You comment that what you provided is skeleton code .. would it be possible to see more details related to your actual operational code? Thanks in advance for any help you might be able to provide.

xaxis->direct_step_pin = _STEP_X;
yaxis->direct_step_pin = _STEP_Y;
zaxis->direct_step_pin = _STEP_Z;

For the arduino, you can specify a pin by a number. but that actually maps to a port and pin number. The default code implies that port B pin 3 is what it is physically connected to it. The arduino library adds a lot of overhead to the code to make it simple to use. by accessing the pin directly, it is much faster for a time critical application.

STDOUT by default goes to whatever function you configure with fdevopen(). If nothing is configured then it gets dropped.

Regards.

I successfully got an Arduino sketch to build with floating point support linked into stdio. But it didn’t generate any serial I/O and I settled for sprintf-ing my formatted data to a buffer and then serial.print-ing the resulting string.
(Btw, where does STDOUT go by default in the Arduino/avr-Libc environment?)

The Arduino build process isn’t as obscure as it first appears. The IDE just doesn’t provide a convenient way to modify it.

Under File->Preferences, turn on “Verbose” output for both “compilation” and “upload”.
Now when you build and upload your sketch, the text in the status window will contain the command lines used to build and upload the program.
You can copy and paste from the window, extract the relevant lines, and modify them to make your own custom build script.

The build process goes like this:
1. avr-gcc to build a SketchName.cpp.elf object file.
2. avr-objcopy builds a SketchName.cpp.eep file from the .elf file.
3. avr-objcopy builds a Sketchname.cpp.hex file from the .elf file.
4. avrdude uploads the .hex file to the Atmega chip.

After you get the sketch built conventionally, with the IDE, its Temp directory will be full of various object files.
I re-built my sketch, from the Windows command line, adding the following to the avr-gcc invocation:
-Wl,-u,vfprintf,-lprintf_flt,-lm
(Those commas are probably important. See this AVR Libc manual section:
http://www.nongnu.org/avr-libc/user-manual/using_tools.html#using_pass_ld_opts )

That was inserted between the first and second existing arguments:
“avr-gcc -Os -Wl,–gc-sections” Wl OPTIONS HERE “-mmcu=atmega328p”.

And I changed the name of the .elf file from SketchName.cpp.elf to NewName.elf.

Then I changed the filenames in the avr-objcopy commands to match.
And I uploaded my NewName.hex file by similarly modifying the filename in the avrdude command.

So, it CAN be done. I just wish the IDE made it easier.