+X/Open does not define any error conditions.
+This implementation returns an error
+.bP
+if the window pointer is null or
+.bP
+if it is not possible to add a complete character in the window.
+.PP
+The latter may be due to different causes:
+.bP
+If \fBscrollok\fR is not enabled,
+writing a character at the lower right margin succeeds.
+However, an error is returned because
+it is not possible to wrap to a new line
+.bP
+If an error is detected when converting a multibyte character to a sequence
+of bytes,
+or if it is not possible to add all of the resulting bytes in the window,
+an error is returned.
+.PP