The Make and Ninja generators behave quite differently when dealing with ExternalProjects. I wanted to document the Ninja behaviour here since I would consider it buggy/un-intuitive.
Suppose we have a 'superbuild' project that has a hierarchy like this:
.
├── bar
│ ├── CMakeLists.txt
│ └── main.cpp
├── CMakeLists.txt
└── foo
├── CMakeLists.txt
├── include/
└── src/
The top-level cmake file sets up two external projects, foo and bar. bar depends on foo. The foo project installs libraries into a staging path and bar pulls the configuration from that path to import the foo library (and headers).
If I use the make generator and build from the top cmake file. I see that foo is built and installed, then bar is built and linked to foo.
Scanning dependencies of target foo
[ 6%] Creating directories for 'foo'
[ 12%] No download step for 'foo'
[ 18%] No patch step for 'foo'
[ 25%] No update step for 'foo'
[ 31%] Performing configure step for 'foo'
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/foo-build
[ 37%] Performing build step for 'foo'
Scanning dependencies of target foo
[ 50%] Building CXX object CMakeFiles/foo.dir/src/foo.cpp.o
[100%] Linking CXX static library libfoo.a
[100%] Built target foo
[ 43%] Performing install step for 'foo'
[100%] Built target foo
Install the project...
-- Install configuration: ""
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/libfoo.a
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config.cmake
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config-noconfig.cmake
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/include/foo.h
[ 50%] Completed 'foo'
[ 50%] Built target foo
Scanning dependencies of target bar
[ 56%] Creating directories for 'bar'
[ 62%] No download step for 'bar'
[ 68%] No patch step for 'bar'
[ 75%] No update step for 'bar'
[ 81%] Performing configure step for 'bar'
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/bar-build
[ 87%] Performing build step for 'bar'
Scanning dependencies of target bar
[ 50%] Building CXX object CMakeFiles/bar.dir/main.cpp.o
[100%] Linking CXX executable bar
[100%] Built target bar
[ 93%] No install step for 'bar'
[100%] Completed 'bar'
[100%] Built target bar
If I don't change anything and run it again, then I see the foo install step run, no changes are needed and then the bar build step run (bar doesn't get relinked).
[ 6%] Performing build step for 'foo'
[100%] Built target foo
[ 12%] Performing install step for 'foo'
[100%] Built target foo
Install the project...
-- Install configuration: ""
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/libfoo.a
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config.cmake
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config-noconfig.cmake
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/include/foo.h
[ 18%] Completed 'foo'
[ 50%] Built target foo
[ 56%] Performing configure step for 'bar'
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/bar-build
[ 62%] Performing build step for 'bar'
[100%] Built target bar
[ 68%] No install step for 'bar'
[ 75%] Completed 'bar'
[100%] Built target bar
Now if I touch something in foo, we can see that foo is rebuilt. The library is reinstalled and bar is relinked. This results in the correct executable.
[ 6%] Performing build step for 'foo'
Scanning dependencies of target foo
[ 50%] Building CXX object CMakeFiles/foo.dir/src/foo.cpp.o
[100%] Linking CXX static library libfoo.a
[100%] Built target foo
[ 12%] Performing install step for 'foo'
[100%] Built target foo
Install the project...
-- Install configuration: ""
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/libfoo.a
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config.cmake
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/lib/foo/foo-config-noconfig.cmake
-- Up-to-date: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/staging/include/foo.h
[ 18%] Completed 'foo'
[ 50%] Built target foo
[ 56%] Performing configure step for 'bar'
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/build/bar-build
[ 62%] Performing build step for 'bar'
[ 50%] Linking CXX executable bar
[100%] Built target bar
[ 68%] No install step for 'bar'
[ 75%] Completed 'bar'
[100%] Built target bar
If I use the ninja generator then the behaviour is different. On the first invocation of 'ninja, foo is built and installed, then bar is built and linked to foo.
[5/16] Performing configure step for 'foo'
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler using: Ninja
-- Check for working C compiler using: Ninja -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler using: Ninja
-- Check for working CXX compiler using: Ninja -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/foo-build
[6/16] Performing build step for 'foo'
[1/2] Building CXX object CMakeFiles/foo.dir/src/foo.cpp.o
[2/2] Linking CXX static library libfoo.a
[7/16] Performing install step for 'foo'
[1/1] Install the project...
-- Install configuration: ""
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/staging/lib/foo/libfoo.a
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/staging/lib/foo/foo-config.cmake
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/staging/lib/foo/foo-config-noconfig.cmake
-- Installing: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/staging/include/foo.h
[13/16] Performing configure step for 'bar'
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler using: Ninja
-- Check for working C compiler using: Ninja -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler using: Ninja
-- Check for working CXX compiler using: Ninja -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /media/data/home/lbuchy/work/cmake/cmake-ninja-install-out/ninja/bar-build
[14/16] Performing build step for 'bar'
[1/2] Building CXX object CMakeFiles/bar.dir/main.cpp.o
[2/2] Linking CXX executable bar
[16/16] Completed 'bar'
On subsequent invocations (with no changes to bar), there is no work to do. Checking that the installed files are there is not done. But it sure is quick to run a no-op build!
[1/7] Performing build step for 'foo'
ninja: no work to do.
[2/4] Performing build step for 'bar'
ninja: no work to do.
But if I touch a file in foo, then foo is rebuilt but not reinstalled. bar is not relinked and thus the bar executable is stale.
[1/7] Performing build step for 'foo'
[1/2] Building CXX object CMakeFiles/foo.dir/src/foo.cpp.o
[2/2] Linking CXX static library libfoo.a
[2/4] Performing build step for 'bar'
ninja: no work to do.