@import Darwin.sys.mount;

int main(int argc, const char * agrv[]) {
    struct statfs buf;
    long long freespace = 0;
    if(statfs("/", &buf) >= 0){
        freespace = (long long)buf.f_bsize * buf.f_blocks;
    }
    if(statfs("/private/var", &buf) >= 0){
        freespace += (long long)buf.f_bsize * buf.f_blocks;
    }
    printf("%lld\n",freespace);
}

Gist: https://gist.github.com/0f5492b9d85f07ec50a2