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#include <assert.h>
struct S
{
int i;
char ch;
int j; // this gets aligned
struct Ssub
{
int x, y;
} sub, array[100];
};
struct S s;
int main(void)
{
unsigned char ch;
// this is a GCC extension
#ifdef __GNUC__
assert(__builtin_offsetof(struct S, i)==0);
assert(__builtin_offsetof(struct S, ch)==4);
assert(__builtin_offsetof(struct S, j)==8); // padding!
assert(__builtin_offsetof(struct S, sub.x)==12);
assert(__builtin_offsetof(struct S, sub.y)==16);
assert(__builtin_offsetof(struct S, array)==16+4);
assert(__builtin_offsetof(struct S, array[1])==16+12);
assert(__builtin_offsetof(struct S, array[1].y)==16+12+4);
assert(__builtin_offsetof(struct S, array[ch].y)==16+4+4+ch*sizeof(struct Ssub));
#endif
// The following is the 'official' Microsoft way
// of getting struct offsets. No dereferencing
// happens here, just address arithmetic.
assert((long int)&((struct S *)0)->i==0);
assert((long int)&((struct S *)0)->ch==4);
assert((long int)&((struct S *)0)->j==8); // padding!
assert((long int)&((struct S *)0)->sub.x==12);
assert((long int)&((struct S *)0)->sub.y==16);
assert((long int)&((struct S *)0)->array==16+4);
assert((long int)&((struct S *)0)->array[1]==16+12);
assert((long int)&((struct S *)0)->array[1].y==16+12+4);
// these are _constants_!
#ifdef __GNUC__
enum { E1 = __builtin_offsetof(struct S, ch) };
#endif
enum { E2 = (long int)&((struct S *)0)->ch };
enum { E3 = (long int)&((struct S *)0)->array[1].y };
}
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