The FATFS structure holds dynamic work area of the logical drive and a file. It is given by application program and registerd/unregisterd to the Petit FatFs module with pf_mount function. There is no member that can be changed by application programs.
typedef struct {
BYTE fs_type; /* FAT sub type */
BYTE csize; /* Number of sectors per cluster */
BYTE flag; /* File status flags */
BYTE pad1;
WORD n_rootdir; /* Number of root directory entries (0 on FAT32) */
CLUST n_fatent; /* Number of FAT entries (= number of clusters + 2) */
DWORD fatbase; /* FAT start sector */
DWORD dirbase; /* Root directory start sector (Cluster# on FAT32) */
DWORD database; /* Data start sector */
DWORD fptr; /* File read/write pointer */
DWORD fsize; /* File size */
CLUST org_clust; /* File start cluster */
CLUST curr_clust; /* File current cluster */
DWORD dsect; /* File current data sector */
} FATFS;