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(libc.info)Buffering Concepts

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Buffering Concepts
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   There are three different kinds of buffering strategies:

   * Characters written to or read from an "unbuffered" stream are
     transmitted individually to or from the file as soon as possible.

   * Characters written to a "line buffered" stream are transmitted to
     the file in blocks when a newline character is encountered.

   * Characters written to or read from a "fully buffered" stream are
     transmitted to or from the file in blocks of arbitrary size.

   Newly opened streams are normally fully buffered, with one
exception: a stream connected to an interactive device such as a
terminal is initially line buffered.  Note: Controlling Buffering,
for information on how to select a different kind of buffering.
Usually the automatic selection gives you the most convenient kind of
buffering for the file or device you open.

   The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want.  Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with `fflush', as described in
Note: Flushing Buffers.