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Wiki MarkupImmutable objects should be {{const}}\-qualified. Enforcing object immutability using {{const}}\- qualification helps ensures ensure the correctness and security of applications. ISO/IEC PDTR 24772 \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\], for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments. [STR05-A. Prefer making string literals const-qualified] describes a specialized case of this recommendation. TR 24772, for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments [ISO/IEC TR 24772]. STR05-C. Use pointers to const when referring to string literals describes a specialized case of this recommendation.

Adding const qualification may propagate through a program; as you add const, qualifiers , become still more become necessary. This phenomenon is sometimes called "const-poisoning." Const-poisoning poisoning, which can frequently lead to violations of EXP05-AC. Do not cast away a const qualification. While Although const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

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A macro or an enumeration constant may also be used instead of a const-qualified object. DCL06-C. Use meaningful symbolic constants to represent literal values describes the relative merits of using const-qualified objects, enumeration constants, and object-like macros. However, adding a const qualifier to an existing variable is a better first step than replacing the variable with an enumeration constant or macro because the compiler will issue warnings on any code that changes your const-qualified variable. Once you have verified that a const-qualified variable is not changed by any code, you may consider changing it to an enumeration constant or macro, as best fits your design.

Noncompliant Code Example

In this non-compliant noncompliant code example, pi is declared as a float. Although pi is a mathematical constant, its value is not protected from accidental modification.

float pi = 3.14159f;
float degrees;
float radians;
/* ... */
radians = degrees * pi / 180;

Compliant Solution

In this compliant solution, pi is declared as a const-qualified object.:

const float pi = 3.14159f;
float degrees;
float radians;
/* ... */
radians = degrees * pi / 180;

Risk Assessment

Failing to const-qualify immutable objects can result in a constant being modified at runtime.

Automated Detection

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.3.2.1, "Lvalues, arrays, and function designators," Section 6.7.2.2, "Enumeration specifiers," and Section 6.10.3, "Macro replacement"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "CSJ Passing parameters and return values"
\[[Saks 00|AA. C References#Saks 00]\] Dan Saks. [Numeric Literals|http://www.embedded.com/2000/0009/0009pp.htm]. Embedded Systems Programming.  September, 2000.

Related Guidelines

 Bibliography

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Image Added Image Added Image Added02. Declarations and Initialization (DCL)      02. Declarations and Initialization (DCL)       DCL01-A. Do not reuse variable names in subscopes