an elementary question concerning double indirection

[Robert G Kaires]

Dear C users:

Thanks to all who answered my question entitled "A very elementary
question concerning indirection". Your answers were most helpful (and
plentiful). Sorry I could answer each response individually.
This next question is maybe not so elementary (hence I left off the
"very"). I am trying to write a code fragment which gets a floating
point number from the user and checks to see if it contains any illegal
characters. The atof() function can change the input string to a
floating point number and stops converting at the first illegal
character. Searching through my Turbo C manual (2.0), I find the
function strtod() which seems to be better suited for my purpose. This
function not only stops at the first illegal character but also returns
a pointer (or is it a pointer to a pointer?). I don't really understand
why this function has the syntax:
             double strtod(const char *s, char **endptr);

Why is endptr a pointer to a pointer (and not just a pointer)?
Unfortunately the description of double indirection is poorly explained
in my manuals, and no description of how to use this function is given.
However, I plunged ahead and tried to use it anyway. Here is my short 
program:

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

main()
{
char string[30]={""};
char **ptr;
double ans;
clrscr();
while(1)
   {
   gets(string);
   if (*string == 'q') break;
   ans=strtod(string,ptr);
   if ( ( string+strlen(string) ) != *ptr )   /* <--- warning here  */
      printf("format error\n");
   else
      printf("You typed the number: %f\n",ans);
   }
}
This program seems to work OK (try it). However I do get warnings on the
line indicated that "I am trying to use ptr before it is defined". Even
worst, when I use this fragment in a much larger program, all hell
breaks loose. It seems that I am writing over sections of memory that
belong to other varaiables. Please tell me what is wrong (without too
much scolding PLEASE, I am still a beginner).
Thanks one and all.
Bob Kaires