int closesocket_callback(void *clientp, curl_socket_t item);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CLOSESOCKETFUNCTION, closesocket_callback);
DESCRIPTIONPass a pointer to your callback function, which should match the prototype shown above.
This callback function gets called by libcurl instead of the close(3) or closesocket(3) call when sockets are closed (not for any other file descriptors). This is pretty much the reverse to the CURLOPT_OPENSOCKETFUNCTION(3) option. Return 0 to signal success and 1 if there was an error.
The clientp pointer is set with CURLOPT_CLOSESOCKETDATA(3). item is the socket libcurl wants to be closed.