合并排序(pthreads）C++

我对pthreads有点陌生，我正在尝试创建一个对100万个随机生成的整数进行排序的程序。我似乎对线程失去了一点控制。当第一次运行时，代码只生成一个线程，但当随后运行该线程时，线程数量会失控。由于我真的不知道问题出在哪里，所以我提供了下面的代码。

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <iostream>
#define N           8          /* # of thread */
#define NUM_INTS    10000      //ideally should be able to sort 1,000,000
int int_list[NUM_INTS];
/* structure for array index
 * used to keep low/high end of sub arrays
 */
typedef struct Arr {
    int low;
    int high;
} ArrayIndex;
void merge(int low, int high) {
    int mid = (low+high)/2;
    int left = low;
    int right = mid+1;
    int list_b[high-low+1];
    volatile int i, cur = 0;
    while((left <= mid) && (right <= high)) {
        if (int_list[left] > int_list[right])
            list_b[cur++] = int_list[right++];
        else
            list_b[cur++] = int_list[right++];
    }
    while(left <= mid)
        list_b[cur++] = int_list[left++];
    while(right <= high)
        list_b[cur++] = int_list[left++];
    for (i = 0; i < (high-low+1) ; i++)
        int_list[low+i] = list_b[i];
}
void * mergesort(void *a){
    ArrayIndex *pa = (ArrayIndex *)int_list;
    int mid = (pa->low + pa->high)/2;
    ArrayIndex aIndex[N];
    pthread_t thread[N];
    aIndex[0].low = pa->low;
    aIndex[0].high = mid;
    aIndex[1].low = mid+1;
    aIndex[1].high = pa->high;
    if (pa->low >= pa->high)
        return 0;
    volatile int i;
    for(i = 0; i < N; i++)
        pthread_create(&thread[i], NULL, mergesort, &aIndex[i]);
    for(i = 0; i < N; i++)
        pthread_join(thread[i], NULL);
    merge(pa->low, pa->high);
    pthread_exit(NULL);
}
int main(){
    volatile int i;
    struct timeval start_time, end_time;
    srand(getpid());
    for(i=0; i<NUM_INTS; i++)
        int_list[i] = rand();
    ArrayIndex ai;
    ai.low = 0;
    ai.high = NUM_INTS/sizeof(int_list[0])-1;
    pthread_t thread;
    pthread_create(&thread, NULL, mergesort, &ai);
    pthread_join(thread, NULL);
    return 0;
}

gdb输出：

(gdb) run
Starting program: /.../sort.o 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff6fd5700 (LWP 25801)]
[Thread 0x7ffff6fd5700 (LWP 25801) exited]
Computation Time: 38006 micro-seconds.
[Inferior 1 (process 25797) exited normally]
(gdb) run
Starting program: /.../sort.o 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff6fd5700 (LWP 25804)]
[New Thread 0x7ffff67d4700 (LWP 25805)]
[New Thread 0x7ffff5fd3700 (LWP 25806)]
[New Thread 0x7ffff57d2700 (LWP 25807)]
[New Thread 0x7ffff4fd1700 (LWP 25808)]
[New Thread 0x7fffef7fe700 (LWP 25811)]
[New Thread 0x7fffeeffd700 (LWP 25810)]
...
[New Thread 0x7ffeca6ec700 (LWP 26148)]
Program received signal SIGINT, Interrupt.
[Switching to Thread 0x7ffee8728700 (LWP 26088)]
__GI___nptl_create_event () at events.c:25
25  events.c: No such file or directory.