DCL00-A. Declare immutable values using enum or const

Immutable (constant) values should be declared as enumeration constants, const-qualified objects (unmodifiable lvalues), or as a last resort, a #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.  However, for integer constants, it is preferable to use an enumeration constant instead of a const-qualified object, as this eliminates the possibility of taking the address of the integer constant and does not require that storage is allocated for the value.

Unfortunately, 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 (except in the case of variable length arrays)
• 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.

This recommendation is related to EXP10-A. Do not diminish the benefits of constants by assuming their values in expressions.

Non-Compliant Code (object-like macro)

A preprocessing directive of the form:

# define identifier replacement-list new-line

defines an object-like macro that causes each subsequent instance of the macro name to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive [[ISO/IEC 9899-1999]].

In this non-compliant code example, PI is defined as an object-like macro. Following the definition, each subsequent occurrence of the string "PI" is replaced by the string "3.14159f" by textual substitution.

#define 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, allowing the constant to have scope and to have its value inspected by a debugger.

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

While inadequate in some ways, this is the best that can be done for non-integer constants.

Non-Compliant Code Example (immutable integer values)

In this non-compliant code example, max is declared as a const-qualified object. While declaring non-integer constants as const-qualified objects is the best that can be done in C, for integer constants we can do better. Declaring immutable integer values as const-qualified objects still allows the programmer to take the address of the object. Also, const-qualified integers cannot be used in locations where an integer constant is required, such as the value of a case constant.

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

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

Most C compilers allocate memory for const-qualified objects.

Compliant Solution (enum)

This compliant solution declares max as an enumeration constant rather than a const-qualified object or a macro definition.

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

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

Risk Assessment

Using ordinary variables to hold constants instead of using enumeration constants or const-qualified objects can result in a value intended to be constant being changed at runtime.

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

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"