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>>>> unsigned long size_words; >>>> -} __packed; >>>> +}; >>>> > > Does "__packed" really do anything between a pointer and a long? If that structure is allocated at a non-void-ptr-aligned address, the packed attribute will ensure that the compiler don't emit instructions that require aligned loads/stores when accessing those fields. It does not change the layout of the structure per se in this specific case, but it informs the compiler about the lack of guarantees about alignment for the entire structure. x86 32/64 cannot care less about this, but it's relevant on other architectures. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com