (match(text, if (match(text, /** Match all occurrences of /wi../ **/
num_matches = matches(text,
for (i = 0; i /** Remove all vowels from text **/
num_matches = substitute(
/** Attempt a substitution **/
if (!substitute(
SvREFCNT_dec(text);
PL_perl_destruct_level = 1; perl_destruct(my_perl); perl_free(my_perl); } 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
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...
static !PerlInterpreter? *my_perl; static void
{ 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(
int main (int argc, char **argv, char **env)
{ char *my_argv[? = {
my_perl = perl_alloc();
perl_construct( my_perl );
perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
perl_run(my_perl);
PerlPower?(3, 4); /*** Compute 3 ** 4 ***/
perl_destruct(my_perl);
perl_free(my_perl); }
3 to the 4th power is 81.
Maintaining a persistent interpreter
When developing interactive and/or potentially long-running applications, it's a good idea to maintain a persistent interpreter rather than allocating and constructing a new 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 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 perl_parse(), compiled into a newly constructed interpreter, and subsequently cleaned out with perl_destruct() afterwards, you're shielded from most namespace troubles.
One way to avoid namespace collisions in this scenario is to translate the filename into a guaranteed-unique package name, and then compile the code into that package using ``eval in perlfunc. In the example below, each file will only be compiled once. Or, the 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 AUTOLOADed 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 ``my'' in perlfunc whenever possible.
package Embed::Persistent;
our %Cache; use Symbol qw(delete_package);
sub valid_package_name {
my($string) = @_; $string = s/([^A-Za-z0-9/?)/sprintf(
- Dress it up as a real package name
$string = s/::g; return
sub eval_file {
my($filename, $delete) = @_; my $package = valid_package_name($filename); my $mtime = -M $filename; if(defined $Cache{$package}{mtime}
- wrap the code into a subroutine inside our unique package
my $eval = qq{package $package; sub handler { $sub; }}; {
my($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- delete_package($package) if $delete;
- take a look if you want
main(int argc, char **argv, char **env) { char *embedding[? = {
if((perl = perl_alloc()) == NULL) {
fprintf(stderr,
exitstatus = perl_parse(perl, NULL, 2, embedding, NULL); if(!exitstatus) {
exitstatus = perl_run(perl);
while(printf(
/* call the subroutine, passing it the filename as an argument */
args[0? = filename; call_argv(
/* check $@ */
if(SvTRUE(ERRSV)) fprintf(stderr,
PL_perl_destruct_level = 0;
perl_destruct(perl); perl_free(perl); exit(exitstatus); }
my $string =
sub foo {
Enter file name: test.pl foo says: hello Enter file name: test.pl already compiled Embed::test_2epl-
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
... /* reset global variables here with PL_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! */ } When 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 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.
Using -Dusethreads -Duseithreads rather than
to run multiple interpreters concurrently in different threads, because it enables support for linking in the thread libraries of your system with the interpreter.
/* we're going to embed two interpreters */
/* we're going to embed two interpreters */
int main(int argc, char **argv, char **env)
{
char *one_args[? = {
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); } Note the calls to PERL_SET_CONTEXT (). These are necessary to initialize the global state that tracks which interpreter is the ``current'' one on the particular process or thread that may be running it. It should always be used if you have more than one interpreter and are making perl API calls on both interpreters in an interleaved fashion.
PERL_SET_CONTEXT (interp) should also be called whenever interp is used by a thread that did not create it (using either perl_alloc(), or the more esoteric perl_clone()).
Hi, I'm one_perl Hi, I'm two_perl
Using Perl modules, which themselves use C libraries, from your C program
If you've played with the examples above and tried to embed a script that use()s a Perl module (such as Socket) which itself uses a C or C ++
(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 calling perl_parse(), handing it
EXTERN_C void boot_Socket (pTHX_ CV* cv);
EXTERN_C void
xs_init(pTHX) { char file = FILE; / !DynaLoader? is a special case */ newXS( Simply put: for each extension linked with your Perl executable (determined during its initial configuration on your computer or when adding a new extension), a Perl subroutine is created to incorporate the extension's routines. Normally, that subroutine is named Module::bootstrap() and is invoked when you say use Module. In turn, this hooks into an XSUB , boot_Module, which creates a Perl counterpart for each of the extension's XSUBs. Don't worry about this part; leave that to the xsubpp and extension authors. If your extension is dynamically loaded, !DynaLoader? creates Module::bootstrap() for you on the fly. In fact, if you have a working !DynaLoader? then there is rarely any need to link in any other extensions statically.
Once you have this code, slap it into the second argument of
use Socket; use !SomeDynamicallyLoadedModule?;
!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.
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
Create a new project and Insert -C:perllibCORE, 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 -
You can sometimes write faster code in C, but you can always write code faster in Perl. Because you can use each from the other, combine them as you wish.
Jon Orwant orwant@tpj.com dougm@osf.org
Doug !MacEachern? has an article on embedding in Volume 1,
Issue 4 of The Perl Journal (http://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. Oracle, Binary Evolution, !ActiveState?,
and Ben Sugars's nsapi_perl have used this model for Oracle,
Netscape and Internet Information Server Perl
plugins.
July 22, 1998
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 documentation into another language, under the above conditions for modified versions.
2 pages link to perlembed(1):
