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The C language provides several different kinds of constants: integer constants, such as 10 and 0x1C; floating constants, such as 1.0 and 6.022e+23; and character constants, such as 'a' and '\x10'. C also provides string literals, such as "hello, world" and "\n". These constants can all be referred to as literals.

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 can be Avoid the use of magic numbers in code when possible. Magic numbers are constant values that represent either an arbitrary value (such as a determined appropriate buffer size) or a malleable concept (such as the age at which a person is considered an adult, which could 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 2002].

The C programming language has several mechanisms for creating named, symbolic constants: const-qualified objects, enumeration constants, and object-like macro definitions. Each of these mechanisms has associated advantages and disadvantages.

const-Qualified Objects

Objects that are const-qualified have scope and can be type-checked by the compiler. Because they are named objects (unlike macro definitions), some debugging tools can show the name of the object. The object also consumes memory.

A const-qualified object allows you to specify the exact type of the constant. For example,

const unsigned int buffer_size = 256;

defines buffer_size as a constant whose type is unsigned int.

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 (which would be an rvalue) must be used.

const-qualified objects allow the programmer to take the address of the object:

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

/* A const-qualified object can have its address taken */
p = &max;

const-qualified objects are likely to incur some runtime overhead [Saks 2001b]. Most C compilers, for example, allocate memory for const-qualified objects. const-qualified objects declared inside a function body can 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.

Enumeration Constants

Enumeration constants 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 consume memory. No storage is allocated for the value, so it is not possible to take the address of an enumeration constant.

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

p = &max; /* Error: "&" on enum constant */

Enumeration constants do not allow the type of the value to be specified. An enumeration constant whose value can be represented as an int is always an int.

Object-like Macros

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.

C programmers frequently define symbolic constants as object-like macros. For example, the code

#define buffer_size 256

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.

Macro names do not observe the scope rules that apply to other names.

...

Therefore, macros could substitute in unanticipated places with unexpected results.

Object-like macros do not consume memory; consequently, it is not possible to create a pointer to one. Macros do not provide for type checking because they are textually replaced by the preprocessor.

Macros can be passed as compile-time arguments.

Summary

The following table summarizes some of the differences between const-qualified objects, enumeration constants, and object-like macro definitions.

Noncompliant Code Example

The meaning of the numeric integer literal 18 is not clear in this example.:

/* ... */
if (age >= 18) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* ... */

Compliant Solution

The This compliant solution replaces the integer literal 18 with the symbolic constant ADULT_AGE to clarify the meaning of the code.When declaring immutable symbolic values, such as ADULT_AGE, it is best to declare them as a constant in accordance with DCL00-A. Declare immutable values using enum or const.:

enum { ADULT_AGE=18 };
/* ... */
if (age >= ADULT_AGE) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* ... */

...

Noncompliant Code Example

Magic numbers Integer literals are frequently used when referring to array dimensions, as shown in this non-compliant coding example.noncompliant code example:

char buffer[256];
/* ... */
fgets(buffer, 256, stdin);

This use of magic numbers integer literals can easily result in buffer overflows if, if for example, the buffer size is reduced but the magic number integer literal used in the call to fgets() is not.

Compliant Solution (enum)

In this compliant solution, the magic number integer literal is replaced with an enumeration constant. (see See DCL00-AC. Declare immutable values using enum or const).Const-qualify immutable objects.)

enum { BUFFER_SIZE=256 };

char buffer[BUFFER_SIZE];
/* ... */
fgets(buffer, BUFFER_SIZE, stdin);

Enumeration constants can safely be used anywhere a constant expression is required.

Compliant Solution (sizeof)

A 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 EXP09-AC. Use sizeof to determine the size of a type or variable.).

char buffer[256];
/* ... */
fgets(buffer, sizeof(buffer), stdin);

Using the sizeof expression in this example reduces the total number of names declared in the program, which is generally a good idea [Saks 2002]. The sizeof operator is almost always a good thing to do.

Exceptions

evaluated at compile time (except in the case of variable-length arrays).

When working with sizeof(), keep in mind ARR01-C. Do not apply the sizeof operator to a pointer when taking the size of an array.

Noncompliant Code Example

In this noncompliant code example, the string literal "localhost" and integer constant 1234 are embedded directly in program logic and are consequently difficult to change:

LDAP *ld = ldap_init("localhost", 1234);
if (ld == NULL) {
  perror("ldap_init");
  return(1);
}

Compliant Solution

In this compliant solution, the host name and port number are both defined as object-like macros, so they can be passed as compile-time arguments:

#ifndef PORTNUMBER     /* Might be passed on compile line */
#  define PORTNUMBER 1234
#endif

#ifndef HOSTNAME       /* Might be passed on compile line */
#  define HOSTNAME "localhost"
#endif

/* ... */

LDAP *ld = ldap_init(HOSTNAME, PORTNUMBER);
if (ld == NULL) {
  perror("ldap_init");
  return(1);
}

Exceptions

DCL06-C-EX1: Although DCL06-EX1: While replacing numeric constants with a symbolic constant is often a good practice, it can be taken too far. Remember that the goal is to improve readability. Exceptions can be made for constants that are themselves the abstraction you want to represent, as in this compliant solution.

x = (-b + sqrt(b*b - 4*a*c)) / (2*a);

Replacing numeric constants with symbolic constants in this example does nothing to improve the readability of the code , and may in fact can actually make the code more difficult to read:.

enum { TWO = 2 };     /* aA scalar */
enum { FOUR = 4 };    /* aA scalar */
enum { SQUARE = 2 };  /* anAn exponent */
x = (-b + sqrt(pow(b, SQUARE) - FOUR*a*c))/ (TWO * a);

When implementing recommendations, it is always necessary to use sound judgment.

(Note that this example does not prevent overflow or check for invalid operations (taking the sqrt() of a negative number). ) See INT32-C. Ensure that integer operations do not result in an overflow and FLP32-C. Prevent or detect domain and range errors in math functions for more information on detecting domain and range errors in math functions.

Risk Assessment

Using numeric literals makes code more difficult to read and understand. Buffer overruns are frequently a consequence of a magic number being changed in one place (like such as in an array declaration) but not elsewhere (like such as in a loop through an array).

Automated Detection

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Tool	Version	Checker	Description
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-DCL06
Compass/ROSE			Could detect violations of this recommendation merely by searching for the use of "magic numbers" and magic strings in the code itself. That is, any number (except a few canonical numbers: −1, 0, 1, 2) that appears in the code anywhere besides where assigned to a variable is a magic number and should instead be assigned to a const integer, enum, or macro. Likewise, any string literal (except "" and individual characters) that appears in the code anywhere besides where assigned to a char* or char[] is a magic string
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.DCL06	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3120, C3121, C3122, C3123, C3131, C3132
LDRA tool suite	Include Page LDRA_V LDRA_V	201 S	Fully implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-DCL06-a	Use meaningful symbolic constants to represent literal values
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. DCL06-C	Checks for: Hard-coded buffer size Hard-coded loop boundary Rec. fully covered.

Related Vulnerabilities

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

References

\[[Henricson 92|AA. C References#Henricson 92]\] Chapter 10, "[Constants|http://www.doc.ic.ac.uk/lab/cplus/c++.rules/chap10.html]"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7, "Declarations"

Related Guidelines

Bibliography

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Image Added Image Added Image AddedDCL05-A. Use typedefs to improve code readability      02. Declarations and Initialization (DCL)       DCL07-A. Include the appropriate type information in function declarators