On Tue, Aug 30, 2022 at 09:10:37PM +0000, Joseph Myers wrote:
> I'm seeing build failures of glibc for powerpc64, as illustrated by the
> following C code:
>
> #if 0
> \NARG
> #endif
>
> (the actual sysdeps/powerpc/powerpc64/sysdep.h code is inside #ifdef
> __ASSEMBLER__).
>
> This shows some problems with this feature - and with delimited escape
> sequences - as it affects C. It's fine to accept it as an extension
> inside string and character literals, because \N or \u{...} would be
> invalid in the absence of the feature (i.e. the syntax for such literals
> fails to match, meaning that the rule about undefined behavior for a
> single ' or " as a pp-token applies). But outside string and character
> literals, the usual lexing rules apply, the \ is a pp-token on its own and
> the code is valid at the preprocessing level, and with expansion of macros
> appearing before or after the \ (e.g. u defined as a macro in the \u{...}
> case) it may be valid code at the language level as well. I don't know
> what older C++ versions say about this, but for C this means e.g.
>
> #define z(x) 0
> #define a z(
> int x = a\NARG);
>
> needs to be accepted as expanding to "int x = 0;", not interpreted as
> using the \N feature in an identifier and produce an error.
Thanks, will look at it tomorrow.
Jakub
