how to run only when debugging

[John (Eljay) Love-Jensen]

Hi Manal,

> I am looking for an ANSI C preprocessor directive that can make code run only when I am in debugging mode (compiled with gcc -g option)

gcc -g does not introduce a define, you have to do that yourself.  As you can see from this diff:

diff <(gcc -g -E -dM -x c++ - <<<'' | sort) <(gcc -E -dM -x c++ - <<<'' | sort)

A rather widespread convention dating back to the misty dawn of time is that debug compiles *LACK* the NDEBUG preprocessor define (e.g., -UNDEBUG), and that release compiles declare the NDEBUG preprocessor define (e.g., -DNDEBUG).

So to compile your debug code, do this:

gcc -g

And to compile your release code, do this:

gcc -DNDEBUG

> I found something like:
#if DEBUG
  Do this in debug;
#else
  do this optionally if not in debug
#endif

Change that to:

#ifndef NDEBUG
  Do this in debug;
#else
  do this optionally if not in debug
#endif

> but it didn't work, as DEBUG is not defined,

Correct, since you did not define it.

For your team, you can adopt any of these other fairly prevalent conventions:

debug:  gcc -g -DDEBUG
debug:  gcc -g -D_DEBUG
release: gcc -DRELEASE
release: gcc -D_RELEASE

Note: _DEBUG is often seen in Microsoft C, C++, Visual C++ project debug builds, and NDEBUG in their release builds.

I prefer to use the NDEBUG indicating release, and absence of NDEBUG indicating debug.

debug: gcc -g -UNDEBUG
release: gcc -DNDEBUG

The <assert.h> or <cassert> headers are affected by the presence or absence of NDEBUG.

HTH,
--Eljay