quicksort for integers, segmentation fault in rand()

Question

I'm writing a quicksort algorithm for integers and I get a strange segfault error in srand functions. Here is the code from sort.h:

int distributePivot (int *a, int left, int pivot, int right) {
    int i, j;
    if (pivot != right)
        swapInt(&pivot, &right);
    i = left;
    j = right - 1;
    while (i < j) {
        while (i < j && a[i] <= a[right])
            i++;
        while (j > i && a[j] >= a[right])
            j--;
        if (i < j)
            swapInt(&a[i], &a[j]);
    }
    if (i < right)
        swapInt(&a[i], &a[right]);
    return i;
}

void intArrayQuickSort (int *a, int left, int right) {
    int pivot;
    if (left < right) {
            pivot = rand() % (right - left +1) + left;
        pivot = distributePivot(a, left, pivot, right);
        intArrayQuickSort (a, left, pivot -1);
        intArrayQuickSort (a, pivot, right);
    }
}

And here is the calling from sort-test.c:

    srand(time(NULL));
    intArrayQuickSort(temp, 0, n - 1);

Where temp is a pointer to integer.

And here is the error I get when i execute it in gdb:

    Program received signal SIGSEGV, Segmentation fault.
    0x00007ffff77e9884 in rand () from /lib64/libc.so.6

Can you please help me?

Thank you very much.

EDIT: This is the swapInt function:

void swapInt (int *a, int *b) {
    int aux = *a;
    *a = *b;
    *b = aux;
}

Answer 1

There is an error in the program logic.
Eg
in main
array = [1,2]
call intArrayQuickSort(array, 0, 1);// a:array, left:0, right:1
in intArrayQuickSort
pivot = 1 //provisional result of rand() % (right - left +1) + left;
call distributePivot(a, 0, 1, 1)
in distributePivot
not swap (pivot, right) because pivot == righit
i = 0 //left
j = 0 //right - 1
not execute while block because i == j
execute swap (a[i],a[right]) because i < right // 0 < 1
//a = [2, 1] //!!NG
return 0
//Already illegal state
in intArrayQuickSort
pivot = 0 ;//from return value : 0
call intArrayQuickSort (a, 0, -1);//left:0, pivot -1 :-1
no operation return
call intArrayQuickSort (a, 1, 1);//pivot + 1:1, right : 1
no operation return in main
result:a = [2, 1] //NG!

Answer 2

Actually I've found a solution, but I don't know why it works. The second recursive call should be:

intArrayQuickSort (a, pivot + 1, right);

It makes sense for the algorithm, but I can't understand why the error is with rand(). Any explanation?

Answer 3

I think a modified version of

int distributePivot (int *a, int left, int pivot, int right) {
    int i, j;
    if (pivot != right)
        swapInt(&a[pivot], &a[right]);
    i = left;
    j = right - 1;
    while (1) {
        while (i < right && a[i] < a[right])
            i++;
        while (left <= j && a[j] >= a[right])
            j--;
        if (i < j)
            swapInt(&a[i], &a[j]);
        else
            break;
    }
    if(i < right)
        swapInt(&a[i], &a[right]);
    return i;
}
void intArrayQuickSort (int *a, int left, int right) {
    int pivot;
    if (left < right) {
        pivot = rand() % (right - left +1) + left ;
        pivot = distributePivot(a, left, pivot, right);
        intArrayQuickSort (a, left, pivot - 1);
        intArrayQuickSort (a, pivot + 1, right);
    }
}

Answer 4

pivot = rand() % (right - left +1) + left;

Should be:

pivot = left + rand() % (right - left +1);

Or maybe even:

pivot = left +1 + rand() % (right - left);