uuid_generate, uuid_generate_random, uuid_generate_time, uuid_generate_time_safe
- create a new unique UUID value
#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);
The
uuid_generate() function creates a new universally unique identifier
(UUID). The uuid will be generated based on high-quality randomness from
getrandom(2),
/dev/urandom, or
/dev/random 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 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 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() 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 a global clock state counter (if the process has permissions
to gain exclusive access to this file) and/or the
uuidd(8) 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() function 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.
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
<
https://tools.ietf.org/html/rfc4122>.
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.
This library generates UUIDs compatible with OSF DCE 1.1, and hash based UUIDs
V3 and V5 compatible with
RFC-4122
<
https://tools.ietf.org/html/rfc4122>.
Theodore Y. Ts’o
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)
For bug reports, use the issue tracker at
<
https://github.com/util-linux/util-linux/issues>.
The
libuuid library is part of the util-linux package since version
2.15.1. It can be downloaded from
Linux Kernel Archive
<
https://www.kernel.org/pub/linux/utils/util-linux/>.