libblkid(3) — Linux manual page


LIBBLKID(3)                Programmers Manual                LIBBLKID(3)

NAME         top

       libblkid - block device identification library

SYNOPSIS         top

       #include <blkid.h>

       cc file.c -lblkid

DESCRIPTION         top

       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. See list of all available
       tags in TAGS section.

       The low-level part of the library also allows the extraction of
       information about partitions and block device topology.

       The high-level part of the library keeps information about block
       devices 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

       The high-level part of the library supports two methods to
       determine LABEL/UUID. It reads information directly from a block
       device or reads 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. For more
       details about the config file see blkid(8) man page.

TAGS         top

       All available tags are listed below. Not all tags are supported
       for all file systems. To enable a tag, set one of the following
       flags with blkid_probe_set_superblocks_flags():


           •   TYPE - filesystem type


           •   SEC_TYPE - secondary filesystem type


           •   LABEL - filesystem label


           •   LABEL_RAW - raw label from FS superblock


           •   UUID - filesystem UUID (lower case)

           •   UUID_SUB - subvolume uuid (e.g. btrfs)

           •   LOGUUID - external log UUID (e.g. xfs)


           •   UUID_RAW - raw UUID from FS superblock


           •   USAGE - usage string: "raid", "filesystem", etc.


           •   VERSION - filesystem version


           •   SBMAGIC - super block magic string

           •   SBMAGIC_OFFSET - offset of SBMAGIC


           •   FSSIZE - size of filesystem. Note that for XFS this will
               return the same value as lsblk (without XFS’s metadata),
               but for ext4 it will return the size with metadata and
               for BTRFS will not count overhead of RAID configuration
               (redundant data).

           •   FSLASTBLOCK - last fsblock/total number of fsblocks

           •   FSBLOCKSIZE - file system block size

       The following tags are always enabled

           •   BLOCK_SIZE - minimal block size accessible by file system

           •   MOUNT - cluster mount name (ocfs only)

           •   EXT_JOURNAL - external journal UUID

           •   SYSTEM_ID - ISO9660 system identifier

           •   VOLUME_SET_ID - ISO9660 volume set identifier

           •   DATA_PREPARER_ID - ISO9660 data identifier

           •   PUBLISHER_ID - ISO9660 publisher identifier

           •   APPLICATION_ID - ISO9660 application identifier

           •   BOOT_SYSTEM_ID - ISO9660 boot system identifier

AUTHORS         top

       libblkid was written by Andreas Dilger for the ext2 filesystem
       utilities, 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.

COPYING         top

       libblkid is available under the terms of the GNU Library General
       Public License (LGPL), version 2 (or at your discretion any later

SEE ALSO         top

       blkid(8), findfs(8)

REPORTING BUGS         top

       For bug reports, use the issue tracker at

AVAILABILITY         top

       The libblkid library is part of the util-linux package since
       version 2.15. It can be downloaded from Linux Kernel Archive
       <>. This page
       is part of the util-linux (a random collection of Linux
       utilities) project. Information about the project can be found at
       ⟨⟩. If you have
       a bug report for this manual page, send it to This page was obtained from the
       project's upstream Git repository
       ⟨git://⟩ on
       2023-12-22. (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-14.) If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in
       this COLOPHON (which is not part of the original manual page),
       send a mail to

util-linux 2.39.594-1e0ad      2023-07-19                    LIBBLKID(3)

Pages that refer to this page: open_by_handle_at(2)blkid(8)mount(8)wipefs(8)