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sockopt
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
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