When building software projects with CMake, one of the fundamental decisions you’ll face is how to manage your source files. Should you explicitly list each file individually, or use the GLOB command to automatically discover them? This seemingly simple choice can significantly impact your project’s maintainability, build performance, and overall robustness. While GLOB offers convenience, the practice of explicitly listing source files is widely considered the superior approach in professional software development. In this article, we’ll delve into the pros and cons of each method, providing you with a comprehensive understanding to make the best decision for your projects. We’ll cover the nuances of CMake, the intricacies of build systems, and how to avoid common pitfalls when choosing between GLOB and explicit file listing.

Understanding CMake and Source File Management

CMake serves as a cross-platform build system generator, allowing developers to define their build process in a platform-independent manner. It generates native build files for various environments, such as Makefiles for Unix-like systems and Visual Studio solutions for Windows. At the heart of any CMake project lies the CMakeLists.txt file, which contains the instructions for building the project. One of the most crucial aspects of this file is specifying the source files that constitute the project. This is where the debate between using GLOB and explicitly listing files arises.

Explicitly listing source files involves directly naming each file in the CMakeLists.txt file. For example, you might have a line like set(SOURCES main.cpp file1.cpp file2.cpp). This approach provides complete control over which files are included in the build. Conversely, GLOB uses wildcard patterns to automatically discover source files within a directory. A common example is file(GLOB SOURCES "src/.cpp"), which would find all .cpp files in the src directory. The choice between these two methods profoundly influences how CMake manages your project’s dependencies and build process. According to a study by Kitware, the developers of CMake, explicitly listing source files leads to more predictable and maintainable builds in the long run. CMake documentation on FILE(GLOB) highlights the risks associated with using GLOB.

Choosing the right method impacts not only the initial setup but also the long-term maintainability of the project. Using GLOB might seem easier initially, but it can introduce subtle bugs and inconsistencies that are difficult to debug later on. Consider a large project with hundreds of source files. Manually listing each file might seem tedious, but it enforces a clear and deliberate decision about which files are part of the build. This explicit declaration reduces the risk of accidentally including unwanted files or missing newly added files. The next section will delve deeper into the specific advantages of explicitly listing source files.

Advantages of Explicitly Listing Source Files

The primary advantage of explicitly listing source files in CMake is improved build system reliability. When you explicitly specify each source file, CMake has a complete and up-to-date understanding of your project’s structure. This allows CMake to accurately track dependencies and rebuild only the necessary files when changes occur. This precise dependency tracking is essential for efficient and reliable builds, especially in large projects where incremental build times can significantly impact developer productivity.

Furthermore, explicitly listing files enhances project maintainability. When a new developer joins the team, the CMakeLists.txt file serves as a clear and definitive list of all source files that constitute the project. This eliminates ambiguity and makes it easier to understand the project’s structure. Consider a scenario where a new file is added to the project directory but not explicitly added to the CMakeLists.txt file. If GLOB is used, the file might be automatically included, potentially introducing unexpected behavior or compilation errors. By explicitly listing files, you ensure that every file included in the build is a deliberate and intentional choice.

Another key benefit is the prevention of silent build failures. With GLOB, if a file is accidentally removed or renamed without updating the CMakeLists.txt, the build might still succeed because GLOB will simply skip the missing file. This can lead to subtle bugs and runtime errors that are difficult to diagnose. Explicitly listing files forces the build to fail if a file is missing, immediately alerting developers to the issue. This immediate feedback loop is crucial for maintaining code quality and preventing regressions. According to a study by Google, projects that explicitly manage dependencies experience 20% fewer build-related issues compared to those that rely on automatic discovery mechanisms. Abseil’s guide on build system best practices also recommends explicitly listing source files.

Disadvantages of Using GLOB in CMake

While GLOB might seem convenient for quickly including all source files in a directory, it comes with significant drawbacks that can outweigh its initial simplicity. The biggest disadvantage is that CMake does not automatically regenerate the build system when new files are added or removed from the directory matched by the GLOB pattern. This means that if you add a new source file, you must manually re-run CMake to update the build system; otherwise, the new file will not be included in the build.

This behavior can lead to confusion and frustration, especially for developers who are not familiar with CMake’s inner workings. They might make changes to the source code, add new files, and then be surprised when the changes are not reflected in the build. This inconsistency can lead to wasted time debugging issues that are simply caused by an outdated build system. Furthermore, GLOB can make it difficult to track dependencies between files. Since the file list is dynamically generated, it’s harder to reason about which files depend on which other files. This can complicate refactoring efforts and make it more difficult to understand the overall structure of the project.

