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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | SEE ALSO | COLOPHON |
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setbuf(3) Library Functions Manual setbuf(3)
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering
operations
Standard C library (libc, -lc)
#include <stdio.h>
int setvbuf(size_t size;
FILE *restrict stream, char buf[restrict size],
int mode, size_t size);
void setbuf(FILE *restrict stream, char *restrict buf);
void setbuffer(size_t size;
FILE *restrict stream, char buf[restrict size],
size_t size);
void setlinebuf(FILE *stream);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
setbuffer(), setlinebuf():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_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. 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:
_IONBF unbuffered
_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 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
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 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.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ setbuf(), setbuffer(), setlinebuf(), │ Thread safety │ MT-Safe │
│ setvbuf() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
setbuf()
setvbuf()
C11, POSIX.1-2008.
setbuf()
setvbuf()
C89, POSIX.1-2001.
POSIX notes that the value of errno is unspecified after a call to
setbuf() and further notes that, since the value of errno is not
required to be unchanged after a successful call to setbuf(),
applications should instead use setvbuf() in order to detect
errors.
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:
#include <stdio.h>
int
main(void)
{
char buf[BUFSIZ];
setbuf(stdout, buf);
printf("Hello, world!\n");
return 0;
}
stdbuf(1), fclose(3), fflush(3), fopen(3), fread(3), malloc(3),
printf(3), puts(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 setbuf(3)
Pages that refer to this page: fclose(3), fcloseall(3), fflush(3), fpurge(3), open_memstream(3), procio(3), stdin(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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