How can I hide the declaration of a struct in C?

Question

In the question Why should we typedef a struct so often in C?, unwind answered that:

In this latter case, you cannot return the Point by value, sinc

Answer 1

In the header file:

typedef struct _point * Point;

After the compiler sees this it knows:

• There is a struct called _point.
• There is a pointer type Point that can refer to a struct _point.

The compiler does not know:

• What the struct _point looks like.
• What members struct _point contains.
• How big struct _point is.

Not only does the compiler not know it - we as programmers don't know it either. This means we can't write code that depends on those properties of struct _point, which means that our code may be more portable.

Given the above code, you can write functions like:

Point f() {
   ....
}

because Point is a pointer and struct pointers are all the same size and the compiler doesn't need to know anything else about them. But you can't write a function that returns by value:

struct _point f() {
  ....
}

because the compiler does not know anything about struct _point, specifically its size, which it needs in order to construct the return value.

Thus, we can only refer to struct _point via the Point type, which is really a pointer. This is why Standard C has types like FILE, which can only be accessed via a pointer - you can't create a FILE structure instance in your code.