Returning more than one row crashes on PostgreSQL

Question

on PostgreSQL I implemented the following stored procedure in C:

extern "C" DLLEXPORT Datum
selectServeralRows(PG_FUNCTION_ARGS)
{
    FuncCallContext     *funcctx;
    int                  call_cntr;
    int                  max_calls;
    TupleDesc            tupdesc;
    AttInMetadata       *attinmeta;

    /* stuff done only on the first call of the function */
    if (SRF_IS_FIRSTCALL())
    {
        MemoryContext   oldcontext;

        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();

        /* switch to memory context appropriate for multiple function calls */
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

        /* total number of tuples to be returned */
        funcctx->max_calls = 1;

        /* Build a tuple descriptor for our result type */
        if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("function returning record called in context "
                        "that cannot accept type record")));

        /*
         * generate attribute metadata needed later to produce tuples from raw
         * C strings
         */
        attinmeta = TupleDescGetAttInMetadata(tupdesc);
        funcctx->attinmeta = attinmeta;

        MemoryContextSwitchTo(oldcontext);
    }

    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();

    if (funcctx->call_cntr < funcctx->max_calls)    /* do when there is more left to send */
    {
        Datum* val = (Datum*)palloc(2 * sizeof(Datum));
        HeapTuple    tuple;
        Datum        result;
        bool    nulls[2]={false,false};

        char * n = new char[2]; 
        n[0] = '1';
        n[1] = '\0';
        char * m = new char[2];
        m[0] = '2';
        n[1] = '\0';

        val[0] = CStringGetTextDatum(m);
        val[1] = CStringGetTextDatum(n);

        /* build a tuple */
        tuple = heap_form_tuple(tupdesc, val, nulls);

        /* make the tuple into a datum */
        result = TupleGetDatum(funcctx->slot, tuple);

        /* clean up (this is not really necessary) */

       SRF_RETURN_NEXT(funcctx, result);
    }
else
       SRF_RETURN_DONE(funcctx);
}

The code is almost the same as here: http://www.postgresql.org/docs/8.4/static/xfunc-c.html

If the procedure return just one row everything is fine and a table with one row is as desired. But if one line is changed like this

/* total number of tuples to be returned */
funcctx->max_calls = 2;

the execution of the query crashes with this message:

Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address: 0x000000000000041a 0x0000000100002d8b in heap_form_tuple ()

Stepping through the code shows that nothing until the crash is an invalid pointer so I am a bit clueless. Is there anything else which I have overseen?

EDIT: The function is called like this in psql:

select (selectServeralRows()).*

EDIT: SQL Definition of the function:

CREATE OR REPLACE FUNCTION selectServeralRows()
RETURNS TABLE(k character varying(20), j character varying(20)) AS
'/opt/local/lib/postgresql84/Debug/libSeveralRows', 'selectServeralRows'
LANGUAGE c STABLE STRICT;

I see a bug in line: Datum* val = (Datum*)palloc(2 + sizeof(Datum)); probably you would to use * instead + — Pavel Stehule
– Pavel Stehule, Commented Apr 29, 2012 at 14:01
@wildplasser, actually this is compiled with gcc. Nevertheless I changed it to use malloc and it had the same effect. — mistapink
– mistapink, Commented Apr 29, 2012 at 14:11
@PavelStehule Thanks. I changed that one too, but it didn't help at the end. — mistapink
– mistapink, Commented Apr 29, 2012 at 14:11
Why don't you use palloc insted of malloc? BTW: the palloc(2 + sizeof(Datum)); looks suspect, too. — wildplasser
– wildplasser, Commented Apr 29, 2012 at 14:17

wildplasser · Accepted Answer · 2012-04-29 16:11:15Z

2

It appears you have omitted a

 tuple = BuildTupleFromCStrings(funcctx->attinmeta, val);

call before the TupleGetDatum(...) call. The tuple variable is still uninitialised, except on the first call.

Also, Datum* val = (Datum*)palloc(2 * sizeof(Datum)); should probably be

char **val;
val = palloc (2 * sizeof *val);

And the n and m arrays could just as well be;

char n[2] ="1", m[2] = "2";
val[0] = n;
val[1] = m;

And you can free the memory after the call to heap_form_tuple(tupdesc, val, nulls);

pfree(val);

IMHO 'val' could also be an automatic ("stack") variable, just like n[] and m[].

answered Apr 29, 2012 at 16:11

wildplasser

44.5k9 gold badges72 silver badges116 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Returning more than one row crashes on PostgreSQL

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest