CURLOPT_CLOSESOCKETFUNCTION (3) - Linux Man Pages
CURLOPT_CLOSESOCKETFUNCTION: callback to socket close replacement function
NAMECURLOPT_CLOSESOCKETFUNCTION - callback to socket close replacement function
int closesocket_callback(void *clientp, curl_socket_t item);
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.