SETBUF(3) Linux Programmer's Manual SETBUF(3)NAME
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
void setbuf(FILE *stream, char *buf);
void setbuffer(FILE *stream, char *buf, size_t size);
void setlinebuf(FILE *stream);
int setvbuf(FILE *stream, char *buf, int mode, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
setbuffer(), setlinebuf(): _BSD_SOURCE
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
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 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 default.
The setvbuf() function may be used on any open stream to change its
buffer. The mode argument 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 cur‐
rent 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 be used only 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 buf‐
fer 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 function setvbuf() returns 0 on success. It returns nonzero on
failure (mode is invalid or the request cannot be honored). It may set
errno on failure.
The other functions do not return a value.
Multithreading (see pthreads(7))
The setbuf(), setbuffer(), setlinebuf(), and setvbuf() functions are
The setbuf() and setvbuf() functions conform to C89 and C99.
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 the space that buf points to still exists by
the time stream is closed, which also happens at program termination.
For example, the following is invalid:
SEE ALSOfclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)COLOPHON
This page is part of release 3.65 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2014-02-18 SETBUF(3)