The FIL structure (file object) holds state of an open file. It is created by f_open function and discarded by f_close function. There is no member that can be changed by the application program except for cltbl. Note that a sector buffer is defined in this structure under non-tiny configuration so that the FIL structures should not be defined as auto variable.
typedef struct {
FATFS* fs; /* Pointer to the owner file system object */
WORD id; /* Owner file system mount ID */
BYTE flag; /* File status flags */
BYTE pad1;
DWORD fptr; /* File read/write pointer (Byte offset origin from top of the file) */
DWORD fsize; /* File size */
DWORD org_clust; /* File start cluster */
DWORD curr_clust; /* Current cluster */
DWORD dsect; /* Current data sector */
#if !_FS_READONLY
DWORD dir_sect; /* Sector containing the directory entry */
BYTE* dir_ptr; /* Ponter to the directory entry in the window */
#endif
#if _USE_FASTSEEK
DWORD* cltbl; /* Pointer to the cluster link map table */
#endif
#if _FS_SHARE
UINT lockid; /* File lock ID */
#endif
#if !_FS_TINY
BYTE buf[_MAX_SS]; /* Data read/write buffer */
#endif
} FIL;