How to share memory between processes created by fork()?

Changing a global variable is not possible because the new created process (child)is having it's own address space.

So it's better to use shmget(),shmat() from POSIX api

Or You can use pthread , since pthreadsare sharing the globaldata and the changes in global variable is reflected in parent.

Then read some Pthreads tutorial.

You can use shared memory (shm_open(), shm_unlink(), mmap(), etc.).

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

static int *glob_var;

int main(void)
{
    glob_var = mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE, 
                    MAP_SHARED | MAP_ANONYMOUS, -1, 0);

    *glob_var = 1;

    if (fork() == 0) {
        *glob_var = 5;
        exit(EXIT_SUCCESS);
    } else {
        wait(NULL);
        printf("%d\n", *glob_var);
        munmap(glob_var, sizeof *glob_var);
    }
    return 0;
}