How does one programmatically determine if “write” system call is atomic on a particular file?

Question

In some cases the coder cannot rely on system calls being atomic, e.g. if the file is on a NFS filesystem. (c.f. NFS Overview, FAQ and HOWTO Documents). But atomic system

Answer 1

The write call as defined in POSIX has no atomicity guarantee at all. So you don't need to confirm anything, it's not atomic.

It doesn't even guarantee that the data will have reached the hard drive (if there is a drive at all) if it completes successfully. Successfully reading back the data doesn't give you any guarantees either.

You'll need to use the sync family of functions to get some durability guarantees.