在前面一篇文章android系统进程间通信（ipc）机制binder中的server和client获得service manager接口之路中，介绍了在android系统中binder进程间通信机制中的server角色是如何获得service manager远程接口的，即defaultservicemanager函数的实现。server获得了service manager远程接口之后，就要把自己的service添加到service manager中去，然后把自己启动起来，等待client的请求。本文将通过分析源代码了解server的启动过程是怎么样的。
本文通过一个具体的例子来说明binder机制中server的启动过程。我们知道，在android系统中，提供了多媒体播放的功能，这个功能是以服务的形式来提供的。这里，我们就通过分析mediaplayerservice的实现来了解media server的启动过程。
首先，看一下mediaplayerservice的类图，以便我们理解下面要描述的内容。
我们将要介绍的主角mediaplayerservice继承于bnmediaplayerservice类，熟悉binder机制的同学应该知道bnmediaplayerservice是一个binder native类，用来处理client请求的。bnmediaplayerservice继承于bninterface<imediaplayerservice>类，bninterface是一个模板类，它定义在frameworks/base/include/binder/iinterface.h文件中：
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template<typename interface>
class bninterface : public interface, public bbinder
{
public:
virtual sp<iinterface> querylocalinterface(const string16& _descriptor);
virtual const string16& getinterfacedescriptor() const;
protected:
virtual ibinder* onasbinder();
};
这里可以看出，bnmediaplayerservice实际是继承了imediaplayerservice和bbinder类。imediaplayerservice和bbinder类又分别继承了iinterface和ibinder类，iinterface和ibinder类又同时继承了refbase类。
实际上，bnmediaplayerservice并不是直接接收到client处发送过来的请求，而是使用了ipcthreadstate接收client处发送过来的请求，而ipcthreadstate又借助了processstate类来与binder驱动程序交互。有关ipcthreadstate和processstate的关系，可以参考上一篇文章android系统进程间通信（ipc）机制binder中的server和client获得service manager接口之路，接下来也会有相应的描述。ipcthreadstate接收到了client处的请求后，就会调用bbinder类的transact函数，并传入相关参数，bbinder类的transact函数最终调用bnmediaplayerservice类的ontransact函数，于是，就开始真正地处理client的请求了。
了解了mediaplayerservice类结构之后，就要开始进入到本文的主题了。
首先，看看mediaplayerservice是如何启动的。启动mediaplayerservice的代码位于frameworks/base/media/mediaserver/main_mediaserver.cpp文件中：
int main(int argc, char** argv)
{
sp<processstate> proc(processstate::self());
sp<iservicemanager> sm = defaultservicemanager();
logi("servicemanager: %p", sm.get());
audioflinger::instantiate();
mediaplayerservice::instantiate();
cameraservice::instantiate();
audiopolicyservice::instantiate();
processstate::self()->startthreadpool();
ipcthreadstate::self()->jointhreadpool();
}
这里我们不关注audioflinger和cameraservice相关的代码。
先看下面这句代码：
sp<processstate> proc(processstate::self());
这句代码的作用是通过processstate::self()调用创建一个processstate实例。processstate::self()是processstate类的一个静态成员变量，定义在frameworks/base/libs/binder/processstate.cpp文件中：
sp<processstate> processstate::self()
{
if (gprocess != null) return gprocess;
automutex _l(gprocessmutex);
if (gprocess == null) gprocess = new processstate;
return gprocess;
}
这里可以看出，这个函数作用是返回一个全局唯一的processstate实例gprocess。全局唯一实例变量gprocess定义在frameworks/base/libs/binder/static.cpp文件中：
mutex gprocessmutex;
sp<processstate> gprocess;
再来看processstate的构造函数：
processstate::processstate()
: mdriverfd(open_driver())
, mvmstart(map_failed)
, mmanagescontexts(false)
, mbindercontextcheckfunc(null)
, mbindercontextuserdata(null)
, mthreadpoolstarted(false)
, mthreadpoolseq(1)
{
if (mdriverfd >= 0) {
// xxx ideally, there should be a specific define for whether we
// have mmap (or whether we could possibly have the kernel module
// availabla).
#if !defined(have_win32_ipc)
// mmap the binder, providing a chunk of virtual address space to receive transactions.
mvmstart = mmap(0, binder_vm_size, prot_read, map_private | map_noreserve, mdriverfd, 0);
if (mvmstart == map_failed) {
// *sigh*
loge("using /dev/binder failed: unable to mmap transaction memory.\n");
close(mdriverfd);
mdriverfd = -1;
}
#else
mdriverfd = -1;
#endif
}
if (mdriverfd < 0) {
// need to run without the driver, starting our own thread pool.
}
}
这个函数有两个关键地方，一是通过open_driver函数打开binder设备文件/dev/binder，并将打开设备文件描述符保存在成员变量mdriverfd中；二是通过mmap来把设备文件/dev/binder映射到内存中。
先看open_driver函数的实现，这个函数同样位于frameworks/base/libs/binder/processstate.cpp文件中：
static int open_driver()
{
if (gsingleprocess) {
return -1;
}
int fd = open("/dev/binder", o_rdwr);
if (fd >= 0) {
fcntl(fd, f_setfd, fd_cloexec);
int vers;
#if defined(have_android_os)
status_t result = ioctl(fd, binder_version, &vers);
#else
status_t result = -1;
errno = eperm;
#endif
if (result == -1) {
loge("binder ioctl to obtain version failed: %s", strerror(errno));
close(fd);
fd = -1;
}
if (result != 0 || vers != binder_current_protocol_version) {
loge("binder driver protocol does not match user space protocol!");
close(fd);
fd = -1;
}
#if defined(have_android_os)
size_t maxthreads = 15;
result = ioctl(fd, binder_set_max_threads, &maxthreads);
if (result == -1) {
loge("binder ioctl to set max threads failed: %s", strerror(errno));
}
#endif
} else {
logw("opening '/dev/binder' failed: %s\n", strerror(errno));
}
return fd;
}
这个函数的作用主要是通过open文件操作函数来打开/dev/binder设备文件，然后再调用ioctl文件控制函数来分别执行binder_version和binder_set_max_threads两个命令来和binder驱动程序进行交互，前者用于获得当前binder驱动程序的版本号，后者用于通知binder驱动程序，mediaplayerservice最多可同时启动15个线程来处理client端的请求。
open在binder驱动程序中的具体实现，请参考前面一篇文章浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路，这里不再重复描述。打开/dev/binder设备文件后，binder驱动程序就为mediaplayerservice进程创建了一个struct binder_proc结构体实例来维护mediaplayerservice进程上下文相关信息。
我们来看一下ioctl文件操作函数执行binder_version命令的过程：
status_t result = ioctl(fd, binder_version, &vers);
这个函数调用最终进入到binder驱动程序的binder_ioctl函数中，我们只关注binder_version相关的部分逻辑：
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
int ret;
struct binder_proc *proc = filp->private_data;
struct binder_thread *thread;
unsigned int size = _ioc_size(cmd);
void __user *ubuf = (void __user *)arg;
/*printk(kern_info "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/
ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
if (ret)
return ret;
mutex_lock(&binder_lock);
thread = binder_get_thread(proc);
if (thread == null) {
ret = -enomem;
goto err;
}
switch (cmd) {
......
