.\" $NetBSD: uuidgen.2,v 1.5 2019/07/24 06:11:57 riastradh Exp $
.\" $FreeBSD: src/lib/libc/sys/uuidgen.2,v 1.7 2003/06/27 13:41:29 yar Exp $
.\" Copyright (c) 2002 Marcel Moolenaar
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.\" $FreeBSD: src/lib/libc/sys/uuidgen.2,v 1.7 2003/06/27 13:41:29 yar Exp $
.\"
.Dd May 26, 2002
.Dt UUIDGEN 2
.Os
.Sh NAME
.Nm uuidgen
.Nd generate universally unique identifiers
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/uuid.h
.Ft int
.Fn uuidgen "struct uuid *store" "int count"
.Sh DESCRIPTION
The
.Fn uuidgen
system call generates
.Fa count
universally unique identifiers (UUIDs) and writes them to the buffer
pointed to by
.Fa store .
The identifiers are randomly generated according to UUID version 4.
.Pp
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:
.Bd -literal
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];
};
.Ed
.Bl -tag -width ".Va clock_seq_hi_and_reserved"
.It Va time_low
The least significant 32 bits of a 60-bit timestamp.
This field is stored in the native byte-order.
.It Va 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.
.It Va 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.
.It Va 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
.Fn uuidgen
system call have variant value 10b.
the variant value is stored in the most significant bits of the field.
.It Va clock_seq_low
The least significant 8 bits of a 14-bit sequence number.
.It Va 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.
.El
.Pp
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.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn uuidgen
system call can fail with:
.Bl -tag -width Er
.It Bq Er EFAULT
The buffer pointed to by
.Fa store
could not be written to for any or all identifiers.
.It Bq Er EINVAL
The
.Fa count
argument is less than 1 or larger than the hard upper limit of 2048.
.El
.Sh SEE ALSO
.Xr uuidgen 1 ,
.Xr uuid 3
.Rs
.%A P. Leach
.%A M. Mealling
.%A R. Salz
.%T A Universally Unique IDentifier (UUID) URN Namespace
.%R RFC 4122
.%I IETF
.%D July 2005
.Re
.Sh 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
.Fn uuidgen
system call is itself not part of the specification.
.Sh HISTORY
The
.Fn uuidgen
system call first appeared in
.Fx 5.0
and was subsequently added to
.Nx 2.0 .
It was changed to use version 4 UUIDs, i.e. randomly generated UUIDs,
in
.Nx 8.0 .