Android系統進程間通信（IPC）機制Binder中的Server啓動過程源代碼分析

在前面一篇文章淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路中，介紹了在Android系統中Binder進程間通信機制中的Server角色是如何獲得Service Manager遠程接口的，即defaultServiceManager函數的實現。Server獲得了Service Manager遠程接口之後，就要把自己的Service添加到Service Manager中去，然後把自己啓動起來，等待Client的請求。本文將通過分析源代碼瞭解Server的啓動過程是怎麼樣的。

        本文通過一個具體的例子來說明Binder機制中Server的啓動過程。我們知道，在Android系統中，提供了多媒體播放的功能，這個功能是以服務的形式來提供的。這裏，我們就通過分析MediaPlayerService的實現來了解Media Server的啓動過程。

        首先，看一下MediaPlayerService的類圖，以便我們理解下面要描述的內容。

        我們將要介紹的主角MediaPlayerService繼承於BnMediaPlayerService類，熟悉Binder機制的同學應該知道BnMediaPlayerService是一個Binder Native類，用來處理Client請求的。BnMediaPlayerService繼承於BnInterface<IMediaPlayerService>類，BnInterface是一個模板類，它定義在frameworks/base/include/binder/IInterface.h文件中：

template<typename INTERFACE>
class BnInterface : public INTERFACE, public BBinder
{
public:
    virtual sp<IInterface>      queryLocalInterface(const String16& _descriptor);
    virtual const String16&     getInterfaceDescriptor() const;

protected:
    virtual IBinder*            onAsBinder();
};

       這裏可以看出，BnMediaPlayerService實際是繼承了IMediaPlayerService和BBinder類。IMediaPlayerService和BBinder類又分別繼承了IInterface和IBinder類，IInterface和IBinder類又同時繼承了RefBase類。

       實際上，BnMediaPlayerService並不是直接接收到Client處發送過來的請求，而是使用了IPCThreadState接收Client處發送過來的請求，而IPCThreadState又藉助了ProcessState類來與Binder驅動程序交互。有關IPCThreadState和ProcessState的關係，可以參考上一篇文章淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路，接下來也會有相應的描述。IPCThreadState接收到了Client處的請求後，就會調用BBinder類的transact函數，並傳入相關參數，BBinder類的transact函數最終調用BnMediaPlayerService類的onTransact函數，於是，就開始真正地處理Client的請求了。

      瞭解了MediaPlayerService類結構之後，就要開始進入到本文的主題了。

      首先，看看MediaPlayerService是如何啓動的。啓動MediaPlayerService的代碼位於frameworks/base/media/mediaserver/main_mediaserver.cpp文件中：

int main(int argc, char** argv)
{
    sp<ProcessState> proc(ProcessState::self());
    sp<IServiceManager> sm = defaultServiceManager();
    LOGI("ServiceManager: %p", sm.get());
    AudioFlinger::instantiate();
    MediaPlayerService::instantiate();
    CameraService::instantiate();
    AudioPolicyService::instantiate();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

       這裏我們不關注AudioFlinger和CameraService相關的代碼。

       先看下面這句代碼：

sp<ProcessState> proc(ProcessState::self());

       這句代碼的作用是通過ProcessState::self()調用創建一個ProcessState實例。ProcessState::self()是ProcessState類的一個靜態成員變量，定義在frameworks/base/libs/binder/ProcessState.cpp文件中：

sp<ProcessState> ProcessState::self()
{
    if (gProcess != NULL) return gProcess;

    AutoMutex _l(gProcessMutex);
    if (gProcess == NULL) gProcess = new ProcessState;
    return gProcess;
}

       這裏可以看出，這個函數作用是返回一個全局唯一的ProcessState實例gProcess。全局唯一實例變量gProcess定義在frameworks/base/libs/binder/Static.cpp文件中：

Mutex gProcessMutex;
sp<ProcessState> gProcess;

       再來看ProcessState的構造函數：

ProcessState::ProcessState()
    : mDriverFD(open_driver())
    , mVMStart(MAP_FAILED)
    , mManagesContexts(false)
    , mBinderContextCheckFunc(NULL)
    , mBinderContextUserData(NULL)
    , mThreadPoolStarted(false)
    , mThreadPoolSeq(1)
{
    if (mDriverFD >= 0) {
        // XXX Ideally, there should be a specific define for whether we
        // have mmap (or whether we could possibly have the kernel module
        // availabla).
#if !defined(HAVE_WIN32_IPC)
        // mmap the binder, providing a chunk of virtual address space to receive transactions.
        mVMStart = mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
        if (mVMStart == MAP_FAILED) {
            // *sigh*
            LOGE("Using /dev/binder failed: unable to mmap transaction memory.\n");
            close(mDriverFD);
            mDriverFD = -1;
        }
#else
        mDriverFD = -1;
#endif
    }
    if (mDriverFD < 0) {
        // Need to run without the driver, starting our own thread pool.
    }
}

        這個函數有兩個關鍵地方，一是通過open_driver函數打開Binder設備文件/dev/binder，並將打開設備文件描述符保存在成員變量mDriverFD中；二是通過mmap來把設備文件/dev/binder映射到內存中。

        先看open_driver函數的實現，這個函數同樣位於frameworks/base/libs/binder/ProcessState.cpp文件中：

static int open_driver()
{
    if (gSingleProcess) {
        return -1;
    }

    int fd = open("/dev/binder", O_RDWR);
    if (fd >= 0) {
        fcntl(fd, F_SETFD, FD_CLOEXEC);
        int vers;
#if defined(HAVE_ANDROID_OS)
        status_t result = ioctl(fd, BINDER_VERSION, &vers);
#else
        status_t result = -1;
        errno = EPERM;
#endif
        if (result == -1) {
            LOGE("Binder ioctl to obtain version failed: %s", strerror(errno));
            close(fd);
            fd = -1;
        }
        if (result != 0 || vers != BINDER_CURRENT_PROTOCOL_VERSION) {
            LOGE("Binder driver protocol does not match user space protocol!");
            close(fd);
            fd = -1;
        }
#if defined(HAVE_ANDROID_OS)
        size_t maxThreads = 15;
        result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);
        if (result == -1) {
            LOGE("Binder ioctl to set max threads failed: %s", strerror(errno));
        }
#endif

    } else {
        LOGW("Opening '/dev/binder' failed: %s\n", strerror(errno));
    }
    return fd;
}

        這個函數的作用主要是通過open文件操作函數來打開/dev/binder設備文件，然後再調用ioctl文件控制函數來分別執行BINDER_VERSION和BINDER_SET_MAX_THREADS兩個命令來和Binder驅動程序進行交互，前者用於獲得當前Binder驅動程序的版本號，後者用於通知Binder驅動程序，MediaPlayerService最多可同時啓動15個線程來處理Client端的請求。

        open在Binder驅動程序中的具體實現，請參考前面一篇文章淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路，這裏不再重複描述。打開/dev/binder設備文件後，Binder驅動程序就爲MediaPlayerService進程創建了一個struct binder_proc結構體實例來維護MediaPlayerService進程上下文相關信息。

        我們來看一下ioctl文件操作函數執行BINDER_VERSION命令的過程：

status_t result = ioctl(fd, BINDER_VERSION, &vers);

        這個函數調用最終進入到Binder驅動程序的binder_ioctl函數中，我們只關注BINDER_VERSION相關的部分邏輯：

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    int ret;
    struct binder_proc *proc = filp->private_data;
    struct binder_thread *thread;
    unsigned int size = _IOC_SIZE(cmd);
    void __user *ubuf = (void __user *)arg;

    /*printk(KERN_INFO "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/

    ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
    if (ret)
        return ret;

    mutex_lock(&binder_lock);
    thread = binder_get_thread(proc);
    if (thread == NULL) {
        ret = -ENOMEM;
        goto err;
    }

    switch (cmd) {
    ......
    case BINDER_VERSION:
        if (size != sizeof(struct binder_version)) {
            ret = -EINVAL;
            goto err;
        }
        if (put_user(BINDER_CURRENT_PROTOCOL_VERSION, &((struct binder_version *)ubuf)->protocol_version)) {
            ret = -EINVAL;
            goto err;
        }
        break;
    ......
    }
    ret = 0;
err:
        ......
    return ret;
}

        很簡單，只是將BINDER_CURRENT_PROTOCOL_VERSION寫入到傳入的參數arg指向的用戶緩衝區中去就返回了。BINDER_CURRENT_PROTOCOL_VERSION是一個宏，定義在kernel/common/drivers/staging/android/binder.h文件中：

/* This is the current protocol version. */
#define BINDER_CURRENT_PROTOCOL_VERSION 7

       這裏爲什麼要把ubuf轉換成struct binder_version之後，再通過其protocol_version成員變量再來寫入呢，轉了一圈，最終內容還是寫入到ubuf中。我們看一下struct binder_version的定義就會明白，同樣是在kernel/common/drivers/staging/android/binder.h文件中：

/* Use with BINDER_VERSION, driver fills in fields. */
struct binder_version {
    /* driver protocol version -- increment with incompatible change */
    signed long protocol_version;
};

        從註釋中可以看出來，這裏是考慮到兼容性，因爲以後很有可能不是用signed long來表示版本號。

        這裏有一個重要的地方要注意的是，由於這裏是打開設備文件/dev/binder之後，第一次進入到binder_ioctl函數，因此，這裏調用binder_get_thread的時候，就會爲當前線程創建一個struct binder_thread結構體變量來維護線程上下文信息，具體可以參考淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路一文。

        接着我們再來看一下ioctl文件操作函數執行BINDER_SET_MAX_THREADS命令的過程：

result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);

        這個函數調用最終進入到Binder驅動程序的binder_ioctl函數中，我們只關注BINDER_SET_MAX_THREADS相關的部分邏輯：

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    int ret;
    struct binder_proc *proc = filp->private_data;
    struct binder_thread *thread;
    unsigned int size = _IOC_SIZE(cmd);
    void __user *ubuf = (void __user *)arg;

    /*printk(KERN_INFO "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/

    ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
    if (ret)
        return ret;

    mutex_lock(&binder_lock);
    thread = binder_get_thread(proc);
    if (thread == NULL) {
        ret = -ENOMEM;
        goto err;
    }

    switch (cmd) {
    ......
    case BINDER_SET_MAX_THREADS:
        if (copy_from_user(&proc->max_threads, ubuf, sizeof(proc->max_threads))) {
            ret = -EINVAL;
            goto err;
        }
        break;
    ......
    }
    ret = 0;
err:
    ......
    return ret;
}

        這裏實現也是非常簡單，只是簡單地把用戶傳進來的參數保存在proc->max_threads中就完畢了。注意，這裏再調用binder_get_thread函數的時候，就可以在proc->threads中找到當前線程對應的struct binder_thread結構了，因爲前面已經創建好並保存在proc->threads紅黑樹中。

