From the build system’s perspective, the most noticeable change is that now it supports building binaries for two target CPU architectures (64-bit and 32-bit) in the same build. That’s also known as Multilib build.
For native static libraries and shared libraries, the build system sets up
rules to build binaries for both architectures. The product configuration
PRODUCT_PACKAGES), together with the dependency graph, determines which
binaries are built and installed to the system image.
For executables and apps, the build system builds only the 64-bit version by
default, but you can override this setting by using a global
BoardConfig.mk variable or a module-scoped variable.
Caution: If an app exposes an API to other
apps that can be either 32- or 64-bit, the app must have the
android:multiarch property set to a value of
within its manifest to avoid potential errors.
BoardConfig.mk, we added the following variables to
configure the second CPU architecture and ABI:
TARGET_2ND_ARCH TARGET_2ND_ARCH_VARIANT TARGET_2ND_CPU_VARIANT TARGET_2ND_CPU_ABI TARGET_2ND_CPU_ABI2
You can see an example in
If you want the build system to build 32-bit executables and apps by default, set the following variable:
TARGET_PREFER_32_BIT := true
However, you can override this setting by using module-specific variables in
In a Multilib build, module names in
both the 32-bit and 64-bit binaries, as long as they are defined by the build
system. For libraries pulled in by dependency, a 32-bit library is installed
only if it’s required by another 32-bit library or executable. The same is true
for 64-bit libraries.
However, module names on the
make command line cover only the
64-bit version. For example, after running
make libc builds only the 64-bit libc. To
build the 32-bit libc, you need to run
Module Definition in Android.mk
You can use the
LOCAL_MULTILIB variable to configure your build
for 32-bit and/or 64-bit and override the global
LOCAL_MULTILIB to one of the following:
- "both”: build both 32-bit and 64-bit.
- “32”: build only 32-bit.
- “64”: build only 64-bit.
- “first”: build for only the first arch (32-bit in 32-bit devices and 64-bit in 64-bit devices).
- “”: the default; the build system decides what arch to build based on the
module class and other
LOCAL_variables, such as
In a Multilib build, conditionals like
ifeq $(TARGET_ARCH) don’t work any
If you want to build your module for some specific arch(s), the following variables can help you:
It can be set to a list of archs, something like “arm x86 arm64”. Only if the arch being built is among that list will the current module be included by the build system.
The opposite of
LOCAL_MODULE_TARGET_ARCH. Only if the arch being built is not among the list, the current module will be included.
There are minor variants of the above two variables:
The build system will give warning if the current module is skipped due to archs limited by them.
To set up arch-specific build flags, use the arch-specific
LOCAL_ variables. An
LOCAL_ variable is a normal
LOCAL_ variable with an arch suffix,
Those variables will be applied only if a binary is currently being built for that arch.
Sometimes it’s more convenient to set up flags based on whether the binary is
currently being built for 32-bit or 64-bit. In that case you can use the
variable with a
_64 suffix, for example:
Note that not all of the
LOCAL_ variables support the arch-specific variants.
For an up-to-date list of such variables, refer to
In the past, you could use
LOCAL_MODULE_PATH to install a library to a
location other than the default one. For example,
In Multilib build, use
LOCAL_MODULE_RELATIVE_PATH := hw
so that both the 64-bit and 32-bit libraries can be installed to the right place.
If you build an executable as both 32-bit and 64-bit, you’ll need to use one of the following variables to distinguish the install path:
Specifies the installed file name.
Specifies the install path.
In a Multilib build, if you generate source files to
with explicit variables), it won’t reliably work any more. That’s
because the intermediate generated sources will be required by both 32-bit and
64-bit build, but
$(local-intermediates-dir) only points to one of
the two intermediate directories.
Happily, the build system now provides a dedicated, Multilib-friendly,
intermediate directory for generating sources. You can call
$(generated-sources-dir-for) to get the directory’s path. Their
usages are similar to
If a source file is generated to the new dedicated directory and picked up
LOCAL_GENERATED_SOURCES, it is built for both 32-bit and 64-bit
in multilib build.
In Multilib, you can’t use
TARGET_ARCH (or together with
TARGET_2ND_ARCH) to tell the build system what arch the prebuilt
binary is targeted for. Use the aforementioned
With these variables, the build system can choose the corresponding 32-bit prebuilt binary even if it’s currently doing a 64-bit Multilib build.
If you want to use the chosen arch to compute the source path for the prebuilt
binary , you can call
For 64-bit devices, by default we generate both 32-bit and 64-bit odex files
for the boot image and any Java libraries. For APKs, by default we generate
odex only for the primary 64-bit arch. If an app will be launched in both
32-bit and 64-bit processes, please use
LOCAL_MULTILIB := both to make sure
both 32-bit and 64-bit odex files are generated. That flag also tells the build
system to include both 32-bit and 64-bit JNI libraries, if the app has any.