2.3.1.5 HNPAGEMAP
The HNPAGEMAP is the last item in the variable length data portion of the block immediately following the last heap item. There can be anywhere from 0 to 63 bytes of padding between the HNPAGEMAP and the block trailer. The beginning of the HNPAGEMAP is aligned on a 2-byte boundary so there can be an additional 1 byte of padding between the last heap item and the HNPAGEMAP.
The HNPAGEMAP structure contains the information about the allocations in the page. The HNPAGEMAP is located using the ibHnpm field in the HNHDR, HNPAGEHDR and HNBITMAPHDR records.
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cAlloc |
cFree |
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rgibAlloc (variable) |
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cAlloc (2 bytes): Allocation count. This represents the number of items (allocations) in the HN.
cFree (2 bytes): Free count. This represents the number of freed items in the HN.
rgibAlloc (variable): Allocation table. This
contains cAlloc + 1 entries. Each entry is a WORD value that is the byte
offset to the beginning of the allocation. An extra entry exists at the cAlloc
+ 1st position to mark the offset of the next available slot.
Therefore, the nth allocation starts at offset rgibAlloc[n]
(from the beginning of the HN header), and its size is calculated as rgibAlloc[n + 1] – rgibAlloc[n] bytes.