| perlembed - how to embed perl in your C program |
perlembed - how to embed perl in your C program
Do you want to:
system and exec in the perlfunc manpage.
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 -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-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.
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 embedding:
#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();
exit(EXIT_SUCCESS);
}
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;
they should only be called once regardless of how many interpreters you
create or destroy. Call PERL_SYS_INIT3() before you create your first
interpreter, and PERL_SYS_TERM() after you free your last interpreter.
Since PERL_SYS_INIT3() may change env, it may be more appropriate to
provide env as an argument to perl_parse().
Also notice that no matter what arguments you pass to perl_parse(),
PERL_SYS_INIT3() must be invoked on the C main() argc, argv and env and
only once.
Mind that argv[argc] must be NULL, same as those passed to a main function in C.
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.
To call individual Perl subroutines, you can use any of the call_*
functions documented in the perlcall manpage.
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();
exit(EXIT_SUCCESS);
}
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 \
`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 manipulate the Perl stack. That's demonstrated in
Fiddling with the Perl stack from your C program.
Perl provides two API functions to evaluate pieces of Perl code. These are perlapi/eval_sv and perlapi/eval_pv.
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 the perlfunc manpage, require in the perlfunc manpage, and do in the perlfunc manpage 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)
{
char *embedding[] = { "", "-e", "0", NULL };
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", 0)));
/** Treat $a as a float **/
eval_pv("$a = 3.14; $a **= 2", TRUE);
printf("a = %f\n", SvNV(get_sv("a", 0)));
/** Treat $a as a string **/
eval_pv(
"$a = 'rekcaH lreP rehtonA tsuJ'; $a = reverse($a);", TRUE);
printf("a = %s\n", SvPV_nolen(get_sv("a", 0)));
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 the perlguts manpage 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'ed expression. It is also possible and in most cases a better strategy to fetch the return value from eval_pv() instead. Example:
...
SV *val = eval_pv("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
printf("%s\n", SvPV_nolen(val));
...
This way, we avoid namespace pollution by not creating global variables and we've simplified our code as well.
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.
SSize_t 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;
PUSHMARK(SP);
eval_sv(sv, G_SCALAR);
SPAGAIN;
retval = POPs;
PUTBACK;
if (croak_on_error && SvTRUE(ERRSV))
croak_sv(ERRSV);
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(0), *retval;
sv_setpvf(command, "my $string = '%s'; $string =~ %s",
SvPV_nolen(string), pattern);
retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
return SvIV(retval);
}
/** substitute(string, pattern) ** ** Used for =~ operations that ** modify their left-hand side (s/// and tr///) ** ** Returns the number of successful matches, and ** modifies the input string if there were any. **/
I32 substitute(SV **string, char *pattern)
{
SV *command = newSV(0), *retval;
sv_setpvf(command, "$string = '%s'; ($string =~ %s)",
SvPV_nolen(*string), pattern);
retval = my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
*string = get_sv("string", 0);
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 **/
SSize_t matches(SV *string, char *pattern, AV **match_list)
{
SV *command = newSV(0);
SSize_t num_matches;
sv_setpvf(command, "my $string = '%s'; @array = ($string =~ %s)",
SvPV_nolen(string), pattern);
my_eval_sv(command, TRUE);
SvREFCNT_dec(command);
*match_list = get_av("array", 0);
num_matches = av_top_index(*match_list) + 1;
return num_matches;
}
main (int argc, char **argv, char **env)
{
char *embedding[] = { "", "-e", "0", NULL };
AV *match_list;
I32 num_matches, i;
SV *text;
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(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_nolen(*av_fetch(match_list, i, FALSE)));
printf("\n");
/** Remove all vowels from text **/
num_matches = substitute(&text, "s/[aeiou]//gi");
if (num_matches) {
printf("substitute: s/[aeiou]//gi...%lu substitutions made.\n",
(unsigned long)num_matches);
printf("Now text is: %s\n\n", SvPV_nolen(text));
}
/** 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.