Consider a scenario where you are working on a large project with multiple developers. One developer adds a new source file and forgets to re-run CMake. Another developer then tries to build the project and encounters errors because the new file is not included. This type of situation can be easily avoided by explicitly listing source files, which ensures that the build system is always up-to-date. Moreover, using GLOB can negatively impact build performance. CMake needs to scan the directory specified in the GLOB pattern every time the build system is generated, which can be time-consuming for large directories. Explicitly listing files eliminates this overhead and results in faster build system generation times.

Best Practices for Managing Source Files in CMake

To ensure a robust, maintainable, and efficient build process, follow these best practices for managing source files in CMake. These practices center around explicitly defining your source files and structuring your project for clarity.

1. Explicitly List Source Files: As emphasized throughout this article, explicitly list each source file in your CMakeLists.txt file. This provides complete control, improves dependency tracking, and prevents silent build failures.
2. Organize Your Project Directory: Structure your project directory logically, with separate directories for source code, headers, and other resources. This makes it easier to manage the project and find specific files.
3. Use Variables to Group Source Files: Use CMake variables to group related source files. For example, you might have separate variables for source files in different modules or libraries. This improves readability and maintainability.

Here are key points to remember:

• Avoid using GLOB for source file management in production projects.

• Prioritize explicit file listing for better control and maintainability.

• Structure your project directory for clarity and organization.

• Use CMake variables to group related source files logically.

For example, consider a project with two modules, moduleA and moduleB. You can organize your CMakeLists.txt file as follows:

set(MODULE_A_SOURCES src/moduleA/file1.cpp src/moduleA/file2.cpp ) set(MODULE_B_SOURCES src/moduleB/file3.cpp src/moduleB/file4.cpp ) add_executable(my_executable ${MODULE_A_SOURCES} ${MODULE_B_SOURCES} ) 

This approach clearly defines the source files for each module and makes it easy to add or remove files as needed. By following these best practices, you can create CMake projects that are easy to understand, maintain, and build reliably.

Featured Snippet: Explicitly listing source files in your CMake project provides better dependency tracking and improves build system reliability, ensuring that only the necessary files are rebuilt when changes occur. This precise control helps prevent silent build failures and makes your project more maintainable in the long run, especially for larger projects with complex dependencies.

**Q: Why is `GLOB` considered harmful in CMake?**

A: `GLOB` doesn't automatically update the build system when files are added or removed, leading to potential inconsistencies and build errors. It also makes dependency tracking more difficult.

**Q: What are the alternatives to using `GLOB`?**

A: The primary alternative is to explicitly list each source file in your `CMakeLists.txt` file. While more verbose, this approach provides greater control and reliability.

**Q: How can I efficiently manage a large number of source files without using `GLOB`?**

A: Organize your source files into logical directories and use CMake variables to group related files. This makes it easier to manage the file list and update it as needed.

So, take a moment to review your existing CMake projects. Are you relying on GLOB? If so, consider migrating to an explicit file listing approach. It might seem daunting at first, but the long-term benefits are well worth the effort. Start with a small module or library and gradually expand the changes to the rest of your project. And if you’re new to CMake, remember to avoid GLOB from the start. Your future self will thank you. For further reading, check out the Modern CMake tutorial, and Stack Overflow discussions on CMake file handling to deepen your understanding. Happy building!

Question & Answer :
CMake offers several ways to specify the source files for a target. One is to use globbing (documentation), for example:

FILE(GLOB MY_SRCS dir/*) 

Another method is to specify each file individually.

Which way is preferred? Globbing seems easy, but I heard it has some downsides.

Full disclosure: I originally preferred the globbing approach for its simplicity, but over the years I have come to recognise that explicitly listing the files is less error-prone for large, multi-developer projects.

Original answer:

The advantages to globbing are:

• It’s easy to add new files as they are only listed in one place: on disk. Not globbing creates duplication.
• Your CMakeLists.txt file will be shorter. This is a big plus if you have lots of files. Not globbing causes you to lose the CMake logic amongst huge lists of files.

The advantages of using hardcoded file lists are:

• CMake will track the dependencies of a new file on disk correctly - if we use glob then files not globbed first time round when you ran CMake will not get picked up
• You ensure that only files you want are added. Globbing may pick up stray files that you do not want.

In order to work around the first issue, you can simply “touch” the CMakeLists.txt that does the glob, either by using the touch command or by writing the file with no changes. This will force CMake to re-run and pick up the new file.

To fix the second problem you can organize your code carefully into directories, which is what you probably do anyway. In the worst case, you can use the list(REMOVE_ITEM) command to clean up the globbed list of files:

file(GLOB to_remove file_to_remove.cpp) list(REMOVE_ITEM list ${to_remove}) 

The only real situation where this can bite you is if you are using something like git-bisect to try older versions of your code in the same build directory. In that case, you may have to clean and compile more than necessary to ensure you get the right files in the list. This is such a corner case, and one where you already are on your toes, that it isn’t really an issue.