rtems-docs/filesystem/call_development.rst

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

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

System Call Development Notes
*****************************

This set of routines represents the application's interface to files and
directories under the RTEMS filesystem. All routines are compliant with POSIX
standards if a specific interface has been established. The list below
represents the routines that have been included as part of the application's
interface.

#. access()

#. chdir()

#. chmod()

#. chown()

#. close()

#. closedir()

#. dup()

#. dup2()

#. fchmod()

#. fcntl()

#. fdatasync()

#. fpathconf()

#. fstat()

#. ioctl()

#. link()

#. lseek()

#. mkdir()

#. mkfifo()

#. mknod()

#. mount()

#. open()

#. opendir()

#. pathconf()

#. read()

#. readdir()

#. unmount()

The sections that follow provide developmental information concerning each of
these functions.

access
======

File:
    ``access.c``

Processing:
    This routine is layered on the stat() function. It acquires the current
    status information for the specified file and then determines if the caller
    has the ability to access the file for read, write or execute according to
    the mode argument to this function.

Development Comments:
    This routine is layered on top of the stat() function. As long as the
    st_mode element in the returned structure follow the standard UNIX
    conventions, this function should support other filesystems without
    alteration.

chdir
=====

File:
    ``chdir.c``

Processing:
    This routine will determine if the pathname that we are attempting to make
    that current directory exists and is in fact a directory. If these
    conditions are met the global indication of the current directory
    (rtems_filesystem_current) is set to the rtems_filesystem_location_info_t
    structure that is returned by the rtems_filesystem_evaluate_path() routine.

Development Comments:
    This routine is layered on the rtems_filesystem_evaluate_path() routine and
    the filesystem specific OP table function node_type().

    The routine ``node_type()`` must be a routine provided for each filesystem
    since it must access the filesystems node information to determine which of
    the following types the node is:

    - RTEMS_FILESYSTEM_DIRECTORY

    - RTEMS_FILESYSTEM_DEVICE

    - RTEMS_FILESYSTEM_HARD_LINK

    - RTEMS_FILESYSTEM_MEMORY_FILE

    This acknowledges that the form of the node management information can vary
    from one filesystem implementation to another.

    RTEMS has a special global structure that maintains the current directory
    location. This global variable is of type rtems_filesystem_location_info_t
    and is called rtems_filesystem_current. This structure is not always
    valid. In order to determine if the structure is valid, you must first test
    the node_access element of this structure. If the pointer is NULL, then the
    structure does not contain a valid indication of what the current directory
    is.

chmod
=====

File:
    ``chmod.c``

Processing:
    This routine is layered on the ``open()``, ``fchmod()`` and ``close()``
    functions. As long as the standard interpretation of the mode_t value is
    maintained, this routine should not need modification to support other
    filesystems.

Development Comments:
    The routine first determines if the selected file can be open with
    read/write access.  This is required to allow modification of the mode
    associated with the selected path.

    The ``fchmod()`` function is used to actually change the mode of the path
    using the integer file descriptor returned by the ``open()`` function.

    After mode modification, the open file descriptor is closed.

chown
=====

File:
    ``chown.c``

Processing:
    This routine is layered on the ``rtems_filesystem_evaluate_path()`` and the
    file system specific ``chown()`` routine that is specified in the OPS table
    for the file system.

Development Comments:
    ``rtems_filesystem_evaluate_path()`` is used to determine if the path
    specified actually exists. If it does a
    ``rtems_filesystem_location_info_t`` structure will be obtained that allows
    the shell function to locate the OPS table that is to be used for this
    filesystem.

    It is possible that the ``chown()`` function that should be in the OPS
    table is not defined. A test for a non-NULL OPS table ``chown()`` entry is
    performed before the function is called.

    If the ``chown()`` function is defined in the indicated OPS table, the
    function is called with the ``rtems_filesystem_location_info_t`` structure
    returned from the path evaluation routine, the desired owner, and group
    information.

close
=====

File:
    ``close.c``

Processing:
    This routine will allow for the closing of both network connections and
    file system devices. If the file descriptor is associated with a network
    device, the appropriate network function handler will be selected from a
    table of previously registered network functions (``rtems_libio_handlers``)
    and that function will be invoked.

