Android系统进程间通信（IPC）机制Binder中的Client获得Server远程接口过程源代码分析
在上一篇文章中，我们分析了Android系统进程间通信机制Binder中的Server在启动过程使用Service Manager的addService接口把自己添加到Service Manager守护过程中接受管理。在这一篇文章中，我们将深入到Binder驱动程序源代码去分析Client是如何通过Service Manager的getService接口中来获得Server远程接口的。Client只有获得了Server的远程接口之后，才能进一步调用Server提供的服务。
函数首先通过defaultServiceManager函数来获得Service Manager的远程接口，实际上就是获得BpServiceManager的IServiceManager接口，具体可以参考浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路一文。总的来说，这里的语句：
相当于是：
这里的0表示Service Manager的远程接口的句柄值是0。
BpServiceManager::getService通过BpServiceManager::checkService执行操作。
首先是调用函数writeTransactionData写入将要传输的数据到IPCThreadState的成员变量mOut中去：
结构体binder_transaction_data在上一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析已经介绍过，这里不再累述，这个结构体是用来描述要传输的参数的内容的。这里着重描述一下将要传输的参数tr里面的内容，handle = 0，code = CHECK_SERVICE_TRANSACTION，cmd = BC_TRANSACTION，data里面的数据分别为：
这是在BpServiceManager::checkService函数里面写进去的，其中前两个是RPC头，Service Manager在收到这个请求时会验证这两个参数是否正确，这点前面也提到了。IPCThread->getStrictModePolicy默认返回0，STRICT_MODE_PENALTY_GATHER定义为：
我们不关心这个参数的含义，这不会影响我们分析下面的源代码，有兴趣的读者可以研究一下。这里要注意的是，要传输的参数不包含有Binder对象，因此tr.offsets_size = 0。要传输的参数最后写入到IPCThreadState的成员变量mOut中，包括cmd和tr两个数据。
进入到waitForResponse函数中：
这个函数通过IPCThreadState::talkWithDriver与驱动程序进行交互：
这里的needRead为true，因此，bwr.read_size大于0；outAvail也大于0，因此，bwr.write_size也大于0。函数最后通过：
进入到Binder驱动程序的binder_ioctl函数中。注意，这里的mProcess->mDriverFD是在我们前面调用defaultServiceManager函数获得Service Manager远程接口时，打开的设备文件/dev/binder的文件描述符，mProcess是IPCSThreadState的成员变量。
这里的filp->private_data的值是在defaultServiceManager函数创建ProcessState对象时，在ProcessState构造函数通过open文件操作函数打开设备文件/dev/binder时设置好的，它表示的是调用open函数打开设备文件/dev/binder的进程上下文信息，这里将它取出来保存在proc本地变量中。
这里可以看出，BINDER_WRITE_READ命令的参数类型为struct binder_write_read：
这个结构体的含义可以参考浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。这里首先是通过copy_from_user函数把用户传进来的参数的内容拷贝到本地变量bwr中。
从上面的调用过程，我们知道，这里bwr.write_size是大于0的，因此进入到binder_thread_write函数中，我们只关注BC_TRANSACTION相关的逻辑：
这里再次把用户传出来的参数拷贝到本地变量tr中，tr的类型为struct binder_transaction_data，这个就是前面我们在IPCThreadState::writeTransactionData写入的内容了。
注意，这里的参数reply = 0，表示这是一个BC_TRANSACTION命令。
前面我们提到，传给驱动程序的handle值为0，即这里的tr->target.handle = 0，表示请求的目标Binder对象是Service Manager，因此有：
这样表明当前线程还有事务要处理。
最后，Service Manager有事情可做了，就要唤醒它了：
前面我们提到，此时Service Manager正在等待Client的请求，也就是Service Manager此时正在进入到Binder驱动程序的binder_thread_read函数中，并且休眠在target->wait上，具体参考浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。
这里，我们暂时忽略Service Manager被唤醒之后的情景，继续看当前线程的执行。
函数binder_transaction执行完成之后，就一路返回到binder_ioctl函数里去了。函数binder_ioctl从binder_thread_write函数调用处返回后，发现bwr.read_size大于0，于是就进入到binder_thread_read函数去了：
函数首先是写入一个操作码BR_NOOP到用户传进来的缓冲区中去。
最后就返回到IPCThreadState::talkWithDriver函数中了。
返回后，首先是清空之前写入Binder驱动程序的内容：
接着是设置从Binder驱动程序读取的内容：
