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
13Added-in: 7.64.1
14---
15
16# NAME
17
18CURLOPT_ALTSVC_CTRL - control alt-svc behavior
19
20# SYNOPSIS
21
22~~~c
23#include <curl/curl.h>
24
25#define CURLALTSVC_READONLYFILE (1<<2)
26#define CURLALTSVC_H1           (1<<3)
27#define CURLALTSVC_H2           (1<<4)
28#define CURLALTSVC_H3           (1<<5)
29
30CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC_CTRL, long bitmask);
31~~~
32
33# DESCRIPTION
34
35Populate the long *bitmask* with the correct set of features to instruct
36libcurl how to handle Alt-Svc for the transfers using this handle.
37
38libcurl only accepts Alt-Svc headers over a Secure Transport, meaning
39HTTPS. It also only completes a request to an alternative origin if that
40origin is properly hosted over HTTPS. These requirements are there to make
41sure both the source and the destination are legitimate.
42
43Alternative services are only used when setting up new connections. If there
44exists an existing connection to the host in the connection pool, then that is
45preferred.
46
47If CURLOPT_ALTSVC(3) is set, CURLOPT_ALTSVC_CTRL(3) gets a default value
48corresponding to CURLALTSVC_H1 | CURLALTSVC_H2 | CURLALTSVC_H3 - the HTTP/2
49and HTTP/3 bits are only set if libcurl was built with support for those
50versions.
51
52Setting any bit enables the alt-svc engine.
53
54## CURLALTSVC_READONLYFILE
55
56Do not write the alt-svc cache back to the file specified with
57CURLOPT_ALTSVC(3) even if it gets updated. By default a file specified
58with that option is read and written to as deemed necessary.
59
60## CURLALTSVC_H1
61
62Accept alternative services offered over HTTP/1.1.
63
64## CURLALTSVC_H2
65
66Accept alternative services offered over HTTP/2. This is only used if libcurl
67was also built to actually support HTTP/2, otherwise this bit is ignored.
68
69## CURLALTSVC_H3
70
71Accept alternative services offered over HTTP/3. This is only used if libcurl
72was also built to actually support HTTP/3, otherwise this bit is ignored.
73
74# DEFAULT
75
760 - Alt-Svc handling is disabled
77
78# %PROTOCOLS%
79
80# EXAMPLE
81
82~~~c
83int main(void)
84{
85  CURL *curl = curl_easy_init();
86  if(curl) {
87    curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, (long)CURLALTSVC_H1);
88    curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
89    curl_easy_perform(curl);
90  }
91}
92~~~
93
94# %AVAILABILITY%
95
96# RETURN VALUE
97
98Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
99