    If the file descriptor refers to an entry in the filesystem, the
    appropriate handler will be selected using information that has been placed
    in the file control block for the device (``rtems_libio_t`` structure).

Development Comments:
    ``rtems_file_descriptor_type`` examines some of the upper bits of the file
    descriptor index. If it finds that the upper bits are set in the file
    descriptor index, the device referenced is a network device.

    Network device handlers are obtained from a special registration table
    (``rtems_libio_handlers``) that is set up during network
    initialization. The network handler invoked and the status of the network
    handler will be returned to the calling process.

    If none of the upper bits are set in the file descriptor index, the file
    descriptor refers to an element of the RTEMS filesystem.

    The following sequence will be performed for any filesystem file descriptor:

    #. Use the ``rtems_libio_iop()`` function to obtain the ``rtems_libio_t``
       structure for the file descriptor

    #. Range check the file descriptor using ``rtems_libio_check_fd()``

    #. Determine if there is actually a function in the selected handler table
       that processes the ``close()`` operation for the filesystem and node
       type selected.  This is generally done to avoid execution attempts on
       functions that have not been implemented.

    #. If the function has been defined it is invoked with the file control
       block pointer as its argument.

    #. The file control block that was associated with the open file descriptor
       is marked as free using ``rtems_libio_free()``.

    #. The return code from the close handler is then passed back to the
       calling program.

closedir
========

File:
    ``closedir.c``

Processing:
    The code was obtained from the BSD group. This routine must clean up the
    memory resources that are required to track an open directory. The code is
    layered on the ``close()`` function and standard memory ``free()``
    functions. It should not require alterations to support other filesystems.

Development Comments:
    The routine alters the file descriptor and the index into the DIR structure
    to make it an invalid file descriptor. Apparently the memory that is about
    to be freed may still be referenced before it is reallocated.

    The dd_buf structure's memory is reallocated before the control structure
    that contains the pointer to the dd_buf region.

    DIR control memory is reallocated.

    The ``close()`` function is used to free the file descriptor index.

dup()      Unimplemented
========================

File:
    ``dup.c``

Processing:

Development Comments:

dup2()      Unimplemented
=========================

File:
    ``dup2.c``

Processing:

Development Comments:

fchmod
======

File:
    ``fchmod.c``

Processing:
    This routine will alter the permissions of a node in a filesystem. It is
    layered on the following functions and macros:

    - rtems_file_descriptor_type()

    - rtems_libio_iop()

    - rtems_libio_check_fd()

    - rtems_libio_check_permissions()

    - fchmod() function that is referenced by the handler table in the file
      control block associated with this file descriptor

Development Comments:
    The routine will test to see if the file descriptor index is associated
    with a network connection. If it is, an error is returned from this
    routine.

    The file descriptor index is used to obtain the associated file control
    block.

    The file descriptor value is range checked.

    The file control block is examined to determine if it has write permissions
    to allow us to alter the mode of the file.

    A test is made to determine if the handler table that is referenced in the
    file control block contains an entry for the ``fchmod()`` handler
    function. If it does not, an error is returned to the calling routine.

    If the ``fchmod()`` handler function exists, it is called with the file
    control block and the desired mode as parameters.

fcntl()
=======

File:
    ``fcntl.c``

Processing:
    This routine currently only interacts with the file control block. If the
    structure of the file control block and the associated meanings do not
    change, the partial implementation of ``fcntl()`` should remain unaltered
    for other filesystem implementations.

Development Comments:
    The only commands that have been implemented are the F_GETFD and F_SETFD.
    The commands manipulate the LIBIO_FLAGS_CLOSE_ON_EXEC bit in the``flags``
    element of the file control block associated with the file descriptor
    index.

