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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 a volatile object by using a non-volatile value, but the resulting behavior is undefined. According to ISO/IEC 9899:1999The C standard, Section 6.7.3 "Type Qualifiers"[ISO/IEC 9899:2011]

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 62 65 of Appendix J.)

Noncompliant Code Example

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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 it is not possible that i != 0 if i is not volatile.

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This example does not compile on MS Visual Studio 2008. The error message is

error C2440: '=' : cannot convert from 'int **' to 'volatile int **'

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

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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) {
  /* ... */
}

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Related Vulnerabilities

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

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CERT C++ Secure Coding Standard: EXP32-CPP. Do not access a volatile object through a non-volatile reference

ISO/IEC 9899:19992011 Section 6.7.3, "Type qualifiers," and Section 6.5.16.1, "Simple assignment"

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