[m-users.] MR_GC_free ?

[Volker Wysk]

Am Donnerstag, dem 09.11.2023 um 00:33 +1100 schrieb Julien Fischer:
> On Wed, 8 Nov 2023, Volker Wysk wrote:
> 
> > > MR_NEW and MR_free allocate and free memory using malloc() and free()
> > > respectively, not with the GC.  It's safer to use the GC versions.
> > > Additionally, foreign_procs that mix memory allocators, especially a
> > > manual allocator, is likely be difficult to debug and maintain.
> > 
> > This leads me to the same question as Peter's answer: Can the garbage
> > collector see inside a C structure, if it is allocated with MR_GC_NEW?
> 
> Yes it can.  Bear in mind that the Boehm GC is designed to be garbage
> collector for C; so far as it is concerned there is no distinction
> between Mercury and C data structures.

I'm wondering how it can, but okay. 

There's the ODBC library, which I'm trying to make thread-safe. I want to
package the former global variables in a new structure like this:

typedef struct {
    MR_jmp_buf odbc_trans_jmp_buf;
    SQLHENV odbc_env_handle;
    SQLHDBC odbc_connection;
    SQLRETURN odbc_ret_code;
    MR_Word odbc_message_list;
} MODBC_Global;

The pointer to this data is to be passed to all the procedures, which used
the global variables before. 

Now, there are two ODBC handles in this structure: odbc_env_handle and
odbc_connection. These are just pointers. But they should NOT be deallocated
by the garbage collector. They need to be freed by the relevant ODBC
"SQLFree..." procedure.

So, I think I should allocate/release the MODBC_Global explicitly with
MR_NEW /MR_free. Is that correct?

Regards,
Volker