UUID_GENERATE(3) Programmer's Manual 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);
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 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>.
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
<https://tools.ietf.org/html/rfc4122>.
AUTHORS
Theodore Y. Ts’o
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)
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
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/>.
util-linux 2.38.1 2022-05-11 UUID_GENERATE(3)
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