f = va_arg(argp, double) returns 0

Projects Programming
1

Sorry guys, probably it's not directly related to Arduino, but to avr-gcc.

I'm trying to write custom printf function.

f = va_arg(argp, double)

returns 0.

Tried both on device(ATMega328P) and Atmel Studio 6 simulator, same result.

Sounds like something to do with linker options?

2

I'm trying to write custom printf function...returns 0.

Nothing in that snippet proves that there is any kind of problem.

3

I've tried to isolate the problem into separate program, but now Atmel Studio won't let me put breakpoint to see the value of f.
Here is the isolated code:

#include <avr/io.h>
#include <stdlib.h>
#include <stdarg.h>

void debug_print(const char *fmt, ...)
{
	const char *p;
	va_list argp;
	double f;
	char *s;
	char fmtbuf[80];
		
	va_start(argp, fmt);
	for(p = fmt; *p != '\0'; p++) 
	{
		if(*p != '%')
			continue;
		switch(*++p)
		{
			case 'f':
				f = va_arg(argp, double);
				s = dtostrf(f, 10, 6, fmtbuf);
				break;
		}			
	}		
	va_end(argp);
}	

int main(void)
{
	debug_print("%f", 123.456);
    while(1)
	{
    }
}

Do you know why Atmel Studio declines breakpoint? Device is set correct as in real project file, where breakpoints works just fine.

4

In that code, you may be assigning the correct value to f (it looks like you are), but I don't see you actually using that value.

5

Turns out to be optimization issue.

When I set compiler optimization to O1, then I got 0. When I set it to None, then I got correct result. Now I will update the code so you can see the usage and optimization will let simulator work.

6

With optimization -O1 you will see the problem, debugger said f optimized out and without optimization it works as expected.

So what exactly I have to do in order to avoid f optimizing out?

7

So what exactly I have to do in order to avoid f optimizing out?

Use the value. Presumably, you want to print it. So, print it.

8

Solved. Debugger is showing cached value (0) but actually it contains correct data.