这个函数的实现很简单，就是把MediaPlayerService这个Binder实体的引用写到一个struct svcinfo结构体中，主要是它的名称和句柄值，然后插入到链接svclist的头部去。这样，Client来向Service Manager查询服务接口时，只要给定服务名称，Service Manger就可以返回相应的句柄值了。
这个函数执行完成后，返回到svcmgr_handler函数，函数的最后，将一个错误码0写到reply变量中去，表示一切正常：
- bio_put_uint32(reply, 0);
svcmgr_handler函数执行完成后，返回到binder_parse函数，执行下面语句：
- binder_send_reply(bs, &reply, txn->data, res);
我们看一下binder_send_reply的实现，从函数名就可以猜到它要做什么了，告诉Binder驱动程序，它完成了Binder驱动程序交给它的任务了。
- void binder_send_reply(struct binder_state *bs,
- struct binder_io *reply,
- void *buffer_to_free,
- int status)
- {
- struct {
- uint32_t cmd_free;
- void *buffer;
- uint32_t cmd_reply;
- struct binder_txn txn;
- } __attribute__((packed)) data;
- data.cmd_free = BC_FREE_BUFFER;
- data.buffer = buffer_to_free;
- data.cmd_reply = BC_REPLY;
- data.txn.target = 0;
- data.txn.cookie = 0;
- data.txn.code = 0;
- if (status) {
- data.txn.flags = TF_STATUS_CODE;
- data.txn.data_size = sizeof(int);
- data.txn.offs_size = 0;
- data.txn.data = &status;
- data.txn.offs = 0;
- } else {
- data.txn.flags = 0;
- data.txn.data_size = reply->data - reply->data0;
- data.txn.offs_size = ((char*) reply->offs) - ((char*) reply->offs0);
- data.txn.data = reply->data0;
- data.txn.offs = reply->offs0;
- }
- binder_write(bs, &data, sizeof(data));
- }
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MediaPlayerService，它的addService操作已经完成了，错误码是0，保存在data.txn.data中。
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，我们首先看第一个命令BC_FREE_BUFFER：
- int
- binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
- void __user *buffer, int size, signed long *consumed)
- {
- uint32_t cmd;
- void __user *ptr = buffer + *consumed;
- void __user *end = buffer + size;
- while (ptr < end && thread->return_error == BR_OK) {
- if (get_user(cmd, (uint32_t __user *)ptr))
- return -EFAULT;
- ptr += sizeof(uint32_t);
- if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
- binder_stats.bc[_IOC_NR(cmd)]++;
- proc->stats.bc[_IOC_NR(cmd)]++;
- thread->stats.bc[_IOC_NR(cmd)]++;
- }
- switch (cmd) {
- ......
- case BC_FREE_BUFFER: {
- void __user *data_ptr;
- struct binder_buffer *buffer;
- if (get_user(data_ptr, (void * __user *)ptr))
- return -EFAULT;
- ptr += sizeof(void *);
- buffer = binder_buffer_lookup(proc, data_ptr);
- if (buffer == NULL) {
- binder_user_error("binder: %d:%d "
- "BC_FREE_BUFFER u%p no match\n",
- proc->pid, thread->pid, data_ptr);
- break;
- }
- if (!buffer->allow_user_free) {
- binder_user_error("binder: %d:%d "
- "BC_FREE_BUFFER u%p matched "
- "unreturned buffer\n",
- proc->pid, thread->pid, data_ptr);
- break;
- }
- if (binder_debug_mask & BINDER_DEBUG_FREE_BUFFER)
- printk(KERN_INFO "binder: %d:%d BC_FREE_BUFFER u%p found buffer %d for %s transaction\n",
- proc->pid, thread->pid, data_ptr, buffer->debug_id,
- buffer->transaction ? "active" : "finished");
- if (buffer->transaction) {
- buffer->transaction->buffer = NULL;
- buffer->transaction = NULL;
- }
- if (buffer->async_transaction && buffer->target_node) {
- BUG_ON(!buffer->target_node->has_async_transaction);
- if (list_empty(&buffer->target_node->async_todo))
- buffer->target_node->has_async_transaction = 0;
- else
- list_move_tail(buffer->target_node->async_todo.next, &thread->todo);
- }
- binder_transaction_buffer_release(proc, buffer, NULL);
- binder_free_buf(proc, buffer);
- break;
- }
- ......
