Invalid read of size 4

Question

Hello there I have the following code:

struct Edge
{
    Node * nodeA, * nodeB;
    int weight;
};
.
.

This is typedef'd to simply Edge elsewhere.

.
.

int cmp(const void *b, const void *a)
{
    Edge * e1 = (Edge*)a;
    Edge * e2 = (Edge*)b;
    printf(" %d++", getWeight(e1));
    printf(" %d++", getWeight(e2));
    if (getWeight(e1) != getWeight(e2))
        return (getWeight(e1) < getWeight(e2)) ? -1 : 1;
    return 0;
}

void sortEdges(Edge ** edgeList, int numEdges)
{
    qsort(edgeList, numEdges, sizeof(Edge*), cmp);
}

Valgrind tells me that I have an Invalid read of size 4, and when I output the value for the the edge weights I get totally unexpected results. I can only conclude that there is something wrong with my cast to an Edge* or that the sizeof is somehow not doing what I think it is. If you need any information please let me know.

Valgrind is generally quite descriptive. It will generally point the exact place of error. Can you post the relevant portion of Valgrind Report? — Jay, Mar 08 '12 at 04:30
Your code looks good to me. I suspect if initialization of `edgeList` has any problems. Btw I recommend you to use `sizeof(edgeList[0])` instead of `sizeof(Edge*)`. The former works regardless of the type of `edgeList`. — nodakai, Mar 08 '12 at 04:34

lmatt · Accepted Answer · 2012-03-08T04:52:55.203

There are two ways to correct your code.
1. if edgeList is a array of (Edge *).

int cmp(const void *b, const void *a)
{
    Edge ** e1 = (Edge**)a;//notice
    Edge ** e2 = (Edge**)b;//notice
    printf(" %d++", getWeight(*e1));//notice
    printf(" %d++", getWeight(*e2));//notice
    if (getWeight(*e1) != getWeight(*e2))//notice
        return (getWeight(*e1) < getWeight(*e2)) ? -1 : 1;//notice
    return 0;
}

void sortEdges(Edge ** edgeList, int numEdges)
{
    qsort(edgeList, numEdges, sizeof(Edge*), cmp);
}

2, if edgeList is a array of Edge.

int cmp(const void *b, const void *a)
{
    Edge * e1 = (Edge*)a;
    Edge * e2 = (Edge*)b;
    printf(" %d++", getWeight(e1));
    printf(" %d++", getWeight(e2));
    if (getWeight(e1) != getWeight(e2))
        return (getWeight(e1) < getWeight(e2)) ? -1 : 1;
    return 0;
}

void sortEdges(Edge * edgeList, int numEdges)//notice
{
    qsort(edgeList, numEdges, sizeof(struct Edge), cmp);//notice
}

Oops my knowledge on plain C had rusted somewhat... Your answer is correct. "The _compar_ argument is a pointer to the comparison function, which is called with two arguments that **point** to the elements being compared." http://pubs.opengroup.org/onlinepubs/009695399/functions/qsort.html — nodakai, Mar 08 '12 at 05:02

Invalid read of size 4

1 Answers1