This callback gets called by libcurl just before the initialization of a TLS connection after having processed all other TLS related options to give a last chance to an application to modify the behavior of the TLS initialization. The ssl_ctx parameter passed to the callback in the second argument is actually a pointer to the SSL library's SSL_CTX for OpenSSL or wolfSSL, and a pointer to mbedtls_ssl_config for mbedTLS. If an error is returned from the callback no attempt to establish a connection is made and the operation will return the callback's error code. Set the userptr argument with the CURLOPT_SSL_CTX_DATA option.