Overlapping dependencies between libraries in CMake

Question

Let's say there's the following directory structure: ``` projects | +--lib1 | | | +-CMakeFiles.txt | +--lib2 | | | +-CMakeFiles.txt | +--test | +-CMakeFiles.txt ``` lib1/CMakeFiles.txt: ``` cmake_minimum_required(VERSION 2.0) add_library(lib1 STATIC lib1.cpp) ``` lib2/CMakeFiles.txt: ``` cmake_minimum_required(VERSION 2.0) add_subdirectory(../lib1 ${CMAKE_CURRENT_BINARY_DIR}/lib1) add_library(lib2 STATIC lib2.cpp) target_link_libraries(lib2 lib1) ``` test/CMakeFiles.txt: ``` cmake_minimum_required(VERSION 2.0) project(test) add_subdirectory(../lib1 ${CMAKE_CURRENT_BINARY_DIR}/lib1) add_subdirectory(../lib2 ${CMAKE_CURRENT_BINARY_DIR}/lib2) add_executable(test main.cpp) target_link_libraries(test lib1 lib2) ``` I.e. `lib2` depends on `lib1` and `test` depends on both of them. (I know that technically static libraries don't "link", but that's just an example.) The problem is that with the current setup, lib1 compiles twice - the first time it is within the "test" build directory, and a…

Accepted Answer

With CMake, library dependencies are transitive, so you shouldn't call `add_subdirectory` twice in `test/CMakeFiles.txt` (nor do you need to list `lib1` as a dependency of `test` since it is already a dependency of `lib2`'s).

So you could modify `test`'s CMakeFiles.txt to:

```
cmake_minimum_required(VERSION 2.8.7)  # Prefer the most current version possible
project(test)

add_subdirectory(../lib2 ${CMAKE_CURRENT_BINARY_DIR}/lib2)

add_executable(test main.cpp)
target_link_libraries(test lib2)
```

Also, you should probably remove the `cmake_minimum_required` calls from your non-project CMakeFiles.txt files (the lib ones). For further info, run:

```
cmake --help-policy CMP0000
```

This setup will still cause all libs to be recompiled if you add a similar test2 subdirectory and project which depends on `lib1` and `lib2`. If you really don't want to have a top-level CMakeFiles.txt in `projects/`, then you're stuck with what you're doing, or you could use either the `export` or `install` command.

`export` would create a file which could be `include`d by other projects and which imports the targets into the project which calls `include`.

`install` could install the libraries to another common subdirectory of `projects/`. Depending on your source directory structure, this could have the benefit of only making the intended library API headers available to dependent projects.

However, both of these options require the dependent library projects to be rebuilt (and installed) if they are modified, whereas your current setup includes all the dependent targets in your project, so any change to a source file in a dependent library will cause your `test` target to go out of date.

For further details about `export` and `install`, run:

```
cmake --help-command export
cmake --help-command install
```

Overlapping dependencies between libraries in CMake

Solution

Problem