A Globally Unique Identifier (GUID, /ˈɡwɪd/ or pron.: /ˈɡuːɪd/) is a unique reference number used as an identifier in computer software. The term GUID typically refers to various implementations of the universally unique identifier (UUID) standard.
GUIDs are usually stored as 128-bit values, and are commonly displayed as 32 hexadecimal digits with groups separated by hyphens, such as {21EC2020-3AEA-1069-A2DD-08002B30309D}. GUIDs generated from random numbers contain 6 fixed bits saying they are random and 122 random bits; the total number of unique such GUIDs is 2 or 5.3×10. This number is so large that the probability of the same number being generated randomly twice is negligible; however other GUID versions have different uniqueness properties and probabilities, ranging from guaranteed uniqueness to likely non-uniqueness.
A GUID can be stored as a 16-byte (128-bit) number; Microsoft defines a format which is split into four fields, defined as follows. Note that this format differs from the UUID standard only in the byte order of the first 3 fields.
| Bits | Bytes | Name | Endianness
(Microsoft GUID Structure) |
Endianness |
|---|---|---|---|---|
| 32 | 4 | Data1 | Native | Big |
| 16 | 2 | Data2 | Native | Big |
| 16 | 2 | Data3 | Native | Big |
| 64 | 8 | Data4 | Big | Big |
This endianness applies only to the way in which a GUID is stored, and not to the way in which it is represented in text. GUIDs and RFC 4122 UUIDs should be identical when displayed textually.
One to three of the most significant bits of the first byte in Data 4 define the type variant of the GUID:
| Pattern | Description |
|---|---|
| 0xx | Network Computing System backward compatibility |
| 10x | Standard |
| 110 | Microsoft Component Object Model backward compatibility; this includes the GUIDs for important interfaces like IUnknown and IDispatch |
| 111 | Reserved for future use |
For the "standard" variant, the most significant four bits of Data3 define the version number, and the algorithm used.
A GUID is most commonly written in text as a sequence of hexadecimal digits separated into five groups, such as:
This text notation contains the following fields, separated by hyphens:
| Hex digits | Description |
|---|---|
| 8 | Data1 |
| 4 | Data2 |
| 4 | Data3 |
| 4 | Initial two bytes from Data4 |
| 12 | Remaining six bytes from Data4 |
For the first three fields, the most significant digit is on the left. The last two fields are treated as eight separate bytes, each having their most significant digit on the left, and they follow each other from left to right. Note that the digit order of the fourth field may be unexpected, since it's treated differently than in the structure.
Often braces are added to enclose the above format, such as:
This is sometimes known as "registry format".
When printing fewer characters is desired, GUIDs are sometimes encoded into a base64 or Ascii85 string.
A base64-encoded GUID consists of 22 to 24 characters (depending on padding), for instance:
4AQlP4lP0xGaDAMF6CwzAQ4AQlP4lP0xGaDAMF6CwzAQ==and Ascii85 encoding gives 20 characters, for example:
5:$Hj:Pf\4RLB9%kU\LjIn Uniform Resource Names (URN), GUIDs have namespace identifier "uuid", e.g.:
urn:uuid:3F2504E0-4F89-11D3-9A0C-0305E82C3301In the OSF-specified algorithm for generating new (V1) GUIDs, the user's network card MAC address is used as a base for the last group of GUID digits, which means, for example, that a document can be tracked back to the computer that created it. This privacy hole was used when locating the creator of the Melissa virus. Most of the other digits are based on the time while generating the GUID.
V1 GUIDs which contain a MAC address and time can be identified by the digit "1" in the first position of the third group of digits, for example {2F1E4FC0-81FD-11DA-9156-00036A0F876A}.
V4 GUIDs use the later algorithm, which is a pseudo-random number. These have a "4" in the same position, for example {38A52BE4-9352-453E-AF97-5C3B448652F0}. More specifically, the 'data3' bit pattern would be 0001xxxxxxxxxxxx in the first case, and 0100xxxxxxxxxxxx in the second. Cryptanalysis of the WinAPI GUID generator shows that, since the sequence of V4 GUIDs is pseudo-random, given full knowledge of the internal state, it is possible to predict previous and subsequent values.
Certain GUIDs turn up again and again, both intentionally, and otherwise. In a GUID Partition Table (GPT), it's not appropriate for more than one disk to have the same Disk GUID, or for more than one partition to have the same Unique partition GUID, however it is appropriate for multiple partitions to use the same Partition type GUID. So only Linux swap partitions, and all Linux swap partitions on GPT-formatted disks can be counted on to have the GUID 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F, for example.
