Android的binder机制

service manager是一个用于查询其他binder服务的特殊binder服务。

1. 在init解析init.rc中启动的，是一个c程序，在路径/frameworks/native/cmds/servicemanager下，由两个文件构成service_manager.c和binder.c

#init.rc
service servicemanager /system/bin/servicemanager
class core
user system
group system
critical
#当servicemanager由于致命错误导致重启，其他系统服务zygote,media,surfaceflinger,drm也会重启
onrestart restart zygote
onrestart restart media
onrestart restart surfaceflinger
onrestart restart drm


工作流程

1. 打开binder设备，做相关初始化

//service_manager.c
//main()
struct binder_state *bs;//该结构体包含binder相关信息
bs=binder_open(128*1024);
//binder.c
//binder_open()
struct binder_state *bs;
bs = malloc(sizeof(*bs));
bs->fd = open("/dev/binder",O_RDWR)//打开binder驱动
bs->mapsize = mapsize;
bs->mapped = mmap(NULL, mapsize, PROT_READ,MAP_PRIVATE,bs->fd,0);//mmap用于将设备(bs->fd=/dev/binder)中指定的内存(0=起始地址)映射到进程空间(bs->mapped)中
return bs;


2. 将自己设置为binder大管家

//service_manager.c
//main()
binder_become_context_manager(bs);
//binder_become_context_manager()
return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR,0);//ioctl向binder驱动发送命BINDER_SET_CONTEXT_MGR


3. 进入处理循环，开始等待和处理客户端请求

//service_manager.c
//main()
binder_loop(bs,BINDER_SERVICE_MANAGER)
//binder_loop()
1. 从binder驱动读取消息==ioctl
2. 处理消息==binder_parse
3. 不断循环


例子

service

#include
#include
#include
#include

#include

#define LOG_TAG "binderserver"

using namespace android;

class MyBinderService : public BBinder{
//override
status_t onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
printf("MyBinderService onTransact code = %d\n", code);

if(code == 123)
{
int readInt = 0;

readInt = data.readInt32();

printf("MyBinderService onTransact readInt = %d\n", readInt);

reply->writeInt32(234);
}
printf("return NO_ERROR\n");

return NO_ERROR;
}
};

int main(int argc, char** argv)
{
sp serverBinder = new MyBinderService();

defaultServiceManager()->addService(String16("mybindertag"), serverBinder);

printf("main addService \n");

sp

proc(ProcessState::self());
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();

printf("never return!!! \n");
return 0;
}c


#######################################Android.mk
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
#需要编译的cpp文件
LOCAL_SRC_FILES:= mybinderserver.cpp
LOCAL_C_INCLUDES := \
external/skia/include/core \
bionic \
external/stlport/stlport
#编译为可执行文件
LOCAL_MODULE:= mybinderserver

LOCAL_MODULE_TAGS := optional
#添加依赖库一定要有libbinder
LOCAL_SHARED_LIBRARIES := \
libcutils \
libutils \
libbinder \
libgui \
libskia \
libui

include $(BUILD_EXECUTABLE)


client

#include
#include
#include
#include
#include

#define LOG_TAG "binderclient"

using namespace android;

int main(int argc, char** argv)
{
sp sm = defaultServiceManager();
//向servicemanager查询id为mybindertag的binger服务器
sp binder = sm->checkService(String16("mybindertag"));

if (binder == 0)
{
printf("service is not found !\n");
return 0;
}
else
{
//获取service
sp binder = defaultServiceManager()->getService(String16("mybindertag"));
}
//业务相关
while(1)
{
Parcel data, reply;

int transCode = 0;
int writeInt = 0;
int replyInt = 0;

printf("please input transCode : \n");
scanf("%d", &transCode);
getchar();

if(123 == transCode)
{
printf("please input int you need transfer: \n");
scanf("%d", &writeInt);
getchar();
data.writeInt32(writeInt);
}
//使用binder的transact方法来传递
binder->transact(transCode, data, &reply);

replyInt = reply.readInt32();
printf("get reply data = %d\n", replyInt);

}
return 0;
}


#######################################Android.mk
LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)

#需要编译的cpp文件
LOCAL_SRC_FILES:= mybinderclient.cpp

LOCAL_C_INCLUDES := \
external/skia/include/core \
bionic \
external/stlport/stlport

#编译为可执行文件
LOCAL_MODULE:= mybinderclient

#LOCAL_MODULE_PATH := $(TARGET_OUT_OPTIONAL_EXECUTABLES)
LOCAL_MODULE_TAGS := optional

#添加依赖库一定要有libbinder
LOCAL_SHARED_LIBRARIES := \
libcutils \
libutils \
libbinder \
libgui \
libskia \
libui

include $(BUILD_EXECUTABLE)


编译测试

在 frameworks\base\cmds 创建相应的文件夹mybinderserver，把Android.mk和mybinderserver.cpp拷贝进去编译的时候

1. 执行. build/envsetup.sh

2. lunch 选择对应的版本

3. mmm frameworks/base/cmds/mybinderserver/

4. adb root

5. adb remount

6. adb push out/target/product/rk3368/system/bin/mybinderserver system/bin

同理mybinderclient 也是这样操作。