volatile??

what is the difference with

unsigned const volatile data;
and
unsigned volatile data;

Code:

---

unsigned const volatile data;

---

You can't change the value of 'data', but volatile indicates that something else may change its value.

Code:

---

unsigned volatile data;

---

You can change the value of 'data', and volatile indicates that something else may change its value too.

Code:

---

#include <stdio.h>
int main(void)
{
  unsigned const volatile a = 1; /* initialization, ok */
  unsigned      volatile b = 1; /* initialization, ok */
  a = 4; /* assignment, error - 'cannot modify a const object' */
  b = 5; /* assignment, ok */
  while(a && b)
  {
      printf("a = %u, b = %u\n", a, b);
  }
  return 0;
}

---

A lot of uses of "volatile" are with systems that interface directly to hardware, where they're talking to something other than RAM, like a device control register. So, to answer your question, the difference between the 2 is that the "const" means you're accessing a read-only memory address.