int FL_more_msgs(mailbox mbox);
NOTE, that this function CANNOT be used as an I/O polling function to check if a receive call should be done. If this function returns zero there still might be a message ready to receive on the connection: FL_poll, and file descriptor selects/polls can detect if there is activity (not necessarily a message) on a connection, while DONT_BLOCK receive semantics can detect whether or not there is a message on the connection and FL_more_msgs can only answer if there are any buffered messages already on the connection.
This function is merely a helper function to re-get the current status of the more_messes parameter returned from a call to FL_receive.