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When used in program logic, literals can reduce the readability of source code. As a result, literals in general, and integer constants in particular, are frequently called _magic numbers_ because their purpose is often obscured. Magic numbers may be constant values that represent either an arbitrary value (such as a determined appropriate buffer size) or a malleable concept (such as the age a person is considered an adult, which can change between geopolitical boundaries). Rather than embed literals in program logic, use appropriately named symbolic constants to clarify the intent of the code. In addition, if a specific value needs to be changed, reassigning a symbolic constant once is more efficient and less error prone than replacing every instance of the value \[[Saks 022002|AA. Bibliography#Saks 02]\]. |
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{{const}}\-qualified objects are likely to incur some runtime overhead \[[Saks 01b2001b|AA. Bibliography#Saks 02]\]. Most C compilers, for example, allocate memory for {{const}}\-qualified objects. {{const}}\-qualified objects declared inside a function body may have automatic storage duration. If so, the compiler will allocate storage for the object, and it will be on the stack. As a result, this storage will need to be allocated and initialized each time the containing function is invoked. |
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defines buffer_size as a macro whose value is 256. The preprocessor substitutes macros before the compiler does any other symbol processing. Later compilation phases never see macro symbols, such as buffer_size; they see only the source text after macro substitution. As a result, many compilers do not preserve macro names among the symbols they pass on to their debuggers.
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In this compliant solution, the integer literal is replaced with an enumeration constant. (see See guideline DCL00-C. Const-qualify immutable objects.).
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enum { BUFFER_SIZE=256 };
char buffer[BUFFER_SIZE];
/* ... */
fgets(buffer, BUFFER_SIZE, stdin);
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Frequently, it is possible to obtain the desired readability by using a symbolic expression composed of existing symbols rather than by defining a new symbol. For example, a sizeof expression can work just as well as an enumeration constant. (see See guideline EXP09-C. Use sizeof to determine the size of a type or variable.).
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char buffer[256]; /* ... */ fgets(buffer, sizeof(buffer), stdin); |
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Using the {{sizeof}} expression in this example reduces the total number of names declared in the program, which is generally a good idea \[[Saks 022002|AA. Bibliography#Saks 02]\]. The {{sizeof}} operator is almost always evaluated at compile time (except in the case of variable-length arrays). |
When working with sizeof(), keep in mind guideline ARR01-C. Do not apply the sizeof operator to a pointer when taking the size of an array.
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Note that this example does not check for invalid operations (taking the sqrt() of a negative number). See guideline FLP32-C. Prevent or detect domain and range errors in math functions, for more information on detecting domain and range errors in math functions.
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Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
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DCL06-C | low | unlikely | medium | P2 | L3 |
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Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
Related Guidelines
This rule appears in the C++ Secure Coding Standard as : DCL06-CPP. Use meaningful symbolic constants to represent literal values in program logic.
Bibliography
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\[[Henricson 921992|AA. Bibliography#Henricson 92]\] Chapter 10, "[Constants|http://www.doc.ic.ac.uk/lab/cplus/c++.rules/chap10.html]" \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.3.2.1, "Lvalues, arrays, and function designators," Section 6.7, "Declarations," Section 6.7.2.2, "Enumeration specifiers," and Section 6.10.3, "Macro replacement" \[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "BRS Leveraging human experience" \[[MITRE 072007|AA. Bibliography#MITRE 07]\] [CWE ID 547|http://cwe.mitre.org/data/definitions/547.html], "Use of Hard-coded, Security-relevant Constants" \[[Saks 01a2001a|AA. Bibliography#Saks 01]\] \[[Saks 01b2001b|AA. Bibliography#Saks 02]\] \[[Saks 022002|AA. Bibliography#Saks 02]\] \[[Summit 052005|AA. Bibliography#Summit 05]\] [Question 10.5b|http://c-faq.com/cpp/constvsdefine.html] |
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