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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_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 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 at any time 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 be used at any time, but can only
change the mode of a stream when it is not ‘‘active’’: that is, before
any I/O, or immediately after a call to fflush.
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);
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 may not be available under Linux. 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)