CURLOPT_ALTSVC_CTRL(3) Library Functions Manual CURLOPT_ALTSVC_CTRL(3)
NAME
CURLOPT_ALTSVC_CTRL - control alt-svc behavior
SYNOPSIS
#include <curl/curl.h>
#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);
DESCRIPTION
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 only accepts Alt-Svc headers over a Secure Transport, meaning
HTTPS. It also only completes 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.
Alternative services are only used when setting up new connections. If
there exists an existing connection to the host in the connection pool,
then that is preferred.
If CURLOPT_ALTSVC(3) is set, CURLOPT_ALTSVC_CTRL(3) gets 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.
Setting any bit enables the alt-svc engine.
CURLALTSVC_READONLYFILE
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 is read and written to as deemed nec-
essary.
CURLALTSVC_H1
Accept alternative services offered over HTTP/1.1.
CURLALTSVC_H2
Accept alternative services offered over HTTP/2. This is only
used if libcurl was also built to actually support HTTP/2, other-
wise this bit is ignored.
CURLALTSVC_H3
Accept alternative services offered over HTTP/3. This is only
used if libcurl was also built to actually support HTTP/3, other-
wise this bit is ignored.
DEFAULT
0 - Alt-Svc handling is disabled
PROTOCOLS
This functionality affects http only
EXAMPLE
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, (long)CURLALTSVC_H1);
curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
curl_easy_perform(curl);
}
}
AVAILABILITY
Added in curl 7.64.1
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_ALTSVC(3), CURLOPT_CONNECT_TO(3), CURLOPT_RESOLVE(3)
libcurl 2026-07-12 CURLOPT_ALTSVC_CTRL(3)
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