This option tells libcurl to use protocol if the URL is missing a scheme
Use one of these protocol (scheme) names:
dict, file, ftp, ftps, gopher, http, https, imap, imaps, ldap, ldaps, pop3,
pop3s, rtsp, scp, sftp, smb, smbs, smtp, smtps, telnet, tftp
An unknown or unsupported protocol causes error
CURLE_UNSUPPORTED_PROTOCOL when libcurl parses a schemeless URL. Parsing
happens when curl_easy_perform(3) or curl_multi_perform(3) is
called. The protocols supported by libcurl will vary depending on how it was
built. Use curl_version_info(3) if you need a list of protocol names
supported by the build of libcurl that you are using.
This option does not change the default proxy protocol (http).
Without this option libcurl would make a guess based on the host, see
CURLOPT_URL(3) for details.
curl = curl_easy_init();
/* set a URL without a scheme */
curl_easy_setopt(curl, CURLOPT_URL, "example.com");
/* set the default protocol (scheme) for schemeless URLs */
curl_easy_setopt(curl, CURLOPT_DEFAULT_PROTOCOL, "https");
/* Perform the request */