Thread: How do I construct a varlena?
Can you tell me, please, how to construct a varlena (to be returned by a function). I see it is declared as an int and a 1-byte array; I don't know how to assign a longer string in place of that 1-byte array; the compiler complains about incompatible types, whatever I do. Thank you. -- Oliver Elphick Oliver.Elphick@lfix.co.uk Isle of Wight http://www.lfix.co.uk/oliver PGP key from public servers; key ID 32B8FAA1 ======================================== "And not only so, but we glory in tribulations also; knowing that tribulation worketh patience; And patience, experience; and experience, hope." Romans 5:3,4
On Tue, 4 Aug 1998, Oliver Elphick wrote:
> Can you tell me, please, how to construct a varlena (to be returned by
> a function).
>
> I see it is declared as an int and a 1-byte array; I don't know how to
> assign a longer string in place of that 1-byte array; the compiler
> complains about incompatible types, whatever I do.
If I'm correct:
malloc(sizeof(VARLENA) + l);
where 'l' is the length of the string you want to put in (without the
trailing '\0').
Maarten
_____________________________________________________________________________
| TU Delft, The Netherlands, Faculty of Information Technology and Systems |
| Department of Electrical Engineering |
| Computer Architecture and Digital Technique section |
| M.Boekhold@et.tudelft.nl |
-----------------------------------------------------------------------------
Maarten Boekhold wrote:
>On Tue, 4 Aug 1998, Oliver Elphick wrote:
>
>> Can you tell me, please, how to construct a varlena (to be returned by
>> a function).
>>
>> I see it is declared as an int and a 1-byte array; I don't know how to
>> assign a longer string in place of that 1-byte array; the compiler
>> complains about incompatible types, whatever I do.
>
>If I'm correct:
>
> malloc(sizeof(VARLENA) + l);
>
>where 'l' is the length of the string you want to put in (without the
>trailing '\0').
Thanks for your reply; I've been away a few days so I've only just seen it.
My problem is in how to get the compiler to treat the malloced space as
a varlena.
I have this (abridged) C code, to be used with
CREATE FUNCTION cname(bpchar, bpchar, bpchar) returns bpchar ...:
char *cxname;
text cname (text s, text t, text f)
{
text *result;
...
cxname = realloc((void *) cxname, strlen(tmp)+sizeof(struct varlena));
strcpy(cxname+sizeof(int32), tmp);
-> result = &((struct varlena) cxname);
result->vl_len = strlen(tmp);
return *result;
}
but the compiler gives the error `conversion to non-scalar type requested'
at the marked line.
Once I know how to do this, I will add it to the examples in the CREATE
FUNCTION documentation, since it will no doubt be helpful to others.
--
Oliver Elphick Oliver.Elphick@lfix.co.uk
Isle of Wight http://www.lfix.co.uk/oliver
PGP key from public servers; key ID 32B8FAA1
========================================
"...ask, and ye shall receive, that your joy may be
full." John 16:24
> My problem is in how to get the compiler to treat the malloced space
> as a varlena.
> I have this (abridged) C code, to be used with
> CREATE FUNCTION cname(bpchar, bpchar, bpchar) returns bpchar ...:
>
> char *cxname;
>
> text cname (text s, text t, text f)
text *cname (text s, text t, text f)
> {
> text *result;
> ...
> cxname = realloc((void *) cxname,
> strlen(tmp)+sizeof(struct varlena));
strcpy is dangerous because it will copy the trailing null, while text
and other varlena types are not guaranteed to be null-terminated. Better
to use memmove() or strncpy().
> strcpy(cxname+sizeof(int32), tmp);
strcpy(cxname+sizeof(result->vl_len), tmp);
or
strcpy(cxname+sizeof(VARHDRSZ), tmp);
not sure where tmp came from...
> -> result = &((struct varlena) cxname);
cxname is already a pointer. And why not make it a pointer to text
instead of a pointer to char?
result = ((text *) cxname);
> result->vl_len = strlen(tmp);
>
> return *result;
> }
> Once I know how to do this, I will add it to the examples in the
> CREATE FUNCTION documentation, since it will no doubt be helpful to
> others.
There is already documentation on this (though it could stand to be
cleaned up and augmented) in doc/src/sgml/xfunc.sgml. This appears in
the Programmer's Guide in the chapter titled "Extending SQL: Functions".
If you want, it's probably OK to keep the SQL reference docs pretty
simple, and if there is an example of C source code it could be shown
but then described more completely in the Programmer's Guide. Or we
could just mention that there _are_ examples in the Programmer's Guide
and leave it at that.
There is some code following which gives a (simpler) example...
- Tom
From the varlena.c file in backend/utils/adt/:
/*
* textin - converts "..." to internal representation
*/
text *
textin(char *inputText)
{
text *result;
int len;
if (inputText == NULL)
return (NULL);
len = strlen(inputText) + VARHDRSZ;
result = (text *) palloc(len);
VARSIZE(result) = len;
memmove(VARDATA(result), inputText, len - VARHDRSZ);
#ifdef CYR_RECODE
convertstr(VARDATA(result), len - VARHDRSZ, 0);
#endif
return (result);
}
> My problem is in how to get the compiler to treat the malloced space as
> a varlena.
>
> I have this (abridged) C code, to be used with
> CREATE FUNCTION cname(bpchar, bpchar, bpchar) returns bpchar ...:
>
>
> char *cxname;
>
> text cname (text s, text t, text f)
> {
> text *result;
> ...
> cxname = realloc((void *) cxname, strlen(tmp)+sizeof(struct varlena));
> strcpy(cxname+sizeof(int32), tmp);
> -> result = &((struct varlena) cxname);
> result->vl_len = strlen(tmp);
>
> return *result;
> }
>
> but the compiler gives the error `conversion to non-scalar type requested'
> at the marked line.
I gues something like this should work:
struct varlena *cxname
cxname = (struct varlena *)
realloc(cxname, strlen(tmp) + VARHDRSZ);
strcpy(cxname->vl_dat, tmp); /* maybe '&cxname->vl_dat' */
return cxname;
I don't think it's possible to 'return *cxname', cos the compiler will
only return the part of your data.
Most of this is from head, so check on things if they don't work immediately.
Maarten
_____________________________________________________________________________
| TU Delft, The Netherlands, Faculty of Information Technology and Systems |
| Department of Electrical Engineering |
| Computer Architecture and Digital Technique section |
| M.Boekhold@et.tudelft.nl |
-----------------------------------------------------------------------------