The result of a modulo operation is negative

Question

Why does the following C code produce negative numbers as output? And how do I prevent this from happening?

    #include <stdio.h>

    int main()
    {
            int i;
            char buf[1024];
            for (i = 0; i < 1024; i++)
                    buf[i] = i%256;

            for (i=0; i<1024; i++) {
                    printf("%d ", buf[i]);
                    if (i%32==31)
                            printf("\n");
            }
    }

Answer 1

Let's look at this line of code:

buf[i] = i%256;

Here, i % 256 is computed as a value of type int. However, buf is an array of chars, so when the value is assigned into the array, it's truncated to a char. If the result of the modulus is outside of the range of positive values that can be stored in a char, it may end up wrapping around and being stored as a negative number instead.

In other words, it's not that the modulus produced a negative value as much as you stored the result in a type that can't hold it. Try changing the array to an int array or unsigned char array and see if that fixes things.

Hope this helps!