Please consider the following code:

// program.cpp

#include <iostream>

int main() {
#ifdef LINUX
    std::cout << "Linux\n";
#elif MAC
    std::cout << "Mac\n";
#elif WINDOWS
    std::cout << "Windows\n";
#elif BSD
    std::cout << "BSD\n";
#else
    std::cout << "Something else\n";
#endif
    return 0;
}

If I compile it with both clang and gcc,

clang   -Wundef -DBSD -o program program.cpp
# or
g   -Wundef -DBSD -o program program.cpp

I will get warnings for not defining symbols MAC and WINDOWS, but no warning for the symbol LINUX:

program.cpp:6:7: warning: 'MAC' is not defined, evaluates to 0 [-Wundef]
#elif MAC
      ^
program.cpp:8:7: warning: 'WINDOWS' is not defined, evaluates to 0 [-Wundef]
#elif WINDOWS
      ^
2 warnings generated.

According to the gcc man page:

-Wundef 
    Warn if an undefined identifier is evaluated in an "#if" directive.
    Such identifiers are replaced with zero.

• It says in an #if directive. Is it because the LINUX is not inside that structure?

• If that's the case, how can I tell compiler to emit warnings for the undefined symbol LINUX?

• clang version 12.0.1
• gcc (GCC) 11.1.0
• Target: x86_64-pc-linux-gnu (artixlinux)

CodePudding user response：

The reason is that your preprocessor code asks if LINUX is defined. But for MAC, WINDOWS and BSD you don’t bother checking whether the symbol is defined; instead, your code assumes it is defined and asks for its value.

Change your code to use #elif defined(…) instead of #elif … to fix the warning.