[vlc-devel] UCRT support in MinGW-w64

[Martin Storsjö]

Hi Stephen,

On Thu, 21 Nov 2019, Stephen Kitt wrote:

> Hi,
>
> I’m the maintainer of the MinGW-w64 toolchain in Debian. I believe VLC uses
> that for its Windows builds; is that correct?
>
> I’m wondering what your thoughts are regarding UCRT support. Currently the
> MinGW-w64 toolchain in Debian targets MSVCRT by default; this can be
> overridden by rewriting spec files, but the result isn’t entirely
> satisfactory since the GCC DLLs still use MSVCRT.

Such a solution isn't generally enough; not only do the GCC DLLs (libgcc 
and libstdc++) still use msvcrt, but statically linking either, in 
particular libstdc++, will fail, as the object files in the static library 
were built with headers targeting msvcrt, which doesn't work if linking 
the final exe/dll against ucrt.

And for the case if the prebuilt DLLs use a different CRT; that could be 
fine if there's no sharing of CRT data structures (file descriptors, FILE* 
handles, etc) across the DLL boundary. I don't know for sure if libstdc++ 
does that or if none of those ABI details leak through the DLL interface.

Within mingw-w64, we've tried to make sure that libmingwex.a should work 
with both msvcrt and ucrt, but that's about as far as the object file 
compatibility goes. I think, but haven't verified, that libgcc doesn't do 
anything particularly fancy, so a static build of that might work fine 
with either crt.

So for simple C code that doesn't link to anything prebuilt fancy, one can 
in principle choose at build time just by setting the right msvcrt version 
defines for all built objects (since a couple months, the recommended way 
for hardcoding the version is to either do -D_UCRT or 
-D__MSVCRT_VERSION__=0), and linking with either -lucrt or -lmsvcrt-os; 
then you know what you get regardless of which one was the toolchain 
default.

> So I’m pondering switching the default to UCRT, to better match what 
> Visual Studio does nowadays, and allow targeting UWP without too many 
> contorsions...

The main gotcha regarding this, is that UCRT isn't available out of the 
box before Windows 10, but it requires installing a separate 
redistributable. With MSVC, this isn't quite as much of an issue as you 
can choose to statically link the CRT there, but that's not possible in 
mingw-w64.

And I've understood that this is a blocker for using UCRT for the VLC 
releases.

> Another possibility, albeit rather more complex on my end, would be to add -m
> options to GCC, with multiple builds of the DLLs. That would also allow me to
> provide DW2 and SJLJ support for Win32.

I'm afraid something like that is necessary, if you want to allow the end 
user to choose.

// Martin