let string = "message"
let array = Array(string.utf8)
let bytes = Data(string.utf8)

CALL_C_FUNC(string, Int32(string.count)) //works
CALL_C_FUNC(array, Int32(array.count)) //works
CALL_C_FUNC(bytes, Int32(bytes.count)) //does not work

//instead use:
bytes.withUnsafeBytes { (pBytes: UnsafePointer<UInt8>) in
    CALL_C_FUNC(pBytes, Int32(bytes.count))
}