PostgreSQL's libpq: Encoding for binary transport of ARRAY[]-data?

Question

after hours of documentations/boards/mailinglists and no progress I may ask you: How do I 'encode' my data to use it for binary transport using libpq's PQexecParams(.) ?

Simple variables are just in big endian order:

PGconn *conn;
PGresult *res;
char *paramValues[1];
int paramLengths[1];
int paramFormats[1];

conn = PQconnectdb(CONNINFO);

// -- (1) -- send a float value
float val_f = 0.12345678901234567890; // float precision: ~7 decimal digits
// alloc some memory & write float (in big endian) into
paramValues[0] = (char *) malloc(sizeof(val_f));
*((uint32_t*) paramValues[0]) = htobe32(*((uint32_t*) &val_f)); // host to big endian

paramLengths[0] = sizeof(val_f);
paramFormats[0] = 1; // binary

res = PQexecParams(conn, "SELECT $1::real ;", //
        1, // number parameters
        NULL, // let the backend deduce param type
        paramValues, //
        paramLengths, //
        paramFormats, //
        0); // return text
printf("sent float: %s \n", PQgetvalue(res, 0, 0));
// --> sent float: 0.123457

and like this also double, int, etc ...

But how about ARRAYs?

    float vals_f[] = {1.23, 9.87};
    // alloc some memory
    paramValues[0] = (char *) malloc(sizeof(float) * 2);

//  ???? paramValues[0] = ??????

    paramLengths[0] = sizeof(float) * 2;
    paramFormats[0] = 1; // binary


    res = PQexecParams(conn, "SELECT $1::real[] ;", //
            1, // number parameters
            NULL, // let the backend deduce param type
            paramValues, //
            paramLengths, //
            paramFormats, //
            0); // return text
    printf("sent float array: %s \n", PQgetvalue(res, 0, 0));

Is there any working example of transfering ARRAY data in PostgreSQL's binary format? The code in backend/utils/adt/ doesn't help me much (except I now know there is a ARRAYTYPE, but not how to use them) :-(

I just need a function char* to_PQbin(float [] input, int length) for passing to paramValues[.] ...

Thanks a lot, Tebas

PS: What is the suggested way of converting simple variables (rather than my htobe32(.))?

Answer 1

As ccuter already mentioned, you need to create your own API. The following code extracts a 1-dimensional array of int4's ignoring any NULL values.

#define   INT4OID   23

/*! Structure of array header to determine array type */
struct array_int4 {
  int32_t ndim; /* Number of dimensions */
  int32_t _ign; /* offset for data, removed by libpq */
  Oid elemtype; /* type of element in the array */

  /* First dimension */
  int32_t size; /* Number of elements */
  int32_t index; /* Index of first element */
  int32_t first_value; /* Beginning of integer data */
};

static int extract_int4_array (char *raw_array, 
                               int32_t **values, 
                               int *num_values) {
  /* Array information header */
  struct array_int4 *array = (struct array_int4 *) raw_array; 
  /* Pointer to traverse int array */
  int32_t *p_value = &(array->first_value);
  /* int value in host byte order */
  int32_t hval;

  /* Check if we have a 1-dimensional INT4 array */
  if (ntohl(array->ndim) != 1 
  || ntohl(array->elemtype) != INT4OID) {
    return -1;
  }
  /* Number of elements including NULLs */
  int array_elements = ntohl (array->size);

  *num_values = 0;
  /* Get size of array */
  for (int i=0; i<array_elements; ++i) {
    /* Check size to see if this is a NULL value */
    hval = ntohl (*p_value);
    if (hval != -1) {
      ++p_value;
      (*num_values) += 1;
    } 

    ++p_value;
  }
  *values = malloc (*num_values * sizeof **values);

  /* Fill output int array. Skip every other value as it contains the size of 
   * the element */
  *num_values = 0; /* Use num_values as the index of the output array */
  p_value = &(array->first_value);
  for (int i=0; i<array_elements; ++i) {
    /* Check size to see if this is a NULL value */
    hval = ntohl (*p_value);
    if (hval != -1) {
      ++p_value;
  (*values)[*num_values] = ntohl (*p_value);
      (*num_values) += 1;
    } 

    ++p_value;
  }

  return 0;
}

There also appears to be a library named libpqtypes which helps for this kind of conversion.