判断ch中的字符是否与str所指串中的某个字符相同；若相同什么也不做。若不同，插在最后

#include <stdio.h>
#include <string.h>
void fun(char *str, char ch )
{   while (  *str && *str != ch ) str++;
if  (  *str != ch  )
{  str [ 0 ] = ch;
str[1] = '\0';
}
}

main( )
{    char  s[81],  c ;
printf( "\nPlease enter a string:\n" );  gets ( s );
printf ("\n Please enter the character to search : " );
c = getchar();
fun(s, c) ;
printf( "\nThe result  is %s\n",  s);
}