Re: OK to use anonymous union (C11 feature)? Opinions wanted

[Erik L. <eri...@gm...>]

Hi Ethan,

There is no objection from the macOS side: The compiler I use to generate
universal binaries defaults to c17/c++17.
Let me know if you wish me to try out a development version.

Best,

Erik



On Tue, Jul 1, 2025 at 1:32 AM Ethan A Merritt <me...@uw...> wrote:

> Bug reports 2812 and 2813 report and diagnose a problem with color-handling
> that manifests on ARM architectures but not on x86.  More detail is
> attached
> to bug 2812 if you want gory details.
>
> The basic issue is that different code is needed if color is represented by
> a signed integer (e.g. "plot foo lt 2") or by an unsigned 32-bit ARGB value
> (e.g. "splot foo with pm3d fillstyle transparent solid 0.25").
>
> It is possible to modify the code by inserting an appropiate cast to either
>    color.lt = (int)value     or    color.lt = (unsigned int)value
> everywhere that it matters.
>
> However IMO a cleaner approach is to modify the t_colorspec structure
> using an anonymous union to distinguish between the two cases:
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>   typedef struct t_colorspec {
>       colortype type;                 /* TC_<type> definitions below */
>       union {
>           int lt;                               /* used for TC_LT,
> TC_LINESTYLE */
>           unsigned rgbcolor;      /* used for TC_RGB */
>       };
>       double value;                    /* used for TC_CB and TC_FRAC */
>   } t_colorspec;
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>
> That allows to write code like
>     if (color.type == TC_RGB)
>         color.rgbcolor = value;
>     else
>         color.lt = value;
> I have a tested this out in a private git branch and it works as intended.
>
> HOWEVER, anonymous unions were only added to the C standard in C11,
> although both gcc and clang already supported them well before that.
>
> So the question is, is it worth adding a requirement for C compiler that
> supports
> C11 or at least supports anonymous unions?
> As of gnuplot version 6 we're requiring c99 (version 5 was ok with c89).
> MSVisualStudio claims to support c11 as of 2019.   I don't know what other
> compilers people might be using, or what their level of support might be.
>
> What do you think?
> Add it to the development version and back it out if compiler problems
> are reported?   What about the stable version?
>
>         Ethan
>
>
>
>
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