Does Compiler Packing Guarantee Cross-Platform Bit Field Order and Alignment?

Cross-Platform Bit Field Order and Alignment: A Myth

While the order of bit fields within a structure may seem platform-specific, the question remains whether platform-specific compiler packing options can guarantee consistent data storage.

Consider the following scenario:

struct Message {
  unsigned int version : 3;
  unsigned int type : 1;
  unsigned int id : 5;
  unsigned int data : 6;
} __attribute__ ((__packed__));

On an Intel processor with GCC, the fields are laid out in memory as written: version first, followed by type.

However, cross-platform portability is still elusive. Packing options are compiler extensions and not fully portable. Furthermore, C99 §6.7.2.1, paragraph 10 explicitly states that the bit field allocation order (high-order to low-order or vice versa) is implementation-defined.

Even the same compiler can exhibit different bit field layouts depending on the target platform's endianness. Therefore, relying on compiler-specific packing options to guarantee cross-platform data order and alignment is not a reliable approach.

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