Recipe: C program to read data from downloaded file

Back to G5NR Data Access Guide.

Problem

We want to read a downloaded data file using C.

Solution

For the purpose of this example, we assume that the file c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4 has already been downloaded from the ftp server. For more information about file naming conventions, and how to download a file from the ftp server, please follow the links in the #See Also section. Here we use the NetCDF-4 library to read the downloaded file (since NetCDF-4 uses HDF-5 as the underlying data format, HDF-5 library can also be used directly). First, we ensure that out NetCDF-4 library has been built with HDF5 support. If the query

> nc-config --has-hdf5

returns "yes", we have a compatible NetCDF-4 library. Here nc-config is a utility bundled with NetCDF-4 package.

#include<stdio.h>
#include<stdlib.h>
#include<netcdf.h>  // for reading NR files

/* Handle errors by printing an error message and exiting with a
 * non-zero status. */
#define ERRCODE 2
#define ERR(e) {printf("Error: %s\n", nc_strerror(e)); exit(ERRCODE);}

int main(void){
  // file name
  char* T_file = "http://opendap.nccs.nasa.gov:9090/dods/OSSE/GEOS-5.12/BETA9/0.5000_deg/inst/inst01hr_3d_T_Cv";
  
  // netCDF ID for the file and data variable
  int ncid, varid;
 
  // global 4D array: (time,lev,lat,lon), one time step
  const int IM = 720;
  const int JM = 361;
  const int LM = 72;
  const int asyz = 1*LM*JM*IM;
  float *T = NULL;

  // hypercube for reading one array for a given time
  size_t start[4] = {36, 0, 0, 0}; // time step 37
  size_t count[4] = {1, LM, JM, IM}; // 1 time step, 3D (lon,lat,lev) array

  // return code
  int rc;

  // min/max values
  float minval, maxval;

  // misc counters
  int ctr;

  // allocate memory for T
  T = malloc(asyz*sizeof(float));

  // read the data file
  printf("Reading T..."); fflush(stdout);
  if (rc = nc_open(T_file, NC_NOWRITE, &ncid)) ERR(rc);
  if (rc = nc_inq_varid(ncid, "t", &varid)) ERR(rc);
  if (rc = nc_get_vara_float(ncid, varid, start, count, T)) ERR(rc);
  printf("done.\n"); fflush(stdout);

  // min/max of T
  minval = 1.0e15;
  maxval = -1.0e15;
  for (ctr=0; ctr<asyz; ctr++){
    if (T[ctr]<minval){
      minval = T[ctr];
    }
    if (T[ctr]>maxval){
      maxval = T[ctr];
    }
  }
  printf("min(T): %f\n", minval);
  printf("max(T): %f\n", maxval);

  // free memory
  free(T);
  
  return 0;
}