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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Cake - C23 to C99 transpiler
Date: Wed, 14 Sep 2022 06:40:12 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> David Brown <david.brown@hesbynett.no> writes:
>
>> I believe (but I might have this wrong) that the use of dollar signs
>> in identifiers is undefined behaviour according to the standard.
>
> I don't think it's undefined.  Recent C drafts permit "other
> implementation defined characters" in the syntax, so I'm not sure how it
> could be undefined.

If the implementation's documentation lists dollar sign amoung
the set of implementation-defined characters for identifiers,
the behavior is defined.  Otherwise, the presence of dollar
sign (in source that hasn't been filtered out by preprocessor
directives) results in a syntax error, which makes the behavior
undefined.

> The C23 draft permits an XID_Start character followed by XID_Continue
> characters.  These may, in fact, include $ but I got lost down the
> rabbit hole of referenced standards so I can't be sure.

Looking at some Unicode reference material and also looking
at some web search results, the evidence seems pretty strong
that $ and @ are not included in the XID_Start and XID_Continue
default sets.  However, it isn't clear (at least it isn't to
me) whether the C23 draft admits the possibility that $ and @
may be accepted under an implementation-defined umbrella.