Understanding HDF5's slabs

Question

First of all, forgive my very basic question: I'm going through the tutorials and I've successfully written my first HDF5 dataset. Now, I'm moving to slabs, but I'm finding difficulties in doing this.

As far as I understand, I need to obtain a valid memory space, select a slab, and then write my data. But obviously I'm doing something wrong, since I get errors:

HDF5-DIAG: Error detected in HDF5 (1.8.14) thread 0:
  #000: H5Dio.c line 271 in H5Dwrite(): can't prepare for writing data
    major: Dataset
    minor: Write failed
  #001: H5Dio.c line 352 in H5D__pre_write(): can't write data
    major: Dataset
    minor: Write failed
  #002: H5Dio.c line 690 in H5D__write(): src and dest data spaces have different sizes
    major: Invalid arguments to routine
    minor: Bad value

Obviously I'm trying to write data with wrong dimensions, but I don't know how to correct that. My goal is to create a 20x3 matrix, and set the second row to { 10, 20, 30 }:

 0  0  0
10 20 30
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0
 0  0  0

Can you help me in understanding this issue? My buggy code is below!

Thanks!

#include "hdf5.h"

#define FILE "dset.h5"
#define DSET "/dset"

int main() {

  hid_t        file_id, dataset_id, dataspace_id, filespace, memspace;
  hsize_t      dims[2], offset[2], count[2];
  herr_t       status;

  unsigned int dset_data[3] = { 10, 20, 30 };

  /* Create a new file using default properties. */
  file_id = H5Fcreate(FILE, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);

  /* Create the data space for the dataset. */
  dims[0] = 20; /* ROWS */
  dims[1] = 3;    /* COLS */
  dataspace_id = H5Screate_simple(2, dims, NULL);

  /* Create the dataset. */
  dataset_id = H5Dcreate2(file_id, DSET, H5T_STD_I32BE, dataspace_id,
                          H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);

  /* Get the memory space */
  memspace = H5Dget_space (dataset_id);

  offset[0] = 1; /* ROWS */
  offset[1] = 0; /* COLS */

  count[0]  = 1; /* ROWS */
  count[1]  = 3; /* COLS */

  /* Select the slab */
  status = H5Sselect_hyperslab(memspace, H5S_SELECT_SET,
                               offset, NULL, count, NULL);

  /* Write the dataset. */
  status = H5Dwrite(dataset_id, H5T_NATIVE_INT,
                    memspace, H5S_ALL, H5P_DEFAULT,
                    dset_data);

  /* End access to the dataset and release resources used by it. */
  status = H5Dclose(dataset_id);

  /* Terminate access to the data space. */
  status = H5Sclose(dataspace_id);

  /* Close the file. */
  status = H5Fclose(file_id);
} 

Answer 1

Your data is contiguous in memory (memspace) but it's an hyperslab in the file's space (dataspace_id). So you need to do your selection on dataset_id, not memspace.

And you need to size memspace to contain just enough space for your row. Here I would just create a simple 1D dataspace of 3 elements. Like this the size of the hyperslab (3 elements) and the size in memory will match.