Why isn't the size of union the size of the largest member? [duplicate]

Question

union data {
    double number2;
    char name[20];
};

int main() {

  printf("%i\n", sizeof(union data));

  return 0;
}

I expected it to be 20 because it is the largest, but the result is 24.

Anderlecht

Answer 1

Besides the size of the largest member, the union must also consider the alignment of the member with the strictest alignment requirements. double must be aligned on 8-byte boundary, so the union must have size that's a multiple of 8. Otherwise in an array of data, some instances of number2 would be misaligned.

Retainer

There can also be other ABI requirements, e.g. in ARM OABI with gcc, struct s { char x[2]; } has size 4. – Odyssey