Random Alphanumeric String Linux Swift 3

Im having 2 problems when trying to generate a random string in Linux with Swift 3.

1. arc4random_uniform is not available in Linux only on BSD. SO i was able to get away with using random() function. And this worked when i was generating random numbers of a variable size (See code below)

   func generateRandomNumber() -> Int
   {
      var place = 1
   
      var finalNumber = 0;
   
   #if os(Linux)
   for _ in 0..<5
   {
       place *= 10
   
       let randomNumber = Int(random() % 10) + 1
   
       finalNumber += randomNumber * place
   }
   #else
   for _ in 0..<5
   {
       place *= 10
   
       let randomNumber = Int(arc4random_uniform(10))
   
       finalNumber += randomNumber * place
   }
   #endif
   
     return finalNumber
   }
   
   

And that WORKS. Edit: it works but it gives me the same number every time :(

2. When trying to generate random alphanumeric string I'm limited to using Swift String and NOT NSSTRING. Linux throws this error

original pre Linux block of code:

   func randomString(_ length: Int) -> String
   {

      let letters : NSString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
      let len = UInt32(letters.length)

      var randomString = ""

      for _ in 0 ..< length {
    let rand = arc4random_uniform(len)
    var nextChar = letters.character(at: Int(rand))
    randomString += NSString(characters: &nextChar, length: 1) as String
      }

       return randomString
    }

And the actual error I get when using above code

    error: cannot convert value of type 'NSString' to type 'String' in coercion
        randomString += NSString(characters: &nextChar, length: 1) as String

modified for linux block of code.

    func randomString(_ length: Int) -> String
    {

let letters : String = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
let len = letters.characters.count

var randomString = ""

#if os(Linux)

    for _ in 0..<length
    {
        let randomValue = (random() % len) + 1

        randomString += "\(letters[letters.index(letters.startIndex, offsetBy: Int(randomValue))])"
    }

    #else
    for _ in 0 ..< length
    {
        let rand = arc4random_uniform(UInt32(len))

        randomString += "\(letters[letters.index(letters.startIndex, offsetBy: Int(rand))])"
    }
    #endif


      return randomString
}          

but this time the error is weird it only says Illegal instruction with no extra information. I ran the docker container in interactive mode and i saw my server running and printing out when calling other functions etc.

but the thing is the function actually WORKS when i ran it in IBMs swift sandbox

and I'm assuming its using linux also. Im very stuck and confused any help would be greatly appreciated.