CURLOPT_SOCKOPTDATA(3) on macOS
The text 7fc04c8bad35, as macOS 10.14.6, macOS 10.14.5, macOS 10.14.4, macOS 10.14.3, macOS 10.14.2, macOS 10.14.1, macOS 10.14, macOS 10.13.6, macOS 10.13.5, macOS 10.13.4, macOS 10.13.3, macOS 10.13.2, macOS 10.13.1, macOS 10.13, macOS 10.12.6 shipped it.
custom pointer to pass to sockopt callback