Re: configure: --extra-cflags are passed along C++ linker via QEMU_LDFLAGS

[Paolo Bonzini]

On 01/12/20 13:48, Philippe Mathieu-Daudé wrote:
> Hi,
>
> I rebased an old series I started more than 1 year ago and tried
> to build it using '../configure --extra-cflags=-Wint-conversion'
> but got:
>
> cc1plus: error: command-line option ‘-Wint-conversion’ is valid for
> C/ObjC but not for C++ [-Werror]
>
> QEMU_CXXFLAGS are sanitized in update_cxxflags() but when linking
> with the C++ linker the C specific flags are propagated via
> QEMU_LDFLAGS:
>
>    --extra-cflags=*) QEMU_CFLAGS="$QEMU_CFLAGS $optarg"
>                      QEMU_LDFLAGS="$QEMU_LDFLAGS $optarg"
>    ;;
>    --extra-cxxflags=*) QEMU_CXXFLAGS="$QEMU_CXXFLAGS $optarg"
>    ;;
>    --extra-ldflags=*) QEMU_LDFLAGS="$QEMU_LDFLAGS $optarg"
>                       EXTRA_LDFLAGS="$optarg"
>
> Maybe the change comes from commit db5adeaa84d:
> ("build-sys: clean up flags included in the linker command line").
>
> Any suggestion where to filter EXTRA_LDFLAGS for C++?
> Should I add a filtered EXTRA_LDXXFLAGS (similar to QEMU_CXXFLAGS)?

EXTRA_LDFLAGS is only used in a special case for Xen, so you can ignore it.

configure already specifies $QEMU_CFLAGS or $QEMU_CXXFLAGS on the linker 
command line.  Therefore, you can remove the assignment to QEMU_LDFLAGS 
from --extra-cflags=*.

Instead, in meson.build, you can use the link_language variable to add 
one of QEMU_CFLAGS or QEMU_CXXFLAGS to the linker, just before the 
existing call to add_project_link_arguments.  It's assigned a bit later 
but it's not an issue to pull the assignment earlier.

Paolo