6. Filesystem Implementation Requirements — RTEMS Filesystem Design Guide 5.4-rc2 (21st August 2026) documentation

# 6. Filesystem Implementation Requirements

This chapter details the behavioral requirements that all filesystem implementations must adhere to.

## 6.1. General

The RTEMS filesystem framework was intended to be compliant with the POSIX Files and Directories interface standard. The following filesystem characteristics resulted in a functional switching layer.

    Figure of the Filesystem Functional Layering goes here.
    This figure includes networking and disk caching layering.

# Application programs are presented with a standard set of POSIX compliant

  functions that allow them to interface with the files, devices and directories in the filesystem. The interfaces to these routines do not reflect the type of subordinate filesystem implementation in which the file will be found.

# The filesystem framework developed under RTEMS allows for mounting filesystem

  of different types under the base filesystem.

# The mechanics of locating file information may be quite different between

  filesystem types.

# The process of locating a file may require crossing filesystem boundaries.

# The transitions between filesystem and the processing required to access

  information in different filesystem is not visible at the level of the POSIX function call.

# The POSIX interface standard provides file access by character pathname to

  the file in some functions and through an integer file descriptor in other functions.

# The nature of the integer file descriptor and its associated processing is

  operating system and filesystem specific.

# Directory and device information must be processed with some of the same

  routines that apply to files.

# The form and content of directory and device information differs greatly from

  that of a regular file.

# Files, directories and devices represent elements (nodes) of a tree

  hierarchy.

# The rules for processing each of the node types that exist under the

  filesystem are node specific but are still not reflected in the POSIX interface routines.

    Figure of the Filesystem Functional Layering goes here.
    This figure focuses on the Base Filesystem and IMFS.

    Figure of the IMFS Memfile control blocks

## 6.2. File and Directory Removal Constraints

The following POSIX constraints must be honored by all filesystems.

- If a node is a directory with children it cannot be removed.

- The root node of any filesystem, whether the base filesystem or a mounted filesystem, cannot be removed.

- A node that is a directory that is acting as the mount point of a file system cannot be removed.

- On filesystems supporting hard links, a link count is maintained. Prior to node removal, the node’s link count is decremented by one. The link count must be less than one to allow for removal of the node.

## 6.3. API Layering

### 6.3.1. Mapping of Generic System Calls to Filesystem Specific Functions

The list of generic system calls includes the routines open(), read(), write(), close(), etc..

The Files and Directories section of the POSIX Application Programs Interface specifies a set of functions with calling arguments that are used to gain access to the information in a filesystem. To the application program, these functions allow access to information in any mounted filesystem without explicit knowledge of the filesystem type or the filesystem mount configuration. The following are functions that are provided to the application:

1. access()

2. chdir()

3. chmod()

4. chown()

5. close()

6. closedir()

7. fchmod()

8. fcntl()

9. fdatasync()

10. fpathconf()

11. fstat()

12. fsync()

13. ftruncate()

14. link()

15. lseek()

16. mkdir()

17. mknod()

18. mount()

19. open()

20. opendir()

21. pathconf()

22. read()

23. readdir()

24. rewinddir()

25. rmdir()

26. rmnod()

27. scandir()

28. seekdir()

29. stat()

30. telldir()

31. umask()

32. unlink()

33. unmount()

34. utime()

35. write()

The filesystem’s type as well as the node type within the filesystem determine the nature of the processing that must be performed for each of the functions above. The RTEMS filesystem provides a framework that allows new filesystem to be developed and integrated without alteration to the basic framework.

To provide the functional switching that is required, each of the POSIX file and directory functions have been implemented as a shell function. The shell function adheres to the POSIX interface standard. Within this functional shell, filesystem and node type information is accessed which is then used to invoke the appropriate filesystem and node type specific routine to process the POSIX function call.

### 6.3.2. File/Device/Directory function access via file control block - rtems_libio_t structure

The POSIX open() function returns an integer file descriptor that is used as a reference to file control block information for a specific file. The file control block contains information that is used to locate node, file system, mount table and functional handler information. The diagram in Figure 8 depicts the relationship between and among the following components.

File Descriptor Table:

  This is an internal RTEMS structure that tracks all currently defined file descriptors in the system. The index that is returned by the file open() operation references a slot in this table. The slot contains a pointer to the file descriptor table entry for this file. The rtems_libio_t structure represents the file control block.

Allocation of entry in the File Descriptor Table:

  Access to the file descriptor table is controlled through a semaphore that is implemented using the rtems_libio_allocate() function. This routine will grab a semaphore and then scan the file control blocks to determine which slot is free for use. The first free slot is marked as used and the index to this slot is returned as the file descriptor for the open() request. After the alterations have been made to the file control block table, the semaphore is released to allow further operations on the table.

