Posix (and I think windows) guarantee this atomically and durably overwrites a file:

tempfile = mkstemp(filename-XXXX)

write(tempfile)

fsync(tempfile)

close(tempfile)

rename(tempfile, filename)

sync()

Assume the entire write failed if any of the above return an error.

In some systems (nfs and ext3 come to mind), you can skip the fsync and/or sync, but don’t do that. It doesn’t make things significantly faster on the systems where it’s safe, but it definitely will lose data on other systems.

The only loophole I know of is that the final sync can fail, then return anyway. If that happens, the file system is probably hosed anyway.