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,
int 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 (typicallystdin). 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 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:
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);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
stdbuf(1), fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
STANDARDS
The setbuf() and
setvbuf() functions conform to
ISO/IEC 9899:1990
(“ISO C90”).
BUGS
setbuf() usually uses a suboptimal buffer
size and should be avoided.