wgrib2: Calling wgrib2 from C¶

Introduction¶

There 3 basic ways to call wgrib2 from a C program. The simpler way is to call wgrib2 and when wgrib2 finishes execution, control is returned to the calling program. A more complicated way to run wgrib2 in another process and communicate between the two processes.

Simple: system call¶

Calling wgrib2 from C can be as simple as calling the system function

    system("wgrib2 IN.grb -match ":TMP:2 m above ground:anl:" -csv tmp2m.csv");

This approach is simple but there is overhead of initializing and running wgrib2 as well as opening and closing of the files for every call to wgrib2. All the communication with wgrib2 is through the file system.

Wgrib2 as a subroutine, CW2¶

Wgrib2 v2.0.2 introduced the ability to call wgrib2 as a subroutine (CW2). This avoids the overhead of loading wgrib2 into memory every time you run wgrib2. Time is also saved by keeping the files open between calls. Memory files were introduced to wgrib2, so that the calling program can encode and decode buffers and avoid using the disk to communicate with wgrib2.

    int argc;
    char argv[MAX_ARGC][MAX_STRLEN];
    ...
    ierr = wgrib2(argc, argv);

More information is given in callable_wgrib2. This facility was used to create a fortran api to read and write grib2.

Wgrib2 as a Process (Filter)¶

A program can run wgrib2 as a process and communicate to wgrib2 by a pipe. In this example, the main program has grib data and it wants the data regridded or converted into netcdf.

#include <stdio.h>
#include <stdlib.h>

/*
 * how to call wgrib2 from a C program.
 *
 * in this example, the main program writes grib data to a pipe,
 * wgrib2 reads from the pipe, regrids the data and saves in a file.
 *
 * This example shows how to call wgrib2 from a C program
 *
 */

#define INPUT_FILE  "../examples/gep19.t00z.pgrb2af180"


int main() {

   FILE  *input, *wgrib2_input;
   int c,err;

   /* open a file with "test" data */
   input = fopen(INPUT_FILE,"rb");
   if (input == NULL) exit(1);

   /* run wgrib2 to read from stdin and interpolate to a new grid */

   wgrib2_input = popen("wgrib2 - -new_grid_winds earth -new_grid ncep grid 221 out.221.grb","w");
   if (wgrib2_input == NULL) exit(2);

   while ( (c = fgetc(input))  != EOF) {
    fputc(c, wgrib2_input);
   }

   /* close the wgrib2 program */
   err=pclose(wgrib2_input);
   printf("all done, err=%d\n",err);

   return 0;
}

A grib encoder was developed using wgrib2 as a process. The main program would write data to a pipe and wgrib2 would read the pipe and write a grib file. The advantage of this approach is the the writing is multitasking. The disadvantage is that it was difficult to develop a grib reader using this approach. Eventually CW2 was developed to solve the grib reader problem and CW2 became the appropriate approach for writing grib.

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Description: calling wgrib2 from C

Docs derived from https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/calling_wgrib2.html