然后就返回到IPCThreadState::waitForResponse去了。IPCThreadState::waitForResponse函数的处理也很简单，就是处理刚才从Binder驱动程序读入内容了。从前面的分析中，我们知道，从Binder驱动程序读入的内容就是两个整数了，分别是BR_NOOP和BR_TRANSACTION_COMPLETE。对BR_NOOP的处理很简单，正如它的名字所示，什么也不做；而对BR_TRANSACTION_COMPLETE的处理，就分情况了，如果这个请求是异步的，那个整个BC_TRANSACTION操作就完成了，如果这个请求是同步的，即要等待回复的，也就是reply不为空，那么还要继续通过IPCThreadState::talkWithDriver进入到Binder驱动程序中去等待BC_TRANSACTION操作的处理结果。
进入休眠状态了，等待Service Manager的唤醒。
这里就是从语句中唤醒了：
Service Manager唤醒过来看，继续往下执行，进入到while循环中。首先是从proc->todo中取回待处理事项w。这个事项w的类型是BINDER_WORK_TRANSACTION，这是上面调用binder_transaction的时候设置的，于是通过w得到待处理事务t：
接下来的内容，就把cmd和t->buffer的内容拷贝到用户传进来的缓冲区去了，这里就是Service Manager从用户空间传进来的缓冲区了：
注意，这里先是把t->buffer的内容拷贝到本地变量tr中，再拷贝到用户空间缓冲区去。关于t->buffer内容的拷贝，请参考Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文，它的一个关键地方是Binder驱动程序和Service Manager守护进程共享了同一个物理内存的内容，拷贝的只是这个物理内存在用户空间的虚拟地址回去：
对于Binder驱动程序这次操作来说，这个事项就算是处理完了，就要从todo队列中删除了：
紧接着，还不放删除这个事务，因为它还要等待Service Manager处理完成后，再进一步处理，因此，放在thread->transaction_stack队列中：
还要注意的一个地方是，上面写入的cmd = BR_TRANSACTION，告诉Service Manager守护进程，它要做什么事情，后面我们会看到相应的分析。
最后，就返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数去了：
这里就是从下面的语句：
返回来了。接着就进入binder_parse函数处理从Binder驱动程序里面读取出来的数据：
前面我们说过，Binder驱动程序写入到用户空间的缓冲区中的cmd为BR_TRANSACTION，因此，这里我们只关注BR_TRANSACTION相关的逻辑。
接着又调用bio_init_from_txn来初始化msg变量：
最后，真正进行处理的函数是从参数中传进来的函数指针func，这里就是定义在frameworks/base/cmds/servicemanager/service_manager.c文件中的svcmgr_handler函数：
这里， Service Manager要处理的code是SVC_MGR_CHECK_SERVICE，这是在前面的BpServiceManager::checkService函数里面设置的。
这里的语句：
其中，会验证一下传进来的第二个参数，即"android.os.IServiceManager"是否正确，这个是验证RPC头，注释已经说得很清楚了。
这里又调用了find_svc函数：
就是在svclist列表中查找对应名称的svcinfo了。
这里很简单，就是把一个类型为BINDER_TYPE_HANDLE的binder_object写入到reply缓冲区中去。这里的binder_object就是相当于是flat_binder_obj了，具体可以参考Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文。
最后返回到binder_parse函数中，调用binder_send_reply函数将操作结果反馈给Binder驱动程序：
注意，这里的status参数为0。从这里可以看出，binder_send_reply告诉Binder驱动程序执行BC_FREE_BUFFER和BC_REPLY命令，前者释放之前在binder_transaction分配的空间，地址为buffer_to_free，buffer_to_free这个地址是Binder驱动程序把自己在内核空间用的地址转换成用户空间地址再传给Service Manager的，所以Binder驱动程序拿到这个地址后，知道怎么样释放这个空间；后者告诉Binder驱动程序，它的SVC_MGR_CHECK_SERVICE操作已经完成了,要查询的服务的句柄值也是保存在data.txn.data，操作结果码是0，也是保存在data.txn.data中。
再来看binder_write函数：
这里可以看出，只有写操作，没有读操作，即read_size为0。
这里又是一个ioctl的BINDER_WRITE_READ操作。直入到驱动程序的binder_ioctl函数后，执行BINDER_WRITE_READ命令，这里就不累述了。
最后，从binder_ioctl执行到binder_thread_write函数，首先是执行BC_FREE_BUFFER命令，这个命令的执行在前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析已经介绍过了，这里就不再累述了。
又再次进入到binder_transaction函数：
这次进入binder_transaction函数的情形和上面介绍的binder_transaction函数的情形基本一致，只是这里的proc、thread和target_proc、target_thread调换了角色，这里的proc和thread指的是Service Manager进程，而target_proc和target_thread指的是刚才请求SVC_MGR_CHECK_SERVICE的进程。
