snprintf not working as expected with avr-gcc

Question

During a debugging session, I found out that snprintf is not working as expected when compiling the code with avr-gcc. The example code should simply convert the floating point value 3999.9f into its character representation.

Here is a minimal test case:

    int TestSnprintf(void)
    {
       const float inputValue = 3999.9f;
       /* Print with a fixed width of 6 characters (5 numbers and 1 dot).
       The buffer must have a length of 7, because snprintf appends a '\0' at the 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;
   }

Compiling this example code with avr-gcc and running it with Atmel Studio 7 gives a return value of -2. This means snprintf is not working.

What I have tried so far:

I tested the code on 32 and 64 bit Linux and it works as expected (TestSnprintf return the value 0).
I tested the code with Visual Studio 2015 and it worked as expected (TestSnprintf return the 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 is performed on the device using the JTAG interface. I tried the simulator as well, with the same result.
No compiler optimization is activated. The code is compiled and debugged with -O0.

Here is a screenshot from the debugging session, that demonstrates that the return value is -2.

This demonstrates that buf is in scope during debugging:

Question

What am I doing wrong?

SOLUTION

First of all thank you all very much for your help! As pointed out by @manilo the following linker options were missing:

-Wl,-u,vfprintf -lprintf_flt -lm

And the C standard library is not part of gcc! Which library do you use? — too honest for this site, Jun 14 '16 at 15:08
Why the downvote ? The question is well formatted, easy to understand, offending code is included, OP stated what he has tried.. — Jabberwocky, Jun 14 '16 at 15:15
According to this: http://www.nongnu.org/avr-libc/user-manual/group__avr__stdio.html#gaa3b98c0d17b35642c0f3e4649092b9f1 you need `-lm` did you link it in? — fukanchik, Jun 14 '16 at 15:22
@EugeneSh. By running the debugging the code via JTAG directly on the device. I also tried running in the simulator, same result. I tried changing the device, with the same result. — orbitcowboy, Jun 14 '16 at 15:23
@orbitcowboy Are you sure you are breaking in the right place? Do you have any other variable named `buf` around? Can you show the *full* code? — Eugene Sh., Jun 14 '16 at 15:25
BTW, it can be very well optimized away, as your test code is doing nothing... — Eugene Sh., Jun 14 '16 at 15:28
@EugeneSh. Good point, i added a screenshot that demonstrated that the break point is placed correctly. But Olaf told me is should'nt add them. I'll re-add it. — orbitcowboy, Jun 14 '16 at 15:29
Are you sure the `printf` family is implemented? In general on such small targets like AVR, using these heavyweights is not a good idea. Better write custom functions for simple conversions and use them. — too honest for this site, Jun 14 '16 at 15:29
@orbitcowboy Well, Olaf meant that the code should not come in pictures. But if there is some extra info that can be shown by picture only - it should be OK.. — Eugene Sh., Jun 14 '16 at 15:30
@orbitcowboy: I did not tell to remove information. Just there is no need to haver the screenshot! Use **text**! — too honest for this site, Jun 14 '16 at 15:30
@orbitcowboy In the place you are showing `buf` is out of scope. — Eugene Sh., Jun 14 '16 at 15:30
@EugeneSh.: There should be no problem showing the breakpoint in a comment. — too honest for this site, Jun 14 '16 at 15:31
@EugeneSh. I will add another screenshot with the break point at return -2, that demonstrates buf is not out of scope. — orbitcowboy, Jun 14 '16 at 15:33

manlio · Accepted Answer · 2021-04-13T08:17:20.077

11

There are three different implementations of printf() (and friends). The default doesn't implement float output.

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

Also, according to the documentation, you have to add the linker options -Wl,-u,vfprintf (e.g. here).

The sequence of the linker flags is important: -Wl,-u,vfprintf -lprintf_flt -lm

edited Apr 13 '21 at 08:17

answered Jun 14 '16 at 15:32

manlio

18,345
14
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126

score -1 · Answer 2 · answered Jun 14 '16 at 15:34

-1

"Also note that by default the Arduino IDE does not set the AVR linker options to support floating point in the xxprintf() routines. So while that saves quite a bit of code space on the AVR builds, it means that printf() functions cannot be used for floating point output on the AVR. Floating support is included by default for the other processors." http://playground.arduino.cc/Main/Printf

answered Jun 14 '16 at 15:34

jeton

841
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15

I am using Atmel Studio not an Arduino IDE. – orbitcowboy Jun 14 '16 at 15:37

snprintf not working as expected with avr-gcc

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