Compiler Warning (level 1) C4401

  • Article

'bitfield' : member is bit field

Inline assembly code tries to access a bit-field member. Inline assembly cannot access bit-field members, so the last packing boundary before the bit-field member is used.

To avoid this warning, cast the bit field to an appropriate type before making the reference in inline assembly code. The following sample generates C4401:

// C4401.cpp
// compile with: /W1
// processor: x86
typedef struct bitfield {
   signed bit : 1;
} mybitfield;

int main() {
   mybitfield bf;
   bf.bit = 0;
   __asm {
      mov bf.bit,0;   // C4401
   }

   /* use the following __asm block to resolve the warning
   int i = (int)bf.bit;
   __asm {
      mov i,0;
   }
   */
}