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SETBUF(3) NetBSD Library Functions Manual SETBUF(3)
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setbuf, setbuffer, setlinebuf, setvbuf -- stream buffering operations
Standard C Library (libc, -lc)
setbuf(FILE * restrict stream, char * restrict buf);
setbuffer(FILE *stream, char *buf, size_t size);
setvbuf(FILE * restrict stream, char * restrict buf, int mode,
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 out-
put or input is read from any stream attached to a terminal device (typi-
cally 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 initially
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:
_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
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);
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
fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
The setbuf() and setvbuf() functions conform to ANSI X3.159-1989
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.
NetBSD 5.0 June 4, 1993 NetBSD 5.0