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 nonvolatile value, but the resulting behavior is undefined. The C Standard, Section 6.7.3 [ISO/IEC 9899:2011], states:

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.

(See also undefined behavior 65 in Appendix J of the C Standard.)

Noncompliant 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; /* produces warnings in modern compilers */
ipp = (int**) &ip; /* constraint violation, also produces warnings */
*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 **'

Versions 3.2.2 and 4.1.3 of the GCC compiler generate a warning but compile successfully.

Compliant Solution

In this compliant solution, ip is declared 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) {
  /* ... */
}

Risk Assessment

Casting away volatile allows access to an object through a nonvolatile reference. This can result in undefined and perhaps unintended program behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography