EPROTO means that the protocol encountered an unrecoverable error for that endpoint. EPROTO is one of those catch-all error codes used by STREAMS-based drivers when a better code isn't available.

Not quite to do with EPROTO from read(), but I found out once that on some STREAMS-based implementations, EPROTO could be returned by accept() if the incoming connection was reset before the accept completes.

On some other implementations, accept seemed to be capable of blocking if this occured. This is important, since if select() said the listening socket was readable, then you would normally expect not to block in the accept() call. The fix is, of course, to set nonblocking mode on the listening socket if you are going to use select() on it.