/* Includes */
#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <sys/wait.h>   /* Wait for Process Termination */
#include <stdlib.h>     /* General Utilities */
 
int main()
{
    pid_t childpid; /* variable to store the child's pid */
    int retval;     /* child process: user-provided return code */

    /* only 1 int variable is needed because each process would have its
       own instance of the variable
       here, 2 int variables are used for clarity */
        
    /* now create new process */
    childpid = fork();
    
    if (childpid >= 0) /* fork succeeded */
    {
        if (childpid == 0) /* fork() returns 0 to the child process */
        {
            printf("CHILD: I am the child process!\n");
            printf("CHILD: Here's my PID: %d\n", getpid());
            printf("CHILD: My parent's PID is: %d\n", getppid());
            printf("CHILD: The value of my copy of childpid is: %d\n", childpid);
            printf("CHILD: Sleeping for 1 second...\n");
            sleep(1); /* sleep for 1 second */
            printf("CHILD: Enter an exit value (0 to 255): ");
            scanf(" %d", &retval);
            printf("CHILD: Goodbye!\n");    
            _exit(retval); /* child exits with user-provided return code */
        }
        else /* fork() returns new pid to the parent process */
        {
            printf("PARENT: I am the parent process!\n");
            printf("PARENT: Here's my PID: %d\n", getpid());
            printf("PARENT: The value of my copy of childpid is %d\n", childpid);
            printf("PARENT: I will now wait for my child to exit.\n");
            wait(&retval); /* wait for child to exit, and store its status */
            printf("PARENT: Child's exit code is: %d\n", WEXITSTATUS(retval));
            printf("PARENT: Goodbye!\n");             
            exit(0);  /* parent exits */       
        }
    }
    else /* fork returns -1 on failure */
    {
        perror("fork"); /* display error message */
        exit(0); 
    }
}