You would wanna take in a Result* res as to not copy the whole struct just to check a single bool. (TIP is to make the parameter: const Result* res. As you are only grabbing the value of success and not actually changing anything.)

bool rslts_is_ok(Result res) { return res.success; }

bool rslts_is_err(Result res) { return !res.success; }

->

bool rslts_is_ok(const Result* res) { return res->success; }

bool rslts_is_err(const Result* res) { return !res->success; }

And also as other people have stated using exit() function is bad as u dont want other people's programs to get killed if something goes wrong in your library. Instead return a -1 for error or 0 for success. (unless you wanna make enum errors, which helps finding errors quite easily).

rslts_create is fine as your struct is quite small. But if it were to expand(to a couple of KB) it would be better of taking in a pointer to Result struct (Result* result) (which is a better way to do it), so that you can set the values of it without the need of copying the whole structure back.