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>Writing a New Filesystem</TITLE
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CLASS="CHAPTER"
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><H1
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><A
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NAME="FILEIO-WRITING">Chapter 29. Writing a New Filesystem</H1
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><P
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>To create a new filesystem it is necessary to define the fstab entry
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and the file IO operations. The easiest way to do this is to copy an
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existing filesystem: either the test filesystem in the FILEIO package,
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or the RAM or ROM filesystem packages.</P
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><P
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>To make this clearer, the following is a brief tour of the FILEIO
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relevant parts of the RAM filesystem.</P
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><P
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>First, it is necessary to provide forward definitions of the functions
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that constitute the filesystem interface:</P
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><TABLE
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>//==========================================================================
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// Forward definitions
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// Filesystem operations
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static int ramfs_mount    ( cyg_fstab_entry *fste, cyg_mtab_entry *mte );
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static int ramfs_umount   ( cyg_mtab_entry *mte );
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static int ramfs_open     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             int mode,  cyg_file *fte );
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static int ramfs_unlink   ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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static int ramfs_mkdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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static int ramfs_rmdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
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static int ramfs_rename   ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
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                             cyg_dir dir2, const char *name2 );
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static int ramfs_link     ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
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                             cyg_dir dir2, const char *name2, int type );
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static int ramfs_opendir  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             cyg_file *fte );
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static int ramfs_chdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             cyg_dir *dir_out );
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static int ramfs_stat     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             struct stat *buf);
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static int ramfs_getinfo  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             int key, void *buf, int len );
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static int ramfs_setinfo  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
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                             int key, void *buf, int len );
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// File operations
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static int ramfs_fo_read      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
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static int ramfs_fo_write     (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
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static int ramfs_fo_lseek     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
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static int ramfs_fo_ioctl     (struct CYG_FILE_TAG *fp, CYG_ADDRWORD com,
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                                CYG_ADDRWORD data);
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static int ramfs_fo_fsync     (struct CYG_FILE_TAG *fp, int mode );
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static int ramfs_fo_close     (struct CYG_FILE_TAG *fp);
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static int ramfs_fo_fstat     (struct CYG_FILE_TAG *fp, struct stat *buf );
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static int ramfs_fo_getinfo   (struct CYG_FILE_TAG *fp, int key, void *buf, int len );
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static int ramfs_fo_setinfo   (struct CYG_FILE_TAG *fp, int key, void *buf, int len );
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// Directory operations
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static int ramfs_fo_dirread      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
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static int ramfs_fo_dirlseek     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );</PRE
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></TD
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></TR
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></TABLE
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><P
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>We define all of the fstab entries and all of the file IO
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operations. We also define alternatives for the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>fo_read</I
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></TT
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> and
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<TT
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CLASS="STRUCTFIELD"
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><I
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>fo_lseek</I
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></TT
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> file IO operations.</P
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><P
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>We can now define the filesystem table entry. There is a macro,
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<TT
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CLASS="LITERAL"
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>FSTAB_ENTRY</TT
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> to do this:</P
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><TABLE
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><PRE
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CLASS="PROGRAMLISTING"
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>//==========================================================================
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// Filesystem table entries
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// -------------------------------------------------------------------------
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// Fstab entry.
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// This defines the entry in the filesystem table.
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// For simplicity we use _FILESYSTEM synchronization for all accesses since
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// we should never block in any filesystem operations.
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FSTAB_ENTRY( ramfs_fste, "ramfs", 0,
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             CYG_SYNCMODE_FILE_FILESYSTEM|CYG_SYNCMODE_IO_FILESYSTEM,
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             ramfs_mount,
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             ramfs_umount,
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             ramfs_open,
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             ramfs_unlink,
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             ramfs_mkdir,
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             ramfs_rmdir,
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             ramfs_rename,
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             ramfs_link,
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             ramfs_opendir,
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             ramfs_chdir,
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             ramfs_stat,
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             ramfs_getinfo,
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             ramfs_setinfo);</PRE
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></TD
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></TR
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></TABLE
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><P
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>The first argument to this macro gives the fstab entry a name, the
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remainder are initializers for the field of the structure.</P
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><P
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>We must also define the file operations table that is installed in all
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open file table entries:</P
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>// -------------------------------------------------------------------------
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// File operations.
