JET_USERDEFINEDDEFAULT Structure

Article
08/26/2021

Applies to: Windows | Windows Server

JET_USERDEFINEDDEFAULT Structure

The JET_USERDEFINEDDEFAULT structure is specified in conjunction with JET_bitColumnUserDefinedDefault to give a new column a default value that is determined using a callback. This technique can be used to implement computed columns.

Windows XP: The JET_USERDEFINEDDEFAULT structure is introduced in Windows XP.

    typedef struct tag_JET_USERDEFINEDDEFAULT {
      tchar* szCallback;
      unsigned char* pbUserData;
      unsigned long cbUserData;
      tchar* szDependantColumns;
    } JET_USERDEFINEDDEFAULT;

Members

szCallback

The export name of the function that implements the callback in "module!function" format.

The callback persists as a part of the column schema. The actual host executable and export name of the function must persist to enable the lookup of the true address of the function at run time.

The module name is the name of the host binary that contains the function. The function name is the name of the export for that function. These two pieces of information will be used by the database engine at runtime to locate the true address of the callback by executing a LoadLibrary call on the module name followed by a GetProcAddress call on the function name.

For example, if the callback were implemented in a DLL called MyCallback.DLL and that DLL were stored in C:\MyApplication and the function implementing the callback were exported from the DLL as UserDefinedDefaultCallback, then the required string would be "C:\MyApplication\MyCallback.DLL!UserDefinedDefaultCallback".

Note Embedded "!" characters in the module portion of the callback name are not supported.

It is highly recommended that you use a callback name that is not a function of the host architecture. For example, do not use exports decorated as stdcall (UserDefinedDefaultCallback@32) because this calling convention is only supported on x86 machines. If such a callback were used then the database could only be used on an x86 machine. An alias should be used to make a platform-agnostic export in this case.

It is highly recommended that you use the full path of the module name that is implementing the callback. If a relative path is used then the process that is hosting the database might be susceptible to attack by a rogue binary.

The application should pass only trusted callbacks to the database engine. The database engine will load the binary to verify its existence when the associated column is created. If the path to the binary is not validated or known to be trusted then it could attack the process that is hosting the database.

pbUserData

A self-contained block of user-defined data to be passed to the callback when invoked.The data block that is provided will persist as a part of the column schema. As a result, the data block must be entirely self-contained and cannot refer to any data that is outside the scope of the database.

If pbUserData is zero then the value of cbUserData is ignored. In this case, no user-defined data will be passed to the callback when invoked.

cbUserData

See pbUserData.

szDependantColumns

Reserved for future use.

This member should always be set to NULL.

Requirements

Requirement	Value
Client	Requires Windows Vista, Windows XP, or Windows 2000 Professional.
Server	Requires Windows Server 2008, Windows Server 2003, or Windows 2000 Server.
Header	Declared in Esent.h.
Unicode	Implemented as JET_ USERDEFINEDDEFAULT_W (Unicode) and JET_ USERDEFINEDDEFAULT_A (ANSI).

JET_USERDEFINEDDEFAULT Structure