CMakeLists: Adding source files from Github with ExternalProject - c++

Question

I'm writing a small catkin wrapper for the Beckhoff ADS library. I would like to install the files from the AdsLib folder but without using the CMake file from Beckhoff.
I just want to copy the files in order to add them to my library in my own CMakeLists. This works fine if I copy the files manually. But I would like to build directly using the latest files from the Github source.
I tried every possible combination I could find here on stackoverflow, but somehow couldn't make it work.
cmake_minimum_required(VERSION 2.8.3)
project(ads_catkin)
find_package(catkin_simple REQUIRED)
catkin_simple()
include(ExternalProject)
ExternalProject_Add(ads
PREFIX ${CMAKE_BINARY_DIR}/ads
GIT_REPOSITORY https://github.com/Beckhoff/ADS.git
GIT_TAG master
CONFIGURE_COMMAND ""
#GIT_TAG 6b3a03009a757cf651fe44d8be7b6df698028f0e
UPDATE_COMMAND ""
BUILD_COMMAND ""
INSTALL_COMMAND ""
)
ExternalProject_Get_Property(ads source_dir)
cs_add_library(${PROJECT_NAME}
${source_dir}/AdsLib/AdsDef.cpp
)
cs_install()
cs_export()
The first time I'm building with catkin_tools, this gives me:
Errors << ads_catkin:cmake /home/xxx/xxx_ws/logs/ads_catkin/build.cmake.000.log
CMake Error at /home/xxx/xxx_ws/devel/share/catkin_simple/cmake/catkin_simple-extras.cmake:150 (add_library):
Cannot find source file:
/home/xxx/xxx/build/ads_catkin/ads/src/ads/AdsLib/AdsDef.cpp
Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
.hxx .in .txx
Call Stack (most recent call first):
CMakeLists.txt:23 (cs_add_library)
CMake Error: CMake can not determine linker language for target: ads_catkin
CMake Error: Cannot determine link language for target "ads_catkin".
Then, the second time I run the build tool over the same code, the files get actually cloned, but I end up with this error:
Errors << ads_catkin:make /home/xxx/xxx_ws/logs/ads_catkin/build.make.000.log
make[2]: *** No rule to make target 'CMakeFiles/ads_catkin.dir/build'. Stop.
make[1]: *** [CMakeFiles/ads_catkin.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Cloning into 'ads'...
Already on 'master'
make: *** [all] Error 2
Btw: cs_add_library is the catkin_simple form for add_library and target_link_libraries.
Update:
Getting closer... adapting the solution from #Tsyvarev to my file:
cmake_minimum_required(VERSION 2.8.3)
project(ads_catkin)
find_package(catkin_simple REQUIRED)
catkin_simple()
include(ExternalProject)
ExternalProject_Add(ads
PREFIX ${CATKIN_DEVEL_PREFIX}/ads
GIT_REPOSITORY https://github.com/Beckhoff/ADS.git
GIT_TAG 6b3a03009a757cf651fe44d8be7b6df698028f0e
#GIT_TAG master
CONFIGURE_COMMAND ""
UPDATE_COMMAND ""
BUILD_COMMAND ""
INSTALL_COMMAND ""
BUILD_BYPRODUCTS
<SOURCE_DIR>/AdsLib/AdsDef.cpp
<SOURCE_DIR>/AdsLib/AdsLib.cpp
<SOURCE_DIR>/AdsLib/AmsConnection.cpp
<SOURCE_DIR>/AdsLib/AmsPort.cpp
<SOURCE_DIR>/AdsLib/AmsRouter.cpp
<SOURCE_DIR>/AdsLib/Frame.cpp
<SOURCE_DIR>/AdsLib/Log.cpp
<SOURCE_DIR>/AdsLib/NotificationDispatcher.cpp
<SOURCE_DIR>/AdsLib/Sockets.cpp
<SOURCE_DIR>/AdsLib/AdsDef.h
<SOURCE_DIR>/AdsLib/AdsLib.h
<SOURCE_DIR>/AdsLib/AdsNotification.h
<SOURCE_DIR>/AdsLib/AmsConnection.h
<SOURCE_DIR>/AdsLib/AmsHeader.h
<SOURCE_DIR>/AdsLib/AmsPort.h
<SOURCE_DIR>/AdsLib/AmsRouter.h
<SOURCE_DIR>/AdsLib/Frame.h
<SOURCE_DIR>/AdsLib/Log.h
<SOURCE_DIR>/AdsLib/NotificationDispatcher.h
<SOURCE_DIR>/AdsLib/RingBuffer.h
<SOURCE_DIR>/AdsLib/Router.h
<SOURCE_DIR>/AdsLib/Semaphore.h
<SOURCE_DIR>/AdsLib/Sockets.h
<SOURCE_DIR>/AdsLib/wrap_endian.h
<SOURCE_DIR>/AdsLib/wrap_socket.h
)
ExternalProject_Get_Property(ads SOURCE_DIR)
include_directories(
${SOURCE_DIR}/AdsLib
)
cs_add_library(AdsLib
${SOURCE_DIR}/AdsLib/AdsDef.cpp
${SOURCE_DIR}/AdsLib/AdsLib.cpp
${SOURCE_DIR}/AdsLib/AmsConnection.cpp
${SOURCE_DIR}/AdsLib/AmsPort.cpp
${SOURCE_DIR}/AdsLib/AmsRouter.cpp
${SOURCE_DIR}/AdsLib/Frame.cpp
${SOURCE_DIR}/AdsLib/Log.cpp
${SOURCE_DIR}/AdsLib/NotificationDispatcher.cpp
${SOURCE_DIR}/AdsLib/Sockets.cpp
)
add_dependencies(AdsLib ads)
cs_install()
install(DIRECTORY ${SOURCE_DIR}/AdsLib/
DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
PATTERN ".cpp" EXCLUDE
)
cs_export()
I tried to include the header files in my AdsLib library. But I still get an error:
In file included from /home/xxx/xxx_ws/src/xxx/nav_controller/src/controller.cpp:19:0:
/home/xxx/xxx_ws/src/xxx/nav_controller/include/nav_controller/controller.h:27:10: fatal error: AdsLib.h: No such file or directory
#include "AdsLib.h"
^~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/nav_controller.dir/src/controller.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
In file included from /home/xxx/xxx_ws/src/xxx/nav_controller/src/test.cpp:22:0:
/home/xxx/xxx_ws/src/xxx/nav_controller/include/nav_controller/controller.h:27:10: fatal error: AdsLib.h: No such file or directory
#include "AdsLib.h"
^~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/nav_controller.dir/src/test.cpp.o] Error 1
make[1]: *** [CMakeFiles/nav_controller.dir/all] Error 2
make: *** [all] Error 2

