how to convert float to string correctly in avr c [duplicate]

Question

I need to convert a float value to a string in my avr project. Therefore I tried it with the following code:

char buf[10];
float emg1 = 33.42;
sprintf(buf, "%f", emg1);
uart_puts(buf);

But the output is only a "?". I tried to change the char format from %f to %g but I only got "?".

Is there another way to simply convert float to string, or can somebody tell me where the mistake is?