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

Have a care for those who follow in your footsteps

It's perfectly normal to protect the file descriptor with a mutex semaphore. It removes any dependence on kernel behaviour so your message boundaries are now certain. You then don't have to cite the last paragraph at the bottom of a 15,489 line manpage which explains why the mutex isn't necessary (I exaggerated, but you get my meaning)

It also makes it clear to anyone reading your code that the file descriptor is being used by more than one thread.

Fringe Benefit

There is a fringe benefit to using a mutex that way. Suppose you've got different messages coming from the different threads and some of those messages are more important than others. All you need to do is set the thread priorities to reflect their messages' importance. That way the OS will ensure that your messages will be sent in order of importance for minimal effort on your part.