A union gives several members the same memory location. Unlike a structure, it does not collect separately stored values; it overlays different type interpretations on one object representation. TAKEAWAY: A union changes how one stored object is viewed, not how many independent objects are stored. section 12.2
2 #include <inttypes.h>
3
4 typedef union unsignedInspect unsignedInspect;
5 union unsignedInspect {
6 unsigned val;
7 unsigned char bytes[sizeof(unsigned)];
8 };
9 unsignedInspect twofold = { .val = 0xAABBCCDD, };
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Inspect the complete declaration: an unsigned value and an unsigned-char array occupy the same union object. section 12.2
Inspecting an unsigned value
Declare `unsignedInspect twofold = { .val = 0xAABBCCDD };`, then print `twofold.val` and each element of `twofold.bytes`. The value prints as `0xAABBCCDD`; on the demonstrated machine, the bytes print `0xDD`, `0xCC`, `0xBB`, and `0xAA`. The array exposes the same storage as individual unsigned-char representation bytes. TAKEAWAY: The observed byte order belongs to that implementation, not to a universal C layout rule. section 12.2
Keep these distinctions in view:
- A union overlays members; a structure stores its members as separate parts of a larger object. section 12.2
- An object can be examined as an array of unsigned char, but byte order and representation details can depend on the implementation. section 12.8
- Unions are introduced after the memory model because their useful interpretation requires understanding object representation. section 6.0
