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 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
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
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 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
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_generate_time_safe returns zero if the UUID has been
generated in a safe manner, -1 otherwise.
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)