CMake: rebuild external project - c++

Question

I have a project that uses an external library. The project's CMakeLists.txt looks like this:
cmake_minimum_required(VERSION 3.6.0)
project(my-project)
set(CMAKE_CXX_STANDARD 14)
include(ExternalProject)
find_package(Git REQUIRED)
ExternalProject_Add(library
PREFIX ${my-project_SOURCE_DIR}/lib/library
GIT_REPOSITORY https://github.com/vendor/library
GIT_TAG master
UPDATE_COMMAND ""
INSTALL_COMMAND ""
)
link_directories(${my-project_SOURCE_DIR}/lib/library/src/library-build/src)
add_subdirectory(src)
And src/CMakeLists.txt like this:
include_directories(../lib/library/src/library/include)
add_executable(my-project
main.cpp
)
add_dependencies(my-project library)
target_link_libraries(my-project liblibrary.a)
It pulls the library from Git and builds it for the first time without any problems.
I want to edit source code of the library and have the library .a file be recompiled automatically. What is the cleanest way I can achieve that? It currently ignores any updates to the source code, because it already has .a file of the library.
When I try to add
add_subdirectory(lib/library/src/library/src)
to my main CMakeLists.txt, I get the following error:
CMake Error at lib/library/src/library/src/CMakeLists.txt:55 (add_library): add_library cannot create target "library" because another target with the same name already exists. The existing target is a custom target created in source directory "/cygdrive/c/Code/my-project". See documentation for policy CMP0002 for more details.
I guess it's caused by calling
add_library(gram STATIC ${SOURCE_FILES})
in the library CMakeLists.txt and then calling
ExternalProject_Add(library ...)
in the project CMakeLists.txt.
Any ideas?

Answer 1

As your main goal for using ExternalProject_Add is to download the dependency from an external source without configuring and building it, you could define the CMAKE_COMMAND, CONFIGURE_COMMAND and BUILD_COMMAND as empty strings. Same as you already did for the UPDATE_COMMAND and INSTALL_COMMAND. That way, ExternalProject_Add will only clone the repository once.
To overcome the name clash, just use a different one for the first argument of ExternalProject_Add, e.g. library_src:
ExternalProject_Add(library_src
PREFIX ${my-project_SOURCE_DIR}/lib/library
GIT_REPOSITORY https://github.com/vendor/library
GIT_TAG master
UPDATE_COMMAND ""
CONFIGURE_COMMAND ""
CMAKE_COMMAND ""
BUILD_COMMAND ""
INSTALL_COMMAND ""
)
ExternalProject_Get_Property(library_src SOURCE_DIR)
add_subdirectory(${SOURCE_DIR})
The second command (ExternalProject_Get_Property) will give you the named properties for the given external project. The output variables are of the same name as the properties. That way, you are immune against changes in the behaviour of ExternalProject_Add where it places the actual source tree.

Answer 2

add this command in ExternalProject_Add maybe help you:
UPDATE_COMMAND ""
https://gitlab.kitware.com/cmake/cmake/issues/16419