Xmake: Fixing Get_stdmodules Crash On Android

Nov 2, 2025 by SLV Team 46 views

Hey guys! Today we're diving deep into a specific bug that popped up in Xmake, version 3.0.4 (dev), affecting users on Windows when building for Android. If you've been experiencing a crash, especially when no runtime is selected for the Android platform, this post is for you. We'll break down the issue, explain why it happens, and walk through how to get things running smoothly again. So, grab your favorite beverage and let's get this sorted!

Understanding the `get_stdmodules` Crash

Alright, let's get straight to the heart of the matter. The crash occurs within the get_stdmodules function in Xmake's C++ module support. The core problem stems from a situation where the get_cpplibrary_name function fails to return a value. This is particularly evident on the Android platform when you haven't explicitly selected a runtime library. Think of it like trying to put a square peg in a round hole; if a crucial piece of information isn't there, the whole process can grind to a halt. The traceback points to specific lines in the Xmake source code: support.lua and scanner.lua. These are the areas where Xmake is trying to figure out which standard C++ modules to include for your build. When get_cpplibrary_name doesn't give it the expected library name (because, for instance, no runtime was specified), it leads to a table index is nil error. This basically means Xmake is trying to access something that doesn't exist, causing it to throw a fit and crash. It’s a pretty common type of error in programming – you expect a certain value, and if it's missing, things go south pretty quickly. The error log clearly shows this: error: @programdir\core\main.lua:272: @programdir\actions\build\main.lua:161: @programdir\core\cache\memcache.lua:69: table index is nil. This chain of errors tells us exactly where the system broke down, starting from trying to build and ending in a cache-related issue that’s triggered by the missing library name. We'll explore the configuration that can lead to this and how to fix it.

The Culprit: Android Platform Without a Runtime

So, what exactly triggers this nasty get_stdmodules crash? The primary suspect, as hinted at earlier, is the Android platform combined with a lack of a specified C++ runtime library. When you're building for Android using Xmake, it needs to know which C++ standard library implementation to link against. This could be something like libstdc++ or libc++. If you don't tell Xmake which one to use, either by explicitly setting it or through other configuration that implies a choice, the get_cpplibrary_name function won't have any information to return. This absence of a return value is what breaks the get_stdmodules function, leading to the table index is nil error. Imagine you're packing for a trip, and you forget to pack your shoes. You might have everything else, but you can't quite complete the task, right? It's similar here; Xmake is missing a fundamental piece of information needed to proceed with scanning and processing your C++ modules.

Consider the provided project configuration:

add_rules("mode.debug", "mode.release")
set_languages("c++23")

add_requires("ndk")
set_toolchains("ndk@ndk")

set_policy("build.c++.modules.std", false)

target("module_target")
    set_kind("moduleonly")
    add_files("src/*.ixx")

This configuration sets up a debug/release build, specifies C++23, adds the Android NDK as a requirement, and sets it as the toolchain. However, it doesn't explicitly define which C++ runtime library to use. When Xmake encounters this, especially when configuring for the Android platform (xmake f --plat=android), it tries to figure out the necessary details. Since the runtime isn't specified, get_cpplibrary_name returns nil, and boom – crash!

Reproducing the Issue: A Step-by-Step Guide

For those of you who want to see this bug in action or try to fix it yourself, here’s how you can reproduce the crash. It's a pretty straightforward process, so follow along:

Set up your project: Make sure you have a basic Xmake project structure. You'll need a xmake.lua file and some source files (like src/main.cpp or src/my_module.ixx if you're testing C++ modules). The example configuration provided in the bug report is a good starting point.
```
-- xmake.lua
add_rules("mode.debug", "mode.release")
set_languages("c++23")

add_requires("ndk")
set_toolchains("ndk@ndk")

set_policy("build.c++.modules.std", false)

target("module_target")
    set_kind("moduleonly")
    add_files("src/*.ixx")
```
Configure for Android: Open your terminal or command prompt in your project directory and run the configuration command, specifically targeting the Android platform. If you don't have the NDK set up correctly, Xmake might warn you about not finding the SDK or NDK directories, which is expected in this scenario.
```
xmake f --plat=android
```
At this stage, Xmake might output warnings like:
```
checking for architecture ... armeabi-v7a
checking for Android SDK directory ... no
checking for NDK directory ... no
```
Don't worry too much about these warnings for now; they indicate that Xmake is struggling to find specific Android development tools, but the core issue we're trying to reproduce is independent of their presence at this exact configuration step. The important part is that the configuration process itself will attempt to resolve module dependencies.
Attempt to Build: Now, try to build your project. Again, use verbose and debug flags (-vD) to get the most detailed output, which is crucial for observing the error stack.
```
xmake build -vD
```

Observe the Crash: If your setup mirrors the bug report, you should now see the familiar error message:

error: @programdir\core\main.lua:272: @programdir\actions\build\main.lua:161: @programdir\core\cache\memcache.lua:69: table index is nil
stack traceback:
    [@programdir\core\cache\memcache.lua:69]: in function 'set'
    [@programdir\core\cache\memcache.lua:77]: in function 'set2'
    [@programdir\rules\c++\modules\clang\..\support.lua:313]: in function 'get_stdmodules'
    [@programdir\rules\c++\modules\scanner.lua:269]: in function '_get_targetdeps_modules'
    ...

