The sockopt_callback function must match this prototype:
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int sockopt_callback(void *clientp,
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curl_socket_t curlfd,
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curlsocktype purpose);
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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: