As well as using a #define preprocessor directive, identifiers may effectively be #define’d in other, implementation-defined, ways. For example some implementations support:

Using a compiler command-line option, such as -D to allow identifiers to be defined prior to translation;
Using environment variables to achieve the same effect;
Pre-defined identifiers provided by the compiler.

Rationale

If an attempt is made to use a macro identifier in a preprocessor directive, and that identifier has not been defined, then the preprocessor will assume that it has a value of zero. This may not meet developer expectations.

Example

The following examples assume that the macro M is undefined.

#if M == 0                 /* Non-compliant                               */ 
                           /* Does 'M' expand to zero or is it undefined? */ 
#endif

#if defined ( M )          /* Compliant - M is not evaluated              */ 
#if M == 0                 /* Compliant - M is known to be defined        */ 
                           /* 'M' must expand to zero.                    */ 
#endif 
#endif

/* Compliant - B is only evaluated in ( B == 0 ) if it is defined         */ 
#if defined ( B ) && ( B == 0 ) 
#endif

MISRA-C 2004 Rule 19.11 (required): All macro identifiers in preprocessor directives shall be defined before use, except in #ifdef and #ifndef preprocessor directives and the defined() operator.

Undefined macros in #if directive.

This rule is also covered by MISRA.ELIF.UNDEF.

If an attempt is made to use an identifier in a preprocessor directive, and that identifier has not been defined, the preprocessor will sometimes not give any warning but will assume the value zero. #ifdef, #ifndef and defined() are provided to test the existence of a macro, and are therefore excluded.

Example

#if x < 0 /* x assumed to be zero if not defined */

Consideration should be given to the use of a #ifdef test before an identifier is used.

Note that preprocessing identifiers may be defined either by use of #define directives or by options specified at compiler invocation. However the use of the #define directive is preferred.

MISRA-C++ 2008 Rule 16-0-7 (required): Undefined macro identifiers shall not be used in #if or #elif preprocessor directives, except as operands to the defined operator.

This rule is also covered by MISRA.ELIF.UNDEF.

Rationale

If an attempt is made to use an identifier in a preprocessor directive, and that identifier has not been defined, the preprocessor will assume the value zero. #ifdef, #ifndef and defined() are provided to test the existence of a macro, and are therefore excluded.

Example

#if x < 0 // Non-compliant - x assumed to be zero as it is not defined

Consideration should be given to the use of a #ifdef test before an identifier is used.

Note that preprocessing identifiers may be defined either by use of #define directives or by options specified at compiler invocation. However, the use of the #define directive is preferred.