write/fwrite in append mode

[Matthew Farwell]

This is a generic Unix question (ie the version of unix isn't really
important because I'm trying to devise a fairly portable way to do this)

I have an application which requires a set of structures (indexed by tty
name) to be maintained in a file.  The structures are simple and 68
bytes long.  This is assuming that two processes will not be running
this application from the same tty.

I've settled on a utmp like structure to the file ie a process searches
through the file looking for its tty name.  If it finds it, it uses
that.  If it doesn't find it, then it closes the file, reopens it in
append mode and writes the structure onto the end of the file.

My question is this.  Is the write guaranteed to be atomic?  The main
concern is that the write will half-finish and the second process will
seek to the wrong place, something like this

Process 1			Process 2
starts
writes 25 bytes
switches to process 2
				starts
				seeks to the end of the file (ie 25)
				writes 68 bytes
				switches to process 1
restarts
writes the rest of the struct,
corrupting the file.

Are my fears well founded or complete dog-poo?  Marc Rochkind basically
says that this way of write should be atomic, but not to trust the
semantics of write in append mode because its not actually documented
anywhere (pp.  28).  I realise this is good advice but I want to avoid
actually implmenting some sort of locking if I can.  How many unices is
write broken in this way?  Does anyone have a reliable way to test for
this behaviour? To make things more complex, I'm actually using stdio
in this case ie fwrite + fflush instead of write.

Does POSIX fix this?

Comments welcome. Also comments on any different way to implement this
type of thing would also be welcome.

Thanks.

Dylan.
-- 
Matthew J Farwell: dylan at ibmpcug.co.uk || ...!uunet!ukc!ibmpcug!dylan
                  You can't kill me, I'm a genius!!!!