// Subdomain data
int L; // number of points in the subdomain
int G; // number of global points  G = \sum L for all subdomains
vector<double> u(L), v(L), w(L);
vector<int> global_indices(L);


// MPI datatype for single field in memory
MPI_Datatype memory_t;
MPI_Type_contiguous( L, MPI_DOUBLE, &memory_t );

// MPI datatype for single field in file
MPI_Datatype file_t;
MPI_Type_create_indexed_block(
 L, //  count
 1, //  blocklength
 &global_indices[0], //  array_of_displacements
 MPI_DOUBLE,
 &file_t
);


//  +--------------+
//  |   Option 1   |
//  +--------------+
//   Repeated Write_all for each field

/* file open etc. */

// Set view and output for each fields in order

MPI_File_set_view( fh, 0, MPI_DOUBLE, file_t, "native", MPI_INFO_NULL    );
MPI_File_write_all(   fh, &u[0], 1, memory_t, MPI_STATUS_IGNORE  );
MPI_File_set_view( fh, G, MPI_DOUBLE, file_t, "native", MPI_INFO_NULL    );
MPI_File_write_all(   fh, &v[0], 1, memory_t, MPI_STATUS_IGNORE  );
MPI_File_set_view( fh, 2*G, MPI_DOUBLE, file_t, "native", MPI_INFO_NULL  );
MPI_File_write_all(   fh, &w[0], 1, memory_t, MPI_STATUS_IGNORE  );


//  +--------------+
//  |   Option 2   |
//  +--------------+
//  Single shot collective I/O for all 3 fields

// Memory type aggregating all fields
MPI_Datatype all_memory_t;
int blocklengths[3];
MPI_Aint disps[3];
disps[0] = MPI_Get_address( &u[0], &disps[0] );
disps[1] = MPI_Get_address( &v[0], &disps[1] );
disps[2] = MPI_Get_address( &w[0], &disps[2] );

MPI_Type_create_hindexed(
  3,
  blocklengths,
  disps,
  memory_t,
  &all_memory_t
);

// file type
int file_disps[] = { 0, G, 2*G };
MPI_Datatype all_file_t;
MPI_Type_create_indexed_block(
 3,
 1,
 file_disps,
 file_t,
 &all_file_t
);


/* file open etc. */
// Set one shot view
MPI_File_set_view( fh, 0, MPI_DOUBLE, all_file_t, "native", MPI_INFO_NULL );
// Output all at once
MPI_File_write_all( fh, MPI_BOTTOM, 1, all_memory_t, MPI_STATUS_IGNORE);