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DESCRIPTION

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.