cmake & gcc compiles every file every time

Question

Hi folks, I'm a learning c++ developer writing a game initially on the Mac platform using XCode, but now moving to cross platform by leveraging CMake. So far I can get it compiled on my ickle linux netbook and I'm putting together a dev environment on this machine for on the go coding. However I'm finding that gcc recompiles every file whenever I make a change. Clearly I need some additional configuration to the CMakeLists.txt . My current one is very simple. Like so;

cmake_minimum_required (VERSION 2.8)
set (source
Creature.cpp
DisplayManager.cpp
Engine.cpp
EngineState.cpp
Entity.cpp
GameWorld.cpp
GfxSFML.cpp
Item.cpp
Map.cpp
Position.cpp
Projectile.cpp
ScreenTile.cpp
SquadAI.cpp
Terrain.cpp
UIButton.cpp
UICharPanel.cpp
UIView.cpp
Utility.cpp
Weapon.cpp
fov.cpp
main.cpp
)

find_package (OpenAL)
find_package (OpenGL)
find_package (SFML)

set(CMAKE_CXX_FLAGS "-g -Wall -pg")
add_executable (tractionedge ${source})
target_link_libraries(tractionedge ${SFML_LIBRARY} ${OPENGL_LIBRARY} ${OPENAL_LIBRARY})

I've concentrated so far on C++ as a language rather than build systems by sticking with XCode for everything. My knowledge of Autotools (make?) and Gcc is very limited. How do I have gcc only recompile the changed source?

Answer 1

Rebuilding only the modified sources SHOULD be the default behavior. Of course if you change a central header included by nearly all dependent cpp files it'll trigger a nearly complete rebuild. Look at what happens if you only modify one cpp file (adding a comment or alike), if more than that compilation unit is compiling then I propose you to invest more time investigating it eventually giving you my EMail to have a deeper look at the configuration.

Another possibility is that you are compiling under windows and using a 2.8 cmake that has a bug regarding this. Look at a 2.9 version to see if that defect is away then: http://www.mail-archive.com/[email protected]/msg24876.html

Answer 2

I would rewrite your CMakeLists.txt using glob (maybe move the files in a "src" directory if you have other *.cpp files around) and give your project a name (this sets some important variables):

cmake_minimum_required (VERSION 2.8)
project(TRACTION)
file (GLOB TRACTION_SOURCES *.cpp)
find_package (OpenAL)
find_package (OpenGL)
find_package (SFML)

set(CMAKE_CXX_FLAGS "-g -Wall -pg")
add_executable (tractionedge ${TRACTION_SOURCES})
target_link_libraries(tractionedge ${SFML_LIBRARY} ${OPENGL_LIBRARY} ${OPENAL_LIBRARY})

Answer 3

Are you rerunning cmake every time? If you just modify one source file, you should be able to simply rerun make, and it should rebuild just the one object file before linking. If you rerun cmake, it might mark all of the source files dirty and rebuild everything.

Only rerun cmake if you change the actual list of source files being used, or other major changes like that.