Compile C/C++ ARM binary (Static Library) for Android using Standalone toolchain

This post is intended for those who wants to compile and reuse their existing C/C++ code or who wants to compile an open source code for their Android applications. The idea came into my mind after putting time and effort for compiling and using an open source library CS-MAP in Android application. Hopefully this will help you to save a lot of time and efforts.

Android NDK provides set of tools for compiling and using C and/or C++ source files and generate the native ARM binaries (static, shared and executable) on Linux, OS X and Windows, which can be further used in Android application through Java Native Interface (JNI) calls.
I will explain how we can use the standalone cross-compilers for creating ARM binary (a static library) from C++ source files.

Step 1: Extract the standalone cross-compiler toolchain.

• Install ANDROID-NDK from http://developer.android.com/sdk/ndk/index.html
• Add an environment variable NDK and add path of NDK root directory as its value.
• Run the script for extracting android NDK standalone toolchain binaries and other stuff.                                                                                                                                $NDK/build/tools/make-standalone-toolchain.sh –platform=android-14 –install-dir = /cygdrive/e/android/standalone-14

The above script creates a directory name “standalone-14” in “E:\android\” directory of my Window box

Note: As I am extracting the toolchain on Windows Box, so I passed the /cygwin/e/android/standalone-14 in my install directory path. For Linux and OS X one need to customize the above command accordingly.

Step 2: Identify the C/C++ Sources for compilation

• Create a project directory having name “StaticLibraryTest”
• Create a source directory having name “src” in StaticLibraryTest directory which contains the C/C++ source files
• Test1.cpp

// Contents of Test1.cpp
#include "Test1.h"
int  test1(int  x, int  y){
int temp = 44;
return (x + y )+ temp ;
}


• Test2.cpp

// Content of Test2.cpp
#include "Test2.h"
int  test2(int  x, int  y, int z){
int temp = 9;
return (x * y) - temp + z;
}


• Create a header directory having name “include” in StaticLibraryTest directory which contains the C/C++ header files
  • Test1.h

// Contents of Test1.h
#ifndef TEST1_H
#define TEST1_H
extern int test1(int  x, int  y);
#endif /* TEST1_H */


• Test2.h

// Contents of Test2.h
#ifndef TEST2_H
#define TEST2_H
extern int test2(int  x, int  y, int z);
#endif /* TEST2_H */

Step 3: Create a make file for building the source and create static library

• Create a file having name “make-static-library.mak” in “src” directory. This directory also contains the C/C++ source files. The content of make script are given below.


# This variable contains the path of bin directory which contains all the gnu tools required for compilation
# I have extracted the standalone toolchain for Android platform 14 in "e:\android\standalone-14"
ANDROID_NDK_BIN:=/cygdrive/e/android/standalone-14/bin
# Compilation Tools
CC := $(ANDROID_NDK_BIN)/arm-linux-androideabi-gcc
CPP := $(ANDROID_NDK_BIN)/arm-linux-androideabi-g++
AR := $(ANDROID_NDK_BIN)/arm-linux-androideabi-ar
# Compiler flags
# Specify all the flags below which you want to use for your compilation, For this simple example, we only need to specify the include directory path
CFLAGS          := -I../include
# Rule for making C++ object file
.cpp.o:
$(CPP) $(CFLAGS) -c  $<
# Rule for making C object file
.c.o:
$(CC) $(CFLAGS) -c  $<
STATIC_LIB_SRC = Test1.cpp \
Test2.cpp
STATIC_LIB_OBJ = Test1.o \
Test2.o
OUT = ccStaticLib.a
$(OUT): $(STATIC_LIB_OBJ)
$(AR) -crs $(OUT) $(STATIC_LIB_OBJ)

Step 4: Compilation using the make script

• Open the console (Cygwin for Windows) and change the directory to the “src” directory which contains the C/C++ source files (Test1.cpp and Test2.cpp) and the make file (make-static-library.mak)
• Execute the following command to compile the sources and generate the static library from these sources. “make –f make-static-library.mak”

This will create the static library in the src directory name “ccStaticLib.a”. I have added the “cc” (Cross Compiler) as prefix in the name of static Library generated from make script.

Now static library “ccStaticLib.a” (ARM Binary for Android) is created and our next mission is to use this static library in Android project.

In my upcoming post I will share how we can use this in an Android project.

Updated code part to monospace font as suggested by Venkatesh Shukla.