or input is read from any stream attached to a terminal device
(typically
.Dv stdin ) .
+.Pp
+The default buffer settings can be overwritten for stdout
+.Dv ( STDBUF1 )
+or for all descriptors
+.Dv ( STDBUF ) .
+The environment variable value is a letter followed by an optional numeric
+value indicating the size of the buffer.
+Valid sizes range from 0B to 64KB.
+Valid letters are:
+.Bl -tag -width X -indent
+.It Dv Li U
+Unbuffered.
+.It Dv Li L
+Line-buffered.
+.It Dv Li F
+Fully-buffered.
+.El
+.Pp
The function
.Xr fflush 3
may be used to force the block out early.