My problem with this definition is what "tests a bit of code" means. For example, in a codebase I work on we use a custom sort function, and memory manager. We

My problem with this definition is what "tests a bit of code" means. For example, in a codebase I work on we use a custom sort function, and memory manager. We have tests for both (as you would expect). However, both sorting and allocation are fairly large pieces of code. Does that mean any test of any function which calls sort or allocates memory isn't a unit test, as it's (indirectly) using something much bigger than a single function? But "stubbing" out either sort, or memory allocation, seems stupid (the stub would just have to do the same thing).