CODEWITHSUNDEEP
cunsigned-integer
user4298677
user4298677

Reputation:

C - unsigned int going negative (-ve)

What I know is - UNSIGNED INT cannot take negative values.
If I take the maximum value of an UNSIGNED INT and increment it, I should get ZERO i.e. the minimum value and if I take the minimum value and decrement it, I should get the maximum value.
Then, why is this happening ?

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int main()
{
    unsigned int ui;
    ui = UINT_MAX;
    ui ++;
    printf("ui = %d", ui);

    ui = 0;
    ui --;
    printf("\n");
    printf("ui = %d", ui);

    return EXIT_SUCCESS;
}

Output:

ui = 0
ui = -1

Upvotes: 0

Views: 162

Answers (4)

Joakim Ericsson
Joakim Ericsson

Reputation: 4696

printf("%u") should be used for unsigned ints.

Upvotes: 2

John O&#39;M.
John O&#39;M.

Reputation: 510

From 'man 3 printf':

d, i The int argument is converted to signed decimal notation

So, although the type of ui is unsigned int, printf is interpreting it as a signed int and showing it as such.

Upvotes: 3

harper
harper

Reputation: 13690

You pass the value to an ellipsis function (printf). You should expect nothing about the signedness here.

The %d in the format string controls the sign of the displayed value. There is a cast inside the printf function since you selected the %d. That's why you see a signed value that is equivalent to the binary value FFFFFFFF1.

1 Assuming a 32 bit width for integer.

Upvotes: 1

Max Komarychev
Max Komarychev

Reputation: 2856

That is because you using %d format specifier that says printf to treat your number as a signed integer. Try using %u to output unsigned value and you get the desired result.

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int main()
{
    unsigned int ui;
    ui = UINT_MAX;
    ui ++;
    printf("ui = %u", ui);
    ui = 0;
    ui --;
    printf("\n");
    printf("ui = %u", ui);
    return EXIT_SUCCESS;
}

output:

ui = 0
ui = 4294967295

Check out the reference on possible format specifiers.

Upvotes: 2

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