How to initialize the size of an array-CodePudding

int main()
{
    char s1[]={0};
    char s2[]={0};
    scanf("%s",s1);
    scanf("%s",s2);
    printf("%s",s2);
}

My question： When type some letter into S1 and S2 ,S2 will copy my S1. Why is this problem and can you initialize an array without identifying its size.

CodePudding user response：

The [] means that the compiler will pick the size after the amount of items provided in the initializer list. In this case you have one item 0 so the arrays get size one. You cannot change the size afterwards. Therefore your code has undefined behavior bugs - you allocated too little memory and the scanf calls will write out-of-bounds of the arrays.

In case you need to change an array size in run-time, you need to either use dynamic memory allocation with malloc or use a variable-length array (VLA).

CodePudding user response：

You can get the size of an array at run time too by declaring it in a variable and then getting it as a input using scanf.

But in your program you have already declared the array so the size of array is also fixed as 1.