case binder_version:
if (size != sizeof(struct binder_version)) {
ret = -einval;
goto err;
}
if (put_user(binder_current_protocol_version, &((struct binder_version *)ubuf)->protocol_version)) {
ret = -einval;
goto err;
}
break;
......
}
ret = 0;
err:
......
return ret;
}
很简单，只是将binder_current_protocol_version写入到传入的参数arg指向的用户缓冲区中去就返回了。binder_current_protocol_version是一个宏，定义在kernel/common/drivers/staging/android/binder.h文件中：
/* this is the current protocol version. */
#define binder_current_protocol_version 7
这里为什么要把ubuf转换成struct binder_version之后，再通过其protocol_version成员变量再来写入呢，转了一圈，最终内容还是写入到ubuf中。我们看一下struct binder_version的定义就会明白，同样是在kernel/common/drivers/staging/android/binder.h文件中：
/* use with binder_version, driver fills in fields. */
struct binder_version {
/* driver protocol version -- increment with incompatible change */
signed long protocol_version;
};
从注释中可以看出来，这里是考虑到兼容性，因为以后很有可能不是用signed long来表示版本号。
这里有一个重要的地方要注意的是，由于这里是打开设备文件/dev/binder之后，第一次进入到binder_ioctl函数，因此，这里调用binder_get_thread的时候，就会为当前线程创建一个struct binder_thread结构体变量来维护线程上下文信息，具体可以参考浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路一文。
接着我们再来看一下ioctl文件操作函数执行binder_set_max_threads命令的过程：
result = ioctl(fd, binder_set_max_threads, &maxthreads);
这个函数调用最终进入到binder驱动程序的binder_ioctl函数中，我们只关注binder_set_max_threads相关的部分逻辑：
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
int ret;
struct binder_proc *proc = filp->private_data;
struct binder_thread *thread;
unsigned int size = _ioc_size(cmd);
void __user *ubuf = (void __user *)arg;
/*printk(kern_info "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/
ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
if (ret)
return ret;
mutex_lock(&binder_lock);
thread = binder_get_thread(proc);
if (thread == null) {
ret = -enomem;
goto err;
}
switch (cmd) {
......
case binder_set_max_threads:
if (copy_from_user(&proc->max_threads, ubuf, sizeof(proc->max_threads))) {
ret = -einval;
goto err;
}
break;
......
}
ret = 0;
err:
......
return ret;
}
这里实现也是非常简单，只是简单地把用户传进来的参数保存在proc->max_threads中就完毕了。注意，这里再调用binder_get_thread函数的时候，就可以在proc->threads中找到当前线程对应的struct binder_thread结构了，因为前面已经创建好并保存在proc->threads红黑树中。
回到processstate的构造函数中，这里还通过mmap函数来把设备文件/dev/binder映射到内存中，这个函数在浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路一文也已经有详细介绍，这里不再重复描述。宏binder_vm_size就定义在processstate.cpp文件中：
#define binder_vm_size ((1*1024*1024) - (4096 *2))
mmap函数调用完成之后，binder驱动程序就为当前进程预留了binder_vm_size大小的内存空间了。
这样，processstate全局唯一变量gprocess就创建完毕了，回到frameworks/base/media/mediaserver/main_mediaserver.cpp文件中的main函数，下一步是调用defaultservicemanager函数来获得service manager的远程接口，这个已经在上一篇文章浅谈android系统进程间通信（ipc）机制binder中的server和client获得service manager接口之路有详细描述，读者可以回过头去参考一下。
再接下来，就进入到mediaplayerservice::instantiate函数把mediaplayerservice添加到service manger中去了。这个函数定义在frameworks/base/media/libmediaplayerservice/mediaplayerservice.cpp文件中：
void mediaplayerservice::instantiate() {
defaultservicemanager()->addservice(
string16("media.player"), new mediaplayerservice());
}
我们重点看一下iservicemanger::addservice的过程，这有助于我们加深对binder机制的理解。
在上一篇文章浅谈android系统进程间通信（ipc）机制binder中的server和client获得service manager接口之路中说到，defaultservicemanager返回的实际是一个bpservicemanger类实例，因此，我们看一下bpservicemanger::addservice的实现，这个函数实现在frameworks/base/libs/binder/iservicemanager.cpp文件中：
class bpservicemanager : public bpinterface<iservicemanager>
{
public:
bpservicemanager(const sp<ibinder>& impl)
: bpinterface<iservicemanager>(impl)
{
}
......
virtual status_t addservice(const string16& name, const sp<ibinder>& service)
{
parcel data, reply;
data.writeinterfacetoken(iservicemanager::getinterfacedescriptor());
data.writestring16(name);
data.writestrongbinder(service);
status_t err = remote()->transact(add_service_transaction, data, &reply);
return err == no_error ? reply.readexceptioncode()
}
......
};
这里的parcel类是用来于序列化进程间通信数据用的。
先来看这一句的调用：
data.writeinterfacetoken(iservicemanager::getinterfacedescriptor());
iservicemanager::getinterfacedescriptor()返回来的是一个字符串，即"android.os.iservicemanager"，具体可以参考iservicemanger的实现。我们看一下parcel::writeinterfacetoken的实现，位于frameworks/base/libs/binder/parcel.cpp文件中：
// write rpc headers. (previously just the interface token)
status_t parcel::writeinterfacetoken(const string16& interface)
{
writeint32(ipcthreadstate::self()->getstrictmodepolicy() |
strict_mode_penalty_gather);
// currently the interface identification token is just its name as a string
return writestring16(interface);
}
它的作用是写入一个整数和一个字符串到parcel中去。
再来看下面的调用：
data.writestring16(name);
这里又是写入一个字符串到parcel中去，这里的name即是上面传进来的“media.player”字符串。
往下看：
data.writestrongbinder(service);
这里定入一个binder对象到parcel去。我们重点看一下这个函数的实现，因为它涉及到进程间传输binder实体的问题，比较复杂，需要重点关注，同时，也是理解binder机制的一个重点所在。注意，这里的service参数是一个mediaplayerservice对象。
status_t parcel::writestrongbinder(const sp<ibinder>& val)
{
return flatten_binder(processstate::self(), val, this);
}
看到flatten_binder函数，是不是似曾相识的感觉？我们在前面一篇文章浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路中，曾经提到在binder驱动程序中，使用struct flat_binder_object来表示传输中的一个binder对象，它的定义如下所示：
/*
* this is the flattened representation of a binder object for transfer
* between processes. the 'offsets' supplied as part of a binder transaction
* contains offsets into the data where these structures occur. the binder
* driver takes care of re-writing the structure type and data as it moves
* between processes.