        回到ProcessState的構造函數中，這裏還通過mmap函數來把設備文件/dev/binder映射到內存中，這個函數在淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路一文也已經有詳細介紹，這裏不再重複描述。宏BINDER_VM_SIZE就定義在ProcessState.cpp文件中：

#define BINDER_VM_SIZE ((1*1024*1024) - (4096 *2))

        mmap函數調用完成之後，Binder驅動程序就爲當前進程預留了BINDER_VM_SIZE大小的內存空間了。

        這樣，ProcessState全局唯一變量gProcess就創建完畢了，回到frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函數，下一步是調用defaultServiceManager函數來獲得Service Manager的遠程接口，這個已經在上一篇文章淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路有詳細描述，讀者可以回過頭去參考一下。

        再接下來，就進入到MediaPlayerService::instantiate函數把MediaPlayerService添加到Service Manger中去了。這個函數定義在frameworks/base/media/libmediaplayerservice/MediaPlayerService.cpp文件中：

void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}

        我們重點看一下IServiceManger::addService的過程，這有助於我們加深對Binder機制的理解。

        在上一篇文章淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路中說到，defaultServiceManager返回的實際是一個BpServiceManger類實例，因此，我們看一下BpServiceManger::addService的實現，這個函數實現在frameworks/base/libs/binder/IServiceManager.cpp文件中：

class BpServiceManager : public BpInterface<IServiceManager>
{
public:
    BpServiceManager(const sp<IBinder>& impl)
        : BpInterface<IServiceManager>(impl)
    {
    }

    ......

    virtual status_t addService(const String16& name, const sp<IBinder>& service)
    {
        Parcel data, reply;
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        data.writeString16(name);
        data.writeStrongBinder(service);
        status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
        return err == NO_ERROR ? reply.readExceptionCode()
    }

    ......

};

         這裏的Parcel類是用來於序列化進程間通信數據用的。

         先來看這一句的調用：

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

         IServiceManager::getInterfaceDescriptor()返回來的是一個字符串，即"android.os.IServiceManager"，具體可以參考IServiceManger的實現。我們看一下Parcel::writeInterfaceToken的實現，位於frameworks/base/libs/binder/Parcel.cpp文件中：

// Write RPC headers.  (previously just the interface token)
status_t Parcel::writeInterfaceToken(const String16& interface)
{
    writeInt32(IPCThreadState::self()->getStrictModePolicy() |
               STRICT_MODE_PENALTY_GATHER);
    // currently the interface identification token is just its name as a string
    return writeString16(interface);
}

         它的作用是寫入一個整數和一個字符串到Parcel中去。

         再來看下面的調用：

data.writeString16(name);

        這裏又是寫入一個字符串到Parcel中去，這裏的name即是上面傳進來的“media.player”字符串。

        往下看：

data.writeStrongBinder(service);

        這裏定入一個Binder對象到Parcel去。我們重點看一下這個函數的實現，因爲它涉及到進程間傳輸Binder實體的問題，比較複雜，需要重點關注，同時，也是理解Binder機制的一個重點所在。注意，這裏的service參數是一個MediaPlayerService對象。

status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{
    return flatten_binder(ProcessState::self(), val, this);
}

        看到flatten_binder函數，是不是似曾相識的感覺？我們在前面一篇文章淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路中，曾經提到在Binder驅動程序中，使用struct flat_binder_object來表示傳輸中的一個binder對象，它的定義如下所示：

/*
 * This is the flattened representation of a Binder object for transfer
 * between processes.  The 'offsets' supplied as part of a binder transaction
 * contains offsets into the data where these structures occur.  The Binder
 * driver takes care of re-writing the structure type and data as it moves
 * between processes.
 */
struct flat_binder_object {
    /* 8 bytes for large_flat_header. */
    unsigned long       type;
    unsigned long       flags;

    /* 8 bytes of data. */
    union {
        void        *binder;    /* local object */
        signed long handle;     /* remote object */
    };

    /* extra data associated with local object */
    void            *cookie;
};

        各個成員變量的含義請參考資料Android Binder設計與實現。

        我們進入到flatten_binder函數看看：

status_t flatten_binder(const sp<ProcessState>& proc,
    const sp<IBinder>& binder, Parcel* out)
{
    flat_binder_object obj;

    obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    if (binder != NULL) {
        IBinder *local = binder->localBinder();
        if (!local) {
            BpBinder *proxy = binder->remoteBinder();
            if (proxy == NULL) {
                LOGE("null proxy");
            }
            const int32_t handle = proxy ? proxy->handle() : 0;
            obj.type = BINDER_TYPE_HANDLE;
            obj.handle = handle;
            obj.cookie = NULL;
        } else {
            obj.type = BINDER_TYPE_BINDER;
            obj.binder = local->getWeakRefs();
            obj.cookie = local;
        }
    } else {
        obj.type = BINDER_TYPE_BINDER;
        obj.binder = NULL;
        obj.cookie = NULL;
    }

    return finish_flatten_binder(binder, obj, out);
}

        首先是初始化flat_binder_object的flags域：

obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;

        0x7f表示處理本Binder實體請求數據包的線程的最低優先級，FLAT_BINDER_FLAG_ACCEPTS_FDS表示這個Binder實體可以接受文件描述符，Binder實體在收到文件描述符時，就會在本進程中打開這個文件。

       傳進來的binder即爲MediaPlayerService::instantiate函數中new出來的MediaPlayerService實例，因此，不爲空。又由於MediaPlayerService繼承自BBinder類，它是一個本地Binder實體，因此binder->localBinder返回一個BBinder指針，而且肯定不爲空，於是執行下面語句：

obj.type = BINDER_TYPE_BINDER;
obj.binder = local->getWeakRefs();
obj.cookie = local;

        設置了flat_binder_obj的其他成員變量，注意，指向這個Binder實體地址的指針local保存在flat_binder_obj的成員變量cookie中。

        函數調用finish_flatten_binder來將這個flat_binder_obj寫入到Parcel中去：

inline static status_t finish_flatten_binder(
    const sp<IBinder>& binder, const flat_binder_object& flat, Parcel* out)
{
    return out->writeObject(flat, false);
}

       Parcel::writeObject的實現如下：

status_t Parcel::writeObject(const flat_binder_object& val, bool nullMetaData)
{
    const bool enoughData = (mDataPos+sizeof(val)) <= mDataCapacity;
    const bool enoughObjects = mObjectsSize < mObjectsCapacity;
    if (enoughData && enoughObjects) {
restart_write:
        *reinterpret_cast<flat_binder_object*>(mData+mDataPos) = val;

        // Need to write meta-data?
        if (nullMetaData || val.binder != NULL) {
            mObjects[mObjectsSize] = mDataPos;
            acquire_object(ProcessState::self(), val, this);
            mObjectsSize++;
        }

        // remember if it's a file descriptor
        if (val.type == BINDER_TYPE_FD) {
            mHasFds = mFdsKnown = true;
        }

        return finishWrite(sizeof(flat_binder_object));
    }

    if (!enoughData) {
        const status_t err = growData(sizeof(val));
        if (err != NO_ERROR) return err;
    }
    if (!enoughObjects) {
        size_t newSize = ((mObjectsSize+2)*3)/2;
        size_t* objects = (size_t*)realloc(mObjects, newSize*sizeof(size_t));
        if (objects == NULL) return NO_MEMORY;
        mObjects = objects;
        mObjectsCapacity = newSize;
    }

    goto restart_write;
}

        這裏除了把flat_binder_obj寫到Parcel裏面之內，還要記錄這個flat_binder_obj在Parcel裏面的偏移位置：

mObjects[mObjectsSize] = mDataPos;

       這裏因爲，如果進程間傳輸的數據間帶有Binder對象的時候，Binder驅動程序需要作進一步的處理，以維護各個Binder實體的一致性，下面我們將會看到Binder驅動程序是怎麼處理這些Binder對象的。

       再回到BpServiceManager::addService函數中，調用下面語句：

status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);

       回到淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路一文中的類圖中去看一下，這裏的remote成員函數來自於BpRefBase類，它返回一個BpBinder指針。因此，我們繼續進入到BpBinder::transact函數中去看看：

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;
}

       這裏又調用了IPCThreadState::transact進執行實際的操作。注意，這裏的mHandle爲0，code爲ADD_SERVICE_TRANSACTION。ADD_SERVICE_TRANSACTION是上面以參數形式傳進來的，那mHandle爲什麼是0呢？因爲這裏表示的是Service Manager遠程接口，它的句柄值一定是0，具體請參考淺談Android系統進程間通信（IPC）機制Binder中的Server和Client獲得Service Manager接口之路一文。
       再進入到IPCThreadState::transact函數，看看做了些什麼事情：

status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err = data.errorCheck();

    flags |= TF_ACCEPT_FDS;

    IF_LOG_TRANSACTIONS() {
        TextOutput::Bundle _b(alog);
        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "
            << handle << " / code " << TypeCode(code) << ": "
            << indent << data << dedent << endl;
    }

    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 0
        if (code == 4) { // relayout
            LOGI(">>>>>> CALLING transaction 4");
        } else {
            LOGI(">>>>>> CALLING transaction %d", code);
        }
        #endif
        if (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        #if 0
        if (code == 4) { // relayout
            LOGI("<<<<<< RETURNING transaction 4");
        } else {
            LOGI("<<<<<< RETURNING transaction %d", code);
        }
        #endif

        IF_LOG_TRANSACTIONS() {
            TextOutput::Bundle _b(alog);
            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "
                << handle << ": ";
            if (reply) alog << indent << *reply << dedent << endl;
            else alog << "(none requested)" << endl;
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }

    return err;
}

        IPCThreadState::transact函數的參數flags是一個默認值爲0的參數，上面沒有傳相應的實參進來，因此，這裏就爲0。

        函數首先調用writeTransactionData函數準備好一個struct binder_transaction_data結構體變量，這個是等一下要傳輸給Binder驅動程序的。struct binder_transaction_data的定義我們在淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路一文中有詳細描述，讀者不妨回過去讀一下。這裏爲了方便描述，將struct binder_transaction_data的定義再次列出來：

struct binder_transaction_data {
    /* The first two are only used for bcTRANSACTION and brTRANSACTION,
     * identifying the target and contents of the transaction.
     */
    union {
        size_t  handle; /* target descriptor of command transaction */
        void    *ptr;   /* target descriptor of return transaction */
    } target;
    void        *cookie;    /* target object cookie */
    unsigned int    code;       /* transaction command */

    /* General information about the transaction. */
    unsigned int    flags;
    pid_t       sender_pid;
    uid_t       sender_euid;
    size_t      data_size;  /* number of bytes of data */
    size_t      offsets_size;   /* number of bytes of offsets */

