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UUID_GENERATE(3) Libuuid API UUID_GENERATE(3)
NAME
uuid_generate, uuid_generate_random, uuid_generate_time, uuid_generate_time_safe - create
a new unique UUID value
SYNOPSIS
#include <uuid.h>
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);
DESCRIPTION
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 cur-
rent 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 gen-
erator may compromise the uniqueness of UUIDs generated in this fashion.
The 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 pri-
vacy problems in some applications, so the 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
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 pro-
cesses 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 synchronization 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 particles 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.
RETURN VALUE
The newly created UUID is returned in the memory location pointed to by out. uuid_gener-
ate_time_safe returns zero if the UUID has been generated in a safe manner, -1 otherwise.
CONFORMING TO
OSF DCE 1.1
AUTHOR
Theodore Y. Ts'o
AVAILABILITY
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/.
SEE ALSO
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), uuid_unparse(3)
util-linux May 2009 UUID_GENERATE(3)
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