Manual Pages  — SETBUF

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 always unbuffered. Note that these defaults may be altered using the stdbuf(1) utility.

The setvbuf() function may be used to alter the buffering behavior of a stream. The mode argument must be one of the following three macros:

_IONBF
	unbuffered
_IOLBF
	line buffered
_IOFBF
	fully buffered

The size argument 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 buf is not NULL, it is the caller's responsibility to free(3) this buffer after closing the stream. (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);