How to install subdirectories to different locations with CMake? - installation

Question

I have a couple of CMake projects which have their own install logic defined. Now, I want to add a global CMakeLists.txt that adds those projects using add_subdirectory() in order to build and install all of them at once. How can I provide a different ${CMAKE_INSTALL_PREFIX} to each of them from the outside?
What I tried is changing the install prefix between the calls to add the subdirectories but they are installed when actually calling make install and at that time, the install prefix is already set to the last assigned value.

Answer 1

If your subprojects require same variable to have different values(at global scope) you cannot build them using add_subdirectory within single global project.
Instead, global project may install them using cmake call either via execute_process or ExternalProject_Add. Sometimes, install(SCRIPT) command may be used for that.
Installing using execute_process makes subproject available immediately at configuration stage.
It can be used when subproject has some packaging mechanism (see CMake tutorials about packaging), so global object may execute find_package(<subproject-name>) and link with libraries, defined in subproject, in the most simple way(using variables, filled outside).
# Configure subproject into <subproject-build-dir>
execute_process(COMMAND ${CMAKE_COMMAND} -D<var>=<value> <subproject-source-dir>
WORKING_DIRECTORY <subproject-build-dir>)
# Build and install subproject
execute_process(COMMAND ${CMAKE_COMMAND} --build <subproject-build-dir> --target install)
# Extract variables, related to subproject for link with it.
find_package(<subproject-name>)
# Create target in global project and link with subproject
include_directories(${<subproject-name>_INCLUDE_DIRS})
link_directories(${<subproject-name>_LIBRARY_DIRS})
add_executable(global_program main.cpp)
target_link_libraries(global_program ${<subproject-name>_LIBRARIES})
Installing using ExternalProject_add assigns target to the subproject, which will be installed at build stage. Linking with subproject in that case is also possible, but requires to fill variables manually.
# Configure, build and install subproject
ExternalProject_Add(<subproject_target>
SOURCE_DIR <subproject-source-dir>
CMAKE_CACHE_ARGS -D<var>=<value>
BINARY_DIR <subproject-build-dir>
INSTALL_DIR <CMAKE_INSTALL_PREFIX-for-subproject>
)
# Create target in global project and link with subproject
include_directories(<subproject-include-files-location>)
link_directories(<subproject-libraries-location>)
add_executable(global_program main.cpp)
target_link_libraries(global_program <subproject-libraries-name>)
# Make sure that subproject will be built before executable.
add_dependencies(global_program <subproject_target>)
Installing using install(SCRIPT) executes script at install stage. This approach can be used when there is no build-dependencies between global project and subproject.
subproject_install.cmake:
# Configure subproject into <subproject-build-dir>
execute_process(COMMAND ${CMAKE_COMMAND} -D<var>=<value> <subproject-source-dir>
WORKING_DIRECTORY <subproject-build-dir>)
# Build and install subproject
execute_process(COMMAND ${CMAKE_COMMAND} --build <subproject-build-dir> --target install)
CMakeLists.txt:
install(SCRIPT subproject_install.cmake)