struct aligned_struct
{
	char a [5];
	int b;
};

It is tempting to say that b is located at the 6th byte from the beginning of the struct because a has already occupied the first 5 bytes. But, actually b is located at the 9th byte from the beginning of the struct because b is aligned at every 4 bytes. Therefore, to know exactly the offset of a member of a struct (i.e., at what n-th byte from the beginning of the struct the member is located), simply use the following technique that I found in Linux kernel 2.6.21.5 source code file in include/linux/stddef.h:

((size_t) &(((struct aligned_struct *)0)->b))

This technique is interesting because it turned out that you can treat a constant as a memory address.

In general as written in the source code file:

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

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