multilevel Debug using c preprocessor macros - c

Question

I have excerpts of code for a multi-level debug implementation, which I don't know how to make it work.
I have tried some of the suggestions posted here about using the do {...} while (0), and I also tried to declare a global variable called Debug, but none of them seem to work.
What should I do?
#include <stdio.h>
#include <stdlib.h>
//#define DEBUG(fmt, ...) fprintf (stderr, fmt, __VA_ARGS__ )
#ifdef DEBON
#define DEBUG(level, fmt, ...) \
if (Debug >= level) \
fprintf (stderr, fmt, __VA_ARGS__)
#else
#define DEBUG(level, fmt, ...)
#endif
int process ( int i1, int i2)
{
int val;
DEBUG (1, "process (%i, %i)\n", i1, i2);
val = i1 * i2;
DEBUG (3, "return %i\n", val);
return val;
}
int main ( int argc, char *argv[])
{
int arg1 = 0, arg2 = 0;
if (argc > 1)
arg1 = atoi (argv[1]);
if ( argc == 3)
arg2 = atoi (argv[2]);
DEBUG (1, "processed %i arguments\n", argc -1 );
DEBUG (3, "arg1 = %i, arg2 = %i\n" , arg1, arg2);
printf ("%d\n", process (arg1, arg2));
return 0;
}
Below is a simpler file I put together to see if I can get this working. No luck yet.
#define DEBUG(level, fmt, ...) if (Debug >= level ) fprintf (stderr, fmt, __VA_ARGS__)
#include <stdio.h>
int Debug;
int main()
{
int i1 = 1;
int i2 = 2;
DEBUG(3, "process (%i, %i)\n", i1, i2);
}

Answer 1

You last example works after minor tweaks. The idea in the book is to have your program parse the command line options and set the value of a global variable named Debug.
#include <stdio.h>
#include <stdlib.h>
#define DEBUG(level, fmt, ...) do if (Debug >= level) fprintf(stderr, fmt, __VA_ARGS__); while (0)
int Debug = 1; // default debug level == 1
int main(int argc, char *argv[])
{
int i1 = 1;
int i2 = 2;
size_t i;
for(i = 1; i < argc; ++i)
if ((argv[i][0] == '-') && (argv[i][1] == 'd'))
Debug = strtol(argv[i] + 2, 0, 10);
DEBUG(3, "process (%i, %i)\n", i1, i2);
return 0;
}