    /* If this transaction is inline, the data immediately
     * follows here; otherwise, it ends with a pointer to
     * the data buffer.
     */
    union {
        struct {
            /* transaction data */
            const void  *buffer;
            /* offsets from buffer to flat_binder_object structs */
            const void  *offsets;
        } ptr;
        uint8_t buf[8];
    } data;
};

         writeTransactionData函數的實現如下：

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;
    tr.code = code;
    tr.flags = binderFlags;

    const status_t err = data.errorCheck();
    if (err == NO_ERROR) {
        tr.data_size = data.ipcDataSize();
        tr.data.ptr.buffer = data.ipcData();
        tr.offsets_size = data.ipcObjectsCount()*sizeof(size_t);
        tr.data.ptr.offsets = data.ipcObjects();
    } else if (statusBuffer) {
        tr.flags |= TF_STATUS_CODE;
        *statusBuffer = err;
        tr.data_size = sizeof(status_t);
        tr.data.ptr.buffer = statusBuffer;
        tr.offsets_size = 0;
        tr.data.ptr.offsets = NULL;
    } else {
        return (mLastError = err);
    }

    mOut.writeInt32(cmd);
    mOut.write(&tr, sizeof(tr));

    return NO_ERROR;
}

        注意，這裏的cmd爲BC_TRANSACTION。 這個函數很簡單，在這個場景下，就是執行下面語句來初始化本地變量tr：

tr.data_size = data.ipcDataSize();
tr.data.ptr.buffer = data.ipcData();
tr.offsets_size = data.ipcObjectsCount()*sizeof(size_t);
tr.data.ptr.offsets = data.ipcObjects();

       回憶一下上面的內容，寫入到tr.data.ptr.buffer的內容相當於下面的內容：

writeInt32(IPCThreadState::self()->getStrictModePolicy() |
               STRICT_MODE_PENALTY_GATHER);
writeString16("android.os.IServiceManager");
writeString16("media.player");
writeStrongBinder(new MediaPlayerService());

       其中包含了一個Binder實體MediaPlayerService，因此需要設置tr.offsets_size就爲1，tr.data.ptr.offsets就指向了這個MediaPlayerService的地址在tr.data.ptr.buffer中的偏移量。最後，將tr的內容保存在IPCThreadState的成員變量mOut中。
       回到IPCThreadState::transact函數中，接下去看，(flags & TF_ONE_WAY) == 0爲true，並且reply不爲空，所以最終進入到waitForResponse(reply)這條路徑來。我們看一下waitForResponse函數的實現：

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    int32_t cmd;
    int32_t err;

    while (1) {
        if ((err=talkWithDriver()) < NO_ERROR) break;
        err = mIn.errorCheck();
        if (err < NO_ERROR) break;
        if (mIn.dataAvail() == 0) continue;

        cmd = mIn.readInt32();

        IF_LOG_COMMANDS() {
            alog << "Processing waitForResponse Command: "
                << getReturnString(cmd) << endl;
        }

        switch (cmd) {
        case BR_TRANSACTION_COMPLETE:
            if (!reply && !acquireResult) goto finish;
            break;

        case BR_DEAD_REPLY:
            err = DEAD_OBJECT;
            goto finish;

        case BR_FAILED_REPLY:
            err = FAILED_TRANSACTION;
            goto finish;

        case BR_ACQUIRE_RESULT:
            {
                LOG_ASSERT(acquireResult != NULL, "Unexpected brACQUIRE_RESULT");
                const int32_t result = mIn.readInt32();
                if (!acquireResult) continue;
                *acquireResult = result ? NO_ERROR : INVALID_OPERATION;
            }
            goto finish;

        case BR_REPLY:
            {
                binder_transaction_data tr;
                err = mIn.read(&tr, sizeof(tr));
                LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");
                if (err != NO_ERROR) goto finish;

                if (reply) {
                    if ((tr.flags & TF_STATUS_CODE) == 0) {
                        reply->ipcSetDataReference(
                            reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t),
                            freeBuffer, this);
                    } else {
                        err = *static_cast<const status_t*>(tr.data.ptr.buffer);
                        freeBuffer(NULL,
                            reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t), this);
                    }
                } else {
                    freeBuffer(NULL,
                        reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                        tr.data_size,
                        reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                        tr.offsets_size/sizeof(size_t), this);
                    continue;
                }
            }
            goto finish;

        default:
            err = executeCommand(cmd);
            if (err != NO_ERROR) goto finish;
            break;
        }
    }

finish:
    if (err != NO_ERROR) {
        if (acquireResult) *acquireResult = err;
        if (reply) reply->setError(err);
        mLastError = err;
    }

    return err;
}

        這個函數雖然很長，但是主要調用了talkWithDriver函數來與Binder驅動程序進行交互：

status_t IPCThreadState::talkWithDriver(bool doReceive)
{
    LOG_ASSERT(mProcess->mDriverFD >= 0, "Binder driver is not opened");

    binder_write_read bwr;

    // Is the read buffer empty?
    const bool needRead = mIn.dataPosition() >= mIn.dataSize();

    // We don't want to write anything if we are still reading
    // from data left in the input buffer and the caller
    // has requested to read the next data.
    const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;

    bwr.write_size = outAvail;
    bwr.write_buffer = (long unsigned int)mOut.data();

    // This is what we'll read.
    if (doReceive && needRead) {
        bwr.read_size = mIn.dataCapacity();
        bwr.read_buffer = (long unsigned int)mIn.data();
    } else {
        bwr.read_size = 0;
    }

    IF_LOG_COMMANDS() {
        TextOutput::Bundle _b(alog);
        if (outAvail != 0) {
            alog << "Sending commands to driver: " << indent;
            const void* cmds = (const void*)bwr.write_buffer;
            const void* end = ((const uint8_t*)cmds)+bwr.write_size;
            alog << HexDump(cmds, bwr.write_size) << endl;
            while (cmds < end) cmds = printCommand(alog, cmds);
            alog << dedent;
        }
        alog << "Size of receive buffer: " << bwr.read_size
            << ", needRead: " << needRead << ", doReceive: " << doReceive << endl;
    }

    // Return immediately if there is nothing to do.
    if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;

    bwr.write_consumed = 0;
    bwr.read_consumed = 0;
    status_t err;
    do {
        IF_LOG_COMMANDS() {
            alog << "About to read/write, write size = " << mOut.dataSize() << endl;
        }
#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
        IF_LOG_COMMANDS() {
            alog << "Finished read/write, write size = " << mOut.dataSize() << endl;
        }
    } while (err == -EINTR);

    IF_LOG_COMMANDS() {
        alog << "Our err: " << (void*)err << ", write consumed: "
            << bwr.write_consumed << " (of " << mOut.dataSize()
            << "), read consumed: " << bwr.read_consumed << endl;
    }

    if (err >= NO_ERROR) {
        if (bwr.write_consumed > 0) {
            if (bwr.write_consumed < (ssize_t)mOut.dataSize())
                mOut.remove(0, bwr.write_consumed);
            else
                mOut.setDataSize(0);
        }
        if (bwr.read_consumed > 0) {
            mIn.setDataSize(bwr.read_consumed);
            mIn.setDataPosition(0);
        }
        IF_LOG_COMMANDS() {
            TextOutput::Bundle _b(alog);
            alog << "Remaining data size: " << mOut.dataSize() << endl;
            alog << "Received commands from driver: " << indent;
            const void* cmds = mIn.data();
            const void* end = mIn.data() + mIn.dataSize();
            alog << HexDump(cmds, mIn.dataSize()) << endl;
            while (cmds < end) cmds = printReturnCommand(alog, cmds);
            alog << dedent;
        }
        return NO_ERROR;
    }

    return err;
}

        這裏doReceive和needRead均爲1，有興趣的讀者可以自已分析一下。因此，這裏告訴Binder驅動程序，先執行write操作，再執行read操作，下面我們將會看到。

        最後，通過ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)進行到Binder驅動程序的binder_ioctl函數，我們只關注cmd爲BINDER_WRITE_READ的邏輯：

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    int ret;
    struct binder_proc *proc = filp->private_data;
    struct binder_thread *thread;
    unsigned int size = _IOC_SIZE(cmd);
    void __user *ubuf = (void __user *)arg;

    /*printk(KERN_INFO "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/

    ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
    if (ret)
        return ret;

    mutex_lock(&binder_lock);
    thread = binder_get_thread(proc);
    if (thread == NULL) {
        ret = -ENOMEM;
        goto err;
    }

    switch (cmd) {
    case BINDER_WRITE_READ: {
        struct binder_write_read bwr;
        if (size != sizeof(struct binder_write_read)) {
            ret = -EINVAL;
            goto err;
        }
        if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
            ret = -EFAULT;
            goto err;
        }
        if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)
            printk(KERN_INFO "binder: %d:%d write %ld at %08lx, read %ld at %08lx\n",
            proc->pid, thread->pid, bwr.write_size, bwr.write_buffer, bwr.read_size, bwr.read_buffer);
        if (bwr.write_size > 0) {
            ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
            if (ret < 0) {
                bwr.read_consumed = 0;
                if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                    ret = -EFAULT;
                goto err;
            }
        }
        if (bwr.read_size > 0) {
            ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
            if (!list_empty(&proc->todo))
                wake_up_interruptible(&proc->wait);
            if (ret < 0) {
                if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                    ret = -EFAULT;
                goto err;
            }
        }
        if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)
            printk(KERN_INFO "binder: %d:%d wrote %ld of %ld, read return %ld of %ld\n",
            proc->pid, thread->pid, bwr.write_consumed, bwr.write_size, bwr.read_consumed, bwr.read_size);
        if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
            ret = -EFAULT;
            goto err;
        }
        break;
    }
    ......
    }
    ret = 0;
err:
    ......
    return ret;
}

         函數首先是將用戶傳進來的參數拷貝到本地變量struct binder_write_read bwr中去。這裏bwr.write_size > 0爲true，因此，進入到binder_thread_write函數中，我們只關注BC_TRANSACTION部分的邏輯：

binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
                    void __user *buffer, int size, signed long *consumed)
{
    uint32_t cmd;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    while (ptr < end && thread->return_error == BR_OK) {
        if (get_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
            binder_stats.bc[_IOC_NR(cmd)]++;
            proc->stats.bc[_IOC_NR(cmd)]++;
            thread->stats.bc[_IOC_NR(cmd)]++;
        }
        switch (cmd) {
            .....
        case BC_TRANSACTION:
        case BC_REPLY: {
            struct binder_transaction_data tr;

            if (copy_from_user(&tr, ptr, sizeof(tr)))
                return -EFAULT;
            ptr += sizeof(tr);
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
        }
        ......
        }
        *consumed = ptr - buffer;
    }
    return 0;
}

         首先將用戶傳進來的transact參數拷貝在本地變量struct binder_transaction_data tr中去，接着調用binder_transaction函數進一步處理，這裏我們忽略掉無關代碼：

static void
binder_transaction(struct binder_proc *proc, struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
    struct binder_transaction *t;
    struct binder_work *tcomplete;
    size_t *offp, *off_end;
    struct binder_proc *target_proc;
    struct binder_thread *target_thread = NULL;
    struct binder_node *target_node = NULL;
    struct list_head *target_list;
    wait_queue_head_t *target_wait;
    struct binder_transaction *in_reply_to = NULL;
    struct binder_transaction_log_entry *e;
    uint32_t return_error;

        ......

    if (reply) {
         ......
    } else {
        if (tr->target.handle) {
            ......
        } else {
            target_node = binder_context_mgr_node;
            if (target_node == NULL) {
                return_error = BR_DEAD_REPLY;
                goto err_no_context_mgr_node;
            }
        }
        ......
        target_proc = target_node->proc;
        if (target_proc == NULL) {
            return_error = BR_DEAD_REPLY;
            goto err_dead_binder;
        }
        ......
    }
    if (target_thread) {
        ......
    } else {
        target_list = &target_proc->todo;
        target_wait = &target_proc->wait;
    }

    ......

