Why below code wont generate a random string every-time it is run?

Program uses rand function to generate random index. But it always produces same string no matter what! Why this wont work?

rand_string()
{
        int a[10];
        int i;
        char b[] = "qwertyuiopasdfghjklzxcvbnm1234567890QWERTYUIOPASDFGHJKLZXCVBNM";
        printf("%d", sizeof(b)/sizeof(b[0]));


        for(i=0;i<10;i++)
        {
        int gen_rand = rand() % 63; 
                a[i] = b[gen_rand];
        }

        for(i=0;i<10;i++)
                printf("%c", a[i]);
}

Thanks!