Solving the Mysterious Case of Passing CMake Command with Cache Entry Not Working: A Step-by-Step Guide

Are you frustrated with CMake’s stubborn refusal to acknowledge your carefully crafted command line arguments? Do you find yourself stuck in an infinite loop of trying to pass a command with a cache entry, only to have it ignored? Fear not, dear developer, for we’re about to embark on a thrilling adventure to unravel the secrets of CMake’s behavior and uncover the hidden solutions to this perplexing problem.

Understanding the Problem: CMake’s Cache System

Before we dive into the fixing part, let’s take a step back and understand the root cause of the issue. CMake’s cache system is designed to store values that are generated during the configuration process. These values are stored in a file called `CMakeCache.txt` and are used to speed up the build process by reusing previously computed results.

However, this caching mechanism can sometimes lead to unexpected behavior, especially when trying to pass command line arguments with cache entries. It’s not uncommon for CMake to ignore these arguments or overwrite them with cached values, leaving you scratching your head and wondering what went wrong.

Scenario 1: Passing Command Line Arguments with `-D` Option

Lets say you want to pass a command line argument to CMake using the `-D` option. For example, you want to set the `CMAKE_BUILD_TYPE` variable to `Debug`:

cmake -DCMAKE_BUILD_TYPE=Debug ..

In most cases, this should work as expected. However, if you have a cached value for `CMAKE_BUILD_TYPE` from a previous run, CMake might ignore the new value and use the cached one instead.

Solution: Forced Override using `CACHE` Keyword

To force CMake to override the cached value, you can add the `CACHE` keyword to your command line argument:

cmake -DCMAKE_BUILD_TYPE:STRING=Debug ..

By adding `:STRING` to the argument, you’re telling CMake to treat the value as a string and override any cached value. This should ensure that your new value takes precedence.

Scenario 2: Passing Command Line Arguments with `-C` Option

Alternatively, you might want to pass a command line argument using the `-C` option, which tells CMake to load a pre-existing cache file. For example:

cmake -Cpath/to/cache/file ..

Again, this should work as expected, but what if you want to override a specific cache entry? Let’s say you want to change the `CMAKE_CXX_COMPILER` cache entry:

cmake -Cpath/to/cache/file -DCMAKE_CXX_COMPILER=g++ ..

In this case, CMake might still use the cached value for `CMAKE_CXX_COMPILER` instead of the new value.

Solution: Creating a New Cache File

To avoid this issue, you can create a new cache file using the `-CClear` option:

cmake -CClear -DCMAKE_CXX_COMPILER=g++ ..

This will clear the cache and create a new one with the updated value for `CMAKE_CXX_COMPILER`.

Scenario 3: Using `cmake` with `-i` Option

Another common scenario is using the `-i` option to specify a script file for CMake to execute. For example:

cmake -i script.cmake ..

What if you want to pass a command line argument to this script file? For example, you want to set a variable `FOO` to `bar`:

cmake -i script.cmake -DFOO=bar ..

Again, CMake might ignore the new value and use a cached one instead.

Solution: Using `set` Command in Script File

To avoid this issue, you can use the `set` command inside your script file to set the variable:

set(FOO "bar" CACHE STRING "Description")

This will ensure that the variable is set correctly and overrides any cached value.

Best Practices for Avoiding Cache-Related Issues

To minimize the risk of encountering cache-related issues, follow these best practices:

• Use the `–wipe-cache` option to clear the cache before running CMake:

cmake --wipe-cache ..

• Avoid using the same cache file for different build configurations:

cmake -DCMAKE_BUILD_TYPE=Debug ..
cmake -DCMAKE_BUILD_TYPE=Release ..

Instead, use separate cache files for each configuration:

cmake -DCMAKE_BUILD_TYPE=Debug -CDebugCache ..
cmake -DCMAKE_BUILD_TYPE=Release -CReleaseCache ..

• Use the `CACHE` keyword when passing command line arguments:

cmake -DCMAKE_BUILD_TYPE:STRING=Debug ..

• Verify that your cache file is not corrupted or outdated:

rm CMakeCache.txt
cmake ..

Conclusion

In conclusion, passing CMake command line arguments with cache entries can be a complex process, but by understanding the underlying cache system and using the right techniques, you can avoid common pitfalls and ensure that your build process runs smoothly. Remember to use the `CACHE` keyword, create new cache files when necessary, and follow best practices to minimize the risk of cache-related issues.

With these tips and tricks, you’ll be well on your way to taming the beast that is CMake’s cache system. Happy building!

Scenario	Solution
Passing command line arguments with `-D` option	Use `CACHE` keyword to force override
Passing command line arguments with `-C` option	Create a new cache file using `-CClear` option
Using `cmake` with `-i` option	Use `set` command in script file to set variables

1. Clear the cache using `–wipe-cache` option
2. Avoid using the same cache file for different build configurations
3. Use `CACHE` keyword when passing command line arguments
4. Verify that cache file is not corrupted or outdated

Remember, the key to success lies in understanding the cache system and using the right techniques to overcome common pitfalls.

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