    /* TODO: reuse incoming transaction for reply */
    t = kzalloc(sizeof(*t), GFP_KERNEL);
    if (t == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_alloc_t_failed;
    }
    ......

    tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
    if (tcomplete == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_alloc_tcomplete_failed;
    }

    ......

    if (!reply && !(tr->flags & TF_ONE_WAY))
        t->from = thread;
    else
        t->from = NULL;
    t->sender_euid = proc->tsk->cred->euid;
    t->to_proc = target_proc;
    t->to_thread = target_thread;
    t->code = tr->code;
    t->flags = tr->flags;
    t->priority = task_nice(current);
    t->buffer = binder_alloc_buf(target_proc, tr->data_size,
        tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
    if (t->buffer == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_binder_alloc_buf_failed;
    }
    t->buffer->allow_user_free = 0;
    t->buffer->debug_id = t->debug_id;
    t->buffer->transaction = t;
    t->buffer->target_node = target_node;
    if (target_node)
        binder_inc_node(target_node, 1, 0, NULL);

    offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

    if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
        ......
        return_error = BR_FAILED_REPLY;
        goto err_copy_data_failed;
    }
    if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
        ......
        return_error = BR_FAILED_REPLY;
        goto err_copy_data_failed;
    }
    ......

    off_end = (void *)offp + tr->offsets_size;
    for (; offp < off_end; offp++) {
        struct flat_binder_object *fp;
        ......
        fp = (struct flat_binder_object *)(t->buffer->data + *offp);
        switch (fp->type) {
        case BINDER_TYPE_BINDER:
        case BINDER_TYPE_WEAK_BINDER: {
            struct binder_ref *ref;
            struct binder_node *node = binder_get_node(proc, fp->binder);
            if (node == NULL) {
                node = binder_new_node(proc, fp->binder, fp->cookie);
                if (node == NULL) {
                    return_error = BR_FAILED_REPLY;
                    goto err_binder_new_node_failed;
                }
                node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;
                node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
            }
            if (fp->cookie != node->cookie) {
                ......
                goto err_binder_get_ref_for_node_failed;
            }
            ref = binder_get_ref_for_node(target_proc, node);
            if (ref == NULL) {
                return_error = BR_FAILED_REPLY;
                goto err_binder_get_ref_for_node_failed;
            }
            if (fp->type == BINDER_TYPE_BINDER)
                fp->type = BINDER_TYPE_HANDLE;
            else
                fp->type = BINDER_TYPE_WEAK_HANDLE;
            fp->handle = ref->desc;
            binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE, &thread->todo);
            ......

        } break;
        ......
        }
    }

    if (reply) {
        ......
    } else if (!(t->flags & TF_ONE_WAY)) {
        BUG_ON(t->buffer->async_transaction != 0);
        t->need_reply = 1;
        t->from_parent = thread->transaction_stack;
        thread->transaction_stack = t;
    } else {
        ......
    }
    t->work.type = BINDER_WORK_TRANSACTION;
    list_add_tail(&t->work.entry, target_list);
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
    ......
}

       注意，這裏傳進來的參數reply爲0，tr->target.handle也爲0。因此，target_proc、target_thread、target_node、target_list和target_wait的值分別爲：

target_node = binder_context_mgr_node;
target_proc = target_node->proc;
target_list = &target_proc->todo;
target_wait = &target_proc->wait;

       接着，分配了一個待處理事務t和一個待完成工作項tcomplete，並執行初始化工作：

/* TODO: reuse incoming transaction for reply */
t = kzalloc(sizeof(*t), GFP_KERNEL);
if (t == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_t_failed;
}
......

tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
if (tcomplete == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_tcomplete_failed;
}

......

if (!reply && !(tr->flags & TF_ONE_WAY))
    t->from = thread;
else
    t->from = NULL;
t->sender_euid = proc->tsk->cred->euid;
t->to_proc = target_proc;
t->to_thread = target_thread;
t->code = tr->code;
t->flags = tr->flags;
t->priority = task_nice(current);
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
    tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
if (t->buffer == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_binder_alloc_buf_failed;
}
t->buffer->allow_user_free = 0;
t->buffer->debug_id = t->debug_id;
t->buffer->transaction = t;
t->buffer->target_node = target_node;
if (target_node)
    binder_inc_node(target_node, 1, 0, NULL);

offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
    ......
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
    ......
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}

         注意，這裏的事務t是要交給target_proc處理的，在這個場景之下，就是Service Manager了。因此，下面的語句：

t->buffer = binder_alloc_buf(target_proc, tr->data_size,
        tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));

         就是在Service Manager的進程空間中分配一塊內存來保存用戶傳進入的參數了：

if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
    ......
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
    ......
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}

         由於現在target_node要被使用了，增加它的引用計數：

if (target_node)
        binder_inc_node(target_node, 1, 0, NULL);

        接下去的for循環，就是用來處理傳輸數據中的Binder對象了。在我們的場景中，有一個類型爲BINDER_TYPE_BINDER的Binder實體MediaPlayerService：

   switch (fp->type) {
   case BINDER_TYPE_BINDER:
   case BINDER_TYPE_WEAK_BINDER: {
struct binder_ref *ref;
struct binder_node *node = binder_get_node(proc, fp->binder);
if (node == NULL) {
    node = binder_new_node(proc, fp->binder, fp->cookie);
    if (node == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_binder_new_node_failed;
    }
    node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;
    node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
}
if (fp->cookie != node->cookie) {
    ......
    goto err_binder_get_ref_for_node_failed;
}
ref = binder_get_ref_for_node(target_proc, node);
if (ref == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_binder_get_ref_for_node_failed;
}
if (fp->type == BINDER_TYPE_BINDER)
    fp->type = BINDER_TYPE_HANDLE;
else
    fp->type = BINDER_TYPE_WEAK_HANDLE;
fp->handle = ref->desc;
binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE, &thread->todo);
......

} break;

        由於是第一次在Binder驅動程序中傳輸這個MediaPlayerService，調用binder_get_node函數查詢這個Binder實體時，會返回空，於是binder_new_node在proc中新建一個，下次就可以直接使用了。

        現在，由於要把這個Binder實體MediaPlayerService交給target_proc，也就是Service Manager來管理，也就是說Service Manager要引用這個MediaPlayerService了，於是通過binder_get_ref_for_node爲MediaPlayerService創建一個引用，並且通過binder_inc_ref來增加這個引用計數，防止這個引用還在使用過程當中就被銷燬。注意，到了這裏的時候，t->buffer中的flat_binder_obj的type已經改爲BINDER_TYPE_HANDLE，handle已經改爲ref->desc，跟原來不一樣了，因爲這個flat_binder_obj是最終是要傳給Service Manager的，而Service Manager只能夠通過句柄值來引用這個Binder實體。

        最後，把待處理事務加入到target_list列表中去：

list_add_tail(&t->work.entry, target_list);

        並且把待完成工作項加入到本線程的todo等待執行列表中去：

list_add_tail(&tcomplete->entry, &thread->todo);

        現在目標進程有事情可做了，於是喚醒它：

if (target_wait)
    wake_up_interruptible(target_wait);

       這裏就是要喚醒Service Manager進程了。回憶一下前面淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路這篇文章，此時， Service Manager正在binder_thread_read函數中調用wait_event_interruptible進入休眠狀態。

       這裏我們先忽略一下Service Manager被喚醒之後的場景，繼續MedaPlayerService的啓動過程，然後再回來。

       回到binder_ioctl函數，bwr.read_size > 0爲true，於是進入binder_thread_read函數：

static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
                   void  __user *buffer, int size, signed long *consumed, int non_block)
{
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    int ret = 0;
    int wait_for_proc_work;

    if (*consumed == 0) {
        if (put_user(BR_NOOP, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
    }

retry:
    wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);

    .......

    if (wait_for_proc_work) {
        .......
    } else {
        if (non_block) {
            if (!binder_has_thread_work(thread))
                ret = -EAGAIN;
        } else
            ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
    }

    ......

    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;

        if (!list_empty(&thread->todo))
            w = list_first_entry(&thread->todo, struct binder_work, entry);
        else if (!list_empty(&proc->todo) && wait_for_proc_work)
            w = list_first_entry(&proc->todo, struct binder_work, entry);
        else {
            if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
                goto retry;
            break;
        }

        if (end - ptr < sizeof(tr) + 4)
            break;

        switch (w->type) {
        ......
        case BINDER_WORK_TRANSACTION_COMPLETE: {
            cmd = BR_TRANSACTION_COMPLETE;
            if (put_user(cmd, (uint32_t __user *)ptr))
                return -EFAULT;
            ptr += sizeof(uint32_t);

            binder_stat_br(proc, thread, cmd);
            if (binder_debug_mask & BINDER_DEBUG_TRANSACTION_COMPLETE)
                printk(KERN_INFO "binder: %d:%d BR_TRANSACTION_COMPLETE\n",
                proc->pid, thread->pid);

            list_del(&w->entry);
            kfree(w);
            binder_stats.obj_deleted[BINDER_STAT_TRANSACTION_COMPLETE]++;
                                               } break;
        ......
        }

        if (!t)
            continue;

        ......
    }

done:
    ......
    return 0;
}

        這裏，thread->transaction_stack和thread->todo均不爲空，於是wait_for_proc_work爲false，由於binder_has_thread_work的時候，返回true，這裏因爲thread->todo不爲空，因此，線程雖然調用了wait_event_interruptible，但是不會睡眠，於是繼續往下執行。

