Writing a sequence of characters across the interface I'm proposing is a request by the writing program for those characters to appear at their destination "soon". Ideally, "soon" would be "immediately"; however, the characters' appearance may deliberately be delayed ("buffered"), for efficiency, as long as such delay is "unobtrusive" to a human user of the program. Buffering timeouts would depend on the device; for a terminal, perhaps 50-100 ms would be appropriate. Such an interval would tend not to be noticeable to a human user but would be long enough to effectively collect, say, an entire line of output for output "in one piece". The use of a reasonable timeout would avoid the confusing behavior where a newline-less prompt doesn't appear until the prompted data is entered.
So, let me see if I've understood: the situation would be exactly as now, except that the buffer automatically gets flushed if it stays non-empty for more than a certain time interval. I don't think I like it: whatever time interval you choose will be too long. If you're doing communication with another program, for example, you really don't want to have the dialog stalled for 100ms doing nothing. Sure, you could insert a flush, but only a programmer knowledgeable about buffering will know to do that, or a programmer who has debugged the program in detail to find out why it is sitting idle most of the time. So, in the end the programmer still needs to know about buffering. I'd prefer it if the program didn't work at all when the flush is missing, because it tells you immediately that something is wrong. Cheers, Simon