Re: Storing graph data structure with MDB_FIXEDMAP

[Howard Chu <hyc@symas.com>]

Aris Setyawan wrote:
> Hi,
>
> I'm new in this list.
>
> I plan to store graph data structure (vertex and edge) in LMDB. The
> "edge" data value of a record will contain a pointer which is pointing
> to the address of "vertex" data value. So, I must make it sure that
> data value address is not changing during operation. According to the
> doc, I can use MDB_FIXEDMAP.
>
> And from the doc:
>
> "... the memory map will always reside at the same virtual address and
> pointers used to reference data items in the database will be constant
> across multiple invocations. ..."
>
> Spesifically: "... pointers used to reference data items in the
> database will be constant ..."
>
> Is this mean that I must allocate memory to write data using mdb_put
> with MDB_RESERVE flag?

Yes, but from what you've described, this will not do what you want. The 
address of a record is not guaranteed to remain constant, so you cannot safely 
reference the address of one record from another record. The only safe way to 
reference one record from another is using its key.

If you want to create a record which has fields referencing the addresses of 
other fields within the same record, that is the goal for FIXEDMAP but this 
feature requires the relocation functions, which currently are not implemented.

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/