再接着往下看，由于Service Manager返回来了一个Binder引用，所以这里要处理一下，就是中间的for循环了。这是一个BINDER_TYPE_HANDLE类型的Binder引用，这是前面设置的。先把t->buffer->data的内容转换为一个struct flat_binder_object对象fp，这里的fp->handle值就是这个Service在Service Manager进程里面的引用值了。接通过调用binder_get_ref函数得到Binder引用对象struct binder_ref类型的对象ref：
这里一定能找到，因为前面MediaPlayerService执行IServiceManager::addService的时候把自己添加到Service Manager的时候，会在Service Manager进程中创建这个Binder引用，然后把这个Binder引用的句柄值返回给Service Manager用户空间。
然后增加引用计数：
这样，返回数据中的Binder对象就处理完成了。注意，这里会把fp->handle的值改为在target_proc中的引用值：
这里就相当于是把t->buffer->data里面的Binder对象的句柄值改写了。因为这是在另外一个不同的进程里面的Binder引用，所以句柄值当然要用新的了。这个值最终是要拷贝回target_proc进程的用户空间去的。
这里reply等于1，执行binder_pop_transaction函数把当前事务in_reply_to从target_thread->transaction_stack队列中删掉，这是上次调用binder_transaction函数的时候设置的，现在不需要了，所以把它删掉。
这样，Service Manger回复调用SVC_MGR_CHECK_SERVICE请求就算完成了，重新回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数等待下一个Client请求的到来。事实上，Service Manger回到binder_loop函数再次执行ioctl函数时候，又会再次进入到binder_thread_read函数。这时个会发现thread->todo不为空，这是因为刚才我们调用了：
把一个工作项tcompelete放在了在thread->todo中，这个tcompelete的type为BINDER_WORK_TRANSACTION_COMPLETE，因此，Binder驱动程序会执行下面操作：
binder_loop函数执行完这个ioctl调用后，才会在下一次调用ioctl进入到Binder驱动程序进入休眠状态，等待下一次Client的请求。
上面讲到调用请求SVC_MGR_CHECK_SERVICE操作的线程被唤醒了，于是，重新执行binder_thread_read函数：
就是从下面这个调用：
被唤醒过来了。在while循环中，从thread->todo得到w，w->type为BINDER_WORK_TRANSACTION，于是，得到t。从上面可以知道，Service Manager返回来了一个Binder引用和一个结果码0回来，写在t->buffer->data里面，现在把t->buffer->data加上proc->user_buffer_offset，得到用户空间地址，保存在tr.data.ptr.buffer里面，这样用户空间就可以访问这个数据了。由于cmd不等于BR_TRANSACTION，这时就可以把t删除掉了，因为以后都不需要用了。
执行完这个函数后，就返回到binder_ioctl函数，执行下面语句，把数据返回给用户空间：
接着返回到用户空间IPCThreadState::talkWithDriver函数，最后返回到IPCThreadState::waitForResponse函数，最终执行到下面语句：
注意，这里的tr.flags等于0，这个是在上面的binder_send_reply函数里设置的。接着就把结果保存在reply了：
我们简单看一下Parcel::ipcSetDataReference函数的实现：
上面提到，返回来的数据中有一个Binder引用，因此，这里的mObjectSize等于1，这个Binder引用对应的位置记录在mObjects成员变量中。
这里就是从：
返回来了。我们接着看一下reply.readStrongBinder函数的实现：
这里调用了unflatten_binder函数来构造一个Binder对象：
这里的flat->type是BINDER_TYPE_HANDLE，因此调用ProcessState::getStrongProxyForHandle函数：
这里我们可以看到，ProcessState会把使用过的Binder远程接口（BpBinder）缓存起来，这样下次从Service Manager那里请求得到相同的句柄（Handle）时就可以直接返回这个Binder远程接口了，不用再创建一个出来。这里是第一次使用，因此，e->binder为空，于是创建了一个BpBinder对象：
最后，函数返回到IMediaDeathNotifier::getMediaPlayerService这里，从这个语句返回：
这里，就相当于是：
最后，函数调用：
到了这里，我们可以参考一下前面一篇文章浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager，就会知道，这里的interface_cast实际上最终调用了IMediaPlayerService::asInterface函数：
这里的obj就是BpBinder，而BpBinder::queryLocalInterface返回NULL，因此就创建了一个BpMediaPlayerService对象：
因此，我们最终就得到了一个BpMediaPlayerService对象，达到我们最初的目标。
这里，我们仍然是通过Android系统中自带的多媒体播放器为例子来说明Client是如何通过IServiceManager::getService接口来获得MediaPlayerService这个Server的远程接口的。假设计读者已经阅读过前面三篇文章浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路、浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路和Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，即假设Service Manager和MediaPlayerService已经启动完毕，Service Manager现在等待Client的请求。
