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

Tool	Version	Checker	Description
Compass/ROSE
GCC			Can detect violations of this rule when the -Wcast-qual flag is used
LDRA tool suite		344 S	Fully implemented
PRQA QA-C		0312	Fully implemented

Related Vulnerabilities

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

Related Guidelines

Bibliography