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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.