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。
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<imediaplayerservice>类，即bpmediaplayerservice继承了imediaplayerservice类和bprefbase类，这两个类又分别继续了refbase类。bprefbase类有一个成员变量mremote，它的类型为ibinder，实际是一个bpbinder对象。bpbinder类使用了ipcthreadstate类来与binder驱动程序进行交互，而ipcthreadstate类有一个成员变量mprocess，它的类型为processstate，ipcthreadstate类借助processstate类来打开binder设备文件/dev/binder，因此，它可以和binder驱动程序进行交互。
bpmediaplayerservice的构造函数有一个参数impl，它的类型为const sp<ibinder>&，从上面的描述中，这个实际上就是一个bpbinder对象。这样，要创建一个bpmediaplayerservice对象，首先就要有一个bpbinder对象。再来看bpbinder类的构造函数，它有一个参数handle，类型为int32_t，这个参数的意义就是请求mediaplayerservice这个远程接口的进程对mediaplayerservice这个binder实体的引用了。因此，获取mediaplayerservice这个远程接口的本质问题就变为从service manager中获得mediaplayerservice的一个句柄了。
现在，我们就来看一下imediadeathnotifier::getmeidaplayerservice的实现：
// establish binder interface to mediaplayerservice
/*static*/const sp<imediaplayerservice>&
imediadeathnotifier::getmediaplayerservice()
{
logv("getmediaplayerservice");
mutex::autolock _l(sservicelock);
if (smediaplayerservice.get() == 0) {
sp<iservicemanager> sm = defaultservicemanager();
sp<ibinder> 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<imediaplayerservice>(binder);
}
loge_if(smediaplayerservice == 0, "no media player service!?");
return smediaplayerservice;
}
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：
sp<iservicemanager> sm = defaultservicemanager();
相当于是：
sp<iservicemanager> sm = new bpservicemanager(new bpbinder(0));
这里的0表示service manager的远程接口的句柄值是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<iservicemanager>
{
......
virtual sp<ibinder> getservice(const string16& name) const
{
unsigned n;
for (n = 0; n < 5; n++){
sp<ibinder> svc = checkservice(name);
if (svc != null) return svc;
logi("waiting for service %s...\n", string8(name).string());
sleep(1);
}
return null;
}
virtual sp<ibinder> 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();
}
......
};
bpservicemanager::getservice通过bpservicemanager::checkservice执行操作。
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;
}
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：
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;
}
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：
writeint32(ipcthreadstate::self()->getstrictmodepolicy() | strict_mode_penalty_gather);
writestring16("android.os.iservicemanager");
writestring16("media.player");
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：
// note: must be kept in sync with android/os/strictmode.java's penalty_gather
#define strict_mode_penalty_gather 0x100
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。
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);
进入到waitforresponse函数中：
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;
}
这个函数通过ipcthreadstate::talkwithdriver与驱动程序进行交互：
status_t ipcthreadstate::talkwithdriver(bool doreceive)
{
log_assert(mprocess->mdriverfd >= 0, "binder driver is not opened");
binder_write_read bwr;
// is the read buffer empty?
const bool needread = min.dataposition() >= min.datasize();
// we don't want to write anything if we are still reading
// from data left in the input buffer and the caller
// has requested to read the next data.
const size_t outavail = (!doreceive || needread) ? mout.datasize() : 0;
bwr.write_size = outavail;
bwr.write_buffer = (long unsigned int)mout.data();
// this is what we'll read.
if (doreceive && needread) {
bwr.read_size = min.datacapacity();
bwr.read_buffer = (long unsigned int)min.data();
} else {
bwr.read_size = 0;
}
......
// 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;
}
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：
ioctl(mprocess->mdriverfd, binder_write_read, &bwr)
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。
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)
这里可以看出，binder_write_read命令的参数类型为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;
};
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：
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;
}
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。
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;
......