Answer 1

The CMake error Cannot find source file means a simple thing: Nothing tells CMake that given source file is generated and the file itself is absent.
Because the file is generated in ExternalProject_Add, you need to adjust corresponded target-level dependency:
add_dependency(ads_catkin ads)
This command should be issued after both add_library() and ExternalProject_Add calls which create corresponded targets. This command tells CMake that the library should be built only after all steps for the external project has been performed.
You still need to tell CMake that the source file is generated. There are two ways for doing this.
Set GENERATED property:
set_source_files_properties(${source_dir}/AdsLib/AdsDef.cpp PROPERTIES GENERATED TRUE)
This prevents CMake to search the file on the configuration stage.
List file in the BYPRODUCTS option for the target which generates it. For the target created by ExternalProject_Add command this is achieved by additional option to that command
BUILD_BYPRODUCTS <SOURCE_DIR>/AdsLib/AdsDef.cpp
This also sets GENERATED property for the source file, as in the first case. But BYPRODUCTS also makes your project usable for Ninja users.
(In the option above expression <SOURCE_DIR> is a special way to refer to ExternalProject's source directory. It is usable for some ExternalProject's options, and BUILD_BYPRODUCTS is one of them).
Technically, CMake should be smart enough for adding target-level dependencies (the effect of add_dependency(ads_catkin ads)) automatically when it sees corresponded BYPRODUCTS option. But this feature is described only for 3.16 version, and I don't know whether it works in older versions.

Answer 2

In the end, even with the good help from #Tsyvarev, I couldn't make it work using ExternalProject_Add. Part of the solution is to manually export the library with cs_export(INCLUDE_DIRS ${ads}), which didn't work with ExternalProject_Add, because the files were not available at this time.
In the end, I included the DownloadProject module, which is a predecessor of FetchContent, in my package. This does what it should.