library is used to identify block devices (disks) as to their content (e.g.
filesystem type) as well as extracting additional information such as
filesystem labels/volume names, unique identifiers/serial numbers, etc.
A common use is to allow use of LABEL= and UUID= tags instead of hard-coding
specific block device names into configuration files.
Block device information is normally kept in a cache file
and is verified to still be valid before being returned to the user
(if the user has read permission on the raw block device, otherwise not).
The cache file also allows unprivileged users (normally anyone other
than root, or those not in the "disk" group) to locate devices by label/id.
The standard location of the cache file can be overridden by the
environment variable BLKID_FILE.
In situations where one is getting information about a single known device,
it does not impact performance whether the cache is used or not (unless you
are not able to read the block device directly). If you are dealing with
multiple devices, use of the cache is highly recommended (even if empty) as
devices will be scanned at most one time and the on-disk cache will be
updated if possible. There is rarely a reason not to use the cache.
In some cases (modular kernels), block devices are not even visible until
after they are accessed the first time, so it is critical that there is
some way to locate these devices without enumerating only visible devices,
so the use of the cache file is
in this situation.
The standard location of the
config file can be overridden by the environment variable BLKID_CONF.
The following options control the libblkid library:
Sends uevent when
symlink does not match with LABEL or UUID on the device. Default is "yes".
Overrides the standard location of the cache file. This setting can be
overridden by the environment variable BLKID_FILE. Default is
Defines LABEL and UUID evaluation method(s). Currently, the libblkid library
supports "udev" and "scan" methods. More than one methods may be specified in
a comma separated list. Default is "udev,scan". The "udev" method uses udev
symlinks and the "scan" method scans all block devices from the
was written by Andreas Dilger for the ext2 filesystem utilties, with input
from Ted Ts'o. The library was subsequently heavily modified by Ted Ts'o.
The low-level probing code was rewritten by Karel Zak.
caches data extracted from each recognized block device
is available under the terms of the GNU Library General Public License (LGPL),
version 2 (or at your discretion any later version). A copy of the LGPL
should be included with this library in the file COPYING. If not, write to
Free Software Foundation, Inc.
51 Franklin St
Boston, MA 02110-1301 USA