Anthony Bryan reported and I corrected two typos.

 curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);

You can control what data your function get in the forth argument by setting

another property:

You can control what data your callback function gets in the fourth argument

by setting another property:

 curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);