fork1.c

// Compile: gcc -o fork1 fork1.c
// Run:     ./fork1

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

int main() {
    pid_t childpid; /* variable to store the child's pid */
    int retval;     /* user-provided return code for child process */
    int status;     /* child's exit status for parent process */

    /* create new process */
    childpid = fork();
    if (childpid >= 0)        /* fork succeeded */
    {
        if (childpid == 0)    /* fork() return 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 fork return is: % d\n", childpid);
            printf("CHILD: Sleep for 1 second...\n");
            sleep(1);
            printf("CHILD: Enter an exit value (0~255): ");
            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 child's PID is: %d\n", childpid);
            printf("PARENT: I will now wait for my child to exit.\n");
            wait(&status);                                                    /* wait for child to exit,and store its status*/
            printf("PARENT: Child's exit code is: %d\n", WEXITSTATUS(status));
            printf("PARENT: Goodbye! \n");
            exit(0);                                                          /* parent exits */
        }
    } else                                                                    /* fork returns -1 on failure */
    {
        perror("fork error!");                                                /* display error messsage */
        exit(0);
    }
}