*/
struct flat_binder_object {
/* 8 bytes for large_flat_header. */
unsigned long type;
unsigned long flags;
/* 8 bytes of data. */
union {
void *binder; /* local object */
signed long handle; /* remote object */
};
/* extra data associated with local object */
void *cookie;
};
各个成员变量的含义请参考资料android binder设计与实现。
我们进入到flatten_binder函数看看：
status_t flatten_binder(const sp<processstate>& proc,
const sp<ibinder>& binder, parcel* out)
{
flat_binder_object obj;
obj.flags = 0x7f | flat_binder_flag_accepts_fds;
if (binder != null) {
ibinder *local = binder->localbinder();
if (!local) {
bpbinder *proxy = binder->remotebinder();
if (proxy == null) {
loge("null proxy");
}
const int32_t handle = proxy ? proxy->handle() : 0;
obj.type = binder_type_handle;
obj.handle = handle;
obj.cookie = null;
} else {
obj.type = binder_type_binder;
obj.binder = local->getweakrefs();
obj.cookie = local;
}
} else {
obj.type = binder_type_binder;
obj.binder = null;
obj.cookie = null;
}
return finish_flatten_binder(binder, obj, out);
}
首先是初始化flat_binder_object的flags域：
obj.flags = 0x7f | flat_binder_flag_accepts_fds;
0x7f表示处理本binder实体请求数据包的线程的最低优先级，flat_binder_flag_accepts_fds表示这个binder实体可以接受文件描述符，binder实体在收到文件描述符时，就会在本进程中打开这个文件。
传进来的binder即为mediaplayerservice::instantiate函数中new出来的mediaplayerservice实例，因此，不为空。又由于mediaplayerservice继承自bbinder类，它是一个本地binder实体，因此binder->localbinder返回一个bbinder指针，而且肯定不为空，于是执行下面语句：
obj.type = binder_type_binder; obj.binder = local->getweakrefs(); obj.cookie = local;
设置了flat_binder_obj的其他成员变量，注意，指向这个binder实体地址的指针local保存在flat_binder_obj的成员变量cookie中。
函数调用finish_flatten_binder来将这个flat_binder_obj写入到parcel中去：
inline static status_t finish_flatten_binder(
const sp<ibinder>& binder, const flat_binder_object& flat, parcel* out)
{
return out->writeobject(flat, false);
}
parcel::writeobject的实现如下：
status_t parcel::writeobject(const flat_binder_object& val, bool nullmetadata)
{
const bool enoughdata = (mdatapos+sizeof(val)) <= mdatacapacity;
const bool enoughobjects = mobjectssize < mobjectscapacity;
if (enoughdata && enoughobjects) {
restart_write:
*reinterpret_cast<flat_binder_object*>(mdata+mdatapos) = val;
// need to write meta-data?
if (nullmetadata || val.binder != null) {
mobjects[mobjectssize] = mdatapos;
acquire_object(processstate::self(), val, this);
mobjectssize++;
}
// remember if it's a file descriptor
if (val.type == binder_type_fd) {
mhasfds = mfdsknown = true;
}
return finishwrite(sizeof(flat_binder_object));
}
if (!enoughdata) {
const status_t err = growdata(sizeof(val));
if (err != no_error) return err;
}
if (!enoughobjects) {
size_t newsize = ((mobjectssize+2)*3)/2;
size_t* objects = (size_t*)realloc(mobjects, newsize*sizeof(size_t));
if (objects == null) return no_memory;
mobjects = objects;
mobjectscapacity = newsize;
}
goto restart_write;
}
这里除了把flat_binder_obj写到parcel里面之内，还要记录这个flat_binder_obj在parcel里面的偏移位置：
mobjects[mobjectssize] = mdatapos;
这里因为，如果进程间传输的数据间带有binder对象的时候，binder驱动程序需要作进一步的处理，以维护各个binder实体的一致性，下面我们将会看到binder驱动程序是怎么处理这些binder对象的。
再回到bpservicemanager::addservice函数中，调用下面语句：
status_t err = remote()->transact(add_service_transaction, data, &reply);
回到浅谈android系统进程间通信（ipc）机制binder中的server和client获得service manager接口之路一文中的类图中去看一下，这里的remote成员函数来自于bprefbase类，它返回一个bpbinder指针。因此，我们继续进入到bpbinder::transact函数中去看看：
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;
}
这里又调用了ipcthreadstate::transact进执行实际的操作。注意，这里的mhandle为0，code为add_service_transaction。add_service_transaction是上面以参数形式传进来的，那mhandle为什么是0呢？因为这里表示的是service manager远程接口，它的句柄值一定是0，具体请参考浅谈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。
再进入到ipcthreadstate::transact函数，看看做了些什么事情：
status_t ipcthreadstate::transact(int32_t handle,
uint32_t code, const parcel& data,
parcel* reply, uint32_t flags)
{
status_t err = data.errorcheck();
flags |= tf_accept_fds;
if_log_transactions() {
textoutput::bundle _b(alog);
alog << "bc_transaction thr " << (void*)pthread_self() << " / hand "
<< handle << " / code " << typecode(code) << ": "
<< indent << data << dedent << endl;
}
if (err == no_error) {
log_oneway(">>>> send from pid %d uid %d %s", getpid(), getuid(),
(flags & tf_one_way) == 0 ? "read reply" : "one way");
err = writetransactiondata(bc_transaction, flags, handle, code, data, null);
}
if (err != no_error) {
if (reply) reply->seterror(err);
return (mlasterror = err);
}
if ((flags & tf_one_way) == 0) {
#if 0
if (code == 4) { // relayout
logi(">>>>>> calling transaction 4");
} else {
logi(">>>>>> calling transaction %d", code);
}
#endif
if (reply) {
err = waitforresponse(reply);
} else {
parcel fakereply;
err = waitforresponse(&fakereply);
}
#if 0
if (code == 4) { // relayout
logi("<<<<<< returning transaction 4");
} else {
logi("<<<<<< returning transaction %d", code);
}
#endif
if_log_transactions() {
textoutput::bundle _b(alog);
alog << "br_reply thr " << (void*)pthread_self() << " / hand "
<< handle << ": ";
if (reply) alog << indent << *reply << dedent << endl;
else alog << "(none requested)" << endl;
}
} else {
err = waitforresponse(null, null);
}
return err;
}
ipcthreadstate::transact函数的参数flags是一个默认值为0的参数，上面没有传相应的实参进来，因此，这里就为0。
函数首先调用writetransactiondata函数准备好一个struct binder_transaction_data结构体变量，这个是等一下要传输给binder驱动程序的。struct binder_transaction_data的定义我们在浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路一文中有详细描述，读者不妨回过去读一下。这里为了方便描述，将struct binder_transaction_data的定义再次列出来：
struct binder_transaction_data {
/* the first two are only used for bctransaction and brtransaction,
* identifying the target and contents of the transaction.
