File System (lv_fs_drv)

LVGL has a "File system" abstraction module that enables you to attach any type of file system. A file system is identified by an assigned identifier letter. For example, if an SD card is associated with the letter 'S', a file can be reached using "S:/path/to/file.txt". See details under Identifier Letters.

Note

If you want to skip the drive-letter prefix in Unix-like paths, you can use the LV_FS_DEFAULT_DRIVER_LETTER config parameter.

Ready-to-use drivers

LVGL contains prepared drivers for the API of POSIX, standard C, Windows, and FATFS. Learn more here.

Identifier Letters

As mentioned above, a file system is identified by an assigned identifier letter. This identifier is merely a way for the LVGL File System abtraction logic to look up the appropriate registered file-system driver for a given path.

How it Works:

You register a driver for your file system and assign it an identifier letter. This letter must be unique among all registered file-system drivers, and in the range [A-Z] or the character '/'. See Adding a Driver for how this is done.

Later, when using paths to files on your file system, you prefix the path with that identifier character plus a colon (':').

Note

Do not confuse this with a Windows or DOS drive letter.

Example:

Let's use the letter 'Z' as the identifier character, and "path_to_file" as the path, then the path strings you pass to lv_fs_...() functions would look like this:

"Z:path_to_file"
 ^ ^^^^^^^^^^^^
 |        |
 |        +-- This part gets passed to the OS-level file-system functions.
 |
 +-- This part LVGL strips from path string, and uses it to find the appropriate
     driver (i.e. set of functions) that apply to that file system.

Note also that the path can be a relative path or a "rooted path" (beginning with /), though rooted paths are recommended since the driver does not yet provide a way to set the default directory.

Examples for Unix-like file systems:

• "Z:/etc/images/splash.png"

• "Z:/etc/images/left_button.png"

• "Z:/etc/images/right_button.png"

• "Z:/home/users/me/wip/proposal.txt"

Examples for Windows/DOS-like file systems:

• "Z:C:/Users/me/wip/proposal.txt"

• "Z:/Users/me/wip/proposal.txt" (if the default drive is known to be C:)

• "Z:C:/Users/Public/Documents/meeting_notes.txt"

• "Z:D:/to_print.docx"

Reminder: Note carefully that the prefixed "Z:" has nothing to do with the "C:" and "D:" Windows/DOS drive letters in 3 of the above examples, which are part of the path. "Z:" is used to look up the driver for that file system in the list of all file-system drivers registered with LVGL.

Adding a Driver

Registering a driver

To add a driver, a lv_fs_drv_t object needs to be initialized and registered in a way similar to the code below. The lv_fs_drv_t variable needs to be static, global or dynamically allocated and not a local variable, since its contents need to remain valid as long as the driver is in use.

static lv_fs_drv_t drv;                   /* Needs to be static or global */
lv_fs_drv_init(&drv);                     /* Basic initialization */

drv.letter = 'S';                         /* An uppercase letter to identify the drive */
drv.cache_size = my_cache_size;           /* Cache size for reading in bytes. 0 to not cache. */

drv.ready_cb = my_ready_cb;               /* Callback to tell if the drive is ready to use */
drv.open_cb = my_open_cb;                 /* Callback to open a file */
drv.close_cb = my_close_cb;               /* Callback to close a file */
drv.read_cb = my_read_cb;                 /* Callback to read a file */
drv.write_cb = my_write_cb;               /* Callback to write a file */
drv.seek_cb = my_seek_cb;                 /* Callback to seek in a file (Move cursor) */
drv.tell_cb = my_tell_cb;                 /* Callback to tell the cursor position  */

drv.dir_open_cb = my_dir_open_cb;         /* Callback to open directory to read its content */
drv.dir_read_cb = my_dir_read_cb;         /* Callback to read a directory's content */
drv.dir_close_cb = my_dir_close_cb;       /* Callback to close a directory */

drv.user_data = my_user_data;             /* Any custom data if required */

lv_fs_drv_register(&drv);                 /* Finally register the drive */

Any of the callbacks can be NULL to indicate that operation is not supported.

Implementing the callbacks

Open callback

The prototype of open_cb looks like this:

void * (*open_cb)(lv_fs_drv_t * drv, const char * path, lv_fs_mode_t mode);

path is the path after the drive letter (e.g. "S:path/to/file.txt" -> "path/to/file.txt"). mode can be LV_FS_MODE_WR or LV_FS_MODE_RD to open for writes or reads.

The return value is a pointer to a file object that describes the opened file or NULL if there were any issues (e.g. the file wasn't found). The returned file object will be passed to other file system related callbacks. (See below.)

Other callbacks

The other callbacks are quite similar. For example write_cb looks like this:

lv_fs_res_t (*write_cb)(lv_fs_drv_t * drv, void * file_p, const void * buf, uint32_t btw, uint32_t * bw);

For file_p, LVGL passes the return value of open_cb, buf is the data to write, btw is the number of "bytes to write", bw is the number of "bytes written" (written to during the function call).

For a list of prototypes for these callbacks see lv_fs_template.c. This file also provides a template for new file-system drivers you can use if the one you need is not already provided.

Drivers that come with LVGL

As of this writing, the list of already-available file-system drivers can be enabled by setting one or more of the following macros to a non-zero value in lv_conf.h. The drivers are as implied by the macro names.

If you use more than one, each associated identifier letter you use must be unique.

