setbuf(3) BSD Library Functions Manual setbuf(3)
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, int size);
int
setlinebuf(FILE *stream);
int
setvbuf(FILE *restrict stream, char *restrict buf, int type,
size_t size);
DESCRIPTION
Three types of buffering are available: 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 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 always unbuffered.
The setvbuf() function may be used to alter the buffering behavior of a
stream. The type argument must be one of the following three macros:
_IONBF unbuffered
_IOLBF line buffered
_IOFBF fully buffered
The size argument 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
buf is not NULL, it is the caller's responsibility to free(3) this buffer
after closing the stream. (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 ISO/IEC 9899:1990
(``ISO C90'').
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.
BSD June 4, 1993 BSD
Mac OS X 10.8 - Generated Thu Aug 30 11:42:24 CDT 2012