libblkid — block device identification library
#include <blkid.h> cc file.c −lblkid
The libblkid
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. A common use is to allow use of
LABEL= and UUID= tags instead of hard-coding specific block
device names into configuration files.
The low-level part of the library also allows to extract infomation about partitions and block device topology.
The high-level part of the library keeps information about
block devices in a cache file /etc/blkid.tab
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).
The high-level part of the library supports two methods to evaluate LABEL/UUID. It reads information directly from a block device or read information from /dev/disk/by-* udev symlinks. The udev is preferred method by default.
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.
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 required
in this
situation.
The standard location of the /etc/blkid.conf
config file can be
overridden by the environment variable BLKID_CONF. The
following options control the libblkid library:
SEND_UEVENT=<yes|not>
Sends uevent when /dev/disk/by-{label,uuid}/
symlink
does not match with LABEL or UUID on the device.
Default is "yes".
CACHE_FILE=<path>
Overrides the standard location of the cache file.
This setting can be overridden by the environment
variable BLKID_FILE. Default is /etc/blkid.tab.
EVALUATE=<methods>
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 /dev/disk/by-*
symlinks and the
"scan" method scans all block devices from the
/proc/partitions
file.
libblkid
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.
/etc/blkid.tab
caches data extracted from each recognized block device
/etc/blkid.conf
configuration file
libblkid
is part of the
util-linux package since version 2.15 and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.