This article or section is in need of attention from an expert on the subject. Please help recruit one or improve this article yourself. See the talk page for details. Please consider using {{Expert-subject}} to associate this request with a WikiProject

A Globally Unique Identifier or GUID (IPA pronunciation: [gwɪd] or ['gu.ɪd]) is a pseudo-random number used in software applications. While each generated GUID is not guaranteed to be unique, the total number of unique keys (2¹²⁸ or 3.40282366×10³⁸) is so large that the probability of the same number being generated twice is very small. For an application using 10 billion random GUIDs, the probability of a coincidence is on the order of 1 in a quintillion. Image File history File links Information_icon. ... For information on how to read IPA transcriptions of English words see here. ... A pseudo-random number is a number belonging to a sequence which appears to be random, but can in fact be generated by a finite computation. ... Computer software (or simply software) refers to one or more computer programs and data held in the storage of a computer for some purpose. ... Main article: Names of large numbers A quintillion is a number written as either: a 1 followed by 18 zeros (10 to the 18th power, as used in the short scale system of numeration. ...

The term GUID usually refers to Microsoft's implementation of the Universally Unique Identifier (UUID) standard, however, many other pieces of software use the term GUID including Oracle Database, dBase and Novell eDirectory. Microsoft is one of few companies engaging itself in the console wars Where they are up against sony, nintendo, and of course sharps new console which may cause a threat. ... A Universally Unique Identifier is an identifier standard used in software construction, standardized by the Open Software Foundation (OSF) as part of the Distributed Computing Environment (DCE). ... An Oracle database consists of a collection of data managed by an Oracle database management system. ... dBASE III The correct title of this article is dBASE. The initial letter is capitalized because of technical restrictions. ... Novell eDirectory (formerly called Novell Directory Services [NDS]) is an X.500 compatible directory service software product released in 1993 by Novell, Inc. ...

Basic structure

The GUID is a 16-byte (128-bit) number. The most commonly used structure of the data type is: In computer science a byte is a unit of measurement of information storage, often containing eight bits. ...

 GUID STRUCT Data1 dd Data2 dw Data3 dw Data4 db 8 GUID ENDS 

The definition of guid from guiddef.h is as shown below:

 typedef struct _GUID { unsigned long Data1; unsigned short Data2; unsigned short Data3; unsigned char Data4[8]; } GUID; 

i.e. a concatenation of a 4-bytenumber, two 2-byte numbers and eight single bytes.

The C# object for creating and storing a guid (which hides the internal structure) is:

 System.Guid (.NET v1.0+) 

Text encoding

Guids are most commonly written in text as a sequence of hexadecimal digits such as: In mathematics and computer science, hexadecimal, base-16, or simply hex, is a numeral system with a radix, or base, of 16, usually written using the symbols 0–9 and A–F, or a–f. ...

3F2504E0-4F89-11D3-9A0C-0305E82C3301

This text notation follows from the data structure defined above. The sequence is

1. Data1 (8 characters)
2. Hyphen
3. Data2 (4 characters)
4. Hyphen
5. Data3 (4 characters)
6. Hyphen
7. Initial two items from Data4 (4 characters)
8. Hyphen
9. Remaining six items from Data4 (12 characters)

Often braces are added to enclose the above format, as such:

{3F2504E0-4F89-11D3-9A0C-0305E82C3301}

When printing fewer characters is desired guids are sometimes encoded into a base64 string of 22 to 24 characters (depending on padding). For instance: Base 64 is a positional numeral system using a base of 64. ...

7QDBkvCA1+B9K/U0vrQx1A

7QDBkvCA1+B9K/U0vrQx1A==

Algorithm

The OSF-specified algorithm used by Microsoft for generating new GUIDs has been widely criticized. In these (V1) GUIDs, the user's network card MAC address was used as a base for the last group of GUID digits, which meant, for example, that a document could be tracked back to the computer that created it. This privacy hole was used when locating the creator of the Melissa worm. In mathematics, computing, linguistics, and related disciplines, an algorithm is a procedure (a finite set of well-defined instructions) for accomplishing some task which, given an initial state, will terminate in a defined end-state. ... A network card, network adapter or NIC (network interface controller) is a piece of computer hardware designed to allow computers to communicate over a computer network. ... In computer networking a Media Access Control address (MAC address) is a unique identifier attached to most network adapters (NICs). ... how would you feel if teh govenment could watch you while you were on the toilet? Its getting really bad adn the politicians dont understnad that there are other ways! If you can do something about th eintrusion, do it . ... The Melissa worm, also known as Mailissa, Simpsons, Kwyjibo, or Kwejeebo, is a computer worm that also functions as a macro virus, hence making it a multipartite virus. // History First found on March 26, 1999, Melissa shut down Internet mail systems that got clogged with infected e-mails propogating from...

