.\"	$OpenBSD: fflush.3,v 1.17 2024/08/30 03:44:48 guenther Exp $
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.Dd $Mdocdate: August 30 2024 $
.Dt FFLUSH 3
.Os
.Sh NAME
.Nm fflush ,
.Nm fpurge
.Nd flush a stream
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn fflush "FILE *stream"
.Ft int
.Fn fpurge "FILE *stream"
.Sh DESCRIPTION
The function
.Fn fflush
forces a write of all buffered data for the given output or update
.Fa stream
via the stream's underlying write function.
If
.Fa stream
is a stream opened for reading with
.Xr fdopen 3 ,
.Xr fopen 3 ,
or
.Xr freopen 3
of a seekable file and it is not already at EOF then
.Fn fflush
sets the seek position of the file to the file position of the
stream and discards any text pushed back via
.Xr ungetc 3
or
.Xr ungetwc 3 .
The open status of the stream is unaffected.
.Pp
If the
.Fa stream
argument is
.Dv NULL ,
.Fn fflush
flushes
.Em all
open output streams.
.Pp
The function
.Fn fpurge
erases any input or output buffered in the given
.Fa stream .
For output streams this discards any unwritten output.
For input streams this discards any input read from the underlying object
but not yet obtained via
.Xr getc 3 ;
this includes any text pushed back via
.Xr ungetc 3 .
.Sh RETURN VALUES
Upon successful completion 0 is returned.
Otherwise,
.Dv EOF
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
.Fa stream
is not an open stream or, in the case of
.Fn fflush ,
not a stream open for writing.
.El
.Pp
The function
.Fn fflush
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr write 2 .
.Sh SEE ALSO
.Xr write 2 ,
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr setvbuf 3
.Sh STANDARDS
The
.Fn fflush
function conforms to
.St -ansiC .
.Sh HISTORY
A predecessor
.Fn flush
first appeared in
.At v1 .
The
.Fn fflush
function first appeared in
.At v4 .