SETBUF (3)
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 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:
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);
SEE ALSO
STANDARDS
The
setbuf
and
setvbuf
functions conform to ANSI C3.159-1989 (``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;
}
man3/setbuf.3
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