# Object libraries and the Ninja generator

**URL:** <https://discourse.cmake.org/t/object-libraries-and-the-ninja-generator/9138>\
**Category:** Code\
**Tags:** gen:ninja\
**Created:** [October 4, 2023, 7:46am UTC](https://discourse.cmake.org/t/object-libraries-and-the-ninja-generator/9138 "2023-10-04T07:46:46Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![plevold](https://discourse.cmake.org/user_avatar/discourse.cmake.org/plevold/32/3892_2.png) [@plevold](https://discourse.cmake.org/u/plevold)\
**Post date:** [October 4, 2023, 7:46am UTC](https://discourse.cmake.org/t/object-libraries-and-the-ninja-generator/9138/1 "2023-10-04T07:46:46Z")

</div>

I’m struggling with using the Ninja generator for a project that contains an object library. This seems to be exactly what’s been discussed in [Issue 24411](https://gitlab.kitware.com/cmake/cmake/-/issues/24411) and in [How to add dependencies to an object library [for ninja generator]?](https://discourse.cmake.org/t/how-to-add-dependencies-to-an-object-library-for-ninja-generator/7462). If I understand those posts correctly then the conclusion is that for object libraries, `target_link_libraries` or `add_dependencies` should only make a _link time_ dependency between the object library and the target it depends on and not a _compile time_ dependency.

For classic C++ projects this seems to be fair, but I cannot see that this is enough for a Fortran project where the _compilation_ step of a library might produce .mod files which again might be needed by other sources which `use` this module. I don’t know C++ modules well enough, but if I understand them correctly then this is an issue there as well.

So my questions are: Am I overseeing something or is this a limitation in the current CMake implementation of object libraries? Is there a way to force compilation of object library dependencies before the object library itself even for the Ninja generator?

Here’s a minimal example:

CMakeListst.txt:

```cmake
cmake_minimum_required(VERSION 3.27 FATAL_ERROR)
project(objlib_test LANGUAGES Fortran)

add_library(static_lib STATIC static_lib.f90)

add_library(obj_lib OBJECT obj_lib.f90)
target_link_libraries(obj_lib static_lib)

```

static\_lib.f90:

```fortran
module static_lib
    implicit none

    integer, parameter :: answer = 42

end module

```

obj\_lib.f90:

```fortran
module obj_lib
    use static_lib, only: answer
    implicit none

    private
    public print_answer

contains

    subroutine print_answer
        write(*,*) 'The answer is ', answer
    end subroutine

end module

```

Building `obj_lib` with `make` works:

```auto
$ cmake ../ && make obj_lib
-- The Fortran compiler identification is GNU 9.4.0
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Check for working Fortran compiler: /usr/bin/f95 - skipped
-- Configuring done (11.4s)
-- Generating done (0.2s)
-- Build files have been written to: /mnt/c/Users/palle/dev/fortran-cosimulation/test-proj/build/make
[33%] Building Fortran object CMakeFiles/static_lib.dir/static_lib.f90.o
[66%] Linking Fortran static library libstatic_lib.a
[66%] Built target static_lib
[100%] Building Fortran object CMakeFiles/obj_lib.dir/obj_lib.f90.o
[100%] Built target obj_lib

```

Building `obj_lib` with `ninja` fails:

```auto
$ cmake ../ -GNinja && ninja obj_lib
-- The Fortran compiler identification is GNU 9.4.0
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Check for working Fortran compiler: /usr/bin/f95 - skipped
-- Configuring done (10.5s)
-- Generating done (0.1s)
-- Build files have been written to: /mnt/c/Users/palle/dev/fortran-cosimulation/test-proj/build/ninja
[3/3] Building Fortran object CMakeFiles/obj_lib.dir/obj_lib.f90.o
FAILED: CMakeFiles/obj_lib.dir/obj_lib.f90.o obj_lib.mod
/usr/bin/f95 -I/mnt/c/Users/palle/dev/fortran-cosimulation/test-proj -fpreprocessed -c CMakeFiles/obj_lib.dir/obj_lib.f90-pp.f90 
-o CMakeFiles/obj_lib.dir/obj_lib.f90.o
/mnt/c/Users/palle/dev/fortran-cosimulation/test-proj/obj_lib.f90:2:8:

    2 | use static_lib, only: answer
      | 1
Fatal Error: Cannot open module file ‘static_lib.mod’ for reading at (1): No such file or directory
compilation terminated.
ninja: build stopped: subcommand failed.

```

Note: Building the `all` target with `ninja` works in this case, but fails for my proper project.

Software versions I’ve used:

- CMake: 3.27.2
- Make: 4.2.1
- Ninja 1.10.2

---

<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [October 4, 2023, 10:01am UTC](https://discourse.cmake.org/t/object-libraries-and-the-ninja-generator/9138/2 "2023-10-04T10:01:33Z")

</div>

> [@plevold](#):
>
> CMake: 3.27.2

There were a number of issues from a reworking of some dependency paths in the collation step (most/all discovered and reported by @scivision). Please try CMake 3.27.6.

---

<div class="post-metadata">

**Author:** ![plevold](https://discourse.cmake.org/user_avatar/discourse.cmake.org/plevold/32/3892_2.png) [@plevold](https://discourse.cmake.org/u/plevold)\
**Post date:** [October 4, 2023, 10:18am UTC](https://discourse.cmake.org/t/object-libraries-and-the-ninja-generator/9138/3 "2023-10-04T10:18:15Z")

</div>

Thanks! CMake version 3.27.6 does indeed resolve the issues I was having 👍