- *consumed = ptr - buffer;
- }
- return 0;
- }
首先通过看这个语句：
- get_user(data_ptr, (void * __user *)ptr)
这个是获得要删除的Buffer的用户空间地址，接着通过下面这个语句来找到这个地址对应的struct binder_buffer信息：
- buffer = binder_buffer_lookup(proc, data_ptr);
因为这个空间是前面在binder_transaction里面分配的，所以这里一定能找到。
最后，就可以释放这块内存了：
- binder_transaction_buffer_release(proc, buffer, NULL);
- binder_free_buf(proc, buffer);
再来看另外一个命令BC_REPLY：
- int
- binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
- void __user *buffer, int size, signed long *consumed)
- {
- uint32_t cmd;
- void __user *ptr = buffer + *consumed;
- void __user *end = buffer + size;
- while (ptr < end && thread->return_error == BR_OK) {
- if (get_user(cmd, (uint32_t __user *)ptr))
- return -EFAULT;
- ptr += sizeof(uint32_t);
- if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
- binder_stats.bc[_IOC_NR(cmd)]++;
- proc->stats.bc[_IOC_NR(cmd)]++;
- thread->stats.bc[_IOC_NR(cmd)]++;
- }
- switch (cmd) {
- ......
- case BC_TRANSACTION:
- case BC_REPLY: {
- struct binder_transaction_data tr;
- if (copy_from_user(&tr, ptr, sizeof(tr)))
- return -EFAULT;
- ptr += sizeof(tr);
- binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
- break;
- }
- ......
- *consumed = ptr - buffer;
- }
- return 0;
- }
又再次进入到binder_transaction函数：
- static void
- binder_transaction(struct binder_proc *proc, struct binder_thread *thread,
- struct binder_transaction_data *tr, int reply)
- {
- struct binder_transaction *t;
- struct binder_work *tcomplete;
- size_t *offp, *off_end;
- struct binder_proc *target_proc;
- struct binder_thread *target_thread = NULL;
- struct binder_node *target_node = NULL;
- struct list_head *target_list;
- wait_queue_head_t *target_wait;
- struct binder_transaction *in_reply_to = NULL;
- struct binder_transaction_log_entry *e;
- uint32_t return_error;
- ......
- if (reply) {
- in_reply_to = thread->transaction_stack;
- if (in_reply_to == NULL) {
- ......
- return_error = BR_FAILED_REPLY;
- goto err_empty_call_stack;
- }
- binder_set_nice(in_reply_to->saved_priority);
- if (in_reply_to->to_thread != thread) {
- .......
- goto err_bad_call_stack;
- }
- thread->transaction_stack = in_reply_to->to_parent;
- target_thread = in_reply_to->from;
- if (target_thread == NULL) {
- return_error = BR_DEAD_REPLY;
- goto err_dead_binder;
- }
- if (target_thread->transaction_stack != in_reply_to) {
- ......
- return_error = BR_FAILED_REPLY;
- in_reply_to = NULL;
- target_thread = NULL;
- goto err_dead_binder;
- }
- target_proc = target_thread->proc;
- } else {
- ......
- }
- if (target_thread) {
- e->to_thread = target_thread->pid;
- target_list = &target_thread->todo;
- target_wait = &target_thread->wait;
- } else {
- ......
- }
- /* TODO: reuse incoming transaction for reply */
- t = kzalloc(sizeof(*t), GFP_KERNEL);
- if (t == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_alloc_t_failed;
- }
- tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
- if (tcomplete == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_alloc_tcomplete_failed;
- }
- if (!reply && !(tr->flags & TF_ONE_WAY))
- t->from = thread;
- else
- t->from = NULL;
- t->sender_euid = proc->tsk->cred->euid;
- t->to_proc = target_proc;
- t->to_thread = target_thread;
- t->code = tr->code;
- t->flags = tr->flags;
- t->priority = task_nice(current);
- t->buffer = binder_alloc_buf(target_proc, tr->data_size,
- tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
- if (t->buffer == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_binder_alloc_buf_failed;
- }
- t->buffer->allow_user_free = 0;
- t->buffer->debug_id = t->debug_id;
- t->buffer->transaction = t;
- t->buffer->target_node = target_node;
- if (target_node)
- binder_inc_node(target_node, 1, 0, NULL);
- offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));
- if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
- binder_user_error("binder: %d:%d got transaction with invalid "
- "data ptr\n", proc->pid, thread->pid);
- return_error = BR_FAILED_REPLY;
- goto err_copy_data_failed;
- }
- if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
- binder_user_error("binder: %d:%d got transaction with invalid "
- "offsets ptr\n", proc->pid, thread->pid);
- return_error = BR_FAILED_REPLY;
- goto err_copy_data_failed;
- }
- ......
