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In this example, a volatile object is accessed through a non-volatile-qualified reference resulting in undefined behavior.

int main(void) {
  static volatile int **ipp;
  static int *ip;
  static volatile int i = 0;;

  printf("i = %d.\n", i);

  ipp = &ip; //* constraint violation */
  *ipp = &i; //* valid */
  if (*ip != 0) { //* valid
	// i had been changed */
     ...
  }
}

The first assignment ipp = &ip is unsafe because it would allow the following valid code to reference the value of the volatile object i through a the non-volatile qualified reference ip. In this example, the compiler may optimize out the entire if block because there it is not possible that i == 0 if i is not volatile.

Implementation Specific Details

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In this compliant solution the int * , ip is declared as volatile.

int main(void) {
  static volatile int **ipp;
  static volatile int *ip;
  static volatile int i = 0;;

  printf("i = %d.\n", i);

  ipp = &ip; // constraint violation
  *ipp = &i; // valid
  if (*ip != 0) { // valid
    /* i has changed */...
  }

Priority:

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P6 Level:

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L2

Accessing a volatile object through a non-volatile reference results in undefined behavior.

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