CMake with protobuf file in subdirectory - c++

Question

I have seen a lot of similar questions and answers, but until now it seems not so obvious to get it working. I am quite new to CMake and until now everything was easy except the integration with protocol buffers.
I have a project with subdirectories, where each subdirectory has its own CMakeLists.txt
One of the subdirectory contains a .proto file. If the PROTOBUF_GENERATE_CPP macro is executed it generates the sources and the headers files. This macro is invoked from the CMakeLists.txt in the subdirectory containing the .proto file.
It seems however the make file is not invoked because no sources are added to the target. I can not add the sources to the target, because the files do not exist, they exist after generation, so this results in an error when CMake runs.
Setting the file properties to generated seems also not to help. In general, before the build process starts the macro should have been run to generated the source files.
How to do this, any working examples ?
Example:
./src/externals/protodef (from other repository, only contains .proto files)
./src/generated (supposed for the generated c and header files by protoc)
CMakeLists-1 (project root)
cmake_minimum_required (VERSION 2.6)
PROJECT (prototest)
ADD_SUBDIRECTORY("${PROJECT_SOURCE_DIR}/src/externals/protodef")
ADD_SUBDIRECTORY("${PROJECT_SOURCE_DIR}/src")
SET_SOURCE_FILES_PROPERTIES(${PROTO_SOURCES} ${PROTO_HEADERS} PROPERTIES GENERATED TRUE)
ADD_EXECUTABLE(prototest ${PROTO_SOURCES} ${SOURCE} )
TARGET_LINK_LIBRARIES(prototest ${EXTERNAL_LIBS} )
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
CMakeLists-2 (src)
SET(SOURCE ${SOURCE}
${CMAKE_CURRENT_SOURCE_DIR}/main.cpp
PARENT_SCOPE
)
CMakeLists-3 (src/externals/protodef)
SET(PROTOBUF_PATH "D:/protobuf-3.0.0/" )
SET(CMAKE_PREFIX_PATH ${CMAKE_PREFIX_PATH} "${PROTOBUF_PATH}")
# Changing PROTO_SRCS and PROTO_HDRS does not work for setting the location
# of the generated files.
# Those variable are ignored by CMake for compiling the proto files.
# Using a dedicated CMakeLists.txt and settng CURRENT_BINARY dir is a
# workaround to get them where we want.
SET(GENERATED_DIR ${PROJECT_SOURCE_DIR}/src/generated )
SET(CMAKE_CURRENT_BINARY_DIR ${GENERATED_DIR} )
INCLUDE(FindProtobuf)
FIND_PACKAGE(Protobuf REQUIRED)
PROTOBUF_GENERATE_CPP(PROTO_SRCS PROTO_HDRS${CMAKE_CURRENT_SOURCE_DIR}/test1.proto)
SET( EXTERNAL_LIBS ${PROTOBUF_PATH}/lib/libprotobuf.a PARENT_SCOPE)
# Propagate sources to the parant project
SET(PROTO_SOURCES ${PROTO_SRCS}
PARENT_SCOPE
)
SET(PROTO_HEADERS ${PROTO_HDRS}
PARENT_SCOPE
)

Answer 1

First generate the protobuf files, then add them to a CMake target.
CMakeLists (src) :
# Generate h/cpp proto files (./src/externals/protodef) into ./src/generated folder
PROTOBUF_GENERATE_CPP(...)
# Process subdir
ADD_SUBDIRECTORY(generated)
It seems that PROTOBUF_GENERATE_CPP can only be used in the same subdirectory. A possible workaround is to invoke protoc directly instead :
FILE(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/src/externals/protodef PROTOMODEL_PATH)
FILE(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/src/generated PROTOBINDING_PATH)
FILE(GLOB DATAMODEL_PROTOS "${CMAKE_CURRENT_SOURCE_DIR}/externals/protodef/*.proto")
FOREACH(proto ${DATAMODEL_PROTOS})
FILE(TO_NATIVE_PATH ${proto} proto_native)
EXECUTE_PROCESS(COMMAND ${PROTOBUF_PROTOC_EXECUTABLE} --proto_path=${PROTOMODEL_PATH} --cpp_out=${PROTOBINDING_PATH} ${proto_native}
RESULT_VARIABLE rv)
# Optional, but that can show the user if something have gone wrong with the proto generation
IF(${rv})
MESSAGE("Generation of data model returned ${rv} for proto ${proto_native}")
ENDIF()
ENDFOREACH(proto)
CMakeLists (src/generated) :
## List generated sources files
FILE(GLOB HDRS "*.h")
FILE(GLOB SRCS "*.cc")
ADD_LIBRARY(protoBinding ${HDRS} ${SRCS})
# ${PROTOBUF_LIBRARIES} should be defined by FIND_PACKAGE(Protobuf REQUIRED)
TARGET_LINK_LIBRARIES(protoBinding ${PROTOBUF_LIBRARIES})
This way CMake will first generate the header/source files, and only then add the generated files to a CMake target.
You can then use protoBinding target to link the generated files to an other target (e.g at the end of src's CMakeLists.txt) :
ADD_LIBRARY(myModel ${myFiles})
TARGET_LINK_LIBRARIES(myModel protoBinding)