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();

lv_fs_close(&f);

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) {
        my_error_handling();
        break;
    }

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

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

lv_fs_dir_close(&dir);

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

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 the 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 the 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 the 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 the cache is enabled)}

The driver's tell will not actually be called.

API

lv_fs.h