Nuttx相關的歷史文章：

Nuttx Task Schedule

Nuttx信號機制

Nuttx編譯系統(tǒng)

Nuttx消息隊列機制

Nuttx工作隊列機制

Nuttx信號量機制

Nuttx內存管理

Nuttx驅動機制

1. 介紹

文件系統(tǒng)，是對一個存儲設備上的數(shù)據(jù)和元數(shù)據(jù)進行組織的機制，它是操作系統(tǒng)管理持久性數(shù)據(jù)的子系統(tǒng)，提供數(shù)據(jù)存儲和訪問功能。
將一個文件系統(tǒng)與一個存儲設備關聯(lián)起來的過程叫做掛載（mount），掛載時會將一個文件系統(tǒng)附著到當前文件系統(tǒng)層次結構中（根），在執(zhí)行掛載時，需要提供文件系統(tǒng)類型、文件系統(tǒng)和一個掛載點。

1.1 Nuttx文件系統(tǒng)介紹

Nuttx包含了一個可選的、可擴展的文件系統(tǒng)，這個文件系統(tǒng)可以完全省略掉，Nuttx不依賴于任何文件系統(tǒng)的存在。

偽根文件系統(tǒng)
可以通過將CONFIG_NFILE_DESCRIPTOS設置成非零值，來使能這個內存中的偽文件系統(tǒng)。它是一個內存文件系統(tǒng)，因為它不需要任何存儲介質或塊驅動程序的支持。文件系統(tǒng)內容是通過標準文件系統(tǒng)操作（open, close, read, write, etc.）實時生成的。在這個意義上，它是一個偽文件系統(tǒng)（Linux的/proc也稱為偽文件系統(tǒng)）。
可以通過偽文件系統(tǒng)訪問用戶提供的任何數(shù)據(jù)或邏輯。支持對字符設備驅動及塊設備驅動節(jié)點在偽文件系統(tǒng)任何目錄中的內建，不過按照慣例，都習慣放在/dev偽文件系統(tǒng)目錄中。
文件系統(tǒng)掛載
簡單的內存文件系統(tǒng)，可通過掛載塊設備來擴展，這些塊設備提供大容量存儲設備支持以實現(xiàn)真正的文件系統(tǒng)訪問。Nuttx支持標準的mount()命令，該命令允許塊驅動程序將文件系統(tǒng)綁定到偽文件系統(tǒng)中的掛載點上。目前，Nuttx支持VFAT文件系統(tǒng)。
與Linux比較
從編程的角度來看，Nuttx文件系統(tǒng)看起來與Linux文件系統(tǒng)非常類似，但是，有一個根本的區(qū)別：Nuttx根文件系統(tǒng)是一個偽文件系統(tǒng)，而真正的文件系統(tǒng)可以掛載在偽文件系統(tǒng)中；相比之下，在典型的Linux安裝中，Linux根文件系統(tǒng)是一個真正的文件系統(tǒng)，偽文件系統(tǒng)掛載在真正的根文件系統(tǒng)中。Nuttx選擇的方法，旨在提供從非常小的平臺到中等平臺等的支持，以便具備更好的可擴展性。

2. 數(shù)據(jù)結構

2.1 struct inode

inode是文件系統(tǒng)中最重要的結構，存放基本的數(shù)據(jù)：

struct inode
{
  FAR struct inode *i_peer;     /* Link to same level inode */
  FAR struct inode *i_child;    /* Link to lower level inode */
  int16_t           i_crefs;    /* References to inode */
  uint16_t          i_flags;    /* Flags for inode */
  union inode_ops_u u;          /* Inode operations */
#ifdef CONFIG_FILE_MODE
  mode_t            i_mode;     /* Access mode flags */
#endif
  FAR void         *i_private;  /* Per inode driver private data */
  char              i_name[1];  /* Name of inode (variable) */
};

i_peer和i_child會將inode組織成樹狀結構；
i_flags用于表明文件類型，比如Character driver/Block driver/Mount point/Special OS type/Named semaphore/Message Queue/Shared memory region/Soft link等
i_private在驅動中，通常用于存放私有數(shù)據(jù)；
union inode_ops_u u，存放對inode的操作函數(shù)集，而針對不同的inode類型，對應不同的操作函數(shù)；

