curl/docs/libcurl/opts/CURLOPT_RESOLVER_START_FUNCTION.md

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---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_RESOLVER_START_FUNCTION
Section: 3
Source: libcurl
See-also:
- CURLOPT_PREREQFUNCTION (3)
- CURLOPT_RESOLVER_START_DATA (3)
Protocol:
- All
Added-in: 7.59.0
---
# NAME
CURLOPT_RESOLVER_START_FUNCTION - callback called before a new name resolve is started
# SYNOPSIS
~~~c
#include <curl/curl.h>
int resolver_start_cb(void *resolver_state, void *reserved, void *userdata);
CURLcode curl_easy_setopt(CURL *handle,
CURLOPT_RESOLVER_START_FUNCTION,
resolver_start_cb);
~~~
# DESCRIPTION
Pass a pointer to your callback function, which should match the prototype
shown above.
This callback function gets called by libcurl every time before a new resolve
request is started.
*resolver_state* points to a backend-specific resolver state. Currently only
the ares resolver backend has a resolver state. It can be used to set up any
desired option on the ares channel before it is used, for example setting up
socket callback options.
*reserved* is reserved.
*userdata* is the user pointer set with the
CURLOPT_RESOLVER_START_DATA(3) option.
The callback must return 0 on success. Returning a non-zero value causes the
resolve to fail.
# DEFAULT
NULL (No callback)
# %PROTOCOLS%
# EXAMPLE
~~~c
static int start_cb(void *resolver_state, void *reserved,
void *userdata)
{
(void)reserved;
printf("Received resolver_state=%p userdata=%p\n",
resolver_state, userdata);
return 0;
}
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_RESOLVER_START_FUNCTION, start_cb);
curl_easy_setopt(curl, CURLOPT_RESOLVER_START_DATA, curl);
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
~~~
# %AVAILABILITY%
# RETURN VALUE
Returns CURLE_OK