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

http://git.postgresql.org/gitweb?p=postgresql.git;a=blob;f=src/include/utils/array.h;h=7f7e744cb12bc872f628f90dad99dfdf074eb314;hb=master describes Postgres's binary format for arrays. When using libpq, omit the vl_len_ portion. For example, a an array of 4 integers would look like:

0x00000001 0x00000000 0x00000017 0x00000004 0x00000001 0x00000004 0x00000004 0x00000004 0x00000004

This has OID 1007 (INT4ARRAYOID). The first integer is 1 dimension, the second integer is no NULL bitmap (so none of the array's values are NULL), the third integer is the OID of the elements (23, INT4OID), the fourth integer is how big the first dimension is (4), the fifth integer is the starting index of the first dimension. After that is raw array data, in sequential order, each element prefixed by it's length (4 bytes for each integer).

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.