Postgres UDT in C with nested struct

Question

Hi Im trying to create a postgres UDT written in C. It's a fraction type. Im trying to experiment with the struct mixednumber with a int64 and struct fraction inside.

#include "postgres.h"
#include "fmgr.h"
#include <stdbool.h>

PG_MODULE_MAGIC;

typedef struct Fraction
{
    int64 numerator; 
    int64 denominator;
} Fraction;

PG_FUNCTION_INFO_V1(fraction_in);

Datum
fraction_in(PG_FUNCTION_ARGS)
{
    char *input = PG_GETARG_CSTRING(0);
    int64 n, d;
    bool valid;

    Fraction *result;

    valid = sscanf(input, "(%ld/%ld)", &n, &d) == 2;

    if (!valid)
        ereport(ERROR, (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
             errmsg("invalid input syntax for fraction: \"%s\"", input)));

    if (d == 0) 
        ereport(ERROR, (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("denominator cannot be \"%ld\" in \"%s\"", d, input)));

    result = (Fraction *) palloc(sizeof(Fraction));

    result->numerator    = n;
    result->denominator  = d;

    PG_RETURN_POINTER(result);
}

PG_FUNCTION_INFO_V1(fraction_out);

Datum
fraction_out(PG_FUNCTION_ARGS)
{
    Fraction *fraction = (Fraction *) PG_GETARG_POINTER(0);
    char          *result;

    result = psprintf("(%ld/%ld)", fraction->numerator, fraction->denominator);

    PG_RETURN_CSTRING(result);
}

//////////////////////////////////////
// Mixed Fractions or Mixed Numbers //
//////////////////////////////////////

typedef struct MixedNumber
{
    int64 wholeNumber; 
    Fraction *fraction;
} MixedNumber;

PG_FUNCTION_INFO_V1(mixednumber_in);

Datum
mixednumber_in(PG_FUNCTION_ARGS)
{
    char *input = PG_GETARG_CSTRING(0);
    int64 w, n, d;
    bool valid;

    MixedNumber *mixed;

    valid = sscanf(input, "(%ld+(%ld/%ld))", &w, &n, &d)
            == 3;

    if (!valid)
        ereport(ERROR, (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
             errmsg("invalid input syntax for fraction: \"%s\"", input)));

    if (d == 0) 
        ereport(ERROR, (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("denominator cannot be \"%ld\" in \"%s\"", d, input)));

    mixed    = (MixedNumber *) palloc(sizeof(MixedNumber));

    mixed->wholeNumber           = w;
    mixed->fraction              = (Fraction *) palloc(sizeof(Fraction));
    mixed->fraction->numerator   = n;
    mixed->fraction->denominator = d;

    PG_RETURN_POINTER(mixed);
}

PG_FUNCTION_INFO_V1(mixednumber_out);

Datum
mixednumber_out(PG_FUNCTION_ARGS)
{
    MixedNumber *mixed = (MixedNumber *) PG_GETARG_POINTER(0);

    char *result;

    result = psprintf("(%ld+(%ld/%ld))", 
        mixed->wholeNumber, mixed->fraction->numerator, mixed->fraction->denominator);

    PG_RETURN_CSTRING(result);
}

The Problem is when I retrieve a mixednumber column the value of the fraction part is wrong.. Ex

CREATE TABLE mixednumber_test (val mixednumber);

INSERT INTO mixednumber_test VALUES ('(1+(7/8))'), ('(-1+(-7/8))'), ('(+1+(7/-8))'), ('(0+(-7/-8))'), ('(-0+(+7/8))'), ('(2+(7/+8))'), ('(9+(+7/+8))');

SELECT * FROM mixednumber_test;

result is :..

"(1+(0/0))"
"(-1+(32/4294967952))"
"(1+(94284056329736/16))"
"(0+(94284055669488/128))"
"(0+(0/94284056646312))"
"(2+(524/94284056644432))"
"(9+(94284055669488/16))"

Can anyone experts help me understand what's going on?

Ctx · Accepted Answer · 2017-02-24 12:47:15Z

3

The user data type cannot contain nested structures referenced by a pointer, since the data (especially the Fraction *-pointer inside the MixedNumber structure) is stored "as is" (and not recursively the data which it points to). So, if the pointer is dereferenced after importing a saved value it will probably no longer contain the original values.

You will have to provide all data in a contiguous memory block, perhaps by altering the definition to

typedef struct MixedNumber {
   int64 wholeNumber; 
   Fraction fraction;
} MixedNumber;

and changing the member access accordingly. Also don't forget to enlarge the internal length to now 24 byte (sizeof(MixedNumber)).

answered Feb 24, 2017 at 12:47

Ctx

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