Re: [HACKERS] How do I construct a varlena?

> 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);
}