focal (3) uuid_generate_random.3.gz
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);
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 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 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.
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.
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)