File system

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 drive letter. For example, if an SD card is associated with the letter 'S', a file can be reached using "S:path/to/file.txt".

Ready to use drivers

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

Adding a driver

Registering a driver

To add a driver, a lv_fs_drv_t needs to be initialized like below. The lv_fs_drv_t needs to be static, global or dynamically allocated and not a local variable.

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 Bytes To Write, bw is the actually written bytes.

For a template of these callbacks see lv_fs_template.c.

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();


The mode in lv_fs_open() can be LV_FS_MODE_WR to open for writes 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) {

    /*fn is empty, if not more files to read*/
    if(strlen(fn) == 0) {

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


Use drives for images

Image objects can be opened from files too (besides variables stored in the compiled program).

To use files in image widgets the following callbacks are required:

  • open

  • close

  • read

  • seek

  • tell