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).
--
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
