Why must the first 31 characters of an identifier be unique?

c misra
Andreas picture Andreas · Nov 11, 2013 · Viewed 6.9k times · Source

MISRA 2004 rule 5.1 states that all identifiers must have the first 31 characters unique. What is the reason for this rule? Is it a technical limitation with some compilers?

Answer

Arkku picture Arkku · Nov 11, 2013

The C standards only guarantee that a certain number of initial characters in identifiers are significant. For C99 this is 31 characters for external identifiers. Even this is a huge step up from ANSI/IS C, which guarantees only 6 significant characters for external identifiers… (So if you're wondering why so many old C functions have unpronounceable names, this is one reason.)

In practice compilers tend to support a higher number of significant characters in identifiers (and IIRC the C standard even has a footnote encouraging this), but MISRA probably wanted to pick a “safe” limit already guaranteed by the then-most-recent C standard, C99, without imposing the limit of 6 that would be guaranteed by C90 which MISRA 2004 otherwise follows.

edit: Since it has been questioned twice in the comments, let me clarify: MISRA 2004 does not follow C99, and there is no hard evidence that the C99 standard contributed to MISRA's chosen limit of specifically 31 characters. However, the limit does not come from C90 (ISO C), because C90 specifies a limit of 6 characters. So, one must either accept that MISRA just picked the number 31 independently with no known reason, or (in my opinion more likely) followed the example of C99 in this particular decision.

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