+SSL_sendfile() writes B<size> bytes from offset B<offset> in the file
+descriptor B<fd> to the specified SSL connection B<s>. This function provides
+efficient zero-copy semantics. SSL_sendfile() is available only when
+Kernel TLS is enabled, which can be checked by calling BIO_get_ktls_send().
+It is provided here to allow users to maintain the same interface.
+The meaning of B<flags> is platform dependent.
+Currently, under Linux it is ignored.
+