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// This set of file operations are used for normal open files.
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static cyg_fileops ramfs_fileops =
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{
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    ramfs_fo_read,
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    ramfs_fo_write,
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    ramfs_fo_lseek,
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    ramfs_fo_ioctl,
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    cyg_fileio_seltrue,
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    ramfs_fo_fsync,
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    ramfs_fo_close,
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    ramfs_fo_fstat,
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    ramfs_fo_getinfo,
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    ramfs_fo_setinfo
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};</PRE
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></TD
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></TR
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></TABLE
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><P
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>These all point to functions supplied by the filesystem except the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>fo_select</I
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></TT
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> field which is filled with a
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pointer to <TT
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CLASS="FUNCTION"
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>cyg_fileio_seltrue()</TT
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>. This is provided
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by the FILEIO package and is a select function that always returns
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true to all operations.</P
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><P
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>Finally, we need to define a set of file operations for use when
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reading directories. This table only defines the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>fo_read</I
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></TT
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> and
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<TT
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CLASS="STRUCTFIELD"
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><I
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>fo_lseek</I
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></TT
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> operations. The rest are filled
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with stub functions supplied by the FILEIO package that just return an
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error code.</P
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><TABLE
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CLASS="PROGRAMLISTING"
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>// -------------------------------------------------------------------------
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// Directory file operations.
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// This set of operations are used for open directories. Most entries
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// point to error-returning stub functions. Only the read, lseek and
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// close entries are functional.
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static cyg_fileops ramfs_dirops =
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{
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    ramfs_fo_dirread,
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    (cyg_fileop_write *)cyg_fileio_enosys,
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    ramfs_fo_dirlseek,
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    (cyg_fileop_ioctl *)cyg_fileio_enosys,
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    cyg_fileio_seltrue,
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    (cyg_fileop_fsync *)cyg_fileio_enosys,
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    ramfs_fo_close,
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    (cyg_fileop_fstat *)cyg_fileio_enosys,
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    (cyg_fileop_getinfo *)cyg_fileio_enosys,
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    (cyg_fileop_setinfo *)cyg_fileio_enosys
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};</PRE
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></TD
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></TR
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></TABLE
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><P
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>If the filesystem wants to have an instance automatically mounted on
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system startup, it must also define a mount table entry. This is done
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with the <TT
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CLASS="LITERAL"
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>MTAB_ENTRY</TT
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> macro. This is an example from
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the test filesystem of how this is used:</P
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><TABLE
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><PRE
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CLASS="PROGRAMLISTING"
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>MTAB_ENTRY( testfs_mte1,
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                   "/",
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                   "testfs",
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                   "",
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                   0);</PRE
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></TD
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></TR
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></TABLE
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><P
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>The first argument provides a name for the table entry. The following
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arguments provide initialization for the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>name</I
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></TT
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>, <TT
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CLASS="STRUCTFIELD"
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><I
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>fsname</I
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></TT
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>,
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<TT
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CLASS="STRUCTFIELD"
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><I
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>devname</I
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></TT
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> and <TT
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CLASS="STRUCTFIELD"
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><I
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>data</I
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></TT
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>
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fields respectively.</P
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><P
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>These definitions are adequate to let the new filesystem interact
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with the FILEIO package. The new filesystem now needs to be fleshed
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out with implementations of the functions defined above. Obviously,
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the exact form this takes will depend on what the filesystem is
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intended to do. Take a look at the RAM and ROM filesystems for
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examples of how this has been done.</P
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