The pkg-config files should define PROTOBUF_USE_DLLS if the libraries are shared

[coryan]

What version of protobuf and what language are you using?

Version: 22.4
Language: C++

What operating system (Linux, Windows, ...) and version?

Linux

What runtime / compiler are you using (e.g., python version or gcc version)

GCC 12

What did you do?

Compile protobuf as a shared library and then try to link it using pkg-config to find the necessary compiler and link flags.

What did you expect to see

A successful build.

What did you see instead?

make: Entering directory '/workspace/google/cloud/profiler/quickstart'
g++  -DGOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY -DHAVE_ABSEIL -pthread -DNOMINMAX -I/tmp/tmp.eh1b3XZLXZ/lib64/pkgconfig/../../include -I/usr/local/include -L/tmp/tmp.eh1b3XZLXZ/lib64/pkgconfig/../../lib64 -L/usr/local/lib64 -L/usr/local/lib -L/usr/lib64 -o /workspace/cmake-out/quickstart/makefile-profiler/quickstart quickstart.cc -lgoogle_cloud_cpp_profiler -lgoogle_cloud_cpp_grpc_utils -lgoogle_cloud_cpp_iam_protos -lgoogle_cloud_cpp_iam_v1_iam_policy_protos -lgoogle_cloud_cpp_iam_v1_options_protos -lgoogle_cloud_cpp_iam_v1_policy_protos -lgoogle_cloud_cpp_type_expr_protos -lgoogle_cloud_cpp_longrunning_operations_protos -lgoogle_cloud_cpp_rpc_status_protos -lgoogle_cloud_cpp_rpc_error_details_protos -lgoogle_cloud_cpp_common -lgoogle_cloud_cpp_profiler_protos -lgoogle_cloud_cpp_api_annotations_protos -lgoogle_cloud_cpp_api_client_protos -lgoogle_cloud_cpp_api_field_behavior_protos -lgoogle_cloud_cpp_api_http_protos -lgoogle_cloud_cpp_api_launch_stage_protos -lgoogle_cloud_cpp_api_resource_protos -lgrpc++ -lgrpc -lgpr -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 -lssl -lcrypto -lprotobuf -labsl_log_internal_check_op -labsl_leak_check -labsl_die_if_null -labsl_log_internal_conditions -labsl_log_internal_message -labsl_examine_stack -labsl_log_internal_format -labsl_log_internal_proto -labsl_log_internal_nullguard -labsl_log_internal_log_sink_set -labsl_log_sink -labsl_log_entry -labsl_flags -labsl_flags_internal -labsl_flags_marshalling -labsl_flags_reflection -labsl_flags_private_handle_accessor -labsl_flags_commandlineflag -labsl_flags_commandlineflag_internal -labsl_flags_config -labsl_flags_program_name -labsl_log_initialize -labsl_log_globals -labsl_log_internal_globals -labsl_hash -labsl_city -labsl_low_level_hash -labsl_raw_hash_set -labsl_hashtablez_sampler -labsl_statusor -labsl_status -labsl_cord -labsl_cordz_info -labsl_cord_internal -labsl_cordz_functions -labsl_exponential_biased -labsl_cordz_handle -labsl_crc_cord_state -labsl_crc32c -labsl_crc_internal -labsl_crc_cpu_detect -labsl_bad_optional_access -labsl_str_format_internal -labsl_strerror -labsl_synchronization -labsl_graphcycles_internal -labsl_stacktrace -labsl_symbolize -labsl_debugging_internal -labsl_demangle_internal -labsl_malloc_internal -labsl_time -labsl_civil_time -labsl_time_zone -labsl_bad_variant_access -lutf8_validity -lutf8_range -labsl_strings -labsl_strings_internal -lrt -labsl_base -labsl_spinlock_wait -labsl_int128 -labsl_throw_delegate -labsl_raw_logging_internal -labsl_log_severity -lz -lcares
make: Leaving directory '/workspace/google/cloud/profiler/quickstart'

-- stderr output is:
/usr/bin/ld: /tmp/ccZ3DEOv.o: in function `google::protobuf::internal::ThreadSafeArena::thread_cache()':
quickstart.cc:(.text._ZN6google8protobuf8internal15ThreadSafeArena12thread_cacheEv[_ZN6google8protobuf8internal15ThreadSafeArena12thread_cacheEv]+0x10): undefined reference to `google::protobuf::internal::ThreadSafeArena::thread_cache_'
collect2: error: ld returned 1 exit status
make: *** [Makefile:35: /workspace/cmake-out/quickstart/makefile-profiler/quickstart] Error 1

Anything else we should know about your project / environment

The error compiles from this file:

https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/profiler/quickstart/Makefile

The google_cloud_cpp_profiler pkg-config module depends on protobuf. protobuf.pc does not add -DPROTOBUF_USE_DLLS:

protobuf/cmake/protobuf.pc.cmake

Line 11 in da2c4a6

Cflags: -I${includedir}

But the ABI requires this to be defined if the library was built as a .so:

protobuf/src/google/protobuf/thread_safe_arena.h

Lines 264 to 271 in da2c4a6

	#elif defined(PROTOBUF_USE_DLLS)
	// Thread local variables cannot be exposed through DLL interface but we can
	// wrap them in static functions.
	static ThreadCache& thread_cache();
	#else
	PROTOBUF_CONSTINIT static PROTOBUF_THREAD_LOCAL ThreadCache thread_cache_;
	static ThreadCache& thread_cache() { return thread_cache_; }
	#endif

I will send a PR to add the necessary flag to the .pc file. But maybe a better fix would be to remove the #ifdef PROTOBUF_USE_DLLS from this header.