serial port - Floating point value not being displayed using printf function -

i have been trying display floating point value using printf function using serial port of atmega8 instead of displaying floating point value, '?' character displayed. output is

float: ?

here code

#include <stdio.h> #include <float.h> #include <avr/io.h>  int printchar(char character, file *stream)  {     while(!(ucsra&0x20));     udr=data;     return 0; } file uart_str = fdev_setup_stream(printchar, null, _fdev_setup_rw);  int main(void) {     float fl = 1.3;     stdout = &uart_str;              ucsrb=0x18; // rxen=1, txen=1      ucsrc=0x06; // no parit, 1-bit stop, 8-bit data     ubrrh=0;     ubrrl=71;     //9600 baud rate     while(1)     {                        printf("\r\nfloat: %f",fl);     } } 

by default, minimalistic printf library used, doesn't support floating point numbers , results in "?" placeholder value. have tell linker use floating point library.

for example (from gnu makefile)

printf_lib_min = -wl,-u,vfprintf -lprintf_min 

you have use floating point version so:

printf_lib_float = -wl,-u,vfprintf -lprintf_flt 

depending on base makefile, there should section this:

# if left blank, use standard printf version. printf_lib =  #printf_lib = $(printf_lib_min) #printf_lib = $(printf_lib_float) 

just uncomment line containing printf_lib_float.

edit:
if use avrstudio4 without custom makefile this:

1. use project - configuration
2. select icon ("libraries")
3. highlight "libm.a" , add right panel
4. do same "libprintf_flt.a"
5. under 5th icon "custom"
6. select "[linker]" optins
7. type in "-wl,-u,vfprintf" , [add]