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