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

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：

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

// 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：

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;
    }
}