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The C Standard, subclause 6.7.34, paragraph 6 7 [ISO/IEC 9899:20112024], states:

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.

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See also undefined behavior

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61.

There are existing compiler implementations that allow const-qualified objects to be modified without generating a warning message.

Avoid casting away const qualification because doing so makes it easier possible to modify const-qualified objects without issuing diagnostics.   (See EXP05-C. Do not cast away a const qualification and STR30-C. Do not attempt to modify string literals for more details.)

Noncompliant Code Example

The following well-formed, but noncompliant, This noncompliant code example allows a constant object to be modified:

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If ipp, ip, and i are declared as automatic variables, this example compiles without warning with Microsoft Visual Studio 2013 when compiled in C mode (/TC) and the resulting program changes the value of i. GCC 4.8.1 generates a warning but compiles, and the resulting program changes the value of i.

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int **ipp;
int *ip;
int i = 42;

void func(void) {
  ipp = &ip; /* Constraint violationValid */
  *ipp = &i; /* Valid */
  *ip = 0; /* Valid */
}

If the intent is that the value of i is not meant to change, then do not write noncompliant code that attempts to modify it.  This may require relying on additional analysis tools to catch constraint violations in cases where the compiler does not emit a diagnostic.   

Risk Assessment

Modifying constant objects through non-constant references results in nonconstant references is undefined behavior 61.

Automated Detection

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)

Bibliography

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