source: rtems/doc/filesystem/syscalls.t @ daf1622

@c
@c  COPYRIGHT (c) 1988-1998.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id$
@c

@chapter System Call Development Notes

This set of routines represents the application's interface to files and directories 
under the RTEMS file system. 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.

@enumerate
@item access()
@item chdir()
@item chmod()
@item chown()
@item close()
@item closedir()
@item dup()
@item dup2()
@item fchmod()
@item fcntl()
@item fdatasync()
@item fpathconf()
@item fstat()
@item ioctl()
@item link()
@item lseek()
@item mkdir()
@item mkfifo()
@item mknod()
@item mount()
@item open()
@item opendir()
@item pathconf()
@item read()
@item readdir()
@item unmount()
@end enumerate

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

 
@page
@section access

@subheading File:

access.c

@subheading 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.

@subheading 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 file systems without alteration.

@page
@section chdir

@subheading File:

chdir.c


@subheading 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.

@subheading Development Comments:

This routine is layered on the rtems_filesystem_evaluate_path() routine and the 
file system specific OP table function node_type().

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

@itemize @bullet
@item RTEMS_FILESYSTEM_DIRECTORY
@item RTEMS_FILESYSTEM_DEVICE
@item RTEMS_FILESYSTEM_HARD_LINK
@item RTEMS_FILESYSTEM_MEMORY_FILE
@end itemize

This acknowledges that the form of the node management information can vary 
from one file system 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.

@page
@section chmod

@subheading File:

chmod.c

@subheading 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 file systems.

@subheading 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.

@page
@section chown

@subheading File:

chown.c

@subheading 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.

@subheading 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 file system.

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.

@page
@section close

@subheading File:

close.c

@subheading 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 file system, the appropriate handler 
will be selected using information that has been placed in the file control block for 
the device (rtems_libio_t structure).

@subheading 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 file system.

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

@enumerate

@item Use the rtems_libio_iop() function to obtain the rtems_libio_t structure for the 
file descriptor

@item Range check the file descriptor using rtems_libio_check_fd()

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

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

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

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

@end enumerate

@page
@section closedir

@subheading File:

closedir.c

@subheading 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 file systems.

@subheading 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.


@page
@section dup()      Unimplemented

@subheading File:

dup.c

@subheading Processing: 


@subheading Development Comments:





@page
@section dup2()      Unimplemented

@subheading File:

dup2.c

@subheading Processing: 


@subheading Development Comments:






@page
@section fchmod

@subheading File:

fchmod.c

@subheading Processing: 

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

@itemize @bullet
@item rtems_file_descriptor_type()

@item rtems_libio_iop()

@item rtems_libio_check_fd()

@item rtems_libio_check_permissions()

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

@end itemize

@subheading 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.

@page
@section fcntl()

@subheading File:

fcntl.c

@subheading 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 file system 
implementations.

@subheading 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:

@enumerate

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

@item Obtain the file control block that is associated with the file descriptor index

@item Perform a range check on the file descriptor index.

@end enumerate



@page
@section fdatasync

@subheading File:

fdatasync.c

@subheading Processing: 

This routine is a template in the in memory file system that will route us to the 
appropriate handler function to carry out the fdatasync() processing. In the in 
memory file system 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:

@itemize @bullet

@item rtems_file_descriptor_type()

@item rtems_libio_iop()

@item rtems_libio_check_fd()

@item rtems_libio_check_permissions()

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

@end itemize

@subheading 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.

@page
@section fpathconf

@subheading File:

fpathconf.c

@subheading Processing: 

This routine is layered on the following functions and macros:

@itemize @bullet

@item rtems_file_descriptor_type()

@item rtems_libio_iop()

@item rtems_libio_check_fd()

@item rtems_libio_check_permissions()

@end itemize

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

@itemize @bullet

@item PCLINKMAX

@item PCMAXCANON

@item PCMAXINPUT

@item PCNAMEMAX

@item PCPATHMAX

@item PCPIPEBUF

@item PCCHOWNRESTRICTED

@item PCNOTRUNC

@item PCVDISABLE

@item PCASYNCIO

@item PCPRIOIO

@item PCSYNCIO


@end itemize

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 file system specific constants that can be accessed by individual 
identifiers. 

@subheading 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 
file system 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 file system. 

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


@page
@section fstat

@subheading File:

fstat.c

@subheading 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 file system, the fstat() 
routine will map to the fstat() function taken from the node handler table. 

@subheading 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 file system, the following 
steps are performed:

@enumerate

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

@item Range check the file descriptor index.

@item 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.

@end enumerate

@page
@section ioctl

@subheading File:

ioctl.c

@subheading 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 file system specific handlers. The 
development of new file systems should not alter the basic processing performed 
by this routine.