V1 GUIDs which contain a MAC address can be identified by the digit "1" in the first position of the third group of digits, for example {2f1e4fc0-81fd-11da-9156-00036a0f876a}. GUIDs using the later algorithm, which has a random suffix, 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.

Uses

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. Component Object Model (COM) is a Microsoft platform for software componentry introduced by Microsoft in 1993. ... Software component representations: above the representation used in UML, below the representation commonly used by Microsofts COM objects. ...

GUIDs are also inserted into documents from Microsoft Office programs, as these are regarded as objects as well. Even audio or video streams in the Advanced Systems Format (ASF) are identified by their GUIDs. Microsoft Office is a suite of productivity programs created or purchased by Microsoft and developed for Microsoft Windows, and Apple Computers Mac OS and Mac OS X operating systems. ... Advanced Systems Format (formerly Advanced Streaming Format) is Microsofts proprietary digital audio/digital video container format, especially meant for streaming media. ...

GUIDs are frequently stored in files as text, but sometimes they are stored in binary. A GUID written in binary format can be written several different ways. The simplest format is the 16 bytes, in order, as they are in memory. Another binary format used in some files, including Advanced Systems Format (ASF) files, is a series of values in little endian byte order -- a 32-bit unsigned integer, followed by two 16-bit unsigned integers, followed by eight unsigned bytes (note that this does not match the text display format, because the first 32-bit value e.g. 0x12AB would be stored as 0xB-0xA-0x2-0x1). Any software running on big endian hardware which reads such GUIDs and creates and writes new ones or compares them with GUIDs created locally needs to reverse the byte order of the first three values in order to ensure uniqueness. Advanced Systems Format (formerly Advanced Streaming Format) is Microsofts proprietary digital audio/digital video container format, especially meant for streaming media. ... When integers or any other data are represented with multiple bytes, there is no unique way of ordering of those bytes in memory or in a transmission over some medium, and so the order is subject to arbitrary convention. ... When integers or any other data are represented with multiple bytes, there is no unique way of ordering of those bytes in memory or in a transmission over some medium, and so the order is subject to arbitrary convention. ...

Subtypes

There are several flavors of GUIDs used in COM:

• IID - interface identifier;
• CLSID - class identifier;
• LIBID - type library identifier;
• CATID - category identifier; (its presence on a class identifies it as belonging to certain class categories)

DCOM introduces many additional GUID subtypes:

• AppID - application identifier;
• MID - machine identifier;
• IPID - interface pointer identifier; (applicable to an interface engaged in RPC)
• CID - causality identifier; (applicable to a RPC session)
• OID - object identifier; (applicable to an object instance)
• OXID - object exporter identifier; (applicable to an instance of the system object that performs RPC)
• SETID - ping set identifier; (applicable to a group of objects)

These GUID subspaces may overlap, as the context of GUID usage defines its subtype. For example, there might be a class using same GUID for its CLSID as another class is using for its IID –- all without a problem. On the other hand, two classes using same CLSID couldn't co-exist.

XML syndication formats

There is also a guid element in some versions of the RSS specification, and 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 post URL. Atom's IDs need to be valid URIs (usually URLs pointing to the entry, or URNs containing any other unique identifier). For RSS feeds from Wikipedia, see Wikipedia:Syndication. ... The name Atom applies to a pair of related standards. ... A Uniform Resource Locator, URL (spelled out as an acronym, not pronounced as earl), or Web address, is a standardized address name layout for resources (such as documents or images) on the Internet (or elsewhere). ... A Uniform Resource Identifier (URI), is a compact string of characters used to identify or name a resource. ... A Uniform Resource Locator, URL (spelled out as an acronym, not pronounced as earl), or Web address, is a standardized address name layout for resources (such as documents or images) on the Internet (or elsewhere). ... A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the urn scheme, and does not imply availability of the identified resource. ...

See also

• Universally Unique Identifier (UUID)

A Universally Unique Identifier is an identifier standard used in software construction, standardized by the Open Software Foundation (OSF) as part of the Distributed Computing Environment (DCE). ...

External links

• International Standard "Generation and registration of Universally Unique Identifiers (UUIDs) and their use as ASN.1 Object Identifier components" (ITU-T Rec. X.667 | ISO/IEC 9834-8, technically compatible with IETF RFC 4122)
• A Universally Unique IDentifier (UUID) URN Namespace (IETF RFC 4122)
• UUID (GUID) Generator on the Web
• UUID Generator on the ITU-T website
• (GUID) Generator for Firefox XPI Extensions
• DmaId for InstanceId Values (DCE Universally Unique IDentifiers, UUIDs)
• Syntax and semantics of the DCE variant of Universal Unique Identifiers (UUIDs)