Original bug ID: 1399
Reporter: administrator
Status: closed
Resolution: fixed
Priority: normal
Severity: minor
Category: ~DO NOT USE (was: OCaml general)

Bug description

Ok, so I think that I now have a bug report. In the file
otherlibs/db/dbstubs.c in the distribution, the following definitions
can be found:

#define Camldb_db(v) (((struct camldb *)(Bp_val(v)))->db)
#define Camldb_info(v) (((struct camldb *)(Bp_val(v)))->info)
#define Camldb_closed(v) (((struct camldb *)(Bp_val(v)))->closed)

These are allocated as finalized blocks, but my understanding is that
finalized blocks and custom blocks now have the same format, so this
code looks wrong. I don't think this code is actually compiled into the
distribution, but it seems like it should either be removed, fixed, or
at least marked as buggy somehow so someone doesn't try to use it as a
model for new code, as I did.

Thanks,
y

On Fri, 2002-09-20 at 05:56, Xavier Leroy wrote:

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
  --
  |--------/ Yaron M. Minsky --------|
  |--------\ http://www.cs.cornell.edu/home/yminsky/ /--------|

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