Thread: 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
_________________________________________________________________
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"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
_________________________________________________________________
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Thanks for all your answers, I begin to study. Enrico -- Enrico <scotty@linuxtime.it>