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* Copyright (C) 2012, Northwestern University and Argonne National Laboratory
* See COPYRIGHT notice in top-level directory.
/* $Id$ */
/* simple demonstration of pnetcdf
* text attribute on dataset
* rank 0 reads into 1-d array, broadcasts to all. This is a dumb way
* to do parallel I/O but folks do this sometimes...
* This program reads the files generated from its counterpart program
* pnetcdf-write-nfiles.c. See comments in pnetcdf-write-nfiles.c for
* the contents of the netCDF files.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mpi.h>
#include <pnetcdf.h>
static void handle_error(int status, int lineno)
fprintf(stderr, "Error at line %d: %s\n", lineno, ncmpi_strerror(status));
#define DSET_NAME_LEN 1024
int main(int argc, char **argv) {
int i, j, rank, nprocs, ret;
int ncfile, ndims, nvars, ngatts, unlimited;
int var_ndims, var_natts;;
MPI_Offset *dim_sizes, var_size;
MPI_Offset *count;
char filename[DSET_NAME_LEN];
char basename[256];
char varname[NC_MAX_NAME+1];
int *dimids=NULL;
nc_type type;
int *data=NULL;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
if (argc > 2) {
if (rank == 0) printf("Usage: %s file_base_name\n", argv[0]);
if (argc > 1) snprintf(basename, 256, "%s", argv[1]);
else strcpy(basename, "testfile");
/* the most significant challenge with the "one file per processor"
* approach is the challenge in reading back on a different number of
* processors. For example, 4k processors output data during a simulation.
* Later on, 100 processors do analysis or visualization of the data.
* Stitching together the many smaller files into a form usable by other
* programs poses a challenge */
ret = snprintf(filename, DSET_NAME_LEN, "", basename, rank, nprocs);
if (ret >= DSET_NAME_LEN) {
fprintf(stderr, "name too long \n");
ret = ncmpi_open(MPI_COMM_SELF, filename, NC_NOWRITE, MPI_INFO_NULL, &ncfile);
if (ret != NC_NOERR) handle_error(ret, __LINE__);
/* reader knows nothing about dataset, but we can interrogate with query
* routines: ncmpi_inq tells us how many of each kind of "thing"
* (dimension, variable, attribute) we will find in the file */
/* In the "one file per processor case" all processors open a file and
* interrogate it */
ret = ncmpi_inq(ncfile, &ndims, &nvars, &ngatts, &unlimited);
if (ret != NC_NOERR) handle_error(ret, __LINE__);
/* we do not really need the name of the dimension or the variable for
* reading in this example. we could, in a different example, take the
* name of a variable on the command line and read just that one */
dim_sizes = (MPI_Offset*) calloc(ndims, sizeof(MPI_Offset));
/* netcdf dimension identifiers are allocated sequentially starting
* at zero; same for variable identifiers */
for (i=0; i<ndims; i++) {
ret = ncmpi_inq_dimlen(ncfile, i, &(dim_sizes[i]) );
if (ret != NC_NOERR) handle_error(ret, __LINE__);
for (i=0; i<nvars; i++) {
/* obtain the number of dimensions of variable i, so we can allocate
* the dimids array */
ret = ncmpi_inq_varndims(ncfile, i, &var_ndims);
if (ret != NC_NOERR) handle_error(ret, __LINE__);
dimids = (int*) malloc(var_ndims * sizeof(int));
/* much less coordination in this case compared to rank 0 doing all
* the i/o: everyone already has the necessary information */
ret = ncmpi_inq_var(ncfile, i, varname, &type, &var_ndims, dimids,
if (ret != NC_NOERR) handle_error(ret, __LINE__);
count = (MPI_Offset*) calloc(var_ndims, sizeof(MPI_Offset));
/* as long as the number of readers is identical to the number of
* writers, we can simply read entire variables back */
count[0] = dim_sizes[dimids[0]];
var_size = count[0];
for (j=1; j<var_ndims; j++) {
count[j] = dim_sizes[dimids[j]];
var_size *= count[j];
switch(type) {
case NC_INT:
data = (int*) calloc(var_size, sizeof(int));
ret = ncmpi_get_var_int_all(ncfile, i, data);
if (ret != NC_NOERR) handle_error(ret, __LINE__);
/* we can do this for all the known netcdf types but this
* example is already getting too long */
fprintf(stderr, "unsupported NetCDF type \n");
ret = ncmpi_close(ncfile);
if (ret != NC_NOERR) handle_error(ret, __LINE__);
return 0;
*vim: ts=8 sts=4 sw=4 noexpandtab */
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