UUID Binary Representation¶

According to the DCE 1.1, ISO/IEC 11578:1996 and IETF RFC-4122 standards, a DCE 1.1 variant UUID is a 128 bit number defined out of 7 fields, each field a multiple of an octet in size and stored in network byte order:

                                                    [4]
                                                   version
                                                 -->|  |<--
                                                    |  |
                                                    |  |  [16]
                [32]                      [16]      |  |time_hi
              time_low                  time_mid    | _and_version
    |<---------------------------->||<------------>||<------------>|
    | MSB                          ||              ||  |           |
    | /                            ||              ||  |           |
    |/                             ||              ||  |           |
    +------++------++------++------++------++------++------++------+~~
    |  15  ||  14  ||  13  ||  12  ||  11  ||  10  |####9  ||   8  |
    | MSO  ||      ||      ||      ||      ||      |####   ||      |
    +------++------++------++------++------++------++------++------+~~
    7654321076543210765432107654321076543210765432107654321076543210
  ~~+------++------++------++------++------++------++------++------+
    ##* 7  ||   6  ||   5  ||   4  ||   3  ||   2  ||   1  ||   0  |
    ##*    ||      ||      ||      ||      ||      ||      ||  LSO |
  ~~+------++------++------++------++------++------++------++------+
    7654321076543210765432107654321076543210765432107654321076543210
    | |    ||      ||                                             /|
    | |    ||      ||                                            / |
    | |    ||      ||                                          LSB |
    |<---->||<---->||<-------------------------------------------->|
    |clk_seq clk_seq                      node
    |_hi_res _low                         [48]
    |[5-6]    [8]
    | |
 -->| |<--
  variant
   [2-3]

An example of a UUID binary representation is the octet stream 0xF8 0x1D 0x4F 0xAE 0x7D 0xEC 0x11 0xD0 0xA7 0x65 0x00 0xA0 0xC9 0x1E 0x6B 0xF6. The binary representation format is exactly what the OSSP uuid API functions uuid_import() and uuid_export() deal with under UUID_FMT_BIN.

UUID ASCII String Representation¶

According to the DCE 1.1, ISO/IEC 11578:1996 and IETF RFC-4122 standards, a DCE 1.1 variant UUID is represented as an ASCII string consisting of 8 hexadecimal digits followed by a hyphen, then three groups of 4 hexadecimal digits each followed by a hyphen, then 12 hexadecimal digits. Formally, the string representation is defined by the following grammar:

 uuid                        = <time_low> "-"
                               <time_mid> "-"
                               <time_high_and_version> "-"
                               <clock_seq_high_and_reserved>
                               <clock_seq_low> "-"
                               <node>
 time_low                    = 4*<hex_octet>
 time_mid                    = 2*<hex_octet>
 time_high_and_version       = 2*<hex_octet>
 clock_seq_high_and_reserved = <hex_octet>
 clock_seq_low               = <hex_octet>
 node                        = 6*<hex_octet>
 hex_octet                   = <hex_digit> <hex_digit>
 hex_digit                   = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
                              |"a"|"b"|"c"|"d"|"e"|"f"
                              |"A"|"B"|"C"|"D"|"E"|"F"

An example of a UUID string representation is the ASCII string "f81d4fae-7dec-11d0-a765-00a0c91e6bf6". The string representation format is exactly what the OSSP uuid API functions uuid_import() and uuid_export() deal with under UUID_FMT_STR.

Notice: a corresponding URL can be generated out of a ASCII string representation of an UUID by prefixing with "urn:uuid:" as in "urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6".

UUID Single Integer Value Representation¶

According to the ISO/IEC 11578:1996 and ITU-T Rec. X.667 standards, a DCE 1.1 variant UUID can be also represented as a single integer value consisting of a decimal number with up to 39 digits.

An example of a UUID single integer value representation is the decimal number "329800735698586629295641978511506172918". The string representation format is exactly what the OSSP uuid API functions uuid_import() and uuid_export() deal with under UUID_FMT_SIV.

Notice: a corresponding ISO OID can be generated under the "{joint-iso-itu-t(2) uuid(25)}" arc out of a single integer value representation of a UUID by prefixing with "2.25.". An example OID is "2.25.329800735698586629295641978511506172918". Additionally, an URL can be generated by further prefixing with "urn:oid:" as in "urn:oid:2.25.329800735698586629295641978511506172918".