这里，我们要举例子说明的Client便是MediaPlayer了，它声明和实现在frameworks/base/include/media/mediaplayer.h和frameworks/base/media/libmedia/mediaplayer.cpp文件中。MediaPlayer继承于IMediaDeathNotifier类，这个类声明和实现在frameworks/base/include/media/IMediaDeathNotifier.h和frameworks/base/media/libmedia//IMediaDeathNotifier.cpp文件中，里面有一个静态成员函数getMeidaPlayerService，它通过IServiceManager::getService接口来获得MediaPlayerService的远程接口。
在介绍IMediaDeathNotifier::getMeidaPlayerService函数之前，我们先了解一下这个函数的目标。看来前面浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路这篇文章的读者知道，我们在获取Service Manager远程接口时，最终是获得了一个BpServiceManager对象的IServiceManager接口。类似地，我们要获得MediaPlayerService的远程接口，实际上就是要获得一个称为BpMediaPlayerService对象的IMediaPlayerService接口。现在，我们就先来看一下BpMediaPlayerService的类图：
从这个类图可以看到，BpMediaPlayerService继承于BpInterface
BpMediaPlayerService的构造函数有一个参数impl，它的类型为const sp
现在，我们就来看一下IMediaDeathNotifier::getMeidaPlayerService的实现：
- // establish binder interface to MediaPlayerService
- /*static*/const sp
& - IMediaDeathNotifier::getMediaPlayerService()
- {
- LOGV("getMediaPlayerService");
- Mutex::Autolock _l(sServiceLock);
- if (sMediaPlayerService.get() == 0) {
- sp
sm = defaultServiceManager(); - sp
binder; - do {
- binder = sm->getService(String16("media.player"));
- if (binder != 0) {
- break;
- }
- LOGW("Media player service not published, waiting...");
- usleep(500000); // 0.5 s
- } while(true);
- if (sDeathNotifier == NULL) {
- sDeathNotifier = new DeathNotifier();
- }
- binder->linkToDeath(sDeathNotifier);
- sMediaPlayerService = interface_cast
(binder); - }
- LOGE_IF(sMediaPlayerService == 0, "no media player service!?");
- return sMediaPlayerService;
- }
- sp
sm = defaultServiceManager();
- sp
sm = new BpServiceManager(new BpBinder(0));
接下去的while循环是通过sm->getService接口来不断尝试获得名称为“media.player”的Service，即MediaPlayerService。为什么要通过这无穷循环来得MediaPlayerService呢？因为这时候MediaPlayerService可能还没有启动起来，所以这里如果发现取回来的binder接口为NULL，就睡眠0.5秒，然后再尝试获取，这是获取Service接口的标准做法。
我们来看一下BpServiceManager::getService的实现：
- class BpServiceManager : public BpInterface
- {
- ......
- virtual sp
getService( const String16& name) const - {
- unsigned n;
- for (n = 0; n < 5; n++){
- sp
svc = checkService(name); - if (svc != NULL) return svc;
- LOGI("Waiting for service %s...\n", String8(name).string());
- sleep(1);
- }
- return NULL;
- }
- virtual sp
checkService( const String16& name) const - {
- Parcel data, reply;
- data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
- data.writeString16(name);
- remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
- return reply.readStrongBinder();
- }
- ......
