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Question about '<limits.h>' and availability of 'LLONG_MAX'

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Question about '<limits.h>' and availability of 'LLONG_MAX'

Martin J. O'Riordan via cfe-dev
I was looking at an issue recently where a programmer was using 'LLONG_MAX'
in a program such as the following:

   #include <limits.h>
   int foo(long long x) { return x == LLONG_MAX; }

If this is a C program, then it compiles okay; but if it is a C++ program it
does not, though this was really simply fixed by adding '-std=c++11
-U__STRICT_ANSI__' to the options.


clang seems to default to a relaxed C90 if '-std=' is not used, so 'long
long' and 'LLONG_MAX' are available.  Adding '-std=c90' results in a more
strict Standard violation error as expected.

But when compiled for C++ the equivalent relaxation does not happen and it
is an error even when '-std=' is not used

The test in '<limits.h>' (from 'clang/lib/Headers') is:

   #if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L

and I wonder would something like:

   #if !defined(__STRICT_ANSI__) || __STDC_VERSION__ >= 199901L ||
__cplusplus >= 201103L

be more consistent with how ISO Standards are enforced when '-std=' is not
used, and it would appear that this would align better with GCC too (though
GCC seems to also allow it when '-std=c++98' or '-std=c90' is selected which
I do not suggest 'clang' should follow).

Thanks,

        MartinO


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Re: Question about '<limits.h>' and availability of 'LLONG_MAX'

Martin J. O'Riordan via cfe-dev
On 8 May 2017, at 17:02, Martin J. O'Riordan via cfe-dev <[hidden email]> wrote:

>
> I was looking at an issue recently where a programmer was using 'LLONG_MAX'
> in a program such as the following:
>
>   #include <limits.h>
>   int foo(long long x) { return x == LLONG_MAX; }
>
> If this is a C program, then it compiles okay; but if it is a C++ program it
> does not, though this was really simply fixed by adding '-std=c++11
> -U__STRICT_ANSI__' to the options.
>
>
> clang seems to default to a relaxed C90 if '-std=' is not used, so 'long
> long' and 'LLONG_MAX' are available.
You are incorrect there.  For C sources, recent versions of clang
default to -std=gnu11 (except on PS4), and before that, the default was
always -std=gnu99.  Under both, long long is available.

For C++, clang defaults to -std=gnu++98 (except again on PS4), and that
might be giving you grief.  AFAIK C++98 does not officially know about
long long.


> Adding '-std=c90' results in a more
> strict Standard violation error as expected.
>
> But when compiled for C++ the equivalent relaxation does not happen and it
> is an error even when '-std=' is not used
>
> The test in '<limits.h>' (from 'clang/lib/Headers') is:
>
>   #if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L
>
> and I wonder would something like:
>
>   #if !defined(__STRICT_ANSI__) || __STDC_VERSION__ >= 199901L ||
> __cplusplus >= 201103L
Does __STRICT_ANSI__ make any sense in regard to C++11 or higher?

-Dimitry


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