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.
#. To mount the filesystem using the same drive number which was passed to :cpp:func:`esp_vfs_fat_register`, call the FatFs function :cpp:func:`f_mount`. If the filesystem is not present on the target logical drive, :cpp:func:`f_mount` will fail with the ``FR_NO_FILESYSTEM`` error. In such case, call :cpp:func:`f_mkfs` to create a fresh FatFS structure on the drive first, and then call :cpp:func:`f_mount` again. Note that SD cards need to be partitioned with :cpp:func:`f_fdisk` prior to previously described steps. For more information, see `FatFs documentation <http://elm-chan.org/fsw/ff/doc/mount.html>`_.
#. 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. Please refer to `FatFs filenames <http://elm-chan.org/fsw/ff/doc/filename.html>`_ for more details.
#. 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.
#. 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.
Because FAT filesystem does not support hardlinks, :cpp:func:`link` copies contents of the file instead. (This only applies to files on FatFs volumes.)
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.
*:ref:`CONFIG_FATFS_USE_FASTSEEK` - If enabled, the POSIX :cpp:func:`lseek` function will be performed 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.
*:ref:`CONFIG_FATFS_IMMEDIATE_FSYNC` - If enabled, the FatFs will automatically call :cpp:func:`f_sync` to flush recent file changes after each call of :cpp:func:`write`, :cpp:func:`pwrite`, :cpp:func:`link`, :cpp:func:`truncate` and :cpp:func:`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.
*:ref:`CONFIG_FATFS_LINK_LOCK` - If enabled, this option guarantees the API thread safety, while disabling this option might be necessary for applications that require fast frequent small file operations (e.g., logging to a file). Note that if this option is disabled, the copying performed by :cpp:func:`link` will be non-atomic. In such case, using :cpp:func:`link` on a large file on the same volume in a different task is not guaranteed to be thread safe.
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.
#. 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.
#. 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``.
#. 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).
#. flag ``ONE_FAT`` - optionally, users can still choose to generate a FATFS volume with a single FAT (file allocation table) instead of two. This makes the free space in the FATFS volume a bit larger (by ``number of sectors used by FAT * sector size``) but also more prone to corruption.
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.
