Section: curl_easy_setopt options (3)
Updated: February 11, 2020
CURLOPT_ALTSVC_CTRL - control alt-svc behavior
#define CURLALTSVC_IMMEDIATELY (1<<0)
#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 explicitly telling configure with --enable-alt-svc
. You are
advised to not ship this in production before the experimental label is
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. (Not currently supported).
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
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
Alt-Svc handling is disabled by default. If CURLOPT_ALTSVC(3)
has a default value corresponding to
CURLALTSVC_H1 | CURLALTSVC_H2 | CURLALTSVC_H3 - the HTTP/2 and HTTP/3 bits are
only set if libcurl was built with support for those versions.
CURL *curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, CURLALTSVC_H1);
curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
Added in 7.64.1
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.