---
name: "CURLOPT_RESOLVER_START_FUNCTION"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_RESOLVER_START_FUNCTION"
---

# CURLOPT\_RESOLVER\_START\_FUNCTION(3)

callback called before a new name resolve is started

## 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)](/CURLOPT_RESOLVER_START_DATA/3.md)* 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)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

[**CURLOPT\_PREREQFUNCTION**(3)](/CURLOPT_PREREQFUNCTION/3.md), [**CURLOPT\_RESOLVER\_START\_DATA**(3)](/CURLOPT_RESOLVER_START_DATA/3.md)