# V2623. MISRA. Macro identifiers should be distinct.

This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).

This diagnostic rule is relevant only to C programs. The macro identifier should be distinct from the identifiers of previously defined macros. The macro parameter identifier should be also distinct from the macro identifier itself and from all other parameters.

The minimum requirement for distinction of the macro identifiers and their parameters depends on the version of the C Standard:

- C90: 31 first characters
- C99: 63 first characters

In practice, compilers can exceed these limits. However, the diagnostic rule requires that macro identifiers should be distinct within the limits recommended by standard.

The following examples are relevant to C90.

The example of incorrect code with the macro identifier:

    //      1234567890123456789012345678901
    #define average_winter_air_temperature_monday              awt_m
    #define average_winter_air_temperature_tuesday             awt_t

The first macro identifier is indistinct from the second one, if we take the first 31 characters. Here is the correct code:

    //      1234567890123456789012345678901
    #define average_winter_air_temp_monday                     awt_m
    #define average_winter_air_temp_tuesday                    awt_t

The example of incorrect code with indistinct macro identifier and its parameters:

    #define average_winter_air_temp(average_winter_air_temp)   awt_m

The correct code:

    #define average_winter_air_temp(winter_air_temp)           awt_m

The example of incorrect code with indistinct identifiers of macro parameters:

    #define air_temp(winter_air_temp, winter_air_temp)         awt_m

The correct code:

    #define air_temp(average_winter_air_temp, winter_air_temp) awt_m

This diagnostic is classified as:

MISRA-C-2012-5.4 MISRA-C-2023-5.4