Using - C
  
 Source Code
  

    
      #include<stdio.h>

      void main() {
        char str1[1024], str2[1024];
        int i=0, j, length=0;
        printf("Enter String-1:  ");
        gets(str1);
        printf("Enter String-2:  ");
        gets(str2);
        
        while(str1[i] != '\0') {
          length++;
          i++;
        }
        
        for (j = 0; str2[j] != '\0'; ++j, ++length) {
          str1[length] = str2[j];
        }
        str1[length] = '\0';
      
        printf("After Concatenation:  %s",str1);
      }
    

Enter String-1:  Hello
Enter String-2:  World
After Concatenation:  HelloWorld

        
Using - C++
        
 Source Code
        

        
          #include<iostream>
using namespace std;

int main() {
	string str1, str2, concat_result;
	cout<< "Enter String-1:  ";
	getline(cin,str1);
	cout<< "Enter String-2:  ";
	getline(cin,str2);

	concat_result = str1 + str2;

	cout<< "After Concatenation:  "<< concat_result;

	return 0;
}
      

Enter String-1:  Anuj
Enter String-2:  Das
After Concatenation:  Anuj Das