C++ 代码的分段错误（核心转储）

Question

I wrote code for multithreaded sort application in C++ that does merge sort with array of random number but I couldn't test this because there's "segmentation fault (core dumped)". I think there's problem in main function but I have no idea for which part of code have problem.

#include <iostream> 
#include <cstdlib>
#include <pthread.h>

#define MAX 20
#define THREAD_MAX 2

using namespace std;

int a[MAX]; //array for test
int part = 0;

void merge(int low, int mid, int high)
{
    //merge function for merge 2 parts
}

void merge_sort(int low, int high)
{ 
    //merge sort function
}

void* merge_sort(void* arg)
{
    //thread function for multithreading
}


int main()
{
    for (int i = 0; i < MAX; i++)
        a[i] = rand() % 100;

    pthread_t threads[THREAD_MAX];

    for (int i = 0; i < THREAD_MAX; i++)
        pthread_create(&threads[i], NULL, merge_sort, (void*)NULL);

    for (int i = 0; i < 4; i++)
        pthread_join(threads[i], NULL);

    merge(0, (MAX / 2 - 1) / 2, MAX / 2 - 1);
    merge(MAX / 2, MAX / 2 + (MAX - 1 - MAX / 2) / 2, MAX - 1);
    merge(0, (MAX - 1) / 2, MAX - 1);

    // displaying sorted array 
    cout << "Sorted array: ";
    for (int i = 0; i < MAX; i++)
        cout << a[i] << " ";

    return 0;
}

I tried to test on linux and used this command for compile

g++ -pthread filename.cpp

Answer 1

#define THREAD_MAX 2

    pthread_t threads[THREAD_MAX];

    for (int i = 0; i < THREAD_MAX; i++)
        pthread_create(&threads[i], NULL, merge_sort, (void*)NULL);

    for (int i = 0; i < 4; i++)
        pthread_join(threads[i], NULL);

Creating two threads but waiting for 4 threads to join??? Accessing threads[2] will give you core dump.

Try to generate debug binary with -g option, enable core dump on your system and analyze core dump with gdb debugger. gdb will show you exact line that is causing the core dump.