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The C Standard, subclause 6.7.34, paragraph 6 7 [ISOISO/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 values objects to be modified without generating a warning message.

It is also a recommended practice not to cast away a const qualification (as in 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) because doing so makes it easier to modify a const-qualified value without warning. and STR30-C. Do not attempt to modify string literals for more details.)

Noncompliant Code Example

The following well-formed but noncompliant code example borrowed from subclause 6.5.16.1 of the C Standard allows a constant value This noncompliant code example allows a constant object to be modified:

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

cppvoid func(void) {
  ipp = &cpip; /* Constraint violation */
  *cppipp = &ci; /* Valid */
  *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.

Noncompliant Code Example (Modifying a String Literal)

Similarly to the previous example, the following well-formed but noncompliant code example modifies a constant object after casting away its constness. Compiling the program on a Linux/x64 system does not produce any diagnostics even at high warning levels, but the generated executable program fails at runtime with SIGESGV.

const char s[] = "foo";
int main() {
  *(char*)s = '\0';
}

Implementation-Specific Details

i.

Implementation Details

If ipp, ip, and i are declared as automatic If cpp, cp, and c are declared as automatic (stack) variables, this example compiles without warning on all versions of warning with Microsoft Visual Studio when 2013 when compiled in C mode (/TC) . In both cases, and the resulting program changes the value of c i. GCC 3GCC 4.28.2 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 compiles without warning and terminates abnormally with Microsoft Visual Studio 2013, and GCC 3.2.2compiles 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 noncompliant code that attempts to modify it.  

Risk Assessment

Modifying constant objects through nonconstant references results in 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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