A difference between Seed and Rand.Seed

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The golang docs says that

Seed, unlike the Rand.Seed method, is safe for concurrent use.

The rand.Seed is actually from math/rand package, but what is Seed? If Seed is another function then it's not present in math/rand so it's unclear from where that function comes from?

update: I'm exploring the demo program where in main we execute

rand.Seed(time.Now().UnixNano())
go process(...)
go process(...)

where the process is determined like

func process(...) {
    time.Sleep(time.Duration(rand.Intn(30)) * time.Second)
    ...
}

We are using the same seed in two different threads, so is such using of rand.Seed considered to be a thread non-safe?

go random
1 Answers

There is a rand.Seed() function, and there is a Rand.Seed() method. Your quote originates from the documentation of the Rand.Seed() method.

Global functions of the math/rand package operate on a global rand.Rand instance. If you check the source code of rand.Seed():

func Seed(seed int64) { globalRand.Seed(seed) }

The global functions are safe for concurrent use, so all other packages can use it (in a shared manner). The global rand.Rand instance is provided for convenience, you can use it "out-of-the box" without any preparation (except the need to properly seed it) and without any synchronization.

Instances of rand.Rand are not safe for concurrent use, each goroutine that needs a rand.Rand for deterministic random sequences should create one, and seed it appropriately. Or if a rand.Rand is to be shared between multiple goroutines, explicit synchronization is required.

Pros of using the global rand.Rand (via the global functions) are: (1) ease of use (it's implicitly shared with everyone) and (2) no synchronization is needed.

Pros of creating and using a custom rand.Rand instance: (1) it's faster (it's not synchronized implicitly) and (2) you are in control of who has access to it, so you can use it to repeat pseudo-random sequences (you can't do that with the global instance as "anyone" may use it concurrently with you).

Edit:

We are using the same seed in two different threads, so is such using of rand.Seed considered to be a thread non-safe?

You only call rand.Seed once, so actually it doesn't even matter if it's thread safe or not, it is not called concurrently. If rand.Seed() would be called from multiple goroutines concurrently, only then would it matter whether it's safe for concurrent use. And as stated earlier in my answer: "The global functions are safe for concurrent use..."

What you do call from multiple goroutines concurrently is rand.Intn(), but again, it's safe to do that.

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