cairo_user_data_key_t is used for attaching user data to cairo data structures. The actual contents of the struct is never used, and there is no need to initialize the object; only the unique address of a cairo_data_key_t object is used. Typically, you would just use the address of a static cairo_data_key_t object. int unused; not used; ignore. Since 1.0

struct cairo_user_data_key_t {
int unused;