- };
在BpServiceManager::checkService中，首先是通过Parcel::writeInterfaceToken往data写入一个RPC头，这个我们在Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析一文已经介绍过了，就是写往data里面写入了一个整数和一个字符串“android.os.IServiceManager”， Service Manager来处理CHECK_SERVICE_TRANSACTION请求之前，会先验证一下这个RPC头，看看是否正确。接着再往data写入一个字符串name，这里就是“media.player”了。回忆一下Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析这篇文章，那里已经往Service Manager中注册了一个名字为“media.player”的MediaPlayerService。
这里的remote()返回的是一个BpBinder，具体可以参考浅谈Android系统进程间通信（IPC）机制Binder中的Server和Client获得Service Manager接口之路一文，于是，就进行到BpBinder::transact函数了：
- 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;
- }
这里的mHandle = 0，code = CHECK_SERVICE_TRANSACTION，flags = 0。
这里再进入到IPCThread::transact函数中：
- 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;
- }
- 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;
- }
- writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
- writeString16("android.os.IServiceManager");
- writeString16("media.player");
- // Note: must be kept in sync with android/os/StrictMode.java's PENALTY_GATHER
- #define STRICT_MODE_PENALTY_GATHER 0x100
回到IPCThread::transact函数中，由于(flags & TF_ONE_WAY) == 0为true，即这是一个同步请求，并且reply != NULL，最终调用：
- err = waitForResponse(reply);
- 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;
- }
- ......
- // 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 defined(HAVE_ANDROID_OS)
- if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
- err = NO_ERROR;
- else
- err = -errno;
- #else
- err = INVALID_OPERATION;
- #endif
- ......
- } while (err == -EINTR);
- ......
- 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);
- }
- ......
- return NO_ERROR;
- }
- return err;
- }
- ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)
Binder驱动程序的binder_ioctl函数中，我们只关注BINDER_WRITE_READ命令相关的逻辑：
- 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;
- }
- ......
- default:
- ret = -EINVAL;
- goto err;
- }
- ret = 0;
- err:
- ......
- return ret;
- }
这里的thread本地变量表示当前线程上下文信息，通过binder_get_thread函数获得。在前面执行ProcessState构造函数时，也会通过ioctl文件操作函数进入到这个函数，那是第一次进入到binder_ioctl这里，因此，调用binder_get_thread时，表示当前进程上下文信息的proc变量还没有关于当前线程的上下文信息，因此，会为proc创建一个表示当前线程上下文信息的thread，会保存在proc->threads表示的红黑树结构中。这里调用binder_get_thread就可以直接从proc找到并返回了。
进入到BINDER_WRITE_READ相关的逻辑。先看看BINDER_WRITE_READ的定义：
- #define BINDER_WRITE_READ _IOWR('b', 1, struct binder_write_read)
- struct binder_write_read {
- signed long write_size; /* bytes to write */
- signed long write_consumed; /* bytes consumed by driver */
- unsigned long write_buffer;
- signed long read_size; /* bytes to read */
- signed long read_consumed; /* bytes consumed by driver */
- unsigned long read_buffer;
- };
从上面的调用过程，我们知道，这里bwr.write_size是大于0的，因此进入到binder_thread_write函数中，我们只关注BC_TRANSACTION相关的逻辑：
- 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;
- }
- ......
- default:
- printk(KERN_ERR "binder: %d:%d unknown command %d\n", proc->pid, thread->pid, cmd);
- return -EINVAL;
- }
- *consumed = ptr - buffer;
- }
- return 0;
- }
接着进入到binder_transaction函数中，不相关的代码我们忽略掉：
- 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 (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
- ......
- }
- }
- 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;
- }
- binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;
- tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
- if (tcomplete == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_alloc_tcomplete_failed;
- }
- binder_stats.obj_created[BINDER_STAT_TRANSACTION_COMPLETE]++;
- t->debug_id = ++binder_last_id;
- ......
- 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 (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;
- ......
