C DEBUG macro (stop me if you've heard this)

[utzoo!decvax!harpo!floyd!vax135!ariel!houti!hogpc!houxm!houxz!hocda!spanky!burl!duke!mcnc!unc!tim]

I thought everyone knew this trick, but I've found out I'm wrong. This
is submitted for the benefit of those who haven't seen it before, and
others will kindly refrain from flaming.

Place the following piece of code at the beginning of a C source file,
or put it in a file and "# include" the file at the top of your program.

# ifdef DEBUG
# undef DEBUG
# define DEBUG printf
# else
# define DEBUG(arg)
# endif

Throughout your program, you should place lines of the form

DEBUG(msg,arg1,...) ;

The file is now capable of being compiled in two different ways.
Say that the file is "prog.c" for these examples.

(1) If you compile with the flag "-DDEBUG" on the command line, e.g.,
% cc -DDEBUG prog.c
then the DEBUG statements will turn into printf's, and print your
debugging information on the standard output when the program runs.

(2) If you compile without the "-DDEBUG" flag, then the DEBUG statements
will become null statements, for which the compiler generates no code.

Thus you can insert debugging probes all through any C functions, yet
never have to remove them, or compile in a special way, for the finished
version of your program. The only overhead (typing the "-DDEBUG" flag
and macroexpanding the DEBUG's) is negligible, and there is no extra
cost at execution time.

Of course, there are other ways of using this trick, but I decided to
submit a complete and usable example, and let people work out their
own variations if they wished.

Tim Maroney