How do I distinguish -std=c 17 and -std=gnu 17 at compile time? checking macros?-CodePudding

I am using the __int128 extension of g . The problem with -std=c 17 is that some of the C library does not have all the support for that extension (i.e. std::make_unsigned<> fails). When using -std=gnu 17 it works fine.

I've added a header file that allows for the <limit> to work with __int128 when using -std=c 17 and I'd like to keep it for now, but when using -std=gnu 17 it breaks (because it is already defined). So I was thinking to add a condition like so:

#if !(<something>)
...
#endif

if the compiler already supports the limits with __int128.

My question is: what is that <something> I could check to distinguish between the standard and the GNU c 17 libraries?

CodePudding user response：

I did this:

$ diff <(g  -11 -std=c  17 -E -dM -x c   /dev/null|LC_ALL=C sort) \
       <(g  -11 -std=gnu  17 -E -dM -x c   /dev/null|LC_ALL=C sort)

And the output was:

180a181,182
> #define __GLIBCXX_BITSIZE_INT_N_0 128
> #define __GLIBCXX_TYPE_INT_N_0 __int128
315d316
< #define __STRICT_ANSI__ 1
424a426,427
> #define linux 1
> #define unix 1

That's not definitive, of course, but it's maybe a start.

So you could check for __STRICT_ANSI__ (indicating that there are no Gnu extensions), but perhaps the undocumentable __GLIBCXX_BITSIZE_INT_N_0 is more direct.