$ chmod 666 datafile
$ cp datafile1 datafile
$ ./locktest &
[1]     1302
$ cp datafile2 datafile
$ datafile2 line 1
datafile2 line 2
$
[1] +  Done                    ./locktest &
$
$
$ cp datafile1 datafile
$ chmod 2666 datafile
$ ./locktest &
[1]     1330
$ cp datafile2 datafile
cp: cannot create datafile: Resource temporarily unavailable
$ datafile1 line 1
datafile1 line 2
$
[1] +  Done                    ./locktest &
$
$ uname -srv
HP-UX B.11.00 A
$ cat locktest.c
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

main(void)
{
        char buff[250];
        FILE *F;
        struct flock mylock;
        int fd, iret;
        fd = open("datafile", O_RDWR);

        mylock.l_type = F_WRLCK;
        mylock.l_whence = SEEK_SET;
        mylock.l_start = 0;
        mylock.l_len = 0;
        iret = fcntl(fd,F_SETLK, &mylock);
        if (iret == -1) {
                 if ((errno == EACCES) || (errno == EAGAIN)) {
                        printf("locked \n");
                        exit(1);
                } else {
                        printf("fcntl failed\n");
                        exit(1);
                }
        }
        sleep(30);
        F = fdopen(fd, "r");
        while (fgets(buff,250,F)) {
                fputs(buff,stdout);
        }
        exit(0);
}