SETBUF(3) | Library Functions Manual | SETBUF(3) |
setbuf
, setbuffer
,
setlinebuf
, setvbuf
—
#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);
The default buffer settings can be overwritten per descriptor
(STDBUF
n, 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);
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.
setbuf
() and setvbuf
()
functions conform to ANSI X3.159-1989
(“ANSI C89”).
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.
setbuf
() function usually uses a suboptimal buffer
size and should be avoided.
June 4, 1993 | NetBSD 9.2 |