零长度数组

#include <stdlib.h>
#include <stdio.h>
struct device{
??? int num;
??? int count;
??? int reserve[0];?? //reserve是一个数组名；该数组没有元素；该数组的其实地址紧随结构device之后；这种声明方法可以巧妙的实现C语言里的数组扩展
};

int main()
{
??? struct device * p_dev =
??????? (struct device *) malloc (sizeof(struct device) + sizeof(int)*25);
??? //sizeof(int)*25是数组reserve的具体空间(25个元素)
??? p_dev->reserve[0] = 100;
??? p_dev->reserve[24] = 0;
??? printf("p_dev->reserve[24] = %d\n", p_dev->reserve[24]);
??? printf("sizeof(struct device) = %d\n",sizeof(struct device));

// 将结构体device之后的第一个内容(int值，其实就是reserve[0]的值)赋值给变量a
// int a = *((&(p_dev->count)) + 1);
??? int a = *(&p_dev->count + 1);
??? printf("a = %d\n", a);
}

# ./a.out
p_dev->reserve[0] = 100
p_dev->reserve[24] = 0
sizeof(struct device) = 8
a = 100

-------------------------------------------

p_dev ---> |--------------|
?????????? |int num?????? |
?????????? |int count???? |
?????????? |--------------|????? reserve是数组的名字;
?? reserve |????????????? |????? 由于数组没有元素，该数组在该结构体中分配占用空间
?????????? |????????????? |????? 所以sizeof(struct device) = 8
?????????? |????????????? |
?????????? |????????????? |
?????????? |--------------|