CMake target dependency to compile protobuf files - cmake

Question

I want to build a c++ static lib out of some protobuf definitions with cmake/make.
I made a custom COMMAND to compile the protobuf to c++, and I set it as a PRE_BUILD dependency to my static lib.
project(mylib)
set(PROTO_PATH "${CMAKE_CURRENT_SOURCE_DIR}/proto_definitions")
file(GLOB PROTO_FILES "${PROTO_PATH}/*.proto")
foreach(PROTO_FILE in ${PROTO_FILES})
string(REGEX REPLACE "[.]proto$" ".pb.cc" OUTPUT_SOURCE ${PROTO_FILE})
list(APPEND OUTPUT_SOURCES ${OUTPUT_SOURCE})
endforeach()
add_custom_command(TARGET ${PROJECT_NAME}
PRE_BUILD
COMMAND protoc --cpp_out=${CMAKE_CURRENT_SOURCE_DIR}/compiled_proto ${PROTO_FILES}
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
COMMENT "some comment")
add_library(${PROJECT_NAME} STATIC ${OUTPUT_SOURCES})
target_include_directories(${PROJECT_NAME} PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
I get the following error when running cmake:
CMake Error: cannot determine link language for target "mylib"
Regardless of this error, the makefiles are generated, but when I make mylib, it does not trigger any proto compilation

Answer 1

The approach more native to CMake would be to add custom commands with the OUTPUT signature to generate the .cc files, and then use them as sources for the library normally. That way, they CMake will know what they are and how to produce them:
project(mylib)
set(PROTO_PATH "${CMAKE_CURRENT_SOURCE_DIR}/proto_definitions")
file(GLOB PROTO_FILES "${PROTO_PATH}/*.proto")
foreach(PROTO_FILE in ${PROTO_FILES})
string(REGEX REPLACE "[.]proto$" ".pb.cc" OUTPUT_SOURCE ${PROTO_FILE})
list(APPEND OUTPUT_SOURCES ${OUTPUT_SOURCE})
endforeach()
add_custom_command(OUTPUT ${OUTPUT_SOURCES}
COMMAND protoc --cpp_out=${CMAKE_CURRENT_SOURCE_DIR}/compiled_proto ${PROTO_FILES}
DEPENDS ${PROTO_FILES}
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
COMMENT "some comment")
add_library(${PROJECT_NAME} STATIC ${OUTPUT_SOURCES})
target_include_directories(${PROJECT_NAME} PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
Done this way, there will be one command which reads all the .proto files and produces all the .cc files — which means that if any of the .proto file changes, all the .cc files will be re-generated. I am not familiar with Protobuffers so I cannot know whether that's sane or not. If they are independent, it would be better to introduce one add_custom_command for each output file.
Also, given the arguments you're passing to protocc, you might have to modify the paths in OUTPUT_SOURCES to correctly point to the generated files.
Also note that CMake comes with a FindProtobuf module which defines a protobuf_generate_cpp() command, so you might want to use that instead of hand-coding the Protobuf support.

Answer 2

Comment for the answer above: There is no in in cmake foreach, which spend me some time to solve it.
After studying his answer(very thanks!), I summarized a method that works for me, which has the following characteristics:
Proto dir and Output dir can be different
Generate grpc_output
Create a custom_command for each proto
My project strcuture is:
hwpb/ (a library for super project)
cpp/
hwpb/ (store *.pb.cc, *.pb.h)
CMakeLists.txt
proto/ (store protos)
go/ (and other languages, not mentioned here)
And this is my CMakeListx.txt:
set(PROTO_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../proto")
set(OUTPUT_DIR "${CMAKE_CURRENT_SOURCE_DIR}/hwpb")
file(GLOB PROTO_FILES "${PROTO_DIR}/*.proto")
foreach(PROTO_FILE ${PROTO_FILES})
get_filename_component (FILENAME ${PROTO_FILE} NAME_WLE)
set(PROTO_SRC "${OUTPUT_DIR}/${FILENAME}.pb.cc")
set(PROTO_HDR "${OUTPUT_DIR}/${FILENAME}.pb.h")
set(GRPC_SRC "${OUTPUT_DIR}/${FILENAME}.grpc.pb.cc")
set(GRPC_HDR "${OUTPUT_DIR}/${FILENAME}.grpc.pb.h")
add_custom_command(
OUTPUT "${PROTO_SRC}" "${PROTO_HDR}" "${GRPC_SRC}" "${GRPC_HDR}"
COMMAND ${_PROTOBUF_PROTOC}
ARGS --plugin=protoc-gen-grpc=${_GRPC_CPP_PLUGIN_EXECUTABLE}
--cpp_out="${OUTPUT_DIR}" --grpc_out="${OUTPUT_DIR}"
-I"${PROTO_DIR}" "${PROTO_FILE}"
DEPENDS ${PROTO_FILE}
)
list(APPEND OUTPUT_SOURCES ${PROTO_SRC} ${GRPC_HDR})
endforeach()
add_library(hwpb ${OUTPUT_SOURCES})
target_link_libraries(hwpb ${_GRPC_GRPCPP_UNSECURE} ${_PROTOBUF_LIBPROTOBUF})
target_include_directories(hwpb PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})