[置顶] Android IPC 通讯机制源码分析【下】

client获取Service的远程IBinder接口

以CameraService为例(camera.cpp):

const sp<ICameraService>& Camera::getCameraService()

{

    Mutex::Autolock _l(mLock);

    if (mCameraService.get() == 0) {

        sp<IServiceManager> sm = defaultServiceManager();

        sp<IBinder> binder;

        do {

            binder = sm->getService(String16("media.camera"));

            if (binder != 0)

                break;

            LOGW("CameraService not published, waiting...");

            usleep(500000); // 0.5 s

        } while(true);

        if (mDeathNotifier == NULL) {

            mDeathNotifier = new DeathNotifier();

        }

        binder->linkToDeath(mDeathNotifier);

        mCameraService = interface_cast<ICameraService>(binder);

    }

    LOGE_IF(mCameraService==0, "no CameraService!?");

    return mCameraService;

}

由前面的分析可知sm是BpCameraService对象:

    virtual sp<IBinder> getService(const String16& name) const

    {

        unsigned n;

        for (n = 0; n < 5; n++){

            sp<IBinder> svc = checkService(name);

            if (svc != NULL) return svc;

            LOGI("Waiting for sevice %s...\n", String8(name).string());

            sleep(1);

        }

        return NULL;

    }

    virtual sp<IBinder> checkService( const String16& name) const

    {

        Parcel data, reply;

        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());

        data.writeString16(name);

        remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);

        return reply.readStrongBinder();

}

这里的remote就是我们前面得到BpBinder对象。所以checkService将调用BpBinder中的transact函数:

status_t BpBinder::transact(

    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)

{

    // Once a binder has died, it will never come back to life.

    if (mAlive) {

        status_t status = IPCThreadState::self()->transact(

            mHandle, code, data, reply, flags);

        if (status == DEAD_OBJECT) mAlive = 0;

        return status;

    }

    return DEAD_OBJECT;

}

mHandle为0,BpBinder继续往下调用IPCThreadState:transact函数将数据发给与mHandle相关联的Service Manager Process。

status_t IPCThreadState::transact(int32_t handle,

                                  uint32_t code, const Parcel& data,

                                  Parcel* reply, uint32_t flags)

{

   ............................................................

    if (err == NO_ERROR) {

        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),

            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");

        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);

    }

    if (err != NO_ERROR) {

        if (reply) reply->setError(err);

        return (mLastError = err);

    }

    if ((flags & TF_ONE_WAY) == 0) {

        if (reply) {

            err = waitForResponse(reply);

        } else {

            Parcel fakeReply;

            err = waitForResponse(&fakeReply);

        }

       ..............................

  

    return err;

}

通过writeTransactionData构造要发送的数据

status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags,

    int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer)

{

    binder_transaction_data tr;

    tr.target.handle = handle; //这个handle将传递到service_manager

    tr.code = code;

    tr.flags = bindrFlags;

。。。。。。。。。。。。。。

}

waitForResponse将调用talkWithDriver与对Binder kernel进行读写操作。当Binder kernel接收到数据后,service_mananger线程的ThreadPool就会启动,service_manager查找到 CameraService服务后调用binder_send_reply,将返回的数据写入Binder kernel,Binder kernel。

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)

{

    int32_t cmd;

    int32_t err;

    while (1) {

        if ((err=talkWithDriver()) < NO_ERROR) break;

..............................................   

}

status_t IPCThreadState::talkWithDriver(bool doReceive)

{

   ............................................

#if defined(HAVE_ANDROID_OS)

        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)

            err = NO_ERROR;

        else

            err = -errno;

#else

        err = INVALID_OPERATION;

#endif

...................................................

}

通过上面的ioctl系统函数中BINDER_WRITE_READ对binder kernel进行读写。


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