    The current implementation of the function performs the sequence of
    operations below:

    #. Test to see if we are trying to operate on a file descriptor associated
       with a network connection

    #. Obtain the file control block that is associated with the file
       descriptor index

    #. Perform a range check on the file descriptor index.

fdatasync
=========

File:
    ``fdatasync.c``

Processing:
    This routine is a template in the in memory filesystem that will route us
    to the appropriate handler function to carry out the fdatasync()
    processing. In the in memory filesystem this function is not necessary. Its
    function in a disk based file system that employs a memory cache is to
    flush all memory based data buffers to disk. It is layered on the following
    functions and macros:

    - rtems_file_descriptor_type()

    - rtems_libio_iop()

    - rtems_libio_check_fd()

    - rtems_libio_check_permissions()

    - fdatasync() function that is referenced by the handler table in the file
      control block associated with this file descriptor

Development Comments:
    The routine will test to see if the file descriptor index is associated
    with a network connection. If it is, an error is returned from this
    routine.

    The file descriptor index is used to obtain the associated file control
    block.

    The file descriptor value is range checked.

    The file control block is examined to determine if it has write permissions
    to the file.

    A test is made to determine if the handler table that is referenced in the
    file control block contains an entry for the fdatasync() handler function.
    If it does not an error is returned to the calling routine.

    If the fdatasync() handler function exists, it is called with the file
    control block as its parameter.

fpathconf
=========

File:
    ``fpathconf.c``

Processing:
    This routine is layered on the following functions and macros:

    - rtems_file_descriptor_type()

    - rtems_libio_iop()

    - rtems_libio_check_fd()

    - rtems_libio_check_permissions()

    When a filesystem is mounted, a set of constants is specified for the
    filesystem.  These constants are stored with the mount table entry for the
    filesystem. These constants appear in the POSIX standard and are listed
    below.

    - PCLINKMAX

    - PCMAXCANON

    - PCMAXINPUT

    - PCNAMEMAX

    - PCPATHMAX

    - PCPIPEBUF

    - PCCHOWNRESTRICTED

    - PCNOTRUNC

    - PCVDISABLE

    - PCASYNCIO

    - PCPRIOIO

    - PCSYNCIO

    This routine will find the mount table information associated the file
    control block for the specified file descriptor parameter. The mount table
    entry structure contains a set of filesystem specific constants that can be
    accessed by individual identifiers.

Development Comments:
    The routine will test to see if the file descriptor index is associated
    with a network connection. If it is, an error is returned from this
    routine.

    The file descriptor index is used to obtain the associated file control
    block.

    The file descriptor value is range checked.

    The file control block is examined to determine if it has read permissions
    to the file.

    Pathinfo in the file control block is used to locate the mount table entry
    for the filesystem associated with the file descriptor.

    The mount table entry contains the pathconf_limits_and_options element.
    This element is a table of constants that is associated with the
    filesystem.

    The name argument is used to reference the desired constant from the
    pathconf_limits_and_options table.

fstat
=====

File:
    ``fstat.c``

Processing:
    This routine will return information concerning a file or network
    connection. If the file descriptor is associated with a network connection,
    the current implementation of ``fstat()`` will return a mode set to
    ``S_IFSOCK``. In a later version, this routine will map the status of a
    network connection to an external handler routine.

    If the file descriptor is associated with a node under a filesystem, the
    fstat() routine will map to the fstat() function taken from the node
    handler table.

Development Comments:
    This routine validates that the struct stat pointer is not NULL so that the
    return location is valid.

    The struct stat is then initialized to all zeros.

    rtems_file_descriptor_type() is then used to determine if the file
    descriptor is associated with a network connection. If it is, network
    status processing is performed. In the current implementation, the file
    descriptor type processing needs to be improved. It currently just drops
    into the normal processing for file system nodes.

    If the file descriptor is associated with a node under a filesystem, the
    following steps are performed:

    #. Obtain the file control block that is associated with the file descriptor
       index.

    #. Range check the file descriptor index.

    #. Test to see if there is a non-NULL function pointer in the handler table
       for the fstat() function. If there is, invoke the function with the file
       control block and the pointer to the stat structure.

ioctl
=====

File:
    ``ioctl.c``

Processing:
    Not defined in the POSIX 1003.1b standard but commonly supported in most
    UNIX and POSIX system. Ioctl() is a catchall for I/O operations. Routine is
    layered on external network handlers and filesystem specific handlers.  The
    development of new filesystems should not alter the basic processing
    performed by this routine.

Development Comments:
    The file descriptor is examined to determine if it is associated with a
    network device. If it is processing is mapped to an external network
    handler. The value returned by this handler is then returned to the calling
    program.

