如何在C函数中改变全局变量？

#include <stdio.h>

int globalVar = 10;

void changeGlobalVar(int* ptr) {
    *ptr = 20;
}

int main() {
    printf("Before change: %d\n", globalVar);
    changeGlobalVar(&globalVar);
    printf("After change: %d\n", globalVar);
    return 0;
}

Before change: 10
After change: 20

#include <stdio.h>

extern int globalVar;

void changeGlobalVar() {
    globalVar = 30;
}

int globalVar = 10;

int main() {
    printf("Before change: %d\n", globalVar);
    changeGlobalVar();
    printf("After change: %d\n", globalVar);
    return 0;
}

Before change: 10
After change: 30

#include <stdio.h>

int globalVar = 10;

void changeGlobalVar(int& ref) {
    ref = 40;
}

int main() {
    printf("Before change: %d\n", globalVar);
    changeGlobalVar(globalVar);
    printf("After change: %d\n", globalVar);
    return 0;
}

Before change: 10
After change: 40