Endianness is the ordering convention used to encode the bytes of a multi-byte value in memory or another representation. For an unsigned value such as 0xAABBCCDD, the bytes may appear from low-order to high-order, or in the reverse order. The C standard does not normalize this ordering; the observed arrangement is implementation-defined. section 12.2
Inspecting an integer's bytes
A union can overlay an unsigned value with an array of unsigned char, allowing its object representation to be inspected. With twofold.val set to 0xAABBCCDD, one observed machine prints byte[0] as 0xDD, followed by 0xCC, 0xBB, and 0xAA. This arrangement places the low-order representation digits before the high-order digits. section 12.2
typedef union unsignedInspect unsignedInspect;
union unsignedInspect {
unsigned val;
unsigned char bytes[sizeof(unsigned)];
};
unsignedInspect twofold = { .val = 0xAABBCCDD, };
Use the union definition to inspect an unsigned value through its individual unsigned-char bytes. section 12.2
Endianness matters when representations cross program or platform boundaries:
- Treat raw binary files as portable only between executions and platforms using the same representation, including the same endianness. section 14.6
- Open streams in binary mode when using fread or fwrite; otherwise, platform-specific transformations can undermine reliable transport of representations. section 14.6
- Do not assume that different platforms, operating systems, or program executions encode objects with the same byte ordering. section 14.6
