1---
2c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
3SPDX-License-Identifier: curl
4Title: CURLOPT_RESOLVER_START_FUNCTION
5Section: 3
6Source: libcurl
7See-also:
8  - CURLOPT_PREREQFUNCTION (3)
9  - CURLOPT_RESOLVER_START_DATA (3)
10---
11
12# NAME
13
14CURLOPT_RESOLVER_START_FUNCTION - callback called before a new name resolve is started
15
16# SYNOPSIS
17
18~~~c
19#include <curl/curl.h>
20
21int resolver_start_cb(void *resolver_state, void *reserved, void *userdata);
22
23CURLcode curl_easy_setopt(CURL *handle,
24                          CURLOPT_RESOLVER_START_FUNCTION,
25                          resolver_start_cb);
26~~~
27
28# DESCRIPTION
29
30Pass a pointer to your callback function, which should match the prototype
31shown above.
32
33This callback function gets called by libcurl every time before a new resolve
34request is started.
35
36*resolver_state* points to a backend-specific resolver state. Currently only
37the ares resolver backend has a resolver state. It can be used to set up any
38desired option on the ares channel before it is used, for example setting up
39socket callback options.
40
41*reserved* is reserved.
42
43*userdata* is the user pointer set with the
44CURLOPT_RESOLVER_START_DATA(3) option.
45
46The callback must return 0 on success. Returning a non-zero value causes the
47resolve to fail.
48
49# DEFAULT
50
51NULL (No callback)
52
53# PROTOCOLS
54
55All
56
57# EXAMPLE
58
59~~~c
60static int start_cb(void *resolver_state, void *reserved,
61                    void *userdata)
62{
63  (void)reserved;
64  printf("Received resolver_state=%p userdata=%p\n",
65         resolver_state, userdata);
66  return 0;
67}
68
69int main(void)
70{
71  CURL *curl = curl_easy_init();
72  if(curl) {
73    curl_easy_setopt(curl, CURLOPT_RESOLVER_START_FUNCTION, start_cb);
74    curl_easy_setopt(curl, CURLOPT_RESOLVER_START_DATA, curl);
75    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
76    curl_easy_perform(curl);
77    curl_easy_cleanup(curl);
78  }
79}
80~~~
81
82# AVAILABILITY
83
84Added in 7.59.0
85
86# RETURN VALUE
87
88Returns CURLE_OK
89