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
fflush(3) may be used to force the block out early. (See
fclose(3).)
Normally all files are block buffered. When the first I/O operation occurs on a file,
malloc(3) is called, and an optimally-sized buffer is obtained. If a stream refers to a terminal (as
stdout normally does) it is line buffered. The standard error stream
stderr is initially unbuffered.
The
setvbuf() function may be used to alter the buffering behavior of a stream. The
mode parameter must be one of the following three macros:
The
size 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
buf argument should point to a buffer at least
size bytes long; this buffer will be used instead of the current buffer. (If the
size argument is not zero but
buf is
NULL, 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
NULL buffer.)
The
setvbuf() 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 I/O is performed.
The other three calls are, in effect, simply aliases for calls to
setvbuf(). Except for the lack of a return value, the
setbuf() function is exactly equivalent to the call
setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
The
setbuffer() function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default
BUFSIZ. The
setlinebuf() function is exactly equivalent to the call:
setvbuf(stream, (char *)NULL, _IOLBF, 0);