The sockopt callback is set with CURLOPT_SOCKOPTFUNCTION:

curl_easy_setopt(handle, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);

The sockopt_callback function must match this prototype:

int sockopt_callback(void *clientp,
                     curl_socket_t curlfd,
                     curlsocktype purpose);

This callback function gets called by libcurl when a new socket has been created but before the connect call, to allow applications to change specific socket options.

The clientp pointer points to the private data set with CURLOPT_SOCKOPTDATA:

curl_easy_setopt(handle, CURLOPT_SOCKOPTDATA, custom_pointer);

This callback should return:

  • CURL_SOCKOPT_OK on success
  • CURL_SOCKOPT_ERROR to signal an unrecoverable error to libcurl
  • CURL_SOCKOPT_ALREADY_CONNECTED to signal success but also that the socket is in fact already connected to the destination