Page History

Immutable (constant values) objects should be declared as const-qualified objects (unmodifiable lvalues), enumerations values, or as a last resort, using #define.

In general, it is preferable to declare immutable values as const-qualified objects rather than as macro definitions. Using a const declared value means that the compiler is able to check the type of the object, the object has scope, and (certain) debugging tools can show the name of the object. Const-qualified objects cannot be used where compile-time integer constants are required, namely to define the:

• size of a bit-field member of a structure
• size of an array
• value of an enumeration constant
• value of a case constant.

If any of these are required, then an integer constant (an rvalue) must be used. For integer constants, it is preferable to use an enum instead of a const-qualified object as this eliminates the possibility of taking the address of the integer constant and does not required that storage is allocated for the value.

Non-Compliant Code Example 1

. Enforcing object immutability using const qualification helps ensure the correctness and security of applications. ISO/IEC 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 necessary. This phenomenon is sometimes called const poisoning, which can frequently lead to violations of EXP05-C. Do not cast away a const qualification. Although const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

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 noncompliant code, pi is declared as a float. Although pi is a mathematical constant, its value is not protected from accidental modificationIn this example, PI is defined using a macro. In the code, the value is introduced by textual subsitution.

float pi =
#define PI 3.14159
...14159f;
float degrees;
float radians;
/* ... */
radians = degrees *PI pi / 180;

Compliant Solution

...

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

Code Block
bgColor #ccccff lang c
const float float const pi = 3.1415914159f; ... float degrees; float radians; /* ... */ radians = degrees * pi / 180;

Non-Compliant Code Example 2

Delcaring immutable integer values as const-qualified objects still allows the programmer to take the address of the object. Also, the constant cannot be used in locations where an integer constant is required, such as the size of an array.

int const max = 15;
int a[max]; /* invalid declaration */
int const *p;

p = &max; /* legal to take the address of a const-qualified object */

Most C compilers will also allocate memory for the const-qualified object.

Compliant Solution 2

This compliant solution uses an enum rather than a const-qualified object or a macro definition.

enum { max = 15 };
int a[max]; /* OK */
int const *p;

p = &max; /* error: '&' on constant */

Risk Assessment

References

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.

Related Guidelines

 Bibliography

...

Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.3.2.1, "Lvalues, arrays, and function designators," Section 6.7.2.2, "Enumeration specifiers," and Section 6.10.3, "Macro replacement"