extern keyword before a symbol (a var or function) tells the linker that it(the source file) uses an external symbol. This can be seen by running nm -a on such an object file (.o) which uses or assigns a value to a extern var (remember to declare a extern symbol on top like this extern int x or still better, use a header file with extern before vars and functions can be without extern; then in main assign a value to it like this x=5;), i find undefined bss info (letter B written) against such an extern var(symbol). This means x is still unresolved and will be resolved when ld is run (during link-time).

why always use extern in headers? If i don't use extern, just declare int x, the declaration becomes sort-of strong and without extern, and this redifines the same variable in every source that includes the header, effectively shadowing the original variable. therefore with just int x in a.h header, I redefine a new global variable x in every source that include this a.h. This var in the source, this without-extern var decl in headers shadows(it doesn't shadow exactly, it's redifining a global variable x in every source code that includes the header with just int x, without extern, when i include such header and try to compile .o from such files, every .o has its own definition of this global variable x which was included in the header without extern, and at the time of linking, I get the error multiple definition of variable or symbol x) an important variable defined somewhere of somewhere else in the source files. Important! it is necessary to use extern before vars in headers. Functions are already extern by-default.

Extern的使用使得一个first.c文件可以完全访问另一个secondc文件中的全局参数。

extern可以在first.c文件中声明，也可以在first.c包含的任何头文件中声明。

Extern关键字与变量一起使用，以将其标识为全局变量。

它还表示可以使用使用extern声明的变量 关键字在任何文件中，尽管它在其他文件中声明/定义。

我使用了一个非常简短的解决方案，允许头文件包含对象的extern引用或实际实现。实际包含该对象的文件只是#define GLOBAL_FOO_IMPLEMENTATION。然后，当我向这个文件添加一个新对象时，它也会显示在那个文件中，而不需要我复制和粘贴定义。

我在多个文件中使用这种模式。因此，为了保持内容尽可能自包含，我只是在每个头文件中重用单个GLOBAL宏。我的标题是这样的:

//file foo_globals.h
#pragma once  
#include "foo.h"  //contains definition of foo

#ifdef GLOBAL  
#undef GLOBAL  
#endif  

#ifdef GLOBAL_FOO_IMPLEMENTATION  
#define GLOBAL  
#else  
#define GLOBAL extern  
#endif  

GLOBAL Foo foo1;  
GLOBAL Foo foo2;


//file main.cpp
#define GLOBAL_FOO_IMPLEMENTATION
#include "foo_globals.h"

//file uses_extern_foo.cpp
#include "foo_globals.h

Extern仅仅意味着变量在其他地方定义(例如，在另一个文件中)。

简而言之，extern意味着变量是在其他模块中定义的，它的地址在链接时是已知的。编译器不会在当前模块中保留内存，并且知道变量类型。要理解extern，至少要有很少的汇编经验。