When I execute the following loop :

 unsigned m;
 for( m = 10; m >= 0; --m ){
        printf("%d\n",m);
    }

the loop doesn't stop at m==0, it keeps executing interminably, so I thought that reason was that an unsigned cannot be compared to 0. But when I did the following test

unsigned m=9;
if(m >= 0)
    printf("m is positive\n");
else
    printf("m is negative\n");

I got this result:

m is positive 

which means that the unsigned variable m was successfully compared to 0.
Why doesn't the comparison of m to 0 work in the for loop and works fine elsewhere?