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.\"

.\"     @(#)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  2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

setbuf, setbuffer, setlinebuf, setvbuf \- stream buffering operations

.SH SYNOPSIS

.nf

.B #include <stdio.h>

.PP

.BI "void setbuf(FILE *" stream ", char *" buf );

.PP

.BI "void setbuffer(FILE *" stream ", char *" buf ", size_t "  size );

.PP

.BI "void setlinebuf(FILE *" stream );

.PP

.BI "int setvbuf(FILE *" stream ", char *" buf ", int " mode \

", size_t " size );

.fi

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR setbuffer (),

.BR setlinebuf ():

    Since glibc 2.19:

        _DEFAULT_SOURCE

    Glibc 2.19 and earlier:

        _BSD_SOURCE

.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).

.TS

allbox;

lbw23 lb lb

l l l.

Interface	Attribute	Value

T{

.BR setbuf (),

.BR setbuffer (),

.br

.BR setlinebuf (),

.BR setvbuf ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

The

.BR setbuf ()

and

.BR setvbuf ()

functions conform to C89 and C99.

.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

.EX

#include <stdio.h>

int

main(void)

{

    char buf[BUFSIZ];

    setbuf(stdin, buf);

    printf("Hello, world!\\n");

    return 0;

}

.EE

.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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.