diff --git a/io.cpp b/io.cpp
new file mode 100644
index 0000000..6cacd25
--- /dev/null
+++ b/io.cpp
@@ -0,0 +1,81 @@
+#include "hdf5.h"
+#include <string>
+struct double3 {double x,y,z;};
+
+void h5_write(const std::string file_name, const int step_num, const double t, const int N, const double *m, const double3 *x, const double3 *v, const double *pot, const double3 *acc, const double3 *jrk, const int write_mode=0, const bool use_double_precision=true)
+{
+    char group_name[32], dataset_path[32];
+    bool write_pot = (write_mode   )%2;
+    bool write_acc = (write_mode>>1)%2;
+    bool write_jrk = (write_mode>>2)%2;
+
+    hid_t file_id, group_id, attribute_id, dataset_id, dataspace_id;  /* identifiers */
+    hsize_t dims[2] = {(hsize_t)N, 3};
+    herr_t status;
+
+    hid_t h5_float_type;
+    if (use_double_precision) h5_float_type = H5T_IEEE_F64LE;
+    else h5_float_type = H5T_IEEE_F32LE;
+
+    file_id = H5Fcreate(file_name.c_str(), H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
+    sprintf(group_name, "/Step#%d", step_num);
+    group_id = H5Gcreate2(file_id, group_name, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    dataspace_id = H5Screate(H5S_SCALAR);
+    attribute_id = H5Acreate2 (group_id, "Time", H5T_IEEE_F64LE, dataspace_id, H5P_DEFAULT, H5P_DEFAULT);
+    H5Awrite(attribute_id, H5T_NATIVE_DOUBLE, &t);
+
+    dataspace_id = H5Screate_simple(1, dims, NULL);
+    sprintf(dataset_path, "%s/MASS", group_name);
+    dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, m);
+
+    dataspace_id = H5Screate_simple(2, dims, NULL);
+    sprintf(dataset_path, "%s/X", group_name);
+    dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, x);
+
+    dataspace_id = H5Screate_simple(2, dims, NULL);
+    sprintf(dataset_path, "%s/V", group_name);
+    dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, v);
+
+    if (write_pot) {
+        dataspace_id = H5Screate_simple(1, dims, NULL);
+        sprintf(dataset_path, "%s/POT", group_name);
+        dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+        H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, pot);
+    }
+
+    if (write_acc) {
+        dataspace_id = H5Screate_simple(2, dims, NULL);
+        sprintf(dataset_path, "%s/ACC", group_name);
+        dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+        H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, acc);
+    }
+
+    if (write_jrk) {
+        dataspace_id = H5Screate_simple(2, dims, NULL);
+        sprintf(dataset_path, "%s/JRK", group_name);
+        dataset_id = H5Dcreate2(file_id, dataset_path, h5_float_type, dataspace_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+        H5Dwrite(dataset_id, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, H5P_DEFAULT, jrk);
+    }
+
+    H5Dclose(dataset_id);
+    H5Gclose(group_id);
+    H5Sclose(dataspace_id);
+    H5Fclose(file_id);
+}
+
+int main()
+{
+    double3 aaa;
+    double pot;
+    int N = 25;
+    double3 x[N];
+    for (int i=0; i<N; i++) {
+        x[i].x=i+0.01;
+        x[i].y=i+0.02;
+        x[i].z=i+0.03;
+    }
+    h5_write("new_file.h5", 34, 987.654, N, (double*)x, x, x, (double*)x, x, x, 7);
+}