What happens when I range over an uninitialized pointer to array in golang

Question

I have this code

var j *[33]byte

for i := range j {
    fmt.Println(j[i])
}

Now when I run this code I get nil pointer dereference error when I try access values in j. I'm not sure why I was even able to enter the loop in the first place considering my pointer is uninitialized.

I know an uninitialized array has all its values set to their zero value. That is

var a [5]int

Will have a default value of [0, 0, 0, 0, 0].

But I don't understand what golang does when you don't initialize a pointer to an array. Why is range able to range over it even though its nil?

Answer 1

From the Go spec Range Clause:

... For an array, pointer to array, or slice value a, the index iteration values are produced in increasing order...

so as a convenience the Go language is dereferencing the pointer with the intent to iterating over its elements. The fact that the pointer is nil is a simple programming error. If this can occur, one should have a runtime check in place to guard against it.

Static analysis may be able to detect this type of bug ahead of time - but what if the variable j is accessible from another goroutine - how would the compiler know for sure that another goroutine may update it to a non-nil value right before the range loop is reached?