source: rtems-docs/filesystem/mounting_and_unmounting.rst

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 1988, 2002 On-Line Applications Research Corporation (OAR)

Mounting and Unmounting Filesystems
***********************************

Mount Points
============

The following is the list of the characteristics of a mount point:

- The mount point must be a directory. It may have files and other directories
  under it. These files and directories will be hidden when the filesystem is
  mounted.

- The task must have read/write/execute permissions to the mount point or the
  mount attempt will be rejected.

- Only one filesystem can be mounted to a single mount point.

- The Root of the mountable filesystem will be referenced by the name of the
  mount point after the mount is complete.

Mount Table Chain
=================

The mount table chain is a dynamic list of structures that describe mounted
filesystems a specific points in the filesystem hierarchy. It is initialized to
an empty state during the base filesystem initialization.  The mount operation
will add entries to the mount table chain. The un-mount operation will remove
entries from the mount table chain.

Each entry in the mount table chain is of the following type:

.. code-block:: c

    struct rtems_filesystem_mount_table_entry_tt
    {
        Chain_Node                             Node;
        rtems_filesystem_location_info_t       mt_point_node;
        rtems_filesystem_location_info_t       mt_fs_root;
        int                                    options;
        void                                  *fs_info;
        rtems_filesystem_limits_and_options_t  pathconf_limits_and_options;
        /*
         *  When someone adds a mounted filesystem on a real device,
         *  this will need to be used.
         *
         *  The best option long term for this is probably an
         *  open file descriptor.
         */
         char                                  *dev;
    };

*Node*
    The Node is used to produce a linked list of mount table entry nodes.

*mt_point_node*
    The mt_point_node contains all information necessary to access the
    directory where a filesystem is mounted onto.  This element may contain
    memory that is allocated during a path evaluation of the filesystem
    containing the mountpoint directory.  The generic code allows this memory
    to be returned by unmount when the filesystem identified by mt_fs_root is
    unmounted.

*mt_fs_root*
    The mt_fs_root contains all information necessary to identify the root of
    the mounted filesystem. The user is never allowed access to this node by
    the generic code, but it is used to identify to the mounted filesystem
    where to start evaluation of pathnames at.

*options*
    XXX

*fs_info*
    The fs_info element is a location available for use by the mounted file
    system to identify unique things applicable to this instance of the file
    system.  For example the IMFS uses this space to provide node
    identification that is unique for each instance (mounting) of the
    filesystem.

*pathconf_limits_and_options*
    XXX

*dev*
    This character string represents the device where the filesystem will reside.

Adding entries to the chain during mount
========================================

When a filesystem is mounted, its presence and location in the file system
hierarchy is recorded in a dynamic list structure known as a chain.  A unique
rtems_filesystem_mount_table_entry_tt structure is logged for each filesystem
that is mounted. This includes the base filesystem.

Removing entries from the chain during unmount
==============================================

When a filesystem is dismounted its entry in the mount table chain is extracted
and the memory for this entry is freed.