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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 non-volatile lvalue is undefined behavior. The C Standard, 6.7.4 paragraph 7 [ISO/IEC 9899:2024], 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 65.

Noncompliant

Do not cast away a volatile qualification on a variable type. Casting away the volatile qualification permits the compiler to optimize away operations on the volatile type, thereby negating the use of the volatile keyword in the first place.

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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; 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 one to access a object through 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 -qualified type is undefined behavior.

Automated Detection

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.3, "Type qualifiers," and Section 6.5.16.1, "Simple assignment"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "HFC Pointer casting and pointer type changes"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 11.5

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added Image AddedEXP31-C. Do not modify constant values      03. Expressions (EXP)       EXP33-C. Do not reference uninitialized memory