2.2 union inode_ops_u

union inode_ops_u
{
  FAR const struct file_operations     *i_ops;    /* Driver operations for inode */
#ifndef CONFIG_DISABLE_MOUNTPOINT
  FAR const struct block_operations    *i_bops;   /* Block driver operations */
  FAR const struct mountpt_operations  *i_mops;   /* Operations on a mountpoint */
#endif
#ifdef CONFIG_FS_NAMED_SEMAPHORES
  FAR struct nsem_inode_s              *i_nsem;   /* Named semaphore */
#endif
#ifndef CONFIG_DISABLE_MQUEUE
  FAR struct mqueue_inode_s            *i_mqueue; /* POSIX message queue */
#endif
#ifdef CONFIG_PSEUDOFS_SOFTLINKS
  FAR char                             *i_link;   /* Full path to link target */
#endif
};

主要有三個操作函數(shù)集，此外由于VFS也維護了像Named semaphores/Message Queues/Shared memory等資源，但是這些資源又不像其他類型的文件有函數(shù)操作集，算是special case，也放在這個結構中。

struct file_operations，存放對驅動的操作，一般在實現(xiàn)驅動程序中，都會去實現(xiàn)對應的函數(shù)操作；

struct file_operations
{
  /* The device driver open method differs from the mountpoint open method */

  int     (*open)(FAR struct file *filep);

  /* The following methods must be identical in signature and position because
   * the struct file_operations and struct mountp_operations are treated like
   * unions.
   */

  int     (*close)(FAR struct file *filep);
  ssize_t (*read)(FAR struct file *filep, FAR char *buffer, size_t buflen);
  ssize_t (*write)(FAR struct file *filep, FAR const char *buffer, size_t buflen);
  off_t   (*seek)(FAR struct file *filep, off_t offset, int whence);
  int     (*ioctl)(FAR struct file *filep, int cmd, unsigned long arg);

  /* The two structures need not be common after this point */

#ifndef CONFIG_DISABLE_POLL
  int     (*poll)(FAR struct file *filep, struct pollfd *fds, bool setup);
#endif
#ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
  int     (*unlink)(FAR struct inode *inode);
#endif
};

struct block_operations，存放塊設備的操作函數(shù)集，用于文件系統(tǒng)轉換；

struct block_operations
{
  int     (*open)(FAR struct inode *inode);
  int     (*close)(FAR struct inode *inode);
  ssize_t (*read)(FAR struct inode *inode, FAR unsigned char *buffer,
            size_t start_sector, unsigned int nsectors);
  ssize_t (*write)(FAR struct inode *inode, FAR const unsigned char *buffer,
            size_t start_sector, unsigned int nsectors);
  int     (*geometry)(FAR struct inode *inode, FAR struct geometry *geometry);
  int     (*ioctl)(FAR struct inode *inode, int cmd, unsigned long arg);
#ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
  int     (*unlink)(FAR struct inode *inode);
#endif
};

struct mountpt_operations，由一個文件系統(tǒng)提供，用于描述掛載點；

struct inode;
struct fs_dirent_s;
struct stat;
struct statfs;
struct mountpt_operations
{
  /* The mountpoint open method differs from the driver open method
   * because it receives (1) the inode that contains the mountpoint
   * private data, (2) the relative path into the mountpoint, and (3)
   * information to manage privileges.
   */

  int     (*open)(FAR struct file *filep, FAR const char *relpath,
            int oflags, mode_t mode);

  /* The following methods must be identical in signature and position
   * because the struct file_operations and struct mountp_operations are
   * treated like unions.
   */

  int     (*close)(FAR struct file *filep);
  ssize_t (*read)(FAR struct file *filep, FAR char *buffer, size_t buflen);
  ssize_t (*write)(FAR struct file *filep, FAR const char *buffer,
            size_t buflen);
  off_t   (*seek)(FAR struct file *filep, off_t offset, int whence);
  int     (*ioctl)(FAR struct file *filep, int cmd, unsigned long arg);

