Add How to Build for Android

How-to-build-for-Android.md

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+# How to use the C++ REST SDK on Android (2.3+)
+
+The easiest way to use the C++ REST SDK on android is to add the "C++ REST SDK - Android" NuGet package available via Visual Studio 2015 (including the free Community Edition). Go to [Using NuGet to add the C++ REST SDK to a VS project](https://casablanca.codeplex.com/wikipage?title=Using%20NuGet%20to%20add%20the%20C%2b%2b%20REST%20SDK%20to%20a%20VS%20project&referringTitle=Use%20on%20Android) for general information about adding NuGet packages.  
+
+After adding the package, you will need to enable exceptions, rtti, c++11, and the GNU standard library (gnustl_static). These options are all found under <span class="codeInline">Configuration Properties</span> as follows:  
+
+*   General -> Use of STL = gnustl_static
+*   C++ -> Code Generation -> Enable C++ Exceptions = Unwind Tables (-funwind-tables)
+*   C++ -> Language -> Enable Run-Time Type Information = Yes
+*   C++ -> Language -> C++ Language Standard = C++11
+
+Finally, you will need to add some small initialization code detailed under [StaticLinking](#StaticLinking). In addition to the instructions mentioned here, you can get a walkthrough with a simple sample application from this [blog post](http://blogs.msdn.com/b/vcblog/archive/2015/01/06/targeting-android-with-the-c-rest-sdk.aspx).  
+
+## Non-NuGet
+
+First, follow the steps here: [Setup and Build on Android](https://casablanca.codeplex.com/wikipage?title=Setup%20and%20Build%20on%20Android&referringTitle=Use%20on%20Android)  
+
+You will need to link against the following from your project:  
+
+*   build.armv7.debug/Binaries/libcpprest.a
+*   Boost-for-Android/build/lib/libboost_*-clang-mt-1_55.a
+*   libiconv/armeabi-v7a/lib/libiconv.a (removed in version 2.6.0)
+*   openssl/armeabi-v7a/lib/libssl.a
+*   openssl/armeabi-v7a/lib/libcrypto.a
+*   libm (math library, link with -lm)
+*   liblog (logging library, link with -llog)
+
+You will also need to include the headers for boost and the C++ REST SDK:  
+
+*   ../../Release/include
+*   Boost-for-Android/build/include/boost-1_55
+
+Use the included documentation with your NDK to add these prebuilt libraries and include paths. An outdated version of the relevant documentation is hosted at [http://www.kandroid.org/ndk/docs/PREBUILTS.html](http://www.kandroid.org/ndk/docs/PREBUILTS.html).  
+
+<a name="StaticLinking"></a>
+
+## Static Linking and Initialization
+
+The C++ REST SDK prefers static linking for Android since version 2.3\. This makes it much simpler for both Native Activities and Java-based Activities since there is only one shared object file in the final app bundle.  
+
+However, in order to prevent conflicts with other code, you must provide a pointer to the JVM to the SDK before it can initialize. This pointer is provided through the method <span class="codeInline">cpprest_init(vm)</span> which is located in the <span class="codeInline">pplx/pplxtasks.h</span> header file.  
+
+**Note: If you do not do this, your application will terminate upon attempting to create a task.**  
+
+### Native Activity (android_main)
+
+For most NativeActivity projects, if you have an entry point like  
+
+```c++
+void android_main(struct android_app* state) {
+```
+
+You can add the following code at the top of the <span class="codeInline">android_main</span> function:  
+
+```c++
+cpprest_init(state->activity->vm);
+```
+
+### Other Shared Libraries (JNI_OnLoad)
+
+For most shared library projects, you will simply need to copy the following code into your main cpp file:  
+
+```c++
+extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved)
+{
+    JNIEnv* env;
+    if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK)
+    {
+        return -1;
+    }
+
+    cpprest_init(vm);
+    return JNI_VERSION_1_6;
+}
+```