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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.

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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.

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# 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.

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

Noncompliant Code Example

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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     /* 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 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 (such as in an array declaration) but not elsewhere (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

Bibliography

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