[Buildroot] [PATCHv2] core: alternate solution to disable C++

Peter Korsgaard peter at korsgaard.com
Fri Mar 30 12:25:04 UTC 2018


>>>>> "Yann" == Yann E MORIN <yann.morin.1998 at free.fr> writes:

 > Some packages that use libtool really need some love to be able to
 > disable C++ support.

 > This is because libtool will want to call AC_PROG_CXXCPP as soon as CXX
 > is set non-empty to something different from 'no'. Then, AC_PROG_CXXCPP
 > will want a C++ preprocessor that works on valid input *and* fail on
 > invalid input.

 > So, providing 'false' as the C++ compiler will then require that we do
 > have a working C++ preprocessor. Which is totally counter-productive
 > since we do not have a C++ compiler to start with...

 > bd39d11d2e (core/infra: fix build on toolchain without C++) was a
 > previous attempt at fixing this, by using the host's C++ preprocessor.

 > However, that is very incorrect (that's my code, I can say so!) because
 > the set of defines will most probably be different for the host and the
 > target, thus causign all sorts of trouble. For example, on ARM we'd have
 > to include different headers for soft-float vs hard-float, which is
 > decided based on a macro, which is not defined for x86, and thus may
 > redirect to the wrong (and missing) header.

 > Instead, we notice that libtool uses the magic value 'no' to decide that
 > a C++ compiler is not available, in which case it skeips the call to
 > AC_PROG_CXXCPP.

 > Given that 'no' is not provided by any package in Debian and
 > derivatives, as well as in Fedora, we can assume that no system will
 > have an executable called 'no'. Hence, we use that as a magic value to
 > disable C++ detection altogether.

 > Fixes: #10846 (again)

 > Reported-by: Damien Riegel <damien.riegel at savoirfairelinux.com>
 > Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
 > Cc: Damien Riegel <damien.riegel at savoirfairelinux.com>
 > Cc: Peter Seiderer <ps.report at gmx.net>
 > Cc: Vivien Didelot <vivien.didelot at savoirfairelinux.com>
 > Cc: Peter Korsgaard <peter at korsgaard.com>
 > Cc: Thomas Petazzoni <thomas.petazzoni at bootlin.com>

 > ---
 > Changes v1 -> v2:
 >   - add big fat comment...

Committed, thanks.

-- 
Bye, Peter Korsgaard



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