        由於thread->todo不爲空，執行下列語句：

if (!list_empty(&thread->todo))
     w = list_first_entry(&thread->todo, struct binder_work, entry);

        w->type爲BINDER_WORK_TRANSACTION_COMPLETE，這是在上面的binder_transaction函數設置的，於是執行：

   switch (w->type) {
   ......
   case BINDER_WORK_TRANSACTION_COMPLETE: {
cmd = BR_TRANSACTION_COMPLETE;
if (put_user(cmd, (uint32_t __user *)ptr))
    return -EFAULT;
ptr += sizeof(uint32_t);

       ......
list_del(&w->entry);
kfree(w);

} break;
......
   }

        這裏就將w從thread->todo刪除了。由於這裏t爲空，重新執行while循環，這時由於已經沒有事情可做了，最後就返回到binder_ioctl函數中。注間，這裏一共往用戶傳進來的緩衝區buffer寫入了兩個整數，分別是BR_NOOP和BR_TRANSACTION_COMPLETE。

        binder_ioctl函數返回到用戶空間之前，把數據消耗情況拷貝回用戶空間中：

if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
    ret = -EFAULT;
    goto err;
}

        最後返回到IPCThreadState::talkWithDriver函數中，執行下面語句：

    if (err >= NO_ERROR) {
        if (bwr.write_consumed > 0) {
            if (bwr.write_consumed < (ssize_t)mOut.dataSize())
                mOut.remove(0, bwr.write_consumed);
            else
                mOut.setDataSize(0);
        }
        if (bwr.read_consumed > 0) {
<pre name="code" class="cpp">            mIn.setDataSize(bwr.read_consumed);
            mIn.setDataPosition(0);

} ...... return NO_ERROR; }        首先是把mOut的數據清空：

mOut.setDataSize(0);

        然後設置已經讀取的內容的大小：

mIn.setDataSize(bwr.read_consumed);
mIn.setDataPosition(0);

        然後返回到IPCThreadState::waitForResponse函數中。在IPCThreadState::waitForResponse函數，先是從mIn讀出一個整數，這個便是BR_NOOP了，這是一個空操作，什麼也不做。然後繼續進入IPCThreadState::talkWithDriver函數中。
        這時候，下面語句執行後：

const bool needRead = mIn.dataPosition() >= mIn.dataSize();

        needRead爲false，因爲在mIn中，尚有一個整數BR_TRANSACTION_COMPLETE未讀出。

       這時候，下面語句執行後：

const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;

        outAvail等於0。因此，最後bwr.write_size和bwr.read_size均爲0，IPCThreadState::talkWithDriver函數什麼也不做，直接返回到IPCThreadState::waitForResponse函數中。在IPCThreadState::waitForResponse函數，又繼續從mIn讀出一個整數，這個便是BR_TRANSACTION_COMPLETE：

switch (cmd) {
case BR_TRANSACTION_COMPLETE:
       if (!reply && !acquireResult) goto finish;
       break;
......
}

        reply不爲NULL，因此，IPCThreadState::waitForResponse的循環沒有結束，繼續執行，又進入到IPCThreadState::talkWithDrive中。

        這次，needRead就爲true了，而outAvail仍爲0，所以bwr.read_size不爲0，bwr.write_size爲0。於是通過：

ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)

        進入到Binder驅動程序中的binder_ioctl函數中。由於bwr.write_size爲0，bwr.read_size不爲0，這次直接就進入到binder_thread_read函數中。這時候，thread->transaction_stack不等於0，但是thread->todo爲空，於是線程就通過：

wait_event_interruptible(thread->wait, binder_has_thread_work(thread));

        進入睡眠狀態，等待Service Manager來喚醒了。

        現在，我們可以回到Service Manager被喚醒的過程了。我們接着前面淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路這篇文章的最後，繼續描述。此時， Service Manager正在binder_thread_read函數中調用wait_event_interruptible_exclusive進入休眠狀態。上面被MediaPlayerService啓動後進程喚醒後，繼續執行binder_thread_read函數：

static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
                   void  __user *buffer, int size, signed long *consumed, int non_block)
{
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    int ret = 0;
    int wait_for_proc_work;

    if (*consumed == 0) {
        if (put_user(BR_NOOP, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
    }

retry:
    wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);

    ......

    if (wait_for_proc_work) {
        ......
        if (non_block) {
            if (!binder_has_proc_work(proc, thread))
                ret = -EAGAIN;
        } else
            ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
    } else {
        ......
    }

    ......

    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;

        if (!list_empty(&thread->todo))
            w = list_first_entry(&thread->todo, struct binder_work, entry);
        else if (!list_empty(&proc->todo) && wait_for_proc_work)
            w = list_first_entry(&proc->todo, struct binder_work, entry);
        else {
            if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
                goto retry;
            break;
        }

        if (end - ptr < sizeof(tr) + 4)
            break;

        switch (w->type) {
        case BINDER_WORK_TRANSACTION: {
            t = container_of(w, struct binder_transaction, work);
                                      } break;
        ......
        }

        if (!t)
            continue;

        BUG_ON(t->buffer == NULL);
        if (t->buffer->target_node) {
            struct binder_node *target_node = t->buffer->target_node;
            tr.target.ptr = target_node->ptr;
            tr.cookie =  target_node->cookie;
            ......
            cmd = BR_TRANSACTION;
        } else {
            ......
        }
        tr.code = t->code;
        tr.flags = t->flags;
        tr.sender_euid = t->sender_euid;

        if (t->from) {
            struct task_struct *sender = t->from->proc->tsk;
            tr.sender_pid = task_tgid_nr_ns(sender, current->nsproxy->pid_ns);
        } else {
            tr.sender_pid = 0;
        }

        tr.data_size = t->buffer->data_size;
        tr.offsets_size = t->buffer->offsets_size;
        tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;
        tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));

        if (put_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (copy_to_user(ptr, &tr, sizeof(tr)))
            return -EFAULT;
        ptr += sizeof(tr);

        ......

        list_del(&t->work.entry);
        t->buffer->allow_user_free = 1;
        if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
            t->to_parent = thread->transaction_stack;
            t->to_thread = thread;
            thread->transaction_stack = t;
        } else {
            t->buffer->transaction = NULL;
            kfree(t);
            binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;
        }
        break;
    }

done:

    ......
    return 0;
}

        Service Manager被喚醒之後，就進入while循環開始處理事務了。這裏wait_for_proc_work等於1，並且proc->todo不爲空，所以從proc->todo列表中得到第一個工作項：

w = list_first_entry(&proc->todo, struct binder_work, entry);

        從上面的描述中，我們知道，這個工作項的類型爲BINDER_WORK_TRANSACTION，於是通過下面語句得到事務項：

t = container_of(w, struct binder_transaction, work);

       接着就是把事務項t中的數據拷貝到本地局部變量struct binder_transaction_data tr中去了：

if (t->buffer->target_node) {
    struct binder_node *target_node = t->buffer->target_node;
    tr.target.ptr = target_node->ptr;
    tr.cookie =  target_node->cookie;
    ......
    cmd = BR_TRANSACTION;
} else {
    ......
}
tr.code = t->code;
tr.flags = t->flags;
tr.sender_euid = t->sender_euid;

if (t->from) {
    struct task_struct *sender = t->from->proc->tsk;
    tr.sender_pid = task_tgid_nr_ns(sender, current->nsproxy->pid_ns);
} else {
    tr.sender_pid = 0;
}

tr.data_size = t->buffer->data_size;
tr.offsets_size = t->buffer->offsets_size;
tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;
tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));

        這裏有一個非常重要的地方，是Binder進程間通信機制的精髓所在：

tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;
tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));

        t->buffer->data所指向的地址是內核空間的，現在要把數據返回給Service Manager進程的用戶空間，而Service Manager進程的用戶空間是不能訪問內核空間的數據的，所以這裏要作一下處理。怎麼處理呢？我們在學面嚮對象語言的時候，對象的拷貝有深拷貝和淺拷貝之分，深拷貝是把另外分配一塊新內存，然後把原始對象的內容搬過去，淺拷貝是並沒有爲新對象分配一塊新空間，而只是分配一個引用，而個引用指向原始對象。Binder機制用的是類似淺拷貝的方法，通過在用戶空間分配一個虛擬地址，然後讓這個用戶空間虛擬地址與 t->buffer->data這個內核空間虛擬地址指向同一個物理地址，這樣就可以實現淺拷貝了。怎麼樣用戶空間和內核空間的虛擬地址同時指向同一個物理地址呢？請參考前面一篇文章淺談Service Manager成爲Android進程間通信（IPC）機制Binder守護進程之路，那裏有詳細描述。這裏只要將t->buffer->data加上一個偏移值proc->user_buffer_offset就可以得到t->buffer->data對應的用戶空間虛擬地址了。調整了tr.data.ptr.buffer的值之後，不要忘記也要一起調整tr.data.ptr.offsets的值。

        接着就是把tr的內容拷貝到用戶傳進來的緩衝區去了，指針ptr指向這個用戶緩衝區的地址：

if (put_user(cmd, (uint32_t __user *)ptr))
    return -EFAULT;
ptr += sizeof(uint32_t);
if (copy_to_user(ptr, &tr, sizeof(tr)))
    return -EFAULT;
ptr += sizeof(tr);

         這裏可以看出，這裏只是對作tr.data.ptr.bufferr和tr.data.ptr.offsets的內容作了淺拷貝。

         最後，由於已經處理了這個事務，要把它從todo列表中刪除：

list_del(&t->work.entry);
t->buffer->allow_user_free = 1;
if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
    t->to_parent = thread->transaction_stack;
    t->to_thread = thread;
    thread->transaction_stack = t;
} else {
    t->buffer->transaction = NULL;
    kfree(t);
    binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;
}

         注意，這裏的cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)爲true，表明這個事務雖然在驅動程序中已經處理完了，但是它仍然要等待Service Manager完成之後，給驅動程序一個確認，也就是需要等待回覆，於是把當前事務t放在thread->transaction_stack隊列的頭部：

t->to_parent = thread->transaction_stack;
t->to_thread = thread;
thread->transaction_stack = t;

         如果cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)爲false，那就不需要等待回覆了，直接把事務t刪掉。

         這個while最後通過一個break跳了出來，最後返回到binder_ioctl函數中：

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    int ret;
    struct binder_proc *proc = filp->private_data;
    struct binder_thread *thread;
    unsigned int size = _IOC_SIZE(cmd);
    void __user *ubuf = (void __user *)arg;

    ......

    switch (cmd) {
    case BINDER_WRITE_READ: {
        struct binder_write_read bwr;
        if (size != sizeof(struct binder_write_read)) {
            ret = -EINVAL;
            goto err;
        }
        if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
            ret = -EFAULT;
            goto err;
        }
        ......
        if (bwr.read_size > 0) {
            ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
            if (!list_empty(&proc->todo))
                wake_up_interruptible(&proc->wait);
            if (ret < 0) {
                if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                    ret = -EFAULT;
                goto err;
            }
        }
        ......
        if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
            ret = -EFAULT;
            goto err;
        }
        break;
        }
    ......
    default:
        ret = -EINVAL;
        goto err;
    }
    ret = 0;
err:
    ......
    return ret;
}

