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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 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; /* 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 -qualified type is undefined behavior.

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	pointer-qualifier-cast-volatile pointer-qualifier-cast-volatile-implicit	Supported indirectly via MISRA C 2012 Rule 11.8
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-EXP32	Fully implemented
Clang	Include Page Clang_V Clang_V	-Wincompatible-pointer-types-discards-qualifiers
Compass/ROSE
Coverity	Include Page Coverity_V Coverity_V	MISRA C 2012 Rule 11.8	Implemented
Cppcheck Premium	Include Page Cppcheck Premium_V Cppcheck Premium_V	premium-cert-exp32-c	Partially implemented
GCC	Include Page GCC_V GCC_V		Can detect violations of this rule when the -Wcast-qual flag is used
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C0312, C0562, C0563, C0673, C0674	Fully implemented
Klocwork	Include Page Klocwork_V Klocwork_V	CERT.EXPR.VOLATILE.ADDR CERT.EXPR.VOLATILE.ADDR.PARAM CERT.EXPR.VOLATILE.

...

PTRPTR	Fully implemented
LDRA tool suite	Include Page LDRA_V LDRA_V	344 S	Partially implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-EXP32-a	A cast shall not remove any 'const' or 'volatile' qualification from the type of a pointer or reference
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rule EXP32-C	Checks for cast to pointer that removes const or volatile qualification (rule fully covered)
RuleChecker	Include Page RuleChecker_V RuleChecker_V	pointer-qualifier-cast-volatile pointer-qualifier-cast-volatile-implicit	Supported indirectly via MISRA C 2012 Rule 11.8

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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)

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Other Languages

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References

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Bibliography

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