📄 fileio-mount-table.html
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><A
NAME="FILEIO-MOUNT-TABLE"
></A
>Chapter 16. Mount Table</H1
><P
>The mount table records the filesystems that are actually active.
These can be seen as being analogous to mount points in Unix systems.</P
><P
>There are two sources of mount table entries. Filesystems (or other
components) may export static entries to the table using the
<TT
CLASS="LITERAL"
>MTAB_ENTRY()</TT
> macro. Alternatively, new entries may
be installed at run time using the <CODE
CLASS="FUNCTION"
>mount()</CODE
>
function. Both types of entry may be unmounted with the
<CODE
CLASS="FUNCTION"
>umount()</CODE
> function.</P
><P
>A mount table entry has the following structure:</P
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><PRE
CLASS="PROGRAMLISTING"
>struct cyg_mtab_entry
{
const char *name; // name of mount point
const char *fsname; // name of implementing filesystem
const char *devname; // name of hardware device
CYG_ADDRWORD data; // private data value
cyg_bool valid; // Valid entry?
cyg_fstab_entry *fs; // pointer to fstab entry
cyg_dir root; // root directory pointer
};</PRE
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><P
>The <CODE
CLASS="STRUCTFIELD"
>name</CODE
> field identifies the mount
point. This is used to direct rooted filenames (filenames that
begin with "/") to the correct filesystem. When a file
name that begins with "/" is submitted, it is matched
against the <CODE
CLASS="STRUCTFIELD"
>name</CODE
> fields of all valid mount
table entries. The entry that yields the longest match terminating
before a "/", or end of string, wins and the appropriate
function from the filesystem table entry is then passed the remainder
of the file name together with a pointer to the table entry and the
value of the <CODE
CLASS="STRUCTFIELD"
>root</CODE
> field as the directory
pointer.</P
><P
>For example, consider a mount table that contains the following
entries:</P
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CLASS="PROGRAMLISTING"
> { "/", "msdos", "/dev/hd0", ... }
{ "/fd", "msdos", "/dev/fd0", ... }
{ "/rom", "romfs", "", ... }
{ "/tmp", "ramfs", "", ... }
{ "/dev", "devfs", "", ... }</PRE
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><P
>An attempt to open "/tmp/foo" would be directed to the RAM
filesystem while an open of "/bar/bundy" would be directed
to the hard disc MSDOS filesystem. Opening "/dev/tty0" would
be directed to the device management filesystem for lookup in the
device table.</P
><P
>Unrooted file names (those that do not begin with a '/') are passed
straight to the filesystem that contains the current directory. The
current directory is represented by a pair consisting of a mount table
entry and a directory pointer.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>fsname</CODE
> field points to a string that
should match the <CODE
CLASS="STRUCTFIELD"
>name</CODE
> field of the
implementing filesystem. During initialization the mount table is
scanned and the <CODE
CLASS="STRUCTFIELD"
>fsname</CODE
> entries looked up in
the filesystem table. For each match, the filesystem's _mount_
function is called and if successful the mount table entry is marked
as valid and the <CODE
CLASS="STRUCTFIELD"
>fs</CODE
> pointer installed.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>devname</CODE
> field contains the name of the
device that this filesystem is to use. This may match an entry in the
device table (see later) or may be a string that is specific to the
filesystem if it has its own internal device drivers.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>data</CODE
> field is a private data value. This
may be installed either statically when the table entry is defined, or
may be installed during the <CODE
CLASS="FUNCTION"
>mount()</CODE
> operation.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>valid</CODE
> field indicates whether this mount
point has actually been mounted successfully. Entries with a false
<CODE
CLASS="STRUCTFIELD"
>valid</CODE
> field are ignored when searching for a
name match.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>fs</CODE
> field is installed after a successful
<CODE
CLASS="FUNCTION"
>mount()</CODE
> operation to point to the implementing
filesystem.</P
><P
>The <CODE
CLASS="STRUCTFIELD"
>root</CODE
> field contains a directory pointer
value that the filesystem can interpret as the root of its directory
tree. This is passed as the <CODE
CLASS="PARAMETER"
>dir</CODE
> argument of
filesystem functions that operate on rooted filenames. This field must
be initialized by the filesystem's <CODE
CLASS="FUNCTION"
>mount()</CODE
>
function.</P
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