全局变量

全局变量

• 定义在函数外面的变量是全局变量
• 全局变量具有全局的生存期和作用域
  • 它们与任何函数都无关
  • 在任何函数内部都可使用它们

int f(viod);
int gAll = 12;

int main(){
	printf("in main gAll=%d\n",gAll);
	f();
	printf("agin in main gAll=%d\n",gAll);
	
	return 0;
}

int f(void){
	printf("in f gAll=%d\n",gAll);
	gAll +=2;
	printf("agin in f gAll=%d\n",gAll);
	
	return gAll;
}

in main gAll=12
in f gAll=12
agin in f gAll=14
agin in main gAll=14

全局变量初始

• 没有初始化的全局变量会得到0值
  • 指针会得到NULL值
• 只能用编译时刻已知的值来初始化全局变量
• 它们的初始化发生在main函数之前

#include<stdio.h>

int f(viod);
int gAll;//未初始化，默认为0，指针则默认为NULL 

//int gAll =12;		//error：initializer element is not a compile-time constant
//int g2 = gAll;	//只有在gAll前+ const 才行 
int main(){
	printf("in main gAll=%d\n",gAll);
	f();
	printf("agin in main gAll=%d\n",gAll);
	
	return 0;
}

int f(void){
	printf("in f gAll=%d\n",gAll);
	gAll +=2;
	printf("agin in f gAll=%d\n",gAll);
	
	return gAll;
}

in main gAll=0
in f gAll=0
agin in f gAll=2
agin in main gAll=2

被隐藏的全局变量

• 如果函数内部存在与全局变量同名的变量，则全局变量被隐藏

#include<stdio.h>

int f(viod);
int gAll = 12;

int main(){
	printf("in main gAll=%d\n",gAll);
	f();
	printf("agin in main gAll=%d\n",gAll);
	
	return 0;
}

int f(void){
	int gAll = 1;//函数里 
	{
		int gAll = 0;//段里 
		printf("in daun gAll=%d\n",gAll);
		gAll +=2;
		printf("agin in duan gAll=%d\n",gAll);
	}
	printf("in f gAll=%d\n",gAll);
	gAll +=2;
	printf("agin in f gAll=%d\n",gAll);
	
	return gAll;
}

in main gAll=12
in daun gAll=0
agin in duan gAll=2
in f gAll=1
agin in f gAll=3
agin in main gAll=12