strnset(): setting n chars of a string

#include<stdio.h>
#include<string.h>
int main()
{
   char str1[20] = "plus2net";
   char str2='*';
   printf("Original string : %s \n", str1);
   strnset(str1,str2,4);
   printf("After  strset() : %s",str1);
   return 0;
}

Original string :plus2net
After strnset() : ****2net

strnset(main_string, set_string,n);