}
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：
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;
这样表明当前线程还有事务要处理。
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);
最后，service manager有事情可做了，就要唤醒它了：
wake_up_interruptible(target_wait);
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：
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;
}
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。
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;
}
最后就返回到ipcthreadstate::talkwithdriver函数中了。
ipcthreadstate::talkwithdriver函数从下面语句：
ioctl(mprocess->mdriverfd, binder_write_read, &bwr)
返回后，首先是清空之前写入binder驱动程序的内容：
if (bwr.write_consumed > 0) {
if (bwr.write_consumed < (ssize_t)mout.datasize())
mout.remove(0, bwr.write_consumed);
else
mout.setdatasize(0);
}
接着是设置从binder驱动程序读取的内容：
if (bwr.read_consumed > 0) {
min.setdatasize(bwr.read_consumed);
min.setdataposition(0);
}
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。
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));
进入休眠状态了，等待service manager的唤醒。
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;
}
这里就是从语句中唤醒了：
ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
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：
t = container_of(w, struct binder_transaction, work);
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：
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);
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：
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 *));
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：
list_del(&t->work.entry);
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：
t->to_parent = thread->transaction_stack;
t->to_thread = thread;
thread->transaction_stack = t;
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。
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;
}
最后，就返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数去了：
void binder_loop(struct binder_state *bs, binder_handler func)
{
int res;
struct binder_write_read bwr;
unsigned readbuf[32];
bwr.write_size = 0;
bwr.write_consumed = 0;
bwr.write_buffer = 0;
readbuf[0] = bc_enter_looper;
binder_write(bs, readbuf, sizeof(unsigned));
for (;;) {
bwr.read_size = sizeof(readbuf);
bwr.read_consumed = 0;
bwr.read_buffer = (unsigned) readbuf;
res = ioctl(bs->fd, binder_write_read, &bwr);
if (res < 0) {
loge("binder_loop: ioctl failed (%s)\n", strerror(errno));
break;
}
res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
if (res == 0) {
loge("binder_loop: unexpected reply?!\n");
break;
}
if (res < 0) {
loge("binder_loop: io error %d %s\n", res, strerror(errno));
break;
}
}
}
这里就是从下面的语句：
res = ioctl(bs->fd, binder_write_read, &bwr);
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：
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;
}
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。
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;
}
接着又调用bio_init_from_txn来初始化msg变量：
void bio_init_from_txn(struct binder_io *bio, struct binder_txn *txn)
{
bio->data = bio->data0 = txn->data;
bio->offs = bio->offs0 = txn->offs;
bio->data_avail = txn->data_size;
bio->offs_avail = txn->offs_size / 4;
bio->flags = bio_f_shared;
}
最后，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：
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;
}
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。
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");
这里的语句：
strict_policy = bio_get_uint32(msg); s = bio_get_string16(msg, &len); s = bio_get_string16(msg, &len);
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。
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;
}
}
这里又调用了find_svc函数：
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;
}
就是在svclist列表中查找对应名称的svcinfo了。
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;
}
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。
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);
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：
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));
}
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：
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，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;
}
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;
}
......
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;
......
}
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。
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;
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：
struct binder_ref *ref = binder_get_ref(proc, fp->handle);
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。
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);
然后增加引用计数：
binder_inc_ref(new_ref, fp->type == binder_type_handle, null);
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：
fp->handle = new_ref->desc;
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。
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 {
......
}
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。
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);
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：
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;
}
就是从下面这个调用：
ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
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：
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。接着就把结果保存在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);
我们简单看一下parcel::ipcsetdatareference函数的实现：
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<uint8_t*>(data);
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();
}
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。
从这里层层返回，最后回到bpservicemanager::checkservice函数中：
virtual sp<ibinder> 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();
}
这里就是从：
remote()->transact(check_service_transaction, data, &reply);
返回来了。我们接着看一下reply.readstrongbinder函数的实现：
sp<ibinder> parcel::readstrongbinder() const
{
sp<ibinder> val;
unflatten_binder(processstate::self(), *this, &val);
return val;
}
这里调用了unflatten_binder函数来构造一个binder对象：
status_t unflatten_binder(const sp<processstate>& proc,
const parcel& in, sp<ibinder>* out)
{
const flat_binder_object* flat = in.readobject(false);
if (flat) {
switch (flat->type) {
case binder_type_binder:
*out = static_cast<ibinder*>(flat->cookie);
return finish_unflatten_binder(null, *flat, in);
case binder_type_handle:
*out = proc->getstrongproxyforhandle(flat->handle);
return finish_unflatten_binder(
static_cast<bpbinder*>(out->get()), *flat, in);
}
}
return bad_type;
}
这里的flat->type是binder_type_handle，因此调用processstate::getstrongproxyforhandle函数：
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;
}
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：
b = new bpbinder(handle); e->binder = b; if (b) e->refs = b->getweakrefs(); result = b;
最后，函数返回到imediadeathnotifier::getmediaplayerservice这里，从这个语句返回：
binder = sm->getservice(string16("media.player"));
这里，就相当于是：
binder = new bpbinder(handle);
最后，函数调用：
smediaplayerservice = interface_cast<imediaplayerservice>(binder);
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：
android::sp<imediaplayerservice> imediaplayerservice::asinterface(const android::sp<android::ibinder>& obj)
{
android::sp<iservicemanager> intr;
if (obj != null) {
intr = static_cast<imediaplayerservice*>(
obj->querylocalinterface(imediaplayerservice::descriptor).get());
if (intr == null) {
intr = new bpmediaplayerservice(obj);
}
}
return intr;
}
这里的obj就是bpbinder，而bpbinder::querylocalinterface返回null，因此就创建了一个bpmediaplayerservice对象：
intr = new bpmediaplayerservice(new bpbinder(handle));
因此，我们最终就得到了一个bpmediaplayerservice对象，达到我们最初的目标。
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。
以上就是对android ipc binder client获得server 远程接口过程的源码分析，后续继续补充相关文章，谢谢大家对本站的支持！