--- Libc/Libc-825.25/stdio/FreeBSD/setbuf.3
+++ Libc/Libc-1669.40.2/stdio/FreeBSD/setbuf.3
@@ -79,6 +79,28 @@
 or input is read from any stream attached to a terminal device
 (typically
 .Dv stdin ) .
+.Pp
+The default buffer settings can be overwritten for stdin
+.Dv ( STDBUF0 , _STDBUF_I ),
+stdout
+.Dv ( STDBUF1 , _STDBUF_O ),
+stderr
+.Dv ( STDBUF2 , _STDBUF_E ),
+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 16MB (suffixes 'k' and 'M' are accepted).
+Valid letters are:
+.Bl -tag -width X -offset indent
+.It Dv Li U
+Unbuffered.
+.It Dv Li L
+Line-buffered.
+.It Dv Li B , Dv Li F
+Fully-buffered.
+.El
+.Pp
 The function
 .Xr fflush 3
 may be used to force the block out early.