On Mon, Oct 20, 2014 at 12:12:06AM +0200, Bogdan Cristea wrote:
>
> > On 20 Oct 2014, at 00:10, John de la Garza <john@xxxxxxxxx> wrote:
> >
> > On Sun, Oct 19, 2014 at 11:42:26PM +0200, Bogdan Cristea wrote:
> >>
> >>> On 19 Oct 2014, at 23:32, John de la Garza <john@xxxxxxxxx> wrote:
> >>>
> >>> Can anone see why these two pieces of code would behave differently?
> >>> They are used in a mergesort merge
> >>>
> >>> the top one ends up with c (cp points to an offset in c)
> >>> 1 2 3 1
> >>>
> >>> the bottom one (not commented out)
> >>> results in 1 2 3 4 (correct)
> >>>
> >>> /*
> >>> if (alen == 0)
> >>> memcpy(cp, bp, blen);
> >>> else if (blen == 0){
> >>> memcpy(cp, ap, alen);
> >>> }
> >>> */
> >>> if (alen == 0)
> >>> while (blen--)
> >>> *cp++ = *bp++;
> >>> else
> >>> while (alen--)
> >>> *cp++ = *ap++;
> >>>
> >>>
> >>> if you want to read it with more context see below
> >>> ---------------------------------------------------
> >>>
> >>> #include <stdio.h>
> >>> #include <string.h>
> >>> #include <stdlib.h>
> >>>
> >>> void pr_array(int *a, int len)
> >>> {
> >>> int i;
> >>> for (i = 0; i < len; i++)
> >>> printf("%d ", a[i]);
> >>> printf("\n");
> >>> }
> >>> void merge(int *a, int alen, int *b, int blen, int *c)
> >>> {
> >>> int *ap = a;
> >>> int *bp = b;
> >>> int *cp = c;
> >>>
> >>> while (alen && blen) {
> >>> if (*ap <= *bp) {
> >>> *cp++ = *ap++;
> >>> alen--;
> >>> } else{
> >>> *cp++ = *bp++;
> >>> blen--;
> >>> }
> >>> }
> >>> if (alen == 0)
> >>> memcpy(cp, bp, blen);
> >>> else if (blen == 0){
> >>> memcpy(cp, ap, alen);
> >>> }
> >>> /*
> >>> if (alen == 0)
> >>> while (blen--)
> >>> *cp++ = *bp++;
> >>> else
> >>> while (alen--)
> >>> *cp++ = *ap++;
> >>> */
> >>> }
> >>>
> >>> void msort(int *arr, int len)
> >>> {
> >>> int mid = len / 2;
> >>> int llen = mid;
> >>> int rlen = len - mid;
> >>> int *l = malloc(llen);
> >>> int *r = malloc(rlen);
> >>> int i;
> >>>
> >>> if (len <= 1)
> >>> return;
> >>> for (i = 0; i < llen; i++)
> >>> l[i] = arr[i];
> >>> for (i = mid; i < mid + rlen; i++)
> >>> r[i-mid] = arr[i];
> >>> msort(l, llen);
> >>> msort(r, rlen);
> >>> merge(l, llen, r, rlen, arr);
> >>> free(l);
> >>> free(r);
> >>> }
> >>> int main()
> >>> {
> >>> int arr[] = {4,3, 2, 1};
> >>> int len = sizeof(arr) /sizeof(int);
> >>> int x[] = {1,3,5,7};
> >>> int y[] = {2,4,6,8};
> >>>
> >>> msort(arr, len);
> >>> pr_array(arr, len);
> >>> return(0);
> >>> }
> >>
> >> memcpy does not work for overlapping memory, use memmove instead
> >>
> > Where does it overlap? I found a big problem, I was passing in size
> > instead of size * sizeof(int).
>
> My intention was to point a possible error source, I haven’t looked carefully at your code.
>
thanks
I just ran valgrind and realized I made the same mistake in many places
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