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Why must flush all line-buffered output streams?

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# 1  
Old 03-27-2011
Question Why must flush all line-buffered output streams?
Hi,
Mentioned in Stevens & Rago "Advanced Programming in the UNIX"
Quote:
whenever input is requested through the standard I/O library from either (a) an unbuffered stream or (b) a line-buffered stream (that requires data to be requested from the kernel), all line-buffered output streams are flushed. The reason for the qualifier on (b) is that the requested data may already be in the buffer, which doesn't require data to be read from the kernel. Obviously, any input from an unbuffered stream, item (a), requires data to be obtained from the kernel.
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  
Old 03-27-2011
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  
Old 03-27-2011
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
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