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Tip of the Day
Expertise: Intermediate
Sep 23, 1999



Application Security Testing: An Integral Part of DevOps

The offsetof Macro

The macro offsetof (defined in <cstdlib> or <stdlib.h> in C) calculates the offset in bytes of a struct's data member. The following example uses this macro to calculate the offset of two data members in a struct named S:
#include <stdlib.h>
struct S
  int a;
  int b;
  void * p;

int main()
  int n = offsetof(S, a);  // 0
  n = offsetof(S, p);  //8
Note that offsetof works with POD (Plain Old Data) structs and POD unions. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined.
Danny Kalev
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