Скачать или смотреть How to Use add_custom_command in CMake for Pre-processing Header Files

Discover how to efficiently set up `add_custom_command` in CMake to automate pre-processing of header files from .def files, ensuring a seamless build process.
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Automating Header File Pre-processing in CMake

When working on software projects, especially those involving CMake, you often encounter challenges that can disrupt the build process. One such challenge is the pre-processing of header files from other sources, like .def files, before compiling your C source files. This guide will guide you through the solution to this common problem by effectively using add_custom_command in CMake.

The Problem

You may have a structured project where both C source files and header files reside in the same directory alongside .def files. In a typical Makefile, you might have used a simple rule to preprocess these files:

[[See Video to Reveal this Text or Code Snippet]]

However, transitioning to CMake can be tricky, especially in ensuring that your header file is generated before your source code tries to reference it. Users often confuse the working directory and shell directives when employing add_custom_command, leading to build failures due to missing header files.

Understanding the Issues

There are two common problems that arise when setting up add_custom_command in CMake:

Working Directory: By default, CMake commands are executed in the build directory rather than the source directory, which may lead to path issues when trying to access your .def files.

Reliance on Shell Features: Using shell-specific functionalities, such as redirection, can make your CMake scripts less portable and prone to errors on different platforms.

The Solution

To efficiently preprocess header files in CMake, we can create a separate CMake script that uses execute_process, which allows us to avoid shell-specific syntax. Here’s how to implement it step-by-step:

Step 1: Create a Preprocessing Script

Create a new file named preprocess_def.cmake with the following content:

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Step 2: Update your CMakeLists.txt

Next, you’ll need to adjust your CMakeLists.txt file to use the preprocessing script. Here’s how:

[[See Video to Reveal this Text or Code Snippet]]

Key Points

Separation of Script: Keeping the preprocessing logic in a separate script (preprocess_def.cmake) improves clarity and reusability.

Safety Sources: Using absolute paths helps the command find the necessary files, ensuring smooth execution.

Generated Properties: Declaring the output file (vvr_const.h) as generated helps CMake understand its lifecycle.

Conclusion

Incorporating custom rules in CMake to pre-process header files from .def files is crucial for maintaining a smooth and error-free build environment. By utilizing add_custom_command alongside a dedicated preprocessing script, you can streamline this process effectively. This approach not only resolves common issues associated with working directories and shell commands but also paves the way for clearer, more maintainable CMake configurations.

With this guide, you should now have the tools to preprocess your header files automatically, eliminating build disruptions and increasing productivity in your projects. Happy coding!