Neal_the_real has asked for the wisdom of the Perl Monks concerning the following question:

Hello out there.

My question is quit simple but is is very difficult (for me) to implement it.

So, i want to read some config_data out of a perl module. In perl it looks like that.

The module: 

package MyConfig;

use strict;
use Exporter;
use vars qw(@ISA @EXPORT $var);

@ISA = qw(Exporter);
@EXPORT = qw($var);

$var = "SOME_FOO";

1;

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And the programm: 
#!/usr/bin/perl -w

use strict;
use MyConfig;

printf(">%s<\n",$MyConfig::var);

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So far, so good. But now I want the same in C. I tried a little bid and thats my C code 
#include <stdio.h>
#include <stdlib.h>
#include <EXTERN.h>
#include <perl.h>

static PerlInterpreter *my_perl;

int main(void) {
  char *var;

  my_perl = perl_alloc();
  perl_construct(my_perl);

  var = SvPV_nolen(get_sv("MyConfig::var", TRUE));

  printf(">%s<\n", var);

  return(EXIT_SUCCESS);
}

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Makefile: 
PERL_CCOPTS = `perl -MExtUtils::Embed -e ccopts`
PERL_LDOPTS = `perl -MExtUtils::Embed -e ldopts`

all:
        gcc -g -o read_config.o -c read_config.c $(PERL_CCOPTS)
        gcc -g -o read_config read_config.o $(PERL_LDOPTS)

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I get's a Segmentation fault. Hope anyone can help me out :-)

Thanks a lot. Regards Neal

P.S. I know, my english is not the best. So please forgive me.

Replies are listed 'Best First'.
Re: Config Module for C and Perl
by samtregar (Abbot) on Feb 20, 2006 at 19:40 UTC
    Follow the first rule of C - always check your return values! I suspect that either get_sv() or SvPV_nolen is returning NULL. Trying to dereference a NULL pointer is a great way to get a Segmentation Fault.

    As to why they'd return NULL, my guess is it's because you're not loading the MyConfig module. Try something like this before trying to fetch the variable:

      eval_pv("use MyConfig", 1);

    -sam

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[d/l]
      Hello samtregar. Your suggestion was a step in the right way. But there was also a few lines lacking for the perl-allocation. Now, I can represent a working C code. 
      #include <stdio.h>
#include <stdlib.h>
#include <EXTERN.h>
#include <perl.h>

static PerlInterpreter *my_perl;

int main(void) {
  char *var;
  char *embedding[] = {"", "-e", "0"};

  my_perl = perl_alloc();
  perl_construct(my_perl);
  perl_parse(my_perl, NULL, 3, embedding, NULL);
  perl_run(my_perl);

  eval_pv("use MyConfig", TRUE);
  var = SvPV_nolen(get_sv("MyConfig::var", TRUE));

  printf(">%s<\n", var);

  return(EXIT_SUCCESS);
}

      [download]
[reply]
[d/l]
      Hello samtregar.

      Your suggestion was a step in the right way. But there was also a few lines lacking for the perl-allocation. Now, I can represent a working C code.

      #include <stdio.h>
#include <stdlib.h>
#include <EXTERN.h>
#include <perl.h>

static PerlInterpreter *my_perl;

int main(void) {
  char *var;
  char *embedding[] = {"", "-e", "0"};

  my_perl = perl_alloc();
  perl_construct(my_perl);
  perl_parse(my_perl, NULL, 3, embedding, NULL);
  perl_run(my_perl);

  eval_pv("use MyConfig", TRUE);
  var = SvPV_nolen(get_sv("MyConfig::var", TRUE));

  printf(">%s<\n", var);

  return(EXIT_SUCCESS);
}

      [download]
[reply]
[d/l]

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