uuid_generate(3)

1UUID_GENERATE(3)                  Libuuid API                 UUID_GENERATE(3)
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NAME

6       uuid_generate,  uuid_generate_random,  uuid_generate_time,  uuid_gener‐
7       ate_time_safe - create a new unique UUID value
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SYNOPSIS

10       #include <uuid.h>
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12       void uuid_generate(uuid_t out);
13       void uuid_generate_random(uuid_t out);
14       void uuid_generate_time(uuid_t out);
15       int uuid_generate_time_safe(uuid_t out);
16       void uuid_generate_md5(uuid_t out, const uuid_t ns, const char *name, size_t len);
17       void uuid_generate_sha1(uuid_t out, const uuid_t ns, const char *name, size_t len);
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DESCRIPTION

20       The uuid_generate function creates a new universally unique  identifier
21       (UUID).   The  uuid  will be generated based on high-quality randomness
22       from  /dev/urandom,  if  available.   If  it  is  not  available,  then
23       uuid_generate  will use an alternative algorithm which uses the current
24       time, the local ethernet MAC address (if available),  and  random  data
25       generated using a pseudo-random generator.
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27       The uuid_generate_random function forces the use of the all-random UUID
28       format,  even  if  a  high-quality  random  number   generator   (i.e.,
29       /dev/urandom) is not available, in which case a pseudo-random generator
30       will be substituted.  Note that the use of  a  pseudo-random  generator
31       may compromise the uniqueness of UUIDs generated in this fashion.
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33       The uuid_generate_time function forces the use of the alternative algo‐
34       rithm which uses the current time and the local  ethernet  MAC  address
35       (if available).  This algorithm used to be the default one used to gen‐
36       erate UUID, but because of the use of the ethernet MAC address, it  can
37       leak information about when and where the UUID was generated.  This can
38       cause privacy problems in some applications, so the uuid_generate func‐
39       tion only uses this algorithm if a high-quality source of randomness is
40       not available.  To guarantee uniqueness of UUIDs generated  by  concur‐
41       rently  running  processes,  the  uuid  library uses global clock state
42       counter (if the process has permissions to  gain  exclusive  access  to
43       this  file) and/or the uuidd daemon, if it is running already or can be
44       spawned by the process (if installed and the process has enough permis‐
45       sions  to  run it).  If neither of these two synchronization mechanisms
46       can be used, it is theoretically possible that two concurrently running
47       processes  obtain  the same UUID(s).  To tell whether the UUID has been
48       generated in a safe manner, use uuid_generate_time_safe.
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50       The uuid_generate_time_safe is similar  to  uuid_generate_time,  except
51       that  it  returns a value which denotes whether any of the synchroniza‐
52       tion mechanisms (see above) has been used.
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54       The UUID is  16  bytes  (128  bits)  long,  which  gives  approximately
55       3.4x10^38  unique values (there are approximately 10^80 elementary par‐
56       ticles in the universe according to Carl Sagan's Cosmos).  The new UUID
57       can  reasonably  be  considered  unique  among all UUIDs created on the
58       local system, and among UUIDs created on other systems in the past  and
59       in the future.
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61       The  uuid_generate_md5 and uuid_generate_sha1 functions generate an MD5
62       and SHA1 hashed (predictable) UUID based on a well-known UUID providing
63       the  namespace  and an arbitrary binary string. The UUIDs confirm to V3
64       and V5 UUIDs per RFC-4122.
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RETURN VALUE

67       The newly created UUID is returned in the memory location pointed to by
68       out.   uuid_generate_time_safe returns zero if the UUID has been gener‐
69       ated in a safe manner, -1 otherwise.
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CONFORMING TO

72       This library generates UUIDs compatible OSF DCE  1.1,  and  hash  based
73       UUIDs V3 and V5 compatible with RFC-4122.
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AUTHOR

76       Theodore Y. Ts'o
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AVAILABILITY

79       libuuid  is  part of the util-linux package since version 2.15.1 and is
80       available from https://www.kernel.org/pub/linux/utils/util-linux/.
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