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(), writev(), read(), readv() can generate partial writes/reads where the amount of data transferred is smaller than what was requested.

Quoting the Linux man page for writev():

Note that is not an error for a successful call to transfer fewer bytes than requested

Quoting the POSIX man page:

If write() is interrupted by a signal after it successfully writes some data, it shall return the number of bytes written.

AFAIU, O_APPEND does not help in this regard because it does not prevent partial writes: it only ensures that whatever data is written is appended at the end of the file.

See this bug report from the Linux kernel:

A process is writing a messages to the file. [...] the writes [...] can be split in two. [...] So if the signal arrives [...] the write is interrupted. [...] this is perfectly valid behavior as far as spec (POSIX, SUS,...) is concerned

FIFOs and PIPE writes smaller than PIPE_MAX however are guaranteed to be atomic.