PROTOBUF_GENERATE_CPP not generating src and header files - cmake

Question

My cmake configuration is not generating any protobuf src and header files.
I've already checked if the proto files can be found.
Cmakelists.txt
cmake_minimum_required(VERSION 3.0.2)
..
include(FindProtobuf REQUIRED)
file(GLOB PROTO_DEF "${CMAKE_CURRENT_SOURCE_DIR}/protobuf/*/*.proto")
foreach(file ${PROTO_DEF})
if(EXISTS ${file})
MESSAGE("YES")
else()
MESSAGE("NO")
endif()
endforeach()
SET(PROTOBUF_GENERATE_CPP_APPEND_PATH PROTOBUF)
SET(PROTOBUF_PROTOC_EXECUTABLE protoc.exe)
..
PROTOBUF_GENERATE_CPP(PROTO_SRC PROTO_INCL ${PROTO_DEF})
..
add_library(${PROJECT_NAME} STATIC ${INCLUDES} ${INTERNAL_INCLUDES} ${SRC} ${PROTO_SRC} ${PROTO_INCL})
target_link_libraries(${PROJECT_NAME} ${PROTOBUF_LIBRARIES})
I have checked the FindProtobuf.cmake and half way through:
foreach(FIL ${ARGN})
get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
get_filename_component(FIL_WE ${FIL} NAME_WE)
list(APPEND ${SRCS} "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.cc")
list(APPEND ${HDRS} "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.h")
MESSAGE(1 ${CMAKE_CURRENT_BINARY_DIR})
MESSAGE(2 ${_protobuf_include_path})
MESSAGE(3 ${ABS_FIL})
MESSAGE(4 ${PROTOBUF_PROTOC_EXECUTABLE})
add_custom_command(
OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.cc"
"${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.h"
COMMAND ${PROTOBUF_PROTOC_EXECUTABLE}
ARGS --cpp_out ${CMAKE_CURRENT_BINARY_DIR} ${_protobuf_include_path} ${ABS_FIL}
DEPENDS ${ABS_FIL}
COMMENT "Running C++ protocol buffer compiler on ${FIL}"
VERBATIM )
endforeach()
You can see i've added the 4 message commands, the script reaches this point and variables show good values.
The proto files do have a depencency to the library, thus the command should get executed !?
Any ideas on this problem?
update
replacing the add_custom_command with
EXEC_PROGRAM(${PROTOBUF_PROTOC_EXECUTABLE} ARGS --cpp_out ${CMAKE_CURRENT_BINARY_DIR} ${_protobuf_include_path} ${ABS_FIL}
does generate the source and header files, must i manually activate the custom_commmands?
Regards Auke

Answer 1

The add_custom_command is fired (for my project) during compilation time.
Adding
SET_SOURCE_FILES_PROPERTIES(${PROTO_SRC} ${PROTO_INCL} PROPERTIES GENERATED TRUE)
gives cmake info that the files will be generated.

Answer 2

Alternative if you have an external proto folder:
file(GLOB PROTOBUF_FILELIST ${PROTO_INCLUDE_DIR}/*.proto)
foreach( proto_file ${PROTOBUF_FILELIST} )
get_filename_component(proto_name ${proto_file} NAME_WE)
get_filename_component(proto_path ${PROTO_INCLUDE_DIR} ABSOLUTE)
set_source_files_properties("${proto_path}/${proto_name}.pb.cc"
"${proto_path}/${proto_name}.pb.h"
PROPERTIES GENERATED TRUE)
endforeach()

Answer 3

See that:
https://cmake.org/cmake/help/v3.20/module/FindProtobuf.html
Example:
find_package(Protobuf REQUIRED)
include_directories(${Protobuf_INCLUDE_DIRS})
include_directories(${CMAKE_CURRENT_BINARY_DIR})
protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS foo.proto)
protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS EXPORT_MACRO DLL_EXPORT foo.proto)
protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS DESCRIPTORS PROTO_DESCS foo.proto)
protobuf_generate_python(PROTO_PY foo.proto)
add_executable(bar bar.cc ${PROTO_SRCS} ${PROTO_HDRS})
target_link_libraries(bar ${Protobuf_LIBRARIES})
Note The protobuf_generate_cpp and protobuf_generate_python functions and add_executable() or add_library() calls only work properly within the same directory.

Answer 4

This problem only happens when the target is a library (using add_library, add_executable seems to be fine). The PROTOBUF_GENERATE_CPP will defer the generation of protobuf sources and headers when building the library. A workaround is to define a custom target and add it as a dependency to the lib target. e.g.
add_custom_target(proto_dep DEPENDS ${PROTO_SRCS} ${PROTO_HDRS})
add_library(${PROJECT_NAME} STATIC ${OTHER_SRCS})
add_dependencies(${PROJECT_NAME} proto_dep)
target_link_libraries(${PROJECT_NAME} PRIVATE gRPC::grpc++ gRPC::grpc++_reflection protobuf::libprotobuf)