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

Of course the kernel will handle it correctly, for the kernel’s idea of correctness — which is by definition correct.

If you have a set of coöperating flockers, then you can use the kernel to queue everyone up. But remember that flock has nothing to do with I/O: it will not stop someone else from writing the file. It will at most only interfere with other flockers.