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
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".
Note
If you want to skip the drive prefix from the path, you can use the LV_FS_DEFAULT_DRIVE_LETTER config parameter.
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 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.