What happens if a write system call is called on same file by 2 different processes simultaneously

Question

Does the OS handle it correctly?

Or will I have to call flock()?

Answer 1

write (and writev, too) guarantee atomicity.

Which means if two threads or processes write simultaneously, you do not have a guarantee which one writes first. But you do have the guarantee that anything that is in one syscall will not be intermingled with data from the other one.

Insofar it will always work correctly, but not necessarily in the way you expect (if you assume that process A comes before process B).