  Maximum number of entries in the file descriptor table is configurable through the src/exec/sapi/headers/confdefs.h file. If the `CONFIGURE_LIBIO_MAXIMUM_FILE_DESCRIPTORS` constant is defined its value will represent the maximum number of file descriptors that are allowed. If `CONFIGURE_LIBIO_MAXIMUM_FILE_DESCRIPTORS` is not specified a default value of 20 will be used as the maximum number of file descriptors allowed.

File control block - rtems_libio_t structure:

      struct rtems_libio_tt {
          rtems_driver_name_t              *driver;
          off_t                             size;
          off_t                             offset;
          unsigned32                        flags;
          rtems_filesystem_location_info_t  pathinfo;
          Objects_Id                        sem;
          unsigned32                        data0;
          void                              data1;
          void                              file_info;
          rtems_filesystem_file_handlers_r  handlers;
      };

  A file control block can exist for regular files, devices and directories. The following fields are important for regular file and directory access:

  - Size - For a file this represents the number of bytes currently stored in a file. For a directory this field is not filled in.

  - Offset - For a file this is the byte file position index relative to the start of the file. For a directory this is the byte offset into a sequence of dirent structures.

  - Pathinfo - This is a structure that provides a pointer to node information, OPS table functions, Handler functions and the mount table entry associated with this node.

  - file_info - A pointer to node information that is used by Handler functions

  - handlers - A pointer to a table of handler functions that operate on a file, device or directory through a file descriptor index

### 6.3.3. File/Directory function access via rtems_filesystem_location_info_t structure

The `rtems_filesystem_location_info_tt` structure below provides sufficient information to process nodes under a mounted filesystem.

    struct rtems_filesystem_location_info_tt {
        void                                     *node_access;
        rtems_filesystem_file_handlers_r         *handlers;
        rtems_filesystem_operations_table        *ops;
        rtems_filesystem_mount_table_entry_t     *mt_entry;
    };

It contains a void pointer to filesystem specific nodal structure, pointers to the OPS table for the filesystem that contains the node, the node type specific handlers for the node and a reference pointer to the mount table entry associated with the filesystem containing the node

## 6.4. Operation Tables

Filesystem specific operations are invoked indirectly. The set of routines that implement the filesystem are configured into two tables. The Filesystem Handler Table has routines that are specific to a filesystem but remain constant regardless of the actual file type. The File Handler Table has routines that are both filesystem and file type specific.

### 6.4.1. Filesystem Handler Table Functions

OPS table functions are defined in a `rtems_filesystem_operations_table` structure. It defines functions that are specific to a given filesystem. One table exists for each filesystem that is supported in the RTEMS configuration. The structure definition appears below and is followed by general developmental information on each of the functions contained in this function management structure.

    typedef struct {
        rtems_filesystem_evalpath_t        evalpath;
        rtems_filesystem_evalmake_t        evalformake;
        rtems_filesystem_link_t            link;
        rtems_filesystem_unlink_t          unlink;
        rtems_filesystem_node_type_t       node_type;
        rtems_filesystem_mknod_t           mknod;
        rtems_filesystem_rmnod_t           rmnod;
        rtems_filesystem_chown_t           chown;
        rtems_filesystem_freenode_t        freenod;
        rtems_filesystem_mount_t           mount;
        rtems_filesystem_fsmount_me_t      fsmount_me;
        rtems_filesystem_unmount_t         unmount;
        rtems_filesystem_fsunmount_me_t    fsunmount_me;
        rtems_filesystem_utime_t           utime;
        rtems_filesystem_evaluate_link_t   eval_link;
        rtems_filesystem_symlink_t         symlink;
    } rtems_filesystem_operations_table;

#### 6.4.1.1. evalpath Handler

Corresponding Structure Element:

  `evalpath`

Arguments:

      const char                        *pathname,      /* IN     */
      int                                flags,         /* IN     */
      rtems_filesystem_location_info_t  *pathloc        /* IN/OUT */

Description:

  This routine is responsible for evaluating the pathname passed in based upon the flags and the valid `rthems_filesystem_location_info_t`. Additionally, it must make any changes to pathloc necessary to identify the pathname node. This should include calling the evalpath for a mounted filesystem, if the given filesystem supports the mount command.

  This routine returns a 0 if the evaluation was successful. Otherwise, it returns a -1 and sets errno to the correct error.

  This routine is required and should NOT be set to NULL.

