Re: [PATCH bpf-next 3/5] selftests/bpf: fix test_cpp compilation failure with clang

[Yonghong Song <yhs@fb.com>]

On 4/12/21 9:32 PM, Andrii Nakryiko wrote:
> On Sat, Apr 10, 2021 at 9:49 AM Yonghong Song <yhs@fb.com> wrote:
>>
>> With clang compiler:
>>    make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
>>    make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
>> the test_cpp build failed due to the failure:
>>    warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated [-Wdeprecated]
>>    clang-13: warning: cannot specify -o when generating multiple output files
>>
>> test_cpp compilation flag looks like:
>>    clang++ -g -Og -rdynamic -Wall -I<...> ... \
>>    -Dbpf_prog_load=bpf_prog_test_load -Dbpf_load_program=bpf_test_load_program \
>>    test_cpp.cpp <...>/test_core_extern.skel.h <...>/libbpf.a <...>/test_stub.o \
>>    -lcap -lelf -lz -lrt -lpthread -o <...>/test_cpp
>>
>> The clang++ compiler complains the header file in the command line.
>> Let us remove the header file from the command line which is not intended
>> any way, and this fixed the problem.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   tools/testing/selftests/bpf/Makefile | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>> index 6448c626498f..bbd61cc3889b 100644
>> --- a/tools/testing/selftests/bpf/Makefile
>> +++ b/tools/testing/selftests/bpf/Makefile
>> @@ -481,7 +481,7 @@ $(OUTPUT)/test_verifier: test_verifier.c verifier/tests.h $(BPFOBJ) | $(OUTPUT)
>>   # Make sure we are able to include and link libbpf against c++.
>>   $(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
>>          $(call msg,CXX,,$@)
>> -       $(Q)$(CXX) $(CFLAGS) $^ $(LDLIBS) -o $@
>> +       $(Q)$(CXX) $(CFLAGS) test_cpp.cpp $(BPFOBJ) $(LDLIBS) -o $@
> 
> see what we do for other binaries:
> 
> $(filter %.a %.o %.c,$^)
> 
> It's more generic. Add %.cpp, of course.

Okay, although I will remove %.c.

[yhs@devbig003.ftw2 ~/tmp/tmp2]$ clang++ -c t.c
clang-13: warning: treating 'c' input as 'c++' when in C++ mode, this 
behavior is deprecated [-Wdeprecated]
[yhs@devbig003.ftw2 ~/tmp/tmp2]$

clang++ will warn if trying to compile .c file.
g++ 8.4.1 doesn't warn. Not sure about latest g++. But it is not
a good practice to compile .c files with c++ compiler any way.

> 
>>
>>   # Benchmark runner
>>   $(OUTPUT)/bench_%.o: benchs/bench_%.c bench.h
>> --
>> 2.30.2
>>