unsigned int类型的函数返回负数

Question

Wow I thought I knew my C++ but this is weird

This function returns an unsigned int so I thought that means I will never get a negative number returned right?

The function determines how many hours ahead or behind of UTC you are. So for me I'm in Australia, Sydney so I am +10 GMT which means I am UTC = LocalTime + (-10). Therefore the GetTimeZoneInformation correctly determines I am -10.

BUT my function returns an unsigned int so shouldn't it return 10 not -10?

unsigned int getTimeZoneBias()
{
    TIME_ZONE_INFORMATION tzInfo;
    DWORD res  = GetTimeZoneInformation( &tzInfo );

    if ( res == TIME_ZONE_ID_INVALID )
    {
        return (INT_MAX/2); 
    }

    return (unsigned int(tzInfo.Bias / 60));  // convert from minutes to hours         
}

TCHAR ch[200];
_stprintf( ch, _T("A: %d\n"), getTimeZoneBias()); // this prints out A: -10
debugLog += _T("Bias: ") + tstring(ch) + _T("\r\n");

Answer 1

You are trying to print an unsigned int as a signed int . Change %d to %u

_stprintf( ch, _T("A: %u\n"), getTimeZoneBias());
                       ^

The problem is that integers aren't positive or negative by themselves for most computers . It's in the way they are interpreted.

So a large integer might be indistinguishable from a small (absolute value) negative one.

Answer 2

Here's what I think is happening:

The value of tzInfo.Bias is actually -10. ( 0xFFFFFFF6 ) On most systems, casting a signed integer to an unsigned integer of the same size does nothing to the representation.

So the function still returns 0xFFFFFFF6 .

But when you print it out, you're printing it back as a signed integer. So it prints -10 . If you printed it as an unsigned integer, you'll probably get 4294967286 .

What you're probably trying to do is to get the absolute value of the time difference. So you want to convert this -10 into a 10. In which you should return abs(tzInfo.Bias / 60) .

Answer 3

One error is in your _T call. It should be:

_T("A: %u\n")

The function does return a non-negative integer. However, by using the wrong printf specifier, you're causing it to get popped off the stack as an integer. In other words, the bits are interpreted wrong. I believe this is also undefined behavior.

Answer 4

As other people have pointed out, when you cast to an unsigned int , you are actually telling the compiler to use the pattern of bits in the int and use it as an unsigned int . If your computer uses two's complement , as most do, then your number will be interpreted as UINT_MAX-10 instead of 10 as you expected. When you use the %d format specifier, the compiler goes back to using the same bit pattern as an int instead of an unsigned int . This is why you are still getting -10 .

If you want the absolute value of an integer, you should try to get it mathematically instead of using a cast.