How to use static

#include <stdio.h>

void add()
{
    static int i = 5;
    i++;
    printf("i in add = %d\n", i);
}

void sub()
{
    static int i = 10;
    i--;
    printf("i in sub = %d\n", i);
}

void main(void)
{
    int i = 1;
    add();
    sub();
    printf("i in main = %d\n", i);
    add();
    sub();
    printf("i in main = %d\n", i);
}
执行结果：

i in add = 6
i in sub = 9
i in main = 1
i in add = 7
i in sub = 8
i in main = 1
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