Comment by hddqsb | Hacker News Reader

Comment by hddqsb | Hacker News Reader

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From https://www.open-std.org/JTC1/sc22/wg14/www/docs/n2625.pdf:

> The goal of the future language and library reservations is to alert C programmers of the potential for future standards to use a given identifier as a keyword, macro, or entity with external linkage so that WG14 can add features with less fear of conflict with identifiers in user’s code. However, the mechanism by which this is accomplished is overly restrictive – it introduces unbounded runtime undefined behavior into programs using a future language/library reserved identifier despite there not being any actual conflict between the identifier chosen and the current release of the standard. ...

> Instead of making the future language/library identifiers be reserved identifiers, causing their use to be runtime unbounded undefined behavior per 7.1.3p1, we propose introducing the notion of a potentially reserved identifier to describe the future language and library identifiers (but not the other kind of reservations like __name or _Name). These potentially reserved identifiers would be an informative (rather than normative) mechanism for alerting users to the potential for the committee to use the identifiers in a future release of the standard. Once an identifier is standardized, the identifier stops being potentially reserved and becomes fully reserved (and its use would then be undefined behavior per the existing wording in C17 7.1.3p2). These potentially reserved identifiers could either be listed in Annex A/B (as appropriate), Annex J, or within a new informative annex. Additionally, it may be reasonable to add a recommended practice for implementations to provide a way for users to discover use of a potentially reserved identifier. By using an informative rather than normative restriction, the committee can continue to caution users as to future identifier usage by the standard without adding undue burden for developers targeting a specific version of the standard.

So... instead of mandating implementations to warn about (re)defining a reserved identifier, they introduce another class of "not yet reserved indentifiers" and advise implementations to warn about defining such identifiers in the user code — even though it's completely legal, — until the moment the implementation itself actually uses/defines such an identifier at which point warning about such redefinition in the user code — now illegal and UB — is no longer necessary or advised.

Am I completely misreading this or is this actually insane? Besides, there is already a huge swath of reserved identifiers in C, why do they feel the need to make an even larger chunk of names unavailable to the programmers?

The problem is that the traditional wording of C meant that any variable named 'top' was technically UB, because it begins with `to'.

In practical terms, what compilers will do is, if C2y adds a 'togoodness' function, they will add a warning to C89-C2x modes saying "this is now a library function in C2y," or maybe even have an extension to use the new thing in earlier modes. This is what they already do in large part; it's semantic wording to make this behavior allowable without resorting to the full unlimited power of UB.

> Besides, there is already a huge swath of reserved identifiers in C, why do they feel the need to make an even larger chunk of names unavailable to the programmers?

The C23 change was mostly to downgrade some of the existing reserved identifiers from "reserved" to "potentially reserved". (It also added some new reserved and potentially reserved identifiers, but they seem reasonable to me.)

I still fail to see any practical difference between these two categories, except that the implementations are recommended to diagnose illegal-in-the-future uses of potentially reserved identifiers but are neither required nor recommended to diagnose actually illegal uses of reserved identifiers. There is also no way to distinguish p.r.i from r.i.

It also means that if an identifier becomes potentially reserved in C23 and reserved in C3X, then compiling a valid C11 program that uses it as C23 will give you a warning, which you can fix and then compile resulting valid C23 program as C3X without any problem; but compiling such a C11 program straight up as C3X will give you no warning and a program with UB.

Seriously, it boggles my mind. Just a) require diagnostics for invalid uses of reserved identifiers starting from C23, b) don't introduce new reserved identifiers, there is already a huge amount of them.

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