Is it possible to alter CMAKE_MODULE_PATH from CMake commandline?

Question

Edit: The accepted answer actually shows that it is pretty normally possible to set CMAKE_MODULE_PATH as any other CMake variable e.g. via the -DCMAKE_MODULE_PATH path CLI parameter. It seems that in my case there is some included CMake script that calls set(CMAKE_MODULE_PATH /library_path), which erases all previous paths set to the variable. That's why I couldn't get the variable to do what I wanted it to do. I'll leave the question here in case anybody else faces this kind of situation.

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I'm building a (3rd party) project that uses the Protobuf library (but this question is general). My system has a system-wide install of a newer version of Protobuf than the project is compatible with. So I've downloaded and compiled from source an older version of Protobuf.

The project uses CMake, and in its CMakeLists.txt, there is:

find_package(Protobuf REQUIRED)

Which, however, finds the (incompatible) system install. Of course, CMake doesn't know about my custom build of Protobuf. But how do I tell it?

I've created a FindProtobuf.cmake file in, say, ~/usr/share/cmake-3.0/Modules/ and want the build process to use this one for finding Protobuf. But I haven't succeeded forcing CMake to pick up this one and not the system one. I think the reason is quite obvious from the CMake docs of find_package:

The command has two modes by which it searches for packages: “Module” mode and “Config” mode. Module mode is available when the command is invoked with the above reduced signature. CMake searches for a file called Find<package>.cmake in the CMAKE_MODULE_PATH followed by the CMake installation. If the file is found, it is read and processed by CMake. ... If no module is found and the MODULE option is not given the command proceeds to Config mode.

So until I succeed to change CMAKE_MODULE_PATH, CMake will just pick up the FindProtobuf.cmake installed to the default system path and won't ever proceed to the "Config" mode where I could probably make use of CMAKE_PREFIX_PATH.

It's important for me to not edit the CMakeLists.txt since it belongs to a 3rd party project I don't maintain.

What I've tried (all without success):

• calling CMAKE_MODULE_PATH=~/usr/share/cmake-3.0/Modules cmake ... (the env. variable is not "transferred" to the CMake variable with the same name)
• calling cmake -DCMAKE_MODULE_PATH=~/usr/share/cmake-3.0/Modules ... (doesn't work, probably by design?)
• calling Protobuf_DIR=path/to/my/protobuf cmake ... (the project doesn't support this kind of override for Protobuf)

It seems to me that, unfortunately, the only way to alter the CMAKE_MODULE_PATH used by find_package is to alter it from within CMakeLists.txt, which is exactly what I want to avoid.

Do you have any ideas/workarounds on how not to touch the CMakeLists.txt and still convince find_package to find my custom Protobuf?

For reference, the CMake part of this project is on github .

Answer 1

As a direct answer to your question, yes, you can set CMAKE_MODULE_PATH at the command line by running cmake -DCMAKE_MODULE_PATH=/some/path -S /path/to/src -B /path/to/build.

But that probably doesn't do what you want it to do; see below.

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The Bitbucket link you supplied is dead, but here are a few suggestions that might help.

1. Avoid writing your own find modules, especially when the upstream supplies CMake config modules.
2. You can direct CMake to your custom Protobuf installation by setting one of CMAKE_PREFIX_PATH or Protobuf_ROOT (v3.12+) to the Protobuf install root.
3. You can tell find_package to try CONFIG mode first by setting CMAKE_FIND_PACKAGE_PREFER_CONFIG to true (v3.15+). Then set Protobuf_DIR to the directory containing ProtobufConfig.cmake.
4. Failing all else, you can manually set the variables documented in CMake's own FindProtobuf module, here: https://cmake.org/cmake/help/latest/module/FindProtobuf.html

All these variables can be set at the configure command line with the -D flag.

There are very few environment variables that populate CMake variables to start and I would avoid relying on them. There is an exhaustive list here: https://cmake.org/cmake/help/latest/manual/cmake-env-variables.7.html. CMAKE_MODULE_PATH is not among them.