netdev - Re: [PATCH bpf-next v2] libbpf: make sure bpf headers are c++ include-able

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Date:   Tue, 20 Nov 2018 17:59:52 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, ys114321@...il.com
Subject: Re: [PATCH bpf-next v2] libbpf: make sure bpf headers are c++
 include-able

On 11/20, Alexei Starovoitov wrote:
> On Tue, Nov 20, 2018 at 04:05:55PM -0800, Stanislav Fomichev wrote:
> > On 11/20, Alexei Starovoitov wrote:
> > > On Tue, Nov 20, 2018 at 01:37:23PM -0800, Stanislav Fomichev wrote:
> > > > Wrap headers in extern "C", to turn off C++ mangling.
> > > > This simplifies including libbpf in c++ and linking against it.
> > > > 
> > > > v2 changes:
> > > > * do the same for btf.h
> > > > 
> > > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > > ---
> > > >  tools/lib/bpf/bpf.h    | 9 +++++++++
> > > >  tools/lib/bpf/btf.h    | 8 ++++++++
> > > >  tools/lib/bpf/libbpf.h | 9 +++++++++
> > > >  3 files changed, 26 insertions(+)
> > > > 
> > > > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> > > > index 26a51538213c..9ea3aec82d8a 100644
> > > > --- a/tools/lib/bpf/bpf.h
> > > > +++ b/tools/lib/bpf/bpf.h
> > > > @@ -27,6 +27,10 @@
> > > >  #include <stdbool.h>
> > > >  #include <stddef.h>
> > > >  
> > > > +#ifdef __cplusplus
> > > > +extern "C" {
> > > > +#endif
> > > 
> > > Acked-by: Alexei Starovoitov <ast@...nel.org>
> > > 
> > > was wondering whether it's possible to make it testable.
> > > HOSTCXX is available, but I don't see much of the kernel tree
> > > using it...
> > By testable you mean compile some dummy c++ main and link against libbpf?
> 
> yes. something like this.
> to make sure that it keeps being functional and no one introduces 'int new'
> in some function argument list by accident.
I tried something like the patch below, it does seem to work locally (building
in the same directory, no cross-compile). Who would be the best to
review that kind of stuff?

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index 425b480bda75..4c0e58628aad 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -66,7 +66,7 @@ ifndef VERBOSE
 endif
 
 FEATURE_USER = .libbpf
-FEATURE_TESTS = libelf libelf-mmap bpf reallocarray
+FEATURE_TESTS = libelf libelf-mmap bpf reallocarray cxx
 FEATURE_DISPLAY = libelf bpf
 
 INCLUDES = -I. -I$(srctree)/tools/include -I$(srctree)/tools/arch/$(ARCH)/include/uapi -I$(srctree)/tools/include/uapi
@@ -148,6 +148,10 @@ LIB_FILE := $(addprefix $(OUTPUT),$(LIB_FILE))
 
 CMD_TARGETS = $(LIB_FILE)
 
+ifeq ($(feature-cxx), 1)
+	CMD_TARGETS += $(OUTPUT)test_libbpf
+endif
+
 TARGETS = $(CMD_TARGETS)
 
 all: fixdep all_cmd
@@ -175,6 +179,9 @@ $(OUTPUT)libbpf.so: $(BPF_IN)
 $(OUTPUT)libbpf.a: $(BPF_IN)
 	$(QUIET_LINK)$(RM) $@; $(AR) rcs $@ $^
 
+$(OUTPUT)test_libbpf: test_libbpf.cpp $(OUTPUT)libbpf.a
+	$(QUIET_LINK)$(CXX) $^ -lelf -o $@
+
 define do_install
 	if [ ! -d '$(DESTDIR_SQ)$2' ]; then		\
 		$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$2';	\