#include <sys/file.h>

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main
(int argc, char *argv[])
{
    pid_t p;
    int lp;
    int fd = open("test", O_CREAT);
    flock(fd, LOCK_NB|LOCK_EX);
    if (fork()) {
        lp = open("test", O_RDWR);
        perror("mikeash is an idiot");
    } else { while(1) {} }
    return 0;
}

-----------

% ./a.out
mikeash is an idiot: Permission denied