void uuid_generate(uuid_t out);
void uuid_generate_random(uuid_t out);
void uuid_generate_time(uuid_t out);
int uuid_generate_time_safe(uuid_t out);
The uuid_generate function creates a new universally unique identifier
(UUID). The uuid will be generated based on high-quality randomness
from /dev/urandom, if available. If it is not available, then
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.
The uuid_generate_random function forces the use of the all-random UUID
format, even if a high-quality random number generator (i.e.,
/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.
The uuid_generate_time function forces the use of the alternative algo-
rithm which uses the current time and the local ethernet MAC address
(if available). This algorithm used to be the default one used to gen-
erate 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 uuid_generate func-
tion only uses this algorithm if a high-quality source of randomness is
not available. To guarantee uniqueness of UUIDs generated by concur-
rently 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 uuidd daemon, if it is running already or can be
spawned by the process (if installed and the process has enough permis-
sions 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 uuid_generate_time_safe.
The uuid_generate_time_safe is similar to uuid_generate_time, except
that it returns a value which denotes whether any of the synchroniza-
tion mechanisms (see above) has been used.
The UUID is 16 bytes (128 bits) long, which gives approximately
3.4x10^38 unique values (there are approximately 10^80 elementary par-
ticles in the universe according to Carl Sagan's 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.
The newly created UUID is returned in the memory location pointed to by
out. uuid_generate_time_safe returns zero if the UUID has been gener-
ated in a safe manner, -1 otherwise.
uuid(3), uuidgen(1), uuidd(8), uuid_clear(3), uuid_compare(3),
uuid_copy(3), uuid_is_null(3), uuid_parse(3), uuid_time(3),
util-linux May 2009 UUID_GENERATE(3)
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