add Binder 的理解

blogs/Android/Framework/Interview/进程间通信相关/Binder 的理解.md

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+---
+谈谈你对 Binder 的理解？
+---
+
+1. binder 是干嘛的？
+2. binder 存在的意义是什么？
+3. binder 的架构原理是怎样的？
+
+#### Binder 是干嘛的？
+
+进程间通信。
+
+#### Binder 存在的意义是什么？
+
+1. 性能
+2. 方便易用
+3. 安全
+
+#### 架构原理
+
+通信架构：
+
+![](https://i.loli.net/2020/03/27/REqCWzQSnokHKFw.png)
+
+#### 进程如何启用 Binder 机制？
+
+1. 打开 binder 驱动
+2. 内存映射，分配缓冲区
+3. 启动 binder 线程
+
+```c++
+// ServiceManager#main
+int main(int argc, char **argv){
+    struct binder_state *bs;
+    bs = binder_open(128*1024);
+    binder_become_context_manager(bs);
+    binder_loop(bs, svcmgr_handler);
+    return 0;
+}
+void binder_loop(struct binder_state *bs, binder_handler func){
+    // 把当前线程注册为 binder 线程，也就是 SM 的主线程
+    readbuf[0] = BC_ENTER_LOOPER;
+    binder_write(bs, readbuf, sizeof(uint32_t));
+    for(;;){
+        bwr.read_size = sizeof(readbuf);
+        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
+        res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);
+    }
+}
+```
+
+以 SurfaceFlinger 注册服务为例：
+
+```c++
+int main(int, char**){
+    // 启动 binder 机制
+	sp<ProcessState> ps(ProcessState::self());
+    ps->startThreadPool();
+    // 初始化服务
+    sp<SurfaceFlinger> flinger = new SurfaceFlinger();
+    flinger->init();
+    // 注册服务
+    sp<IServiceManager> sm(defaultServiceManager());
+    sm->addService(String16(SurfaceFlinger::getServiceName(), flinger, false));
+    flinger->run();
+    return 0;
+}
+sp<IServiceManager> defaultServiceManager(){
+    while(gDefaultServiceManager==NULL){
+        ProcessState::self()->getContextObject(NULL);
+        if(gDefaultServiceManager==NULL){
+            sleep(1);
+        }
+    }
+    return gDefaultServiceManager;
+}
+status_t addService(const String16& name, const sp<IBinder>& service, ...){
+    Parcel data, reply;
+    data.writeInterfaceToken(IServiceManager::getInterfaceDescriptior());
+    data.writeString16(name);
+    data.writeStrongBinder(service);
+    data.writeInt32(allowlsolated?1:0);
+    status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
+    return err == NO_ERROR?reply.readExceptionCode():err;
+}
+status_t IPCThreadState::transact(int32_t handle, unit32_t code, ...){
+    err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, ...);
+    if((flag&TF_ONE_WAY)==0){
+        if(reply){
+            err=waitForResponse(reply);
+        }else{
+            Parcel fakeReply;
+            err = waitForResponse(&fakeReply);
+        }
+    }else{
+        err = waitForResponse(NULL, NULL);
+    }
+    return err;
+}
+status_t BnServiceManager::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){
+    switch(code){
+        case ADD_SERVICE_TRANSACTION:{
+            CHECK_INTERFACE(IServiceManager, data, reply);
+            String16 which = data.readString16();
+            sp<IBinder> b = data.readStrongBinder();
+            status_t err = addService(which, b);
+            reply->writeInt32(err);
+            return NO_ERROR;
+        }break;
+    }
+}
+```
+
+![](https://i.loli.net/2020/03/28/1qUCWh5B7vSVzAe.png)
+