TI 89 Input routine with comma, backspace and sign

Anyone programming his TI calc in c here`s a fine input routine to enter numbers.Only problem is,that at the end, function atof will not convert the string properly when compiled with TIGCC IDE.
The reason is header #define MIN_AMS 100.
It works fine without it!
took me a week to figure that out!

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);

                         }

code tags added by Hammer

I have a TI 92-plus at school and i know u can program on it but do you know if its possible to program on it in C. Because the functions and commands used in the TI-92 suck (IMO).