UUIDGEN(2)

NAME
uuidgen - generate universally unique identifiers

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/uuid.h>
int
uuidgen(struct uuid *store, int count);

DESCRIPTION
The uuidgen() system call generates count universally unique identifiers (UUIDs) and writes them to the buffer pointed to by store. The identifiers are randomly generated according to UUID version 4.

Universally unique identifiers, also known as globally unique identifiers (GUIDs), have a binary representation of 128-bits. The grouping and meaning of these bits is based on historical methods of generation from on timestamps and IEEE 802 MAC addresses, and is described by the following structure and its description of the fields that follow it:

    struct uuid {
    	uint32_t	time_low;
    	uint16_t	time_mid;
    	uint16_t	time_hi_and_version;
    	uint8_t		clock_seq_hi_and_reserved;
    	uint8_t		clock_seq_low;
    	uint8_t		node[_UUID_NODE_LEN];
    };

time_low
    The least significant 32 bits of a 60-bit timestamp. This field is stored in the native byte-order.

time_mid
    The least significant 16 bits of the most significant 28 bits of the 60-bit timestamp. This field is stored in the native byte-order.

time_hi_and_version
    The most significant 12 bits of the 60-bit timestamp multiplexed with a 4-bit version number. The version number is stored in the most significant 4 bits of the 16-bit field. This field is stored in the native byte-order.

clock_seq_hi_and_reserved
    The most significant 6 bits of a 14-bit sequence number multiplexed with a 2-bit variant value. Note that the width of the variant value is determined by the variant itself. Identifiers generated by the uuidgen() system call have variant value 10b. the variant value is stored in the most significant bits of the field.

clock_seq_low
    The least significant 8 bits of a 14-bit sequence number.

node
    The 6-byte IEEE 802 (MAC) address of one of the interfaces of the node. If no such interface exists, a random multi-cast address is used instead.

The binary representation is sensitive to byte ordering. Any multi-byte field is to be stored in the local or native byte-order and identifiers must be converted when transmitted to hosts that do not agree on the byte-order. The specification does not however document what this means in concrete terms and is otherwise beyond the scope of this system call.

RETURN VALUES

ERRORS
The uuidgen() system call can fail with:

[EFAULT]
    The buffer pointed to by store could not be written to for any or all identifiers.

[EINVAL]
    The count argument is less than 1 or larger than the hard upper limit of 2048.

SEE ALSO
uuidgen(1), uuid(3) P. Leach, M. Mealling, R. Salz, A Universally Unique IDentifier (UUID) URN Namespace, RFC 4122, IETF, July 2005.

STANDARDS
The identifiers are represented and generated in conformance with IETF RFC 4122, based on the historic DCE 1.1 RPC specification of the Open Software Foundation (now the Open Group). The uuidgen() system call is itself not part of the specification.

HISTORY
The uuidgen() system call first appeared in FreeBSD 5.0 and was subsequently added to NetBSD 2.0. It was changed to use version 4 UUIDs, i.e. randomly generated UUIDs, in NetBSD 8.0.