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Wiki MarkupAn object that has volatile-qualified type may be modified in ways unknown to the [implementation|BB. Definitions#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|BB. Definitions#undefined behavior]. According to C99 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.3, "Type qualifiers," Paragraph 5: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.

This also applies to objects that behave as if they were defined with qualified types, such as an object at a memory-mapped input/output address.

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Risk Assessment

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; /* May produce a warning diagnostic */
  ipp = (int**) &ip; /* Constraint violation; may produce a warning diagnostic */
  *ipp = &i; /* Valid */
  if (*ip != 0) { /* Valid */
    /* ... */
  }
}

The assignment ipp = &ip is not safe because it 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 *ip != 0 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++ mode (/TP).

GCC 4.8.1 generates a warning but compiles successfully.

Compliant Solution

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

Accessing an object with a volatile-qualified type through a reference with a non-volatile-qualified type is undefined Accessing a volatile object through a non-volatile reference can result in undefined and perhaps unintended program 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"

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 variables