• LV_USE_FS_FATFS

• LV_USE_FS_STDIO

• LV_USE_FS_POSIX

• LV_USE_FS_WIN32

• LV_USE_FS_MEMFS

• LV_USE_FS_LITTLEFS

• LV_USE_FS_ARDUINO_ESP_LITTLEFS

• LV_USE_FS_ARDUINO_SD

Limiting Directory Access

If you are using one of the following file-system drivers:

• LV_USE_FS_STDIO

• LV_USE_FS_POSIX

• LV_USE_FS_WIN32

you will have a LV_FS_xxx_PATH macro available to you in lv_conf.h that you can use to provide a path that gets dynamically prefixed to the path_to_file portion of of the path strings provided to lv_fs_...() functions when files and directories are opened. This can be useful to limit directory access (e.g. when a portion of a path can be typed by an end user), or simply to reduce the length of the path strings provided to lv_fs_...() functions.

Do this by filling in the full path to the directory you wish his access to be limited to in the applicable LV_FS_xxx_PATH macro in lv_conf.h. Do not prefix the path with the driver-identifier letter, and do append a directory separator character at the end.

Examples for Unix-like file systems:

#define LV_FS_WIN32_PATH   "/home/users/me/"

Examples for Windows/DOS-like file systems:

#define LV_FS_WIN32_PATH   "C:/Users/me/"

Then in both cases, path strings passed to lv_fs_...() functions in the application get reduced to:

• "Z:wip/proposal.txt"

Usage Example

The example below shows how to read from a file:

lv_fs_file_t f;
lv_fs_res_t res;
res = lv_fs_open(&f, "S:folder/file.txt", LV_FS_MODE_RD);
if(res != LV_FS_RES_OK) my_error_handling();

uint32_t read_num;
uint8_t buf[8];
res = lv_fs_read(&f, buf, 8, &read_num);
if(res != LV_FS_RES_OK || read_num != 8) my_error_handling();

lv_fs_close(&f);

The mode in lv_fs_open() can be LV_FS_MODE_WR to open for writes only, LV_FS_MODE_RD for reads only, or LV_FS_MODE_RD | LV_FS_MODE_WR for both.

This example shows how to read a directory's content. It's up to the driver how to mark directories in the result but it can be a good practice to insert a '/' in front of each directory name.

lv_fs_dir_t dir;
lv_fs_res_t res;
res = lv_fs_dir_open(&dir, "S:/folder");
if(res != LV_FS_RES_OK) my_error_handling();

char fn[256];
while(1) {
    res = lv_fs_dir_read(&dir, fn, sizeof(fn));
    if(res != LV_FS_RES_OK) {
        my_error_handling();
        break;
    }

    /* fn is empty if there are no more files to read. */
    if(strlen(fn) == 0) {
        break;
    }

    printf("%s\n", fn);
}

lv_fs_dir_close(&dir);

Use Drives for Images

Image Widgets can be opened from files as well (besides variables stored in the compiled program).

To use files in Image Widgets the following callbacks are required:

• open

• close

• read

• seek

• tell

Optional File Buffering/Caching

Files will buffer their reads if the corresponding LV_FS_*_CACHE_SIZE config option is set to a value greater than zero. Each open file will buffer up to that many bytes to reduce the number of FS driver calls.

Generally speaking, file buffering can be optimized for different kinds of access patterns. The one implemented here is optimal for reading large files in chunks, which is what the image decoder does. It has the potential to call the driver's read fewer times than lv_fs_read is called. In the best case where the cache size is >= the size of the file, read will only be called once. This strategy is good for linear reading of large files but less helpful for short random reads across a file bigger than the buffer since data will be buffered that will be discarded after the next seek and read. The cache should be sufficiently large or disabled in that case. Another case where the cache should be disabled is if the file contents are expected to change by an external factor like with special OS files.

The implementation is documented below. Note that the FS functions make calls to other driver FS functions when the cache is enabled. i.e., lv_fs_read may call the driver's seek so the driver needs to implement more callbacks when the cache is enabled.

lv_fs_read _{(behavior when cache is enabled)}

        %%{init: {'theme':'neutral'}}%%
flowchart LR
    A["call lv_fs_read and
       the cache is enabled"] --> B{{"is there cached data
                                      at the file position?"}}
    B -->|yes| C{{"does the cache have
                   all required bytes available?"}}
    C -->|yes| D["copy all required bytes from
                  the cache to the destination
                  buffer"]
    C -->|no| F["copy the available
                 required bytes
                 until the end of the cache
                 into the destination buffer"]
          --> G["seek the real file to the end
                 of what the cache had available"]
          --> H{{"is the number of remaining bytes
                  larger than the size of the whole cache?"}}
    H -->|yes| I["read the remaining bytes
                  from the real file to the
                  destination buffer"]
    H -->|no| J["eagerly read the real file
                 to fill the whole cache
                 or as many bytes as the
                 read call can"]
          --> O["copy the required bytes
                 to the destination buffer"]
    B -->|no| K["seek the real file to
                 the file position"]
          --> L{{"is the number of required
                  bytes greater than the
                  size of the entire cache?"}}
    L -->|yes| M["read the real file to
                  the destination buffer"]
    L -->|no| N["eagerly read the real file
                 to fill the whole cache
                 or as many bytes as the
                 read call can"]
          --> P["copy the required bytes
                 to the destination buffer"]
    

lv_fs_write _{(behavior when cache is enabled)}

The part of the cache that coincides with the written content will be updated to reflect the written content.

lv_fs_seek _{(behavior when cache is enabled)}

The driver's seek will not actually be called unless the whence is LV_FS_SEEK_END, in which case seek and tell will be called to determine where the end of the file is.

lv_fs_tell _{(behavior when cache is enabled)}

The driver's tell will not actually be called.

API

lv_types.h

lv_fsdrv.h

lv_fs.h