Multiple serial port misbehavior

Which set of functions are you using, the fopen(),fread(),fwrite() or the open(),read(),write() ?

As I understand it, the fopen(),etc does thread-safe checking and would have more overhead than the open(),etc.

Increasing the buffer size is a great idea - but I had the impression from the program behavior that there already was a large print buffer for printf(). The reason I say that is the a multi-task app which generates a lot of output from several tasks, I never see interleaved print output. A 64 char buffer would fill up fast. I'd expect to see either interleaved print output or lost chars on my main terminal. I don't see this.