ptrdiff_t is the result of
subtracting two pointers.
size_t is the unsigned integer result of
the sizeof keyword.
wchar_t is an integer type of the size
of a wide character constant.
NULL is the value of a null pointer
This results in a constant integer of type
size_t which is the offset in bytes of a
structure member from the beginning of the structure. The
member is given by member-designator, and the name of
the structure is given in type.
The output should be:
printf("level is the %d byte in the user structure.\n"),
level is the 100 byte in the user structure.