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The first interpretation is that when this value is used as an rvalue (e.g., lvalue = rvalue), the type is "unsigned int" as declared.   An unsigned int cannot be represented as an int, so integer promotions require that this be an unsigned int, and hence "unsigned".

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struct {
    unsigned long long a:8;
} ull = {255};

<< The following examples need to be completed and tested >>

Non-Compliant Code Example 1

In the following non-compliant code example, cBlocks is multiplied by 16 and the result is stored in the unsigned long long int alloc.  The result of this multiplication can overflow because it is a 32 bit operation, and the resulting value stored in alloc is invalid.

struct {
    unsigned long long size: 24;
} ull;

void* AllocBlocks(ull cBlocks) {
  if (cBlocks == 0) return NULL;
  unsigned long long alloc = cBlocks.size * 16;
  return (alloc < UINT_MAX)
    ? malloc(cBlocks * 16)
    : NULL;
}

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On architectures where unsigned long long int is guaranteed to have 2x the number of bits as size_tupcast the variable used in the multiplication to a 64-bit value.  This ensures that the multiplication operation is performed.

void* AllocBlocks(size_t cBlocks) {
  if (cBlocks == 0) return NULL;
  unsigned long long alloc =
           (unsigned long long)cBlocks.size * 16;
  return (alloc < UINT_MAX)
    ? malloc(cBlocks * 16)
    : NULL;
}

The assumption concerning the relationship of unsigned long long int and size_t must be documented in the header for each file that depends on this assumption for correct execution.

References

• ISO/IEC 9899-1999 Section 6.7.2 Type specifiers
• MISRA 04 Rule 3.5