IMFS TAR load.

[Jake Janovetz]

Hello...

   I believe several people are using tar files to construct the 
initial filesystem on their RTEMS devices.  Joel and I have added
a new call: rtems_tarfs_load() to the IMFS to mount a tar file so
that its storage may be used rather than copying the data to new
memory file storage.  Here is a brief summary.  It would be nice
if some people could try this out and give me feedback.

int rtems_tarfs_load(char *mountpoint,
                     unsigned char *tar_image,
                     unsigned long tar_size)

   [mountpoint] is a string which describes the path at which the 
                new "tarfs" will be mounted.  Tarfs is in quotes because
                it is not really a filesystem, but sort of an extension
                to the IMFS.
   [tar_imag] is a pointer to the TAR image in ROM or RAM.  The TAR image
              is never modified, so no writes will be made to this memory.
   [tar_size] is the size of the TAR image.  This is needed because 
              (to my knowledge), there is no other way to determine the
              end of a TAR file.  (or at least discriminate between the
              end of the file and an invalid TAR header)

rtems_tarfs_load first evaluates the path "mountpoint" and makes sure
it is valid.  It does not create a directory for the mount.  You must
do this.  It then goes through the TAR file and generates IMFS nodes
accordingly:
   1. Directories are created as standard IMFS directories.
   2. Files are created in two ways:
      a. If write permission is granted to the file in the TAR, then
         a new file is created using creat() and written to.  This file
         is then a standard IMFS file and contains a _copy_ of the data
         in the TAR.
      b. If write permission is not granted, then a new node is created
         as a "linear file" node (new to IMFS).  The pointer to the data
         is set to point to the data in the TAR file.  Of course, the
         file is created with read-only permission.

This new "linear file" shares most of the same handlers as the "memory file"
originally in IMFS.  The differences are handled in the function calls
themselves.  Notably, memfile_read() performs reads in a much more
simplistic manner because files are not fragmented as they are in IMFS
memory files.

chmod() has a little different functionality, though.  If the file
is a "linear file" -- that is, it points to the TAR file, then it
is read-only.  If you chmod() it to be read/write, then the call 
changes the node from a "linear file" to a "memory file" and copies
the contents.

Oh, and in case you were wondering, "rtems_tarfs_load("/", ...)"
does, indeed, work.  TARFS is not really a filesystem.  The call
just loads the TAR file into the filesystem.


    Cheers,
    Jake