- if (reply) {
- BUG_ON(t->buffer->async_transaction != 0);
- binder_pop_transaction(target_thread, in_reply_to);
- } else if (!(t->flags & TF_ONE_WAY)) {
- ......
- } else {
- ......
- }
- t->work.type = BINDER_WORK_TRANSACTION;
- list_add_tail(&t->work.entry, target_list);
- tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
- list_add_tail(&tcomplete->entry, &thread->todo);
- if (target_wait)
- wake_up_interruptible(target_wait);
- return;
- ......
- }
注意，这里的reply为1，我们忽略掉其它无关代码。
前面Service Manager正在binder_thread_read函数中被MediaPlayerService启动后进程唤醒后，在最后会把当前处理完的事务放在thread->transaction_stack中：
- if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
- t->to_parent = thread->transaction_stack;
- t->to_thread = thread;
- thread->transaction_stack = t;
- }
所以，这里，首先是把它这个binder_transaction取回来，并且放在本地变量in_reply_to中：
- in_reply_to = thread->transaction_stack;
接着就可以通过in_reply_to得到最终发出这个事务请求的线程和进程：
- target_thread = in_reply_to->from;
- target_proc = target_thread->proc;
然后得到target_list和target_wait：
- target_list = &target_thread->todo;
- target_wait = &target_thread->wait;
下面这一段代码：
- /* TODO: reuse incoming transaction for reply */
- t = kzalloc(sizeof(*t), GFP_KERNEL);
- if (t == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_alloc_t_failed;
- }
- tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
- if (tcomplete == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_alloc_tcomplete_failed;
- }
- if (!reply && !(tr->flags & TF_ONE_WAY))
- t->from = thread;
- else
- t->from = NULL;
- t->sender_euid = proc->tsk->cred->euid;
- t->to_proc = target_proc;
- t->to_thread = target_thread;
- t->code = tr->code;
- t->flags = tr->flags;
- t->priority = task_nice(current);
- t->buffer = binder_alloc_buf(target_proc, tr->data_size,
- tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
- if (t->buffer == NULL) {
- return_error = BR_FAILED_REPLY;
- goto err_binder_alloc_buf_failed;
- }
- t->buffer->allow_user_free = 0;
- t->buffer->debug_id = t->debug_id;
- t->buffer->transaction = t;
- t->buffer->target_node = target_node;
- if (target_node)
- binder_inc_node(target_node, 1, 0, NULL);
- offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));
- if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
- binder_user_error("binder: %d:%d got transaction with invalid "
- "data ptr\n", proc->pid, thread->pid);
- return_error = BR_FAILED_REPLY;
- goto err_copy_data_failed;
- }
- if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
- binder_user_error("binder: %d:%d got transaction with invalid "
- "offsets ptr\n", proc->pid, thread->pid);
- return_error = BR_FAILED_REPLY;
- goto err_copy_data_failed;
- }
我们在前面已经分析过了，这里不再重复。但是有一点要注意的是，这里target_node为NULL，因此，t->buffer->target_node也为NULL。
函数本来有一个for循环，用来处理数据中的Binder对象，这里由于没有Binder对象，所以就略过了。到了下面这句代码：
- binder_pop_transaction(target_thread, in_reply_to);
我们看看做了什么事情
- static void
- binder_pop_transaction(
- struct binder_thread *target_thread, struct binder_transaction *t)
- {
- if (target_thread) {
- BUG_ON(target_thread->transaction_stack != t);
- BUG_ON(target_thread->transaction_stack->from != target_thread);
- target_thread->transaction_stack =
- target_thread->transaction_stack->from_parent;
- t->from = NULL;
- }
- t->need_reply = 0;
- if (t->buffer)
- t->buffer->transaction = NULL;
- kfree(t);
- binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;
- }
由于到了这里，已经不需要in_reply_to这个transaction了，就把它删掉。
回到binder_transaction函数：
- t->work.type = BINDER_WORK_TRANSACTION;
- list_add_tail(&t->work.entry, target_list);
- tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
- list_add_tail(&tcomplete->entry, &thread->todo);
和前面一样，分别把t和tcomplete分别放在target_list和thread->todo队列中，这里的target_list指的就是最初调用IServiceManager::addService的MediaPlayerService的Server主线程的的thread->todo队列了，而thread->todo指的是Service Manager中用来回复IServiceManager::addService请求的线程。
