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Unfortunately, this solution is not portable. C99 does not require that implementations support sizeof, offsetof, or enumeration constants in #if conditions. According to Section 6.10.1, "Conditional inclusion," all identifiers in the expression that controls conditional inclusion either are or are not macro names. Some compilers allow these constructs in conditionals as an extension, but most do not.
Compliant Solution
This portable compliant solution mimics the behavior of uses static_assert in a portable manner.
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struct timer {
uint8_t MODE;
uint32_t DATA;
uint32_t COUNT;
};
static_assert(offsetof(struct timer, DATA) == 4, "Member DATA is not 4-byte aligned");
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