'\" t
.\" Copyright (c) 1980, 1991 Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the American National Standards Committee X3, on Information
.\" Processing Systems.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     @(#)setbuf.3	6.10 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 14:55:24 1993, faith@cs.unc.edu
.\" Added section to BUGS, Sun Mar 12 22:28:33 MET 1995,
.\"                   Thomas.Koenig@ciw.uni-karlsruhe.de
.\" Correction,  Sun, 11 Apr 1999 15:55:18,
.\"     Martin Vicente <martin@netadmin.dgac.fr>
.\" Correction,  2000-03-03, Andreas Jaeger <aj@suse.de>
.\" Added return value for setvbuf, aeb,
.\"
.TH setbuf 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
setbuf, setbuffer, setlinebuf, setvbuf \- stream buffering operations
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "int setvbuf(FILE *restrict " stream ", char " buf "[restrict ." size ],
.BI "            int " mode ", size_t " size );
.PP
.BI "void setbuf(FILE *restrict " stream ", char *restrict " buf );
.BI "void setbuffer(FILE *restrict " stream ", char " buf "[restrict ." size ],
.BI "            size_t "  size );
.BI "void setlinebuf(FILE *" stream );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR setbuffer (),
.BR setlinebuf ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE
.fi
.SH DESCRIPTION
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 \fIstdin\fP).
The function
.BR fflush (3)
may be used to force the block out early.
(See
.BR fclose (3).)
.PP
Normally all files are block buffered.
If a stream refers to a terminal (as
.I stdout
normally does), it is line buffered.
The standard error stream
.I stderr
is always unbuffered by default.
.PP
The
.BR setvbuf ()
function may be used on any open stream to change its buffer.
The
.I mode
argument must be one of the following three macros:
.RS
.TP
.B _IONBF
unbuffered
.TP
.B _IOLBF
line buffered
.TP
.B _IOFBF
fully buffered
.RE
.PP
Except for unbuffered files, the
.I buf
argument should point to a buffer at least
.I size
bytes long; this buffer will be used instead of the current buffer.
If the argument
.I buf
is NULL,
only the mode is affected; a new buffer will be allocated on the next read
or write operation.
The
.BR setvbuf ()
function may be used only after opening a stream and before any other
operations have been performed on it.
.PP
The other three calls are, in effect, simply aliases for calls to
.BR setvbuf ().
The
.BR setbuf ()
function is exactly equivalent to the call
.PP
.in +4n
setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
.in
.PP
The
.BR setbuffer ()
function is the same, except that the size of the buffer is up to the
caller, rather than being determined by the default
.BR BUFSIZ .
The
.BR setlinebuf ()
function is exactly equivalent to the call:
.PP
.in +4n
setvbuf(stream, NULL, _IOLBF, 0);
.in
.SH RETURN VALUE
The function
.BR setvbuf ()
returns 0 on success.
It returns nonzero on failure
.RI ( mode
is invalid or the request cannot be honored).
It may set
.I errno
on failure.
.PP
The other functions do not return a value.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR setbuf (),
.BR setbuffer (),
.BR setlinebuf (),
.BR setvbuf ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
.TP
.BR setbuf ()
.TQ
.BR setvbuf ()
C11, POSIX.1-2008.
.SH HISTORY
.TP
.BR setbuf ()
.TQ
.BR setvbuf ()
C89, POSIX.1-2001.
.SH CAVEATS
POSIX notes
.\" https://www.austingroupbugs.net/view.php?id=397#c799
.\" 0000397: setbuf and errno
that the value of
.I errno
is unspecified after a call to
.BR setbuf ()
and further notes that, since the value of
.I errno
is not required to be unchanged after a successful call to
.BR setbuf (),
applications should instead use
.BR setvbuf ()
in order to detect errors.
.SH BUGS
.\" The
.\" .BR setbuffer ()
.\" and
.\" .BR 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,
.\" .BR setbuf ()
.\" always uses a suboptimal buffer size and should be avoided.
.\".PP
You must make sure that the space that
.I buf
points to still exists by the time
.I stream
is closed, which also happens at program termination.
For example, the following is invalid:
.PP
.\" [[invalid]] SRC BEGIN (setbuf.c)
.EX
#include <stdio.h>

int
main(void)
{
    char buf[BUFSIZ];

    setbuf(stdout, buf);
    printf("Hello, world!\en");
    return 0;
}
.EE
.\" SRC END
.SH SEE ALSO
.BR stdbuf (1),
.BR fclose (3),
.BR fflush (3),
.BR fopen (3),
.BR fread (3),
.BR malloc (3),
.BR printf (3),
.BR puts (3)