- }
前面我们提到，传给驱动程序的handle值为0，即这里的tr->target.handle = 0，表示请求的目标Binder对象是Service Manager，因此有：
- target_node = binder_context_mgr_node;
- target_proc = target_node->proc;
- target_list = &target_proc->todo;
- target_wait = &target_proc->wait;
其中binder_context_mgr_node是在Service Manager通知Binder驱动程序它是守护过程时创建的。
接着创建一个待完成事项tcomplete，它的类型为struct binder_work，这是等一会要保存在当前线程的todo队列去的，表示当前线程有一个待完成的事务。紧跟着创建一个待处理事务t，它的类型为struct binder_transaction，这是等一会要存在到Service Manager的todo队列去的，表示Service Manager当前有一个事务需要处理。同时，这个待处理事务t也要存放在当前线程的待完成事务transaction_stack列表中去：
- t->from_parent = thread->transaction_stack;
- thread->transaction_stack = t;
继续往下看，就是分别把tcomplete和t放在当前线程thread和Service Manager进程的todo队列去了：
- 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);
- wake_up_interruptible(target_wait);
这里，我们暂时忽略Service Manager被唤醒之后的情景，继续看当前线程的执行。
函数binder_transaction执行完成之后，就一路返回到binder_ioctl函数里去了。函数binder_ioctl从binder_thread_write函数调用处返回后，发现bwr.read_size大于0，于是就进入到binder_thread_read函数去了：
- 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;
- }
回忆一下上面的binder_transaction函数，这里的thread->transaction_stack != NULL，并且thread->todo也不为空，所以线程不会进入休眠状态。
进入while循环中，首先是从thread->todo队列中取回待处理事项w，w的类型为BINDER_WORK_TRANSACTION_COMPLETE，这也是在binder_transaction函数里面设置的。对BINDER_WORK_TRANSACTION_COMPLETE的处理也很简单，只是把一个操作码BR_TRANSACTION_COMPLETE写回到用户传进来的缓冲区中去。这时候，用户传进来的缓冲区就包含两个操作码了，分别是BR_NOOP和BINDER_WORK_TRANSACTION_COMPLETE。
binder_thread_read执行完之后，返回到binder_ioctl函数中，将操作结果写回到用户空间中去：
- if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
- ret = -EFAULT;
- goto err;
- }
IPCThreadState::talkWithDriver函数从下面语句：
- ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)
- 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);
- }
这里属于后一种情况，于是再次通过IPCThreadState::talkWithDriver进入到Binder驱动程序的binder_ioctl函数中。不过这一次在binder_ioctl函数中，bwr.write_size等于0，而bwr.read_size大于0，于是再次进入到binder_thread_read函数中。这时候thread->transaction_stack仍然不为NULL，不过thread->todo队列已经为空了，因为前面我们已经处理过thread->todo队列的内容了，于是就通过下面语句：
- ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
现在，我们终于可以回到Service Manager被唤醒之后的过程了。前面我们说过，Service Manager此时正在binder_thread_read函数中休眠中：
- 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;
- t->saved_priority = task_nice(current);
- if (t->priority < target_node->min_priority &&
- !(t->flags & TF_ONE_WAY))
- binder_set_nice(t->priority);
- else if (!(t->flags & TF_ONE_WAY) ||
- t->saved_priority > target_node->min_priority)
- binder_set_nice(target_node->min_priority);
- 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.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 {
- ......
- }
- break;
- }
- done:
- *consumed = ptr - buffer;
- ......
- return 0;
- }
- ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
- t = container_of(w, struct binder_transaction, work);
- 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);
- 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 *));
- list_del(&t->work.entry);
- t->to_parent = thread->transaction_stack;
- t->to_thread = thread;
- thread->transaction_stack = t;
这样，binder_thread_read函数就处理完了，回到binder_ioctl函数中，同样是操作结果写回到用户空间的缓冲区中去：
- if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
- ret = -EFAULT;
- goto err;
- }
- 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;
- }
- }
- }
- res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
- 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++;
- switch(cmd) {
- ......
- case BR_TRANSACTION: {
- struct binder_txn *txn = (void *) ptr;
- ......
- 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;
- }
这里用到的两个数据结构struct binder_txn和struct binder_io可以参考前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，这里就不复述了。
接着往下看，函数调bio_init来初始化reply变量：
- 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;
- }
- 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;
- }
- 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;
- // LOGI("target=%p code=%d pid=%d uid=%d\n",
- // txn->target, txn->code, txn->sender_pid, txn->sender_euid);
- 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_GET_SERVICE:
- case SVC_MGR_CHECK_SERVICE:
- s = bio_get_string16(msg, &len);
- ptr = do_find_service(bs, s, len);
- if (!ptr)
- break;
- bio_put_ref(reply, ptr);
- return 0;
- ......
