C read and thread safety (linux)

Question

What would happen if you call read (or write, or both) in two different thread, on the same file descriptor (lets says we are interested about a lo

Answer 1

The result would depend on how the threads are scheduled to run at that particular instant in time.

One way to potentially avoid undefined behavior with multi-threading is to assume that you are doing memory operations. E.g. updating a linked list or changing a variable, etc.

If you use mutex/semaphores/lock or some other synchronization mechanism, it should work as intended.