A basic question about File System Library

Where can I find guidance about how to use the function in FS Library? I'm trying to use these functions in rl_fs_lib.h (from MDK-Middleware 7.11.1) but don't know how to pass the pointer properly

extern int32_t  fat_handle_get(fsFAT_Volume *vol);
extern fsStatus fat_init      (fsFAT_Volume *vol);
extern fsStatus fat_uninit    (fsFAT_Volume *vol);
extern fsStatus fat_mount     (fsFAT_Volume *vol);
extern fsStatus fat_unmount   (fsFAT_Volume *vol);
extern fsStatus fat_open      (int32_t handle, const char *name, int32_t openmode);
extern fsStatus fat_close     (int32_t handle);
extern int32_t  fat_read      (int32_t handle, uint8_t *buf, uint32_t len);
extern int32_t  fat_write     (int32_t handle, const uint8_t *buf, uint32_t len);
extern fsStatus fat_flush     (int32_t handle);
extern int32_t  fat_flen      (int32_t handle);
extern fsStatus fat_seek      (int32_t handle, uint32_t pos);

and also I'm trying to partition a TF card into several volumes and drive them seperately by change the DrvLet string in the code below:

typedef struct _fsFAT_Volume {
  uint32_t    DrvLet;                   /* 4-byte encoded drive letter string */
  FS_MUTEX    Mutex;                    /* Volume mutex                       */
  FAT_DRV    *Drv;                      /* Registered FAT Device Driver       */
  uint32_t    Status;                   /* Volume Status                      */
  uint32_t   *CaBuf;                    /* Cache Buffer (FAT + Data)          */
  uint32_t    CaSize;                   /* Cache Buffer size                  */
  uint32_t    free_clus_cnt;            /* FAT32: Number of free clusters     */
  uint32_t    free_clus;                /* FAT32: First free cluster          */
  uint32_t    cdir_clus;                /* Current directory cluster          */
  NCACHE     *ncache;                   /* Name cache memory                  */
  FSJOUR     *fsj;                      /* File System Journal                */
  FATINFO     cfg;                      /* FAT Volume configuration           */
  FCACHE      fat;                      /* FAT table cache control            */
  DCACHE      ca;                       /* Data cache control                 */
  uint16_t    RsvdS;                    /* Reserved sectors used by journal   */
  uint8_t     Reserved[2];              /* Reserved for future use            */
} fsFAT_Volume;

Does someone have insight about what function can be used to assign values to this DrvLet?