int factorial(int);
int main(void)
{
  char x=5;
  printf("%d!=%d",x,factorial(x));
  return 0;
}

int factorial(int n)
{
  if (n==1)
    return (1);
  else
    return (n*factorial(n-1));
}

char x=5;
int n;
n = x;
//あるいは
n = (int)x;

test.c: In function 'main':
test.c:7:31: warning: passing argument 1 of 'factorial' makes integer from pointer without a cast [-Wint-conversion]
   printf("%d!=%d",x,factorial(&x));
                               ^
test.c:3:5: note: expected 'int' but argument is of type 'char *'
 int factorial(int);
     ^