/* Source: https://www.thegeekstuff.com/2012/03/c-process-control-functions/ */

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

int global = 0; 

int main() {
    pid_t child_pid;
    int status;
    int local = 0;

    /* now create new process */
    child_pid = fork();

    if (child_pid >= 0) /* fork succeeded */ {
        if (child_pid == 0) /* fork() returns 0 for the child process */ {
            printf("child process!\n");

            /* Increment the local and global variables */
            local++;
            global++;

            printf("child PID =  %d, parent pid = %d\n", getpid(), getppid());
            printf("\nchild's local = %d, child's global = %d\n",local,global);

            char * cmd[] = {"whoami",(char*)0};
            return execv("/usr/bin/whoami",cmd); // call whoami command

         }
         else /* parent process */ {
         
             printf("parent process!\n");
             printf("parent PID =  %d, child pid = %d\n", getpid(), child_pid);
             wait(&status); /* wait for child to exit, and store child's exit status */
             printf("Child exit code: %d\n", WEXITSTATUS(status));
             /* The change in local and global variable in child process should not reflect here in parent process. */
             printf("\nParent'z local = %d, parent's  global = %d\n",local,global);
             printf("Parent says bye!\n");
             exit(0);  /* parent exits */ }}
    else /* failure */ {
        perror("fork");
        exit(0);
    }
}