Re: [dpdk-dev] [PATCH 4/6] build: MinGW-w64 support for Meson

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Feb 05, 2020 at 11:41:05PM +0300, Dmitry Kozlyuk wrote:
> > > > > > +if is_windows
> > > > > > +	# Require platform SDK for Windows 7 and above.
> > > > > > +	add_project_arguments('-D_WIN32_WINNT=0x0601', language: 'c')    
> > > > > 
> > > > > Please explain. Why Windows 7 is needed? What this define is doing?  
> > > > 
> > > > Yes, Windows 7 and above is need for already existing code in eal_lcore.c,
> > > > specifically for GetLogicalProcessorInformation() call.
> > > > 
> > > > When including <windows.h>, one must define minimum API version the
> > > > application is compiled against [0]. MSVC and Clang default to the version of
> > > > platform SDK (that is, maximum supported). MinGW defaults to Windows XP, so
> > > > this definition must be either in <rte_os.h> before #include <windows.h> or
> > > > here. Because other files may include <windows.h>, I'd prefer to have a
> > > > global definition via compiler command-line.
> > > > 
> > > > [0]:
> > > > https://docs.microsoft.com/en-us/windows/win32/WinProg/using-the-windows-headers  
> > > 
> > > OK, thanks.
> > > Please reword the comment with something like
> > > 	"Minimum supported API is Windows 7."
> > >   
> > For this, as an alternative to putting it as a project argument, you can just
> > add it to dpdk_conf which means it will end up as a define in the global
> > rte_build_config.h and so be directly included in each compilation unit
> > ahead of any other headers. (rte_config.h includes rte_build_config.h)
> 
> Can you please explain why using dpdk_conf is a better alternative? In
> lib/meson.build I can see add_project_arguments('_D_GNU_SOURCE', ...), which
> serves a similar purpose on POSIX systems. Compiler option also makes it
> impossible to forget or redefine this constant in code by mistake.
> 
I'm not necessarily saying it's better, it's just an alternative to
consider. :-) Having it in rte_config.h makes the define available to any
external apps using DPDK, which may or may not be desirable.