curl/docs/libcurl/opts/CURLOPT_ALTSVC_CTRL.md

---

c: Copyright (C) Daniel Stenberg, daniel@haxx.se, et al. SPDX-License-Identifier: curl Title: CURLOPT_ALTSVC_CTRL Section: 3 Source: libcurl See-also:

• CURLOPT_ALTSVC (3)
• CURLOPT_CONNECT_TO (3)
• CURLOPT_RESOLVE (3) Protocol:
• HTTP Added-in: 7.64.1 ---

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 necessary.

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, otherwise 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, otherwise this bit is ignored.

DEFAULT

0 - Alt-Svc handling is disabled

%PROTOCOLS%

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%

RETURN VALUE

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