*/
union {
size_t handle; /* target descriptor of command transaction */
void *ptr; /* target descriptor of return transaction */
} target;
void *cookie; /* target object cookie */
unsigned int code; /* transaction command */
/* general information about the transaction. */
unsigned int flags;
pid_t sender_pid;
uid_t sender_euid;
size_t data_size; /* number of bytes of data */
size_t offsets_size; /* number of bytes of offsets */
/* if this transaction is inline, the data immediately
* follows here; otherwise, it ends with a pointer to
* the data buffer.
*/
union {
struct {
/* transaction data */
const void *buffer;
/* offsets from buffer to flat_binder_object structs */
const void *offsets;
} ptr;
uint8_t buf[8];
} data;
};
writetransactiondata函数的实现如下：
status_t ipcthreadstate::writetransactiondata(int32_t cmd, uint32_t binderflags,
int32_t handle, uint32_t code, const parcel& data, status_t* statusbuffer)
{
binder_transaction_data tr;
tr.target.handle = handle;
tr.code = code;
tr.flags = binderflags;
const status_t err = data.errorcheck();
if (err == no_error) {
tr.data_size = data.ipcdatasize();
tr.data.ptr.buffer = data.ipcdata();
tr.offsets_size = data.ipcobjectscount()*sizeof(size_t);
tr.data.ptr.offsets = data.ipcobjects();
} else if (statusbuffer) {
tr.flags |= tf_status_code;
*statusbuffer = err;
tr.data_size = sizeof(status_t);
tr.data.ptr.buffer = statusbuffer;
tr.offsets_size = 0;
tr.data.ptr.offsets = null;
} else {
return (mlasterror = err);
}
mout.writeint32(cmd);
mout.write(&tr, sizeof(tr));
return no_error;
}
注意，这里的cmd为bc_transaction。 这个函数很简单，在这个场景下，就是执行下面语句来初始化本地变量tr：
tr.data_size = data.ipcdatasize(); tr.data.ptr.buffer = data.ipcdata(); tr.offsets_size = data.ipcobjectscount()*sizeof(size_t); tr.data.ptr.offsets = data.ipcobjects();
回忆一下上面的内容，写入到tr.data.ptr.buffer的内容相当于下面的内容：
writeint32(ipcthreadstate::self()->getstrictmodepolicy() |
strict_mode_penalty_gather);
writestring16("android.os.iservicemanager");
writestring16("media.player");
writestrongbinder(new mediaplayerservice());
其中包含了一个binder实体mediaplayerservice，因此需要设置tr.offsets_size就为1，tr.data.ptr.offsets就指向了这个mediaplayerservice的地址在tr.data.ptr.buffer中的偏移量。最后，将tr的内容保存在ipcthreadstate的成员变量mout中。
回到ipcthreadstate::transact函数中，接下去看，(flags & tf_one_way) == 0为true，并且reply不为空，所以最终进入到waitforresponse(reply)这条路径来。我们看一下waitforresponse函数的实现：
status_t ipcthreadstate::waitforresponse(parcel *reply, status_t *acquireresult)
{
int32_t cmd;
int32_t err;
while (1) {
if ((err=talkwithdriver()) < no_error) break;
err = min.errorcheck();
if (err < no_error) break;
if (min.dataavail() == 0) continue;
cmd = min.readint32();
if_log_commands() {
alog << "processing waitforresponse command: "
<< getreturnstring(cmd) << endl;
}
switch (cmd) {
case br_transaction_complete:
if (!reply && !acquireresult) goto finish;
break;
case br_dead_reply:
err = dead_object;
goto finish;
case br_failed_reply:
err = failed_transaction;
goto finish;
case br_acquire_result:
{
log_assert(acquireresult != null, "unexpected bracquire_result");
const int32_t result = min.readint32();
if (!acquireresult) continue;
*acquireresult = result ? no_error : invalid_operation;
}
goto finish;
case br_reply:
{
binder_transaction_data tr;
err = min.read(&tr, sizeof(tr));
log_assert(err == no_error, "not enough command data for brreply");
if (err != no_error) goto finish;
if (reply) {
if ((tr.flags & tf_status_code) == 0) {
reply->ipcsetdatareference(
reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
tr.data_size,
reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
tr.offsets_size/sizeof(size_t),
freebuffer, this);
} else {
err = *static_cast<const status_t*>(tr.data.ptr.buffer);
freebuffer(null,
reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
tr.data_size,
reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
tr.offsets_size/sizeof(size_t), this);
}
} else {
freebuffer(null,
reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
tr.data_size,
reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
tr.offsets_size/sizeof(size_t), this);
continue;
}
}
goto finish;
default:
err = executecommand(cmd);
if (err != no_error) goto finish;
break;
}
}
finish:
if (err != no_error) {
if (acquireresult) *acquireresult = err;
if (reply) reply->seterror(err);
mlasterror = err;
}
return err;
}
这个函数虽然很长，但是主要调用了talkwithdriver函数来与binder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;
}
if_log_commands() {
textoutput::bundle _b(alog);
if (outavail != 0) {
alog << "sending commands to driver: " << indent;
const void* cmds = (const void*)bwr.write_buffer;
const void* end = ((const uint8_t*)cmds)+bwr.write_size;
alog << hexdump(cmds, bwr.write_size) << endl;
while (cmds < end) cmds = printcommand(alog, cmds);
alog << dedent;
}
alog << "size of receive buffer: " << bwr.read_size
<< ", needread: " << needread << ", doreceive: " << doreceive << endl;
}
// return immediately if there is nothing to do.
