Question

I implement a file’s struct in my program but for some arrays in the struct I don’t know the size. The size of the array is stored in another variable but it’s unknown before the struct is filled in.

struct Vertex {
    float x;
    float y;
    float z;
};
struct myFile {
    ulong nVertices;
    Vertex vertices[nVertices];
};

That gives an error: “error C2065: ‘nVertices’ : undeclared identifier”.

Answer 1

You should store a pointer in your struct:

Vertex *vertices;

Then allocate the memory at runtime:

myFile f;
f.vertices = malloc(nVertices * sizeof(Vertex));
if (f.vertices == 0)
    handle_out_of_memory();

f.nVertices = nVertices;

Remember to free the memory when done:

free(f.vertices);