Reading large files from NSInputstream is not working in iOS

- (void)readAllData { 
   
  if ( _readData == nil) { 
  _readData = [[NSMutableData data] retain]; 
  } 
   
  while ([[ _session inputStream] hasBytesAvailable]) 
  { 
  unsigned int bytesRead = 0; 
  bytesRead = [[ _session inputStream] read:buf maxLength: @"1024"]; 
   
  if (bytesRead) { 
   
  NSMutableString * _string = [NSMutableString stringWithString:@""]; 
  for (int i = 0; i < _readData.length; i++) { 
  unsigned char _byte; 
  [_readData getBytes:&_byte range:NSMakeRange(i, 1)]; 
  i f ( _byte >= 32 && _byte < 127) { 
  [ _string appendFormat:@"%c", _byte]; 
  } else { 
  [ _string appendFormat:@"[%d]", _byte]; 
  } 
  } 
   
  [ _readData appendBytes:(const void *)buf length:bytesRead]; 
   
  } 
   
  } 
   
  NSString *string=[NSString stringWithFormat:@"%d", _readData.length]; 
  UIAlertView *sesView = [[UIAlertView alloc] initWithTitle:@"readDatalength" 
  message:string 
  delegate:self 
  cancelButtonTitle:nil otherButtonTitles:@"OK" , nil]; 
  [sesView show]; 
  }