B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
with the same arguments.
+When calling SSL_write() with num=0 bytes to be sent the behaviour is
+undefined.
+
=head1 RETURN VALUES
The following return values can occur:
=item 0
-The write operation was not successful. Call SSL_get_error() with the return
-value B<ret> to find out, whether an error occurred.
+The write operation was not successful. Probably the underlying connection
+was closed. Call SSL_get_error() with the return value B<ret> to find out,
+whether an error occurred or the connection was shut down cleanly
+(SSL_ERROR_ZERO_RETURN).
+
+SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
+only be detected, whether the underlying connection was closed. It cannot
+be checked, why the closure happened.
=item E<lt>0