source: rtems/doc/filesystem/basefs.t @ 6408000

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@c  COPYRIGHT (c) 1988-1998.
@c  On-Line Applications Research Corporation (OAR).
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@chapter Base File System

RTEMS initially mounts a RAM based file system known as the base file system. 
The root directory of this file system tree serves as the logical root of the directory 
hierarchy (Figure 3). Under the root directory a `/dev' directory is created under which all 
I/O device directories and files are registered as part of the file system hierarchy. 

A RAM based file system draws its management resources from memory. File 
and directory nodes are simply allocated blocks of memory. Data associated with regular 
files is stored in collections of memory blocks. When the system is turned off or restarted 
all memory-based components of the file system are lost.

The base file system serves as a starting point for the mounting of file systems 
that are resident on semi-permanent storage media. Examples of such media include non-
volatile memory, flash memory and IDE hard disk drives (Figure 3). File systems of other 
types will be mounted onto mount points within the base file system or other file systems 
that are subordinate to the base file system. The framework set up under the base file 
system will allow for these new file system types and the unique data and functionality 
that is required to manage the future file systems.

@section Base File System Mounting

At present, the first file system to be mounted is the `In Memory File System'. It 
is mounted using a standard MOUNT() command in which the mount point is NULL. 
This flags the mount as the first file system to be registered under the operating system 
and appropriate initialization of file system management information is performed (See 
figures 4 and 5). If a different file system type is desired as the base file system, 
alterations must be made to base_fs.c. This routine handles the mount of the base file 
system.


@example
Figure 4
@end example


Once the root of the base file system has been established and it has been 
recorded as the mount point of the base file system, devices are integrated into the base 
file system. For every device that is configured into the system (See ioman.c) a device 
registration process is performed. Device registration produces a unique dev_t handle that 
consists of a major and minor device number. In addition, the configuration information 
for each device contains a text string that represents the fully qualified pathname to that 
device's place in the base file system's hierarchy. A file system node is created for the 
device along the specified registration path.



@example
Figure 5
@end example


Note: Other file systems can be mounted but they are mounted onto points (directory 
mount points) in the base file system.


@subsection Base File System Node Structure and Function

Each regular file, device, hard link, and directory is represented by a data 
structure called a @code{jnode}. The -jnode- is formally represented by the structure:

@example
struct IMFS_jnode_tt @{
  Chain_Node                 Node;             /* for chaining them together */
  IMFS_jnode_t              *Parent;           /* Parent node */
  char                       name[NAME_MAX+1]; /* "basename" */
  mode_t                     st_mode;          /* File mode */
  nlink_t                    st_nlink;         /* Link count */
  ino_t                      st_ino;           /* inode */

  uid_t                      st_uid;           /* User ID of owner */
  gid_t                      st_gid;           /* Group ID of owner */

  time_t                     st_atime;         /* Time of last access */
  time_t                     st_mtime;         /* Time of last modification */
  time_t                     st_ctime;         /* Time of last status change */
  IMFS_jnode_types_t         type;             /* Type of this entry */
  IMFS_typs_union            info;
@};
@end example

The key elements of this structure are listed below together with a brief explanation of 
their role in the file system.

@table @b

@item node
This element exists simply to allow the entire @code{jnode} structure to be 
included in a chain.

@item parent
A pointer to another @code{jnode} structure that is the logical parent of the 
node in which it appears. There are circumstances that will produce a null parent 
pointer within a @code{jnode}. This can occur when a hard link is created to a file and 
the file is then removed without removing the hard link.

@item name
The name of this node within the file system hierarchical tree. Example: 
If the fully qualified pathname to the @code{jnode} was /a/b/c, the -jnode- name field 
would contain the null terminated string "c"

@item st_mode
The standard Unix access permissions for the file or directory.

@item st_nlink
The number of hard links to this file. When a @code{jnode} is first created its 
link count is set to 1. A @code{jnode} and its associated resources cannot be deleted 
unless its link count is less than 1.

@item st_ino
A unique node identification number

@item st_uid
The user ID of the file's owner

@item st_gid
The group ID of the file's owner

@item st_atime
The time of the last access to this file

@item st_mtime
The time of the last modification of this file

@item st_ctime
The time of the last status change to the file

@item type
The indication of node type must be one of the following states:

@itemize @bullet
@item IMFS_DIRECTORY 
@item IMFS_MEMORY_FILE 
@item IMFS_HARD_LINK
@item IMFS_SYM_LINK
@item IMFS_DEVICE 
@end itemize


@item info
This contains a structure that is unique to file type(See IMFS_typs_union in 
imfs.h )

@itemize @bullet

@item IMFS_DIRECTORY
An in memory file system directory contains a 
dynamic chain structure that records all files and directories that are 
subordinate to the directory node. 

