联合体的内存问题

构体（struct）

简单来说，结构体的数据成员在内存中的顺序是确定的，但是每个数据成员前后有可能插入一定数量的位（bit）来保证内存是对齐的。如下图所示：

每个方格右上角的数字即变量的起始地址偏移量。

From C99 §6.7.2.1:

12 Each non-bit-field member of a structure or union object is aligned in an implementation- defined manner appropriate to its type.

13 Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

联合体（union）

简单来说，联合体中的所有成员的地址都是相同的。如下图所示：

三个变量的起始地址偏移量都是 0。

From the C99 standard §6.7.2.1:

14 The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.

参考

•C struct memory layout - Stack Overflow

•Memory layout of union of different sized member - Stack Overflow

•ELF ABI §3.1