libcxx + MingW: correct printout of long doubles

[Marshall Clow]

On Nov 15, 2013, at 2:03 PM, Yaron Keren <yaron.keren at gmail.com> wrote:

> (this is slightly modfied version of patch posted on 4 Oct 2013)
> 
> Long ago Microsoft had decided (http://msdn.microsoft.com/en-us/library/9cx8xs15.aspx) to make thelong double type in Visual C++ same as double, 64 bit. This was an incredibly stupid decision, at least for those of us who actually care about numeric accuracy.
> 
> MingW, gcc-based port for Windows, had kept 80-bit long doubles and so does clang. This is a good decision which enable using these compilers in numeric code where 80 bit long doubles are required.
> 
> However, MingW uses the Microsoft runtime DLL msvcrt.dll. Here lies a problem: while gcc creates 80 bits long doubles, the MS runtime accepts 64 bit long doubles only. The problem is usually not obvious since the long double is casted to a double, as in:
> 
>  exp(10.0L)
> 
> where the 10.0L will be silently cast to 10.0 and the routine will seem to work.
> 
> However, in the *printf and *scanf family of functions arguments are not downcasted from long double to double (float does cast to double). For instance, naively compiling
> 
>  printf("%Lf\n",2345.0L);
> 
> with gcc or clang with the MingW runtime will print garbage results.
> 
> The MingW developers did develop a solution to the problem, namely alternative __mingw_printf and __mingw_scanf family of functions that accept long double 80 bit arguments properly.  In order to use the correct routines, one has to
> 
> #define __USE_MINGW_ANSI_STDIO 1
> 
> which is not the default in libc but it is defined in gcc libstdc++ library header "os_defines.h" as default. 
> 
> However, for snprintf_l there is no MingW alternative, it is simply #defined to _snprintf_l, a function from msvcrt.dll which does not support 80 bit long doubles. 
> 
> This patch implements snprintf_l  function in a way similar to the other functions in src/support/win32/locale_win32.cpp and locale_win32.h, calling upon vsnprintf for which there is a MingW correct alternative.
> 
> Note! __USE_MINGW_ANSI_STDIO is not modified in this patch. In order to use the __mingw version it must be defined before including the MingW headers.

As far as I can tell (not having a MinGW setup here) this patch looks fine.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

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