Some flawed GUID-generating implementations rely on pseudo-random number generators that use random number seed sources that turn out to be predictable. Standard Valid GUIDs are not chosen at random; they are chosen by standardized algorithms. (See, e.g. RFC 4122.) These algorithms result in GUIDs that are more reliably unique than ones chosen using even a hypothetically perfect random number generator, and far more reliably unique than numbers chosen by pseudorandom number generators.
Operating systems (including Windows, Mac OS X and Linux) are designed based on the expectation that a given disk will never have the same Disk GUID as another. However, the so-called FireWire GUIDs (which are called GUIDs, but are a non-standard 64 bits) of every unit of several common models of hard drive/drive case that each use the same manufacturer's chipset, Oxford Semiconductor, all have the same GUID - 0x30E002E0454647 (sometimes displayed in decimal form as Connection ID 13757101839304263) including NewerTech, Vantec, and Cavalry, and this causes problems when such drives are daisy-chained or otherwise connected to the same system. The manufacturers were supposed to serialize them, but many didn't.[when? clarification needed]
GUIDs are commonly used as the primary key of database tables, and with that, often the table has a clustered index on that attribute. This presents a performance issue when inserting records because a fully random GUID means the record may need to be inserted anywhere within the table rather than merely appended near the end of it.
As a way of mitigating this issue while still providing enough randomness to effectively prevent duplicate number collisions, several algorithms have been used to generate sequential GUIDs.
The oldest technique, present as a feature in early version of Microsoft's GUIDGEN SDK tool, works by simply outputting the set of MAC-based version 1 GUIDs corresponding to a time interval, taking advantage of the fact that the time field in v1 GUIDs has a resolution of 100 ns, which allows a million sequential GUIDs to be generated by simply locking out other GUID generators on the computer for a tenth of a second (or 10000 GUIDs in a millisecond). These sequential GUIDs are unique, but the increment happens in the Data1 field, not at the end of the GUID.
The second technique, described by Jimmy Nilsson in August 2002 and referred to as a "COMB" ("combined guid/timestamp"), replaces the last 6 bytes of Data4 in a random (version 4) GUID with the least-significant 6 bytes of the current system date/time. While this can result in GUIDs that are generated out of order within the same fraction of a second, his tests showed this had little real-world[clarify] impact on insertion. One side effect of this approach is that the date and time of insertion can be easily extracted from the value later, if desired. The COMB technique tries to compensate for the reduced clustering in database indexes caused by switching to an OS version that uses random GUIDs rather than MAC-based GUIDs, and is useful only when it is not possible to revert to version 1 GUIDs.
Starting with Microsoft SQL Server version 2005, Microsoft added a function to the Transact-SQL language called NEWSEQUENTIALID(), which essentially provides access to the traditional version 1 GUIDs (or something so close it fits the same description), with all their advantages and disadvantages.
In 2006, a programmer found that the SYS_GUID function provided by Oracle was returning sequential GUIDs on some platforms, but this appears to be a bug rather than a feature.
In the Microsoft Component Object Model (COM), GUIDs are used to uniquely distinguish different software component interfaces. This means that two (possibly incompatible) versions of a component can have exactly the same name but still be distinguishable by their GUIDs. For example, in the creation of components for Microsoft Windows using COM, all components must implement the IUnknown interface to allow client code to find all other interfaces and features of that component, and they do this by creating a GUID which may be called upon to provide an entry point. The IUnknown interface is defined as a GUID with the value of {00000000-0000-0000-C000-000000000046}, and rather than having a named entry point called "IUnknown", the preceding GUID is used, thus every component that provides an IUnknown entry point gives the same GUID, and every program that looks for an IUnknown interface in a component always uses that GUID to find the entry point, knowing that an application using that particular GUID must always consistently implement IUnknown in the same manner and the same way.
GUIDs are also inserted into documents from Microsoft Office programs. Even audio or video streams in the Advanced Systems Format (ASF) are identified by their GUIDs.
A GUID's representation can be little endian or big endian, so all APIs need to ensure that the correct data structure is used.
There are several flavors of GUIDs used in COM:
DCOM introduces many additional GUID subtypes:
These GUID subspaces may overlap, as the context of GUID usage defines its subtype. For example, there might be a class using the same GUID for its CLSID as another class is using for its IID - all without a problem. On the other hand, two classes using the same CLSID could not co-exist.
There is also a guid element in some versions of the RSS specification, and a mandatory id element in Atom, which should contain a unique identifier for each individual article or weblog post. In RSS the contents of the GUID can be any text, and in practice is typically a copy of the article URL. Atoms' IDs need to be valid URIs (usually URLs pointing to the entry, or URNs containing any other unique identifier).
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