Block device information is normally kept in a cache file 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.
void blkid_gc_cache (
Removes garbage (non-existing devices) from the cache.
int blkid_get_cache (
const char *filename);
Allocates and initialize library cache handler.
pointer to return cache handler
path to the cache file or NULL for the default path
void blkid_put_cache (
Saves changes to cache file.
int blkid_probe_all (
Probes all block devices.
int blkid_probe_all_removable (
The libblkid probing is based on devices from /proc/partitions by default. This file usually does not contain removable devices (e.g. CDROMs) and this kind of devices are invisible for libblkid.
This function adds removable block devices to
(probing is based on
information from the /sys directory). Don't forget that removable devices
(floppies, CDROMs, ...) could be pretty slow. It's very bad idea to call
this function by default.
Note that devices which were detected by this function won't be written to blkid.tab cache file.
int blkid_probe_all_new (
Probes all new block devices.