First call of rand() without seed() --> same number?

Question

I am looking through ac program that is available for several OSs and produces inconsistent results among platforms for the exact same data. Looking because I lack some libraries to compile and debug it on my system.

I noticed that it calls rand() during initialization without calling srand(), probably because it needs only 1 random value. Now, I was wondering how does rand() behave in this case? Does it takes the operating system initial value of the random library ? I am currently hoping that this might explain the different results per platform, but I lack background knowledge to rand() and what happens if it is called without srand().

If it always returns an default-init value of the OS, it would explain why different platforms produces different results.

Thx for any input and sorry for the theoretic question - no code ^^.

Answer 1

From rand() 's manpage:

If no seed value is provided, the rand() function is automatically seeded with a value of 1.

So, in a given system, it will always give you the same sequence. However, even if the seed is the same, different platforms may implement different random generators, and thus yield different sequences.