SEI CERT C Coding Standard

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Wiki Markup
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 referred to as _magic numbers_ because their purpose is often obscured. Magic numbers aremay 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 could 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. \[[Saks 02|AA. C References#Saks 02]\].

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Method

Evaluated at

Consumes Memory

Viewable by Debuggers

Type Checking

Compile-time constant expression

Enumerations

compile time

no

yes

yes

no yes

const-qualified

run time

yes

yes

yes

no

Macros

preprocessor

no

no

no

yes

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Wiki Markup
Using the {{sizeof}} expression in this example reduces the total number of names declared in the program, which is generally a good idea \[[Saks 02|AA. C References#Saks 02]\].  The {{sizeof}} operator is almost always evaluated at compile time (except in the case of variable length arrays).

Non-Compliant Code Example

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

Compliant Solution

Code Block
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if ( (ld = ldap_init("localhost", 1234)) == 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 that Constant values that may be passed as compile-time arguments must be macro definitions, as shown by this example:.

Code Block
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#ifndef PORTNUMBER     /* might be passed on compile line */
#  define PORTNUMBER 1234
#endif

#ifndef MYPORTNUMBERHOSTNAME        /* might be passed on compile line */
#  define MYPORTNUMBERHOSTNAME 1234
#endif"localhost"
#endif

/* ... */ 

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

Exceptions

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.

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