Question

I have what looks to me an innocent cycle which iterates on elements of an array whose type is unknown at compile time; my array is named mesh->vertices and is a pointer to void. Depending on the truth value of mesh->textured I need to consider the array differently. Incidentally, the code in the if and the else in the code segment below is similar, but I do need to distinguish two cases.

void TransformMesh(struct Mesh *mesh, struct Matrix4 *t)
{
    for (int i = 0; i < mesh->nVertices; ++i)
    {
        if (mesh->textured)
        {
            struct TexturedVertex *ptr = ((struct TexturedVertex *)mesh->vertices) + i;

            ptr[i].position = MatrixPointMultiply3(t, &ptr->position);
            ptr[i].normal = MatrixPointMultiply3(t, &ptr->normal);
        }
        else
        {
            struct Vertex *ptr = ((struct Vertex *)mesh->vertices) + i;

            ptr[i].position = MatrixPointMultiply3(t, &ptr->position);
            ptr[i].normal = MatrixPointMultiply3(t, &ptr->normal);
        }
    }
}

I guess I created the project with the Automatic Reference Counting option, thinking that it would not have affected C code, but now I feel like I’m wrong (by the way, how can I check which option I chose?).

Well, it looks like this function is doing something wrong with another array, called mesh->triangles, probably freeing it. When I try to use the vector I get an EXC_BAD_ACCESS error:

glDrawElements(GL_TRIANGLES, mesh->nTriangles * 3, GL_UNSIGNED_INT, mesh->triangles);

It looks like iterating on the mesh->vertices elements, casting them and doing the pointer arithmetic, is corrupting the memory. I think my problem is ARC, so I tried to do what described here but with no luck.

EDIT:

The code above was wrong, as pointed out by Conrad Shultz; the following is correct:

            ptr->position = MatrixPointMultiply3(t, &ptr->position);
            ptr->normal = MatrixPointMultiply3(t, &ptr->normal);

Answer 1

I seriously doubt ARC has anything to do with this – ARC only manages Objective-C objects. (It doesn’t even know how to handle Core Foundation types, which leads to the requirement for using the __bridge... keywords.)

I’m struggling to understand your code. Admittedly, I don’t do a great deal of straight C programming, but I don’t get what you’re trying to do by adding i to ptr, which is presumably the pointer arithmetic of which you speak.

Are you trying to just access the ith struct TexturedVertex in mesh->vertices? If so, just use your ptr[i] construct as written.

It looks to me like you are doing arithmetic such that ptr ends up pointing to the ith struct TexturedVertex, then by accessing ptr[i] you are reading i elements past the ith struct TexturedVertex. If nVertices refers to the count of vertices (as would seem logical, given the name and C array conventions), you are then reading past the end of vertices, a classic buffer overflow error, which would unsurprisingly lead to EXC_BAD_ACCESS and all sorts of other fun errors.