File access questions

I have one Cocoa-based process that needs to publish a small amount of =
data (32 bytes) about 1/sec to n other C-based processes that are busy =
doing calculations. I was thinking of writing to a small file in /tmp, =
and then the other process could read it (if/when they choose to).=20

Q: Does a file opened for write-only "w" access preclude others from =
reading it? Will an open "r" call to a file that's being written simply =
hang until the writer closes it? Will a write-only open call on a file =
that's being read hang?

Perhaps there's a better approach to this...