#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>

FILE *fp;

void main() {
    unlink("a"); unlink("b");

    fp = fopen("a", "wb"); // file a

    printf("lock a\n");
    flock(fileno(fp), LOCK_EX);

    fwrite("TEST", sizeof(char), 4, fp);

    printf("unlock a\n");
    flock(fileno(fp), LOCK_UN);
    fclose(fp);

    fp = fopen("b", "wb");  // file b

    printf("lock b\n");
    flock(fileno(fp), LOCK_EX);  // hang

    fwrite("TEST", sizeof(char), 4, fp);

    printf("unlock b\n");
    flock(fileno(fp), LOCK_UN);
    fclose(fp);
}