最后，唤醒等待在target_wait队列上的线程了，就是最初调用IServiceManager::addService的MediaPlayerService的Server主线程了，它最后在binder_thread_read函数中睡眠在thread->wait上，就是这里的target_wait了：
- if (target_wait)
- wake_up_interruptible(target_wait);
这样，Service Manger回复调用IServiceManager::addService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：
- list_add_tail(&tcomplete->entry, &thread->todo);
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：
- switch (w->type) {
- case BINDER_WORK_TRANSACTION_COMPLETE: {
- cmd = BR_TRANSACTION_COMPLETE;
- if (put_user(cmd, (uint32_t __user *)ptr))
- return -EFAULT;
- ptr += sizeof(uint32_t);
- list_del(&w->entry);
- kfree(w);
- } break;
- ......
- }
binder_loop函数执行完这个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IServiceManager::addService的MediaPlayerService的Server主线程被唤醒了，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;
- }
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反回了一个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返回码了。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：
- if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
- ret = -EFAULT;
- goto err;
- }
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：
- status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
- {
- int32_t cmd;
- int32_t err;
- while (1) {
- if ((err=talkWithDriver()) < NO_ERROR) break;
- ......
- cmd = mIn.readInt32();
- ......
- switch (cmd) {
- ......
- case BR_REPLY:
- {
- binder_transaction_data tr;
- err = mIn.read(&tr, sizeof(tr));
- LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");
- if (err != NO_ERROR) goto finish;
- if (reply) {
- if ((tr.flags & TF_STATUS_CODE) == 0) {
- reply->ipcSetDataReference(
- reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
- tr.data_size,
- reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
- tr.offsets_size/sizeof(size_t),
- freeBuffer, this);
- } else {
- ......
- }
- } else {
- ......
- }
- }
- goto finish;
- ......
- }
- }
- finish:
- ......
- return err;
- }
注意，这里的tr.flags等于0，这个是在上面的binder_send_reply函数里设置的。最终把结果保存在reply了：
- reply->ipcSetDataReference(
- reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
- tr.data_size,
- reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
- tr.offsets_size/sizeof(size_t),
- freeBuffer, this);
这个函数我们就不看了，有兴趣的读者可以研究一下。
从这里层层返回，最后回到MediaPlayerService::instantiate函数中。
至此，IServiceManager::addService终于执行完毕了。这个过程非常复杂，但是如果我们能够深刻地理解这一过程，将能很好地理解Binder机制的设计思想和实现过程。这里，对IServiceManager::addService过程中MediaPlayerService、ServiceManager和BinderDriver之间的交互作一个小结：
回到frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函数，接下去还要执行下面两个函数：
- ProcessState::self()->startThreadPool();
- IPCThreadState::self()->joinThreadPool();
首先看ProcessState::startThreadPool函数的实现：
- void ProcessState::startThreadPool()
- {
- AutoMutex _l(mLock);
- if (!mThreadPoolStarted) {
- mThreadPoolStarted = true;
- spawnPooledThread(true);
- }
- }
这里调用spwanPooledThread：
- void ProcessState::spawnPooledThread(bool isMain)
- {
- if (mThreadPoolStarted) {
- int32_t s = android_atomic_add(1, &mThreadPoolSeq);
- char buf[32];
- sprintf(buf, "Binder Thread #%d", s);
- LOGV("Spawning new pooled thread, name=%s\n", buf);
- sp<Thread> t = new PoolThread(isMain);
- t->run(buf);
- }
- }
这里主要是创建一个线程，PoolThread继续Thread类，Thread类定义在frameworks/base/libs/utils/Threads.cpp文件中，其run函数最终调用子类的threadLoop函数，这里即为PoolThread::threadLoop函数：
- virtual bool threadLoop()
- {
- IPCThreadState::self()->joinThreadPool(mIsMain);
- return false;
- }
这里和frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函数一样，最终都是调用了IPCThreadState::joinThreadPool函数，它们的区别是，一个参数是true，一个是默认值false。我们来看一下这个函数的实现：
- void IPCThreadState::joinThreadPool(bool isMain)
- {
- LOG_THREADPOOL("**** THREAD %p (PID %d) IS JOINING THE THREAD POOL\n", (void*)pthread_self(), getpid());
- mOut.writeInt32(isMain ? BC_ENTER_LOOPER : BC_REGISTER_LOOPER);
- ......
