How do I generate an error or warning in the C preprocessor?

Question

I have a program that must be compiled only in DEBUG mode. (testing purpose)

How can I have the preprocessor prevent compilation in RELEASE mode?

nobugz_17034 · Answer

Place anywhere:

#ifndef DEBUG
#error Only Debug builds are supported
#endif

Philippe_18804 · Answer

C provide a #error statement, and most compilers add a #warning statement. The gcc documentation recommends to quote the message.

user_121278 · Answer

You can use a error directive for that. The following code will throw an error at compile time if DEBUG is not defined:

#ifndef DEBUG
#error This is an error message
#endif

Admin · Answer

If you simply want to report an error:

#ifdef RELEASE
  #error Release mode not allowed
#endif

will work with most compilers.