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
   

    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\n", 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$

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

    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\n", 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$