1---
2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
3SPDX-License-Identifier: curl
4Title: CURLOPT_ALTSVC_CTRL
5Section: 3
6Source: libcurl
7See-also:
8  - CURLOPT_ALTSVC (3)
9  - CURLOPT_CONNECT_TO (3)
10  - CURLOPT_RESOLVE (3)
11Protocol:
12  - HTTP
13---
14
15# NAME
16
17CURLOPT_ALTSVC_CTRL - control alt-svc behavior
18
19# SYNOPSIS
20
21~~~c
22#include <curl/curl.h>
23
24#define CURLALTSVC_READONLYFILE (1<<2)
25#define CURLALTSVC_H1           (1<<3)
26#define CURLALTSVC_H2           (1<<4)
27#define CURLALTSVC_H3           (1<<5)
28
29CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC_CTRL, long bitmask);
30~~~
31
32# DESCRIPTION
33
34Populate the long *bitmask* with the correct set of features to instruct
35libcurl how to handle Alt-Svc for the transfers using this handle.
36
37libcurl only accepts Alt-Svc headers over a secure transport, meaning
38HTTPS. It also only completes a request to an alternative origin if that
39origin is properly hosted over HTTPS. These requirements are there to make
40sure both the source and the destination are legitimate.
41
42Alternative services are only used when setting up new connections. If there
43exists an existing connection to the host in the connection pool, then that is
44preferred.
45
46Setting any bit enables the alt-svc engine.
47
48## CURLALTSVC_READONLYFILE
49
50Do not write the alt-svc cache back to the file specified with
51CURLOPT_ALTSVC(3) even if it gets updated. By default a file specified
52with that option is read and written to as deemed necessary.
53
54## CURLALTSVC_H1
55
56Accept alternative services offered over HTTP/1.1.
57
58## CURLALTSVC_H2
59
60Accept alternative services offered over HTTP/2. This is only used if libcurl
61was also built to actually support HTTP/2, otherwise this bit is ignored.
62
63## CURLALTSVC_H3
64
65Accept alternative services offered over HTTP/3. This is only used if libcurl
66was also built to actually support HTTP/3, otherwise this bit is ignored.
67
68# DEFAULT
69
70Alt-Svc handling is disabled by default. If CURLOPT_ALTSVC(3) is set,
71CURLOPT_ALTSVC_CTRL(3) has a default value corresponding to
72CURLALTSVC_H1 | CURLALTSVC_H2 | CURLALTSVC_H3 - the HTTP/2 and HTTP/3 bits are
73only set if libcurl was built with support for those versions.
74
75# EXAMPLE
76
77~~~c
78int main(void)
79{
80  CURL *curl = curl_easy_init();
81  if(curl) {
82    curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, (long)CURLALTSVC_H1);
83    curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
84    curl_easy_perform(curl);
85  }
86}
87~~~
88
89# AVAILABILITY
90
91Added in 7.64.1
92
93# RETURN VALUE
94
95Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
96