From: William Djaja Tjokroaminata <billtj@...>
Date: 2002-08-16T05:55:16+09:00
Subject: Re: Data_Make_Struct Considered Dangerous?

Two things:
1) First, it is not the intention to make a clear-cut judgment whether
Data_Make_Struct IS dangerous or Data_Make_Struct IS NOT dangerous.  As
written below, "... it is only dangerous when used incorrectly".  By the
same token, statement such as "Goto Considered Harmful" is then also
false, Goto is only harmful when used incorrectly or unwisely.  The
intention is to bring up the issue that may occur during this memory
allocation with respect to garbage collection.  As written below, in C++
we have to use Data_Wrap_Struct instead of Data_Make_Struct.

2) Second, the "danger" in Data_Make_Struct comes from ALLOC and even from
the Data_Wrap_Struct itself, which may invoke the garbage
collector.  However, I still do not get the answer why I ever need to use
ALLOC instead of malloc and its derivatives.  For example, ignoring
run-out-of-memory problem, a C statement such as

    ptr = ALLOC (my_struct);

may create segmentation fault at this point (if some of the memory values
are not proper) while a C statement such as

    ptr = (my_struct*) malloc (sizeof (my_struct));

will never create segmentation fault (at this point).  Why do we ever need
to use ALLOC (which is tied to a garbage collector) instead of malloc and
its derivatives?

Regards,

Bill
=========================================================================
Paul Brannan <pbrannan@atdesk.com> wrote:
> On Thu, Aug 15, 2002 at 11:44:59PM +0900, ts wrote:
>>  Data_Make_Struct() is *not* dangerous. The goal of Data_Make_Struct()
>>  is to call ALLOC(), initialize all members of the struct to zero then
>>  call Data_Wrap_Struct()

> This is a problem, though, because setting a struct to all zeroes
> is not the same as initializing all members of the struct.  It is
> impossible to write a macro in C that can properly initialize all C
> structs in a generic manner.

> In C++, for example, a class is not initialized until its constructor
> has completed.  I might find it convenient to use Data_Make_Struct and
> then to use placement new to initialize the object, but this would be a
> bug.  Instead, I should call ALLOC, use placement new on the allocated
> structure, then call Data_Wrap_Struct to wrap the object.  I have to be
> very careful of exception-safety issues if I do this.

> The same applies in C to any structure that has non-trivial
> initialization.  If you have non-trivial initialization, then perhaps
> Data_Wrap_Struct is more appropriate for your application.  I agree with
> ts that THIS DOES NOT make Data_Make_Struct dangerous; it is only
> dangerous when used incorrectly.

> Paul