Before you do anything libcurl related in your program, you should do a global libcurl initialize call with
curl_global_init(). This is necessary because some underlying libraries that libcurl might be using need a call ahead to get setup and initialized properly.
curl_global_init() is, unfortunately, not thread safe, so you must ensure that you only do it once and never simultaneously with another call. It initializes global state so you should only call it once, and once your program is completely done using libcurl you can call
curl_global_cleanup() to free and clean up the associated global resources the init call allocated.
libcurl is built to handle the situation where you skip the
curl_global_init() call, but it does so by calling it itself instead (if you did not do it before any actual file transfer starts) and it then uses its own defaults. But beware that it is still not thread safe even then, so it might cause some "interesting" side effects for you. It is much better to call curl_global_init() yourself in a controlled manner.