core at curl_multi_socket_action function libcurl

See more:

core-data

when i init the multi_curl and one easy_curl, i set CURLMOPT_SOCKETFUNCTION,and add easy_curl to the mult_curl, and i call "curl_multi_socket_action(multi_handle, CURL_SOCKET_TIMEOUT, 0, &still_running)" , the program core . why????
==============================================================================================

curl_multi_setopt(multi_handle, CURLMOPT_SOCKETFUNCTION, multi_sock_cb);
curl_multi_setopt(multi_handle, CURLMOPT_SOCKETDATA, cb_data);
curl_easy_setopt(easy_curl, CURLOPT_URL, url.c_str());
curl_easy_setopt(easy_curl, CURLOPT_WRITEFUNCTION, &http_get_callback);
curl_easy_setopt(easy_curl, CURLOPT_WRITEDATA, pResponse);

CURLMcode rc;
rc = curl_multi_add_handle(multi_handle, easy_curl);
if ( CURLM_OK != rc ) {
ERROR("curl_multi_add_handle failed");
return -1;
}
int still_running;
return CURLM_OK == curl_multi_socket_action(multi_handle, CURL_SOCKET_TIMEOUT, 0, &still_running) ? 0 : -1; //core at this.
==========================================================================================
can somebody tell me why?? thanks a lot. :)