Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析
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在前面一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路中，介绍了在android系统中Binder进程间通信机制中的Server角色是如何获得Service Manager远程接口的，即defaultServiceManager函数的实现。Server获得了Service Manager远程接口之后，就要把自己的Service添加到Service Manager中去，然后把自己启动起来，等待Client的请求。本文将通过分析源代码了解Server的启动过程是怎么样的。
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本文通过一个具体的例子来说明Binder机制中Server的启动过程。我们知道，在Android系统中，提供了多媒体播放的功能，这个功能是以服务的形式来提供的。这里，我们就通过分析MediaPlayerService的实现来了解Media Server的启动过程。
首先，看一下MediaPlayerService的类图，以便我们理解下面要描述的内容。
我们将要介绍的主角MediaPlayerService继承于BnMediaPlayerService类，熟悉Binder机制的同学应该知道BnMediaPlayerService是一个Binder Native类，用来处理Client请求的。BnMediaPlayerService继承于BnInterface<IMediaPlayerService>类，BnInterface是一个模板类，它定义在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();
;实际上，BnMediaPlayerService并不是直接接收到Client处发送过来的请求，而是使用了IPCThreadState接收Client处发送过来的请求，而IPCThreadState又借助了ProcessState类来与Binder驱动程序交互。有关IPCThreadState和ProcessState的关系，可以参考上一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路，接下来也会有相应的描述。IPCThreadState接收到了Client处的请求后，就会调用BBinder类的transact函数，并传入相关参数，BBinder类的transact函数最终调用BnMediaPlayerService类的onTransact函数，于是，就开始真正地处理Client的请求了。
了解了MediaPlayerService类结构之后，就要开始进入到本文的主题了。
首先，看看MediaPlayerService是如何启动的。启动MediaPlayerService的代码位于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();
先看下面这句代码：
sp<ProcessState> proc(ProcessState::self());sp<ProcessState> ProcessState::self()
if (gProcess != NULL) return gProcess;
AutoMutex _l(gProcessMutex);
if (gProcess == NULL) gProcess = new ProcessState;
return gProcess;
Mutex gProcessMutex;
sp<ProcessState> gProcess;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.
先看open_driver函数的实现，这个函数同样位于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;
open在Binder驱动程序中的具体实现，请参考前面一篇文章浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路，这里不再重复描述。打开/dev/binder设备文件后，Binder驱动程序就为MediaPlayerService进程创建了一个struct binder_proc结构体实例来维护MediaPlayerService进程上下文相关信息。
我们来看一下ioctl文件操作函数执行BINDER_VERSION命令的过程：
status_t result = ioctl(fd, BINDER_VERSION, &vers);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;
很简单，只是将BINDER_CURRENT_PROTOCOL_VERSION写入到传入的参数arg指向的用户缓冲区中去就返回了。BINDER_CURRENT_PROTOCOL_VERSION是一个宏，定义在kernel/common/drivers/staging/android/binder.h文件中：
/* This is the current protocol version. */
#define BINDER_CURRENT_PROTOCOL_VERSION 7/* Use with BINDER_VERSION, driver fills in fields. */
struct binder_version
/* driver protocol version -- increment with incompatible change */
signed long protocol_version;
;这里有一个重要的地方要注意的是，由于这里是打开设备文件/dev/binder之后，第一次进入到binder_ioctl函数，因此，这里调用binder_get_thread的时候，就会为当前线程创建一个struct binder_thread结构体变量来维护线程上下文信息，具体可以参考浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。
接着我们再来看一下ioctl文件操作函数执行BINDER_SET_MAX_THREADS命令的过程：
result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);这个函数调用最终进入到Binder驱动程序的binder_ioctl函数中，我们只关注BINDER_SET_MAX_THREADS相关的部分逻辑：
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;
回到ProcessState的构造函数中，这里还通过mmap函数来把设备文件/dev/binder映射到内存中，这个函数在浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文也已经有详细介绍，这里不再重复描述。宏BINDER_VM_SIZE就定义在ProcessState.cpp文件中：
#define BINDER_VM_SIZE ((1*1024*1024) - (4096 *2))这样，ProcessState全局唯一变量gProcess就创建完毕了，回到frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函数，下一步是调用defaultServiceManager函数来获得Service Manager的远程接口，这个已经在上一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路有详细描述，读者可以回过头去参考一下。
再接下来，就进入到MediaPlayerService::instantiate函数把MediaPlayerService添加到Service Manger中去了。这个函数定义在frameworks/base/media/libmediaplayerservice/MediaPlayerService.cpp文件中：
void MediaPlayerService::instantiate()
defaultServiceManager()->addService(
String16("media.player"), new MediaPlayerService());
在上一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路中说到，defaultServiceManager返回的实际是一个BpServiceManger类实例，因此，我们看一下BpServiceManger::addService的实现，这个函数实现在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()
......
