c - Can I treat a struct like an array?

Question

I have a struct for holding a 4D vector

struct {
    float x;
    float y;
    float z;
    float w;
} vector4f

And I'm using a library that has some functions that operate on vectors but take float pointers as their arguments.

Is it legal to call something like doSomethingWithVectors( (float *) &myVector)?

Answer 1

It might work but it is not portable, the compiler is free to align things so that one float does not neccessarily immediately follow the other.