stream buffering operations
*stream, char *buf,
*stream, char *buf,
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
\n’) is output or input is read from
any stream attached to a terminal device (typically
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.
function may be used to alter the buffering behavior of a stream. The
mode parameter must be one of the following three
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
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
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
Except for the lack of a return value, the
function is exactly equivalent to the call
setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
function is the same, except that the size of the buffer is up to the
caller, rather than being determined by the default
function is exactly equivalent to the call:
setvbuf(stream, NULL, _IOLBF, 0);
setvbuf() function returns 0 on
EOF if the request cannot be honored
(note that the stream is still functional in this case).
setlinebuf() function returns what the
setvbuf() would have returned.
fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
setvbuf() functions conform to ANSI
X3.159-1989 (“ANSI C89”).
setbuf() first appeared in
Version 7 AT&T UNIX.
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.