UUID Variants and Versions¶

A UUID has a variant and version. The variant defines the layout of the UUID. The version defines the content of the UUID. The UUID variant supported in OSSP uuid is the DCE 1.1 variant only. The DCE 1.1 UUID variant versions supported in OSSP uuid are:

Version 1 (time and node based)

These are the classical UUIDs, created out of a 60-bit system time, a 14-bit local clock sequence and 48-bit system MAC address. The MAC address can be either the real one of a physical network interface card (NIC) or a random multi-cast MAC address. Version 1 UUIDs are usually used as one-time global unique identifiers.

Version 3 (name based, MD5)

These are UUIDs which are based on the 128-bit MD5 message digest of the concatenation of a 128-bit namespace UUID and a name string of arbitrary length. Version 3 UUIDs are usually used for non-unique but repeatable message digest identifiers.

Version 4 (random data based)

These are UUIDs which are based on just 128-bit of random data. Version 4 UUIDs are usually used as one-time local unique identifiers.

Version 5 (name based, SHA-1)

These are UUIDs which are based on the 160-bit SHA-1 message digest of the concatenation of a 128-bit namespace UUID and a name string of arbitrary length. Version 5 UUIDs are usually used for non-unique but repeatable message digest identifiers.

UUID Uniqueness¶

Version 1 UUIDs are guaranteed to be unique through combinations of hardware addresses, time stamps and random seeds. There is a reference in the UUID to the hardware (MAC) address of the first network interface card (NIC) on the host which generated the UUID -- this reference is intended to ensure the UUID will be unique in space as the MAC address of every network card is assigned by a single global authority (IEEE) and is guaranteed to be unique. The next component in a UUID is a timestamp which, as clock always (should) move forward, will be unique in time. Just in case some part of the above goes wrong (the hardware address cannot be determined or the clock moved steps backward), there is a random clock sequence component placed into the UUID as a "catch-all" for uniqueness.

Version 3 and version 5 UUIDs are guaranteed to be inherently globally unique if the combination of namespace and name used to generate them is unique.

Version 4 UUIDs are not guaranteed to be globally unique, because they are generated out of locally gathered pseudo-random numbers only. Nevertheless there is still a high likelihood of uniqueness over space and time and that they are computationally difficult to guess.

Nil UUID¶

There is a special Nil UUID consisting of all octets set to zero in the binary representation. It can be used as a special UUID value which does not conflict with real UUIDs.

CONSTANTS¶

The following constants are provided:

UUID_VERSION

The hexadecimal encoded OSSP uuid version. This allows compile-time checking of the OSSP uuid version. For run-time checking use uuid_version() instead.

The hexadecimal encoding for a version "$v.$r$t$l" is calculated with the GNU shtool version command and is (in Perl-style for concise description) "sprintf('0x%x%02x%d%02x', $v, $r, {qw(s 9 . 2 b 1 a 0)}->{$t}, ($t eq 's' ? 99 : $l))", i.e., the version 0.9.6 is encoded as "0x009206".

UUID_LEN_BIN, UUID_LEN_STR, UUID_LEN_SIV

The number of octets of the UUID binary and string representations. Notice that the lengths of the string representation (UUID_LEN_STR) and the lengths of the single integer value representation (UUID_LEN_SIV) does not include the necessary NUL termination character.

UUID_MAKE_V1, UUID_MAKE_V3, UUID_MAKE_V4, UUID_MAKE_V5, UUID_MAKE_MC

The mode bits for use with uuid_make(). The UUID_MAKE_VN specify which UUID version to generate. The UUID_MAKE_MC forces the use of a random multi-cast MAC address instead of the real physical MAC address in version 1 UUIDs.

UUID_RC_OK, UUID_RC_ARG, UUID_RC_MEM, UUID_RC_SYS, UUID_RC_INT, UUID_RC_IMP

The possible numerical return-codes of API functions. The UUID_RC_OK indicates success, the others indicate errors. Use uuid_error() to translate them into string versions.

UUID_FMT_BIN, UUID_FMT_STR, UUID_FMT_SIV, UUID_FMT_TXT

The fmt formats for use with uuid_import() and uuid_export(). The UUID_FMT_BIN indicates the UUID binary representation (of length UUID_LEN_BIN), the UUID_FMT_STR indicates the UUID string representation (of length UUID_LEN_STR), the UUID_FMT_SIV indicates the UUID single integer value representation (of maximum length UUID_LEN_SIV) and the UUID_FMT_TXT indicates the textual description (of arbitrary length) of a UUID.