@item IMFS_MEMORY_FILE
Under the in memory file system regular files hold 
data. Data is dynamically allocated to the file in 128 byte chunks of memory. 
The individual chunks of memory are tracked by arrays of pointers that record 
the address of the allocated chunk of memory. Single, double, and triple 
indirection pointers are used to record the locations of all segments of the file. 
These memory-tracking techniques are graphically depicted in figures XXX 
and XXX of appendix A.

@item IMFS_HARD_LINK
The IMFS file system supports the concept of hard 
links to other nodes in the IMFS file system. These hard links are actual 
pointers to the memory associated with other nodes in the file system. This 
type of link cannot cross-file system boundaries.

@item IMFS_SYM_LINK
The IMFS file system supports the concept of symbolic 
links to other nodes in any file system. A symbolic link consists of a pointer to 
a character string that represents the pathname to the target node. This type of 
link can cross-file system boundaries. 

@item IMFS_DEVICE
All RTEMS devices now appear as files under the in 
memory file system. On system initialization, all devices are registered as 
nodes under the file system.

@end itemize

@end table


@subsection Node removal constraints for the base files system

@itemize @bullet 

@item If a node is a directory with children it cannot be removed.
@item The root node of the base file system or the mounted file system 
cannot be removed.

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

@item Prior to node removal, decrement the node's link count by one. The 
link count must be less than one to allow for removal of the node.

@end itemize

@subsection Housekeeping

@itemize @bullet 

@item If the global variable rtems_filesystem_current refers to the node that 
we are trying to remove, the node_access element of this structure 
must be set to NULL to invalidate it.

@item If the node was of IMFS_MEMORY_FILE type, free the memory 
associated with the memory file before freeing the node. Use the 
IMFS_memfile_remove() function.

@end itemize


@section IMFS

@subsection OPS Table Functions for the In Memory File System (IMFS)

@example
OPS Table Functions             File                    Routine Name

Evalpath Imfs_eval.c IMFS_eval_path()
Evalformake Imfs_eval.c IMFS_evaluate_for_make()
Link Imfs_link.c IMFS_link()
Unlink Imfs_unlink.c IMFS_unlink()
Node_type Imfs_ntype.c IMFS_node_type()
Mknod Imfs_mknod.c IMFS_mknod()
Rmnod Imfs_rmnod.c IMFS_rmnod()
Chown Imfs_chown.c IMFS_chown()
Freenod Imfs_free.c IMFS_freenodinfo()
Mount Imfs_mount.c IMFS_mount()
Fsmount_me Imfs_init.c IMFS_initialize()
Unmount Imfs_unmount.c IMFS_unmount()
Fsunmount_me Imfs_init.c IMFS_fsunmount()
Utime Imfs_utime.c IMFS_utime()
Eval_link Imfs_eval.c IMFS_evaluate_link()
Symlink Imfs_symlink.c IMFS_symlink()
Readlink Imfs_readlink.c IMFS_readlink()
@end example



@subsection Handler Functions for Regular Files of In Memory File System

@example
Handler Function            File            Routine Name

Open Memfile.c Memfile_open()
Close Memfile.c Memfile_close()
Read Memfile.c Memfile_read()
Write Memfile.c Memfile_write()
Ioctl Memfile.c Memfile_ioctl()
Lseek Memfile.c Memfile_lseek()
Fstat Imfs_stat.c IMFS_stat()
Fchmod Imfs_fchmod.c IMFS_fchmod()
Ftruncate Memfile.c Memfile_ftruncate()
Fpathconf NA NULL
Fsync NA NULL
Fdatasync NA NULL
@end example


@subsection Handler Functions for Directories of In Memory File System

@example
Handler Function             File                Routine Name

Open imfs_directory.c Imfs_dir_open()
Close imfs_directory.c Imfs_dir_close()
Read imfs_directory.c Imfs_dir_read()
Write imfs_directory.c NULL
Ioctl imfs_directory.c NULL
Lseek imfs_directory.c Imfs_dir_lseek()
Fstat imfs_directory.c Imfs_dir_fstat()
Fchmod imfs_fchmod.c IMFS_fchmod()
Ftruncate NA NULL
Fpathconf NA NULL
Fsync NA NULL
Fdatasync NA NULL
@end example



@subsection Handler Functions for Devices of In Memory File System

@example
Handler Function          File                  Routine Name

Open deviceio.c Device_open()
Close deviceio.c Device_close()
Read deviceio.c Device_read()
Write deviceio.c Device_write()
Ioctl deviceio.c Device_ioctl()
Lseek deviceio.c Device_lseek()
Fstat imfs_stat.c IMFS_stat()
Fchmod imfs_fchmod.c IMFS_fchmod()
Ftruncate NA NULL
Fpathconf NA NULL
Fsync NA NULL
Fdatasync NA NULL
@end example