invalid use of void expression

Question

void dd(char* c)
{
int i=0;int j;
while (c[i]) i++;
i=i-1;
for(j=0;j<i/2;j++)
swap(c[j],c[i-j]);
}
int main()
{
char c[]="123";
printf("%s",dd(c));
}

Answer 1

printf("%s",dd(c));
->
dd(c);
printf("%s",c);