在c++中进行线程处理，在2d数组中找到最大的条目

我在使用线程函数的2d数组中查找最大条目时遇到了一些问题。我已经粘在屏幕上好几个小时了，我需要一些帮助。

这是代码：

#include <iostream>
#include <sys/time.h>
#include <stdio.h>
#include <stdlib.h>
#include <random>
#include <ctime>
#include <future>
#include <thread>
using namespace std;
// A vanilla random number generator
double genRandNum(double min, double max){
  return min + (rand() / (RAND_MAX / (max - min)));
}
double get_wallTime() {
    struct timeval tp;
    gettimeofday(&tp, NULL);
    return (double) (tp.tv_sec + tp.tv_usec/1000000.0);
} 
void getLargest(double** anArray, double largestEntry, int dimLower, int dimUpper, int dim) {
    for (int i = dimLower; i < dimUpper; i++) {
        for (int  j = 0; j < dim; j++) {
            if (anArray[i][j] > largestEntry) {
                largestEntry = anArray[i][j]; 
            }
        }
    }
}
// Main routine
int main(){
  // Seed the random number generator
  srand( time(NULL));
  // 2D array dimension
  int dim = 30000;
  // Specify max values
  double max = (double) (dim * dim * dim);
  double min = (double) (dim * dim * dim * -1.0);
  double t1 = get_wallTime();
  // Create a 2D array
  double **myArray = new double*[dim];
  for (int i=0; i<dim; i++){
    myArray[i] = new double[dim];
    for (int j=0; j<dim; j++){
      // generate random number
      myArray[i][j] = genRandNum(min, max);
    }
  }
  double largestEntry = 0.0;
  double largestEntry2 = 0.0;
  double largestEntry3 = 0.0;
  double largestEntry4 = 0.0;
  double largestEntry5 = 0.0;
  int portion = dim / 5;
  std::future<void> thread1 = std::async (std::launch::async, getLargest, myArray, largestEntry, 0, portion, dim);
  std::future<void> thread2 = std::async (std::launch::async, getLargest, myArray, largestEntry2, portion, (portion * 2), dim);
  std::future<void> thread3 = std::async (std::launch::async, getLargest, myArray, largestEntry3, (portion * 2), (portion * 3), dim);
  std::future<void> thread4 = std::async (std::launch::async, getLargest, myArray, largestEntry4, (portion * 3), (portion * 4), dim);
  std::future<void> thread5 = std::async (std::launch::async, getLargest, myArray, largestEntry5, (portion *4), dim, dim);
  thread1.get();
  thread2.get();
  thread3.get();
  thread4.get();
  thread5.get();

  if (largestEntry2 > largestEntry) {
     largestEntry = largestEntry2;
  } 
  if (largestEntry3 > largestEntry) {
    largestEntry = largestEntry3;
  }
  if (largestEntry4 > largestEntry) {
    largestEntry = largestEntry4;
  }
  if (largestEntry5 > largestEntry) {
    largestEntry = largestEntry5;
  }
  double t2 = get_wallTime();
  double t3 = t2 - t1;
  cout << " The largest entry is " << largestEntry << endl;  
  cout << "runtime : " <<  t3 << "n";
}

我相信我已经正确地设置了所有内容，但我刚刚学会了如何使用异步，所以我确信有些地方不正确。当运行时，我得到零作为最大条目的输出，这是错误的，并且我得到的运行时间比我预期的要长得多。

这是输出：

 The largest entry is 0
runtime : 13.8261

我认为我将largestEntry错误地传递给线程函数，但我不知道如何修复它。线程函数getLargest应该找到它所提供的数组中最大的条目，当我创建线程时，我会为每个线程分配数组的一部分。

任何人能提供的建议都将不胜感激。