Question

This might be a pretty basic question but I am a newbie in C so please bear with me.

I am going through some documentation for a firmware written in C where I came across this:

typedef struct {
                 int x;
                 int arr[10];
               } sx;

Then I have a function where a pointer to that type is being passed.

int f(sx xtype *sp, int i){
                            return sp -> arr[i];
                          }

I am not sure where about two things:
From my Java experience I think the first snippet is equivalent to a superclass much like Object class in Java and then the second snippet is using a subclass of sx. However I am not sure. Can anyone please confirm?
From my little C experience so far, I think more easy-to-understand way to pass a pointer to the type would be as follows:

typedef struct xtype {
                 int x;
                 int arr[10];
                 } sx;

int f(sx *sp, int i){
                     return sp -> arr[i];
                    }

But I just suspect I am missing something from this picture.
Thanks in advance!

Answer 1

This is unusual syntax. I suspect that xtype is a macro defined somewhere and could look something like this

#define xtype const

At any rate, the declaration of a variable sx xtype *sp does not say that sp refers to a subtype xtype of sx or this is the worst pre-processor magic I’ve seen in a long time.

---

As for your second example, this doesn’t model sub-classing, either. What this:

typedef struct A {
  /* ... */
} B;

says is that struct A is the same as B. It’s a typedef: It just defines a new type, a short rephrased version of the longer name.

void func(struct A);
void func(B); // exactly equivalent

It has nothing to do with inheritance.