- }
- default:
- LOGE("unknown code %d\n", txn->code);
- return -1;
- }
- bio_put_uint32(reply, 0);
- return 0;
- }
回忆一下，在BpServiceManager::checkService时，传给Binder驱动程序的参数为：
- writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
- writeString16("android.os.IServiceManager");
- writeString16("media.player");
- strict_policy = bio_get_uint32(msg);
- s = bio_get_string16(msg, &len);
- s = bio_get_string16(msg, &len);
最后，就是调用do_find_service函数查找是存在名称为"media.player"的服务了。回忆一下前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，MediaPlayerService已经把一个名称为"media.player"的服务注册到Service Manager中，所以这里一定能找到。我们看看do_find_service这个函数：
- void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len)
- {
- struct svcinfo *si;
- si = find_svc(s, len);
- // LOGI("check_service('%s') ptr = %p\n", str8(s), si ? si->ptr : 0);
- if (si && si->ptr) {
- return si->ptr;
- } else {
- return 0;
- }
- }
- struct svcinfo *find_svc(uint16_t *s16, unsigned len)
- {
- struct svcinfo *si;
- for (si = svclist; si; si = si->next) {
- if ((len == si->len) &&
- !memcmp(s16, si->name, len * sizeof(uint16_t))) {
- return si;
- }
- }
- return 0;
- }
然后返回到do_find_service函数中。回忆一下前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析，这里的si->ptr就是指MediaPlayerService这个Binder实体在Service Manager进程中的句柄值了。
回到svcmgr_handler函数中，调用bio_put_ref函数将这个Binder引用写回到reply参数。我们看看bio_put_ref的实现：
- void bio_put_ref(struct binder_io *bio, void *ptr)
- {
- struct binder_object *obj;
- if (ptr)
- obj = bio_alloc_obj(bio);
- else
- obj = bio_alloc(bio, sizeof(*obj));
- if (!obj)
- return;
- obj->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
- obj->type = BINDER_TYPE_HANDLE;
- obj->pointer = ptr;
- obj->cookie = 0;
- }
再回到svcmgr_handler函数中，最后，还写入一个0值到reply缓冲区中，表示操作结果码：
- bio_put_uint32(reply, 0);
- 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));
- }
再来看binder_write函数：
- 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;
- }
这里又是一个ioctl的BINDER_WRITE_READ操作。直入到驱动程序的binder_ioctl函数后，执行BINDER_WRITE_READ命令，这里就不累述了。
最后，从binder_ioctl执行到binder_thread_write函数，首先是执行BC_FREE_BUFFER命令，这个命令的执行在前面一篇文章Android系统进程间通信（IPC）机制Binder中的Server启动过程源代码分析已经介绍过了，这里就不再累述了。
我们重点关注BC_REPLY命令的执行：
- 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;
- }
- 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;
- }
- ......
- thread->transaction_stack = in_reply_to->to_parent;
- target_thread = in_reply_to->from;
- ......
- 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;
- }
- binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;
- 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;
- }
- ......
- 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_HANDLE:
- case BINDER_TYPE_WEAK_HANDLE: {
- struct binder_ref *ref = binder_get_ref(proc, fp->handle);
- if (ref == NULL) {
- ......
- return_error = BR_FAILED_REPLY;
- goto err_binder_get_ref_failed;
- }
- if (ref->node->proc == target_proc) {
- ......
- } else {
- struct binder_ref *new_ref;
- new_ref = binder_get_ref_for_node(target_proc, ref->node);
- if (new_ref == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_binder_get_ref_for_node_failed;
- }
- fp->handle = new_ref->desc;
- binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
- ......
- }
- } break;
- ......
- }
- }
- 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;
- ......
- }
那么，这次是如何找到target_proc和target_thread呢。首先，我们注意到，这里的reply等于1，其次，上面我们提到，Binder驱动程序在唤醒Service Manager，告诉它有一个事务t要处理时，事务t虽然从Service Manager的todo队列中删除了，但是仍然保留在transaction_stack中。因此，这里可以从thread->transaction_stack找回这个等待回复的事务t，然后通过它找回target_proc和target_thread：
- in_reply_to = thread->transaction_stack;
- target_thread = in_reply_to->from;
- target_list = &target_thread->todo;
- target_wait = &target_thread->wait;
- struct binder_ref *ref = binder_get_ref(proc, fp->handle);
这里面的ref->node->proc不等于target_proc，因为这个Binder实体是属于创建MediaPlayerService的进程的，而不是请求这个服务的远程接口的进程的，因此，这里调用binder_get_ref_for_node函数为这个Binder实体在target_proc创建一个引用：
- struct binder_ref *new_ref;
- new_ref = binder_get_ref_for_node(target_proc, ref->node);
- binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
- fp->handle = new_ref->desc;
再往下看：
- 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 {
- ......
