01  #include <stdio.h>
02  #include <string.h>
03  
04  int main()
05  
06  {
07   char s1[80];
08   char s2[80];
09   unsigned int len;
10  
11   printf("Enter the first string :");
12   gets(s1);
13   printf("Enter the second string :");
14   gets(s2);
15  
16    /* compute how many charaters will actually fit */
17   len = 79- strlen(s1);
18   strncat(s1,s2,len);
19   puts(s1);
20   
21   return 0;
22  }

 OUTPUT :
 
 Enter the first string :Hello
 
 Enter the second string :World
 
 HelloWorld