The three types of stream 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 line buffered, characters are saved up until a newline
("\en")
is output or input is read from any stream attached to a terminal device
(typically
stdin).
The
fflush(3)
function may be used to force the block out early.
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, 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.
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.