SETBUF(3) Linux Programmer's Manual SETBUF(3)
NAME
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
SYNOPSIS
#include<stdio.h>
voidsetbuf(FILE*stream,char*buf);
voidsetbuffer(FILE*stream,char*buf,size_tsize);
voidsetlinebuf(FILE*stream);
intsetvbuf(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 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 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 cur-
rent 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 BUF-
SIZ. The setlinebuf function is exactly equivalent to the call:
setvbuf(stream, (char *)NULL, _IOLBF, 0);
RETURNVALUE
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.
CONFORMINGTO
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 termina-
tion.
For example, the following is illegal:
#include <stdio.h>
int main()
{
char buf[BUFSIZ];
setbuf(stdin, buf);
printf("Hello, world!\n");
return 0;
}
SEEALSO
fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
Linux 2001-06-09 SETBUF(3)
Take our 
