Access the flags without inline assembly?

Question

I have the following method in C that takes two 16-bit short ints and:

• Adds the two integers
• If the carry flag is set, add 1 to the result
• Negate (NOT) all the bits in the final results

• Return the result:

  short __declspec(naked) getchecksum(short s1, short s2)
  {
      __asm
      {
          mov ax, word ptr [esp+4]
          mov bx, word ptr [esp+8]
          add ax, bx
          jnc skip_add
          add ax, 1
          skip_add:
          not ax      
          ret
      }
  }
  

I had to write this in inline assembly because I do not know any way to test the carry flag without using assembler. Does anyone know of a way to do this?

Answer 1

No (C has no notion of flags at all) but that doesn't mean you can't get the same result. If you use 32bit integers to do addition, the 17th bit is the carry. So you can write it like this:

uint16_t getchecksum(uint16_t s1, uint16_t s2)
{
    uint32_t u1 = s1, u2 = s2;
    uint32_t sum = u1 + u2;
    sum += sum >> 16;
    return ~sum;
}

I've made the types unsigned to prevent trouble. That may not be necessary on your platform.