> 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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