#### 6.4.1.2. evalformake Handler

Corresponding Structure Element:

  `evalformake`

Arguments:

      const char                       *path,       /* IN */
      rtems_filesystem_location_info_t *pathloc,    /* IN/OUT */
      const char                      **name        /* OUT */

Description:

  This method is given a path to evaluate and a valid start location. It is responsible for finding the parent node for a requested make command, setting pathloc information to identify the parent node, and setting the name pointer to the first character of the name of the new node. Additionally, if the filesystem supports the mount command, this method should call the evalformake routine for the mounted filesystem.

  This routine returns a 0 if the evaluation was successful. Otherwise, it returns a -1 and sets errno to the correct error.

  This routine is required and should NOT be set to NULL. However, if the filesystem does not support user creation of a new node, it may set errno to ENOSYS and return -1.

#### 6.4.1.3. link Handler

Corresponding Structure Element:

  `link`

Arguments:

      rtems_filesystem_location_info_t    *to_loc,      /* IN */
      rtems_filesystem_location_info_t    *parent_loc,  /* IN */
      const char                          *token        /* IN */

Description:

  This routine is used to create a hard-link.

  It will first examine the st_nlink count of the node that we are trying to. If the link count exceeds LINK_MAX an error will be returned.

  The name of the link will be normalized to remove extraneous separators from the end of the name.

  This routine is not required and may be set to NULL.

#### 6.4.1.4. unlink Handler

Corresponding Structure Element:

  `unlink`

Arguments:

  XXX

Description:

  XXX

#### 6.4.1.5. node_type Handler

Corresponding Structure Element:

  `node_type()`

Arguments:

      rtems_filesystem_location_info_t    *pathloc        /* IN */

Description:

  XXX

#### 6.4.1.6. mknod Handler

Corresponding Structure Element:

  `mknod()`

Arguments:

      const char                          *token,        /* IN */
      mode_t                               mode,         /* IN */
      dev_t                                dev,          /* IN */
      rtems_filesystem_location_info_t    *pathloc       /* IN/OUT */

Description:

  XXX

#### 6.4.1.7. rmnod Handler

Corresponding Structure Element:

  `rmnod()`

Arguments:

  XXX

Description:

  XXX

#### 6.4.1.8. chown Handler

Corresponding Structure Element:

  `chown()`

Arguments:

      rtems_filesystem_location_info_t    *pathloc        /* IN */
      uid_t                                owner          /* IN */
      gid_t                                group          /* IN */

Description:

  XXX

#### 6.4.1.9. freenod Handler

Corresponding Structure Element:

  `freenod()`

Arguments:

      rtems_filesystem_location_info_t      *pathloc       /* IN */

Description:

  This routine is used by the generic code to allow memory to be allocated during the evaluate routines, and set free when the generic code is finished accessing a node. If the evaluate routines allocate memory to identify a node this routine should be utilized to free that memory.

  This routine is not required and may be set to NULL.

#### 6.4.1.10. mount Handler

Corresponding Structure Element:

  `mount()`

Arguments:

      rtems_filesystem_mount_table_entry_t   *mt_entry

Description:

  XXX

#### 6.4.1.11. fsmount_me Handler

Corresponding Structure Element:

  `imfs_fsmount_me`

Arguments:

      rtems_filesystem_mount_table_entry_t   *mt_entry

Description:

  This function is provided with a filesystem to take care of the internal filesystem management details associated with mounting that filesystem under the RTEMS environment.

  It is not responsible for the mounting details associated the filesystem containing the mount point.

  The rtems_filesystem_mount_table_entry_t structure contains the key elements below:

      rtems_filesystem_location_info_t         *mt_point_node,

  This structure contains information about the mount point. This allows us to find the ops-table and the handling functions associated with the filesystem containing the mount point.

      rtems_filesystem_location_info_t         *fs_root_node,

  This structure contains information about the root node in the file system to be mounted. It allows us to find the ops-table and the handling functions associated with the filesystem to be mounted.

  rtems_filesystem_options_t options,

  Read only or read/write access

      void                                         *fs_info,

  This points to an allocated block of memory the will be used to hold any filesystem specific information of a global nature. This allocated region if important because it allows us to mount the same filesystem type more than once under the RTEMS system. Each instance of the mounted filesystem has its own set of global management information that is separate from the global management information associated with the other instances of the mounted filesystem type.

      rtems_filesystem_limits_and_options_t    pathconf_info,

  The table contains the following set of values associated with the mounted filesystem:

  - link_max

  - max_canon

  - max_input

  - name_max

  - path_max

  - pipe_buf

  - posix_async_io

  - posix_chown_restrictions

  - posix_no_trunc

  - posix_prio_io

  - posix_sync_io

  - posix_vdisable

  These values are accessed with the pathconf() and the fpathconf () functions.

      const char                                   *dev

  The is intended to contain a string that identifies the device that contains the filesystem information. The filesystems that are currently implemented are memory based and don’t require a device specification.

