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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 C99 Section behavior. The C Standard, 6.7.3, "Type qualifiers," Paragraph 5:4 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 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

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.

...

Risk Assessment

Accessing a volatile object through a non-volatile reference results in undefined behavior.

Automated Detection

Related Vulnerabilities

Search for Examples of vulnerabilities resulting from the violation of this rule can be found on the CERT website.

References

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[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"