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。

Extern告诉编译器信任你，这个变量的内存在其他地方声明了，所以它不会尝试分配/检查内存。

因此，您可以编译一个引用extern的文件，但是如果没有在某个地方声明该内存，则不能进行链接。

对于全局变量和库很有用，但是危险，因为链接器不进行类型检查。

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.

In C a variable inside a file say example.c is given local scope. The compiler expects that the variable would have its definition inside the same file example.c and when it does not find the same , it would throw an error.A function on the other hand has by default global scope . Thus you do not have to explicitly mention to the compiler "look dude...you might find the definition of this function here". For a function including the file which contains its declaration is enough.(The file which you actually call a header file). For example consider the following 2 files : example.c

#include<stdio.h>
extern int a;
main(){
       printf("The value of a is <%d>\n",a);
}

example1.c

int a = 5;

现在，当你一起编译这两个文件时，使用以下命令:

步骤1)cc -o ex example.c 步骤2)。/交货

输出结果如下:a的值<5>

GCC ELF Linux实现

其他答案已经涵盖了语言使用方面的观点，所以现在让我们看看它是如何在这个实现中实现的。

c

#include <stdio.h>

int not_extern_int = 1;
extern int extern_int;

void main() {
    printf("%d\n", not_extern_int);
    printf("%d\n", extern_int);
}

编译和反编译:

gcc -c main.c
readelf -s main.o

输出包含:

Num:    Value          Size Type    Bind   Vis      Ndx Name
 9: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    3 not_extern_int
12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND extern_int

System V ABI Update ELF规范“符号表”章节解释:

这个节表索引表示该符号未定义。当链接编辑器将这个目标文件与另一个定义指定符号的文件结合在一起时，这个文件对符号的引用将被链接到实际的定义。

这基本上是C标准赋予extern变量的行为。

从现在开始，制作最终程序是链接器的工作，但是extern信息已经从源代码中提取到目标文件中。

在GCC 4.8上测试。

c++ 17内联变量

在c++ 17中，你可能想要使用内联变量而不是extern变量，因为它们使用简单(可以在头文件中定义一次)并且更强大(支持constexpr)。参见:“const static”在C和c++中是什么意思?

extern变量是在另一个翻译单元中定义的变量的声明(感谢sbi的更正)。这意味着变量的存储空间被分配到另一个文件中。

假设您有两个.c文件test1.c和test2.c。如果你定义一个全局变量int test1_var;在test1.c中你想访问test2.c中的这个变量你必须使用extern int test1_var;在test2.c。

完整的示例:

$ cat test1.c 
int test1_var = 5;
$ cat test2.c
#include <stdio.h>

extern int test1_var;

int main(void) {
    printf("test1_var = %d\n", test1_var);
    return 0;
}
$ gcc test1.c test2.c -o test
$ ./test
test1_var = 5