Poster of Linux kernelThe best gift for a Linux geek
LIBBLKID

LIBBLKID

Section: C Library Functions (3) Updated: May 2009
Local index Up
 

NAME

libblkid - block device identification library  

SYNOPSIS

#include <blkid/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, 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 /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). 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 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-ng package since version 2.15 and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.  

COPYING

libblkid 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
Fifth Floor
Boston, MA 02110-1301 USA

or visit http://www.gnu.org/licenses/licenses.html#LGPL  

SEE ALSO

blkid(8) findfs(8)


 

Index

NAME
SYNOPSIS
DESCRIPTION
CONFIGURATION FILE
AUTHOR
FILES
AVAILABILITY
COPYING
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 21:49:17 GMT, April 16, 2011