How to set project root when "No project() command is present"?

Usage
1

I am a just few days into CMake, and I need to understand if it is possible to reference project root from CMakeLists.txt in subdirectories to get project variables right?

This is a typical C project with Python binding.

./
├── c/
│   ├── Makefile.am
│   ├── prog.h
│   └── prog.cpp
├── include/
│   ├── prog/
│   └── prog.h
├── python/
│   ├── CMakeLists.txt
│   └── setup.py.in
├── src/
├── tests/
├── CMakeLists.txt
└── README.md

I want to build the python binding, so I do cd python && cmake . to get setup.py and some warnings. I know that I can do `cmake

CMake Warning (dev) in CMakeLists.txt:
  No project() command is present.  The top-level CMakeLists.txt file must
  contain a literal, direct call to the project() command.  Add a line of
  code such as

    project(ProjectName)

  near the top of the file, but after cmake_minimum_required().

  CMake is pretending there is a "project(Project)" command on the first
  line.
This warning is for project developers.  Use -Wno-dev to suppress it.

-- The C compiler identification is GNU 11.2.1
...
-- Generating done
-- Build files have been written to: /tmp/proj/python

The problem is that values in generated setup.py are wrong. Here is the lines from setup.py.in that are substituted.

      version="@PACKAGE_VERSION@",
...
          ["@CURRENT_SOURCE_DIR@/prog.py"],
          include_dirs = ['@PROJECT_SOURCE_DIR@/include'],
...
          library_dirs = ['@CURRENT_BINARY_DIR@', '@PROJECT_BINARY_DIR@']

And here is the result.

      version="",
          ["/tmp/prog/python/prog.py"],
          include_dirs = ['/tmp/prog/python/include'],
...
          library_dirs = ['/tmp/prog/python', '/tmp/prog/python']

version is not set, include_dirs should point to /tmp/prog/python. The contents of the python/CMakeFile.txt that does this.

✗ cat python/CMakeLists.txt 

set(CURRENT_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
set(CURRENT_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})

configure_file(setup.py.in setup.py)

add_custom_target(python ALL
    COMMAND python3 setup.py build
    DEPENDS prog prog.py)

install(CODE "execute_process(COMMAND python3 setup.py install --root=\$ENV{DESTDIR}/ WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})")

As I see the problem is that this CMakeFile.txt build gets project root wrong. That’s why it can not read version and other data.

So, is it possible to have a workflow of doing cd python && cmake .?

2

Yes, this is possible. Just write the CMakeLists.txt file in that directory to be a standalone project. Add a project command to the top of it, then you should be able to build from that directory.

3

I don’t want to duplicate what is already written in the root file. How to reuse it?

4

If you want to configure directly from the python directory, it must have a project command. This is a requirement of CMake. Besides, it shouldn’t be duplication, you can have different details there. For example, you might have:

Top level repo root:

project (MyCoolProject
   VERSION 1.0.0
   DESCRIPTION "A cool project")

Python directory:

project (MyCoolProjectPythonBindings
   VERSION 1.0.0
   DESCRIPTION "Python bindings for MyCoolProject")
5

So there is no way to refer to root CMakeLists.txt from CMakeLists.txt in subdirectory?

So that @PACKAGE_VERSION@, @PROJECT_SOURCE_DIR@ and @PROJECT_BINARY_DIR@ won’t have to be redefined when run from subdir.

6

No. If you configure from the top level directory, then any subdirectories would have the top-level variables defined. But if you want to run cmake in one of the subdirectories, that subdirectory needs to support that use case by setting itself up as a top level standalone project.

1 Like
7

Thanks for confirming. Any best practices how to make cmake in subdirectory fail with instruction to rerun it from top directory?

8

CMake already warned you in that output you posted above:

CMake Warning (dev) in CMakeLists.txt:
  No project() command is present.  The top-level CMakeLists.txt file must
  contain a literal, direct call to the project() command.  Add a line of
  code such as

    project(ProjectName)

  near the top of the file, but after cmake_minimum_required().

  CMake is pretending there is a "project(Project)" command on the first
  line.
This warning is for project developers.  Use -Wno-dev to suppress it.

To make this an actual error, you can make dev warnings errors. I forget the specific command line flag, I’d have to check.