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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