EXP40-C. Do not modify constant objects

The C Standard, subclause 6.7.3, paragraph 6 [ISO/IEC 9899:2011], 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.

(See also undefined behavior 64 in Appendix J of the C Standard.)

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 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, code example allows a constant object to be modified:

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

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

The first assignment is unsafe because it would allow the valid code that follows to attempt to change the value of the const object i.

Implementation Details

If ipp, ip, and i are declared as automatic variables, this example compiles without warning with Microsoft Visual Studio 2012 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.

If ipp, ip, and i are declared with static storage duration, this program compiles without warning and terminates abnormally with Microsoft Visual Studio 2012, and compiles with warning and terminates abnormally with GCC 4.8.1.

Compliant Solution

The compliant solution depends on the intent of the programmer. If the intent is that the value of i is modifiable, then it should not be declared as a constant, as in this compliant solution:

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

void func(void) {
  ipp = &ip; /* Constraint violation */
  *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 undefined behavior.

Related Vulnerabilities

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

Related Guidelines

Bibliography