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.
3. Call the FatFs function :cpp:func:`f_mount`, and optionally :cpp:func:`f_fdisk`, :cpp:func:`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://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`, you can use the POSIX lseek function to perform it faster. The fast seek does 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.
6. Optionally, by enabling the option :ref:`CONFIG_FATFS_IMMEDIATE_FSYNC`, you can enable automatic calling of :cpp:func:`f_sync` to flush recent file changes after each call of :cpp:func:`vfs_fat_write`, :cpp:func:`vfs_fat_pwrite`, :cpp:func:`vfs_fat_link`, :cpp:func:`vfs_fat_truncate` and :cpp:func:`vfs_fat_ftruncate` functions. This feature improves file-consistency and size reporting accuracy for the FatFs, at a price on decreased performance due to frequent disk operations.
10. 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.
11. 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 :cpp:func:`esp_vfs_fat_sdmmc_mount`, :cpp:func:`esp_vfs_fat_sdspi_mount`, and :cpp:func:`esp_vfs_fat_sdcard_unmount` wrap the steps described above and also handle SD card initialization. These 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 functions 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_spiflash_mount_ro` and :cpp:func:`esp_vfs_fat_spiflash_unmount_ro`. 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.
These APIs 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`.
We provide a partition generator for FatFs (:component_file:`wl_fatfsgen.py <fatfs/wl_fatfsgen.py>`) which is integrated into the build system and could be easily used in the user project.
The script is based on the partition generator (:component_file:`fatfsgen.py <fatfs/fatfsgen.py>`). Apart from generating partition, it can also initialize wear levelling.
The latest version supports both short and long file names, FAT12 and FAT16. The long file names are limited to 255 characters and can contain multiple periods (``.``) characters within the filename and additional characters ``+``, ``,``, ``;``, ``=``, ``[`` and ``]``.
If you decide for any reason to use ``fatfs_create_rawflash_image`` (without wear levelling support), beware that it supports mounting only in read-only mode in the device.
2. base_dir - the directory that will be encoded to FatFs partition and optionally flashed into the device. Beware that you have to specify the suitable size of the partition in the partition table.
3. flag ``FLASH_IN_PROJECT`` - optionally, users can have the image automatically flashed together with the app binaries, partition tables, etc. on ``idf.py flash -p <PORT>`` by specifying ``FLASH_IN_PROJECT``.
4. flag ``PRESERVE_TIME`` - optionally, users can force preserving the timestamps from the source folder to the target image. Without preserving the time, every timestamp will be set to the FATFS default initial time (1st January 1980).
If FLASH_IN_PROJECT is not specified, the image will still be generated, but you will have to flash it manually using ``esptool.py`` or a custom build system target.
It is a reverse tool of (:component_file:`fatfsgen.py <fatfs/fatfsgen.py>`), i.e., it can generate the folder structure on the host based on the FatFs image.
