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Here, the compiler could implement arg.b = 2 in the following way,:

• setting the low byte of a 32-bit register to 2, leaving the high bytes unchanged, and
• storing all 32 bits of the register into memory,

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GCC allows specifying attributes of variables and structures using the keyword __attribute__((__packed__)). This means that the GCC will not add any padding bytes (for memory alignment) and will make variables or fields immediately next to each other.

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For example, in an IA-32 machine, the explicit insertion of the padding bytes into the struct should ensure that no "invisible" padding bytes are added in by the compiler, and consequently the expression in static_assert should be true.

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However, if you were compiling for a different architecture and the compiler adds added padding bytes, then the memory would not be contiguous and the expression would return 0.

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