Defining a Packed Union

You can use _Packed to qualify a union. However, the memory layout of the union members is not affected. Each member starts at offset zero. The _Packed qualifier does affect the total alignment restriction of the whole union.

C++ does not support the _Packed qualifier. To change the alignment of unions, use the #pragma pack directive or the

• align
• /Sp

compiler option. Both of these methods are also supported by C.

In the following example, each of the elements in the nonpacked n_array is of type union uu.

          union uu {
            short    a;
            struct {
              char x;
              char y;
              char z;
            } b;
          };

          union uu           n_array[2];
          _Packed union uu   p_array[2];

Because it is not packed, each element in the array has an alignment restriction of 2 bytes (the largest alignment requirement among the union members is that of short a), and there is 1 byte of padding at the end of each element to enforce this requirement.

Now consider the packed array p_array. Because each of its elements is of type _Packed union uu, the alignment restriction of every element is the byte boundary. Therefore, each element has a length of only 3 bytes, instead of the 4 bytes in the previous example.