Android匿名内存怎么实现

寻技术 Android 2023年07月25日 86

本篇内容主要讲解“Android匿名内存怎么实现”,感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷,实用性强。下面就让小编来带大家学习“Android匿名内存怎么实现”吧!

    Android 匿名内存解析

    有了binder机制为什么还需要匿名内存来实现IPC呢?我觉得很大的原因就是binder传输是有大小限制的,不说应用层的限制。在驱动中binder的传输大小被限制在了4M,分享一张图片可能就超过了这个限制。匿名内存的主要解决思路就是通过binder传输文件描述符,使得两个进程都能访问同一个地址来实现共享。

    MemoryFile使用

    在平常开发中android提供了MemoryFile来实现匿名内存。看下最简单的实现。

    Service端

    const val GET_ASH_MEMORY = 1000
    class MyService : Service() {
        val ashData = "AshDemo".toByteArray()
        override fun onBind(intent: Intent): IBinder {
            return object : Binder() {
                override fun onTransact(code: Int, data: Parcel, reply: Parcel?, flags: Int): Boolean {
                    when(code){
                        GET_ASH_MEMORY->{//收到客户端请求的时候会烦
                            val descriptor = createMemoryFile()
                            reply?.writeParcelable(descriptor, 0)
                            reply?.writeInt(ashData.size)
                            return true
                        }
                        else->{
                            return super.onTransact(code, data, reply, flags)
                        }
                    }
                }
            }
        }
        private fun createMemoryFile(): ParcelFileDescriptor? {
            val file = MemoryFile("AshFile", 1024)//创建MemoryFile
            val descriptorMethod = file.javaClass.getDeclaredMethod("getFileDescriptor")
            val fd=descriptorMethod.invoke(file)//反射拿到fd
            file.writeBytes(ashData, 0, 0,ashData.size)//写入字符串
            return ParcelFileDescriptor.dup(fd as FileDescriptor?)//返回一个封装的fd
        }
    }

    Server的功能很简单收到GET_ASH_MEMORY请求的时候创建一个MemoryFile,往里写入一个字符串的byte数组,然后将fd和字符长度写入reply中返回给客户端。

    Client端

    class MainActivity : AppCompatActivity() {
        val connect = object :ServiceConnection{
            override fun onServiceConnected(name: ComponentName?, service: IBinder?) {
                val reply = Parcel.obtain()
                val sendData = Parcel.obtain()
                service?.transact(GET_ASH_MEMORY, sendData, reply, 0)//传输信号GET_ASH_MEMORY
                val pfd = reply.readParcelable<ParcelFileDescriptor>(javaClass.classLoader)
                val descriptor = pfd?.fileDescriptor//拿到fd
                val size = reply.readInt()//拿到长度
                val input = FileInputStream(descriptor)
                val bytes = input.readBytes()
                val message = String(bytes, 0, size, Charsets.UTF_8)//生成string
                Toast.makeText(this@MainActivity,message,Toast.LENGTH_SHORT).show()
            }
    
            override fun onServiceDisconnected(name: ComponentName?) {
            }
    
        }
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
            findViewById<TextView>(R.id.intent).setOnClickListener {
              //启动服务
                bindService(Intent(this,MyService::class.java),connect, Context.BIND_AUTO_CREATE)
            }
        }
    }

    客户端也很简单,启动服务,发送一个获取MemoryFile的请求,然后通过reply拿到fd和长度,用FileInputStream读取fd中的内容,最后通过toast可以验证这个message已经拿到了。

    AshMemory 创建原理

        public MemoryFile(String name, int length) throws IOException {
            try {
                mSharedMemory = SharedMemory.create(name, length);
                mMapping = mSharedMemory.mapReadWrite();
            } catch (ErrnoException ex) {
                ex.rethrowAsIOException();
            }
        }

    MemoryFile就是对SharedMemory的一层封装,具体的工能都是SharedMemory实现的。看SharedMemory的实现。

        public static @NonNull SharedMemory create(@Nullable String name, int size)
                throws ErrnoException {
            if (size <= 0) {
                throw new IllegalArgumentException("Size must be greater than zero");
            }
            return new SharedMemory(nCreate(name, size));
        }
      private static native FileDescriptor nCreate(String name, int size) throws ErrnoException;

