string to float,with sign,comma,backspace

I`m using the following code and it works fine till the last line.atof should convert a string to float.It works fine with atoi(ansi to int).With the atof it turns out weird numbers in the range of thousands. What am I doing wrong? I`d appreciate any help.
code:

Code:

void InputStr(char *buffer, unsigned short maxlen)
{
  SCR_STATE ss1;
  signed short key;
  unsigned short i = 0;
  buffer[0] = 0;
  SaveScrState (&ss1);  
 do{	 
    MoveTo (ss1.CurX, ss1.CurY);       
     SaveScrState (&ss1);
      if ( i==3 ) {printf ("%s  ", buffer);}    
       else                                                 printf ("%s_  ", buffer);             
     key = ngetchx ();                          
     if (key >= '0' && key <= '9' && i < maxlen)                                            //48-57 
      buffer[i++] = key;                        
      else if (key == KEY_BACKSPACE )   i--;                                                  //257 
     else if (key==173 && i==0) buffer[i++] = key;                                              //(-) 
     else if (key==46)buffer[i++] = key;   //comma
     buffer[i] = 0;                               
   } while (key != KEY_ENTER);

      Input = atof(buffer);//buffer is string      
}

[code][/code]tagged by Salem

You don't like whitespaces or readability in general isn't it?

Anyway, I assume you have included stdlib.h. What type is Input, it should be double. I see a lot of functions I don't know about, but it seems I don't have to care about that.

Actually I have nothing against white spaces and I am concerned about readability but it looks like that I`m not very good at it.Input is float type, and her`s the eintire code:Its for my TI calculator.

Code:
#define USE_TI89 // Compile for TI-89
#define USE_TI92PLUS // Compile for TI-92 Plus
#define USE_V200 // Compile for V200

#define MIN_AMS 100 // Compile for AMS 1.00 or higher

#define SAVE_SCREEN // Save/Restore LCD Contents

#include <tigcclib.h> // Include All Header Files
/**************Glob_Var*************************/
float Input; // converted enters tring -any input
/****************SubRoutines*********************** *********/
void InputStr(char *buffer, unsigned short maxlen);

/****************VoidInputstring******************* *********/
void InputStr(char *buffer, unsigned short maxlen)
{
SCR_STATE ss1;
signed short key;
unsigned short i = 0;
buffer[0] = 0;

SaveScrState (&ss1);
do{

MoveTo (ss1.CurX, ss1.CurY);
SaveScrState (&ss1);

if ( i==3 ) {printf ("%s ", buffer);}
else
printf ("%s_ ", buffer);
key = ngetchx ();
if (key >= '0' && key <= '9' && i < maxlen)
buffer[i++] = key;
else if (key == KEY_BACKSPACE ) i--;
else if (key==173 && i==0) buffer[i++] = key;//(-)
else if (key==46)buffer[i++] = key; //comma
buffer[i] = 0;

} while (key != KEY_ENTER);
Input = atof(buffer);


}

void _main(void)
{

int key;
char str[7];
do
{
clrscr ();
InputStr (str,7);
printf("\nconverted float=%3.3f",Input);
key=ngetchx();

}while (key!=KEY_ESC);

}