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(grub.info)Filesystem interface

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The generic interface for filesystems
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   For any particular partition, it is presumed that only one of the
"normal" filesystems such as FAT, FFS, or ext2fs can be used, so there
is a switch table managed by the functions in `disk_io.c'. The notation
is that you can only "mount" one at a time.

   The block list filesystem has a special place in the system. In
addition to the "normal" filesystem (or even without one mounted), you
can access disk blocks directly (in the indicated partition) via the
block list notation. Using the block list filesystem doesn't effect any
other filesystem mounts.

   The variables which can be read by the filesystem backend are:

`current_drive'
     The current BIOS drive number (numbered from 0, if a floppy, and
     numbered from 0x80, if a hard disk).

`current_partition'
     The current partition number.

`current_slice'
     The current partition type.

`saved_drive'
     The "drive" part of the root device.

`saved_partition'
     The "partition" part of the root device.

`part_start'
     The current partition starting address, in sectors.

`part_length'
     The current partition length, in sectors.

`print_possibilities'
     True when the `dir' function should print the possible completions
     of a file, and false when it should try to actually open a file of
     that name.

`FSYS_BUF'
     Filesystem buffer which is 32K in size, to use in any way which the
     filesystem backend desires.

   The variables which need to be written by a filesystem backend are:

`filepos'
     The current position in the file, in sectors.

     *Caution:* the value of FILEPOS can be changed out from under the
     filesystem code in the current implementation. Don't depend on it
     being the same for later calls into the backend code!

`filemax'
     The length of the file.

`disk_read_func'
     The value of DISK_READ_HOOK _only_ during reading of data for the
     file, not any other fs data, inodes, FAT tables, whatever, then
     set to `NULL' at all other times (it will be `NULL' by default).
     If this isn't done correctly, then the `testload' and `install'
     commands won't work correctly.

   The functions expected to be used by the filesystem backend are:

`devread'
     Only read sectors from within a partition. Sector 0 is the first
     sector in the partition.

`grub_read'
     If the backend uses the block list code, then `grub_read' can be
     used, after setting BLOCK_FILE to 1.

`print_a_completion'
     If PRINT_POSSIBILITIES is true, call `print_a_completion' for each
     possible file name. Otherwise, the file name completion won't work.

   The functions expected to be defined by the filesystem backend are
described at least moderately in the file `filesys.h'. Their usage is
fairly evident from their use in the functions in `disk_io.c', look for
the use of the FSYS_TABLE array.

   *Caution:* The semantics are such that then `mount'ing the
filesystem, presume the filesystem buffer `FSYS_BUF' is corrupted, and
(re-)load all important contents. When opening and reading a file,
presume that the data from the `mount' is available, and doesn't get
corrupted by the open/read (i.e. multiple opens and/or reads will be
done with only one mount if in the same filesystem).