(Secure) Random string?

In Lua, one would usually generate random values, and/or strings by using math.random & math.randomseed, where os.time is used for math.randomseed.

This method however has one major weakness; The returned number is always just as random as the current time, AND the interval for each random number is one second, which is way too long if one needs many random values in a very short time.

This issue is even pointed out by the Lua Users wiki: http://lua-users.org/wiki/MathLibraryTutorial, and the corresponding RandomStringS receipe: http://lua-users.org/wiki/RandomStrings.

So I've sat down and wrote a different algorithm (if it even can be called that), that generates random numbers by (mis-)using the memory addresses of tables:

math.randomseed(os.time())
function realrandom(maxlen)
    local tbl = {}
    local num = tonumber(string.sub(tostring(tbl), 8))
    if maxlen ~= nil then
        num = num % maxlen
    end
    return num
end

function string.random(length,pattern)
    local length = length or 11
    local pattern = pattern or '%a%d'
    local rand = ""
    local allchars = ""
    for loop=0, 255 do
        allchars = allchars .. string.char(loop)
    end
    local str=string.gsub(allchars, '[^'..pattern..']','')
    while string.len(rand) ~= length do
        local randidx = realrandom(string.len(str))
        local randbyte = string.byte(str, randidx)
        rand = rand .. string.char(randbyte)
    end

    return rand
end

At first, everything seems perfectly random, and I'm sure they are... at least for the current program.

So my question is, how random are these numbers returned by realrandom really?

Or is there an even better way to generate