C :: How To Create Private Fields In A Struct

How can you actually create private fields in a C struct? I got the concept of VTable and inheriting methods aswell overriding them, but private variables can be obtained with static keyword in sourcefile. If you have variables in a struct, you can access them as soon as you got a ref to the struct. The functions are easy:

Code:

struct classA {
pointerToFunction *p;
} static void* thePrivateFoo() { }
void* publicFoo() {
thePrivateFoo(); /* or something like that */
} ...
/*in init code somewhere in the c file */ classAInstance->p = publicFoo;

But I was thinking about the variables... How is this achievable? I was thinking of a struct with only get/set functions and with no datamembers at all. All vars to be static outside the struct. But this kind of destroys the encapsulation.