    通过一个JNI获得fd,从这里可以推断出java层也只是一个封装,拿到的已经是创建好的fd。

    //frameworks/base/core/jni/android_os_SharedMemory.cpp
    jobject SharedMemory_nCreate(JNIEnv* env, jobject, jstring jname, jint size) {
        const char* name = jname ? env->GetStringUTFChars(jname, nullptr) : nullptr;
        int fd = ashmem_create_region(name, size);//创建匿名内存块
        int err = fd < 0 ? errno : 0;
        if (name) {
            env->ReleaseStringUTFChars(jname, name);
        }
        if (fd < 0) {
            jniThrowErrnoException(env, "SharedMemory_create", err);
            return nullptr;
        }
        jobject jifd = jniCreateFileDescriptor(env, fd);//创建java fd返回
        if (jifd == nullptr) {
            close(fd);
        }
        return jifd;
    }

    通过cutils中的ashmem_create_region函数实现的创建

    //system/core/libcutils/ashmem-dev.cpp
    int ashmem_create_region(const char *name, size_t size)
    {
        int ret, save_errno;
    
        if (has_memfd_support()) {//老版本兼容用
            return memfd_create_region(name ? name : "none", size);
        }
    
        int fd = __ashmem_open();//打开Ashmem驱动
        if (fd < 0) {
            return fd;
        }
        if (name) {
            char buf[ASHMEM_NAME_LEN] = {0};
            strlcpy(buf, name, sizeof(buf));
            ret = TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_SET_NAME, buf));//通过ioctl设置名字
            if (ret < 0) {
                goto error;
            }
        }
        ret = TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_SET_SIZE, size));//通过ioctl设置大小
        if (ret < 0) {
            goto error;
        }
        return fd;
    error:
        save_errno = errno;
        close(fd);
        errno = save_errno;
        return ret;
    }

    标准的驱动交互操作

    1.open打开驱动

    2.通过ioctl与驱动进行交互

    下面看下open的流程

    static int __ashmem_open()
    {
        int fd;
    
        pthread_mutex_lock(&__ashmem_lock);
        fd = __ashmem_open_locked();
        pthread_mutex_unlock(&__ashmem_lock);
    
        return fd;
    }
    
    /* logistics of getting file descriptor for ashmem */
    static int __ashmem_open_locked()
    {
        static const std::string ashmem_device_path = get_ashmem_device_path();//拿到Ashmem驱动路径
        if (ashmem_device_path.empty()) {
            return -1;
        }
        int fd = TEMP_FAILURE_RETRY(open(ashmem_device_path.c_str(), O_RDWR | O_CLOEXEC));
        return fd;
    }

    回到MemoryFile的构造函数中,拿到了驱动的fd之后调用了mapReadWrite

        public @NonNull ByteBuffer mapReadWrite() throws ErrnoException {
            return map(OsConstants.PROT_READ | OsConstants.PROT_WRITE, 0, mSize);
        }
     public @NonNull ByteBuffer map(int prot, int offset, int length) throws ErrnoException {
            checkOpen();
            validateProt(prot);
            if (offset < 0) {
                throw new IllegalArgumentException("Offset must be >= 0");
            }
            if (length <= 0) {
                throw new IllegalArgumentException("Length must be > 0");
            }
            if (offset + length > mSize) {
                throw new IllegalArgumentException("offset + length must not exceed getSize()");
            }
            long address = Os.mmap(0, length, prot, OsConstants.MAP_SHARED, mFileDescriptor, offset);//调用了系统的mmap
            boolean readOnly = (prot & OsConstants.PROT_WRITE) == 0;
            Runnable unmapper = new Unmapper(address, length, mMemoryRegistration.acquire());
            return new DirectByteBuffer(length, address, mFileDescriptor, unmapper, readOnly);
        }

