How can I convert an int64 into a byte array in go?

You can use this too:

var num int64 = -123456789

b := []byte(strconv.FormatInt(num, 10))

fmt.Printf("num is: %v, in string is: %s", b, string(b))

Output:

num is: [45 49 50 51 52 53 54 55 56 57], in string is: -123456789

Converting between int64 and uint64 doesn't change the sign bit, only the way it's interpreted.

You can use Uint64 and PutUint64 with the correct ByteOrder

http://play.golang.org/p/wN3ZlB40wH

i := int64(-123456789)

fmt.Println(i)

b := make([]byte, 8)
binary.LittleEndian.PutUint64(b, uint64(i))

fmt.Println(b)

i = int64(binary.LittleEndian.Uint64(b))
fmt.Println(i)

output:

-123456789
[235 50 164 248 255 255 255 255]
-123456789