ESP-IDF uses `FatFs <http://elm-chan.org/fsw/ff/00index_e.html>`_ library to work with FAT filesystems. FatFs library resides in ``fatfs`` component. Although it can be used directly, many of its features can be accessed via VFS using C standard library and POSIX APIs.
Additionally, FatFs has been modified to support run-time pluggable disk IO layer. This allows mapping of FatFs drives to physical disks at run-time.
:component_file:`fatfs/src/esp_vfs_fat.h` header file defines functions to connect FatFs with VFS. :cpp:func:`esp_vfs_fat_register` function allocates a ``FATFS`` structure, and registers a given path prefix in VFS. Subsequent operations on files starting with this prefix are forwarded to FatFs APIs. :cpp:func:`esp_vfs_fat_unregister_path` function deletes the registration with VFS, and frees the ``FATFS`` structure.
1. Call :cpp:func:`esp_vfs_fat_register`, specifying path prefix where the filesystem has to be mounted (e.g. ``"/sdcard"``, ``"/spiflash"``), FatFs drive number, and a variable which will receive a pointer to ``FATFS`` structure.
3. Call FatFs ``f_mount`` function (and optionally ``f_fdisk``, ``f_mkfs``) to mount the filesystem using the same drive number which was passed to :cpp:func:`esp_vfs_fat_register`. See `FatFs documentation for more details <http://www.elm-chan.org/fsw/ff/doc/mount.html>`.
4. Call POSIX and C standard library functions to open, read, write, erase, copy files, etc. Use paths starting with the prefix passed to :cpp:func:`esp_vfs_register` (such as ``"/sdcard/hello.txt"``).
9. Call :cpp:func:`esp_vfs_fat_unregister_path` with the path where the file system is mounted to remove FatFs from VFS, and free the ``FATFS`` structure allocated on step 1.
Convenience functions, ``esp_vfs_fat_sdmmc_mount`` and ``esp_vfs_fat_sdmmc_unmount``, which wrap these steps and also handle SD card initialization, are described in the next section.
:component_file:`fatfs/src/esp_vfs_fat.h` header file also provides a convenience function to perform steps 1–3 and 7–9, and also handle SD card initialization::cpp:func:`esp_vfs_fat_sdmmc_mount`. This function does only limited error handling. Developers are encouraged to look at its source code and incorporate more advanced versions into production applications. :cpp:func:`esp_vfs_fat_sdmmc_unmount` function unmounts the filesystem and releases resources acquired by :cpp:func:`esp_vfs_fat_sdmmc_mount`.
Convenience functions, :cpp:func:`esp_vfs_fat_rawflash_mount` and :cpp:func:`esp_vfs_fat_rawflash_unmount`, are provided by :component_file:`fatfs/src/esp_vfs_fat.h` header file in order to perform steps 1-3 and 7-9 for read-only FAT partitions. These are particularly helpful for data partitions written only once during factory provisioning and need not be changed by production application throughout the lifetime.
Implementation of disk IO functions for SD/MMC cards is provided. It can be registered for the given FatFs drive number using :cpp:func:`ff_diskio_register_sdmmc` function.