  /* The two structures need not be common after this point. The following
   * are extended methods needed to deal with the unique needs of mounted
   * file systems.
   *
   * Additional open-file-specific mountpoint operations:
   */

  int     (*sync)(FAR struct file *filep);
  int     (*dup)(FAR const struct file *oldp, FAR struct file *newp);
  int     (*fstat)(FAR const struct file *filep, FAR struct stat *buf);

  /* Directory operations */

  int     (*opendir)(FAR struct inode *mountpt, FAR const char *relpath,
            FAR struct fs_dirent_s *dir);
  int     (*closedir)(FAR struct inode *mountpt,
            FAR struct fs_dirent_s *dir);
  int     (*readdir)(FAR struct inode *mountpt,
            FAR struct fs_dirent_s *dir);
  int     (*rewinddir)(FAR struct inode *mountpt,
            FAR struct fs_dirent_s *dir);

  /* General volume-related mountpoint operations: */

  int     (*bind)(FAR struct inode *blkdriver, FAR const void *data,
            FAR void **handle);
  int     (*unbind)(FAR void *handle, FAR struct inode **blkdriver,
            unsigned int flags);
  int     (*statfs)(FAR struct inode *mountpt, FAR struct statfs *buf);

  /* Operations on paths */

  int     (*unlink)(FAR struct inode *mountpt, FAR const char *relpath);
  int     (*mkdir)(FAR struct inode *mountpt, FAR const char *relpath,
            mode_t mode);
  int     (*rmdir)(FAR struct inode *mountpt, FAR const char *relpath);
  int     (*rename)(FAR struct inode *mountpt, FAR const char *oldrelpath,
            FAR const char *newrelpath);
  int     (*stat)(FAR struct inode *mountpt, FAR const char *relpath,
            FAR struct stat *buf);

  /* NOTE:  More operations will be needed here to support:  disk usage
   * stats file stat(), file attributes, file truncation, etc.
   */
};

2.3 struct file

一個打開的文件對應一個struct file結構，在該結構中包含了inode，用于描述文件的類型以及對應的函數(shù)操作集。

struct file
{
  int               f_oflags;   /* Open mode flags */
  off_t             f_pos;      /* File position */
  FAR struct inode *f_inode;    /* Driver or file system interface */
  void             *f_priv;     /* Per file driver private data */
};

在每個進程中，struct tcb_s結構中都有一個struct filelist結構，用于維護打開的文件，當一個進程調用POSIX接口open來打開時，會得到文件描述符，文件描述符對應的就是這個文件數(shù)組的索引值。

struct filelist
{
  sem_t   fl_sem;               /* Manage access to the file list */
  struct file fl_files[CONFIG_NFILE_DESCRIPTORS];
}

3. 原理分析

3.1 框架分析

架構框圖如下所示：

文件系統(tǒng)框圖

用戶層，通過系統(tǒng)調用調到VFS層的通用接口；
VFS層，相當于一個適配層，用于對接不同的實際文件系統(tǒng)；
實際文件系統(tǒng)層，典型的情況下一個文件系統(tǒng)都需要綁定到塊設備驅動程序上，而一些不太典型的情況是不需要塊設備驅動，比如偽文件系統(tǒng)（BINFS、PROCFS）和MTD文件系統(tǒng)（NXFFS）。在Nuttx中，需要塊設備驅動的文件系統(tǒng)為：FAT、ROMFS、SMARTFS；而不需要塊設備驅動的文件系統(tǒng)為：NXFFS、BINFS、PROCFS、NFS、TMPFS等；
MTD，Memory Technology Devices，向上提供MTD接口，向下對接不同的硬件設備；

