In xc/programs/Xserver/os/auth.c we have this code:

/* A random number generator that is more unpredictable
   than that shipped with some systems.
   This code is taken from the C standard. */

static unsigned long int next = 1;

static int
xdm_rand(void)
{
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768;
}

static void
xdm_srand(unsigned int seed)
{
    next = seed;
}

void
GenerateRandomData (int len, char *buf)
{
    static int seed;
    int value;
    int i;

    seed += GetTimeInMillis();
    xdm_srand (seed);
    for (i = 0; i < len; i++)
    {
        value = xdm_rand ();
        buf[i] ^= (value & 0xff00) >> 8;
    }

    /* XXX add getrusage, popen("ps -ale") */
}

#endif /* XCSECURITY */

which is not a very good random generator.