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SETBUF(3) Linux Programmer’s Manual SETBUF(3)
NAME
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
SYNOPSIS
#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);
DESCRIPTION
The three types of buffering available are unbuffered, block buffered, and line
buffered. When an output stream is unbuffered, information appears on the destina-
tion 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 termi-
nal 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 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 func-
tion 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 sys-
tems, 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)
Linux 2001-06-09 SETBUF(3)
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