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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 Referencing a volatile object by using a nonvolatile value, but the resulting behavior non-volatile lvalue is undefined behavior. The C Standard, Section 6.7.3 4 paragraph 7 [ISO/IEC 9899:20112024], 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 noncompliant code example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior:

#include <stdio.h>
 
void func(void) {
  static volatile int **ipp;
  static int *ip;
  static volatile int i = 0;

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

  ipp = &ip; /* producesMay warningsproduce ina modernwarning compilersdiagnostic */
  ipp = (int**) &ip; /* constraintConstraint violation, also produces warnings; may produce a warning diagnostic */
  *ipp = &i; /* validValid */
  if (*ip != 0) { /* validValid */
    /* ... */
  }
}

The assignment ipp = &ip is unsafe not safe because it would allow allows 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 *ip != 0 if i must be false if the object to which ip points is not volatile.

Implementation Details

This example compiles without warning on Microsoft Visual Studio 2013 when compiled in C mode (/TC) but causes errors when compiled in C++ .NET mode (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 **'

/TP).

GCC 4.8.1 generates a warning but compiles 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:

#include <stdio.h>

void func(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) {
    /* ... */
  }

}

Risk Assessment

Casting away volatile allows access to Accessing an object with a volatile-qualified type through a nonvolatile reference. This can result in undefined and perhaps unintended program reference with a non-volatile-qualified type is undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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