c - Implementation of sizeof operator

Question

I have tried implementing the sizeof operator. I have done in this way:

#define my_sizeof(x) ((&x + 1) - &x)

But it always ended up in giving the result as '1' for either of the data type.

I have then googled it, and I found the following code:

#define my_size(x) ((char *)(&x + 1) - (char *)&x)

And the code is working if it is typecasted, I don't understand why. This code is also PADDING a STRUCTURE perfectly.

It is also working for:

#define my_sizeof(x) (unsigned int)(&x + 1) - (unsigned int)(&x)

Can anyone please explain how is it working if typecasted?

Answer 1

The result of pointer subtraction is in elements and not in bytes. Thus the first expression evaluates to 1 by definition.

This aside, you really ought to use parentheses in macros:

#define my_sizeof(x) ((&x + 1) - &x)
#define my_sizeof(x) ((char *)(&x + 1) - (char *)&x)

Otherwise attempting to use my_sizeof() in an expression can lead to errors.