C Vertex Array wrong values

Question

I have this declarations in my cc.h

Vertex *graphVertices2;

typedef struct
{
    float XYZW[4];
    float RGBA[4];
} Vertex;

And in my cc.c i do the following:

float vert [] = {306, 319, 360, 357, 375, 374, 387, 391, 391, 70, 82, 94, 91, 108, 114, 125, 127, 131};
            graphVertices2 = transformIntoVertex(vert, 18);

Vertex *transformIntoVertex(float *v, int size){
    int i;
    float x_axis = x_0 + (x_Max/size);
    Vertex graphVertices[18];

    for(i = 0; i < size; ++i){      
        graphVertices[i].XYZW[0] = x_axis; // x
        graphVertices[i].XYZW[1] = v[i]; // y
        graphVertices[i].XYZW[2] = 0.0f; // z
        graphVertices[i].XYZW[3] = 1.0f; // w
        if(size <= 9){
        graphVertices[i].RGBA[0] = 1.0f;
        graphVertices[i].RGBA[1] = 0.0f; // g
        graphVertices[i].RGBA[2] = 0.0f; // b
        graphVertices[i].RGBA[3] = 0.0f; // a
        }else{
            graphVertices[i].RGBA[0] = 0.0f; // r
            graphVertices[i].RGBA[1] = 1.0f; // g
            graphVertices[i].RGBA[2] = 0.0f; // b
            graphVertices[i].RGBA[3] = 0.0f; // a
        }
        x_axis = x_axis + x_axis;
        return graphVertices;
    }

But I am getting wrong values when I print graphVertices2. The problem doesn't come from the function I think, I have printed the for loop and everything is with the right values. The values start to get weird at the middle of the vertex. I can't figure it out why.

This line is doing the right attributions:

graphVertices[i].XYZW[1] = v[i];

I have printed it and checked. But at the middle of the vertex, the values get hugely big.

You're returning a local scope variable address, which is no longer valid once you leave `transformIntoVertex()`. Either dynamically allocate the return memory, use a local `static`, or use a global. Oh, and *don't* use a global =P — WhozCraig, Apr 22 '13 at 17:38

score 2 · Accepted Answer · edited Nov 25 '13 at 15:56

2

The problem doesn't come from the function I think,

It does.

Vertex graphVertices[18];
// ...
// do stuff
// ...
return graphVertices;

You're returning an automatic array - which will be out of scope the moment the function returns. Your program invokes undefined behavior, so anything can happen. The usual advice for solving this: make it static (and the read what that keyword does), or malloc()ate some dynamic memory for it (in which case you will also have to free() it after use).

edited Nov 25 '13 at 15:56

BenMorel

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answered Apr 22 '13 at 17:38

Ok. So don you mean, when i create a new Vertex array inside the function, i should use malloc so that the memory reserved for that temporary array is not destroyed after function call? – Nick Spot Apr 22 '13 at 17:49

C Vertex Array wrong values

1 Answers1