How do get the array length?

[system]

int x [] = {}; // empty array
char buffer [20];
void setup ()
{
  Serial.begin (115200);
  sprintf (buffer, "%p", x);
  Serial.print ("x@ ");
  Serial.println (buffer);

  sprintf (buffer, "%p", buffer);
  Serial.print ("buffer@ ");
  Serial.println (buffer);
}

void loop ()
{}

Enjoy.