# V2618. MISRA. Identifiers declared in the same scope and name space should be distinct.

This diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) manual for software development.

This rule only applies to C. The code has two identifiers that differ only in non-significant characters. If you declare these identifiers in the same scope, it leads to undefined behavior. Besides, long identifiers make it difficult to read the code. Long identifiers can also be confused with auto-generated ones.

Before C99 standard only the first 31 characters were significant. Starting from the C99 standard the first 63 characters are significant. The rest of the characters are non-significant.

This rule doesn't apply to code if both identifiers have external linkage. For this case, the V2614 diagnostic is intended.

This rule doesn't apply to code if one of the identifiers is a macro.

Look at the example for C90:

    //         1234567890123456789012345678901***
    static int very_long_long_long_long__test_var1;  // (1)
    extern int very_long_long_long_long__test_var2;  // (2)

Identifiers 1 and 2 differ only in the non-significant characters ('var1' and 'var2'). The significant part — 'long_long_long_long_long__test_' — is the same. If you want to avoid undefined behavior, shorten the length of the identifier:

    //         1234567890123456789012345678901***
    static int not_very_long__test_var1;
    extern int not_very_long__test_var2;

Look at the second example:

    //         1234567890123456789012345678901***
    static int long_long_long_long_long__test_var3;  // (3)

    void foo()
    {
    //    1234567890123456789012345678901***
      int long_long_long_long_long__test_var4;       // (4)
    }

Here identifiers 3 and 4 also differ in non-significant characters. However, they are in different scopes, so there's no rule violation.

This diagnostic is classified as:

MISRA-C-2012-5.2 MISRA-C-2023-5.2