Why is the first random number always the same on some platforms in lua?

Question

Consider the following lua code snippet :

local time = os.time()
for _= 1, 10 do
    time = time + 1
    print('Seeding with ' .. time)
    math.randomseed(time)
    for i = 1, 5 do
        print('\t' .. math.random(100))
    end
end

On a Linux machine, the result is, as expected, random numbers. But it seems that at least on Mac OS X, the first random number after changing the seed is always the same !

I guess this is related to the fact that Lua relies on the C rand() function for generating random numbers, but does anybody have an explanation ?

EDIT: here is an extract of the output of the above code on a linux machine (ie the output is as expected) :

$ lua test.lua
Seeding with 1232472273
    69
    30
    83
    59
    84
Seeding with 1232472274
    5
    21
    63
    91
    27
[...]

On an OS X machine, the first number after "Seeding with ..." was always 66.

Answer 1

Lua's random used to use C's rand(3) and srand(3) functions (see here). UPDATE: newer Lua versions use random(3) where available.

Both the C90 standard and POSIX suggest an cross-platform implementation of rand and srand that isn't the best. It especially lacks randomness in the lower bits.

Some platforms like Linux moved off of the standard recommendation to a better implementation (e.g. random(3)).

OS/X remains true to the classic rand implementation, and Lua inherits it.