To use parallel access, open or create the file with nc_open_par (see nc_open_par) or nc_create_par (see nc_create_par).
The mode flag NC_PNETCDF will be automatically turned on for classic or 64-bit offset files opened or created with the parallel access functions.
The following example shows the creation of a file using parallel access with a netCDF-4/HDF5 file, and how a program might write data to such a file.
#include "netcdf.h" #include <mpi.h> #include <assert.h> #include "hdf5.h" #include <string.h> #include <stdlib.h> #define BAIL(e) do { \ printf("Bailing out in file %s, line %d, error:%s.\n", __FILE__, __LINE__, nc_strerror(e)); \ return e; \ } while (0) #define FILE "test_par.nc" #define NDIMS 2 #define DIMSIZE 24 #define QTR_DATA (DIMSIZE*DIMSIZE/4) #define NUM_PROC 4 int main(int argc, char **argv) { /* MPI stuff. */ int mpi_namelen; char mpi_name[MPI_MAX_PROCESSOR_NAME]; int mpi_size, mpi_rank; MPI_Comm comm = MPI_COMM_WORLD; MPI_Info info = MPI_INFO_NULL; /* Netcdf-4 stuff. */ int ncid, v1id, dimids[NDIMS]; size_t start[NDIMS], count[NDIMS]; int data[DIMSIZE*DIMSIZE], j, i, res; /* Initialize MPI. */ MPI_Init(&argc,&argv); MPI_Comm_size(MPI_COMM_WORLD, &mpi_size); MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank); MPI_Get_processor_name(mpi_name, &mpi_namelen); printf("mpi_name: %s size: %d rank: %d\n", mpi_name, mpi_size, mpi_rank); /* Create a parallel netcdf-4 file. */ if ((res = nc_create_par(FILE, NC_NETCDF4|NC_MPIIO, comm, info, &ncid))) BAIL(res); /* Create two dimensions. */ if ((res = nc_def_dim(ncid, "d1", DIMSIZE, dimids))) BAIL(res); if ((res = nc_def_dim(ncid, "d2", DIMSIZE, &dimids[1]))) BAIL(res); /* Create one var. */ if ((res = nc_def_var(ncid, "v1", NC_INT, NDIMS, dimids, &v1id))) BAIL(res); if ((res = nc_enddef(ncid))) BAIL(res); /* Set up slab for this process. */ start[0] = mpi_rank * DIMSIZE/mpi_size; start[1] = 0; count[0] = DIMSIZE/mpi_size; count[1] = DIMSIZE; printf("mpi_rank=%d start[0]=%d start[1]=%d count[0]=%d count[1]=%d\n", mpi_rank, start[0], start[1], count[0], count[1]); /* Create phony data. We're going to write a 24x24 array of ints, in 4 sets of 144. */ printf("mpi_rank*QTR_DATA=%d (mpi_rank+1)*QTR_DATA-1=%d\n", mpi_rank*QTR_DATA, (mpi_rank+1)*QTR_DATA); for (i=mpi_rank*QTR_DATA; i<(mpi_rank+1)*QTR_DATA; i++) data[i] = mpi_rank; /*if ((res = nc_var_par_access(ncid, v1id, NC_COLLECTIVE))) BAIL(res);*/ if ((res = nc_var_par_access(ncid, v1id, NC_INDEPENDENT))) BAIL(res); /* Write slabs of phony data. */ if ((res = nc_put_vara_int(ncid, v1id, start, count, &data[mpi_rank*QTR_DATA]))) BAIL(res); /* Close the netcdf file. */ if ((res = nc_close(ncid))) BAIL(res); /* Shut down MPI. */ MPI_Finalize(); return 0; }