NAME¶
libblkid - block device identification library
SYNOPSIS¶
#include <blkid.h>
cc file.c -lblkid
DESCRIPTION¶
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.
CONFIGURATION FILE¶
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.
AUTHOR¶
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.
FILES¶
- /etc/blkid.tab
- caches data extracted from each recognized block
device
- /etc/blkid.conf
- configuration file
AVAILABILITY¶
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/.
COPYING¶
libblkid is available under the terms of the GNU Library General Public
License (LGPL), version 2 (or at your discretion any later version).
SEE ALSO¶
blkid(8) findfs(8)