How to generate random letters out of random numbers?

Hi,
I create random numbers with a C-function like that:

        FILE *dev_random;
        char rand_num[30];
	int count = 0;

        if ((dev_random = fopen("/dev/urandom","r"))== NULL)
                                print_error("Can't open /dev/random for
reading.");
        fread(&rand_num,1,1,dev_random);
        fclose(dev_random);

        while (rand_num2[count] != '\0'){
                count++;
                printf("%i\n",rand_num2[count]);
                }
        return 0;

So it seems I'm getting numbers in the range of -126 upto 126. How can I
now randomly generate letters out of that?