EXP32-C. Do not access a volatile object through a non-volatile reference

An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. It is possible to reference a volatile object by using a non-volatile value, but the resulting behavior is undefined. According to C99 Section 6.7.3, "Type qualifiers," Paragraph 5:

If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.

This also applies to objects that behave as if they were defined with qualified types, such as an object at a memory-mapped input/output address.

Non-Compliant Code Example

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 */
     ...
  }
}

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

Implementation Specific Details

This example compiles without warning on Microsoft Visual C++ .NET (2003) and on MS Visual Studio 2005. Version 3.2.2 of the gcc compiler generates a warning but compiles.

Compliant Solution

In this compliant solution, 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;
  *ipp = &i;
  if (*ip != 0) {
    ...
  }

Priority: P6 Level: L2

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

References

• ISO/IEC 9899-1999 Section 6.7.3 Type qualifiers, Section 6.5.16.1 Simple assignment