if ((bwr.write_size == 0) && (bwr.read_size == 0)) return no_error;
bwr.write_consumed = 0;
bwr.read_consumed = 0;
status_t err;
do {
if_log_commands() {
alog << "about to read/write, write size = " << mout.datasize() << endl;
}
#if defined(have_android_os)
if (ioctl(mprocess->mdriverfd, binder_write_read, &bwr) >= 0)
err = no_error;
else
err = -errno;
#else
err = invalid_operation;
#endif
if_log_commands() {
alog << "finished read/write, write size = " << mout.datasize() << endl;
}
} while (err == -eintr);
if_log_commands() {
alog << "our err: " << (void*)err << ", write consumed: "
<< bwr.write_consumed << " (of " << mout.datasize()
<< "), read consumed: " << bwr.read_consumed << endl;
}
if (err >= no_error) {
if (bwr.write_consumed > 0) {
if (bwr.write_consumed < (ssize_t)mout.datasize())
mout.remove(0, bwr.write_consumed);
else
mout.setdatasize(0);
}
if (bwr.read_consumed > 0) {
min.setdatasize(bwr.read_consumed);
min.setdataposition(0);
}
if_log_commands() {
textoutput::bundle _b(alog);
alog << "remaining data size: " << mout.datasize() << endl;
alog << "received commands from driver: " << indent;
const void* cmds = min.data();
const void* end = min.data() + min.datasize();
alog << hexdump(cmds, min.datasize()) << endl;
while (cmds < end) cmds = printreturncommand(alog, cmds);
alog << dedent;
}
return no_error;
}
return err;
}
这里doreceive和needread均为1，有兴趣的读者可以自已分析一下。因此，这里告诉binder驱动程序，先执行write操作，再执行read操作，下面我们将会看到。
最后，通过ioctl(mprocess->mdriverfd, binder_write_read, &bwr)进行到binder驱动程序的binder_ioctl函数，我们只关注cmd为binder_write_read的逻辑：
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
int ret;
struct binder_proc *proc = filp->private_data;
struct binder_thread *thread;
unsigned int size = _ioc_size(cmd);
void __user *ubuf = (void __user *)arg;
/*printk(kern_info "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/
ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
if (ret)
return ret;
mutex_lock(&binder_lock);
thread = binder_get_thread(proc);
if (thread == null) {
ret = -enomem;
goto err;
}
switch (cmd) {
case binder_write_read: {
struct binder_write_read bwr;
if (size != sizeof(struct binder_write_read)) {
ret = -einval;
goto err;
}
if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
ret = -efault;
goto err;
}
if (binder_debug_mask & binder_debug_read_write)
printk(kern_info "binder: %d:%d write %ld at %08lx, read %ld at %08lx\n",
proc->pid, thread->pid, bwr.write_size, bwr.write_buffer, bwr.read_size, bwr.read_buffer);
if (bwr.write_size > 0) {
ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
if (ret < 0) {
bwr.read_consumed = 0;
if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
ret = -efault;
goto err;
}
}
if (bwr.read_size > 0) {
ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & o_nonblock);
if (!list_empty(&proc->todo))
wake_up_interruptible(&proc->wait);
if (ret < 0) {
if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
ret = -efault;
goto err;
}
}
if (binder_debug_mask & binder_debug_read_write)
printk(kern_info "binder: %d:%d wrote %ld of %ld, read return %ld of %ld\n",
proc->pid, thread->pid, bwr.write_consumed, bwr.write_size, bwr.read_consumed, bwr.read_size);
if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
ret = -efault;
goto err;
}
break;
}
......
}
ret = 0;
err:
......
return ret;
}
函数首先是将用户传进来的参数拷贝到本地变量struct binder_write_read bwr中去。这里bwr.write_size > 0为true，因此，进入到binder_thread_write函数中，我们只关注bc_transaction部分的逻辑：
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;
}
首先将用户传进来的transact参数拷贝在本地变量struct binder_transaction_data tr中去，接着调用binder_transaction函数进一步处理，这里我们忽略掉无关代码：
static void
binder_transaction(struct binder_proc *proc, struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
struct binder_transaction *t;
struct binder_work *tcomplete;
size_t *offp, *off_end;
struct binder_proc *target_proc;
struct binder_thread *target_thread = null;
struct binder_node *target_node = null;
struct list_head *target_list;
wait_queue_head_t *target_wait;
struct binder_transaction *in_reply_to = null;
struct binder_transaction_log_entry *e;
uint32_t return_error;
......
if (reply) {
......
} else {
if (tr->target.handle) {
......
} else {
target_node = binder_context_mgr_node;
if (target_node == null) {
return_error = br_dead_reply;
goto err_no_context_mgr_node;
}
}
......
target_proc = target_node->proc;
if (target_proc == null) {
return_error = br_dead_reply;
goto err_dead_binder;
}
......
}
if (target_thread) {
......
} else {
target_list = &target_proc->todo;
target_wait = &target_proc->wait;
}
......
/* todo: reuse incoming transaction for reply */
t = kzalloc(sizeof(*t), gfp_kernel);
if (t == null) {
return_error = br_failed_reply;
goto err_alloc_t_failed;
}
......
tcomplete = kzalloc(sizeof(*tcomplete), gfp_kernel);
if (tcomplete == null) {
return_error = br_failed_reply;
goto err_alloc_tcomplete_failed;
}
......
if (!reply && !(tr->flags & tf_one_way))
t->from = thread;
else
t->from = null;
t->sender_euid = proc->tsk->cred->euid;
t->to_proc = target_proc;
t->to_thread = target_thread;
t->code = tr->code;
t->flags = tr->flags;
t->priority = task_nice(current);
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
tr->offsets_size, !reply && (t->flags & tf_one_way));
if (t->buffer == null) {
return_error = br_failed_reply;
goto err_binder_alloc_buf_failed;
}
t->buffer->allow_user_free = 0;
t->buffer->debug_id = t->debug_id;
t->buffer->transaction = t;
t->buffer->target_node = target_node;
if (target_node)
binder_inc_node(target_node, 1, 0, null);
offp = (size_t *)(t->buffer->data + align(tr->data_size, sizeof(void *)));
if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
......
off_end = (void *)offp + tr->offsets_size;
for (; offp < off_end; offp++) {
struct flat_binder_object *fp;
......
