setbuf, setbuffer, setlinebuf, setvbuf — stream buffering operations
#include <stdio.h>
void
setbuf( |
FILE *stream, |
char *buf) ; |
void
setbuffer( |
FILE *stream, |
char *buf, | |
size_t size) ; |
void
setlinebuf( |
FILE *stream) ; |
int
setvbuf( |
FILE *stream, |
char *buf, | |
int mode, | |
size_t size) ; |
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Note | ||
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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 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 a
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 by default.
The setvbuf
() function may
be used on any open stream to change its buffer. The
mode
argument must be
one of the following three macros:
_IONBF
unbuffered
_IOLBF
line buffered
_IOFBF
fully buffered
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 argument buf
is NULL, only the mode is
affected; a new buffer will be allocated on the next read or
write operation. The setvbuf
()
function may only be used after opening a stream and before
any other operations have been performed on it.
The other three calls are, in effect, simply aliases for
calls to setvbuf
(). 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);
The function setvbuf
()
returns 0 on success. It returns nonzero on failure
(mode
is invalid or
the request cannot be honored). It may set errno
on failure.
The other functions do not return a value.
The setbuffer
() and
setlinebuf
() functions are not
portable to versions of BSD before 4.2BSD, and are available
under Linux since libc 4.5.21. On 4.2BSD and 4.3BSD systems,
setbuf
() always uses a
suboptimal buffer size and should be avoided.
You must make sure that the space that buf
points to still exists by
the time stream
is
closed, which also happens at program termination. For
example, the following is invalid:
#include <stdio.h> int main(void) { char buf[BUFSIZ]; setbuf(stdin, buf); printf("Hello, world!\n"); return 0; }