extern 数组---不能通过sizeof计算数组大小

extern int a[];  //size of array

extern数组时，链接器并不知道数组的大小信息；

When compiling b.c, the compiler doesn't know the size of a - you haven't told it. All it knows that an int array called a exists in some other module. The linking process resolves object addresses, but not sizes.

如果extern声明数组的时候没有显示告诉链接器数组大小，链接器就不会知道数组大小，因此使用sizeof计算时编译器就会报错。

有以下两种方式extern，其中有一种会报错：

extern声明时显示指定数组大小，则编译器不会报错

 1 python@ubuntu:~/Documents/c_fundamental/extern_array$ cat provide_array.c
 2 int iArray[] = {1,2,3,4,5};
 3 python@ubuntu:~/Documents/c_fundamental/extern_array$ cat call_array.c
 4 #include<stdio.h>
 5 
 6 extern int iArray[5];
 7 int main(void)
 8 {
 9         for(int i = 0; i < 5; i++)
10         {
11                 printf("%d
", iArray[i]);
12         }
13         printf("size of array: %zu", sizeof(iArray));
14 
15         return 0;
16 }
17 python@ubuntu:~/Documents/c_fundamental/extern_array$ gcc provide_array.c  call_array.c  -o extern_array
18 python@ubuntu:~/Documents/c_fundamental/extern_array$ ./extern_array
19 1
20 2
21 3
22 4
23 5
24 size of array: 20python@ubuntu:~/Documents/c_fundamental/extern_array$

extern声明时未显示声明数组大小，则调用sizeof计算数组大小编译会报错

 1 python@ubuntu:~/Documents/c_fundamental/extern_array$ cat call_array.c
 2 #include<stdio.h>
 3 
 4 extern int iArray[];
 5 int main(void)
 6 {
 7         for(int i = 0; i < 5; i++)
 8         {
 9                 printf("%d
", iArray[i]);
10         }
11         printf("size of array: %zu", sizeof(iArray));
12 
13         return 0;
14 }
15 python@ubuntu:~/Documents/c_fundamental/extern_array$ gcc provide_array.c  call_array.c  -o extern_array
16 call_array.c: In function ‘main’:
17 call_array.c:10:37: error: invalid application of ‘sizeof’ to incomplete type ‘int[]’
18   printf("size of array: %zu", sizeof(iArray));
19                                      ^
20 python@ubuntu:~/Documents/c_fundamental/extern_array$