GLTessellator crashing

I'v followed a tutorial to get the GLU tesselator working. It works except the interpolation for colors of new points causes a crash after creating a random polygon开发者_StackOverflow中文版(error reading from memory...)

This is my callback where it crashes:

void CALLBACK combineCallback(GLdouble coords[3], GLdouble *vertex_data[4],
         GLfloat weight[4], GLdouble **dataOut)
{
 GLdouble *vertex;
 int i;

 vertex = (GLdouble *) malloc(6 * sizeof(GLdouble));
 vertex[0] = coords[0];
 vertex[1] = coords[1];
 vertex[2] = coords[2];

 //crashes here
 **for (int i = 3; i < 6; i++)
 {
  vertex[i] = weight[0] * vertex_data[0][i] +
   weight[1] * vertex_data[1][i] +
   weight[2] * vertex_data[2][i] +
   weight[3] * vertex_data[3][i];
 }**
 //crashes here

 *dataOut = vertex;

}

I looked at memory when it crashes but can't put my finger on exactly what triggers it. I followed this tutorial: http://www.flipcode.com/archives/Polygon_Tessellation_In_OpenGL.shtml

Thanks

My guess is that vertex_data is wrong. It is the same pointer that you pass to gluTessVertex function. What does that pointer point to?