SETBUF(3) Linux Programmer's Manual SETBUF(3)
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
void setbuf(FILE *stream, char *buf);
void setbuffer(FILE *stream, char *buf, size_tsize);
void setlinebuf(FILE *stream);
int setvbuf(FILE *stream, char *buf, int mode , size_t size);
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 termi-
nal 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 func-
tion 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 a 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 by
The setvbuf function may be used on any open stream to change its buffer. The mode param-
eter must be one of the following three macros:
_IOLBF line buffered
_IOFBF fully buffered
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 argument buf is
NULL, only the mode is affected; a new buffer will be allocated on the next read or write
operation. The setvbuf function may only be used after opening a stream and before any
other operations have been performed on it.
The other three calls are, in effect, simply aliases for calls to setvbuf. 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 call-
er, rather than being determined by the default BUFSIZ. The setlinebuf function is
exactly equivalent to the call:
setvbuf(stream, (char *)NULL, _IOLBF, 0);
The function setvbuf returns 0 on success. It can return any value on failure, but
returns nonzero when mode is invalid or the request cannot be honoured. It may set errno
on failure. The other functions are void.
The setbuf and setvbuf functions conform to ANSI X3.159-1989 (``ANSI C'').
The setbuffer and setlinebuf functions are not portable to versions of BSD before 4.2BSD,
and are available under Linux since libc 4.5.21. On 4.2BSD and 4.3BSD systems, setbuf
always uses a suboptimal buffer size and should be avoided.
You must make sure that both buf and the space it points to still exist by the time stream
is closed, which also happens at program termination.
For example, the following is illegal:
fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
Linux 2001-06-09 SETBUF(3)