How do you create a debug only function that takes a variable argument list? Like printf()

Question

I\'d like to make a debug logging function with the same parameters as printf. But one that can be removed by the pre-processor during optimized builds.

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Answer 1

@CodingTheWheel:

There is one slight problem with your approach. Consider a call such as

XTRACE("x=%d", x);

This works fine in the debug build, but in the release build it will expand to:

("x=%d", x);

Which is perfectly legitimate C and will compile and usually run without side-effects but generates unnecessary code. The approach I usually use to eliminate that problem is:

1. Make the XTrace function return an int (just return 0, the return value doesn't matter)

2. Change the #define in the #else clause to:

   0 && XTrace
   

Now the release version will expand to:

0 && XTrace("x=%d", x);

and any decent optimizer will throw away the whole thing since short-circuit evaluation would have prevented anything after the && from ever being executed.

Of course, just as I wrote that last sentence, I realized that perhaps the original form might be optimized away too and in the case of side effects, such as function calls passed as parameters to XTrace, it might be a better solution since it will make sure that debug and release versions will behave the same.

Answer 2

What platforms are they not available on? stdarg is part of the standard library:

http://www.opengroup.org/onlinepubs/009695399/basedefs/stdarg.h.html

Any platform not providing it is not a standard C implementation (or very, very old). For those, you will have to use varargs:

http://opengroup.org/onlinepubs/007908775/xsh/varargs.h.html

Answer 3

Another fun way to stub out variadic functions is:

#define function sizeof

Answer 4

Having come across the problem today, my solution is the following macro:

    static TCHAR __DEBUG_BUF[1024];
    #define DLog(fmt, ...)  swprintf(__DEBUG_BUF, fmt, ##__VA_ARGS__); OutputDebugString(__DEBUG_BUF) 
  

You can then call the function like this:

    int value = 42;
    DLog(L"The answer is: %d\n", value);

Answer 5

This is how I do debug print outs in C++. Define 'dout' (debug out) like this:

#ifdef DEBUG
#define dout cout
#else
#define dout 0 && cout
#endif

In the code I use 'dout' just like 'cout'.

dout << "in foobar with x= " << x << " and y= " << y << '\n';

If the preprocessor replaces 'dout' with '0 && cout' note that << has higher precedence than && and short-circuit evaluation of && makes the whole line evaluate to 0. Since the 0 is not used the compiler generates no code at all for that line.

Answer 6

This is what I use:

inline void DPRINTF(int level, char *format, ...)
{
#    ifdef _DEBUG_LOG
        va_list args;
        va_start(args, format);
        if(debugPrint & level) {
                vfprintf(stdout, format, args);
        }
        va_end(args);
#    endif /* _DEBUG_LOG */
}

which costs absolutely nothing at run-time when the _DEBUG_LOG flag is turned off.