Defines a few generally useful types and constants. Refer to the following table:
| Constant | Description |
|---|---|
| ptrdiff_t | The result type of the subtraction of two pointers. |
| size_t | Unsigned type for the result of sizeof. |
| wchar_t | Integral type for wide characters. |
| #define NULL ((void *) 0) | |
| size_t offsetof ( type, struct_member) | Returns the offset of field struct_member in struct type. |