CURLOPT\_RESOLVER\_START\_FUNCTION(3) Library Functions Manual CURLOPT\_RESOLVER\_START\_FUNCTION(3)

# [NAME](#NAME)

CURLOPT\_RESOLVER\_START\_FUNCTION - callback called before a new name resolve is started

# [SYNOPSIS](#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](#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)*](/man/os/curl/CURLOPT_RESOLVER_START_DATA.3) option.

The callback must return 0 on success. Returning a non-zero value causes the resolve to fail.

# [DEFAULT](#DEFAULT)

NULL (No callback)

# [PROTOCOLS](#PROTOCOLS)

This functionality affects all supported protocols

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.59.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

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

2026-10-31 libcurl

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Document: CURLOPT_RESOLVER_START_FUNCTION(3) from curl 8.23.0-r3 (Chainguard OS)
Source: https://github.com/curl/curl.git