fp = (struct flat_binder_object *)(t->buffer->data + *offp);
switch (fp->type) {
case binder_type_binder:
case binder_type_weak_binder: {
struct binder_ref *ref;
struct binder_node *node = binder_get_node(proc, fp->binder);
if (node == null) {
node = binder_new_node(proc, fp->binder, fp->cookie);
if (node == null) {
return_error = br_failed_reply;
goto err_binder_new_node_failed;
}
node->min_priority = fp->flags & flat_binder_flag_priority_mask;
node->accept_fds = !!(fp->flags & flat_binder_flag_accepts_fds);
}
if (fp->cookie != node->cookie) {
......
goto err_binder_get_ref_for_node_failed;
}
ref = binder_get_ref_for_node(target_proc, node);
if (ref == null) {
return_error = br_failed_reply;
goto err_binder_get_ref_for_node_failed;
}
if (fp->type == binder_type_binder)
fp->type = binder_type_handle;
else
fp->type = binder_type_weak_handle;
fp->handle = ref->desc;
binder_inc_ref(ref, fp->type == binder_type_handle, &thread->todo);
......
} break;
......
}
}
if (reply) {
......
} else if (!(t->flags & tf_one_way)) {
bug_on(t->buffer->async_transaction != 0);
t->need_reply = 1;
t->from_parent = thread->transaction_stack;
thread->transaction_stack = t;
} else {
......
}
t->work.type = binder_work_transaction;
list_add_tail(&t->work.entry, target_list);
tcomplete->type = binder_work_transaction_complete;
list_add_tail(&tcomplete->entry, &thread->todo);
if (target_wait)
wake_up_interruptible(target_wait);
return;
......
}
注意，这里传进来的参数reply为0，tr->target.handle也为0。因此，target_proc、target_thread、target_node、target_list和target_wait的值分别为：
target_node = binder_context_mgr_node; target_proc = target_node->proc; target_list = &target_proc->todo; target_wait = &target_proc->wait;
接着，分配了一个待处理事务t和一个待完成工作项tcomplete，并执行初始化工作：
/* todo: reuse incoming transaction for reply */
t = kzalloc(sizeof(*t), gfp_kernel);
if (t == null) {
return_error = br_failed_reply;
goto err_alloc_t_failed;
}
......
tcomplete = kzalloc(sizeof(*tcomplete), gfp_kernel);
if (tcomplete == null) {
return_error = br_failed_reply;
goto err_alloc_tcomplete_failed;
}
......
if (!reply && !(tr->flags & tf_one_way))
t->from = thread;
else
t->from = null;
t->sender_euid = proc->tsk->cred->euid;
t->to_proc = target_proc;
t->to_thread = target_thread;
t->code = tr->code;
t->flags = tr->flags;
t->priority = task_nice(current);
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
tr->offsets_size, !reply && (t->flags & tf_one_way));
if (t->buffer == null) {
return_error = br_failed_reply;
goto err_binder_alloc_buf_failed;
}
t->buffer->allow_user_free = 0;
t->buffer->debug_id = t->debug_id;
t->buffer->transaction = t;
t->buffer->target_node = target_node;
if (target_node)
binder_inc_node(target_node, 1, 0, null);
offp = (size_t *)(t->buffer->data + align(tr->data_size, sizeof(void *)));
if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
注意，这里的事务t是要交给target_proc处理的，在这个场景之下，就是service manager了。因此，下面的语句：
t->buffer = binder_alloc_buf(target_proc, tr->data_size, tr->offsets_size, !reply && (t->flags & tf_one_way));
就是在service manager的进程空间中分配一块内存来保存用户传进入的参数了：
if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
......
return_error = br_failed_reply;
goto err_copy_data_failed;
}
由于现在target_node要被使用了，增加它的引用计数：
if (target_node) binder_inc_node(target_node, 1, 0, null);
接下去的for循环，就是用来处理传输数据中的binder对象了。在我们的场景中，有一个类型为binder_type_binder的binder实体mediaplayerservice：
switch (fp->type) {
case binder_type_binder:
case binder_type_weak_binder: {
struct binder_ref *ref;
struct binder_node *node = binder_get_node(proc, fp->binder);
if (node == null) {
node = binder_new_node(proc, fp->binder, fp->cookie);
if (node == null) {
return_error = br_failed_reply;
goto err_binder_new_node_failed;
}
node->min_priority = fp->flags & flat_binder_flag_priority_mask;
node->accept_fds = !!(fp->flags & flat_binder_flag_accepts_fds);
}
if (fp->cookie != node->cookie) {
......
goto err_binder_get_ref_for_node_failed;
}
ref = binder_get_ref_for_node(target_proc, node);
if (ref == null) {
return_error = br_failed_reply;
goto err_binder_get_ref_for_node_failed;
}
if (fp->type == binder_type_binder)
fp->type = binder_type_handle;
else
fp->type = binder_type_weak_handle;
fp->handle = ref->desc;
binder_inc_ref(ref, fp->type == binder_type_handle, &thread->todo);
......
} break;
由于是第一次在binder驱动程序中传输这个mediaplayerservice，调用binder_get_node函数查询这个binder实体时，会返回空，于是binder_new_node在proc中新建一个，下次就可以直接使用了。
现在，由于要把这个binder实体mediaplayerservice交给target_proc，也就是service manager来管理，也就是说service manager要引用这个mediaplayerservice了，于是通过binder_get_ref_for_node为mediaplayerservice创建一个引用，并且通过binder_inc_ref来增加这个引用计数，防止这个引用还在使用过程当中就被销毁。注意，到了这里的时候，t->buffer中的flat_binder_obj的type已经改为binder_type_handle，handle已经改为ref->desc，跟原来不一样了，因为这个flat_binder_obj是最终是要传给service manager的，而service manager只能够通过句柄值来引用这个binder实体。
最后，把待处理事务加入到target_list列表中去：
list_add_tail(&t->work.entry, target_list);
并且把待完成工作项加入到本线程的todo等待执行列表中去：
list_add_tail(&tcomplete->entry, &thread->todo);
现在目标进程有事情可做了，于是唤醒它：
if (target_wait)
wake_up_interruptible(target_wait);
这里就是要唤醒service manager进程了。回忆一下前面这篇文章，此时， service manager正在binder_t浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路hread_read函数中调用wait_event_interruptible进入休眠状态。
这里我们先忽略一下service manager被唤醒之后的场景，继续medaplayerservice的启动过程，然后再回来。
回到binder_ioctl函数，bwr.read_size > 0为true，于是进入binder_thread_read函数：
static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
void __user *buffer, int size, signed long *consumed, int non_block)
{
void __user *ptr = buffer + *consumed;
void __user *end = buffer + size;
int ret = 0;
int wait_for_proc_work;
if (*consumed == 0) {
if (put_user(br_noop, (uint32_t __user *)ptr))
return -efault;
ptr += sizeof(uint32_t);
}
retry:
wait_for_proc_work = thread->transaction_stack == null && list_empty(&thread->todo);
.......
if (wait_for_proc_work) {
.......
