Обсуждение: return varchar from C function

Hi, 
just for fun, I wrote a little postgresql contrib,
who has a C function called myfun inside it.
The function myfun returns a value , now I return
a cstring type value, and it works fine if 
I run from psql shell:

select value from myfun(paramteres);

but I can't do an insert like:

insert (charfield) select value from myfun(paramteres); 

becuse I have an error, exactly casting error.

I want that myfun returns a char,varchar, or text type.

Where I can find more documentation about differences between
cstring,char,text etc...? 

Regards,
Enrico
 

Hello

cstring is clasic c (zero terminated) string and is used only in some 
PostgreSQL functions. This type isn't compatible with text and you have to 
explicit cast trick with textin function.

root=# select textin(('abc'::cstring));
textin
--------
abc
(1 row)

Standard is using VARLENA types like text, varchar, ... You can find info in 
PostgreSQL FAQ. These types are similar Pascal string -> first four bytes 
cary length and next bytes are data without spec. ending symbol. 
http://www.varlena.com/GeneralBits/68.php

using text type in C function is simple:

Datum *const_fce(PG_FUNCTION_ARGS)
{   text *txt = palloc(5 + VARHDRSZ);   memcpy(VARDATA(txt), "pavel", 5);   VARATT_SIZE(txt) = 5 + VARHDRSZ;
   PG_RETURN_TEXT_P(txt);
}

please look to source code my orafce contrib module (you can find it on 
pgfoundry).

Regards
Pavel Stehule

_________________________________________________________________
Najdete si svou lasku a nove pratele na Match.com. http://www.msn.cz/

"Pavel Stehule" <pavel.stehule@hotmail.com> writes:

>
> Datum *const_fce(PG_FUNCTION_ARGS)
> {
>    text *txt = palloc(5 + VARHDRSZ);
>    memcpy(VARDATA(txt), "pavel", 5);
>    VARATT_SIZE(txt) = 5 + VARHDRSZ;
>
>    PG_RETURN_TEXT_P(txt);
> }

Much better practice is to use the input function of the data type you want to
convert to:

{text *txt = DatumGetTextP(DirectFunctionCall1(textin, CStringGetDatum("pavel")));PG_RETURN_TEXT_P(txt);
}



--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com

On Sun, Feb 18, 2007 at 12:56:08PM -0500, scotty@linuxtime.it wrote:
> Hi,
> just for fun, I wrote a little postgresql contrib,
> who has a C function called myfun inside it.
> The function myfun returns a value , now I return
> a cstring type value, and it works fine if
> I run from psql shell:

You don't provide the definition you used, but:

> select value from myfun(paramteres);

This isn't the usual way to return things, unless it is a set returning
function. Did you provide the return type at declaration time?

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.

>"Pavel Stehule" <pavel.stehule@hotmail.com> writes:
>
> >
> > Datum *const_fce(PG_FUNCTION_ARGS)
> > {
> >    text *txt = palloc(5 + VARHDRSZ);
> >    memcpy(VARDATA(txt), "pavel", 5);
> >    VARATT_SIZE(txt) = 5 + VARHDRSZ;
> >
> >    PG_RETURN_TEXT_P(txt);
> > }
>
>Much better practice is to use the input function of the data type you want 
>to
>convert to:
>
>{
>    text *txt = DatumGetTextP(DirectFunctionCall1(textin, 
>CStringGetDatum("pavel")));
>    PG_RETURN_TEXT_P(txt);
>}
>

Generally we want to do something with text value (concat, trim, ..) and 
then call textin isn't practic. Respective, for bussness processes 
implemented in C is textin and similar functions right. For string toolkit 
is better direct work with VARLENA struct.

Regards and thank you note

Pavel

_________________________________________________________________
Emotikony a pozadi programu MSN Messenger ozivi vasi konverzaci. 
http://messenger.msn.cz/

Thanks for all your answers,
I begin to study.

Enrico
-- 
Enrico <scotty@linuxtime.it>