Convert byte array of ASCII numeric characters to int using GO language

@Volker is right in his comment, you don't have a binary number in your array, you have an ASCII string. Yet, you try to decode it as binary. Note there's no need to validate any input (maybe except the length) if you were dealing with binary number, as all single byte values are valid.

@Ainar-G gave you a way of converting ASCII number into integer.

Compare these two approaches: (http://play.golang.org/p/_wufZ4P_aE)

buf := []byte{54, 54, 49, 49}

x, _ := strconv.Atoi(string(buf))
fmt.Println(x)

This prints 6611; but look at this:

var y int32
_ = binary.Read(bytes.NewReader(buf), binary.BigEndian, &y)
fmt.Println(y)

This prints 909521201, so exactly what you got (and didn't expect). As a side note, you're manually decoding it as BigEndian, so this is not "such a simple conversion" at the end, because there're some more factors to consider.

Your handcrafted conversion from ASCII would look more or less as follows:

var x int32
for _, c := range in {
    x = x*10 + int32(c - '0')
}
return x

But using strconv is the way to go.

If all you need is to convert a simple positive int value that comes in a []byte, like in this example []byte{'6', '6', '1', '1'} or []byte{54, 54, 49, 49} (which is the same), a very simple for loop on the []byte adding to an int, will do the trick, just like this:

var (
    myInt, i int
    myBytes = []byte{'6', '6', '1', '1'}
    v byte
)
for ; i < len(myBytes); i++ {
            v = myBytes[i] - '0'
            myInt *= 10
            myInt += int(v)
}

...and that's all.

See the full working code in the Playground

Convert your bytes to string and use strconv.Atoi.

b := []byte{54, 54, 49, 49}
s := string(b)
i, err := strconv.Atoi(s)
if err != nil {
    panic(err)
}
fmt.Println(i)

Playground: http://play.golang.org/p/NiobWHZ9gd

Adding to @tomasz's answer, if you want to store a number in a binary format and don't care about endianess you can use pointers:

b := []byte{239, 190, 173, 222}
v := *(*uint32)(unsafe.Pointer(&b[0]))
fmt.Printf("0x%X\n", v)
fmt.Printf("%v", *(*[4]byte)(unsafe.Pointer(&v)))

playground