c++,multithreading,sorting,recursion , EXC_BAD_ACCESS error occurring when running recursive merge sort method in a thread


EXC_BAD_ACCESS error occurring when running recursive merge sort method in a thread

Question:

Tag: c++,multithreading,sorting,recursion

I'm having trouble with my C++ code in xCode. I've tried debugging and I can't fix it. I know it's roughly when my mergeSort() method calls my mergeNumbers() method. I know it's not the method itself because I've run the method without threading and it works just fine. It's when I try to use it in the thread that I'm getting a problem.

I tried looking online and the closest thing I could find to my problem was that it may be a problem with my method calling an object that was deleted or no longer exists. I would appreciate any feedback on this. Thank you.

#include <cstdlib>
#include <pthread.h>
#include <stdio.h>
#include <fstream>
#include <iostream>
#include <string>


using namespace std;
void openFile();
void* mergeSort(void *threadid);
void mergeNumbers(int *myArray, int minCount, int maxCount, int midCount);
void mergeSort(int *myArray, int minCount, int maxCount);
void trial2();

int array1[500000];
int array2[500000];
int array3[250000];
int array4[250000];
int array5[250000];
int array6[250000];


int main(int argc, char *argv[]) {

    openFile();
    //trial1();
    trial2();
    return 0; 
}

void trial2(){
    int i, t1;
    void *status;
    pthread_t thread1, thread2, thread3, thread4;
    t1 = pthread_create(&thread1, NULL, mergeSort, (void *)i);
    pthread_join(thread1, &status);
}

void* mergeSort(void *threadid){
    long tid = (long)threadid;
    cout << "I am performing the Merge Sort" << endl;
    mergeSort(array6, 0, 249999);
    pthread_exit(NULL);
}

void mergeSort(int *myArray, int minCount, int maxCount)
{
    int midCount;
    if (minCount < maxCount)
    {
        midCount=(minCount+maxCount)/2;
        mergeSort(myArray,minCount,midCount);
        mergeSort(myArray,midCount + 1,maxCount);

        mergeNumbers(myArray, minCount, maxCount, midCount);
    }
    return;
}

void mergeNumbers(int *myArray, int minCount, int maxCount, int midCount)
{
    int newArray[maxCount + 1];
    int arrayNum = minCount;
    int num1 = minCount;
    int num2 = midCount + 1;


    while (num1 <= midCount && num2 <= maxCount)
    {
        if (myArray[num1] < myArray[num2])
        {
            newArray[arrayNum] = myArray[num1];
            num1++;
            arrayNum++;
        }
        else
        {
            newArray[arrayNum] = myArray[num2];
            num2++;
            arrayNum++;
        }
    }

    while (num1 <= midCount)
    {
        newArray[arrayNum] = myArray[num1];
        num1++;
        arrayNum++;
    }

    while (num2 <= maxCount)
    {
        newArray[arrayNum] = myArray[num2];
        num2++;
        arrayNum++;
    }

    for (num1 = minCount; num1 < arrayNum; num1++)
    {
        myArray[num1] = newArray[num1];
    }
}

Answer:

You problem originates in this line:

int newArray[maxCount + 1];

You are trying to allocate ~250000 ints on the stack (on most 32 bit platforms it will take ~1MB of memory). Your thread's stack may not be able to do this.

By the way, you should not do this - creating arrays of size known at runtime should be done via heap memory allocation:

int* newArray = (int*)malloc(sizeof(int) * (maxCount + 1));

and at the end of the function:

free(newArray);

The best solution, however, would be to use std::vector:


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