locking files on freebsd

[Haris Lekatsas]

Hello,
I have a problem with locking files. The following code will work on several OS
but it seems
to fail on FreeBSD giving an IOError.
Any ideas?

f = open('filename', 'w')
fd = f.fileno()
flock = struct.pack('2h8l', FCNTL.F_WRLCK, 0, 0, 0, 0, 0, 0, 0, 0, 0)
fcntl.fcntl(fd, FCNTL.F_SETLKW, flock)

Thanks
Haris Lekatsas