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Re: Array elements (urgent help needed)

From: Akbar Mokhtarani <akbarm(at)slac(dot)stanford(dot)edu>
To: Brett Schwarz <brett_schwarz(at)yahoo(dot)com>
Cc: PostgreSQL-interfaces <pgsql-interfaces(at)postgresql(dot)org>
Subject: Re: Array elements (urgent help needed)
Date: 2004-10-04 04:15:34
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-interfaces
Yes the result from PQgetvalue is a char*. I looked at the array.h and
used the ArrayType structure defined there to walk through the memory
area. The first 4 bytes gives you the dimension of the array, the second 8
bytes are not used and the third 6 bytes gives the number of elements, and the
rest are 4 bytes pointers to the data type stored in the array.
Using this I was
able to parse arrays of int, double, and char BUT not string. This worked
fine on a cluster of intel based machines. However, when I moved the code
to another intel based machine, the program crashed. I have not been able
to parse the memory on this machine. It is an AMD processor which should
be the same as intel processor.
This made me believe that there must be a better way of doing this without
knowing the intenal structure of the memory.

Akbar Mokhtarani

On Sun, 3 Oct 2004, Brett Schwarz wrote:

> I really haven't work with arrays that much at the C
> level, but I thought that the result from PQgetvalue
> was just a char string. If it is an array, I thought
> that the string would look like:  {elem1, elem2,
> elem3, ...}.
> I don't think there are any helper functions for this,
> so you need to manually walk that char string to peel
> out the least as far as I am aware. You
> might want to look at the how the operators for arrays
> access the elements, e.g. the <> operator for arrays.
> Again, I'm not expert, but this is what I had thought
> is the case. I hope I understood your question.
> HTH,
>     --brett
> --- Akbar Mokhtarani <akbarm(at)slac(dot)stanford(dot)edu> wrote:
> > I am desparate to find a way to access array
> > elements from my c/c++
> > program. I have stored int and double data as arrays
> > in my tables and need
> > to retrieve them from the database. I use a binary
> > cursor to store the
> > query results and PQgetvalue gives me a char*
> > pointer to the storage area
> > where the data is stored (this is according to the
> > documentation). I have
> > tried to understand the internal structure of the
> > storage area, but it
> > changes with data type. I suspect there is a more
> > reliable/elegant way to
> > get access to the elements. What is the correct way
> > of doing this?
> >
> > Thanks in advance.
> >
> > Akbar Mokhtarani
> >
> >
> > ---------------------------(end of
> > broadcast)---------------------------
> > TIP 4: Don't 'kill -9' the postmaster
> >
> ---------------------------(end of broadcast)---------------------------
> TIP 6: Have you searched our list archives?

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