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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: __func__ is not a keyword
Date: Sun, 16 Mar 2025 13:29:00 -0700
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Kaz Kylheku <643-408-1753@kylheku.com> writes:
> On 2025-03-16, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>> source code and its behavior indicate that gcc treats __func__ as
>> a keyword, which is inconsistent with the info page.  For example,
>> one would expect this:
>>
>> int main(void) {
>>     {
>>         int __func__;
>>     }
>> }
>>
>> to be accepted, with the inner definition of __func__ hiding the
>> implicit static declaration, but gcc reports a syntax error.
>>
>> It's not a conformance issue, since __func__ is a reserved identifier
>> and any code that can tell whether it's a keyword has undefined
>> behavior.
>
> But __func__ is not a reserved identifier! Inside a function, it's a
> documented identifier with specified properties, and those properties do
> not support an interpretation that it may be a keyword.

"All identifiers that begin with a double underscore (__) or begin with
an underscore (_) followed by an uppercase letter are reserved for any
use, except those identifiers which are lexically identical to
keywords."

My interpretation is that the fact that the language defines a meaning
for __func__ doesn't exclude it from the set of reserved identifiers.

> We have to distinguish between specific, defined, standard identifiers
> allocated from a reserved namespace, and the reserved namespace itself.
>
> If gcc (in c99 mode or later) *allowed*  int _Bool = 42;   would you
> call that conforming, because _B* is in the reserved namespace, so
> any behavior is okay?

No, that's a syntax error because _Bool is a keyword, and keywords
are not identifiers.  "When preprocessing tokens are converted to
tokens during translation phase 7, if a preprocessing token could be
converted to either a keyword or an identifier, it is converted to
a keyword except in an attribute token."  Keywords and identifiers
are disjoint sets.

> Since __func__ is not described as existing outside of a function,
> there, it is just an identifier landing in the reserved namespace.

As far as the language is concerned yes, but gcc doesn't treat it
that way.

Are you suggesting that a conforming compiler must accept the above
code (defining __func__ in an inner scope)?  If so, I disagree.

gcc treating __func__ as a keyword arguably violates the intent of
the standard, but not in a way that affects conformance (unless I'm
missing something).  It behaves *as if* __func__ were an identifier
in the reserved space, with some instances of undefined behavior
behaving in odd but conforming ways.

(Practically, not being able to define my own entity named __func__
is no great loss.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */