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CURLOPT_ALTSVC_CTRL - control alt-svc behavior

#include <curl/curl.h>

#define CURLALTSVC_IMMEDIATELY (1<<0)
#define CURLALTSVC_ALTUSED (1<<1)
#define CURLALTSVC_READONLYFILE (1<<2)
#define CURLALTSVC_H1 (1<<3)
#define CURLALTSVC_H2 (1<<4)
#define CURLALTSVC_H3 (1<<5)

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC_CTRL, long bitmask);

Warning: this feature is early code and is marked as experimental. It can only be enabled by explictly telling configure with --enable-alt-svc. You are advised to not ship this in production before the experimental label is removed. Populate the long bitmask with the correct set of features to instruct libcurl how to handle Alt-Svc for the transfers using this handle. libcurl will only accept Alt-Svc headers over a secure transport, meaning HTTPS. It will also only complete a request to an alternative origin if that origin is properly hosted over HTTPS. These requirements are there to make sure both the source and the destination are legitimate. Setting any bit will enable the alt-svc engine. If an Alt-Svc: header is received, this instructs libcurl to switch to one of those alternatives asap rather than to save it and use for the next request. Issue the Alt-Used: header in all requests that have been redirected by alt-svc. Do not write the alt-svc cache back to the file specified with CURLOPT_ALTSVC(3) even if it gets updated. By default a file specified with that option will be read and written to as deemed necessary. Accept alternative services offered over HTTP/1.1. Accept alternative services offered over HTTP/2. This will only be used if libcurl was also built to actually support HTTP/2, otherwise this bit will be ignored. Accept alternative services offered over HTTP/3. This will only be used if libcurl was also built to actually support HTTP/3, otherwise this bit will be ignored. 0. No Alt-Svc treatment. 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 "(3), " CURLOPT_CONNECT_TO "(3), " CURLOPT_RESOLVE "(3), "