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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: VAX
Date: Tue, 06 Jan 2026 21:14:48 -0800
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Michael S <already5chosen@yahoo.com> writes:
>
>> On Mon, 04 Aug 2025 12:09:32 GMT
>> anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:
>
> [...]
>
>>> typedef ump unsigned _BitInt(65535);
>
> The correct syntax is :
>
> typedef unsigned _BitInt(65535) ump;
>
>>> ump sum3(ump a, ump b, ump c)
>>> {
>>>    return a+b+c;
>>> }
>
> [...]
>
>> 1.  Both gcc and clang happily* accept _BitInt() syntax even when
>> -std=c17 or lower.  Is not here a potential name clash for existing
>> sources that use _BitInt() as a name of the function?  I should think
>> more about it.
>
> In C17 and earlier, _BitInt is a reserved identifier.  Any attempt to
> use it has undefined behavior.  [...]

Not so.  The C standard says if a program declares or defines a
reserved identifier (in a context where it is reserved) then the
behavior is undefined.  The C standard does /not/ say that simply
using (as opposed to declaring or defining) a reserved identifier
has undefined behavior (in any context).

The C23 standard is different, but I believe what it says is
consistent with the previous paragraph.

Furthermore it is always possible to use any reserved identifier
in a way that has defined behavior, as for example:

  #include <stdio.h>

  #define XX(a) X(a)
  #define X(a) #a

  #define show(a) printf( "The id %s expands to %s\n", X(a), XX(a) )

  int
  main( int argc, char *argv[] ){
      show( _BitInt );
      return  0;
  }

This program works without complaint under all of C90, C99, C11, and
C17.