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

int main(int argc, char* argv[])
{
    int fd;
    struct flock lck;

    fd = open("file.txt", O_WRONLY | O_CREAT | O_TRUNC, 0600);

    lck.l_type      = F_WRLCK;
    lck.l_whence    = SEEK_SET;
    lck.l_start     = 10;
    lck.l_len       = 5;

    if ( -1 == (fcntl(fd, F_SETLKW, &lck) ) ) {
        exit(3);
    }

    write(fd, argv[1], 1);

    sleep(5);
    close(fd);
    return 0;
}