NAME
setbuf, setbuffer,
setlinebuf, setvbuf —
stream buffering operations
LIBRARY
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:
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);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 ANSI
X3.159-1989 (“ANSI C89”).
HISTORY
The setbuf() function first appeared in
Version 7 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.