    File descriptors that are associated with a filesystem undergo the
    following processing:

    #. The file descriptor index is used to obtain the associated file control
       block.

    #. The file descriptor value is range checked.

    #. A test is made to determine if the handler table that is referenced in
       the file control block contains an entry for the ioctl() handler
       function. If it does not, an error is returned to the calling routine.

    #. If the ioctl() handler function exists, it is called with the file
       control block, the command and buffer as its parameters.

    #. The return code from this function is then sent to the calling routine.

link
====

File:
    ``link.c``

Processing:
    This routine will establish a hard link to a file, directory or a device.
    The target of the hard link must be in the same filesystem as the new link
    being created. A link to an existing link is also permitted but the
    existing link is evaluated before the new link is made. This implies that
    links to links are reduced to links to files, directories or devices before
    they are made.

Development Comments:
    Calling parameters:

    .. code-block:: c

        const char   *existing
        const char   *new

    link() will determine if the target of the link actually exists using
    rtems_filesystem_evaluate_path()

    rtems_filesystem_get_start_loc() is used to determine where to start the
    path evaluation of the new name. This macro examines the first characters
    of the name to see if the name of the new link starts with a
    rtems_filesystem_is_separator. If it does the search starts from the root
    of the RTEMS filesystem; otherwise the search will start from the current
    directory.

    The OPS table evalformake() function for the parent's filesystem is used to
    locate the node that will be the parent of the new link. It will also
    locate the start of the new path's name. This name will be used to define a
    child under the parent directory.

    If the parent is found, the routine will determine if the hard link that we
    are trying to create will cross a filesystem boundary. This is not
    permitted for hard-links.

    If the hard-link does not cross a filesystem boundary, a check is performed
    to determine if the OPS table contains an entry for the link() function.

    If a link() function is defined, the OPS table link() function will be
    called to establish the actual link within the filesystem.

    The return code from the OPS table link() function is returned to the
    calling program.

lseek
=====

File:
    ``lseek.c``

Processing:
    This routine is layered on both external handlers and filesystem / node
    type specific handlers. This routine should allow for the support of new
    filesystems without modification.

Development Comments:
    This routine will determine if the file descriptor is associated with a
    network device. If it is lseek will map to an external network handler.
    The handler will be called with the file descriptor, offset and whence as
    its calling parameters. The return code from the external handler will be
    returned to the calling routine.

    If the file descriptor is not associated with a network connection, it is
    associated with a node in a filesystem. The following steps will be
    performed for filesystem nodes:

    #. The file descriptor is used to obtain the file control block for the
       node.

    #. The file descriptor is range checked.

    #. The offset element of the file control block is altered as indicated by
       the offset and whence calling parameters

    #. The handler table in the file control block is examined to determine if
       it contains an entry for the lseek() function. If it does not an error
       is returned to the calling program.

    #. The lseek() function from the designated handler table is called with
       the file control block, offset and whence as calling arguments

    #. The return code from the lseek() handler function is returned to the
       calling program

mkdir
=====

File:
    ``mkdir.c``

Processing:
    This routine attempts to create a directory node under the filesystem. The
    routine is layered the mknod() function.

Development Comments:
    See mknod() for developmental comments.

mkfifo
======

File:
    ``mkfifo.c``

Processing:
    This routine attempts to create a FIFO node under the filesystem. The
    routine is layered the mknod() function.

Development Comments:
    See mknod() for developmental comments

.. COMMENT: @page

mknod
=====

File:
    ``mknod.c``

Processing:
    This function will allow for the creation of the following types of nodes
    under the filesystem:

    - directories

    - regular files

    - character devices

    - block devices

    - fifos

    At the present time, an attempt to create a FIFO will result in an ENOTSUP
    error to the calling function. This routine is layered the filesystem
    specific routines evalformake and mknod. The introduction of a new
    filesystem must include its own evalformake and mknod function to support
    the generic mknod() function.  Under this condition the generic mknod()
    function should accommodate other filesystem types without alteration.

