offsetof
#include <stddef.h>
size_t offsetof(type, member);
The macro offsetof() returns the offset of the field member from the start of the structure type.
struct s {
int i;
char c;
double d;
char a[];
};
/* Output is compiler dependent */
printf("offsets: i=%zd; c=%zd; d=%zd a=%zd\n",
offsetof(struct s, i), offsetof(struct s, c),
offsetof(struct s, d), offsetof(struct s, a));
printf("sizeof(struct s)=%zd\n", sizeof(struct s));
exit(EXIT_SUCCESS);
size_t vs. uintptr_t
in C99, the standard introduced the intptr_t and uintptr_t types, which are signed and unsigned types guaranteed to be able to hold pointers.
size_t has to be big enough to contain the size of the largest possible object. uintptr_t must be big enough to contain any pointer. Given this, it is more or less guaranteed that sizeof(uintptr_t) >= sizeof(size_t) (since all of the bytes in the largest possible object must be addressable), but not more.
On machines with linear addressing, they probably will be the same size. On segmented architectures, on the other hand, it is usual for uintptr_t to be bigger than size_t, since an object must be in a single segment, but a pointer must be able to address all of the memory.
