question on conditional compilation

Applications Programming
1

Hey, can I #define something outside the source file ?

I have a C program which uses #ifdef..

#ifdef ABC
 ... do this..
#else
 ... that ...
#endif

The usual way that I know of defining ABC is in the source/header file

#define ABC

But is there any other way to do that ? Maybe as an option to pass to the compiler ?

The reason is, I just want to run one script/makefile which will compile both versions of the progra, with ABC defined and without ABC defined.. without having to make changes in the source code.

2

Indeed there are more options.

  1. Put the define in a common include file, such as 'config.h'

  2. Put the define in a compiler option, normally it's

$(CC) $(CFLAGS)  -DMYDEFINE[=myvalue] ....
  1. Put the define in a makefile variable
CFLAGS=$(CFLAGS) -DMYDEFINE[=myvalue]
3

Thanks buddy !!