Compiling the native static libraries for Android_Android NDK Game Development Cookbook-QQ阅读男生轻小说网

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Compiling the native static libraries for Android

Android NDK includes a number of GCC and Clang toolchains for each kind of the supported processors.

Getting ready

When building a static library from the source code, we follow the steps similar to the Windows version.

How to do it...

Create a folder named jni and create the Application.mk file with the appropriate compiler switches, and set the name of the library accordingly. For example, one for the FreeImage library should look like the following:
```
APP_OPTIM := release
APP_PLATFORM := android-8
APP_STL := gnustl_static
APP_CPPFLAGS += -frtti 
APP_CPPFLAGS += -fexceptions
APP_CPPFLAGS += -DANDROID
APP_ABI := armeabi-v7a x86
APP_MODULES := FreeImage
```

The Android.mk file is similar to the ones we have written for the sample applications in the previous chapter, yet with a few exceptions. At the top of the file, some required variables must be defined. Let us see what the Android.mk file for the FreeImage library may look like:

# Android API level
TARGET_PLATFORM := android-8
# local directory
LOCAL_PATH := $(call my-dir)
# the command to reset the compiler flags to the empty state
include $(CLEAR_VARS)
# use the complete ARM instruction set
LOCAL_ARM_MODE := arm
# define the library name and the name of the .a file
LOCAL_MODULE     := FreeImage
# add the include directories
LOCAL_C_INCLUDES += src \
# add the list of source files
LOCAL_SRC_FILES += <ListOfSourceFiles>

Define some common compiler options: treat all warnings as errors (-Werror), the ANDROID pre-processing symbol is defined, and the system include directory is set:
```
COMMON_CFLAGS := -Werror -DANDROID -isystem $(SYSROOT)/usr/include/
```

The compilation flags are fixed, according to the selected CPU architecture:

ifeq ($(TARGET_ARCH),x86)
  LOCAL_CFLAGS   := $(COMMON_CFLAGS)
else
  LOCAL_CFLAGS   := -mfpu=vfp -mfloat-abi=softfp -fno-short-enums $(COMMON_CFLAGS)
endif

Since we are building a static library, we need the following line at the end of the makefile:
```
include $(BUILD_STATIC_LIBRARY)
```

How it works...

The Android NDK developers provide their own set of rules to build applications and libraries. In the previous chapter we saw how to build a shared object file with the .so extension. Here we just replace the BUILD_SHARED_LIBRARY symbol to the BUILD_STATIC_LIBRARY and explicitly list the source files required to build each object file.

Note

Of course, you can build a shared library and link your application dynamically against it. However, this usually is a good choice when the library is located in the system and is shared between several applications. In our case, since our application is the sole user of the library, the static linking will make it easier to link and debug the project.