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FAT Filesystem Support

:link_to_translation:zh_CN:[中文]

ESP-IDF uses the FatFs 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.

Using FatFs with VFS

The header file :component_file:`fatfs/vfs/esp_vfs_fat.h` defines the functions for connecting FatFs and VFS.

The function :cppesp_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 :cppesp_vfs_fat_unregister_path deletes the registration with VFS, and frees the FATFS structure.

Most applications use the following workflow when working with esp_vfs_fat_ functions:

  1. Call :cppesp_vfs_fat_register to specify:
    • Path prefix where to mount the filesystem (e.g., "/sdcard", "/spiflash")
    • FatFs drive number
    • A variable which receives the pointer to the FATFS structure
  2. Call :cppff_diskio_register to register the disk I/O driver for the drive number used in Step 1.
  3. To mount the filesystem using the same drive number which was passed to :cppesp_vfs_fat_register, call the FatFs function :cppf_mount. If the filesystem is not present on the target logical drive, :cppf_mount will fail with the FR_NO_FILESYSTEM error. In such case, call :cppf_mkfs to create a fresh FatFS structure on the drive first, and then call :cppf_mount again. Note that SD cards need to be partitioned with :cppf_fdisk prior to previously described steps. For more information, see FatFs documentation.
  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 :cppesp_vfs_register (for example, "/sdcard/hello.txt"). The filesystem uses 8.3 filenames format (SFN) by default. If you need to use long filenames (LFN), enable the CONFIG_FATFS_LONG_FILENAMES option. Please refer to FatFs filenames for more details.
  5. Optionally, call the FatFs library functions directly. In this case, use paths without a VFS prefix, for example, "/hello.txt".
  6. Close all open files.
  7. Call the FatFs function :cppf_mount for the same drive number with NULL FATFS* argument to unmount the filesystem.
  8. Call the FatFs function :cppff_diskio_register with NULL ff_diskio_impl_t* argument and the same drive number to unregister the disk I/O driver.
  9. Call :cppesp_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 :cppesp_vfs_fat_sdmmc_mount, :cppesp_vfs_fat_sdspi_mount, and :cppesp_vfs_fat_sdcard_unmount wrap the steps described above and also handle SD card initialization. These functions are described in the next section.

Note

Because FAT filesystem does not support hardlinks, :cpplink copies contents of the file instead. (This only applies to files on FatFs volumes.)

Using FatFs with VFS and SD Cards

The header file :component_file:`fatfs/vfs/esp_vfs_fat.h defines convenience functions :cpp:func:`esp_vfs_fat_sdmmc_mount, :cppesp_vfs_fat_sdspi_mount, and :cppesp_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 :cppesp_vfs_fat_sdmmc_unmount unmounts the filesystem and releases the resources acquired by :cppesp_vfs_fat_sdmmc_mount.

Using FatFs with VFS in Read-Only Mode

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 :cppesp_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.

Configuration options

The following configuration options are available for the FatFs component:

  • CONFIG_FATFS_USE_FASTSEEK - If enabled, the POSIX :cpplseek 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.
  • CONFIG_FATFS_IMMEDIATE_FSYNC - If enabled, the FatFs will automatically call :cppf_sync to flush recent file changes after each call of :cppwrite, :cpppwrite, :cpplink, :cpptruncate and :cppftruncate functions. This feature improves file-consistency and size reporting accuracy for the FatFs, at a price on decreased performance due to frequent disk operations.
  • 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 :cpplink will be non-atomic. In such case, using :cpplink on a large file on the same volume in a different task is not guaranteed to be thread safe.

FatFS Disk IO Layer

FatFs has been extended with API functions that register the disk I/O driver at runtime.

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 :cppff_diskio_register_sdmmc.

ff_diskio_register

ff_diskio_impl_t

ff_diskio_register_sdmmc

ff_diskio_register_wl_partition

ff_diskio_register_raw_partition

FatFs Partition Generator

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 tool is used to create filesystem images on a host and populate it with content of the specified host folder.

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 ].

An in-depth description of the FatFs partition generator and analyzer can be found at Generating and parsing FAT partition on host <fatfsgen>.

Build System Integration with FatFs Partition Generator

It is possible to invoke FatFs generator directly from the CMake build system by calling fatfs_create_spiflash_image:

fatfs_create_spiflash_image(<partition> <base_dir> [FLASH_IN_PROJECT])

If you prefer generating partition without wear levelling support, you can use fatfs_create_rawflash_image:

fatfs_create_rawflash_image(<partition> <base_dir> [FLASH_IN_PROJECT])

fatfs_create_spiflash_image respectively fatfs_create_rawflash_image must be called from project's CMakeLists.txt.

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.

The arguments of the function are as follows:

  1. partition - the name of the partition as defined in the partition table (e.g., :example_file:`storage/fatfsgen/partitions_example.csv`).
  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).
  5. 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.

For example:

fatfs_create_spiflash_image(my_fatfs_partition my_folder FLASH_IN_PROJECT)

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.

For an example, see storage/fatfsgen.

FatFs Partition Analyzer

(:component_file:`fatfsparse.py <fatfs/fatfsparse.py>`) is a partition analyzing tool for FatFs.

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.

Usage:

./fatfsparse.py [-h] [--wl-layer {detect,enabled,disabled}] [--verbose] fatfs_image.img

Parameter --verbose prints detailed information from boot sector of the FatFs image to the terminal before folder structure is generated.

High-level API Reference

inc/esp_vfs_fat.inc