Cache

Cache — basic routines to work with libblkid cache

Synopsis

typedef             blkid_cache;
void                blkid_gc_cache                      (blkid_cache cache);
int                 blkid_get_cache                     (blkid_cache *cache,
                                                         const char *filename);
void                blkid_put_cache                     (blkid_cache cache);
int                 blkid_probe_all                     (blkid_cache cache);
int                 blkid_probe_all_removable           (blkid_cache cache);
int                 blkid_probe_all_new                 (blkid_cache cache);
blkid_dev           blkid_verify                        (blkid_cache cache,
                                                         blkid_dev dev);

Description

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.

Details

blkid_cache

typedef struct blkid_struct_cache *blkid_cache;

information about all system devices

blkid_gc_cache ()

void                blkid_gc_cache                      (blkid_cache cache);

Removes garbage (non-existing devices) from the cache.

cache :

cache handler

blkid_get_cache ()

int                 blkid_get_cache                     (blkid_cache *cache,
                                                         const char *filename);

Allocates and initialize librray cache handler.

`cache` :	pointer to return cache handler
`filename` :	path to the cache file or NULL for the default path
Returns :	0 on success or number less than zero in case of error.

blkid_put_cache ()

void                blkid_put_cache                     (blkid_cache cache);

Saves changes to cache file.

cache :

cache handler

blkid_probe_all ()

int                 blkid_probe_all                     (blkid_cache cache);

Probes all block devices.

`cache` :	cache handler
Returns :	0 on success, or number less than zero in case of error.

blkid_probe_all_removable ()

int                 blkid_probe_all_removable           (blkid_cache cache);

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 cache (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.

`cache` :	cache handler
Returns :	0 on success, or number less than zero in case of error.

blkid_probe_all_new ()

int                 blkid_probe_all_new                 (blkid_cache cache);

Probes all new block devices.

`cache` :	cache handler
Returns :	0 on success, or number less than zero in case of error.

blkid_verify ()

blkid_dev           blkid_verify                        (blkid_cache cache,
                                                         blkid_dev dev);

			libblkid Reference Manual
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