This sequence of events confirms that the crash is reproducible under the specified conditions: building for Android without a defined C++ runtime library. It highlights a gap in Xmake's handling of module dependencies when essential configuration like the C++ runtime is missing.

The Fix: Specifying the C++ Runtime

Now for the good news, guys! Fixing this crash is usually quite straightforward. The key is to provide Xmake with the information it's missing: the C++ runtime library. For Android development with the NDK, you typically have two main choices: libstdc++ (GNU Standard C++ Library) or libc++ (LLVM Standard C++ Library).

Here’s how you can explicitly tell Xmake which runtime to use. You add this line to your xmake.lua file, typically near your target definition or at the top level:

-- Option 1: Use libstdc++
add_cpplibs("libstdc++")

-- Option 2: Use libc++
add_cpplibs("libc++")

Let's integrate this into the example configuration. If you choose to use libc++, your xmake.lua would look like this:

add_rules("mode.debug", "mode.release")
set_languages("c++23")

add_requires("ndk")
set_toolchains("ndk@ndk")

set_policy("build.c++.modules.std", false)

-- *** FIX: Specify the C++ runtime library ***
add_cpplibs("libc++") 

target("module_target")
    set_kind("moduleonly")
    add_files("src/*.ixx")

Alternatively, you could use libstdc++:

add_rules("mode.debug", "mode.release")
set_languages("c++23")

add_requires("ndk")
set_toolchains("ndk@ndk")

set_policy("build.c++.modules.std", false)

-- *** FIX: Specify the C++ runtime library ***
add_cpplibs("libstdc++") 

target("module_target")
    set_kind("moduleonly")
    add_files("src/*.ixx")

Why does this work?

By adding add_cpplibs("libc++") or add_cpplibs("libstdc++"), you are explicitly informing Xmake about the C++ runtime library that should be used for your target. This provides the necessary information to the get_cpplibrary_name function, ensuring it returns a valid name. Consequently, the get_stdmodules function receives the expected input, and the table index is nil error is avoided. This small addition effectively plugs the hole that was causing the crash, allowing Xmake to correctly process your module dependencies and proceed with the build without any hiccups. Remember to choose the C++ runtime that aligns with your project's requirements or the recommendations for your specific NDK version.

Verifying the Fix

After applying the fix – that is, after adding add_cpplibs("libc++") or add_cpplibs("libstdc++") to your xmake.lua – it's crucial to verify that the crash is indeed resolved. The process is simple: re-run the same steps that previously caused the crash.

Re-configure: First, clean your previous configuration and re-configure Xmake for the Android platform. This ensures that Xmake picks up the new setting.
```
xmake clean
xmake f --plat=android
```
You should still see the same warnings about not finding the SDK or NDK if they aren't properly configured on your system, but these are tangential to the build crash itself.
Build the Project: Now, attempt to build your project again. Use the verbose and debug flags to monitor the process closely.
```
xmake build -vD
```

What to expect:

If the fix is successful, you should no longer see the error: ... table index is nil message originating from get_stdmodules. Instead, the build process should proceed smoothly. Xmake will correctly identify and process the C++ modules, and your build should complete without errors related to this specific issue. You might see other build-related messages or warnings, but the critical crash should be gone.

This successful build demonstrates that providing the C++ runtime library information resolved the underlying problem. It reinforces the importance of explicitly configuring all necessary build components when using tools like Xmake, especially when dealing with platform-specific requirements like those for Android development.

Conclusion: Keeping Your Builds Smooth

And there you have it, folks! We've successfully identified, reproduced, and fixed a pesky crash in Xmake's get_stdmodules function that specifically affects Android builds when the C++ runtime isn't specified. By simply adding add_cpplibs("libc++") or add_cpplibs("libstdc++") to your xmake.lua, you provide Xmake with the crucial information it needs to proceed, preventing the table index is nil error and ensuring your build completes without a hitch. It's a great reminder that even small configuration details can have a big impact on your build process. Keep experimenting, keep building, and if you run into any more Xmake mysteries, you know where to find us! Happy coding, everyone!