How to use UIDs

A UID is a globally unique identifier consisting of a 32-bit number.

In Symbian OS, objects are identified by a compound identifier, known as the UID type, which is constructed from three 32 bit identifiers. The individual UIDs making up the UID type are referred to as UID1, UID2 and UID3.

Symbian OS makes widespread use of UIDs:

• UIDs are used to identify the types of objects at runtime and loadtime, for example identifying executables, DLLs, filestores.

• UIDs are used to verify that objects present compatible and expected interfaces at runtime and loadtime, for example verifying that DLLs or filestores are of the expected type.

• UIDs are fundamental to the association of applications with documents. For example, associating a document with an application allows the system to launch the application when the document is opened.

A file's three UIDs are stored in the first 12 bytes of the file.

To program successfully, developers need to understand why and how to use UIDs in their programs.

From a user's point of view, conventional file names are preferable to UIDs for file identification. Symbian OS therefore supports a flexible, long filename, file naming scheme.

However from a system point of view, guaranteed unique, 32-bit numbers provide for much safer, systematic, and more lightweight identification.

UID1, UID2 and UID3 have the following general characteristics:

• UID1 indicates the structure of the file, for example, whether it is an executable, a DLL or a file store.

• The meaning of UID2 depends on the type of object it applies to. For polymorphic interface (plug-in framework) DLLs, UID2 identifies the interface that the DLL implements. For static interface (shared library) DLLs that others link to, the UID2 value is always the same. For executables (with the exception of GUI applications), the UID2 value is ignored.

• UID3 distinguishes between objects with the same UID2 and can be thought of as a project identifier. For example, for an application, the same UID3 is shared by the executable, registration file (which defines the application's icon, caption, and some capability information), and all documents.

  In OS versions with platform security, the Secure ID (SID) for a binary is taken to be the same as the third UID for that file. If an application (EXE) does not specify a UID3 or specifies it as zero, then the Secure ID is undefined for the application. This has several consequences, including lack of privacy for data used by that application.

The UID type is a TUidType object and is constructed from a combination of all or some of the three possible UIDs. The UID type can be queried to return its component UID1, UID2 and UID3 values.

If no UIDs are attached to an object, then all three component UIDs are returned as KUidNull.

An object in Symbian OS and more particularly files in Symbian OS may have all, some, or none of the three possible UIDs defined.

Symbian OS predefines all possible UID1 values and the UID2 values used for GUI applications and static interface DLLs at system level. Developers refer to them by their constant names, although in project (.mmp) files, hexadecimal numbers are used.

• UID1: examples include KExecutableImageUid, KDynamicLibraryUid and KDirectFileStoreLayoutUid

• UID2: examples include KSharedLibraryUid (0x1000008d) for a static interface DLL and KUidApp (0x100039CE) for a GUI application

• Developers are responsible for ensuring that where UID3 values are required, they are properly allocated. See the Symbian Signed web site for information on how to allocate UIDs (free registration is required to see the FAQ).

Note that in project (.mmp) files, UID2 and UID3 values can be specified, but UID1 values cannot; the UID1 value is implied by the project's target type.

All executable targets have a UID1 of KExecutableImageUid. This is defined by Symbian OS and is automatically built into any executable target based on the exe target type declared in the project file.

A UID2 value should be specified by GUI applications (.EXE) and the application architecture expects this to be KUidApp (0x100039CE). Console applications (also .EXE), which are often used for testing and as example code, do not need to specify a UID2, and if they do, it is ignored. All executables should however specify a UID3 value, in part because it is used as the secure ID (SID) of the process, which is used as the executable's private directory name.

All DLLs, whether they have static or polymorphic interfaces, have a UID1 of KDynamicLibraryUid. This is defined by Symbian OS and is automatically built into any DLL target based on the dll target type declared in the project file.

For static interface DLLs, the UID2 is KSharedLibraryUid. The UID3, which is used to identify the interface to the library, must be allocated by Symbian Signed.

For polymorphic DLLs (for instance ECom plugins, device drivers and front end processors), UID2 identifies the interface that the DLL implements. UID3 (which, if required, must be allocated by Symbian Signed) may be used to identify a specific implementation of that interface.

In Symbian OS, documents are really file-stores: stores which can be closed and re-opened. There are two different kinds of file store, direct file stores which are write-once-read-many, and permanent file stores which are write-many-read-many.

A document's UID1 will therefore be one of KDirectFileStoreLayoutUid or KPermanentFileStoreLayoutUid. The UID2 and/or UID3 will be application dependent.

Every native document must have an appropriate UID1 which should be set by the application which creates the document. Typically documents may have a UID2 of KUidAppDllDoc and a UID3 shared with the creating application. More precisely, a document’s UID3 should match that of the application which will open it.

Only the UID1 is required, but in most cases developers will want to specify second and third UIDs for the documents their applications create and use. These UIDs are used by the application architecture framework to manage associations between applications and their documents. This allows an application to be found and launched when a file is opened, and it also allows an application icon to be associated with documents in system shell displays. Conversely, it allows an application, when opening files, to select only applicable files.

Symbian OS also allows default file associations with the implication that in some cases users may want to select a different application to open a file. Applications which support this must therefore be able to open documents whose third UID differs from their own.

Applications may also want to open non-native documents which have no UIDs, and may wish to be specified as default applications for these documents.

Package (.PKG) files are used in Symbian OS to provide the information required to create Symbian installation (.SIS) files. Each SIS file contains a UID, which is defined in the package file's package header. This UID should be allocated in the same way as other UIDs, through Symbian Signed.

Because UIDs are fundamental to Symbian OS, it is important that they are used correctly when developing programs. To ensure uniqueness, it is essential that UIDs are properly allocated.

Uniqueness is guaranteed by managing allocation centrally from a single database. All UIDs must therefore be assigned to users by a central allocating authority.

With the introduction of platform security, UIDs are also split into protected and unprotected ranges. Any UID values falling below 0x7FFFFFFF are classed as "protected" and are only intended for use with signed applications (or those pre-installed in ROM). The software installer will refuse to install an unsigned application if it uses a package UID in the protected range.

Developers can request UIDs through https://www.symbiansigned.com. For more information, see the Symbian Signed FAQ (free registration required). Note that from version 9 of Symbian OS, UIDs are no longer requested from [email protected].

During development, or for test code, temporary UIDs may be chosen from the unprotected test range 0xExxxxxxx. These UIDs can be used during development for unsigned applications but must not be used in released software. Note that such applications may not be installable via a SIS file. See the Symbian Signed website for more information.

Care must still be taken to avoid clashes within development teams and between multiple projects, including old projects which may still be installed on an emulator or native platforms. UID clashes may stop a program from loading correctly, typically leading to Not Found errors.