UUID_GENERATE(3) Libuuid API UUID_GENERATE(3)
NAME
uuid_generate, uuid_generate_random, uuid_generate_time, uuid_gener-
ate_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);
void uuid_generate_md5(uuid_t out, const uuid_t ns, const char *name, size_t len);
void uuid_generate_sha1(uuid_t out, const uuid_t ns, const char *name, size_t len);
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 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 UUIDs, 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 a 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 function is similar to uuid_generate_time,
except that it returns a value which denotes whether any of the syn-
chronization 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 lo-
cal system, and among UUIDs created on other systems in the past and in
the future.
The uuid_generate_md5 and uuid_generate_sha1 functions generate an MD5
and SHA1 hashed (predictable) UUID based on a well-known UUID providing
the namespace and an arbitrary binary string. The UUIDs conform to V3
and V5 UUIDs per RFC-4122.
RETURN VALUE
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.
CONFORMING TO
This library generates UUIDs compatible with OSF DCE 1.1, and hash
based UUIDs V3 and V5 compatible with RFC-4122.
AUTHOR
Theodore Y. Ts'o
AVAILABILITY
libuuid is part of the util-linux package since version 2.15.1 and is
available from https://www.kernel.org/pub/linux/utils/util-linux/.
SEE ALSO
uuidgen(1), uuid(3), uuid_clear(3), uuid_compare(3), uuid_copy(3),
uuid_is_null(3), uuid_parse(3), uuid_time(3), uuid_unparse(3), uuidd(8)
util-linux May 2009 UUID_GENERATE(3)