Re: [PATCH v13 2/7] net/vmnet: add vmnet backends to qapi/net

[Roman Bolshakov <roman@roolebo.dev>]

On Mon, Jan 24, 2022 at 12:27:40PM +0100, Christian Schoenebeck wrote:
> On Montag, 24. Januar 2022 10:56:00 CET Roman Bolshakov wrote:
> > On Thu, Jan 13, 2022 at 08:22:14PM +0300, Vladislav Yaroshchuk wrote:
> > >  net/vmnet-bridged.m |  25 +++++++++
> > >  net/vmnet-common.m  |  20 +++++++
> > 
> > It seems the last two files should have .c extension rather than .m.
> 
> I would not do that. Mind cross-compilers, please.
> 

Hi Christian,

Cross-compilers for Apple platforms can be constructed using à la carte
approach where toolchain comes from the source, SDK from Apple and a
port of cctools from GitHub (mind all library dependencies of QEMU).
That's quite an effort!

I very much doubt this is a relevant and typical case for QEMU on macOS.
And if cross-compiler is constructed properly it'll pass required flags
that enable blocks and will link block runtime in its default build
recipe like all cross-compilers do for the platform of interest.

Gladly, there's osxcross [1] and crossbuild image with Darwin support [2].
They can deal with blocks just fine:

  # CROSS_TRIPLE=i386-apple-darwin
  $ cc block.c && file a.out
  a.out: Mach-O i386 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|NO_HEAP_EXECUTION>

  # CROSS_TRIPLE=x86_64-apple-darwin
  $ cc block.c && file a.out
  $ file a.out
  a.out: Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>

> > Unlike Cocoa UI code, the files do not contain Objective-C classes. They are
> > just C code with blocks (which is supported by compilers shipped with Xcode
> > SDK), e.g this program can be compiled without extra compiler flags:
> > 
> > $ cat block.c
> > int main() {
> >         int (^x)(void) = ^{
> >                 return 0;
> >         };
> > 
> >         return x();
> > }
> > $ cc block.c && ./a.out
> > $
> > 
> 
> Such blocks are still Objective-C language specific, they are not C and 
> therefore won't work with GCC.
> 

I'm not sure why blocks are Objective-C specific. All the data I have
shows the opposite [3][4][5]. They're just extensively used in Apple APIs.

> $ gcc block.c
> 
> block.c: In function ‘main’:
> block.c:2:14: error: expected identifier or ‘(’ before ‘^’ token
>          int (^x)(void) = ^{
>               ^
> block.c:6:16: warning: implicit declaration of function ‘x’ [-Wimplicit-
> function-declaration]
>          return x();
>                 ^

You might do this on Linux and it'll work:

$ clang -g -fblocks -lBlocksRuntime block.c && ./a.out

However, vmnet code won't be compiled on non-Apple platforms because the
compilation happens only if vmnet is available which happens only if
appleframeworks dependency is available, that is not available on
non-OSX hosts [6]:

  "These dependencies can never be found for non-OSX hosts."

1. https://github.com/tpoechtrager/osxcross
2. https://github.com/multiarch/crossbuild
3. http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1370.pdf
4. https://clang.llvm.org/docs/BlockLanguageSpec.html
5. https://clang.llvm.org/docs/Block-ABI-Apple.html
6. https://mesonbuild.com/Dependencies.html#appleframeworks

Regards,
Roman