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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. 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 also undefined behavior 65 in Annex 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:

#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 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 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

Related Vulnerabilities

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

Related Guidelines

Bibliography