Taming the CMake Beast: Including CMake from a vcpkg Port in a Patch
Problem: You're working on a vcpkg port, and you need to make changes to the way CMake builds the package. This often involves adding new options, modifying build scripts, or even introducing new CMake modules. But how do you include these changes directly within your vcpkg port without affecting the original package source?
Solution: Patches! vcpkg allows you to apply patches directly to the source code of a package, allowing you to make modifications without altering the upstream repository. This article will guide you through the process of integrating your CMake changes into a vcpkg patch.
Scenario: Let's imagine you're working on a port for the popular OpenCV library. You want to add a new CMake option to enable a specific feature for your project.
Original Code:
# CMakeLists.txt (Original)
cmake_minimum_required(VERSION 3.10)
project(opencv)
# ... rest of the CMake code ...
Patch:
To add your CMake option, you can create a patch file named cmake-options.patch inside your vcpkg port directory:
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,7 @@
project(opencv)
# ... rest of the CMake code ...
+add_compile_options(-DOPENCV_CUSTOM_FEATURE=ON)
Explanation:
- Patch Structure: The patch uses the
diffformat, where lines starting with+represent additions and lines starting with-represent deletions. In this case, we're adding a new line to the CMake file. add_compile_options: This CMake command adds the-DOPENCV_CUSTOM_FEATURE=ONcompiler flag, effectively enabling the new feature in your build.vcpkg.jsonConfiguration: You need to include the patch in yourvcpkg.jsonfile:
{
"name": "opencv",
"version": "4.5.5",
"dependencies": [
# ...
],
"patches": [
"cmake-options.patch"
]
}
Additional Insights:
- Patching Strategies: While adding new CMake options is common, patches can handle more complex modifications:
- Updating CMake Modules: Patching existing modules to add functionality or correct bugs.
- Modifying Build Scripts: Adjusting build configurations or adding custom build steps.
- Testing Your Patch: Always test your patch thoroughly after applying it to ensure it doesn't break the original functionality.
- Version Control: Use a version control system like Git to manage your patches and track changes. This helps you revert to previous versions if needed.
Conclusion:
Using patches within your vcpkg ports provides a powerful way to modify CMake configuration and build processes without impacting the original source code. By understanding the structure and usage of patches, you can tailor your vcpkg packages to meet specific project requirements and leverage the flexibility of CMake.
References:
