Try adding some prints to stderr through my earlier test program then and see if you can find where it stops giving you output. Does output work before curl_easy_init? After it? Somewhere later on?

Note that I did update the program to add the line with CURLOPT_ERRORBUFFER -- that's not strictly needed, but might provide more debug info if something goes wrong later in the program. (Forgot to add the setup line initially despite writing the rest of it... 🤦‍♂️️)

You could also try adding curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L); to get it to explain more details about what it's doing internally if you can get it to print output at all.