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It is possible to assign the value of a constant object by using a non-constant value, but the resulting behavior is undefined. According to C99 Section The C Standard, 6.7.3, "Type qualifiers," Paragraph 5:4, paragraph 7 [ISO/IEC 9899:2024], 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 61.

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

It is also a recommended practice not to cast away a {{const}} qualification (\[[EXP05-A. Do not cast away a const qualification]\]), as this makes it easier to modify a {{const}}\-qualified value without warning.

Non-Compliant Code Example

Avoid casting away const qualification because doing so makes it 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

This noncompliant code example allows a constant object to be modified:This non-compliant code example allows a constant value to be modified.

const int
char const **cppipp;
charint *cpip;
charconst constint ci = 'A'42;


cppvoid func(void) {
  ipp = &cpip; /* constraintConstraint violation */
  *cppipp = &ci; /* validValid */
  *cpip = 'B'0;   /* valid Modifies constant i (was 42) */
}

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

Implementation

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Details

If cpp ipp, cp ip, and c and i are declared as automatic (stack) variables, this example compiles without warning on warning with Microsoft Visual C++ .NET (2003) and on MS Visual Studio 2005. In both cases, Studio 2013 when compiled in C mode (/TC) and the resulting program changes the value of c i. Version 3GCC 4.2.2 of the gcc compiler 8.1 generates a warning but compiles. The , and the resulting program changes the value of c i.

If cpp ipp, cp ip, and c and i are declared with static storage duration, this program terminates abnormally for both MS Visual Studio and gcc Version 3.2.2compiles without warning and terminates abnormally with Microsoft Visual Studio 2013, and compiles with warning and terminates abnormally with GCC 4.8.1.

Compliant Solution

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

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

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

If the intent is that the value of c of i is not meant to change, then do not write non-compliant noncompliant code that attempts to modify it.  

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.

References

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Related Guidelines

Key here (explains table format and definitions)

Bibliography

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