Hacker News Reader

Comment by valleyer | Hacker News Reader

Names beginning with double underbar (or single underbar + capital letter) are reserved. Single underbar + lowercase is not. C23 §6.4.2.1.

View on HN

Also reserved as identifier with file scope, just not for "any use". In any case, the program used underbar + capital letter.

Ah, I hadn't noticed _SimpleSetNode.

This leaves out part of the clause.

  All identifiers that begin with an underscore are reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Single underscore followed by non-uppercase is allowed, but not in file scope. This means that you can use them in structs and as local variables, but never as globals.

You're right, and I guess I've been breaking that rule for a while. What's the purpose there? The double-underbar and underbar-capital rules seem to be allowing for non-conflicting introduction of keywords. Is the single-underbar rule to protect standard library headers or something?

Reserved identifiers in general are not only for future use of the standard, but also for internal use of implementations and for extensions. The single-under bar rule is to give the programmer some part of the _... namespace which is otherwise reserved for the implementation. I.e. for use in macros, variables, or struct members. The intention is for use in hidden names in some API.

Thank you for pointing this out, and to those adding clarifications to this statement as well. I've fixed the issue by renaming the reserved identifiers.

Reply on news.ycombinator.com