         從binder_thread_read返回來後，再看看proc->todo是否還有事務等待處理，如果是，就把睡眠在proc->wait隊列的線程喚醒來處理。最後，把本地變量struct binder_write_read bwr的內容拷貝回到用戶傳進來的緩衝區中，就返回了。

        這裏就是返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函數了：

void binder_loop(struct binder_state *bs, binder_handler func)
{
    int res;
    struct binder_write_read bwr;
    unsigned readbuf[32];

    bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;

    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(unsigned));

    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (unsigned) readbuf;

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);

        if (res < 0) {
            LOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

        res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
        if (res == 0) {
            LOGE("binder_loop: unexpected reply?!\n");
            break;
        }
        if (res < 0) {
            LOGE("binder_loop: io error %d %s\n", res, strerror(errno));
            break;
        }
    }
}

       返回來的數據都放在readbuf中，接着調用binder_parse進行解析：

int binder_parse(struct binder_state *bs, struct binder_io *bio,
                 uint32_t *ptr, uint32_t size, binder_handler func)
{
    int r = 1;
    uint32_t *end = ptr + (size / 4);

    while (ptr < end) {
        uint32_t cmd = *ptr++;
        ......
        case BR_TRANSACTION: {
            struct binder_txn *txn = (void *) ptr;
            if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
                LOGE("parse: txn too small!\n");
                return -1;
            }
            binder_dump_txn(txn);
            if (func) {
                unsigned rdata[256/4];
                struct binder_io msg;
                struct binder_io reply;
                int res;

                bio_init(&reply, rdata, sizeof(rdata), 4);
                bio_init_from_txn(&msg, txn);
                res = func(bs, txn, &msg, &reply);
                binder_send_reply(bs, &reply, txn->data, res);
            }
            ptr += sizeof(*txn) / sizeof(uint32_t);
            break;
                             }
        ......
        default:
            LOGE("parse: OOPS %d\n", cmd);
            return -1;
        }
    }

    return r;
}

        首先把從Binder驅動程序讀出來的數據轉換爲一個struct binder_txn結構體，保存在txn本地變量中，struct binder_txn定義在frameworks/base/cmds/servicemanager/binder.h文件中：

struct binder_txn
{
    void *target;
    void *cookie;
    uint32_t code;
    uint32_t flags;

    uint32_t sender_pid;
    uint32_t sender_euid;

    uint32_t data_size;
    uint32_t offs_size;
    void *data;
    void *offs;
};

       函數中還用到了另外一個數據結構struct binder_io，也是定義在frameworks/base/cmds/servicemanager/binder.h文件中：

struct binder_io
{
    char *data;            /* pointer to read/write from */
    uint32_t *offs;        /* array of offsets */
    uint32_t data_avail;   /* bytes available in data buffer */
    uint32_t offs_avail;   /* entries available in offsets array */

    char *data0;           /* start of data buffer */
    uint32_t *offs0;       /* start of offsets buffer */
    uint32_t flags;
    uint32_t unused;
};

       接着往下看，函數調bio_init來初始化reply變量：

void bio_init(struct binder_io *bio, void *data,
              uint32_t maxdata, uint32_t maxoffs)
{
    uint32_t n = maxoffs * sizeof(uint32_t);

    if (n > maxdata) {
        bio->flags = BIO_F_OVERFLOW;
        bio->data_avail = 0;
        bio->offs_avail = 0;
        return;
    }

    bio->data = bio->data0 = data + n;
    bio->offs = bio->offs0 = data;
    bio->data_avail = maxdata - n;
    bio->offs_avail = maxoffs;
    bio->flags = 0;
}

       接着又調用bio_init_from_txn來初始化msg變量：

void bio_init_from_txn(struct binder_io *bio, struct binder_txn *txn)
{
    bio->data = bio->data0 = txn->data;
    bio->offs = bio->offs0 = txn->offs;
    bio->data_avail = txn->data_size;
    bio->offs_avail = txn->offs_size / 4;
    bio->flags = BIO_F_SHARED;
}

      最後，真正進行處理的函數是從參數中傳進來的函數指針func，這裏就是定義在frameworks/base/cmds/servicemanager/service_manager.c文件中的svcmgr_handler函數：

int svcmgr_handler(struct binder_state *bs,
                   struct binder_txn *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    unsigned len;
    void *ptr;
    uint32_t strict_policy;

    if (txn->target != svcmgr_handle)
        return -1;

    // Equivalent to Parcel::enforceInterface(), reading the RPC
    // header with the strict mode policy mask and the interface name.
    // Note that we ignore the strict_policy and don't propagate it
    // further (since we do no outbound RPCs anyway).
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
            fprintf(stderr,"invalid id %s\n", str8(s));
            return -1;
    }

    switch(txn->code) {
    ......
    case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = bio_get_ref(msg);
        if (do_add_service(bs, s, len, ptr, txn->sender_euid))
            return -1;
        break;
    ......
    }

    bio_put_uint32(reply, 0);
    return 0;
}

         回憶一下，在BpServiceManager::addService時，傳給Binder驅動程序的參數爲：

writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
writeString16("android.os.IServiceManager");
writeString16("media.player");
writeStrongBinder(new MediaPlayerService());

         這裏的語句：

strict_policy = bio_get_uint32(msg);
s = bio_get_string16(msg, &len);
s = bio_get_string16(msg, &len);
ptr = bio_get_ref(msg);

         就是依次把它們讀取出來了，這裏，我們只要看一下bio_get_ref的實現。先看一個數據結構struct binder_obj的定義：

struct binder_object
{
    uint32_t type;
    uint32_t flags;
    void *pointer;
    void *cookie;
};

        這個結構體其實就是對應struct flat_binder_obj的。

        接着看bio_get_ref實現：

void *bio_get_ref(struct binder_io *bio)
{
    struct binder_object *obj;

    obj = _bio_get_obj(bio);
    if (!obj)
        return 0;

    if (obj->type == BINDER_TYPE_HANDLE)
        return obj->pointer;

    return 0;
}

       _bio_get_obj這個函數就不跟進去看了，它的作用就是從binder_io中取得第一個還沒取獲取過的binder_object。在這個場景下，就是我們最開始傳過來代表MediaPlayerService的flat_binder_obj了，這個原始的flat_binder_obj的type爲BINDER_TYPE_BINDER，binder爲指向MediaPlayerService的弱引用的地址。在前面我們說過，在Binder驅動驅動程序裏面，會把這個flat_binder_obj的type改爲BINDER_TYPE_HANDLE，handle改爲一個句柄值。這裏的handle值就等於obj->pointer的值。

        回到svcmgr_handler函數，調用do_add_service進一步處理：

int do_add_service(struct binder_state *bs,
                   uint16_t *s, unsigned len,
                   void *ptr, unsigned uid)
{
    struct svcinfo *si;
//    LOGI("add_service('%s',%p) uid=%d\n", str8(s), ptr, uid);

    if (!ptr || (len == 0) || (len > 127))
        return -1;

    if (!svc_can_register(uid, s)) {
        LOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",
             str8(s), ptr, uid);
        return -1;
    }

    si = find_svc(s, len);
    if (si) {
        if (si->ptr) {
            LOGE("add_service('%s',%p) uid=%d - ALREADY REGISTERED\n",
                 str8(s), ptr, uid);
            return -1;
        }
        si->ptr = ptr;
    } else {
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {
            LOGE("add_service('%s',%p) uid=%d - OUT OF MEMORY\n",
                 str8(s), ptr, uid);
            return -1;
        }
        si->ptr = ptr;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
        si->name[len] = '\0';
        si->death.func = svcinfo_death;
        si->death.ptr = si;
        si->next = svclist;
        svclist = si;
    }

    binder_acquire(bs, ptr);
    binder_link_to_death(bs, ptr, &si->death);
    return 0;
}

        這個函數的實現很簡單，就是把MediaPlayerService這個Binder實體的引用寫到一個struct svcinfo結構體中，主要是它的名稱和句柄值，然後插入到鏈接svclist的頭部去。這樣，Client來向Service Manager查詢服務接口時，只要給定服務名稱，Service Manger就可以返回相應的句柄值了。

        這個函數執行完成後，返回到svcmgr_handler函數，函數的最後，將一個錯誤碼0寫到reply變量中去，表示一切正常：

bio_put_uint32(reply, 0);

       svcmgr_handler函數執行完成後，返回到binder_parse函數，執行下面語句：

binder_send_reply(bs, &reply, txn->data, res);

       我們看一下binder_send_reply的實現，從函數名就可以猜到它要做什麼了，告訴Binder驅動程序，它完成了Binder驅動程序交給它的任務了。

void binder_send_reply(struct binder_state *bs,
                       struct binder_io *reply,
                       void *buffer_to_free,
                       int status)
{
    struct {
        uint32_t cmd_free;
        void *buffer;
        uint32_t cmd_reply;
        struct binder_txn txn;
    } __attribute__((packed)) data;

    data.cmd_free = BC_FREE_BUFFER;
    data.buffer = buffer_to_free;
    data.cmd_reply = BC_REPLY;
    data.txn.target = 0;
    data.txn.cookie = 0;
    data.txn.code = 0;
    if (status) {
        data.txn.flags = TF_STATUS_CODE;
        data.txn.data_size = sizeof(int);
        data.txn.offs_size = 0;
        data.txn.data = &status;
        data.txn.offs = 0;
    } else {
        data.txn.flags = 0;
        data.txn.data_size = reply->data - reply->data0;
        data.txn.offs_size = ((char*) reply->offs) - ((char*) reply->offs0);
        data.txn.data = reply->data0;
        data.txn.offs = reply->offs0;
    }
    binder_write(bs, &data, sizeof(data));
}

       從這裏可以看出，binder_send_reply告訴Binder驅動程序執行BC_FREE_BUFFER和BC_REPLY命令，前者釋放之前在binder_transaction分配的空間，地址爲buffer_to_free，buffer_to_free這個地址是Binder驅動程序把自己在內核空間用的地址轉換成用戶空間地址再傳給Service Manager的，所以Binder驅動程序拿到這個地址後，知道怎麼樣釋放這個空間；後者告訴MediaPlayerService，它的addService操作已經完成了，錯誤碼是0，保存在data.txn.data中。

       再來看binder_write函數：

int binder_write(struct binder_state *bs, void *data, unsigned len)
{
    struct binder_write_read bwr;
    int res;
    bwr.write_size = len;
    bwr.write_consumed = 0;
    bwr.write_buffer = (unsigned) data;
    bwr.read_size = 0;
    bwr.read_consumed = 0;
    bwr.read_buffer = 0;
    res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
    if (res < 0) {
        fprintf(stderr,"binder_write: ioctl failed (%s)\n",
                strerror(errno));
    }
    return res;
}

