Why doesn’t append work every time? [scary bug]

What’s up with the append function?

a := []byte("ba")

a1 := append(a, 'd')
a2 := append(a, 'g')

fmt.Println(string(a1)) // bag
fmt.Println(string(a2)) // bag

Answer

If there is room for more elements, append reuses the underlying array. Let's take a look:

a := []byte("ba")
fmt.Println(len(a), cap(a)) // 2 32

This means that the slices a, a1 and a2 will refer to the same underlying array in our example.

To avoid this, we need to use two separate byte arrays.

const prefix = "ba"

a1 := append([]byte(prefix), 'd')
a2 := append([]byte(prefix), 'g')

fmt.Println(string(a1)) // bad
fmt.Println(string(a2)) // bag

The scary case: It “worked” for me

In some Go implementations []byte("ba") only allocates two bytes, and then the code seems to work: the first string is "bad" and the second one "bag".

Unfortunately the code is still wrong, even though it seems to work. The program may behave differently when you run it in another environment.

See How to append anything for more about the built-in append function in Go.

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