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(libc.info)Stream Buffering

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Stream Buffering
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   Characters that are written to a stream are normally accumulated and
transmitted asynchronously to the file in a block, instead of appearing
as soon as they are output by the application program.  Similarly,
streams often retrieve input from the host environment in blocks rather
than on a character-by-character basis.  This is called "buffering".

   If you are writing programs that do interactive input and output
using streams, you need to understand how buffering works when you
design the user interface to your program.  Otherwise, you might find
that output (such as progress or prompt messages) doesn't appear when
you intended it to, or displays some other unexpected behavior.

   This section deals only with controlling when characters are
transmitted between the stream and the file or device, and _not_ with
how things like echoing, flow control, and the like are handled on
specific classes of devices.  For information on common control
operations on terminal devices, see Note: Low-Level Terminal
Interface.

   You can bypass the stream buffering facilities altogether by using
the low-level input and output functions that operate on file
descriptors instead.  Note: Low-Level I/O.

Buffering Concepts

Terminology is defined here.

Flushing Buffers

How to ensure that output buffers are flushed.

Controlling Buffering

How to specify what kind of buffering to use.