Q: text palloc() size vs. SET_VARSIZE()

[Albrecht Dreß <albrecht.dress@xxxxxxxx>]

Hi all,

I have a (hopefully not too dumb) question regarding the size allocation of a text return value in a C user-defined function.

Basically, the function is somewhat similar to the copytext() example on <https://www.postgresql.org/docs/10/static/xfunc-c.html>.  However, the function shall perform some “decoding” of the input text, so the result is either as long as the input, or shorter.

In order to avoid time-consuming double-scanning of the input or re-allocation of memory, the idea is to allocate the result to the maximum possible size, which may or may not be filled completely.  Copied from the example in the manual:

---8<--------------------------------------------------------------------------
Datum
decode_text(PG_FUNCTION_ARGS)
{
    text     *t = PG_GETARG_TEXT_PP(0);
    size_t    out_len = 0U;

    // allocate to the max. possible output size
    text     *new_t = (text *) palloc(VARSIZE_ANY_EXHDR(t) + VARHDRSZ);

    // copy data to VARDATA(new_t), and count bytes in out_len

    // set output size which is out_len <= VARSIZE_ANY_EXHDR(t)
    SET_VARSIZE(new_t, out_len + VARHDRSZ);
    PG_RETURN_TEXT_P(new_t);
}
---8<--------------------------------------------------------------------------

From the docs, for me it is not clear whether the value assigned using SET_VARSIZE() must be the *exact* size of the newly allocated return value, or just the length of the text plus the header size.  IOW would the code above create a memory leak if out_len < VARSIZE_ANY_EXHDR(t)?

If this approach is wrong, would it be possible in the example above to just re-size new_t to the correct size by calling repalloc()?

Thanks in advance,
Albrecht.

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