Android IPC机制详解

文章出处：http://www.limodev.cn/blog

http://blog.csdn.net/stevenliyong/archive/2009/08/27/4489525.aspx

IBinder接口

IBinder接口是对跨进程的对象的抽象。普通对象在当前进程可以访问，如果希望对象能被其它进程访问，那就必须实现IBinder接口。IBinder接口可以指向本地对象，也可以指向远程对象，调用者不需要关心指向的对象是本地的还是远程。

transact是IBinder接口中一个比较重要的函数，它的函数原型如下：

 

virtual status_t transact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags = 0) = 0;

android中的IPC的基本模型是基于客户/服务器(C/S)架构的。

 

客户端请求通过内核模块中转服务端

如果IBinder指向的是一个客户端代理，那transact只是把请求发送给服务器。服务端的IBinder的transact则提供了实际的服务。

o 客户端

BpBinder是远程对象在当前进程的代理，它实现了IBinder接口。它的transact函数实现如下：

 

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;
}

参数说明：

code 是请求的ID号。

data 是请求的参数。

reply 是返回的结果。

flags 一些额外的标识，如FLAG_ONEWAY。通常为0。

transact只是简单的调用了IPCThreadState::self()的transact，在IPCThreadState::transact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 (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }

        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::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(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t),
                            freeBuffer, this);
                    } else {
                        err = *static_cast(tr.data.ptr.buffer);
                        freeBuffer(NULL,
                            reinterpret_cast(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t), this);
                    }
                } else {
                    freeBuffer(NULL,
                        reinterpret_cast(tr.data.ptr.buffer),
                        tr.data_size,
                        reinterpret_cast(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;
}

这里transact把请求经内核模块发送了给服务端，服务端处理完请求之后，沿原路返回结果给调用者。这里也可以看出请求是同步操作，它会等待直到结果返回为止。

在BpBinder之上进行简单包装，我们可以得到与服务对象相同的接口，调用者无需要关心调用的对象是远程的还是本地的。拿ServiceManager来说：
(frameworks/base/libs/utils/IServiceManager.cpp)

 

class BpServiceManager : public BpInterface
{
public:
    BpServiceManager(const sp& impl)
        : BpInterface(impl)
    {
    }
...
    virtual status_t addService(const String16& name, const sp& 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.readInt32() : err;
    }
...
};

BpServiceManager实现了 IServiceManager和IBinder两个接口，调用者可以把BpServiceManager的对象看作是一个 IServiceManager对象或者IBinder对象。当调用者把BpServiceManager对象当作IServiceManager对象使用时，所有的请求只是对BpBinder::transact的封装。这样的封装使得调用者不需要关心IServiceManager对象是本地的还是远程的了。

客户通过defaultServiceManager函数来创建BpServiceManager对象：
(frameworks/base/libs/utils/IServiceManager.cpp)

 

sp<IServiceManager> defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager; 

    {
        AutoMutex _l(gDefaultServiceManagerLock);
        if (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
        }
    } 

    return gDefaultServiceManager;
}

先通过ProcessState::self()->getContextObject(NULL)创建一个Binder对象，然后通过 interface_cast和IMPLEMENT_META_INTERFACE(ServiceManager, “android.os.IServiceManager”)把Binder对象包装成 IServiceManager对象。原理上等同于创建了一个BpServiceManager对象。

ProcessState::self()->getContextObject调用ProcessState::getStrongProxyForHandle创建代理对象：

 

sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp<IBinder> 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;
}

如果handle为空，默认为context_manager对象，context_manager实际上就是ServiceManager。
o 服务端
服务端也要实现IBinder接口，BBinder类对IBinder接口提供了部分默认实现，其中transact的实现如下：

 

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;
}

PING_TRANSACTION请求用来检查对象是否还存在，这里简单的把 pingBinder的返回值返回给调用者。其它的请求交给onTransact处理。onTransact是BBinder里声明的一个 protected类型的虚函数，这个要求它的子类去实现。比如CameraService里的实现如下：

 

status_t CameraService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // permission checks...
    switch (code) {
        case BnCameraService::CONNECT:
            IPCThreadState* ipc = IPCThreadState::self();
            const int pid = ipc->getCallingPid();
            const int self_pid = getpid();
            if (pid != self_pid) {
                // we're called from a different process, do the real check
                if (!checkCallingPermission(
                        String16("android.permission.CAMERA")))
                {
                    const int uid = ipc->getCallingUid();
                    LOGE("Permission Denial: "
                            "can't use the camera pid=%d, uid=%d", pid, uid);
                    return PERMISSION_DENIED;
                }
            }
            break;
    }

    status_t err = BnCameraService::onTransact(code, data, reply, flags);

    LOGD("+++ onTransact err %d code %d", err, code);

    if (err == UNKNOWN_TRANSACTION || err == PERMISSION_DENIED) {
        // the 'service' command interrogates this binder for its name, and then supplies it
        // even for the debugging commands.  that means we need to check for it here, using
        // ISurfaceComposer (since we delegated the INTERFACE_TRANSACTION handling to
        // BnSurfaceComposer before falling through to this code).

