Office 2013 and later

Last modified: July 23, 2011

Applies to: Outlook

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Queue of long-term ID structures used by the Personal Folders file (PST) store provider to assign an Entry ID for a new message or folder in offline mode.

typedef struct { 
    ULONG    ulVersion; 
    MAPIUID  muidReserved; 
    ULONG    ulReserved; 
    DWORD    dwAlloc; 
    DWORD    dwNextAlloc; 
    LTID     ltidAlloc; 
    LTID     ltidNextAlloc; 


  • Version number for the structure.


  • This member is reserved for the internal use of Outlook and is not supported.


  • This member is reserved for the internal use of Outlook and is not supported.


  • The number of entries that are available for allocation. These entries share the same globally unique identifier (GUID).


  • The number of entries that are available next for allocation. These entries share the same GUID.


  • The long-term ID structure, LTID, identifying the entry currently available for allocation. The long-term ID structure contains a GUID and an index identifying an object in the store. Together, the GUID and the index can form a unique Entry ID for an object.


  • Long-term ID structure identifying the next available entry.

An Entry ID is a 4-byte MAPI entry identifier for a folder or a message. For more information, see ENTRYID.

When a PST store provider assigns an Entry ID to a new object, it first needs a GUID that identifies the server, and an index that identifies the object in the store. Even though the GUID is not unique across all Entry IDs, the GUID and the index combined provide a unique entry. This GUID and index pair is tracked by a long-term ID structure, LTID, which is part of the OLFI structure.

The PST store provider does not physically keep in OLFI an LTID structure for each GUID-index pair. It keeps one LTID structure, ltidAlloc, for the currently first available GUID-index pair; a count, dwAlloc, of the number of available entries that share this same GUID; and a second LTID structure, ltidNextAlloc, for the next available GUID-index pair that has a different GUID. The PST store provider uses the OLFI structure to track the GUIDs and indexes that it has handed out. At a virtual level, the provider maintains a reserve of a number of LTID structures that are ready to be allocated. dwAlloc maintains a count of the available LTID structures.

Requests for Entry IDs come in blocks. When there is a request for a block, the PST store provider checks if there is sufficient reserve on hand by comparing the requested size with dwAlloc. If there is sufficient reserve, it returns the GUID and index in ltidAlloc for allocation. It then decreases dwAlloc by the requested size, and increments the index in ltidAlloc by the requested size. This prepares the PST store provider to allocate ltidAlloc on the next request for another block of Entry IDs. Note that the GUID remains the same for the next request.

If the size of a request is larger than dwAlloc, the PST store provider tries to use what it has next in reserve, as specified by dwNextAlloc and ltidNextAlloc. It copies dwNextAlloc and ltidNextAlloc to dwAlloc and ltidAlloc respectively, and sets dwNextAlloc and ltidNextAlloc to NULL.

A provider that wraps the PST store provider should periodically check ltidNextAlloc to see if it is NULL. If it is, the provider should populate it with a new GUID and reset dwNextAlloc so that more entry IDs can be allocated.