LLVM libc++ Android C++ library support for Xamarin Android #4949
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Hey @raghav-alevoor we don't want to use the full libstdc++ in Xamarin.Android proper because of the size increase it would cause for all the APKs. Xamarin.Android's C++ code uses no libstdc++ types that require linking against the "full" library and we want it to stay this way for as long as it's possible. If, however, we were to use it, we would link it into the runtime statically. The reason is that the application developed with Xamarin.Android can ship its own version of libstdc++ and we'd end up with symbol conflicts and a situation of potentially disastrous side effects arising from two different versions of libstdc++ living in one process. On the other hand, we must link against something and currently this is the |
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So, linking |
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I don't see a way to hide the |
The libstdc++ runtime library may be removed in a future Android release Android 10 VTS test prohibits linkage to library libstdc++. This commit replaces libstdc++ STL linkage with NDK's libc++_static.
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Hi @grendello, thanks for your comments. I've created a pull request #4952 Thank you. |
Fixes: dotnet#4949 Context: dotnet#4952 Context: android/ndk#744 Xamarin.Android uses the "system" version of the C++ standard library (which is, in reality, a very basic stub containing only weak definitions of the `new` and `delete` operator functions) in order to avoid possible conflicts with Xamarin.Android applications which could make use of their own version of the C++ standard library. The conflict would arise from the fact that the `libc++` library shipped with the current versions of Android NDK is built in a way that would expose a large number of its symbols which would conflict with any other `libc++` used by the application. Additionally, linking `libc++` into Xamarin.Android runtime would result in a binary 4-6 times bigger than what we currently ship (when linking statically) or by 6.2MB if the shared library version of `libc++` were used. The "system" version of `libstdc++` is deprecated in favor of `libc++` and may be removed from the NDK in a near future. In order to prepare for this we need to switch to one of the supported STL implementations. The static and dynamic linking options are not viable, as described above, which leaves us with the `none` STL option. However, using this variety causes Android NDK's cmake toolchain to pass the `-nostdlib++ -nostdinc++` options to the compiler. The first of them doesn't affect us, since we don't use any types from the `libc++` library, but the second of them causes the standard C++ files, which we do use, to not be found. This commit enables the `none` STL variety and works around the `-nostdinc++` flag by detecting and specifying the include path to the standard C++ headers manually on the command line.
Fixes: #4949 Context: #4952 Context: android/ndk#744 Xamarin.Android uses the "system" version of the C++ standard library (which is, in reality, a very basic stub containing only weak definitions of the `new` and `delete` operator functions) in order to avoid possible conflicts with Xamarin.Android applications which could make use of their own version of the C++ standard library. The conflict would arise from the fact that the `libc++` library shipped with the current versions of Android NDK is built in a way that would expose a large number of its symbols which would conflict with any other `libc++` used by the application. Additionally, linking `libc++` into Xamarin.Android runtime would result in a binary 4-6 times bigger than what we currently ship (when linking statically) or by 6.2MB if the shared library version of `libc++` were used. The "system" version of `libstdc++` is deprecated in favor of `libc++` and may be removed from the NDK in a near future. In order to prepare for this we need to switch to one of the supported STL implementations. The static and dynamic linking options are not viable, as described above, which leaves us with the `none` STL option. However, using this variety causes Android NDK's `cmake` toolchain to pass the `-nostdlib++ -nostdinc++` options to the compiler. The first of them doesn't affect us, since we don't use any types from the `libc++` library, but the second of them causes the standard C++ files, which we do use, to not be found. Use the `none` STL variety and work around the `-nostdinc++` flag by detecting and specifying the include path to the standard C++ headers manually on the command line. This allows us to no longer link against `libstdc++.so`, while continuing to use "header-file only" functionality.
Fixes: #4949 Context: #4952 Context: android/ndk#744 Xamarin.Android uses the "system" version of the C++ standard library (which is, in reality, a very basic stub containing only weak definitions of the `new` and `delete` operator functions) in order to avoid possible conflicts with Xamarin.Android applications which could make use of their own version of the C++ standard library. The conflict would arise from the fact that the `libc++` library shipped with the current versions of Android NDK is built in a way that would expose a large number of its symbols which would conflict with any other `libc++` used by the application. Additionally, linking `libc++` into Xamarin.Android runtime would result in a binary 4-6 times bigger than what we currently ship (when linking statically) or by 6.2MB if the shared library version of `libc++` were used. The "system" version of `libstdc++` is deprecated in favor of `libc++` and may be removed from the NDK in a near future. In order to prepare for this we need to switch to one of the supported STL implementations. The static and dynamic linking options are not viable, as described above, which leaves us with the `none` STL option. However, using this variety causes Android NDK's `cmake` toolchain to pass the `-nostdlib++ -nostdinc++` options to the compiler. The first of them doesn't affect us, since we don't use any types from the `libc++` library, but the second of them causes the standard C++ files, which we do use, to not be found. Use the `none` STL variety and work around the `-nostdinc++` flag by detecting and specifying the include path to the standard C++ headers manually on the command line. This allows us to no longer link against `libstdc++.so`, while continuing to use "header-file only" functionality.
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Release status update A new Preview version of Xamarin.Android has now been published that includes the fix for this item. The fix is not yet included in a Release version. I will update this item again when a Release version is available that includes the fix. Fix included in Xamarin.Android SDK version 11.0.99.34. Fix included on Windows in Visual Studio 2019 version 16.8 Preview 2. To try the Preview version that includes the fix, check for the latest updates in Visual Studio Preview. Fix included on macOS in Visual Studio 2019 for Mac version 8.8 Preview 2. To try the Preview version that includes the fix, check for the latest updates on the Preview updater channel. |
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Release status update A new Release version of Xamarin.Android has now been published that includes the fix for this item. Fix included in Xamarin.Android SDK version 11.1.0.17. Fix included on Windows in Visual Studio 2019 version 16.8. To get the new version that includes the fix, check for the latest updates or install the most recent release from https://visualstudio.microsoft.com/downloads/. Fix included on macOS in Visual Studio 2019 for Mac version 8.8. To get the new version that includes the fix, check for the latest updates on the Stable updater channel. |
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The current build configs for Xamarin Android uses "libstdc++" runtime with the cmake configuration -DANDROID_STL="system"
This runtime library may be removed in a future release as discussed in android/ndk#744
Also, the LLVM libc++ is the standard C++ library since Android Lollipop. Full details https://developer.android.com/ndk/guides/cpp-support
The cmake configuration for Xamarin-Android build should also be updated to use libc++_shared.so with the configuration -DDANDROID_STL="c++_shared", so that the binaries are compatible with the current and future version of Android.
The Android Vendor Test Suite ,a requirement for Android compatibility testing, test case "VtsVndkDependencyTest" forbids
any linkage against "libstdc++" for ROM installed libraries. Because of the linkage to "libstdc++" in "libmonodroid" any ROM installed Xamarin apk will fail the test.
Can the C++ runtime libraries be updated to LLVM libc++ implementation from the current libstdc++.
Thank you.
Regards,
Raghav
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