Why must flush all line-buffered output streams?

Applications Programming
1

Hi,
Mentioned in Stevens & Rago "Advanced Programming in the UNIX"

I don't understand why must flush all line-buffered output streams when (a)an unbuffered or (b)a line-buffered stream require data from kernel?

2

What if you're using both ends of a pipe? If you write to one end and it ends up in a buffer, then you try to read it from the other end, it'll hang until something flushes it.

3

The behavior mentioned in that Stevens quote exists in many c stdio implementations to ensure that prompts without a newline have been emitted before input is read. I don't believe that's standardized even though it's probably common behavior. It's best to use an explicit fflush as necessary.

Regards,
Alister