CURLOPT_XFERINFOFUNCTION man page:

[coralw]

It seems that clientp and userp mismatch in the code example:

 static size_t progress_callback(void *clientp,
                                 curl_off_t dltotal,
                                 curl_off_t dlnow,
                                 curl_off_t ultotal,
                                 curl_off_t ulnow)
 {
   struct memory *progress = (struct progress *)userp;
 
   /* use the values */
 
   return 0; /* all is good */
 }

The man page is found here:
CURLOPT_XFERINFOFUNCTION explained
https://curl.se/libcurl/c/CURLOPT_XFERINFOFUNCTION.html