gistfile1.m

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long)NULL), ^(void) {
  char line[80];
  FILE *fr;
  NSString *path = @"/path/to/long/file";

  struct stat info;
  stat([path UTF8String], &info);
  off_t lastSize = info.st_size;
  
  fr = fopen([path UTF8String], "rt");
  fseek(fr, 0, SEEK_END);
  
  while (1) {
    stat([path UTF8String], &info);
    off_t nowSize = status.st_size;
    
    if (nowSize > lastSize) {
      lastSize = nowSize;
      
      while (ftell(fr) < nowSize) {
        memset(&line[0], 0, sizeof(line));
        fgets(line, 80, fr);
        NSData *data = [NSData dataWithBytes:line length:sizeof(line)];
        NSString *string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"%@", string);
      }
    }
  }
  
  fclose(fr);
});