Question

I’ve just migrated a somewhat large project from Visual Studio solutions to CMake and I’ve noticed a weird behavior. I have something like the following structure:

project/CMakeLists.txt
project/code/CMakeLists.txt

project/code/library-1/CMakeLists.txt
project/code/library-1/*.hpp
project/code/library-1/*.cpp
project/code/library-2/CMakeLists.txt
project/code/library-2/*.hpp
project/code/library-2/*.cpp
...
project/code/library-n/CMakeLists.txt
project/code/library-n/*.hpp
project/code/library-n/*.cpp

project/demo/CMakeLists.txt
project/demo/demo-1/CMakeLists.txt
project/demo/demo-1/*.hpp
project/demo/demo-1/*.cpp
project/demo/demo-2/CMakeLists.txt
project/demo/demo-2/*.hpp
project/demo/demo-2/*.cpp
...
project/demo/demo-n/CMakeLists.txt
project/demo/demo-n/*.hpp
project/demo/demo-n/*.cpp

1. The root CMakeLists.txt file configures the compilation flags, macro definitions, etc. and uses CMake’s add_subdirectory() to include targets defined by the libraries and demo projects.
2. The code sub-folder contains a flat list of sub-folders with each containing source code for a static library (as well as its target defined in a CMakeLists.txt file).
3. The demo sub-folder contains a flat list of sub-folders. Each contains source code for an executable and associated CMakeLists.txt file.
4. Each library is a standalone component and builds independently from all other libraries and demo projects.
5. Each demo program depends on one or more of the different libraries in the code sub-folder.

This setup is really nice. If I want to change build options, I only need to modify the root CMakeLists.txt and everything re-compiles with the new settings. If I modify any source code anywhere in the tree, the appropriate libraries, if any, are recompiled and all dependent demo programs are also re-built.

However, if I modify any CMakeLists.txt file anywhere in the tree, the entire tree of libraries and programs is re-compiled without respect of dependencies. To give an idea of what I mean, here a few parts of the CMake build scripts.

---

project/demo/CMakeLists.txt

# Resolve libraries built in `code` sub-folder.
link_directories(${LIBRARY_OUTPUT_PATH})

set(demo-projects
  demo-1
  demo-2
  ...
  demo-n
)
foreach(demo-project ${demo-projects})
  add_subdirectory(${demo-project})
endforeach()

---

project/demo/demo-n/CMakeLists.txt

# Find all source code in the same folder.
file(GLOB ${demo-project}_headers
  ${CMAKE_CURRENT_SOURCE_DIR}/*.hpp
)
file(GLOB ${demo-project}_sources
  ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp
)

# Select libraries to link with.
set(${demo-project}_libraries
  library-1
  library-2
  library-5
)

# Build the demo program.
add_executable(${demo-project}
  ${${demo-project}_headers}
  ${${demo-project}_sources}
)
if(${demo-project}_libraries)
  target_link_libraries(${demo-project} ${${demo-project}_libraries})
endif()

# Manually register some dependencies on other targets.
if(${demo-project}_dependencies)
  add_dependencies(${demo-project} ${${demo-project}_dependencies})
endif()

---

If I happen to modify project/demo/demo-n/CMakeLists.txt to add an extra library, like this:

set(${demo-project}_libraries
  library-1
  library-2
  library-5
  library-6
)

Then the entire source code for all libraries and demo programs in the project is re-compiled. Why is this so? Is there a better way to structure my scripts in order to avoid this?

Answer 1

It happens that my problem was cause by a totally unrelated issue. I applied Bill Hoffman’s suggestion and the modifying any “CMakeLists.txt” file in the project ended up modifying the CXX_FLAGS (C++ compiler flags) variable in all generated Makefiles.

I traced that back to my root “CMakeLists.txt” file, which had something like the following:

if(MSVC)
  # ...
  set(CMAKE_CXX_FLAGS_DEBUG
    "${CMAKE_CXX_FLAGS_DEBUG} /WX /wd4355" // depends on cached value.
    CACHE STRING "Debug compiler flags" FORCE)
  # ...
endif()

I changed it to the following.

if(MSVC)
  # ...
  set(CMAKE_CXX_FLAGS_DEBUG
    "/DWIN32 /D_WINDOWS /WX /wd4355" // no longer depends on cached value.
    CACHE STRING "Debug compiler flags" FORCE)
  # ...
endif()

CMake no longer repeats the /WX /wd4355 flags when updating the build scripts and my project no longer re-compiles from scratch at each modification!