offsetof

The offsetof macro is a standard C and C++ preprocessor macro defined in the <stddef.h> header (or <cstddef> in C++). It provides a way to determine the offset, in bytes, of a particular member within a structure or union.

Purpose

The primary purpose of offsetof is to provide a portable and standardized method for determining the memory layout of structures and unions. This is particularly useful in situations where direct memory manipulation is required, such as:

• Interfacing with hardware that expects data in a specific structure format.
• Implementing generic data structures or algorithms that need to work with different structure types.
• Serializing and deserializing data structures for storage or transmission.

Usage

The offsetof macro takes two arguments:

1. A structure or union type.
2. A member of that structure or union type.

It returns a size_t value representing the offset of the specified member from the beginning of the structure or union.

Technical Details

The offsetof macro works by creating a null pointer of the specified structure or union type and then accessing the member through that pointer. The address of the member is then subtracted from the address of the beginning of the structure, resulting in the offset. Because it relies on pointer arithmetic with a null pointer, its correct implementation relies on specific compiler behavior that is defined by the C and C++ standards.

Portability

The offsetof macro is highly portable because it is a standard part of the C and C++ languages. However, its behavior is only guaranteed to be well-defined if the member is directly accessible using the . operator. It should not be used with bit-fields, or with members of structures that have virtual functions or inherit from virtual base classes (in C++). Using offsetof with a packed struct may yield compiler-specific results. The macro's implementation depends on the specific compiler and target architecture, but the standard guarantees that it will provide consistent and correct results within the defined limitations.