[Solved] Passing a struct pointer containing NULL all of a sudden contains a struct instead of NULL

by 


List test = NULL;
one(&test);

test is a pointer to struct node. We pass the address of that
pointer to one, allowing one to change test (letting it return a
new pointer is a cleaner way to do this).

one then executes this:

two(&self);

two is now being passed the address of the address of test. This is
probably not what you intended. Passing the address of test still lets
you change test if that’s what you want to do:

two(self);

1

solved Passing a struct pointer containing NULL all of a sudden contains a struct instead of NULL