3.2 mount流程

mount()函數(shù)用于將source塊設備指定的文件系統(tǒng)與根文件系統(tǒng)中target指定的路徑名關聯(lián)在一起。
在講mount()之前，需要先了解一下數(shù)據(jù)結構：

struct fsmap_t

struct fsmap_t
{
  FAR const char                      *fs_filesystemtype;
  FAR const struct mountpt_operations *fs_mops;
};

這個結構完成的就是文件系統(tǒng)名字和對應的操作函數(shù)集的映射，在mount()函數(shù)中會根據(jù)對應的文件系統(tǒng)名字去查找struct mountpt_operations。

struct mountpt_operations
這個數(shù)據(jù)結構在上文中介紹過了，有一個函數(shù)需要特別注意一下：

int     (*bind)(FAR struct inode *blkdriver, FAR const void *data, FAR void **handle);

該函數(shù)用于將文件系統(tǒng)與某個塊設備驅動進行綁定。

mount()關鍵代碼如下：

int mount(FAR const char *source, FAR const char *target,
          FAR const char *filesystemtype, unsigned long mountflags,
          FAR const void *data)
{
#if defined(BDFS_SUPPORT) || defined(NONBDFS_SUPPORT)
#ifdef BDFS_SUPPORT
  FAR struct inode *blkdrvr_inode = NULL;
#endif
  FAR struct inode *mountpt_inode;
  FAR const struct mountpt_operations *mops;
#ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
  struct inode_search_s desc;
#endif
  void *fshandle;
  int errcode;
  int ret;

  /* Verify required pointer arguments */

  DEBUGASSERT(target && filesystemtype);

  /* Find the specified filesystem.  Try the block driver file systems first */

#ifdef BDFS_SUPPORT
  if (source && (mops = mount_findfs(g_bdfsmap, filesystemtype)) != NULL)
    {
      /* Make sure that a block driver argument was provided */

      DEBUGASSERT(source);

      /* Find the block driver */

      ret = find_blockdriver(source, mountflags, &blkdrvr_inode);
      if (ret < 0)
        {
          ferr("ERROR: Failed to find block driver %s\n", source);
          errcode = -ret;
          goto errout;
        }
    }
  else
#endif /* BDFS_SUPPORT */
#ifdef NONBDFS_SUPPORT
  if ((mops = mount_findfs(g_nonbdfsmap, filesystemtype)) != NULL)
    {
    }
  else
#endif /* NONBDFS_SUPPORT */
    {
      ferr("ERROR: Failed to find file system %s\n", filesystemtype);
      errcode = ENODEV;
      goto errout;
    }

  inode_semtake();

#ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
  /* Check if the inode already exists */

  SETUP_SEARCH(&desc, target, false);

  ret = inode_find(&desc);
  if (ret >= 0)
    {
      /* Successfully found.  The reference count on the inode has been
       * incremented.
       */

      mountpt_inode = desc.node;
      DEBUGASSERT(mountpt_inode != NULL);

      /* But is it a directory node (i.e., not a driver or other special
       * node)?
       */

      if (INODE_IS_SPECIAL(mountpt_inode))
        {
          ferr("ERROR: target %s exists and is a special node\n", target);
          errcode = -ENOTDIR;
          inode_release(mountpt_inode);
          goto errout_with_semaphore;
        }
    }
  else
#endif

  /* Insert a dummy node -- we need to hold the inode semaphore
   * to do this because we will have a momentarily bad structure.
   * NOTE that the new inode will be created with an initial reference
   * count of zero.
   */

    {
      ret = inode_reserve(target, &mountpt_inode);
      if (ret < 0)
        {
          /* inode_reserve can fail for a couple of reasons, but the most likely
           * one is that the inode already exists. inode_reserve may return:
           *
           *  -EINVAL - 'path' is invalid for this operation
           *  -EEXIST - An inode already exists at 'path'
           *  -ENOMEM - Failed to allocate in-memory resources for the operation
           */

          ferr("ERROR: Failed to reserve inode for target %s\n", target);
          errcode = -ret;
          goto errout_with_semaphore;
        }
    }

