How Much Memory Does a C/C Union Allocate?

Understanding Union Memory Allocation in C/C

In C/C , a union is a data type that allows variables to share the same memory space. This raises the question, how much memory is allocated for a union?

Is it the Size of the Largest Member?

Answer: Yes. A union always occupies the size of its largest member. This holds true regardless of which member is currently active.

Compiler Calculation for Stack Pointer Movement

Despite sharing the same memory space, unions allow individual members to be accessed independently. This raises another question: how does the compiler determine how much stack space to move in case a smaller member is active?

As mentioned earlier, a union's memory allocation is based on the size of its largest member. When accessing a smaller member, the compiler reserves the same amount of memory that is allocated for the union's largest member. This ensures that the union's memory space is appropriately utilized.

Example

Consider the following union:

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

An instance of this union will always occupy a memory space equal to that of a long long (the largest member), even if only x is active. This means that the compiler moves the stack pointer by the size of a long long in such scenarios.

Note: Alignment

It's important to consider that the actual memory size of any data type (including unions) may vary based on compiler optimizations and alignment requirements. This can introduce slight variations in the size occupied by a union compared to the size of its largest member. However, the principle remains the same: a union always allocates space equal to the size of its largest member.

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