1 .\" Copyright 1999 Andreas Dilger (adilger@enel.ucalgary.ca)
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31 .\" Created Wed Mar 10 17:42:12 1999, Andreas Dilger
32 .TH UUID_GENERATE 3 "May 2009" "util-linux" "Libuuid API"
34 uuid_generate, uuid_generate_random, uuid_generate_time,
35 uuid_generate_time_safe \- create a new unique UUID value
38 .B #include <uuid/uuid.h>
40 .BI "void uuid_generate(uuid_t " out );
41 .BI "void uuid_generate_random(uuid_t " out );
42 .BI "void uuid_generate_time(uuid_t " out );
43 .BI "int uuid_generate_time_safe(uuid_t " out );
48 function creates a new universally unique identifier (UUID). The uuid will
49 be generated based on high-quality randomness from
51 if available. If it is not available, then
53 will use an alternative algorithm which uses the current time, the
54 local ethernet MAC address (if available), and random data generated
55 using a pseudo-random generator.
58 .B uuid_generate_random
59 function forces the use of the all-random UUID format, even if
60 a high-quality random number generator (i.e.,
62 is not available, in which case a pseudo-random
63 generator will be substituted. Note that the use of a pseudo-random
64 generator may compromise the uniqueness of UUIDs
65 generated in this fashion.
69 function forces the use of the alternative algorithm which uses the
70 current time and the local ethernet MAC address (if available).
71 This algorithm used to be the default one used to generate UUID, but
72 because of the use of the ethernet MAC address, it can leak
73 information about when and where the UUID was generated. This can cause
74 privacy problems in some applications, so the
76 function only uses this algorithm if a high-quality source of
77 randomness is not available. To guarantee uniqueness of UUIDs generated
78 by concurrently running processes, the uuid library uses global
79 clock state counter (if the process has permissions to gain exclusive access
80 to this file) and/or the
82 daemon, if it is running already or can be be spawned by the process (if
83 installed and the process has enough permissions to run it). If neither of
84 these two synchronization mechanisms can be used, it is theoretically possible
85 that two concurrently running processes obtain the same UUID(s). To tell
86 whether the UUID has been generated in a safe manner, use
87 .BR uuid_generate_time_safe .
90 .B uuid_generate_time_safe
92 .BR uuid_generate_time ,
93 except that it returns a value which denotes whether any of the synchronization
94 mechanisms (see above) has been used.
96 The UUID is 16 bytes (128 bits) long, which gives approximately 3.4x10^38
97 unique values (there are approximately 10^80 elementary particles in
98 the universe according to Carl Sagan's
100 The new UUID can reasonably be considered unique among all UUIDs created
101 on the local system, and among UUIDs created on other systems in the past
104 The newly created UUID is returned in the memory location pointed to by
106 .B uuid_generate_time_safe
107 returns zero if the UUID has been generated in a safe manner, -1 otherwise.
114 is part of the util-linux package since version 2.15.1 and is available from
115 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
121 .BR uuid_compare (3),
123 .BR uuid_is_null (3),