NAME
uuidgen —
generate universally unique
identifiers
LIBRARY
Standard C Library (libc, -lc)
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 generated according to the syntax and semantics of the DCE version 1
variant of universally unique identifiers. See below for a more in-depth
description of the identifiers. When no IEEE 802 address is available for the
node field, a random multi-cast address is generated for each invocation of
the system call. According to the algorithm of generating time-based UUIDs,
this will also force a new random clock sequence, thereby increasing the
likelihood for the identifier to be unique.
When multiple identifiers are to be generated, the
uuidgen()
system call will generate a set of identifiers that is dense in such a way
that there is no identifier that is larger than the smallest identifier in the
set and smaller than the largest identifier in the set and that is not already
in the set.
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 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
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
errno
is set to indicate the error.
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)
STANDARDS
The identifiers are represented and generated in conformance with the DCE 1.1
RPC specification. 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.