| bigbiff | 7b4c7a6 | 2015-01-01 19:44:14 -0500 | [diff] [blame] | 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" |
| 33 | .SH NAME |
| 34 | uuid_generate, uuid_generate_random, uuid_generate_time, |
| 35 | uuid_generate_time_safe \- create a new unique UUID value |
| 36 | .SH SYNOPSIS |
| 37 | .nf |
| 38 | .B #include <uuid.h> |
| 39 | .sp |
| 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 ); |
| 44 | .fi |
| 45 | .SH DESCRIPTION |
| 46 | The |
| 47 | .B uuid_generate |
| 48 | function creates a new universally unique identifier (UUID). The uuid will |
| 49 | be generated based on high-quality randomness from |
| 50 | .IR /dev/urandom , |
| 51 | if available. If it is not available, then |
| 52 | .B uuid_generate |
| 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. |
| 56 | .sp |
| 57 | The |
| 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., |
| 61 | .IR /dev/urandom ) |
| 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. |
| 66 | .sp |
| 67 | The |
| 68 | .B uuid_generate_time |
| 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 |
| 75 | .B uuid_generate |
| 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 |
| 81 | .B uuidd |
| 82 | daemon, if it is running already or can 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 . |
| 88 | .sp |
| 89 | The |
| 90 | .B uuid_generate_time_safe |
| 91 | is similar to |
| 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. |
| 95 | .sp |
| 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 |
| 99 | .IR Cosmos ). |
| 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 |
| 102 | and in the future. |
| 103 | .SH RETURN VALUE |
| 104 | The newly created UUID is returned in the memory location pointed to by |
| 105 | .IR out . |
| 106 | .B uuid_generate_time_safe |
| 107 | returns zero if the UUID has been generated in a safe manner, \-1 otherwise. |
| 108 | .SH "CONFORMING TO" |
| 109 | OSF DCE 1.1 |
| 110 | .SH AUTHOR |
| 111 | Theodore Y.\& Ts'o |
| 112 | .SH AVAILABILITY |
| 113 | .B libuuid |
| 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/. |
| 116 | .SH "SEE ALSO" |
| 117 | .BR uuid (3), |
| 118 | .BR uuidgen (1), |
| 119 | .BR uuidd (8), |
| 120 | .BR uuid_clear (3), |
| 121 | .BR uuid_compare (3), |
| 122 | .BR uuid_copy (3), |
| 123 | .BR uuid_is_null (3), |
| 124 | .BR uuid_parse (3), |
| 125 | .BR uuid_time (3), |
| 126 | .BR uuid_unparse (3) |