Enable npn
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_ENABLE_NPN, long npn);
Pass a long as parameter, 0 or 1 where 1 is for enable and 0 for disable. This option enables/disables NPN in the SSL handshake (if the SSL backend libcurl is built to use supports it), which can be used to negotiate http2.
1, enabled
HTTP
TODO
Added in 7.36.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.