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libblkid - a library to handle device identification and token extraction
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Basic usage is as follows - there are two normal usage patterns:
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For cases where a program wants information about multiple devices, or
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expects to be doing multiple token searches, the program should
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directly initialize cache file via (second parameter is cache
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filename, NULL = default):
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blkid_cache cache = NULL;
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if (blkid_get_cache(&cache, NULL) < 0)
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/* error reading the cache file, not really fatal */
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Note that if no cache file exists, an empty cache struct is still
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allocated. Usage of libblkid functions will use the cache to avoid
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needless device scans.
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The model of the blkid cache is that each device has a number of
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attributes that can be associated with it. Currently the attributes
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which are supported (and set) by blkid are:
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LABEL filesystem label
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How to use libblkid? Normally, you either want to find a device with
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a specific NAME=value token, or you want to output token(s) from a
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device. To find a device that matches a following attribute, you
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simply call the blkid_get_devname() function:
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if ((devname = blkid_get_devname(cache, attribute_name, value))) {
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/* do something with devname */
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The cache parameter is optional; if it is NULL, then the blkid library
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will load the default blkid.tab cache file, and then release the cache
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before function call returns. The return value is an allocated string
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which holds the resulting device name (if it is found). If the value
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is NULL, then attribute_name is parsed as if it were
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"<attribute_name>=<value>"; if it cannot be so parsed, then the
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original attribute_name is returned in a copied allocated string.
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This is a convenience to allow user programs to want to translate user
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input, whether it is of the form: "/dev/hda1", "LABEL=root",
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"UUID=082D-26E3", and get back a device name that it can use.
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Alternatively, of course, the programmer can pass an attribute name of
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"LABEL", and value of "root", if that is more convenient.
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Another common usage is to retrieve the value of a specific attribute
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for a particular device. This can be used to determine the filesystem
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type, or label, or uuid for a particular device:
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if ((value = blkid_get_tag_value(cache, attribute_name, devname))) {
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/* do something with value */
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If a program needs to call multiple blkid functions, then passing in a
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cache value of NULL is not recommended, since the /etc/blkid.tab file
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will be repeatedly parsed over and over again, with memory allocated
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and deallocated. To initialize the blkid cache, blkid_get_cache()
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if (blkid_get_cache(&cache, NULL) < 0)
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The second parameter of blkid_get_cache (if non-zero) is the alternate
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filename of the blkid cache file (where the default is
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/etc/blkid.tab). Normally, programs should just pass in NULL.
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If you have called blkid_get_cache(), you should call blkid_put_cache()
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when you are done using the blkid library functions. This will save the
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cache to the blkid.tab file, if you have write access to the file. It
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will also free all associated devices and tags:
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blkid_put_cache(cache);
added
removed
libs/blkid/README.blkid
