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Find an Offset into a Structure

Find an Offset into a Structure

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Here’s the code:

#define offset(a,b)  ((int) (&( ((a*)(0)) -> b)))

Step by step, here’s what it does:

  1. Type-cast 0, or NULL, into a pointer of type a. So a must be a typename.
  2. Member-select b from that pointer. So a must be a structure typename, and b some member property.
  3. Take the address of b. Since the a* used points to address 0, and b is some offset above the start of the a-structure, this means you’re getting the offset of b within a plus the address of a, which is zero. That is, you get the offset of b within a.
  4. Typecast that number to an integer.
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