OpenServer question (returning parameters)

Hi,

I am trying to write an extended store procedure that returns its data via
an
output parameter, similar to what one can do with regular stored
procedures.
However, I've had no luck with it. Perhaps somebody with a better
understand
of the Sybase internals can help me out. It seems it ain't possible (wish
ASE
was open source) but I cannot believe that (I am using ASE 12.5.3)
Anyway, I get stuck with the first call to srv_descfmt. The server
replies:
"XP Server Error: 16165/10/0: srv_descfmt(): CS_SET for SRV_RPCDATA not
valid"

Here an exceprt of my code.
First I create the xp_test in ASE:

create procedure xp_test @[EMAIL PROTECTED]
 INTEGER OUTPUT AS external name "test.so"

The call I make:
declare @[EMAIL PROTECTED]
 integer 
select @[EMAIL PROTECTED]
 
exec xp_test @[EMAIL PROTECTED]
 OUTPUT 
select @[EMAIL PROTECTED]
 ESP code:
    
    CS_DATAFMT  paramfmt;
    memset(&paramfmt, (CS_INT) 0, (CS_INT) sizeof(CS_DATAFMT));

    CS_INT data = 23;

    paramfmt.namelen   = 0;
    paramfmt.datatype  = CS_INT_TYPE;
    paramfmt.format    = 0;
    paramfmt.maxlength = 4;
    paramfmt.count     = 1;
    paramfmt.locale    = NULL;
    paramfmt.status    = CS_RETURN;

    srv_descfmt(srvproc, CS_SET, SRV_RPCDATA, 1, &paramfmt);
    
    CS_INT      len = sizeof(CS_INT);
    CS_SMALLINT outlen = CS_GOODDATA;

srv_bind(srvproc, roc, CS_SET, SRV_RPCDATA, q, &paramfmt, (CS_BYTE *)
&data,
&len, &outlen);

    srv_xferdata(srvproc, CS_SET, SRV_RPCDATA)

Thanks