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.\"     @(#)fflush.3	8.1 (Berkeley) 6/4/93
.\"
.Dd May 1, 2020
.Dt FFLUSH 3
.Os
.Sh NAME
.Nm fflush ,
.Nm fflush_unlocked ,
.Nm fpurge
.Nd flush a stream
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn fflush "FILE *stream"
.Ft int
.Fn fflush_unlocked "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.
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
.Fn fflush_unlocked
function is equivalent to
.Fn fflush ,
except that the caller is responsible for locking the stream with
.Xr flockfile 3
before calling it.
This function may be used to avoid the overhead of locking the stream and to
prevent races when multiple threads are operating on the same stream.
.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
The
.Fa stream
argument
is not an open stream.
.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 ,
except that in case of
.Fa stream
being a read-only descriptor,
.Fn fflush
returns 0.
.Sh SEE ALSO
.Xr write 2 ,
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr setbuf 3
.Sh STANDARDS
The
.Fn fflush
function
conforms to
.St -isoC .
.Sh HISTORY
The
.Fn fflush
function first appeared in
.At v4 .
The
.Fn fpurge
function first appeared in
.Bx 4.4 .