[layer8]

You can declare a type without (fully) defining it, like in

    typedef struct foo foo_t;

and then have code that (for example) works with pointers to it (*foo_t). If you include a standard header containing such a forward declaration, and also declare foo_t yourself, no compilation error might be triggered, but other translation units might use differing definitions of struct foo, leading to unpredictable behavior in the linked program.