File Systems

Zephyr RTOS Virtual Filesystem Switch (VFS) allows applications to mount multiple file systems at different mount points (e.g., /fatfs and /lfs). The mount point data structure contains all the necessary information required to instantiate, mount, and operate on a file system. The File system Switch decouples the applications from directly accessing an individual file system’s specific API or internal functions by introducing file system registration mechanisms.

In Zephyr, any file system implementation or library can be plugged into or pulled out through a file system registration API. Each file system implementation must have a globally unique integer identifier; use FS_TYPE_EXTERNAL_BASE to avoid clashes with in-tree identifiers.

int fs_register(int type, const struct fs_file_system_t *fs);

int fs_unregister(int type, const struct fs_file_system_t *fs);

Zephyr RTOS supports multiple instances of a file system by making use of the mount point as the disk volume name, which is used by the file system library while formatting or mounting a disk.

A file system is declared as:

static struct fs_mount_t mp = {
.type = FS_FATFS,
.mnt_point = FATFS_MNTP,
.fs_data = &fat_fs,
};

where

  • FS_FATFS is the file system type like FATFS or LittleFS.

  • FATFS_MNTP is the mount point where the file system will be mounted.

  • fat_fs is the file system data which will be used by fs_mount() API.

Sample

A sample of how the file system can be used is supplied in samples/subsys/fs.

API Reference

group file_system_api

File System APIs.

fs_open open and creation mode flags

FS_O_READ

Open for read flag

FS_O_WRITE

Open for write flag

FS_O_RDWR

Open for read-write flag combination

FS_O_MODE_MASK

Bitmask for read and write flags

FS_O_CREATE

Create file if it does not exist

FS_O_APPEND

Open/create file for append

FS_O_FLAGS_MASK

Bitmask for open/create flags

FS_O_MASK

Bitmask for open flags

fs_seek whence parameter values

FS_SEEK_SET

Seek from the beginning of file

FS_SEEK_CUR

Seek from a current position

FS_SEEK_END

Seek from the end of file

Defines

FS_MOUNT_FLAG_NO_FORMAT

Flag prevents formatting device if requested file system not found

FS_MOUNT_FLAG_READ_ONLY

Flag makes mounted file system read-only

Enums

enum fs_dir_entry_type

Values:

enumerator FS_DIR_ENTRY_FILE

Identifier for file entry

enumerator FS_DIR_ENTRY_DIR

Identifier for directory entry

enum [anonymous]

Enumeration to uniquely identify file system types.

Zephyr supports in-tree file systems and external ones. Each requires a unique identifier used to register the file system implementation and to associate a mount point with the file system type. This anonymous enum defines global identifiers for the in-tree file systems.

External file systems should be registered using unique identifiers starting at FS_TYPE_EXTERNAL_BASE. It is the responsibility of applications that use external file systems to ensure that these identifiers are unique if multiple file system implementations are used by the application.

Values:

enumerator FS_FATFS

Identifier for in-tree FatFS file system.

enumerator FS_LITTLEFS

Identifier for in-tree LittleFS file system.

enumerator FS_TYPE_EXTERNAL_BASE

Base identifier for external file systems.

Functions

int fs_open(struct fs_file_t *zfp, const char *file_name, fs_mode_t flags)

Open or create file.

Opens or possibly creates a file and associates a stream with it.

flags can be 0 or a binary combination of one or more of the following identifiers:

  • FS_O_READ open for read

  • FS_O_WRITE open for write

  • FS_O_RDWR open for read/write (FS_O_READ | FS_O_WRITE)

  • FS_O_CREATE create file if it does not exist

  • FS_O_APPEND move to end of file before each write

If flags are set to 0 the function will attempt to open an existing file with no read/write access; this may be used to e.g. check if the file exists.

Parameters

  • zfp: Pointer to a file object

  • file_name: The name of a file to open

  • flags: The mode flags

Return Value

  • 0: on success;

  • -EINVAL: when a bad file name is given;

  • -EROFS: when opening read-only file for write, or attempting to create a file on a system that has been mounted with the FS_MOUNT_FLAG_READ_ONLY flag;

  • -ENOENT: when the file path is not possible (bad mount point);

  • <0: an other negative errno code, depending on a file system back-end.

int fs_close(struct fs_file_t *zfp)

Close file.

Flushes the associated stream and closes the file.

Parameters

  • zfp: Pointer to the file object

Return Value

  • 0: on success;

  • <0: a negative errno code on error.

Unlink file.

Deletes the specified file or directory

Parameters

  • path: Path to the file or directory to delete

Return Value

  • 0: on success;

  • -EROFS: if file is read-only, or when file system has been mounted with the FS_MOUNT_FLAG_READ_ONLY flag;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_rename(const char *from, const char *to)

Rename file or directory.