  If the mt_point_node.node_access is NULL then we are mounting the base file system.

  The routine will create a directory node for the root of the IMFS file system.

  The node will have read, write and execute permissions for owner, group and others.

  The node’s name will be a null string.

  A filesystem information structure(fs_info) will be allocated and initialized for the IMFS filesystem. The fs_info pointer in the mount table entry will be set to point the filesystem information structure.

  The pathconf_info element of the mount table will be set to the appropriate table of path configuration constants (LIMITS_AND_OPTIONS).

  The fs_root_node structure will be filled in with the following:

  - pointer to the allocated root node of the filesystem

  - directory handlers for a directory node under the IMFS filesystem

  - OPS table functions for the IMFS

  A 0 will be returned to the calling routine if the process succeeded, otherwise a 1 will be returned.

#### 6.4.1.12. unmount Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

#### 6.4.1.13. fsunmount_me Handler

Corresponding Structure Element:

  `imfs_fsunmount_me()`

Arguments:

      rtems_filesystem_mount_table_entry_t   *mt_entry

Description:

  XXX

#### 6.4.1.14. utime Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

#### 6.4.1.15. eval_link Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

#### 6.4.1.16. symlink Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

### 6.4.2. File Handler Table Functions

Handler table functions are defined in a `rtems_filesystem_file_handlers_r` structure. It defines functions that are specific to a node type in a given filesystem. One table exists for each of the filesystem’s node types. The structure definition appears below. It is followed by general developmental information on each of the functions associated with regular files contained in this function management structure.

    typedef struct {
        rtems_filesystem_open_t           open;
        rtems_filesystem_close_t          close;
        rtems_filesystem_read_t           read;
        rtems_filesystem_write_t          write;
        rtems_filesystem_ioctl_t          ioctl;
        rtems_filesystem_lseek_t          lseek;
        rtems_filesystem_fstat_t          fstat;
        rtems_filesystem_fchmod_t         fchmod;
        rtems_filesystem_ftruncate_t      ftruncate;
        rtems_filesystem_fpathconf_t      fpathconf;
        rtems_filesystem_fsync_t          fsync;
        rtems_filesystem_fdatasync_t      fdatasync;
        rtems_filesystem_fcntl_t          fcntl;
    } rtems_filesystem_file_handlers_r;

#### 6.4.2.1. open Handler

Corresponding Structure Element:

  `open()`

Arguments:

      rtems_libio_t   *iop,
      const char      *pathname,
      unsigned32       flag,
      unsigned32       mode

Description:

  XXX

##### 6.4.2.1.1. close Handler

Corresponding Structure Element:

  `close()`

Arguments:

      rtems_libio_t     *iop

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.2. read Handler

Corresponding Structure Element:

  `read()`

Arguments:

      rtems_libio_t     *iop,
      void              *buffer,
      unsigned32         count

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.3. write Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.4. ioctl Handler

Corresponding Structure Element:

  XXX

Arguments:

      rtems_libio_t     *iop,
      unsigned32       command,
      void              *buffer

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.5. lseek Handler

Corresponding Structure Element:

  `lseek()`

Arguments:

      rtems_libio_t     *iop,
      off_t              offset,
      int                whence

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.6. fstat Handler

Corresponding Structure Element:

  `fstat()`

Arguments:

      rtems_filesystem_location_info_t   *loc,
      struct stat                        *buf

Description:

  The following information is extracted from the filesystem specific node and placed in the `stat` structure:

  - st_mode

  - st_nlink

  - st_ino

  - st_uid

  - st_gid

  - st_atime

  - st_mtime

  - st_ctime

NOTES:

  Both the `stat()` and `lstat()` services are implemented directly using the `fstat()` handler. The difference in behavior is determined by how the path is evaluated prior to this handler being called on a particular file entity.

  The `fstat()` system call is implemented directly on top of this filesystem handler.

##### 6.4.2.1.7. fchmod Handler

Corresponding Structure Element:

  `fchmod()`

Arguments:

      rtems_libio_t     *iop
      mode_t             mode

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.8. ftruncate Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.9. fpathconf Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.10. fsync Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.11. fdatasync Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX

##### 6.4.2.1.12. fcntl Handler

Corresponding Structure Element:

  XXX

Arguments:

  XXX

Description:

  XXX

NOTES:

  XXX