Return an array from a Postgres C function?

Question

This code compiles without error but it does not return anything. Any ideas on what's missing? #include #include #include

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

Datum my_c_function(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(my_c_function);

Datum my_c_function(PG_FUNCTION_ARGS) {
    float4 var1, var2, var3, var4;
    Datum* vals;
    var1 = PG_GETARG_FLOAT8(0);
    var2 = PG_GETARG_FLOAT8(1);
    var3 = PG_GETARG_FLOAT8(2);
    var4 = PG_GETARG_FLOAT8(3);

    vals = palloc(sizeof(Datum) * 4);

    if (var1 < var4) {
        vals[0] = (int) 1;
        vals[1] = (int) (0.5 + (100 * ((var2 - var4) / (var2 - var3))));
        vals[2] = (int) (0.5 + (100 * ((var4 - var1) / (var2 - var3))));
        vals[3] = (int) (0.5 + (100 * ((var1 - var3) / (var2 - var3))));
    }
    else if (var1 > var4) {
        vals[0] = (int) -1;
        vals[1] = (int) (0.5 + (100 * ((var2 - var1) / (var2 - var3))));
        vals[2] = (int) (0.5 + (100 * ((var1 - var4) / (var2 - var3))));
        vals[3] = (int) (0.5 + (100 * ((var4 - var3) / (var2 - var3))));
    }
    else if (var2 == var3) {
        PG_RETURN_NULL();
    }
    else {
        vals[0] = (int) 0;
        vals[1] = (int) (0.5 + (100 * ((var2 - var4) / (var2 - var3))));
        vals[2] = (int) 0;
        vals[3] = (int) (0.5 + (100 * ((var4 - var3) / (var2 - var3))));
    }

    PG_RETURN_ARRAYTYPE_P(vals);

}

Corrected version:

#include <postgres.h>
#include <fmgr.h>
#include <utils/array.h>
#include <catalog/pg_type.h>

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

Datum cget_bar_structure2(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(cget_bar_structure2);

Datum
cget_bar_structure2(PG_FUNCTION_ARGS) {
    float4   var1, var2, var3, var4;
    Datum    * vals = (Datum*) palloc(sizeof(Datum) * 4);
    ArrayType* result;

    var1 = PG_GETARG_FLOAT8(0);
    var2 = PG_GETARG_FLOAT8(1);
    var3 = PG_GETARG_FLOAT8(2);
    var4 = PG_GETARG_FLOAT8(3);

    if (var1 < var4) {
        vals[0] = Int32GetDatum(1);
        vals[1] = Int32GetDatum((0.5 + (100 * ((var2 - var4) / (var2 - var3)))));
        vals[2] = Int32GetDatum((0.5 + (100 * ((var4 - var1) / (var2 - var3)))));
        vals[3] = Int32GetDatum((0.5 + (100 * ((var1 - var3) / (var2 - var3)))));
    }
    else if (var1 > var4) {
        vals[0] = Int32GetDatum(-1);
        vals[1] = Int32GetDatum((0.5 + (100 * ((var2 - var1) / (var2 - var3)))));
        vals[2] = Int32GetDatum((0.5 + (100 * ((var1 - var4) / (var2 - var3)))));
        vals[3] = Int32GetDatum((0.5 + (100 * ((var4 - var3) / (var2 - var3)))));
    }
    else if (var2 == var3) {
        PG_RETURN_NULL();
    }
    else {
        vals[0] = Int32GetDatum(0);
        vals[1] = Int32GetDatum((0.5 + (100 * ((var2 - var4) / (var2 - var3)))));
        vals[2] = Int32GetDatum(0);
        vals[3] = Int32GetDatum((0.5 + (100 * ((var4 - var3) / (var2 - var3)))));
    }

    result = construct_array(vals, 4, INT4OID, sizeof(int4), true, 'i');

    PG_RETURN_ARRAYTYPE_P(result);

}

"Does not return anything". Huh? Can you show the commands you used to CREATE FUNCTION and what happens when you invoke it? If you SELECT pg_backend_pid(), attach gdb to the backend, break my_c_function then c and run the SQL, does gdb actually stop execution at the start of your function? — Craig Ringer
– Craig Ringer, Commented May 9, 2014 at 6:34

Pavel Stehule · Accepted Answer · 2014-05-09 07:58:11Z

2

The fundamental problem is in fact, so PostgreSQL arrays are not compatible with C arrays.

Good google keyword is a "construct_md_array" or "construct_array"

I found one fragment that should to help

const int *data = array.data(); // C array
Datum *d = (Datum *) palloc(sizeof(Datum) * size);
ArrayType *a;

for (int i = 0; i < size; i++)
     d[i] = Int32GetDatum(data[i]);

a = construct_array(d, size, INT4OID, sizeof(int4), true, 'i');

PG_RETURN_ARRAYTYPE_P(a)

answered May 9, 2014 at 7:58

Pavel Stehule

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2 Comments

Nick Over a year ago

error: ‘INT4OID’ undeclared (first use in this function)

Nick Over a year ago

I added <catalog/pg_type.h> which removed the undeclared error. Works now. Thanks!

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