Android module paths (sharing code made easy):
==============================================
Starting from r5, the Android NDK comes with a cool feature that allows
you to share and reuse other people's modules more easily.
I. Overview:
------------
The main idea behind this feature are:
- You can install NDK modules outside of your main project source tree.
- You can easily 'import' them into your project with a one-line command.
In practice, here's how this works:
1. Your NDK_MODULE_PATH environment variable will contain a list
of search paths on your system to lookup for modules.
It is up to you to set the variable, and to copy other modules
to the directories you listed in it.
2. To import a module, place a line like the following to, preferably at
the *end* of, your Android.mk:
$(call import-module,<tag>)
This will look for <tag>/Android.mk under any of the directories
listed in your NDK_MODULE_PATH.
(The reason why it must be at the end is to avoid messing with
the results of the 'my-dir' function. See its description in
docs/ANDROID-MK.html for details).
3. Declare that your project's modules depend on the imported one by
listing them in either your LOCAL_STATIC_LIBRARIES or
LOCAL_SHARED_LIBRARIES. For example:
LOCAL_STATIC_LIBRARIES += <tag>
4. Rebuild!
Remember that NDK r5 also added the ability for a module to "export"
declarations to other modules that depend on it (for example, see the
definition of LOCAL_EXPORT_CFLAGS in docs/ANDROID-MK.html).
A well-written module will correctly export all the things its
dependees need, et voila.
Now for the full details:
I. NDK_MODULE_PATH:
-------------------
The NDK_MODULE_PATH variable must contain a list of directories.
* Due to GNU Make limitations, NDK_MODULE_PATH must not contain any space.
The NDK will complain if this is not the case.
* Use ':' as the path separator.
* On Windows, use '/' as the directory separator.
The directories of NDK_MODULE_PATH will be searched in order. The first
<path>/<tag>/Android.mk file that is found during the lookup will be
included automatically.
As a convenience, $NDK/sources is appended to your NDK_MODULE_PATH
definition by the NDK build system. This allows you to easily import
the helper libraries that come with it (see docs/CPU-FEATURES.html for
a practical example).
II. Writing an import module:
-----------------------------
Writing an import module is trivial and very similar to what you do when
writing project modules:
1. Create a sub-directory from one of your NDK_MODULE_PATH directories.
For example, if NDK_MODULE_PATH is defined to /home/user/ndk-modules,
then create the directory /home/user/ndk-modules/my-module/
2. Place an Android.mk and eventual source code there.
Just like you would for a project module, where these files normally
go to $PROJECT_PATH/Android.mk. In the example above, this would go
to /home/user/ndk-modules/my-module/Android.mk
NOTE: Any Application.mk file here will be ignored.
3. Any module that depends on your new module, would import by calling
the import-module function. For example:
$(call import-module,my-first-module)
Import modules *can* import other modules, but circular dependencies are
not permitted and will be detected. Dependencies are transitive and the build
system will compute all the things that need to be built for you.
The NDK build system will not place object files or executables in your
import module directory (they will all be placed under the project's
build directory, e.g. $PROJECT_PATH/obj/).
You can however distribute prebuilt binaries in your import module with
the new PREBUILT_STATIC_LIBRARIES or PREBUILT_SHARED_LIBRARIES feature
(see docs/ANDROID-MK.html).
This makes it easy to package and redistribute your import module directory
to third-parties.
III. Naming an import module:
-----------------------------
It is important to understand a few things related to the naming of
your import module:
- What 'import-module' does is really search for a file named Android.mk
using the list provided by NDK_MODULE_PATH, then include while performing
very little bit of house-keeping.
Your imported Android.mk can define any number of modules, with
any name. As a consequence, there is no direct relationship between
<name> in the following line:
$(call import-module,<tag>/<name>)
And the names of the modules defined under <tag>/<name>/Android.mk.
IN CASE OF DOUBT, KEEP IT SIMPLE!
If you only plan to provide one import module, just name it like the
base import directory.
On the other hand, you may want to provide a static and a shared
version of your module: use distinct names under the same top-level
Android.mk. Consider the following build script:
$NDK_MODULE_PATH/foo/bar/Android.mk:
LOCAL_PATH := $(call my-dir)
# Static version of the library is named 'bar_static'
include $(CLEAR_VARS)
LOCAL_MODULE := bar_static
LOCAL_SRC_FILES := bar.c
# Ensure our dependees can include <bar.h> too
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(BUILD_STATIC_LIBRARY)
# Shared version of the library is named 'bar_shared'
LOCAL_MODULE := bar_shared
LOCAL_SRC_FILES := bar.c
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(BUILD_SHARED_LIBRARY)
Another module would refer to it by doing the following:
1. Import 'foo/bar', as in:
$(call import-module,foo/bar)
2. To use the static library:
...
LOCAL_STATIC_LIBRARIES := bar_static
3. Or to use the shared library:
...
LOCAL_SHARED_LIBRARIES := bar_shared
- The module namespace is flat, so try to give your modules names that
are likely to not collide with other. Note that your can use
LOCAL_MODULE_FILENAME to give the name of your module's binary
file, independently from its LOCAL_MODULE (see docs/ANDROID-MK.html
for definition and usage). For example:
include $(CLEAR_VARS)
LOCAL_MODULE := super_foo
LOCAL_MODULE_FILENAME := foo # will give libfoo.so
LOCAL_SRC_FILES := foo-src.c
LOCAL_CFLAGS := -DVOLUME=11
include $(BUILD_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := normal_foo
LOCAL_MODULE_FILENAME := foo # will also give libfoo.so
LOCAL_SRC_FILES := foo-src.c
include $(BUILD_SHARED_LIBRARY)
Defines two modules named "super_foo" and "normal_foo"
which both produce a shared library named 'libfoo.so'
As a consequence, only one of them can be used by your project or
a conflict will happen at build time. This allows you to select either
the normal or optimized version in your NDK build scripts, while
keeping the same simple loading instruction in your Java sources as:
static {
System.loadLibrary("foo");
}
IV. Tips & Recommendations:
---------------------------
* You don't need to import a module before you can reference it!
* Use import-module at the *end* of your Android.mk to avoid messing with
the result of 'my-dir'. See the description of this function in
docs/ANDROID-MK.html to understand why.
* It is *strongly* suggested to use a subdirectory for your import tags,
that describes its origin, as in:
$(call import-module,gtk/glib)
> or something like:
$(call import-module,com.example/awesomelib)
IMPORTANT: THE 'android' IMPORT DIRECTORY, AND ANY OF ITS SUB-DIRECTORIES
IS *RESERVED* FOR NDK USAGE. FEEL FREE TO ORGANIZE YOUR OTHER
IMPORT MODULES AS YOU WANT OTHERWISE.