rtems-docs/filesystem/call_development.rst

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

.. COMMENT: COPYRIGHT (c) 1988-2002.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

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
====

**File:**

XXX

**Processing:**

XXX

**Development Comments:**

XXX

getdentsc
=========

**File:**

XXX

**Processing:**

XXX

**Development Comments:**

XXX