This series is about code in various languages that will calculate the distance between two latitudes and longitudes. This article is for the C code.

Code courtsey : GeoDataSource

/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::                                                             :*/
/*::  This routine calculates the distance between two           :*/
/*::  points (given the latitude/longitude of those points).     :*/
/*::  It is being used to calculate the distance between two     :*/
/*::  locations using GeoDataSource(TM) Products                 :*/
/*::                                                             :*/
/*::  Definitions:                                               :*/
/*::    South latitudes are negative,                            :*/
/*::    east longitudes are positive                             :*/
/*::                                                             :*/
/*::  Passed to function:                                        :*/
/*::    lat1, lon1 = Latitude and Longitude of point 1           :*/
/*::                 (in decimal degrees)                        :*/
/*::    lat2, lon2 = Latitude and Longitude of point 2           :*/
/*::                 (in decimal degrees)                        :*/
/*::    unit = the unit you desire for results                   :*/
/*::           where: 'M' is statute miles                       :*/
/*::                  'K' is kilometers (default)                :*/
/*::                  'N' is nautical miles                      :*/
/*::  Worldwide cities and other features databases with         :*/
/*::  latitude longitude are available at                        :*/
/*::  http://www.geodatasource.com                               :*/ 
/*::                                                             :*/
/*::  For enquiries, please contact [email protected]      :*/
/*::                                                             :*/
/*::  Official Web site: http://www.geodatasource.com            :*/
/*::                                                             :*/
/*::         GeoDataSource.com (C) All Rights Reserved 2014      :*/
/*::                                                             :*/
/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/

#include 

#define pi 3.14159265358979323846

double distance(double lat1, double lon1, double lat2, double lon2, char unit) 
{
  double theta, dist;
  theta = lon1 - lon2;

  dist = sin(deg2rad(lat1)) * sin(deg2rad(lat2)) + cos(deg2rad(lat1)) * cos(deg2rad(lat2)) * cos(deg2rad(theta));

  dist = acos(dist);
  dist = rad2deg(dist);
  dist = dist * 60 * 1.1515;
  switch(unit) 
 {
    case 'M':
      break;
    case 'K':
      dist = dist * 1.609344;
      break;
    case 'N':
      dist = dist * 0.8684;
      break;
  }
  return (dist);
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts decimal degrees to radians             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double deg2rad(double deg) {
  return (deg * pi / 180);
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts radians to decimal degrees             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double rad2deg(double rad) {
  return (rad * 180 / pi);
}

printf("%f",distance(32.9697, -96.80322, 29.46786, -98.53506, "K"));
puts(" Kilometers");