...
int queens = 8;
main(argc, argv) char **argv;
{
        int i;
        for(i=0;i<queens;i++){ col[i] = qpos[i] = 0; }
        for(i= -queens;i<queens;i++){ up[i] = down[i] = 0; }
        extend(0);
}
extend(n)
{
        int c;
        for(c=0;c<queens;c++){
                if(!col[c] && !up[n+c] && !down[n-c]){
                        qpos[n] = c;
                        if(n+1 >= queens)
                                printqueens();
                        else {
                                col[c] = up[n+c] = down[n-c] = 1;
                                extend(n+1);
                                col[c] = up[n+c] = down[n-c] = 0;
                        }
                }
        }
}