Multithreading Seg fault

[sadelbrid]

I am new to multithreading and I tried to implement it using pthreads on my merge sort. mergeSort4 recursively divides the given array into two halves and sort them. Instead of calling mergeSort4 back to back, I create two new threads. Anyway the program is seg faulting. When debugging it says its seg faulting when I call pthread_join(). Now I don't fully understand how to join threads. I don't want merge2 to be called until both threads have terminated. Am I not doing that correctly?

EDIT: The program works fine without using threads, so I am convinced my problem is in mergeSort4 when managing the 2 threads.

Code:

void merge2(int *a, int f1, int l1, int f2, int l2)
{
	int smallest;
	int first = f1;
	int size = l2-f1 + 1;
	int *temp = new int[size];
	int i = 0;
	while(l1-f1 >= 0 || l2-f2 >=0)
	{
		if(l1-f1 < 0)smallest = a[f2++];//first is empty
		else if(l2-f2 < 0)smallest = a[f1++];//second is empty
		else{ //both non empty
			if(a[f1] < a[f2]) smallest = a[f1++];
			else smallest = a[f2++];
		}
		temp[i++] = smallest;
	}

	//transfer
	for(int j = 0; j<size; j++){
		a[first+j] = temp[j];
	}
	delete [] temp;
}

void *mergeSort4(void *args)
{	
	struct merge_args *mergeargs = (struct merge_args*)args;
	//Divide
	if(mergeargs->last - mergeargs->first > 1)
	{
		int midpoint = (mergeargs->first + mergeargs->last)/2;
		//Split into two new threads
		pthread_t t1, t2;
		struct merge_args mergeargs1, mergeargs2;
		mergeargs1.a = mergeargs->a;
		mergeargs1.first = mergeargs->first;
		mergeargs1.last = midpoint;

		mergeargs2.a = mergeargs->a;
		mergeargs2.first = midpoint+1;
		mergeargs2.last = mergeargs->last;

		//CREATE THREADS
		pthread_create(&t1, NULL, mergeSort4, (void*)&mergeargs1);
		pthread_create(&t2, NULL, mergeSort4, (void*)&mergeargs2);
                //Wait for threads to terminate?
		pthread_join(t1, NULL);
		pthread_join(t2, NULL);
		//Merge
		merge2(mergeargs->a, mergeargs->first, midpoint, midpoint+1, mergeargs->last);		
	}
	else if(mergeargs->last - mergeargs->first == 1)
	{
		if(mergeargs->a[mergeargs->first] > mergeargs->a[mergeargs->last])
			std::swap(mergeargs->a[mergeargs->first], mergeargs->a[mergeargs->last]);
	}
}

[koothkeeper]

First of all, please check your return codes.

Also, why are you creating your threads from within the same thread? Perhaps you need to call pthread_create() in main or some other function.