72 lines
3.2 KiB
Text
72 lines
3.2 KiB
Text
This interface is most closely based on the DB4 C api and tries to
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maintain close interface proximity.
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That API is published by sleepycat at
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http://www.sleepycat.com/docs/api_c/frame.html
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function all arguments that are systematically omitted are leading
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DB handles and TXN handles. A few calls omit the flags parameter when the
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documenation indicates that no flag values are used. cursor.close is one.
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the defines generator is imperfect and includes some defines that are not
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flags. while it could be improved, it is easier to delete the incorrect ones.
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thus, if you decide to rebuild the defines, you will need to edit the resulting
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file. this may be necessary if using a different release of DB4 than the one
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I used.
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I have put all possible caution into ensuring that DB and Ruby cooperate.
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The memory access was one apsect carefully considered. Since Ruby copies
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when doing String#new, all key/data retrieval from DB is done with a 0 flag,
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meaning that DB will be responsible. See the copied news group posting about
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the effect of that.
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The only other design consideration of consequence was associate. The prior
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version used a Ruby thread local variable and kept track of the current
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database in use. I decided to take a simpler approach since Ruby is green
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threads. A global array stores the VALUE of the Proc for a given association
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by the file descriptor number of the underlying database. This is looked
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up when the first layer callback is made. It would have been better considered
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if DB allowed the passing of a (void *) user data into the alloc that would
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be supplied during callback. So far this design has not produced any problems.
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[This is a message from comp.databases.berkeley-db]
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http://groups.google.com/group/comp.databases.berkeley-db/browse_frm/thread/4f70a9999b64ce6a/c06b94692e3cbc41?tvc=1&q=dbt+malloc#c06b94692e3cbc41
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Subject: Some questions about BerkeleyDB
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Patrick Schaaf
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Sep 16 2004, 9:31 am show options
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Hi Cylin,
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I'm only replying to one point; I'm not so sure about the others.
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>> >4.In http://www.sleepycat.com/docs/api_c/dbt_class.html#DB_DBT_MALLOC
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>I mean when we query by a key, and get return data( or key).
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>If we set DB_DBT_MALLOC or not ,what is the difference about
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>data.data/key.data?
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DB_DBT_MALLOC is only relevant for the DBT in which Berkeley DB
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gives you back data from the database.
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Without DB_DBT_MALLOC, i.e. with DBT.flags set to 0, after the successful
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query call, you will find in data.data a pointer into memory which is
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under the responsibility of Berkeley DB. It may be (I don't know for sure)
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a pointer into the memory mapped shared environment. You can copy from
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there to some place safe, and I think you must NOT assume that the pointer
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will be valid after another call to a retrieval function.
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On the other hand, when you set (before the retrieval call) that DBT's
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.flags field to DB_DBT_MALLOC, then the retrieval function of Berkeley DB
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will automatically call malloc() to get _new_ memory for the retrieved
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data, and you will find after the retrieval that data.data points
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to that memory. As it is newly malloc()ed, you can access it for as
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long as you want. IMPORTANT: it is also your responsibility to
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use free() when you don't need it any more.
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best regards
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Patrick
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