Command line utility to calculate great circle distance using the spherical law of cosines

greatcircle.c

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

static double getRadians(const char * str, double limit, const char * msg) {
    char * endptr;
    double val = strtod(str, &endptr);
    if(endptr != str && val < limit && val > -limit)
        return val * 0.0174533; // convert to radians
    fprintf(stderr, "Invalid value for %s: %s\n", msg, str);
    exit(1);
}
int main(int argc, char ** argv) {
    if(argc < 5) {
        fprintf(stderr, "Usage: greatcircle lat1 lon1 lat2 lon2\n");
        exit(1);
    }
    double y1 = getRadians(argv[1], 90.0, "start latitude");
    double x1 = getRadians(argv[2], 180.0, "start longitude");
    double y2 = getRadians(argv[3], 90.0, "end latitude");
    double x2 = getRadians(argv[4], 180.0, "end longitude");
    double angle1 = acos(sin(x1) * sin(x2)  + cos(x1) * cos(x2) * cos(y1 - y2));
    double distance = angle1 * 60.0 / 0.0174533 * 1852;
    printf("%f\n", distance); // distance in metres
    exit(0);
}