dlopen help

Applications Programming
1 
//foo.c

#include<stdio.h>

int pen(int a)
{
    printf("%d",a);
}

$cc -c foo.c
$ls -shared -o libfoo.so foo.o

///////////now libfoo.so formed 
//i have already designed libfoo.so
//main.c

#include<stdio.h>
#include <dlfcn.h>
int main()
{
   int (*p)(int)=NULL ;
   void *handle;
   handle=dlopen("libfoo.so",RTLD_LAZY);
   p=dlsym(handle,"pen");
   (*p)(5);
   return 0;
}

/////////////////////////////////////////////////////////////////////////////////////////

but this gives error

$ cc main.c -ldl
$ ./a.out
Segmentation fault
2

You have no error checking, for starters. This will tell you where things go wrong.

#include<stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
int main()
{
   int (*p)(int)=NULL ;
   void *handle;
   handle=dlopen("libfoo.so",RTLD_LAZY);
   if (!handle) 
  {
	fputs (dlerror(), stderr);
	exit(1);
   }

   p=dlsym(handle,"pen");
   {
          char *err=dlerror();
          if(err!=NULL)
          {
                fputs(err,stderr);
                exit(1);                
           }
    }

   (*p)(5);
   return 0;
}
3

In addition to error checking you need to cast the operands and function return values to the proper type before assignment. Source code changes highlighted in red.

#include <stdio.h>
#include <dlfcn.h>

int main(void)
{
   int (*p)(int)=NULL;
   int i;
   void *handle;
   handle=dlopen("libfoo.so",RTLD_LAZY);

   /* cast the return value from dlsym() to the proper type before assignment to p
      p is a pointer to a function that takes an int argument and return an int */
   p=((int) (*)(int)) dlsym(handle,"pen");
   
   /* capture the integer return value from the shared library */
   i = (*p)(5); 
   return 0;
}