CURLOPT_HTTP_VERSION(3) Library Functions Manual CURLOPT_HTTP_VERSION(3)

CURLOPT_HTTP_VERSION - 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 still prioritizes to reuse existing connections so it might then reuse a connection using an HTTP version you have not asked for.

We do not care about what version the library uses. libcurl uses whatever it thinks fit.
Enforce HTTP 1.0 requests.
Enforce HTTP 1.1 requests.
Attempt HTTP 2 requests. libcurl falls back to HTTP 1.1 if HTTP 2 cannot be negotiated with the server. (Added in 7.33.0)

When libcurl uses HTTP/2 over HTTPS, it does not itself insist on TLS 1.2 or higher even though that is required by the specification. A user can add this version requirement with CURLOPT_SSLVERSION(3).

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 falls back to HTTP 1.1 if HTTP 2 cannot be negotiated with the HTTPS server. For clear text HTTP servers, libcurl uses 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 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) This option makes libcurl attempt to use HTTP/3 to the host given in the URL, with fallback to earlier HTTP versions if needed.
(Added in 7.88.0) Setting this makes libcurl attempt to use HTTP/3 directly to server given in the URL and does not downgrade to earlier HTTP version if the server does not support HTTP/3.

Since curl 7.62.0: CURL_HTTP_VERSION_2TLS

Before that: CURL_HTTP_VERSION_1_1

HTTP

int main(void)
{
  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,
                     (long)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_ALTSVC(3), CURLOPT_HTTP09_ALLOWED(3), CURLOPT_HTTP200ALIASES(3), CURLOPT_SSLVERSION(3)

2024-05-22 libcurl