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Information in this article applies to:

  • C51 All Versions


I have a problem with the following application code that uses the scanf function and a customized version of the getkey function.

#include <reg52.h>
#include <stdio.h>

void Setup (void) {
    SCON  = 0x50;  // SCON: mode 1, 8-bit UART, enable rcvr
    TMOD |= 0x20;  // TMOD: timer 1, mode 2, 8-bit reload
    TH1   = 221;   // TH1:  reload value for 1200 baud @ 16Mz
    TR1   = 1;     // TR1:  timer 1 run
    TI    = 1;     // TI:   set TI to send first char of UART

char _getkey(void) {
   char c;
   while (!RI);
   c = SBUF;
   RI = 0;

void main(void) {
   float flt;

   Setup(); //Initialize serial port
   while(1)  {
      printf("\r\nEnter a Number (try 123.45): ");
      scanf("%f",&flt);    // scanf fails here

When using the '%f' format parameter, the scanf function fails at OPTIMIZE level 2 and level 3. What is the reason for this problem?


The scanf function does not use a temporary buffer when processing input. Instead, it directly processes the input returned by the getkey function.

Unfortunately, there is a problem with the floating-point processing buffer that only appears when the getkey function requires overlayable data. This problem also occurs when getkey is compiled with OPTIMIZE level 2 or level 3 (no CPU register optimization is performed at these optimizer levels).


This problem may be solved by any of the following:

  • Use the CPU register optimization of the C51 Compiler (OPTIMIZE level 4 or higher) so that getkey does not require overlayable data.
  • Disable data overlaying for the getkey function using the Linker/Locater OVERLAY directive with the parameter:
    OVERLAY (* ! _getkey)
  • Use an input buffer and process the input with sscanf instead.


This problem will not be corrected.

Last Reviewed: Friday, June 17, 2005

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