CODEWITHSUNDEEP
c++cpointersstruct
Michel Martel
Michel Martel

Reputation: 11

struct member gets overwritten by garbage value after successful initialization

I've been debugging this function for quite a while now and can't wrap my head around what could be going on with this piece of code.

    void make_points(DocSpec instance, Tree *myTree, Point *p){
int i, j, k, index = 0;

for(i = 0; i < instance.numPt; i++)
    {
        p[i].x = instance.xCoordinates[i];
        p[i].y = instance.yCoordinates[i];
        p[i].parent = myTree[i].parent;
        p[i].num_children = 0;
        for(k = 0; k < MAX_NUM_CHILDREN; k++)
            {
                p[i].child[k] = 0;
            }
        for(j = 0; j < instance.numPt; j++)
            {
                if((i != j) && (myTree[j].parent == i))
                    {
                        p[i].num_children++;
                        p[i].child[index] = j;
                        index++;
                    }
            }
         p[i].overlap_hv = -1;
         p[i].overlap_vh = -1;
         index = 0;
     }

printf("p[1].index = %d;\n", p[1].index);
printf("p[1].x = %d;\n", p[1].x);
printf("p[1].y = %d;\n", p[1].y);
printf("p[1].parent = %d;\n", p[1].parent);
printf("p[1].num_children = %d;\n", p[1].num_children);

printf("p[1].child[8] = {");
index = 0;
for(i = 0; i < MAX_NUM_CHILDREN; i++)
    {
        if(p[1].child[i] != 0 && index == 0)
            {
                printf("%d", p[1].child[i]);
            }
        elseif(p[1].child[i] != 0)
            printf(", %d", p[1].child[i]);
    }
print("};\n");
printf("p[1].overlap_hv = %d;\n", p[1].overlap_hv);
printf("p[1].overlap_vh = %d;\n", p[1].overlap_vh);
}

The output I'm getting after running the function is the following:

p[1].index = 1;
p[1].x = 0;
p[1].y = 90;
p[1].parent = 5;
p[1].num_children = 0;
p[1].child[8] = {1563515760, 1768448814, 945513580, 540876893};
p[1].overlap_hv = 909455739;
p[1].overlap_vh = 892679225;

But it should be:

p[1].index = 1;
p[1].x = 0;
p[1].y = 90;
p[1].parent = 5;
p[1].num_children = 0;
p[1].child[8] = {};
p[1].overlap_hv = -1;
p[1].overlap_vh = -1;

When I ran gdb on my program, I noticed that the values of p[1] are initialized properly, but when 

printf("p[1].x = %d;\n", p[1].x);

is executed - p[1].child[4], p[1].child[5], p[1].child[6], p[1].child[7], p[1].overlap_hv, p[1].overlap_vh all get overwritten with the garbage values.

I have no idea why or how a printf statement could change the values of my struct members.

Any help would be greatly appreciated.

Upvotes: 0

Views: 687

Answers (2)

Michel Martel
Michel Martel

Reputation: 11

Turns out I didn't use the proper typecast when reallocating memory. A quick check with Valgrind led me right to the culprit.

had 

p = (Point*) realloc(p, instance.numPt * sizeof(p));

and this fixed it

p = (Point*) realloc(p, instance.numPt * sizeof(Point));

Thanks to all for the suggestions.

Upvotes: 1

sg7
sg7

Reputation: 6298

My guess is that index is crossing child boundary and j overwrites next struct members p[1].overlap_hv and p[1].overlap_vh: 

                    p[i].child[index] = j;
                    index++;

Upvotes: 0

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