Please read this: https://docs.codewars.com/training/troubleshooting#post-discourse

With a declaration of int *array_diff you will need to return a dynamically allocated array as opposed to one only local to the scope of the function. To make the expectations more clear, I have added some comments to the solution set-up, one of which refers to setting *z as the array length as opposed to z. Use malloc to define your return, and the test suite will free the memory.

Please, mark your post as having spoiler content next time, also, check how to use markdown formatting when posting code here, there is a link in the post below.

• It's not formatted properly (https://docs.codewars.com/training/troubleshooting#post-discourse)
• It always returns None to the caller (https://docs.codewars.com/training/troubleshooting#expected-the-same)