;先来看这一句的调用：
data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());// 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);
再来看下面的调用：
data.writeString16(name);往下看：
data.writeStrongBinder(service);status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
return flatten_binder(ProcessState::self(), val, this);
/*
* 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;
;我们进入到flatten_binder函数看看：
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);
obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;传进来的binder即为MediaPlayerService::instantiate函数中new出来的MediaPlayerService实例，因此，不为空。又由于MediaPlayerService继承自BBinder类，它是一个本地Binder实体，因此binder->localBinder返回一个BBinder指针，而且肯定不为空，于是执行下面语句：
obj.type = BINDER_TYPE_BINDER;
obj.binder = local->getWeakRefs();
obj.cookie = local;函数调用finish_flatten_binder来将这个flat_binder_obj写入到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);
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;
mObjects[mObjectsSize] = mDataPos;再回到BpServiceManager::addService函数中，调用下面语句：
status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);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;
再进入到IPCThreadState::transact函数，看看做了些什么事情：
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;
函数首先调用writeTransactionData函数准备好一个struct binder_transaction_data结构体变量，这个是等一下要传输给Binder驱动程序的。struct binder_transaction_data的定义我们在浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文中有详细描述，读者不妨回过去读一下。这里为了方便描述，将struct binder_transaction_data的定义再次列出来：
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;
;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，这里的cmd为BC_TRANSACTION。 这个函数很简单，在这个场景下，就是执行下面语句来初始化本地变量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();writeInt32(IPCThreadState::self()->getStrictModePolicy() |
STRICT_MODE_PENALTY_GATHER);
writeString16("android.os.IServiceManager");
writeString16("media.player");
writeStrongBinder(new MediaPlayerService());回到IPCThreadState::transact函数中，接下去看，(flags & TF_ONE_WAY) == 0为true，并且reply不为空，所以最终进入到waitForResponse(reply)这条路径来。我们看一下waitForResponse函数的实现：
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;
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;
最后，通过ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)进行到Binder驱动程序的binder_ioctl函数，我们只关注cmd为BINDER_WRITE_READ的逻辑：
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;
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;
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;
......
target_node = binder_context_mgr_node;
target_proc = target_node->proc;
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;
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
tr->offsets_size, !reply && (t->flags & TF_ONE_WAY)); 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;
if (target_node)
binder_inc_node(target_node, 1, 0, NULL); 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;现在，由于要把这个Binder实体MediaPlayerService交给target_proc，也就是Service Manager来管理，也就是说Service Manager要引用这个MediaPlayerService了，于是通过binder_get_ref_for_node为MediaPlayerService创建一个引用，并且通过binder_inc_ref来增加这个引用计数，防止这个引用还在使用过程当中就被销毁。注意，到了这里的时候，t->buffer中的flat_binder_obj的type已经改为BINDER_TYPE_HANDLE，handle已经改为ref->desc，跟原来不一样了，因为这个flat_binder_obj是最终是要传给Service Manager的，而Service Manager只能够通过句柄值来引用这个Binder实体。
最后，把待处理事务加入到target_list列表中去：
list_add_tail(&t->work.entry, target_list);list_add_tail(&tcomplete->entry, &thread->todo);if (target_wait)
wake_up_interruptible(target_wait);这里我们先忽略一下Service Manager被唤醒之后的场景，继续MedaPlayerService的启动过程，然后再回来。
回到binder_ioctl函数，bwr.read_size > 0为true，于是进入binder_thread_read函数：
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;
这里，thread->transaction_stack和thread->todo均不为空，于是wait_for_proc_work为false，由于binder_has_thread_work的时候，返回true，这里因为thread->todo不为空，因此，线程虽然调用了wait_event_interruptible，但是不会睡眠，于是继续往下执行。
由于thread->todo不为空，执行下列语句：
if (!list_empty(&thread->todo))
w = list_first_entry(&thread->todo, struct binder_work, entry); 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_ioctl函数返回到用户空间之前，把数据消耗情况拷贝回用户空间中：
if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
ret = -EFAULT;
goto err;
首先是把mOut的数据清空：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)...... return NO_ERROR;mIn.setDataSize(bwr.read_consumed); mIn.setDataPosition(0);
mOut.setDataSize(0); mIn.setDataSize(bwr.read_consumed);
mIn.setDataPosition(0);这时候，下面语句执行后：
const bool needRead = mIn.dataPosition() >= mIn.dataSize();这时候，下面语句执行后：
const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;switch (cmd)
case BR_TRANSACTION_COMPLETE:
if (!reply && !acquireResult) goto finish;
break;
......
这次，needRead就为true了，而outAvail仍为0，所以bwr.read_size不为0，bwr.write_size为0。于是通过：
ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)wait_event_interruptible(thread->wait, binder_has_thread_work(thread)); 现在，我们可以回到Service Manager被唤醒的过程了。我们接着前面浅谈Service Manager成为And 以上是关于Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析的主要内容，如果未能解决你的问题，请参考以下文章 Android进程间通信（IPC）机制Binder简要介绍和学习计划