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[Caml-list] Bp_val and Data_custom_val
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Date: 2002-09-18 (19:54)
From: Yaron M. Minsky <yminsky@C...>
Subject: [Caml-list] Bp_val and Data_custom_val
I'm wondering if someone can help me out in understanding the difference 
between Bp_val and Data_custom_val.  These are both macros for accessing 
caml values from C.  I lifted and adapted some code from the 
distribution for building a berkeley DB interface.  In that code, Bp_val 
is used to access the contents of finalized blocks.  I adapted that code 
but shifted to custom blocks, which according to the docs have the same 
layout.  The docs also suggest using Data_custom_val for custom blocks. 
  But when I try to do that, I end up with segfaults.  I'm wondering 
what precisely the difference is.  Here's a snippet of my data type 
definitions and the macros I use for accessing the data contained in the 
custom blocks holding that data:

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)

#define UW_txn(v) (((struct camltxn *)Bp_val(v))->txn)
#define UW_txn_closed(v) (((struct camltxn *)Bp_val(v))->closed)

Anything fishy about that?  And any problem with replacing Bp_val with 
Data_custom_val here?

By the way, in a separate post, I put a URL for where you can download 
the entire wrapper in question.  Here it is again:


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