Re: DLM returning a pointer...

[Randall Skelton <rhskelto(at)atm.ox.ac.uk>]

After reading Craig's email, I am somewhat confused... 

What I really need to know is how do I allocate a memory block in my C
function such that I am sure that it cannot be overwritten or otherwise
corrupted.  I want a routine that can establish a connection to a given
'port' which requires me to allocate some memory and return a pointer to
this memory block.  I then want to be very sure that the memory allocated
is 'safe' while now that I am back in IDL.  Then I want to be able to pass
this pointer from IDL back to C so as the database connection must be
established before data can be sent or received.  Finally, from IDL I
would close the connection (again requiring me to pass this pointer) and
de-allocate the memory.  Obviously, such an implementation is risky as it
could lead to memory leaks in IDL if the programmer fails to close the
connection properly.  I am open to other ideas, but I want to separate the
open and close connection functions.  I am thinking about putting a
time-out on the connection so that if idle for more than n minutes it
deallocates.  I fear, however, that a time-out would likely lead to
problems and would be rather tricky to implement.  It would be nice if
there were some way to ensure that there was a matching 'open' and 'close'
connection function with the IDL compiler...

Will the memory allocated with the IDL_GetScratch function span the forked
C process life?  i.e. if I use IDL_GetScratch to allocate memory, will IDL
(potentially) cleanup and deallocate the memory before I call
IDL_Deltmp()?  What about IDL_MemAlloc and IDL_MemFree?  Should I just
consider defining an list say 10 of these structures with IDL_MemAllocPerm
(giving me 10 possible connections) and forget about reclaiming the
memory?

(I am assuming here that since IDL is calling the C program, this is a
unix fork process giving C access to IDL's memory space alone.  I am
reluctant to use malloc directly in C as I doubt that IDL would respect
the memory it allocates when I return to IDL).

All comments, suggestions and queries are greatly appreciated!

Randall Skelton

NB: just wait until I start asking how to make this multi threaded with
asynchronous output from simultaneous connections ;)

On Tue, 24 Apr 2001, Martin Schultz wrote:

> > > Hi all,
> > >
> > > I am trying to write a few IDL functions which mirror those of a C library
> > > [...]
> > 
> > I don't think it would be wise to return a pointer, although
> > technically it is possible.  You could in principle cast the pointer
> > to an integer, and return the integer. 
> 
> ... you probably meant an unsigned 64-bit value (in IDL speak
> ULONG64).
> 
> Martin