CURLOPT_ALTSVC man page

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC, char *filename);

Warning: this feature is early code and is marked as experimental. It can only be enabled by explicitly telling configure with --enable-alt-svc. You are advised to not ship this in production before the experimental label is removed.

Pass in a pointer to a filename to instruct libcurl to use that file as the Alt-Svc cache to read existing cache contents from and possibly also write it back to a after a transfer, unless CURLALTSVC_READONLYFILE is set in CURLOPT_ALTSVC_CTRL(3).

Specify a blank file name ("") to make libcurl not load from a file at all.

NULL. The alt-svc cache is not read nor written to file.

HTTPS

CURL *curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, CURLALTSVC_H1);
  curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
  curl_easy_perform(curl);
}

Added in 7.64.1

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

CURLOPT_ALTSVC_CTRL(3), CURLOPT_CONNECT_TO(3), CURLOPT_RESOLVE(3), CURLOPT_COOKIEFILE(3),

curl_easy_setopt(3), CURLOPT_ALTSVC_CTRL(3), CURLOPT_HTTP_VERSION(3).