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NAME
uuidgen — generate universally unique identifiers
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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 multicast 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.