Performs a rename and / or move of the specified source path to the specified destination. The source path can refer to either a file or a directory. All intermediate directories in the destination path must already exist. If the source path refers to a file, the destination path must contain a full filename path, rather than just the new parent directory. If an object already exists at the specified destination path, this function causes it to be unlinked prior to the rename (i.e., the destination gets clobbered).

Note

Current implementation does not allow moving files between mount points.

Parameters

  • from: The source path

  • to: The destination path

Return Value

  • 0: on success;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error.

ssize_t fs_read(struct fs_file_t *zfp, void *ptr, size_t size)

Read file.

Reads up to size bytes of data to ptr pointed buffer, returns number of bytes read. A returned value may be lower than size if there were fewer bytes available than requested.

Parameters

  • zfp: Pointer to the file object

  • ptr: Pointer to the data buffer

  • size: Number of bytes to be read

Return Value

  • >=0: a number of bytes read, on success;

  • <0: a negative errno code on error.

ssize_t fs_write(struct fs_file_t *zfp, const void *ptr, size_t size)

Write file.

Attempts to write size number of bytes to the specified file. If a negative value is returned from the function, the file pointer has not been advanced. If the function returns a non-negative number that is lower than size, the global errno variable should be checked for an error code, as the device may have no free space for data.

Parameters

  • zfp: Pointer to the file object

  • ptr: Pointer to the data buffer

  • size: Number of bytes to be written

Return Value

  • >=0: a number of bytes written, on success;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_seek(struct fs_file_t *zfp, off_t offset, int whence)

Seek file.

Moves the file position to a new location in the file. The offset is added to file position based on the whence parameter.

Parameters

  • zfp: Pointer to the file object

  • offset: Relative location to move the file pointer to

  • whence: Relative location from where offset is to be calculated.

    • FS_SEEK_SET for the beginning of the file;

    • FS_SEEK_CUR for the current position;

    • FS_SEEK_END for the end of the file.

Return Value

  • 0: on success;

  • -ENOTSUP: if not supported by underlying file system driver;

  • <0: an other negative errno code on error.

off_t fs_tell(struct fs_file_t *zfp)

Get current file position.

Retrieves and returns the current position in the file stream.

The current revision does not validate the file object.

Parameters

  • zfp: Pointer to the file object

Return Value

  • >=: 0 a current position in file;

  • -ENOTSUP: if not supported by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_truncate(struct fs_file_t *zfp, off_t length)

Truncate or extend an open file to a given size.

Truncates the file to the new length if it is shorter than the current size of the file. Expands the file if the new length is greater than the current size of the file. The expanded region would be filled with zeroes.

Note

In the case of expansion, if the volume got full during the expansion process, the function will expand to the maximum possible length and return success. Caller should check if the expanded size matches the requested length.

Parameters

  • zfp: Pointer to the file object

  • length: New size of the file in bytes

Return Value

  • 0: on success;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_sync(struct fs_file_t *zfp)

Flush cached write data buffers of an open file.

The function flushes the cache of an open file; it can be invoked to ensure data gets written to the storage media immediately, e.g. to avoid data loss in case if power is removed unexpectedly.

Note

Closing a file will cause caches to be flushed correctly so the function need not be called when the file is being closed.

Parameters

  • zfp: Pointer to the file object

Return Value

  • 0: on success;

  • <0: a negative errno code on error.

int fs_mkdir(const char *path)

Directory create.

Creates a new directory using specified path.

Parameters

  • path: Path to the directory to create

Return Value

  • 0: on success;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error

int fs_opendir(struct fs_dir_t *zdp, const char *path)

Directory open.

Opens an existing directory specified by the path.

Parameters

  • zdp: Pointer to the directory object

  • path: Path to the directory to open

Return Value

  • 0: on success;

  • <0: a negative errno code on error.

int fs_readdir(struct fs_dir_t *zdp, struct fs_dirent *entry)

Directory read entry.

Reads directory entries of an open directory. In end-of-dir condition, the function will return 0 and set the entry->name[0] to 0.

Note

: Most existing underlying file systems do not generate POSIX special directory entries “.” or “..”. For consistency the abstraction layer will remove these from lower layer results so higher layers see consistent results.

Parameters

  • zdp: Pointer to the directory object

  • entry: Pointer to zfs_dirent structure to read the entry into

Return Value

  • 0: on success or end-of-dir;;

  • <0: a negative errno code on error.

int fs_closedir(struct fs_dir_t *zdp)

Directory close.

Closes an open directory.

Parameters

  • zdp: Pointer to the directory object

Return Value

  • 0: on success;

  • <0: a negative errno code on error.

int fs_mount(struct fs_mount_t *mp)

Mount filesystem.

