1 libblkid - a library to handle device identification and token extraction 2 3 Basic usage is as follows - there are two normal usage patterns: 4 5 For cases where a program wants information about multiple devices, or 6 expects to be doing multiple token searches, the program should 7 directly initialize cache file via (second parameter is cache 8 filename, NULL = default): 9 10 blkid_cache cache = NULL; 11 if (blkid_get_cache(&cache, NULL) < 0) 12 /* error reading the cache file, not really fatal */ 13 14 Note that if no cache file exists, an empty cache struct is still 15 allocated. Usage of libblkid functions will use the cache to avoid 16 needless device scans. 17 18 The model of the blkid cache is that each device has a number of 19 attributes that can be associated with it. Currently the attributes 20 which are supported (and set) by blkid are: 21 22 TYPE filesystem type 23 UUID filesystem uuid 24 LABEL filesystem label 25 26 27 How to use libblkid? Normally, you either want to find a device with 28 a specific NAME=value token, or you want to output token(s) from a 29 device. To find a device that matches a following attribute, you 30 simply call the blkid_get_devname() function: 31 32 if ((devname = blkid_get_devname(cache, attribute_name, value))) { 33 /* do something with devname */ 34 string_free(devname); 35 } 36 37 The cache parameter is optional; if it is NULL, then the blkid library 38 will load the default blkid.tab cache file, and then release the cache 39 before function call returns. The return value is an allocated string 40 which holds the resulting device name (if it is found). If the value 41 is NULL, then attribute_name is parsed as if it were 42 "<attribute_name>=<value>"; if it cannot be so parsed, then the 43 original attribute_name is returned in a copied allocated string. 44 This is a convenience to allow user programs to want to translate user 45 input, whether it is of the form: "/dev/hda1", "LABEL=root", 46 "UUID=082D-26E3", and get back a device name that it can use. 47 48 Alternatively, of course, the programmer can pass an attribute name of 49 "LABEL", and value of "root", if that is more convenient. 50 51 Another common usage is to retrieve the value of a specific attribute 52 for a particular device. This can be used to determine the filesystem 53 type, or label, or uuid for a particular device: 54 55 if ((value = blkid_get_tag_value(cache, attribute_name, devname))) { 56 /* do something with value */ 57 string_free(value); 58 } 59 60 If a program needs to call multiple blkid functions, then passing in a 61 cache value of NULL is not recommended, since the /etc/blkid.tab file 62 will be repeatedly parsed over and over again, with memory allocated 63 and deallocated. To initialize the blkid cache, blkid_get_cache() 64 function is used: 65 66 if (blkid_get_cache(&cache, NULL) < 0) 67 goto errout; 68 69 The second parameter of blkid_get_cache (if non-zero) is the alternate 70 filename of the blkid cache file (where the default is 71 /etc/blkid.tab). Normally, programs should just pass in NULL. 72 73 If you have called blkid_get_cache(), you should call blkid_put_cache() 74 when you are done using the blkid library functions. This will save the 75 cache to the blkid.tab file, if you have write access to the file. It 76 will also free all associated devices and tags: 77 78 blkid_put_cache(cache); 79
