Why might a process block in fcntl(fd, F_SETLKW, ...) on OSX?

A new experimental feature of SQLite uses a blocking posix lock at one point. I have a test case where a process is blocked in this fcntl() call on OSX, but I can't see why. The same test works on Linux, but that could just be lucky timing.

The call is:

fcntl(fd, F_SETLKW, &f);

where "f" is:

$1 = {
  l_start = 120,
  l_len = 1,
  l_pid = 0,
  l_type = 3,
  l_whence = 0
}

According to [lsof] my process is the only one that has the file open:

bash-3.2$ sudo lsof test.db-shm
COMMAND     PID USER   FD   TYPE DEVICE SIZE/OFF      NODE NAME
testfixtu 21393  dan   10u   REG   14,2    32768 228781731 test.db-shm

It is possible that some other process was holding a conflicting lock when fcntl() was first called though.

gdb shows that the file descriptor being passed to fcntl() is "10", so I think it is valid.

So my question is, why might fcntl() be blocking if no other processes have the file open?