DCL00-A. Const-qualify immutable objects

Enforcing invariants such as object immutability using existing const-qualification helps ensures the correctness and security of applications. For example, ISO/IEC PDTR 24772 [[ISO/IEC PDTR 24772]] recommends labeling parameters as constant to avoid the unintentional modification of function arguments.

The C programming language has two mechanisms for enforcing immutability: constants and const-qualified objects. Constants include integer constants, floating constants
enumeration constants, and character constants.

Immutable (constant) values should be declared as const-qualified objects (unmodifiable lvalues), macro definitions using #define, or enumeration constants.

The C programming language has several mechanisms for creating symbolic constants: const-qualified objects, enumeration constants, and macro definitions.

Objects that are const-qualified have scope and can be type-checked by the compiler. Because these are named objects (unlike macro definitions), (certain) debugging tools can show the name of the object. The objects also consumes memory (though this is not too important). 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

An enumeration constant is a member of an enumeration. Enumeration constant can be used to represent an integer constant expression that has a value representable as an int. Unlike const-qualified objects, enumeration constants do not require that storage is allocated for the value so it is not possible to take the address of an enumeration constant.

#define:

• operates at compile time
• consumes no memory (though this is not too important)
• can use in compile-time constant expression
• uses different syntax; can make mistake with ;
• can't create pointers to
• no type checking

const:

• operates at run time
• consumes memory (though this is not too important)
• can't use in compile-time constant expression
• uses consistent syntax
• can create pointers to
• does type checking

If any of these are required, then an integer constant (which would be an rvalue) must be used.

This recommendation is related to EXP07-A. Do not diminish the benefits of constants by assuming their values in expressions.

Non-Compliant Code Example (Object-Like Macro)

A preprocessing directive of the form:

# define identifier replacement-list

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; /* a const-qualified object can have its address taken */

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 */

Exceptions

DCL00-EX1 Constant values that may be passed as compile-time arguments must be macro definitions, as shown by this example:

#ifndef MYPORTNUMBER        /* might be passed on compile line */
#  define MYPORTNUMBER 1234
#endif

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.

Related Vulnerabilities

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"
[[ISO/IEC PDTR 24772]] "CSJ Passing parameters and return values"
[[Saks 00]] Dan Saks. Numeric Literals. Embedded Systems Programming. September, 2000.
[[Summit 05]] Question 10.5b

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