.. _file-system:

===========
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 <http://elm-chan.org/fsw/ff/00index_e.html>`__.
Learn more `here </libs/fsdrv>`__.

Adding a driver
***************

Registering a driver
--------------------

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

.. code:: c

   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:

.. code:: c

   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 :cpp:enumerator:`LV_FS_MODE_WR` or :cpp:enumerator:`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:

.. code:: c

   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 <https://github.com/lvgl/lvgl/blob/master/examples/porting/lv_port_fs_template.c>`__.

Usage example
*************

The example below shows how to read from a file:

.. code:: c

   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 :cpp:func:`lv_fs_open` can be :cpp:enumerator:`LV_FS_MODE_WR` to open for writes
only or :cpp:enumerator:`LV_FS_MODE_RD` ``|`` :cpp:enumerator:`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.

.. code:: c

   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);
       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/img>`__ 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

API
***
