#include <stdio.h>

int
main()
{
    char buf[80];

    fork();

    // both parent and child will read from the 
    // same terminal - it's undefined who's get
    // the input.
    // (And, you need to type ^D twice to exit
    // both programs.)
    while (fgets(buf, sizeof buf, stdin)) {
        printf("pid %d: ", getpid());
        fputs(buf, stdout);
    }
}