Simple dlopen-example

        A simple sample for checking whether a .so in Linux both dynamically linked & dynamically loaded-linked in a process will share the same address space. also how the cases of multiple loads are handled.

https://github.com/tvijayas/dlopen-example/blob/main/README.md

main.cpp

int main(void){

   void *handle1,*handle2;

   void (*IncCount)(void);

   void (*GetCount)(void);

   char *error;

   /*lib reference count=1*/

   handle1 = dlopen("libmylib.so", RTLD_LAZY);

   

   if (!handle1) {

      fprintf(stderr, "%s\n", dlerror());

      exit(EXIT_FAILURE);

   }

                 

   dlerror();/* Clear any existing error */

   

   *(void **) (&IncCount) = dlsym(handle1, "increment_count");

   *(void **) (&GetCount) = dlsym(handle1, "get_count");

   

   IncCount();

   

   error = dlerror();

   if (error != NULL) {

      fprintf(stderr, "%s\n", error);

      exit(EXIT_FAILURE);

   }       

                          

   cout<< "I'm in main" <<endl;

   increment_count();

   cout<< get_count() <<endl;

   

   /*lib reference count=2*/

   handle2 = dlopen("libmylib.so", RTLD_LAZY);

   

   if (!handle2) {

      fprintf(stderr, "%s\n", dlerror());

      exit(EXIT_FAILURE);

   }

   

   cout<< "handle1 = " <<(unsigned int)handle1 <<endl;

   cout<< "handle2 = " <<(unsigned int)handle2 <<endl;

   

   /*lib reference count reduced from 2 to 1*/

   dlclose(handle1);

   

   /*handle1 still works as the reference to the library not 0*/

   *(void **) (&IncCount) = dlsym(handle1, "increment_count");

   increment_count();

   cout<< get_count() <<endl;

   

   /*handle2 also works*/

   *(void **) (&IncCount) = dlsym(handle2, "increment_count");

   increment_count();

   cout<< get_count() <<endl;

   

   /*lib reference count reduced from 1 to 0*/

   dlclose(handle1);

      

   return 0;

}

SharedSrc.c

#include "stdio.h"

static int count = 0;

int get_count(void){

  return count;

}

void increment_count(void){

  count++;

}

__attribute__((constructor)) void lib_init(void){

  fprintf(stderr,"%s \n","mylib loaded");   

}

__attribute__((destructor)) void lib_exit(void){

  fprintf(stderr,"%s \n","mylib unload");

}

Output: 

       mylib loaded 

       I'm in main 

       2 

       handle1 = 2439537088 

       handle2 = 2439537088 

       3 

       4 

       mylib unload