Fix cmake_c_compiler and cmake_cxx_compiler Not Set After NDK Update

If you’ve recently upgraded your Android Studio NDK and encountered errors indicating that cmake_c_compiler and cmake_cxx_compiler are not set, you’re not alone. These errors typically occur because the build system no longer recognizes the compiler paths after the upgrade, leading to build failures. This guide walks you through the necessary steps to resolve this issue, ensuring your project builds smoothly with the latest NDK. Before starting, ensure you have the latest Android Studio installed, along with the updated NDK version, and that your project is configured to use the correct SDK and NDK paths.

What You’ll Need

Android Studio (latest version recommended)
Updated Android NDK installed via SDK Manager
Your project files and build configuration (CMakeLists.txt, gradle files)
Basic knowledge of Android Studio and Gradle build system

Step-by-Step

Verify NDK Installation and Paths
Open Android Studio, go to Preferences > Appearance & Behavior > System Settings > Android SDK. Under the SDK Tools tab, ensure the latest NDK is installed. Note the NDK path, typically something like /Users/yourname/Library/Android/sdk/ndk/21.4.7075529.
Update Your CMakeLists.txt and Gradle Files
Check your project’s build.gradle (app module) and ensure you are specifying the correct NDK version. For example, in defaultConfig, set:
```
externalNativeBuild {
    cmake {
        arguments "-DANDROID_NDK=/path/to/your/ndk"
    }
}
```
Replace /path/to/your/ndk with the actual path noted earlier. Also, ensure your cmake section points to the correct CMake version compatible with your NDK.


Configure the Toolchain File

    In your CMakeLists.txt, explicitly specify the toolchain file to ensure CMake uses the correct compiler paths. Add or modify the following line at the top:
set(CMAKE_TOOLCHAIN_FILE $ENV{ANDROID_NDK}/build/cmake/android.toolchain.cmake)
    Make sure the ANDROID_NDK environment variable is correctly set to the NDK path, or replace it directly with the absolute path.
Set Compiler Paths Explicitly

    If CMake still doesn't recognize the compilers, manually specify the compiler paths within your CMakeLists.txt:
set(CMAKE_C_COMPILER /path/to/ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/clang)
set(CMAKE_CXX_COMPILER /path/to/ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++)
    Replace the paths with the actual locations in your NDK directory.
Clean and Rebuild the Project

    In Android Studio, go to Build > Clean Project, then Build > Rebuild Project. This clears cached build files and forces CMake to regenerate build scripts with the updated configuration.
Sync Gradle and Reconfigure CMake

    Click on File > Sync Project with Gradle Files. Then, run a build. If errors persist, check the Gradle Console for detailed error messages and verify paths.


Common Mistakes

Not updating or correctly setting the ANDROID_NDK environment variable
Using an incompatible CMake version with the new NDK
Forgetting to specify the toolchain file in CMakeLists.txt
Incorrect paths in build configuration files or environment variables
Failing to clean and rebuild after configuration changes

Tips

Always verify the NDK path after upgrading and update your project configuration accordingly.
Use absolute paths in your build scripts during troubleshooting to eliminate path-related errors.
Check the build logs for specific errors related to missing or misconfigured compilers.
Update your CMake version in Android Studio to match the one recommended for your NDK version.
If using Windows, ensure that environment variables are set correctly in your system settings or within Android Studio’s terminal.

Troubleshooting
If the above steps do not resolve your issue, consider the following troubleshooting tips:

Verify Environment Variables: Ensure ANDROID_NDK and PATH environment variables point to the correct directories. On Unix-based systems, you can set them in your terminal or add them to your shell profile (e.g., .bashrc or .zshrc).
Check CMake Version Compatibility: Sometimes, newer NDK versions require newer CMake versions. Confirm compatibility and update CMake via SDK Manager or manually.
Explicitly Specify Compiler Paths: As a temporary workaround, manually set CMAKE_C_COMPILER and CMAKE_CXX_COMPILER in your CMakeLists.txt.
Update Android Studio and Plugins: Ensure your Android Studio, Gradle plugin, and SDK tools are all up to date, as incompatibilities can cause build issues.
Delete CMake Cache: Sometimes, stale cache files cause issues. Delete the build directory and restart the build process.
Check for Known Issues: Visit the Android developer forums or issue trackers for known bugs related to your NDK version and build tools.

Frequently Asked Questions
1. Why are my CMake compiler variables not set after upgrading the NDK?
This often happens because the new NDK version has different default paths or the build scripts are not configured to recognize the updated paths. Explicitly setting the compiler paths or updating environment variables usually solves this.
2. How do I verify which NDK version my project is using?
Check your local.properties or build.gradle files for the ndk.dir entry, or inspect the SDK Manager in Android Studio to see the installed NDK versions.
3. Can I use a custom version of CMake with the new NDK?
Yes. You can specify a custom CMake version in your project's SDK settings or configure it manually in your build scripts, provided it is compatible with your NDK version.
4. How do I set environment variables for the Android NDK on Windows?
Open the System Properties > Environment Variables, then add or edit ANDROID_NDK and Path variables to include your NDK directory.
5. Should I downgrade my NDK if I encounter issues?
Downgrading is not recommended unless necessary. Instead, try updating your build scripts and tools to support the latest NDK, as older versions may lack features or have compatibility issues.
6. How often should I update my NDK and build tools?
It's advisable to keep your tools up to date to benefit from bug fixes and new features, but always test updates thoroughly before integrating into production projects.
Following these steps carefully will help you resolve the common issue of cmake_c_compiler and cmake_cxx_compiler not being set after an NDK upgrade. Proper configuration ensures your Android projects build reliably with the latest tools and SDKs, saving you time and frustration in the long run.
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Fix cmake_c_compiler and cmake_cxx_compiler Not Set After NDK Update