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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.

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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 these they are named objects (unlike macro definitions), some debugging tools can show the name of the object. The object also consumes memory.

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Unfortunately, const-qualified objects cannot be used where compile-time integer constants are required, namely to define the

• size Size of a bit-field member of a structure.
• size Size of an array (except in the case of variable length arrays).
• value Value of an enumeration constant.
• value 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]; /* invalidInvalid declaration outside of a function */
const int *p;

/* aA 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; /* errorError: '"&'" 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 [ISO/IEC 9899:2011].

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

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Object-like macros do not consume memory, and, ; 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 integer literal 18 is not clear in this example.:

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

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This compliant solution replaces the integer literal 18 with the symbolic constant ADULT_AGE to clarify the meaning of the code.:

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

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Integer literals are frequently used when referring to array dimensions, as shown in this noncompliant code example.:

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

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

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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);
}

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

#ifndef HOSTNAME        /* mightMight 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 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.

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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);

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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

Related Vulnerabilities

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

Related Guidelines

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ISO/IEC 9899:2011 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 TR 24772 "BRS Leveraging human experience"

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Bibliography

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