} else {
if (non_block) {
if (!binder_has_thread_work(thread))
ret = -eagain;
} else
ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
}
......
while (1) {
uint32_t cmd;
struct binder_transaction_data tr;
struct binder_work *w;
struct binder_transaction *t = null;
if (!list_empty(&thread->todo))
w = list_first_entry(&thread->todo, struct binder_work, entry);
else if (!list_empty(&proc->todo) && wait_for_proc_work)
w = list_first_entry(&proc->todo, struct binder_work, entry);
else {
if (ptr - buffer == 4 && !(thread->looper & binder_looper_state_need_return)) /* no data added */
goto retry;
break;
}
if (end - ptr < sizeof(tr) + 4)
break;
switch (w->type) {
......
case binder_work_transaction_complete: {
cmd = br_transaction_complete;
if (put_user(cmd, (uint32_t __user *)ptr))
return -efault;
ptr += sizeof(uint32_t);
binder_stat_br(proc, thread, cmd);
if (binder_debug_mask & binder_debug_transaction_complete)
printk(kern_info "binder: %d:%d br_transaction_complete\n",
proc->pid, thread->pid);
list_del(&w->entry);
kfree(w);
binder_stats.obj_deleted[binder_stat_transaction_complete]++;
} break;
......
}
if (!t)
continue;
......
}
done:
......
return 0;
}
这里，thread->transaction_stack和thread->todo均不为空，于是wait_for_proc_work为false，由于binder_has_thread_work的时候，返回true，这里因为thread->todo不为空，因此，线程虽然调用了wait_event_interruptible，但是不会睡眠，于是继续往下执行。
由于thread->todo不为空，执行下列语句：
if (!list_empty(&thread->todo)) w = list_first_entry(&thread->todo, struct binder_work, entry);
w->type为binder_work_transaction_complete，这是在上面的binder_transaction函数设置的，于是执行：
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;
......
}
这里就将w从thread->todo删除了。由于这里t为空，重新执行while循环，这时由于已经没有事情可做了，最后就返回到binder_ioctl函数中。注间，这里一共往用户传进来的缓冲区buffer写入了两个整数，分别是br_noop和br_transaction_complete。
binder_ioctl函数返回到用户空间之前，把数据消耗情况拷贝回用户空间中：
if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
ret = -efault;
goto err;
}
最后返回到ipcthreadstate::talkwithdriver函数中，执行下面语句：
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) {
<pre code_snippet_id="134056" snippet_file_name="blog_20131230_54_6706870" name="code" class="cpp"> min.setdatasize(bwr.read_consumed);
min.setdataposition(0);</pre> } ...... return no_error; }
首先是把mout的数据清空：
mout.setdatasize(0);
然后设置已经读取的内容的大小：
min.setdatasize(bwr.read_consumed);
min.setdataposition(0);
然后返回到ipcthreadstate::waitforresponse函数中。在ipcthreadstate::waitforresponse函数，先是从min读出一个整数，这个便是br_noop了，这是一个空操作，什么也不做。然后继续进入ipcthreadstate::talkwithdriver函数中。
这时候，下面语句执行后：
const bool needread = min.dataposition() >= min.datasize();
needread为false，因为在min中，尚有一个整数br_transaction_complete未读出。
这时候，下面语句执行后：
const size_t outavail = (!doreceive || needread) ? mout.datasize() : 0;
outavail等于0。因此，最后bwr.write_size和bwr.read_size均为0，ipcthreadstate::talkwithdriver函数什么也不做，直接返回到ipcthreadstate::waitforresponse函数中。在ipcthreadstate::waitforresponse函数，又继续从min读出一个整数，这个便是br_transaction_complete：
switch (cmd) {
case br_transaction_complete:
if (!reply && !acquireresult) goto finish;
break;
......
}
reply不为null，因此，ipcthreadstate::waitforresponse的循环没有结束，继续执行，又进入到ipcthreadstate::talkwithdrive中。
这次，needread就为true了，而outavail仍为0，所以bwr.read_size不为0，bwr.write_size为0。于是通过：
ioctl(mprocess->mdriverfd, binder_write_read, &bwr)
进入到binder驱动程序中的binder_ioctl函数中。由于bwr.write_size为0，bwr.read_size不为0，这次直接就进入到binder_thread_read函数中。这时候，thread->transaction_stack不等于0，但是thread->todo为空，于是线程就通过：
[cpp] view plain copy 在code上查看代码片派生到我的代码片
wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
进入睡眠状态，等待service manager来唤醒了。
现在，我们可以回到service manager被唤醒的过程了。我们接着前面浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路这篇文章的最后，继续描述。此时， service manager正在binder_thread_read函数中调用wait_event_interruptible_exclusive进入休眠状态。上面被mediaplayerservice启动后进程唤醒后，继续执行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) {
......
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;
......
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.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)) {
t->to_parent = thread->transaction_stack;
t->to_thread = thread;
thread->transaction_stack = t;
} else {
t->buffer->transaction = null;
kfree(t);
binder_stats.obj_deleted[binder_stat_transaction]++;
}
break;
}
done:
......
return 0;
}
service manager被唤醒之后，就进入while循环开始处理事务了。这里wait_for_proc_work等于1，并且proc->todo不为空，所以从proc->todo列表中得到第一个工作项：
w = list_first_entry(&proc->todo, struct binder_work, entry);
从上面的描述中，我们知道，这个工作项的类型为binder_work_transaction，于是通过下面语句得到事务项：
t = container_of(w, struct binder_transaction, work);
接着就是把事务项t中的数据拷贝到本地局部变量struct binder_transaction_data tr中去了：
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;
......