Perform steps needed for mounting a file system like calling the file system specific mount function and adding the mount point to mounted file system list.

Parameters

  • mp: Pointer to the fs_mount_t structure. Referenced object is not changed if the mount operation failed. A reference is captured in the fs infrastructure if the mount operation succeeds, and the application must not mutate the structure contents until fs_unmount is successfully invoked on the same pointer.

Return Value

  • 0: on success;

  • -ENOENT: when file system type has not been registered;

  • -ENOTSUP: when not supported by underlying file system driver;

  • -EROFS: if system requires formatting but FS_MOUNT_FLAG_READ_ONLY has been set;

  • <0: an other negative errno code on error.

int fs_unmount(struct fs_mount_t *mp)

Unmount filesystem.

Perform steps needed to unmount a file system like calling the file system specific unmount function and removing the mount point from mounted file system list.

Parameters
Return Value

  • 0: on success;

  • -EINVAL: if no system has been mounted at given mount point;

  • -ENOTSUP: when not supported by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_readmount(int *index, const char **name)

Get path of mount point at index.

This function iterates through the list of mount points and returns the directory name of the mount point at the given index. On success index is incremented and name is set to the mount directory name. If a mount point with the given index does not exist, name will be set to NULL.

Parameters

  • index: Pointer to mount point index

  • name: Pointer to pointer to path name

Return Value

  • 0: on success;

  • -ENOENT: if there is no mount point with given index.

int fs_stat(const char *path, struct fs_dirent *entry)

File or directory status.

Checks the status of a file or directory specified by the path.

Note

The file on a storage device may not be updated until it is closed.

Parameters

  • path: Path to the file or directory

  • entry: Pointer to the zfs_dirent structure to fill if the file or directory exists.

Return Value

  • 0: on success;

  • <0: negative errno code on error.

int fs_statvfs(const char *path, struct fs_statvfs *stat)

Retrieves statistics of the file system volume.

Returns the total and available space in the file system volume.

Parameters

  • path: Path to the mounted directory

  • stat: Pointer to the zfs_statvfs structure to receive the fs statistics

Return Value

  • 0: on success;

  • -ENOTSUP: when not implemented by underlying file system driver;

  • <0: an other negative errno code on error.

int fs_register(int type, const struct fs_file_system_t *fs)

Register a file system.

Register file system with virtual file system.

Parameters

  • type: Type of file system (ex: FS_FATFS)

  • fs: Pointer to File system

Return Value

  • 0: on success;

  • <0: negative errno code on error.

int fs_unregister(int type, const struct fs_file_system_t *fs)

Unregister a file system.

Unregister file system from virtual file system.

Parameters

  • type: Type of file system (ex: FS_FATFS)

  • fs: Pointer to File system

Return Value

  • 0: on success;

  • <0: negative errno code on error.

struct fs_mount_t
#include <fs.h>

File system mount info structure.

Parameters

  • node: Entry for the fs_mount_list list

  • type: File system type

  • mnt_point: Mount point directory name (ex: “/fatfs”)

  • fs_data: Pointer to file system specific data

  • storage_dev: Pointer to backend storage device

  • mountp_len: Length of Mount point string

  • fs: Pointer to File system interface of the mount point

  • flags: Mount flags

struct fs_dirent
#include <fs.h>

Structure to receive file or directory information.

Used in functions that reads the directory entries to get file or directory information.

Parameters

  • dir_entry_type: Whether file or directory

    • FS_DIR_ENTRY_FILE

    • FS_DIR_ENTRY_DIR

  • name: Name of directory or file

  • size: Size of file. 0 if directory

struct fs_statvfs
#include <fs.h>

Structure to receive volume statistics.

Used to retrieve information about total and available space in the volume.

Parameters

  • f_bsize: Optimal transfer block size

  • f_frsize: Allocation unit size

  • f_blocks: Size of FS in f_frsize units

  • f_bfree: Number of free blocks

struct fs_file_system_t
#include <fs_sys.h>

File System interface structure.

Parameters

  • open: Opens or creates a file, depending on flags given

  • read: Reads nbytes number of bytes

  • write: Writes nbytes number of bytes

  • lseek: Moves the file position to a new location in the file

  • tell: Retrieves the current position in the file

  • truncate: Truncates/expands the file to the new length

  • sync: Flushes the cache of an open file

  • close: Flushes the associated stream and closes the file

  • opendir: Opens an existing directory specified by the path

  • readdir: Reads directory entries of an open directory

  • closedir: Closes an open directory

  • mount: Mounts a file system

  • unmount: Unmounts a file system

  • unlink: Deletes the specified file or directory

  • rename: Renames a file or directory

  • mkdir: Creates a new directory using specified path

  • stat: Checks the status of a file or directory specified by the path

  • statvfs: Returns the total and available space on the file system volume