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.

对于xc8，必须小心声明变量 在每个文件中都是相同的类型， 在一个文件中声明int型，在另一个文件中声明char型。 这可能会导致变量的破坏。

这个问题在大约15年前的一个微芯片论坛上得到了优雅的解决 /*参见“http:www.htsoft.com”/ /“论坛/ / showflat.php /猫/ 0 /数字/ 18766 / / /页面/ 0 # 18766”

但这种联系似乎不再起作用了……

所以我很快就会解释一下; 创建一个名为global.h的文件。

在其中声明以下内容

#ifdef MAIN_C
#define GLOBAL
 /* #warning COMPILING MAIN.C */
#else
#define GLOBAL extern
#endif
GLOBAL unsigned char testing_mode; // example var used in several C files

现在在文件main.c中

#define MAIN_C 1
#include "global.h"
#undef MAIN_C

这意味着在main.c中变量将被声明为unsigned char。

现在在其他文件中简单地包括global.h将 将其声明为该文件的extern。

extern unsigned char testing_mode;

但是它将被正确地声明为unsigned char。

旧的论坛帖子可能解释得更清楚一点。 但在使用编译器时，这是一个真正的潜在问题 这允许您在一个文件中声明一个变量，然后在另一个文件中将其声明为不同的类型。这些问题与 也就是说，如果你在另一个文件中将testing_mode声明为int类型 它会认为它是一个16位的变量，并覆盖ram的其他部分，可能会破坏另一个变量。很难调试!

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>

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

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是用于声明变量本身驻留在另一个翻译单元中的关键字。

因此，您可以决定在一个转换单元中使用一个变量，然后从另一个转换单元访问它，然后在第二个转换单元中将其声明为extern，符号将由链接器解析。

如果不将其声明为extern，则会得到两个名称相同但完全不相关的变量，并且变量的多个定义存在错误。