Which one does NOT concatenate two String s1 and s2? code example

Example 1: concatenate two strings in c

#include <stdio.h>
#include <string.h>
int main()
{
  char a[1000], b[1000];

  printf("Enter the first string\n");
  gets(a);

  printf("Enter the second string\n");
  gets(b);

  strcat(a, b);

  printf("String obtained on concatenation: %s\n", a);

  return 0;
}

Example 2: program to concatenate two strings in c

// this program is to contenate strings without using string function

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

void concatenate(char *str1, char *str2)
{
    int i = strlen(str1), j = 0;

    while (str2[j] != '\0')
    {
        str1[i] = str2[j];
        i++;
        j++;
    }

    str1[i] = '\0';  // declaring the end of the string
}

int main()
{
  char string1[20], string2[20];
  gets(string1);
  gets(string2);
  
  concatenate(string1, string2);
  
  printf("String 1 : ");
  puts(string1);
  
  return 0;
}