The FATFS structure holds dynamic work area of individual logical drives. It is given by application program and registerd/unregisterd to the FatFs module with f_mount function. Following members are in standard configuration. There is no member that can be changed from the application program.
typedef struct {
BYTE fs_type; /* FAT sub-type (0:Not mounted) */
BYTE drv; /* Physical drive number */
BYTE csize; /* Sectors per cluster (1,2,4...128) */
BYTE n_fats; /* Number of FAT copies (1,2) */
BYTE wflag; /* win[] dirty flag */
BYTE fsi_flag; /* fsinfo dirty flag */
WORD id; /* File system mount ID */
WORD n_rootdir; /* Number of root directory entries (FAT12/16) */
#if _MAX_SS != 512
WORD ssize; /* Sector size (512,1024,2048,4096) */
#endif
#if _FS_REENTRANT
_SYNC_t sobj; /* Identifier of sync object */
#endif
#if !_FS_READONLY
DWORD last_clust; /* Last allocated cluster */
DWORD free_clust; /* Number of free clusters */
DWORD fsi_sector; /* fsinfo sector (FAT32) */
#endif
#if _FS_RPATH
DWORD cdir; /* Current directory cluster (0:root) */
#endif
DWORD n_fatent; /* Number of FAT entries (= number of clusters + 2) */
DWORD fsize; /* Sectors per FAT */
DWORD fatbase; /* FAT area start sector */
DWORD dirbase; /* Root directory area start sector (FAT32: cluster#) */
DWORD database; /* Data area start sector */
DWORD winsect; /* Current sector appearing in the win[] */
BYTE win[_MAX_SS]; /* Disk access window for Directory, FAT (and Data on tiny cfg) */
} FATFS;