[Bug c/53182] GNU C: attributes without underscores should be discouraged / no longer be documented e.g. as examples

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53182

--- Comment #8 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Jonathan Wakely from comment #7)
> (In reply to Vincent Lefèvre from comment #6)
> > Also, note that identifiers that are not reserved should not be used,
> > because they could be defined as macros by the developer, who has not tested
> > his code with GCC (I'm saying GCC here, but this applies to any compiler).
> 
> Should not be used by who?

by non-user code, but this applies in particular to what's inside
__attribute__.

Here's an example. In MPFR's mpfr.h file, we use:

#define __MPFR_SENTINEL_ATTR
#if defined (__GNUC__)
# if __GNUC__ >= 4
#  undef __MPFR_SENTINEL_ATTR
#  define __MPFR_SENTINEL_ATTR __attribute__ ((sentinel))
# endif
#endif

But if in my personal C code, I have

#define sentinel 1

before the #include's or, in a similar way, compile my program with -Dsentinel,
then I get a compilation failure. Here the end user is not supposed to know
that the identifier "sentinel" (which is not reserved) is used internally.