@subheading 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 file system undergo the following 
processing:

@enumerate

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

@item The file descriptor value is range checked.

@item 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.

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

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

@end enumerate


@page
@section link

@subheading File:

link.c

@subheading 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 file system 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.

@subheading Development Comments:

Calling parameters:
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 file system; otherwise the 
search will start from the current directory.

The OPS table evalformake() function for the parent's file system 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 file system boundary. This is not permitted for hard-links.

If the hard-link does not cross a file system 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 file system.

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

@page
@section lseek

@subheading File:

lseek.c

@subheading Processing: 

This routine is layered on both external handlers and file system / node type 
specific handlers. This routine should allow for the support of new file systems 
without modification.

@subheading 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 file system. The following steps will be performed for file system nodes:

@enumerate

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

@item The file descriptor is range checked.

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

@item 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.

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

@item The return code from the lseek() handler function is returned to the 
calling program

@end enumerate


@page
@section mkdir

@subheading File:

mkdir.c

@subheading Processing: 

This routine attempts to create a directory node under the file system. The routine 
is layered the mknod() function. 

@subheading Development Comments:

See mknod() for developmental comments.

@page
@section mkfifo

@subheading File:

mkfifo.c

@subheading Processing: 

This routine attempts to create a FIFO node under the file system. The routine is 
layered the mknod() function. 

@subheading Development Comments:

See mknod() for developmental comments

@page
@section mknod

@subheading File:

mknod.c

@subheading Processing: 

This function will allow for the creation of the following types of nodes under the 
file system:

@itemize @bullet

@item directories

@item regular files

@item character devices

@item block devices

@item fifos

@end itemize

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 file system specific routines 
evalformake and mknod. The introduction of a new file system must include its 
own evalformake and mknod function to support the generic mknod() function. 
Under this condition the generic mknod() function should accommodate other 
file system types without alteration. 

@subheading Development Comments:

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

@example
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);

@end example

Where:

@itemize @bullet
@item S_IFREG (0100000) - Creation of a regular file
@item S_IFCHR (0020000) - Creation of a character device
@item S_IFBLK (0060000) - Creation of a block device
@item S_IFIFO (0010000) - Creation of a FIFO
@end itemize

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 file system 
specific mknod() function exists.

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

@page
@section mount

@subheading File:

mount.c


Arguments (Not a standard POSIX call): 

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. 

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 file systems that we wish to use into the 
RTEMS build. Unused file systems types will not be drawn into the build.

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"

char                      *device,

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

char                      *mount_point

This is a pathname to a directory in a currently mounted file system that 
allows read, write and execute permissions.

@subheading Processing: 

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



@subheading 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 file system 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 file system to be mounted.

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

If a mount function is defined in the ops table for the file system 
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 
file system.

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

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

@page
@section open

@subheading File:

open.c

@subheading Processing: 

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

@subheading 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, file system 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 file system 
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 
file system data in detail.

If an open() function exists in the file system 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.

@page
@section opendir

@subheading File:

opendir.c

@subheading 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 file system specific directory 
processing routines. 

@subheading Development Comments:

The BSD group provided this routine.

@page
@section pathconf

@subheading File:

pathconf.c

@subheading 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 file system 
types.

@subheading 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.

@page
@section read

@subheading File:

deviceio.c

@subheading Processing: 

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

@subheading 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 file system the following sequence 
will be performed:

@enumerate

@item Obtain the file control block associated with the file descriptor

@item Range check the file descriptor

@item Determine that the buffer pointer is not invalid

@item Check that the count is not zero

@item Check the file control block to see if we have permissions to read

@item If  there is a read function in the handler table, invoke the handler table 
read() function

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

@item Return the number of bytes read to the calling program

@end enumerate

@page
@section readdir

@subheading File:

readdir.c

@subheading Processing: 

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

@subheading Development Comments:

The routine calls a customized getdents() function that is provided by the user. 
This routine provides the file system 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.

@page
@section unmount

@subheading File:

unmount.c

@subheading Processing: 

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

@subheading Development Comments:

@itemize @bullet

@item This routine will determine if there are any file systems currently mounted 
under the file system that we are trying to dismount. This would prevent the 
dismount of the file system.

@item It will test to see if the current directory is in the file system that we are 
attempting to dismount. This would prevent the dismount of the file system.

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

@end itemize

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

@enumerate

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

@item Call the file system specific fsunmount_me() function for the mounted 
file system 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.

@item Extract the mount table entry for the file system that was just dismounted 
from the mount table chain.

@item Free the memory associated with the extracted mount table entry.

@end enumerate

@page
@section eval

@subheading File:

XXX

@subheading Processing: 

XXX

@subheading Development Comments:

XXX

@page
@section getdentsc

@subheading File:

XXX

@subheading Processing: 

XXX

@subheading Development Comments:

XXX