#include <stdlib.h>
    double atof(const char *str);

       The atof() function converts the initial portion of the string pointed to by
       nptr to double.  The behavior is the same as
 
           strtod(nptr, (char **) NULL);
 
       except that atof() does not detect errors.

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <stdlib.h>
 
int main(void){
 
    char string[]="1234";
    float i;
 
printf("string = %s \n", string );
printf("int = %f \n", atof(string) );
 
i = atof(string);
i = i + i;
printf("i + i = %f\n", i);
 
return 0;
}

  Output:
  user@host:~$ ./atof 
  string = 1234 
  int = 1234.000000 
  i + i = 2468.000000