    到这里就有一个疑问,Linux就有共享内存,android为什么要自己搞一套,只能看下Ashmemory驱动的实现了。

    驱动第一步看init和file_operations

    static int __init ashmem_init(void)
    {
        int ret = -ENOMEM;
    
        ashmem_area_cachep = kmem_cache_create("ashmem_area_cache",
                               sizeof(struct ashmem_area),
                               0, 0, NULL);//创建
        if (!ashmem_area_cachep) {
            pr_err("failed to create slab cache
    ");
            goto out;
        }
    
        ashmem_range_cachep = kmem_cache_create("ashmem_range_cache",
                            sizeof(struct ashmem_range),
                            0, SLAB_RECLAIM_ACCOUNT, NULL);//创建
        if (!ashmem_range_cachep) {
            pr_err("failed to create slab cache
    ");
            goto out_free1;
        }
    
        ret = misc_register(&ashmem_misc);//注册为了一个misc设备
        ........
        return ret;
    }

    创建了两个内存分配器ashmem_area_cachep和ashmem_range_cachep用于分配ashmem_area和ashmem_range

    //common/drivers/staging/android/ashmem.c
    static const struct file_operations ashmem_fops = {
        .owner = THIS_MODULE,
        .open = ashmem_open,
        .release = ashmem_release,
        .read_iter = ashmem_read_iter,
        .llseek = ashmem_llseek,
        .mmap = ashmem_mmap,
        .unlocked_ioctl = ashmem_ioctl,
    #ifdef CONFIG_COMPAT
        .compat_ioctl = compat_ashmem_ioctl,
    #endif
    #ifdef CONFIG_PROC_FS
        .show_fdinfo = ashmem_show_fdinfo,
    #endif
    };

    open调用的就是ashmem_open

    static int ashmem_open(struct inode *inode, struct file *file)
    {
        struct ashmem_area *asma;
        int ret;
    
        ret = generic_file_open(inode, file);
        if (ret)
            return ret;
    
        asma = kmem_cache_zalloc(ashmem_area_cachep, GFP_KERNEL);//分配一个ashmem_area
        if (!asma)
            return -ENOMEM;
    
        INIT_LIST_HEAD(&asma->unpinned_list);//初始化unpinned_list
        memcpy(asma->name, ASHMEM_NAME_PREFIX, ASHMEM_NAME_PREFIX_LEN);//初始化一个名字
        asma->prot_mask = PROT_MASK;
        file->private_data = asma;
        return 0;
    }

    ioctl设置名字和长度调用的就是ashmem_ioctl

    static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
    {
        struct ashmem_area *asma = file->private_data;
        long ret = -ENOTTY;
    
        switch (cmd) {
        case ASHMEM_SET_NAME:
            ret = set_name(asma, (void __user *)arg);
            break;
        case ASHMEM_SET_SIZE:
            ret = -EINVAL;
            mutex_lock(&ashmem_mutex);
            if (!asma->file) {
                ret = 0;
                asma->size = (size_t)arg;
            }
            mutex_unlock(&ashmem_mutex);
            break;
        }
      ........
      }

    实现也都很简单就是改变了一下asma里的值。接下来就是重点mmap了,具体是怎么分配内存的。

    static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
    {
        static struct file_operations vmfile_fops;
        struct ashmem_area *asma = file->private_data;
        int ret = 0;
    
        mutex_lock(&ashmem_mutex);
    
        /* user needs to SET_SIZE before mapping */
        if (!asma->size) {//判断设置了size
            ret = -EINVAL;
            goto out;
        }
    
        /* requested mapping size larger than object size */
        if (vma->vm_end - vma->vm_start > PAGE_ALIGN(asma->size)) {//判断大小是否超过了虚拟内存
            ret = -EINVAL;
            goto out;
        }
    
        /* requested protection bits must match our allowed protection mask */
        if ((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask, 0)) &
            calc_vm_prot_bits(PROT_MASK, 0)) {//权限判断
            ret = -EPERM;
            goto out;
        }
        vma->vm_flags &= ~calc_vm_may_flags(~asma->prot_mask);
    
        if (!asma->file) {//是否创建过临时文件,没创建过进入
            char *name = ASHMEM_NAME_DEF;
            struct file *vmfile;
            struct inode *inode;
    
            if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '
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