- status_t result;
- do {
- int32_t cmd;
- .......
- // now get the next command to be processed, waiting if necessary
- result = talkWithDriver();
- if (result >= NO_ERROR) {
- size_t IN = mIn.dataAvail();
- if (IN < sizeof(int32_t)) continue;
- cmd = mIn.readInt32();
- ......
- }
- result = executeCommand(cmd);
- }
- ......
- } while (result != -ECONNREFUSED && result != -EBADF);
- .......
- mOut.writeInt32(BC_EXIT_LOOPER);
- talkWithDriver(false);
- }
这个函数最终是在一个无穷循环中，通过调用talkWithDriver函数来和Binder驱动程序进行交互，实际上就是调用talkWithDriver来等待Client的请求，然后再调用executeCommand来处理请求，而在executeCommand函数中，最终会调用BBinder::transact来真正处理Client的请求：
- status_t IPCThreadState::executeCommand(int32_t cmd)
- {
- BBinder* obj;
- RefBase::weakref_type* refs;
- status_t result = NO_ERROR;
- switch (cmd) {
- ......
- case BR_TRANSACTION:
- {
- binder_transaction_data tr;
- result = mIn.read(&tr, sizeof(tr));
- ......
- Parcel reply;
- ......
- if (tr.target.ptr) {
- sp<BBinder> b((BBinder*)tr.cookie);
- const status_t error = b->transact(tr.code, buffer, &reply, tr.flags);
- if (error < NO_ERROR) reply.setError(error);
- } else {
- const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);
- if (error < NO_ERROR) reply.setError(error);
- }
- ......
- }
- break;
- .......
- }
- if (result != NO_ERROR) {
- mLastError = result;
- }
- return result;
- }
接下来再看一下BBinder::transact的实现：
- status_t BBinder::transact(
- uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
- {
- data.setDataPosition(0);
- status_t err = NO_ERROR;
- switch (code) {
- case PING_TRANSACTION:
- reply->writeInt32(pingBinder());
- break;
- default:
- err = onTransact(code, data, reply, flags);
- break;
- }
- if (reply != NULL) {
- reply->setDataPosition(0);
- }
- return err;
- }
最终会调用onTransact函数来处理。在这个场景中，BnMediaPlayerService继承了BBinder类，并且重载了onTransact函数，因此，这里实际上是调用了BnMediaPlayerService::onTransact函数，这个函数定义在frameworks/base/libs/media/libmedia/IMediaPlayerService.cpp文件中：
- status_t BnMediaPlayerService::onTransact(
- uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
- {
- switch(code) {
- case CREATE_URL: {
- ......
- } break;
- case CREATE_FD: {
- ......
- } break;
- case DECODE_URL: {
- ......
- } break;
- case DECODE_FD: {
- ......
- } break;
- case CREATE_MEDIA_RECORDER: {
- ......
- } break;
- case CREATE_METADATA_RETRIEVER: {
- ......
- } break;
- case GET_OMX: {
- ......
- } break;
- default:
- return BBinder::onTransact(code, data, reply, flags);
- }
- }
至此，我们就以MediaPlayerService为例，完整地介绍了Android系统进程间通信Binder机制中的Server启动过程。Server启动起来之后，就会在一个无穷循环中等待Client的请求了。在下一篇文章中，我们将介绍Client如何通过Service Manager远程接口来获得Server远程接口，进而调用Server远程接口来使用Server提供的服务，敬请关注。
