One value, four observed bytes
On a platform where sizeof(unsigned) is 4, an unsigned value of 0xAABBCCDD was inspected through a union containing both an unsigned member and an unsigned char array. The printed bytes were DD, CC, BB, AA, in that order. The value did not change; the array exposed the order of its stored bytes on that platform. section 12.2
An object's in-memory format, or object representation, is its representation in storage. C models each object as a collection of bytes that can be viewed as an array of unsigned char. This differs from a type's binary representation: that model describes possible values and determines operations, but does not completely specify storage in memory or on disk. section 12.0 section 5.1.3
Figure 12.1. The different levels of the value-memory model for an int32_t. Example of a platform that maps this type to a 32-bit signed int that has two’s complement sign representation and little-endian object representation.
The diagram separates an int32_t value's binary representation from its little-endian object representation; both are shown for a specified example platform. section 12.0
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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The union overlays an unsigned value and an unsigned char array of exactly sizeof(unsigned) bytes, providing two views of the same object. section 12.2
0 ~/build/modernC% code/endianness
1 value is 0xAABBCCDD
2 byte[0]: 0xDD
3 byte[1]: 0xCC
4 byte[2]: 0xBB
5 byte[3]: 0xAA
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The printed byte indices show low-order digits first on the tested machine, not a storage order guaranteed for every implementation. section 12.2
TAKEAWAY: Do not infer a universal byte order from one run. The in-memory order of an arithmetic type's representation digits is implementation-defined: low-order digits first is little-endian; high-order digits first is big-endian. A cast to unsigned char* offers another way to inspect an object's bytes, but pointer conversions require care because a different pointer type can change how an object is interpreted. section 12.2 section 12.5
