CURLINFO_LASTSOCKET man page
CURLINFO_LASTSOCKET — get the last socket used
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_LASTSOCKET, long *socket);
Deprecated since 7.45.0. Use CURLINFO_ACTIVESOCKET(3) instead.
Pass a pointer to a long to receive the last socket used by this curl session. If the socket is no longer valid, -1 is returned. When you finish working with the socket, you must call curl_easy_cleanup() as usual 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).
NOTE: this API is deprecated since it is not working on win64 where the SOCKET type is 64 bits large while its 'long' is 32 bits. Use the CURLINFO_ACTIVESOCKET(3) instead, if possible.
Added in 7.15.2
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_getinfo(3), curl_easy_setopt(3), CURLINFO_ACTIVESOCKET(3),