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.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 *));
这里有一个非常重要的地方，是binder进程间通信机制的精髓所在：
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 *));
t->buffer->data所指向的地址是内核空间的，现在要把数据返回给service manager进程的用户空间，而service manager进程的用户空间是不能访问内核空间的数据的，所以这里要作一下处理。怎么处理呢？我们在学面向对象语言的时候，对象的拷贝有深拷贝和浅拷贝之分，深拷贝是把另外分配一块新内存，然后把原始对象的内容搬过去，浅拷贝是并没有为新对象分配一块新空间，而只是分配一个引用，而个引用指向原始对象。binder机制用的是类似浅拷贝的方法，通过在用户空间分配一个虚拟地址，然后让这个用户空间虚拟地址与 t->buffer->data这个内核空间虚拟地址指向同一个物理地址，这样就可以实现浅拷贝了。怎么样用户空间和内核空间的虚拟地址同时指向同一个物理地址呢？请参考前面一篇文章浅谈service manager成为android进程间通信（ipc）机制binder守护进程之路，那里有详细描述。这里只要将t->buffer->data加上一个偏移值proc->user_buffer_offset就可以得到t->buffer->data对应的用户空间虚拟地址了。调整了tr.data.ptr.buffer的值之后，不要忘记也要一起调整tr.data.ptr.offsets的值。
接着就是把tr的内容拷贝到用户传进来的缓冲区去了，指针ptr指向这个用户缓冲区的地址：
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);
这里可以看出，这里只是对作tr.data.ptr.bufferr和tr.data.ptr.offsets的内容作了浅拷贝。
最后，由于已经处理了这个事务，要把它从todo列表中删除：
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 {
t->buffer->transaction = null;
kfree(t);
binder_stats.obj_deleted[binder_stat_transaction]++;
}
注意，这里的cmd == br_transaction && !(t->flags & tf_one_way)为true，表明这个事务虽然在驱动程序中已经处理完了，但是它仍然要等待service manager完成之后，给驱动程序一个确认，也就是需要等待回复，于是把当前事务t放在thread->transaction_stack队列的头部：
t->to_parent = thread->transaction_stack; t->to_thread = thread; thread->transaction_stack = t;
如果cmd == br_transaction && !(t->flags & tf_one_way)为false，那就不需要等待回复了，直接把事务t删掉。
这个while最后通过一个break跳了出来，最后返回到binder_ioctl函数中：
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;
......
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 (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 (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
ret = -efault;
goto err;
}
break;
}
......
default:
ret = -einval;
goto err;
}
ret = 0;
err:
......
return ret;
}
从binder_thread_read返回来后，再看看proc->todo是否还有事务等待处理，如果是，就把睡眠在proc->wait队列的线程唤醒来处理。最后，把本地变量struct binder_write_read bwr的内容拷贝回到用户传进来的缓冲区中，就返回了。
这里就是返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数了：
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;
}
}
}
返回来的数据都放在readbuf中，接着调用binder_parse进行解析：
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++;
......
case br_transaction: {
struct binder_txn *txn = (void *) ptr;
if ((end - ptr) * sizeof(uint32_t) < sizeof(struct binder_txn)) {
loge("parse: txn too small!\n");
return -1;
}
binder_dump_txn(txn);
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;
}
首先把从binder驱动程序读出来的数据转换为一个struct binder_txn结构体，保存在txn本地变量中，struct binder_txn定义在frameworks/base/cmds/servicemanager/binder.h文件中：
struct binder_txn
{
void *target;
void *cookie;
uint32_t code;
uint32_t flags;
uint32_t sender_pid;
uint32_t sender_euid;
uint32_t data_size;
uint32_t offs_size;
void *data;
void *offs;
};
函数中还用到了另外一个数据结构struct binder_io，也是定义在frameworks/base/cmds/servicemanager/binder.h文件中：
struct binder_io
{
char *data; /* pointer to read/write from */
uint32_t *offs; /* array of offsets */
uint32_t data_avail; /* bytes available in data buffer */
uint32_t offs_avail; /* entries available in offsets array */
char *data0; /* start of data buffer */
uint32_t *offs0; /* start of offsets buffer */
uint32_t flags;
uint32_t unused;
};
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;
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_add_service:
s = bio_get_string16(msg, &len);
ptr = bio_get_ref(msg);
if (do_add_service(bs, s, len, ptr, txn->sender_euid))
return -1;
break;
......
}
bio_put_uint32(reply, 0);
return 0;
}
回忆一下，在bpservicemanager::addservice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");
writestrongbinder(new mediaplayerservice());
这里的语句：
strict_policy = bio_get_uint32(msg); s = bio_get_string16(msg, &len); s = bio_get_string16(msg, &len); ptr = bio_get_ref(msg);
就是依次把它们读取出来了，这里，我们只要看一下bio_get_ref的实现。先看一个数据结构struct binder_obj的定义：
struct binder_object
{
uint32_t type;
uint32_t flags;
void *pointer;
void *cookie;
};
这个结构体其实就是对应struct flat_binder_obj的。
接着看bio_get_ref实现：
void *bio_get_ref(struct binder_io *bio)
{
struct binder_object *obj;
obj = _bio_get_obj(bio);
if (!obj)
return 0;
if (obj->type == binder_type_handle)
return obj->pointer;
return 0;
}
_bio_get_obj这个函数就不跟进去看了，它的作用就是从binder_io中取得第一个还没取获取过的binder_object。在这个场景下，就是我们最开始传过来代表mediaplayerservice的flat_binder_obj了，这个原始的flat_binder_obj的type为binder_type_binder，binder为指向mediaplayerservice的弱引用的地址。在前面我们说过，在binder驱动驱动程序里面，会把这个flat_binder_obj的type改为binder_type_handle，handle改为一个句柄值。这里的handle值就等于obj->pointer的值。
回到svcmgr_handler函数，调用do_add_service进一步处理：
int do_add_service(struct binder_state *bs,
uint16_t *s, unsigned len,
void *ptr, unsigned uid)
{
struct svcinfo *si;
// logi("add_service('%s',%p) uid=%d\n", str8(s), ptr, uid);
if (!ptr || (len == 0) || (len > 127))
return -1;
if (!svc_can_register(uid, s)) {
loge("add_service('%s',%p) uid=%d - permission denied\n",
str8(s), ptr, uid);
return -1;
}
si = find_svc(s, len);
if (si) {
if (si->ptr) {
loge("add_service('%s',%p) uid=%d - already registered\n",
str8(s), ptr, uid);
return -1;
}
si->ptr = ptr;
} else {
si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
if (!si) {
loge("add_service('%s',%p) uid=%d - out of memory\n",
str8(s), ptr, uid);
return -1;
}
si->ptr = ptr;
si->len = len;
memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
si->name[len] = '\0';
si->death.func = svcinfo_death;
si->death.ptr = si;
si->next = svclist;
svclist = si;
}
binder_acquire(bs, ptr);
binder_link_to_death(bs, ptr, &si->death);
return 0;
}
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。
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