path: root/doc/filesystem/applayering.t

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@c  COPYRIGHT (c) 1988-1998.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
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@chapter Applications and Functional Layering

@section General

The RTEMS file system framework was intended to be compliant with the
POSIX Files and Directories interface standard. The following file system
characteristics resulted in a functional switching layer (See figures 6
and 7).


@example
Figure 6
@end example

@enumerate

@item 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 file system. The interfaces to these routines do
not reflect the type of subordinate file system implementation in which
the file will be found.

@item The file system framework developed under RTEMS allows for mounting
file systems of different types under the base file system.( Figure 3 )

@item The mechanics of locating file information may be quite different
between file system types.

@item The process of locating a file may require crossing file system
boundaries.

@item The transitions between file systems and the processing required to
access information in different file systems is not visible at the level
of the POSIX function call.

@item The POSIX interface standard provides file access by character
pathname to the file in some functions and through an integer file
descriptor (Figure 8) in other functions.

@item The nature of the integer file descriptor and its associated
processing is operating system and file system specific.

@item Directory and device information must be processed with some of the
same routines that apply to files.

@item The form and content of directory and device information differs
greatly from that of a regular file.

@item Files, directories and devices represent elements (nodes) of a tree
hierarchy.

@item The rules for processing each of the node types that exist under the
file system are node specific but are still not reflected in the POSIX
interface routines.

@end enumerate


@example
Figure 7
@end example





@example
Figure 8
@end example





@section Mapping of Generic System Calls to File System 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 file system. To the
application program, these functions allow access to information in any
mounted file system without explicit knowledge of the file system type or
the file system mount configuration. The following are functions that are
provided to the application:

@enumerate
@item access()
@item chdir()
@item chmod()
@item chown()
@item close()
@item closedir()
@item fchmod()
@item fcntl()
@item fdatasync()
@item fpathconf()
@item fstat()
@item fsync()
@item ftruncate()
@item link()
@item lseek()
@item mkdir()
@item mknod()
@item mount()
@item open()
@item opendir()
@item pathconf()
@item read()
@item readdir()
@item rewinddir()
@item rmdir()
@item rmnod()
@item scandir()
@item seekdir()
@item stat()
@item telldir()
@item umask()
@item unlink()
@item unmount()
@item utime()
@item write()
@end enumerate

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

@subsection Use of function pointers for functional redirection

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, file system and node type information is accessed which
is then used to invoke the appropriate file system and node type specific
routine to process the POSIX function call.

@subsection 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.

@enumerate

@item 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.

@item 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.

@item 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.

@item File control block - rtems_libio_t structure

@example
struct rtems_libio_tt @{
  rtems_driver_name_t               *driver;
  off_t                              size;      /* size of file */
  off_t                              offset;    /* current offset into file */
  unsigned32                         flags;
  rtems_filesystem_location_info_t   pathinfo;
  Objects_Id                         sem;
  unsigned32                         data0;     /* private to "driver" */
  void                               data1;     / . */
  void                               file_info; /used by file handlers/
  rtems_filesystem_file_handlers_r   handlers;  /type specific handlers/
@};
@end example

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

@itemize @bullet

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

@item 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.

@item 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.

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

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

@end itemize

@end enumerate

@subsection 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 file system.

@example
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;
@};
@end example

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

@subsection File System Functional Interface to Files and Directories

@itemize @bullet

@item Access using relative or absolute path names to file

@item OP Table for File System - rtems_filesystem_operations_table

@enumerate
@item evalpath()
@item evalformake()
@item link()
@item unlink()
@item node_type()
@item mknod()
@item rmnod()
@item chown()
@item freenod()
@item mount()
@item fsmount_me()
@item unmount()
@item fsunmount_me()
@item utime()
@item eval_link()
@item symlink()
@item readlink()
@end enumerate

@item Access using file handle

@item Handlers for file system node types ( regular file, directories, and devices ) - rtems_filesystem_file_handlers_r

@enumerate
@item open()
@item close()
@item read()
@item write()
@item ioctl()
@item lseek()
@item fstat()
@item fchmod()
@item ftruncate()
@item fpathconf()
@item fsync()
@item fdatasync()
@end enumerate

@end itemize

@section POSIX Directory Access Functions

BSD/newlib has provided a set of POSIX directory access routines. These
routines allow directories to be open and the entries under the directory
read. The getdents routine allows for customization of the BSD routines to
a particular file system implementation. Some of the original BSD routines
were modified, but maintain the same calling interface as the original
BSD/newlib routine. The following directory access routines are included
in the BSD set:

@itemize @bullet
@item opendir()
@item closedir()
@item readdir() 
@item getdents()
@item closedir() 
@item rewinddir() (not BSD)
@item scandir() (built on fstat() )
@item seekdir()  (not BSD)
@item telldir()
@end itemize

XXX Jennifer will fill this section in.