Why rand() is generating negative numbers?

I have a function to generate numbers between 0-255 but rand() is also generating negative values. Any advice, please.

char* generateRandomNumbers()
{
    static unsigned char random[9];
    srand(0);
    static int i = 0;
    for(;i<9;i++)
    {
        random[i] = rand() % (256);
    }

    return (char*)random;
}