NAME
setbuf,
setbuffer,
setlinebuf,
setvbuf —
stream buffering operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdio.h>
void
setbuf(
FILE *
restrict stream,
char *
restrict buf);
void
setbuffer(
FILE
*stream,
char *buf,
size_t size);
int
setlinebuf(
FILE
*stream);
int
setvbuf(
FILE *
restrict stream,
char *
restrict 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 default buffer settings can be overwritten per descriptor
(
STDBUFn
) where
n
is the
numeric value of the file descriptor represented by the stream, or for all
descriptors (
STDBUF
). The environment variable value
is a letter followed by an optional numeric value indicating the size of the
buffer. Valid sizes range from 0B to 1MB. Valid letters are:
-
-
U
- Unbuffered.
-
-
L
- Line-buffered.
-
-
F
- Fully-buffered.
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
an optimally-sized buffer is obtained. If a stream refers to a terminal (as
stdout normally does) it is line buffered. The standard
error stream
stderr is initially unbuffered.
The
setvbuf() function may be used to alter the buffering
behavior of a stream. The
mode parameter must be one of
the following three macros:
-
-
_IONBF
- unbuffered
-
-
_IOLBF
- line buffered
-
-
_IOFBF
- fully buffered
The
size parameter may be given as zero to obtain deferred
optimal-size buffer allocation as usual. If it is not zero, then 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
size
argument is not zero but
buf is
NULL
, a buffer of the given size will be allocated
immediately, and released on close. This is an extension to ANSI C; portable
code should use a size of 0 with any
NULL
buffer.)
The
setvbuf() function may be used at any time, but may have
peculiar side effects (such as discarding input or flushing output) if the
stream is ``active''. Portable applications should call it only once on any
given stream, and before any I/O is performed.
The other three calls are, in effect, simply aliases for calls to
setvbuf(). Except for the lack of a return value, 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);
RETURN VALUES
The
setvbuf() function returns 0 on success, or
EOF
if the request cannot be honored (note that the
stream is still functional in this case).
The
setlinebuf() function returns what the equivalent
setvbuf() would have returned.
SEE ALSO
fclose(3),
fopen(3),
fread(3),
malloc(3),
printf(3),
puts(3)
STANDARDS
The
setbuf() and
setvbuf() functions conform
to
ANSI X3.159-1989 (“ANSI C89”).
BUGS
The
setbuffer() and
setlinebuf() functions
are not portable to versions of
BSD before
4.2BSD. On
4.2BSD and
4.3BSD systems,
setbuf() always uses
a suboptimal buffer size and should be avoided.