Thread: pointer scope and memory contexts

I am working on a set returning function and have a question about
switching memory contexts. Basically, what I want to know is whether
memory allocated in one context can be referenced when a different
context is current.  The docs give examples like:
   if (SRF_IS_FIRSTCALL())   {       funcctx = SRF_FIRSTCALL_INIT();       oldcontext =
MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
       /* allocate some memory that needs to span calls */       my_ptr = (a_type *) palloc(size);
       MemoryContextSwitchTo(oldcontext);   }
   funcctx = SRF_PERCALL_SETUP();
   /* can I now dereference here? Is it necessary to manually switch
to multi_call_memory_ctx? */   some_function(my_ptr[0]);   ...

In a related question, do I only need to switch to
multi_call_memory_ctx when allocating memory that needs to be
referenced in subsequent calls? Would the following make sense?
   if (SRF_IS_FIRSTCALL())   {       funcctx = SRF_FIRSTCALL_INIT();
       /* this will be deallocated at the end of the first call, right? */       scratch_ptr = (a_type *)
palloc(size);
       oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);       /* allocate some memory that needs to
spancalls */       my_ptr = (a_type *) palloc(size);       MemoryContextSwitchTo(oldcontext);
 
       /* can I dereference my_ptr here? */       func_called_once(scratch_ptr, my_ptr);
       ...
   }
   /* per call stuff follows... */

In other words, if I have allocations that only are needed during the
first call, can I allocate them outside of the multi_call_memory_ctx
(so that they do not live past the first call) and only allocate the
memory needed for multiple calls within the multi_call_memory_ctx?

One last question: if I call SPI_finish, on the first call, do I need
to switch contexts in the per-call section? (I saw some example code
that suggested one needs to switch contexts back to
multi_call_memory_ctx after SPI_finish.)

Thanks.

THK

-- 
Timothy H. Keitt
http://www.keittlab.org/

On Thu, Nov 6, 2008 at 7:01 AM, Tim Keitt <tkeitt@keittlab.org> wrote:
> I am working on a set returning function and have a question about
> switching memory contexts. Basically, what I want to know is whether
> memory allocated in one context can be referenced when a different
> context is current.

You can safely refer the memory allocated in a different context as
long as the other memory context is still alive.


Thanks,
Pavan

-- 
Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com

"Tim Keitt" <tkeitt@keittlab.org> writes:
> [questions]

Switching memory contexts, in itself, only switches which context a bare
palloc() will allocate from (as opposed to MemoryContextAlloc).  It
cannot have some magic impact on the validity of existing pointers.

> One last question: if I call SPI_finish, on the first call, do I need
> to switch contexts in the per-call section? (I saw some example code
> that suggested one needs to switch contexts back to
> multi_call_memory_ctx after SPI_finish.)

I believe SPI_finish will switch back to the context that was current
when SPI_connect was called.
        regards, tom lane