- }
再往后的逻辑就跟前面执行binder_transaction函数时候一样了，这里不再介绍。最后的结果就是唤醒请求SVC_MGR_CHECK_SERVICE操作的线程：
- if (target_wait)
- wake_up_interruptible(target_wait);
- list_add_tail(&tcomplete->entry, &thread->todo);
- 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;
- ......
- }
上面讲到调用请求SVC_MGR_CHECK_SERVICE操作的线程被唤醒了，于是，重新执行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;
- }
- ......
- 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;
- }
- ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
执行完这个函数后，就返回到binder_ioctl函数，执行下面语句，把数据返回给用户空间：
- if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
- ret = -EFAULT;
- goto err;
- }
- 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;
- }
- 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);
- void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize,
- const size_t* objects, size_t objectsCount, release_func relFunc, void* relCookie)
- {
- freeDataNoInit();
- mError = NO_ERROR;
- mData = const_cast
- mDataSize = mDataCapacity = dataSize;
- //LOGI("setDataReference Setting data size of %p to %lu (pid=%d)\n", this, mDataSize, getpid());
- mDataPos = 0;
- LOGV("setDataReference Setting data pos of %p to %d\n", this, mDataPos);
- mObjects = const_cast<size_t*>(objects);
- mObjectsSize = mObjectsCapacity = objectsCount;
- mNextObjectHint = 0;
- mOwner = relFunc;
- mOwnerCookie = relCookie;
- scanForFds();
- }
从这里层层返回，最后回到BpServiceManager::checkService函数中：
- virtual sp
BpServiceManager::checkService( const String16& name) const - {
- Parcel data, reply;
- data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
- data.writeString16(name);
- remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
- return reply.readStrongBinder();
- }
- remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
- sp
Parcel::readStrongBinder() const - {
- sp
val; - unflatten_binder(ProcessState::self(), *this, &val);
- return val;
- }
- status_t unflatten_binder(const sp
& proc, - const Parcel& in, sp
* out) - {
- const flat_binder_object* flat = in.readObject(false);
- if (flat) {
- switch (flat->type) {
- case BINDER_TYPE_BINDER:
- *out = static_cast
- return finish_unflatten_binder(NULL, *flat, in);
- case BINDER_TYPE_HANDLE:
- *out = proc->getStrongProxyForHandle(flat->handle);
- return finish_unflatten_binder(
- static_cast
- }
- }
- return BAD_TYPE;
- }
- sp
ProcessState::getStrongProxyForHandle(int32_t handle) - {
- sp
result; - AutoMutex _l(mLock);
- handle_entry* e = lookupHandleLocked(handle);
- if (e != NULL) {
- // We need to create a new BpBinder if there isn't currently one, OR we
- // are unable to acquire a weak reference on this current one. See comment
- // in getWeakProxyForHandle() for more info about this.
- IBinder* b = e->binder;
- if (b == NULL || !e->refs->attemptIncWeak(this)) {
- b = new BpBinder(handle);
- e->binder = b;
- if (b) e->refs = b->getWeakRefs();
- result = b;
- } else {
- // This little bit of nastyness is to allow us to add a primary
- // reference to the remote proxy when this team doesn't have one
- // but another team is sending the handle to us.
- result.force_set(b);
- e->refs->decWeak(this);
- }
- }
- return result;
- }
- b = new BpBinder(handle);
- e->binder = b;
- if (b) e->refs = b->getWeakRefs();
- result = b;
- binder = sm->getService(String16("media.player"));
- binder = new BpBinder(handle);
- sMediaPlayerService = interface_cast
(binder);
- android::sp
IMediaPlayerService::asInterface( const android::sp - {
- android::sp
intr; - if (obj != NULL) {
- intr = static_cast
- obj->queryLocalInterface(IMediaPlayerService::descriptor).get());
- if (intr == NULL) {
- intr = new BpMediaPlayerService(obj);
- }
- }
- return intr;
- }
- intr = new BpMediaPlayerService(new BpBinder(handle));
有了这个BpMediaPlayerService这个远程接口之后，MediaPlayer就可以调用MediaPlayerService的服务了。
至此，Android系统进程间通信（IPC）机制Binder中的Client如何通过Service Manager的getService函数获得Server远程接口的过程就分析完了，Binder机制的学习就暂告一段落了。
不过，细心的读者可能会发现，我们这里介绍的Binder机制都是基于C/C++语言实现的，但是我们在编写应用程序都是基于Java语言的，那么，我们如何使用Java语言来使用系统的Binder机制来进行进程间通信呢？这就是下一篇文章要介绍的内容了，敬请关注。