+=head1 RETURN VALUES
+
+BIO_get_callback_ex() and BIO_get_callback() return the callback function
+previously set by a call to BIO_set_callback_ex() and BIO_set_callback()
+respectively.
+
+BIO_get_callback_arg() returns a B<char> pointer to the value previously set
+via a call to BIO_set_callback_arg().
+
+BIO_debug_callback() returns 1 or B<ret> if it's called after specific BIO
+operations.
+