Immutable objects should be {{const}}\-qualified.  Enforcing object immutability using {{const}}\-qualification helps ensures 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.

Non-Compliant Code Example

In this non-compliant 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;

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 */

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

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] 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.
\[[Summit 05|AA. C References#Summit 05]\] [Question 10.5b|http://c-faq.com/cpp/constvsdefine.html]

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