Confused with the char array when scanf

Question

I am confused with one tiny program.

#include <stdio.h>

#define LEN 10

int main()
{
    char str1[LEN] = "\0";
    char str2[LEN] = "\0";

    scanf("%s", str1);
    scanf("%s", str2);

    printf("%s\n", str1);
    printf("%s\n", str2);

    return 0;
}

If my input are:

mangobatao
mangobatao123456

Why should the output be:

123456
mangobatao123456

And not:

mangobatao
mangobatao123456

How has the char array has been allocated in the memory?

Solution

Well, a 10 character char array won't fit "mangobatao", since it has 10 characters - there's no room for the null terminator. That means you've caused undefined behaviour, so anything could happen.

In this case, it looks like your compiler has laid out str2 before str1 in memory, so when you call scanf to fill str2, the longer string overwrites the beginning of str1. That's why you see the end of what you think should be in str2 when trying to print str1. Your example will work fine if you use a length of 100.