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. See 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 Flushing Buffers.
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