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makefileg++c-preprocessorconditional-compilationpreprocessor-directive

Preprocessors directives have no effect

I am experiencing technical difficulties with preprocessor directives : 

#ifdef, #define

I have a program built by a Makefile, and I have 2 options two build it : standalone or embedded.
I did something like :

#ifdef _mdimode_
   //code for embedded
#else
   //code for standalone (default)
#endif

And in my main file when I compile in embedded purpose I wrote a :

#define _mdimode_

But it seems that g++ does not recognize or understand it. It always go in the else and never compile the code for embedded version.

Infos:
GNU Make 3.82
g++ (GCC) 4.6.1 20110908 (Red Hat 4.6.1-9)
file suffix : .C

Solution

  • This is a bit of a guess without more information. I assume the code is in a different file than the main file. If this is so then the #define doesn't propagate to that part of the project. You would have to set it in the file that contains the code, or in a header that is #included in it.

    You can also choose to set the #define in the options of the compilation command:

    g++ -c -D_mdimode_ mycode.C