libc/stdio/setbuf.3

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.\"     @(#)setbuf.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/stdio/setbuf.3,v 1.17 2007/01/09 00:28:07 imp Exp $
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.Dd January 2, 2021
.Dt SETBUF 3
.Os
.Sh NAME
.Nm setbuf ,
.Nm setbuffer ,
.Nm setlinebuf ,
.Nm setvbuf
.Nd stream buffering operations
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn setbuf "FILE * restrict stream" "char * restrict buf"
.Ft void
.Fn setbuffer "FILE *stream" "char *buf" "int size"
.Ft int
.Fn setlinebuf "FILE *stream"
.Ft int
.Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size"
.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
.Dv stdin ) .
The function
.Xr fflush 3
may be used to force the block out early.
(See
.Xr fclose 3 . )
.Pp
Normally all files are block buffered.
When the first
.Tn I/O
operation occurs on a file,
.Xr malloc 3
is called,
and an optimally-sized buffer is obtained.
If a stream refers to a terminal
(as
.Dv stdout
normally does) it is line buffered.
The standard error stream
.Dv stderr
is always unbuffered.
Note that these defaults may be altered using the
.Xr stdbuf 1
utility.
.Pp
The
.Fn setvbuf
function
may be used to alter the buffering behavior of a stream.
The
.Fa mode
argument must be one of the following three macros:
.Bl -tag -width _IOFBF -offset indent
.It Dv _IONBF
unbuffered
.It Dv _IOLBF
line buffered
.It Dv _IOFBF
fully buffered
.El
.Pp
The
.Fa size
argument may be given as zero
to obtain deferred optimal-size buffer allocation as usual.
If it is not zero,
then except for unbuffered files, the
.Fa buf
argument should point to a buffer at least
.Fa size
bytes long;
this buffer will be used instead of the current buffer.
If
.Fa buf
is not
.Dv NULL ,
it is the caller's responsibility to
.Xr free 3
this buffer after closing the stream.
(If the
.Fa size
argument
is not zero but
.Fa buf
is
.Dv NULL ,
a buffer of the given size will be allocated immediately,
and released on close.
This is an extension to ANSI C;
portable code should use a size of 0 with any
.Dv NULL
buffer.)
.Pp
The
.Fn setvbuf
function may be used at any time,
but may have peculiar side effects
(such as discarding input or flushing output)
if the stream is ``active''.
Portable applications should call it only once on any given stream,
and before any
.Tn I/O
is performed.
.Pp
The other three calls are, in effect, simply aliases for calls to
.Fn setvbuf .
Except for the lack of a return value, the
.Fn setbuf
function is exactly equivalent to the call
.Pp
.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
.Pp
The
.Fn setbuffer
function
is the same, except that the size of the buffer is up to the caller,
rather than being determined by the default
.Dv BUFSIZ .
The
.Fn setlinebuf
function
is exactly equivalent to the call:
.Pp
.Dl "setvbuf(stream, NULL, _IOLBF, 0);"
.Sh RETURN VALUES
The
.Fn setvbuf
function returns 0 on success, or
.Dv EOF
if the request cannot be honored
(note that the stream is still functional in this case).
.Pp
The
.Fn setlinebuf
function returns what the equivalent
.Fn setvbuf
would have returned.
.Sh SEE ALSO
.Xr stdbuf 1 ,
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr fread 3 ,
.Xr malloc 3 ,
.Xr printf 3 ,
.Xr puts 3
.Sh STANDARDS
The
.Fn setbuf
and
.Fn setvbuf
functions
conform to
.St -isoC .
.Sh HISTORY
The
.Fn setbuf
function first appeared in
.At v7 .
The
.Fn setbuffer
function first appeared in
.Bx 4.1c .
The
.Fn setlinebuf
function first appeared in
.Bx 4.2 .
The
.Fn setvbuf
function first appeared in
.Bx 4.4 .
.Sh BUGS
.Fn setbuf
usually uses a suboptimal buffer size and should be avoided.