c - snprintf not working as expected with avr-gcc -

during debugging session, found out snprintf not working expected when comiling code avr-gcc. example code should convert floating point value 3999.9f character representation.

here minimal test case:

    int testsnprintf(void)     {        const float inputvalue = 3999.9f;        /* print fixed width of 6 characters (5 numbers , 1 dot).        buffer must have length of 7, because snprintf appends '\0' @ end. */        char buf[7u] = {0, 0, 0, 0, 0, 0, 0};        const uint8_t buffersize = 7u;        if(6 != snprintf(buf, buffersize, "%06.1f", inputvalue))        {           return -1;        }        if( buf[0] != '3'             || buf[1] != '9'             || buf[2] != '9'             || buf[3] != '9'             || buf[4] != '.'             || buf[5] != '9'             || buf[6] != '\0')       {           return -2;       }        return 0;    }     int main(void)    {      int retval = testsnprintf();      return 0;    } 

compling example code avr-gcc , running atmel studio 7 gives return value of -2. means snprintf not working.

what if have have tried far?

• i tested code on 32 , 64 linux , works expected (testsnprintf return value 0).
• i tested code visual studio 2015 , worked expected (testsnprintf return value 0).

• the content of buf is

  buf[0] = 32;     buf[1] = 32;     buf[2] = 32;     buf[3] = 32;     buf[4] = 32;     buf[5] = 63;     buf[6] = 0; 

• the testing performed on device using jtag interface. tried simulator well, same result.

• no compiler optimization activated. code compiled , debugged -o0.

here screenshot debugging session, demonstrates return value -2.

this demonstrates buf in scope during debugging:

question

what doing wrong?

solution

first of thank help! pointed out @manilo following linker options missing:

-wl,-u,vfprintf -lprintf_flt -lm 

there 3 different implementations of printf() (and friends). default doesn't implement float output.

snprintf won't work without linking libprintf_flt.a (-lprintf_flt) , libm.a (-lm).

also, according documentation, have add linker options -wl,-u,vfprintf (e.g. http://winavr.scienceprog.com/avr-gcc-tutorial/using-sprintf-function-for-float-numbers-in-avr-gcc.html).

the sequence of linker flags important: -wl,-u,vfprintf -lprintf_flt -lm