       這裏可以看出，只有寫操作，沒有讀操作，即read_size爲0。

       這裏又是一個ioctl的BINDER_WRITE_READ操作。直入到驅動程序的binder_ioctl函數後，執行BINDER_WRITE_READ命令，這裏就不累述了。

       最後，從binder_ioctl執行到binder_thread_write函數，我們首先看第一個命令BC_FREE_BUFFER：

int
binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
                    void __user *buffer, int size, signed long *consumed)
{
    uint32_t cmd;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    while (ptr < end && thread->return_error == BR_OK) {
        if (get_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
            binder_stats.bc[_IOC_NR(cmd)]++;
            proc->stats.bc[_IOC_NR(cmd)]++;
            thread->stats.bc[_IOC_NR(cmd)]++;
        }
        switch (cmd) {
        ......
        case BC_FREE_BUFFER: {
            void __user *data_ptr;
            struct binder_buffer *buffer;

            if (get_user(data_ptr, (void * __user *)ptr))
                return -EFAULT;
            ptr += sizeof(void *);

            buffer = binder_buffer_lookup(proc, data_ptr);
            if (buffer == NULL) {
                binder_user_error("binder: %d:%d "
                    "BC_FREE_BUFFER u%p no match\n",
                    proc->pid, thread->pid, data_ptr);
                break;
            }
            if (!buffer->allow_user_free) {
                binder_user_error("binder: %d:%d "
                    "BC_FREE_BUFFER u%p matched "
                    "unreturned buffer\n",
                    proc->pid, thread->pid, data_ptr);
                break;
            }
            if (binder_debug_mask & BINDER_DEBUG_FREE_BUFFER)
                printk(KERN_INFO "binder: %d:%d BC_FREE_BUFFER u%p found buffer %d for %s transaction\n",
                proc->pid, thread->pid, data_ptr, buffer->debug_id,
                buffer->transaction ? "active" : "finished");

            if (buffer->transaction) {
                buffer->transaction->buffer = NULL;
                buffer->transaction = NULL;
            }
            if (buffer->async_transaction && buffer->target_node) {
                BUG_ON(!buffer->target_node->has_async_transaction);
                if (list_empty(&buffer->target_node->async_todo))
                    buffer->target_node->has_async_transaction = 0;
                else
                    list_move_tail(buffer->target_node->async_todo.next, &thread->todo);
            }
            binder_transaction_buffer_release(proc, buffer, NULL);
            binder_free_buf(proc, buffer);
            break;
                             }

        ......
        *consumed = ptr - buffer;
    }
    return 0;
}

       首先通過看這個語句：

get_user(data_ptr, (void * __user *)ptr)

       這個是獲得要刪除的Buffer的用戶空間地址，接着通過下面這個語句來找到這個地址對應的struct binder_buffer信息：

buffer = binder_buffer_lookup(proc, data_ptr);

       因爲這個空間是前面在binder_transaction裏面分配的，所以這裏一定能找到。

       最後，就可以釋放這塊內存了：

binder_transaction_buffer_release(proc, buffer, NULL);
binder_free_buf(proc, buffer);

       再來看另外一個命令BC_REPLY：

int
binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
                    void __user *buffer, int size, signed long *consumed)
{
    uint32_t cmd;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    while (ptr < end && thread->return_error == BR_OK) {
        if (get_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
            binder_stats.bc[_IOC_NR(cmd)]++;
            proc->stats.bc[_IOC_NR(cmd)]++;
            thread->stats.bc[_IOC_NR(cmd)]++;
        }
        switch (cmd) {
        ......
        case BC_TRANSACTION:
        case BC_REPLY: {
            struct binder_transaction_data tr;

            if (copy_from_user(&tr, ptr, sizeof(tr)))
                return -EFAULT;
            ptr += sizeof(tr);
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
                       }

        ......
        *consumed = ptr - buffer;
    }
    return 0;
}

       又再次進入到binder_transaction函數：

static void
binder_transaction(struct binder_proc *proc, struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
    struct binder_transaction *t;
    struct binder_work *tcomplete;
    size_t *offp, *off_end;
    struct binder_proc *target_proc;
    struct binder_thread *target_thread = NULL;
    struct binder_node *target_node = NULL;
    struct list_head *target_list;
    wait_queue_head_t *target_wait;
    struct binder_transaction *in_reply_to = NULL;
    struct binder_transaction_log_entry *e;
    uint32_t return_error;

    ......

    if (reply) {
        in_reply_to = thread->transaction_stack;
        if (in_reply_to == NULL) {
            ......
            return_error = BR_FAILED_REPLY;
            goto err_empty_call_stack;
        }
        binder_set_nice(in_reply_to->saved_priority);
        if (in_reply_to->to_thread != thread) {
            .......
            goto err_bad_call_stack;
        }
        thread->transaction_stack = in_reply_to->to_parent;
        target_thread = in_reply_to->from;
        if (target_thread == NULL) {
            return_error = BR_DEAD_REPLY;
            goto err_dead_binder;
        }
        if (target_thread->transaction_stack != in_reply_to) {
            ......
            return_error = BR_FAILED_REPLY;
            in_reply_to = NULL;
            target_thread = NULL;
            goto err_dead_binder;
        }
        target_proc = target_thread->proc;
    } else {
        ......
    }
    if (target_thread) {
        e->to_thread = target_thread->pid;
        target_list = &target_thread->todo;
        target_wait = &target_thread->wait;
    } else {
        ......
    }


    /* TODO: reuse incoming transaction for reply */
    t = kzalloc(sizeof(*t), GFP_KERNEL);
    if (t == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_alloc_t_failed;
    }


    tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
    if (tcomplete == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_alloc_tcomplete_failed;
    }

    if (!reply && !(tr->flags & TF_ONE_WAY))
        t->from = thread;
    else
        t->from = NULL;
    t->sender_euid = proc->tsk->cred->euid;
    t->to_proc = target_proc;
    t->to_thread = target_thread;
    t->code = tr->code;
    t->flags = tr->flags;
    t->priority = task_nice(current);
    t->buffer = binder_alloc_buf(target_proc, tr->data_size,
        tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
    if (t->buffer == NULL) {
        return_error = BR_FAILED_REPLY;
        goto err_binder_alloc_buf_failed;
    }
    t->buffer->allow_user_free = 0;
    t->buffer->debug_id = t->debug_id;
    t->buffer->transaction = t;
    t->buffer->target_node = target_node;
    if (target_node)
        binder_inc_node(target_node, 1, 0, NULL);

    offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

    if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
        binder_user_error("binder: %d:%d got transaction with invalid "
            "data ptr\n", proc->pid, thread->pid);
        return_error = BR_FAILED_REPLY;
        goto err_copy_data_failed;
    }
    if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
        binder_user_error("binder: %d:%d got transaction with invalid "
            "offsets ptr\n", proc->pid, thread->pid);
        return_error = BR_FAILED_REPLY;
        goto err_copy_data_failed;
    }

    ......

    if (reply) {
        BUG_ON(t->buffer->async_transaction != 0);
        binder_pop_transaction(target_thread, in_reply_to);
    } else if (!(t->flags & TF_ONE_WAY)) {
        ......
    } else {
        ......
    }
    t->work.type = BINDER_WORK_TRANSACTION;
    list_add_tail(&t->work.entry, target_list);
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
    ......
}

       注意，這裏的reply爲1，我們忽略掉其它無關代碼。

       前面Service Manager正在binder_thread_read函數中被MediaPlayerService啓動後進程喚醒後，在最後會把當前處理完的事務放在thread->transaction_stack中：

if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
    t->to_parent = thread->transaction_stack;
    t->to_thread = thread;
    thread->transaction_stack = t;
}

       所以，這裏，首先是把它這個binder_transaction取回來，並且放在本地變量in_reply_to中：

in_reply_to = thread->transaction_stack;

       接着就可以通過in_reply_to得到最終發出這個事務請求的線程和進程：

target_thread = in_reply_to->from;
target_proc = target_thread->proc;

        然後得到target_list和target_wait：

target_list = &target_thread->todo;
target_wait = &target_thread->wait;

       下面這一段代碼：

/* TODO: reuse incoming transaction for reply */
t = kzalloc(sizeof(*t), GFP_KERNEL);
if (t == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_t_failed;
}


tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
if (tcomplete == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_tcomplete_failed;
}

if (!reply && !(tr->flags & TF_ONE_WAY))
    t->from = thread;
else
    t->from = NULL;
t->sender_euid = proc->tsk->cred->euid;
t->to_proc = target_proc;
t->to_thread = target_thread;
t->code = tr->code;
t->flags = tr->flags;
t->priority = task_nice(current);
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
    tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
if (t->buffer == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_binder_alloc_buf_failed;
}
t->buffer->allow_user_free = 0;
t->buffer->debug_id = t->debug_id;
t->buffer->transaction = t;
t->buffer->target_node = target_node;
if (target_node)
    binder_inc_node(target_node, 1, 0, NULL);

offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
    binder_user_error("binder: %d:%d got transaction with invalid "
        "data ptr\n", proc->pid, thread->pid);
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
    binder_user_error("binder: %d:%d got transaction with invalid "
        "offsets ptr\n", proc->pid, thread->pid);
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
}

          我們在前面已經分析過了，這裏不再重複。但是有一點要注意的是，這裏target_node爲NULL，因此，t->buffer->target_node也爲NULL。

          函數本來有一個for循環，用來處理數據中的Binder對象，這裏由於沒有Binder對象，所以就略過了。到了下面這句代碼：

binder_pop_transaction(target_thread, in_reply_to);

          我們看看做了什麼事情：

static void
binder_pop_transaction(
    struct binder_thread *target_thread, struct binder_transaction *t)
{
    if (target_thread) {
        BUG_ON(target_thread->transaction_stack != t);
        BUG_ON(target_thread->transaction_stack->from != target_thread);
        target_thread->transaction_stack =
            target_thread->transaction_stack->from_parent;
        t->from = NULL;
    }
    t->need_reply = 0;
    if (t->buffer)
        t->buffer->transaction = NULL;
    kfree(t);
    binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;
}

        由於到了這裏，已經不需要in_reply_to這個transaction了，就把它刪掉。

        回到binder_transaction函數：

t->work.type = BINDER_WORK_TRANSACTION;
list_add_tail(&t->work.entry, target_list);
tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
list_add_tail(&tcomplete->entry, &thread->todo);

         和前面一樣，分別把t和tcomplete分別放在target_list和thread->todo隊列中，這裏的target_list指的就是最初調用IServiceManager::addService的MediaPlayerService的Server主線程的的thread->todo隊列了，而thread->todo指的是Service Manager中用來回復IServiceManager::addService請求的線程。

