c - 在 C 中初始化数组

Question

下面的 C 程序创建了一个具有固定数量元素的数组。即使我们再分配三个超过最大数组元素数量的值也能正常工作。

#include <stdio.h>

int main(){

//declares a fixed sized array
int balance[5];

//initializing
balance[0] = 1;
balance[1] = 2;
balance[2] = 3;
balance[3] = 4;
balance[4] = 5;
//three more
balance[5] = 6;
balance[6] = 7;
balance[7] = 8;


printf("%d \n", balance[0]);
printf("%d \n", balance[1]);
printf("%d \n", balance[2]);
printf("%d \n", balance[3]);
printf("%d \n", balance[4]);
printf("%d \n", balance[5]);
printf("%d \n", balance[7]);

//and it works fine!

return 0;
}

但是当我们尝试分配三个以上的元素时它会抛出异常

#include <stdio.h>

int main(){

//declares a fixed sized array
int balance[5];

//initializing
balance[0] = 1;
balance[1] = 2;
balance[2] = 3;
balance[3] = 4;
balance[4] = 5;
//three more
balance[5] = 6;
balance[6] = 7;
balance[7] = 8;
//one more again
balance[8] = 9;


printf("%d \n", balance[0]);
printf("%d \n", balance[1]);
printf("%d \n", balance[2]);
printf("%d \n", balance[3]);
printf("%d \n", balance[4]);
printf("%d \n", balance[5]);
printf("%d \n", balance[7]);
printf("%d \n", balance[8]);

//throws an exception!

return 0;
}

但是在使用整数文字声明和初始化数组后，我们可以插入更多元素而不会出错。

#include <stdio.h>

int main(){

//declares and initialize a fixed sized array
int balance[5] = {1,2,3,4,5};

//let's add few more
balance[5] = 6;
balance[6] = 7;

//and even more
balance[7] = 8;
balance[8] = 9;
balance[9] = 10;

printf("%d \n", balance[0]);
printf("%d \n", balance[1]);
printf("%d \n", balance[2]);
printf("%d \n", balance[3]);
printf("%d \n", balance[4]);
printf("%d \n", balance[5]);
printf("%d \n", balance[6]);
printf("%d \n", balance[7]);
printf("%d \n", balance[8]);
printf("%d \n", balance[9]);

//and it works fine!

return 0;
}

C语言出现这种行为的原因是什么？

Answer 1

当您尝试越界访问数组时，您的代码会调用未定义行为，这是不安全的内存，因此结果是未定义的。