FUNCTIONS¶

The following functions are provided:

uuid_rc_t uuid_create(uuid_t **uuid);

Create a new UUID object and store a pointer to it in *uuid. A UUID object consists of an internal representation of a UUID, the internal PRNG and MD5 generator contexts, and cached MAC address and timestamp information. The initial UUID is the Nil UUID.

uuid_rc_t uuid_destroy(uuid_t *uuid);

Destroy UUID object uuid.

uuid_rc_t uuid_clone(const uuid_t *uuid, uuid_t **uuid_clone);

Clone UUID object uuid and store new UUID object in uuid_clone.

uuid_rc_t uuid_isnil(const uuid_t *uuid, int *result);

Checks whether the UUID in uuid is the Nil UUID. If this is the case, it returns true in *result. Else it returns false in *result.

uuid_rc_t uuid_compare(const uuid_t *uuid, const uuid_t *uuid2, int *result);

Compares the order of the two UUIDs in uuid1 and uuid2 and returns the result in *result: -1 if uuid1 is smaller than uuid2, 0 if uuid1 is equal to uuid2 and +1 if uuid1 is greater than uuid2.

uuid_rc_t uuid_import(uuid_t *uuid, uuid_fmt_t fmt, const void *data_ptr, size_t data_len);

Imports a UUID uuid from an external representation of format fmt. The data is read from the buffer at data_ptr which contains at least data_len bytes.

The format of the external representation is specified by fmt and the minimum expected length in data_len depends on it. Valid values for fmt are UUID_FMT_BIN, UUID_FMT_STR and UUID_FMT_SIV.

uuid_rc_t uuid_export(const uuid_t *uuid, uuid_fmt_t fmt, void *data_ptr, size_t *data_len);

Exports a UUID uuid into an external representation of format fmt. Valid values for fmt are UUID_FMT_BIN, UUID_FMT_STR, UUID_FMT_SIV and UUID_FMT_TXT.

The data is written to the buffer whose location is obtained by dereferencing data_ptr after a "cast" to the appropriate pointer-to-pointer type. Hence the generic pointer argument data_ptr is expected to be a pointer to a "pointer of a particular type", i.e., it has to be of type "unsigned char **" for UUID_FMT_BIN and "char **" for UUID_FMT_STR, UUID_FMT_SIV and UUID_FMT_TXT.

The buffer has to be room for at least *data_len bytes. If the value of the pointer after "casting" and dereferencing data_ptr is NULL, data_len is ignored as input and a new buffer is allocated and returned in the pointer after "casting" and dereferencing data_ptr (the caller has to free(3) it later on).

If data_len is not NULL, the number of available bytes in the buffer has to be provided in *data_len and the number of actually written bytes are returned in *data_len again. The minimum required buffer length depends on the external representation as specified by fmt and is at least UUID_LEN_BIN for UUID_FMT_BIN, UUID_LEN_STR for UUID_FMT_STR and UUID_LEN_SIV for UUID_FMT_SIV. For UUID_FMT_TXT a buffer of unspecified length is required and hence it is recommended to allow OSSP uuid to allocate the buffer as necessary.

uuid_rc_t uuid_load(uuid_t *uuid, const char *name);

Loads a pre-defined UUID value into the UUID object uuid. The following name arguments are currently known:

uuid_rc_t uuid_make(uuid_t *uuid, unsigned int mode, ...);

Generates a new UUID in uuid according to mode and optional arguments (dependent on mode).

If mode contains the UUID_MAKE_V1 bit, a DCE 1.1 variant UUID of version 1 is generated. Then optionally the bit UUID_MAKE_MC forces the use of random multi-cast MAC address instead of the real physical MAC address (the default). The UUID is generated out of the 60-bit current system time, a 12-bit clock sequence and the 48-bit MAC address.

If mode contains the UUID_MAKE_V3 or UUID_MAKE_V5 bit, a DCE 1.1 variant UUID of version 3 or 5 is generated and two additional arguments are expected: first, a namespace UUID object (uuid_t *). Second, a name string of arbitrary length (const char *). The UUID is generated out of the 128-bit MD5 or 160-bit SHA-1 from the concatenated octet stream of namespace UUID and name string.

If mode contains the UUID_MAKE_V4 bit, a DCE 1.1 variant UUID of version 4 is generated. The UUID is generated out of 128-bit random data.

char *uuid_error(uuid_rc_t rc);

Returns a constant string representation corresponding to the return-code rc for use in displaying OSSP uuid errors.

unsigned long uuid_version(void);

Returns the hexadecimal encoded OSSP uuid version as compiled into the library object files. This allows run-time checking of the OSSP uuid version. For compile-time checking use UUID_VERSION instead.