Why doesn't this work?

Question

I'v tried to solve a memory leak in the GLU callback by creating a global variable but now it dos not draw anything:

GLdouble *gluptr = NULL;

void CALLBACK combineCallback(GLdouble coords[3], GLdouble *vertex_data[4],
                              GLfloat weight[4], GLdouble **dataOut)
{
    GLdouble *vertex;
    if(gluptr == NULL)
    {
        gluptr = (GLdouble *) malloc(6 * sizeof(GLdouble));
    }
    vertex = (GLdouble*)gluptr;
    vertex[0] = coords[0];
    vertex[1] = coords[1];
    vertex[2] = coords[2];


    for (int i = 3; i < 6; i++)
    {

        vertex[i] = weight[0] * vertex_data[0][i] +
            weight[1] * vertex_data[0][i] +
            weight[2] * vertex_data[0][i] +
            weight[3] * vertex_data[0][i];
    }


    *dataOut = vertex;


}

basically instead of doing malloc each time in the loop (thus the memory leak) im using a global pointer, but this doesn't work (drawing to the screen not working) which means dataOut is not receiving the vertex data pointed to by my pointer. Why would using malloc to a pointer created in the function work any different than a global variable?

Thanks

Answer 1

You allocate the data only once -- but GLUtesselator needs more than one set of data at a time!

What you do here, is putting all the vertex data into a single place in memory, where in the original code, you had memory per vertex. GLUtesselator needs more then one vertex to function properly.

You do call

void gluDeleteTess(GLUtesselator *tessobj);

...afterwards, do you?

Answer 2

Most likely the reason is that something outside of your callback is holding on to the returned data across calls to combineCallback(), and subsequent calls to combineCallback() clobber now the data from the older calls.