UUID_GENERATE(3)                 Libuuid API                UUID_GENERATE(3)

NAME         top

       uuid_generate,  uuid_generate_random, uuid_generate_time, uuid_gener‐
       ate_time_safe - create a new unique UUID value

SYNOPSIS         top

       #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         top

       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

       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         top

       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         top

       OSF DCE 1.1

AUTHOR         top

       Theodore Y. Ts'o

AVAILABILITY         top

       libuuid is part of the util-linux package since version 2.15.1 and is
       available from

SEE ALSO         top

       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),

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨⟩.  If you have a
       bug report for this manual page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨git://⟩ on
       2018-02-02.  (At that time, the date of the most recent commit that
       was found in the repository was 2018-02-01.)  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

util-linux                        May 2009                  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)