What is the output of this c++ code which has strcat and strspn function?

Question

I have this code. the output of compiler were 11 but I didn't undrestand why?
#include <iostream>
#include<string.h>
using namespace std;

int main() {

char s1[]="abcdefgh";
char s2[]="ijkl";
strcpy(s2,s1);
strcat(s2,s1);
cout<<strspn(s1,s2);
return 0;
}

Answer 1

references

http://www.cplusplus.com/reference/cstring/strcpy/

http://www.cplusplus.com/reference/cstring/strcat/

Answer 2

note one thing strcpy() can copy address not content.

Answer 3

compiler ptoduce 11. when length of destination of strcpy is less than source string,what happened?