An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Referencing a volatile object by using a nonvolatile value results in undefined behavior. The C Standard, subclause 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 undefined behavior 65.

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; /* 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 not safe because it 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 i != 0 must be false if i 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++ mode (/TP).

GCC 4.8.1 generates a warning but compiles 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 an object through a nonvolatile reference and 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