Re: C Stored Function with ECPG

Sorry if this double posts. My first attempt was stalled because I was
not a member of the list. So now I am.

I'm starting to experiment with writing some stored functions in C and
find it really handy. I have a simple C function that takes two
integers and returns the sum. This works fine. Now I want to create a
function that makes us of some embedded ECPG. So far I have failed at
my first attempt. Below are the steps and outputs of what I did. I
assume it's feasible to embedd ECPG in a Postgres Stored function
written in C?

My foo1.pcg file looks like this.
#include "postgres.h"
#include <string.h>
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(foo1);

Datum foo1(PG_FUNCTION_ARGS)
{
EXEC SQL CREATE TABLE foo1 (c1 integer NOT NULL, c2 varchar(50) NOT NULL);

PG_RETURN_INT32(0);
}

1. I run ecpg foo1.pcg and this generates my foo1.c file. I get no
warnings or errors.
[postgres(at)PortJackson ~]$ ecpg foo1.pgc
[postgres(at)PortJackson ~]$

2. I compile foo1.c and get some warnings. Are these warnings ok?
[postgres(at)PortJackson ~]$ gcc -g -fpic -c foo1.c
-I/usr/home/postgres/postgresql-8.3.5/src/include/
-I/home/postgres/postgresql-8.3.5/src/include/libpq/
-I/usr/home/postgres/postgresql-8.3.5/src/interfaces/ecpg/include/
-I/usr/local/pgsql/include/
foo1.pgc: In function 'foo1':
foo1.pgc:14: warning: passing argument 5 of 'ECPGdo' makes integer
from pointer without a cast
foo1.pgc:14: warning: passing argument 7 of 'ECPGdo' makes pointer
from integer without a cast
[postgres(at)PortJackson ~]$

3.I link foo1. With now warnings or errors.
[postgres(at)PortJackson ~]$ gcc -shared -o foo1.so foo1.o
-L/usr/local/pgsql/lib -lecpg
[postgres(at)PortJackson ~]$

4. I create a function in psql to call my new library. And get a could
not load library error for file libecpg.so.6. How do I fix this
error?
dev=# CREATE OR REPLACE FUNCTION foo1(integer)
RETURNS integer AS
'/usr/home/postgres/foo1', 'foo1'
LANGUAGE 'c' VOLATILE STRICT
COST 1;
ERROR: could not load library "/usr/home/postgres/foo1.so": dlopen
(/usr/home/postgres/foo1.so) failed: Shared object "libecpg.so.6" not
found, required by "foo1.so"
dev=#

I'm new to C functions and to ECPG. So mixing the two at this point
for me is probably asking for trouble. Any help would be much
appreciated.

Thanks
Bob

Bob Henkel <bob(dot)henkel(at)gmail(dot)com> writes:
> I assume it's feasible to embedd ECPG in a Postgres Stored function
> written in C?

No, I don't think so; or at least that's not the expected usage. ECPG
expects to connect to an external server. The normal way to issue
SQL from C-level backend code is to use the SPI interface:
http://www.postgresql.org/docs/8.3/static/spi.html

regards, tom lane

On Mon, Feb 23, 2009 at 07:04:04PM -0600, Bob Henkel wrote:
> assume it's feasible to embedd ECPG in a Postgres Stored function
> written in C?

As Tom already pointed out it is not. I always played with the idea of
implementing an additional ecpg interface based on spi instead of libpq but
never started working on it. Sorry.

Michael
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