Re: Foreign memory context read

On 23.05.2011 13:44, Vaibhav Kaushal wrote:
> Hello,
>
> I made some code changes, compilation went fine but the database could not
> start with the message:
>
> LOG:  server process (PID 17684) was terminated by signal 11: Segmentation
> fault
>
> I think this is because of memory allocation outside of any memory context.

There's always a memory context active, it just might not be the correct 
one.

> Is it possible to create some variable in a memory context (say
> "cut_context") and then access the variable in that context from a piece of
> code which is working with variables in a different context (say the
> "per_query" context)?
>
> If yes, then how?

Sure, for accessing a variable, it doesn't matter which memory context 
it was allocated in. As long as you make sure you allocate things in 
sufficiently long-lived memory contexts, so that your allocations are 
not free'd too early, while they're still needed by some code.

> My first guess is a Memory Context switch. But then, I need to bring in the
> value of the variable from the cut_context (which was formed earlier) to the
> per_query context which was created later on.
>
> Precisely I am trying to create a small array of Datums (before the ExecQual
> is called inside ExecScan) and then use the array inside the ExecEvalVar
> (which obviously is inside the executer).

Switching to the right memory context before the palloc() call is the 
key. Sounds like you want to allocate your array in the per-query memory 
context. If you need to move a value from one memory context to another, 
like if you need to take a Datum that's already been allocated in some 
other memory context, and store it in that array, you need to copy the 
Datum to the right memory context.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com