The NDK supports multiple C++ runtime libraries. This document provides information about these libraries, the tradeoffs involved, and how to use them.
C++ runtime libraries
Table 1. NDK C++ Runtimes and Features.
| Name | Features |
|---|---|
| libc++ | Modern C++ support. |
| system | new and delete. (Deprecated in r18.) |
| none | No headers, limited C++. |
libc++ is available as both a static and shared library.
libc++
LLVM's libc++ is the C++ standard library that has been used by the Android OS since Lollipop, and as of NDK r18 is the only STL available in the NDK.
CMake defaults to whatever version of C++ clang defaults to (currently C++14),
so you'll need to set the standard CMAKE_CXX_STANDARD to the appropriate value
in your CMakeLists.txt file to use C++17 or later features. See the CMake
documentation for CMAKE_CXX_STANDARD
for more details.
ndk-build also leaves the decision to clang by default, so ndk-build users
should use APP_CPPFLAGS to add -std=c++17 or whatever they want instead.
The shared library for libc++ is libc++_shared.so, and the static
library is libc++_static.a. In typical cases the build system will handle
using and packaging these libraries as needed for the user. For atypical cases
or when implementing your own build system, see the Build System Maintainers
Guide or the guide for using other build systems.
The LLVM Project is under the Apache License v2.0 with LLVM Exceptions. For more information, see the license file.
system
The system runtime refers to /system/lib/libstdc++.so. This library should not
be confused with GNU's full-featured libstdc++. On Android, libstdc++ is just
new and delete. Use libc++ for a full-featured C++ standard library.
The system C++ runtime provides support for the basic C++ Runtime ABI.
Essentially, this library provides new and delete. In contrast to the other
options available in the NDK, there is no support for exception handling or
RTTI.
There is no standard library support aside from the C++ wrappers for the C
library headers such as <cstdio>. If you want an STL, you should use one of
the other options presented on this page.
none
There is also the option to have no STL. There are no linking or licensing requirements in that case. No C++ standard headers are available.
Selecting a C++ Runtime
CMake
The default for CMake is c++_static.
You can specify c++_shared, c++_static, none, or system using the
ANDROID_STL variable in your module-level build.gradle file. To learn more,
see the documentation for ANDROID_STL in
CMake.
ndk-build
The default for ndk-build is none.
You can specify c++_shared, c++_static, none, or system using the
APP_STL variable in your Application.mk file. For example:
APP_STL := c++_shared
ndk-build only allows you to select one runtime for your app, and can only do in Application.mk.
Use clang directly
If you're using clang directly in your own build system, clang++ will use
c++_shared by default. To use the static variant, add -static-libstdc++ to
your linker flags. Note that although the option uses the name "libstdc++" for
historical reasons, this is correct for libc++ as well.
Important considerations
Static runtimes
If all of your application's native code is contained in a single shared library, we recommend using the static runtime. This allows the linker to inline and prune as much unused code as possible, leading to the most optimized and smallest application possible. It also avoids PackageManager and dynamic linker bugs in old versions of Android that make handling multiple shared libraries difficult and error-prone.
That said, in C++, it is not safe to define more than one copy of the same function or object in a single program. This is one aspect of the One Definition Rule present in the C++ standard.
When using a static runtime (and static libraries in general), it is easy to accidentally break this rule. For example, the following application breaks this rule:
# Application.mk
APP_STL := c++_static
# Android.mk
include $(CLEAR_VARS)
LOCAL_MODULE := foo
LOCAL_SRC_FILES := foo.cpp
include $(BUILD_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := bar
LOCAL_SRC_FILES := bar.cpp
LOCAL_SHARED_LIBRARIES := foo
include $(BUILD_SHARED_LIBRARY)
In this situation, the STL, including and global data and static constructors, will be present in both libraries. The runtime behavior of this application is undefined, and in practice crashes are very common. Other possible issues include:
- Memory allocated in one library, and freed in the other, causing memory leakage or heap corruption.
- Exceptions raised in
libfoo.sogoing uncaught inlibbar.so, causing your app to crash. - Buffering of
std::coutnot working properly.
Beyond the behavioral issues involved, linking the static runtime into multiple libraries will duplicate the code in each shared library, increasing the size of your application.
In general, you can only use a static variant of the C++ runtime if you have one and only one shared library in your application.
Shared runtimes
If your application includes multiple shared libraries, you should use
libc++_shared.so.
On Android, the libc++ used by the NDK is not the same as the one that's part
of the OS. This gives NDK users access to the latest libc++ features and bug
fixes even when targeting old versions of Android. The trade-off is that if you
use libc++_shared.so, you must include it in your app. If you're building your
application with Gradle this is handled automatically.
Old versions of Android had bugs in PackageManager and the dynamic linker
that caused installation, update, and loading of native libraries to be
unreliable. In particular, if your app targets a version of Android earlier
than Android 4.3 (Android API level 18), and you use libc++_shared.so, you
must load the shared library before any other library that depends on it.
The ReLinker project offers workarounds for all known native library loading problems, and is usually a better choice than writing your own workarounds.
One STL per app
Historically the NDK supported GNU libstdc++ and STLport in addition to libc++. If your application depends on prebuilt libraries that were built against an NDK different than the one used to build your application, you will need to ensure that it does so in a compatible manner.
An application should not use more than one C++ runtime. The various STLs are
not compatible with one another. As an example, the layout of std::string
in libc++ is not the same as it is in gnustl. Code written against one STL will
not be able to use objects written against another. This is just one example;
the incompatibilities are numerous.
This rule extends beyond your code. All of your dependencies must use the same STL that you have selected. If you depend on a closed source third-party dependency that uses the STL and does not provide a library per STL, you do not have a choice in STL. You must use the same STL as your dependency.
It is possible that you will depend on two mutually incompatible libraries. In this situation the only solutions are to drop one of the dependencies or ask the maintainer to provide a library built against the other STL.
C++ Exceptions
C++ exceptions are supported by libc++, but they are disabled by default in ndk-build. This is because historically C++ exceptions were not available in the NDK. CMake and standalone toolchains have C++ exceptions enabled by default.
To enable exceptions across your whole application in ndk-build, add the following line to your Application.mk file:
APP_CPPFLAGS := -fexceptions
To enable exceptions for a single ndk-build module, add the following line to the given module in its Android.mk:
LOCAL_CPP_FEATURES := exceptions
Alternatively, you can use:
LOCAL_CPPFLAGS := -fexceptions
RTTI
As with exceptions, RTTI is supported by libc++, but is disabled by default in ndk-build. CMake and standalone toolchains have RTTI enabled by default.
To enable RTTI across your whole application in ndk-build, add the following line to your Application.mk file:
APP_CPPFLAGS := -frtti
To enable RTTI for a single ndk-build module, add the following line to the given module in its Android.mk:
LOCAL_CPP_FEATURES := rtti
Alternatively, you can use:
LOCAL_CPPFLAGS := -frtti