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The
three types of buffering available are unbuffered, block buffered, and
line buffered. When an output stream is unbuffered, information appears
on the destination file or terminal as soon as written; when it is block
buffered many characters are saved up and written as a block; when it is
line buffered characters are saved up until a newline is output or input
is read from any stream attached to a terminal device (typically stdin).
The function may be used to force the block out early. (See Normally
all files are block buffered. When the first operation occurs on a file,
is called, and an optimally-sized buffer is obtained. If a stream refers
to a terminal (as normally does) it is line buffered. The standard error
stream is always unbuffered. The function may be used to alter the buffering
behavior of a stream. The parameter must be one of the following three
macros: unbuffered line buffered fully buffered The parameter may
be given as zero to obtain deferred optimal-size buffer allocation as usual.
If it is not zero, then except for unbuffered files, the argument should
point to a buffer at least bytes long; this buffer will be used instead
of the current buffer. (If the argument is not zero but is a buffer of
the given size will be allocated immediately, and released on close. This
is an extension to ANSI C; portable code should use a size of 0 with any
buffer.) The function may be used at any time, but may have peculiar
side effects (such as discarding input or flushing output) if the stream
is ‘‘active’’. Portable applications should call it only once on any given stream,
and before any is performed. The other three calls are, in effect, simply
aliases for calls to Except for the lack of a return value, the function
is exactly equivalent to the call The function is the same, except
that the size of the buffer is up to the caller, rather than being determined
by the default The function is exactly equivalent to the call:
The function returns 0 on success, or if the request cannot be
honored (note that the stream is still functional in this case). The function
returns what the equivalent would have returned.
The
and functions conform to
The and functions are not portable to
versions of before On and systems, always uses a suboptimal buffer
size and should be avoided.
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