ESP-IDF uses the `FatFs <http://elm-chan.org/fsw/ff/00index_e.html>`_ library to work with FAT filesystems. FatFs resides in the ``fatfs`` component. Although the library can be used directly, many of its features can be accessed via VFS, using the C standard library and POSIX API functions.
Additionally, FatFs has been modified to support the runtime pluggable disk I/O layer. This allows mapping of FatFs drives to physical disks at runtime.
The function :cpp:func:`esp_vfs_fat_register` 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.
The function :cpp:func:`esp_vfs_fat_unregister_path` deletes the registration with VFS, and frees the ``FATFS`` structure.
3. Call the FatFs function ``f_mount``, 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`. For more information, see `FatFs documentation <http://www.elm-chan.org/fsw/ff/doc/mount.html>`_.
4. Call the C standard library and POSIX API functions to perform such actions on files as open, read, write, erase, copy, etc. Use paths starting with the path prefix passed to :cpp:func:`esp_vfs_register` (for example, ``"/sdcard/hello.txt"``). The filesystem uses `8.3 filenames <https://en.wikipedia.org/wiki/8.3_filename>`_ format (SFN) by default. If you need to use long filenames (LFN), enable the :ref:`CONFIG_FATFS_LONG_FILENAMES` option. More details on the FatFs filenames are available `here <http://elm-chan.org/fsw/ff/doc/filename.html>`_.
5. Optionally, by enabling the option :ref:`CONFIG_FATFS_USE_FASTSEEK`, use the POSIX lseek function to perform it faster, the fast seek will not work for files in write mode, so to take advantage of fast seek, you should open (or close and then reopen) the file in read-only mode.
9. Call the FatFs function :cpp:func:`ff_diskio_register` with NULL ``ff_diskio_impl_t*`` argument and the same drive number to unregister the disk I/O driver.
10. 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 in Step 1.
The convenience functions ``esp_vfs_fat_sdmmc_mount``, ``esp_vfs_fat_sdspi_mount`` and ``esp_vfs_fat_sdcard_unmount`` wrap the steps described above and also handle SD card initialization. These two functions are described in the next section.
The header file :component_file:`fatfs/vfs/esp_vfs_fat.h` defines convenience functions :cpp:func:`esp_vfs_fat_sdmmc_mount`, :cpp:func:`esp_vfs_fat_sdspi_mount` and :cpp:func:`esp_vfs_fat_sdcard_unmount`. These function perform Steps 1–3 and 7–9 respectively and handle SD card initialization, but provide only limited error handling. Developers are encouraged to check its source code and incorporate more advanced features into production applications.
The convenience function :cpp:func:`esp_vfs_fat_sdmmc_unmount` unmounts the filesystem and releases the resources acquired by :cpp:func:`esp_vfs_fat_sdmmc_mount`.
The header file :component_file:`fatfs/vfs/esp_vfs_fat.h` also defines the convenience functions :cpp:func:`esp_vfs_fat_rawflash_mount` and :cpp:func:`esp_vfs_fat_rawflash_unmount`. These functions perform Steps 1-3 and 7-9 respectively for read-only FAT partitions. These are particularly helpful for data partitions written only once during factory provisioning which will not be changed by production application throughout the lifetime of the hardware.
They provide implementation of disk I/O functions for SD/MMC cards and can be registered for the given FatFs drive number using the function :cpp:func:`ff_diskio_register_sdmmc`.
