Best way to generate NSData object with random bytes of a specific length?

Question

If I create a new NSData object of a specific size using dataWithBytes:length:, what is the most efficient way to create the input bytes (20 Mb worth) of random characters, preferably without reading the data in from a file? I need a unique buffer of a specific size each time.

Thanks

Answer 1

You can create a 20*2^20b NSData object, then append a random 4 byte integer to it 20*2^20/4 times with arc4random(). I believe you need to include stdlib.h (via Generating random numbers in Objective-C).

#include <stdlib.h>

-(NSData*)create20mbRandomNSData
{
  int twentyMb           = 20971520;
  NSMutableData* theData = [NSMutableData dataWithCapacity:twentyMb];
  for( unsigned int i = 0 ; i < twentyMb/4 ; ++i )
  { 
    u_int32_t randomBits = arc4random();
    [theData appendBytes:(void*)&randomBits length:4];
  }
  return theData;
}

Answer 2

void * bytes = malloc(numberOfBytes);
NSData * data = [NSData dataWithBytes:bytes length:numberOfBytes];
free(bytes);

The bytes are not 'random', but will contain garbage values (whatever was on the heap before this was run). The advantage being its fast and the code is concise.

Answer 3

Here's a 3-liner swift version:

Swift 2

let length = 2048
let bytes = [UInt32](count: length, repeatedValue: 0).map { _ in arc4random() }
let data = NSData(bytes: bytes, length: bytes.count * sizeof(UInt32))

Swift 3

let bytes = [UInt32](repeating: 0, count: length).map { _ in arc4random() }
let data = Data(bytes: bytes, count: length)

Answer 4

You might consider using CCRandomGenerateBytes function from CommonCrypto to generate random data. Like:

func generateBytes(length : Int) throws -> NSData? {
    var bytes = [UInt8](count: length, repeatedValue: UInt8(0))
    let statusCode = CCRandomGenerateBytes(&bytes, bytes.count)
    if statusCode != CCRNGStatus(kCCSuccess) {
        return nil
    }
    return NSData(bytes: bytes, length: bytes.count)
}