#include <stdlib.h>
    double strtod(const char *str, char **endptr);

strtod converts a string into a double variable

/* 
 * strtod example code
 * http://code-reference.com/c/stdlib.h/strtod
 */
#include <stdio.h>
#include <stdlib.h>
 
int main ( void )
{
  char string[] ="2040 strtod string 5";
  char *endptr;
  double number;
 
  number = strtod(string, &endptr);
  printf("String is %s\n",string);
  printf("Double is %f\n", number);
 
  return 0;
}

  user@host:~$  ./strtod 
  String is 2040 strtod string 5
  Double is 2040.000000