Manual Reference Pages  - UUID_GENERATE (3)


uuid_generate, uuid_generate_random, uuid_generate_time, uuid_generate_time_safe - create a new unique UUID value


Return Value


#include <uuid/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);


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 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 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.


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.




Theodore Y. Ts’o



uuid(3), uuidgen(1), uuidd(8), uuid_clear(3), uuid_compare(3), uuid_copy(3), uuid_is_null(3), uuid_parse(3), uuid_time(3), uuid_unparse(3)

E2fsprogs version 1.39 UUID_GENERATE (3) May 2006
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