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 <binder/IServiceManager.h>
#include <binder/IPCThreadState.h>
#include <binder/Parcel.h>
#include <binder/IInterface.h>
 
#include<stdio.h>
 
#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<IBinder> serverBinder = new MyBinderService();
 
    defaultServiceManager()->addService(String16("mybindertag"), serverBinder);
 
    printf("main addService \n");
 
    sp<ProcessState> 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 <binder/IServiceManager.h>
#include <binder/IPCThreadState.h>
#include <binder/Parcel.h>
#include <binder/IInterface.h>
#include<stdio.h>
 
#define LOG_TAG "binderclient"
 
using namespace android;
 
int main(int argc, char** argv)
{
    sp<IServiceManager> sm = defaultServiceManager();
    //向servicemanager查询id为mybindertag的binger服务器
    sp<IBinder> binder = sm->checkService(String16("mybindertag"));
 
    if (binder == 0)
    {
        printf("service is not found !\n");
        return 0;
    }
    else
    {
        //获取service
        sp<IBinder> 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。