Things About 'extern'

Note: All Learned From Here

C和Objective-C的function前面都有个隐含的extern,对于function来说，有没有extern都无所谓，但变量不一样。

declaration 声明可以无数次但定义只能有一次

声明但不定义：no memory allocation for it

extern int var;

// This could be done as many times as you want

// If we prepend extern in variable as default,then the memory for them will not be allocated ever.

definition 定义且声明一个变量：memory allocated

int var;

Examples that work

int var;

int main(void) {

var = 10;

return 0;

}

extern int var;

int main(void) {

return 0;

}

extern int var = 0;  //如果有初始化 那么内存是被分配了的

int main() {

var = 10;

return 0;

}

#include "somefile.h"

extern int var;

int main() {

var = 10;

return 0;

}

// 如果somefile.h里有var的definition，那么是可行的

Examples that don't work

extern int var;

int main(void) {

var = 10;

return 0;

}

// var 被声明了但没有被定义,也就是没有被分配内存,所以赋值行为是错误的

1. Declaration can be done any number of times but definition only once.

2. “extern” keyword is used to extend the visibility of variables/functions().

3. Since functions are visible through out the program by default. The use of extern is not needed in function declaration/definition. Its use is redundant.

4. When extern is used with a variable, it’s only declared not defined.

5. As an exception, when an extern variable is declared with initialization, it is taken as definition of the variable as well.