Create Shared Library Function Calls in Linux or Unix 3 steps

Creating Shared Library Function Calls in Linux

1. Create programe code ngprogram.c

#include <stdio.h>
#include <unistd.h>
int main() 

{
        puts("aroundng dot com world!n");
    return 0;
}

2. libaroundng.c file library code

#include <stdio.h> 

#include <unistd.h> 

#include <dlfcn.h> 

int puts(const char *message) 

{ 

int (*new_puts)(const char *message);

int result; 

new_puts = dlsym(RTLD_NEXT, "puts"); 

if(strcmp(message, "aroundng dot com world!n") == 0)

   { 

       result = new_puts("Goodbye, aroundng!\n");

    } 

   else 

    { 

         result = new_puts(message);

     } 

   return result; 

}

3. Create and then export library :

$ gcc libaroundng.c -o libaroundng.so -fPIC -shared -ldl -D_GNU_SOURCE

$ export LD_PRELOAD="${PWD}/libexample.so"

Finally, Compile:

$ gcc ngprogram.c -o ngprogram

$ ./ngprogram

Goodbye, aroundng!

$

Here system call is   puts(const char *message)

Feel free ask any queries