gno/usr.man/man3/setbuf.3

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.\"     @(#)setbuf.3	8.1 (Berkeley) 6/4/93
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.TH SETBUF 3 "15 September 1997" GNO "Library Routines"
.SH NAME
.BR setbuf ,
.BR setbuffer ,
.BR setlinebuf ,
.BR setvbuf
\- stream buffering operations
.SH SYNOPSIS
#include <stdio.h>
.sp 1
void \fBsetbuf\fR (FILE *\fIstream\fR, char *\fIbuf\fR);
.br
void \fBsetbuffer\fR (FILE *\fIstream\fR, char *\fIbuf\fR, size_t \fIsize\fR);
.sp 1
int \fBsetlinebuf\fR (FILE *\fIstream\fR);
.sp 1
int \fBsetvbuf\fR (FILE *\fIstream\fR, char *\fIbuf\fR, int \fImode\fR, size_t \fIsize\fR);
.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
.BR fflush (3)
may be used to force the block out early.
(See 
.BR fclose (3).)
.LP
Normally all files are block buffered.
When the first I/O operation occurs on a file,
.BR malloc (3)
is called,
and an optimally-sized buffer is obtained.
If a stream refers to a terminal
(as
.IR stdout 
normally does) it is line buffered.
The standard error stream
.IR stderr 
is always unbuffered.
.LP
The
.BR setvbuf 
function
may be used to alter the buffering behavior of a stream.
The
.I mode
parameter must be one of the following three macros:
.RS
.nf

\fB_IONBF\fR	unbuffered
\fB_IOLBF\fR	line buffered
\fB_IOFBF\fR	fully buffered
.fi
.RE
.LP
The
.I size
parameter 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 
.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
.I size
argument
is not zero but
.I buf
is
.BR 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
.BR NULL
buffer.)
.LP
The
.BR 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 I/O is performed.
.LP
The other three calls are, in effect, simply aliases for calls to
.BR setvbuf .
Except for the lack of a return value, the
.BR setbuf 
function is exactly equivalent to the call
.nf

	setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

.fi
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:
.nf

	setvbuf(stream, (char *)NULL, _IOLBF, 0);

.fi
.SH RETURN VALUES
The
.BR setvbuf 
function returns 0 on success, or
.BR EOF
if the request cannot be honored
(note that the stream is still functional in this case).
.LP
The
.BR setlinebuf 
function returns what the equivalent
.BR setvbuf 
would have returned.
.SH SEE ALSO
.BR fopen (3),
.BR fclose (3),
.BR fread (3),
.BR malloc (3),
.BR puts (3),
.BR printf (3)
.SH STANDARDS
The
.BR setbuf 
and
.BR setvbuf 
functions
conform to ANSI/C.
.SH BUGS
The
.BR setbuffer 
and
.BR setlinebuf 
functions are not portable to versions of BSD before 4.2BSD.
On 4.2BSD and 4.3BSD systems,
.BR setbuf 
always uses a suboptimal buffer size and should be avoided.