Can't understand unsigned int type in C

StackOverflow https://stackoverflow.com/questions/11903028

  •  25-06-2021
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Question

I'm starting to program in C and I'm having a problem to understand some results I'm getting. I'll paste the code here:

#include <stdio.h>
unsigned int main(void)
{
    unsigned int x = 0;

    printf("%u\n",x-1);
    return 0;
}

The terminal is returning 4.294.967.295, and I'm not getting why. I know that this value is the max value of a unsigned int, but actually I was expecting some warning from the compiler that I would have to use a int type not an unsigned int because the result is negative. Anyway, can someone help me?

Was it helpful?

Solution

When you use an unsigned type all of the bits are used to represent non-negative values ( i.e., >= 0). Think of this as the values wrapping around, so when you decrement below the lower limit (0), you wrap around to the largest value (with 32 bits, 2^32 - 1).

Also, note you did not assign a negative value to variable x, you just provided an expression that subtracted 1 from it. I.e.,

x-1

vs

x = x - 1;

though whether you would have gotten a warning about this would probably depend on the compiler and the warning levels set.

OTHER TIPS

It's the fixed number of bits (32), and two's complement that get you here.

unsigned int as it's name implies has no sign.

unsigned int values are represented as a 32 bit values, so when you subtract one it rolls over from 0x00000000 to 0xFFFFFFFF, that gives you the 4294967295 you are getting.

The C Standard says that all unsigned arithmetic happens within the modulo 2^N range, where N is the number of bits in the type. Thus, substracting 1 from 0 in unsigned arithmetic does not result in -1, but in UINT_MAX.

Think of the unsigned values as a ring, where 0 is glued next to UINT_whatever. Regardless of whether you are adding, subtracting, shifting, multiplying, even negating, you'll never leave the ring. So you are perfectly allowed to do things like

 unsigned int x = -1;    /* Assigns UINT_MAX. */

Because unsigned int cannot store negative integers.

Please read up about ones-complement and twos-complex numbers.

What is happening that the top bit is being set along with the others and is then interpreted as a positive number.

You don't get the error because the compiler can't know what will you do in future.

Unsigned means that you have only positive values. Computers stores numbers as binary with some rules. When you do 0 - 1 you got maximum int value. You may read more about it here: http://en.wikipedia.org/wiki/Signed_number_representations

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