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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.

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Non-Compliant Code Example

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

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 Details

This example compiles without warning on Microsoft Visual C++ .NET (2003) and on MS Visual Studio 2005. 

This example does not compile on MS Visual Studio 2008. The error message is

error C2440: '=' : cannot convert from 'int **' to 'volatile int **'

Version 3.2.2 and Version 4.1.3 of the GCC compiler generate a warning, but they compile.

Compliant Solution

In this compliant solution, ip is declared as volatile.

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) {
  /* ... */
}

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Risk Assessment

Accessing a volatile object through a non-volatile reference can result in undefined and perhaps unintended program behavior.

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