        LOGD("+++ onTransact code %d", code);

        CHECK_INTERFACE(ICameraService, data, reply);

        switch(code) {
        case 1000:
        {
            if (gWeakHeap != 0) {
                sp h = gWeakHeap.promote();
                IMemoryHeap *p = gWeakHeap.unsafe_get();
                LOGD("CHECKING WEAK REFERENCE %p (%p)", h.get(), p);
                if (h != 0)
                    h->printRefs();
                bool attempt_to_delete = data.readInt32() == 1;
                if (attempt_to_delete) {
                    // NOT SAFE!
                    LOGD("DELETING WEAK REFERENCE %p (%p)", h.get(), p);
                    if (p) delete p;
                }
                return NO_ERROR;
            }
        }
        break;
        default:
            break;
        }
    }
    return err;
}

由此可见，服务端的onTransact是一个请求分发函数，它根据请求码(code)做相应的处理。

o 消息循环

服务端(任何进程都可以作为服务端)有一个线程监听来自客户端的请求，并循环处理这些请求。

如果在主线程中处理请求，可以直接调用下面的函数：

 

IPCThreadState::self()->joinThreadPool(mIsMain);

如果想在非主线程中处理请求，可以按下列方式：

 

        sp
 proc = ProcessState::self();
        if (proc->supportsProcesses()) {
            LOGV("App process: starting thread pool./n");
            proc->startThreadPool();
        }

startThreadPool的实现原理：

 

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

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
 t = new PoolThread(isMain);
        t->run(buf);
    }
}

这里创建了PoolThread的对象，实现上就是创建了一个线程。所有的线程类都要实现threadLoop虚函数。PoolThread的threadLoop的实现如下：

 

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

上述代码，简而言之就是创建了一个线程，然后在线程里调用 IPCThreadState::self()->joinThreadPool函数。

下面再看joinThreadPool的实现：

 

do
{
...
        result = talkWithDriver();
        if (result >= NO_ERROR) {
            size_t IN = mIn.dataAvail();
            if (IN < sizeof(int32_t)) continue;
            cmd = mIn.readInt32();
            IF_LOG_COMMANDS() {
                alog << "Processing top-level Command: "
                    << getReturnString(cmd) << endl;
            }
            result = executeCommand(cmd);
        }
...
while(...);

这个函数在循环中重复执行下列动作：

1. talkWithDriver 通过ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)读取请求和写回结果。
2. executeCommand 执行相应的请求

在IPCThreadState::executeCommand(int32_t cmd)函数中：

1. 对于控制对象生命周期的请求，像BR_ACQUIRE/BR_RELEASE直接做了处理。
2. 对于BR_TRANSACTION请求，它调用被请求对象的transact函数。

按下列方式调用实际的对象：

 

if (tr.target.ptr) {
    sp<BBinder> b((BBinder*)tr.cookie);
    const status_t error = b->transact(tr.code, buffer, &reply, 0);
    if (error < NO_ERROR) reply.setError(error); 

} else {
    const status_t error = the_context_object->transact(tr.code, buffer, &reply, 0);
    if (error < NO_ERROR) reply.setError(error);
}

如果tr.target.ptr不为空，就把tr.cookie转换成一个Binder对象，并调用它的transact函数。如果没有目标对象，就调用 the_context_object对象的transact函数。奇怪的是，根本没有谁对the_context_object进行初始化，the_context_object是空指针。原因是context_mgr的请求发给了ServiceManager，所以根本不会走到else 语句里来。

o 内核模块

android使用了一个内核模块binder来中转各个进程之间的消息。模块源代码放在binder.c里，它是一个字符驱动程序，主要通过 binder_ioctl与用户空间的进程交换数据。其中BINDER_WRITE_READ用来读写数据，数据包中有一个cmd域用于区分不同的请求：

1. binder_thread_write用于发送请求或返回结果。
2. binder_thread_read用于读取结果。

从binder_thread_write中调用binder_transaction中转请求和返回结果，binder_transaction的实现如下：

对请求的处理：

1. 通过对象的handle找到对象所在的进程，如果handle为空就认为对象是context_mgr，把请求发给context_mgr所在的进程。
2. 把请求中所有的binder对象全部放到一个RB树中。
3. 把请求放到目标进程的队列中，等待目标进程读取。

如何成为context_mgr呢？内核模块提供了BINDER_SET_CONTEXT_MGR调用:

 

