Copy compilation/linking settings from project in CMake object library? - cmake

Question

I'm in a similar situation to Adding object-file dependencies where it is mentioned:
... I need some object files that are part of the same library to be compiled before others.
Don't try to declare dependencies between object files. If there are files that have a dependency, break them out into a separate library with add_library and then declare the dependency with add_dependencies and target_link_libraries. There is no extra cost for doing this. Particularly, consider looking at Object Libraries.
I tried to implement this in my original project, where I'm working with a massive CMakeLists.txt file, where include & linker paths may be conditionally inserted or removed, and I failed. In essence, that CMakeLists.txt file would looks like this simplified:
project(MY_PROGRAM C CXX ASM)
set(MY_PROGRAM_sources
main.c
file_one.c
file_two.c
)
add_executable(MY_PROGRAM ${MY_PROGRAM_sources})
target_include_directories(MY_PROGRAM PRIVATE
${CUSTOM_PATH}/src/custom/include
...
)
add_compile_options(-Wall
-Wno-format
...
)
target_link_libraries(MY_PROGRAM
somelib1
somelib2
)
This builds fine, using Unix Makefiles.
Now, I've tried to extract file_one and file_two objects like this:
project(MY_PROGRAM C CXX ASM)
set(MY_PROGRAM_sources
main.c
#file_one.c
#file_two.c
$<TARGET_OBJECTS:SEPFILE_OBJS>
)
add_library(SEPFILE_OBJS OBJECT
file_one.c
file_two.c
)
add_executable(MY_PROGRAM ${MY_PROGRAM_sources})
target_include_directories(MY_PROGRAM PRIVATE
${CUSTOM_PATH}/src/custom/include
...
)
add_compile_options(-Wall
-Wno-format
...
)
target_link_libraries(MY_PROGRAM
somelib1
somelib2
)
cmake passes fine here, but when I hit make, there is breakage as soon as file_one.c starts compiling:
...
[ 9%] Building C object CMakeFiles/SEPFILE_OBJS.dir/file_one.c.obj
In file included from C:/tmp/file_one.c:14:
C:/tmp/file_one.h:19:10: fatal error: ExoticHeader.h: No such file or directory
19 | #include <ExoticHeader.h>
Clearly, the SEPFILE_OBJS library did not inherit the include libraries nor other compilation/linker settings from the MY_PROGRAM executable.
I could probably repeat all target_* lines for SEPFILE_OBJS - but that looks quite unmanageable, plus my original CMakeLists.txt is huge, and I'd like to avoid that kind of work.
So is there a way to say to CMake "keep the same compilation and linker settings as in executable for MY_PROGRAM, also for the object library SEPFILE_OBJS"?

Answer 1

I would do it the way around. I would first define a library section, including file_one.c and file_two.c, and then a main executable section, which would link against this library target.
I would also follow something along the lines below for a directory structure (ideally, I would put the source files within a src directory):
/ MY_PROGRAM
|- include
| \- MY_PROGRAM
| |- file_one.h
| \- fine_two.h
|- file_one.c
|- file_two.c
|- main.c
\- CMakeLists.txt
Put your header files in an include/${PROJECT_NAME} folder, and set an include_dir var pointing to it.
set(include_dir ${PROJECT_SOURCE_DIR}/include/${PROJECT_NAME})
Define the list of library and application sources:
set(lib_sources
"${CMAKE_CURRENT_SOURCE_DIR}/file_one.c"
"${CMAKE_CURRENT_SOURCE_DIR}/file_two.c"
)
set(app_sources
"${CMAKE_CURRENT_SOURCE_DIR}/main.cpp"
)
Library section. Define a lib_MY_PROGRAM target, and set the include directories, compile options, and link libraries for it. Notice it's better to use the target_... specific commands than the more general ones, such as add_compile_options:
add_library(lib_${PROJECT_NAME} ${lib_sources})
target_include_directories(lib_${PROJECT_NAME} PUBLIC
"$<BUILD_INTERFACE:${include_dir}>"
"$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
)
target_include_directories(lib_${PROJECT_NAME} SYSTEM PUBLIC
${CUSTOM_PATH}/src/custom/include
)
target_compile_options(lib_${PROJECT_NAME} PRIVATE
-Wall
-Wno-format
)
target_link_libraries(lib_${PROJECT_NAME} PRIVATE
somelib1
somelib2
)
Main binary section:
add_executable(${PROJECT_NAME} ${app_sources})
target_include_directories(${PROJECT_NAME} PUBLIC
"$<BUILD_INTERFACE:${include_dir}>"
"$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
)
target_compile_options(lib_${PROJECT_NAME} PRIVATE
-Wall
-Wno-format
)
target_link_libraries(${PROJECT_NAME} PRIVATE lib_${PROJECT_NAME})