Using Removable Devices

Owned by Former user (Deleted)
Last updated: Dec 25, 2020 by WES Admin
2 min read

µC/FS expects that any call to a function that accesses a removable device may fail, since the device may be removed, powered off or suddenly unresponsive. If µC/FS detects such an event, the device will need to be refreshed or closed and re-opened. FSDev_Refresh() refreshes a device:


chngd = FSDev_Refresh((CPU_CHAR *)"ide:0:",  /* <-- device name  */
                      (FS_ERR   *)&err);     /* <-- return error */


There are several cases to consider:

  • If the return error is FS_ERR_NONE and the return value is DEF_YES, then a new device (e.g., SD card) has been inserted. All files and directories that are open on volumes on the device must be closed and all volumes that are open on the device must be closed or refreshed.
  • If the return error is FS_ERR_NONE and the return value is DEF_NO, then the same device (e.g., SD card) is still inserted. The application can continue to access open files, directories and volumes.
  • If the return error is neither FS_ERR_NONE nor FS_ERR_DEV_INVALID_LOW_FMT, then no functioning device is present. The device must be refreshed at a later time.

A device can be refreshed explicitly with FSDev_Refresh(); however, refresh also happens automatically. If a volume access (e.g., FSVol_Fmt(), FSVol_Rd()), entry access (FSEntry_Create(), fs_remove()), file open (fs_fopen() or FSFile_Open()) or directory open (fs_opendir() or FSDir_Open()) is initiated on a device that was not present at the last attempted access, µC/FS attempts to refresh the device information; if that succeeds, it attempts to refresh the volume information.

Files and directories have additional behavior. If a file is opened on a volume, and the underlying device is subsequently removed or changed, all further accesses using the file API (e.g., FSFile_Rd()) will fail with the error code FS_ERR_DEV_CHNGD; all POSIX API functions will return error values. The file should then be closed (to free the file structure).

Similarly, if a directory is opened on a volume, and the underlying device is subsequently removed or changed, all further FSDir_Rd() attempts will fail with the error code FS_ERR_DEV_CHNGD; fs_readdir_r() will return 1. The directory should then be closed (to free the directory structure).