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ISO/IEC 9899:TC2

6.7.2.1 Structure and union specifiers

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Each non-bit-field member of a structure or union object is aligned in an implementation-defined manner appropriate to its type.

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Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.