Development Comments:
    Test for nodal types - I thought that this test should look like the
    following code:

    .. code-block:: c

        if ( (mode & S_IFDIR) = = S_IFDIR) ||
             (mode & S_IFREG) = = S_IFREG) ||
             (mode & S_IFCHR) = = S_IFCHR) ||
             (mode & S_IFBLK) = = S_IFBLK) ||
             (mode & S_IFIFO) = = S_IFIFO))
                Set_errno_and_return_minus_one (EINVAL);

    Where:

    - S_IFREG (0100000) - Creation of a regular file

    - S_IFCHR (0020000) - Creation of a character device

    - S_IFBLK (0060000) - Creation of a block device

    - S_IFIFO (0010000) - Creation of a FIFO

    Determine if the pathname that we are trying to create starts at the root
    directory or is relative to the current directory using the
    ``rtems_filesystem_get_start_loc()`` function.

    Determine if the pathname leads to a valid directory that can be accessed
    for the creation of a node.

    If the pathname is a valid location to create a node, verify that a
    filesystem specific mknod() function exists.

    If the mknod() function exists, call the filesystem specific mknod()
    function.  Pass the name, mode, device type and the location information
    associated with the directory under which the node will be created.

mount
=====

File:
    ``mount.c``

    Arguments (Not a standard POSIX call):

    .. code-block:: c

        rtems_filesystem_mount_table_entry_t   **mt_entry,

    If the mount operation is successful, this pointer to a pointer will be set
    to reference the mount table chain entry that has been allocated for this
    file system mount.

    .. code-block:: c

        rtems_filesystem_operations_table   *fs_ops,

    This is a pointer to a table of functions that are associated with the file
    system that we are about to mount. This is the mechanism to selected file
    system type without keeping a dynamic database of all possible file system
    types that are valid for the mount operation. Using this method, it is only
    necessary to configure the filesystems that we wish to use into the RTEMS
    build. Unused filesystems types will not be drawn into the build.

    .. code-block:: c

        char                      *fsoptions,

    This argument points to a string that selects mounting for read only access
    or read/write access. Valid states are "RO" and "RW"

    .. code-block:: c

        char                      *device,

    This argument is reserved for the name of a device that will be used to
    access the filesystem information. Current filesystem implementations are
    memory based and do not require a device to access filesystem information.

    .. code-block:: c

        char                      *mount_point

    This is a pathname to a directory in a currently mounted filesystem that
    allows read, write and execute permissions.  If successful, the node found
    by evaluating this name, is stored in the mt_entry.

Processing:
    This routine will handle the mounting of a filesystem on a mount point. If
    the operation is successful, a pointer to the mount table chain entry
    associated with the mounted filesystem will be returned to the calling
    function. The specifics about the processing required at the mount point
    and within the filesystem being mounted is isolated in the filesystem
    specific mount() and fsmount_me() functions. This allows the generic
    mount() function to remain unaltered even if new filesystem types are
    introduced.

Development Comments:
    This routine will use get_file_system_options() to determine if the mount
    options are valid ("RO" or "RW").

    It confirms that a filesystem ops-table has been selected.

    Space is allocated for a mount table entry and selective elements of the
    temporary mount table entry are initialized.

    If a mount point is specified: The mount point is examined to determine
    that it is a directory and also has the appropriate permissions to allow a
    filesystem to be mounted.

    The current mount table chain is searched to determine that there is not
    another filesystem mounted at the mount point we are trying to mount onto.

    If a mount function is defined in the ops table for the filesystem
    containing the mount point, it is called at this time.

    If no mount point is specified: Processing if performed to set up the mount
    table chain entry as the base filesystem.

    If the fsmount_me() function is specified for ops-table of the filesystem
    being mounted, that function is called to initialize for the new
    filesystem.

    On successful completion, the temporary mount table entry will be placed on
    the mount table chain to record the presence of the mounted filesystem.

open
====

File:
    ``open.c``

Processing:
    This routine is layered on both RTEMS calls and filesystem specific
    implementations of the open() function. These functional interfaces should
    not change for new filesystems and therefore this code should be stable as
    new file systems are introduced.

Development Comments:
    This routine will allocate a file control block for the file or device that
    we are about to open.

    It will then test to see if the pathname exists. If it does a
    rtems_filesystem_location_info_t data structure will be filled out. This
    structure contains information that associates node information, filesystem
    specific functions and mount table chain information with the pathname.

