CODEWITHSUNDEEP
ctypesintunsigned-integer
René Moraes
René Moraes

Reputation: 43

Can't understand unsigned int type in C

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?

Upvotes: 4

Views: 1127

Answers (7)

Jens
Jens

Reputation: 72619

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. */

Upvotes: 1

Levon
Levon

Reputation: 143027

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.

Upvotes: 3

Marcin Zaluski
Marcin Zaluski

Reputation: 727

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

Upvotes: 0

Bruno Ferreira
Bruno Ferreira

Reputation: 952

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.

Upvotes: 1

Nikola
Nikola

Reputation: 15038

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

Upvotes: 0

Ed Heal
Ed Heal

Reputation: 59987

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.

Upvotes: 0

Nikolai Fetissov
Nikolai Fetissov

Reputation: 84151

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

Upvotes: 1

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