**************************************************************************
* _ _ ____ _
* Project ___| | | | _ \| |
* / __| | | | |_) | |
* | (__| |_| | _ <| |___
* \___|\___/|_| \_\_____|
*
* Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
*
* This software is licensed as described in the file COPYING, which
* you should have received as part of this distribution. The terms
* are also available at https://curl.haxx.se/docs/copyright.html.
*
* You may opt to use, copy, modify, merge, publish, distribute and/or sell
* copies of the Software, and permit persons to whom the Software is
* furnished to do so, under the terms of the COPYING file.
*
* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
* KIND, either express or implied.
*
**************************************************************************

CURLOPT_HTTP_VERSION - specify HTTP protocol version to use #include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTP_VERSION, long version); Pass version a long, set to one of the values described below. They ask libcurl to use the specific HTTP versions. Note that the HTTP version is just a request. libcurl will still prioritize to re-use an existing connection so it might then re-use a connection using a HTTP version you haven't asked for. We don't care about what version the library uses. libcurl will use whatever it thinks fit. Enforce HTTP 1.0 requests. Enforce HTTP 1.1 requests. Attempt HTTP 2 requests. libcurl will fall back to HTTP 1.1 if HTTP 2 can't be negotiated with the server. (Added in 7.33.0) The alias CURL_HTTP_VERSION_2 was added in 7.43.0 to better reflect the actual protocol name. Attempt HTTP 2 over TLS (HTTPS) only. libcurl will fall back to HTTP 1.1 if HTTP 2 can't be negotiated with the HTTPS server. For clear text HTTP servers, libcurl will use 1.1. (Added in 7.47.0) Issue non-TLS HTTP requests using HTTP/2 without HTTP/1.1 Upgrade. It requires prior knowledge that the server supports HTTP/2 straight away. HTTPS requests will still do HTTP/2 the standard way with negotiated protocol version in the TLS handshake. (Added in 7.49.0) (Added in 7.66.0) Setting this value will make libcurl attempt to use HTTP/3 directly to server given in the URL. Note that this cannot gracefully downgrade to earlier HTTP version if the server doesn't support HTTP/3. For more reliably upgrading to HTTP/3, set the preferred version to something lower and let the server announce its HTTP/3 support via Alt-Svc:. See CURLOPT_ALTSVC(3). Since curl 7.62.0: CURL_HTTP_VERSION_2TLS Before that: CURL_HTTP_VERSION_1_1 HTTP

CURL *curl = curl_easy_init();
if(curl) {
 CURLcode ret;
 curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
 curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);
 ret = curl_easy_perform(curl);
 if(ret == CURLE_HTTP_RETURNED_ERROR) {
 /* an HTTP response error problem */
 }
}

Along with HTTP Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not. CURLOPT_SSLVERSION "(3), " CURLOPT_HTTP200ALIASES "(3), " CURLOPT_HTTP09_ALLOWED "(3), " CURLOPT_ALTSVC "(3) "