尝试从动态分配的二维数组 c++ 中读取值并给出 munmap_chunk() 时出现分段错误：尝试删除它时

Question

I'm trying to measure the time taken for insertion sort (avg, best, and worst for certain a size, n times) and storing all results in a 2D array

Here is the code for it:

#include <iostream>
#include <sys/time.h>

void insertionSort(int* array, int size) {
   for (int i = 1; i < size; i++) {
    std::cout << "insetion flag " << i  << std::endl;
    int key = array[i];
    int j = i - 1;
    while (key < array[j] && j >= 0) {
        array[j + 1] = array[j];
        j--;
    }
    array[j + 1] = key;
  }
}

void getRandomArray(int* avg, int* best, int* worst, int size) {
srand((unsigned)time(0));
    for (int i = 0; i < size; i++) {
      //std::cout << "rand flag " << i  << std::endl;
      avg[i] = (rand() % 100) + 1;
    }
    for (int i = 0; i < size; i++) {
        //std::cout << "copy flag " << i  << std::endl;
        best[i] = avg[i];
    }
    insertionSort(best, size);
    for (int i = 0, j = size; i < size; i++, j--) {
        //std::cout << "rev flag " << i  << std::endl;
        worst[j] = best[i];
     }
 }

 double getComplx(int* arry, int size) {
      struct timeval *start, *end;

      std::cout << "comp flag " << std::endl;

      gettimeofday(start, NULL);
      insertionSort(arry, size);
      gettimeofday(end, NULL);

      double timeTaken = (end->tv_sec - start->tv_sec) * 1000000 + (end->tv_usec - start->tv_usec);
      return timeTaken;
  }

  int main(int argc, char** argv) {
      int size = atoi(argv[1]);
      int times = atoi(argv[2]);
      int* avg   = new int[size];
      int* best  = new int[size];
      int* worst = new int[size];

      double** resTable = new double*[times];
      for (int i = 0; i < times; i++) {
          std::cout << i << std::endl;
          resTable[i] = new double[3];
      }

      std::cout << "flag 1" << std::endl;
      for (int i = 0; i < times; i++) {
          getRandomArray(avg, best, worst, size);

          std::cout << "flag 2" << std::endl;
          resTable[i][0] = getComplx(avg, size);
          std::cout << " avg ("<<i<<") (0) " << resTable[i][0] << std::endl;
          resTable[i][1] = getComplx(best, size);
          std::cout << " best ("<<i<<") (0) " << resTable[i][1] << std::endl;
          resTable[i][2] = getComplx(worst, size);
          std::cout << " worst ("<<i<<") (0) " << resTable[i][2] << std::endl;
          printf("|%9.0f  |%9.0f  |%9.0f  |", resTable[i][0], resTable[i][1], resTable[i][2]);
          std::cout << " : loop no : " << i << std::endl;
     }

    std::cout << "after loop" << std::endl;

    delete []avg;
    std::cout << "after avg" << std::endl;
    delete []best;
    std::cout << "after best" << std::endl;
    delete []worst;
    std::cout << "after worst" << std::endl;
    for (int i = 0; i < times; i++) {
      std::cout << "after " << i << std::endl;
      delete[] resTable[i];
   }
   delete []resTable;
   std::cout << "last flag" << std::endl;
   return 0;
}

In the main function I have a dynamically created array and store all data in it but it gives a segmentation fault when it reaches in last row to store data.

So I tried to skip storing in last row and it worked but this time when deleting at the end it again gives me the error "munmap_chunk() : invalid pointer"

I have checked other questions related to this but couldn't find what I am doing wrong here.

I know we can do it better with vectors but I am just trying to get a better understanding of the inner workings of pointers and memory allocation.

Answer 1

You've got two instances of reading/writing outside of allocated memory bounds, and two instances of using an uninitialized pointer.

First, in insersionSort :

while (key < array[j] && j >= 0) {

You use j to index the array before checking if the value is positive. This results in reading before the start of the array. You want to check j first:

while ( j >= 0 && key < array[j]) {

Then you have this in getRandomArray:

for (int i = 0, j = size; i < size; i++, j--) {

You start j at size , then use it to index the array. This writes past the end of the array. You want to start one element earlier:

for (int i = 0, j = size - 1; i < size; i++, j--) {

Finally, there's getComplx :

 double getComplx(int* arry, int size) {
      struct timeval *start, *end;

      std::cout << "comp flag " << std::endl;

      gettimeofday(start, NULL);
      insertionSort(arry, size);
      gettimeofday(end, NULL);

      double timeTaken = (end->tv_sec - start->tv_sec) * 1000000 + (end->tv_usec - start->tv_usec);
      return timeTaken;
  }

The gettimeofday function expects a pointer to a struct timeval , but in both cases you pass it an uninitialized pointer. Rather than making start and end pointers, make them instances of struct timeval and pass their addresses:

 double getComplx(int* arry, int size) {
      struct timeval start, end;

      std::cout << "comp flag " << std::endl;

      gettimeofday(&start, NULL);
      insertionSort(arry, size);
      gettimeofday(&end, NULL);

      // also change -> to .
      double timeTaken = (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec);
      return timeTaken;
  }