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
...
case BINDER_SET_CONTEXT_MGR:
if (binder_context_mgr_node != NULL) {
printk(KERN_ERR "binder: BINDER_SET_CONTEXT_MGR already set/n");
ret = -EBUSY;
goto err;
}
if (binder_context_mgr_uid != -1) {
if (binder_context_mgr_uid != current->euid) {
printk(KERN_ERR "binder: BINDER_SET_"
       "CONTEXT_MGR bad uid %d != %d/n",
       current->euid,
       binder_context_mgr_uid);
ret = -EPERM;
goto err;
}
} else
binder_context_mgr_uid = current->euid;
binder_context_mgr_node = binder_new_node(proc, NULL, NULL);
if (binder_context_mgr_node == NULL) {
ret = -ENOMEM;
goto err;
}
binder_context_mgr_node->local_weak_refs++;
binder_context_mgr_node->local_strong_refs++;
binder_context_mgr_node->has_strong_ref = 1;
binder_context_mgr_node->has_weak_ref = 1;
break;

ServiceManager（frameworks/base/cmds/servicemanager)通过下列方式成为了context_mgr进程：

 

int binder_become_context_manager(struct binder_state *bs)
{
    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}

int main(int argc, char **argv)
{
    struct binder_state *bs;
    void *svcmgr = BINDER_SERVICE_MANAGER;

    bs = binder_open(128*1024);

    if (binder_become_context_manager(bs)) {
        LOGE("cannot become context manager (%s)/n", strerror(errno));
        return -1;
    }

    svcmgr_handle = svcmgr;
    binder_loop(bs, svcmgr_handler);
    return 0;
}

o 如何得到服务对象的handle

1. 服务提供者通过defaultServiceManager得到ServiceManager对象，然后调用addService向服务管理器注册。
2. 服务使用者通过defaultServiceManager得到ServiceManager对象，然后调用getService通过服务名称查找到服务对象的handle。

o 如何通过服务对象的handle找到服务所在的进程

0表示服务管理器的handle，getService可以查找到系统服务的handle。这个handle只是代表了服务对象，内核模块是如何通过handle找到服务所在的进程的呢？

1. 对于ServiceManager: ServiceManager调用了binder_become_context_manager使用自己成为context_mgr，所有handle为0的请求都会被转发给ServiceManager。
2. 对于系统服务和应用程序的Listener，在第一次请求内核模块时(比如调用addService)，内核模块在一个RB树中建立了服务对象和进程的对应关系。
   

   off_end = (void *)offp + tr->offsets_size;
   for (; offp < off_end; offp++) {
   struct flat_binder_object *fp;
   if (*offp > t->buffer->data_size - sizeof(*fp)) {
   binder_user_error("binder: %d:%d got transaction with "
   "invalid offset, %d/n",
   proc->pid, thread->pid, *offp);
   return_error = BR_FAILED_REPLY;
   goto err_bad_offset;
   }
   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) {
   binder_user_error("binder: %d:%d sending u%p "
   "node %d, cookie mismatch %p != %p/n",
   proc->pid, thread->pid,
   fp->binder, node->debug_id,
   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);
   if (binder_debug_mask & BINDER_DEBUG_TRANSACTION)
   printk(KERN_INFO "        node %d u%p -> ref %d desc %d/n",
          node->debug_id, node->ptr, ref->debug_id, ref->desc);
   } break;

3. 请求服务时，内核先通过handle找到对应的进程，然后把请求放到服务进程的队列中。

o C调用JAVA

前面我们分析的是C代码的处理。对于JAVA代码，JAVA调用C的函数通过JNI调用即可。从内核时读取请求是在C代码(executeCommand)里进行了，那如何在C代码中调用那些用JAVA实现的服务呢？

android_os_Binder_init里的JavaBBinder对Java里的Binder对象进行包装。

JavaBBinder::onTransact调用Java里的execTransact函数：

 

        jboolean res = env->CallBooleanMethod(mObject, gBinderOffsets.mExecTransact,
            code, (int32_t)&data, (int32_t)reply, flags);
        jthrowable excep = env->ExceptionOccurred();
        if (excep) {
            report_exception(env, excep,
                "*** Uncaught remote exception!  "
                "(Exceptions are not yet supported across processes.)");
            res = JNI_FALSE; 

            /* clean up JNI local ref -- we don't return to Java code */
            env->DeleteLocalRef(excep);
        }

o 广播消息

binder不提供广播消息，不过可以ActivityManagerService服务来实现广播。
(frameworks/base/core/java/android/app/ActivityManagerNative.java)

接收广播消息需要实现接口BroadcastReceiver，然后调用ActivityManagerProxy::registerReceiver注册。

触发广播调用ActivityManagerProxy::broadcastIntent。(应用程序并不直接调用它，而是调用Context对它的包装)