Why the behavior of same logic code is different?

Why below code is throwing stack smashing detected if str1=”Overflow” & str2=”stack” but not if str1=”stack” & str2=”Overflow” ?

#include <iostream>
#include<cstring>
using namespace std;
void string_Concat (char *ptr1, const char *ptr2)
{
  int length1 = strlen (ptr1);
  int length2 = strlen (ptr2);
  int i, j;

  char *temp = ptr1;
  ptr1 = new char[length1 + length2 + 1];
  ptr1 = temp;

  for (i = length1, j = 0; ptr2[j] != '
#include <iostream>
#include<cstring>
using namespace std;
void string_Concat (char *ptr1, const char *ptr2)
{
int length1 = strlen (ptr1);
int length2 = strlen (ptr2);
int i, j;
char *temp = ptr1;
ptr1 = new char[length1 + length2 + 1];
ptr1 = temp;
for (i = length1, j = 0; ptr2[j] != '\0'; i++, j++)
ptr1[i] = ptr2[j];
ptr1[i] = '\0';
}
int
main ()
{
char str1[] = "Overflow";
char str2[] = "stack";
string_Concat (str1, str2);
std::cout << str1 << std::endl;
return 0;
}
'; i++, j++)
    ptr1[i] = ptr2[j];

  ptr1[i] = '
#include <iostream>
#include<cstring>
using namespace std;
void string_Concat (char *ptr1, const char *ptr2)
{
int length1 = strlen (ptr1);
int length2 = strlen (ptr2);
int i, j;
char *temp = ptr1;
ptr1 = new char[length1 + length2 + 1];
ptr1 = temp;
for (i = length1, j = 0; ptr2[j] != '\0'; i++, j++)
ptr1[i] = ptr2[j];
ptr1[i] = '\0';
}
int
main ()
{
char str1[] = "Overflow";
char str2[] = "stack";
string_Concat (str1, str2);
std::cout << str1 << std::endl;
return 0;
}
';
}

int
main ()
{
  char str1[] = "Overflow";
  char str2[] = "stack";
  string_Concat (str1, str2);
  std::cout << str1 << std::endl;
  return 0;
}

output :
Overflowstack
*** stack smashing detected ***: terminated

Here even if input strings are changed it should not give stack smash issue as we are allocating the sufficient space.

This is all kinds of wrong. You have multiple problems.

First, this code:

char str1[] = "...";

This allocates just enough space to store the string plus the trailing null byte. If you try appending to it, you're eventually going to run into whatever data follows it. This is BAD. You flat out can't.

Then you do this code:

char *temp = ptr1;
ptr1 = new char[length1 + length2 + 1];
ptr1 = temp;

What happens here. temp points to the value of ptr1. Then you allocate new space for ptr1 -- okay, that's cool. You then destroy the pointer and set it back to temp -- the original ptr1. This is a memory leak and accomplishes nothing at all.

The right way to do this, of course, is to use C++ strings. But you're just learning, so the second-right way to do this is almost what you have.

Allocate space the way you did (sort of), and copy both strings to it, and then return that value. Like this:

char * stringConcat(const char * str1, const char * str2) {
     // get the lengths
     char * newStr = new char[length1 + length2 + 1];
     // append the two input strings to newStr kind of like what you did
     return newStr;
}

And in main, you would do keep the returned pointer and print it kind of like you did.