CURLOPT_RESOLVER_START_FUNCTION(3) Library Functions Manual
NAME
CURLOPT_RESOLVER_START_FUNCTION - callback called before a new name
resolve is started
SYNOPSIS
#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. A single transfer can trigger more than one
such request, for example an HTTPS RR lookup is a separate request from
the regular A/AAAA lookup, and each gets its own call to this callback
before it 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
This functionality affects all supported protocols
EXAMPLE
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) {
CURLcode result;
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");
result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
AVAILABILITY
Added in curl 7.59.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_PREREQFUNCTION(3), CURLOPT_RESOLVER_START_DATA(3)
libcurl 2026-10-18
CURLOPT_RESOLVER_START_FUNCTION(3)
curl 8.23.0 - Generated Sun Oct 18 18:16:05 CDT 2026