  /* Bind the block driver to an instance of the file system.  The file
   * system returns a reference to some opaque, fs-dependent structure
   * that encapsulates this binding.
   */

  if (!mops->bind)
    {
      /* The filesystem does not support the bind operation ??? */

      ferr("ERROR: Filesystem does not support bind\n");
      errcode = EINVAL;
      goto errout_with_mountpt;
    }

  /* Increment reference count for the reference we pass to the file system */

#ifdef BDFS_SUPPORT
#ifdef NONBDFS_SUPPORT
  if (blkdrvr_inode)
#endif
    {
      blkdrvr_inode->i_crefs++;
    }
#endif

  /* On failure, the bind method returns -errorcode */

#ifdef BDFS_SUPPORT
  ret = mops->bind(blkdrvr_inode, data, &fshandle);
#else
  ret = mops->bind(NULL, data, &fshandle);
#endif
  if (ret != 0)
    {
      /* The inode is unhappy with the blkdrvr for some reason.  Back out
       * the count for the reference we failed to pass and exit with an
       * error.
       */

      ferr("ERROR: Bind method failed: %d\n", ret);
#ifdef BDFS_SUPPORT
#ifdef NONBDFS_SUPPORT
      if (blkdrvr_inode)
#endif
        {
          blkdrvr_inode->i_crefs--;
        }
#endif
      errcode = -ret;
      goto errout_with_mountpt;
    }

  /* We have it, now populate it with driver specific information. */

  INODE_SET_MOUNTPT(mountpt_inode);

  mountpt_inode->u.i_mops  = mops;
#ifdef CONFIG_FILE_MODE
  mountpt_inode->i_mode    = mode;
#endif
  mountpt_inode->i_private = fshandle;
  inode_semgive();

  /* We can release our reference to the blkdrver_inode, if the filesystem
   * wants to retain the blockdriver inode (which it should), then it must
   * have called inode_addref().  There is one reference on mountpt_inode
   * that will persist until umount2() is called.
   */
...
}

完成的工作主要有：

調用mount_findfs()函數(shù)，根據(jù)傳入的參數(shù)filesystemtype來找到對應的文件系統(tǒng)操作函數(shù)集mops，如果是需要塊設備支持的文件系統(tǒng)，則需要調用find_blockdriver()來查找傳入?yún)?shù)source對應的塊設備驅動；
根據(jù)傳入?yún)?shù)target，來查找需要mount的路徑對應的inode節(jié)點，如果沒有的話需要調用inode_reserve()創(chuàng)建一個mountpt_inode；
調用mops->bind()函數(shù)將文件系統(tǒng)與塊設備驅動進行綁定，如果不需要塊設備支持的文件系統(tǒng)，bind()函數(shù)可能不需要做特殊處理，而在需要塊設備支持的文件系統(tǒng)中，bind()函數(shù)最終會將該文件系統(tǒng)的整體狀態(tài)都傳出來，保存在fshandle中；
更新掛載點mountpt_inode的內容，包括操作函數(shù)集mops，以及將fshandle保存的文件系統(tǒng)的整體狀態(tài)放置到mountpt_inode結構中的i_private字段中。當打開這個掛載點mountpt_inode時，便可以根據(jù)這個字段來取出對應的信息。

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Nuttx文件系統(tǒng)

Nuttx文件系統(tǒng)

1. 介紹

1.1 Nuttx文件系統(tǒng)介紹

2. 數(shù)據(jù)結構

2.1 struct inode

2.2 union inode_ops_u

2.3 `struct file`

3. 原理分析

3.1 框架分析

3.2 mount流程

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Nuttx文件系統(tǒng)

1. 介紹

1.1 Nuttx文件系統(tǒng)介紹

2. 數(shù)據(jù)結構

2.1 struct inode

2.2 union inode_ops_u

2.3 struct file

3. 原理分析

3.1 框架分析

3.2 mount流程

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友情鏈接更多精彩內容

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2.3 `struct file`