How does calling srand more than once affect the quality of randomness?

Question

This comment, which states:

srand(time(0)); I would put this line as the first line in main() instead if calling it multiple times (

Answer 1

Most of the other answers are saying exactly what the question already stated: multiple calls to srand with the same second will produce the same seed. I believe the actual question is the same one that I had, which is: why would it be bad to call srand multiple times, even if it was with a different seed every time?

I can think of three reasons:

1. People are not clear in their language and they actually mean srand should not be called multiple times with time() if you want different sequences of random numbers.

2. It's cryptographically bad because every seed passed to srand is not itself a random number (well, it's probably not). Meaning, every srand is injecting a chance for someone to guess that seed and therefore predict your stream of pseudo-random numbers.

3. It can mess up the distribution of pseudo-random numbers. @vsoftco's answer gave me a clue. If you call srand once, rand can be designed to give you a uniform distribution of pseudo-random numbers over its lifetime. If you call srand in the middle, however, you'll throw off that uniform distribution because it would "start over" with a new seed.

So, if you don't care about any of that, I would think it's okay to call srand more than once. In my case, I want to call it at the start of my program, but call it again after a fork() because the seed is apparently shared across child processes, and I want each child process to have its own sequence of pseudo-random numbers.

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Going back to why it's cryptographically bad, it's easier to guess a seed if it's something like time() because a bad actor can try to guess the time it was seeded. That is why calling srand at the start of a program might be better, because it could be less likely that someone would guess that time as well as, say, when a server request was initiated.

But I would surmise that even passing nanoseconds would be cryptographically dangerous if there's a chance the underlying clock doesn't have that kind of precision. Imagine, for example, that you call srand(get_time_in_ns()) and the underlying clock only returns time to the nearest millisecond.

Now, I'm no crypto expert in any way, but this leads me to wonder if it would be safer than current-time to pass the output of a different pseudo-random generator as seeds to multiple srand calls? For example, can you call each srand with a number from Linux's /dev/random? (I imagine you might want to do that if you want a safer seed than the current time but still want to use rand() so you don't have the overhead of reading from the kernel every time.)