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.Dd $Mdocdate: July 25 2022 $
.Dt SETVBUF 3
.Os
.Sh NAME
.Nm setvbuf
.Nd stream buffering operations
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn setvbuf "FILE *stream" "char *buf" "int mode" "size_t size"
.Sh DESCRIPTION
The three types of stream 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 line buffered, characters are saved up until a newline
.Pq Ql \en
is output or input is read from any stream attached to a terminal device
(typically
.Em stdin ) .
.Pp
The
.Xr fflush 3
function may be used to force the block out early.
.Pp
Normally, all files are block buffered.
When the first 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
.Em stdout
normally does), it is line buffered.
.Pp
The standard error stream
.Em stderr
is initially unbuffered.
.Pp
The
.Fn setvbuf
function may be used to alter the buffering behavior of a stream.
The
.Fa mode
parameter must be one of the following three macros:
.Pp
.Bl -tag -width _IOFBF -offset indent -compact
.It Dv _IONBF
unbuffered
.It Dv _IOLBF
line buffered
.It Dv _IOFBF
fully buffered
.El
.Pp
The
.Fa 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
.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
.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
.Dq active .
Portable applications should call it only once on any given stream,
and before any I/O is performed.
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
If
.Fa mode
is invalid or if the request cannot be honored, a non-zero value is returned,
possibly setting
.Va errno
to indicate the error.
The stream is not modified in the error case.
.Sh ERRORS
The
.Fn setvbuf
function will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa stream
specified is not associated with a valid file descriptor.
.El
.Sh SEE ALSO
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr fread 3 ,
.Xr malloc 3 ,
.Xr printf 3 ,
.Xr puts 3 ,
.Xr setbuf 3
.Sh STANDARDS
The
.Fn setvbuf
function conforms to
.St -isoC-99 .
.Sh HISTORY
The
.Fn setvbuf
function first appeared in
.At V.2
and was reimplemented for
.Bx 4.3 Net/2 .