    If the create option has been it will attempt to create a node for a
    regular file along the specified path. If a file already exists along this
    path, an error will be generated; otherwise, a node will be allocated for
    the file under the filesystem that contains the pathname. When a new node
    is created, it is also evaluated so that an appropriate
    rtems_filesystem_location_info_t data structure can be filled out for the
    newly created node.

    If the file exists or the new file was created successfully, the file
    control block structure will be initialized with handler table information,
    node information and the rtems_filesystem_location_info_t data structure
    that describes the node and filesystem data in detail.

    If an open() function exists in the filesystem specific handlers table for
    the node that we are trying to open, it will be called at this time.

    If any error is detected in the process, cleanup is performed. It consists
    of freeing the file control block structure that was allocated at the
    beginning of the generic open() routine.

    On a successful open(), the index into the file descriptor table will be
    calculated and returned to the calling routine.

opendir
=======

File:
    ``opendir.c``

Processing:
    This routine will attempt to open a directory for read access. It will
    setup a DIR control structure that will be used to access directory
    information. This routine is layered on the generic open() routine and
    filesystem specific directory processing routines.

Development Comments:
    The BSD group provided this routine.

pathconf
========

File:
    ``pathconf.c``

Processing:
    This routine will obtain the value of one of the path configuration
    parameters and return it to the calling routine. It is layered on the
    generic open() and fpathconf() functions. These interfaces should not
    change with the addition of new filesystem types.

Development Comments:
    This routine will try to open the file indicated by path.

    If successful, the file descriptor will be used to access the pathconf
    value specified by ``name`` using the fpathconf() function.

    The file that was accessed is then closed.

read
====

File:
    ``deviceio.c``

Processing:
    This routine is layered on a set of RTEMS calls and filesystem specific
    read operations. The functions are layered in such a way as to isolate them
    from change as new filesystems are introduced.

Development Comments:
    This routine will examine the type of file descriptor it is sent.

    If the file descriptor is associated with a network device, the read
    function will be mapped to a special network handler. The return code from
    the network handler will then be sent as the return code from generic
    read() function.

    For file descriptors that are associated with the filesystem the following
    sequence will be performed:

    #. Obtain the file control block associated with the file descriptor

    #. Range check the file descriptor

    #. Determine that the buffer pointer is not invalid

    #. Check that the count is not zero

    #. Check the file control block to see if we have permissions to read

    #. If there is a read function in the handler table, invoke the handler
       table read() function

    #. Use the return code from the handler table read function(number of bytes
       read) to increment the offset element of the file control block

    #. Return the number of bytes read to the calling program

readdir
=======

File:
    ``readdir.c``

Processing:
    This routine was acquired from the BSD group. It has not been altered from
    its original form.

Development Comments:
    The routine calls a customized getdents() function that is provided by the
    user.  This routine provides the filesystem specific aspects of reading a
    directory.

    It is layered on the read() function in the directory handler table. This
    function has been mapped to the Imfs_dir_read() function.

unmount
=======

File:
    ``unmount.c``

Processing:
    This routine will attempt to dismount a mounted filesystem and then free
    all resources that were allocated for the management of that filesystem.

Development Comments:
    - This routine will determine if there are any filesystems currently
      mounted under the filesystem that we are trying to dismount. This would
      prevent the dismount of the filesystem.

    - It will test to see if the current directory is in the filesystem that we
      are attempting to dismount. This would prevent the dismount of the
      filesystem.

    - It will scan all the currently open file descriptors to determine is
      there is an open file descriptor to a file in the filesystem that we are
      attempting to unmount().

    If the above preconditions are met then the following sequence is
    performed:

    #. Call the filesystem specific unmount() function for the filesystem that
       contains the mount point. This routine should indicate that the mount
       point no longer has a filesystem mounted below it.

    #. Call the filesystem specific fsunmount_me() function for the mounted
       filesystem that we are trying to unmount(). This routine should clean up
       any resources that are no longer needed for the management of the file
       system being un-mounted.

    #. Extract the mount table entry for the filesystem that was just dismounted
       from the mount table chain.

    #. Free the memory associated with the extracted mount table entry.

eval
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getdentsc
=========

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