Detect if I'm building Executable or Dynamic Library? - c++

Question

I have two versions of my app, one works as a dynamic library loaded from a host app and another one works as a standalone executable app.
In my code, there are parts where I need to use different code depending on which version I'm building.
Is there a way to detect if I'm building a dynamic library or a standalone executable app so I can use the same source files for the two versions?
I would appreciate a cross-platform solution.
ADDED:
My dynamic lib or executable code:
#include <iostream>
int main(int argc, const char * argv[])
{
#ifdef IS_DYNAMICLIB
std::cout << "Compiled as a dynamic library!\n";
#else
std::cout << "Compiled as an executable!\n";
#endif
return 0;
}
In the terminal on macOS and I build it using the command:
g++ -dynamiclib -undefined suppress -flat_namespace main.cpp -o Test.dylib
Now, I have Test.dylib
Host app code:
#include <iostream>
#include <dlfcn.h>
int main(int argc, const char * argv[])
{
void* handle;
typedef void (*func_t)();
handle = dlopen("/Path/to/Test.dylib", RTLD_LAZY);
if (!handle) {
printf("failed to open the library\n");
return 0;
}
func_t mainFunc = (func_t) dlsym(handle, "main");
if (!mainFunc) {
printf("failed to find main method\n");
dlclose(handle);
return 0;
}
mainFunc();
return 0;
}
When I build and run this, I get:
Compiled as an executable!
Program ended with exit code: 0
But I would like it to print:
Compiled as a dynamic library!
Program ended with exit code: 0
How can I do this?