Question

I’m still a noob to C. I use it on a regular basis, but generally don’t use the pointer features of C. What I’m trying to do is pass pointers to structures that already exist in local space inside a function. I have this code:

struct WorldCamera p_viewer;
struct Point3D_LLA p_subj;
struct Point2D_CalcRes p_res;
p_viewer.hfov = 25;
p_viewer.vfov = 25;
p_viewer.p.lat = 10.0f;
p_viewer.p.lon = 10.0f;
p_viewer.p.alt = 100.0f;
p_subj.lat = 9.98f;
p_subj.lon = 10.0f;
p_subj.alt = 100.0f;
// Do something interesting here.
compute_3d_transform(&p_viewer, &p_subj, &p_res, 10000.0f);
// Do something interesting here.

The prototype of compute_3d_transform is this:

void compute_3d_transform(struct WorldCamera *p_viewer, struct Point3D_LLA *p_subj, struct Point2D_CalcRes *res, float cliph);

I want to know if I’ve got this right. I am programming on a small microcontroller which I don’t think has much memory protection. Corrupting one piece of memory could cause odd bugs in future which would be very difficult to debug. I’m trying to avoid the use of the malloc/calloc function as these require a dedicated heap.

Answer 1

Use the addressof operator &.

Ah yes you have it right.

The reason why you have it right is that, for e.g., &p_viewer is a pointer that points to the (for your example, local) variable p_viewer, which is a struct WorldCamera. And compute_3d_transform wants the a pointer to a struct WorldCamera, so your intentions as a caller match the intentions of the callee.

Of course, without seeing the whole intention (which can only be gleaned by reading the program, including compute_3d_transform, or at least its documentation), there might a logic error, but the fact that the types match up is already of great comfort (of course, if the type were void * instead then there are more headaches because the intention is not so clear)