Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

UUIDGEN(2)                    System Calls Manual                   UUIDGEN(2)

     uuidgen - generate universally unique identifiers

     Standard C Library (libc, -lc)

     #include <sys/types.h>
     #include <sys/uuid.h>

     uuidgen(struct uuid *store, int count);

     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

     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.

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     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.

     uuidgen(1), uuid(3)

     P. Leach, M. Mealling, and R. Salz, A Universally Unique IDentifier
     (UUID) URN Namespace, IETF, RFC 4122, July 2005.

     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.

     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.

NetBSD 10.99                     May 26, 2002                     NetBSD 10.99