Question

I want to make a program work and compile in both WIN\Linux .
I want to have some information about the OS which my program is ran on.
moreover I want to have a variable to decide about the code i want to be performed.

I thought about a pre-processed code to have an input to my controlling variable which I described.
So, I must have something like:

#  //a preprocess code to detect the os
# define controllingVar // ?

I use C++;

Answer 1

You could check if the WIN32 macro has been defined:

#ifdef WIN32
   // do windows stuff
#else
   // do GNU/Linux stuff
#endif

Note that on some compilers, you might also have to check for _WIN32, as stated in wikipedia.

As an example:

#ifdef WIN32
   void foo() {
       std::cout << "I'm on Windows!\n";
   }
#else
   void foo() {
       std::cout << "I'm on GNU/Linux!\n";
   }
#endif

Edit: since you’re asking about different mains for each OS, here’s an example:

int main() {
#ifdef WIN32
   // do whatever you want when executing in a Windows OS
#else
   // do the same for GNU/Linux OS.
#endif
}

You could also have different mains:

#ifdef WIN32
   int main() {
       //windows main
   } 
#else
   int main() {
       //GNU/Linux main
   } 
#endif