CURLOPT_DEBUGDATA - custom pointer for debug callback
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_DEBUGDATA, void *pointer);
Pass a pointer to whatever you want passed in to your CURLOPT_DEBUGFUNCTION in the last void * argument. This pointer is not used by libcurl, it is only passed to the callback.
http://curl.haxx.se/libcurl/c/debug.html
CURLOPT_STDERR CURLOPT_DEBUGFUNCTION
This HTML page was made with roffit.