| .\" Copyright 1999 Andreas Dilger (adilger@enel.ucalgary.ca) |
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| .\" Created Wed Mar 10 17:42:12 1999, Andreas Dilger |
| .TH UUID_GENERATE 3 "May 2009" "util-linux" "Libuuid API" |
| .SH NAME |
| uuid_generate, uuid_generate_random, uuid_generate_time, |
| uuid_generate_time_safe \- create a new unique UUID value |
| .SH SYNOPSIS |
| .nf |
| .B #include <uuid.h> |
| .sp |
| .BI "void uuid_generate(uuid_t " out ); |
| .BI "void uuid_generate_random(uuid_t " out ); |
| .BI "void uuid_generate_time(uuid_t " out ); |
| .BI "int uuid_generate_time_safe(uuid_t " out ); |
| .fi |
| .SH DESCRIPTION |
| The |
| .B uuid_generate |
| function creates a new universally unique identifier (UUID). The uuid will |
| be generated based on high-quality randomness from |
| .IR /dev/urandom , |
| if available. If it is not available, then |
| .B uuid_generate |
| will use an alternative algorithm which uses the current time, the |
| local ethernet MAC address (if available), and random data generated |
| using a pseudo-random generator. |
| .sp |
| The |
| .B uuid_generate_random |
| function forces the use of the all-random UUID format, even if |
| a high-quality random number generator (i.e., |
| .IR /dev/urandom ) |
| is not available, in which case a pseudo-random |
| generator will be substituted. Note that the use of a pseudo-random |
| generator may compromise the uniqueness of UUIDs |
| generated in this fashion. |
| .sp |
| The |
| .B uuid_generate_time |
| function forces the use of the alternative algorithm which uses the |
| current time and the local ethernet MAC address (if available). |
| This algorithm used to be the default one used to generate UUID, but |
| because of the use of the ethernet MAC address, it can leak |
| information about when and where the UUID was generated. This can cause |
| privacy problems in some applications, so the |
| .B uuid_generate |
| function only uses this algorithm if a high-quality source of |
| randomness is not available. To guarantee uniqueness of UUIDs generated |
| by concurrently running processes, the uuid library uses global |
| clock state counter (if the process has permissions to gain exclusive access |
| to this file) and/or the |
| .B uuidd |
| daemon, if it is running already or can be spawned by the process (if |
| installed and the process has enough permissions to run it). If neither of |
| these two synchronization mechanisms can be used, it is theoretically possible |
| that two concurrently running processes obtain the same UUID(s). To tell |
| whether the UUID has been generated in a safe manner, use |
| .BR uuid_generate_time_safe . |
| .sp |
| The |
| .B uuid_generate_time_safe |
| is similar to |
| .BR uuid_generate_time , |
| except that it returns a value which denotes whether any of the synchronization |
| mechanisms (see above) has been used. |
| .sp |
| The UUID is 16 bytes (128 bits) long, which gives approximately 3.4x10^38 |
| unique values (there are approximately 10^80 elementary particles in |
| the universe according to Carl Sagan's |
| .IR Cosmos ). |
| The new UUID can reasonably be considered unique among all UUIDs created |
| on the local system, and among UUIDs created on other systems in the past |
| and in the future. |
| .SH RETURN VALUE |
| The newly created UUID is returned in the memory location pointed to by |
| .IR out . |
| .B uuid_generate_time_safe |
| returns zero if the UUID has been generated in a safe manner, \-1 otherwise. |
| .SH "CONFORMING TO" |
| OSF DCE 1.1 |
| .SH AUTHOR |
| Theodore Y.\& Ts'o |
| .SH AVAILABILITY |
| .B libuuid |
| is part of the util-linux package since version 2.15.1 and is available from |
| ftp://ftp.kernel.org/pub/linux/utils/util-linux/. |
| .SH "SEE ALSO" |
| .BR uuid (3), |
| .BR uuidgen (1), |
| .BR uuidd (8), |
| .BR uuid_clear (3), |
| .BR uuid_compare (3), |
| .BR uuid_copy (3), |
| .BR uuid_is_null (3), |
| .BR uuid_parse (3), |
| .BR uuid_time (3), |
| .BR uuid_unparse (3) |