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The macro offsetof(type, member-designator) accepts a restricted set of type arguments in this International Standard. If type is not a standard-layout class, the results are undefined. The expression offsetof(type, member-designator) is never type-dependent and it is value-dependent if and only if type is dependent. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined. No operation invoked by the offsetof macro shall throw an exception and noexcept(offsetof(type, member-designator)) shall be true.

When specifying the type argument for the offsetof() macro, pass only a standard-layout class. The full description of a standard-layout class can be found in paragraph 7 of the [class] clause of the C++ Standard, or the type can be checked with the std::is_standard_layout<> type trait. When specifying the member designator argument for the offsetof() macro, do not pass a bit-field, static data member, or function member. Passing an invalid type or member to the offsetof() macro is undefined behavior.

Noncompliant Code Example

In this noncompliant code example, a type that is not a standard-layout class is passed to the offsetof() macro, resulting in undefined behavior.

#include <cstddef>
 
struct D {
  virtual void f() {}
  int i;
};
 
void f() {
  size_t off = offsetof(D, i);
  // ...
}

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In this noncompliant code example, the offset to i is calculated so that a value can be stored at that offset within buffer. However, because i is a static data member of the class, this example results in undefined behavior. According to the C++ Standard, [class.static.data], paragraph 1 [ISO/IEC 14882-2014], static data members are not part of the subobjects of a class.

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Passing an invalid type or member to offsetof() can result in undefined behavior that might be exploited to cause data integrity violations or result in incorrect values from the macro expansion.

Automated Detection 

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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