why does this code show the following output?

Question

code:
#include <stdio.h>
#include <string.h>

int main()
{
char s1[ ] = "The grey fox";
char s2[ ] = " jumped.";

strcat(s1, s2);
printf("%s\n", s1);
printf("Length of s1 is %d\n", strlen(s1));
strcpy(s1, s2);
printf("s1 is now %s \n", s1);

return 0;
}
output:
The grey fox jumped.
Length of s1 is 20
s1 is now jumped.
*** stack smashing detected ***: <unknown> terminated
Aborted (core dumped)

Answer 1

char s1[ ] = "The grey fox";
s1 is char array of 13 bytes
when you call
strcat(s1, s2);
you do buffer overflow because your need 21 byte to store concatenated string but s1 can contain only 13.

declare s1 as
char s1[50] = "The grey fox";
to reserve space for the enlarged string.

Answer 2

test