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 and perlapi.
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.
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", NULL };
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(); exit(EXIT_SUCCESS); }
Compile and run:
% cc -o power power.c `perl -MExtUtils::Embed -e ccopts -e ldopts`
% power
3 to the 4th power is 81.
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 the perlfunc manpage. 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 perlapi/call_argv, 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 the perlfunc manpage 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", NULL };
char *args[] = { "", DO_CLEAN, NULL };
char filename[BUFFER_SIZE];
int failing, exitstatus;
PERL_SYS_INIT3(&argc,&argv,&env);
if((my_perl = perl_alloc()) == NULL) {
fprintf(stderr, "no memory!");
exit(EXIT_FAILURE);
}
perl_construct(my_perl);
PL_origalen = 1; /* don't let $0 assignment update the
proctitle or embedding[0] */
failing = perl_parse(my_perl, NULL, 2, embedding, NULL);
PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
if(!failing)
failing = perl_run(my_perl);
if(!failing) {
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_nolen(ERRSV));
}
}
PL_perl_destruct_level = 0;
exitstatus = 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
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.
When a perl script assigns a value to $0 then the perl runtime will
try to make this value show up as the program name reported by ``ps'' by
updating the memory pointed to by the argv passed to perl_parse() and
also calling API functions like setproctitle() where available. This
behaviour might not be appropriate when embedding perl and can be
disabled by assigning the value 1 to the variable PL_origalen
before perl_parse() is called.
The persistent.c example above is for instance likely to segfault
when $0 is assigned to if the PL_origalen = 1; assignment is
removed. This because perl will try to write to the read only memory
of the embedding[] strings.
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 interpreter variables may
not be initialized correctly between consecutive runs and your
application may crash.
See also Thread-aware system interfaces in the perlxs manpage.
Using -Dusethreads -Duseithreads rather than -Dusemultiplicity
is more appropriate if you intend to run multiple interpreters
concurrently 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 */
#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, NULL };
char *two_args[] = { "two_perl", SAY_HELLO, NULL };
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();
exit(EXIT_SUCCESS);
}
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()).
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
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 calling 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 contact 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 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 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 the perlxs manpage, the perlguts manpage, and perlapi for more details.
(See the perllocale manpage for information about these.)
When a Perl interpreter normally starts up, it tells the system it wants
to use the system's default locale. This is often, but not necessarily,
the ``C'' or ``POSIX'' locale. Absent a "use locale" within the perl
code, this mostly has no effect (but see Not within the scope of ``use locale'' in the perllocale manpage). Also, there is not a problem if the
locale you want to use in your embedded perl is the same as the system
default. However, this doesn't work if you have set up and want to use
a locale that isn't the system default one. Starting in Perl v5.20, you
can tell the embedded Perl interpreter that the locale is already
properly set up, and to skip doing its own normal initialization. It
skips if the environment variable PERL_SKIP_LOCALE_INIT is set (even
if set to 0 or ""). A perl that has this capability will define the
C pre-processor symbol HAS_SKIP_LOCALE_INIT. This allows code that
has to work with multiple Perl versions to do some sort of work-around
when confronted with an earlier Perl.
If your program is using the POSIX 2008 multi-thread locale functionality, you should switch into the global locale and set that up properly before starting the Perl interpreter. It will then properly switch back to using the thread-safe functions.
If you completely hide the short 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 in the perlguts manpage for the explanation
of the aTHX_. ) 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.)
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@media.mit.edu> and Doug MacEachern <dougm@covalent.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. Oracle, Binary Evolution, ActiveState, and Ben Sugars's nsapi_perl have used this model for Oracle, Netscape and Internet Information Server Perl plugins.
Copyright (C) 1995, 1996, 1997, 1998 Doug MacEachern and Jon Orwant. All Rights Reserved.
This document may be distributed under the same terms as Perl itself.
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