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CURLOPT_RE...RT_FUNCTION(3) Library Functions Manual CURLOPT_RE...RT_FUNCTION(3)

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  re-
     solve 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 set-
     ting 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 			   2026-06-11	     CURLOPT_RE...RT_FUNCTION(3)

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