setbuf, setbuffer, setlinebuf, setvbuf - stream buffering
operations
SYNOPSIS
#include <stdio.h>
void setbuf(FILE *stream, char *buf);
void setbuffer(FILE *stream, char *buf, size_tsize);
void setlinebuf(FILE *stream);
int setvbuf(FILE *stream, char *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 new
line 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 parameter 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 equiva
lent to the call
setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUF
SIZ);
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 VALUE
The function setvbuf returns 0 on success. It can return
any value on failure, but returns nonzero when mode is
invalid or the request cannot be honoured. It may set
errno on failure. The other functions are void.
CONFORMING TO
The setbuf and setvbuf functions conform to ANSI
X3.159-1989 (``ANSI C'').
BUGS
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 both buf and the space it points
to still exist by the time stream is closed, which also
happens at program termination.
For example, the following is illegal:
#include <stdio.h>
int main()
{
char buf[BUFSIZ];
setbuf(stdin, buf);
printf("Hello, world!\n");
return 0;
}
SEE ALSO
fclose(3), fflush(3), fopen(3), fread(3), malloc(3),
printf(3), puts(3)
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