[CMake] Again on dependencies tree

[Marco Corvo]

Hi all,

I thought I came up with a solution for this problem, but looks like I'm 
still not doing the right thing.

My project is made of many tens of packages, everyone with its nice 
CMakeLists.txt. What I'd like to do is to have the freedom to build 
every single package, or bunches of packages, independently, that is 
without having to "add_subdirectory" for all packages every time. The 
point, just to give an idea, is that the developers of the different 
packages could checkout from the software repo just their own packages, 
which are then supposed to be built against a software release 
(libraries) which is centrally installed on a development machine.

In this framework I would like to let CMake discover the transitive 
dependencies among libraries.

To make a practical example I prepared a single package A (the one the 
developer checks out from our repo) which has an executable EXE 
depending on library libA.a, which resides in the same package (that is 
EXE and libA.a are all together in a source directory).

Yet libA.a depends on a library libB.a which, in this view, is available 
somewhere in the machine. In CMake syntax I have:

- package A CMakeLists.txt:

     add_library(A src/xxx.cc)
     set(LIB_LINK_LIST B)
     target_link_libraries(${pkgname} ${LIB_LINK_LIST})

     add_executable(EXE bins/EXE.cc)
     set(BIN_LINK_LIST A)
     target_link_libraries(EXE ${BIN_LINK_LIST})

- "external" library LibDeps.cmake:

     add_library(B STATIC IMPORTED)
     add_dependencies(B C)

- main CMakeLists.txt

     include(LibDeps.cmake)
     add_subdirectory(A)

In my view what I should obtain is a link command like:
g++ ...... -o EXE -L/path/to/external/libs -L/path/to/local/libs -L.... 
-L.... libA.a -lB -lC

since Exe depends on A, which depends on B via target_link_libraries, 
which on its side depends on C, via add_dependencies.

Instead what I get is:
g++ ...... -o EXE -L/path/to/external/libs -L/path/to/local/libs -L.... 
-L.... libA.a B-NOTFOUND

It's clear that CMake tries to resolve the deps, otherwise what I would 
get is just "libA.a -lB", but is not able to due to some trick I can't  
find.

target_link_library(B C) instead of add_dependencies(B C) doesn't work. 
I know there are ExternalProject_Add, but these are not external 
projects, just libs residing in some (well known) directories, and 
find_package, but it would be rather complicated  in case my libA.a 
depends on N other libraries.

Am I missing any CMake instructions? Or is it simply the wrong way to 
proceed?

Sorry for the long mail and thanks in advance.

Cheers,

Marco

-- 
Marco Corvo
SuperB experiment
CNRS - Orsay
c/o INFN - Padova