C에서 static variable과 static function의 차이를 알고싶습니다.

#include <stdio.h>
#include "other.h"
 
int main(){
    struct ptr_to_static_t ptr_to_static = ftn();
 
    puts(__func__);
    printf("\tstatic_var : %d\n", *ptr_to_static.p_static_var);
    *ptr_to_static.p_static_var = 42;
    (*ptr_to_static.p_static_ftn)(__func__);
 
    ftn();
 
    return 0;
}

#ifndef __OTHER_H__
#define __OTHER_H__
 
struct ptr_to_static_t{
    int *p_static_var;
    void (*p_static_ftn)(const char *);
};
 
struct ptr_to_static_t ftn();
 
#endif

#include <stdio.h>
#include "other.h"
 
static int static_var = -42;
 
static void static_ftn(const char *from){
    printf("\tstatic_ftn called by %s\n", from);
}
 
struct ptr_to_static_t ftn(){
    puts(__func__);
    printf("\tstatic_var : %d\n", static_var);
    static_ftn(__func__);
 
    return (struct ptr_to_static_t){
        .p_static_var = &static_var,
        .p_static_ftn = &static_ftn
    };
}

ftn
	static_var : -42
	static_ftn called by ftn
main
	static_var : -42
	static_ftn called by main
ftn
	static_var : 42
	static_ftn called by ftn