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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2

wgrib2: a sample function


Wgrib2 is "function" driven. Options on the command line corresponds to a function call. When wgrib2 is started, each option-function is initialized by calling it with mode=-1. For each grib record processed, the option-function is called with mode=verbosity index (verbosity ≥ 0). After all grib records have been processed, the option-functions are told to clean up by calling them with mode=-2. Here is a sample function.

#include <stdio.h>
#include "grb2.h"
#include "wgrib2.h"
#include "fnlist.h"

extern int decode;
extern int need_output_file;

* HEADER:100:min:inv:0:print minimum value
* the above line is needed by each command line option
* HEADER:sort-order:type-of-function:number-of-arguments:description

int f_min(int mode, unsigned char **sec, float *data, int ndata, char *inv_out, void **local) {
   double mn;
   int ok, i;

   if (mode == -1) decode = 1; // decode=1 so wgrib2 will unpack grib data
   else {
     mn = ok = 0;
     for (i = 0; i < ndata; i++) {
       if (!UNDEFINED_VAL(data[i])) {
         if (ok) mn = mn < data[i] ? mn : data[i];
         else { ok = 1; mn = data[i]; }
     if (ok) sprintf(inv_out, "min=%lg",mn);
     else sprintf(inv_out, "min=undefined");
   return 0;
The arguments to f_min() are the standard for options that do not require an argument
  mode = -1: initialization  -2: cleanup  O+: processing verbosity index
  sec = sections of the grib (sub) message
  *data = decoded grid point values
  ndata = number of grid points
  *inv_out = string that written to the inventory
  **local = unique pointer, used for "static" variables.

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Page last modified: July 27, 2016
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