CURLINFO_ACTIVESOCKET man page
CURLINFO_ACTIVESOCKET — get the active socket
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_ACTIVESOCKET,
Pass a pointer to a curl_socket_t to receive the active socket used by this curl session. If the socket is no longer valid, CURL_SOCKET_BAD is returned. When you finish working with the socket, you must call curl_easy_cleanup(3) as usual on the easy handle and let libcurl close the socket and cleanup other resources associated with the handle. This is typically used in combination with CURLOPT_CONNECT_ONLY(3).
This option was added as a replacement for CURLINFO_LASTSOCKET(3) since that one isn't working on all platforms.
Added in 7.45.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_getinfo(3), curl_easy_setopt(3), CURLINFO_LASTSOCKET(3),
curl_easy_getinfo(3), curl_easy_recv(3), curl_easy_send(3), CURLINFO_LASTSOCKET(3), CURLOPT_CONNECT_ONLY(3).