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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | AUTHORS | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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UUID_GENERATE(3) Programmers Manual UUID_GENERATE(3)
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
<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/>. This page is
part of the util-linux (a random collection of Linux utilities)
project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) If you discover any
rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
util-linux 2.42-start-521-ec46 2025-08-09 UUID_GENERATE(3)
Pages that refer to this page: uuid(3), uuid_clear(3), uuid_compare(3), uuid_copy(3), uuid_is_null(3), uuid_parse(3), uuid_time(3), uuid_unparse(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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