Examples for Extracting Time from RAF netCDF files

The C Standard Library contains a function called strptime(3). This function can easily decode the "units" field below into a "struct tm" and the examples below demonstrate this.

For descriptions of strptime and "struct tm", see:

• man strptime
• man ctime

Example netCDF Time variable declaration

        int Time(Time) ;
                Time:long_name = "time of measurement" ;
                Time:standard_name = "time" ;
                Time:units = "seconds since 2001-09-20 14:38:33 +0000" ;
                Time:strptime_format = "seconds since %F %T %z" ;

Test C Program

#include ‹netcdf.h›
#include ‹time.h›

int main(int argc, char *argv[])
{
  int	ncid, varID, nDims, dimids[3];
  int	i, total, len;
  int	*fp;
  time_t  StartFlightUnixTime;

  nc_open(argv[1], NC_NOWRITE, &ncid);

  StartFlightUnixTime = InitFlightTime(ncid);

  nc_inq_varid(ncid, "Time", &varID);
  nc_inq_varndims(ncid, varID, &nDims);
  nc_inq_vardimid(ncid, varID, dimids);

  total = 1;
  for (i = 0; i ‹ nDims; ++i)
  {
     nc_inq_dimlen(ncid, dimids[i], &len);
     total *= len;
  }

  fp = malloc(total * sizeof(int));
  nc_get_var_int(ncid, varID, fp);

  for (i = 0; i ‹ total; ++i)
  {
    printf("%5d %04d/%02d/%02d %02d:%02d:%02d\n", fp[i],
	GetFlightYear(fp[i]),
	GetFlightMonth(fp[i]),
	GetFlightDay(fp[i]),
	GetFlightHour(fp[i]),
	GetFlightMinute(fp[i]),
	GetFlightSecond(fp[i]));
  }

  nc_close(ncid);
}

Example C Routines (raf-time.c)

#include ‹netcdf.h›
#include ‹time.h›
#include ‹stdlib.h›
#include ‹stdio.h›

/* Given an open netCDF file (ncid), return the Unix time (seconds since
 * 01/01/1970) of Time::units.  Subsequent values from 'Time' should be added
 * to this return value.
 */

static time_t start_t = 0;

time_t InitFlightTime(int ncid)
{
  int id;

  putenv("TZ=UTC");	// Force all time routines to work in UTC.

  if (nc_inq_varid(ncid, "Time", &id) == NC_NOERR)
  {
    struct tm StartFlight;
    char t_units[128], units_frmt[128];

    nc_get_att_text(ncid, id, "units", t_units);
    nc_get_att_text(ncid, id, "strptime_format", units_frmt);

    strptime(t_units, units_frmt, &StartFlight);
    start_t = mktime(&StartFlight);
  }
  else
  {
    fprintf(stderr, "No 'Time' variable, this is fatal.\n");
    exit(1);
  }

  return start_t;
}

int GetFlightYear(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_year + 1900;
}

int GetFlightMonth(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_mon + 1;
}

int GetFlightDay(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_mday;
}

int GetFlightHour(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_hour;
}

int GetFlightMinute(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_min;
}

int GetFlightSecond(int currentTimeOffset)
{
  time_t x = start_t + currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_sec;
}

C Routines to compile with Fortran Programs

#include ‹stdio.h›
#include ‹string.h›
#include ‹time.h›

static const char * default_format = "seconds since %F %T %z";

static time_t start_t = 0;

/*
 * Call this function first with the flight start date and time.  This
 * can be found as the "units" attribute in the "Time" variable.
 */
void init_flight_date__(char * start, char * format)
{
  struct tm start_tm;

  putenv("TZ=UTC");	/* Perform all time calculations in/as UTC. */

start[39] = '\0';
format[22] = '\0';
printf("[%s] [%s]\n", start, format);

  if (format == 0 || strlen(format) == 0)
    strptime(start, default_format, &start_tm);
  else
    strptime(start, format, &start_tm);

  start_t = mktime(&start_tm);
}


int get_flight_year__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_year + 1900;
}

int get_flight_month__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_mon + 1;
}

int get_flight_day__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_mday;
}

int get_flight_hour__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_hour;
}

int get_flight_minute__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_min;
}

int get_flight_second__(int * currentTimeOffset)
{
  time_t x = start_t + *currentTimeOffset;
  struct tm * stm = gmtime(&x);
  return stm-›tm_sec;
}