Undefined reference grpc and protobuf error - C++

Question

I am writing a grpc communication code between two entities, matchmaker and host. My makefile looks as below:

CXX = g++
LDFLAGS +=  `pkg-config --cflags --libs protobuf grpc grpc++`\
           -lgrpc++_reflection\
           -ldl


host: comm hosts/host.cc hosts/host.h
    $(CXX) $(LDFLAGS) hosts/host.cc build/matchmaker.grpc.pb.o build/matchmaker.pb.o -g -o build/host.o

comm: comm/matchmaker.grpc.pb.cc comm/matchmaker.grpc.pb.h comm/matchmaker.pb.cc comm/matchmaker.pb.h
    $(CXX) $(LDFLAGS) comm/matchmaker.grpc.pb.cc -g -c -o build/matchmaker.grpc.pb.o
    $(CXX) $(LDFLAGS) comm/matchmaker.pb.cc -g -c -o build/matchmaker.pb.o

I am getting multiple undefined reference errors for grpc and protobuf class codes. Some part of the error is shown below:

/usr/bin/ld: /tmp/cciFPkpo.o: in function `Host::RegisterHost()':
~/Projects/p1/hosts/host.cc:11: undefined reference to `grpc::ClientContext::ClientContext()'
/usr/bin/ld: ~/Projects/p1/hosts/host.cc:11: undefined reference to `grpc::ClientContext::~ClientContext()'
/usr/bin/ld: ~/Projects/p1/hosts/host.cc:11: undefined reference to `grpc::ClientContext::~ClientContext()'
/usr/bin/ld: /tmp/cciFPkpo.o: in function `main':
~/Projects/p1/hosts/host.cc:27: undefined reference to `grpc::InsecureChannelCredentials()'
.
.
.

~/Projects/p1/hosts/../comm/matchmaker.pb.h:895: undefined reference to `google::protobuf::RepeatedField<float>::Set(int, float const&)'

The output for pkg-config --cflags --libs protobuf grpc grpc++ is as below:

-DNOMINMAX -maes -msse4.1 -DNOMINMAX -maes -msse4.1 -DNOMINMAX -I~/grpc/include -L~/grpc/lib -lprotobuf -pthread -lgrpc++ -lgrpc -laddress_sorting -lre2 -lupb -lcares -lz -lgpr -lssl -lcrypto -labsl_raw_hash_set -labsl_hashtablez_sampler -labsl_hash -labsl_city -labsl_low_level_hash -labsl_random_distributions -labsl_random_seed_sequences -labsl_random_internal_pool_urbg -labsl_random_internal_randen -labsl_random_internal_randen_hwaes -labsl_random_internal_randen_hwaes_impl -labsl_random_internal_randen_slow -labsl_random_internal_platform -labsl_random_internal_seed_material -labsl_random_seed_gen_exception -labsl_statusor -labsl_status -labsl_cord -labsl_cordz_info -labsl_cord_internal -labsl_cordz_functions -labsl_exponential_biased -labsl_cordz_handle -labsl_bad_optional_access -labsl_str_format_internal -labsl_synchronization -labsl_graphcycles_internal -labsl_stacktrace -labsl_symbolize -labsl_debugging_internal -labsl_demangle_internal -labsl_malloc_internal -labsl_time -labsl_civil_time -labsl_strings -labsl_strings_internal -lrt -labsl_base -labsl_spinlock_wait -labsl_int128 -labsl_throw_delegate -labsl_time_zone -labsl_bad_variant_access -labsl_raw_logging_internal -labsl_log_severity

grpc and protobuf was previous installed via software center, which I removed. The current grpc and protobuf is built from the source code.

I am not sure how to resolve the undefined reference error. Any pointers will be really helpful. Thanks in advance.

Answer 1

You need to move the $(LDFLAGS) until after the object files that depend on it:

host: comm hosts/host.cc hosts/host.h
    $(CXX) hosts/host.cc build/matchmaker.grpc.pb.o build/matchmaker.pb.o -g -o build/host $(LDFLAGS)