SETBUF(3)

NAME
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations

LIBRARY
libc

SYNOPSIS
#include <stdio.h>
void
setbuf(FILE * restrict stream, char * restrict buf);
void
setbuffer(FILE *stream, char *buf, size_t size);
int
setlinebuf(FILE *stream);
int
setvbuf(FILE * restrict stream, char * restrict buf, int mode, size_t size);

DESCRIPTION
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 default buffer settings can be overwritten per descriptor (STDBUFn, where n is the numeric value of the file descriptor represented by the stream), or for all descriptors (STDBUF). The environment variable value is a letter followed by an optional numeric value indicating the size of the buffer. Valid sizes range from 0B to 1MB. Valid letters are:

U   unbuffered

L   line buffered

F   fully buffered

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:

_IONBF
    unbuffered

_IOLBF
    line buffered

_IOFBF
    fully buffered

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);

RETURN VALUES
The setvbuf() function returns 0 on success, or EOF if the request cannot be honored (note that the stream is still functional in this case).

The setlinebuf() function returns what the equivalent setvbuf() would have returned.

SEE ALSO
fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)

STANDARDS
The setbuf() and setvbuf() functions conform to ansiC.

HISTORY
The setbuf() function first appeared in AT&T UNIX. The setbuffer() function first appeared in 4.1cBSD. The setlinebuf() function first appeared in 4.2BSD. The setvbuf() function first appeared in 4.4BSD.

BUGS
The setbuf() function usually uses a suboptimal buffer size and should be avoided.