How Big is a C/C Union, and Why?

Understanding the Size of a Union in C/C

A union is a versatile data structure that consists of a group of variables that share the same memory location. The size of a union is often a topic of discussion among C/C programmers.

Does the Size of a Union Match the Size of the Largest Datatype Within It?

Yes, the size of a union is equal to the size of its largest member. This is because all the members of a union reside in the same memory location, and the compiler allocates space for the largest member to ensure that there is enough room for any of the other members.

How Does the Compiler Calculate Stack Pointer Movement?

When an element of a union is accessed, the compiler calculates the offset of that element from the beginning of the union's memory location. The stack pointer is then moved accordingly. However, even if a smaller data type within the union is active, the stack pointer adjusts based on the size of the largest data type.

Example:

Consider the following union:

union {
  short x;
  int y;
  long long z;
};

In this case, the size of the union will be equal to the size of a long long (8 bytes). If 'x' is active, which is a short (2 bytes), accessing it will cause the stack pointer to move 8 bytes (the size of a long long) from the union's memory location.

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