usr/src/lib/libc/stdio/setbuf.3

.\" 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.
.\"
.\" %sccs.include.redist.man%
.\"
.\"     @(#)setbuf.3    6.10 (Berkeley) %G%
.\"
.Dd
.Dt SETBUF 3
.Os BSD 4
.Sh NAME
.Nm setbuf ,
.Nm setbuffer ,
.Nm setlinebuf ,
.Nm setvbuf
.Nd stream buffering operations
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft int
.Fn setbuf "FILE *stream" "char *buf"
.Ft int
.Fn setbuffer "FILE *stream" "char *buf" "size_t size"
.Ft int
.Fn setlinebuf "FILE *stream"
.Ft int
.Fn setvbuf "FILE *stream" "char *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 stdin).
The function
.Xr fflush 3
may be used to force the block out early.
(See
.Xr fclose 3 . )
Normally all files are block buffered.
When the first
.Tn I/O
operation occurs on a file,
.Xr malloc 3
is called,
and a buffer is obtained.
If a stream refers to a terminal
(as
.Em stdout
normally does) it is line buffered.
The standard error stream
.Em stderr
is always unbuffered.
.Pp
The
.Fn setvbuf
function
may be used at any time on any open stream
to change its buffer.
The
.Fa mode
parameter 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
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 the argument
.Fa buf
is NULL,
only the mode is affected;
a new buffer will be allocated on the next read or write operation.
The
.Fn setvbuf
function
may be used at any time,
but can only change the mode of a stream
when it is not ``active'':
that is, before any
.Tn I/O ,
or immediately after a call to
.Xr fflush .
.Pp
The other three calls are, in effect, simply aliases
for calls to
.Fn setvbuf .
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, (char *)NULL, _IOLBF, 0);
.Sh SEE ALSO
.Xr fopen 3 ,
.Xr fclose 3 ,
.Xr fread 3 ,
.Xr malloc 3 ,
.Xr puts 3 ,
.Xr printf 3
.Sh STANDARDS
The
.Fn setbuf
and
.Fn setvbuf
functions
conform to
.St -ansiC .
.Sh BUGS
The
.Fn setbuffer
and
.Fn setlinebuf
functions are not portable to versions of
.Bx
before
.Bx 4.2 .
On
.Bx 4.2
and
.Bx 4.3
systems,
.Fn setbuf
always uses a suboptimal buffer size and should be avoided.