c语言学习笔记—关于在头文件中定义extern变量

“1.h”

#include 
extern int a;

“1.c”

#include 
#include "1.h"
int a = 3;
int main(int argc, char const *argv[])
{
 printf("%d\n", a);
 f();
 printf("%d\n", a);
 return 0;
}

“2.c”

#include "1.h"
void f()
{
 a = 5;
}

结果
gcc 1.c 2.c -o main
./main
3
5