curl/docs/libcurl/opts/CURLOPT_ALTSVC_CTRL.md

c	SPDX-License-Identifier	Title	Section	Source	See-also	Protocol	Added-in
Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.	curl	CURLOPT_ALTSVC_CTRL	3	libcurl	CURLOPT_ALTSVC (3) CURLOPT_CONNECT_TO (3) CURLOPT_RESOLVE (3)	HTTP	7.64.1

NAME

CURLOPT_ALTSVC_CTRL - control alt-svc behavior

SYNOPSIS

#include <curl/curl.h>

#define CURLALTSVC_READONLYFILE (1L<<2)
#define CURLALTSVC_H1           (1L<<3)
#define CURLALTSVC_H2           (1L<<4)
#define CURLALTSVC_H3           (1L<<5)
#define CURLALTSVC_NO_RETRY     (1L<<6)

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 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.

If the connection fails when trying to connect to an alternative host, the hostname from the URL is attempted instead.

Failed connection problems that trigger this fallback:

1. bad addresses such as .onion
2. non-responsive servers
3. bad SSL
4. unsupported protocols
5. response appears to be the wrong protocol

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, CURLALTSVC_H1);
    curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
    curl_easy_perform(curl);
  }
}

HISTORY

CURLALTSVC_* macros became long types in 8.16.0, prior to this version a long cast was necessary when passed to curl_easy_setopt(3).

%AVAILABILITY%

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).