c - warning: comparison of unsigned expression >= 0 is always true

Question

I have the following error when compiling a C file:

t_memmove.c: In function ‘ft_memmove’:
ft_memmove.c:19: warning: comparison of unsigned expression >= 0 is always true

Here's the full code, via cat ft_memmove.c:

#include "libft.h"
#include <string.h>

void    *ft_memmove(void *s1, const void *s2, size_t n)
{
    char    *s1c;
    char    *s2c;
    size_t  i;

    if (!s1 || !s2 || !n)
    {
        return s1;
    }
    i = 0;
    s1c = (char *) s1;
    s2c = (char *) s2;
    if (s1c > s2c)
    {
        while (n - i >= 0) // this triggers the error
        {
            s1c[n - i] = s2c[n - i];
            ++i;
        }
    }
    else
    {
        while (i < n)
        {
            s1c[i] = s2c[i];
            ++i;
        }
    }
    return s1;
}

I do understand that size_t is unsigned and that both integers will be >= 0 because of that. But since I'm subtracting one from the other, I don't get it. Why does this error come up?

Answer 1

If you subtract two unsigned integers in C, the result will be interpreted as unsigned. It doesn't automatically treat it as signed just because you subtracted. One way to fix that is use n >= i instead of n - i >= 0.