        最後，喚醒等待在target_wait隊列上的線程了，就是最初調用IServiceManager::addService的MediaPlayerService的Server主線程了，它最後在binder_thread_read函數中睡眠在thread->wait上，就是這裏的target_wait了：

if (target_wait)
    wake_up_interruptible(target_wait);

        這樣，Service Manger回覆調用IServiceManager::addService請求就算完成了，重新回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函數等待下一個Client請求的到來。事實上，Service Manger回到binder_loop函數再次執行ioctl函數時候，又會再次進入到binder_thread_read函數。這時個會發現thread->todo不爲空，這是因爲剛纔我們調用了：

list_add_tail(&tcomplete->entry, &thread->todo);

          把一個工作項tcompelete放在了在thread->todo中，這個tcompelete的type爲BINDER_WORK_TRANSACTION_COMPLETE，因此，Binder驅動程序會執行下面操作：

switch (w->type) {
case BINDER_WORK_TRANSACTION_COMPLETE: {
    cmd = BR_TRANSACTION_COMPLETE;
    if (put_user(cmd, (uint32_t __user *)ptr))
        return -EFAULT;
    ptr += sizeof(uint32_t);

    list_del(&w->entry);
    kfree(w);

    } break;
    ......
}

        binder_loop函數執行完這個ioctl調用後，纔會在下一次調用ioctl進入到Binder驅動程序進入休眠狀態，等待下一次Client的請求。

        上面講到調用IServiceManager::addService的MediaPlayerService的Server主線程被喚醒了，於是，重新執行binder_thread_read函數：

static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
                   void  __user *buffer, int size, signed long *consumed, int non_block)
{
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    int ret = 0;
    int wait_for_proc_work;

    if (*consumed == 0) {
        if (put_user(BR_NOOP, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
    }

retry:
    wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);

    ......

    if (wait_for_proc_work) {
        ......
    } else {
        if (non_block) {
            if (!binder_has_thread_work(thread))
                ret = -EAGAIN;
        } else
            ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
    }

    ......

    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;

        if (!list_empty(&thread->todo))
            w = list_first_entry(&thread->todo, struct binder_work, entry);
        else if (!list_empty(&proc->todo) && wait_for_proc_work)
            w = list_first_entry(&proc->todo, struct binder_work, entry);
        else {
            if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
                goto retry;
            break;
        }

        ......

        switch (w->type) {
        case BINDER_WORK_TRANSACTION: {
            t = container_of(w, struct binder_transaction, work);
                                      } break;
        ......
        }

        if (!t)
            continue;

        BUG_ON(t->buffer == NULL);
        if (t->buffer->target_node) {
            ......
        } else {
            tr.target.ptr = NULL;
            tr.cookie = NULL;
            cmd = BR_REPLY;
        }
        tr.code = t->code;
        tr.flags = t->flags;
        tr.sender_euid = t->sender_euid;

        if (t->from) {
            ......
        } else {
            tr.sender_pid = 0;
        }

        tr.data_size = t->buffer->data_size;
        tr.offsets_size = t->buffer->offsets_size;
        tr.data.ptr.buffer = (void *)t->buffer->data + proc->user_buffer_offset;
        tr.data.ptr.offsets = tr.data.ptr.buffer + ALIGN(t->buffer->data_size, sizeof(void *));

        if (put_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (copy_to_user(ptr, &tr, sizeof(tr)))
            return -EFAULT;
        ptr += sizeof(tr);

        ......

        list_del(&t->work.entry);
        t->buffer->allow_user_free = 1;
        if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
            ......
        } else {
            t->buffer->transaction = NULL;
            kfree(t);
            binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;
        }
        break;
    }

done:
    ......
    return 0;
}

         在while循環中，從thread->todo得到w，w->type爲BINDER_WORK_TRANSACTION，於是，得到t。從上面可以知道，Service Manager反回了一個0回來，寫在t->buffer->data裏面，現在把t->buffer->data加上proc->user_buffer_offset，得到用戶空間地址，保存在tr.data.ptr.buffer裏面，這樣用戶空間就可以訪問這個返回碼了。由於cmd不等於BR_TRANSACTION，這時就可以把t刪除掉了，因爲以後都不需要用了。

         執行完這個函數後，就返回到binder_ioctl函數，執行下面語句，把數據返回給用戶空間：

if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
    ret = -EFAULT;
    goto err;
}

         接着返回到用戶空間IPCThreadState::talkWithDriver函數，最後返回到IPCThreadState::waitForResponse函數，最終執行到下面語句：

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    int32_t cmd;
    int32_t err;

    while (1) {
        if ((err=talkWithDriver()) < NO_ERROR) break;

        ......

        cmd = mIn.readInt32();

        ......

        switch (cmd) {
        ......
        case BR_REPLY:
            {
                binder_transaction_data tr;
                err = mIn.read(&tr, sizeof(tr));
                LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");
                if (err != NO_ERROR) goto finish;

                if (reply) {
                    if ((tr.flags & TF_STATUS_CODE) == 0) {
                        reply->ipcSetDataReference(
                            reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t),
                            freeBuffer, this);
                    } else {
                        ......
                    }
                } else {
                    ......
                }
            }
            goto finish;

        ......
        }
    }

finish:
    ......
    return err;
}

        注意，這裏的tr.flags等於0，這個是在上面的binder_send_reply函數裏設置的。最終把結果保存在reply了：

reply->ipcSetDataReference(
       reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
       tr.data_size,
       reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
       tr.offsets_size/sizeof(size_t),
       freeBuffer, this);

       這個函數我們就不看了，有興趣的讀者可以研究一下。

       從這裏層層返回，最後回到MediaPlayerService::instantiate函數中。

       至此，IServiceManager::addService終於執行完畢了。這個過程非常複雜，但是如果我們能夠深刻地理解這一過程，將能很好地理解Binder機制的設計思想和實現過程。這裏，對IServiceManager::addService過程中MediaPlayerService、ServiceManager和BinderDriver之間的交互作一個小結：

        回到frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函數，接下去還要執行下面兩個函數：

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

        首先看ProcessState::startThreadPool函數的實現：

void ProcessState::startThreadPool()
{
    AutoMutex _l(mLock);
    if (!mThreadPoolStarted) {
        mThreadPoolStarted = true;
        spawnPooledThread(true);
    }
}

       這裏調用spwanPooledThread：

void ProcessState::spawnPooledThread(bool isMain)
{
    if (mThreadPoolStarted) {
        int32_t s = android_atomic_add(1, &mThreadPoolSeq);
        char buf[32];
        sprintf(buf, "Binder Thread #%d", s);
        LOGV("Spawning new pooled thread, name=%s\n", buf);
        sp<Thread> t = new PoolThread(isMain);
        t->run(buf);
    }
}

       這裏主要是創建一個線程，PoolThread繼續Thread類，Thread類定義在frameworks/base/libs/utils/Threads.cpp文件中，其run函數最終調用子類的threadLoop函數，這裏即爲PoolThread::threadLoop函數：

virtual bool threadLoop()
{
    IPCThreadState::self()->joinThreadPool(mIsMain);
    return false;
}

       這裏和frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函數一樣，最終都是調用了IPCThreadState::joinThreadPool函數，它們的區別是，一個參數是true，一個是默認值false。我們來看一下這個函數的實現：

void IPCThreadState::joinThreadPool(bool isMain)
{
    LOG_THREADPOOL("**** THREAD %p (PID %d) IS JOINING THE THREAD POOL\n", (void*)pthread_self(), getpid());

    mOut.writeInt32(isMain ? BC_ENTER_LOOPER : BC_REGISTER_LOOPER);

    ......

    status_t result;
    do {
        int32_t cmd;

        .......

        // now get the next command to be processed, waiting if necessary
        result = talkWithDriver();
        if (result >= NO_ERROR) {
            size_t IN = mIn.dataAvail();
            if (IN < sizeof(int32_t)) continue;
            cmd = mIn.readInt32();
            ......
            }

            result = executeCommand(cmd);
        }

        ......
    } while (result != -ECONNREFUSED && result != -EBADF);

    .......

    mOut.writeInt32(BC_EXIT_LOOPER);
    talkWithDriver(false);
}

        這個函數最終是在一個無窮循環中，通過調用talkWithDriver函數來和Binder驅動程序進行交互，實際上就是調用talkWithDriver來等待Client的請求，然後再調用executeCommand來處理請求，而在executeCommand函數中，最終會調用BBinder::transact來真正處理Client的請求：

status_t IPCThreadState::executeCommand(int32_t cmd)
{
    BBinder* obj;
    RefBase::weakref_type* refs;
    status_t result = NO_ERROR;

    switch (cmd) {
    ......

    case BR_TRANSACTION:
        {
            binder_transaction_data tr;
            result = mIn.read(&tr, sizeof(tr));

            ......

            Parcel reply;

            ......

            if (tr.target.ptr) {
                sp<BBinder> b((BBinder*)tr.cookie);
                const status_t error = b->transact(tr.code, buffer, &reply, tr.flags);
                if (error < NO_ERROR) reply.setError(error);

            } else {
                const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);
                if (error < NO_ERROR) reply.setError(error);
            }

            ......
        }
        break;

    .......
    }

    if (result != NO_ERROR) {
        mLastError = result;
    }

    return result;
}

        接下來再看一下BBinder::transact的實現：

status_t BBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    data.setDataPosition(0);

    status_t err = NO_ERROR;
    switch (code) {
        case PING_TRANSACTION:
            reply->writeInt32(pingBinder());
            break;
        default:
            err = onTransact(code, data, reply, flags);
            break;
    }

    if (reply != NULL) {
        reply->setDataPosition(0);
    }

    return err;
}

       最終會調用onTransact函數來處理。在這個場景中，BnMediaPlayerService繼承了BBinder類，並且重載了onTransact函數，因此，這裏實際上是調用了BnMediaPlayerService::onTransact函數，這個函數定義在frameworks/base/libs/media/libmedia/IMediaPlayerService.cpp文件中：

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        case CREATE_URL: {
            ......
                         } break;
        case CREATE_FD: {
            ......
                        } break;
        case DECODE_URL: {
            ......
                         } break;
        case DECODE_FD: {
            ......
                        } break;
        case CREATE_MEDIA_RECORDER: {
            ......
                                    } break;
        case CREATE_METADATA_RETRIEVER: {
            ......
                                        } break;
        case GET_OMX: {
            ......
                      } break;
        default:
            return BBinder::onTransact(code, data, reply, flags);
    }
}

       至此，我們就以MediaPlayerService爲例，完整地介紹了Android系統進程間通信Binder機制中的Server啓動過程。Server啓動起來之後，就會在一個無窮循環中等待Client的請求了。在下一篇文章中，我們將介紹Client如何通過Service Manager遠程接口來獲得Server遠程接口，進而調用Server遠程接口來使用Server提供的服務，敬請關注。