[SOLVED] Cmake : Copy in library file if not present

Issue

I have a simple C++ project simple_test that depends on an external prebuilt library mylib.

The project file structure looks like this

CMakeLists.txt
main.cpp
external_dependencies/
   libmylib.a

With the Cmake file looking like this

make_minimum_required(VERSION 3.17)
project(simple_test)

set(CMAKE_CXX_STANDARD 17)

add_executable(simple_test main.cpp)

target_link_directories( simple_test PUBLIC
        external_dependencies/
        )

target_link_libraries( simple_test
        mylib
        )

This builds and links fine.

What I would like to do though is have the project detect and obtain the mylib file if not present. When I say obtain, all I am attempting for now is to copy libmylib.a from another location into external_dependencies but a full implementation would involve more steps.

I found these relevant questions on stack overflow and the cmake manuals

• Running a bash command via CMake
• CMake add_custom_command with target_link_libraries

so I am led to believe that I need to use add_custom_command and to add the lib file to a target.

I've created this custom command to copy the library into place and set up a target for the executable to depend on it.

add_custom_command( OUTPUT external_dependencies/libmylib.a
        WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
        COMMAND cp depsrc/libmylib.a external_dependencies/
        COMMENT "CreatedLib"
        )

add_custom_target(mylib_target
        DEPENDS external_dependencies/libmylib.a
       )

add_dependencies( simple_test
        mylib_target
      )

This works to copy the file but is also executed on every build.

I've then tried using an IMPORTED library:

target_link_libraries( simple_test
        mylib_target
        )

add_library(mylib_target STATIC IMPORTED)
set_property(TARGET mylib_target PROPERTY
       IMPORTED_LOCATION external_dependencies/libmylib.a
      )

add_custom_command( OUTPUT external_dependencies/libmylib.a
        WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
        COMMAND cp depsrc/libmylib.a external_dependencies/
        COMMENT "CreatedLib"
        )

but this gives me an "no rule found" error

-- Configuring done
-- Generating done
-- Build files have been written to: /simple_test/cmake-build-debug
gmake[3]: *** No rule to make target `external_dependencies/libmylib.a', needed by `simple_test'.  Stop.

What am I doing wrong?

Solution

Assuming that you keep the initial details relatively the same, you should be able to do something like this:

Set up the path to the source and destination:

set(LIB_SOURCE "depsrc/libmylib.a")
set(LIB_DEST "${CMAKE_SOURCE_DIR}/external_dependencies/libmylib.a")

Check if it exists, then use ${CMAKE_COMMAND} instead of cp for good compatibility practice:

if(NOT EXISTS ${LIB_DEST})
    add_custom_command(
        OUTPUT ${LIB_DEST}
        COMMAND ${CMAKE_COMMAND} -E copy ${LIB_SOURCE} ${LIB_DEST}
        COMMENT "Copying libmylib.a to external_dependencies/"
    )
    add_custom_target(mylib_copy_target DEPENDS ${LIB_DEST})
    add_dependencies(simple_test mylib_copy_target)
endif()

Link the simple_test against the imported mylib:

add_library(mylib STATIC IMPORTED)
set_property(TARGET mylib PROPERTY IMPORTED_LOCATION ${LIB_DEST})

target_link_libraries(simple_test mylib)

Finally, set the directory where the library is located:

target_link_directories(simple_test PUBLIC external_dependencies/)

Answered By - neutron02

Answer Checked By - Terry (WPSolving Volunteer)