Can fwrite & fclose be called parallely from two threads for the same file descriptor?

Question

What will happen if fwrite & fclose are called in parallel from two threads for the same file descriptor?

Answer 1

POSIX requires FILE access to be thread-safe, but since fclose closes the file and invalidates the pointer, there is no way (i.e. it's not just a specification issue but a fundamental API issue that could never be "fixed" or made to go away) to use fclose when another thread might be accessing the FILE still. You'll need to do your own locking.