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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 non-volatile value, but the resulting behavior lvalue is undefined . According to behavior. The C Standard, 6.7.4 paragraph 7 [ISO/IEC 9899:1999, Section 6.7.3 "Type Qualifiers" (see also undefined behavior 62 of Appendix J):2024], 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 62.

Noncompliant Code Example

In this noncompliant code example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior .62:

#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 Version 3.2.2 and Version 4.1.3 of the GCC compiler generate a warning but compile successfully.

Compliant Solution

In this compliant solution, ip is declared as 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 reference with a non-volatile reference. This can result in undefined and perhaps unintended program behavior-qualified type is undefined behavior 62.

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Related Vulnerabilities

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

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Related Guidelines

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References

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Image Added Image Added Image Added "Type qualifiers," and Section 6.5.16.1, "Simple assignment" \[[ISO/IEC PDTR 24772|AA. References#ISO/IEC PDTR 24772]\] "HFC Pointer casting and pointer type changes" and "IHN Type system" \[[MISRA 04|AA. References#MISRA 04]\] Rule 11.5EXP31-C. Avoid side effects in assertions      03. Expressions (EXP)      EXP33-C. Do not reference uninitialized memory