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cinteger-overflowqsortundefined-behavior

Bug in quicksort example (K&R C book)?

This quicksort is supposed to sort "v[left]...v[right] into increasing order"; copied (without comments) from The C Programming Language by K&R (Second Edition):

void qsort(int v[], int left, int right)
{
    int i, last;
    void swap(int v[], int i, int j);

    if (left >= right)
        return;
    swap(v, left, (left + right) / 2);
    last = left;
    for (i = left+1; i <= right; i++)
        if (v[i] < v[left])
            swap(v, ++last, i);
    swap(v, left, last);
    qsort(v, left, last-1);
    qsort(v, last+1, right);
}

I think there's a bug at

(left + right) / 2

Suppose left = INT_MAX - 1 and right = INT_MAX. Wouldn't this result in undefined behavior due to integer overflow?

Solution

  • Yes, you're right. You can use left - (left - right) / 2 to avoid overflows.