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Wiki MarkupImmutable objects should be {{const}}\-qualified. Enforcing object immutability using {{const}}\- qualification helps ensure the correctness and security of applications. ISO/IEC PDTR TR 24772, for example, recommends labeling parameters as constant to avoid the unintentional modification of function arguments \[ [ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\]. [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, which can frequently lead to violations of EXP05-C. 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.

Macros, 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 in program logic 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.

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In this noncompliant code, 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;

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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

In this example, adapted from \[[Dewhurst 02|AA. C++ References#Dewhurst 02]\] Gotcha #25, a macro is used to define a maximum value, but there is no type information associated with the macro.

#define MAX (1<<16)
// ...
void f(int);
void f(long);
// ...
f(MAX);  // which f?

The value 1 << 16 could be an int or a long depending on the platform. As a result, this code becomes platform dependent.

Compliant Solution

Using a constant associates type information with the value and eliminates the possibility of confusion.

int const max = 1<<16;
// ...
void f(int);
void f(long);
// ...
f(max);

Exceptions

DCL00-EX1: It is acceptable to define valueless macros to serve as 'inclusion guards'. That is, the macro serves to control the multiple inclusion of header files, as in the following example:

#ifndef SOME_HEADER_H
#define SOME_HEADER_H
...  // content of header file
#endif

See PRE06-C. Enclose header files in an inclusion guard for more information on inclusion guards.

Risk Assessment

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

Automated Detection

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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.

Other Languages

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Related Guidelines

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References

\[[Dewhurst 02|AA. C References#Dewhurst 02]\] Gotcha #25, "#define Literals"
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.3, "Type qualifiers"
\[[Saks 00|AA. C References#Saks 00]\]

 Bibliography

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