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：

1. extern声明时显示指定数组大小，则编译器不会报错
   

   python@ubuntu:~/Documents/c_fundamental/extern_array$ cat provide_array.c
   int iArray[] = {1,2,3,4,5};
   python@ubuntu:~/Documents/c_fundamental/extern_array$ cat call_array.c
   #include<stdio.h>
   
   extern int iArray[5];
   int main(void)
   {
           for(int i = 0; i < 5; i++)
           {
                   printf("%d
   ", iArray[i]);
           }
           printf("size of array: %zu", sizeof(iArray));
   
           return 0;
   }
   python@ubuntu:~/Documents/c_fundamental/extern_array$ gcc provide_array.c  call_array.c  -o extern_array
   python@ubuntu:~/Documents/c_fundamental/extern_array$ ./extern_array
   1
   2
   3
   4
   5
   size of array: 20python@ubuntu:~/Documents/c_fundamental/extern_array$

2. 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

   python@ubuntu:~/Documents/c_fundamental/extern_array$ cat call_array.c
   #include<stdio.h>
   
   extern int iArray[];
   int main(void)
   {
           for(int i = 0; i < 5; i++)
           {
                   printf("%d
   ", iArray[i]);
           }
           printf("size of array: %zu", sizeof(iArray));
   
           return 0;
   }
   python@ubuntu:~/Documents/c_fundamental/extern_array$ gcc provide_array.c  call_array.c  -o extern_array
   call_array.c: In function ‘main’:
   call_array.c:10:37: error: invalid application of ‘sizeof’ to incomplete type ‘int[]’
     printf("size of array: %zu", sizeof(iArray));
                                        ^
   python@ubuntu:~/Documents/c_fundamental/extern_array$