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.