Count bits in a 32-bit integer

countbits.c

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

/** returns the number of 1 bits in x */
short countBits(int x) {
    short count = 0;
    int mask = 1;
    for(short i = 0; i < sizeof(x); i++) {
        if((mask & x) == mask) {
            count++;
        }
        mask = mask << 1;
    }
    return count;
}

void runTest(int x, short expected) {
    short actual = countBits(x);
    if(actual != expected) {
        printf("Error: Expected %d bits in %d, but counted %d.", expected, x, actual);
    }
}

int main() {
    runTest(0, 0);
    runTest(1, 1);
    runTest(2, 1);
    runTest(3, 2);
    runTest(((int)pow(2, 6))-1, 6);
    runTest(INT_MAX, 31);
    runTest(INT_MIN, 1);
    printf("Run all tests!\n");
}