SETBUF(3) BSD Programmer's Manual SETBUF(3)
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
#include <stdio.h>
void
setbuf(FILE *stream, char *buf);
void
setbuffer(FILE *stream, char *buf, size_t size);
int
setlinebuf(FILE *stream);
int
setvbuf(FILE *stream, char *buf, int mode, size_t size);
The three types of stream buffering available are unbuffered, block buf-
fered, and line buffered. When an output stream is unbuffered, informa-
tion 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 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 oc-
curs on a file, malloc(3) is called, and an optimally sized buffer is ob-
tained. 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 exten-
sion 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);
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.
fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
The setbuf() and setvbuf() functions conform to ANSI X3.159-1989 ("ANSI
C89").
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.
MirBSD #10-current June 4, 1993 1