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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 non-volatile lvalue is undefined behavior. The C Standard, subclause 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 Annex 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; /* Constraint violation; also produces warningsmay produce a warning diagnostic */
  *ipp = &i; /* Valid */
  if (*ip != 0) { /* Valid */
    /* ... */
  }

}

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 i *ip != 0 must be false if i the object to which ip points is not volatile.

Implementation Details

This example compiles without warning on Microsoft Visual Studio 2012 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.

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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 and 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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