-Wundef does not warn about an undefined symbol in front of #ifdef - c++

Question

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)

Answer 1

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.