From: Xavier Leroy <xavier.leroy@inria.fr>
To: "Yaron M. Minsky" <yminsky@CS.Cornell.EDU>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Bp_val and Data_custom_val
Date: Fri, 20 Sep 2002 11:56:57 +0200 [thread overview]
Message-ID: <20020920115657.A15003@pauillac.inria.fr> (raw)
In-Reply-To: <3D88DA09.4010906@cs.cornell.edu>; from yminsky@CS.Cornell.EDU on Wed, Sep 18, 2002 at 03:54:49PM -0400
> I'm wondering if someone can help me out in understanding the difference
> between Bp_val and Data_custom_val.
Caml heap blocks are composed of a GC header and a number of words of data.
Bp_val points to the first word of data:
header data0 data1 data2 ...
^
|
Bp_val
For finalized and custom blocks, the first word of "data" holds a
pointer to a record of functions (finalization functions, comparison
functions, etc), and Data_custom_val points to the second word of data,
i.e. the beginning of the user data space:
header function data1 data2 ...
pointer
^
|
Data_custom_val
> struct camldb {
> DB *db;
> int closed;
> };
>
> struct camltxn {
> DB_TXN *txn;
> int closed;
> };
>
> #define UW_db(v) (((struct camldb *)Bp_val(v))->db)
> #define UW_db_closed(v) (((struct camldb *)Bp_val(v))->closed)
This is incorrect, you should use Data_custom_val(v) instead of
Bp_val(v), otherwise the db field will overwrite the function pointer
at the beginning of the custom/finalized block.
- Xavier Leroy
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prev parent reply other threads:[~2002-09-20 9:57 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-09-18 19:54 Yaron M. Minsky
[not found] ` <3D891635.4030002@htec.demon.co.uk>
2002-09-19 1:41 ` Yaron M. Minsky
2002-09-19 2:32 ` Berkeley DB wrapper is fixed (was: Re: [Caml-list] Bp_val and Data_custom_val) Yaron M. Minsky
2002-09-20 9:56 ` Xavier Leroy [this message]
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